Ukraine 2008. – Pp. 107-108. **Part 2**

**Lubrication Tests and Biodegradable Lubricants** 

172 Tribology - Lubricants and Lubrication

Scientific-Technical Conference "МТ-2008", Kiev, 5–7 June 2008). – Kiev: IPS NAS

**6** 

*Malaysia* 

**Experimental Evaluation on Lubricity of** 

Tiong Chiong Ing1, Mohammed Rafiq Abdul Kadir2, Nor Azwadi Che Sidik3 and Syahrullail Samion3 *1School of Graduates Studies, Universiti Teknologi Malaysia,* 

*3Faculty of Mechanical Engineering, Universiti Teknologi Malaysia,* 

**RBD Palm Olein Using Fourball Tribotester** 

*2Faculty of Biomedical Engineering and Health Science, Universiti Teknologi Malaysia,* 

Tribology is defined as "the science and technology of surface interacting in motion". Thus it is important for us to understand the surface interaction when they are loaded together as to understand the tribology process occurring in the system. The physical, chemical and mechanical properties not only cause the effects to the surface material in tribology behavior but also the near surface material. Apart from that, on the surface of the bulk material, lies a layer formed as a result from the manufacturing process. This deformed layer is covered by a compound layer resulting of chemical reaction of metal with the environmental substance such as air. In addition, the machining process such as cutting lubricants to be trapped may also cause the deformed regions of the surface. The regions on the surface material can critically affect both friction and wear of metals. In addition, the forces which arise from the contact of solid bodies in relative motion also affect both friction and wear. Thus, it is important for us to understand the mechanics contact of solid bodies in order to evaluate the friction and wear on solid bodies. Solid bodies are subjected to an increasing load deform elastically until the stress reaches a limit or maximum yield stress then deform plastically

Friction is known as resistance to motion. Friction can be categorized into five types; which are dry friction, fluid friction, lubricated friction, skin friction and internal friction. The friction forces are divided into two types; static friction force which is required to initiate sliding, and kinetic friction force which is required to maintain sliding. Coefficient of friction is known as the constant of proportionality in which the typical two materials may be similar or dissimilar, sliding against each other under a given set of surfaces and

The first laboratory test device for determining lubricant quality was known as fourball tribotester is proposed by Boerlage in the year of 1993 (Ivan, 1980). The concept of friction for this machine is three stationary balls pressed against a rotating ball. The quality and the characteristics of the lubricant were established by the size of the wear scar or the seizure load and the value of friction obtained. The main elements of fourball machine are vertical driving shaft which hold the moving ball at the lower end with conical devices. Besides that,

**1. Introduction** 

(Gohar and Rahnejat, 2008).

environmental conditions (Arnell and Davies, 1991).

### **Experimental Evaluation on Lubricity of RBD Palm Olein Using Fourball Tribotester**

Tiong Chiong Ing1, Mohammed Rafiq Abdul Kadir2,

Nor Azwadi Che Sidik3 and Syahrullail Samion3 *1School of Graduates Studies, Universiti Teknologi Malaysia, 2Faculty of Biomedical Engineering and Health Science, Universiti Teknologi Malaysia, 3Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, Malaysia* 

#### **1. Introduction**

Tribology is defined as "the science and technology of surface interacting in motion". Thus it is important for us to understand the surface interaction when they are loaded together as to understand the tribology process occurring in the system. The physical, chemical and mechanical properties not only cause the effects to the surface material in tribology behavior but also the near surface material. Apart from that, on the surface of the bulk material, lies a layer formed as a result from the manufacturing process. This deformed layer is covered by a compound layer resulting of chemical reaction of metal with the environmental substance such as air. In addition, the machining process such as cutting lubricants to be trapped may also cause the deformed regions of the surface. The regions on the surface material can critically affect both friction and wear of metals. In addition, the forces which arise from the contact of solid bodies in relative motion also affect both friction and wear. Thus, it is important for us to understand the mechanics contact of solid bodies in order to evaluate the friction and wear on solid bodies. Solid bodies are subjected to an increasing load deform elastically until the stress reaches a limit or maximum yield stress then deform plastically (Gohar and Rahnejat, 2008).

Friction is known as resistance to motion. Friction can be categorized into five types; which are dry friction, fluid friction, lubricated friction, skin friction and internal friction. The friction forces are divided into two types; static friction force which is required to initiate sliding, and kinetic friction force which is required to maintain sliding. Coefficient of friction is known as the constant of proportionality in which the typical two materials may be similar or dissimilar, sliding against each other under a given set of surfaces and environmental conditions (Arnell and Davies, 1991).

The first laboratory test device for determining lubricant quality was known as fourball tribotester is proposed by Boerlage in the year of 1993 (Ivan, 1980). The concept of friction for this machine is three stationary balls pressed against a rotating ball. The quality and the characteristics of the lubricant were established by the size of the wear scar or the seizure load and the value of friction obtained. The main elements of fourball machine are vertical driving shaft which hold the moving ball at the lower end with conical devices. Besides that,

Experimental Evaluation on Lubricity of RBD Palm Olein Using Fourball Tribotester 177

potential of palm oil as fuels for diesel engines (Kinoshita et al, 2003; Bari et al, 2002), hydraulic fluid (Wan Nik et al, 2002), and lubricants (Syahrullail et al., 2011) has been confirmed in previous studies. In addition, Malaysia is one of the world's largest palm oil

Throughout all the previous studies, the characteristics of RBD palm olein were investigated using fourball tribotester. The objective of this experiment is to study the lubricity characteristics of vegetable oils compared to the petroleum based oil. RBD palm olein and additive free paraffinic mineral oil were used as lubricants in this experiment. RBD palm olein is a refined, bleached and deodorized palm olein product and it exists in liquid state at room temperature. Fourball tester was used in this experiment to evaluate the lubricity of those lubricants. The lubricity performance of RBD palm olein and non-aditive paraffinic mineral oil were compared mutually. The experiments were carried out at the temperature of 75°C for one hour duration. Besides that, the load applied on the fourball tester was 40 kg (392.4N). Apart from that, the speed of spindle was set to 1200 rpm. At the end of the experiments, the evaluations of lubricants focused on the friction and wear of each lubricant. From the experiments, the authors confirmed that RBD palm olein showed satisfactory lubrication performance as compared to additive free paraffinic mineral oil, especially in

Ball bearing

Applied force (upward)

The fourball wear tester machine was first described by Boerlage to have acquired the status of an established institution in the fundamental investigation of lubricants characteristics (Boerlage, 1933). In this research, the fourball wear tester was used. This instrument uses four balls, three at the bottom and one on top. The bottom three balls are held firmly in a ball pot containing the lubricant under test and pressed against the top ball. The top ball is made to rotate at the desired speed while the bottom three balls are pressed against it. The important components are ballpot (oil cup) assembly, collet, locknut adaptor and standard

Thermocouple

producers.

terms of friction reduction.

Oil cup

Fig. 1. A schematic sketch of the fourball tribotester

**2. Experimental procedures 2.1 Experimental apparatus** 

Collet

three stationary balls which are fixed by a conical ring and lock nut are pressed by the moving ball. The stationary ball holder is mounted on an axial bearing so that it can rotate and displace in the vertical direction freely. In addition, a lever device is used to apply load on stationary balls. The friction occurring on the fixed stationary balls by the rotating ball is transmitted by means of a lever to the measuring device. The wear is viewed based on the size of the wear scar on the stationary balls. 12.7mm diameter of balls is usually used. These are specially processed to ensure high dimensional accuracy as well as uniform hardness and surface quality. The tested lubricant was immersed into the stationary balls cup hold with desire volume. Apart from that, the speed for rotating ball depends on the type of machine and the experiment conditions. There are several standards and specifications for fourball machine: such as Socialist Republic of Romania State Standard 8618-70; FTM no. 791 a/6503; ASTM D2596-67 and DIN 51350 (Ivan, 1980).

Boundary lubrication is defined as a condition of lubrication in which the friction and wear between two surfaces in relative motion are determined by the properties of lubricant. Lubrication is critical for minimizing the wear in mechanical systems that operate for extended time period. Developing lubricants that can be used in engineering systems without replenishment is very important for increasing the functional lifetime of mechanical components. The additives usually to be added in to the base oil to improve its performance. Joseph Perez stated that the number of additives and the amount present depends on the application (Joseph and Waleska, 2005). They are selected to enhance the base oil performance so that they will meet the system requirement.

The increasing and wide usage of petro and synthetic based oil overwhelm the lubricant industry because the major damage to the environment and the rise of concern about health and environmental damage caused by the mineral oil based lubricant; have created a growing worldwide trend of promoting vegetable oil as based oil in industries. Biodegradable oils are becoming an important alternative to conventional lubricants as a result of awareness of ecological pollution and their detrimental effects on our lives. The use of vegetable oils in industrial sector is not a new idea. They had been used in the construction of monuments in Ancient Egypt (Nosonovsky, 2000). Vegetable oil with high stearic acid content is considered to be potential candidates as the substitute for conventional mineral oil based lubricants because they are biodegradable and non toxic. Besides that, they have better intrinsic boundary lubricant properties because of the presence of long chain fatty acids in their composition (Carcel and Palomares, 2004). Other advantages include very low volatility due to the high molecular weight of triglyceride molecule and excellent temperature viscosity properties. Their polar ester groups are able to adhere to metal surface and therefore possess good lubricating ability. In addition, vegetable oils have high solution power for polar contaminants and additive molecules (Sevim et al, 2006). Vegetable oils show good lubricating abilities as they give rise to low coefficient of friction. However, many researchers report that although the co-efficiency of friction is low with vegetable oil as boundary lubricant, the wear rate is high. This behavior is possible due to the chemical attack on the surface by the fatty acid present in vegetable oil. The metallic soap film is rubbed away during sliding and producing the non-reactive detergents increase in wear (Bowden and Tabor, 2001).

In western country, the common vegetable oils that have been widely used in the tribology test are sunflower oil, rapeseed oil and corn oil. For this research, the authors used RBD palm olein as test oil and evaluated its friction and wear performance using fourball tribotester. Nowadays, palm oil has been widely tested for engineering applications. The

three stationary balls which are fixed by a conical ring and lock nut are pressed by the moving ball. The stationary ball holder is mounted on an axial bearing so that it can rotate and displace in the vertical direction freely. In addition, a lever device is used to apply load on stationary balls. The friction occurring on the fixed stationary balls by the rotating ball is transmitted by means of a lever to the measuring device. The wear is viewed based on the size of the wear scar on the stationary balls. 12.7mm diameter of balls is usually used. These are specially processed to ensure high dimensional accuracy as well as uniform hardness and surface quality. The tested lubricant was immersed into the stationary balls cup hold with desire volume. Apart from that, the speed for rotating ball depends on the type of machine and the experiment conditions. There are several standards and specifications for fourball machine: such as Socialist Republic of Romania State Standard 8618-70; FTM no. 791

Boundary lubrication is defined as a condition of lubrication in which the friction and wear between two surfaces in relative motion are determined by the properties of lubricant. Lubrication is critical for minimizing the wear in mechanical systems that operate for extended time period. Developing lubricants that can be used in engineering systems without replenishment is very important for increasing the functional lifetime of mechanical components. The additives usually to be added in to the base oil to improve its performance. Joseph Perez stated that the number of additives and the amount present depends on the application (Joseph and Waleska, 2005). They are selected to enhance the base oil

The increasing and wide usage of petro and synthetic based oil overwhelm the lubricant industry because the major damage to the environment and the rise of concern about health and environmental damage caused by the mineral oil based lubricant; have created a growing worldwide trend of promoting vegetable oil as based oil in industries. Biodegradable oils are becoming an important alternative to conventional lubricants as a result of awareness of ecological pollution and their detrimental effects on our lives. The use of vegetable oils in industrial sector is not a new idea. They had been used in the construction of monuments in Ancient Egypt (Nosonovsky, 2000). Vegetable oil with high stearic acid content is considered to be potential candidates as the substitute for conventional mineral oil based lubricants because they are biodegradable and non toxic. Besides that, they have better intrinsic boundary lubricant properties because of the presence of long chain fatty acids in their composition (Carcel and Palomares, 2004). Other advantages include very low volatility due to the high molecular weight of triglyceride molecule and excellent temperature viscosity properties. Their polar ester groups are able to adhere to metal surface and therefore possess good lubricating ability. In addition, vegetable oils have high solution power for polar contaminants and additive molecules (Sevim et al, 2006). Vegetable oils show good lubricating abilities as they give rise to low coefficient of friction. However, many researchers report that although the co-efficiency of friction is low with vegetable oil as boundary lubricant, the wear rate is high. This behavior is possible due to the chemical attack on the surface by the fatty acid present in vegetable oil. The metallic soap film is rubbed away during sliding and producing the non-reactive detergents increase

In western country, the common vegetable oils that have been widely used in the tribology test are sunflower oil, rapeseed oil and corn oil. For this research, the authors used RBD palm olein as test oil and evaluated its friction and wear performance using fourball tribotester. Nowadays, palm oil has been widely tested for engineering applications. The

a/6503; ASTM D2596-67 and DIN 51350 (Ivan, 1980).

performance so that they will meet the system requirement.

in wear (Bowden and Tabor, 2001).

potential of palm oil as fuels for diesel engines (Kinoshita et al, 2003; Bari et al, 2002), hydraulic fluid (Wan Nik et al, 2002), and lubricants (Syahrullail et al., 2011) has been confirmed in previous studies. In addition, Malaysia is one of the world's largest palm oil producers.

Throughout all the previous studies, the characteristics of RBD palm olein were investigated using fourball tribotester. The objective of this experiment is to study the lubricity characteristics of vegetable oils compared to the petroleum based oil. RBD palm olein and additive free paraffinic mineral oil were used as lubricants in this experiment. RBD palm olein is a refined, bleached and deodorized palm olein product and it exists in liquid state at room temperature. Fourball tester was used in this experiment to evaluate the lubricity of those lubricants. The lubricity performance of RBD palm olein and non-aditive paraffinic mineral oil were compared mutually. The experiments were carried out at the temperature of 75°C for one hour duration. Besides that, the load applied on the fourball tester was 40 kg (392.4N). Apart from that, the speed of spindle was set to 1200 rpm. At the end of the experiments, the evaluations of lubricants focused on the friction and wear of each lubricant. From the experiments, the authors confirmed that RBD palm olein showed satisfactory lubrication performance as compared to additive free paraffinic mineral oil, especially in terms of friction reduction.

Fig. 1. A schematic sketch of the fourball tribotester

#### **2. Experimental procedures**

#### **2.1 Experimental apparatus**

The fourball wear tester machine was first described by Boerlage to have acquired the status of an established institution in the fundamental investigation of lubricants characteristics (Boerlage, 1933). In this research, the fourball wear tester was used. This instrument uses four balls, three at the bottom and one on top. The bottom three balls are held firmly in a ball pot containing the lubricant under test and pressed against the top ball. The top ball is made to rotate at the desired speed while the bottom three balls are pressed against it. The important components are ballpot (oil cup) assembly, collet, locknut adaptor and standard

Experimental Evaluation on Lubricity of RBD Palm Olein Using Fourball Tribotester 179

started the drive motor which had been set to drive the top ball at 1200 rpm. For the duration of one hour, the heater was turn off and the oil cup assembly was removed from the machine. Then, the test oil in the oil cup was drained off and wear scar area was wiped using tissue. The wear scars on the bottom balls were put on a special base of a microscope

Density of fluids is defined as the unit of mass per volume. A laboratory experiment had been carried out to measure the density of RBD palm olein and paraffinic mineral oil. The result was shown in Table 1. Dynamic viscosity is a measure of the resistance of a fluid which is formed by either shear stress or tensile stress of the fluids. It is also known as the internal friction of the fluids. A viscometer was used to measure the viscosity for both lubricants. Viscometer rotor was immerged into the lubricants to evaluate it fluidity by turning the rotor for 99 seconds. The viscosity of the RBD palm olein and paraffinic mineral oil was shown in the Figure 3. The viscosity of both lubricants dropped as the temperature of the lubricants increase. The lesser the viscosity of the fluids, the easier the particles will

Test oil RBD palm olein Paraffinic mineral oil

RBD palm olein Paraffinic mineral oil

40 60 80 100

Temperature (oC)

Density at 25ºC (kg/m3) 915 848 Flash point (ºC) 315-330 140-180 Pour point (ºC) 18-24 -20

Table 1. Properties of RBD palm olein and paraffinic mineral oil

Fig. 3. Viscosity curves of RBD palm olein and paraffinic mineral oil

that has been designed for the purpose. All tests were repeat several times.

**3. Results and discussions** 

**3.1 Density and viscosity** 

move in the fluids.

0

5

10

15

20

Viscosity (mPa.s)

25

30

35

40

steel balls. The components surface needs to be clean with acetone before the tests. The amount of lubricant test is 10 ml.

#### **2.2 Test lubricants**

The tested lubricants for this experiment were RBD palm olein and additive free paraffinic mineral oil (written as paraffinic mineral oil). The RBD is an abbreviation for refined, bleached and deodorized. As shown in Figure 2, RBD palm olein is the liquid fraction that is obtained by the fractionation of palm oil after crystallization at a controlled temperature (Pantzaris, 2000). In these experiments, a standard grade of RBD palm olein, which was incorporated in the Malaysian Standard MS 816:1991, was used. The amount for all lubricant tests is 10 ml.

Fig. 2. Refining method of RBD palm olein

#### **2.3 Experimental procedures**

The wear tests were carried out under the ASTM method D-4172 method B with the applied load of 392.4 N (40 kg) at a spindle speed of 1200 revolution per minute (rpm). The experiment was carried out for duration of one hour and conducted under the temperature of 75 degree Celsius. The three bottoms stationary balls in the wear test were evaluated the average diameter of the circular scar formed. Besides that, the lubricating ability of the RBD palm olein was also being evaluated based on the friction torque produced compared with the additive free paraffinic mineral oil. All parts in fourball (upper ball, lower balls and oil cup) were cleaned thoroughly using acetone and wiped using a fresh lint free industrial wipe. There should not be any trace of solvent remain when the test oil was introduced and the parts were assembled. The steel balls were placed into the ballpot assembly and to be tightened using torque wrench. This purpose was to prevent the bottoms steel balls from moving during the experiment. The top spinning ball was locked inside the collector and tightened into the spindle. 10 ml of test lubricant (RBD palm olein or paraffinic mineral oil) was to be poured into the ballpot assembly. Apart from that, researcher should note or observe that this oil level filled all the voids in the test cup assembly. The ballpot assembly components were installed onto the non-friction disc in the four-ball machine and avoided shock loading by slowly applying the test load up to 392.4 N. After that, the lubricant used was heated up to 75 degree Celsius. When the set temperature was reached, researcher started the drive motor which had been set to drive the top ball at 1200 rpm. For the duration of one hour, the heater was turn off and the oil cup assembly was removed from the machine. Then, the test oil in the oil cup was drained off and wear scar area was wiped using tissue. The wear scars on the bottom balls were put on a special base of a microscope that has been designed for the purpose. All tests were repeat several times.

#### **3. Results and discussions**

#### **3.1 Density and viscosity**

178 Tribology - Lubricants and Lubrication

steel balls. The components surface needs to be clean with acetone before the tests. The

The tested lubricants for this experiment were RBD palm olein and additive free paraffinic mineral oil (written as paraffinic mineral oil). The RBD is an abbreviation for refined, bleached and deodorized. As shown in Figure 2, RBD palm olein is the liquid fraction that is obtained by the fractionation of palm oil after crystallization at a controlled temperature (Pantzaris, 2000). In these experiments, a standard grade of RBD palm olein, which was incorporated in the Malaysian Standard MS 816:1991, was used. The amount for all lubricant

Fractionation and refining

Liquid fraction Solid fraction

RBD palm olein RBD palm stearin

The wear tests were carried out under the ASTM method D-4172 method B with the applied load of 392.4 N (40 kg) at a spindle speed of 1200 revolution per minute (rpm). The experiment was carried out for duration of one hour and conducted under the temperature of 75 degree Celsius. The three bottoms stationary balls in the wear test were evaluated the average diameter of the circular scar formed. Besides that, the lubricating ability of the RBD palm olein was also being evaluated based on the friction torque produced compared with the additive free paraffinic mineral oil. All parts in fourball (upper ball, lower balls and oil cup) were cleaned thoroughly using acetone and wiped using a fresh lint free industrial wipe. There should not be any trace of solvent remain when the test oil was introduced and the parts were assembled. The steel balls were placed into the ballpot assembly and to be tightened using torque wrench. This purpose was to prevent the bottoms steel balls from moving during the experiment. The top spinning ball was locked inside the collector and tightened into the spindle. 10 ml of test lubricant (RBD palm olein or paraffinic mineral oil) was to be poured into the ballpot assembly. Apart from that, researcher should note or observe that this oil level filled all the voids in the test cup assembly. The ballpot assembly components were installed onto the non-friction disc in the four-ball machine and avoided shock loading by slowly applying the test load up to 392.4 N. After that, the lubricant used was heated up to 75 degree Celsius. When the set temperature was reached, researcher

amount of lubricant test is 10 ml.

Fresh fruit brunches

Mill process

Crude palm oil

Refining

RBD palm oil

Fig. 2. Refining method of RBD palm olein

**2.3 Experimental procedures** 

**2.2 Test lubricants** 

tests is 10 ml.

Density of fluids is defined as the unit of mass per volume. A laboratory experiment had been carried out to measure the density of RBD palm olein and paraffinic mineral oil. The result was shown in Table 1. Dynamic viscosity is a measure of the resistance of a fluid which is formed by either shear stress or tensile stress of the fluids. It is also known as the internal friction of the fluids. A viscometer was used to measure the viscosity for both lubricants. Viscometer rotor was immerged into the lubricants to evaluate it fluidity by turning the rotor for 99 seconds. The viscosity of the RBD palm olein and paraffinic mineral oil was shown in the Figure 3. The viscosity of both lubricants dropped as the temperature of the lubricants increase. The lesser the viscosity of the fluids, the easier the particles will move in the fluids.


Table 1. Properties of RBD palm olein and paraffinic mineral oil

Fig. 3. Viscosity curves of RBD palm olein and paraffinic mineral oil

Experimental Evaluation on Lubricity of RBD Palm Olein Using Fourball Tribotester 181

Paraffinic mineral oil

Fig. 5. Friction torque curves for RBD palm olein and paraffinic mineral oil

0 1200 2400 3600 Time (s)

> PO P2 Test lubricant

Fig. 6. Wear scar diameter for RBD palm olein and paraffinic mineral oil

was recalculated to obtain the mean or average wear scar diameter for each lubricant test. Figure 6 illustrates the average wear scar diameter of fourball tribotester for RBD palm olein and paraffinic mineral oil. The average wear scar diameter measured for RBD palm olein is larger than paraffinic mineral oil in this experiment. RBD palm olein shows 0.828 mm and paraffinic mineral oil shows 0.764 mm in wear scar diameter. In addition, this result is totally opposite with the result of friction. The wear increases as the friction decreases as shown in Figure 4 and Figure 5. This due to the increased shear strength of the adsorbed oil on the surface of the balls and affected chemical attack on the surface by the fatty acid

RBD palm olein

0

0.7

0.75

0.8

Wear scar diameter (mm)

0.85

0.9

present in vegetable oil (Bowden and Tabor, 2001).

0.02

0.04

0.06

0.08

Friction torque (N.m)

0.12

0.14

0.16

0.1

#### **3.2 Friction**

*F N* = μ⋅

The friction coefficient (*μ*) between two solid surfaces is defined as the ratio of the tangential force (*F*) which required sliding, and is divided by the normal force between the surfaces (*N*) (Jamal, 2008). Coefficient of friction for RBD palm olein and paraffinic mineral oil had been obtained using the relevant software. Figure 4 shows the value of coefficient of friction at steady state for both lubricants in the fourball tribotester. The coefficient of friction for RBD palm olein is lower than paraffinic mineral oil. As shown in Figure 5 the steady state friction torque for RBD palm olein is lower than paraffin mineral oil, thus the steady state coefficient of friction also shows the same trend of result. From the experiment, the value of coefficient of friction for RBD palm olein is 0.065 while the value of coefficient of friction for paraffinic mineral oil is 0.075.

Fig. 4. Coefficient of friction for RBD palm olein and paraffinic mineral oil at steady state condition

Few series of wear tests had been conducted using fourball tribotester. Figure 4 illustrates the friction torque obtained for RBD palm olein and paraffinic mineral oil using fourball tribotester along the period of experiments. The trend of graph for both lubricants was similar to each other. The friction torque for both lubricants was increased along the period of experiments. In Figure 5 the friction torque of RBD palm olein is lower than paraffinic mineral oil. The value of friction torque at steady state for RBD palm olein and paraffinic mineral oil is 0.12 Nm and 0.14 Nm respectively. Based on the previous study, the long chain of fatty acids present in the palm oil has the potential to reduce the friction constraint (Abdulquadir and Adeyemi, 2008).

#### **3.3 Wear**

The wear scar on the surface of balls bearing was obtained and measured using the CCD microscope and its specific software. The measured wear scar diameter on the balls bearing

*F N* = μ ⋅ The friction coefficient (*μ*) between two solid surfaces is defined as the ratio of the tangential force (*F*) which required sliding, and is divided by the normal force between the surfaces (*N*) (Jamal, 2008). Coefficient of friction for RBD palm olein and paraffinic mineral oil had been obtained using the relevant software. Figure 4 shows the value of coefficient of friction at steady state for both lubricants in the fourball tribotester. The coefficient of friction for RBD palm olein is lower than paraffinic mineral oil. As shown in Figure 5 the steady state friction torque for RBD palm olein is lower than paraffin mineral oil, thus the steady state coefficient of friction also shows the same trend of result. From the experiment, the value of coefficient of friction for RBD palm olein is 0.065 while the value of coefficient of friction for paraffinic

RBD palm olein Paraffinic mineral oil

Fig. 4. Coefficient of friction for RBD palm olein and paraffinic mineral oil at steady state

Few series of wear tests had been conducted using fourball tribotester. Figure 4 illustrates the friction torque obtained for RBD palm olein and paraffinic mineral oil using fourball tribotester along the period of experiments. The trend of graph for both lubricants was similar to each other. The friction torque for both lubricants was increased along the period of experiments. In Figure 5 the friction torque of RBD palm olein is lower than paraffinic mineral oil. The value of friction torque at steady state for RBD palm olein and paraffinic mineral oil is 0.12 Nm and 0.14 Nm respectively. Based on the previous study, the long chain of fatty acids present in the palm oil has the potential to reduce the friction constraint

The wear scar on the surface of balls bearing was obtained and measured using the CCD microscope and its specific software. The measured wear scar diameter on the balls bearing

**3.2 Friction** 

mineral oil is 0.075.

0.04

(Abdulquadir and Adeyemi, 2008).

0.045

0.05

0.055

0.06

Coefficient of friction

condition

**3.3 Wear** 

0.065

0.07

0.075

0.08

Fig. 5. Friction torque curves for RBD palm olein and paraffinic mineral oil

was recalculated to obtain the mean or average wear scar diameter for each lubricant test. Figure 6 illustrates the average wear scar diameter of fourball tribotester for RBD palm olein and paraffinic mineral oil. The average wear scar diameter measured for RBD palm olein is larger than paraffinic mineral oil in this experiment. RBD palm olein shows 0.828 mm and paraffinic mineral oil shows 0.764 mm in wear scar diameter. In addition, this result is totally opposite with the result of friction. The wear increases as the friction decreases as shown in Figure 4 and Figure 5. This due to the increased shear strength of the adsorbed oil on the surface of the balls and affected chemical attack on the surface by the fatty acid present in vegetable oil (Bowden and Tabor, 2001).

Fig. 6. Wear scar diameter for RBD palm olein and paraffinic mineral oil

Experimental Evaluation on Lubricity of RBD Palm Olein Using Fourball Tribotester 183

mineral oil. Besides that, from the observation of scar view using CCD microscope, the scar surface of balls lubricated with RBD palm olein looks smoother than paraffinic mineral oil.

The authors wish to thank the Faculty of Mechanical Engineering at the Universiti Teknologi Malaysia for their support and cooperation during this study. The authors also wish to thank the Research University Grant from the Universiti Teknologi Malaysia, the Ministry of Higher Education (MOHE) and the Ministry of Science, Technology and Innovation

Abdulquadir, B.A. and Adeyemi, M.B., 2008, "Evaluations of Vegetable Oil-Based as

Arnell, R.D., Davies, P.B. and Halling, J., 1991, "Tribology-Principles and Design

Bari, S., Lim, T.H. and Yu, C.W., 2002, "Effect of Preheating of Crude Palm Oil (CPO) on

Boerlage, G.D., 1933, "Four-ball Testing Apparatus for Extreme-pressure Lubricants,"

Bowden, F.P. and Tabor, D., 2001, "The Nature of Metallic Wear. The Friction and

Carcel, A.D. and Palomares, D., 2004, "Evaluation of Vegetable Oils as Pre-Lube Oils for

Jamal Takadoum, 2008, "Materials and Surface Engineering in Tribology," John Wiley &

Joseph, M.P. and Waleska, C., 2005, "The Effect of Chemical Structure of Basefluids on Antiwear Effectiveness of Additives", Tribology International, Vol. 38, pp.321-326. Kinoshita, E., Hamasaki, K. and Jaqin, C., 2003, "Diesel Combustion of Palm Oil Methyl

Meng Hua and Jian Li, 2008, "Friction and Wear Behavior of SUS 304 Austenitic Stainless

Nosonovsky, M., 2000, "Oil as a Lubricant in the Ancient Middle East", Tribology Online,

Syahrullail, S., Zubil, B.M., Azwadi, C.S.N. and Ridzuan, M.J.M., 2011. Experimental

Pantzaris, T.P., 2000, "Pocketbook of Palm Oil Uses," Malaysian Palm Oil Board.

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Gohar, R and Rahnejat, H., 2008, "Fundamentals of Tribology", Imperial College Press. Ivan Iliuc, 1980, Tribology of Thin Layers", Elsevier Scientific Publishing Company.

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Ester", SAE, 2003, Paper No. 2003-01-1929.

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Lubrication of Solids," Oxford Classic Texts. New York: Oxford University Press;

**5. Acknowledgement** 

**6. References** 

60, pp.242-248.

pp.285-98.

Sons, Inc.

pp.799-810.

Vol. 2-2, pp.44-49.

(MOSTI) of Malaysia for their financial support.

Energy, Vol. 27, pp.339-351.

Engineering, Vol. 136, pp.46-47.

Wear scar track that was lubricated with RBD palm olein and paraffinic mineral oil had been viewed and captured using the microscope. The enlargement of wear scar track for both tested oils is shown in Figure 6. The wear scar track on the ball bearing lubricated with RBD palm olein shows smoother surface than wear scar track lubricated with paraffin mineral oil on the surface of ball bearing. The wear scar worn on the ball bearing lubricated with paraffin mineral oil has more ploughed traces or grooves as the result of material transfer. The narrower and deeper of groove on the wear traces would be the sources of roughening the surface of ball bearing after the experiments (Meng and Jian, 2008).

Fig. 7. Observation of the wear scar condition for RBD palm olein and paraffinic mineral oil

#### **4. Conclusion**

The lubricating ability of RBD palm olein had been evaluated using the fourball tribotester. All the results were compared mutually with the additive free paraffinic mineral oil. For the reduction in friction, RBD palm olein shows better result compared to the additive free paraffinic mineral oil. RBD palm olein shows lower coefficient of friction and friction torque compared to the paraffinic mineral oil. This behavior is related to the long chain fatty acid in the RBD palm olein. However in wear, due to the increasing shear strength of the RBD palm olein on the surface of the balls, it shows larger wear scar diameter compared to the paraffin mineral oil. Besides that, from the observation of scar view using CCD microscope, the scar surface of balls lubricated with RBD palm olein looks smoother than paraffinic mineral oil.

#### **5. Acknowledgement**

182 Tribology - Lubricants and Lubrication

Wear scar track that was lubricated with RBD palm olein and paraffinic mineral oil had been viewed and captured using the microscope. The enlargement of wear scar track for both tested oils is shown in Figure 6. The wear scar track on the ball bearing lubricated with RBD palm olein shows smoother surface than wear scar track lubricated with paraffin mineral oil on the surface of ball bearing. The wear scar worn on the ball bearing lubricated with paraffin mineral oil has more ploughed traces or grooves as the result of material transfer. The narrower and deeper of groove on the wear traces would be the sources of roughening

Fig. 7. Observation of the wear scar condition for RBD palm olein and paraffinic mineral oil

The lubricating ability of RBD palm olein had been evaluated using the fourball tribotester. All the results were compared mutually with the additive free paraffinic mineral oil. For the reduction in friction, RBD palm olein shows better result compared to the additive free paraffinic mineral oil. RBD palm olein shows lower coefficient of friction and friction torque compared to the paraffinic mineral oil. This behavior is related to the long chain fatty acid in the RBD palm olein. However in wear, due to the increasing shear strength of the RBD palm olein on the surface of the balls, it shows larger wear scar diameter compared to the paraffin

**4. Conclusion** 

the surface of ball bearing after the experiments (Meng and Jian, 2008).

The authors wish to thank the Faculty of Mechanical Engineering at the Universiti Teknologi Malaysia for their support and cooperation during this study. The authors also wish to thank the Research University Grant from the Universiti Teknologi Malaysia, the Ministry of Higher Education (MOHE) and the Ministry of Science, Technology and Innovation (MOSTI) of Malaysia for their financial support.

#### **6. References**


**7** 

*Brazil* 

**Biodegradable Lubricants and** 

José André Cavalcanti da Silva

**Their Production Via Chemical Catalysis** 

The primary purpose of this chapter is to describe the differences among biolubricants and petroleum-based lubricants, especially their production and physical and chemical properties. Established production methodology will be described, especially those using chemical catalysis that have been developed at the laboratories of the Petrobras Research

 Today there is growing concern about the future availability of petroleum-based products. In addition, millions of tons of lubricants are dumped into the environment through leakage, exhaust gas and careless disposal. Some of these wastes are resistant to biodegradation and are threats to the environment. Thus, there are two major issues confronting the lubricant industries today: the search for raw materials that are renewable

The oleochemistry represents a significant challenge to biolubricants production by petroleum companies. All the required technologies from seed crushing to oil refining, fractionation and chemical transformation are in place. The main research emphasis has been placed on ways to produce biolubricants with suitable viscosity and liquid-state temperature range. In addition, these lubricants must not corrode the machinery they lubricate and they must be stable under the conditions of their use. These requirements eliminate many simple fatty acid esters. Saturated esters with long enough chains to not be too volatile or lacking in viscosity are solids in the temperature range required by many lubricant applications. Double bonds will lower their melting point but introduce instability to oxygen attack, especially for the typical polyunsaturated fatty acid found in most vegetable oils. Branching will reduce the melting point but such fatty acids are relatively rare in nature. The solution to these problems that will be described on this chapter emphasizes the use of Brazilian raw materials. The well-developed Brazilian program of biodiesel production from soybean and castor oils has led to the choice of ricinoleate esters

Castor oil is produced in the seed of the castor oil plant, *Ricinus communis*, and has been used for medicinal purposes for many years. During the 20th century, a number of industrial

uses were developed including its use as a lubricant (Azevedo & Lima, 2001).

**1. Introduction** 

Center (CENPES), in Rio de Janeiro, Brazil.

and products that are biodegradable.

as potential biolubricant ingredients.

**2. Castor and its derivatives** 

*Petróleo Brasileiro S.A. – Petrobras / Research Center – CENPES* 


## **Biodegradable Lubricants and Their Production Via Chemical Catalysis**

José André Cavalcanti da Silva *Petróleo Brasileiro S.A. – Petrobras / Research Center – CENPES Brazil* 

#### **1. Introduction**

184 Tribology - Lubricants and Lubrication

Sevim, Z.E, Brajendra, K.S. and Joseph, M.P., 2006, "Oxidation and Low Temperature

Wan Nik, W.B., Ani, F.N. and Masjuki, H., 2002, "Thermal Performances of Bio-fluid as

Combustion and Energy Utilization, Kuala Lumpur, Malaysia, pp.558-563.

24, pp.292-299.

Stability of Vegetable Oil-Based Lubricants", Industrial Crops and Products, Vol.

Energy Transport Media", The 6th Asia Pacific International Symposium on

The primary purpose of this chapter is to describe the differences among biolubricants and petroleum-based lubricants, especially their production and physical and chemical properties. Established production methodology will be described, especially those using chemical catalysis that have been developed at the laboratories of the Petrobras Research Center (CENPES), in Rio de Janeiro, Brazil.

 Today there is growing concern about the future availability of petroleum-based products. In addition, millions of tons of lubricants are dumped into the environment through leakage, exhaust gas and careless disposal. Some of these wastes are resistant to biodegradation and are threats to the environment. Thus, there are two major issues confronting the lubricant industries today: the search for raw materials that are renewable and products that are biodegradable.

The oleochemistry represents a significant challenge to biolubricants production by petroleum companies. All the required technologies from seed crushing to oil refining, fractionation and chemical transformation are in place. The main research emphasis has been placed on ways to produce biolubricants with suitable viscosity and liquid-state temperature range. In addition, these lubricants must not corrode the machinery they lubricate and they must be stable under the conditions of their use. These requirements eliminate many simple fatty acid esters. Saturated esters with long enough chains to not be too volatile or lacking in viscosity are solids in the temperature range required by many lubricant applications. Double bonds will lower their melting point but introduce instability to oxygen attack, especially for the typical polyunsaturated fatty acid found in most vegetable oils. Branching will reduce the melting point but such fatty acids are relatively rare in nature. The solution to these problems that will be described on this chapter emphasizes the use of Brazilian raw materials. The well-developed Brazilian program of biodiesel production from soybean and castor oils has led to the choice of ricinoleate esters as potential biolubricant ingredients.

#### **2. Castor and its derivatives**

Castor oil is produced in the seed of the castor oil plant, *Ricinus communis*, and has been used for medicinal purposes for many years. During the 20th century, a number of industrial uses were developed including its use as a lubricant (Azevedo & Lima, 2001).

Biodegradable Lubricants and Their Production Via Chemical Catalysis 187

The hydroxyl group of castor oil increases its polarity and makes it a better solvent for lubricant additives than other vegetable or mineral oils. Besides, castor oil presents high viscosity and low pour point, but its viscosity index is lower than the others vegetable oils,

Castor oil has been used on the manufacturing of more than 800 products, ranging from bullet-proof glasses, contact lenses, lipsticks, metal soaps, special engine and high rotation reactors lubricants, high resistance plastics, polyurethanes, etc. Its odd properties give lubricity to the mineral diesel, like sulfur, becoming a special oil in the current world

The major castor seeds and oil producing nations in order of their production are India, China and Brazil. Germany and Thailand are the greatest castor beans importers (94%), but

The state of Bahia produces 85% of Brazil's production of castor oil, being together with the state of Minas Gerais, the states where are located the main oil extraction companies. Brazil produces about 160,000 metric tons of beans per year. As the internal consumption of castor oil is small (10,000-15,000 metric tons per year), there is an excess of about 45,000-50,000

The term "base oils" refers to the various oils used in the world's technological applications. This chapter will focus on lubricant oils. The base oils are the larger proportion constituents at the lubricants formulations and most of them are derived from petroleum. They can be classified as mineral or synthetic oils, depending on their production history (Lastres, 2003). The first known lubricants used by humans were animal and vegetable based oils. In the 19th Century, the natural triglycerides were replaced by petroleum based oils, called mineral oils. In some lubricants applications, certain performance standards are required that cannot be met by conventional mineral oils. Alternate processes have been devised for their production usually to achieve greater durability or lower environmental impact. Vegetable

Mineral oils are produced through the petroleum distillation and refining. They are classified in paraffinics, naphthenics and aromatics, depending on the hydrocarbon type predominant in its composition. They possess 20 to 50 carbon atoms, on average, per molecule, and these can be paraffinic chains (linear or branching alkanes), naphthenic chains (cicloalkanes with side chains) or aromatic chains (alkyl benzenes), as illustrated on the

The paraffinic base oils owe high pour point and viscosity index. To produce them, the dewax step is very important and the product, even dewaxed, still needs to be additivated with a pour point depressor to avoid the wax crystals growth at low temperatures and to

The naphthenic base oils possess higher levels of carbons in cycle chains (naphthenics) than the paraffinics. The cut of a naphthenic petroleum has low linear wax levels and does not need to be dewaxed. Its pour point can achieve -51°C (base oil NH-10). On the other hand, they have low VI values (becoming very hard their usage on the engine oil formulations). They are more used on the formulations of cutfluids, shock absorbers oils and as isolation fluid to electrical transformers. The aromatic oils are used as extensor oils at the rubber

oils are less expensive than minerals and are produced from renewable resources.

which means that its viscosity changes more with temperature than the other oils.

market.

**3. Base oils** 

figure 2.

industry.

the United States consumes the most castor oil.

metric tons per year for export.

reduce the product flow temperature.

Castor oil was introduced into Brazil by the Portuguese for use as in illumination and as a carriage shaft lubricant. The climate of Brazil is suitable for growing castor plants and it can be found today among the wild flora in many parts of Brazil as well as a drought resistant cultivated plant.

From its seeds industrialization is obtained, as main product, the oil (47%) and, as byproduct, the castor waste that may be used as a fertilizer.

Castor oil posses unusual and has greater density, viscosity, ethanol solubility and lubricity compared with other vegetable oil. This oil also has a wide chemical versatility inside the industry, due to be used as raw material to the synthesis of a large amount of products.

Furthermore, we can obtain biodiesel from castor oil, which replaces the petroleum-derived diesel as fuel. Besides, this oil posses the unusually fatty acid, ricinoleic acid, which makes about 90% of its composition. Ricinoleic acid is similar to the common fatty acid, oleic acid, except it has a hydroxyl group on the 12th carbon of its 18 carbon chain. Like oleic acid, ricinoleic acid has a *cis* double bond between the 9th and 10th carbon, as can be seen in figure 1.

Fig. 1. Castor oil molecular structure (*Ricinus Communis*)

Table 1 presents the main physical-chemical characteristics of this oil.


RPVOT: Rotary pressure vessel oxidation test.

Table 1. Typical castor oil physical-chemical characteristics

The hydroxyl group of castor oil increases its polarity and makes it a better solvent for lubricant additives than other vegetable or mineral oils. Besides, castor oil presents high viscosity and low pour point, but its viscosity index is lower than the others vegetable oils, which means that its viscosity changes more with temperature than the other oils.

Castor oil has been used on the manufacturing of more than 800 products, ranging from bullet-proof glasses, contact lenses, lipsticks, metal soaps, special engine and high rotation reactors lubricants, high resistance plastics, polyurethanes, etc. Its odd properties give lubricity to the mineral diesel, like sulfur, becoming a special oil in the current world market.

The major castor seeds and oil producing nations in order of their production are India, China and Brazil. Germany and Thailand are the greatest castor beans importers (94%), but the United States consumes the most castor oil.

The state of Bahia produces 85% of Brazil's production of castor oil, being together with the state of Minas Gerais, the states where are located the main oil extraction companies. Brazil produces about 160,000 metric tons of beans per year. As the internal consumption of castor oil is small (10,000-15,000 metric tons per year), there is an excess of about 45,000-50,000 metric tons per year for export.

#### **3. Base oils**

186 Tribology - Lubricants and Lubrication

Castor oil was introduced into Brazil by the Portuguese for use as in illumination and as a carriage shaft lubricant. The climate of Brazil is suitable for growing castor plants and it can be found today among the wild flora in many parts of Brazil as well as a drought resistant

From its seeds industrialization is obtained, as main product, the oil (47%) and, as by-

Castor oil posses unusual and has greater density, viscosity, ethanol solubility and lubricity compared with other vegetable oil. This oil also has a wide chemical versatility inside the industry, due to be used as raw material to the synthesis of a large amount of products. Furthermore, we can obtain biodiesel from castor oil, which replaces the petroleum-derived diesel as fuel. Besides, this oil posses the unusually fatty acid, ricinoleic acid, which makes about 90% of its composition. Ricinoleic acid is similar to the common fatty acid, oleic acid, except it has a hydroxyl group on the 12th carbon of its 18 carbon chain. Like oleic acid, ricinoleic acid has a *cis* double bond between the 9th and 10th carbon, as can be seen in figure 1.

product, the castor waste that may be used as a fertilizer.

Fig. 1. Castor oil molecular structure (*Ricinus Communis*)

RPVOT: Rotary pressure vessel oxidation test.

Table 1. Typical castor oil physical-chemical characteristics

Table 1 presents the main physical-chemical characteristics of this oil.

Property Value Iodine Index 84-88 Viscosity at 100°C 20.00 cSt VI (Viscosity Index) 90 Melting Point -23°C Ricinoleic Acid Content 90% Linoleic Acid Content 4.2% Oxidative Stability by RPVOT 25 Min.

cultivated plant.

The term "base oils" refers to the various oils used in the world's technological applications. This chapter will focus on lubricant oils. The base oils are the larger proportion constituents at the lubricants formulations and most of them are derived from petroleum. They can be classified as mineral or synthetic oils, depending on their production history (Lastres, 2003).

The first known lubricants used by humans were animal and vegetable based oils. In the 19th Century, the natural triglycerides were replaced by petroleum based oils, called mineral oils. In some lubricants applications, certain performance standards are required that cannot be met by conventional mineral oils. Alternate processes have been devised for their production usually to achieve greater durability or lower environmental impact. Vegetable oils are less expensive than minerals and are produced from renewable resources.

Mineral oils are produced through the petroleum distillation and refining. They are classified in paraffinics, naphthenics and aromatics, depending on the hydrocarbon type predominant in its composition. They possess 20 to 50 carbon atoms, on average, per molecule, and these can be paraffinic chains (linear or branching alkanes), naphthenic chains (cicloalkanes with side chains) or aromatic chains (alkyl benzenes), as illustrated on the figure 2.

The paraffinic base oils owe high pour point and viscosity index. To produce them, the dewax step is very important and the product, even dewaxed, still needs to be additivated with a pour point depressor to avoid the wax crystals growth at low temperatures and to reduce the product flow temperature.

The naphthenic base oils possess higher levels of carbons in cycle chains (naphthenics) than the paraffinics. The cut of a naphthenic petroleum has low linear wax levels and does not need to be dewaxed. Its pour point can achieve -51°C (base oil NH-10). On the other hand, they have low VI values (becoming very hard their usage on the engine oil formulations). They are more used on the formulations of cutfluids, shock absorbers oils and as isolation fluid to electrical transformers. The aromatic oils are used as extensor oils at the rubber industry.

Biodegradable Lubricants and Their Production Via Chemical Catalysis 189

advantage, in general, higher thermal and oxidative stability, better low temperature properties and lower volatility when compared to mineral oils. However, these base oils are

Applications that require high level of biodegradability need to use vegetable based

Regarding the automotive oils, the American Petroleum Institute, API, classifies the base oils

The lubricant's performance is evaluated by their friction reduction, oxidation resistance, deposits formation minimization, corrosion and wear avoiding abilities. The main problem with lubricants is related to the oil degradation and its contamination by the engine combustion by-products (automotives). Thus, the main causes of engine bad working, regarding the lubricant quality, are due to deposit formation, viscosity increase, high

Deposit formation occurs when the detergent/dispersant power of the lubricant is not enough to keep the contaminants in suspension. The oil thickness results from the lubricant oxidation and the insolubles material accumulation. The viscosity increases due to the oxygenated compounds polymerization and to the insoluble products in suspension, derived from the irregular fuel burning. The sulfur level in the diesel mays cause corrosion and wear on the cylinders and rings, because of the sulfur acids or organic acids attack on the iron surfaces. To avoid this attack, lubricants with a good alkaline reserve must be used. To minimize such problems, lubricants are obtained from the mixture between base oils and additives. These additives have antioxidant, antiwear, detergent and dispersant, and others functions. Therefore, to design a lubricant to play all these roles is a hard task which

The world final lubricants market is about 38,000,000 tons/year (Whitby, 2005). The US market is about 9.5 millions tones, from which 32% are discarded on the environment (Lal & Carrick, 1993). On the other hand, the European biodegradable lubricants market is 172,000

From the 1.3 million tons German lubricants market, 53% are collected as used oil, which is equivalent to 100% of all oil collection of the several applications. These used oils are recycled or used as thermal energy source. The remainder is lost to the environment as leakages, total loss applications or specific systems. Only 5% of all lubricants from the German market are biodegradable (Wagner et al., 2001). To increase this market, one must increase the acceptance and the trust on the biodegradable lubricants and decrease its price. Nearly 13% (Europe) and 32% (USA) of all commercialized lubricants return to the environment with properties and appearance modified (Bartz, 1998). These are used on total loss wear contacts, approximately 40,000 tons/year in Germany, and on circulation systems, which are not collected neither disposed. Besides, one must take account of the lubricants from leakages and the remainder amounts in filters and recipients. Thus, the German environment is exposed to nearly 150,000 tons/year, based on the 13% previously cited. A calculation based on the current lubricants consumption in Germany and on the discard rates for the different lubricants results in nearly 250,000 tons/year. Including the not defined amount (leakages, etc.), the lubricants discarded amount on the German environment may reach 300,000 tons/year. Taking account of the lubricants market share represented by Germany, as well as the fact that in many places around the world the collect

tons/year, concentrated on Germany and Scandinavian countries (Whitby, 2006).

involves a careful evaluation of the base oils and additives properties.

more expensive than mineral oils.

consumption, corrosion and wear.

in five categories as illustrated on the table 2.

synthetic base oils.

**4. Biolubricants** 


Fig. 2. Structure of the mineral oils composition


(1) ASTM D 2007

(2) ASTM D 2622 or ASTM D 4294 or ASTM D 4927 or ASTM D 3120

(3) ASTM D 2270

Table 2. Base oils API classification

Mineral base oils can also be classified by the production process. The most common is the solvent extraction, or conventional process, where compounds like aromatics and compounds that contain heteroatoms, as nitrogen and sulfur, are removed, increasing the VI and improving the products stability. This process also includes dewax steps, in order to reach the specified pour point, and hydrotreatment, to improve the products specifications. The non conventional process includes more severe steps of hydrocracking, where the molecules are cracked and saturated, with very stable and high VI final products.

On the other hand, synthetic base oils are produced through chemical reactions. Approximately 80% of the synthetic lubricant world market is composed by: polyalphaolefins (45%), organic esters (25%) and polyglycols (10%) (Murphy et al., 2002). The most used synthetic base oils are the polyalphaolefins, and the synthetic oils have as an advantage, in general, higher thermal and oxidative stability, better low temperature properties and lower volatility when compared to mineral oils. However, these base oils are more expensive than mineral oils.

Applications that require high level of biodegradability need to use vegetable based synthetic base oils.

Regarding the automotive oils, the American Petroleum Institute, API, classifies the base oils in five categories as illustrated on the table 2.

The lubricant's performance is evaluated by their friction reduction, oxidation resistance, deposits formation minimization, corrosion and wear avoiding abilities. The main problem with lubricants is related to the oil degradation and its contamination by the engine combustion by-products (automotives). Thus, the main causes of engine bad working, regarding the lubricant quality, are due to deposit formation, viscosity increase, high consumption, corrosion and wear.

Deposit formation occurs when the detergent/dispersant power of the lubricant is not enough to keep the contaminants in suspension. The oil thickness results from the lubricant oxidation and the insolubles material accumulation. The viscosity increases due to the oxygenated compounds polymerization and to the insoluble products in suspension, derived from the irregular fuel burning. The sulfur level in the diesel mays cause corrosion and wear on the cylinders and rings, because of the sulfur acids or organic acids attack on the iron surfaces. To avoid this attack, lubricants with a good alkaline reserve must be used.

To minimize such problems, lubricants are obtained from the mixture between base oils and additives. These additives have antioxidant, antiwear, detergent and dispersant, and others functions. Therefore, to design a lubricant to play all these roles is a hard task which involves a careful evaluation of the base oils and additives properties.

#### **4. Biolubricants**

188 Tribology - Lubricants and Lubrication

Oil type Carbon chains type

CATEGORY SATURATES (1) SULFUR, %P (2) VISCOSITY

GROUP I < 90 and / or > 0.03 80 - 120

GROUP II ≥ 90 and ≤ 0.03 80 - 120

GROUP III ≥ 90 and ≤ 0.03 > 120

GROUP V OTHER BASE OILS NOT INCLUDED ON THE GROUPS I, II, III and IV

Mineral base oils can also be classified by the production process. The most common is the solvent extraction, or conventional process, where compounds like aromatics and compounds that contain heteroatoms, as nitrogen and sulfur, are removed, increasing the VI and improving the products stability. This process also includes dewax steps, in order to reach the specified pour point, and hydrotreatment, to improve the products specifications. The non conventional process includes more severe steps of hydrocracking, where the

On the other hand, synthetic base oils are produced through chemical reactions. Approximately 80% of the synthetic lubricant world market is composed by: polyalphaolefins (45%), organic esters (25%) and polyglycols (10%) (Murphy et al., 2002). The most used synthetic base oils are the polyalphaolefins, and the synthetic oils have as an

molecules are cracked and saturated, with very stable and high VI final products.

GROUP IV POLYALPHAOLEFINS (PAO)

(2) ASTM D 2622 or ASTM D 4294 or ASTM D 4927 or ASTM D 3120

INDEX (3)

Paraffinics

Naphthenics

Aromatics

(1) ASTM D 2007

(3) ASTM D 2270

Table 2. Base oils API classification

Fig. 2. Structure of the mineral oils composition

The world final lubricants market is about 38,000,000 tons/year (Whitby, 2005). The US market is about 9.5 millions tones, from which 32% are discarded on the environment (Lal & Carrick, 1993). On the other hand, the European biodegradable lubricants market is 172,000 tons/year, concentrated on Germany and Scandinavian countries (Whitby, 2006).

From the 1.3 million tons German lubricants market, 53% are collected as used oil, which is equivalent to 100% of all oil collection of the several applications. These used oils are recycled or used as thermal energy source. The remainder is lost to the environment as leakages, total loss applications or specific systems. Only 5% of all lubricants from the German market are biodegradable (Wagner et al., 2001). To increase this market, one must increase the acceptance and the trust on the biodegradable lubricants and decrease its price.

Nearly 13% (Europe) and 32% (USA) of all commercialized lubricants return to the environment with properties and appearance modified (Bartz, 1998). These are used on total loss wear contacts, approximately 40,000 tons/year in Germany, and on circulation systems, which are not collected neither disposed. Besides, one must take account of the lubricants from leakages and the remainder amounts in filters and recipients. Thus, the German environment is exposed to nearly 150,000 tons/year, based on the 13% previously cited. A calculation based on the current lubricants consumption in Germany and on the discard rates for the different lubricants results in nearly 250,000 tons/year. Including the not defined amount (leakages, etc.), the lubricants discarded amount on the German environment may reach 300,000 tons/year. Taking account of the lubricants market share represented by Germany, as well as the fact that in many places around the world the collect

Biodegradable Lubricants and Their Production Via Chemical Catalysis 191

oxidative and hydrolytic stabilities of the vegetable oils. The most important modifications occur on the carboxyl groups of the fatty acids, approximately 90%, while oleochemical

Esters, similar substances to triglycerides in terms of chemical structure, are excellent replacements for mineral oils, which possess only 20% of biodegradability (CEC-L-33-A-93). The organic esters are a growing interest in the base lubricants industry and its advantages

• High viscosity index (VI), due to the double bonds and the molecule linearity;

These disadvantages can be minimized by additives, but the biodegradability, the toxicity and the price can be endangered. Thus, the chemical synthesis of these compounds seems to

The additives used traditionally are antioxidant, antiwear, anticorrosion, etc. These agents have low biodegradability. However, the additives industry has increased efforts on the

Organic esters have a wide diversity of applications in the lubricant industry because of the growing awareness of health and environment beneficial aspects, besides the benefits from better products performance: chain-saw, drilling fluids, food industry equipments,

Critical points

reactions on the fatty acid chain are approximately 10%.

compared to mineral base oil are (Lal & Carrick, 1993):

Fig. 3. Typical structure of a vegetable oil and its instability critical points

O

α

β

γ

O

O

• Low toxicity;

• High flash point; • Low volatility;

• High added value;

• Oxidative instability; • Hydrolitic instability; • Low temperature properties.

**5. Esters** 

O

O

O

• Higher biodegradability;

• Obtained from renewable sources;

• High additives solvency power;

• Good lubricity (due to molecule polarity);

However, the main disadvantages of these compounds are:

be the best choice to overcome these disadvantages.

development of biodegradable additives.

and recycling rates of used lubricants are lower than in Europe, the total amount of lubricants returning to the environment is about 12 million tons/year.

Only 10-50% of the lubricants used on the world market are recycled (Kolwzan & Gryglewicz, 2003). The remainder, which represents millions of tons, is disposed irreversibly on the environment through leakages, oil-water emulsions, components exhaust gases, etc. Some of them are carcinogenic and resistant to biodegradation, representing a serious menace to the environment. One of the solutions to modify this situation is replacing mineral oils with biodegradable synthetic lubricants.

In the last decades, there has been an increased worldwide concern about the environmental impact from the petroleum derivatives usage. Although only approximately 1% of all consumed petroleum be used on the lubricants formulations, the most part of these products are disposed in the environment without any treatment and this concern has driven the biodegradable lubricants development.

The pollution potential of the mineral oil is extremely high. For example, 1 liter of mineral oil contaminates 1 million liters of water for the human consumption (Ravasio et al., 2002).

Regarding the 2 strokes engines (currently, the main use of biolubricants), the lubrication mechanism results in the release of unburned oil, together with exhaust gases, promoting the possibility of environmental pollution. Furthermore, when using these engines in rivers, lakes or oceans, the unburned oil, released in the water, can become a possible pollution source. Tractors, agricultural machines, chain-saws, and other forest equipments, may pollute forests and rivers, as well due to the unburned released oil.

Measures to reduce the environmental impact of lubricants, that means to eliminate or decrease the problems caused by lubricant contact, are driven by the following forces: environmental facts, public awareness, government rules, market globalization and economic incentives.

A biolubricant is a biodegradable lubricant. A substance is called biodegradable when it presents the proved capacity of being decomposed within 1 year, through natural biological processes in carbonaceous land, water and carbon dioxide (Whitby, 2005).

In general, biodegradability means a lubricant trend to be metabolized by microorganisms within 1 year. When it is complete, it means that the lubricant has essentially been back to Nature, but when it partially decomposes, one or more lubricant compounds are not biodegradable.

Some of the readily biodegradable lubricants are based on pure unmodified vegetable oils (Wagner et al., 2001), that present a biodegradability of about 99% (CEC L-33-A-93) (Birova et al., 2002). In Europe there is a predominance of sunflower and rapeseed oils, which are esters of glycerin and long chain fatty acids (triglycerides). The fatty acids are specific for each plant, being variable. The fatty acids found in natural vegetable oils differ in chain length and in their double carbon bond number. Moreover, function groups may be present. Natural triglycerides are highly biodegradable and efficient as lubricants. However, their thermal, oxidative and hydrolytic stabilities are limited. Thus, pure vegetable oils are used only on applications with low thermal requirements, as unmolding and chain-saws.

The reasons for the thermal and oxidative instabilities of the vegetable oils are the double bonds in the fatty acid molecule and the group β-CH in the alcohol counterpart (figure 3). Double bonds are especially reactive and react immediately with the air oxygen, while the hydrogen β atom is easily eliminated from the molecule structure. This results in the ester breakage in olefins and acids. A further weak point of the esters is its trend to undergo hydrolysis in the presence of water. Chemical modifications may improve the thermal, oxidative and hydrolytic stabilities of the vegetable oils. The most important modifications occur on the carboxyl groups of the fatty acids, approximately 90%, while oleochemical reactions on the fatty acid chain are approximately 10%.

Fig. 3. Typical structure of a vegetable oil and its instability critical points

Esters, similar substances to triglycerides in terms of chemical structure, are excellent replacements for mineral oils, which possess only 20% of biodegradability (CEC-L-33-A-93). The organic esters are a growing interest in the base lubricants industry and its advantages compared to mineral base oil are (Lal & Carrick, 1993):

• Low toxicity;

190 Tribology - Lubricants and Lubrication

and recycling rates of used lubricants are lower than in Europe, the total amount of

Only 10-50% of the lubricants used on the world market are recycled (Kolwzan & Gryglewicz, 2003). The remainder, which represents millions of tons, is disposed irreversibly on the environment through leakages, oil-water emulsions, components exhaust gases, etc. Some of them are carcinogenic and resistant to biodegradation, representing a serious menace to the environment. One of the solutions to modify this situation is replacing

In the last decades, there has been an increased worldwide concern about the environmental impact from the petroleum derivatives usage. Although only approximately 1% of all consumed petroleum be used on the lubricants formulations, the most part of these products are disposed in the environment without any treatment and this concern has

The pollution potential of the mineral oil is extremely high. For example, 1 liter of mineral oil contaminates 1 million liters of water for the human consumption (Ravasio et al., 2002). Regarding the 2 strokes engines (currently, the main use of biolubricants), the lubrication mechanism results in the release of unburned oil, together with exhaust gases, promoting the possibility of environmental pollution. Furthermore, when using these engines in rivers, lakes or oceans, the unburned oil, released in the water, can become a possible pollution source. Tractors, agricultural machines, chain-saws, and other forest equipments, may

Measures to reduce the environmental impact of lubricants, that means to eliminate or decrease the problems caused by lubricant contact, are driven by the following forces: environmental facts, public awareness, government rules, market globalization and

A biolubricant is a biodegradable lubricant. A substance is called biodegradable when it presents the proved capacity of being decomposed within 1 year, through natural biological

In general, biodegradability means a lubricant trend to be metabolized by microorganisms within 1 year. When it is complete, it means that the lubricant has essentially been back to Nature, but when it partially decomposes, one or more lubricant compounds are not

Some of the readily biodegradable lubricants are based on pure unmodified vegetable oils (Wagner et al., 2001), that present a biodegradability of about 99% (CEC L-33-A-93) (Birova et al., 2002). In Europe there is a predominance of sunflower and rapeseed oils, which are esters of glycerin and long chain fatty acids (triglycerides). The fatty acids are specific for each plant, being variable. The fatty acids found in natural vegetable oils differ in chain length and in their double carbon bond number. Moreover, function groups may be present. Natural triglycerides are highly biodegradable and efficient as lubricants. However, their thermal, oxidative and hydrolytic stabilities are limited. Thus, pure vegetable oils are used

only on applications with low thermal requirements, as unmolding and chain-saws.

The reasons for the thermal and oxidative instabilities of the vegetable oils are the double bonds in the fatty acid molecule and the group β-CH in the alcohol counterpart (figure 3). Double bonds are especially reactive and react immediately with the air oxygen, while the hydrogen β atom is easily eliminated from the molecule structure. This results in the ester breakage in olefins and acids. A further weak point of the esters is its trend to undergo hydrolysis in the presence of water. Chemical modifications may improve the thermal,

lubricants returning to the environment is about 12 million tons/year.

pollute forests and rivers, as well due to the unburned released oil.

processes in carbonaceous land, water and carbon dioxide (Whitby, 2005).

mineral oils with biodegradable synthetic lubricants.

driven the biodegradable lubricants development.

economic incentives.

biodegradable.


However, the main disadvantages of these compounds are:


These disadvantages can be minimized by additives, but the biodegradability, the toxicity and the price can be endangered. Thus, the chemical synthesis of these compounds seems to be the best choice to overcome these disadvantages.

The additives used traditionally are antioxidant, antiwear, anticorrosion, etc. These agents have low biodegradability. However, the additives industry has increased efforts on the development of biodegradable additives.

#### **5. Esters**

Organic esters have a wide diversity of applications in the lubricant industry because of the growing awareness of health and environment beneficial aspects, besides the benefits from better products performance: chain-saw, drilling fluids, food industry equipments,

Biodegradable Lubricants and Their Production Via Chemical Catalysis 193

H+

O H

+ H2O R <sup>C</sup> O R'

O

When one follows the reaction clockwise, this is the direction of a carboxylic acid esterification, catalyzed by acid. If, however, one follows the counterclockwise, this is the mechanism of an ester hydrolysis, catalyzed by acid. The final result will depend on the choice conditions to the reaction. If the goal is to ersterify an acid, one uses an alcohol excess and if it is possible, one promotes the water removal as it is formed. However, if the goal is

The steric hindrance strongly affects the reaction rates of the ester hydrolysis catalyzed by acids. The presence of large groups near to the reaction center in the alcohol component or

Esters can be synthesized through transesterification reactions (figure 6). In this process, the equilibrium is shifted towards the products, allowing the alcohol, with the lower boiling point, to be distilled from the reactant mixture. The transesterification mechanism is similar to the one of a catalyzed by acid esterification (or to the one of a catalyzed by acid ester

+ R

H

O

H

Fig. 4. Esterification reaction scheme between a carboxylic acid and an alcohol

R C

R C

+ R' OH


O

OR' + H2O

R C OH

H O

H O R'

R C O

<sup>H</sup> - H2O

O R''

C

O <sup>+</sup> R' OH H+

H H O

O R'

R C

R C

O H


Fig. 5. Esterification reaction mechanism

the hydrolysis, one uses a large water excess.

in the acid component retards the reaction.

<sup>O</sup> R' R'' OH

Fig. 6. Transesterification reaction between an ester and an alcohol

O

hydrolysis).

C

O

R

O

O

OH <sup>+</sup> R' OH

+ H+ - H<sup>+</sup>

+ H+

hydraulic fluids, boat engines, 2 stroke engines, tractors, agriculture equipments, cut fluids, cooling fluids, etc (Erhan & Asadauskas, 2000).

Esters have been used as lubricants since the beginning of the 19th Century, in the form of natural esters in pig fat and whale oil (Whitby, 1998). During World War II, a large number of synthetic fluids were developed such as alcohol and long chain acids esters, that presented excellent low temperature properties.

Nowadays, the esters represent only 0.8% of the world lubricants market. However, while the global consumption of lubricants has been stagnant, the consumption of synthetic oils has grown approximately 10% per year. This growing esters consumption is due to performance reasons and also to changes on the environmental laws of several European Community countries, mainly Germany.

Esters have a low environmental impact and its metabolization consists of the following steps: ester hydrolysis, beta-oxidation of long chain hydrocarbons and oxygenases attack to aromatic nucleus. The main characteristics that reduce the microbial metabolization or degradability are:


The strongest effect of the ester group on the lubricant physical properties is a decrease in its volatility and increase in its flash point. This is due to the strong dipole moment (London forces) that keeps the ester molecules together. The ester group affects other properties, too such as: thermal and hydrolytic stabilities, solvency, lubricity and biodegradability. Besides, esters, mainly from polyalcohols, as trimethylolpropane (TMP), produce a unimolecular layer on the metal surface, protecting it against wear. This layer is produced by the oxygen atoms which are presents in the ester molecules.

The ester's most important physical-chemistry properties are viscosity, viscosity index (VI), pour point, lubricity, thermal and hydrolytic stabilities and solvency.

The main esters used as biolubricants are: diesters, phthalates, trimethilates, C36 dimerates and polyolesters. The polyolesters are formed from polyols with one quaternary carbon atom (neopentylalcohols), as trimethylolpropane, neopentylglycol and pentaerythritol. This class of compounds is very stable due to the absence of a secondary hydrogen on the β position and to the presence of a central quaternary carbon atom (Wagner et al., 2001). The main applications to the esters are: engine oil, 2 stroke engine oils, compressor oils, cooling fluids, aviation fluids and hydraulic fluids.

#### **5.1 Synthesis of biolubricant esters**

According to (Solomons, 1983), the carboxylic acids react with alcohols to produce esters, through a condensation reaction called esterification (figure 4). This reaction is catalyzed by acids and the equilibrium is achieved in a few hours, when an alcohol and an acid are heated under reflux with a small amount of sulfuric acid or hydrochloric acid. Since the equilibrium constant controls the amount of produced ester, an excess of the carboxylic acid or of the alcohol increases the yield of the ester. The compound choice to use in excess will depend on its availability and cost. The yield of a esterification reaction may be increased also through the removal of one of the products, the water, as it is formed.

The typical mechanism of esterification reactions is the nucleophilic substitution in acyl-carbon, as illustrated on figure 5.

hydraulic fluids, boat engines, 2 stroke engines, tractors, agriculture equipments, cut fluids,

Esters have been used as lubricants since the beginning of the 19th Century, in the form of natural esters in pig fat and whale oil (Whitby, 1998). During World War II, a large number of synthetic fluids were developed such as alcohol and long chain acids esters, that

Nowadays, the esters represent only 0.8% of the world lubricants market. However, while the global consumption of lubricants has been stagnant, the consumption of synthetic oils has grown approximately 10% per year. This growing esters consumption is due to performance reasons and also to changes on the environmental laws of several European

Esters have a low environmental impact and its metabolization consists of the following steps: ester hydrolysis, beta-oxidation of long chain hydrocarbons and oxygenases attack to aromatic nucleus. The main characteristics that reduce the microbial metabolization or

The strongest effect of the ester group on the lubricant physical properties is a decrease in its volatility and increase in its flash point. This is due to the strong dipole moment (London forces) that keeps the ester molecules together. The ester group affects other properties, too such as: thermal and hydrolytic stabilities, solvency, lubricity and biodegradability. Besides, esters, mainly from polyalcohols, as trimethylolpropane (TMP), produce a unimolecular layer on the metal surface, protecting it against wear. This layer is produced by the oxygen

The ester's most important physical-chemistry properties are viscosity, viscosity index (VI),

The main esters used as biolubricants are: diesters, phthalates, trimethilates, C36 dimerates and polyolesters. The polyolesters are formed from polyols with one quaternary carbon atom (neopentylalcohols), as trimethylolpropane, neopentylglycol and pentaerythritol. This class of compounds is very stable due to the absence of a secondary hydrogen on the β position and to the presence of a central quaternary carbon atom (Wagner et al., 2001). The main applications to the esters are: engine oil, 2 stroke engine oils, compressor oils, cooling

According to (Solomons, 1983), the carboxylic acids react with alcohols to produce esters, through a condensation reaction called esterification (figure 4). This reaction is catalyzed by acids and the equilibrium is achieved in a few hours, when an alcohol and an acid are heated under reflux with a small amount of sulfuric acid or hydrochloric acid. Since the equilibrium constant controls the amount of produced ester, an excess of the carboxylic acid or of the alcohol increases the yield of the ester. The compound choice to use in excess will depend on its availability and cost. The yield of a esterification reaction may be increased

The typical mechanism of esterification reactions is the nucleophilic substitution in

• Branching position and degree (that reduce the beta-oxidation);

pour point, lubricity, thermal and hydrolytic stabilities and solvency.

also through the removal of one of the products, the water, as it is formed.

cooling fluids, etc (Erhan & Asadauskas, 2000).

presented excellent low temperature properties.

atoms which are presents in the ester molecules.

fluids, aviation fluids and hydraulic fluids.

**5.1 Synthesis of biolubricant esters** 

acyl-carbon, as illustrated on figure 5.

Community countries, mainly Germany.

• Molecule saturation degree; • Ester molecular weight increase.

degradability are:

R C O OH <sup>+</sup> R' OH H+ R C O OR' + H2O

Fig. 4. Esterification reaction scheme between a carboxylic acid and an alcohol

Fig. 5. Esterification reaction mechanism

When one follows the reaction clockwise, this is the direction of a carboxylic acid esterification, catalyzed by acid. If, however, one follows the counterclockwise, this is the mechanism of an ester hydrolysis, catalyzed by acid. The final result will depend on the choice conditions to the reaction. If the goal is to ersterify an acid, one uses an alcohol excess and if it is possible, one promotes the water removal as it is formed. However, if the goal is the hydrolysis, one uses a large water excess.

The steric hindrance strongly affects the reaction rates of the ester hydrolysis catalyzed by acids. The presence of large groups near to the reaction center in the alcohol component or in the acid component retards the reaction.

Esters can be synthesized through transesterification reactions (figure 6). In this process, the equilibrium is shifted towards the products, allowing the alcohol, with the lower boiling point, to be distilled from the reactant mixture. The transesterification mechanism is similar to the one of a catalyzed by acid esterification (or to the one of a catalyzed by acid ester hydrolysis).

Fig. 6. Transesterification reaction between an ester and an alcohol

Biodegradable Lubricants and Their Production Via Chemical Catalysis 195

(Bondioli et al., 2003) performed the esterification reaction between caprilic acid and TMP, using tin oxide (SnO) as catalyst at 150°C. The yield was 99%, with the continuous removal

(Bondioli, 2004) reported the usage of strong acid ions exchange resins as catalysts in esterification and transesterification reactions. In the case of esterification reactions, the water plays a fundamental role on the catalyst performance. If on the one hand one must remove the produced water to increase the reaction yield, on the other hand the water has a positive effect on the dissociation of the strong acid groups of the resin. Thus, a completely dry resin does not present any catalytic activity, due to the impossibility of the sulfonic

Another limiting factor is the reactant diffusion inside a resin. Fatty materials possess high viscosity, which limits the catalysis using ion exchange resins. In the case of a required high catalytic efficiency, one must choose ion exchange resins with a limited crosslinking degree.

To esters synthesis, one must to use only acid-sulfonic ion exchange resins. Strong basic ion exchange resins may be attractive for transesterification reactions, however they have a limited stability when heated at temperatures higher than 40°C, and are neutralized by low concentrations of fatty acids. Another negative factor is the glycerin production during the

In spite of these negative effects, ion exchange resins, when used as heterogeneous catalysts,

• As solid acids or bases, in a batch process, they can easily be separated from the system

• One may prepare the catalytic bed by packaging and produce a continuous process

Biolubricants esters synthesis may be performed with efficiency using not only chemical catalysts but also biological ones (lipases). However, catalyst choice parameters must be based on the knowledge of each one's limitation. Thus, although the chemical via presents a main advantage because of the lower cost when compared to the enzymatic via, due to its

• More severe operation conditions and higher energy consumption due to higher

Regarding the enzymatic catalysis, it occurs in milder temperatures (60°C), using lipases, triacyl ester hydrolases (glycerol ester hydrolases, E.C. 3.1.1.3). Normally, the lipases catalyze the glycerol ester hydrolysis in lipid/water interphases (Dossat et al., 2002). However, in aqua restrict systems, for example, solvents, lipases catalyze also the synthesis of such esters. Thus, they have been employed on the fat and oil modifications, in aqua restrict systems with or without the presence of organic solvents. Lipases from several

Powder resins are more active than spherical ones on esterification reactions.

reaction, which can make the resin waterproof.

present the following operational advantages:

with higher productivity and catalytic efficiency; • The possibility of regeneration decreases the process costs; • Due to its molecular sieve action, there is a higher selectivity;

• Low catalyst selectivity, with several parallel reactions;

• Foam production (Basic catalysts);

temperatures required.

• These resins are less corrosive than the regular used acids and bases.

higher availability in large amounts, it also presents some disadvantages, such as:

• Corrosion, mainly with sulfuric acid and sodium hydroxide as catalysts; • Low conversion (40% in average), mainly with metal complex catalysts;

• Almost any catalytic activity (H2SO4 and NaOH) with long chain alcohols;

at the reaction end;

of the produced water.

group dissociation.

The methylricinoleate, from a transesterification reaction of the castor oil with methanol, is the main constituent of castor biodiesel. The transesterification of this compound with superior alcohols (TMP, Pentaerythritol or Neo-pentylglycol) (figure 7) allows the production of poliolesters, important synthetic base oils precursors.

Neo Pentylglycol Ester

Fig. 7. Poliolesters molecular structures

The higher the molecule branching degree of this product the better the pour point, the higher the hydrolytic stability, the lower the VI. Regarding linearity, it is verified the opposite way. Regarding the double bonds, the higher the saturation, the better the oxidative stability, the worse the pour point (Wagner et al., 2001). Base oils from these superior alcohols, but with other vegetable oils, can be found in the market, with excellent performance.

To increase the transesterification reactions yield one must promote the reaction equilibrium shift towards the products. This can be reached by using a vacuum, which will remove the formed alcohol from the mixture.

Chemical or enzymatic catalysts may be used on the biolubricants esters synthesis. The chemical catalysis occurs in high temperatures (> 150oC), with the usage of homogeneous or heterogeneous chemical catalysts, with acid or alkaline nature (Abreu et al., 2004). The typical acid homogeneous catalysts are acid p-toluenesulfonic, phosphoric acid and sulfuric acid, while the alkaline are caustic soda, sodium ethoxide and sodium methoxide. The more popular heterogeneous catalysts are tin oxalate and cationic exchange resins.

The methylricinoleate, from a transesterification reaction of the castor oil with methanol, is the main constituent of castor biodiesel. The transesterification of this compound with superior alcohols (TMP, Pentaerythritol or Neo-pentylglycol) (figure 7) allows the production

OC17H33 O

O C17H33O

O C17H33O

O

O

CH3

Neo Pentylglycol Ester

The higher the molecule branching degree of this product the better the pour point, the higher the hydrolytic stability, the lower the VI. Regarding linearity, it is verified the opposite way. Regarding the double bonds, the higher the saturation, the better the oxidative stability, the worse the pour point (Wagner et al., 2001). Base oils from these superior alcohols, but with other vegetable oils, can be found in the market, with excellent

To increase the transesterification reactions yield one must promote the reaction equilibrium shift towards the products. This can be reached by using a vacuum, which will remove the

Chemical or enzymatic catalysts may be used on the biolubricants esters synthesis. The chemical catalysis occurs in high temperatures (> 150oC), with the usage of homogeneous or heterogeneous chemical catalysts, with acid or alkaline nature (Abreu et al., 2004). The typical acid homogeneous catalysts are acid p-toluenesulfonic, phosphoric acid and sulfuric acid, while the alkaline are caustic soda, sodium ethoxide and sodium methoxide. The more

popular heterogeneous catalysts are tin oxalate and cationic exchange resins.

O

O

O

C17H33O

O C17H33O

O

CH2 O C17H33O

O

of poliolesters, important synthetic base oils precursors.

O

O

H3C

Trimethylolpropane Ester Pentaerythritol Ester

CH2 O C17H33O

O C17H33O

Fig. 7. Poliolesters molecular structures

formed alcohol from the mixture.

performance.

O C17H33O

O

(Bondioli et al., 2003) performed the esterification reaction between caprilic acid and TMP, using tin oxide (SnO) as catalyst at 150°C. The yield was 99%, with the continuous removal of the produced water.

(Bondioli, 2004) reported the usage of strong acid ions exchange resins as catalysts in esterification and transesterification reactions. In the case of esterification reactions, the water plays a fundamental role on the catalyst performance. If on the one hand one must remove the produced water to increase the reaction yield, on the other hand the water has a positive effect on the dissociation of the strong acid groups of the resin. Thus, a completely dry resin does not present any catalytic activity, due to the impossibility of the sulfonic group dissociation.

Another limiting factor is the reactant diffusion inside a resin. Fatty materials possess high viscosity, which limits the catalysis using ion exchange resins. In the case of a required high catalytic efficiency, one must choose ion exchange resins with a limited crosslinking degree. Powder resins are more active than spherical ones on esterification reactions.

To esters synthesis, one must to use only acid-sulfonic ion exchange resins. Strong basic ion exchange resins may be attractive for transesterification reactions, however they have a limited stability when heated at temperatures higher than 40°C, and are neutralized by low concentrations of fatty acids. Another negative factor is the glycerin production during the reaction, which can make the resin waterproof.

In spite of these negative effects, ion exchange resins, when used as heterogeneous catalysts, present the following operational advantages:


Biolubricants esters synthesis may be performed with efficiency using not only chemical catalysts but also biological ones (lipases). However, catalyst choice parameters must be based on the knowledge of each one's limitation. Thus, although the chemical via presents a main advantage because of the lower cost when compared to the enzymatic via, due to its higher availability in large amounts, it also presents some disadvantages, such as:


Regarding the enzymatic catalysis, it occurs in milder temperatures (60°C), using lipases, triacyl ester hydrolases (glycerol ester hydrolases, E.C. 3.1.1.3). Normally, the lipases catalyze the glycerol ester hydrolysis in lipid/water interphases (Dossat et al., 2002). However, in aqua restrict systems, for example, solvents, lipases catalyze also the synthesis of such esters. Thus, they have been employed on the fat and oil modifications, in aqua restrict systems with or without the presence of organic solvents. Lipases from several

Biodegradable Lubricants and Their Production Via Chemical Catalysis 197

The same author still promoted the reaction between the rapeseed methyl ester and trimethylolpropane (TMP). This transesterification reaction followed a strategy of individual analyses of each variable behavior involved in the process. Firstly, it was studied the type and the amount of catalyst used, with the best results attributed to sodium methoxide (0.7%). Next, the molar ratio ester:TMP was evaluated, with the best value being 3.2:1 (small ester excess). Finally, the temperature and the pressure were studied, both of these variables have a strong effect on the yield. It was established the values of 85-110°C and 3.3 MPa for a

At last, the author performed the rapeseed methyl ester synthesis through enzymatic catalysis. The yields using lipases were high, but the reaction duration was extremely high

The main properties of a lubricant oil, which are basic requirements to the good performance

a. Viscosity: the viscosity of lubricants is the most important property of these fluids, due to it being directly related to the film formation that protects the metal surfaces from several attacks. In essence, the fluid viscosity is its resistance to the flow, which is a function of the required force to occur slide between its molecule internal layers. For the biolubricants, there is not a pre-defined value, however, due to market reasons, the

Fig. 8. Transesterification batch reactor

yield of 98.9%, in 2.5 hours of reaction.

**6. Biolubrificant properties** 

of it, will be described as follows:

range 8 to 15 cSt at 100°C is the most required;

(46 hours in average).

microorganisms have been studied in the vegetable oil transesterification reactions, such as: *Candida rugosa, Chromobacterium viscosum, Rhizomucor miehei, Pseudomonas fluorescens* and *Candida antarctica*. The most used among these are *Rhizomucor miehei* (immobilized in macroporous anionic resin – Lipozyme) and *Candida rugosa*, in powder. In works made with sunflower oil, the *Candida rugosa* lipase usage showed a higher yield in the transesterification reaction, besides a lower cost than the *Rhizomucor miehei* lipase (Castro et al., 2004).

The transesterification reactions via enzymes may occur with or without the presence of organic solvents. Other interesting variable on this type of reactions is the added amount of alcohol. A large alcohol excess shifts the reaction equilibrium to the production of ester. However, literature data show that a very large excess (higher than 1:6, ester:alcohol) can cause inhibition of the enzymatic activity.

Another interesting characteristic regarding these reactions can be seen in transesterifications directly from the vegetable oils. These reactions have glycerin as subproduct, which, according to some authors, may be adsorbed on the enzyme surface, thus inactivating it (Dossat et al., 2002).

The enzymatic via shows some advantages, as well for example:


A main disadvantage of this via is the high cost of the industrial scale process, due to the high cost of the enzymes. However, the development of more robust biocatalysts through molecular biology techniques or enzymes immobilization can make this process more industrially competitive in a few years.

The biolubricants esters synthesis can be carried out not only in batch reactors, but also in continuous reactors (fixed or fluidized bed). However, due to process simplicity, the batch is the majority choice. One illustrative example of a batch reactor is on figure 8.

(Lämsa, 1995) studied and developed new methods and processes regarding the esters production from vegetable oils, raw-materials for the biodegradable lubricants production, using not only chemical catalysts but also enzymatic catalysts. On the beginning it was synthesized 2-ethyl-1-hexyester of rapeseed oil, from 2-ethyl-1-hexanol and rapeseed oil, ranging catalysts (sodium hydroxide, potassium hydroxide, sodium methoxide, sodium ethoxide and sulfuric acid), molar ratio oil:alcohol (1:3 to 1:6), temperature (80 to 120°C) and pressure (2.0 to 10.6 MPa).

The established optimum conditions were: molar ratio (1:5), 0.5% alkaline catalyst (sodium methoxide), temperature range 80 to 105°C and pressure of 2.7 MPa. The obtained rapeseed yield was 97.6% in five hours of reaction.

The above described synthesis was also studied using *Candida rugosa* lipase as catalyst, with a yield of 87% in five hours of reaction. The best conditions were: molar ratio oil:alcohol (1:2.8), lipase concentration (3.4%), added water (1.0%) and temperature of 37°C.

(Lämsa, 1995) synthesized also a rapeseed methyl ester (biodiesel), reacting rapeseed with methanol (in excess) at 60°C, using 0.5% of alkaline catalyst. After four hours of reaction, the yield was 97%, with the separation of the formed glycerin and the distillation of the excess alcohol.

microorganisms have been studied in the vegetable oil transesterification reactions, such as: *Candida rugosa, Chromobacterium viscosum, Rhizomucor miehei, Pseudomonas fluorescens* and *Candida antarctica*. The most used among these are *Rhizomucor miehei* (immobilized in macroporous anionic resin – Lipozyme) and *Candida rugosa*, in powder. In works made with sunflower oil, the *Candida rugosa* lipase usage showed a higher yield in the transesterification reaction, besides a lower cost than the *Rhizomucor miehei* lipase (Castro et

The transesterification reactions via enzymes may occur with or without the presence of organic solvents. Other interesting variable on this type of reactions is the added amount of alcohol. A large alcohol excess shifts the reaction equilibrium to the production of ester. However, literature data show that a very large excess (higher than 1:6, ester:alcohol) can

Another interesting characteristic regarding these reactions can be seen in transesterifications directly from the vegetable oils. These reactions have glycerin as subproduct, which, according to some authors, may be adsorbed on the enzyme surface, thus inactivating it

A main disadvantage of this via is the high cost of the industrial scale process, due to the high cost of the enzymes. However, the development of more robust biocatalysts through molecular biology techniques or enzymes immobilization can make this process more

The biolubricants esters synthesis can be carried out not only in batch reactors, but also in continuous reactors (fixed or fluidized bed). However, due to process simplicity, the batch is

(Lämsa, 1995) studied and developed new methods and processes regarding the esters production from vegetable oils, raw-materials for the biodegradable lubricants production, using not only chemical catalysts but also enzymatic catalysts. On the beginning it was synthesized 2-ethyl-1-hexyester of rapeseed oil, from 2-ethyl-1-hexanol and rapeseed oil, ranging catalysts (sodium hydroxide, potassium hydroxide, sodium methoxide, sodium ethoxide and sulfuric acid), molar ratio oil:alcohol (1:3 to 1:6), temperature (80 to 120°C) and

The established optimum conditions were: molar ratio (1:5), 0.5% alkaline catalyst (sodium methoxide), temperature range 80 to 105°C and pressure of 2.7 MPa. The obtained rapeseed

The above described synthesis was also studied using *Candida rugosa* lipase as catalyst, with a yield of 87% in five hours of reaction. The best conditions were: molar ratio oil:alcohol

(Lämsa, 1995) synthesized also a rapeseed methyl ester (biodiesel), reacting rapeseed with methanol (in excess) at 60°C, using 0.5% of alkaline catalyst. After four hours of reaction, the yield was 97%, with the separation of the formed glycerin and the distillation of the excess

(1:2.8), lipase concentration (3.4%), added water (1.0%) and temperature of 37°C.

al., 2004).

(Dossat et al., 2002).

• High enzyme selectivity;

• Catalyst biodegradability;

pressure (2.0 to 10.6 MPa).

alcohol.

cause inhibition of the enzymatic activity.

• High yields on the ester conversion;

industrially competitive in a few years.

yield was 97.6% in five hours of reaction.

The enzymatic via shows some advantages, as well for example:

• Lower energy consumption, due to low temperatures;

• Easy recover of the enzymatic catalyst (Dossat et al., 2002).

• Milder reaction conditions, avoiding degradation of reactants and products;

the majority choice. One illustrative example of a batch reactor is on figure 8.

Fig. 8. Transesterification batch reactor

The same author still promoted the reaction between the rapeseed methyl ester and trimethylolpropane (TMP). This transesterification reaction followed a strategy of individual analyses of each variable behavior involved in the process. Firstly, it was studied the type and the amount of catalyst used, with the best results attributed to sodium methoxide (0.7%). Next, the molar ratio ester:TMP was evaluated, with the best value being 3.2:1 (small ester excess). Finally, the temperature and the pressure were studied, both of these variables have a strong effect on the yield. It was established the values of 85-110°C and 3.3 MPa for a yield of 98.9%, in 2.5 hours of reaction.

At last, the author performed the rapeseed methyl ester synthesis through enzymatic catalysis. The yields using lipases were high, but the reaction duration was extremely high (46 hours in average).

#### **6. Biolubrificant properties**

The main properties of a lubricant oil, which are basic requirements to the good performance of it, will be described as follows:

a. Viscosity: the viscosity of lubricants is the most important property of these fluids, due to it being directly related to the film formation that protects the metal surfaces from several attacks. In essence, the fluid viscosity is its resistance to the flow, which is a function of the required force to occur slide between its molecule internal layers. For the biolubricants, there is not a pre-defined value, however, due to market reasons, the range 8 to 15 cSt at 100°C is the most required;

Biodegradable Lubricants and Their Production Via Chemical Catalysis 199

Even though, when compared to the mineral oil market, the biolubricants usage is very small, and, as mentioned before, concentrated in some countries of Europe and in the USA. In order to change the scenario, the biggest challenge to the industries is how to reduce the production costs of such products, therefore making its prices more attractive. The chemical process has low costs, but the yields are a little small. On the other hand, the enzymatic process, with high yields, possesses elevated costs. The newest technologies in lipases development and immobilization may contribute to decrease these costs and make these

Another important matter related to the biolubricants is the quality of their characteristics. On properties as viscosity, viscosity index and pour point, these products overcome the mineral oils based lubricants. But in terms of oxidative stability, efforts have been made to develop products with at least the same level of mineral oils. This can be achieved by chemical modification, acting on the biolubricant molecule, or by adding some special developed additives. The problem is that these additives must be biodegradable too, in order to not damage the biodegradability of the product as a whole. The additives and the lubricants industries have worked together towards the development of environmental

The usage of each country's typical raw materials, like castor oil in Brazil, is used both for an economic reason and a social reason. In the Brazilian case, the small farmers of the poorest country regions are encouraged to plant castor, which is a very easily cultivated crop due to the Brazilian weather. They are able to sell these castor seeds for the oil and biodiesel producers, who can then produce biolubricants. This is a very interesting way to promote the social inclusion in underdeveloped countries. And another interesting feature of this

Finally, the biolubricants have a very important role in the future of mankind, because their potential to contribute to an environment free of pollution and with more equal

Abreu, F. R.; Lima, D. G.; Hamú, E. H.; Wolf C. & Suarez, P. A. Z. (2004). Utilization of Metal

Bartz, W. J. (1998). Lubricant and the Environment. *Tribology International*, Vol. 31, pp. 35-47. Birová, A.; Pavlovicová, A. & Cvengros, J. (2002). Lubricating Oils Base from Chemically

Bondioli, P.; Della Bella, L. & Manglaviti, A. (2003). Synthesis of Biolubricants with High

Bondioli, P. (2004). The Preparation of Fatty Acid Esters by Means of Catalytic Reactions.

Castro, H. F.; Mendes, A. A.; Santos, J. C. & Aguiar, C. L. (2004). Modificação de Óleos e Gorduras por Biotransformação. *Química Nova*, Vol. 27, No. 1, pp. 146-156. Dossat, V.; Combes, D. & Marty, A. (2002). Lipase-Catalysed Transesterification of High Oleic Sunflower Oil. *Enzyme and Microbial Technology*, Vol. 30, pp. 90-94.

Viscosity and High Oxidation Stability. *OCL*, Vol. 10, pp. 150-154.

*Topics in Catalysis*, Vol .27, No. 1-4 (Feb), pp. 77-81.

Complexes as Catalysts in the Transesterification of Brazilian Vegetable Oils with Different Alcohols. *Journal of Molecular Catalysis A: Chemical*, Vol. 209, pp. 29-33. Azevedo, D. M. P. & Lima, E. F. (2001). *O Agronegócio da Mamona no Brasil*, Embrapa, (21st

Modified Vegetable Oils. *Journal of Synthetic Lubrication*, Vol. 18, No. 18-4, pp. 292-

products cheaper.

friendly products.

**8. References** 

299.

crop is that there is not any food competition.

edition)., Brasília, Brazil.

opportunities for the entire World.


#### **7. Conclusion**

The biolubricants market has increased at an approximately 10% per year rate in the last ten years (Erhan et al., 2008). The driven forces of such increase are mainly the growing awareness regarding environmental friendly products and government incentives and regulations.

Even though, when compared to the mineral oil market, the biolubricants usage is very small, and, as mentioned before, concentrated in some countries of Europe and in the USA. In order to change the scenario, the biggest challenge to the industries is how to reduce the production costs of such products, therefore making its prices more attractive. The chemical process has low costs, but the yields are a little small. On the other hand, the enzymatic process, with high yields, possesses elevated costs. The newest technologies in lipases development and immobilization may contribute to decrease these costs and make these products cheaper.

Another important matter related to the biolubricants is the quality of their characteristics. On properties as viscosity, viscosity index and pour point, these products overcome the mineral oils based lubricants. But in terms of oxidative stability, efforts have been made to develop products with at least the same level of mineral oils. This can be achieved by chemical modification, acting on the biolubricant molecule, or by adding some special developed additives. The problem is that these additives must be biodegradable too, in order to not damage the biodegradability of the product as a whole. The additives and the lubricants industries have worked together towards the development of environmental friendly products.

The usage of each country's typical raw materials, like castor oil in Brazil, is used both for an economic reason and a social reason. In the Brazilian case, the small farmers of the poorest country regions are encouraged to plant castor, which is a very easily cultivated crop due to the Brazilian weather. They are able to sell these castor seeds for the oil and biodiesel producers, who can then produce biolubricants. This is a very interesting way to promote the social inclusion in underdeveloped countries. And another interesting feature of this crop is that there is not any food competition.

Finally, the biolubricants have a very important role in the future of mankind, because their potential to contribute to an environment free of pollution and with more equal opportunities for the entire World.

#### **8. References**

198 Tribology - Lubricants and Lubrication

b. Viscosity index (VI): it is an arbitrary dimensionless number used to characterize the range of the kinematic viscosity of a petroleum product with the temperature. A higher viscosity index means a low viscosity decrease when it increases the temperature of a product. Normally, the viscosity index value is determined through calculation (ASTM D2270 method), which takes in account the product viscosities at 40 and 100°C. Oils

c. Pour point: this essay was for a long period of time the only one used to evaluate the lubricants behavior at low temperatures. After pre-heating, the sample is cooled at a specified rate and observed in 3°C intervals to evaluate the flow characteristics. The lowest temperature where is observed movement in the oil is reported as the pour point. The lower the pour point, the better the base oil, having values lower than -36°C a wide market. Some pour point depressants may be used on the biolubricants

d. Corrosion: biolubricants, as mineral lubricants, must not be corrosives. Because of that, they must present 1B result (maximum) on the test ASTM D130, which consists on the observation of the corrosion in a copper plate after this plate is taken out from an oven, where it has been for 3 hours, immersed in the lubricant sample, at 150°C. The values

e. Total acid number (TAN): this essay's goal is to measure the acidity of the lubricant, derived, in general, from the oxidation process, the fuel burning and some additives. In this essay, a sample, with known mass, is previously mixed with titration solvent and titrated in KOH in alcohol. It is determined the KOH mass by sample mass to the titration. It is desired values lower than 0.5 mgKOH/g, since higher TAN values

f. Biodegradability: many vegetable oils and synthetic esters are inherently biodegradable. This means that they are not permanent and undergo physical and chemical changes as a result of its reaction with the biota, which leads to the removal of not favorable environmental characteristics. The negative characteristics are water immiscibility, eco toxicity, bioaccumulation in live organisms and biocide action against such organisms. For some applications, the lubricants must be readily biodegradable. The tests CEC L-33-T-82 and modified STURM are two of the most widely used to measure the lubricants biodegradability. To consider a lubricant as biodegradable, for example, it

g. Oxidative stability: most parts of the vegetable oils are unsaturated and trend to be less stable to oxidation than mineral oils. Low amounts of antioxidants (0.1-0.2%) are effective in mineral oil formulations. However, vegetable oils may require a large amount of such antioxidants (1-5%) to prevent its oxidative degradation. The most used essay to measure the oxidative stability of lubricants is the Rotary Pressure Vessel (RPVOT – ASTM D2272). A good lubricant must present an oxidation times higher than

The biolubricants market has increased at an approximately 10% per year rate in the last ten years (Erhan et al., 2008). The driven forces of such increase are mainly the growing awareness regarding environmental friendly products and government incentives and regulations.

with VI values higher than 130 find a wide diversity of applications;

formulations, but these are less efficient than when used with mineral oils;

1A, 1B, etc., are attributed based on comparison with standards;

contribute to increase the corrosion effects;

180 minutes, on this method.

**7. Conclusion** 

must present a result higher than 67% on the CEC test;


**8** 

*Nasr City, Cairo* 

*Egypt* 

**Lubricating Greases Based on** 

Refaat A. El-Adly and Enas A. Ismail *Egyptian Petroleum Research Institute,* 

**Fatty By-Products and Jojoba Constituents** 

There has been a need since ancient times for lubricating greases. The Egyptians used mutton fat and beef tallow to reduce axle friction in chariots as far back as 1400 BC. More complex lubrications were tried on ancient axle hubs by mixing animal fat and lime, but these crude lubricants were in no way equivalent to the lubricating greases of modern times. Good lubricating greases were not available until the development of petroleum based oils in the late 1800's. Today, there are many different types of lubricating greases, but the basic

In modern industrial years, greases have been increasingly employed to cope with a variety of difficult lubrication problems, particularly those where the liquid lubricant is not feasible. Over the last several decades, greases making technology throughout the world, has undergone rapid change to meet the growing demands of the sophisticated industrial environment. With automation and mechanization of industry, modern greases, like all other lubricants, are designed to last longer, work better under extreme condition and generally expected to provide adequate protection against rust, water, and dust. So, greases are the important items for maintenance and smooth running of various machineries, automobiles, industrial equipments, instruments and other mechanical parts. Industrial development and advances in the field of greases have been geared to satisfy all these

In general, lubricating greases contain a variety of chemical substances ranging from complicated mixtures of natural hydrocarbons in the base oils, well defined soaps and complex organic molecules as additives. Therefore, the more practical greases are lubricating oils which has been thickened in order to remain in contact with the moving surfaces, do not leak out under gravity or centrifugal action or be squeezed out under pressure. The majority of greases in the market are composed of mineral oil blended with soap thickeners. Additives enhance the performance and protect the greases and/or lubricated surfaces. Lubricating greases are used to meet various requirements in machine elements and components, including: valves, seals, gears, threaded connections, plain bearings, chains, contacts, ropes, rolling bearing and shaft/hub connections (Boner, 1954,

**1. Introduction** 

structure of these greases is similar.

diverse expectations (Cann, 1997).

1976).


### **Lubricating Greases Based on Fatty By-Products and Jojoba Constituents**

Refaat A. El-Adly and Enas A. Ismail *Egyptian Petroleum Research Institute, Nasr City, Cairo Egypt* 

#### **1. Introduction**

200 Tribology - Lubricants and Lubrication

Erhan, S. Z. & Asadauskas, S. (2000). Lubricant Basestocks from Vegetable Oils. *Industrial* 

Erhan, S. Z., Sharma, B. K., Liu, Z., Adhvaryu A. (2008). Lubricant Base Stock Potential of Chemically Modified Vegetable Oils. *J. Agric. Food Chem.*, Vol. 56, pp. 8919-8925. Kolwzan, B. & Gryglewicz, S. (2003). Synthesis and Biodegradability of Some Adipic and Sebacic Esters. *Journal of Synthetic Lubrication*, Vol. 20, No. 20-2, pp. 99-107. Lal, K. & Carrick, V. (1993). Performance Testing of Lubricants Based on High Oleic Vegetable Oils. *Journal of Synthetic Lubrication*, No. 11-3, pp. 189-206. Lämsa, M. (1995). *Environmentally Friendly Products Based on Vegetable Oils*. D.Sc. Thesis,

Lastres, L. F. M. (2003). *Lubrificantes e Lubrificação em Motores de Combustão Interna.*

Murphy, W. R.; Blain, D. A. & Galiano-Roth, A. S. (2002). Benefits of Synthetic Lubricants in Industrial Applications. *J. Synthetic Lubrication*, Vol. 18, No. 18-4 (Jan), pp. 301-325. Ravasio, N.; Zaccheria, F.; Gargano, M.; Recchia, S.; Fusi, A.; Poli, N. & Psaro, R. (2002).

Oil over Supported Copper Catalysts. *App. Cat. A: Gen*., Vol. 233, pp. 1-6. Solomons, T. W. G. (1983). *Química Orgânica*, LTC, (1st edition), Rio de Janeiro, Brazil.

Wagner, H.; Luther, R. & Mang, T. (2001). Lubricant Base Fluids Based on Renewable Raw

Whitby, R. D. (1998). Synthetic and VHVI-Based Lubricants Applications, Markets and Price-Performance Competition. Course Notes, Rio de Janeiro, Brazil. Whitby, R. D. (2005). Understanding the Global Lubricants Business – Regional Markets,

Whitby, R. D. (2006). Bio-Lubricants: Applications and Prospects. In: *Proceedings of the 15th*

Economic Issues and Profitability. Course Notes, Oxford, England.

Environmental Friendly Lubricants Through Selective Hydrogenation of Rapeseed

Materials. Their Catalytic Manufacture and Modification. *Applied Catalysis A:* 

*International Colloquium Tribology*, Vol. 1, pp. 150, Ostfildern, Germany, January,

*Crops and Products*, Vol. 11, pp. 277-282.

Helsinki University of Technology, Helsinki, Finland.

Petrobras/CENPES/LPE, Rio de Janeiro, Brazil.

*General*, Vol. 221, pp. 429-442.

2006.

There has been a need since ancient times for lubricating greases. The Egyptians used mutton fat and beef tallow to reduce axle friction in chariots as far back as 1400 BC. More complex lubrications were tried on ancient axle hubs by mixing animal fat and lime, but these crude lubricants were in no way equivalent to the lubricating greases of modern times. Good lubricating greases were not available until the development of petroleum based oils in the late 1800's. Today, there are many different types of lubricating greases, but the basic structure of these greases is similar.

In modern industrial years, greases have been increasingly employed to cope with a variety of difficult lubrication problems, particularly those where the liquid lubricant is not feasible. Over the last several decades, greases making technology throughout the world, has undergone rapid change to meet the growing demands of the sophisticated industrial environment. With automation and mechanization of industry, modern greases, like all other lubricants, are designed to last longer, work better under extreme condition and generally expected to provide adequate protection against rust, water, and dust. So, greases are the important items for maintenance and smooth running of various machineries, automobiles, industrial equipments, instruments and other mechanical parts. Industrial development and advances in the field of greases have been geared to satisfy all these diverse expectations (Cann, 1997).

In general, lubricating greases contain a variety of chemical substances ranging from complicated mixtures of natural hydrocarbons in the base oils, well defined soaps and complex organic molecules as additives. Therefore, the more practical greases are lubricating oils which has been thickened in order to remain in contact with the moving surfaces, do not leak out under gravity or centrifugal action or be squeezed out under pressure. The majority of greases in the market are composed of mineral oil blended with soap thickeners. Additives enhance the performance and protect the greases and/or lubricated surfaces. Lubricating greases are used to meet various requirements in machine elements and components, including: valves, seals, gears, threaded connections, plain bearings, chains, contacts, ropes, rolling bearing and shaft/hub connections (Boner, 1954, 1976).

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 203

more stable than naphthenic oils, hence are less likely to react chemically during grease

**Characteristics Base oil (B1) Bright stock (B2) Test Methods** 

Density, g/ml: at 15.56, °C 0.872 0.8975 ASTM D.1298

Refractive index, nD20 1.5723 1.5988 ASTM D.1218

ASTM-Color 1.0 1.0 ASTM D.1500

Viscosity index 233 225 ASTM D. 189

(20 rpm), cP 2100 2905 ASTM D. 189

Pour point, °C -3 Zero ASTM D.97

Total acid number, mg KOH/g@72 hr 0.12 0.2 ASTM D.664

Flash Point, °C 210 290 ASTM D.92

Molecular Weight 755 890 GPC\*

Predominant, molecular weight 762 898 GPC\*

Polydispersity 1.1023 1.253 GPC\*

Table 1. Physico-chemical properties of the lubricating fluids (Base oil B1&bright stock B2)

%CN (Naphthinic Percentage) 20 12

Mono-aromatic 14.9 13.2

Di-aromatic 12.0 15.5

Poly-aromatic 1.2 1.5

50 9

78

ASTM D-3238

Column chromatography

19 20

61 68

19 ASTM D.445

formulation.

at 40°C at 100°C

Kinematics viscosity, c St.

Dynamic Viscosity, @ 30 °C

Structural group analysis hydrocarbon component, wt %

%CA (Aromatic Percentage)

%CP (Paraffinic Percentage)

GPC\* Gel Permeation Chromatography

Developments in thickeners have been fundamental to the advances in grease technology. The contribution of thickeners has been so central to developments that many types of greases are often classified by the type of thickener used to give the required structured matrix and consistency. The two principal groups of thickeners are metal soaps and inorganic compounds. Soap-based greases are by far the most widespread lubricants.

In soap greases the metallic soap consists of a long-chain fatty acid neutralized by a metal such as lithium, sodium, calcium, aluminum, barium or strontium. A wide variety of fatty materials are used in the manufacture of base lubricating greases. In particular, lithium lubricating greases, first appeared during World War II, were made from lithium stearate pre-formed soap. Nowadays they are usually prepared by reacting lithium hydroxide, as a powder or dissolved in water, with 12-hydroxy stearic acid or its

glycerides in mineral oils or synthetic oils Whether the free acid or its glycerides is preferred depends on the relationship between cost and performance (kinnear & Kranz, 1998; El-Adly, 2004a).

A comprehensive study of all aspects of grease technology with the corresponding literature references is beyond the scope of this short contribution. There are numerous textbooks available on this subject (Vinogradov, 1989; Klamann, 1984; Boner, 1976; Erlich, 1984; Lansdown, 1982).

Within the area of alternate sources of lubricants (El-Adly et al, 1999, 2004a, 2004b, 2005, 2009), a new frontier remains for researchers in the field of lubricating greases. Lithium greases have good multi-purpose properties, e.g. high dropping point, good water resistance and good shear stability. Alternative sources of fatty materials and additives involved in the preparation of such lithium greases will be found later in this chapter. The main objective is to explore the preparation, evaluation and development of lithium lubricating greases from low cost starting materials such as, bone fat, cottonseed soapstock and jojoba meal. The role of the jojoba oil and its meal as novel additives for such greases is also explored (El-Adly et al., 2004b).

#### **2. Raw materials**

The main components of lubricating greases, in general, are lubricating mineral oil, soaps and additives. The mineral oil consists of varying proportions of paraffinic, naphthenic and aromatic hydrocarbons, in addition to minor concentrations of non-hydrocarbon compounds. Soaps may be derived from animal or vegetable fats or fatty acids. Additives are added to lubricating greases, generally in small concentrations, to improve or enhance the desirable properties of the finished product. The use of these ingredients such as fats, fluids and additives, each of which consists of a number of chemical compounds, was originally dictated to a large extent by economic factors and availability. The raw materials mentioned in this chapter are, therefore, according to the following:

#### **2.1 Lubricating fluid**

Mineral oils are most often used as the base stock in grease formulation. About 99% of greases are made with mineral oils. Naphthenic oils are the most popular despite of their low viscosity index. They maintain the liquid phase at low temperatures and easily combine with soaps. Paraffinic oils are poorer solvents for many of the additives used in greases, and with some soaps they may generate at weaker gel structure. On the other hand, they are

Developments in thickeners have been fundamental to the advances in grease technology. The contribution of thickeners has been so central to developments that many types of greases are often classified by the type of thickener used to give the required structured matrix and consistency. The two principal groups of thickeners are metal soaps and inorganic

In soap greases the metallic soap consists of a long-chain fatty acid neutralized by a metal such as lithium, sodium, calcium, aluminum, barium or strontium. A wide variety of fatty materials are used in the manufacture of base lubricating greases. In particular, lithium lubricating greases, first appeared during World War II, were made from lithium stearate pre-formed soap. Nowadays they are usually prepared by reacting lithium hydroxide, as a

glycerides in mineral oils or synthetic oils Whether the free acid or its glycerides is preferred depends on the relationship between cost and performance (kinnear & Kranz, 1998; El-Adly,

A comprehensive study of all aspects of grease technology with the corresponding literature references is beyond the scope of this short contribution. There are numerous textbooks available on this subject (Vinogradov, 1989; Klamann, 1984; Boner, 1976; Erlich, 1984;

Within the area of alternate sources of lubricants (El-Adly et al, 1999, 2004a, 2004b, 2005, 2009), a new frontier remains for researchers in the field of lubricating greases. Lithium greases have good multi-purpose properties, e.g. high dropping point, good water resistance and good shear stability. Alternative sources of fatty materials and additives involved in the preparation of such lithium greases will be found later in this chapter. The main objective is to explore the preparation, evaluation and development of lithium lubricating greases from low cost starting materials such as, bone fat, cottonseed soapstock and jojoba meal. The role of the jojoba oil and its meal as novel additives for such greases is also explored (El-Adly et

The main components of lubricating greases, in general, are lubricating mineral oil, soaps and additives. The mineral oil consists of varying proportions of paraffinic, naphthenic and aromatic hydrocarbons, in addition to minor concentrations of non-hydrocarbon compounds. Soaps may be derived from animal or vegetable fats or fatty acids. Additives are added to lubricating greases, generally in small concentrations, to improve or enhance the desirable properties of the finished product. The use of these ingredients such as fats, fluids and additives, each of which consists of a number of chemical compounds, was originally dictated to a large extent by economic factors and availability. The raw materials mentioned

Mineral oils are most often used as the base stock in grease formulation. About 99% of greases are made with mineral oils. Naphthenic oils are the most popular despite of their low viscosity index. They maintain the liquid phase at low temperatures and easily combine with soaps. Paraffinic oils are poorer solvents for many of the additives used in greases, and with some soaps they may generate at weaker gel structure. On the other hand, they are

compounds. Soap-based greases are by far the most widespread lubricants.

powder or dissolved in water, with 12-hydroxy stearic acid or its

in this chapter are, therefore, according to the following:

2004a).

Lansdown, 1982).

al., 2004b).

**2. Raw materials** 

**2.1 Lubricating fluid** 

more stable than naphthenic oils, hence are less likely to react chemically during grease formulation.


GPC\* Gel Permeation Chromatography

Table 1. Physico-chemical properties of the lubricating fluids (Base oil B1&bright stock B2)

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 205

The results of gas liquid chromatography analysis of the esterified fatty acids in bone fat and hydrolyzed cottonseed soapstock are shown in Table (2). There is a wide variation in their fatty acids composition myristic, palmitic, stearic, oleic, linoleic and linolenic acid. Bone fat is composed of about 52% unsaturated fatty acids, mainly oleic acid, and 47% saturated fatty acids, being palmitic, stearic and myristic acid. However, soapstock contains more unsaturated fatty acids 71% and saturated 29%. This finding was supported by the iodine value measured for both fatty materials. The difference in their fatty constituents leads to

Property (in mole %) Bone Fat Soapstock Test method Saponification number 180 198 ASTM D-1962 Iodine value 45 60.0 ASTM D-2075 Titer, C° 35 45.0 ASTM D-1982

Table 2. Physicochemical properties of bone fat and cottonseed soapstock

Palmitic acid 23.0 27.0 Gas chromatography

The additives used in grease formulation are similar to those used in lubricating oils. Some of them modify the soaps, others improve the oil characteristics. The most common additives include anti-oxidants, rust and corrosion inhibitors, tackiness, and anti-wear and extreme pressure additives. Many studies reported detailed information about lubricating additives (Mang & Dresel, 2001; Shirahama, 1985). This chapter presents the utilization of

Jojoba is known in botanical literatures as *Simmondsia chinenasis* (Link) of the family Buxaceaa and as *Simmondsia californica* Nutall. The first name is the correct one, although it perpetuates a geographical misnomer. In late 1970 sperm whale was included by the US Government in the list of endangered species and imports of oil, meal and other products derived from whales were banned. At that time, sperm oil consumption in the United States was about 40-50 million pounds per year, with half that figure used in lubricant applications. No single natural, or synthetic replacement with the unique qualities of sperm whale oil has yet been found, but enough experimental evidence has accumulated in the last years that jojoba oil is not only an excellent substitute of sperm oil but its potential industrial uses go beyond those of sperm oil (Wisniak, 1994). Sperm oil is widely used in lubricants because of the oiliness and metallic wetting properties, it imparts and its nondrying characteristics that prevent gumming and tackiness in end-use formulations. It is more important as a chemical intermediate since it is sulphonated, oxidized, sulfurized, sulfurchlorinated and chlorinated to give industrial products that were used primarily as wetting

jojoba oil and its meal as additives for the preparation of lithium lubricating greases.

the possibility of producing lithium lubricating grease.

Myristic acid 9.0 trace Oleic acid 48.0 29.0 Stearic acid 15.0 2.0 Linoleic acid 4.0 42.0 Linolenic acid trace trace

**2.3 Additives** 

**2.3.1 Jojoba oil** 

In this respect**,** two types of lube base oils are investigated as fluids part for preparing lithium lubricating greases: the first is a base mineral oil designated B1 and the second is a bright stock designated B2. The Physico-chemical properties of these oils were carried out using ASTM/ IP standard methods of analysis as shown in Table (1). Data in this table reveal that the bright stock could be classified as heavier oil than lube base oil. It may be pointed out, therefore, that the internal friction between oil layers in B2 is greater than in B1. This interpretation agrees with the data of gel permeation chromatography concerning molecular weights of B1 and B2. This is further supported by predominant molecular weights of B1 and B2 which are 762 and 898, respectively. In addition, the polydispersity (i.e., number of average molecular weight divided by mean molecular weight value, Mn/Mw) for bright stock is 1.2530 while it is 1.1023 for base mineral oil. This indicates that B1 and B2 have higher degree of similarity in hydrocarbon constituents (cross sectional areas of molecules are similar) and morphology of structure.

The rheological properties of the above mentioned oils were studied at different temperatures using Brookfield programmable Rheometer LV DV-III ULTRA. Different mathematical model (Herschel Bulkley, Bingham and Casson models) were applied to deduce the viscoelastic parameters. It was found that the fluids under investigation had a Newtonian behavior (El-Adly, 2009).

#### **2.2 Fatty material**

#### **2.2.1 Cottonseed soapstock**

Soapstock is formed by reacting crude vegetable oils with alkali to produce sodium soap as a by-product, which is separated from the oil by centrifuging. Typically, soapstock accounts for 5 to 10 wt. % of the crude oil. In general, soapstock from oilseed refining has been a source of fatty acids and glycerol. These processes are no longer cost effective. Consequently, in cottonseed oil extraction facilities, the treated soapstock is added to the animal meal to increase the energy content, reduce dust and improve pelleting of food products **(**Michael, 1996**).** In general compositional information considering raw and acidulated cottonseed soapstock has been published **(**El-Shattory, 1979; Cherry & Berardi, 1983)**.** 

#### **2.2.2 Bone fat**

The crude bone fat is produced by solvent extraction of crushed bone during the manufacture of animal charcoal. It is considered as by-product for this process. Also, it is extracted by wet rendering under atmospheric pressure from femur epiphyses of cattle, buffaloes and camels. The physical and chemical properties of the above mentioned bone fat was studied (El-Adly, 1999). It has low cost and possesses large-scale availability.

#### **2.2.3 Physicochemical properties of the bone fat and soapstock**

Data in Table (2) show the physicochemical properties of the bone fat and cottonseed soapstock carried out using ASTM/ IP standard methods of analysis. Bone fat and cottonseed soapstock consist primarily of glycerides, that is, of various fatty acid radicals combined with glycerol. It is apparent from Table (2) that the saponification number for bone fat and soapstock are 180.0 and 198.0, respectively. These values were not only used as basis for figuring the amount of alkali required for a particular formulation, but also permitted speculation as to the identity of the fatty acids making up the fatty materials.

The results of gas liquid chromatography analysis of the esterified fatty acids in bone fat and hydrolyzed cottonseed soapstock are shown in Table (2). There is a wide variation in their fatty acids composition myristic, palmitic, stearic, oleic, linoleic and linolenic acid. Bone fat is composed of about 52% unsaturated fatty acids, mainly oleic acid, and 47% saturated fatty acids, being palmitic, stearic and myristic acid. However, soapstock contains more unsaturated fatty acids 71% and saturated 29%. This finding was supported by the iodine value measured for both fatty materials. The difference in their fatty constituents leads to the possibility of producing lithium lubricating grease.


Table 2. Physicochemical properties of bone fat and cottonseed soapstock

#### **2.3 Additives**

204 Tribology - Lubricants and Lubrication

In this respect**,** two types of lube base oils are investigated as fluids part for preparing lithium lubricating greases: the first is a base mineral oil designated B1 and the second is a bright stock designated B2. The Physico-chemical properties of these oils were carried out using ASTM/ IP standard methods of analysis as shown in Table (1). Data in this table reveal that the bright stock could be classified as heavier oil than lube base oil. It may be pointed out, therefore, that the internal friction between oil layers in B2 is greater than in B1. This interpretation agrees with the data of gel permeation chromatography concerning molecular weights of B1 and B2. This is further supported by predominant molecular weights of B1 and B2 which are 762 and 898, respectively. In addition, the polydispersity (i.e., number of average molecular weight divided by mean molecular weight value, Mn/Mw) for bright stock is 1.2530 while it is 1.1023 for base mineral oil. This indicates that B1 and B2 have higher degree of similarity in hydrocarbon constituents (cross sectional

The rheological properties of the above mentioned oils were studied at different temperatures using Brookfield programmable Rheometer LV DV-III ULTRA. Different mathematical model (Herschel Bulkley, Bingham and Casson models) were applied to deduce the viscoelastic parameters. It was found that the fluids under investigation had a Newtonian

Soapstock is formed by reacting crude vegetable oils with alkali to produce sodium soap as a by-product, which is separated from the oil by centrifuging. Typically, soapstock accounts for 5 to 10 wt. % of the crude oil. In general, soapstock from oilseed refining has been a source of fatty acids and glycerol. These processes are no longer cost effective. Consequently, in cottonseed oil extraction facilities, the treated soapstock is added to the animal meal to increase the energy content, reduce dust and improve pelleting of food products **(**Michael, 1996**).** In general compositional information considering raw and acidulated cottonseed

The crude bone fat is produced by solvent extraction of crushed bone during the manufacture of animal charcoal. It is considered as by-product for this process. Also, it is extracted by wet rendering under atmospheric pressure from femur epiphyses of cattle, buffaloes and camels. The physical and chemical properties of the above mentioned bone fat was studied (El-Adly,

Data in Table (2) show the physicochemical properties of the bone fat and cottonseed soapstock carried out using ASTM/ IP standard methods of analysis. Bone fat and cottonseed soapstock consist primarily of glycerides, that is, of various fatty acid radicals combined with glycerol. It is apparent from Table (2) that the saponification number for bone fat and soapstock are 180.0 and 198.0, respectively. These values were not only used as basis for figuring the amount of alkali required for a particular formulation, but also permitted speculation as to the identity of the fatty acids making up the fatty materials.

soapstock has been published **(**El-Shattory, 1979; Cherry & Berardi, 1983)**.** 

1999). It has low cost and possesses large-scale availability.

**2.2.3 Physicochemical properties of the bone fat and soapstock** 

areas of molecules are similar) and morphology of structure.

behavior (El-Adly, 2009).

**2.2.1 Cottonseed soapstock** 

**2.2 Fatty material** 

**2.2.2 Bone fat** 

The additives used in grease formulation are similar to those used in lubricating oils. Some of them modify the soaps, others improve the oil characteristics. The most common additives include anti-oxidants, rust and corrosion inhibitors, tackiness, and anti-wear and extreme pressure additives. Many studies reported detailed information about lubricating additives (Mang & Dresel, 2001; Shirahama, 1985). This chapter presents the utilization of jojoba oil and its meal as additives for the preparation of lithium lubricating greases.

#### **2.3.1 Jojoba oil**

Jojoba is known in botanical literatures as *Simmondsia chinenasis* (Link) of the family Buxaceaa and as *Simmondsia californica* Nutall. The first name is the correct one, although it perpetuates a geographical misnomer. In late 1970 sperm whale was included by the US Government in the list of endangered species and imports of oil, meal and other products derived from whales were banned. At that time, sperm oil consumption in the United States was about 40-50 million pounds per year, with half that figure used in lubricant applications. No single natural, or synthetic replacement with the unique qualities of sperm whale oil has yet been found, but enough experimental evidence has accumulated in the last years that jojoba oil is not only an excellent substitute of sperm oil but its potential industrial uses go beyond those of sperm oil (Wisniak, 1994). Sperm oil is widely used in lubricants because of the oiliness and metallic wetting properties, it imparts and its nondrying characteristics that prevent gumming and tackiness in end-use formulations. It is more important as a chemical intermediate since it is sulphonated, oxidized, sulfurized, sulfurchlorinated and chlorinated to give industrial products that were used primarily as wetting

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 207

Jojoba oil is chemically purer than most natural substances. It is soluble in common organic solvents such as benzene, petroleum ether, chloroform, carbon tetrachloride, and carbon disulfide, but it is immiscible with ethanol, methanol, acetic acid, and acetone (Miwa & Hagemann, 1978). It is usually a low-acidity, light-golden fluid that requires little or no refining. It is non-volatile and free from rancidity. Even after repeated heating to temperatures above 285°C for 4 days it is essentially unchanged (Daugherty et al., 1953). Its boiling point (at a pressure of 757 mmHg, under nitrogen) rises to 418 °C but drops rapidly to a steady 398 °C (Miwa 1973; Wisniak, 1987). Neutralization of the oil is not usually required and bleaching to a water-clear fluid can be done with common commercial

Data in Table (3) reveal that the possibilities for economic development of the oil and its suitability to produce lubricants and lubricant additives for use in the preparation of lubricating greases. This view is in agreement with a study on using of jojoba oil as oxidation, thermal and mechanical stabilities to improve the properties of lithium

**Characteristics Jojoba oil Test Method**  Density, g/ml @ 25/25, °C 0.863 ASTM D-1298 Refractive index, nD20 1.4652 ASTM D-1218

at 40°C 26 ASTM D-445 at 100°C 7.5 ASTM D-445 Viscosity index 257 ASTM D- 189 Dynamic Viscosity, @ 30 °C (rpm 6), cP 58.4 ASTM D-97 TAN, mg KOH/g 2.0 ASTM D-664 Flash Point, °C 310 ASTM D-92 Iodine Value 80 ASTM D-2075 Average Molecular Weight 604 GPC

Oxidation stability test (min) 23 IP 229

A byproduct of jojoba seeds is the meal remaining after the oil has been pressed and extracted. This material constitutes about 50% of the seed and contains 25-30 % crude protein. Table (4), presents the amino acid composition (%by wieght) of deoiled meal of two varieties of jojoba meal (Verbiscar &Banigan, 1978). Basic information on the composition of jojoba meal, polyphenolic compounds, carbohydrate contents, and Simmondsin compounds have been reported **(**Verbiscar et al., 1978; Cardeso et al., 1980; Wisniak, 1994**)**. On the other hand, the possibility of using the meal as fuel has already been considered (Kuester, 1984 & Kuester et al., 1985). El-Adly et.al (2004b) reported the novel application of jojoba meal as

Surface tensions mN/m 24

Table 3. Physico-chemical properties of Jojoba oil (El-Adly et al, 2009)

techniques. Some properties of the oil are listed in Table 3 (El-Adly et al, 2009).

**2.3.1.2 Physical properties** 

lubricating grease (Ismail, 2008).

Kinematics viscosity, c St.

**2.3.2 Jojoba meal** 

additive for sodium lubricating grease**.** 

agents and extreme pressure (EP) additives. The composition and physical properties of Jojoba are close enough to sperm oil to suggest the use of Jojoba oil as a substitute for most of the uses of sperm oil (Miwa & Rothfus, 1978). Sperm oil has been used as an extreme pressure and antiwear additive in lubricants for gears in differentials and transmissions, in hydraulic fluids that need a low coefficient of friction and in cutting and drawing oils. In some of these, sperm oil has been directly, but it is usually Sulfurized (sometimes epoxidized, chlorinated, or fluorinated). Gear lubricants (e.g., in automobile transmissions) commonly contain 5 to 25 percent of Sulfurized sperm oil (Peeler & Hartman, 1972).

Some of the first published results of sulfurized jojoba oil use a lubricant and extreme pressure (EP) additive were reported as patents (Flaxman, 1940; Wells, 1948). Wells pointed out several advantages of jojoba oil over sperm oil. Its slight odor is distinctly more pleasant than the fishy odor of sperm oil. Crude jojoba oil contains no glycerides so that the crude oil needs little or no treatment to prepare it for most industrial purposes.

In general, lubricant technology dealing with jojoba oil and its derivatives in the 70's concentrated on its replacement of sulfurized sperm oil products in such applications as industrial and automotive gear oils, hydraulic oils and metal working lubricants (Heilweil, 1988; Wills, 1985). In the 80's the lubrication industry has developed and research on jojoba has been shifting towards new derivatives with potential application to new technologies and newer areas of lubricant use. A monograph by Wisniak (1987) summarized the chemistry and technology of jojoba oil and jojoba meal.

#### **2.3.1.1 Composition**

The chemical composition of jojoba oil is unique in that it contains little or no glycerin and that most of its components fall in the chain-length range of C36-C42. Linearity and closerange composition are probably the two outstanding properties that give jojoba oil its unique characteristics. The oil is characterized of being a monoester of high molecular weight and straight chain fatty acids and fatty alcohols that has a double bond on each side of the ester. The molecular structure of the oil can be represented by the following general formula:

$$\text{CH}\_3\text{-(}\text{CH}\_2\text{-)}\_7\text{-CH=CH-(CH}\_2\text{)}\_{\text{m}^-}\text{-COO-(}\text{-CH-}\text{)}\_{\text{n}^-}\text{CH=CH-(CH}\_2\text{)}\_{\text{2}^-}\text{-CH}\_3\text{.}$$

Where, m and n are between 8 to 12 (Miwa 1971, 1980; Spencer et al, 1977; Greene & Foster, 1933).

They were the first to report that jojoba nuts contain about 46-50% of liquid oil which resembles sperm whale oil in its analytical characteristics. Qualitative tests suggested that the oil might consist mainly of fatty acid esters of decyl alcohol. Shortly thereafter, detailed analysis of the chemical constituents was reported (Greene & Foster, 1933). The main components were eicosenoic and docosenoic acids and eicosanol and docosanol. Because of the problems of the high resistance of the oil to saponification, the difficulties in isolating pure fractions and the lack of convenient and reliable quantitative analytical techniques the characterization of jojoba oil was developed by Miwa (1971 & 1980).

Jojoba oil is unusually stable towards oxidation especially at high temperature. Kono et al, (1981) mentioned that the oxidative stability of jojoba oil was due, at least in part, to the presence of tocopherol and other natural antioxidants. Also, some of the antioxidants separated and identified by molecular distillation of the oil and analysis of the distillate by gas chromatography/ mass spectrometry. The, **ά , γ** and **δ** isomers of tocopherol are present, in varying quantities depending on the origin of the oil, **γ**-isomer being most abundant.

#### **2.3.1.2 Physical properties**

206 Tribology - Lubricants and Lubrication

agents and extreme pressure (EP) additives. The composition and physical properties of Jojoba are close enough to sperm oil to suggest the use of Jojoba oil as a substitute for most of the uses of sperm oil (Miwa & Rothfus, 1978). Sperm oil has been used as an extreme pressure and antiwear additive in lubricants for gears in differentials and transmissions, in hydraulic fluids that need a low coefficient of friction and in cutting and drawing oils. In some of these, sperm oil has been directly, but it is usually Sulfurized (sometimes epoxidized, chlorinated, or fluorinated). Gear lubricants (e.g., in automobile transmissions)

Some of the first published results of sulfurized jojoba oil use a lubricant and extreme pressure (EP) additive were reported as patents (Flaxman, 1940; Wells, 1948). Wells pointed out several advantages of jojoba oil over sperm oil. Its slight odor is distinctly more pleasant than the fishy odor of sperm oil. Crude jojoba oil contains no glycerides so that the crude oil

In general, lubricant technology dealing with jojoba oil and its derivatives in the 70's concentrated on its replacement of sulfurized sperm oil products in such applications as industrial and automotive gear oils, hydraulic oils and metal working lubricants (Heilweil, 1988; Wills, 1985). In the 80's the lubrication industry has developed and research on jojoba has been shifting towards new derivatives with potential application to new technologies and newer areas of lubricant use. A monograph by Wisniak (1987) summarized the

The chemical composition of jojoba oil is unique in that it contains little or no glycerin and that most of its components fall in the chain-length range of C36-C42. Linearity and closerange composition are probably the two outstanding properties that give jojoba oil its unique characteristics. The oil is characterized of being a monoester of high molecular weight and straight chain fatty acids and fatty alcohols that has a double bond on each side of the ester. The molecular structure of the oil can be represented by the following general

CH3-(-CH2-) 7-CH=CH-(CH2-) m-COO-(-CH-) n-CH=CH-(CH2-) 7-CH3 Where, m and n are between 8 to 12 (Miwa 1971, 1980; Spencer et al, 1977; Greene & Foster,

They were the first to report that jojoba nuts contain about 46-50% of liquid oil which resembles sperm whale oil in its analytical characteristics. Qualitative tests suggested that the oil might consist mainly of fatty acid esters of decyl alcohol. Shortly thereafter, detailed analysis of the chemical constituents was reported (Greene & Foster, 1933). The main components were eicosenoic and docosenoic acids and eicosanol and docosanol. Because of the problems of the high resistance of the oil to saponification, the difficulties in isolating pure fractions and the lack of convenient and reliable quantitative analytical techniques the

Jojoba oil is unusually stable towards oxidation especially at high temperature. Kono et al, (1981) mentioned that the oxidative stability of jojoba oil was due, at least in part, to the presence of tocopherol and other natural antioxidants. Also, some of the antioxidants separated and identified by molecular distillation of the oil and analysis of the distillate by gas chromatography/ mass spectrometry. The, **ά , γ** and **δ** isomers of tocopherol are present, in varying quantities depending on the origin of the oil, **γ**-isomer being most abundant.

characterization of jojoba oil was developed by Miwa (1971 & 1980).

commonly contain 5 to 25 percent of Sulfurized sperm oil (Peeler & Hartman, 1972).

needs little or no treatment to prepare it for most industrial purposes.

chemistry and technology of jojoba oil and jojoba meal.

**2.3.1.1 Composition** 

formula:

1933).

Jojoba oil is chemically purer than most natural substances. It is soluble in common organic solvents such as benzene, petroleum ether, chloroform, carbon tetrachloride, and carbon disulfide, but it is immiscible with ethanol, methanol, acetic acid, and acetone (Miwa & Hagemann, 1978). It is usually a low-acidity, light-golden fluid that requires little or no refining. It is non-volatile and free from rancidity. Even after repeated heating to temperatures above 285°C for 4 days it is essentially unchanged (Daugherty et al., 1953). Its boiling point (at a pressure of 757 mmHg, under nitrogen) rises to 418 °C but drops rapidly to a steady 398 °C (Miwa 1973; Wisniak, 1987). Neutralization of the oil is not usually required and bleaching to a water-clear fluid can be done with common commercial techniques. Some properties of the oil are listed in Table 3 (El-Adly et al, 2009).

Data in Table (3) reveal that the possibilities for economic development of the oil and its suitability to produce lubricants and lubricant additives for use in the preparation of lubricating greases. This view is in agreement with a study on using of jojoba oil as oxidation, thermal and mechanical stabilities to improve the properties of lithium lubricating grease (Ismail, 2008).


Table 3. Physico-chemical properties of Jojoba oil (El-Adly et al, 2009)

#### **2.3.2 Jojoba meal**

A byproduct of jojoba seeds is the meal remaining after the oil has been pressed and extracted. This material constitutes about 50% of the seed and contains 25-30 % crude protein. Table (4), presents the amino acid composition (%by wieght) of deoiled meal of two varieties of jojoba meal (Verbiscar &Banigan, 1978). Basic information on the composition of jojoba meal, polyphenolic compounds, carbohydrate contents, and Simmondsin compounds have been reported **(**Verbiscar et al., 1978; Cardeso et al., 1980; Wisniak, 1994**)**. On the other hand, the possibility of using the meal as fuel has already been considered (Kuester, 1984 & Kuester et al., 1985). El-Adly et.al (2004b) reported the novel application of jojoba meal as additive for sodium lubricating grease**.** 

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 209

Table (5) also reveals that the main anions in jojoba meal are phosphate (12718 ppm) and chloride (1286 ppm) but the main cations are magnesium, calcium, potassium and sodium. This indicates the possibility of using and optimizing the organometalic compounds in

Lithium base lubricating greases can be prepared either by batch or continuous processes. Such products can be manufactured from either preformed soap or soap prepared in *situ*. From the standpoint of economy and versatility, the latter method is preferable and is therefore used by most manufacturers. The exception to this last statement is in the case of synthetic lubricating fluids. Preformed soaps are desirable in such case because some of these fluids such as diesters will hydrolyze in the presence of alkalies and heat (Boner, 1954, 1976). The lithium lubricating greases mentioned in this chapter were prepared using batch processing. The studied greases were prepared in two steps according to the following: a. Saponification process was performed on a mixture of fatty materials and fluids by alkaline slurry within the temperature range 190 to 195oC. The autoclave was charged, while stirring, with a mixture of 25% wt of light mineral oil and 14% wt of fatty materials (bone fat and soapstock). The autoclave was closed and heating started. Then about 3% wt lithium hydroxide/oil slurry is gradually pumped into the autoclave. The temperature of the reaction mixture must be raised to 190-195oC and held at this temperature for approximately 60 min. to ensure complete saponification. After completion of the saponification step, jojoba oil and or jojoba meal in different concentrations was added. A sample was then taken to examine its alkalinity/acidity. Corrections were made by adding fatty materials or Lithium hydroxide oil slurry as

required reaching a neutral product i.e. complete saponification

base oil, grease and antioxidants (Pohlen, 1998; Gatto &Grina, 1999).

attain the required grease consistency.

b. Cooling process was performed after the completion of the saponification reaction. The reaction mixture was cooled gradually while adding the rest of the base lube oil to

The obtained greases were tested and classified according to the standards methods, National Lubricating Greases Institute (NLGI) and the Egyptian Standards (ES). Also, the physico-chemical characteristics of all the prepared greases under investigation were determined using standard methods of analysis. These include penetration, dropping point, apparent viscosity, oxidation stability, total acid number, oil separation and four balls. In general, test methods are used to judge the single or combined and more or less complex properties of the greases. The last summary containing detailed descriptions of ASTM and DIN methods was reported (Schultze, 1962); but the elemental analysis of the greases is nowadays performed by spectroscopic methods, e.g. X-ray fluorescence spectrometry, inductively coupled plasma atomic emission, or atomic absorption spectrometry, with attention being directed mostly to methods of preparation (Robison et al 1993; Kieke,1998). Also, Thermogravimetry and differential scanning calorimetry tools are used to evaluate of

**3.2 Effect of the fatty materials and fluid part concentrations on the prepared greases**  The physical and chemical behaviors of greases are largely controlled by the consistency or hardness. The consistency of grease is its resistance to deformation by an applied force.

jojoba meal as additives for the lubricating greases**.** 

**3. Grease preparation and evaluation** 

**3.1 Lithium greases preparation** 


Table 4. Amino acid composition (%) of deoiled meal of two varieties of jojoba meal (Verbiscar and Banigan, 1978)

Table (5) presents the anion and cation concentrations in jojoba meal determined through the sulphuric acid wet ashing. The anion concentrations are measured using ion chromatography (IC) model DIONEX LC20 equipped with electrochemical detector model DIONEX ED50, while the cation concentrations determined by inductively coupled plasma/ atomic emission (ICP/AE) spectrometer model flame Modula spectra.


Table 5. Anions and cations contents of the jojoba meal

Table (5) also reveals that the main anions in jojoba meal are phosphate (12718 ppm) and chloride (1286 ppm) but the main cations are magnesium, calcium, potassium and sodium. This indicates the possibility of using and optimizing the organometalic compounds in jojoba meal as additives for the lubricating greases**.** 

#### **3. Grease preparation and evaluation**

#### **3.1 Lithium greases preparation**

208 Tribology - Lubricants and Lubrication

**Amino acid** Apache 377 SCJP 977

Table 4. Amino acid composition (%) of deoiled meal of two varieties of jojoba meal

atomic emission (ICP/AE) spectrometer model flame Modula spectra.

Magnesium 2079 Alumminum 33.4 Iron 124 Copper 13.9 Manganese 20.1 Barium 1.51 Zinc 29.8 Cobalt 3.56 Nickel 0.34 Strontium 3.99

Table 5. Anions and cations contents of the jojoba meal

Table (5) presents the anion and cation concentrations in jojoba meal determined through the sulphuric acid wet ashing. The anion concentrations are measured using ion chromatography (IC) model DIONEX LC20 equipped with electrochemical detector model DIONEX ED50, while the cation concentrations determined by inductively coupled plasma/

**Cations Concentration ppm Anions Concentration, ppm** 

Calcium 1178 Phosphate 12718 Lithiuum 1.73 Chloride 1286 Potassium 7304 Sulphate 8600 Sodium 566 fluoride 135

1.05 0.486 1.56 2.18 1.14 1.04 2.40 0.958 1.50 0.832 1.10 0.186 0.777 1.46 1.04 0.919 0.791 0.492 1.11 0.493 1.81 3.11 1.22 1.11 2.79 1.1. 1.41 0.953 1.19 0.210 0.866 1.57 1.05 1.07 0.519 0.559

Lysine Histidine Arginine Aspartic acid Threonine Serine

(Verbiscar and Banigan, 1978)

Glutamic acid Proline Glycine Alanine Valine Methionine Isoleucine Leucine Tyrosine Phenylalanine Cystine+ cystine Tryptophan

Lithium base lubricating greases can be prepared either by batch or continuous processes. Such products can be manufactured from either preformed soap or soap prepared in *situ*. From the standpoint of economy and versatility, the latter method is preferable and is therefore used by most manufacturers. The exception to this last statement is in the case of synthetic lubricating fluids. Preformed soaps are desirable in such case because some of these fluids such as diesters will hydrolyze in the presence of alkalies and heat (Boner, 1954, 1976). The lithium lubricating greases mentioned in this chapter were prepared using batch processing. The studied greases were prepared in two steps according to the following:


The obtained greases were tested and classified according to the standards methods, National Lubricating Greases Institute (NLGI) and the Egyptian Standards (ES). Also, the physico-chemical characteristics of all the prepared greases under investigation were determined using standard methods of analysis. These include penetration, dropping point, apparent viscosity, oxidation stability, total acid number, oil separation and four balls. In general, test methods are used to judge the single or combined and more or less complex properties of the greases. The last summary containing detailed descriptions of ASTM and DIN methods was reported (Schultze, 1962); but the elemental analysis of the greases is nowadays performed by spectroscopic methods, e.g. X-ray fluorescence spectrometry, inductively coupled plasma atomic emission, or atomic absorption spectrometry, with attention being directed mostly to methods of preparation (Robison et al 1993; Kieke,1998). Also, Thermogravimetry and differential scanning calorimetry tools are used to evaluate of base oil, grease and antioxidants (Pohlen, 1998; Gatto &Grina, 1999).

#### **3.2 Effect of the fatty materials and fluid part concentrations on the prepared greases**

The physical and chemical behaviors of greases are largely controlled by the consistency or hardness. The consistency of grease is its resistance to deformation by an applied force.

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 211

Also, it is defined in terms of grease penetration depth by a standard cone under prescribed conditions of time and temperature (ASTM D-217, ASTM D-1403). In order to standardize grease hardness measurements, the National Lubricating Grease Institute (NLGI) has separated grease into nine classification, ranging from the softest, NLGI 000, to the hardest, NLGI 6. On the other hand, the drop point is the temperature at which grease shows a change from a semi-solid to a liquid state under the prescribed conditions. The drop point is the maximum useful operating temperature of the grease. It can be determined in an apparatus in which the sample of grease is heated until a drop of liquid is formed and

In order to evaluate the effect of fatty materials type and fluid on the prepared lithium grease properties, grease blends G1A, G1B, G1C, G1D, G1E, G1F and G1G have been prepared and

Data in Table (6) indicate the effect of different ratios from soapstock, bone fat, base oil and bright stock on the properties of the prepared lithium lubricating greases. It is evident from these results that the dropping point of lithium grease blend made from bone fat or soapstock alone is lower than that of lithium grease containing a mix from each both fatty materials and fluids. This clearly indicates that the most powerful thickener in the saponification process is the equimolar ratio from bone fat and soapstock. In other words, both fatty materials have synergistic effect during the saponification reaction. The mechanical efficiency of the formulated greases is according to the following order G1G > G1F > G1E > G1D > G1C > G1B > G1A. On the other hand, the above mentioned test showed that the difference of penetration values between unworked and worked (60 strokes) greases follows an opposite order. Based on this finding, it is concluded that the most efficient lube oil in saponification is the light base oil (B1). This is attributed to the fact that lighter oil B1 is easily dispersed in fatty materials during saponification step at temperature 190oC and form stable soap texture. After completion of saponification, the bright stock (B2) is suitable in the cooling step which leads to heavier consistency and provides varying resistance to deformation. This reflects the role of the effect of mineral oil viscosity and fatty materials on

It is apparent from the data in Table (6) that the oil separation, oxidation stability, total acid number and mechanical stability for the prepared grease G1G are 2.0, 3.0, 0.68 and 5.0 respectively. This indicates that the best formula is G1G compared with G1A, G1B, G1C, G1D, G1E, and G1F. Based on the above mention results and correlating these results with the apparent viscosity dropping point and penetration, clearly indicates that the suitable and

**3.3 Effect of the jojoba oil additive on properties of the selected prepared grease**  To evaluate the role of jojoba oil as additive for the Selected Prepared Grease G1G, different concentrations from jojoba oil were tested*.* In this respect, three concentrations of jojoba oil of 1wt%, 3 wt% and 5wt% were added to the selected grease G1G yielding G2A, G2B and G2C, respectively, as shown in Table (6). Worth mentioning here, Jojoba oil ratio was added to the prepared greases after the completion of saponification process. Data in Table (7) show that the results of the penetration and dropping point tests for lithium grease prepared G2A, G2B and G2C produced from different ratio of jojoba oil. These results show that the difference of penetration values between unworked and worked (60 double strokes) lithium lubricating greases are in the order G2C <G2B <G2A. This means that the resistance to texture deformation

detaches from the grease (ASTM D-266, ASTM D-2265).

the properties of the prepared grease.

selected formula for the lithium lubricating grease is G1G.

formulated according to the percent ingredient listed in Table (6).


Table 6. Effect of the fatty material and fluid concentrations on characterization of prepared greases

Unworked 300 300 300 290 290 290 285

worked 310 310 310 300 300 300 290

Dropping point, °C 170 173 174 174 175 177 178 ASTM D-

3h/100°C Ia Ia Ia Ia Ia Ia Ia ASTM D-

96h, pressure drop, psi 4.2 4.1 4.5 4.0 4.0 4.1 4.0 ASTM D-

Alkalinity, Wt% 0.3 0.4 0.4 0.5 0.5 0.5 0.5 ASTM D-

72h 0.34 0.34 0.33 0.33 0.32 0.30 0.28 ASTM D-

Oil Separation, Wt% 2.5 2.5 2.3 2.3 2.2 2.2 2 ASTM D-

2 LB

@ 90 °C 39600 39650 39680 39700 39710 39750 39891 ASTM D-

Kg 160 162 165 166 168 169 170 ASTM D-

Table 6. Effect of the fatty material and fluid concentrations on characterization of prepared

2 LB

2 LB

2 LB

2 LB

2 LB

2 LB

Yield stress, D/cm2 60.2 61.3 62.1 62.9 63.6 64.3 65.0

**Test method** 

ASTM D-217

566

4048

942

664

664

1724

189

2596

Ingredient **G1A G1B G1C G1D G1E G1F G1G**

Base oil, Wt % 79.0 79.0 80.0 - - - 30

Brightstock, Wt % - - - 80.0 80.0 80.0 50

Soap stock, Wt % 18 - 8.5 17.0 - 8.5 8.5 Bone fat, Wt % - 18.0 8.5 - 17.0 8.5 8.5 LiOH, Wt % 3.0 3.0 3.0 3.0 2.8-3 2.8-3 2.8-3

Symbol

Penetration

Copper Corrosion

Oxidation Stability 99±

TAN, mg KOH/gm @

Code grease NLGI

Egyptian standard

Four ball weld load,

greases

Apparent Viscosity, cP,

Also, it is defined in terms of grease penetration depth by a standard cone under prescribed conditions of time and temperature (ASTM D-217, ASTM D-1403). In order to standardize grease hardness measurements, the National Lubricating Grease Institute (NLGI) has separated grease into nine classification, ranging from the softest, NLGI 000, to the hardest, NLGI 6. On the other hand, the drop point is the temperature at which grease shows a change from a semi-solid to a liquid state under the prescribed conditions. The drop point is the maximum useful operating temperature of the grease. It can be determined in an apparatus in which the sample of grease is heated until a drop of liquid is formed and detaches from the grease (ASTM D-266, ASTM D-2265).

In order to evaluate the effect of fatty materials type and fluid on the prepared lithium grease properties, grease blends G1A, G1B, G1C, G1D, G1E, G1F and G1G have been prepared and formulated according to the percent ingredient listed in Table (6).

Data in Table (6) indicate the effect of different ratios from soapstock, bone fat, base oil and bright stock on the properties of the prepared lithium lubricating greases. It is evident from these results that the dropping point of lithium grease blend made from bone fat or soapstock alone is lower than that of lithium grease containing a mix from each both fatty materials and fluids. This clearly indicates that the most powerful thickener in the saponification process is the equimolar ratio from bone fat and soapstock. In other words, both fatty materials have synergistic effect during the saponification reaction. The mechanical efficiency of the formulated greases is according to the following order G1G > G1F > G1E > G1D > G1C > G1B > G1A. On the other hand, the above mentioned test showed that the difference of penetration values between unworked and worked (60 strokes) greases follows an opposite order. Based on this finding, it is concluded that the most efficient lube oil in saponification is the light base oil (B1). This is attributed to the fact that lighter oil B1 is easily dispersed in fatty materials during saponification step at temperature 190oC and form stable soap texture. After completion of saponification, the bright stock (B2) is suitable in the cooling step which leads to heavier consistency and provides varying resistance to deformation. This reflects the role of the effect of mineral oil viscosity and fatty materials on the properties of the prepared grease.

It is apparent from the data in Table (6) that the oil separation, oxidation stability, total acid number and mechanical stability for the prepared grease G1G are 2.0, 3.0, 0.68 and 5.0 respectively. This indicates that the best formula is G1G compared with G1A, G1B, G1C, G1D, G1E, and G1F. Based on the above mention results and correlating these results with the apparent viscosity dropping point and penetration, clearly indicates that the suitable and selected formula for the lithium lubricating grease is G1G.

#### **3.3 Effect of the jojoba oil additive on properties of the selected prepared grease**

To evaluate the role of jojoba oil as additive for the Selected Prepared Grease G1G, different concentrations from jojoba oil were tested*.* In this respect, three concentrations of jojoba oil of 1wt%, 3 wt% and 5wt% were added to the selected grease G1G yielding G2A, G2B and G2C, respectively, as shown in Table (6). Worth mentioning here, Jojoba oil ratio was added to the prepared greases after the completion of saponification process. Data in Table (7) show that the results of the penetration and dropping point tests for lithium grease prepared G2A, G2B and G2C produced from different ratio of jojoba oil. These results show that the difference of penetration values between unworked and worked (60 double strokes) lithium lubricating greases are in the order G2C <G2B <G2A. This means that the resistance to texture deformation

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 213

These greases have been prepared and formulated according to the percent ingredient listed

property G3A G3B G3C G3D G3E

G2C, Wt % 99 98 97 96 95 Jojoba meal, Wt % 1 2 3 4 5

Dropping point, °C 188 190 192 195 198

pressure, drop, psi 2.5 2.3 2.0 1.5 1.5

72h, 1.2 1.0 1.0 0.995 0.937

72h 0.821 0.7921 0.7501 0.7023 0.6813

Alkalinity, Wt% 0.12 0.13 .14 0.15 0.15

KOH/g @ 72 h 0.15 0.15 0.14 0.12 0.12

Oil separation, Wt% 1.8 1.8 1.7 1.7 1.6

3h/100°C Ia Ia Ia Ia Ia

90 °C 41820 42032 42232 42611 42652

Yield stress, D/cm2 80.6 82.5 85.0 86.4 86.6

Four ball weld load ,Kg 235 240 245 250 250

2 LB

Table 8. Effect of addition of jojoba meal on properties of the selected prepared grease G2C

2 LB

2 LB

2 LB

2 LB 280 285 278 280 275 277 275 277

282 287 Test method

> ASTM D-217

> ASTM D-566

> ASTM D-942

> ASTM D-942

> ASTM D-942

> ASTM D-664

> ASTM D-664

ASTM D-1724

ASTM D-4048

ASTM D-189

ASTM D-2596

in Table (8).

Ingredient&

Penetration at 25°C

Un worked worked

Oxidation Stability 99± 96h,

Intensity of (C=O) group @

Intensity of (OH) group@

Total acid number, mg

Copper Corrosion

Egyptian Standard

Apparent Viscosity, cP, @

Code grease NLGI

Symbol

decreases with increase of jojoba oil ratio in the prepared grease. It may be indicated also that on increasing the ratio jojoba oil additive to the prepared greases would increase binding and compatibility of the grease ingredient. As a result, the dropping point values for prepared greases G2A, G2B and G2C increased to 178, 180 and 183°C, respectively.

Table (7) shows, in general, the positive effect of all concentrations of jojoba oil additive on the proprieties of G2A, G2B and G2C. In this respect, the 5%wt of additive of jojoba oil showed a marked improvements effect. Such improvements may be attributed to the unique properties of jojoba oil, e.g. high viscosity index 257, surface tension 45 mN/m and its chemical structure **(**Wisniak, 1987)**.** Based on these properties and correlation with the dropping point, penetration, oil separation, oxidation stability, dynamic viscosity, consistency index and yield stress data, its clear that the suitable and selective grease formula is G2C.


Table 7. Effect of addition of Jojoba oil on properties of the selected prepared grease G1G

#### **3.4 Effect of the jojoba meal additive**

Because greases are colloidal systems, they are sensitive to small amounts of additives. To study the effect of jojoba meal additive on the properties of the selected grease G2C, five grades of lithium lubricating greases containing different concentrations of jojoba meal additive were prepared. These concentrations included 1 wt%,, 2 wt%,, 3 wt%, 4 wt% and 5 wt% yielding G3A, G3B, G3C, G3D and G3E greases, respectively.

decreases with increase of jojoba oil ratio in the prepared grease. It may be indicated also that on increasing the ratio jojoba oil additive to the prepared greases would increase binding and compatibility of the grease ingredient. As a result, the dropping point values

Table (7) shows, in general, the positive effect of all concentrations of jojoba oil additive on the proprieties of G2A, G2B and G2C. In this respect, the 5%wt of additive of jojoba oil showed a marked improvements effect. Such improvements may be attributed to the unique properties of jojoba oil, e.g. high viscosity index 257, surface tension 45 mN/m and its chemical structure **(**Wisniak, 1987)**.** Based on these properties and correlation with the dropping point, penetration, oil separation, oxidation stability, dynamic viscosity, consistency index and yield stress data, its clear that the suitable and selective grease

**Test method** 

280 ASTM D-217

for prepared greases G2A, G2B and G2C increased to 178, 180 and 183°C, respectively.

formula is G2C.

Penetration at 25°C

Un worked worked

Oxidation Stability 99±96h,

NLGI Egyptian Standard

**3.4 Effect of the jojoba meal additive** 

Apparent Viscosity, cP, @

Total acid number, mg

Copper Corrosion

Code Grease

Symbol

Ingredient & property **G2A G2B G2C**

G1g, wt% 99 97 95 Jojoba oil, wt% 1 3 5

> 284 289

2 LB

Yield stress, D/cm2 75.6 78.1 80.6

wt% yielding G3A, G3B, G3C, G3D and G3E greases, respectively.

278 282

Dropping point, °C 180 182 187 ASTM D-566

pressure, drop, psi 3.5 3.2 3.0 ASTM D-942 Alkalinity, Wt% 0.16 0.14 0.14 ASTM D-664

KOH/g, @72h 0.20 0.18 0.16 ASTM D-664 Oil separation, Wt% 1.8 1.8 1.7 ASTM D-1724

Four ball weld load, Kg 188 190 195 ASTM D -2596 Table 7. Effect of addition of Jojoba oil on properties of the selected prepared grease G1G

Because greases are colloidal systems, they are sensitive to small amounts of additives. To study the effect of jojoba meal additive on the properties of the selected grease G2C, five grades of lithium lubricating greases containing different concentrations of jojoba meal additive were prepared. These concentrations included 1 wt%,, 2 wt%,, 3 wt%, 4 wt% and 5

3h/100°C Ia Ia Ia ASTM D-4048

2 LB

90 °C 39891 41090 41294 ASTM D-189

277

2 LB


These greases have been prepared and formulated according to the percent ingredient listed in Table (8).

Table 8. Effect of addition of jojoba meal on properties of the selected prepared grease G2C

Four ball weld load ,Kg 235 240 245 250 250

D-2596

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 215

operative forces among lithium soap, lubricating fluid, jojoba oil and its meal. Also, the variety in fatty acids (soapstock and bone fat compositions) lead to the soap particles will arrange themselves to form soap crystallites, which looks a fiber in the grease. These soap fibers are disposed in a random manner within a given volume. This packing will automatically ensure many fiber contacts, and as a result, an oil-retentive pore network is formed, which is usually known as the gel network. When a stress is applied to this network, a sufficient number of contact junctions will rupture to make flow possible. The resistance value associated with the rupture is known as yield stress. Therefore yield stress

> G1G G2C G3D

> > G1G G2C G3D

0 20 40 60 80 100 120 140 160 **Shear rate, s-1**

0 20 40 60 80 100 120 140 160 **Shear rate, S-1**

Fig. 2. Variation of shear stress with shear rate for G1G, G2C and G3D at 120°C

Fig. 1. Variation of shear stress with shear rate for G1G, G2C and G3D at 90°C

can be defined as the stress value required to make a grease flow (Barnes, 1999).

**Shear stress,D/Cm2**

**Shear stress, D/Cm2**

Data in this table reveal that all concentrations of the JM exhibit marked improvements in all properties of the investigated greases compared with the corresponding grease G2C without jojoba meal. In addition, the difference of penetration values between unworked and worked for greases G3A-3E decreased markedly by increasing jojoba meal content in the range of 1wt to 3wt%. Further increase of the jojoba meal concentration up to 4 and 5% by wt shows almost no difference. Parallel data are obtained concerning dropping point, dynamic viscosity, oil separation and total acid number of greases G3A-3E. Such improving effect, as mentioned above, could be attributed to the high polarity of jojoba meal constitutes, which result in increasing both the compatibility and electrostatic forces among the ingredients of the prepared greases under investigation. Based on the improvement in the dynamic viscosity, consistency, dropping point and oil separation of the addition jojoba meal to the selected grease G2C (Table 8), a suggested mechanism for this improvement is illustrated in the Schemes 1& 2**.** This suggested mechanism explains the ability of jojoba meal ingredients (amino-acids and polyphenolic compounds) to act as complexing agents leading to grease G3D which is considered the best among all the investigated greases. This agrees well with previous reported results in this connection **(**El-Adly et al, 2009).

The aforementioned studies on the effects of fatty materials, jojoba oil and meal reveal that the selective greases are G1G, G2C and G3D, respectively.

#### **3.5 Evaluation of the selected greases (G1G, G2C and G3D) 3.5.1 Rheological behavior**

Lubricating grease, according to rheological definition, is a lubricant which under certain loads and within its range of temperature application, exhibits the properties of a solid body, undergoes plastic strain and starts to flow like a liquid should the load reach the critical point, and regains solid body like properties after the removal of stress (Sinitsyn, 1974).

Rheology is the cornerstone of any quantitative analysis of processes involving complex materials. Because grease has rather complex rheological (Wassermann, 1991) properties it has been described as both solid and liquid or as viscoelastic plastic solids. It is not thick oil but thickened oil. The grease matrix is held together by internal binding forces giving the grease a solid character by resisting positional change. This rigidity is commonly referred to as consistency. When the external stress exceed the threshold level of sheer (stress or strain) the yield value-the solid goes through a transitional state of plastic strain before turning into a flowing liquid. Consistency can be seen the most important property of a lubricating grease, the vital difference between grease and oil. Under the force of gravity, grease is normally subjected to shear stresses below the yield and will therefore remain in place a solid body. At higher level of shear, however, the grease will flow. Therefore, it is the utmost important to be able to determine the exact level of yield (Gow, 1997).

The rheological measurement of the selected greases is tested using Brookfield Programmable Rheometer HADV-III ULTRA in conjunction with software RHEOCALC. V.2. All Rheometer functions (rotational speed, instrument % torque scale, time interval, set temperature) are controlled by a computer. The temperature is controlled by connection with bath controller HT-107 and measured by the attached temperature probe. In this respect, the rheological behavior of the selected greases G1G, G2C and G3D are determined at 90 °C and 120 °C. Figures 1 and 2 afford nearly linear plots having different yield values. Also, they indicate that the flow behavior of greases at all temperatures obey plastic flow. This is due to

Data in this table reveal that all concentrations of the JM exhibit marked improvements in all properties of the investigated greases compared with the corresponding grease G2C without jojoba meal. In addition, the difference of penetration values between unworked and worked for greases G3A-3E decreased markedly by increasing jojoba meal content in the range of 1wt to 3wt%. Further increase of the jojoba meal concentration up to 4 and 5% by wt shows almost no difference. Parallel data are obtained concerning dropping point, dynamic viscosity, oil separation and total acid number of greases G3A-3E. Such improving effect, as mentioned above, could be attributed to the high polarity of jojoba meal constitutes, which result in increasing both the compatibility and electrostatic forces among the ingredients of the prepared greases under investigation. Based on the improvement in the dynamic viscosity, consistency, dropping point and oil separation of the addition jojoba meal to the selected grease G2C (Table 8), a suggested mechanism for this improvement is illustrated in the Schemes 1& 2**.** This suggested mechanism explains the ability of jojoba meal ingredients (amino-acids and polyphenolic compounds) to act as complexing agents leading to grease G3D which is considered the best among all the investigated greases. This agrees well with

The aforementioned studies on the effects of fatty materials, jojoba oil and meal reveal that

Lubricating grease, according to rheological definition, is a lubricant which under certain loads and within its range of temperature application, exhibits the properties of a solid body, undergoes plastic strain and starts to flow like a liquid should the load reach the critical point, and regains solid body like properties after the removal of stress (Sinitsyn,

Rheology is the cornerstone of any quantitative analysis of processes involving complex materials. Because grease has rather complex rheological (Wassermann, 1991) properties it has been described as both solid and liquid or as viscoelastic plastic solids. It is not thick oil but thickened oil. The grease matrix is held together by internal binding forces giving the grease a solid character by resisting positional change. This rigidity is commonly referred to as consistency. When the external stress exceed the threshold level of sheer (stress or strain) the yield value-the solid goes through a transitional state of plastic strain before turning into a flowing liquid. Consistency can be seen the most important property of a lubricating grease, the vital difference between grease and oil. Under the force of gravity, grease is normally subjected to shear stresses below the yield and will therefore remain in place a solid body. At higher level of shear, however, the grease will flow. Therefore, it is the

The rheological measurement of the selected greases is tested using Brookfield Programmable Rheometer HADV-III ULTRA in conjunction with software RHEOCALC. V.2. All Rheometer functions (rotational speed, instrument % torque scale, time interval, set temperature) are controlled by a computer. The temperature is controlled by connection with bath controller HT-107 and measured by the attached temperature probe. In this respect, the rheological behavior of the selected greases G1G, G2C and G3D are determined at 90 °C and 120 °C. Figures 1 and 2 afford nearly linear plots having different yield values. Also, they indicate that the flow behavior of greases at all temperatures obey plastic flow. This is due to

utmost important to be able to determine the exact level of yield (Gow, 1997).

previous reported results in this connection **(**El-Adly et al, 2009).

the selective greases are G1G, G2C and G3D, respectively.

**3.5.1 Rheological behavior** 

1974).

**3.5 Evaluation of the selected greases (G1G, G2C and G3D)** 

operative forces among lithium soap, lubricating fluid, jojoba oil and its meal. Also, the variety in fatty acids (soapstock and bone fat compositions) lead to the soap particles will arrange themselves to form soap crystallites, which looks a fiber in the grease. These soap fibers are disposed in a random manner within a given volume. This packing will automatically ensure many fiber contacts, and as a result, an oil-retentive pore network is formed, which is usually known as the gel network. When a stress is applied to this network, a sufficient number of contact junctions will rupture to make flow possible. The resistance value associated with the rupture is known as yield stress. Therefore yield stress can be defined as the stress value required to make a grease flow (Barnes, 1999).

Fig. 1. Variation of shear stress with shear rate for G1G, G2C and G3D at 90°C

Fig. 2. Variation of shear stress with shear rate for G1G, G2C and G3D at 120°C

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 217

combination among Li-soap, amino acids, and polyphenolic compounds scheme (1 &2), in

Al3+, Fe2+, Cu2+, Ba2+, Sr2+, Mn2+, Zn2+, Co2+ and Ni2+) in jojoba meal. These chemical elements are in such a form, that under pressure between metal surfaces they react with the metal to produce a coating film which will either sustain the load or prevent welding of the two metals together. This view introduces the key reasons for the improvements of the loadcarrying properties and agrees well with the data previously reported by El-Adly et al

On other hand, it has been found that some thickening agents used in grease formulation inhibit the action of EP additives (Silver & Stanly 1974). The additives most commonly used as anti-seize and anti-scuffing compounds are graphite and molybdenum disulphide.

The oxidation stability of grease (ASTM D-942) is the ability of the lubricant to resist oxidation. It is also used to evaluate grease stability during its storage. The base oil in grease will oxidize in the same way as lubricating oil of a similar type. The thickener will also oxidize but is usually less prone to oxidation than the base oil. So, anti-oxidant additive must be selected to match the individual grease. Their primary function is to protect the

> 18 38 58 78 98 118 138 **Time,hr**

Oxidative deterioration for the selected greases G1G, G2C and G3D are determined by the total acid number at oxidative times ranging from zero to 120 hours Figures (3). In addition, pressure drop, in psi. at 96 hour for greases G1G, G2C and G3D are 4.0, 3.0 and 1.5 psi respectively. These results give an overview on the efficiency of the jojoba meal and jojoba oil in controlling the oxidation reactions compared with the grease without additive G1G. Jojoba oil in conjunction with jojoba meal additive proves to be successful in controlling and inhibiting the oxidation of the selected grease G3D. Inhibition of oxidation can be accomplished in two main ways: firstly by removal of peroxy radicals, thus breaking the oxidation chain, secondly, by obviating or discouraging free radical formation. A suggested

Fig. 3. Effect of deterioration time on Total Acid Number for selected greases

grease during storage and extend the service life, especially at high temperatures.

and F-

) and cation (Li+, Na+, K+, Ca2+, Mg2+,

G3D G2C G1G

addition to the role of anion (PO43-, SO42-, Cl-

(2004).

**3.5.3 Oxidation stability** 

0.05

0.1

0.15

0.2

0.25

**TAN,mg KOH**

0.3

0.35

0.4

In this respect, Rheological data apparent viscosity and yield stress (Tables 6, 7 & 8), for the selected greases show improvement and reinforcement in the order G3D > G2C > G1G. This is attributed to the ability of jojoba meal to enhance the resistance to flow for G3D, due to the action of the jojoba meal containing amino acids which act as chelating compounds, columbic interactions and hydrogen bonding, with Li-soap Scheme (1& 2). Also, according to the basic information on the composition of the jojoba meal (Verbiscar, et al., 1978; Cardeso, et al., 1980; Wisniak, 1994), amino acids, wax ester, fatty materials, polyphenolic compounds and fatty alcohols in jojoba meal could be acting as natural emulsifiers leading to increase in the compatibility among the grease ingredients. There is evidence that soap and additive have significant effects on the rheological behavior.

The flow and viscoelastic properties of a lubricating grease formed from a thickener composed of lithium hydroxystearate and a high boiling point mineral oil are investigated as a function of thickener concentration (Luckham & Tadros, 2004).

Scheme 1. The role of amino acids as complexing agent with texture of lithium soap grease

#### **3.5.2 Extreme-pressure properties**

Extreme pressure additives (EP) improve, in general, the load-carrying ability in most rolling contact bearing and gears. They react with the surface to form protective films which prevent metal to metal contact and the consequent scoring or welding of the surfaces. The EP additives are intended to improve the performance of grease. In this respect, the selected greases are usually tested in a four ball machine where a rotating ball slides over three stationary balls using ASTM-D 2596 procedure. The weld load data for the selected greases G1G, G2C and **G3D** are 170, 195 and 250 Kg, respectively. These results indicate that the selected grease containing jojoba oil and jojoba meal G3D exhibit remarkable improvement in extreme pressure properties compared with grease without additives G1G and grease G2C with jojoba oil alone. This may be attributed to the synergistic effect of the complex combination among Li-soap, amino acids, and polyphenolic compounds scheme (1 &2), in addition to the role of anion (PO43-, SO42-, Cl and F- ) and cation (Li+, Na+, K+, Ca2+, Mg2+, Al3+, Fe2+, Cu2+, Ba2+, Sr2+, Mn2+, Zn2+, Co2+ and Ni2+) in jojoba meal. These chemical elements are in such a form, that under pressure between metal surfaces they react with the metal to produce a coating film which will either sustain the load or prevent welding of the two metals together. This view introduces the key reasons for the improvements of the loadcarrying properties and agrees well with the data previously reported by El-Adly et al (2004).

On other hand, it has been found that some thickening agents used in grease formulation inhibit the action of EP additives (Silver & Stanly 1974). The additives most commonly used as anti-seize and anti-scuffing compounds are graphite and molybdenum disulphide.

#### **3.5.3 Oxidation stability**

216 Tribology - Lubricants and Lubrication

In this respect, Rheological data apparent viscosity and yield stress (Tables 6, 7 & 8), for the selected greases show improvement and reinforcement in the order G3D > G2C > G1G. This is attributed to the ability of jojoba meal to enhance the resistance to flow for G3D, due to the action of the jojoba meal containing amino acids which act as chelating compounds, columbic interactions and hydrogen bonding, with Li-soap Scheme (1& 2). Also, according to the basic information on the composition of the jojoba meal (Verbiscar, et al., 1978; Cardeso, et al., 1980; Wisniak, 1994), amino acids, wax ester, fatty materials, polyphenolic compounds and fatty alcohols in jojoba meal could be acting as natural emulsifiers leading to increase in the compatibility among the grease ingredients. There is evidence that soap

The flow and viscoelastic properties of a lubricating grease formed from a thickener composed of lithium hydroxystearate and a high boiling point mineral oil are investigated

**O**

**O H O C**

**Glutamic acid**

**Glycine**

**N Li O**

**CH2 CH C**

**H H Li O**

**··**

**O C**

Scheme 1. The role of amino acids as complexing agent with texture of lithium soap grease

Extreme pressure additives (EP) improve, in general, the load-carrying ability in most rolling contact bearing and gears. They react with the surface to form protective films which prevent metal to metal contact and the consequent scoring or welding of the surfaces. The EP additives are intended to improve the performance of grease. In this respect, the selected greases are usually tested in a four ball machine where a rotating ball slides over three stationary balls using ASTM-D 2596 procedure. The weld load data for the selected greases G1G, G2C and **G3D** are 170, 195 and 250 Kg, respectively. These results indicate that the selected grease containing jojoba oil and jojoba meal G3D exhibit remarkable improvement in extreme pressure properties compared with grease without additives G1G and grease G2C with jojoba oil alone. This may be attributed to the synergistic effect of the complex

and additive have significant effects on the rheological behavior.

as a function of thickener concentration (Luckham & Tadros, 2004).

**O H O C**

**N Li O**

**O C H O**

**O Li**

**C=O**

**H2C C**

**H H Li O**

**··**

**O C**

**3.5.2 Extreme-pressure properties** 

**O**

The oxidation stability of grease (ASTM D-942) is the ability of the lubricant to resist oxidation. It is also used to evaluate grease stability during its storage. The base oil in grease will oxidize in the same way as lubricating oil of a similar type. The thickener will also oxidize but is usually less prone to oxidation than the base oil. So, anti-oxidant additive must be selected to match the individual grease. Their primary function is to protect the grease during storage and extend the service life, especially at high temperatures.

Fig. 3. Effect of deterioration time on Total Acid Number for selected greases

Oxidative deterioration for the selected greases G1G, G2C and G3D are determined by the total acid number at oxidative times ranging from zero to 120 hours Figures (3). In addition, pressure drop, in psi. at 96 hour for greases G1G, G2C and G3D are 4.0, 3.0 and 1.5 psi respectively. These results give an overview on the efficiency of the jojoba meal and jojoba oil in controlling the oxidation reactions compared with the grease without additive G1G. Jojoba oil in conjunction with jojoba meal additive proves to be successful in controlling and inhibiting the oxidation of the selected grease G3D. Inhibition of oxidation can be accomplished in two main ways: firstly by removal of peroxy radicals, thus breaking the oxidation chain, secondly, by obviating or discouraging free radical formation. A suggested

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 219

**Intramolecular hydrogen bond**

**C N CH**

**RO**

**O**

**ROCH2 O**

**OR**

**OR**

**OR**

**OR**

**RO**

**ROCH2**

**OH**

**CN**

**OCH3**

**OCH3**

**C**

**OCH3**

**O**

**C N**

**OCH3**

**H**

**O O**

**O O**

**OR**

**O**

**OR**

**RO**

**4. Future research** 

**ROCH2**

2007).

**OR**

**OR**

**1,3 H**

**OR**

**RO**

**RO**

**ROCH2**

**Simmondsin**

**ROCH2**

**OR**

**CH**

**OCH3**

**CH**

**R or ROO**

**OCH3**

**C**

**OCH3**

**OH**

**<sup>O</sup> <sup>O</sup> <sup>O</sup>**

**Intramolecular hydrogen bond**

Base oils used to formulate greases are normally petroleum or synthetic oils. Due to growing environmental awareness and stringent regulations on the petroleum products uses, research and development in the area of eco-friendly grease is now gaining importance. Since biodegradable synthetic ester lubricant is higher in cost, vegetable oils are drawing attention economically as biodegradable alternates for synthetic esters. Looking forward into the next decade, the need for more advanced science in grease technology is essential. The design of special components is becoming increasingly complicated and machines are becoming much smaller and lighter in weight and are required to run faster and withstand heavier loads. To be able to develop the optimal lubricants for these new conditions, the mechanism behind grease lubrication must be further studied and understood. There will be an increased specialization in both products and markets and the survival of individual lubricants companies will depend on their ability to adapt to changing conditions. Not only machines but also new materials will affect the development of greases. Biogreases (El-Adly et al 2010) and nanogrease have better lubricating properties such as, wear protection, corrosion resistance, friction reduction, heat removal, etc. In this respect, anti-friction, antiwear and load-carrying environment friendly additives are prepared from non-traditional vegetable oils and alkyl phenols of agricultural, forest and wasteland origin (Anand, et al,

**CN**

**OCH3**

Scheme 3. The role of the Simmondsin as antioxidant for prepared lithium grease

**OH**

**OCH3**

**O**

**CN**

**OCH3**

mechanism for this inhibition is illustrated in the Schemes (2 & 3). The efficiency of jojoba meal ingredients as antioxidants is here postulated due to the presence of phenolic groups and hyper conjugated effect. Accordingly, Simmondsin derivatives and polyphenolic compounds which are considered the main component of jojoba meal include in their composition electron rich centers, which act as antioxidants by destroying the peroxides without producing radicals or reactive oxygenated products.

**Stable radical**

Scheme 2. The role of Polyphenolic compounds as antioxidant for prepared lithium grease

Scheme 3. The role of the Simmondsin as antioxidant for prepared lithium grease

#### **4. Future research**

218 Tribology - Lubricants and Lubrication

mechanism for this inhibition is illustrated in the Schemes (2 & 3). The efficiency of jojoba meal ingredients as antioxidants is here postulated due to the presence of phenolic groups and hyper conjugated effect. Accordingly, Simmondsin derivatives and polyphenolic compounds which are considered the main component of jojoba meal include in their composition electron rich centers, which act as antioxidants by destroying the peroxides

**C**

**Poly phenolic compound**

 **(Tannic acid)**

**OH**

**OH**

**OH**

**O**

**OH**

**OH2**

**O**

**OH**

**OH**

**RH or ROH**

**OH**

**O**

**· ·**

**· R or RO·**

**O C**

**HC <sup>C</sup> OH**

without producing radicals or reactive oxygenated products.

**O**

**C**

**HC C**

**O**

**C H2 O C**

**H**

**OH**

**H**

**OH**

**O**

**C**

**H**

**OH**

**C**

**HC C**

**HC <sup>C</sup>** <sup>Ο</sup>

**C**

**OH**

**O**

**C OH**

**O C**

**Stable radical**

Scheme 2. The role of Polyphenolic compounds as antioxidant for prepared lithium grease

**O**

**·**

**Rearangment by resonance**

**O C**

**O**

**H**

**OH**

**O**

**C H2 C**

**H**

**OH**

**O**

**C**

**HC C**

**· <sup>O</sup>**

**H**

**OH**

**OH**

**H**

**HO**

**H**

**OH**

**H**

**H2C**

**HO**

**H**

**OH**

**H**

**HO**

**H**

Base oils used to formulate greases are normally petroleum or synthetic oils. Due to growing environmental awareness and stringent regulations on the petroleum products uses, research and development in the area of eco-friendly grease is now gaining importance. Since biodegradable synthetic ester lubricant is higher in cost, vegetable oils are drawing attention economically as biodegradable alternates for synthetic esters. Looking forward into the next decade, the need for more advanced science in grease technology is essential. The design of special components is becoming increasingly complicated and machines are becoming much smaller and lighter in weight and are required to run faster and withstand heavier loads. To be able to develop the optimal lubricants for these new conditions, the mechanism behind grease lubrication must be further studied and understood. There will be an increased specialization in both products and markets and the survival of individual lubricants companies will depend on their ability to adapt to changing conditions. Not only machines but also new materials will affect the development of greases. Biogreases (El-Adly et al 2010) and nanogrease have better lubricating properties such as, wear protection, corrosion resistance, friction reduction, heat removal, etc. In this respect, anti-friction, antiwear and load-carrying environment friendly additives are prepared from non-traditional vegetable oils and alkyl phenols of agricultural, forest and wasteland origin (Anand, et al, 2007).

Lubricating Greases Based on Fatty By-Products and Jojoba Constituents 221

El-Shattory, Y. (1979). Statistical Studies on Physical and Chemical Characteristics,

Greene, R.A. & Foster, E. D. (1933). The Liquid Wax of Seeds of *Simmondsia Californica*, Bot.

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Ismail, I.A. (2008). A Study on the Utilization of Jojoba Oil and Meal as Additives for Lubricating Oils and Greases, Ph.D. Thesis Ain Shams University, Egypt. Gatto, V. J. & Grina, M.A. (1999). Effect of Base Oil Type, Oxidation Test Conditions and

Kieke, M. L. (1998). Microwave Assisted Digestion of Zinc, Phosphorus and Molybdenum in

Kinnear, S. & Kranz, K. (1998). An Economic Evaluation of 12- Hydroxyl Stearic Acid and

Klamann, D. (1984). Lubrications and Related Products: Synthesis, Proprties, Applications,

Kono, Y.; Tomita, K.; Katsura, H. & Ohta, S. (1981). Antioxidant in Jojoba Crude Oil, In:

Kuester, J. L.; Fernandez Carmo,T.C. & Heath, G. (1985). *Fundam. Thermochem .Biomass* 

Lansdown, A. R. (1982). Lubrication, A Practical Guide to Lubricant Selection, Pergamon

Luckham, P. F.& Tadros,Th.F. (2004). Steady Flow and Viscoelastic Properties of Lithium

Mang, T. & Dresel, W. (2001). Lubricants and Lubrication, *WILEY-VCH,* ISBN 3-527-295-36-

Michael, K. Dowd. (1996). Compositional Characterization of Cottonseed Soapstock*,* 

Miwa, T.K, (1980). Chemical Structure and Propreties of Jojoba Oil, In: M. Puebla (Editor),

Miwa, T.K. & Hagemann, J.W. (1978). Physical and Chemical Properties of Jojoba Liquid

Analysis of Lubricating greases, *NLGI Spoksman* Vol.62, pp. 29-35.

Phenolic Antioxidant Structure on the Detection and Magnitude of Hindered Phenol/ Diphenylamine Synergism, Lubrication Engineering, Vol.55, pp.11-20. Gow, G. (1997). Lubricating Greases, in Chemistry and Technology of Lubricants, 2nd edn,

(Eds R.M. Mortiers, S. T. Orszulik), Blackie Academic and Professional, London, pp

Hydrogenated Castor Oil as Raw Materials for Lithium Soap Lubricating Grease,

Puebla, (Editor), Proceedings of the Fourth International Conference on Jojoba,

Grease Containing Various Thickener Concentration, *Journal of colloid and Interface* 

Proceeding of *the Fourth International Conference on Jojoba*, Hermosillo, pp pp 227-

and Solid Waxes, In: Proceedings of *the Second International Conference on jojoba and* 

International Scientists Association, Egypt, March 7-9 2010.

Soapstock, *Rev. Fr. Corps Gras*. Vol, 26, pp.187-190. Erlich, M. (ed). (1984). NLGI Lubricating Grease Guide, *NLGI, Kansas City.*  Flaxman,M.T.( 1940). Sulfurized Lubricating Oil, *U.S.Patant* 2,212,899.

Gaz., Vol.94, pp. 826-828.

307-319.

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*NLGI Spokesman*, Vol.62, No.5 pp.13-19.

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Hermosillo, pp 239-256.

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*Convers*: 875.

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4, New York

235.

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*Iits Uses,* Bnsenada, pp 245-252, 1976.

Research Institute In Cooperation with EURO-Arab Cooperation Center &

Phospholipids and Fatty Acid Constitution of Different Processed Cottonseed

#### **5. Conclusion**

Lubricating grease is an exceptionally complex product incorporating a high degree of technology in all the related sciences. The by-products, soapstock, bone fat, jojoba meal, produced from processing crude vegetable oils are valuable compounds for lubricating greases. Such byproducts have varieties of chemical compounds which show synergistic effect in enhancing and improving the grease properties. Advantages of these byproducts include also their low cost and large scale availability. Research in this area plays a great role in the economic, scientific and environmental fields.

#### **6. References**


Lubricating grease is an exceptionally complex product incorporating a high degree of technology in all the related sciences. The by-products, soapstock, bone fat, jojoba meal, produced from processing crude vegetable oils are valuable compounds for lubricating greases. Such byproducts have varieties of chemical compounds which show synergistic effect in enhancing and improving the grease properties. Advantages of these byproducts include also their low cost and large scale availability. Research in this area plays a great

Anand, O. N; Vijay, k.; Singh, A.K. & Bisht, R.P. (2007). Anti-friction, Anti-Wear and Load-

Boner, C.J. (1954). Manufacture and Application of Lubricating Greases, *New York Reinhold* 

Cann, P.M. (1997). Grease Lubrication Films in Rolling Contacts*, Eurogrease* Nov-Dec 1997,

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Cherry, J. P, & Berardi, I.C. (1983). Cottonseed, *Handbook of Processing and utilization in* 

Daugherty, P.M.; Sineath, H.H. & Wastler, T.A. (1953). Industrial Raw Material of Plant

El-Adly R. A. (1999). Producing Multigrade Lubricating Greases from Animal and Vegetable

El-Adly R. A.; El-Sayed S. M. & Ismail M. M. (2005). Studies on The Synthesis and

El-Adly, R.A & Enas A. Ismail. (2009). Study on Rheological Behavior of Lithium

El-Adly, R.A.; El-Sayed, S.M. & Moustafa, Y.M. (2004). A Novel Application of Jojoba Meal

El-Adly,R.A. (2004). A Comparative Study on the Preparation of Some Lithium Greases from Virgin and Recycled Oils, *Egypt J. Petrol* Vol.13, No, 1. pp. 95-103. El-Adly, R.A.; Enas, A.Ismail. & Modather, F. Houssien. (2010). A Study on Preparation and

*Agriculture*, Vol.II, edited by I.A.wolff, CRC press Inc, Boca Raton

Fat By-products. *J. Synthetic Lubrication*. Vol.16, No.4, pp. 323-332.

Mussoorie, India. February 19-21 2009 ( NLGI India Chapter)

of Jojoba Proteines. In: M. Puebla (Ed.) Proceedinf of *Forth International Conference* 

Origin, IV.A Survey of *Simmondsia Chinensis*, *Bull.Eng. Exp. Sta., Georgia* 

Utilization of Some Schiff's Bases: 1. Schiff's Bases as Antioxidants for Lubricating

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as Additives for Sodium Lubricating Grease, *The 7th International Conference on Petroleum & the Environment,* Egyptian petroleum Research Institute In Cooperation with EURO-Arab Cooperation Center & International Scientists Association, Cairo,

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Carrying Characteristics of Environment Friendly Additive Formulation,

role in the economic, scientific and environmental fields.

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*on Jojoba and its Uses*, Hermosillo, pp 305-316.

Greases. *J. Synthetic Lubrication* Vol.22, pp. 211-223.

**5. Conclusion** 

**6. References** 

*Publishing*.

pp. 6-22.

*Inst.Technol., 15(13).* 

Egypt. March 27-29 2004.

Research Institute In Cooperation with EURO-Arab Cooperation Center & International Scientists Association, Egypt, March 7-9 2010.


**9** 

*Japan* 

**Characterization of Lubricant on** 

When people started wearing eye-glasses from the 13th century until the middle of 20th century, the glass was the only material used for ophthalmic lenses. However, plastic lenses were rapidly developed and began to be widely used when PPG Industries, Inc. developed CR-39® in 1940; CR-39®, i.e., allyl diglycol carbonate (ADC), is a thermosetting resin that can be used as a lens material with a refractive index of 1.5. The features of this material are as follows: (1) it is a lightweight material (its specific gravity is half of that of glass), (2) it has strong impact resistance (i.e., it is shatter proof, which guarantees high safety), (3) it is stainable (i.e., has high fashionability), and (4) it can be used in a variety of frames (i.e., it has high fashionability or high workability). The quest for thinner lenses led to an increase in the refractive index of lenses, and current lenses have a super-high refractive index of 1.74

The biggest drawback of plastic lenses was that they could be "easily scratched," but they were improved sufficiently for practical use, by using a hard coating (HC), i.e., an overcoat formed on the plastic substrate. Subsequently, anti-reflection (AR) coating films were added to increase the clearness of the lens, to reduce the reflection from the ophthalmic lens as viewed by another person, and even to enhance measures for preventing scratches. In recent years, further value-adds have been made to plastic lenses, with the use of lubricants in the top layers for increasing durability, preventing contamination due to scratches on spectacle

Research on lubricants used for the improvement of tribology characteristics has progressed rapidly; it has been supported from the end of the 1980s by the development of surface analysis methods (Kimachi et al., 1987; Mate et al., 1989; Novotny et al., 1989; Newman et al., 1990; Mate et al., 1991; Toney et al., 1991; Novotny et al., 1994; Sakane et al., 1999; Tani, 1999; Tadokoro et al., 2001; Tadokoro et al., 2003) and by the technology for high-density magnetic disc recording used in personal computers. The main lubricant selected was perfluoropolyether (PFPE), because it possesses thermal stability, oxidation stability, low vapor pressure, low surface tension, and good boundary lubricity. It was effective in reducing the frictional wear of the surfaces of the magnetic disc and magnetic head, and thus, hundreds of thousands of stable data read-and-write operations could be conducted. The main parameters that determine lubricant properties are the structure, thickness, and

state of the lubricant, and various methods were used to investigate them.

**1. Introduction** 

or 1.76.

lenses, and facilitating "easy removal" of dirt.

**Ophthalmic Lenses** 

*HOYA corporation/VC Company* 

Nobuyuki Tadokoro


### **Characterization of Lubricant on Ophthalmic Lenses**

Nobuyuki Tadokoro *HOYA corporation/VC Company Japan* 

#### **1. Introduction**

222 Tribology - Lubricants and Lubrication

Miwa, T.K. & Rothfus, J.A. (1978). In-depth Comparison of Sulfurized Jojoba and Sperm

Miwa, T.K. (1971). Jojoba Oil wax Esters and Derived Fatty Acids and Alcohols, Gas

Miwa, T.K. (1973). Chemical Aspects of Jojoba Oil, a Unique Liquid Wax from Desert Shrub

Peeler, R.F.& Hartman, L.M,(1972). Evaluation of Sulfurized Sperm Oil Replacements*, NLGI* 

Pohlen, M. J. (1998). DSC- A Valuable Tool for the Grease Laboratory, NLGI Spokesman,

Robison, P. D.; Salmon, S.G.; Siber, J. R. & Williams, M.C. (1993). Elemintal Analysis of

Schultze, G. R. (1962). Wesen and Eufbau Derschmierfette in Zerbe, C. Mineralole and

Shirahama.(1985). The Effects of Temperature and Additive Interaction on Valve Train

Silver B.H.& Stanley R.I. (1974). Effect of The Thickener on The Efficiency of Load Carrying

Verbiscer,A. J.; Banigen,T. F. ;Weper, C. W. ; Reid, B. L.; Tlei, J. E.& Nelson, E. A. (1978).

Vinogradov, G.V. (1989). Rheological and Thermophysical Properties of Grease, Gordon and

Wills, J. G. (1985). Jojoba, New Crop for Arid Lands, New Material for Industry, National Research Council, National Academy Press, Washington, No,6, pp,130-150. Wisniak, J. (1987). The Chemistry and Technology of Jojoba Oil, *American Oil Chemists* 

Wisniak, J. (1994). Potential Uses of Jojoba Oil and Meal-a Review, *Industrial Crops and* 

Liquid Chromatography/Mass Spectrometry, *J. Am. Oil Chem. Soc.,* vol.54, pp 187-

Detoxification and Analysis of Jojoba Meal. In: D. M. Yermoanos (Ed.) Proceeding of the *Third International Conference on Jojoba and Its Uses*, Riverside, Calif. pp 185-

Chromatographic Analysis , *J.Am.Oil Chem*., Vol.48, pp 299-264

*Simmondsia californica*, Cosmet. Perfum, Vol.88, pp 39-41

verwandte Produkte, *2nd edn, Springer* Berlin, pp .405-432.

Wear, Proc. *JSLE.Int. Trib.Conf*. Tokyo,Japan, pp 331-336 8-10 July

Additives in Greases, *Tribology International*, Vol.7, pp 113-118. Sinitsyn, V. V. (1974). The Choice and Application of Plastic Greases, *Khimiya, Moscow.*  Spencer, G.F.; Plattner, R.D.& Miwa, T. K, (1977). Jojoba Oil Analysis by High Pressure

Wells, F. B. (1948). Process of Making Sulfurized Jojoba oil *U.S. Patent* 2,450,403.

Wassermann, G. From Heraklit to Blair,W. S. (1991). *Rheology,* Vol.91 pp 32-38.

Greases, NLGI Spokesman, Vol.56, pp 157-160.

Breach Science Publications, London.

*Society,* Champaign, Illinois.

*Products* Vol.3 pp, 43-68.

*Uses*, Riverside, Calif., pp 243-267

*Spokesman,* vol.37, No.17

Vol.62, pp 11-16.

189.

197.

Whale Oils a Extreme Pressure Extreme Temperature Lubricants, In: D.M. Yermanos (Editor), Proceeding of *the Third International Conference on Jojoba and Its* 

> When people started wearing eye-glasses from the 13th century until the middle of 20th century, the glass was the only material used for ophthalmic lenses. However, plastic lenses were rapidly developed and began to be widely used when PPG Industries, Inc. developed CR-39® in 1940; CR-39®, i.e., allyl diglycol carbonate (ADC), is a thermosetting resin that can be used as a lens material with a refractive index of 1.5. The features of this material are as follows: (1) it is a lightweight material (its specific gravity is half of that of glass), (2) it has strong impact resistance (i.e., it is shatter proof, which guarantees high safety), (3) it is stainable (i.e., has high fashionability), and (4) it can be used in a variety of frames (i.e., it has high fashionability or high workability). The quest for thinner lenses led to an increase in the refractive index of lenses, and current lenses have a super-high refractive index of 1.74 or 1.76.

> The biggest drawback of plastic lenses was that they could be "easily scratched," but they were improved sufficiently for practical use, by using a hard coating (HC), i.e., an overcoat formed on the plastic substrate. Subsequently, anti-reflection (AR) coating films were added to increase the clearness of the lens, to reduce the reflection from the ophthalmic lens as viewed by another person, and even to enhance measures for preventing scratches. In recent years, further value-adds have been made to plastic lenses, with the use of lubricants in the top layers for increasing durability, preventing contamination due to scratches on spectacle lenses, and facilitating "easy removal" of dirt.

> Research on lubricants used for the improvement of tribology characteristics has progressed rapidly; it has been supported from the end of the 1980s by the development of surface analysis methods (Kimachi et al., 1987; Mate et al., 1989; Novotny et al., 1989; Newman et al., 1990; Mate et al., 1991; Toney et al., 1991; Novotny et al., 1994; Sakane et al., 1999; Tani, 1999; Tadokoro et al., 2001; Tadokoro et al., 2003) and by the technology for high-density magnetic disc recording used in personal computers. The main lubricant selected was perfluoropolyether (PFPE), because it possesses thermal stability, oxidation stability, low vapor pressure, low surface tension, and good boundary lubricity. It was effective in reducing the frictional wear of the surfaces of the magnetic disc and magnetic head, and thus, hundreds of thousands of stable data read-and-write operations could be conducted. The main parameters that determine lubricant properties are the structure, thickness, and state of the lubricant, and various methods were used to investigate them.

Characterization of Lubricant on Ophthalmic Lenses 225

allergy). The results in figure 3 show that changing the surface condition reduces the amount of pollen adhered to the ophthalmic lens brought indoors. As in the example of scratches, the results show the possibility that the surface condition can be controlled to

Fig. 2. Scratch test results for 3types lubricants: the lens was scrubbed 20 times with 2 kg

A B

Fig. 3. Comparison between surface condition and cedar pollen adheres to the lens

Commercial ophthalmic lenses of allyl diglycole carbonate (ADC, CR-39®) were used in this study. In addition, the detailed estimations of lubricants were carried out directly on silicon wafer in order to avoid the influence of surface curvature, roughness, or amorphous states

change the amount of dirt that adheres to the lenses.

steel wool

**2.1 Experimental** 

**2.1.1 Sample preparation** 

On the other hand, the purpose of using a lubricant for ophthalmic lenses is to improve a scratch resistance, to prevent contamination, and to facilitate "easy removal" of dirt; the tribology characteristics of such a lubricant are similar to those of the lubricant used on magnetic discs, and has possibilities of application. There are two differences between lubricants used for ophthalmic lenses and those used for magnetic discs: (1) the film thickness of the lubricant used for magnetic discs does not need to be reduced, because the recording density achieved by using the lubricant for the magnetic disc increases exponentially when the gap between the magnetic disc surface and magnetic head is reduced as much as possible (to approximately 1 nm), and (2) the lubricant for ophthalmic lenses needs to be solid, but magnetic discs can be solid or liquid if stiction, in which a magnetic head sticks to the surface of a magnetic disc does not occur. However, in the case of ophthalmic lenses, dirt, dust, and fingerprints frequently block the view of the user, and the user cleans the lenses with water or rubs them with a soft cloth or paper; therefore, liquid lubricants can cause adhesion problems and does not last for a long time. In reality, conference presentations and papers are limited to information provided by the authors (Tadokoro et al, 2009; Tadokoro et al, 2010; Tadokoro et al, 2011). This chapter discusses tribology, with a focus on the characterization of lubricants, and presents analysis and evaluation results based on the film thickness, structure, distribution, and abrasion resistance of lubricants reported by the authors.

#### **2. Scratches and dirt**

Figure 1 shows optical microscopic pictures of ophthalmic lens returned by a consumer who complained about the quality. The different colors in the picture demonstrate the peeling of the AR coating films along the scratch, and thus, the small scratches become visible. Details on how and when the lenses were used are unknown, but it must be understood that scratches actually occur and this problem must be taken into account; this picture shows the importance of surface reforming based on the use of lubricants. While scratch-free lenses cannot be made only by modifying lubricants, the lubricant is one of the most important factors that affect the formation of scratches. Figure 2 shows the results of an abrasion test conducted by scrubbing a lens 20 times with 20 kg steel wool for different lubricants. The results show that the formation of scratches can be controlled by changing the structure or the distribution state of the lubricant. Finally as an example of the comparison of dirt adhesion, figure 3 shows the adhesion of cedar pollen on the lens. In Japan, hay fever, a seasonal allergy caused by cedar pollen, is very common (30% of the citizens have this

Fig. 1. Damaged ophthalmic lens and scratches

On the other hand, the purpose of using a lubricant for ophthalmic lenses is to improve a scratch resistance, to prevent contamination, and to facilitate "easy removal" of dirt; the tribology characteristics of such a lubricant are similar to those of the lubricant used on magnetic discs, and has possibilities of application. There are two differences between lubricants used for ophthalmic lenses and those used for magnetic discs: (1) the film thickness of the lubricant used for magnetic discs does not need to be reduced, because the recording density achieved by using the lubricant for the magnetic disc increases exponentially when the gap between the magnetic disc surface and magnetic head is reduced as much as possible (to approximately 1 nm), and (2) the lubricant for ophthalmic lenses needs to be solid, but magnetic discs can be solid or liquid if stiction, in which a magnetic head sticks to the surface of a magnetic disc does not occur. However, in the case of ophthalmic lenses, dirt, dust, and fingerprints frequently block the view of the user, and the user cleans the lenses with water or rubs them with a soft cloth or paper; therefore, liquid lubricants can cause adhesion problems and does not last for a long time. In reality, conference presentations and papers are limited to information provided by the authors (Tadokoro et al, 2009; Tadokoro et al, 2010; Tadokoro et al, 2011). This chapter discusses tribology, with a focus on the characterization of lubricants, and presents analysis and evaluation results based on the film thickness, structure, distribution, and abrasion

Figure 1 shows optical microscopic pictures of ophthalmic lens returned by a consumer who complained about the quality. The different colors in the picture demonstrate the peeling of the AR coating films along the scratch, and thus, the small scratches become visible. Details on how and when the lenses were used are unknown, but it must be understood that scratches actually occur and this problem must be taken into account; this picture shows the importance of surface reforming based on the use of lubricants. While scratch-free lenses cannot be made only by modifying lubricants, the lubricant is one of the most important factors that affect the formation of scratches. Figure 2 shows the results of an abrasion test conducted by scrubbing a lens 20 times with 20 kg steel wool for different lubricants. The results show that the formation of scratches can be controlled by changing the structure or the distribution state of the lubricant. Finally as an example of the comparison of dirt adhesion, figure 3 shows the adhesion of cedar pollen on the lens. In Japan, hay fever, a seasonal allergy caused by cedar pollen, is very common (30% of the citizens have this

resistance of lubricants reported by the authors.

Fig. 1. Damaged ophthalmic lens and scratches

**2. Scratches and dirt** 

allergy). The results in figure 3 show that changing the surface condition reduces the amount of pollen adhered to the ophthalmic lens brought indoors. As in the example of scratches, the results show the possibility that the surface condition can be controlled to change the amount of dirt that adheres to the lenses.

Fig. 2. Scratch test results for 3types lubricants: the lens was scrubbed 20 times with 2 kg steel wool

Fig. 3. Comparison between surface condition and cedar pollen adheres to the lens

#### **2.1 Experimental**

#### **2.1.1 Sample preparation**

Commercial ophthalmic lenses of allyl diglycole carbonate (ADC, CR-39®) were used in this study. In addition, the detailed estimations of lubricants were carried out directly on silicon wafer in order to avoid the influence of surface curvature, roughness, or amorphous states

Characterization of Lubricant on Ophthalmic Lenses 227

equation; they provide a set of relations for different classes of material over the energy

where λlub F is the escape depth of F1s photoelectron of lubricants, λm is the escape depth of monolayers for organic materials, Ek is electron kinetic energy, and ρ is the density of material.

Fig. 4. TEM cross-sectional photograph (glue/Cr layer/lubricant/Si wafer) of lubricant B

Fig. 5. TEM photograph of lubricant B on a silicon wafer. (Blue area shows the EDS analysis

λm = 49/Ek2 + 0.11• Ek0.5 (1)

λlub F = λm /ρ (2)

range 1 eV – 6keV (Briggs & Seah, 1990).

area)

of actual ophthalmic lenses. The structures of the ophthalmic lenses were as follows: a sol-gel based underlayer on the plastic lens substrate was deposited by dip coating or spin coating methods. The HC material was made using a silica sol and 3-glycidoxypropyltrimethoxysilane. The thickness of HC was approximately 3500 nm. AR coating layers, composed of a sandwich structure between low-index material (SiO2) and high-index material (Ta2O5), were deposited by vacuum deposition methods after the HC underlayer was cleaned by ultrasonic washing with detergent and de-ionized water. The total film thickness was approximately 620nm. The PFPE lubricants, which were also commercial products, were deposited over the AR coating layers by the vacuum deposition methods. The main structure of lubricants A, B, C, G, and H has (-CF2-CF2-O-)m-(CF2-O-)n, the main structure of lubricants D and F has (-CF (CF3)-CF2-O-)m', the main structure of lubricant E has (-CF2-CF2- CF2-O-)m''.

#### **2.1.2 Analysis and evaluation methods**

The surface morphology and the lubricant film distribution were examined by atomic force microscopy (AFM; Asylum Research, Molecule Force Microscope System MFP-3D). The film thickness, morphology of the cross section, and elemental analysis were used by transmission electron microscopy (TEM-EDS; JEOL, JEM-200FX-2). For the TEM observation, a Cr protective layer was deposited onto the lubricants layer in order to identify a top surface of the lubricants films. The film thickness and the coverage ratio of the lubricant were measured by X-ray photoelectron spectroscopy (XPS; Physical Electronics, PHI ESCA5400MC). Structure analysis was conducted by time-of-flight secondary ion mass spectrometry (TOF-SIMS; ULVAC-PHI, PHI TRIFT-3 or PHI TRIFT-4) and XPS. The wear properties of lubricants were evaluated by contact angle measurement (Kyowa Interface Science Co.,Ltd.; Contact angle meter, model CA-D) and by the use of an abrasion tester (Shinto Scientific Co., Ltd.; Heidon Tribogear, Type 30S). The abrasion test was rubbed in the Dusper K3(Ozu corp.) to have wrapped around the eraser under the condition of 2 kg weight and 600 strokes.

#### **2.2 Results and discussion**

#### **2.2.1 Cross-sectional structure, film thickness and coverage of lubricants**

Figure 4 shows an example of TEM photograph of lubricant B on a silicon wafer. Figure 5 and figure 6 show an EDS analysis area of TEM photograph and an EDS spectrum of lubricant B. Table 1 summarized the lubricant film thickness and coverage ratio by XPS and TEM. The thickness of the lubricant layer was estimated to be 2.6 nm. And also, we recognized fluorine element in this area by TEM-EDS. These data indicate that both the film thicknesses and the coverage ratios were almost identical across all films. Here, we directly measured the film thickness by TEM. Despite the fact that the lubricant layer was comprised of organic materials, the existence of the lubricant film was directly observed and the film thickness was successfully measured by TEM. Generally, the issue of TEM measurement is sample damage by electron beam. For the reason of successful measurement by TEM, it seems that the lubricant damage of ophthalmic lens is stronger than that of the magnetic disk for electron beam.

It is well-known that the film thickness is proportional to a logarithmic function of the intensity ratio of photoelectrons. According to Seah and Dench (1979), they reported the escape depth of electrons of organic materials with electron kinetic energy by the following

of actual ophthalmic lenses. The structures of the ophthalmic lenses were as follows: a sol-gel based underlayer on the plastic lens substrate was deposited by dip coating or spin coating methods. The HC material was made using a silica sol and 3-glycidoxypropyltrimethoxysilane. The thickness of HC was approximately 3500 nm. AR coating layers, composed of a sandwich structure between low-index material (SiO2) and high-index material (Ta2O5), were deposited by vacuum deposition methods after the HC underlayer was cleaned by ultrasonic washing with detergent and de-ionized water. The total film thickness was approximately 620nm. The PFPE lubricants, which were also commercial products, were deposited over the AR coating layers by the vacuum deposition methods. The main structure of lubricants A, B, C, G, and H has (-CF2-CF2-O-)m-(CF2-O-)n, the main structure of lubricants D and F has (-CF (CF3)-CF2-O-)m', the main structure of

The surface morphology and the lubricant film distribution were examined by atomic force microscopy (AFM; Asylum Research, Molecule Force Microscope System MFP-3D). The film thickness, morphology of the cross section, and elemental analysis were used by transmission electron microscopy (TEM-EDS; JEOL, JEM-200FX-2). For the TEM observation, a Cr protective layer was deposited onto the lubricants layer in order to identify a top surface of the lubricants films. The film thickness and the coverage ratio of the lubricant were measured by X-ray photoelectron spectroscopy (XPS; Physical Electronics, PHI ESCA5400MC). Structure analysis was conducted by time-of-flight secondary ion mass spectrometry (TOF-SIMS; ULVAC-PHI, PHI TRIFT-3 or PHI TRIFT-4) and XPS. The wear properties of lubricants were evaluated by contact angle measurement (Kyowa Interface Science Co.,Ltd.; Contact angle meter, model CA-D) and by the use of an abrasion tester (Shinto Scientific Co., Ltd.; Heidon Tribogear, Type 30S). The abrasion test was rubbed in the Dusper K3(Ozu corp.) to have wrapped around the eraser under the condition of 2 kg weight and 600

**2.2.1 Cross-sectional structure, film thickness and coverage of lubricants** 

Figure 4 shows an example of TEM photograph of lubricant B on a silicon wafer. Figure 5 and figure 6 show an EDS analysis area of TEM photograph and an EDS spectrum of lubricant B. Table 1 summarized the lubricant film thickness and coverage ratio by XPS and TEM. The thickness of the lubricant layer was estimated to be 2.6 nm. And also, we recognized fluorine element in this area by TEM-EDS. These data indicate that both the film thicknesses and the coverage ratios were almost identical across all films. Here, we directly measured the film thickness by TEM. Despite the fact that the lubricant layer was comprised of organic materials, the existence of the lubricant film was directly observed and the film thickness was successfully measured by TEM. Generally, the issue of TEM measurement is sample damage by electron beam. For the reason of successful measurement by TEM, it seems that the lubricant damage of ophthalmic lens is stronger than that of the magnetic

It is well-known that the film thickness is proportional to a logarithmic function of the intensity ratio of photoelectrons. According to Seah and Dench (1979), they reported the escape depth of electrons of organic materials with electron kinetic energy by the following

lubricant E has (-CF2-CF2- CF2-O-)m''.

strokes.

**2.2 Results and discussion** 

disk for electron beam.

**2.1.2 Analysis and evaluation methods** 

equation; they provide a set of relations for different classes of material over the energy range 1 eV – 6keV (Briggs & Seah, 1990).

$$
\lambda\_{\rm m} = 49/E\_k^2 + 0.11 \bullet E\_k^{0.5} \tag{1}
$$

λlub F = λm /ρ (2)

where λlub F is the escape depth of F1s photoelectron of lubricants, λm is the escape depth of monolayers for organic materials, Ek is electron kinetic energy, and ρ is the density of material.

Fig. 4. TEM cross-sectional photograph (glue/Cr layer/lubricant/Si wafer) of lubricant B

Fig. 5. TEM photograph of lubricant B on a silicon wafer. (Blue area shows the EDS analysis area)

Characterization of Lubricant on Ophthalmic Lenses 229

modified equation (4) for the coverage of our lubricants using the F1s and the Si2p

A· (Ilub F/ISi) = {r· [1-exp(-T/(λlub F· sinθ))]}/{(1-r)+r· exp(-T/(λSi· sinθ))} (4)

Figure 7 shows an example of the relationship between the logarithmic function of the intensity ratio of photoelectron and the coverage ratio. Table 1 already summarized the lubricant film thickness and coverage ratio by XPS and TEM. The coverage ratio of lubricants by XPS is estimated to be over 98%. However, the coverage ratio of TEM seems to be covered a fully 100 % on Si wafer. In case of an actual XPS measurement, a coverage ratio of 100% is unlikely to occur due to the influence of surface roughness, the density of actual lubricants films, and the photoelectron signal of Si2p. Therefore, it seems that the lubricant layer completely covers on the Si wafer when the coverage ratio is approximately 100%. By using this XPS technique, we can easily monitor the lubricant thickness and coverage ratio

Lub. F1s 689 1.45 1.8x103

B.E (eV) λlub F (nm) ρ (kg/m3)

photoelectrons.

where r is the coverage ratio from 0 to 1.

on a production line for quality control.

Table 2. The escape depth and parameters used

Fig. 7. Coverage calculation results of sample B by XPS measurement

Figure 8 illustrates the lubricant distribution of samples A, B, and C by TOF-SIMS analysis. The image was obtained by detecting the positive ion fragments of C+, C2F4+, and Si+. The ion signal intensity is displayed on a scale of relative brightness; bright areas indicate high intensity of each type of fragment ion. Figure 9 shows the comparison of lubricants fragment

**2.2.2 The distribution state of lubricants** 

Fig. 6. TEM-EDS spectrum for lubricant B on a silicon wafer


Table 1. Film thickness and coverage ratio of lubricant by XPS and TEM

The lubricants film thickness of XPS was calculated by the following equation (3). Table-2 summarizes the parameters used. We experimentally calculated the A factor by using equation (3) from TEM's film thickness and the intensity ration of F1s and Si2p photoelectron (the experimental A factor is 0.116).

$$\mathbf{T} = \lambda\_{\text{hub}\,\text{F}} \cdot \sin \theta \cdot \ln\left[\,\mathbf{A} \cdot (\mathbf{I}\_{\text{hub}\,\text{F}} / \,\mathbf{I}\_{\text{Si}}) + \mathbf{1}\right] \tag{3}$$

where T is the film thickness of lubricants , θ is the detection angle of XPS measurement, Ilub F is the intensity of F1s photoelectrons, ISi is the intensity of Si2p photoelectrons, A is the correction factor (calculated value: 0.116, i.e., lubricants films thickness by TEM).

According to Kimachi et al. (1987), they have derived an expression for the coverage ratio of lubricants on magnetic disks using an island model. In the present study, we propose a

CrKa

Lub. film coverage by TEM (%)

0.00 0.80 1.60 2.40 3.20 4.00 4.80 5.60 keV

Sample A 1.5-1.7 98 over 100 Sample B 2.3-2.7 98 over 100 Sample C 2.3-2.7 98 over 100 Sample D 2.1-2.5 98 over ----- Sample E 1.7-2.2 98 over -----

The lubricants film thickness of XPS was calculated by the following equation (3). Table-2 summarizes the parameters used. We experimentally calculated the A factor by using equation (3) from TEM's film thickness and the intensity ration of F1s and Si2p photoelectron

 T = λlub F · sinθ· ln [ A· (Ilub F / I Si) + 1 ] (3) where T is the film thickness of lubricants , θ is the detection angle of XPS measurement, Ilub F is the intensity of F1s photoelectrons, ISi is the intensity of Si2p photoelectrons, A is the

According to Kimachi et al. (1987), they have derived an expression for the coverage ratio of lubricants on magnetic disks using an island model. In the present study, we propose a

correction factor (calculated value: 0.116, i.e., lubricants films thickness by TEM).

Lub. film coverage by XPS (%)

270

240

CKa

OKa

CrLa

FKa

(the experimental A factor is 0.116).

AlKa

Fig. 6. TEM-EDS spectrum for lubricant B on a silicon wafer

Lub. film thickness (nm)

Table 1. Film thickness and coverage ratio of lubricant by XPS and TEM

SiKa

210

180

150

Counts

120

90

60

30

0

modified equation (4) for the coverage of our lubricants using the F1s and the Si2p photoelectrons.

$$\mathbf{A} \cdot (\mathbf{I}\_{\text{ub} \cdot \text{F}} / \mathbf{I}\_{\overline{\text{si}}}) = \{ \mathbf{r} \cdot [\mathbf{1} \cdot \exp(\mathbf{-T} / (\lambda\_{\text{ub} \cdot \text{F}} \cdot \sin \theta))] \} / \{ (\mathbf{1} \cdot \mathbf{r}) + \mathbf{r} \cdot \exp(\mathbf{-T} / (\lambda\_{\text{si}} \cdot \sin \theta)) \} \tag{4}$$

where r is the coverage ratio from 0 to 1.

Figure 7 shows an example of the relationship between the logarithmic function of the intensity ratio of photoelectron and the coverage ratio. Table 1 already summarized the lubricant film thickness and coverage ratio by XPS and TEM. The coverage ratio of lubricants by XPS is estimated to be over 98%. However, the coverage ratio of TEM seems to be covered a fully 100 % on Si wafer. In case of an actual XPS measurement, a coverage ratio of 100% is unlikely to occur due to the influence of surface roughness, the density of actual lubricants films, and the photoelectron signal of Si2p. Therefore, it seems that the lubricant layer completely covers on the Si wafer when the coverage ratio is approximately 100%. By using this XPS technique, we can easily monitor the lubricant thickness and coverage ratio on a production line for quality control.


Table 2. The escape depth and parameters used

Fig. 7. Coverage calculation results of sample B by XPS measurement

#### **2.2.2 The distribution state of lubricants**

Figure 8 illustrates the lubricant distribution of samples A, B, and C by TOF-SIMS analysis. The image was obtained by detecting the positive ion fragments of C+, C2F4+, and Si+. The ion signal intensity is displayed on a scale of relative brightness; bright areas indicate high intensity of each type of fragment ion. Figure 9 shows the comparison of lubricants fragment

Characterization of Lubricant on Ophthalmic Lenses 231

C4F7O2 C4F8O 225 300Relative intensity (normarized CF ions)

Fragment ions or mass number

fragment image in figure 8, we recognized the homogeneity of lubricant distribution for sample A, sample B and sample C. However, we found that the uniformity or heterogeneity of an image depended upon the sample and the scale, except for topographic images by AFM measurement added some functionality from figures 10, 12, and 13. Here, in the case of sample B, the friction images agree with the phase images and the phase images agree to the force modulation images. Thus, the friction force image reveals the distribution of friction behavior on the surface. Also, the force modulation image indicates the distribution of hardness; the darker areas correspond to softer areas. Thus, the phase image suggests friction or hardness behavior because it assumes the same image form as the friction force

By friction force microscopy (FFM), the twisting angle is proportional to the tip height of the cantilever in the case of the same cantilever shape and the same material (Matsuyama, 1997).

where θo: twisting angle, L: length of cantilever, r: correction factor (calculated value 0.3 to ~0.4), G: shear modulus, w: width of cantilever, t: thickness of cantilever, μ: friction

In previous work (Tadokoro et al., 2001), we observed the morphology of lubricants on the magnetic disk surface by FFM. The images of lubricants obtained by a high-response cantilever of tip height 8.4 μm were clearer than those by a standard cantilever of tip height 3 μm in the same load force. The sensitivity of the high-response cantilever was about 2 to 3 times greater than that of the standard cantilever when compared in the same sample area. These observations seemed to experimentally support the theoretical predictions, and the effects of load force for the standard cantilever agree with the theoretical equation. However, FFM has two disadvantages. If the area is too small (i.e., <1 μm) and is low-friction material, the friction force signal is drastically reduced. Moreover, there might be

C3F5O2 C3F7 C3F7O

θo= μ· FL· (ht + t/2) · L/(r· G· w· t3) (5)

Sample A Sample B Sample C

C CF CFO CF2 CF3 C2F3O C2F4 109 C2F4O C2F5

Fig. 9. The comparison of lubricants fragment ion for sample A, B and C

5

4

3

2

1

0

and force modulation.

coefficient, FL: load force, ht: height of cantilever.

ion for samples A, B and C. From fugure-9, we recognized that these samples have same main structure of (-CF2-CF2-O-)m-(CF2-O-)n. The lubricant distribution determined by this analysis was consistent with the actual lubricant distribution. The behavior of the lubricant distribution obtained is attributable to suggest chemical structure and mechanical property of lubricant. Therefore, in terms of elemental fragment ions, the distribution of the lubricant appears to be homogenous at the 10μm scale from figure 8.

Fig. 8. TOF-SIMS image (C+, C2F4+, and Si+ fragment ions) for each sample

Figure 10 illustrates the lubricant distribution of samples A, B, and C by AFM topographic image and friction force image at the 10 μm scale. Figure 11 shows a frequency analysis of phase separation for sample A and sample B. A red histogram shows the whole area, a blue area shows the phase separation A of lubricants, and a green area shows phase separation B of lubricants. Area distribution of sample 2 has approximately two times larger than that of sample 1. Figure 12 shows the lubricant image of sample B by using phase image and force modulation image. The components between the in-phase (input-i: elasticity) and the quadrature (input-q: viscosity) divided phase image are shown in figure 13. From the TOF-SIMS

ion for samples A, B and C. From fugure-9, we recognized that these samples have same main structure of (-CF2-CF2-O-)m-(CF2-O-)n. The lubricant distribution determined by this analysis was consistent with the actual lubricant distribution. The behavior of the lubricant distribution obtained is attributable to suggest chemical structure and mechanical property of lubricant. Therefore, in terms of elemental fragment ions, the distribution of the lubricant

Sample-A Sample-B Sample-C

C+ C+ C+

C2F4+ C2F4+ C2F4+

Si+ Si+ Si+

Figure 10 illustrates the lubricant distribution of samples A, B, and C by AFM topographic image and friction force image at the 10 μm scale. Figure 11 shows a frequency analysis of phase separation for sample A and sample B. A red histogram shows the whole area, a blue area shows the phase separation A of lubricants, and a green area shows phase separation B of lubricants. Area distribution of sample 2 has approximately two times larger than that of sample 1. Figure 12 shows the lubricant image of sample B by using phase image and force modulation image. The components between the in-phase (input-i: elasticity) and the quadrature (input-q: viscosity) divided phase image are shown in figure 13. From the TOF-SIMS

Fig. 8. TOF-SIMS image (C+, C2F4+, and Si+ fragment ions) for each sample

appears to be homogenous at the 10μm scale from figure 8.

1 *μ* m

Fig. 9. The comparison of lubricants fragment ion for sample A, B and C

fragment image in figure 8, we recognized the homogeneity of lubricant distribution for sample A, sample B and sample C. However, we found that the uniformity or heterogeneity of an image depended upon the sample and the scale, except for topographic images by AFM measurement added some functionality from figures 10, 12, and 13. Here, in the case of sample B, the friction images agree with the phase images and the phase images agree to the force modulation images. Thus, the friction force image reveals the distribution of friction behavior on the surface. Also, the force modulation image indicates the distribution of hardness; the darker areas correspond to softer areas. Thus, the phase image suggests friction or hardness behavior because it assumes the same image form as the friction force and force modulation.

By friction force microscopy (FFM), the twisting angle is proportional to the tip height of the cantilever in the case of the same cantilever shape and the same material (Matsuyama, 1997).

$$
\partial\_o = \mu \cdot \mathbf{F}\_\mathbf{L} \cdot \left(\mathbf{h}\_\mathbf{t} + \mathbf{t}/2\right) \cdot \mathbf{L} / \left(\mathbf{r} \cdot \mathbf{G} \cdot \mathbf{w} \cdot \mathbf{t}^3\right) \tag{5}
$$

where θo: twisting angle, L: length of cantilever, r: correction factor (calculated value 0.3 to ~0.4), G: shear modulus, w: width of cantilever, t: thickness of cantilever, μ: friction coefficient, FL: load force, ht: height of cantilever.

In previous work (Tadokoro et al., 2001), we observed the morphology of lubricants on the magnetic disk surface by FFM. The images of lubricants obtained by a high-response cantilever of tip height 8.4 μm were clearer than those by a standard cantilever of tip height 3 μm in the same load force. The sensitivity of the high-response cantilever was about 2 to 3 times greater than that of the standard cantilever when compared in the same sample area. These observations seemed to experimentally support the theoretical predictions, and the effects of load force for the standard cantilever agree with the theoretical equation. However, FFM has two disadvantages. If the area is too small (i.e., <1 μm) and is low-friction material, the friction force signal is drastically reduced. Moreover, there might be

Characterization of Lubricant on Ophthalmic Lenses 233

Fig. 11. Frequency analysis of phase separation by FFM (top distribution: sample A, bottom distribution: sample B), it shows red histogram for whole area, blue area for lubricant phase

Fig. 12. Phase image (left side), force modulation image (right side; bright area indicates

Fig. 13. In-phase image (input-i: left side) and quadrature image (input-q: right side) of

separation A, and green area for lubricant phase separation B

harder area, darker area indicates softer area) of sample B

sample B divided by phase image

damage to the lens surface because the friction force image is made by contact. On the other hand, the disadvantage of force modulation methods is that the tip can change shape and is a possible source of contamination because it is always pushed into the sample (indentation). Therefore, we believe that it is more convenient to use phase images than friction force images or force modulation images for determining the island structures of shapes with similar surface morphologies.

Fig. 10. Topographic image (left side), FFM image (right side; bright area indicates higher friction, darker area indicates lower friction); upper image is sample A, middle image is sample B, lower image is sample C

damage to the lens surface because the friction force image is made by contact. On the other hand, the disadvantage of force modulation methods is that the tip can change shape and is a possible source of contamination because it is always pushed into the sample (indentation). Therefore, we believe that it is more convenient to use phase images than friction force images or force modulation images for determining the island structures of

Fig. 10. Topographic image (left side), FFM image (right side; bright area indicates higher friction, darker area indicates lower friction); upper image is sample A, middle image is

shapes with similar surface morphologies.

sample B, lower image is sample C

Fig. 11. Frequency analysis of phase separation by FFM (top distribution: sample A, bottom distribution: sample B), it shows red histogram for whole area, blue area for lubricant phase separation A, and green area for lubricant phase separation B

Fig. 12. Phase image (left side), force modulation image (right side; bright area indicates harder area, darker area indicates softer area) of sample B

Fig. 13. In-phase image (input-i: left side) and quadrature image (input-q: right side) of sample B divided by phase image

Characterization of Lubricant on Ophthalmic Lenses 235

ophthalmic lenses vary widely and thus perform differently in terms of wear property and dirt protection. Therefore, the methods described here are useful and suitable for investigation

Fig. 14. Topographic image (left side), phase image (right side) of sample D

Fig. 15. Topographic image (upper left), phase image (upper right), input-i image (lower

left,) and input-q image (lower right) of sample C at the 1 μm scale

of lubricants on ophthalmic lens surfaces.

According to Cleveland et al. (1998), if the amplitude of the cantilever is held constant, the sine of the phase angle of the driven vibration is then proportional to changes in the tipsample energy dissipation. This means that images of the cantilever phase in tappingmode AFM are closely related to maps of dissipation. Our phase images suggest that the bright area corresponds to a higher phase because a phase image is taken in repulsive mode. The bright area is more energy-dissipated than the dark area, which means the bright area is softer or more adhesive. Because the phase image was divided by the components of the in-phase (input-i) and the quadrature (input-q), the relation of the inphase (input-i) and the quadrature (input-q) is converse. It seems that an in-phase image (input-i) has the same tendency as the force modulation image: its darker area corresponds to a softer area. In general, the relation between an in-phase image (input-i) and a quadrature image (input-q) is the relation between elasticity and viscosity. Our observations seem to experimentally support this relation. Figure 13 demonstrates that the bright area of the in-phase image has lower energy dissipation than the darker area, which means the bright area is harder or less adhesive. On the other hand, the darker area in figure 12 (the force modulation image) corresponds to a softer area. If ophthalmic lens surface is sticky, a lot of contaminants can easily attach to the lens surface. Fortunately, the lubricant material is fluorocarbon, which has low surface energy. Thus, the contaminant is easily removed from the lens surface wiping the surface with a cloth. From these results, in the case of sample B, it appears that these island structures are mixtures of soft regions and hard regions at the 10 μm scale.

Figure 14 illustrates the lubricant distribution of sample D by AFM topographic image and phase image at the 10 μm scale. Figure 15 shows the lubricant image of sample C by topographic image, phase image, in-phase image (input-i), and quadrature image (input-q) at the 1 μm scale. Figure 16 shows the lubricant image of sample C by topographic image, phase image, in-phase image (input-i), and quadrature image (input-q) at the 500 nm scale. The topographic image, phase image, in-phase (input-i), and quadrature image (input-q) of sample D at the 1 μm scale are shown in figure 17. Finally, the topographic image, phase image, in-phase (input-i), and quadrature image (input-q) of sample E at the 1 μm scale are shown in figure 18.

In the case of samples C, D, and E at the 10 μm scale, island structures cannot be observed by phase image, although it seems that the lubricant is homogeneous in these areas. However, samples C, D, and E reveal some island structures at smaller scales (i.e., 500 nm scale and 1 μm scale). We earlier discussed the relation between friction force image, force modulation image, and phase image. Nevertheless, the signal mark depends upon the measurement mode; these images reveal island structures in cases of similar morphology.

In the case of sample C, it seems that the grain is too small and some clusters gather with different dissipation energies. The topographic image of sample D reveals unevenness of grain, but the phase image clearly shows the grain boundary. This suggests that the grain boundary in sample D is accumulated lubricants rather than grain. On the other hand, sample E has grain but the grain boundary in the phase image is not clearly apparent. It seems that the lubricant in the grain boundary is in accord with the lubricant on the grain, and the lubricant of sample E is more homogenous than that of sample C or D.

In some ophthalmic lenses, island structures can be observed on the lens surface at the 10 μm scale, whereas in others it is necessary to use the 1 μm or 500 nm scale. From these lubricant images we have determined that the morphologies of the lubricants of commercial

According to Cleveland et al. (1998), if the amplitude of the cantilever is held constant, the sine of the phase angle of the driven vibration is then proportional to changes in the tipsample energy dissipation. This means that images of the cantilever phase in tappingmode AFM are closely related to maps of dissipation. Our phase images suggest that the bright area corresponds to a higher phase because a phase image is taken in repulsive mode. The bright area is more energy-dissipated than the dark area, which means the bright area is softer or more adhesive. Because the phase image was divided by the components of the in-phase (input-i) and the quadrature (input-q), the relation of the inphase (input-i) and the quadrature (input-q) is converse. It seems that an in-phase image (input-i) has the same tendency as the force modulation image: its darker area corresponds to a softer area. In general, the relation between an in-phase image (input-i) and a quadrature image (input-q) is the relation between elasticity and viscosity. Our observations seem to experimentally support this relation. Figure 13 demonstrates that the bright area of the in-phase image has lower energy dissipation than the darker area, which means the bright area is harder or less adhesive. On the other hand, the darker area in figure 12 (the force modulation image) corresponds to a softer area. If ophthalmic lens surface is sticky, a lot of contaminants can easily attach to the lens surface. Fortunately, the lubricant material is fluorocarbon, which has low surface energy. Thus, the contaminant is easily removed from the lens surface wiping the surface with a cloth. From these results, in the case of sample B, it appears that these island structures are mixtures of soft regions and hard regions at the 10

Figure 14 illustrates the lubricant distribution of sample D by AFM topographic image and phase image at the 10 μm scale. Figure 15 shows the lubricant image of sample C by topographic image, phase image, in-phase image (input-i), and quadrature image (input-q) at the 1 μm scale. Figure 16 shows the lubricant image of sample C by topographic image, phase image, in-phase image (input-i), and quadrature image (input-q) at the 500 nm scale. The topographic image, phase image, in-phase (input-i), and quadrature image (input-q) of sample D at the 1 μm scale are shown in figure 17. Finally, the topographic image, phase image, in-phase (input-i), and quadrature image (input-q) of sample E at the 1 μm scale are

In the case of samples C, D, and E at the 10 μm scale, island structures cannot be observed by phase image, although it seems that the lubricant is homogeneous in these areas. However, samples C, D, and E reveal some island structures at smaller scales (i.e., 500 nm scale and 1 μm scale). We earlier discussed the relation between friction force image, force modulation image, and phase image. Nevertheless, the signal mark depends upon the measurement mode; these images reveal island structures in cases of similar morphology. In the case of sample C, it seems that the grain is too small and some clusters gather with different dissipation energies. The topographic image of sample D reveals unevenness of grain, but the phase image clearly shows the grain boundary. This suggests that the grain boundary in sample D is accumulated lubricants rather than grain. On the other hand, sample E has grain but the grain boundary in the phase image is not clearly apparent. It seems that the lubricant in the grain boundary is in accord with the lubricant on the grain,

and the lubricant of sample E is more homogenous than that of sample C or D.

In some ophthalmic lenses, island structures can be observed on the lens surface at the 10 μm scale, whereas in others it is necessary to use the 1 μm or 500 nm scale. From these lubricant images we have determined that the morphologies of the lubricants of commercial

μm scale.

shown in figure 18.

ophthalmic lenses vary widely and thus perform differently in terms of wear property and dirt protection. Therefore, the methods described here are useful and suitable for investigation of lubricants on ophthalmic lens surfaces.

Fig. 14. Topographic image (left side), phase image (right side) of sample D

Fig. 15. Topographic image (upper left), phase image (upper right), input-i image (lower left,) and input-q image (lower right) of sample C at the 1 μm scale

Characterization of Lubricant on Ophthalmic Lenses 237

Fig. 18. Topographic image (upper left), phase image (upper right), input-i image (lower

Figure 19 shows the X-ray damage ratio of F1s spectra for sample F, G, and H as a function of X-ray exposure time under the condition of X-ray power 300W and Mg-Kα source by XPS. Figures 20 - 22 show the changing chemical structure of C1s for samples F-G as a function of exposure time (initial structure shown for reference, structure after 30 min, and structure after 60 min), as determined by XPS. Figure 23 shows the initial structure and of the mass spectra of positive fragment ions, as obtained by TOF-SIMS (upper spectrum: sample F, middle spectrum: sample G, lower spectrum: sample H). Figure 24 shows the mass spectra of positive fragment ions after 60 min X-ray exposure by XPS (upper spectrum: sample F, middle spectrum: sample G, lower spectrum: sample H). Figure 25 shows the mass spectra of negative fragment ions for sample F, as obtained by TOF-SIMS (upper spectrum: initial, lower spectrum: after 60 min, obtained by XPS). Table 3 summarized the film thickness and

From figure 19, we found that the X-ray damage in the case of sample F is greater than that in the case of sample G and sample H. In the case of sample G and sample H, the lubricant component of fluorine remained on the surface; fluorine was kept on approximately 80% on the surface after 60 min of exposure to X-rays. On the other hand, the lubricant component

On the basis of the initial structures shown in figure 23 and figure 25, it is concluded that the main structure of sample F has a side chain structure (-CF (CF3)-CF2-O-)m', similar to that in Fombline Y or Krytox. This periodic relation of 166 amu (C3F6O) continues up till mass numbers of approximately 5000 amu. In the case of magnetic disks, the high molecular structure of the lubricants was realized and maintained by dip coating or spin coating.

left), and input-q image (lower right) of sample E at the 1 μm scale

**2.2.3 X-ray damage of lubricants and chimerical structures** 

coverage ratio of lubricant before and after XPS damage.

of sample F decreased by approximately 40% after exposure for 60 min.

Fig. 16. Topographic image (upper left), phase image (upper right), input-i image (lower left), and input-q image (lower right) of sample C at the 500 nm scale

Fig. 17. Topographic image (upper left), phase image (upper right), input-i image (lower left), and input-q image (lower right) of sample D at the 1 μm scale

Fig. 16. Topographic image (upper left), phase image (upper right), input-i image (lower

Fig. 17. Topographic image (upper left), phase image (upper right), input-i image (lower

left), and input-q image (lower right) of sample D at the 1 μm scale

left), and input-q image (lower right) of sample C at the 500 nm scale

Fig. 18. Topographic image (upper left), phase image (upper right), input-i image (lower left), and input-q image (lower right) of sample E at the 1 μm scale

#### **2.2.3 X-ray damage of lubricants and chimerical structures**

Figure 19 shows the X-ray damage ratio of F1s spectra for sample F, G, and H as a function of X-ray exposure time under the condition of X-ray power 300W and Mg-Kα source by XPS. Figures 20 - 22 show the changing chemical structure of C1s for samples F-G as a function of exposure time (initial structure shown for reference, structure after 30 min, and structure after 60 min), as determined by XPS. Figure 23 shows the initial structure and of the mass spectra of positive fragment ions, as obtained by TOF-SIMS (upper spectrum: sample F, middle spectrum: sample G, lower spectrum: sample H). Figure 24 shows the mass spectra of positive fragment ions after 60 min X-ray exposure by XPS (upper spectrum: sample F, middle spectrum: sample G, lower spectrum: sample H). Figure 25 shows the mass spectra of negative fragment ions for sample F, as obtained by TOF-SIMS (upper spectrum: initial, lower spectrum: after 60 min, obtained by XPS). Table 3 summarized the film thickness and coverage ratio of lubricant before and after XPS damage.

From figure 19, we found that the X-ray damage in the case of sample F is greater than that in the case of sample G and sample H. In the case of sample G and sample H, the lubricant component of fluorine remained on the surface; fluorine was kept on approximately 80% on the surface after 60 min of exposure to X-rays. On the other hand, the lubricant component of sample F decreased by approximately 40% after exposure for 60 min.

On the basis of the initial structures shown in figure 23 and figure 25, it is concluded that the main structure of sample F has a side chain structure (-CF (CF3)-CF2-O-)m', similar to that in Fombline Y or Krytox. This periodic relation of 166 amu (C3F6O) continues up till mass numbers of approximately 5000 amu. In the case of magnetic disks, the high molecular structure of the lubricants was realized and maintained by dip coating or spin coating.

Characterization of Lubricant on Ophthalmic Lenses 239

of the lubricants is 2–3 nm. According to Tani (1999), he found double steps on the lubricant film with 2.9 nm thickness that was almost completely cover the surface by the mean molecular radius of gyration with coil of lubricant molecular. Therefore, it seems that the 2-3

In the case of sample F, the molecular interaction in the side chain structure of CF3 is weaker than that in the straight chain structure of CF2 because in CF3, three-dimensional structures overlap and this leads to repulsion between fluorine atoms. Therefore, the damage due to exposure to X-rays during XPS in the case of sample F is more than that in the case of sample G or that in the case of sample H. It is predicted that the trend observed in the adhesion properties of lubricants will be the same as that observed in the case of these damages.

Fig. 21. Changing chemical structure of C1s spectrum for sample G as a function of X-ray

Fig. 22. Changing chemical structure of C1s spectrum for sample H as a function of X-ray

exposure time by XPS

exposure time

coils of lubricant molecular have been stacked on the surface of the ophthalmic lens.

Fig. 19. Relationship between F1s intensity and X-Ray exposure time during XPS

However, the ophthalmic lens of lubricants was deposited by lamp heating methods into vacuum. Nevertheless, some main structure of lubricants was contained high-polymeric structures. On the other hand, the main structures of sample G and sample H has a straight chain structure without the side chain structures (-CF2-CF2-O-)m-(CF2-O-)n, similar to the main structure of Fombline Z. From figure 20, 24 and 25, we found that the main chemical structure of lubricants for sample F is decreasing and destroying as a function of exposure time by XPS.

Fig. 20. Changing chemical structure of C1s spectrum for sample F as a function of X-ray exposure time by XPS

These observations suggest that the straight chain structure of (-CF2-CF2-O-)m-(CF2-O-)n is more robust to X-ray damage during XPS than the side chain structure (-CF (CF3)-CF2-O-)m'. We attribute this difference in the strength of the structures to the presence or absence of the chemical structure of the side chain. TEM or XPS measurement reveals that the film thickness

0 15 30 45 60 X-Ray exposure time (min)

However, the ophthalmic lens of lubricants was deposited by lamp heating methods into vacuum. Nevertheless, some main structure of lubricants was contained high-polymeric structures. On the other hand, the main structures of sample G and sample H has a straight chain structure without the side chain structures (-CF2-CF2-O-)m-(CF2-O-)n, similar to the main structure of Fombline Z. From figure 20, 24 and 25, we found that the main chemical structure of lubricants for sample F is decreasing and destroying as a function of exposure

Fig. 20. Changing chemical structure of C1s spectrum for sample F as a function of X-ray

These observations suggest that the straight chain structure of (-CF2-CF2-O-)m-(CF2-O-)n is more robust to X-ray damage during XPS than the side chain structure (-CF (CF3)-CF2-O-)m'. We attribute this difference in the strength of the structures to the presence or absence of the chemical structure of the side chain. TEM or XPS measurement reveals that the film thickness

0

0.2

0.4

Relative F1s counts ratio

time by XPS.

exposure time by XPS

0.6

A B C

Fig. 19. Relationship between F1s intensity and X-Ray exposure time during XPS

0.8

1

of the lubricants is 2–3 nm. According to Tani (1999), he found double steps on the lubricant film with 2.9 nm thickness that was almost completely cover the surface by the mean molecular radius of gyration with coil of lubricant molecular. Therefore, it seems that the 2-3 coils of lubricant molecular have been stacked on the surface of the ophthalmic lens.

In the case of sample F, the molecular interaction in the side chain structure of CF3 is weaker than that in the straight chain structure of CF2 because in CF3, three-dimensional structures overlap and this leads to repulsion between fluorine atoms. Therefore, the damage due to exposure to X-rays during XPS in the case of sample F is more than that in the case of sample G or that in the case of sample H. It is predicted that the trend observed in the adhesion properties of lubricants will be the same as that observed in the case of these damages.

Fig. 21. Changing chemical structure of C1s spectrum for sample G as a function of X-ray exposure time by XPS

Fig. 22. Changing chemical structure of C1s spectrum for sample H as a function of X-ray exposure time

Characterization of Lubricant on Ophthalmic Lenses 241

Fig. 24. The mass spectra of positive fragment ions after 60 min X-ray exposure by XPS, as determined by TOF-SIMS (upper spectrum: sample F, middle spectrum: sample G, lower

spectrum: sample H)

Fig. 23. Initial structure of the mass spectra of positive fragment ions, as determined by TOF-SIMS (upper spectrum: sample F, middle spectrum: sample G, lower spectrum: sample H)

Fig. 23. Initial structure of the mass spectra of positive fragment ions, as determined by TOF-SIMS (upper spectrum: sample F, middle spectrum: sample G, lower spectrum: sample H)

Fig. 24. The mass spectra of positive fragment ions after 60 min X-ray exposure by XPS, as determined by TOF-SIMS (upper spectrum: sample F, middle spectrum: sample G, lower spectrum: sample H)

Characterization of Lubricant on Ophthalmic Lenses 243

Fig. 26. Changing chemical structure of C1s spectrum for sample F before and after the

Fig. 27. Changing chemical structure of C1s spectrum for sample G before and after the

Fig. 28. Changing chemical structure of C1s spectrum for sample H before and after the

abrasion test

abrasion test

abrasion test

Fig. 25. Mass spectra of negative fragment ions for sample A, as determined by TOF-SIMS (upper spectrum: initial, lower spectrum: after 60 min X-ray exposure by XPS)


Table 3. Film thickness and coverage ratio of lubricant before and after XPS damage

#### **2.2.4 Abrasion test**

The water contact angle for sample F, sample G, and sample H before and after the abrasion test is listed in table 4. The XPS spectrum for each sample before and after abrasion test is shown in figures 26 – 28. Figures 29 – 31 show the topographic image and the phase image for each sample before and after abrasion test (image on the upper left image: initial topographic image, upper right image: initial phase image, lower left image: topographic image after abrasion test, lower right image: phase image after abrasion test).

The results in table 4 indicate that the water contact angles in the case of sample G and sample H decreased slightly after the abrasion test was performed. In contrast, the water contact angle of sample F decreased drastically from 116° to 89° after the sample was scratched by a 2 kg weight over 600 strokes. In the case of sample F, it seems that the water

Fig. 25. Mass spectra of negative fragment ions for sample A, as determined by TOF-SIMS

Lub. film coverage (%)

Sample F 2.4-2.9 98 over 0.9-1.3 88-91 Sample G 2.3-2.7 98 over 1.8-2.2 94-95 Sample H 2.3-2.7 98 over 1.7-2.1 94-95

Table 3. Film thickness and coverage ratio of lubricant before and after XPS damage

image after abrasion test, lower right image: phase image after abrasion test).

The water contact angle for sample F, sample G, and sample H before and after the abrasion test is listed in table 4. The XPS spectrum for each sample before and after abrasion test is shown in figures 26 – 28. Figures 29 – 31 show the topographic image and the phase image for each sample before and after abrasion test (image on the upper left image: initial topographic image, upper right image: initial phase image, lower left image: topographic

The results in table 4 indicate that the water contact angles in the case of sample G and sample H decreased slightly after the abrasion test was performed. In contrast, the water contact angle of sample F decreased drastically from 116° to 89° after the sample was scratched by a 2 kg weight over 600 strokes. In the case of sample F, it seems that the water

Initial After 60min X-ray explosured

Lub. film thickness (nm)

Lub. film coverage (%)

(upper spectrum: initial, lower spectrum: after 60 min X-ray exposure by XPS)

Lub. film thickness (nm)

**2.2.4 Abrasion test** 

Fig. 26. Changing chemical structure of C1s spectrum for sample F before and after the abrasion test

Fig. 27. Changing chemical structure of C1s spectrum for sample G before and after the abrasion test

Fig. 28. Changing chemical structure of C1s spectrum for sample H before and after the abrasion test

Characterization of Lubricant on Ophthalmic Lenses 245

process. Thus, there is no significant change in the water contact angle. These results indicate that the trend in lubricant damage during XPS agrees with the trend in durability during the abrasion test. Therefore, we found that we can select suitable lubricants for an

Fig. 30. Topographic image and phase image obtained for sample G (upper left image: initial topographic image, upper right image: initial phase image, lower left image: topographic

> Contact angle

Sample F 2.4-2.9 116° 1.1-1.5 89° Sample G 2.3-2.7 110° 2.1-2.5 107° Sample H 2.3-2.7 111° 1.9-2.5 108°

Table 4. Film thickness and water contact angle before and after the abrasion test

Initial After abration test

Lub. film thickness (nm) Contact angle

image after abrasion test, lower right image: phase image after abrasion test)

Lub. film thickness (nm)

ophthalmic lens by XPS measurement.

Fig. 29. Topographic image and phase image obtained for sample F (upper left image: initial topographic image, upper right image: initial phase image, lower left image: topographic image after abrasion test, lower right image: phase image after abrasion test)

repellant of lubricant was declined because it was decreased the lubricants quantity of sample F by abrasion test. A phase image that was obtained by AFM revealed the distribution of unevenness (roughness), the viscosity, elasticity, friction force, adhesion, and soft-hardness from the energy dissipation of interaction between tip and sample. In a previous study, we showed that the energy dissipation in the areas corresponding to bright areas in the phase image is greater than that in the areas corresponding to dark areas in the image. This result, along with a comparison of the phase image and force modulation image, reveals that the bright area is softer or more adhesive than the dark area. The initial phase images for each sample comprise a mixture of small soft areas and small hard areas (or small adhesive areas and small non-adhesive areas). In the case of sample F, a scratch is observed along the scan area in the image obtained after the abrasion test. Just like, the lubricants were removed by rubbing. Therefore, the water contact angle decreased when the lubricants were removed. On the other hand, in the case of sample G and sample H, we observed that the cluster of lubricants was larger than the initial cluster. Further, there is no scratch in the image obtained after the abrasion test. We guess that lubricants repeated the attaching and moving, the mixtures of soft regions and hard regions were grown by rubbing

Fig. 29. Topographic image and phase image obtained for sample F (upper left image: initial topographic image, upper right image: initial phase image, lower left image: topographic

repellant of lubricant was declined because it was decreased the lubricants quantity of sample F by abrasion test. A phase image that was obtained by AFM revealed the distribution of unevenness (roughness), the viscosity, elasticity, friction force, adhesion, and soft-hardness from the energy dissipation of interaction between tip and sample. In a previous study, we showed that the energy dissipation in the areas corresponding to bright areas in the phase image is greater than that in the areas corresponding to dark areas in the image. This result, along with a comparison of the phase image and force modulation image, reveals that the bright area is softer or more adhesive than the dark area. The initial phase images for each sample comprise a mixture of small soft areas and small hard areas (or small adhesive areas and small non-adhesive areas). In the case of sample F, a scratch is observed along the scan area in the image obtained after the abrasion test. Just like, the lubricants were removed by rubbing. Therefore, the water contact angle decreased when the lubricants were removed. On the other hand, in the case of sample G and sample H, we observed that the cluster of lubricants was larger than the initial cluster. Further, there is no scratch in the image obtained after the abrasion test. We guess that lubricants repeated the attaching and moving, the mixtures of soft regions and hard regions were grown by rubbing

image after abrasion test, lower right image: phase image after abrasion test)

process. Thus, there is no significant change in the water contact angle. These results indicate that the trend in lubricant damage during XPS agrees with the trend in durability during the abrasion test. Therefore, we found that we can select suitable lubricants for an ophthalmic lens by XPS measurement.

Fig. 30. Topographic image and phase image obtained for sample G (upper left image: initial topographic image, upper right image: initial phase image, lower left image: topographic image after abrasion test, lower right image: phase image after abrasion test)


Table 4. Film thickness and water contact angle before and after the abrasion test

Characterization of Lubricant on Ophthalmic Lenses 247

The authors would like to thank Ms. Pannakarn, Mr. Parnich, Mr. Takashiba, Mr. Shimizu, Mr. Higuchi, Ms. Khraikratoke, Mr. Kamura, and Mr. Iwata for supplying the samples, measurements, and fruitful discussions for a surface investigation of ophthalmic lenses. Additionally thanks to Mr. Takami and Ms. Moriya (Asylum Technology Japan) for

Briggs, D. & Seah, M. P. (1990). Practical surface analysis 2'nd edition, John Wiley & Sons

Cleveland, J. P., Anczykowski, B., Schmid, A. E., & Elings, V. B. (1998). Applied Physics

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Mate, C. M., Lorenz, M. R. & Novotny, V. J. (1989). J. Chem. Phys., Vol.90, pp. 7550-7555,

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Tadokoro, N. & Osakabe, K. (2001). Proc. Int. Tribol. Conference Nagasaaki 2000, pp. 2191-

Tadokoro, N., Yuki M., & Osakabe, K. (2003). Applied surface science, 203-204, pp. 72-77,

Tadokoro, N., Khraikratoke, S., Jamnongpian, P., Maeda, A., Komine, Y., Pavarinpong, N.,

Tadokoro, N., Pannakarn, S., Khraikratoke S., Kamura, H., & Iwata, N. (2010). Proc. the 8th

Tadokoro, N., Pannakarn, S., Wisuthtatip, J., Kunchoo, S., Parnich, V., Takashiba, K.,

Tani, H. (1999). Magnetics Conference, INTERMAG 99, IEEE. Trans. mag., vol.35, pp.2397-

Suyjantuk, S., & Iwata, N. (2009). Proc. World. Tribol. Congress 2009, pp. 749, ISBN

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Letters, Vol. 72, No. 20, pp. 2613-2615, ISSN 0003-6951

**4. Acknowledgment** 

**5. References** 

1168

ISSN 0021-9606

0734-2101

0734-2101

1096-9918

ISSN 0169-4332

978-4-9900139-9-8

1341-1756

technical discussions on AFM measurement.

Ltd., pp209, ISBN 0471920819

2392- 2394, ISSN 0018-9464

5861-5868, ISSN 0021-9606

2196, ISBN 4-9900139-6-4

2399, ISBN 0-7803-5555-5

vol.35, pp. 2394-2396, ISBN 0-7803-5555-5

ICCG8, pp. 343-348, ISBN 978-3-00-031387-5

Fig. 31. Topographic image and phase image obtained for sample H (upper left image: initial topographic image, upper right image: initial phase image, lower left image: topographic image after abrasion test, lower right image: phase image after abrasion test)

#### **3. Conclusion**

We evaluated various methods for the analysis of lubricants on ophthalmic lenses. The lubricant film thickness can be directly determined by TEM measurement. The coverage ratio, the X-ray damage and the chemical structure can be investigated by XPS analysis. And also, TOF-SIMS analysis was used the investigation of X-ray damage and the chemical structure. In particular, AFM with an additional functional mode is a highly effective method for examining the morphology of lubricants; while determining the island structures of shapes with similar surface morphologies, it is more convenient to use phase images than friction force images and force modulation image. This information can be used to improve the tribological performance of ophthalmic lenses surface in order to meet customer demand.

#### **4. Acknowledgment**

246 Tribology - Lubricants and Lubrication

Fig. 31. Topographic image and phase image obtained for sample H (upper left image: initial topographic image, upper right image: initial phase image, lower left image: topographic

We evaluated various methods for the analysis of lubricants on ophthalmic lenses. The lubricant film thickness can be directly determined by TEM measurement. The coverage ratio, the X-ray damage and the chemical structure can be investigated by XPS analysis. And also, TOF-SIMS analysis was used the investigation of X-ray damage and the chemical structure. In particular, AFM with an additional functional mode is a highly effective method for examining the morphology of lubricants; while determining the island structures of shapes with similar surface morphologies, it is more convenient to use phase images than friction force images and force modulation image. This information can be used to improve the tribological performance of ophthalmic lenses surface in order to meet

image after abrasion test, lower right image: phase image after abrasion test)

**3. Conclusion** 

customer demand.

The authors would like to thank Ms. Pannakarn, Mr. Parnich, Mr. Takashiba, Mr. Shimizu, Mr. Higuchi, Ms. Khraikratoke, Mr. Kamura, and Mr. Iwata for supplying the samples, measurements, and fruitful discussions for a surface investigation of ophthalmic lenses. Additionally thanks to Mr. Takami and Ms. Moriya (Asylum Technology Japan) for technical discussions on AFM measurement.

#### **5. References**


**1. Introduction** 

and corrosion control.

**1.1 Lubrication (Rizvi, 2009)** 

intervening layer of fluid or fluid-like material.

successful operation of a machine and failure.

and exceedingly frequent oil changes were required.

**1.2 Lubricants (Rizvi, 2009; Ludema, 1996; and Leslie, 2003)** 

**Lubricating Oil Additives** 

Nehal S. Ahmed and Amal M. Nassar *Egyptian Petroleum Research Institute* 

The principle of supporting a sliding load on a friction reducing film is known as lubrication (Ludema, 1996). The substance of which the film is composed is a lubricant, and to apply it is to lubricate. These are not new concepts, nor, in their essence, particularly involved ones. Farmers lubricated the axles of their ox carts with animal fat centuries ago. But modern machinery has become many times more complicated since the days of the ox cart, and the demands placed upon the lubricant have become proportionally more exacting. Though the basic principle still prevails the prevention of metal-to-metal contact by means of an

All liquids will provide lubrication of a sort, but some do it a great deal better than others. The difference between one lubricating material and another is often the difference between

Modern equipment must be lubricated in order to prolong its lifetime. A lubricant performs a number of critical functions. These include lubrication, cooling, cleaning and suspending, and protecting metal surfaces against corrosive damage. Lubricant comprises a base fluid and an additive package. The primary function of the base fluid is to lubricate and act as a carrier of additives. The function of additives is either to enhance an already-existing property of the base fluid or to add a new property. The examples of already-existing properties include viscosity, viscosity index, pour point, and oxidation resistance. The examples of new properties include cleaning and suspending ability, antiwear performance,

Engine oil at the dawn of the automotive era was not highly specialized or standardized,

Engine oil lubricants make up nearly one half of the lubricant market and therefore attract a lot of interest. The principal function of the engine oil lubricant is to extend the life of moving parts operating under many different conditions of speed, temperature, and pressure. At low temperatures the lubricant is expected to flow sufficiently in order that moving parts are not starved of oil. At higher temperatures they are expected to keep the

*Egypt* 

Toney, M. F., Mate C. M., & Pocker, D. (1991). IEEE. Trans. mag., Vol.34, pp. 1774-1776, ISSN 0018-9464 **10** 

## **Lubricating Oil Additives**

Nehal S. Ahmed and Amal M. Nassar *Egyptian Petroleum Research Institute Egypt* 

#### **1. Introduction**

248 Tribology - Lubricants and Lubrication

Toney, M. F., Mate C. M., & Pocker, D. (1991). IEEE. Trans. mag., Vol.34, pp. 1774-1776, ISSN

#### **1.1 Lubrication (Rizvi, 2009)**

The principle of supporting a sliding load on a friction reducing film is known as lubrication (Ludema, 1996). The substance of which the film is composed is a lubricant, and to apply it is to lubricate. These are not new concepts, nor, in their essence, particularly involved ones. Farmers lubricated the axles of their ox carts with animal fat centuries ago. But modern machinery has become many times more complicated since the days of the ox cart, and the demands placed upon the lubricant have become proportionally more exacting. Though the basic principle still prevails the prevention of metal-to-metal contact by means of an intervening layer of fluid or fluid-like material.

#### **1.2 Lubricants (Rizvi, 2009; Ludema, 1996; and Leslie, 2003)**

All liquids will provide lubrication of a sort, but some do it a great deal better than others. The difference between one lubricating material and another is often the difference between successful operation of a machine and failure.

Modern equipment must be lubricated in order to prolong its lifetime. A lubricant performs a number of critical functions. These include lubrication, cooling, cleaning and suspending, and protecting metal surfaces against corrosive damage. Lubricant comprises a base fluid and an additive package. The primary function of the base fluid is to lubricate and act as a carrier of additives. The function of additives is either to enhance an already-existing property of the base fluid or to add a new property. The examples of already-existing properties include viscosity, viscosity index, pour point, and oxidation resistance. The examples of new properties include cleaning and suspending ability, antiwear performance, and corrosion control.

Engine oil at the dawn of the automotive era was not highly specialized or standardized, and exceedingly frequent oil changes were required.

Engine oil lubricants make up nearly one half of the lubricant market and therefore attract a lot of interest. The principal function of the engine oil lubricant is to extend the life of moving parts operating under many different conditions of speed, temperature, and pressure. At low temperatures the lubricant is expected to flow sufficiently in order that moving parts are not starved of oil. At higher temperatures they are expected to keep the

Lubricating Oil Additives 251

**F L O W**  **(B) Metal Surface**

**Metal Surface**

**Friction Modifier Molecule Hydrocarbon (Oil) Molecule**

**Metal Surface**

inactive, and they improve fuel efficiency.

Leslie, 2003, agents.

**Hydrocarbon Polar Chain Group**

Fig. 1.1 Adsorption of friction modifiers on metal (A) Steady state (B) Under shear

**1.3.2 Anti-wear agents (A.W.) and extreme-pressure (E.P.) additives** 

Common materials that are used for this purpose include long-chain fatty acids, their derivatives, and the molybdenum compounds. In addition to reducing friction, the friction modifiers also reduce wear, especially at low temperatures where the anti-wear agents are

Anti-wear (AW) (Rizvi, 2009, Ludema, 1996, Leslie, 2003, and Masabumi, 2008 ), agents have a lower activation temperature than the extreme-pressure (EP) agents. The latter are also referred to as anti-seize and anti-scuffing additives. Organosulfur and organo-phosphorus compounds, Figure (1.2), such as organic polysulfides, phosphates, dithiophosphates, and dithiocarbamates are the most commonly used AW and EP, Rizvi, 2009, Ludema, 1996,

 **(A) Metal Surface**

moving parts apart to minimize wear. The lubricant does this by reducing friction and removing heat from moving parts. Contaminants pose an additional problem, as they accumulate in the engine during operation. The contaminants may be wear debris, sludges, soot particles, acids, or peroxides. An important function of the lubricant is to prevent these contaminants from doing any damage.

The lube oil base stock is the building block with respect to which appropriate additives are selected and properly blended to achieve a delicate balance in performance characteristics of the finished lubricant. Various base stock manufacturing processes can all produce base stocks with the necessary characteristics to formulate finished lubricants with the desirable performance levels. The key to achieving the highest levels of performance in finished lubricants is in the understanding of the interactions of base stocks and additives and matching those to requirements of machinery and operating conditions to which they can be subjected.

#### **1.3 Additives**

Additives, (Rizvi, 2009, Ludema, 1996, and Leslie, 2003 ), are chemical compounds added to lubricating oils to impart specific properties to the finished oils. Some additives impart new and useful properties to the lubricant; some enhance properties already present, while some act to reduce the rate at which undesirable changes take place in the product during its service life. Additives, in improving the performance characteristics of lubricating oils, have aided significantly in the development of improved prime movers and industrial machinery.

Modern passenger car engines, automatic transmissions, hypoid gears, railroad and marine diesel engines, high speed gas and steam turbines, and industrial processing machinery, as well as many other types of equipment, would have been greatly retarded in their development were it not for additives and the performance benefits they provide.

Additives for lubricating oils were used first during the 1920s, and their use has since increased tremendously. Today, practically all types of lubricating oil contain at least one additive, and some oils contain additives of several different types. The amount of additive used varies from a few hundredths of a percent to 30% or more.

Over a period of many years, oil additives were identified that solved a variety of engine problems: corrosion inhibition, ability to keep particles such as soot dispersed, ability to prohibit acidic combustion products from plating out as varnish on engine surfaces, and ability to minimize wear by laying down a chemical film on heavily loaded surfaces. In addition, engine oil became specialized so that requirements for diesel engine oils began to diverge from requirements for gasoline engines, since enhanced dispersive capability was needed to keep soot from clumping in the oil of diesel engines.

The more commonly used additives are discussed in the following sections. Although some are multifunctional, as in the case of certain viscosity index improvers that also function as pour point depressants or dispersants or antiwear agents that also function as oxidation inhibitors, they are discussed in terms of their primary function only.

#### **1.3.1 Friction Modifiers (FM) (Ludema, 1996)**

These are additives that usually reduce friction (Battez et al., 2010 & Mel'nikov, 1997). The mechanism of their performance is similar to that of the rust and corrosion inhibitors in that they form durable low resistance lubricant films via adsorption on surfaces and via association with the oil, Figure (1.1).

moving parts apart to minimize wear. The lubricant does this by reducing friction and removing heat from moving parts. Contaminants pose an additional problem, as they accumulate in the engine during operation. The contaminants may be wear debris, sludges, soot particles, acids, or peroxides. An important function of the lubricant is to prevent these

The lube oil base stock is the building block with respect to which appropriate additives are selected and properly blended to achieve a delicate balance in performance characteristics of the finished lubricant. Various base stock manufacturing processes can all produce base stocks with the necessary characteristics to formulate finished lubricants with the desirable performance levels. The key to achieving the highest levels of performance in finished lubricants is in the understanding of the interactions of base stocks and additives and matching those to requirements of machinery and operating conditions to which they can be

Additives, (Rizvi, 2009, Ludema, 1996, and Leslie, 2003 ), are chemical compounds added to lubricating oils to impart specific properties to the finished oils. Some additives impart new and useful properties to the lubricant; some enhance properties already present, while some act to reduce the rate at which undesirable changes take place in the product during its service life. Additives, in improving the performance characteristics of lubricating oils, have aided significantly in the development of improved prime movers and industrial

Modern passenger car engines, automatic transmissions, hypoid gears, railroad and marine diesel engines, high speed gas and steam turbines, and industrial processing machinery, as well as many other types of equipment, would have been greatly retarded in their

Additives for lubricating oils were used first during the 1920s, and their use has since increased tremendously. Today, practically all types of lubricating oil contain at least one additive, and some oils contain additives of several different types. The amount of additive

Over a period of many years, oil additives were identified that solved a variety of engine problems: corrosion inhibition, ability to keep particles such as soot dispersed, ability to prohibit acidic combustion products from plating out as varnish on engine surfaces, and ability to minimize wear by laying down a chemical film on heavily loaded surfaces. In addition, engine oil became specialized so that requirements for diesel engine oils began to diverge from requirements for gasoline engines, since enhanced dispersive capability was

The more commonly used additives are discussed in the following sections. Although some are multifunctional, as in the case of certain viscosity index improvers that also function as pour point depressants or dispersants or antiwear agents that also function as oxidation

These are additives that usually reduce friction (Battez et al., 2010 & Mel'nikov, 1997). The mechanism of their performance is similar to that of the rust and corrosion inhibitors in that they form durable low resistance lubricant films via adsorption on surfaces and via

development were it not for additives and the performance benefits they provide.

used varies from a few hundredths of a percent to 30% or more.

needed to keep soot from clumping in the oil of diesel engines.

**1.3.1 Friction Modifiers (FM) (Ludema, 1996)**

association with the oil, Figure (1.1).

inhibitors, they are discussed in terms of their primary function only.

contaminants from doing any damage.

subjected.

**1.3 Additives** 

machinery.

Common materials that are used for this purpose include long-chain fatty acids, their derivatives, and the molybdenum compounds. In addition to reducing friction, the friction modifiers also reduce wear, especially at low temperatures where the anti-wear agents are inactive, and they improve fuel efficiency.

#### **1.3.2 Anti-wear agents (A.W.) and extreme-pressure (E.P.) additives**

Anti-wear (AW) (Rizvi, 2009, Ludema, 1996, Leslie, 2003, and Masabumi, 2008 ), agents have a lower activation temperature than the extreme-pressure (EP) agents. The latter are also referred to as anti-seize and anti-scuffing additives. Organosulfur and organo-phosphorus compounds, Figure (1.2), such as organic polysulfides, phosphates, dithiophosphates, and dithiocarbamates are the most commonly used AW and EP, Rizvi, 2009, Ludema, 1996, Leslie, 2003, agents.

Lubricating Oil Additives 253

RO S S OR

P Zn P

RO S S OR

The R group may be alkyl or aryl

One of the most important aspects of lubricating oils is that the oxidation stability be maximized. Exposure of hydrocarbons to oxygen and heat will accelerate the oxidation process. The internal combustion engine is an excellent chemical reactor for catalyzing the process of oxidation. Also, the engine's metal parts, such as copper and iron, act as effective oxidation catalysts. Thus, engine oils are probably more susceptible to oxidation than any

The lubricating oils consist of hydrocarbons with (C20 – C70) carbon atoms. At higher temperature these hydrocarbons are oxidized to form fatty acids, fatty alcohols, fatty aldehydes and ketones, fatty esters and fatty peroxides as shown in the following mechanism, Figure (1.4). All these compounds form the solid asphaltic materials. For this reason, the addition of antioxidants is necessary to all lubricating oils to prevent the

Fig. 1.3 Zinc dithiophosphate as antiwear additives / extreme pressure

Initiation

+O2 RH R ⎯⎯⎯→ i

Chain propagation

R O ROO i i + ⎯⎯ <sup>2</sup> →

Chain branching

Termination

Fig. 1.4 Oxidation mechanism of lubricating oils

2R R -R i ⎯⎯→

ROOH RO OH ⎯⎯→ + i i RO RH ROH R i i + ⎯⎯→ + OH RH H O R + ⎯⎯→ + <sup>2</sup> i i

ROO RH ROOH + R i i + ⎯⎯→

**1.3.3 Antioxidant additives (AO)** 

other lubricant application.

formation of such compounds.

**Oxidation mechanism of lubricating oils** 

Fig. 1.2 Common phosphorus derivatives used as antiwear agents / extreme-pressure

As the power of engines has risen, the need for additives to prevent wear has become more important. Initially engines were lightly loaded and could withstand the loading on the bearings and valve train. Corrosive protection of bearing metals was one of the early requirements for engine oils. Fortunately, the additives used to protect bearings usually had mild antiwear properties. These antiwear agents were compounds such as lead salts of longchain carboxylic acids and were often used in combination with sulfur-containing materials. Oil-soluble sulfur-phosphorous and chlorinated compounds also worked well as antiwear agents. However, the most important advance in antiwear chemistry was made during the 1930s and 1940s with the discovery of zinc dialkyldithiophosphates (ZDDP) (Masabumi, et. al., 2008). These compounds were initially used to prevent bearing corrosion but were later found to have exceptional antioxidant and antiwear properties. The antioxidant mechanism of the ZDDP was the key to its ability to reduce bearing corrosion. Since the ZDDP suppresses the formation of peroxides, it prevents the corrosion of Cu/Pb bearings by organic acids. Antiwear and extreme-pressure additives function by thermally decomposing to yield compounds that react with the metal surface. These surface-active compounds form a thin layer that preferentially shears under boundary lubrication conditions.

After the discovery of ZDDP, Figure (1.3) it rapidly became the most widespread antiwear additive used in lubricants. As a result, many interesting studies have been undertaken on ZDDP with many mechanisms proposed for the antiwear and antioxidant action (Masabumi, et. al., 2008).

Extreme pressure additives form extremely durable protective films by thermo-chemically reacting with the metal surfaces. This film can withstand extreme temperatures and mechanical pressures and minimizes direct contact between surfaces, thereby protecting them from scoring and seizing.

**RO**

**P**

**CH R2 CH R2 RS**

**RO**

**Trialkyl Dithiophosphate Dialkyl Phosphonate Dialkyl ThioPhosphonate Trialkyl TetrathioPhosphonate**

As the power of engines has risen, the need for additives to prevent wear has become more important. Initially engines were lightly loaded and could withstand the loading on the bearings and valve train. Corrosive protection of bearing metals was one of the early requirements for engine oils. Fortunately, the additives used to protect bearings usually had mild antiwear properties. These antiwear agents were compounds such as lead salts of longchain carboxylic acids and were often used in combination with sulfur-containing materials. Oil-soluble sulfur-phosphorous and chlorinated compounds also worked well as antiwear agents. However, the most important advance in antiwear chemistry was made during the 1930s and 1940s with the discovery of zinc dialkyldithiophosphates (ZDDP) (Masabumi, et. al., 2008). These compounds were initially used to prevent bearing corrosion but were later found to have exceptional antioxidant and antiwear properties. The antioxidant mechanism of the ZDDP was the key to its ability to reduce bearing corrosion. Since the ZDDP suppresses the formation of peroxides, it prevents the corrosion of Cu/Pb bearings by organic acids. Antiwear and extreme-pressure additives function by thermally decomposing to yield compounds that react with the metal surface. These surface-active compounds form

Fig. 1.2 Common phosphorus derivatives used as antiwear agents / extreme-pressure

a thin layer that preferentially shears under boundary lubrication conditions.

After the discovery of ZDDP, Figure (1.3) it rapidly became the most widespread antiwear additive used in lubricants. As a result, many interesting studies have been undertaken on ZDDP with many mechanisms proposed for the antiwear and antioxidant action

Extreme pressure additives form extremely durable protective films by thermo-chemically reacting with the metal surfaces. This film can withstand extreme temperatures and mechanical pressures and minimizes direct contact between surfaces, thereby protecting

**SR SR**

**RO**

**SH OR**

**S S S**

**RO**

**Dialkyl Hydrogen Phosphite Trialkyl Phosphite Monoalkyl Phosphoric Acid** 

**RO**

**P**

**P**

**OR**

**OH**

**O**

**RO**

**RS**

**RO**

**HO**

**RO**

**P**

**P**

**S S**

**P**

(Masabumi, et. al., 2008).

them from scoring and seizing.

**RO RO**

**OH**

**H**

**O**

**RO**

**RO**

**RO**

**RO**

**RO**

**P**

**RO RO**

**O**

**Dialkyl Phosphoric Acid Dialkyl Dithiophosphoric Acid Trialkyl Phosphate Trialkyl Thiophosphate**

**O O**

**P P OH P OR**

**P**

**RO**

**RO**

**RO**

**RO**

**P**

The R group may be alkyl or aryl

Fig. 1.3 Zinc dithiophosphate as antiwear additives / extreme pressure

#### **1.3.3 Antioxidant additives (AO)**

One of the most important aspects of lubricating oils is that the oxidation stability be maximized. Exposure of hydrocarbons to oxygen and heat will accelerate the oxidation process. The internal combustion engine is an excellent chemical reactor for catalyzing the process of oxidation. Also, the engine's metal parts, such as copper and iron, act as effective oxidation catalysts. Thus, engine oils are probably more susceptible to oxidation than any other lubricant application.

#### **Oxidation mechanism of lubricating oils**

The lubricating oils consist of hydrocarbons with (C20 – C70) carbon atoms. At higher temperature these hydrocarbons are oxidized to form fatty acids, fatty alcohols, fatty aldehydes and ketones, fatty esters and fatty peroxides as shown in the following mechanism, Figure (1.4). All these compounds form the solid asphaltic materials. For this reason, the addition of antioxidants is necessary to all lubricating oils to prevent the formation of such compounds.

```
Initiation
+O2 RH R ⎯⎯⎯→ i
Chain propagation
R O ROO i i + ⎯⎯ 2 →
ROO RH ROOH + R i i + ⎯⎯→
Chain branching
ROOH RO OH ⎯⎯→ + i i
RO RH ROH R i i + ⎯⎯→ +
OH RH H O R + ⎯⎯→ + 2 i i
Termination
2R R -R i ⎯⎯→
```
Fig. 1.4 Oxidation mechanism of lubricating oils

Lubricating Oil Additives 255

During oxidation process, in a median period oil viscosity is increased although its acidity is remaining constant because there are alcohol's of unsaturated hydrocarbons, produced from decomposition of hydroperoxides process, so it neutralizes the effect of acidity formed from

The proceeding lubricant degradation mechanism makes clear several possible counter measures to control lubricant degradation. Blocking the energy source is one path but is effective only for lubricants used in low-shear and temperature situations. However, more practical for most lubricant applications are the trapping of catalytic impurities and the destruction of hydrocarbon radicals, alkyl peroxy radicals, and hydroperoxides. This can be achieved through the use of radical scavengers, peroxide decomposers, and metal

The radical scavengers are known as *primary antioxidants*. They donate hydrogen atoms that react with alkyl radicals and/or alkyl peroxy radicals, interrupting the radical chain mechanism of the auto-oxidation process. The primary antioxidant then becomes a stable radical, the alkyl radical becomes a hydrocarbon, and the alkyl peroxy radical becomes an alkyl hydroperoxide. Hindered phenolics and aromatic amines are the two chemical classes of primary antioxidants for lubricants. The transfer of a hydrogen from the oxygen or nitrogen atom to the radical forms quinones or quinine imines that do not maintain the

The peroxide decomposers are known as *secondary antioxidants* (Rizvi, 2009, Ludema, 1996, and Leslie, 2003). Sulfur and/or phosphorus compounds reduce the alkyl hydroperoxides in the radical chain to alcohols while being oxidized in a sacrificial manner. Zinc dialkyldithiophosphate, phosphites, and thio-ethers are examples of different chemical

There are two types of metal deactivators: chelating agents and film forming agents . The chelating agents will form a stable complex with metal ions, reducing the catalytic activity of the metal ions. Thus, the deactivators can show an antioxidant effect. Film-forming agents act two ways. First, they coat the metal surface, thus preventing metal ions from entering the oil. Second, they minimize corrosive attack of the metal surface by physically restricting

Several effective antioxidants classes have been developed over the years and have seen use in engine oils, automatic transmission fluids, gear oils, turbine oils, compressor oils, greases, hydraulic fluids, and metal-working fluids. The main classes of oil-soluble organic and

The foaming of lubricants, (Rizvi, 2009, Ludema, 1996, and Leslie, 2003), is a very undesirable effect that can cause enhanced oxidation by the intensive mixture with air,

another oxygen bearing compound such as: aldehydes, ketones and acids.

deactivators.

radical chain mechanism.

1. Sulfur compounds 2. Phosphorus compounds 3. Sulfur-phosphorus compounds 4. Aromatic amine compounds 5. Hindered phenolic compounds 6. Organo-alkaline earth salt compounds

7. Organo-zinc compounds 8. Organo-copper compounds 9. Organo-molybdenum compounds

**1.3.4 Anti-foam (A.F.) agents** 

classes of secondary antioxidants.

access of the corrosive species to the metal surface.

organo-metallic antioxidants are the following types:

ROOH RO OH → + i i (1.4)

#### **Inhibition effects of antioxidants on lubricating oil oxidations**

It is known that high temperature, high pressure, high friction, and high metal concentration in motors, lead to oxidation of lubricating oil it is necessary to improve oil stabilities against oxidation. Oxidation generally increase oil viscosity and results in formation of the following compounds:


Thus, addition of antioxidant additives to lubricating oils prevents the formation of all resins, lacquers and acidic compounds.

There is no relationship between the two rates of increasing the viscosity and acidity in the oxidation process. The rate of viscosity increased in direct proportion with the rate of decomposition of an organic peroxides. By studying the antioxidant additives mechanism in turbine aviation oils; it was showen that these antioxidants reacted with oxygenic free radical compounds to form the antioxidant N – oxide derivatives and thus decrease the quantity acids, alcohols, esters in the media. However, any lubricating oil exposed to air and heat will eventually oxidize. Antioxidants are the key additive that protects the lubricant from oxidative degradation, allowing the fluid to meet the demanding requirements for use in engines and industrial applications.

#### **Antioxidant additives mechanism**

To define the oxidation stability for lubricating oils, it is necessary to check the rate of acidity and viscosity increase with oxidation time during the oxidation process. Lubricating oils are susceptible to degradation by oxygen. The oil oxidation (Rizvi, 2009, Ludema, 1996, and Leslie, 2003) process is the major cause of oil thickening. This manifests itself as sludge and varnish formation on engine parts, leading to increased engine wear, poor lubrication, and reduced fuel economy. Antioxidants are essential additives incorporated into lubricant formulations to minimize and delay the onset of lubricant oxidative degradation.

The rate of acidity and viscosity was increased (in the oxidation process for oils) due to the continuous repetition of the oxidation process, where a chain reaction occurs. The oxidation process can be considered to progress in the following manner:

$$\text{RH}^\cdot \rightarrow \text{R}^\bullet \text{+H}^\cdot \tag{1.1}$$

$$\rm{R}^\* + \rm{O}\_2 \rightarrow \rm{ROO}^\* \tag{1.2}$$

$$\text{ROO}^{\cdot} + \text{RH} \rightarrow \text{ROOH} + \text{R}^{\cdot} \tag{1.3}$$

Decomposition of the hydroperoxide molecule caused by the so-called branching reaction leads to the formation of oxygen bearing compounds. Their oxidation products form high – molecular – weight –oil –insoluble polymers that settle as deposit causing an increase of oil viscosity. Decomposition of the hydroperoxide as follow:

It is known that high temperature, high pressure, high friction, and high metal concentration in motors, lead to oxidation of lubricating oil it is necessary to improve oil stabilities against oxidation. Oxidation generally increase oil viscosity and results in formation of the

• Resins, which are oxygen-containing compounds, soluble in oil and can, lead to lacquer

• Lacquers, slightly colored, relatively plastic, and can form deposits on various engine

• Insoluble asphaltic compounds, when associated in the oil with combustion residues

• Acidic compounds and hydroperoxides, which may promote corrosion, particularly of

There is no relationship between the two rates of increasing the viscosity and acidity in the oxidation process. The rate of viscosity increased in direct proportion with the rate of decomposition of an organic peroxides. By studying the antioxidant additives mechanism in turbine aviation oils; it was showen that these antioxidants reacted with oxygenic free radical compounds to form the antioxidant N – oxide derivatives and thus decrease the quantity acids, alcohols, esters in the media. However, any lubricating oil exposed to air and heat will eventually oxidize. Antioxidants are the key additive that protects the lubricant from oxidative degradation, allowing the fluid to meet the demanding requirements for use

To define the oxidation stability for lubricating oils, it is necessary to check the rate of acidity and viscosity increase with oxidation time during the oxidation process. Lubricating oils are susceptible to degradation by oxygen. The oil oxidation (Rizvi, 2009, Ludema, 1996, and Leslie, 2003) process is the major cause of oil thickening. This manifests itself as sludge and varnish formation on engine parts, leading to increased engine wear, poor lubrication, and reduced fuel economy. Antioxidants are essential additives incorporated into lubricant

The rate of acidity and viscosity was increased (in the oxidation process for oils) due to the continuous repetition of the oxidation process, where a chain reaction occurs. The oxidation

R O ROO + →<sup>2</sup>

Decomposition of the hydroperoxide molecule caused by the so-called branching reaction leads to the formation of oxygen bearing compounds. Their oxidation products form high – molecular – weight –oil –insoluble polymers that settle as deposit causing an increase of oil

RH R +H → i i (1.1)

i i (1.2)

ROO RH ROOH + R i i + → (1.3)

formulations to minimize and delay the onset of lubricant oxidative degradation.

process can be considered to progress in the following manner:

viscosity. Decomposition of the hydroperoxide as follow:

Thus, addition of antioxidant additives to lubricating oils prevents the formation of all

**Inhibition effects of antioxidants on lubricating oil oxidations** 

following compounds:

hard alloy bearings.

parts (particularly on piston skirts).

and condensed water from sludge.

resins, lacquers and acidic compounds.

in engines and industrial applications. **Antioxidant additives mechanism** 

formation.

$$\text{ROOH} \rightarrow \text{RO} \cdot + \text{\textasciicircum} \tag{1.4}$$

During oxidation process, in a median period oil viscosity is increased although its acidity is remaining constant because there are alcohol's of unsaturated hydrocarbons, produced from decomposition of hydroperoxides process, so it neutralizes the effect of acidity formed from another oxygen bearing compound such as: aldehydes, ketones and acids.

The proceeding lubricant degradation mechanism makes clear several possible counter measures to control lubricant degradation. Blocking the energy source is one path but is effective only for lubricants used in low-shear and temperature situations. However, more practical for most lubricant applications are the trapping of catalytic impurities and the destruction of hydrocarbon radicals, alkyl peroxy radicals, and hydroperoxides. This can be achieved through the use of radical scavengers, peroxide decomposers, and metal deactivators.

The radical scavengers are known as *primary antioxidants*. They donate hydrogen atoms that react with alkyl radicals and/or alkyl peroxy radicals, interrupting the radical chain mechanism of the auto-oxidation process. The primary antioxidant then becomes a stable radical, the alkyl radical becomes a hydrocarbon, and the alkyl peroxy radical becomes an alkyl hydroperoxide. Hindered phenolics and aromatic amines are the two chemical classes of primary antioxidants for lubricants. The transfer of a hydrogen from the oxygen or nitrogen atom to the radical forms quinones or quinine imines that do not maintain the radical chain mechanism.

The peroxide decomposers are known as *secondary antioxidants* (Rizvi, 2009, Ludema, 1996, and Leslie, 2003). Sulfur and/or phosphorus compounds reduce the alkyl hydroperoxides in the radical chain to alcohols while being oxidized in a sacrificial manner. Zinc dialkyldithiophosphate, phosphites, and thio-ethers are examples of different chemical classes of secondary antioxidants.

There are two types of metal deactivators: chelating agents and film forming agents . The chelating agents will form a stable complex with metal ions, reducing the catalytic activity of the metal ions. Thus, the deactivators can show an antioxidant effect. Film-forming agents act two ways. First, they coat the metal surface, thus preventing metal ions from entering the oil. Second, they minimize corrosive attack of the metal surface by physically restricting access of the corrosive species to the metal surface.

Several effective antioxidants classes have been developed over the years and have seen use in engine oils, automatic transmission fluids, gear oils, turbine oils, compressor oils, greases, hydraulic fluids, and metal-working fluids. The main classes of oil-soluble organic and organo-metallic antioxidants are the following types:


#### **1.3.4 Anti-foam (A.F.) agents**

The foaming of lubricants, (Rizvi, 2009, Ludema, 1996, and Leslie, 2003), is a very undesirable effect that can cause enhanced oxidation by the intensive mixture with air,

Lubricating Oil Additives 257

products already formed in the bulk lubricant. Oxidation inhibitors intercept the oxidation mechanism, and dispersants and detergents perform the suspending part (Kyunghyun, 2010). Detergents are metal salts of organic acids that frequently contain associated excess base, usually in the form of carbonate. Dispersants are metal-free and are of higher molecular weights than detergents. The two types of additives work in conjunction with

The final products of combustion and lubricant decomposition include organic and inorganic acids, aldehydes, ketones, and other oxygenated materials. The acids have the propensity to attack metal surfaces and cause corrosive wear. Detergents, especially basic detergents, contain reserve base that will neutralize the acids to form salts. While this decreases the corrosive tendency of the acids, the solubility of the salts in the bulk lubricant is still low. The organic portion of the detergent, commonly called "soap", has the ability to associate with the salts to keep them suspended in the bulk lubricant. However, in this regard, detergents are not as effective as dispersants because of their lower molecular weight. The soap in detergents and the dispersants also have the ability to suspend nonacidic oxygenated products, such as alcohols, aldehydes, and resinous oxygenates. The

Dispersants and detergents together make up the bulk, about 45–50%, of the total volume of the lubricant additives manufactured. This is a consequence of their major use in engine oils, transmission fluids, and tractor hydraulic fluids, all of which are high-volume lubricants. As mentioned, detergents neutralize oxidation-derived acids as well as help suspend polar oxidation products in the bulk lubricant. Because of this, these additives control rust, corrosion, and resinous buildup in the engine. Like most additives detergents contain a surface-active polar functionality and an oleophilic hydrocarbon group, with an appropriate number of carbon atoms to ensure good oil solubility. Sulfonate, phenate, and carboxylate are the common polar groups present in detergent molecules. However, additives containing salicylate and thiophosphonate functional groups are also sometimes used,

**OIL**

 **Polar oxidation product**

**OIL OIL**

**OIL**

**OIL**

**OIL**

mechanism by which this occurs is depicted in Figure (1.5).

**OIL**

**OIL**

**OIL**

Fig. 1.5 Oil Suspension of polar oxidation products

each other.

Figure (1.6).

cavitation damage as well as insufficient oil transport in circulation systems that can even lead to lack of lubrication. Beside negative mechanical influences the foaming tendency depends very much on the lubricant itself and is influenced by the surface tension of the base oil and especially by the presence of surface-active substances such as detergents, corrosion inhibitors and other ionic compounds.

In many applications, there may be considerable tendency to agitate the oil and cause foaming, while in other cases even small amounts of foam can be extremely troublesome. In these cases, a defoamant may be added to the oil. It is thought that the defoamant droplets attach themselves to the air bubbles and can either spread or form unstable bridges between bubbles, which then coalesce into larger bubbles, which in turn rise more readily to the surface of the foam layer where they collapse, thus releasing the air.

#### **1.3.5 Rust and corrosion inhibitors**

Rust inhibitors, (Rizvi, 2009, Ludema, 1996, and Leslie, 2003), are usually compounds having a high polar attraction toward metal surfaces. By physical or chemical interaction at the metal surface, they form a tenacious, continuous film that prevents water from reaching the metal surface. Typical materials used for this purpose are amine succinates and alkaline earth sulfonates.

Rust inhibitors can be used in most types of lubricating oil, but the selection must be made carefully to avoid problems such as corrosion of nonferrous metals or the formation of troublesome emulsions with water. Because rust inhibitors are adsorbed on metal surfaces, an oil can be depleted of rust inhibitor in time.

A number of kinds of corrosion can occur in systems served by lubricating oils. Probably the two most important types are corrosion by organic acids that develop in the oil itself and corrosion by contaminants that are picked up and carried by the oil.

Corrosion by organic acids can occur, for example, in the bearing inserts used in internal combustion engines. Some of the metals used in these inserts, such as the lead in copperlead or lead-bronze, are readily attacked by organic acids in oil, The corrosion inhibitors form a protective film on the bearing surfaces that prevents the corrosive materials from reaching or attacking the metal. The film may be either adsorbed on the metal or chemically bonded to it. It has been found that the inclusion of highly alkaline materials in the oil will help to neutralize these strong acids as they are formed, greatly reducing this corrosion and corrosive wear.

#### **1.3.6 Detergent and dispersant (D / D) additives**

Modern equipment must be lubricated in order to prolong its lifetime. One of the most critical properties of the automotive lubricants, especially engine oils, is their ability to suspend undesirable products from thermal and oxidative degradation of the lubricant. Such products form when the byproducts of fuel combustion, such as hydroperoxides and free radicals, go past piston rings into the lubricant and, being reactive species, initiate lubricant oxidation. The resulting oxidation products are thermally labile and decompose to highly polar materials with a tendency to separate from the bulk lubricant and form surface deposits and clog small openings.

Oxidation inhibitors, detergents (Rizvi, 2009, Ludema, 1996, Leslie, 2003, and Ming et. al., 2009), and dispersants (Alun, 2010) make up the general class of additives called *stabilizers and deposit control agents*. These additives are designed to control deposit formation, either by inhibiting the oxidative breakdown of the lubricant or by suspending the harmful

cavitation damage as well as insufficient oil transport in circulation systems that can even lead to lack of lubrication. Beside negative mechanical influences the foaming tendency depends very much on the lubricant itself and is influenced by the surface tension of the base oil and especially by the presence of surface-active substances such as detergents,

In many applications, there may be considerable tendency to agitate the oil and cause foaming, while in other cases even small amounts of foam can be extremely troublesome. In these cases, a defoamant may be added to the oil. It is thought that the defoamant droplets attach themselves to the air bubbles and can either spread or form unstable bridges between bubbles, which then coalesce into larger bubbles, which in turn rise more readily to the

Rust inhibitors, (Rizvi, 2009, Ludema, 1996, and Leslie, 2003), are usually compounds having a high polar attraction toward metal surfaces. By physical or chemical interaction at the metal surface, they form a tenacious, continuous film that prevents water from reaching the metal surface. Typical materials used for this purpose are amine succinates and alkaline

Rust inhibitors can be used in most types of lubricating oil, but the selection must be made carefully to avoid problems such as corrosion of nonferrous metals or the formation of troublesome emulsions with water. Because rust inhibitors are adsorbed on metal surfaces,

A number of kinds of corrosion can occur in systems served by lubricating oils. Probably the two most important types are corrosion by organic acids that develop in the oil itself and

Corrosion by organic acids can occur, for example, in the bearing inserts used in internal combustion engines. Some of the metals used in these inserts, such as the lead in copperlead or lead-bronze, are readily attacked by organic acids in oil, The corrosion inhibitors form a protective film on the bearing surfaces that prevents the corrosive materials from reaching or attacking the metal. The film may be either adsorbed on the metal or chemically bonded to it. It has been found that the inclusion of highly alkaline materials in the oil will help to neutralize these strong acids as they are formed, greatly reducing this corrosion and

Modern equipment must be lubricated in order to prolong its lifetime. One of the most critical properties of the automotive lubricants, especially engine oils, is their ability to suspend undesirable products from thermal and oxidative degradation of the lubricant. Such products form when the byproducts of fuel combustion, such as hydroperoxides and free radicals, go past piston rings into the lubricant and, being reactive species, initiate lubricant oxidation. The resulting oxidation products are thermally labile and decompose to highly polar materials with a tendency to separate from the bulk lubricant and form surface

Oxidation inhibitors, detergents (Rizvi, 2009, Ludema, 1996, Leslie, 2003, and Ming et. al., 2009), and dispersants (Alun, 2010) make up the general class of additives called *stabilizers and deposit control agents*. These additives are designed to control deposit formation, either by inhibiting the oxidative breakdown of the lubricant or by suspending the harmful

corrosion inhibitors and other ionic compounds.

**1.3.5 Rust and corrosion inhibitors** 

an oil can be depleted of rust inhibitor in time.

**1.3.6 Detergent and dispersant (D / D) additives** 

deposits and clog small openings.

earth sulfonates.

corrosive wear.

surface of the foam layer where they collapse, thus releasing the air.

corrosion by contaminants that are picked up and carried by the oil.

products already formed in the bulk lubricant. Oxidation inhibitors intercept the oxidation mechanism, and dispersants and detergents perform the suspending part (Kyunghyun, 2010). Detergents are metal salts of organic acids that frequently contain associated excess base, usually in the form of carbonate. Dispersants are metal-free and are of higher molecular weights than detergents. The two types of additives work in conjunction with each other.

The final products of combustion and lubricant decomposition include organic and inorganic acids, aldehydes, ketones, and other oxygenated materials. The acids have the propensity to attack metal surfaces and cause corrosive wear. Detergents, especially basic detergents, contain reserve base that will neutralize the acids to form salts. While this decreases the corrosive tendency of the acids, the solubility of the salts in the bulk lubricant is still low. The organic portion of the detergent, commonly called "soap", has the ability to associate with the salts to keep them suspended in the bulk lubricant. However, in this regard, detergents are not as effective as dispersants because of their lower molecular weight. The soap in detergents and the dispersants also have the ability to suspend nonacidic oxygenated products, such as alcohols, aldehydes, and resinous oxygenates. The mechanism by which this occurs is depicted in Figure (1.5).

Dispersants and detergents together make up the bulk, about 45–50%, of the total volume of the lubricant additives manufactured. This is a consequence of their major use in engine oils, transmission fluids, and tractor hydraulic fluids, all of which are high-volume lubricants.

As mentioned, detergents neutralize oxidation-derived acids as well as help suspend polar oxidation products in the bulk lubricant. Because of this, these additives control rust, corrosion, and resinous buildup in the engine. Like most additives detergents contain a surface-active polar functionality and an oleophilic hydrocarbon group, with an appropriate number of carbon atoms to ensure good oil solubility. Sulfonate, phenate, and carboxylate are the common polar groups present in detergent molecules. However, additives containing salicylate and thiophosphonate functional groups are also sometimes used, Figure (1.6).

Fig. 1.5 Oil Suspension of polar oxidation products

Lubricating Oil Additives 259

preferred bases. For sodium, calcium, and barium detergents, sodium hydroxide, calcium hydroxide, and barium hydroxide are often used. For magnesium detergents, however, magnesium oxide is the preferred base. Dispersants differ from detergents in three

1. Dispersants are metal-free, but detergents contain metals, such as magnesium, calcium, and sometimes barium. This means that on combustion detergents will lead to ash

2. Dispersants have little or no acid-neutralizing ability, but detergents do. This is because dispersants have either no basicity, as is the case in ester dispersants, or low basicity, as is the case in imide / amide dispersants. The basicity of the imide/amide dispersants is due to the presence of the amine functionality. Amines are weak bases and therefore possess minimal acid-neutralizing ability. Conversely, detergents, especially basic detergents, contain reserve metal bases as metal hydroxides and metal carbonates. These are strong bases, with the ability to neutralize combustion and oxidation-derived inorganic acids, such as sulfuric acid and nitric acid, and oxidation-derived organic

3. Dispersants are much higher in molecular weight, approximately 4–15 times higher, than the organic portion (soap) of the detergent. Because of this, dispersants are more

• Associating with colloidal particles, thereby preventing them from agglomerating and

• Modifying soot particles so as to prevent their aggregation. The aggregation will lead to

• Lowering the surface / interfacial energy of the polar species in order to prevent their

At the low-temperature regions, such as the piston skirt, the deposits are not heavy and form only a thin film. For diesel engine pistons, this type of deposit is referred to as "lacquer"; for gasoline engine pistons, this type of deposit is called "varnish". The difference between lacquer and varnish is that lacquer is lubricant-derived and varnish is largely fuelderived. In addition, the two differ in their solubility characteristics. That is, lacquer is water-soluble and varnish is acetone soluble. Lacquer usually occurs on piston skirts, on cylinder walls, and in the combustion chamber. Varnish occurs on valve lifters, piston rings,

Another component of the combustion effluent that must be considered is soot. Soot not only contributes toward some types of deposits, such as carbon and sludge, but it also leads to a viscosity increase. These factors can cause poor lubricant circulation and lubricating

Fuel and lubricant oxidation and degradation products, such as soot, resin, varnish, lacquer, and carbon, are of low lubricant (hydrocarbon) solubility, with a propensity to separate on surfaces. The separation tendency of these materials is a consequence of their particle size. Small particles are more likely to stay in oil than large particles. Therefore, resin and soot particles, which are the two essential components of all deposit-forming species, must grow

effective in fulfilling the suspending and cleaning functions than detergents. The dispersants suspend deposit precursors in oil in a variety of ways. These comprise:

significant ways:

acids.

falling out of solution.

adherence to metal surfaces.

**Deposit control by dispersants** 

formation and dispersants will not.

• Including the undesirable polar species into micelles.

• Suspending aggregates in the bulk lubricant, if they form.

oil thickening, a typical problem in heavy-duty diesel engine oils.

piston skirts, valve covers, and positive crankcase ventilation (PCV) valves.

film formation, both of which will result in wear and catastrophic failure.

Fig. 1.6 Idealized structures of neutral salts (soaps)

As mentioned, common metals that can be used to make neutral or basic detergents include sodium, potassium, magnesium, calcium, and barium. Calcium and magnesium find most extensive use as lubricant additives, with a preference for calcium due to its lower cost. The use of barium-derived detergents is being curbed due to concerns for barium's toxicity. Technically, one can use metal oxides, hydroxides, and carbonates to manufacture neutral (non-overbased) detergents; for non-overbased detergents, oxides and hydroxides are the

 **Metal salt of Metal salt of Metal salt of alkylphenol**

**or**

**Metal salt alkylsalicyclic acid**

**or**

**O**

**R R R R**

**S**

**Metal**

**P O P P P**

**thiophosphonate**

As mentioned, common metals that can be used to make neutral or basic detergents include sodium, potassium, magnesium, calcium, and barium. Calcium and magnesium find most extensive use as lubricant additives, with a preference for calcium due to its lower cost. The use of barium-derived detergents is being curbed due to concerns for barium's toxicity. Technically, one can use metal oxides, hydroxides, and carbonates to manufacture neutral (non-overbased) detergents; for non-overbased detergents, oxides and hydroxides are the

**R**

**M**

**OH**

**O**

*Y*

**R R**

**O O**

**O O**

**S**

**M**

**thiopyrophosphonate**

**Metal**

**O O**

**O M)** *X*

> *X* **= 1 or 2** *Y* **= S or CH** *M* **= Na, Mg, Ca**

**M**

**2**

**O M)** *X*

**SO M3)***<sup>X</sup>* **SO M3)***<sup>X</sup>*

**<sup>R</sup> <sup>R</sup> <sup>R</sup> <sup>R</sup>**

**alkylbenzenesulfonic acid alkylnaphthalenesulfonic acid**

**O**

**O**

**O**

**M**

**R**

*Y*

**R R**

**O**

**Metal phosphonate**

**Sulfur and methylene bridged phenates**

**O O**

Fig. 1.6 Idealized structures of neutral salts (soaps)

**M M**

**O O**

**M**

preferred bases. For sodium, calcium, and barium detergents, sodium hydroxide, calcium hydroxide, and barium hydroxide are often used. For magnesium detergents, however, magnesium oxide is the preferred base. Dispersants differ from detergents in three significant ways:


The dispersants suspend deposit precursors in oil in a variety of ways. These comprise:


At the low-temperature regions, such as the piston skirt, the deposits are not heavy and form only a thin film. For diesel engine pistons, this type of deposit is referred to as "lacquer"; for gasoline engine pistons, this type of deposit is called "varnish". The difference between lacquer and varnish is that lacquer is lubricant-derived and varnish is largely fuelderived. In addition, the two differ in their solubility characteristics. That is, lacquer is water-soluble and varnish is acetone soluble. Lacquer usually occurs on piston skirts, on cylinder walls, and in the combustion chamber. Varnish occurs on valve lifters, piston rings, piston skirts, valve covers, and positive crankcase ventilation (PCV) valves.

Another component of the combustion effluent that must be considered is soot. Soot not only contributes toward some types of deposits, such as carbon and sludge, but it also leads to a viscosity increase. These factors can cause poor lubricant circulation and lubricating film formation, both of which will result in wear and catastrophic failure.

#### **Deposit control by dispersants**

Fuel and lubricant oxidation and degradation products, such as soot, resin, varnish, lacquer, and carbon, are of low lubricant (hydrocarbon) solubility, with a propensity to separate on surfaces. The separation tendency of these materials is a consequence of their particle size. Small particles are more likely to stay in oil than large particles. Therefore, resin and soot particles, which are the two essential components of all deposit-forming species, must grow

Lubricating Oil Additives 261

Probably the most important single property of a lubricating oil is its viscosity. A factor in the formation of lubricating films under both thick and thin film conditions, viscosity (Rizvi, 2009, Ludema, 1996, Leslie, 2003 and Margareth, et. al., 2010), affects heat generation in bearings, cylinders, and gears; it governs the sealing effect of the oil and the rate of consumption or loss; and it determines the ease with which machines may be started under cold conditions. For any piece of equipment, the first essential for satisfactory results is to

In selecting the proper oil for a given application, viscosity is a primary consideration. It must be high enough to provide proper lubricating films but not so high that friction losses in the oil will be excessive. Since viscosity varies with temperature, it is necessary to consider the actual operating temperature of the oil in the machine. Other considerations, such as whether a machine must be started at low ambient temperatures, must also be taken

The kinematic viscosity of a fluid is the quotient of its dynamic viscosity divided by its density, both measured at the same temperature and in consistent units. The most common units for reporting kinematic viscosities now are the stokes (St) or centistokes (cSt; 1 cSt = 0.01 St), or in SI units, square millimeters per second (mm2/s; 1 mm2/s = 1 cSt). The viscosity of any fluid changes with temperature, increasing as the temperature is decreased, and decreasing as the temperature is increased. Thus, it is necessary to have some method of determining the viscosities of lubricating oils at temperatures other than those at which they are measured. This is usually accomplished by measuring the viscosity at two temperatures, then plotting these points on special viscosity–temperature charts developed by ASTM. The two temperatures most used for reporting viscosities are 40ºC

*VI* improvers are long chain, high molecular weight polymers that function by causing the relative viscosity of an oil to increase more at high temperatures than at low temperatures. Generally this result is due to a change in the polymer's physical configuration with increasing temperature of the mixture. It is postulated that in cold oil the molecules of the polymer adopt a coiled form so that their effect on viscosity is minimized. In hot oil, the molecules tend to straighten out, and the interaction between these long molecules and the

As temperature increases, solubility improves, and polymer coils eventually expand to some maximum size and in so doing donate more and more viscosity. The process of coil expansion is entirely reversible as coil contraction occurs with decreasing temperature (see

Different oils have different rates of change of viscosity with temperature. For example, a distillate oil from a naphthenic base crude would show a greater rate of change of viscosity

**Hydrocarbon group**

**Connecting group**

**Nitrogen or Oxygen derived functionality**

**1.3.7 Viscosity index improvers** 

into account.

Figure 1.9).

(104ºF) and 100ºC (212ºF).

Fig. 1.8 Graphic representation of a dispersant molecule

use an oil of proper viscosity to meet the operating conditions.

oil produces a proportionally greater thickening effect.

in size via agglomeration prior to separation. Alternatively, soot particles are caught in the sticky resin, which is shown in parts A and B of Figure (1.7). Dispersants interfere in agglomeration by associating with individual resin and soot particles. The particles with associated dispersant molecules are unable to coalesce because of either steric factors or electrostatic factors. Dispersants consist of a polar group, usually oxygen- or nitrogen-based, and a large non polar group. The polar group associates with the polar particles, and the non polar group keeps such particles suspended in the bulk lubricant. This is shown in parts C and D of Figure (1.7). Neutral detergents, or soaps, operate by an analogous mechanism.

Fig. 1.7 Mechanism of soot-resin-additive interaction

#### **Dispersant structure**

A dispersant molecule consists of three distinct structural features:

A *hydrocarbon group*, a *polar group*, and a connecting group or a *link* (see Figure 1.8). The *hydrocarbon group* is polymeric in nature and, depending on its molecular weight; dispersants can be classified into *polymeric dispersants* and *dispersant polymers*. Polymeric dispersants are of lower molecular weight than dispersant polymers. The molecular weight of polymeric dispersants ranges between 3000 and 7000 as compared to dispersant polymers, which have a molecular weight of 25,000 and higher. While a variety of olefins, such as polyisobutylene, polypropylene, polyalphaolefins, and mixtures thereof, can be used to make polymeric dispersants, the polyisobutylene derived dispersants are the most common.

in size via agglomeration prior to separation. Alternatively, soot particles are caught in the sticky resin, which is shown in parts A and B of Figure (1.7). Dispersants interfere in agglomeration by associating with individual resin and soot particles. The particles with associated dispersant molecules are unable to coalesce because of either steric factors or electrostatic factors. Dispersants consist of a polar group, usually oxygen- or nitrogen-based, and a large non polar group. The polar group associates with the polar particles, and the non polar group keeps such particles suspended in the bulk lubricant. This is shown in parts C and D of Figure (1.7). Neutral detergents, or soaps, operate by an analogous mechanism.

Fig. 1.7 Mechanism of soot-resin-additive interaction

A dispersant molecule consists of three distinct structural features:

A *hydrocarbon group*, a *polar group*, and a connecting group or a *link* (see Figure 1.8). The *hydrocarbon group* is polymeric in nature and, depending on its molecular weight; dispersants can be classified into *polymeric dispersants* and *dispersant polymers*. Polymeric dispersants are of lower molecular weight than dispersant polymers. The molecular weight of polymeric dispersants ranges between 3000 and 7000 as compared to dispersant polymers, which have a molecular weight of 25,000 and higher. While a variety of olefins, such as polyisobutylene, polypropylene, polyalphaolefins, and mixtures thereof, can be used to make polymeric dispersants, the polyisobutylene derived dispersants are the most

**Dispersant structure** 

common.

**Connecting group**

**Hydrocarbon group**

Fig. 1.8 Graphic representation of a dispersant molecule

#### **1.3.7 Viscosity index improvers**

Probably the most important single property of a lubricating oil is its viscosity. A factor in the formation of lubricating films under both thick and thin film conditions, viscosity (Rizvi, 2009, Ludema, 1996, Leslie, 2003 and Margareth, et. al., 2010), affects heat generation in bearings, cylinders, and gears; it governs the sealing effect of the oil and the rate of consumption or loss; and it determines the ease with which machines may be started under cold conditions. For any piece of equipment, the first essential for satisfactory results is to use an oil of proper viscosity to meet the operating conditions.

In selecting the proper oil for a given application, viscosity is a primary consideration. It must be high enough to provide proper lubricating films but not so high that friction losses in the oil will be excessive. Since viscosity varies with temperature, it is necessary to consider the actual operating temperature of the oil in the machine. Other considerations, such as whether a machine must be started at low ambient temperatures, must also be taken into account.

The kinematic viscosity of a fluid is the quotient of its dynamic viscosity divided by its density, both measured at the same temperature and in consistent units. The most common units for reporting kinematic viscosities now are the stokes (St) or centistokes (cSt; 1 cSt = 0.01 St), or in SI units, square millimeters per second (mm2/s; 1 mm2/s = 1 cSt).

The viscosity of any fluid changes with temperature, increasing as the temperature is decreased, and decreasing as the temperature is increased. Thus, it is necessary to have some method of determining the viscosities of lubricating oils at temperatures other than those at which they are measured. This is usually accomplished by measuring the viscosity at two temperatures, then plotting these points on special viscosity–temperature charts developed by ASTM. The two temperatures most used for reporting viscosities are 40ºC (104ºF) and 100ºC (212ºF).

*VI* improvers are long chain, high molecular weight polymers that function by causing the relative viscosity of an oil to increase more at high temperatures than at low temperatures. Generally this result is due to a change in the polymer's physical configuration with increasing temperature of the mixture. It is postulated that in cold oil the molecules of the polymer adopt a coiled form so that their effect on viscosity is minimized. In hot oil, the molecules tend to straighten out, and the interaction between these long molecules and the oil produces a proportionally greater thickening effect.

As temperature increases, solubility improves, and polymer coils eventually expand to some maximum size and in so doing donate more and more viscosity. The process of coil expansion is entirely reversible as coil contraction occurs with decreasing temperature (see Figure 1.9).

Different oils have different rates of change of viscosity with temperature. For example, a distillate oil from a naphthenic base crude would show a greater rate of change of viscosity

Lubricating Oil Additives 263

them into lower molecular weight materials, which are less effective *VI* improvers. This results in a permanent viscosity loss, which can be significant. It is generally the limiting factor controlling the maximum amount of *VI* improver that can be used in a particular oil blend. *VI* improvers are used in engine oils, automatic transmission fluids, multipurpose tractor fluids, and hydraulic fluids. They are also used in automotive gear lubricants. Their use permits the formulation of products that provide satisfactory lubrication over a much

> These are polymerised esters of methacrylic acid. They normally exhibit pour-point depressing activity. Dispersant properties can be obtained by incorporating polar groups in the

These are non-dispersant polymers and they have no effect on the pour point of formulated lubricants. They have limited use in modern formulations.

These are usually co-polymers of ethylene and propylene. Dispersant

The molecular weight distribution is optimised to give good shear stability in crankcase applications. Its uniquely effective thickening power in solution gives an overall thickening efficiency that is superior to other polymers of

properties can be obtained incorporating polar groups in the

equivalent shear stability.

moleculare structure.

molecular structure.

wider temperature range than is possible with straight mineral oils alone.

**C**

**O**

**C**

**CH3**

**(c) Olefin co-polymers (OCP)**

**(d) Styrene/diene co-polymers**

Fig. 1.10 Viscosity index improvers

**1.3.8 Pour point depressants** 

**(CH CH ) (CH CH) <sup>2</sup> <sup>2</sup> <sup>2</sup> <sup>a</sup> <sup>b</sup>**

**CH CH CH CH CH CH <sup>2</sup> <sup>2</sup> <sup>2</sup> <sup>2</sup>**

**CH3**

**CH2**

**CH3**

**(b) Polyisobutenes (PIB)**

**C Hn 2n+1**

**O**

**x**

**x**

**CH3**

**x**

**x y**

The pour point, (Rizvi, 2009, Ludema, 1996, and Leslie, 2003), *PP* of a lubricating oil is the lowest temperature at which it will pour or flow when it is chilled without disturbance

**C**

**CH2**

**CH3**

**(a) Polymethacrylates (PMA)**

with temperature than would a distillate oil from a paraffin crude. The viscosity index is a method of applying a numerical value to this rate of change, based on comparison with the relative rates of change of two arbitrarily selected types of oil that differ widely in this characteristic. A high *VI* indicates a relatively low rate of change of viscosity with temperature; a low *VI* indicates a relatively high rate of change of viscosity with temperature. For example, consider a high *VI* oil and a low *VI* oil having the same viscosity at, say, room temperature: as the temperature increased, the high *VI* oil would thin out less and, therefore, would have a higher viscosity than the low *VI* oil at higher temperatures. The *VI* of an oil is calculated from viscosities determined at two temperatures by means of tables published by ASTM. Tables based on viscosities determined at both 104ºF and 212ºF, and 40ºC and 100ºC are available. Finished mineral-based lubricating oils made by conventional methods range in *VI* from somewhat below 0 to slightly above 100. Mineral oil base stocks refined through special hydroprocessing techniques can have *VIs* well above 100.

Additives called *VI* improvers can be blended into oils to increase *VIs*; however, *VI* improvers are not always stable in lubricating environments exposed to shear or thermal stressing. Accordingly, these additives must be used with due care to assure adequate viscosity over the anticipated service interval for the application for which they are intended.

Among the principal *VI* improvers are methacrylate polymers and copolymers, acrylate polymers, olefin polymers and copolymers, and styrene butadiene copolymers, Figure (1.10). The degree of *VI* improvement from these materials is a function of the molecular weight distribution of the polymer.

The long molecules in *VI* improvers are subject to degradation due to mechanical shearing in service. Shear breakdown occurs by two mechanisms. Temporary shear breakdown occurs under certain conditions of moderate shear stress and results in a temporary loss of viscosity. Apparently, under these conditions the long molecules of the *VI* improver align themselves in the direction of the stress, thus reducing resistance to flow. When the stress is removed, the molecules return to their usual random arrangement and the temporary viscosity loss is recovered. This effect can be beneficial in that it can temporarily reduce oil friction to permit easier starting, as in the cranking of a cold engine. Permanent shear breakdown occurs when the shear stresses actually rupture the long molecules, converting

 **TEMPERATURE SOLVENT POWER**

refined through special hydroprocessing techniques can have *VIs* well above 100.

Additives called *VI* improvers can be blended into oils to increase *VIs*; however, *VI* improvers are not always stable in lubricating environments exposed to shear or thermal stressing. Accordingly, these additives must be used with due care to assure adequate viscosity over the anticipated service interval for the application for which they are

Among the principal *VI* improvers are methacrylate polymers and copolymers, acrylate polymers, olefin polymers and copolymers, and styrene butadiene copolymers, Figure (1.10). The degree of *VI* improvement from these materials is a function of the molecular

The long molecules in *VI* improvers are subject to degradation due to mechanical shearing in service. Shear breakdown occurs by two mechanisms. Temporary shear breakdown occurs under certain conditions of moderate shear stress and results in a temporary loss of viscosity. Apparently, under these conditions the long molecules of the *VI* improver align themselves in the direction of the stress, thus reducing resistance to flow. When the stress is removed, the molecules return to their usual random arrangement and the temporary viscosity loss is recovered. This effect can be beneficial in that it can temporarily reduce oil friction to permit easier starting, as in the cranking of a cold engine. Permanent shear breakdown occurs when the shear stresses actually rupture the long molecules, converting

with temperature than would a distillate oil from a paraffin crude. The viscosity index is a method of applying a numerical value to this rate of change, based on comparison with the relative rates of change of two arbitrarily selected types of oil that differ widely in this characteristic. A high *VI* indicates a relatively low rate of change of viscosity with temperature; a low *VI* indicates a relatively high rate of change of viscosity with temperature. For example, consider a high *VI* oil and a low *VI* oil having the same viscosity at, say, room temperature: as the temperature increased, the high *VI* oil would thin out less and, therefore, would have a higher viscosity than the low *VI* oil at higher temperatures. The *VI* of an oil is calculated from viscosities determined at two temperatures by means of tables published by ASTM. Tables based on viscosities determined at both 104ºF and 212ºF, and 40ºC and 100ºC are available. Finished mineral-based lubricating oils made by conventional methods range in *VI* from somewhat below 0 to slightly above 100. Mineral oil base stocks

Fig. 1.9 Polymer coil expansion

weight distribution of the polymer.

intended.

them into lower molecular weight materials, which are less effective *VI* improvers. This results in a permanent viscosity loss, which can be significant. It is generally the limiting factor controlling the maximum amount of *VI* improver that can be used in a particular oil blend. *VI* improvers are used in engine oils, automatic transmission fluids, multipurpose tractor fluids, and hydraulic fluids. They are also used in automotive gear lubricants. Their use permits the formulation of products that provide satisfactory lubrication over a much wider temperature range than is possible with straight mineral oils alone.

#### **(a) Polymethacrylates (PMA)**

These are polymerised esters of methacrylic acid. They normally exhibit pour-point depressing activity. Dispersant properties can be obtained by incorporating polar groups in the molecular structure.

**(b) Polyisobutenes (PIB)**

**(d) Styrene/diene co-polymers**

Fig. 1.10 Viscosity index improvers

#### **1.3.8 Pour point depressants**

The pour point, (Rizvi, 2009, Ludema, 1996, and Leslie, 2003), *PP* of a lubricating oil is the lowest temperature at which it will pour or flow when it is chilled without disturbance

These are non-dispersant polymers and they have no effect on the pour point of formulated lubricants. They have limited use in modern formulations.

These are usually co-polymers of ethylene and propylene. Dispersant properties can be obtained incorporating polar groups in the moleculare structure.

The molecular weight distribution is optimised to give good shear stability in crankcase applications. Its uniquely effective thickening power in solution gives an overall thickening efficiency that is superior to other polymers of equivalent shear stability.

Lubricating Oil Additives 265

Oils must have pour points (1) below the minimum operating temperature of the system and (2) below the minimum surrounding temperature to which the oil will be exposed. While removal of the residue waxes from the oil is somewhat expensive, pour point depressants are an economical alternative to reduce the pour point of lubricants. The most common pour point depressants are the same additives used for viscosity index improvement. The mechanism through which these molecules reduce pour point is still

It has been suggested that these molecules adsorb into the wax crystals, (Chen et. al., 2010, and Bharambe, 2010) and redirect their growth, forming smaller and more isotropic crystals

 Depending on the type of oil, pour point depression of up to 50ºF (10ºC) can be achieved by these additives, although a lowering of the pour point by about (20Fº – 30Fº) (-6.67Cº to -

C

H C2

COOR

CH

Methacrylate polymers are much used as additives in lubricating oils, as pour point depressants and viscosity index improvers. Although the mechanism of such pour point depression is still controversial, it is thought to be related to the length of the alkyl side chains of the polymethacrylate, and to the nature of the base oil. R in the ester has a major effect on the product, and is usually represented by a normal paraffinic chain of at least 12 carbon atoms. This ensures oil solubility. The molecular weight of the polymer is also very important. Typically these materials are between 7000 and 10,000 number average molecular weights.

C

COOR

CH

<sup>3</sup> <sup>n</sup>

Commercial materials normally contain mixed alkyl chains, which can be branched.

H C2

These are very similar in behavior to the polymethacrylates.

<sup>3</sup> <sup>n</sup>

There is a range of pour point depressant additives of different chemical species (102 - 105).

poorly understood and somewhat controversial.

that interfere less with oil flow.

1.1Cº) is more common.

Polymethacrylates polymers

**Polyacrylates** 

**Polymethacrylates** 

under prescribed conditions. Most mineral oils contain some dissolved wax and, as an oil is chilled, this wax begins to separate as crystal that interlock to form a rigid structure that traps the oil in small pockets in the structure.

When this wax crystal structure becomes sufficiently complete, the oil will no longer flow under the conditions of the test. Since, however, mechanical agitation can break up the wax structure; it is possible to have an oil flow at temperatures considerably below its pour point. Cooling rates also affect wax crystallization; it is possible to cool an oil rapidly to a temperature below its pour point and still have it flow.

While the pour point of most oils is related to the crystallization of wax, certain oils, which are essentially wax free, have viscosity-limited pour points. In these oils the viscosity becomes progressively higher as the temperature is lowered until at some temperature no flow can be observed. The pour points of such oils cannot be lowered with pour point depressants, *PPDs*, since these agents act by interfering with the growth and interlocking of the wax crystal structure.

Certain high molecular weight polymers function by inhibiting the formation of a wax crystal structure that would prevent oil flow at low temperatures, Figure (1.11).

 Crystal Morphology Without Crystal Morphology With Pour Point Depressant Pour Point Depressant

Fig. 1.11 The mechanism of the pour point depressant performance

Two general types of pour point depressant are used:


The additives do not entirely prevent wax crystal growth, but rather lower the temperature at which a rigid structure is formed. Oils used under low-temperature conditions must have low pour points.

under prescribed conditions. Most mineral oils contain some dissolved wax and, as an oil is chilled, this wax begins to separate as crystal that interlock to form a rigid structure that

When this wax crystal structure becomes sufficiently complete, the oil will no longer flow under the conditions of the test. Since, however, mechanical agitation can break up the wax structure; it is possible to have an oil flow at temperatures considerably below its pour point. Cooling rates also affect wax crystallization; it is possible to cool an oil rapidly to a

While the pour point of most oils is related to the crystallization of wax, certain oils, which are essentially wax free, have viscosity-limited pour points. In these oils the viscosity becomes progressively higher as the temperature is lowered until at some temperature no flow can be observed. The pour points of such oils cannot be lowered with pour point depressants, *PPDs*, since these agents act by interfering with the growth and interlocking of

Certain high molecular weight polymers function by inhibiting the formation of a wax crystal structure that would prevent oil flow at low temperatures, Figure (1.11).

 Crystal Morphology Without Crystal Morphology With Pour Point Depressant Pour Point Depressant

1. Alkylaromatic polymers adsorb on the wax crystals as they form, preventing them from

The additives do not entirely prevent wax crystal growth, but rather lower the temperature at which a rigid structure is formed. Oils used under low-temperature conditions must have

Fig. 1.11 The mechanism of the pour point depressant performance

2. Polymethacrylates co-crystallize with wax to prevent crystal growth.

Two general types of pour point depressant are used:

growing and adhering to each other.

low pour points.

traps the oil in small pockets in the structure.

the wax crystal structure.

temperature below its pour point and still have it flow.

Oils must have pour points (1) below the minimum operating temperature of the system and (2) below the minimum surrounding temperature to which the oil will be exposed.

While removal of the residue waxes from the oil is somewhat expensive, pour point depressants are an economical alternative to reduce the pour point of lubricants. The most common pour point depressants are the same additives used for viscosity index improvement. The mechanism through which these molecules reduce pour point is still poorly understood and somewhat controversial.

It has been suggested that these molecules adsorb into the wax crystals, (Chen et. al., 2010, and Bharambe, 2010) and redirect their growth, forming smaller and more isotropic crystals that interfere less with oil flow.

 Depending on the type of oil, pour point depression of up to 50ºF (10ºC) can be achieved by these additives, although a lowering of the pour point by about (20Fº – 30Fº) (-6.67Cº to - 1.1Cº) is more common.

There is a range of pour point depressant additives of different chemical species (102 - 105).

#### **Polymethacrylates**

#### Polymethacrylates polymers

Methacrylate polymers are much used as additives in lubricating oils, as pour point depressants and viscosity index improvers. Although the mechanism of such pour point depression is still controversial, it is thought to be related to the length of the alkyl side chains of the polymethacrylate, and to the nature of the base oil. R in the ester has a major effect on the product, and is usually represented by a normal paraffinic chain of at least 12 carbon atoms. This ensures oil solubility. The molecular weight of the polymer is also very important. Typically these materials are between 7000 and 10,000 number average molecular weights. Commercial materials normally contain mixed alkyl chains, which can be branched.

#### **Polyacrylates**

These are very similar in behavior to the polymethacrylates.

Lubricating Oil Additives 267

A number of additives perform more than one function. Zinc dialkyl dithiophosphates, known mainly for their antiwear action, are also potent oxidation and corrosion inhibitors. Styrene-ester polymers and functionalized polymethacrylates and can act as viscosity modifiers, dispersants, and pour point depressants. Basic sulfonates, in addition to acting as detergents, perform as rust and corrosion inhibitors. They do so by forming protective surface films and by neutralizing acids that arise from fuel combustion, lubricant oxidation,

• Using nanotechnology in preparation of lube oil additives, "synthesis of overbased nanodetergent". Production of stable, efficient nanodetergent system depends on development and new generation of surfactant. These nano-particles are relatively

• In spite of the increasing temperature, loads and other requirements imposed on lubricants, mineral oils are likely to continue to be employed in the foreseeable future for the majority of automotive, industrial and marine applications. However, in the aviation field, synthetic lubricants are extensively used and there are a growing number of critical automotive, industrial and marine application where the use of synthetic

Alun L., Ken B.T., Randy C.B., and Joseph V.M., Large-scale dispersant leaching and

Battez A.H., Viesca J.L., González R., Blanco D., Asedegbega E., and Osorio A., Friction

Bharambe D.P., Designing maleic anhydride-α-olifin copolymeric combs as wax crystal

Chen B., Sun Y., Fang J., Wang J., and Wu Jiang, Effect of cold flow improvers on flow properties of soybean biodiesel; *Biomass and bioenergy*, 34, 1309-1313, (2010). Kyunghyun R., The characteristics of performance and exhaust emissions of a diesel engine

Leslie R.R., Lubricant Additives, "Chemistry and Applications", Marcel Dekker, Inc., 293-

Ludema K.C.; Friction, Wear, Lubrication, A Textbook in Tribology, CRC Press L.L.C., 124-

Margareth J.S., Peter R.S., Carlos R.P.B., and José R.S., Lubricant viscosity and viscosity

Masabumi M., Hiroyasu S., Akihito S.,and Osamu K., Prevention of oxidative degradation

growth nucleators; *Fuel Processing Technology,* 91, 997–1004, (2010).

effectiveness experiments with oils on calm water; *Marine Pollution Bulletin,* 60, 244–

reduction properties of a CuO nanolubricant used as lubricant for a NiCrBSi

using a bio-diesel with antioxidants; using a bio-diesel with antioxidants;

improver additive effects on diesel fuel economy; *Tribology International,* 43, 2298–

of ZnDTP by microcapsulation and verification of its antiwear performance;

**1.3.9 Multifunctional nature of additives (Rizvi, 2009)** 

lubricants van be justified on a cost / performance basis.

coating; *Wear*, 268, 325–328, (2010).

*Bioresource Technology*, 101, 578–582, (2010).

*Tribology International,* 41, 1097–1102, (2008).

and additive degradation.

insensitive to temperature.

**2. Future work** 

**3. References** 

254, (2010).

254, (2003).

134, (1996).

2302, (2010).

#### **Di (tetra paraffin phenol) phthalate**

#### **Condensation products of tetra paraffin phenol**

#### **Condensation product of a chlorinated paraffin wax with naphthalene (107)**

It has been suggested that these molecules adsorb into the wax crystals and redirect their growth, forming smaller and more isotropic crystals that interfere less with oil flow.

Paraffin

Paraffin

Paraffin

Paraffin

Paraffin

Paraffin

Paraffin

Paraffin

Paraffin

n

C

C

O

Paraffin

O

O

O

**Condensation products of tetra paraffin phenol** 

Paraffin

Paraffin

Paraffin

**Condensation product of a chlorinated paraffin wax with naphthalene (107)**

OH

It has been suggested that these molecules adsorb into the wax crystals and redirect their

growth, forming smaller and more isotropic crystals that interfere less with oil flow.

**Di (tetra paraffin phenol) phthalate** 

#### **1.3.9 Multifunctional nature of additives (Rizvi, 2009)**

A number of additives perform more than one function. Zinc dialkyl dithiophosphates, known mainly for their antiwear action, are also potent oxidation and corrosion inhibitors. Styrene-ester polymers and functionalized polymethacrylates and can act as viscosity modifiers, dispersants, and pour point depressants. Basic sulfonates, in addition to acting as detergents, perform as rust and corrosion inhibitors. They do so by forming protective surface films and by neutralizing acids that arise from fuel combustion, lubricant oxidation, and additive degradation.

#### **2. Future work**


#### **3. References**


**Part 3** 

**Solid Lubricants and Coatings** 


**Part 3** 

**Solid Lubricants and Coatings** 

268 Tribology - Lubricants and Lubrication

Mel'nikov V.G., Tribological and Colloid-Chemical Aspects of the Action of Organic

Ming Z., Xiaobo W., Xisheng F., and Yanqiu X., Performance and anti-wear mechanism of

Rizvi, S.Q.A., A comprehensive review of lubricant chemistry, technology, selection, and design, ASTM International, West Conshohocken, PA., 100-112, (2009).

*of Fuels and Oils,* 33, No. 5, 286-295, (1997).

*International,* 42, 1029–1039, (2009).

Fluorine Compounds as Friction Modifiers in Motor Oils; *Chemistry and Technology* 

CaCO3 nanoparticles as a green additive in poly-alpha-olefin; *Tribology* 

**11** 

Thomas Gradt

*Germany* 

**Tribological Behaviour of** 

*BAM Federal Institute for Materials Research and Testing* 

0.08989 kg/m3 1.338 kg/m3 0.0695 kg/m3

28.59 J/(mol K) 20.3 J/(mol K)

**Solid Lubricants in Hydrogen Environment** 

In a future energy supply system based on renewable sources hydrogen technology will play a key role. Because the amount of energy from renewable sources, such as wind or solar power, differs seasonally and regionally, an energy storage method is necessary. Hydrogen, as an environmentally friendly energy carrier, can fill this gap in an ideal way, in particular for mobile applications (Wurster et al., 2009). Already today, in Germany the amount of hydrogen as a byproduct in chemical industry is enough for fuelling about 1 Mio passenger cars1. Excess electrical power can be used to produce hydrogen by electrolysis. On demand, this hydrogen can be used for mobile or stationary fuel cells. Beside this new developing technology, hydrogen is used as fuel for rocket engines and in chemical industry since a long time. Table 1 comprises some physical parameters of hydrogen. It can be seen that hydrogen gas has a very low density which makes storage at high pressure or in liquid

Melting temperature -259.35°C (13.80 K) Boiling temperature (1.013 bar) -252.87°C (20.28 K)

Density of the liquid at boiling temperature 0.07098 kg/l evaporation enthalpy at boiling temperature hl 0.915 kJ/mol Heat conductivity (0°C; 1.013 bar) 0.1739 W/(m K) Heat capacity of the liquid at boiling temperature 9.69 kJ/(kg K)

Critical temperature Tc -240.17°C (32.98 K)

Table 1. Physical parameters of hydrogen (Bulletin M 055, 1991 and Frey & Haefer, 1981)

at 0°C, p = 1.013 bar at boiling temperature density ratio H2/air

> cp cv

1 Supplement "Frankfurter Allgemeine Zeitung", March 22. 2011

Critical pressure pc 12.93 bar

**1. Introduction** 

form (LH2) necessary.

Gas densities

Specific heat (0°C; 1.013 bar)

## **Tribological Behaviour of Solid Lubricants in Hydrogen Environment**

#### Thomas Gradt

*BAM Federal Institute for Materials Research and Testing Germany* 

### **1. Introduction**

In a future energy supply system based on renewable sources hydrogen technology will play a key role. Because the amount of energy from renewable sources, such as wind or solar power, differs seasonally and regionally, an energy storage method is necessary. Hydrogen, as an environmentally friendly energy carrier, can fill this gap in an ideal way, in particular for mobile applications (Wurster et al., 2009). Already today, in Germany the amount of hydrogen as a byproduct in chemical industry is enough for fuelling about 1 Mio passenger cars1. Excess electrical power can be used to produce hydrogen by electrolysis. On demand, this hydrogen can be used for mobile or stationary fuel cells. Beside this new developing technology, hydrogen is used as fuel for rocket engines and in chemical industry since a long time. Table 1 comprises some physical parameters of hydrogen. It can be seen that hydrogen gas has a very low density which makes storage at high pressure or in liquid form (LH2) necessary.


Table 1. Physical parameters of hydrogen (Bulletin M 055, 1991 and Frey & Haefer, 1981)

1 Supplement "Frankfurter Allgemeine Zeitung", March 22. 2011

Tribological Behaviour of Solid Lubricants in Hydrogen Environment 273

fact that oxide layers, which protect many metals against wear and corrosion, are not renewed after they are worn away. In the special case of tribosystems running in liquid hydrogen, the environmental temperature is 20 K (-253°C) and far too low for any liquid lubricant. In such cases, solid lubricants can be employed for reducing friction and wear. Also, in applications such as fuel cells or semiconductor fabrication, gaseous hydrogen of high purity is required and high demands on the outgassing of the materials are made,

Commercial hydrogen gas contains a certain amount of water and oxygen. The influence of residual water in hydrogen gas on the fretting wear behaviour of bearing steel SUJ2 (similar to AISI 52100) was investigated by Izumi et al. (2011). These tests were performed in hydrogen and nitrogen gas with water content between 2 and 70 ppm. In both gases friction

In general, test equipment for hydrogen environment has to meet the safety standards for handling this medium. In particular, explosion safe electrical installations, proper venting, gas tight experimental chambers, filling, and venting tubes are necessary. For liquid hydrogen also cryogenic equipment has to be employed. As examples two cryotribometers which are available at BAM2 are shown in Figures 1 and 2 (Gradt, et al., 2001). Both cryotribometers are appropriate for liquid and gaseous hydrogen. The sample chambers are thermally insulated by vacuum superinsulation and cooled directly by a bath of liquid

In the case of CT 2 (Fig. 1) the liquid coolant is filled directly into the sample chamber (bath cryostat). The complete friction sample is immersed into the liquid cryogen and the environmental temperature is equal to the boiling temperature of the coolant (liquid nitrogen, LN2: 77.3 K; liquid hydrogen, LH2: 20.3 K; liquid helium, LHe: 4.2 K). The advantage of this method is a very effective cooling of the sample by making use of the heat

Most of the tests are carried out by using the standard pin-on-disc configuration where a fixed flat pin or ball is continuously sliding against a rotating disc. The rotation is transmitted via a rotary vacuum feedthrough to a shaft with the sample disc at the lower end. In CT2 loading is performed by means of a gas bellow which acts on a frame with the fixed sample (pin) mounted on its lower beam. The mechanical stability of this assembly allows normal forces up to 500 N. The friction force is measured by means of a torque sensor on top of the motor journal or a beam force transducer integrated in the sample holder. The sample chamber of the tribometer CT 3 is designed for pressures between 10-3 mbar and 20 bar and cooled by a heat exchanger (continuous flow cryostat). The coolant is pumped through the heat exchanger, evaporates and removes heat from the inner vessel. Thus, it is possible to adjust the temperature between 4.2 K (with LHe cooling) and room temperature independently from the pressure. There is not limitation to an equilibrium state of the boiling coolant as in a bath cryostat. In hydrogen environment, the behaviour of tribosystems in gaseous or liquid environment as well as in the vicinity of the critical point can be investigated, which is of importance for the design of high performance hydrogen

2 BAM Federal Institute for Materials Research and Testing, Berlin, Germany

**3. Test devices for friction tests in cryogenic hydrogen environment** 

which usually cannot be met by liquid lubricants.

cryogen or by a heat exchanger.

of evaporation of the liquid.

pumps.

decreases, but wear increases with increasing water content.

With increasing utilisation of hydrogen it will be necessary to optimize components which are in contact with this medium. If these components contain tribosystems directly exposed to hydrogen they are critical in respect of excess wear, because of vanishing protective oxide layers in the presence of a chemically reducing environment. Furthermore, liquid lubricants are often not applicable, because of purity requirements, or very low temperatures in the case of liquid hydrogen. Thus, for numerous components in hydrogen technology, solid lubrication is the only possible method for reducing friction and wear.

Although the tribological behaviour of typical solid lubricants such as graphite, DLC, and MoS2 has been characterized comprehensively (Landsdown, 1999; Donnet & Erdemir, 2004; Gradt et al., 2001), information about their suitability for hydrogen environment is very limited. Therefore, based on available literature and own measurements, an overview of solid lubricants and other materials for tribosystems in hydrogen environment is given in the following.

#### **2. Tribosystems in hydrogen environment**

Prominent examples for extremely stressed components in hydrogen environment are turbopumps for cryogenic propellants in rocket engines. They comprise numerous tribosystems, such as shaft seals or self-lubricated bearings, which have to work in hydrogen environment at low temperatures and high pressures. In the tribo-components of a turbopump various solid lubricants such gold, silver, silver-copper alloy, PTFE, graphite, and MoS2 or wear resistant coatings as WC, Cr, Cr2O3, and TiN are applied (Nosaka, 2011).

The LH2-turbopump of the LE-7 engine for the Japanese H-2 rocket has a flow rate of 510 l/s, a shaft power of 19,700 kW, and a rotational speed of 42,000 rpm. All steel bearings made from AISI 440C with lubrication by PTFE transfer from the retainer to the raceways showed sufficient performance. These bearings were operated at 50,000 rpm without severe wear (Nosaka, 2011). For better performance and rotational speeds up to 100,000 rpm hybrid ceramic bearings with Si3N4 balls and steel rings are used. Such bearings were developed for the space shuttle (Gipson, 2001), the future VINCI launcher in Europe, and the Japanese LE-7 rocket engine (Nosaka et al., 2010). Hybrid ball bearings with ceramic balls can be operated up to 120,000 rpm in liquid hydrogen.

Hydrogen environment influences the friction behaviour of materials such as transition metals and metals that react chemically with hydrogen by building stable hydrides (Fukuda & Sugimura, 2008). In the case of transition metals, chemisorption is the main mechanism. The influence of hydrogen on the tribological properties of steels cannot be derived directly from these mechanisms, although the main components in steels are transition metals (Fukuda et al., 2011).

A general problem for materials, especially metals, exposed to hydrogen is environmentally induced embrittlement, which is also active in tribologically stressed systems. One example is embrittlement of raceways in ball bearings. The effect of hydrogen on the fatigue behaviour of bearing steel AISI 52100 was studied by Fujita et al. (2010). He investigated samples of steel under cyclic loading. Samples precharged with hydrogen showed a significant shorter lifetime, which could be attributed to the occurrence of an increased number of cracks.

However, hydrogen embrittlement is a general materials problem and not specific to tribosystems. Friction induced changes in the structure of steels that lead to embrittlement phenomena are treated in chapter 4. More specific to tribologically stressed surfaces is the

With increasing utilisation of hydrogen it will be necessary to optimize components which are in contact with this medium. If these components contain tribosystems directly exposed to hydrogen they are critical in respect of excess wear, because of vanishing protective oxide layers in the presence of a chemically reducing environment. Furthermore, liquid lubricants are often not applicable, because of purity requirements, or very low temperatures in the case of liquid hydrogen. Thus, for numerous components in hydrogen technology, solid

Although the tribological behaviour of typical solid lubricants such as graphite, DLC, and MoS2 has been characterized comprehensively (Landsdown, 1999; Donnet & Erdemir, 2004; Gradt et al., 2001), information about their suitability for hydrogen environment is very limited. Therefore, based on available literature and own measurements, an overview of solid lubricants and other materials for tribosystems in hydrogen environment is given in

Prominent examples for extremely stressed components in hydrogen environment are turbopumps for cryogenic propellants in rocket engines. They comprise numerous tribosystems, such as shaft seals or self-lubricated bearings, which have to work in hydrogen environment at low temperatures and high pressures. In the tribo-components of a turbopump various solid lubricants such gold, silver, silver-copper alloy, PTFE, graphite, and MoS2 or wear resistant coatings as WC, Cr, Cr2O3, and TiN are applied (Nosaka, 2011). The LH2-turbopump of the LE-7 engine for the Japanese H-2 rocket has a flow rate of 510 l/s, a shaft power of 19,700 kW, and a rotational speed of 42,000 rpm. All steel bearings made from AISI 440C with lubrication by PTFE transfer from the retainer to the raceways showed sufficient performance. These bearings were operated at 50,000 rpm without severe wear (Nosaka, 2011). For better performance and rotational speeds up to 100,000 rpm hybrid ceramic bearings with Si3N4 balls and steel rings are used. Such bearings were developed for the space shuttle (Gipson, 2001), the future VINCI launcher in Europe, and the Japanese LE-7 rocket engine (Nosaka et al., 2010). Hybrid ball bearings with ceramic balls can be

Hydrogen environment influences the friction behaviour of materials such as transition metals and metals that react chemically with hydrogen by building stable hydrides (Fukuda & Sugimura, 2008). In the case of transition metals, chemisorption is the main mechanism. The influence of hydrogen on the tribological properties of steels cannot be derived directly from these mechanisms, although the main components in steels are transition metals

A general problem for materials, especially metals, exposed to hydrogen is environmentally induced embrittlement, which is also active in tribologically stressed systems. One example is embrittlement of raceways in ball bearings. The effect of hydrogen on the fatigue behaviour of bearing steel AISI 52100 was studied by Fujita et al. (2010). He investigated samples of steel under cyclic loading. Samples precharged with hydrogen showed a significant shorter lifetime, which could be attributed to the occurrence of an increased

However, hydrogen embrittlement is a general materials problem and not specific to tribosystems. Friction induced changes in the structure of steels that lead to embrittlement phenomena are treated in chapter 4. More specific to tribologically stressed surfaces is the

lubrication is the only possible method for reducing friction and wear.

**2. Tribosystems in hydrogen environment** 

operated up to 120,000 rpm in liquid hydrogen.

(Fukuda et al., 2011).

number of cracks.

the following.

fact that oxide layers, which protect many metals against wear and corrosion, are not renewed after they are worn away. In the special case of tribosystems running in liquid hydrogen, the environmental temperature is 20 K (-253°C) and far too low for any liquid lubricant. In such cases, solid lubricants can be employed for reducing friction and wear. Also, in applications such as fuel cells or semiconductor fabrication, gaseous hydrogen of high purity is required and high demands on the outgassing of the materials are made, which usually cannot be met by liquid lubricants.

Commercial hydrogen gas contains a certain amount of water and oxygen. The influence of residual water in hydrogen gas on the fretting wear behaviour of bearing steel SUJ2 (similar to AISI 52100) was investigated by Izumi et al. (2011). These tests were performed in hydrogen and nitrogen gas with water content between 2 and 70 ppm. In both gases friction decreases, but wear increases with increasing water content.

#### **3. Test devices for friction tests in cryogenic hydrogen environment**

In general, test equipment for hydrogen environment has to meet the safety standards for handling this medium. In particular, explosion safe electrical installations, proper venting, gas tight experimental chambers, filling, and venting tubes are necessary. For liquid hydrogen also cryogenic equipment has to be employed. As examples two cryotribometers which are available at BAM2 are shown in Figures 1 and 2 (Gradt, et al., 2001). Both cryotribometers are appropriate for liquid and gaseous hydrogen. The sample chambers are thermally insulated by vacuum superinsulation and cooled directly by a bath of liquid cryogen or by a heat exchanger.

In the case of CT 2 (Fig. 1) the liquid coolant is filled directly into the sample chamber (bath cryostat). The complete friction sample is immersed into the liquid cryogen and the environmental temperature is equal to the boiling temperature of the coolant (liquid nitrogen, LN2: 77.3 K; liquid hydrogen, LH2: 20.3 K; liquid helium, LHe: 4.2 K). The advantage of this method is a very effective cooling of the sample by making use of the heat of evaporation of the liquid.

Most of the tests are carried out by using the standard pin-on-disc configuration where a fixed flat pin or ball is continuously sliding against a rotating disc. The rotation is transmitted via a rotary vacuum feedthrough to a shaft with the sample disc at the lower end. In CT2 loading is performed by means of a gas bellow which acts on a frame with the fixed sample (pin) mounted on its lower beam. The mechanical stability of this assembly allows normal forces up to 500 N. The friction force is measured by means of a torque sensor on top of the motor journal or a beam force transducer integrated in the sample holder.

The sample chamber of the tribometer CT 3 is designed for pressures between 10-3 mbar and 20 bar and cooled by a heat exchanger (continuous flow cryostat). The coolant is pumped through the heat exchanger, evaporates and removes heat from the inner vessel. Thus, it is possible to adjust the temperature between 4.2 K (with LHe cooling) and room temperature independently from the pressure. There is not limitation to an equilibrium state of the boiling coolant as in a bath cryostat. In hydrogen environment, the behaviour of tribosystems in gaseous or liquid environment as well as in the vicinity of the critical point can be investigated, which is of importance for the design of high performance hydrogen pumps.

 2 BAM Federal Institute for Materials Research and Testing, Berlin, Germany

Tribological Behaviour of Solid Lubricants in Hydrogen Environment 275

Soft Metals such as gold, silver, lead, and indium can serve as solid lubricants. Thin films with good adhesion can be applied by ion-plating with an optimum thickness of about 1 µm. The tribological properties of soft metals are similar in ambient air and vacuum environment with friction coefficients of about 0.1 and remain unchanged during cooling down to cryogenic temperatures. Furthermore, as they have a f.c.c. crystal structure, they are not affected by hydrogen embrittlement (Moulder & Hust, 1983) and therefore, applicable for tribosystems in gaseous and liquid hydrogen. However, in sliding friction in vacuum these materials have higher friction and wear than lamellar solids (Roberts, 1990,

A large number of ferrous alloys are employed for tribosystems, including those running in hydrogen environment. As many of these materials suffer from hydrogen embrittlement, they are treated in this chapter, although they are no solid lubricants. In particular, ferritic and martensitic steels with b.c.c. lattice are strongly affected by hydrogen. Austenitic FeCrNi alloys with f.c.c. structure don't show hydrogen embrittlement, and therefore, these alloys are the favoured materials in hydrogen technology. As these steels have good mechanical properties even at cryogenic temperatures they are also appropriate for components in contact with liquid hydrogen. However, in highly stressed tribosystems deformation-induced generation of martensite is possible, and the danger of embrittlement in these regions arises. Furthermore, an uptake of hydrogen can intensify the deterioration of the material. In an austenitic lattice solute hydrogen decreases the stacking fault energy (SFE) (Holzworth & Louthan, 1968). As a consequence, the deformation behaviour changes and the martensite generation is facilitated. In Fig. 3 (Butakova, 1973) the generation of martensite in tensile testing in dependence of the SFE for various FeCrNi-alloys is shown. Therefore, it is necessary to investigate the tribological behaviour of austenitic steels in hydrogen-containing environments. The friction and wear behaviour in liquid hydrogen of the austenitic steels 1.4301 (AISI 304), 1.4439 (comparable to AISI 316), 1.4876, and 1.4591 (German materials numbers) was studied by Huebner, et al. (2003a). These FeCrNi alloys

**4. Tribological behaviour of metals in hydrogen environment** 

**4.2 Properties of steels in cryogenic hydrogen environment** 

have different stability of their austenitic structure and are included in Fig. 3.

be sensitive enough to describe the transformation at a crack tip (Bowe et al., 1979).

Steel 1.4301 is a metastable austenite. Its SFE is very low and thus, deformation-induced structure transformation is possible, even at room temperature. Steel 1.4439 is a so-called stable austenitic steel. Transformation is impeded because of its increased SFE. Finally, in materials 1.4876 and 1.4591 with very high contents of Ni, the SFE is rather high, and the generation of martensite should be impossible. As counterbodies Al2O3 ceramic balls were used to avoid metal transfer to the steels samples. The austenitic steels were tested in inert environments at low temperatures and in LH2. After the friction experiments, the transformation to martensite in the wear scars was detected by changes of the materials magnetic properties (magneto-inductive single-pole probe). This method has been shown to

The amount of martensite vs. temperature for 1.4301 is shown in Fig. 4. The amount of martensite strongly depends on the temperature with a maximum at about 30 K. Below this temperature the generation of martensite decreases. For this metastable steel, hydrogen environment was without any influence on the amount of austenite transformed into

**4.1 Soft metals as solid lubricants** 

Subramonian et al., 2005).

martensite (symbol ×).

Fig. 1. Cryotribometer CT 2 (bath cryostat)

While in CT 2 the loading unit is located in the room temperature part of the apparatus, in CT 3 loading and force measurement is performed close to the friction couple in the cold part. Therefore, combined loading and measuring units are employed. The sample holder for the counterbody is directly mounted on a two dimensional beam force transducer for measuring normal and friction forces. Loading is accomplished by pressurized He-gas which acts on a piston that moves the beam with the sample holder upwards and presses the counterbody against the lower face of the rotating disk.

To remove any residual gases and condensed liquids, the sample chamber is evacuated to a pressure below 10-3 mbar and filled with pure He-gas. The pump-down-refill cycle is repeated three times. During the experiment, the sliding force and the displacement of the pin are measured. After the measurement, the wear scars of both bodies can be examined by profilometry, light, electron, or atomic force microscopy.

Fig. 2. Cryotribometer CT 3 ( flow cryostat)

#### **4. Tribological behaviour of metals in hydrogen environment**

#### **4.1 Soft metals as solid lubricants**

274 Tribology - Lubricants and Lubrication

**LHe**

While in CT 2 the loading unit is located in the room temperature part of the apparatus, in CT 3 loading and force measurement is performed close to the friction couple in the cold part. Therefore, combined loading and measuring units are employed. The sample holder for the counterbody is directly mounted on a two dimensional beam force transducer for measuring normal and friction forces. Loading is accomplished by pressurized He-gas which acts on a piston that moves the beam with the sample holder upwards and presses

To remove any residual gases and condensed liquids, the sample chamber is evacuated to a pressure below 10-3 mbar and filled with pure He-gas. The pump-down-refill cycle is repeated three times. During the experiment, the sliding force and the displacement of the pin are measured. After the measurement, the wear scars of both bodies can be examined by

**HEAT EXCHANGER**

**MOTOR**

**COOLANT LN2 RADIATION SHIELD** **MOTOR TORQUE SENSOR**

**SAMPLE CONFIGURATION**

**ROTARY FEEDTHROUGH**

**SAMPLE CONFIGURATION**

**DISC**

**PIN**

**DISC**

**PIN**

**ROTARY FEEDTHROUGH**

**BELLOWS FORCE TRANSDUCER**

the counterbody against the lower face of the rotating disk.

profilometry, light, electron, or atomic force microscopy.

Fig. 2. Cryotribometer CT 3 ( flow cryostat)

Fig. 1. Cryotribometer CT 2 (bath cryostat)

Soft Metals such as gold, silver, lead, and indium can serve as solid lubricants. Thin films with good adhesion can be applied by ion-plating with an optimum thickness of about 1 µm. The tribological properties of soft metals are similar in ambient air and vacuum environment with friction coefficients of about 0.1 and remain unchanged during cooling down to cryogenic temperatures. Furthermore, as they have a f.c.c. crystal structure, they are not affected by hydrogen embrittlement (Moulder & Hust, 1983) and therefore, applicable for tribosystems in gaseous and liquid hydrogen. However, in sliding friction in vacuum these materials have higher friction and wear than lamellar solids (Roberts, 1990, Subramonian et al., 2005).

#### **4.2 Properties of steels in cryogenic hydrogen environment**

A large number of ferrous alloys are employed for tribosystems, including those running in hydrogen environment. As many of these materials suffer from hydrogen embrittlement, they are treated in this chapter, although they are no solid lubricants. In particular, ferritic and martensitic steels with b.c.c. lattice are strongly affected by hydrogen. Austenitic FeCrNi alloys with f.c.c. structure don't show hydrogen embrittlement, and therefore, these alloys are the favoured materials in hydrogen technology. As these steels have good mechanical properties even at cryogenic temperatures they are also appropriate for components in contact with liquid hydrogen. However, in highly stressed tribosystems deformation-induced generation of martensite is possible, and the danger of embrittlement in these regions arises. Furthermore, an uptake of hydrogen can intensify the deterioration of the material. In an austenitic lattice solute hydrogen decreases the stacking fault energy (SFE) (Holzworth & Louthan, 1968). As a consequence, the deformation behaviour changes and the martensite generation is facilitated. In Fig. 3 (Butakova, 1973) the generation of martensite in tensile testing in dependence of the SFE for various FeCrNi-alloys is shown.

Therefore, it is necessary to investigate the tribological behaviour of austenitic steels in hydrogen-containing environments. The friction and wear behaviour in liquid hydrogen of the austenitic steels 1.4301 (AISI 304), 1.4439 (comparable to AISI 316), 1.4876, and 1.4591 (German materials numbers) was studied by Huebner, et al. (2003a). These FeCrNi alloys have different stability of their austenitic structure and are included in Fig. 3.

Steel 1.4301 is a metastable austenite. Its SFE is very low and thus, deformation-induced structure transformation is possible, even at room temperature. Steel 1.4439 is a so-called stable austenitic steel. Transformation is impeded because of its increased SFE. Finally, in materials 1.4876 and 1.4591 with very high contents of Ni, the SFE is rather high, and the generation of martensite should be impossible. As counterbodies Al2O3 ceramic balls were used to avoid metal transfer to the steels samples. The austenitic steels were tested in inert environments at low temperatures and in LH2. After the friction experiments, the transformation to martensite in the wear scars was detected by changes of the materials magnetic properties (magneto-inductive single-pole probe). This method has been shown to be sensitive enough to describe the transformation at a crack tip (Bowe et al., 1979).

The amount of martensite vs. temperature for 1.4301 is shown in Fig. 4. The amount of martensite strongly depends on the temperature with a maximum at about 30 K. Below this temperature the generation of martensite decreases. For this metastable steel, hydrogen environment was without any influence on the amount of austenite transformed into martensite (symbol ×).

Tribological Behaviour of Solid Lubricants in Hydrogen Environment 277

**0 90 180 270 360 Circular segment (o**

Fig. 5. Steel 1.4439, Influence of hydrogen on the generation of martensite during friction

300:1 1000:1

process could only be activated by mechanical energy from sliding.

Fig. 6. Steel 1.4591, SEM images of the wear track; net of brittle cracks in the wear scar after

For influencing the deformation behaviour, it is necessary that atomic hydrogen exists in the material. In LH2 thermally initiated dissociation is not possible. Thus, the dissociation

After the tests in inert environment, extremely deformed wear debris was found all over the wear track. However, these particles did not show any embrittlement. After sliding in hydrogen, the surface showed completely different features. The wear scar exhibits a net of microcracks (Fig. 6). This topography was detected for all austenitic alloys chosen for these experiments, even for the highly alloyed materials 1.4876 and 1.4591. This is clear indication for the occurrence of hydrogen induced embrittlement, even in LH2. These findings could be confirmed by measurements of residual stresses in the deformed zone (Hübner et al., 2003b).

**20 K / He-Gas 20 K / LH2 77 K / LN2**

**)**

**0**

frictional stressing in LH2

**0,1**

**0,2**

**0,3**

**Amount of martensite (a.u.)**

**0,4**

**0,5**

Fig. 3. Influence of the SFE of austenitic FeNiCr alloys on the martensite volume fraction after 80% plastic deformation in tensile testing (according to Butakova, 1973)

Fig. 4. Steel 1.4301, Temperature-dependence of friction-induced generation of martensite

Contrary to steel 1.4301, the transformation behaviour of the steel 1.4439 showed a distinct influence of the environment (Fig. 5). In LN2 and at 20 K in gaseous He, only local magnetisation was detected in the wear scars (symbols: , Δ). It could be shown by scanning electron microscopy that locations with magnetic signals correspond to extremely deformed transfer particles (Hübner, 2001). After a test in liquid hydrogen (symbol: +), magnetic changes were observed in the entire circular wear track.

FeNi15Cr10 FeNi20Cr5

*1.4439*

**0 20 40 60 80 100 120 Stacking fault energy (erg/cm<sup>2</sup>**

**0 50 100 150 200 250 300 Temperature (K)**

Fig. 4. Steel 1.4301, Temperature-dependence of friction-induced generation of martensite Contrary to steel 1.4301, the transformation behaviour of the steel 1.4439 showed a distinct influence of the environment (Fig. 5). In LN2 and at 20 K in gaseous He, only local magnetisation was detected in the wear scars (symbols: , Δ). It could be shown by scanning electron microscopy that locations with magnetic signals correspond to extremely deformed transfer particles (Hübner, 2001). After a test in liquid hydrogen (symbol: +), magnetic

Fig. 3. Influence of the SFE of austenitic FeNiCr alloys on the martensite volume fraction

after 80% plastic deformation in tensile testing (according to Butakova, 1973)

**)**

5 N - 0.2 m/s 10 N - 0.2 m/s

5 N - 0.06 m/s - LH2

FeNi31

*1.4876*

*1.4591*

FeNi29

**0**

**0**

changes were observed in the entire circular wear track.

**1**

**2**

**Amount of martensite (a.u.)**

**3**

**4**

**5**

**20**

**40**

**Martensite content (%)**

**60**

**80**

**100**

FeNi8Cr18

FeNi10Cr15

*1.4301*

Fig. 5. Steel 1.4439, Influence of hydrogen on the generation of martensite during friction

After the tests in inert environment, extremely deformed wear debris was found all over the wear track. However, these particles did not show any embrittlement. After sliding in hydrogen, the surface showed completely different features. The wear scar exhibits a net of microcracks (Fig. 6). This topography was detected for all austenitic alloys chosen for these experiments, even for the highly alloyed materials 1.4876 and 1.4591. This is clear indication for the occurrence of hydrogen induced embrittlement, even in LH2. These findings could be confirmed by measurements of residual stresses in the deformed zone (Hübner et al., 2003b).

300:1 1000:1

Fig. 6. Steel 1.4591, SEM images of the wear track; net of brittle cracks in the wear scar after frictional stressing in LH2

For influencing the deformation behaviour, it is necessary that atomic hydrogen exists in the material. In LH2 thermally initiated dissociation is not possible. Thus, the dissociation process could only be activated by mechanical energy from sliding.

Tribological Behaviour of Solid Lubricants in Hydrogen Environment 279

structure as CaF2, which can be used as solid lubricant. As NbH2 has a lattice structure similar to CaF2 it may also have lubricating properties. Pure Zr and Nb were tested as selfmated pairs in pin-on-disc tests with a sliding speed of 3.49 x 10-2 ms-1 and loads of 25 and 70 N. Both materials showed lower friction coefficients in H2 gas atmosphere than in air, He gas, and vacuum. In H2-gas atmosphere the friction coefficients of the Nb specimens were much higher than those of the Zr specimens. X-ray diffraction analysis showed that the wear particles, which were formed by sliding Zr and Nb specimens in the H2 gas atmosphere, consisted mainly of the ZrH2 phase (ε phase) and NbH phase (β phase), respectively. X-ray diffraction analysis also showed that the wear particles, which were formed by sliding in air,

An amorphous carbon (DLC-), a MoS2-coating, prepared by physical vapour deposition (PVD), and two types of anti-friction coatings (AFC 1 and AFC 2) were tested in dry and humid N2-, H2-, and CH4-environment at BAM (Gradt & Theiler, 2010). In dry gas, the residual water content was in the ppm-range. In humid environment the relative humidity was close to 100%. The solid lubricant in both AF- coatings was PTFE. The tests were performed in ball-on-flat configuration in reciprocating sliding at room temperature. The

> Substrate 100Cr6 (AISI 52100) Coatings DLC, MoS2,

Counterbody uncoated ball, d = 4 mm.

Gas pressure, bar 3

is still intact, and the wear track has a very smooth, polished-like surface.

FN, N 5; 10 Stroke, µm 200 Frequency, Hz 20

Test duration 2 h (144,000 Cycles)

Fig. 8 summarizes the measured friction coefficients in the miscellaneous environments. The carbon coating shows a distinct sensitivity to the humidity. While in dry gases the friction coefficient is about 0.15, it rises to 0.19 to 0.25 in humid environment. Also the wear of this type of DLC-coating rises under high humidity, as can be seen in Figures 9 and 10. Fig. 9 shows an SEM-image of a wear scar after a test in hydrogen of high humidity. The complete coating is worn away, and abrasive wear of the substrate is visible. Fig. 10 shows an image of a wear scar after the same sliding distance in dry hydrogen. It can be seen that the coating

Both AF-coatings showed friction coefficients around 0.15 in dry hydrogen and nitrogen. They have not been tested in these gases with high humidity. Such comparative measurements were done in methane gas. It can be seen that AFC 1 reaches a low friction coefficient of 0.08 in dry and humid CH4. Thus, this coating is sensitive to the particular type

Environment N2-, H2-, CH4-gas, dry/humid

AF-Coatings (lubricant: PTFE)

X90CrMoV18 (AISI 440B)

consisted mainly of the αZr and Nb phases, respectively.

**5. Solid lubricant coatings** 

test parameters are given in Table 2.

Table 2. Test parameters, solid lubricant coatings

The influence of hydrogen on the deformation mechanisms is also visible in the shape of the X-ray diffraction line profiles. Fig. 7 shows the γ311 reflection of the austenitic steel 1.4876 after sliding in air, LHe, and LH2. The reflection profiles of the tests in air and LHe are symmetrical. They exhibit only deformation-induced broadening. However, in LH2 an asymmetry occurs, which is a clear indication for hydrogen uptake. Hydrogen lowers the stacking fault energy of the austenite lattice, which enhances the building of the epsilon phase (Whiteman & Troiano, 1984, Pontini & Hermida, 1997).

Gavriljuk et al. (1995) described in detail how hydrogen influences the transformation behaviour of unstable as well as stable austenitic steels. In so-called unstable steels, already cold working induces phase transformation. Stable steels may be subject to structure changes after charging with hydrogen, which causes a decrease in SFE. These explanations are in good agreement with the results shown in Figures 4 and 5. A significant influence of hydrogen on the austenite-martensite transformation is observed only in the stable steel (Fig. 5), because the metastable steel 1.4301 (Fig. 4) experiences structure changes already during the low-temperature deformation.

Fig. 7. Steel 1.4876, Asymmetry of the γ311 reflection of the after frictional stressing in LH2

Beside deformation enhanced creation of martensite, also other mechanisms can lead to increased wear in austenitic stainless steel. Kubota et al. (2011) reported a reduction of the fretting fatigue limit in hydrogen gas for steel AISI 304. He found that small cracks which were stable in air propagated in hydrogen gas. The reason for this effect was an increased local adhesion in hydrogen environment.

#### **4.3 Other metals**

The tribological properties of Zr and Nb in hydrogen environment were investigated by Murakami et al. (2010). Coatings of Zr-alloys on high strength steels are considered as a diffusion-barrier for hydrogen. Furthermore, Zr forms hydrides which have the same structure as CaF2, which can be used as solid lubricant. As NbH2 has a lattice structure similar to CaF2 it may also have lubricating properties. Pure Zr and Nb were tested as selfmated pairs in pin-on-disc tests with a sliding speed of 3.49 x 10-2 ms-1 and loads of 25 and 70 N. Both materials showed lower friction coefficients in H2 gas atmosphere than in air, He gas, and vacuum. In H2-gas atmosphere the friction coefficients of the Nb specimens were much higher than those of the Zr specimens. X-ray diffraction analysis showed that the wear particles, which were formed by sliding Zr and Nb specimens in the H2 gas atmosphere, consisted mainly of the ZrH2 phase (ε phase) and NbH phase (β phase), respectively. X-ray diffraction analysis also showed that the wear particles, which were formed by sliding in air, consisted mainly of the αZr and Nb phases, respectively.

### **5. Solid lubricant coatings**

278 Tribology - Lubricants and Lubrication

The influence of hydrogen on the deformation mechanisms is also visible in the shape of the X-ray diffraction line profiles. Fig. 7 shows the γ311 reflection of the austenitic steel 1.4876 after sliding in air, LHe, and LH2. The reflection profiles of the tests in air and LHe are symmetrical. They exhibit only deformation-induced broadening. However, in LH2 an asymmetry occurs, which is a clear indication for hydrogen uptake. Hydrogen lowers the stacking fault energy of the austenite lattice, which enhances the building of the epsilon

Gavriljuk et al. (1995) described in detail how hydrogen influences the transformation behaviour of unstable as well as stable austenitic steels. In so-called unstable steels, already cold working induces phase transformation. Stable steels may be subject to structure changes after charging with hydrogen, which causes a decrease in SFE. These explanations are in good agreement with the results shown in Figures 4 and 5. A significant influence of hydrogen on the austenite-martensite transformation is observed only in the stable steel (Fig. 5), because the metastable steel 1.4301 (Fig. 4) experiences structure changes already

Fig. 7. Steel 1.4876, Asymmetry of the γ311 reflection of the after frictional stressing in LH2

Beside deformation enhanced creation of martensite, also other mechanisms can lead to increased wear in austenitic stainless steel. Kubota et al. (2011) reported a reduction of the fretting fatigue limit in hydrogen gas for steel AISI 304. He found that small cracks which were stable in air propagated in hydrogen gas. The reason for this effect was an increased

The tribological properties of Zr and Nb in hydrogen environment were investigated by Murakami et al. (2010). Coatings of Zr-alloys on high strength steels are considered as a diffusion-barrier for hydrogen. Furthermore, Zr forms hydrides which have the same

phase (Whiteman & Troiano, 1984, Pontini & Hermida, 1997).

during the low-temperature deformation.

local adhesion in hydrogen environment.

**4.3 Other metals** 

An amorphous carbon (DLC-), a MoS2-coating, prepared by physical vapour deposition (PVD), and two types of anti-friction coatings (AFC 1 and AFC 2) were tested in dry and humid N2-, H2-, and CH4-environment at BAM (Gradt & Theiler, 2010). In dry gas, the residual water content was in the ppm-range. In humid environment the relative humidity was close to 100%. The solid lubricant in both AF- coatings was PTFE. The tests were performed in ball-on-flat configuration in reciprocating sliding at room temperature. The test parameters are given in Table 2.


Table 2. Test parameters, solid lubricant coatings

Fig. 8 summarizes the measured friction coefficients in the miscellaneous environments. The carbon coating shows a distinct sensitivity to the humidity. While in dry gases the friction coefficient is about 0.15, it rises to 0.19 to 0.25 in humid environment. Also the wear of this type of DLC-coating rises under high humidity, as can be seen in Figures 9 and 10. Fig. 9 shows an SEM-image of a wear scar after a test in hydrogen of high humidity. The complete coating is worn away, and abrasive wear of the substrate is visible. Fig. 10 shows an image of a wear scar after the same sliding distance in dry hydrogen. It can be seen that the coating is still intact, and the wear track has a very smooth, polished-like surface.

Both AF-coatings showed friction coefficients around 0.15 in dry hydrogen and nitrogen. They have not been tested in these gases with high humidity. Such comparative measurements were done in methane gas. It can be seen that AFC 1 reaches a low friction coefficient of 0.08 in dry and humid CH4. Thus, this coating is sensitive to the particular type

Tribological Behaviour of Solid Lubricants in Hydrogen Environment 281

Fig. 10. Smooth wear track of the a DLC-coating after a reciprocating sliding test in dry H2-

The lowest coefficient of friction (COF = 0.03) was observed for MoS2. This coating showed a very smooth sliding behaviour with nearly no running-in. Fig. 11 shows the development of the COF of the three tested MoS2- coatings in comparison to DLC. The DLC-coatings showed a higher COF and a pronounced running-in behaviour. However, the lifetime of the MoS2-coatings was much shorter than that of DLC and not sufficient in the scope of this test series, where more than 100,000 cycles were necessary. Therefore, no further tests in other environments were carried out. Nevertheless, for dry sliding tribosystems, where a lifetime of 10,000 friction cycles is sufficient, MoS2-lubrication seems to be applicable in hydrogen

> **0 10000 20000 30000 40000 Cycles**

Fig. 11. Oscillating friction of DLC- and MoS2-coatings in gaseous hydrogen

**DLC MoS2**

environment (SEM-image of the wear scar)

environment.

**0**

**0.1**

**0.2 0.3**

**0.4 0.5**

**COF**

**0.6 0.7**

**0.8**

of gas and not to general chemical reactivity or humidity. AFC 2 also changes its frictional behaviour in CH4. However, while methane seems to have a beneficial effect on AFC 1, it causes an increasing friction of AFC 2.

Fig. 8. Friction coefficients of several solid lubricants in inert and reactive gaseous environment

Fig. 9. Coating failure of a DLC-coating after a reciprocating sliding test in humid H2 environment (SEM-image of the wear scar)

of gas and not to general chemical reactivity or humidity. AFC 2 also changes its frictional behaviour in CH4. However, while methane seems to have a beneficial effect on AFC 1, it

**0 0,05 0,1 0,15 0,2 0,25**

**DLC AFC 1 AFC 2 MoS2**

**COF**

Fig. 8. Friction coefficients of several solid lubricants in inert and reactive gaseous

Fig. 9. Coating failure of a DLC-coating after a reciprocating sliding test in humid H2-

environment (SEM-image of the wear scar)

causes an increasing friction of AFC 2.

**N2 dry**

**H2 dry**

**N2 humid**

environment

**H2 humid**

**CH4 dry**

**CH4 humid**

Fig. 10. Smooth wear track of the a DLC-coating after a reciprocating sliding test in dry H2 environment (SEM-image of the wear scar)

The lowest coefficient of friction (COF = 0.03) was observed for MoS2. This coating showed a very smooth sliding behaviour with nearly no running-in. Fig. 11 shows the development of the COF of the three tested MoS2- coatings in comparison to DLC. The DLC-coatings showed a higher COF and a pronounced running-in behaviour. However, the lifetime of the MoS2-coatings was much shorter than that of DLC and not sufficient in the scope of this test series, where more than 100,000 cycles were necessary. Therefore, no further tests in other environments were carried out. Nevertheless, for dry sliding tribosystems, where a lifetime of 10,000 friction cycles is sufficient, MoS2-lubrication seems to be applicable in hydrogen environment.

Fig. 11. Oscillating friction of DLC- and MoS2-coatings in gaseous hydrogen

Tribological Behaviour of Solid Lubricants in Hydrogen Environment 283

**MoS2 graphite**

**0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5**

**PI +15%MoS2**

**PEEK+10%PTFE+13%CF**

interface and thus a lower friction (Theiler et al., 2004).

and LH2 is a possible drawback for practical application.

material for hydrogen applications.

**PTFE + 13,5%PEEK+18,2%CF**

**PTFE+9,2%bronze+16,7%CF**

Fig. 13. Sliding friction of polymer composites against steel (Theiler & Gradt, 2007)

**PTFE + 20%PPS**

temperatures is observed for many polymers and is due to the fact that hardness and Young's modulus of the polymers increase with decreasing temperature. Both lead to lower deformation and a smaller real area of contact. This causes a lower shearing force at the

Another tendency is that graphite containing composites have the lowest friction coefficients in liquid hydrogen, in one case even lower than 0.05. On the other hand, composites containing MoS2 don't reach values below 0.2. Thus, for hydrogen applications graphite seems to be a much more efficient component for improving the lubricating properties of polymers. The friction coefficients of the composites without graphite or MoS2 are between 0.1 and 0.2 in LH2 which is sufficient for many applications. All materials of this group contain PTFE, which also acts as a solid lubricant. In some cases, the large difference between ambient air

A comparison of the friction coefficients in liquid hydrogen, hydrogen gas, and ambient air at room temperature for two composites with PTFE- and two with PEEK-matrix is shown in Fig. 14. The materials with PTFE-matrix show a large difference in COF between normal air and hydrogen environment and no significant influence of the temperature. This difference is much smaller for the PEEK materials with additions of PTFE. Additional admixture of graphite leads to a COF of about 0.15, which depends only very little on the environment. Although the other composites exhibit lower friction under certain conditions, this low dependence on the environment makes the graphite containing composite a most suitable

The wear behaviour of the PTFE- and PEEK-composites follows a similar tendency. As shown in Fig. 15, the wear rate of the two materials without graphite is much smaller in hydrogen environment than in air. The wear of the graphite containing material is not significantly influenced by the environment. Furthermore, a wear rate below 10-6 makes this

material suitable for application in sliding bearings or in cages for roller bearings.

**PA6.6 + 30%PTFE**

**PEEK+10%PTFE+10%CF+10%graphite**

**PEEK+5%PTFE+15%CF+ 5% graphite**

**Air, RT LH2**

**EP+15%CF+15%graphite+5%TiO2**

**PEI+5%CF+15%graphite+5%TiO2**

**PA+15%CF+5%graphite+5%TiO2**

**PEEK+10%PTFE+10%CF+10%MoS2**

**friction coefficient**

#### **6. Solid lubricants in polymer composites**

Polymers and polymer composites are widely used as dry sliding materials in friction assemblies where external supply of lubricants is impossible, or not recommended. The field of application of self-lubricating materials in tribological systems is considerably extending also to extreme environments (Gardos, 1986). Over the years, composite materials have replaced many traditional metallic materials in sliding components. They offer not only low weight and corrosion resistance, but also excellent tribological properties. In view of hydrogen technology, numerous polymer composites containing PTFE, MoS2, and graphite respectively have been tested in hydrogen and inert media such as nitrogen and helium (Theiler & Gradt, 2007). Some of these materials were also tested in liquid hydrogen. Fig. 12 shows the test configuration, and Table 3 summarizes the materials and test parameters. The material compositions are given in the figures of the test results below.


Table 3. Materials and test parameters, polymer composites

Fig. 12. Sample configuration for tests of polymer composites

Fig. 13 shows the friction coefficient of various polymer composites against steel in air and liquid hydrogen (Theiler & Gradt, 2007). Except the first one, all tested composites have lower friction in LH2 than in air at room temperature. A decrease of friction at lower

Polymers and polymer composites are widely used as dry sliding materials in friction assemblies where external supply of lubricants is impossible, or not recommended. The field of application of self-lubricating materials in tribological systems is considerably extending also to extreme environments (Gardos, 1986). Over the years, composite materials have replaced many traditional metallic materials in sliding components. They offer not only low weight and corrosion resistance, but also excellent tribological properties. In view of hydrogen technology, numerous polymer composites containing PTFE, MoS2, and graphite respectively have been tested in hydrogen and inert media such as nitrogen and helium (Theiler & Gradt, 2007). Some of these materials were also tested in liquid hydrogen. Fig. 12 shows the test configuration, and Table 3 summarizes the materials and test parameters. The

material compositions are given in the figures of the test results below.

Fibers CF: carbon fibers Fillers PEEK, PPS

Lubricants PTFE, MoS2, graphite

Normal load, N 16; 50 N Sliding speed, m/s 0.2 Sliding distance, m 2000 Table 3. Materials and test parameters, polymer composites

Fig. 12. Sample configuration for tests of polymer composites

Polymer matrix PTFE: polytetrafluoroethylene

PI: polyimide PA: Polyamide PEI: polyetherimide

EP: epoxy

bronze TiO2

**FN**

Fig. 13 shows the friction coefficient of various polymer composites against steel in air and liquid hydrogen (Theiler & Gradt, 2007). Except the first one, all tested composites have lower friction in LH2 than in air at room temperature. A decrease of friction at lower

PEEK: polyetheretherketone

**Pin-on-disc configuration**

**Disc: Steel 52100 Ø 40 mm**

**Pin: Polymer composite 4 x 4 mm²**

**6. Solid lubricants in polymer composites** 

Fig. 13. Sliding friction of polymer composites against steel (Theiler & Gradt, 2007)

temperatures is observed for many polymers and is due to the fact that hardness and Young's modulus of the polymers increase with decreasing temperature. Both lead to lower deformation and a smaller real area of contact. This causes a lower shearing force at the interface and thus a lower friction (Theiler et al., 2004).

Another tendency is that graphite containing composites have the lowest friction coefficients in liquid hydrogen, in one case even lower than 0.05. On the other hand, composites containing MoS2 don't reach values below 0.2. Thus, for hydrogen applications graphite seems to be a much more efficient component for improving the lubricating properties of polymers.

The friction coefficients of the composites without graphite or MoS2 are between 0.1 and 0.2 in LH2 which is sufficient for many applications. All materials of this group contain PTFE, which also acts as a solid lubricant. In some cases, the large difference between ambient air and LH2 is a possible drawback for practical application.

A comparison of the friction coefficients in liquid hydrogen, hydrogen gas, and ambient air at room temperature for two composites with PTFE- and two with PEEK-matrix is shown in Fig. 14. The materials with PTFE-matrix show a large difference in COF between normal air and hydrogen environment and no significant influence of the temperature. This difference is much smaller for the PEEK materials with additions of PTFE. Additional admixture of graphite leads to a COF of about 0.15, which depends only very little on the environment. Although the other composites exhibit lower friction under certain conditions, this low dependence on the environment makes the graphite containing composite a most suitable material for hydrogen applications.

The wear behaviour of the PTFE- and PEEK-composites follows a similar tendency. As shown in Fig. 15, the wear rate of the two materials without graphite is much smaller in hydrogen environment than in air. The wear of the graphite containing material is not significantly influenced by the environment. Furthermore, a wear rate below 10-6 makes this material suitable for application in sliding bearings or in cages for roller bearings.

Tribological Behaviour of Solid Lubricants in Hydrogen Environment 285

For numerous components in hydrogen technology solid lubrication is the only possible method for reducing friction and wear. Solid lubricants such as PTFE, graphite, DLC, and MoS2 applied as coatings, or as components in polymer composites, in general are able to

MoS2-coatings have low friction, but a very short lifetime in hydrogen environment. The tested carbon coating showed higher friction, but a much longer lifetime in dry environment.

PTFE-based anti friction (AF-) coatings exhibit low friction and a negligible sensitivity to humidity. However, the type of gas influences their frictional behaviour, independent of the

In general, friction coefficients and wear rates of polymer composites decrease with decreasing temperature. Also hydrogen has a beneficial effect on the friction behaviour of polymer composites. The addition of graphite leads to a favourable tribological behaviour which is not significantly influenced by the environmental medium. This makes graphite-

Many thanks to the colleagues from BAM divisions 6.2 and 6.4, who participated in the investigations of this paper. Also many thanks to the Institute for Composite Materials IVW GmbH, Kaiserslautern for supplying some polymer composites and the German Research

Bulletin M 055 (1991). Wasserstoff, BGI 612, Berufsgenossenschaft Rohstoffe und chemische

Donnet, C. & Erdemir, A. (2004). Solid lubricant coatings: recent developments and future

Frey, H.; Haefer, R.A. (1981). Tieftemperaturtechnologie, Eder, F.X. (Ed.), VDI-Verlag,

Fujita, S.; Matsuoka, S.; Murakami, Y.; Marquis, G. (2010). Effect of hydrogen on Mode II

Fukuda, K.; Sugimura, J. (2008). Sliding Properties of Pure Metals in Hydrogen

Fukuda, K.; Hashimoto, M.; Sugimura, J. (2011). Friction and Wear of Ferrous Materials in a

Gardos, M.N. (1986). Self lubricating composites for extreme environmental conditions, In:

Gavriljuk, V.G.; Hänninen, H.; Tarasenko, A.V.; Tereshchenko, A.S.; Ullakko, K.. (1995).

Gipson, H. (2001). Lubrication of Space Shuttle Main Engine Turbopump Bearings,

Hydrogen Gas Environment, *Tribology Online,* Vol. 6, pp. 142-147

austenitic stainless steels, *Acta metal. mater*., Vol. 43, pp. 559-568

fatigue crack behaviour of tempered bearing steel and microstructural changes.

*Friction and Wear of Polymer Composites, Composite Materials Series 1*, K. Friedrich

Phase transformations and relaxation phenomena caused by hydrogen in stable

containing PEEK-composites most suitable materials for hydrogen applications.

Bowe, K.H.; Hornbogen, E.; Wittkamp, I. (1979). *Materialprüfung*, Vol. 21, pp. 74

Industrie, Jedermann Verlag, Heidelberg, Germany

*International Journal of Fatigue.* Vol. 32, pp. 934-951

Environment, *Proc. STLE/ASME,* IJTC2008-71210

*Lubrication Engineering,* August 2001, pp. 10-12

Butakova, E.D. (1973). *Fizika Met. Metalloved*. Vol. 35, pp. 662

trends, *Tribology Letters,* Vol. 17, pp. 389-397

Düsseldorf, Germany, ISBN 3-18-400503-8

(Ed.), pp. 397-447

reduce friction and wear in gaseous as well as in liquid hydrogen.

In humid environment this type of coating fails rapidly.

Association (DFG) for supporting parts of this study.

humidity.

**8. Acknowledgements** 

**9. References** 

Fig. 14. Friction of polymer composites in air and H2

Fig. 15. Wear of polymer composites in air and H2

#### **7. Conclusion**

Tribosystems directly exposed to hydrogen are critical in respect of excess wear, because they may experience hydrogen embrittlement, chemical reactions to hydrides, and vanishing protective oxide layers respectively. Furthermore, liquid lubricants are often not applicable, because of purity requirements, or very low temperatures in the case of liquid hydrogen. Hydrogen uptake and material deterioration influences wear processes also in austenitic stainless steels. Hydrogen lowers the stacking fault energy of the austenite lattice, which enhances the building of deformation induced martensite that is prone to hydrogen embrittlement.

For numerous components in hydrogen technology solid lubrication is the only possible method for reducing friction and wear. Solid lubricants such as PTFE, graphite, DLC, and MoS2 applied as coatings, or as components in polymer composites, in general are able to reduce friction and wear in gaseous as well as in liquid hydrogen.

MoS2-coatings have low friction, but a very short lifetime in hydrogen environment. The tested carbon coating showed higher friction, but a much longer lifetime in dry environment. In humid environment this type of coating fails rapidly.

PTFE-based anti friction (AF-) coatings exhibit low friction and a negligible sensitivity to humidity. However, the type of gas influences their frictional behaviour, independent of the humidity.

In general, friction coefficients and wear rates of polymer composites decrease with decreasing temperature. Also hydrogen has a beneficial effect on the friction behaviour of polymer composites. The addition of graphite leads to a favourable tribological behaviour which is not significantly influenced by the environmental medium. This makes graphitecontaining PEEK-composites most suitable materials for hydrogen applications.

#### **8. Acknowledgements**

Many thanks to the colleagues from BAM divisions 6.2 and 6.4, who participated in the investigations of this paper. Also many thanks to the Institute for Composite Materials IVW GmbH, Kaiserslautern for supplying some polymer composites and the German Research Association (DFG) for supporting parts of this study.

#### **9. References**

284 Tribology - Lubricants and Lubrication

**PTFE+ 9,2%Bronze +16,7%CF**

**PEEK+ 10%PTFE+ 13%CF**

**CDH**

**PTFE+13.5%PEEK +18%C F**

Tribosystems directly exposed to hydrogen are critical in respect of excess wear, because they may experience hydrogen embrittlement, chemical reactions to hydrides, and vanishing protective oxide layers respectively. Furthermore, liquid lubricants are often not applicable, because of purity requirements, or very low temperatures in the case of liquid hydrogen. Hydrogen uptake and material deterioration influences wear processes also in austenitic stainless steels. Hydrogen lowers the stacking fault energy of the austenite lattice, which enhances the building of deformation induced martensite that is prone to hydrogen

**PEEK+ 10%PTFE+ 10%CF +10%graphite**

> **RT, air LH2 RT, H2**

**PEEK+10%PTFE+ 10 %CF+10%graphite**

**RT, air H2, RT LH2**

**0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5**

**0.0**

**0.5**

**1.0**

**1.5**

**wear rate [mm³/Nm] 10-6**

**7. Conclusion** 

embrittlement.

**2.0**

**2.5**

**3.0**

**friction coefficient**

**PTFE + 13,5%PEEK +18,2%CF**

Fig. 14. Friction of polymer composites in air and H2

**PEEK+10%PTFE +13%CF**

Fig. 15. Wear of polymer composites in air and H2

Bowe, K.H.; Hornbogen, E.; Wittkamp, I. (1979). *Materialprüfung*, Vol. 21, pp. 74


**12** 

*TEKNIKER-IK4* 

*Spain* 

**Alternative Cr+6-Free** 

**Coatings Sliding Against NBR Elastomer** 

Hexavalent chromium compounds result attractive primarily for industrial activity because they provide manufactured products with enhanced hardness, shininess, durability, color, corrosion resistance, heat resistance, decay resistance and tribological properties. On the other hand, it poses far more health hazards than trivalent chromium. It is a hazardous substance that increases the risk of developing lung cancer if itis inhaled. Ingestion or even simple skin exposure of chromic acid could increase the risk of cancer formation. In this situation, hexavalent chromium is classified by the International Agency for Research on Cancer (IARC) as a known human carcinogen (Group 1) (Working Group on the Evaluation of Carcinogenic Risks to Humans, 1987), where workers have the highest risk of adverse

Hexavalent chromium has been deeply used in tribological applications being friction and wear reduction also one of the main objectives in sliding mechanical parts for minimizing loss of energy and improving systems performance (Flitney, 2007) (Monaghan, 2008). In the last years, in fact, attention in maintenance costs saving also grow up, therefore a key question is to achieve low levels of friction as well as high wear resistance. In the field of elastomeric materials in 1978 A. N. Gent et al. (Gent, 1978) studied wear of metal by rubber attributing those phenomena at the direct attack upon metals of free radical species generated by mechanical rupture of elastomer molecules during abrasion. It suggested that such studies might lead to new metal texturing processes and surface treatment that can have the double effect of improving the tribological performances and protecting from external agents. Furthermore, coating technology is gaining ground thanks to new available technologies and focusing in particular to the need of using new alternative non toxic

The availability of new coating technologies like High Velocity Oxy-Fuel (HVOF) permits to have a wide range of hard coatings, but a deep study of their mechanical and tribological characteristics is needed due to the strong influence of their roughness, hardness, finishing

HVOF thermal spray technique allows depositing variety of materials (alloys and ceramics). The powdered feedstock of deposition material is heated and accelerated to high velocities in oxygen fuel. The material hits and solidifies as high density well adherent coating material on the sample/component. HVOF coatings are also strong and show low residual

**1. Introduction** 

health effects from hexavalent chromium exposure.

surface treatment with equivalent functionality of Cr+6.

and resistance to wear and corrosion.

Beatriz Fernandez-Diaz, Raquel Bayón and Amaya Igartua


## **Alternative Cr+6-Free Coatings Sliding Against NBR Elastomer**

Beatriz Fernandez-Diaz, Raquel Bayón and Amaya Igartua *TEKNIKER-IK4 Spain* 

#### **1. Introduction**

286 Tribology - Lubricants and Lubrication

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Hübner, W.; Pyzalla, A.; Assmus, K.; Wild, E.; Wroblewski, T. (2003b). Phase stability of

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Nosaka, N; Takata, S.; Yoshida, M.; Kikuchi, M; Sudo, T.; Nakamura, S. (2010). Improvement

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*Hydrogen and Fuell Cell Association*, EHA/DWV publication

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304L stainless steels, *Corrosion*, Vol. 244, pp. 110-124

*Show*, Vancouver, Canada

Dowson (Ed.), Elsevier, Amsterdam,

(120,000 rpm), *Tribology Online,* Vol. 5, pp. 60-70

Hydrogen, *Tribology Online,* Vol. 6, 133-141

Nitrogen, *Tribol. Lett.*, Vol. 20, pp. 263-272

*Werkstofftechnik*, Vol. 35, pp. 683-689

*Safety*, San Sebastian, Spain

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Vol. 268, pp. 721–729

37, pp. 1831-1837

*in press* 

tribological stressing. *Tribology Intern.*, Vol. 34, 231-236

of ADLC Coatings in Cryogenic Environment, *Tribology Intern.*, Vol. 34, pp. 225-230

Absorption by Steels during Friction, *Proc. Hydrogen and Fuel Cells Conf. and Trade* 

AISI 304 stainless steel during sliding wear at extremely low temperatures, *Wear*,

fatigue limit caused by hydrogen gas in SUS304 austenitic stainless steel, *Tribol. Int.,* 

wear properties of zirconium and niobium in a hydrogen Environment, *Wear*,

of Durability of Hybrid Ceramic Ball Bearings in Liquid Hydrogen at 3 Million DN

energy Reduction Induced by Hydrogen in an AISI 304 Steel, *Scripta Materialia*, Vol.

and Wear Properties of Solid Lubricant Coatings on SUS440C Steel in Liquid

Polymer Composites at Room and Low Temperatures", *Materialwissenschaft und* 

Hydrogen Environment, *Proceedings of the 2nd International Conference on Hydrogen* 

Hydrogen in Addressing the Challenges in the new Global Energy System. *German* 

Hexavalent chromium compounds result attractive primarily for industrial activity because they provide manufactured products with enhanced hardness, shininess, durability, color, corrosion resistance, heat resistance, decay resistance and tribological properties. On the other hand, it poses far more health hazards than trivalent chromium. It is a hazardous substance that increases the risk of developing lung cancer if itis inhaled. Ingestion or even simple skin exposure of chromic acid could increase the risk of cancer formation. In this situation, hexavalent chromium is classified by the International Agency for Research on Cancer (IARC) as a known human carcinogen (Group 1) (Working Group on the Evaluation of Carcinogenic Risks to Humans, 1987), where workers have the highest risk of adverse health effects from hexavalent chromium exposure.

Hexavalent chromium has been deeply used in tribological applications being friction and wear reduction also one of the main objectives in sliding mechanical parts for minimizing loss of energy and improving systems performance (Flitney, 2007) (Monaghan, 2008). In the last years, in fact, attention in maintenance costs saving also grow up, therefore a key question is to achieve low levels of friction as well as high wear resistance. In the field of elastomeric materials in 1978 A. N. Gent et al. (Gent, 1978) studied wear of metal by rubber attributing those phenomena at the direct attack upon metals of free radical species generated by mechanical rupture of elastomer molecules during abrasion. It suggested that such studies might lead to new metal texturing processes and surface treatment that can have the double effect of improving the tribological performances and protecting from external agents. Furthermore, coating technology is gaining ground thanks to new available technologies and focusing in particular to the need of using new alternative non toxic surface treatment with equivalent functionality of Cr+6.

The availability of new coating technologies like High Velocity Oxy-Fuel (HVOF) permits to have a wide range of hard coatings, but a deep study of their mechanical and tribological characteristics is needed due to the strong influence of their roughness, hardness, finishing and resistance to wear and corrosion.

HVOF thermal spray technique allows depositing variety of materials (alloys and ceramics). The powdered feedstock of deposition material is heated and accelerated to high velocities in oxygen fuel. The material hits and solidifies as high density well adherent coating material on the sample/component. HVOF coatings are also strong and show low residual

Alternative Cr+6-Free Coatings Sliding Against NBR Elastomer 289

respectively. However, differences were observed when shot peening was applied. WCCoCr material had a high hardness so impacts of microballs did not modify its surface and hence the final surface was very similar to the roughness achieved with the "Grinding" process, that is, 0.28. However, the other two materials (AlBronze and NiCrBSi) were strongly affected by the shots, so final roughnesses were 1.36 and 2.06 μm, respectively. These two last high values have to be considered as rough figures, since the surface of the shot peened

**(HV)** 

Plating 850 ±11 Grinding 0.20

**Surface texture process** 

Shot peening+

Shot peening+

Shot peening+

Superfinishing 0.04

Grinding 1.36

+Superfinishing 0.04

Grinding 2.06

+Superfinishing 0.03

Grinding 0.28

**Ra (**μ**m)** 

coatings was very irregular, so high dispersion of values was obtained.

Hard Chromium

AlBronze

NiCrBSi

WCCoCr

AlBronze+G+F Grinding +

NiCrBSi+G+F Grinding

WCCoCr+G+F Grinding

presence of some irregularities in the coatings which were analyzed in detail.

AlBronze+G Grinding 0.22

NiCrBSi+G Grinding 0.16

WCCoCr+G Grinding 0.23

Fig. 2 to Fig. 4 show the cross section of the HVOF coated rods where structure can be examined. For this characterization, rods with shot peening process where selected in order to analyze the deformation suffered by the coating after the glass impacts. The thickness of the coatings was in the range of 120-150 μm. Neither pores nor cracks in the interface of the coating where found in the coatings, which improves corrosion resistance and facilitates proper bonding, respectively. However, the analysis of the SEM images evidences the

In the WCCoCr coating (Fig. 2) Nickel traps form some clusters of material. These clusters could come from previous processes were Nickel was deposited (for example in the preparation of the NiCrBSi coating). It was also identified alumina particles between the substrate and the coating (darker area in Fig. 2) which could come from the machining process. No evidence of craters was present on the surface of the coatings. It seemed that the hard nature of this coating (1115±92 HV) made difficult the creation of craters on its surface. The NiCrBSi coating (Fig. 3) had many clusters of material particles. The pale clusters corresponded to Molybdenum, also detected in the surface of this rod; the dark polygonal clusters corresponded again to alumina. The alumina was detected not only between

(HVOF) 1115±92

(HVOF) 745±15

(HVOF) 260±10

**Rod identification Coating Hardness** 

HCP + G *(reference)* 

AlBronze+SP+G

NiCrBSi+SP+G

WCCoCr+SP+G

Table 1. Tested coated rods

tensile stress or in some cases compressive stress, which enable very much thicker coatings to be applied than previously possible with the other processes.

An investigation is herein proposed considering NBR (Nitrile butadiene rubber) material sliding against HVOF coated steel rod in order to clarify the influence of the surface characteristics (hardness, roughness and texture) on the tribological measurements. Many times, in addition, the metallic parts need to have good corrosion resistance for protecting them from external hostile atmospheres. A study of the corrosion resistance of the HVOF coatings is then presented, in comparison with the reference Hard Chromium Plating (HCP) treatment.

In this situation, the main objective of this work was to investigate and compare the tribological and corrosion behavior of a reference tribopair NBR/HCP versus some alternatives based on NBR/HVOF coatings. These materials combinations simulates contact occurring in sealing systems, where polymer and metallic parts are rubbed each other (Conte, 2006).

#### **2. Materials and methodology**

#### **2.1 Rod coatings**

Three different material powders were sprayed by HVOF on a 15-5PH steel rod (diameter 19 mm, length 33 mm): AlBronze, NiCrBSi and WCCoCr. After the HVOF coating process, the cylinders were subjected to different surface modification processes identified as Grinding (G), Superfinishing (F) and Shot Peening (SP). Shot peening was performed with glass balls of diameter in the range of 90- 150 μm, which were injected on the surface of the rods at a pressure of 7 bar and at approximately a distance of 20 mm from the rod. By combining grinding, finishing and shot peening processes it was possible to create different textures on the surface of the coated rods. In addition, reference surface treatment, Hard Chromium Plating (HCP), was also investigated. Coated rods are shown in Fig. 1 where it can be seen that the coatings have been homogeneously deposited on the surface of the rods.

Fig. 1. Coated rod samples. Image corresponds to rods with Grinding process

Table 1 shows some information about hardness and roughness of the tested coated rods. In all the materials "Grinding" and "Grinding + Superfinishing" processes modified the surface of the rods to an averaged roughness of approximately 0.20 μm and 0.04 μm,

tensile stress or in some cases compressive stress, which enable very much thicker coatings

An investigation is herein proposed considering NBR (Nitrile butadiene rubber) material sliding against HVOF coated steel rod in order to clarify the influence of the surface characteristics (hardness, roughness and texture) on the tribological measurements. Many times, in addition, the metallic parts need to have good corrosion resistance for protecting them from external hostile atmospheres. A study of the corrosion resistance of the HVOF coatings is then presented, in comparison with the reference Hard Chromium Plating (HCP)

In this situation, the main objective of this work was to investigate and compare the tribological and corrosion behavior of a reference tribopair NBR/HCP versus some alternatives based on NBR/HVOF coatings. These materials combinations simulates contact occurring in sealing systems, where polymer and metallic parts are rubbed each other

Three different material powders were sprayed by HVOF on a 15-5PH steel rod (diameter 19 mm, length 33 mm): AlBronze, NiCrBSi and WCCoCr. After the HVOF coating process, the cylinders were subjected to different surface modification processes identified as Grinding (G), Superfinishing (F) and Shot Peening (SP). Shot peening was performed with glass balls of diameter in the range of 90- 150 μm, which were injected on the surface of the rods at a pressure of 7 bar and at approximately a distance of 20 mm from the rod. By combining grinding, finishing and shot peening processes it was possible to create different textures on the surface of the coated rods. In addition, reference surface treatment, Hard Chromium Plating (HCP), was also investigated. Coated rods are shown in Fig. 1 where it can be seen

that the coatings have been homogeneously deposited on the surface of the rods.

Reference HVOF coating

Fig. 1. Coated rod samples. Image corresponds to rods with Grinding process

HCP AlBronze NiCrBSi WCCoCr

Table 1 shows some information about hardness and roughness of the tested coated rods. In all the materials "Grinding" and "Grinding + Superfinishing" processes modified the surface of the rods to an averaged roughness of approximately 0.20 μm and 0.04 μm,

to be applied than previously possible with the other processes.

treatment.

(Conte, 2006).

**2.1 Rod coatings** 

**2. Materials and methodology** 

respectively. However, differences were observed when shot peening was applied. WCCoCr material had a high hardness so impacts of microballs did not modify its surface and hence the final surface was very similar to the roughness achieved with the "Grinding" process, that is, 0.28. However, the other two materials (AlBronze and NiCrBSi) were strongly affected by the shots, so final roughnesses were 1.36 and 2.06 μm, respectively. These two last high values have to be considered as rough figures, since the surface of the shot peened coatings was very irregular, so high dispersion of values was obtained.


Table 1. Tested coated rods

Fig. 2 to Fig. 4 show the cross section of the HVOF coated rods where structure can be examined. For this characterization, rods with shot peening process where selected in order to analyze the deformation suffered by the coating after the glass impacts. The thickness of the coatings was in the range of 120-150 μm. Neither pores nor cracks in the interface of the coating where found in the coatings, which improves corrosion resistance and facilitates proper bonding, respectively. However, the analysis of the SEM images evidences the presence of some irregularities in the coatings which were analyzed in detail.

In the WCCoCr coating (Fig. 2) Nickel traps form some clusters of material. These clusters could come from previous processes were Nickel was deposited (for example in the preparation of the NiCrBSi coating). It was also identified alumina particles between the substrate and the coating (darker area in Fig. 2) which could come from the machining process. No evidence of craters was present on the surface of the coatings. It seemed that the hard nature of this coating (1115±92 HV) made difficult the creation of craters on its surface.

The NiCrBSi coating (Fig. 3) had many clusters of material particles. The pale clusters corresponded to Molybdenum, also detected in the surface of this rod; the dark polygonal clusters corresponded again to alumina. The alumina was detected not only between

Alternative Cr+6-Free Coatings Sliding Against NBR Elastomer 291

the substrate and the coating, but also in the matrix of the coating. In this case, shots of the glass balls did perform craters on the coating, increasing then the roughness of the coating till 2.06 μm. In some areas of the surface of the coating it was appreciated flakes-like irregularities which could had been provoked during the finishing process. These non homogeneous features under severe working conditions could accelerate the fail of the

The superficial appearance of the AlBronze coating (Fig. 4) was similar to the NiCrBSi coating. It showed high roughness (Ra=1.36 μm) because of the combination of its relatively low hardness (260 HV) and the craters performed during the shot peening; flake-like cracks

NBR elastomer samples were obtained from real seals, and had a hardness of 85±1 ShA. The material was analyzed by Thermogravimetry Analysis (TGA) and Scanning Electro Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) techniques. The composition of the tested NBR is shown in Table 2. The analysis of the inorganic part revealed the presence of Magnesium Silicate (talc), Sulphur and Zinc Oxide. Magnesium Silicate is used as compounding material, Sulphur acts as vulcanization agent and Zinc

**Component Quantity (% in weigth)** 

Friction and wear tests were carried out using the cylinder on plate configuration (Fig. 5). Coated rods were put in contact against flat sample of NBR under sliding linear reciprocating conditions. Contacting surfaces were lubricated using AeroShell Fluid 41

During the test, the coated rod was linearly reciprocated at a maximum linear speed of 100 mm/s with a stroke of 2 mm. Testing normal load was applied gradually in order to soften the contact between the metallic rod and the rubber sample: during the first 30 s it was set a normal load of 50 N and then a ramp of load was applied to reach 100 N, the testing normal

Specimens were located in a climate chamber to set temperature and relative humidity at 25 ºC and 50 %RH, respectively. Each material combination was tested at least twice in order to

It was recorded the evolution of the coefficient of friction through time and, after the tests, surface damage on the specimens was analyzed by optical microscopy. It was also considered the evaluation of the mass loss but no significant results were obtained, so it was

Elastomer and plasticizers 49 Carbon black 46 Inorganic filler 5

an alumina clusters were again found within the coating.

Oxide is used for activating this process.

Table 2. Composition of the NBR rubber

load. Tests had a duration of 30 min.

evaluate the dispersion of the results.

**2.3 Tribological tests** 

hydraulic mineral oil.

not reported.

coating.

**2.2 NBR elastomer** 

Fig. 2. WCCoCr + SP+ G coating

Fig. 3. NiCrBSi + SP+ G coating

Fig. 4. Al Bronze + SP+ G coating

the substrate and the coating, but also in the matrix of the coating. In this case, shots of the glass balls did perform craters on the coating, increasing then the roughness of the coating till 2.06 μm. In some areas of the surface of the coating it was appreciated flakes-like irregularities which could had been provoked during the finishing process. These non homogeneous features under severe working conditions could accelerate the fail of the coating.

The superficial appearance of the AlBronze coating (Fig. 4) was similar to the NiCrBSi coating. It showed high roughness (Ra=1.36 μm) because of the combination of its relatively low hardness (260 HV) and the craters performed during the shot peening; flake-like cracks an alumina clusters were again found within the coating.

#### **2.2 NBR elastomer**

290 Tribology - Lubricants and Lubrication

Alumina particles

Nickel

Alumina particles

Crater

Alumina particles

Flake

Fig. 2. WCCoCr + SP+ G coating

Crater

Flakes

Molybdenum

Fig. 3. NiCrBSi + SP+ G coating

Fig. 4. Al Bronze + SP+ G coating

NBR elastomer samples were obtained from real seals, and had a hardness of 85±1 ShA. The material was analyzed by Thermogravimetry Analysis (TGA) and Scanning Electro Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS) techniques. The composition of the tested NBR is shown in Table 2. The analysis of the inorganic part revealed the presence of Magnesium Silicate (talc), Sulphur and Zinc Oxide. Magnesium Silicate is used as compounding material, Sulphur acts as vulcanization agent and Zinc Oxide is used for activating this process.


Table 2. Composition of the NBR rubber

#### **2.3 Tribological tests**

Friction and wear tests were carried out using the cylinder on plate configuration (Fig. 5). Coated rods were put in contact against flat sample of NBR under sliding linear reciprocating conditions. Contacting surfaces were lubricated using AeroShell Fluid 41 hydraulic mineral oil.

During the test, the coated rod was linearly reciprocated at a maximum linear speed of 100 mm/s with a stroke of 2 mm. Testing normal load was applied gradually in order to soften the contact between the metallic rod and the rubber sample: during the first 30 s it was set a normal load of 50 N and then a ramp of load was applied to reach 100 N, the testing normal load. Tests had a duration of 30 min.

Specimens were located in a climate chamber to set temperature and relative humidity at 25 ºC and 50 %RH, respectively. Each material combination was tested at least twice in order to evaluate the dispersion of the results.

It was recorded the evolution of the coefficient of friction through time and, after the tests, surface damage on the specimens was analyzed by optical microscopy. It was also considered the evaluation of the mass loss but no significant results were obtained, so it was not reported.

Alternative Cr+6-Free Coatings Sliding Against NBR Elastomer 293

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

**Coefficient of friction 0**

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

**Coefficient of friction 0**

**Ra=0.04** 

**Ra=0.22** 

AlBronze 260Hv

**Ra=1.36** 

**Surface treatment on the steel cylinder Hardness (Hv)**

**Ra=0.04** 

**Ra=0.16** 

NiCrBSi 745Hv

**Ra=2.06** 

**Ra=0.03** 

**Ra=0.23** 

WCCoCr 1115Hv

**Ra=0.28** 

µ**m** 

µ**m** 

µ**m** 

µ**m** 

µ

**m**

µ**m** 

µ**m** 

µ**m** 

**Ra=0.20** 

HCP (Ref.)

Fig. 7. Mean coefficient of friction, averaged roughness and hardness

850Hv

µ**m** 

G+F G SP+G

µ**m** 

0 5 10 15 20 25 30

**WCCoCr HVOF coating**

WCCoCr+G+F

0 5 10 15 20 25 30 **Time (min)**

WCCoCr+SP+G

WCCoCr+G

AlBronze+G

**AlBronze HVOF coating**

AlBronze+SP+G

AlBronze+G+F

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

> 0 5 10 15 20 25 30 **Time (min)**

> > **NiCrBSi HVOF Coating**

**Time (min)** 0 5 10 15 20 25 30

NiCrBSi+G

NiCrBSi+SP+G

0,45 0,40 0,35 0,30 0,25 0,20 0,15 0,10 0,05 0,00

Mean coefficient of friction 0

Fig. 6. Friction curves

NiCrBSi+G+F

HCP+G

**HCP (Reference)**

**Coefficient of friction 0**

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

**Coefficient of friction 0**

Fig. 5. Scheme of the testing arrangement (Cylinder on Plate configuration) (a) and load history (b)

#### **2.4 Corrosion tests**

Corrosion tests were performed in a conventional electrochemical cell of three electrodes. The reference electrode used for these measurements was a silver/silver chloride electrode (SSC, 0.207V vs SHE), the counter electrode was a platinum wire and the working electrode was the studied surface in each case. The exposed area of the samples was 1.47 cm2. Tests were done at room temperature and under aerated conditions. The aggressive media used was NaCl 0.06M. The electrochemical techniques applied for the corrosion behaviour study were electrochemical impedance spectroscopy in function of immersion time (4 and 24 hours of immersion) and potentiodynamic polarization.

On the other hand, impedance measurements were performed at a frequency range between 100 kHz and 10 mHz (10 freq/decade) with a signal amplitude of 10 mV. Polarization curves were registered from -0.4V versus open circuit potential (OCP) and 0.8 V vs OCP at a scan rate of 0.5mV/s.

#### **3. Friction and wear behaviour of hard coatings and rubber material**

The evolution of friction coefficient through time for the different rods is shown in Fig. 6. The steady-state of the coefficient of friction was reached from the beginning of the tests, that is, the running-in phase is really short. The high values during the first seconds corresponded to the loading phase since the setting of the testing normal load was reached after 50 s.

Considering the mean values of the friction curves it was found that in general, for the three HVOF coatings, the lower the averaged roughness, the higher the mean friction coefficient, independently of the material of the coating (Fig. 7). The effect of reducing roughness by mechanical surface treatments revealed that lowering rod roughness did not promote the formation of the lubrication film in the interphase rod/rubber, resulting in friction force increment. This general tendency was not followed by the AlBronze coating. This material had the lowest hardness so it was very affected by the shot peening process, which generated a very irregular surface with unbalanced tribological effect.

Fig. 6. Friction curves

Holders

Sliding direction

Fig. 5. Scheme of the testing arrangement (Cylinder on Plate configuration) (a) and load

Corrosion tests were performed in a conventional electrochemical cell of three electrodes. The reference electrode used for these measurements was a silver/silver chloride electrode (SSC, 0.207V vs SHE), the counter electrode was a platinum wire and the working electrode was the studied surface in each case. The exposed area of the samples was 1.47 cm2. Tests were done at room temperature and under aerated conditions. The aggressive media used was NaCl 0.06M. The electrochemical techniques applied for the corrosion behaviour study were electrochemical impedance spectroscopy in function of immersion time (4 and 24

On the other hand, impedance measurements were performed at a frequency range between 100 kHz and 10 mHz (10 freq/decade) with a signal amplitude of 10 mV. Polarization curves were registered from -0.4V versus open circuit potential (OCP) and 0.8 V vs OCP at a

The evolution of friction coefficient through time for the different rods is shown in Fig. 6. The steady-state of the coefficient of friction was reached from the beginning of the tests, that is, the running-in phase is really short. The high values during the first seconds corresponded to the loading phase since the setting of the testing normal load was reached

Considering the mean values of the friction curves it was found that in general, for the three HVOF coatings, the lower the averaged roughness, the higher the mean friction coefficient, independently of the material of the coating (Fig. 7). The effect of reducing roughness by mechanical surface treatments revealed that lowering rod roughness did not promote the formation of the lubrication film in the interphase rod/rubber, resulting in friction force increment. This general tendency was not followed by the AlBronze coating. This material had the lowest hardness so it was very affected by the shot peening process, which

**3. Friction and wear behaviour of hard coatings and rubber material** 

generated a very irregular surface with unbalanced tribological effect.

Polymeric simple

Rod

hours of immersion) and potentiodynamic polarization.

Bath oil

history (b)

**2.4 Corrosion tests** 

scan rate of 0.5mV/s.

after 50 s.

Normal force

Fig. 7. Mean coefficient of friction, averaged roughness and hardness

Alternative Cr+6-Free Coatings Sliding Against NBR Elastomer 295

The coated rods did not suffer damage as consequence of the contact with the relatively soft rubber sample; the lubrication film protected effectively the metallic surfaces. On the other hand, strong influence of the counterbody was observed when analyzing the wear

An overview of the SEM images showing the surface damage on the surface of the NBR samples revealed different wear behaviour depending on the tested counterbody. The initial surface texture of the NBR sample had a flake-like shape (Fig. 8 (a)), a texture acquired during the moulding phase of the elastomeric sample. Straight lines were also observed, again a replica of the texture of the mould. As observed in Fig. 8 (b) the reference cylinder coating HCP softened this texture by reducing the microscopic roughness. However, straight lines from the mould remained still visible. Particles on the worn area were analyzed by X-Ray. Spectrum of Fig. 9 (c) indicated they were rubber with a significant amount of Sulphur and Zinc. These elements corresponded to the components used in the vulcanization process of the rubber. They tend to emigrate to surface of the NBR sample and thus, they remain within the matrix of the detached wear particles. Important presence of these two elements was found on the untested area ((Fig. 9 (a)); contrary, the plain worn area had less quantity of these elements as observed in Fig. 9 (b), since the successive cycles

In relation to the tests with the HVOF coated rods, the intensity of the surface damage on the NBR sample was very influenced by the surface texture of the rod. Rods with high roughness (AlBronze+SP+G and NiCrBSi+SP+G) produced important abrasion marks in the sliding direction as observed in Fig. 10 (c) and Fig. 11 (c). With rods of lower roughness this phenomenon was still present, but with lower intensity (Fig. 10 (b) and Fig. 12 (c)). Schallamach waves (Schallamach, 1971) perpendicular to the sliding direction were observed on the NBR after the test with the AlBronze+G (Fig. 10 (b)), which indicated that micro-bonding between contacting surfaces occurred. This material produced light surface damage on the NBR when the surface roughness was low according to the Superfinishing process (Fig. 10 (a)). There is still present the flake-like shape of the texture of the untested rubber, as well as the straight lines from the mould. The same behaviour was observed with the WCCoCr+G+F rod as shown in Fig. 12 (a). On the other hand, the NiCrBSi alloy with the G+F and G processes roughened the NBR surface in very similar way; the rubber failed by

Open circuit measurements registered during the initial 5000 s of immersion in the electrolyte appear in Fig. 13. The potential in case of reference chromed sample differs from

After the first 4 hours of immersion an electrochemical impedance spectroscopy was performed on each surface to evaluate the electrochemical response of the coatings to the selected aggressive media. In this study, EIS (Electrochemical Impedance Spectroscopy) was employed to detect the pinholes in the coatings proposed and assessed their effect on the system corrosion behaviour over longer immersion times. Because of that, a second EIS was additionally measured on each sample after 24 hours of exposure to the aggressive electrolyte. Fig. 14 shows the impedance diagrams registered at 4 h and 24 h of immersion

the rest of coatings showing a more stable and noble open circuit potential.

behaviour of the NBR elastomers.

removed the upper film of the NBR sample.

cracking and fatigue phenomena.

for each coating.

**4. Corrosion resistance of coatings** 

Fig. 8. Not tested area on the NBR elastomeric samples (a) and worn area after tests againts HCP+G reference material (b). White arrow indicates sliding direction. Blue arrows indicate straigth marks from the mould. Red arrows indicate points where X-Ray analysis was done

Fig. 9. X-Ray microanalysis on the NBR sample: not tested surface (a), plain worn area (b) and particle on the worn surface (c)

a) b)

Fig. 8. Not tested area on the NBR elastomeric samples (a) and worn area after tests againts HCP+G reference material (b). White arrow indicates sliding direction. Blue arrows indicate straigth marks from the mould. Red arrows indicate points where X-Ray analysis was done

(a)

(b)

(c)

Fig. 9. X-Ray microanalysis on the NBR sample: not tested surface (a), plain worn area (b)

and particle on the worn surface (c)

The coated rods did not suffer damage as consequence of the contact with the relatively soft rubber sample; the lubrication film protected effectively the metallic surfaces. On the other hand, strong influence of the counterbody was observed when analyzing the wear behaviour of the NBR elastomers.

An overview of the SEM images showing the surface damage on the surface of the NBR samples revealed different wear behaviour depending on the tested counterbody. The initial surface texture of the NBR sample had a flake-like shape (Fig. 8 (a)), a texture acquired during the moulding phase of the elastomeric sample. Straight lines were also observed, again a replica of the texture of the mould. As observed in Fig. 8 (b) the reference cylinder coating HCP softened this texture by reducing the microscopic roughness. However, straight lines from the mould remained still visible. Particles on the worn area were analyzed by X-Ray. Spectrum of Fig. 9 (c) indicated they were rubber with a significant amount of Sulphur and Zinc. These elements corresponded to the components used in the vulcanization process of the rubber. They tend to emigrate to surface of the NBR sample and thus, they remain within the matrix of the detached wear particles. Important presence of these two elements was found on the untested area ((Fig. 9 (a)); contrary, the plain worn area had less quantity of these elements as observed in Fig. 9 (b), since the successive cycles removed the upper film of the NBR sample.

In relation to the tests with the HVOF coated rods, the intensity of the surface damage on the NBR sample was very influenced by the surface texture of the rod. Rods with high roughness (AlBronze+SP+G and NiCrBSi+SP+G) produced important abrasion marks in the sliding direction as observed in Fig. 10 (c) and Fig. 11 (c). With rods of lower roughness this phenomenon was still present, but with lower intensity (Fig. 10 (b) and Fig. 12 (c)). Schallamach waves (Schallamach, 1971) perpendicular to the sliding direction were observed on the NBR after the test with the AlBronze+G (Fig. 10 (b)), which indicated that micro-bonding between contacting surfaces occurred. This material produced light surface damage on the NBR when the surface roughness was low according to the Superfinishing process (Fig. 10 (a)). There is still present the flake-like shape of the texture of the untested rubber, as well as the straight lines from the mould. The same behaviour was observed with the WCCoCr+G+F rod as shown in Fig. 12 (a). On the other hand, the NiCrBSi alloy with the G+F and G processes roughened the NBR surface in very similar way; the rubber failed by cracking and fatigue phenomena.

#### **4. Corrosion resistance of coatings**

Open circuit measurements registered during the initial 5000 s of immersion in the electrolyte appear in Fig. 13. The potential in case of reference chromed sample differs from the rest of coatings showing a more stable and noble open circuit potential.

After the first 4 hours of immersion an electrochemical impedance spectroscopy was performed on each surface to evaluate the electrochemical response of the coatings to the selected aggressive media. In this study, EIS (Electrochemical Impedance Spectroscopy) was employed to detect the pinholes in the coatings proposed and assessed their effect on the system corrosion behaviour over longer immersion times. Because of that, a second EIS was additionally measured on each sample after 24 hours of exposure to the aggressive electrolyte. Fig. 14 shows the impedance diagrams registered at 4 h and 24 h of immersion for each coating.

Alternative Cr+6-Free Coatings Sliding Against NBR Elastomer 297

a)

b)

c)

Fig. 11. Worn areas on NBR elastomeric samples against NiCrBSi coatings: G+F (a), G (b)

and SP+G (c). White arrows indicate sliding direction

Fig. 10. Worn areas on NBR elastomeric samples against AlBronze coatings: G+F (a), G (b) and SP+G (c). White arrows indicate sliding direction

a)

b)

c)

Fig. 10. Worn areas on NBR elastomeric samples against AlBronze coatings: G+F (a), G (b)

and SP+G (c). White arrows indicate sliding direction

Fig. 11. Worn areas on NBR elastomeric samples against NiCrBSi coatings: G+F (a), G (b) and SP+G (c). White arrows indicate sliding direction

Alternative Cr+6-Free Coatings Sliding Against NBR Elastomer 299

0 1000 2000 3000 4000 5000

15-5PH + HCP (Ref.) 15-5PH + AlBronze 15-5PH + NiCrBSi 15-5PH + WCCoCr

**Time(s)**

b)

0

**Zim(Ohm)**

Fig. 14. Impedance diagrams at 4 h and 24 h of immersion in NaCl 0.6M; a) chromed

reference, b) AlBronze coating; c) NiCrBSi coating and d) WCCoCr coating

d)

0 10000 20000 30000 40000 50000

0 2000 4000 6000 8000 10000

15-5pH+WCCoCr 4h 15-5pH+WCCoCr 24h

**Zre(Ohm)**

15-5PH+AlBronze 4h 15-5PH+AlBronze 24h

**EIS 15-5PH+AlBronze**

**Zre(Ohm)**

**EIS 15-5PH+WCCoCr**

5000

10000

**Zim(Ohm)**

15000

20000

25000

Fig. 13. Open circuit potential measurements of coated rods in NaCl 0.06M


0 50000 100000 150000 200000

15-5PH+HCP (Ref) 4 h 15-5PH+HCP (Ref) 24 h

**EIS 15-5PH+ HCP (Ref)**

**Zre(Ohm)**

**EIS 15-5PH+NiCrBSi**

0 2000 4000 6000 8000 10000 12000

15-5pH+NiCrBSi 4h 15-5pH+NiCrBSi 24h

**Zre(Ohm)**




**E(V vs Ag/AgCl)**

**Zim(Ohm)**

c)

**Zim(Ohm)**

a)


0.000

0.050

0.100

Fig. 12. Worn areas on NBR elastomeric samples against WCCoCr coatings: G+F (a), G (b) and SP+G (c). White arrows indicate sliding direction

a)

b)

c)

Fig. 12. Worn areas on NBR elastomeric samples against WCCoCr coatings: G+F (a), G (b)

and SP+G (c). White arrows indicate sliding direction

Fig. 13. Open circuit potential measurements of coated rods in NaCl 0.06M

Fig. 14. Impedance diagrams at 4 h and 24 h of immersion in NaCl 0.6M; a) chromed reference, b) AlBronze coating; c) NiCrBSi coating and d) WCCoCr coating

Alternative Cr+6-Free Coatings Sliding Against NBR Elastomer 301

element of the coating, R2 is Rpo, the resistance through the coating pores, CPE-s is CPE-2, the constant phase element of the substrate and Rct corresponds to R2, the charge transfer

Time (h) 4 24 4 24 4 24 4 24 Eoc (V) 0.025 0.050 -0.087 -0.183 -0.192 -0.258 -0.171 -0.174 Rs (Ω.cm2) 68.2 46.6 89.3 88.2 56.9 42.9 52.6 39.1 R1 (KΩ.cm2) 238.0 242.1 38.2 11.26 6.7 16 12.1 24.9 Y0-CPE-1 (10-4F/cm2) 0.127 0.123 0.203 0.566 2.751 3.337 1.973 10.21 N1 0.885 0.888 0.742 0.687 0.716 0.668 0.73 0.691 Zw (10-3 Ω-1.cm-2.s1/2) 0.039 0.049 8.769 / 0.701 / 0.845 / R2 (KΩ.cm2) / / / 9.9 / 5.9 / 8 Y0-CPE-2 (10-4F/cm2) / / / 1.646 / 3.681 / 1.165 n2 / / / 0.762 / 0.758 / 0.843 Table 3. Electrochemical parameters obtained from EIS tests using the equivalent circuits of

Fig. 16. Equivalent circuits used to simulate impedance experimental data. Circuit A) used in all cases at 4 hours of immersion time, and at 24h in case of chromed reference sample. Circuit B) used at 24h of immersion time for the three alternative coatings: AlBronze,

**HCP AlBronze NiCrBSi WCCrCr** 

resistance in the interface substrate/electrolyte.

Fig. 16 in NaCl 0.06M

NiCrBSi and WCCoCr

Fig. 15 gives the Bode plots from the coated samples over the two immersion times in NaCl. According to the impedance diagram, after 4 h immersion, only one semi-circle was shown in all cases, corresponding to the coatings time constant. Low immersion periods were too short to reveal any contribution of the 15-5PH substrate. When the immersion period was increased to 24 h, the phase shift was different to that of 4 h in all alternative coatings, except in case of reference HCP film, whose Bode spectra remains stable and very similar to the first one registered at 4 h of exposure time.

At 4 h of immersion time, all coatings showed diffusion processes in the low frequency range and the experimental data could be fitted by using the equivalent circuit (A) drawn in Fig. 16. The electrochemical parameters obtained using this circuit are listened in Table 3. In this case, CPE1 is the constant phase element of the coating (CPE-c) which impedance can be written as ZCPE=1/Yo(iω)n. R1 is the charge transfer resistance (Rct)in the interface coating/electrolyte and W is the diffusion element (Zw).

Fig. 15. Impedance data (Bode diagrams) of reference and alternative coatings for 15-5PH alloy at 4 h and 24 h of immersion in NaCl 0.06M

After 24 h of immersion, impedance data of the three alternative coatings (AlBronze, NiCrBSi and WCCoCr) presented two time constants due to the contribution of the substrate through the coatings micropores or defects. In this case, the experimental data could be fitted with the equivalent circuit (B) where CPE-c corresponds to CPE1, the constant phase

Fig. 15 gives the Bode plots from the coated samples over the two immersion times in NaCl. According to the impedance diagram, after 4 h immersion, only one semi-circle was shown in all cases, corresponding to the coatings time constant. Low immersion periods were too short to reveal any contribution of the 15-5PH substrate. When the immersion period was increased to 24 h, the phase shift was different to that of 4 h in all alternative coatings, except in case of reference HCP film, whose Bode spectra remains stable and very similar to the

At 4 h of immersion time, all coatings showed diffusion processes in the low frequency range and the experimental data could be fitted by using the equivalent circuit (A) drawn in Fig. 16. The electrochemical parameters obtained using this circuit are listened in Table 3. In this case, CPE1 is the constant phase element of the coating (CPE-c) which impedance can be written as ZCPE=1/Yo(iω)n. R1 is the charge transfer resistance (Rct)in the interface

**-Phase (º)**

Fig. 15. Impedance data (Bode diagrams) of reference and alternative coatings for 15-5PH

After 24 h of immersion, impedance data of the three alternative coatings (AlBronze, NiCrBSi and WCCoCr) presented two time constants due to the contribution of the substrate through the coatings micropores or defects. In this case, the experimental data could be fitted with the equivalent circuit (B) where CPE-c corresponds to CPE1, the constant phase

**log |Z| (ohm cm2)**


**EIS 24 h**

15-5pH+ HCP (Ref) 15-5pH+Al-Bronze 15-5Ph+NiCrBSi 15-5pH+WCCoCr

> 15-5pH+ HCP (Ref) 15-5pH+Al-Bronze 15-5Ph+NiCrBSi 15-5pH+WCCoCr

**log f (Hz)**

**EIS 24 h**


**log f (Hz)**

first one registered at 4 h of exposure time.

**EIS 4 h**

**-Phase (º)**

**log |Z| (ohm cm2)**

coating/electrolyte and W is the diffusion element (Zw).

15-5pH+ HCP (Ref) 15-5pH+Al-Bronze 15-5Ph+NiCrBSi 15-5pH+WCCoCr

> 15-5pH+ HCP (Ref) 15-5pH+Al-Bronze 15-5Ph+NiCrBSi 15-5pH+WCCoCr


**log f (Hz)**

**EIS 4 h**


**log f (Hz)**

alloy at 4 h and 24 h of immersion in NaCl 0.06M


element of the coating, R2 is Rpo, the resistance through the coating pores, CPE-s is CPE-2, the constant phase element of the substrate and Rct corresponds to R2, the charge transfer resistance in the interface substrate/electrolyte.

Table 3. Electrochemical parameters obtained from EIS tests using the equivalent circuits of Fig. 16 in NaCl 0.06M

Fig. 16. Equivalent circuits used to simulate impedance experimental data. Circuit A) used in all cases at 4 hours of immersion time, and at 24h in case of chromed reference sample. Circuit B) used at 24h of immersion time for the three alternative coatings: AlBronze, NiCrBSi and WCCoCr

Alternative Cr+6-Free Coatings Sliding Against NBR Elastomer 303

Tribological tests under lubricated conditions were performed in order to compare the friction and wear behaviour of reference HCP and some alternative HVOF coatings applied on 15-5PH steel rods, sliding in against NBR elastomer. Additionally a corrosion resistance study was carried out on the coated rods. According to the obtained results the following




The authors would like to acknowledge the EU for their financial support (KRISTAL: Knowledge-based Radical Innovation Surfacing for Tribology and Advanced Lubrication, Contract Nr.: NMP3-CT-2005-515837 (www.kristal-project.org)). We also wish to acknowledge Mr. A. Straub (Liebherr Aerospace Lindenberg Gmbh, Lindenberg, Germany) and Dr. M. Meyer from EADS, Ottobrunn, Germany) for their valuable collaboration on this research. Finally, we thank our colleagues Xana Fernández, Gemma Mendoza, Roberto Teruel, Virginia Sáenz de Viteri, Elena Fuentes and Marcello Conte for their support in the

Conte, M. (2006), Interaction between seals and counterparts in pneumatic and hydraulic

Flitney, B. (2007). Alternatives to chrome for hydraulic actuators. *Sealing Technology*, Vol

Gent A.N., Pulford C.T.R. (1978). Wear of steel by rubber. *Wear*, Vol. 49, Issue 1, (July 1978),

Monaghan, K. J. & Straub, A. (2008). Comparison of seal friction on chrome and HVOF

*Technology*, Vol 2008, Issue 11, (November 2008), pp. 9-14

coated rods under conditions of short stroke reciprocating motion. *Sealing* 

This phenomenon suggested significant temperature rise in the contact.

**5. Conclusions** 

conclusions can be drawn:

behaviour of these coatings.

**6. Acknowledgment** 

experimental work.

pp. 135-139

**7. References** 

good enough for protecting the substrate material.

components. *PhD Thesis* (June 2009)

2007, Issue 10, (October 2007), pp.8-12

According to this results, it was seen that the HCP coating was a very good reference for corrosion protection in chloride media since it showed the most constant and stable behaviour after 24 hours of immersion time, as well as high corrosion resistance in comparison to the other alternative coatings.

After 24 hours of exposure, a potentiodynamic polarization curve was performed on the different coated rods. The potential-current curves are exposed in Fig. 17. The results of polarization tests were in agreement with impedance measurements. Chromed rod showed the lowest corrosion current over the whole potential range analyzed, whereas in the case of AlBronze and NiCrBSi coatings the current progressively increased when potential went to more anodic values which involved a more active behaviour in these cases. WCCoCr coating showed more stable and lower corrosion current than the other two alternatives but the corrosion resistances were worst than those measured in case of reference coating (Table 4).


Table 4. Tafel analysis of potential-current curves

Fig. 17. Potentiodynamic polarization curves of coated 15-5PH samples after 24 hours of immersion in NaCl 0.06M

### **5. Conclusions**

302 Tribology - Lubricants and Lubrication

According to this results, it was seen that the HCP coating was a very good reference for corrosion protection in chloride media since it showed the most constant and stable behaviour after 24 hours of immersion time, as well as high corrosion resistance in

After 24 hours of exposure, a potentiodynamic polarization curve was performed on the different coated rods. The potential-current curves are exposed in Fig. 17. The results of polarization tests were in agreement with impedance measurements. Chromed rod showed the lowest corrosion current over the whole potential range analyzed, whereas in the case of AlBronze and NiCrBSi coatings the current progressively increased when potential went to more anodic values which involved a more active behaviour in these cases. WCCoCr coating showed more stable and lower corrosion current than the other two alternatives but the corrosion resistances were worst than those measured in case of

> 15-5PH+HCP (Ref) -0.095 0.13 417 15-5PH+AlBronze -0.209 12.50 7 15-5PH+NiCrBSi -0.269 1.79 21 15-5PH+WCCoCr -0.271 1.40 38

> > **NaCl 0.06M**


Fig. 17. Potentiodynamic polarization curves of coated 15-5PH samples after 24 hours of

Ecorr (V) Icorr (10-6A) Rp (KΩ)

15-5pH+Cr-Ref 15-5pH+Al-Bronze 15-5pH+NiCrBSi 15-5pH+WCCoCr

**E(V vs Ag/AgCl)**

comparison to the other alternative coatings.

Table 4. Tafel analysis of potential-current curves

reference coating (Table 4).

 0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10

immersion in NaCl 0.06M

1×10 1×10 1×10 1×10 1×10 1×10 1×10 1×10 1×10 1×10 1×10

**log l**

Tribological tests under lubricated conditions were performed in order to compare the friction and wear behaviour of reference HCP and some alternative HVOF coatings applied on 15-5PH steel rods, sliding in against NBR elastomer. Additionally a corrosion resistance study was carried out on the coated rods. According to the obtained results the following conclusions can be drawn:


#### **6. Acknowledgment**

The authors would like to acknowledge the EU for their financial support (KRISTAL: Knowledge-based Radical Innovation Surfacing for Tribology and Advanced Lubrication, Contract Nr.: NMP3-CT-2005-515837 (www.kristal-project.org)). We also wish to acknowledge Mr. A. Straub (Liebherr Aerospace Lindenberg Gmbh, Lindenberg, Germany) and Dr. M. Meyer from EADS, Ottobrunn, Germany) for their valuable collaboration on this research. Finally, we thank our colleagues Xana Fernández, Gemma Mendoza, Roberto Teruel, Virginia Sáenz de Viteri, Elena Fuentes and Marcello Conte for their support in the experimental work.

#### **7. References**


**13** 

*Poland* 

**The New Methods for Scuffing and** 

Remigiusz Michalczewski, Witold Piekoszewski,

**Pitting Investigation of Coated Materials** 

**for Heavy Loaded, Lubricated Elements** 

Waldemar Tuszyński, Marian Szczerek and Jan Wulczyński

*Institute for Sustainable Technologies - National Research Institute (ITeE-PIB)* 

In modern technology due to the increase of the unit pressure, velocities, and hence temperatures in the tribosystems of machines, a risk of two very dangerous forms of wear

Scuffing is a form of wear typical of highly-loaded surfaces working at high relative speeds. Scuffing is considered to be a localised damage caused by the occurrence of solid-phase welding between sliding gear flanks, due to excessive heat generated by friction, and it is characterised by the transfer of material between sliding surfaces. This condition occurs during metal-to-metal contact and due to the removal of the protective oxide layer of the

A typical scuffing zone of gear teeth (Michalczewski et al., 2010) is illustrated in Fig. 1.

Another form of wear is rolling contact fatigue (pitting). Pitting is a form of wear typical of highly-loaded surfaces working at a sliding-rolling and rolling contact, e.g. such components in transmissions like toothed gears and rolling bearings (Torrance et al, 1996). It is caused by the cyclic contact stress, which leads to cracks initiation (Libera et al., 2005). The lubricant is

**1. Introduction** 

exists. These forms are scuffing and pitting.

metal surfaces (Burakowski et. al., 2004).

Fig. 1. A typical scuffing wear of gear teeth

Schallamach, A. (1971), How does rubber slide?, *Wear*, Vol. 17, Issue 4, pp.301–312 Working Group on the Evaluation of Carcinogenic Risks to Humans (1987), *IARC Monographs on the evaluation of the Carcinogenic Risks to Humans*, Supplement 7, International Agency for Research on Cancer (IARC), ISBN 9283214110, Lyon

## **The New Methods for Scuffing and Pitting Investigation of Coated Materials for Heavy Loaded, Lubricated Elements**

 Remigiusz Michalczewski, Witold Piekoszewski, Waldemar Tuszyński, Marian Szczerek and Jan Wulczyński *Institute for Sustainable Technologies - National Research Institute (ITeE-PIB) Poland* 

#### **1. Introduction**

304 Tribology - Lubricants and Lubrication

Working Group on the Evaluation of Carcinogenic Risks to Humans (1987), *IARC* 

*Monographs on the evaluation of the Carcinogenic Risks to Humans*, Supplement 7, International Agency for Research on Cancer (IARC), ISBN 9283214110, Lyon

Schallamach, A. (1971), How does rubber slide?, *Wear*, Vol. 17, Issue 4, pp.301–312

In modern technology due to the increase of the unit pressure, velocities, and hence temperatures in the tribosystems of machines, a risk of two very dangerous forms of wear exists. These forms are scuffing and pitting.

Scuffing is a form of wear typical of highly-loaded surfaces working at high relative speeds. Scuffing is considered to be a localised damage caused by the occurrence of solid-phase welding between sliding gear flanks, due to excessive heat generated by friction, and it is characterised by the transfer of material between sliding surfaces. This condition occurs during metal-to-metal contact and due to the removal of the protective oxide layer of the metal surfaces (Burakowski et. al., 2004).

A typical scuffing zone of gear teeth (Michalczewski et al., 2010) is illustrated in Fig. 1.

Fig. 1. A typical scuffing wear of gear teeth

Another form of wear is rolling contact fatigue (pitting). Pitting is a form of wear typical of highly-loaded surfaces working at a sliding-rolling and rolling contact, e.g. such components in transmissions like toothed gears and rolling bearings (Torrance et al, 1996). It is caused by the cyclic contact stress, which leads to cracks initiation (Libera et al., 2005). The lubricant is

The New Methods for Scuffing and Pitting Investigation


PVD/CVD coatings is presented in Fig. 3.

devices have been developed:

**T-02U** 

developed. They are as follows:

of Coated Materials for Heavy Loaded, Lubricated Elements 307

For the purpose of the tribological research in the areas mentioned above, two tribological

The set of methods and devices intended for the comprehensive tribological evaluation of

**MODEL TESTS COMPONENT TESTS**

**SCUFFING SCUFFING** 

**PITTING PITTING**

Fig. 3. The tribological methods and devices developed intended for comprehensive



wear of low-friction and antiwear PVD/CVD coatings are described below.

By means of this set, low-friction and antiwear PVD/CVD coatings can be evaluated from

Using new devices, five test methods, giving the possibility of comprehensive testing of various low-friction and antiwear PVD/CVD coatings intended for machine elements, were

The new test methods and the new devices for the experimental evaluation of friction and

tribological evaluation of elements covered with PCD/CVD coatings

micro to macroscale in model and component tests (Antonov et al., 2009).

**T-12U** 

pressed into the cracks at a very high pressure (elastohydrodynamic lubrication), making them propagate. Finally, cyclic stress results in breaking a piece of material off the surface. Examples of a gear and a race worn due to pitting (Michalczewski et al., 2010) are presented in Fig. 2.

Fig. 2. The pitting wear: a) on a pinion gear, b) on a bearing race

(a) (b)

For many engineering materials, further improvement of their properties through a modification of their microstructure, chemical composition, and phase composition, is practically impossible. In this situation the most effective way of improving mechanical properties of various engineering components is the modification of surface properties by the deposition of PVD/CVD coatings (Michalczewski, 2008). One of the most important characteristics of these coatings is the fact that its thickness, usually in the range from 1 to 5 µm, is located in the field of dimensional tolerances of typical machine elements.

There are many successful applications of thin hard PVD/CVD coatings in various technical devices like engines, pumps, compressors. However the problem of application of such coatings for heavy-loaded friction parts (e.g. gears, bearings) is still open - the share of mechanical components that are coated is extremely small (less than 2%). Why? The service life of heavy-loaded machine parts is essentially determined by two types of tribological failures: scuffing which is a severe form of mechanical wear, and pitting which is a surface fatigue phenomenon. Up to now, there was a lack of verified laboratory test methods intended for correlated determination of coating material and lubricating media on scuffing and pitting resistance of heavy-loaded system. So, the selection of coating material and technology was realised mainly basing on very expensive and long-term practical component research and the results are frequently contradictory (Szczerek, Michalczewski, & Piekoszewski, 2009).

The evaluation of friction and wear characteristics of PVD/CVD coatings is only possible on the way of experimental research. The experimental research of friction and wear of interacting surfaces is realised by means of a special device called tribotester. The new test methods and testing machines have been developed based on the achievements of the System for Tribological Research (SBT) implemented in the Tribology Department at the Institute for Sustainable Technologies – National Research Institute (ITeE-PIB), Radom, Poland (Szczerek, 1996). The SBT system was developed on the basis of the combinatorics that enables to reduce the tendency which is widely known as "testing rush".

pressed into the cracks at a very high pressure (elastohydrodynamic lubrication), making them propagate. Finally, cyclic stress results in breaking a piece of material off the surface. Examples of a gear and a race worn due to pitting (Michalczewski et al., 2010) are presented in Fig. 2.

For many engineering materials, further improvement of their properties through a modification of their microstructure, chemical composition, and phase composition, is practically impossible. In this situation the most effective way of improving mechanical properties of various engineering components is the modification of surface properties by the deposition of PVD/CVD coatings (Michalczewski, 2008). One of the most important characteristics of these coatings is the fact that its thickness, usually in the range from 1 to

There are many successful applications of thin hard PVD/CVD coatings in various technical devices like engines, pumps, compressors. However the problem of application of such coatings for heavy-loaded friction parts (e.g. gears, bearings) is still open - the share of mechanical components that are coated is extremely small (less than 2%). Why? The service life of heavy-loaded machine parts is essentially determined by two types of tribological failures: scuffing which is a severe form of mechanical wear, and pitting which is a surface fatigue phenomenon. Up to now, there was a lack of verified laboratory test methods intended for correlated determination of coating material and lubricating media on scuffing and pitting resistance of heavy-loaded system. So, the selection of coating material and technology was realised mainly basing on very expensive and long-term practical component research and the results are frequently contradictory (Szczerek, Michalczewski,

The evaluation of friction and wear characteristics of PVD/CVD coatings is only possible on the way of experimental research. The experimental research of friction and wear of interacting surfaces is realised by means of a special device called tribotester. The new test methods and testing machines have been developed based on the achievements of the System for Tribological Research (SBT) implemented in the Tribology Department at the Institute for Sustainable Technologies – National Research Institute (ITeE-PIB), Radom, Poland (Szczerek, 1996). The SBT system was developed on the basis of the combinatorics

that enables to reduce the tendency which is widely known as "testing rush".

5 µm, is located in the field of dimensional tolerances of typical machine elements.

(a) (b)

Fig. 2. The pitting wear: a) on a pinion gear, b) on a bearing race

& Piekoszewski, 2009).

For the purpose of the tribological research in the areas mentioned above, two tribological devices have been developed:


The set of methods and devices intended for the comprehensive tribological evaluation of PVD/CVD coatings is presented in Fig. 3.

Fig. 3. The tribological methods and devices developed intended for comprehensive tribological evaluation of elements covered with PCD/CVD coatings

By means of this set, low-friction and antiwear PVD/CVD coatings can be evaluated from micro to macroscale in model and component tests (Antonov et al., 2009).

Using new devices, five test methods, giving the possibility of comprehensive testing of various low-friction and antiwear PVD/CVD coatings intended for machine elements, were developed. They are as follows:


The new test methods and the new devices for the experimental evaluation of friction and wear of low-friction and antiwear PVD/CVD coatings are described below.

The New Methods for Scuffing and Pitting Investigation

**Pt**

poz – limiting pressure of seizure, N/mm2,

tribosystems are presented in Fig. 7.

**0**

the equation (1):

Poz – seizure load [N],

where:

**Poz**

**Friction torque, Mt**

**Applied load, P**

**Mt = 10 Nm**

load at this moment is called the *scuffing load* and denoted Pt.

d – average wear scar diameter measured on the stationary balls, mm.

of Coated Materials for Heavy Loaded, Lubricated Elements 309

**Mt**

**1**

Fig. 5. Simplified friction torque curve (Mt) obtained at continuously increasing load (P); 1 – scuffing initiation, 1-2 – scuffing propagation, 2 – seizure (exceeding 10 Nm friction torque) Scuffing initiation occurs at the time of a sudden increase in the friction torque – point 1. The

According to the new test method, the load still increases (over a value of Pt) until seizure occurs (i.e. friction torque exceeds 10 N m – point 2). The load at this moment will be called the *seizure load* and denoted Poz. If 10 Nm is not reached, maximum load (c.a. 7200 N) is considered to be the seizure load (although in such a case there is no seizure). For every tested lubricant the so-called *limiting pressure of seizure* (denoted poz) should be calculated. This value reflects the lubricant behaviour under scuffing conditions and is equal to the nominal pressure exerted on the wear scar surface at the moment of seizure or at the end of the run (when seizure has not appeared). The limiting pressure of seizure is calculated from

The 0.52 coefficient results from the force distribution in the four-ball tribosystem. The

The developed test methods were successfully used for testing the scuffing resistance of components with thin hard coatings (thickness of 2 µm) deposited by PVD/CVD method.

Wear scars images on lower balls from scuffing tests for steel-steel and CrN-CrN

The developed test methods have the resolution, not achieved by the other methods, good enough to differentiate between coatings, engineering materials and lubricants (Piekoszewski, Szczerek & Tuszynski, 2001). What is more, they are fast and inexpensive. So, these test methods can be effectively used to select the optimum substrate-coating-lubricant combinations best suited for highly loaded machine components (Michalczewski et al., 2009a).

higher poz value, the better action of the tested lubricant under scuffing conditions is.

The example of their application (Michalczewski et al., 2010) is presented in Fig. 6.

**P**

*seizure*

*scuffing initiation*

<sup>2</sup> 0.52 *Poz poz <sup>d</sup>* <sup>=</sup> (1)

**2**

**Time**

#### **2. Model methods and T-02U Universal Four-Ball Testing Machine for evaluation of scuffing and pitting resistance of PVD/CVD coatings**

#### **2.1 Model scuffing tests in four-ball and cone-three balls tribosystems**

For evaluation of scuffing resistance of lubricants, coatings, and engineering materials two tribosystems were employed: four-ball and cone-three balls. In typical four-ball test balls are made of chrome alloy 100Cr6 bearing steel, with diameter of 12.7 mm (0.5 in.). Surface roughness is Ra = 0.032 µm and hardness 60 to 65 HRC. In the new method the investigated coating can be deposited on the ball or on the cone. Furthermore the cone can be made of various engineering material, not only of bearing steel.

The four-ball and cone-three balls tribosystems are presented in Fig. 4.

Fig. 4. Model tribosystems for testing scuffing: a) four-ball tribosystem: 1- top ball, 2- lower balls, 3- ball chuck, 4 – ball pot, b) cone-three balls tribosystem; 1 – top cone, 2 – bottom balls, 3 – ball chuck, 4-ball pot

The three stationary, bottom balls (2), having a diameter of 0.5 in., are fixed in the ball pot (4) and pressed against the top ball or cone (1) at the continuously increasing load P. The top ball/cone is fixed in the ball chuck (3) and rotates at the constant speed n. The tribosystem is immersed in the tested lubricant. During the run the friction torque is observed until seizure occurs.

The test conditions are as follows: rotational speed: 500 rpm, speed of continuous load increase: 409 N/s, initial applied load: 0 N, maximum load: 7200 ± 100 N.

The methods are described in detail in works (Szczerek & Tuszynski, 2002) and patented (Polish Patent No. 179123 - B1 – G01N 33/30). A friction torque curve (Mt) obtained at the continuously increasing load (P) is shown in Fig. 5.

Scuffing initiation occurs at the time of a sudden increase in the friction torque – point 1. The load at this moment is called the *scuffing load* and denoted Pt.

According to the new test method, the load still increases (over a value of Pt) until seizure occurs (i.e. friction torque exceeds 10 N m – point 2). The load at this moment will be called the *seizure load* and denoted Poz. If 10 Nm is not reached, maximum load (c.a. 7200 N) is considered to be the seizure load (although in such a case there is no seizure). For every tested lubricant the so-called *limiting pressure of seizure* (denoted poz) should be calculated. This value reflects the lubricant behaviour under scuffing conditions and is equal to the nominal pressure exerted on the wear scar surface at the moment of seizure or at the end of the run (when seizure has not appeared). The limiting pressure of seizure is calculated from the equation (1):

where:

308 Tribology - Lubricants and Lubrication

For evaluation of scuffing resistance of lubricants, coatings, and engineering materials two tribosystems were employed: four-ball and cone-three balls. In typical four-ball test balls are made of chrome alloy 100Cr6 bearing steel, with diameter of 12.7 mm (0.5 in.). Surface roughness is Ra = 0.032 µm and hardness 60 to 65 HRC. In the new method the investigated coating can be deposited on the ball or on the cone. Furthermore the cone can be made of

Fig. 4. Model tribosystems for testing scuffing: a) four-ball tribosystem: 1- top ball, 2- lower balls, 3- ball chuck, 4 – ball pot, b) cone-three balls tribosystem; 1 – top cone, 2 – bottom

The three stationary, bottom balls (2), having a diameter of 0.5 in., are fixed in the ball pot (4) and pressed against the top ball or cone (1) at the continuously increasing load P. The top ball/cone is fixed in the ball chuck (3) and rotates at the constant speed n. The tribosystem is immersed in the tested lubricant. During the run the friction torque is observed until seizure

The test conditions are as follows: rotational speed: 500 rpm, speed of continuous load

The methods are described in detail in works (Szczerek & Tuszynski, 2002) and patented (Polish Patent No. 179123 - B1 – G01N 33/30). A friction torque curve (Mt) obtained at the

increase: 409 N/s, initial applied load: 0 N, maximum load: 7200 ± 100 N.

continuously increasing load (P) is shown in Fig. 5.

**2. Model methods and T-02U Universal Four-Ball Testing Machine for evaluation of scuffing and pitting resistance of PVD/CVD coatings 2.1 Model scuffing tests in four-ball and cone-three balls tribosystems** 

various engineering material, not only of bearing steel.

a)

b)

balls, 3 – ball chuck, 4-ball pot

occurs.

The four-ball and cone-three balls tribosystems are presented in Fig. 4.

poz – limiting pressure of seizure, N/mm2,

Poz – seizure load [N],

d – average wear scar diameter measured on the stationary balls, mm.

The 0.52 coefficient results from the force distribution in the four-ball tribosystem. The higher poz value, the better action of the tested lubricant under scuffing conditions is.

<sup>2</sup> 0.52 *Poz poz <sup>d</sup>* <sup>=</sup> (1)

The developed test methods were successfully used for testing the scuffing resistance of components with thin hard coatings (thickness of 2 µm) deposited by PVD/CVD method. The example of their application (Michalczewski et al., 2010) is presented in Fig. 6.

Wear scars images on lower balls from scuffing tests for steel-steel and CrN-CrN tribosystems are presented in Fig. 7.

The developed test methods have the resolution, not achieved by the other methods, good enough to differentiate between coatings, engineering materials and lubricants (Piekoszewski, Szczerek & Tuszynski, 2001). What is more, they are fast and inexpensive. So, these test methods can be effectively used to select the optimum substrate-coating-lubricant combinations best suited for highly loaded machine components (Michalczewski et al., 2009a).

The New Methods for Scuffing and Pitting Investigation

three balls tribosystem is presented in Fig. 8.

the vibration level is monitored until pitting occurs.

resistance of the tested material to pitting is.

coatings are presented in Fig. 9.

0

100

200

300

**L10 [min.]**

400

500

600

of Coated Materials for Heavy Loaded, Lubricated Elements 311

**2.2 Model method for evaluation of pitting wear in cone-three balls tribosystem**  The cone-three balls test method is generally based on IP 300 standard (Rolling contact fatigue tests for fluids in a modified four-ball machine). The main change is the geometry of the contact of the rolling elements. The upper ball was replaced with a special cone (Michalczewski & Piekoszewski, 2006). The cone can be made of any material. The cone-

Fig. 8. Cone-three balls tribosystem: a) scheme, b) photograph; 1- cone, 2 - balls, 3 – race

The tribosystem consists of a rotating cone (1) loaded against three balls (2) which are able to rotate in the race (3). The specimens are immersed in the tested lubricant. During the run

The tested cones are made of the tested material. The test balls are made of 100Cr6 chrome alloy bearing steel. For each test the new set of balls should be used. According to the method the test conditions are 3924 N (400 kg) load and 1450 rpm top cone speed. 24 top cone failures are necessary to assess the performance of the lubricant and the material. The tested materials can be compared on the basis of L10 or L50 values as well as scatter factor K. The value of L10 represents the life at which 10% of a large number of cones made of the tested material would be expected to have failed. The value of L50 relates in a corresponding manner to the failure of 50% of tested cones. The higher L10 and L50 value, the better the

The developed test method was successfully used for testing the fatigue life of components

The results from pitting tests for uncoated steel and steel coated with single and low-friction

**100Cr6 WC/C MoS2/Ti MoS2 TiN CrN**

Fig. 9. Results from pitting tests for 100Cr6 steel covered with thin, hard coating

with thin hard coatings deposited by PVD/CVD method and presented in.

a) b)

Fig. 6. Results from scuffing tests for lubricants, engineering materials and thin hard coatings: a) modified four-ball scuffing test, b) cone-three balls scuffing test

Fig. 7. Wear scars images on lower balls from scuffing tests for: a) steel-steel, b) CrN-CrN (four ball test, mineral base oil without lubricating additives)

Base oil + EP

Base oil +EP

coatings: a) modified four-ball scuffing test, b) cone-three balls scuffing test

**Scuffing load, Pt [N]**

Base oil + AW

Fig. 6. Results from scuffing tests for lubricants, engineering materials and thin hard

> Mineral base oil

(a) (b)

Fig. 7. Wear scars images on lower balls from scuffing tests for: a) steel-steel, b) CrN-CrN

(four ball test, mineral base oil without lubricating additives)

0

2000

4000

**Scuffing load, Pt [N]**

6000

8000

Base oil + AW

Mineral base oil Steel-steel WC/C-WC/C TiN-TiN CrN-CrN

Bearing steel Tool steel Nitrided steel Carburized steel WC/C

a)

b)

#### **2.2 Model method for evaluation of pitting wear in cone-three balls tribosystem**

The cone-three balls test method is generally based on IP 300 standard (Rolling contact fatigue tests for fluids in a modified four-ball machine). The main change is the geometry of the contact of the rolling elements. The upper ball was replaced with a special cone (Michalczewski & Piekoszewski, 2006). The cone can be made of any material. The conethree balls tribosystem is presented in Fig. 8.

Fig. 8. Cone-three balls tribosystem: a) scheme, b) photograph; 1- cone, 2 - balls, 3 – race

The tribosystem consists of a rotating cone (1) loaded against three balls (2) which are able to rotate in the race (3). The specimens are immersed in the tested lubricant. During the run the vibration level is monitored until pitting occurs.

The tested cones are made of the tested material. The test balls are made of 100Cr6 chrome alloy bearing steel. For each test the new set of balls should be used. According to the method the test conditions are 3924 N (400 kg) load and 1450 rpm top cone speed. 24 top cone failures are necessary to assess the performance of the lubricant and the material. The tested materials can be compared on the basis of L10 or L50 values as well as scatter factor K. The value of L10 represents the life at which 10% of a large number of cones made of the tested material would be expected to have failed. The value of L50 relates in a corresponding manner to the failure of 50% of tested cones. The higher L10 and L50 value, the better the resistance of the tested material to pitting is.

The developed test method was successfully used for testing the fatigue life of components with thin hard coatings deposited by PVD/CVD method and presented in.

The results from pitting tests for uncoated steel and steel coated with single and low-friction coatings are presented in Fig. 9.

Fig. 9. Results from pitting tests for 100Cr6 steel covered with thin, hard coating

The New Methods for Scuffing and Pitting Investigation

**2.3 T-02U Universal Four-Ball Testing Machine** 

computer-aided system of control and measurements.

of Coated Materials for Heavy Loaded, Lubricated Elements 313

The methods for evaluation of pitting and scuffing resistance of PVD/CVD coatings is realised by means of T-02U Universal Four-Ball Testing Machine (Michalczewski et al., 2009b). The photo of the machine is presented in Fig. 11. The tribotester is equipped with a

(a)

(b)

Fig. 11. T-02U Universal Four-Ball Testing Machine: a) photograph, b) computer screen

during data acquisition

The SEM images of wear on the test cone from pitting tests for 100Cr6 steel covered with WC/C coating are presented in Fig. 10.

(a)

Fig. 10. The pitting wear on the test cone: a) upper view, b) cross-section, c) enlargement of selected fragment (WC/C coated cone, RL-144/4 mineral oil)

The results indicate beneficial impact of low friction coatings on pitting wear (e.g. MoS2/Ti coating).

The presented method for testing pitting in cone-three balls tribosystem can be applied to testing fatigue wear of various materials, surface coatings as well as various lubricants. In comparison to other existing methods the new method gives better resolution and is timeand cost-effective.

#### **2.3 T-02U Universal Four-Ball Testing Machine**

312 Tribology - Lubricants and Lubrication

The SEM images of wear on the test cone from pitting tests for 100Cr6 steel covered with

(a)

Fig. 10. The pitting wear on the test cone: a) upper view, b) cross-section, c) enlargement of

The results indicate beneficial impact of low friction coatings on pitting wear (e.g. MoS2/Ti

The presented method for testing pitting in cone-three balls tribosystem can be applied to testing fatigue wear of various materials, surface coatings as well as various lubricants. In comparison to other existing methods the new method gives better resolution and is time-

(b) (c)

selected fragment (WC/C coated cone, RL-144/4 mineral oil)

coating).

and cost-effective.

WC/C coating are presented in Fig. 10.

The methods for evaluation of pitting and scuffing resistance of PVD/CVD coatings is realised by means of T-02U Universal Four-Ball Testing Machine (Michalczewski et al., 2009b). The photo of the machine is presented in Fig. 11. The tribotester is equipped with a computer-aided system of control and measurements.

Fig. 11. T-02U Universal Four-Ball Testing Machine: a) photograph, b) computer screen during data acquisition

The New Methods for Scuffing and Pitting Investigation

test rig manufactured by many producers.

10 1538 wide scuffing

lubricated with eco-oil (A/8.3/90 method)

**Failure load stage**

Hertzian stress at pitch point pmax

Load stage

vibration level and motor load during the test can be measured.

of Coated Materials for Heavy Loaded, Lubricated Elements 315

oil is heated up to 90°C. Loading of the gear teeth is raised in stages. During the running time of each load stage the oil temperature is allowed to rise freely. After load stage 4 the pinion gear teeth flanks are inspected for damage and any changes in tooth appearance are noted. The maximum load stage is 12. If the summed total width of the damaged areas on all the pinion gear teeth faces is estimated to equal or exceed one gear tooth width then this load stage should be taken as the failure load stage (FLS). Additionally the oil temperature,

The main advantage of the method is the possibility of scuffing testing of various materials, surface coatings as well as various lubricants intended for heavy-loaded friction joints. Furthermore the test can be realised by means of the worldwide popular back-to-back gear

 [MPa] uncoated steel a-C:H:W a-C:Cr a-C:H 4 621 light grooves light scars face polished none 5 773 light grooves light scars face polished none 6 927 light grooves light scars face polished none 7 1080 light grooves light scars face polished light scars 8 1232 grooves light scars face polished light scars 9 1386 scuffing strips light scars face polished light scars

areas light scars wide scuffing

11 1691 light scars numerous scars 12 1841 light scars numerous scars Table 1. The teeth failure at load stage for various DLC coatings (gears lubricated with eco-oil)

 **uncoated WC/C (a-C:H:W) DLC (a-C:Cr) DLC (a-C:H)**

Fig. 13. Failure load stages for uncoated steel gears and for teeth coated with DLC coatings

The type for tooth failure

areas light scars

A very wide range of lubricants can be tested using the T-02U Machine, e.g.: gear oils, hydraulic-gear oils, motor oils, eco-lubricants, non-toxic lubricants, new EP additives, cutting fluids, and greases. Many test methods described in international and national standards can be performed - ISO 20623, ASTM D 2783, D 2596, D 4172, D 2266, D 5183, DIN 51350, IP 239, IP 300, PN-76/C-04147. They concern the determination of the influence of the tested lubricants on scuffing, pitting, friction coefficient, and sliding wear, at ambient and elevated temperatures.

#### **3. Component methods and T-12U Universal Back-to-back Gear Test Rig for evaluation of scuffing resistance and rolling contact fatigue of PVD/CVD coated gears**

In research where high reliability is at stake, there is a tendency to use such test specimens that are similar to real machine components. The gear testing is incomparably more expensive and time consuming than tests carried out on simple specimens. But the main advantage is better reliability of the results obtained.

Concerning the most dangerous kinds of wear of gear wheels, two types can be specified: scuffing and pitting. These forms have been described previously in this study.

#### **3.1 Component method for evaluation of scuffing resistance of gears**

The test method for the evaluation of scuffing resistance of gears has been originally developed by FZG (Gear Research Centre) at the Technical University of Munich. This method was adapted for investigation of PVD/CVD coated gears at ITeE-PIB.

All test gears are case carburised, with HRC 60 to 62 surface hardness and case depth of 0.6 to 0.9 mm. "A" test gears are cross-Maag's ground, and their tips are especially shaped to achieve high sliding velocities, hence the tendency to scuffing. The tested PVD/CVD coating can be deposited on one or both gears – Fig. 12.

Fig. 12. Coated test gears used for testing scuffing - type A

The only limitation is the deposition temperature that should be below 180°C, which is connected with thermal stability of gear material.

Special coated gears (e.g. A20 type) are run in the test lubricant, at constant speed for a fixed time, in dip lubrication system. From load stage 4 the initial temperature is controlled. The

A very wide range of lubricants can be tested using the T-02U Machine, e.g.: gear oils, hydraulic-gear oils, motor oils, eco-lubricants, non-toxic lubricants, new EP additives, cutting fluids, and greases. Many test methods described in international and national standards can be performed - ISO 20623, ASTM D 2783, D 2596, D 4172, D 2266, D 5183, DIN 51350, IP 239, IP 300, PN-76/C-04147. They concern the determination of the influence of the tested lubricants on scuffing, pitting, friction coefficient, and sliding wear, at ambient and

**3. Component methods and T-12U Universal Back-to-back Gear Test Rig for evaluation of scuffing resistance and rolling contact fatigue of PVD/CVD** 

In research where high reliability is at stake, there is a tendency to use such test specimens that are similar to real machine components. The gear testing is incomparably more expensive and time consuming than tests carried out on simple specimens. But the main

Concerning the most dangerous kinds of wear of gear wheels, two types can be specified:

The test method for the evaluation of scuffing resistance of gears has been originally developed by FZG (Gear Research Centre) at the Technical University of Munich. This

All test gears are case carburised, with HRC 60 to 62 surface hardness and case depth of 0.6 to 0.9 mm. "A" test gears are cross-Maag's ground, and their tips are especially shaped to achieve high sliding velocities, hence the tendency to scuffing. The tested PVD/CVD

The only limitation is the deposition temperature that should be below 180°C, which is

Special coated gears (e.g. A20 type) are run in the test lubricant, at constant speed for a fixed time, in dip lubrication system. From load stage 4 the initial temperature is controlled. The

scuffing and pitting. These forms have been described previously in this study.

method was adapted for investigation of PVD/CVD coated gears at ITeE-PIB.

**3.1 Component method for evaluation of scuffing resistance of gears** 

elevated temperatures.

advantage is better reliability of the results obtained.

coating can be deposited on one or both gears – Fig. 12.

Fig. 12. Coated test gears used for testing scuffing - type A

connected with thermal stability of gear material.

**coated gears** 

oil is heated up to 90°C. Loading of the gear teeth is raised in stages. During the running time of each load stage the oil temperature is allowed to rise freely. After load stage 4 the pinion gear teeth flanks are inspected for damage and any changes in tooth appearance are noted. The maximum load stage is 12. If the summed total width of the damaged areas on all the pinion gear teeth faces is estimated to equal or exceed one gear tooth width then this load stage should be taken as the failure load stage (FLS). Additionally the oil temperature, vibration level and motor load during the test can be measured.

The main advantage of the method is the possibility of scuffing testing of various materials, surface coatings as well as various lubricants intended for heavy-loaded friction joints. Furthermore the test can be realised by means of the worldwide popular back-to-back gear test rig manufactured by many producers.


Table 1. The teeth failure at load stage for various DLC coatings (gears lubricated with eco-oil)

Fig. 13. Failure load stages for uncoated steel gears and for teeth coated with DLC coatings lubricated with eco-oil (A/8.3/90 method)

The New Methods for Scuffing and Pitting Investigation

in an FZG type gear test rig, using C-PT gears – Fig. 15.

Fig. 15. Coated test gears used for testing pitting – type C-PT

three valid runs are necessary to calculate the LC50 parameter.

is presented below.

gears was obtained – Fig. 16.

application of DLC coatings for gears.

of Coated Materials for Heavy Loaded, Lubricated Elements 317

Similarly to scuffing gear tests, the method for evaluation of pitting wear of gears has been originally developed by FZG (Gear Research Centre) in the Technical University of Munich. This method was also adapted for the investigation of PVD/CVD coated gears at ITeE-PIB. The experiments are performed using the single-stage pitting test procedure (PT C/10/90)

Special coated gears (C-PT type) are run in the lubricant test, at constant speed for a fixed time, in dip lubrication system. The load stage is 9 or 10 giving 302 Nm and 372 of torque respectively. The oil is heated up to 90°C. The oil temperature is controlled and kept at

The result of the tests is the LC50 fatigue life, related to 50% probability of failure. LC50 is defined as the number of load cycles when the damage area of the most damaged tooth flanks exceeds 4% (about 5 mm2). The total test time of each run is limited to 40 millions load cycles at pinion (300 operating hours). In some cases other criteria can be used. At least

The main advantage of the method is the possibility of comprehensive testing on various low-friction and antiwear PVD/CVD coatings intended for heavy-loaded machine elements. The method is realised by means of the worldwide popular back-to-back gear test rig. The test method has been successfully used for extensive research to determine the effect of low-friction and antiwear coatings on pitting wear. An example of the research on gear oils

The results indicate that for the coated/coated pair (pinion and wheel coated) and coated pinion/steel wheel pair a significant decrease in the fatigue life compared to the uncoated

The best results were obtained in the case of the steel pinion/W-DLC coated wheel – even fourfold increase in the fatigue life was observed. This shows a very high potential of the

Thanks to the component gear method for the evaluation of pitting wear of gears, it was possible to overcome the main factor hampering application of thin coatings on heavy loaded elements for many years i.e. their poor behaviour under cyclic stress conditions. This new method will allow for selection of low-friction and antiwear PVD/CVD coatings intended for manufacturing of steel heavy-loaded machine components. This will increase the service life of components and allow for the application of environmentally friendly oils.

This will increase the reliability of machines and reduce environmental pollution.

**3.2 Component gear method for evaluation of pitting wear of gears** 

constant level. The inspection of gears is performed every 7 or 14 hours.

The test method has been successfully used for extensive research to determine the effect of ecological gear oils on scuffing resistance of coated gears and for the selection of coating types for gear applications. An example of the research on gear oils is presented below.

The method has been applied for selecting a proper DLC coating for increasing the scuffing resistance of gears. The results from gear tests are presented in Table 1 and Fig. 13.

For uncoated gears lubricated eco-oil the 10th failure load stage only was achieved. The application of the coating (a-C:H:W or a-C:H) increased the scuffing resistance of gears. They passed the maximum 12th stage without scuffing. Only a-C:Cr coating did not improve the scuffing resistance of the tested gears.

The photographs of teeth surfaces after tests for tested DLC coatings (gears lubricated with eco-oil) are presented in Fig. 14.

Fig. 14. The photographs of teeth surfaces after tests for various DLC coatings (gears lubricated with eco-oil)

The presented component method for evaluation of scuffing resistance of gears have been applied for developing a new solution for manufacturing steel heavy-loaded machine components covered with low friction coatings that enables increase service life of components and allows lubricating with environmentally friendly oils. This will increase the reliability of machines and reduce pollution of the environment by oil.

The test method has been successfully used for extensive research to determine the effect of ecological gear oils on scuffing resistance of coated gears and for the selection of coating types for gear applications. An example of the research on gear oils is presented below. The method has been applied for selecting a proper DLC coating for increasing the scuffing

For uncoated gears lubricated eco-oil the 10th failure load stage only was achieved. The application of the coating (a-C:H:W or a-C:H) increased the scuffing resistance of gears. They passed the maximum 12th stage without scuffing. Only a-C:Cr coating did not improve

The photographs of teeth surfaces after tests for tested DLC coatings (gears lubricated with

resistance of gears. The results from gear tests are presented in Table 1 and Fig. 13.

Fig. 14. The photographs of teeth surfaces after tests for various DLC coatings (gears

reliability of machines and reduce pollution of the environment by oil.

The presented component method for evaluation of scuffing resistance of gears have been applied for developing a new solution for manufacturing steel heavy-loaded machine components covered with low friction coatings that enables increase service life of components and allows lubricating with environmentally friendly oils. This will increase the

the scuffing resistance of the tested gears.

eco-oil) are presented in Fig. 14.

WC/C (a-C:H:W)

DLC (a-C:Cr)

DLC (a-C:H)

lubricated with eco-oil)

#### **3.2 Component gear method for evaluation of pitting wear of gears**

Similarly to scuffing gear tests, the method for evaluation of pitting wear of gears has been originally developed by FZG (Gear Research Centre) in the Technical University of Munich. This method was also adapted for the investigation of PVD/CVD coated gears at ITeE-PIB.

The experiments are performed using the single-stage pitting test procedure (PT C/10/90) in an FZG type gear test rig, using C-PT gears – Fig. 15.

Special coated gears (C-PT type) are run in the lubricant test, at constant speed for a fixed time, in dip lubrication system. The load stage is 9 or 10 giving 302 Nm and 372 of torque respectively. The oil is heated up to 90°C. The oil temperature is controlled and kept at constant level. The inspection of gears is performed every 7 or 14 hours.

Fig. 15. Coated test gears used for testing pitting – type C-PT

The result of the tests is the LC50 fatigue life, related to 50% probability of failure. LC50 is defined as the number of load cycles when the damage area of the most damaged tooth flanks exceeds 4% (about 5 mm2). The total test time of each run is limited to 40 millions load cycles at pinion (300 operating hours). In some cases other criteria can be used. At least three valid runs are necessary to calculate the LC50 parameter.

The main advantage of the method is the possibility of comprehensive testing on various low-friction and antiwear PVD/CVD coatings intended for heavy-loaded machine elements. The method is realised by means of the worldwide popular back-to-back gear test rig.

The test method has been successfully used for extensive research to determine the effect of low-friction and antiwear coatings on pitting wear. An example of the research on gear oils is presented below.

The results indicate that for the coated/coated pair (pinion and wheel coated) and coated pinion/steel wheel pair a significant decrease in the fatigue life compared to the uncoated gears was obtained – Fig. 16.

The best results were obtained in the case of the steel pinion/W-DLC coated wheel – even fourfold increase in the fatigue life was observed. This shows a very high potential of the application of DLC coatings for gears.

Thanks to the component gear method for the evaluation of pitting wear of gears, it was possible to overcome the main factor hampering application of thin coatings on heavy loaded elements for many years i.e. their poor behaviour under cyclic stress conditions. This new method will allow for selection of low-friction and antiwear PVD/CVD coatings intended for manufacturing of steel heavy-loaded machine components. This will increase the service life of components and allow for the application of environmentally friendly oils. This will increase the reliability of machines and reduce environmental pollution.

The New Methods for Scuffing and Pitting Investigation

deposition of low-friction coatings.

institutes, technical universities).

of toxic lubricating additives have been obtained.

testing spur gears and rolling bearings under extreme conditions.

**4. Conclusion** 

**5. References** 

358, ISSN 1736-6038

0-8247-4873-5, New York-Basel

205–215, ISSN 0208-7774

of Coated Materials for Heavy Loaded, Lubricated Elements 319

performed - ISO 14635-1, 14635-2, 14635-3, CEC L-07-A-95, L-84-02, DIN 51354, IP 334, ASTM D 5182, D 4998, PN-78/C-04169, FVA information sheets: 2/IV (1997), 54/7 (1993), 243 (2000). For the last few years, the T-12U Rig has been successfully used at ITeE-PIB for the extensive research to determine an effect of modern gear oils (including ecological oils) on different forms of gear tooth wear, as well as possibility of improving the gear life by the

Presented methods give the possibility of comprehensive testing on various low-friction and antiwear PVD/CVD coatings intended for machine elements. All the presented methods and both tribotesters i.e. T-02U Universal Four-Ball Testing Machine, T-12U Universal Backto-back Gear Test Rig have been implemented at the Tribology Laboratory of ITeE-PIB and successfully verified. They are employed to perform various kinds of projects e.g. grants, R&D projects, ordered by the Polish government and international projects (COST Actions, 6th EU Framework Programme). They are also used to realise research orders from Polish industry (especially small and medium size enterprises) and the scientific sector (research

The new methods exhibit very good resolution and precision comparable to standardised test methods and are time and cost effective. Furthermore the cone-three ball method gives the possibility of testing fatigue wear of any coating and substrate material. Basing on the elaborated methods the optimal selection and development of PVD/CVD technologies applied for extension of the life of the heavy-loaded friction joints as well as the elimination

The further development of tribological devices is performed in the frame of Strategic Programme "Innovative Systems of Technical Support for Sustainable Development of Economy," which is currently realised at the Institute for Sustainable Technologies-National Research Institute (ITeE-PIB) in Radom, in Poland. The Programme is realised within the framework of the Innovative Economy Operational Programme co-funded from European structural funds. The greatest emphasis is put on the development of advanced machines for

Antonov, M., Michalczewski, R., Pasaribu, R. & Piekoszewski W. (2009). Assessment of the

Burakowski, T.; Szczerek, M. & Tuszynski, W. (2004). Scuffing and seizure - characterization

Libera, M., Piekoszewski, W. & Waligóra W. (2005). The influence of operational conditions

potential of lubricated contact conditions laboratory testing and surface analysis for improving the performance of machine elements. Comparision of model and real components test methods. *Estonian Journal of Engineering*, Vol. 15. No. 4, pp. 349-

and investigation, In: Mechanical tribology. Materials, characterization, and applications, Totten, G.E. & Liang, H., (Ed.), pp. 185-234, Marcel Dekker, Inc., ISBN

of rolling bearings elements on surface fatigue scatter. *Tribologia*. 2005, No. 3, pp.

#### **Gear material combination (pinion/wheel)**

Fig. 16. Fatigue life LC50 for various pinion/wheel gear material

#### **3.3 T-12U Universal Back-to-back Gear Test Rig**

The T-12U Universal Back-to-back Gear Test Rig makes it possible to investigate both aforementioned forms of wear. The photo of the tester is presented in Fig. 17.

Fig. 17. T-12U Universal Back-to-back Gear Test Rig

The tribotester is equipped with a microprocessor-aided controller and as an option, it may also be equipped with a computer-aided measuring system.

A very wide range of lubricants can be tested using the T-12U Test Rig, e.g.: gear oils, hydraulic-gear oils, eco-oils, non-toxic oils, and new EP additives. What is more, there is a possibility of testing modern engineering materials and surface coatings intended for gear manufacturing. Many test methods described in international and national standards can be performed - ISO 14635-1, 14635-2, 14635-3, CEC L-07-A-95, L-84-02, DIN 51354, IP 334, ASTM D 5182, D 4998, PN-78/C-04169, FVA information sheets: 2/IV (1997), 54/7 (1993), 243 (2000). For the last few years, the T-12U Rig has been successfully used at ITeE-PIB for the extensive research to determine an effect of modern gear oils (including ecological oils) on different forms of gear tooth wear, as well as possibility of improving the gear life by the deposition of low-friction coatings.

#### **4. Conclusion**

318 Tribology - Lubricants and Lubrication

**steel / steel WC/C / steel WC/C / WC/C steel / WC/C**

**Gear material combination (pinion/wheel)** 

The T-12U Universal Back-to-back Gear Test Rig makes it possible to investigate both

The tribotester is equipped with a microprocessor-aided controller and as an option, it may

A very wide range of lubricants can be tested using the T-12U Test Rig, e.g.: gear oils, hydraulic-gear oils, eco-oils, non-toxic oils, and new EP additives. What is more, there is a possibility of testing modern engineering materials and surface coatings intended for gear manufacturing. Many test methods described in international and national standards can be

aforementioned forms of wear. The photo of the tester is presented in Fig. 17.

Fig. 16. Fatigue life LC50 for various pinion/wheel gear material

**3.3 T-12U Universal Back-to-back Gear Test Rig** 

Fig. 17. T-12U Universal Back-to-back Gear Test Rig

also be equipped with a computer-aided measuring system.

**LC50 [million cycles]**

Presented methods give the possibility of comprehensive testing on various low-friction and antiwear PVD/CVD coatings intended for machine elements. All the presented methods and both tribotesters i.e. T-02U Universal Four-Ball Testing Machine, T-12U Universal Backto-back Gear Test Rig have been implemented at the Tribology Laboratory of ITeE-PIB and successfully verified. They are employed to perform various kinds of projects e.g. grants, R&D projects, ordered by the Polish government and international projects (COST Actions, 6th EU Framework Programme). They are also used to realise research orders from Polish industry (especially small and medium size enterprises) and the scientific sector (research institutes, technical universities).

The new methods exhibit very good resolution and precision comparable to standardised test methods and are time and cost effective. Furthermore the cone-three ball method gives the possibility of testing fatigue wear of any coating and substrate material. Basing on the elaborated methods the optimal selection and development of PVD/CVD technologies applied for extension of the life of the heavy-loaded friction joints as well as the elimination of toxic lubricating additives have been obtained.

The further development of tribological devices is performed in the frame of Strategic Programme "Innovative Systems of Technical Support for Sustainable Development of Economy," which is currently realised at the Institute for Sustainable Technologies-National Research Institute (ITeE-PIB) in Radom, in Poland. The Programme is realised within the framework of the Innovative Economy Operational Programme co-funded from European structural funds. The greatest emphasis is put on the development of advanced machines for testing spur gears and rolling bearings under extreme conditions.

#### **5. References**


Michalczewski, R. (2008). Chemomechanical synergy of PVD/CVD coatings and

Michalczewski, R. & Piekoszewski, W. (2006). The method for assessment of rolling contact

Michalczewski, R., Piekoszewski, W., Szczerek, M. & Tuszynski W. (2009a) The lubricant-

Michalczewski, R., Piekoszewski, W., Szczerek, M., Tuszyński, W. & Wulczyński, J. (2010).

Michalczewski, R., Szczerek, M., Tuszynski, W. & Wulczyński, J. (2009b). A four-ball

Piekoszewski, W.; Szczerek, M. & Tuszynski, W. (2001). The action of lubricants under

Szczerek, M. (1996) Metodologiczne problemy systematyzacji eksperymentalnych badañ

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### *Edited by Chang-Hung Kuo*

In the past decades, significant advances in tribology have been made as engineers strive to develop more reliable and high performance products. The advancements are mainly driven by the evolution of computational techniques and experimental characterization that leads to a thorough understanding of tribological process on both macro- and microscales. The purpose of this book is to present recent progress of researchers on the hydrodynamic lubrication analysis and the lubrication tests for biodegradable lubricants.

Tribology - Lubricants and Lubrication

Tribology

Lubricants and Lubrication

*Edited by Chang-Hung Kuo*

Photo by Okea / iStock