**2. Polymers, and materials with polymers for tribological applications**

**Table 1** presents the polymers used in tribological applications, several features and usual components made of them.


**Table 1.** *Tribological characteristics of thermoplastic polymers [5–8, 20].*

Semi-crystalline polymers can be used even above their glass transition temperature (Tg), another added advantage against chemical constancy.

What do the engineers want from polymeric materials when introducing in tribological applications? A set of characteristics including thermal, mechanical and

• higher softening temperatures, sometimes obtained by adding short glass

• higher toughness; reinforcement could rise the flexural modulus till 11,000 MPa, a value that is overpass only by PPS in the thermoplastic

• good strength al negative temperature, including impact resistance;

• good ability for compounding (mixing), when adding materials for

reinforcement, solid lubricants, anti-ignition agents etc.,

• chemical resistance at fluids circulated in application (as lubricant or/and

• good processing capability (uniform flow, fast solidification and acceptably

Based on important works on tribology of polymer-based materials [3, 20, 23–25]. **Figure 4** presents a classification of adding materials taking into account the function of these materials in polymers. Generally, reinforcements [24–27] and solid lubricants in polymer-based materials improve their tribological behavior, but it is not a rule and the new recipes should be tested at laboratory scale and then the designed components at actual scale and under functioning conditions. Some solid lubricants, especially with sheet-like aspects (graphite, graphite, sulphides etc.) weaken the bulk materials as they reduce the superficial energy, but the mechanical properties are diminishing. Reinforcements in polymers make their resistance greater, but generate a more intense abrasive wear on the counterpart surface and

• low or acceptable friction and high wear resistance;

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

• no or very less liquid absorption (including water)

• good dimensional stability; low thermal expansion;

low cost and improvement by treatment).

*A classification of materials in polymeric-based materials.*

tribological ones:

fibers;

polymers;

environment);

**Figure 4.**

**69**

Various inorganic nanofillers, e.g., from metals (Cu, Fe), metallic and nonmetallic oxides (CuO, ZnO, TiO2, ZrO2, SiO2) and salts as silicon nitride (Si3N4), have been proved to not only enhancing mechanical properties, but also to lowering the friction coefficient and the rate of wear under various sliding circumstances. In particular, PEEK, PPS, and PTFE are the most widely studied polymers for different tribological applications and they are often blended with TiO2, SiC, Si3N4, and carbon fiber fillers [19]. Nevertheless, it is also noted that there are no single or combined polymers or fillers that provide the best tribological performance in all conditions. Being a result of "system responses", friction and wear always depend on both the intrinsic material properties and the external environmental conditions. The beneficial effect of adding a certain material in a polymeric matrix is exemplified by tests did by Kurdi et al. [21] (**Figure 3**), 5–15% of TiO2 reducing friction and wear at room temperature, but not at elevated temperature. Thus, functioning conditions are tremendously important when selecting a pair of materials for a good or at least acceptable tribological behavior.

Hanchi et al. [13] reported results on friction and wear under dry sliding of injection molded blends of PEEK and PEI, at temperatures from 20–232°C, on a pinon-disk tribotester. It was found that tan δ peaks corresponding to α transitions occurring in the vicinity of the glass transition temperature (Tg) coincided with catastrophic tribological failure in the case of PEI and the amorphous PEEK/PEI blends. PEEK and the annealed 70% PEEK/30% PEI blend exhibited marked increases in friction and wear above the Tg. The absence of catastrophic tribological failure in PEEK and the annealed 70/30 blend in the vicinity of Tg corresponded to a transition of significantly lower strength those observed in PEI and the amorphous blends. Between 90°C and 105°C for PEI and 45°C and 70°C for the PEEK/PEI 50/50 blend, severe to mild friction and wear transitions were observed. It appeared that a substantial change in ductility associated with these β transitions resulted in the transitional tribological behavior.

Unal et al. reported the influence of test speed and load values on the friction and wear behavior of PTFE, POM and PEI, on a pin-on-disc tribotester. Tests were carried out at room temperature, under 5 N, 10 N and 15N and at 0,5 m/s, 0,75 m/s and 1m/s. The specific wear rates were deduced from mass loss. The results showed that, for all tested polymers, the coefficient of friction increases linearly with the increase in load. For the load and speed range of this investigation, the wear rate showed very low sensitivity to the applied load and large sensitivity to speed, particularly at high load values [22].

#### **Figure 3.**

*Influence of percentage of TiO2 on (a) friction coefficient and (b) specific wear rate, for a pin-on-disk configuration, in sliding at v = 0.1 m/s, average pressure p = 1 MPa, for 2 hours [21].*

Semi-crystalline polymers can be used even above their glass transition temper-

Various inorganic nanofillers, e.g., from metals (Cu, Fe), metallic and nonmetallic oxides (CuO, ZnO, TiO2, ZrO2, SiO2) and salts as silicon nitride (Si3N4), have been proved to not only enhancing mechanical properties, but also to lowering the friction coefficient and the rate of wear under various sliding circumstances. In particular, PEEK, PPS, and PTFE are the most widely studied polymers for different tribological applications and they are often blended with TiO2, SiC, Si3N4, and carbon fiber fillers [19]. Nevertheless, it is also noted that there are no single or combined polymers or fillers that provide the best tribological performance in all conditions. Being a result of "system responses", friction and wear always depend on both the intrinsic material properties and the external environmental conditions. The beneficial effect of adding a certain material in a polymeric matrix is exemplified by tests did by Kurdi et al. [21] (**Figure 3**), 5–15% of TiO2 reducing friction and wear at room temperature, but not at elevated temperature. Thus, functioning conditions are tremendously important when selecting a pair of materials for a good

Hanchi et al. [13] reported results on friction and wear under dry sliding of injection molded blends of PEEK and PEI, at temperatures from 20–232°C, on a pinon-disk tribotester. It was found that tan δ peaks corresponding to α transitions occurring in the vicinity of the glass transition temperature (Tg) coincided with catastrophic tribological failure in the case of PEI and the amorphous PEEK/PEI blends. PEEK and the annealed 70% PEEK/30% PEI blend exhibited marked increases in friction and wear above the Tg. The absence of catastrophic tribological failure in PEEK and the annealed 70/30 blend in the vicinity of Tg corresponded to a transition of significantly lower strength those observed in PEI and the amorphous blends. Between 90°C and 105°C for PEI and 45°C and 70°C for the PEEK/PEI 50/50 blend, severe to mild friction and wear transitions were observed. It appeared that a substantial change in ductility associated with these β transitions resulted in the

Unal et al. reported the influence of test speed and load values on the friction and wear behavior of PTFE, POM and PEI, on a pin-on-disc tribotester. Tests were carried out at room temperature, under 5 N, 10 N and 15N and at 0,5 m/s, 0,75 m/s and 1m/s. The specific wear rates were deduced from mass loss. The results showed that, for all tested polymers, the coefficient of friction increases linearly with the increase in load. For the load and speed range of this investigation, the wear rate showed very low sensitivity to the applied load and large sensitivity to speed,

*Influence of percentage of TiO2 on (a) friction coefficient and (b) specific wear rate, for a pin-on-disk*

*configuration, in sliding at v = 0.1 m/s, average pressure p = 1 MPa, for 2 hours [21].*

ature (Tg), another added advantage against chemical constancy.

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

or at least acceptable tribological behavior.

transitional tribological behavior.

particularly at high load values [22].

**Figure 3.**

**68**

What do the engineers want from polymeric materials when introducing in tribological applications? A set of characteristics including thermal, mechanical and tribological ones:


Based on important works on tribology of polymer-based materials [3, 20, 23–25]. **Figure 4** presents a classification of adding materials taking into account the function of these materials in polymers. Generally, reinforcements [24–27] and solid lubricants in polymer-based materials improve their tribological behavior, but it is not a rule and the new recipes should be tested at laboratory scale and then the designed components at actual scale and under functioning conditions. Some solid lubricants, especially with sheet-like aspects (graphite, graphite, sulphides etc.) weaken the bulk materials as they reduce the superficial energy, but the mechanical properties are diminishing. Reinforcements in polymers make their resistance greater, but generate a more intense abrasive wear on the counterpart surface and

#### **Figure 4.**

*A classification of materials in polymeric-based materials.*

the friction coefficient is higher and the surface quality of both rubbing surfaces becomes worse. Reinforcements reduce or even damage the protective transfer films [28]. They could generate a sliding regime characterized by severe and third body wear [29]. For instance, the composite PA + 50% glass beads [11] exhibit a third body friction and wear, especially at low velocity (see **Figures 35b** and **c**).

**Figure 4** points out that adding materials in polymers have different roles (sometimes, they could act in two or more directions) and the influence of the set added in the basic polymer could be synergic [30], difficult to enclose in formula, thus, testing is a necessity. **Figure 5** presents several reinforcements: a) micros glass beads with large dispersion of the bead radius (this is a favorable aspect because this large distribution allow for the small beads to fix the matrix next the bigger ones and wear is considerably reduced), b) short glass fibers with diameters of 8 … 20 μm and length of hundred microns [31] (similarly, carbon fibers are added in 10 … 20% wt), c) aramid fibers [16, 32] (more flexible and with nail-shape ends that help them to fix the polymer matrix).

Harder polymers and polymer composites with hard components are helped to reduce friction by adding solid lubricants with plaquette-like shape (graphite [33], graphene, disulphides [33], several examples being given in **Figure 6**) or polymers as PTFE, with more uniform transfer and having very low friction coefficient.

Tribologists considers that short fibers are more beneficial for tribological application, but recently, the polymers with long fibers were also introduced as materials for moving parts due to the advances in fibers and polymer technology. There is a short discussion about fiber architecture. Usually, short and tangle fibers are randomly organized in the material, they rarely could be oriented, but the cost will increase. Long fibers could be organized in woven, unidirectional, multi-axial,

**Figure 5.**

*Aspect of reinforcements in polymers. (a) Glass beads used in [11, 15]. (b) Short glass fibers from LANXESS [31]. (c) Short aramid fibers Twaron [16].*

depending on the other requirements besides the tribological one. Being organized, the wear of materials of long fibers is usually in steps, characterizing the damage of each layer of fibers. As fibers could have 5 to 50 microns, the wear of the first layers or two ones will end the life of the triboelement. The nature of the fibers is natural, synthetic or combination. For tribological application, there are used carbon fibers, carbon nanotubes, glass fibers (if short, from tens microns to hundreds of millimeters but more efficient being those of several hundred microns to several millimeters), polymer fibers, more recently, aramid fibers [16]. Particles as reinforcement could have different shapes, from almost spherical (as for glass beads) to sheet-like or plaquettes (one dimension being very small as compared to the other two). A particular aspect of wearing polymeric composites or blends is the initially preferential wear of the softer material, the result being an increase concentration of harder particles or fibers; then the counterpart body will "attack" these harder materials; they could be fragmented and embedded into the soft matrix or they are torn off becoming wear debris, "traveling" in contact and induces oscillations of friction coefficient, but when their concentration increases, the component of abrasive wear becomes dominant and wear is greater; when the tribolayer loses its hard particles, the cycle is repeating. Thus, wear is a dynamic process, in steps,

**Figure 7** presents a process of consolidation of the tribolayers by embedding the fragments broken from short glass fibers a) PTFE +25% glass fibers, water lubrication, partial bearing (Ø60 mm, 30 mm width) and steel shaft: some glass fibers within the superficial layer cannot bear the local load and were broken; the frag-

Sometimes, adding materials in polymers could worsen the tribological behavior. For instance, too much concentration of glass fibers increases both and friction

coefficient and wear (especially abrasive wear on both surfaces in contact). A relation between mechanical characteristics in tensile tests and tribological one could be triky. Tensile strength could be improved by adding reinforcements, but strain at break is usually decreased. In sliding contact, a deformability ensures the contact conformability and in fluid lubrication helps generating the fluid film. But, even from 1979, Evans and Lancaster [35] reported that fibers in polymers have beneficial effects on wear and only rarely worsen this parameter. Some adding materials could have the role of a reinforcement but also could help for heat evacuating. A greater interest in using polymer composites and blends pointed out that the designer of the material has to do compromises that have to be accepted only by experimental results, models for predicting tribological behavior being difficult to

*Consolidation of the soft polymer matrix by glass fiber fragments, water lubrication, composites PTFE+glass fibers, large contact, partial bearing (120°) (Ø 60 mm x 30 mm width) [9]. (a) PTFE+15% glass fibers.*

depending on local concentration of material constituents [9].

ments are embedded into the PTFE matrix [9].

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

establish in quantities [36].

*(b) Detail of (a). (c) PTFE+25% glass fibers.*

**Figure 7.**

**71**

#### **Figure 6.**

*Aspect of several solid lubricants introduced in polymers. Graphite [33]. Hexagonal plates of WS2 [34]. Hexagonal boron nitride (h-BN) [33].*

### *Tribological Behavior of Polymers and Polymer Composites DOI: http://dx.doi.org/10.5772/intechopen.94264*

the friction coefficient is higher and the surface quality of both rubbing surfaces becomes worse. Reinforcements reduce or even damage the protective transfer films [28]. They could generate a sliding regime characterized by severe and third body wear [29]. For instance, the composite PA + 50% glass beads [11] exhibit a third body friction and wear, especially at low velocity (see **Figures 35b** and **c**). **Figure 4** points out that adding materials in polymers have different roles (sometimes, they could act in two or more directions) and the influence of the set added in the basic polymer could be synergic [30], difficult to enclose in formula, thus, testing is a necessity. **Figure 5** presents several reinforcements: a) micros glass beads with large dispersion of the bead radius (this is a favorable aspect because this large distribution allow for the small beads to fix the matrix next the bigger ones and wear is considerably reduced), b) short glass fibers with diameters of 8 … 20 μm and length of hundred microns [31] (similarly, carbon fibers are added in 10 … 20% wt), c) aramid fibers [16, 32] (more flexible and with nail-shape ends that help

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

Harder polymers and polymer composites with hard components are helped to reduce friction by adding solid lubricants with plaquette-like shape (graphite [33], graphene, disulphides [33], several examples being given in **Figure 6**) or polymers as PTFE, with more uniform transfer and having very low friction coefficient.

Tribologists considers that short fibers are more beneficial for tribological application, but recently, the polymers with long fibers were also introduced as materials for moving parts due to the advances in fibers and polymer technology. There is a short discussion about fiber architecture. Usually, short and tangle fibers are randomly organized in the material, they rarely could be oriented, but the cost will increase. Long fibers could be organized in woven, unidirectional, multi-axial,

*Aspect of reinforcements in polymers. (a) Glass beads used in [11, 15]. (b) Short glass fibers from LANXESS*

*Aspect of several solid lubricants introduced in polymers. Graphite [33]. Hexagonal plates of WS2 [34].*

them to fix the polymer matrix).

*[31]. (c) Short aramid fibers Twaron [16].*

*Hexagonal boron nitride (h-BN) [33].*

**Figure 5.**

**Figure 6.**

**70**

depending on the other requirements besides the tribological one. Being organized, the wear of materials of long fibers is usually in steps, characterizing the damage of each layer of fibers. As fibers could have 5 to 50 microns, the wear of the first layers or two ones will end the life of the triboelement. The nature of the fibers is natural, synthetic or combination. For tribological application, there are used carbon fibers, carbon nanotubes, glass fibers (if short, from tens microns to hundreds of millimeters but more efficient being those of several hundred microns to several millimeters), polymer fibers, more recently, aramid fibers [16]. Particles as reinforcement could have different shapes, from almost spherical (as for glass beads) to sheet-like or plaquettes (one dimension being very small as compared to the other two). A particular aspect of wearing polymeric composites or blends is the initially preferential wear of the softer material, the result being an increase concentration of harder particles or fibers; then the counterpart body will "attack" these harder materials; they could be fragmented and embedded into the soft matrix or they are torn off becoming wear debris, "traveling" in contact and induces oscillations of friction coefficient, but when their concentration increases, the component of abrasive wear becomes dominant and wear is greater; when the tribolayer loses its hard particles, the cycle is repeating. Thus, wear is a dynamic process, in steps, depending on local concentration of material constituents [9].

**Figure 7** presents a process of consolidation of the tribolayers by embedding the fragments broken from short glass fibers a) PTFE +25% glass fibers, water lubrication, partial bearing (Ø60 mm, 30 mm width) and steel shaft: some glass fibers within the superficial layer cannot bear the local load and were broken; the fragments are embedded into the PTFE matrix [9].

Sometimes, adding materials in polymers could worsen the tribological behavior. For instance, too much concentration of glass fibers increases both and friction coefficient and wear (especially abrasive wear on both surfaces in contact). A relation between mechanical characteristics in tensile tests and tribological one could be triky. Tensile strength could be improved by adding reinforcements, but strain at break is usually decreased. In sliding contact, a deformability ensures the contact conformability and in fluid lubrication helps generating the fluid film. But, even from 1979, Evans and Lancaster [35] reported that fibers in polymers have beneficial effects on wear and only rarely worsen this parameter. Some adding materials could have the role of a reinforcement but also could help for heat evacuating. A greater interest in using polymer composites and blends pointed out that the designer of the material has to do compromises that have to be accepted only by experimental results, models for predicting tribological behavior being difficult to establish in quantities [36].

#### **Figure 7.**

*Consolidation of the soft polymer matrix by glass fiber fragments, water lubrication, composites PTFE+glass fibers, large contact, partial bearing (120°) (Ø 60 mm x 30 mm width) [9]. (a) PTFE+15% glass fibers. (b) Detail of (a). (c) PTFE+25% glass fibers.*

The addition of short carbon fibers (SCF) in a concentration from 5% to 20 vol% can improve the wear resistance of neat PEI remarkably, especially at high temperature and under high working pv-factor. The increased test temperature from room temperature to 150°C leads to a seven times increase in the wear rate of neat PEI and five times for the composites. SCF/PEI can withstand much higher pv-factor than that of neat PEI. When the pv-factor increased from 1 to 9 MPa m/s, the timerelated wear rate of SCF/PEI almost linearly enhanced from 1.5 <sup>10</sup><sup>3</sup> to <sup>7</sup> <sup>10</sup><sup>2</sup> m/h. However, the wear rate of neat PEI increased from 0.214 to 3.42 m/h when the pv-factor was only increased from 0.25 to 3 MPa m/s. The micrographs of the worn counterface and specimens indicated that the sliding of neat PEI against metal counterface did not form a transfer film, and wear mechanisms varied from fatigue wear to plastic plowing at the increased temperatures. The presence of short carbon fibers helped generating transfer films both on the counterface and worn surface of specimens. The transfer film became more continuous with the increased test temperature. The composite wear was mainly undertaken by fibers [37].

• family of tested pairs of materials

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

*Tribological Behavior of Polymers and Polymer Composites*

implants, but for testing plastics there is.

parameters.

sliding motion.

microtribology measurements.

"severe" and "mild".

A logical order will be.

**Figure 8.**

**73**

• the actual working conditions

In the ISO standard collection, the word wear is mentioned in 118 items, the test methology being adapted to the application, as, for instance, road and tire wear,

ISO 7148-2:2012 Plain bearings — Testing of the tribological behavior of bearing

ISO 6601:2002. Plastics — Friction and wear by sliding — Identification of test

ISO 20329 Plastics — Determination of abrasive wear by reciprocating linear

ISO/TR 11811:2012 Nanotechnologies — Guidance on methods for nano- and

The selection of tests necessary for assessing tribological behavior of a material

• the research level (laboratory, application under development, design of new

• the characteristics of the tribosystems, distinct regimes of sliding wear are

Many different approaches could be seen in literature for assessing the tribological behavior of a system, differentiate in scale and complexity of the tested system.

In the same direction there are increasing complexity and costs, but first types of

Laboratory tests ! Model tests! Component bench tests ! System bench

Depending of the novelty degree of the solution, one or more of the stages mentioned above could be omitted. New materials and original design solutions ask

for all, but they have to be solved quite rapidly in order to gain the market.

ISO 9352:2012 Plastics — Determination of resistance to wear by abrasive wheels. ISO/DIS 7148–2 Plain bearings — Testing of the tribological behavior of bearing

materials — Part 2: Testing of polymer-based bearing materials.

materials — Part 2: Testing of polymer-based bearing materials.

pair including polymer-based materials depends on

tests! Machine bench test! Machine field test [41, 42].

tests have increasing control and scale investigation and flexibility.

*Characteristics and relevant parameters for the tribotester block-on-ring [15, 16].*

materials, failure investigation),

Even if the process of wearing the polymeric composites comprises same stages, the aspect, dimensions and the concentration of added materials make the aspects of worn tribolayers very different. When sliding two bodies one against the other, the matrix is more deformable and the adding materials are like pebbles in the bottom of a shallow river. A partial detaching between matrix and particles/fibers could happen, the fibers change their position and the particles could roll or be dragged on the surface. The space left behind the hard element accumulate fine wear debris from both bodies or even from lubricant (when lubricating), stiffening the tribolayer. The random position of the hard materials and their agglomeration by wearing the soft material increase the probability of detaching conglomerates. This is why an optimum concentration of hard reinforcement in polymer-based material is around 15 … 25% and depends on the nature of reinforcement. For instance, 20 … 25% wt is an optimum in PTFE [9, 38, 39], but short aramid fibers are usually added at 10% wt due to the difficulty of injection molding as they block the injection nozzle [16]. As for particles with similar dimensions in all directions Georgescu [15] and Maftei [11] proved that 20% is the optimum concentration for glass beads of micron size.

If one analyzes the soft phases introduces in polymer-based materials, usually a solid lubricant, and with particular reference to PTFE [40] as solid lubricant, this concentration varies from 5 to 15% wt depending on the nature of the involved material. In PBT, the best concentration of PTFE was 5 … 10% the preferred criterion being the wear rate of the polymeric blend on steel [15].

### **3. Testing rigs, standard and non-standard testing methodologies**

Laboratory tests, on simplified specimens, are useful for ranking materials, but these results could not be extrapolated to actual component, especially for polymeric materials.

Test campaign has to answer how the material pair behaves in a series of parameters


*Tribological Behavior of Polymers and Polymer Composites DOI: http://dx.doi.org/10.5772/intechopen.94264*

• family of tested pairs of materials

The addition of short carbon fibers (SCF) in a concentration from 5% to 20 vol% can improve the wear resistance of neat PEI remarkably, especially at high temperature and under high working pv-factor. The increased test temperature from room temperature to 150°C leads to a seven times increase in the wear rate of neat PEI and five times for the composites. SCF/PEI can withstand much higher pv-factor than that of neat PEI. When the pv-factor increased from 1 to 9 MPa m/s, the time-

<sup>7</sup> <sup>10</sup><sup>2</sup> m/h. However, the wear rate of neat PEI increased from 0.214 to 3.42 m/h when the pv-factor was only increased from 0.25 to 3 MPa m/s. The micrographs of the worn counterface and specimens indicated that the sliding of neat PEI against metal counterface did not form a transfer film, and wear mechanisms varied from fatigue wear to plastic plowing at the increased temperatures. The presence of short carbon fibers helped generating transfer films both on the counterface and worn surface of specimens. The transfer film became more continuous with the increased test temperature. The composite wear was mainly undertaken by

Even if the process of wearing the polymeric composites comprises same stages, the aspect, dimensions and the concentration of added materials make the aspects of worn tribolayers very different. When sliding two bodies one against the other, the matrix is more deformable and the adding materials are like pebbles in the bottom of a shallow river. A partial detaching between matrix and particles/fibers could happen, the fibers change their position and the particles could roll or be dragged on the surface. The space left behind the hard element accumulate fine wear debris from both bodies or even from lubricant (when lubricating), stiffening the tribolayer. The random position of the hard materials and their agglomeration by wearing the soft material increase the probability of detaching conglomerates. This is why an optimum concentration of hard reinforcement in polymer-based material is around 15 … 25% and depends on the nature of reinforcement. For instance, 20 … 25% wt is an optimum in PTFE [9, 38, 39], but short aramid fibers are usually added at 10% wt due to the difficulty of injection molding as they block the injection nozzle [16]. As for particles with similar dimensions in all directions Georgescu [15] and Maftei [11] proved that 20% is the optimum concentration for

If one analyzes the soft phases introduces in polymer-based materials, usually a solid lubricant, and with particular reference to PTFE [40] as solid lubricant, this concentration varies from 5 to 15% wt depending on the nature of the involved material. In PBT, the best concentration of PTFE was 5 … 10% the preferred

Laboratory tests, on simplified specimens, are useful for ranking materials, but these results could not be extrapolated to actual component, especially for poly-

criterion being the wear rate of the polymeric blend on steel [15].

**3. Testing rigs, standard and non-standard testing methodologies**

Test campaign has to answer how the material pair behaves in a series of

related wear rate of SCF/PEI almost linearly enhanced from 1.5 <sup>10</sup><sup>3</sup> to

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

fibers [37].

glass beads of micron size.

meric materials.

• lubrication regime,

• working regime (load, speed etc.),

• environment

parameters

**72**

In the ISO standard collection, the word wear is mentioned in 118 items, the test methology being adapted to the application, as, for instance, road and tire wear, implants, but for testing plastics there is.

ISO 7148-2:2012 Plain bearings — Testing of the tribological behavior of bearing materials — Part 2: Testing of polymer-based bearing materials.

ISO 6601:2002. Plastics — Friction and wear by sliding — Identification of test parameters.

ISO 20329 Plastics — Determination of abrasive wear by reciprocating linear sliding motion.

ISO 9352:2012 Plastics — Determination of resistance to wear by abrasive wheels. ISO/DIS 7148–2 Plain bearings — Testing of the tribological behavior of bearing materials — Part 2: Testing of polymer-based bearing materials.

ISO/TR 11811:2012 Nanotechnologies — Guidance on methods for nano- and microtribology measurements.

The selection of tests necessary for assessing tribological behavior of a material pair including polymer-based materials depends on


Many different approaches could be seen in literature for assessing the tribological behavior of a system, differentiate in scale and complexity of the tested system. A logical order will be.

Laboratory tests ! Model tests! Component bench tests ! System bench tests! Machine bench test! Machine field test [41, 42].

In the same direction there are increasing complexity and costs, but first types of tests have increasing control and scale investigation and flexibility.

Depending of the novelty degree of the solution, one or more of the stages mentioned above could be omitted. New materials and original design solutions ask for all, but they have to be solved quite rapidly in order to gain the market.

A testing campaign is suggestively given in **Figure 8**. This plan was elaborated by Georgescu [15], but also used by Botan [16]. It was the result of consulting adapted from [42]. Polymeric blocks have the dimensions (10 mm x 16.5 mm x 4 mm). The values are quite small and it is very probably not to have actual component of such dimensions, but such a test campaign is very useful for ranking the materials and to investigate modifications in the tribolayers by the help of electron scanning microscopy, AFM, Raman microscopy as the test specimens are small.

## **4. Tribological parameters and evaluation of experimental results**

The set of tribological parameters are characterizing the materials Laboratory tests, on simplified specimens, are useful for ranking materials, but these results could not be extrapolated to actual component, especially for polymeric materials.

When designing a test campaign, for assessing the tribological behavior of a material pair, the tribosystem has to be identified as one in **Figure 9** [42], this simplified initial system being tested at laboratory level with as many as possible parameters closer to those from actual application.

The coefficient of friction is a convenient method for reporting friction force, since in many cases Ff is approximately linearly proportional to F over quite large ranges of N. The equation, known as Amonton's law is

$$\mathbf{F}\_{\mathbf{f}} = \mu \mathbf{F} \tag{1}$$

load, speed, temperature. This component of friction could be reduced by introducing a lubricant in contact or/and by re-design the system to have rolling or rolling-sliding motion and by an adequate cutting (usually grinding, honing) of the

*Positioning of polymers and polymer composites in a space hardness-wear rate constant [http://www.mie.uth.g*

The adhesion is present both in static friction and dynamic friction of polymeric materials: at the interface motion generates shear and deformation of a very thin layer of the polymeric material, directly in contact with the counterpart. As adhesion and transfer on the counter part are developed in steps, the friction loss, and consequently, the friction coefficient will vary in time, especially for sliding

Values of friction coefficient are given by producers, researchers but they are depending on test conditions. Thus, they could give a ranking of the tested materials under the same conditions, but they could not be the same with actual components. Sometimes, especially under low load, negative values of μ may be noticed: they are rather artificial, due to contact separation and inertia of the tester components; values of μ greater than 1 are physically logical, especially in material processing, in the interaction between a car tire and a dry road. Sampling could vary depending on the gauge measuring the resistance force. Researchers usually use a moving average to draw the curve of friction force or coefficient in time. For instance, the curve in **Figure 11** was done by moving average of 200 values with sampling 2 values per second. But extreme values are also important as they limit a range that could explain failure mechanisms as adhesion or local melting, especially for polymeric

In most cases, a single value of coefficient of friction is not adequate. This can be seen from the examples in **Figure 11**, depicting the evolution of friction coefficient for three sliding distance. The aspect of evolution is kept for PBT, but these three tests gave values between 0.16 … 0.19, with stable evolution, a characteristic of

The evolution of COF in **Figure 11** points out that, for polymer on steel in dry regime, it is less sensitive to time, but these conclusion has to mention the time range for which the researchers had obtained this results, here for 2500 … 7000 m. Czichos [41] modeled the evolution of COF for a dry regime in four stages: 1 increasing trend as the surfaces accommodate by wear, 2 - shorter stage of maximum values of COF, 3 - decrease of COF by the generation of a tribolayer favorable

polymer sliding as compared to metal–metal contacts.

metallic counterpart.

*r/ekp\_yliko/2\_materials-charts-2009.pdf] [43].*

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

contacts.

**Figure 10.**

materials.

**75**

where the value of μ depends significantly on working regime (lubricated or not), the composition, topography and history of the tribolayers, the environment in which they are working and the loading conditions. Ashby [43] gave a suggestive diagram, positioning the polymeric materials with lowest wear rate, but wear rate values could scan o two-order of magnitude. He also suggests by this diagram that wear rate field could be extended, especially towards low values by filling the polymers. A special position is noticed for PTFE (**Figure 10**), unique polymer as tribological behavior (the lowest friction coefficient, high wear rate, high working temperature and very resistant in aggressive media).

Usually, when a component if made of polymeric material, the other is harder, made of steel, but recently contact could be between the same polymeric materials of different. Thus, friction has to be treated for these cases.

In the case of harder counterpart, the friction polymer-metal has the following components: plowing as a form of abrasion with larger elasto-plastic deformation and micro-cracks and adhesion [3, 8]. These processes are severely depending on many factors including the hardness and asperity shape of the counterpart, contact

**Figure 9.**

*Testers for assessment of tribological behavior of polymers and polymer composites [42].*

*Tribological Behavior of Polymers and Polymer Composites DOI: http://dx.doi.org/10.5772/intechopen.94264*

#### **Figure 10.**

A testing campaign is suggestively given in **Figure 8**. This plan was elaborated by Georgescu [15], but also used by Botan [16]. It was the result of consulting adapted from [42]. Polymeric blocks have the dimensions (10 mm x 16.5 mm x 4 mm). The values are quite small and it is very probably not to have actual component of such dimensions, but such a test campaign is very useful for ranking the materials and to investigate modifications in the tribolayers by the help of electron scanning microscopy, AFM, Raman microscopy as the test specimens are

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

**4. Tribological parameters and evaluation of experimental results**

parameters closer to those from actual application.

ranges of N. The equation, known as Amonton's law is

temperature and very resistant in aggressive media).

of different. Thus, friction has to be treated for these cases.

*Testers for assessment of tribological behavior of polymers and polymer composites [42].*

The set of tribological parameters are characterizing the materials Laboratory tests, on simplified specimens, are useful for ranking materials, but these results could not be extrapolated to actual component, especially for polymeric materials. When designing a test campaign, for assessing the tribological behavior of a material pair, the tribosystem has to be identified as one in **Figure 9** [42], this simplified initial system being tested at laboratory level with as many as possible

The coefficient of friction is a convenient method for reporting friction force, since in many cases Ff is approximately linearly proportional to F over quite large

where the value of μ depends significantly on working regime (lubricated or not), the composition, topography and history of the tribolayers, the environment in which they are working and the loading conditions. Ashby [43] gave a suggestive diagram, positioning the polymeric materials with lowest wear rate, but wear rate values could scan o two-order of magnitude. He also suggests by this diagram that wear rate field could be extended, especially towards low values by filling the polymers. A special position is noticed for PTFE (**Figure 10**), unique polymer as tribological behavior (the lowest friction coefficient, high wear rate, high working

Usually, when a component if made of polymeric material, the other is harder, made of steel, but recently contact could be between the same polymeric materials

In the case of harder counterpart, the friction polymer-metal has the following components: plowing as a form of abrasion with larger elasto-plastic deformation and micro-cracks and adhesion [3, 8]. These processes are severely depending on many factors including the hardness and asperity shape of the counterpart, contact

Ff ¼ μF (1)

small.

**Figure 9.**

**74**

*Positioning of polymers and polymer composites in a space hardness-wear rate constant [http://www.mie.uth.g r/ekp\_yliko/2\_materials-charts-2009.pdf] [43].*

load, speed, temperature. This component of friction could be reduced by introducing a lubricant in contact or/and by re-design the system to have rolling or rolling-sliding motion and by an adequate cutting (usually grinding, honing) of the metallic counterpart.

The adhesion is present both in static friction and dynamic friction of polymeric materials: at the interface motion generates shear and deformation of a very thin layer of the polymeric material, directly in contact with the counterpart. As adhesion and transfer on the counter part are developed in steps, the friction loss, and consequently, the friction coefficient will vary in time, especially for sliding contacts.

Values of friction coefficient are given by producers, researchers but they are depending on test conditions. Thus, they could give a ranking of the tested materials under the same conditions, but they could not be the same with actual components. Sometimes, especially under low load, negative values of μ may be noticed: they are rather artificial, due to contact separation and inertia of the tester components; values of μ greater than 1 are physically logical, especially in material processing, in the interaction between a car tire and a dry road. Sampling could vary depending on the gauge measuring the resistance force. Researchers usually use a moving average to draw the curve of friction force or coefficient in time. For instance, the curve in **Figure 11** was done by moving average of 200 values with sampling 2 values per second. But extreme values are also important as they limit a range that could explain failure mechanisms as adhesion or local melting, especially for polymeric materials.

In most cases, a single value of coefficient of friction is not adequate. This can be seen from the examples in **Figure 11**, depicting the evolution of friction coefficient for three sliding distance. The aspect of evolution is kept for PBT, but these three tests gave values between 0.16 … 0.19, with stable evolution, a characteristic of polymer sliding as compared to metal–metal contacts.

The evolution of COF in **Figure 11** points out that, for polymer on steel in dry regime, it is less sensitive to time, but these conclusion has to mention the time range for which the researchers had obtained this results, here for 2500 … 7000 m.

Czichos [41] modeled the evolution of COF for a dry regime in four stages: 1 increasing trend as the surfaces accommodate by wear, 2 - shorter stage of maximum values of COF, 3 - decrease of COF by the generation of a tribolayer favorable

a very thin soften/melted layer of polymer. This is obvious in another study [11],

This example point out the influence of the nature of polymeric materials: the composite (the composites with hard micro-particles in a PBT matrix have higher and rough aspect of the curve, the blends PBT+ PTFE having lower values even the polymer PBT, considered a polymeric blends with soft drops of PTFE in PBT matrix). **Figure 14** presents the influence of sliding velocity on the friction coefficient, and the curves in **Figure 15** show the friction coefficient evolution in time depending on the highest load and velocity. The last plot is given only once as it could be related both to load and velocity dependence. The abbreviations for the materials are: PF5 - PBT + 5% PTFE, PF10 - PBT + 10% PTFE, PF15 - PBT + 15% PTFE). The composition of the hybrid composite GB10 + PF10 (having 10% glass beads and 10% PTFE) makes the friction coefficient to be higher at low velocity (0.25 m/s), but for the other two tested velocity, this tribological parameter evolves in a similar manner, but with higher oscillations, probably because of hard glass

Wear is not only a process of material removal in moving contacts, but a more complex one, defined recently as damage of the solid bodies caused by working or testing conditions, generally involving progressive loss of material, elasto-plastic deformations, tribo-chemical reactions caused by local pressure and heat generation in friction and their synergic interactions [8, 20]. In majority cases, the relative motion is intentional: for example, in plain bearings, pistons in cylinders, automotive brake disks interacting with brake pads, or in material processing (cutting, injection, rolling or extrusion). But in some cases, there are also undesired motion(s), resulted because of particular working conditions, as in the small cyclic displacements, known as fretting, produced by vibrations, elasto-plastic and tribological behavior of components in contact. If solid particles are passing through the contact, as contaminants in lubricant or, intentionally, as abrasive material for processing, then they will have a tremendous influence on wear process and, thus,

Wear is a complex process, quantified by the volume or mass of removed material, from each body in contact, the change in some linear dimension after a time period of functioning. Thus, wear is obviously a function of material pair, working time and conditions and it is related to a particular tribosystem (materials,

using pin-on-disk tribotester.

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

beads in the tribolayer (**Figure 16**).

on system durability.

**Figure 14.**

**77**

dimensions, shapes and working conditions).

*Influence of sliding velocity, at F = 5 N, for PBT, PF5, PF10, PF15 [15].*

**Figure 11.** *Friction coefficient for three tests block-on-ring, with different sliding distances [15].*

to reduce friction, for instance, a soften or molten layer of polymer, transfer films on harder surface etc. and the abrasive wear and deformation intensities decrease, 4 - stable evolution of friction. For polymer on steel or even on themselves, the authors will add a stage, 5 - slowly or sudden increase of COF meaning worsening the surface in contact due to severe wear, fatigue etc., in many times this increase announcing the life end of at least one triboelement (**Figures 12** and **13**).

Too low load makes the friction coefficient to have higher oscillations as superficial layer of the polymer is not compresses and hard asperities will easier tear up micro-sheets or plaquettes. As the load increases, the tribolayer is compacting and the energy loss by tearing decreases. This phenomenon of oscillating the friction coefficient in dry contact of polymers have been notice also by Jones in 1971 [44]. Higher concentration of reinforcement increases the friction coefficient and makes its evolution wavy (high amplitude could mean an increase of the glass bead concentration in the tribolayer and low values could happen when the tribolayer is richer in polymer.

Convergence of the curves for higher velocity (in **Figure 13**, for sliding speed of 0.5 m/s and 0.75 m/s) means that friction process is similar, very possible involving

**Figure 12.** *Influence of load at the same sliding velocity (GB10 - PBT +10% glass beads, GB20 - PBT +20% glass beads) [15].*

**Figure 13.** *Influence of sliding velocity under the same load [15].*

a very thin soften/melted layer of polymer. This is obvious in another study [11], using pin-on-disk tribotester.

This example point out the influence of the nature of polymeric materials: the composite (the composites with hard micro-particles in a PBT matrix have higher and rough aspect of the curve, the blends PBT+ PTFE having lower values even the polymer PBT, considered a polymeric blends with soft drops of PTFE in PBT matrix). **Figure 14** presents the influence of sliding velocity on the friction coefficient, and the curves in **Figure 15** show the friction coefficient evolution in time depending on the highest load and velocity. The last plot is given only once as it could be related both to load and velocity dependence. The abbreviations for the materials are: PF5 - PBT + 5% PTFE, PF10 - PBT + 10% PTFE, PF15 - PBT + 15% PTFE). The composition of the hybrid composite GB10 + PF10 (having 10% glass beads and 10% PTFE) makes the friction coefficient to be higher at low velocity (0.25 m/s), but for the other two tested velocity, this tribological parameter evolves in a similar manner, but with higher oscillations, probably because of hard glass beads in the tribolayer (**Figure 16**).

Wear is not only a process of material removal in moving contacts, but a more complex one, defined recently as damage of the solid bodies caused by working or testing conditions, generally involving progressive loss of material, elasto-plastic deformations, tribo-chemical reactions caused by local pressure and heat generation in friction and their synergic interactions [8, 20]. In majority cases, the relative motion is intentional: for example, in plain bearings, pistons in cylinders, automotive brake disks interacting with brake pads, or in material processing (cutting, injection, rolling or extrusion). But in some cases, there are also undesired motion(s), resulted because of particular working conditions, as in the small cyclic displacements, known as fretting, produced by vibrations, elasto-plastic and tribological behavior of components in contact. If solid particles are passing through the contact, as contaminants in lubricant or, intentionally, as abrasive material for processing, then they will have a tremendous influence on wear process and, thus, on system durability.

Wear is a complex process, quantified by the volume or mass of removed material, from each body in contact, the change in some linear dimension after a time period of functioning. Thus, wear is obviously a function of material pair, working time and conditions and it is related to a particular tribosystem (materials, dimensions, shapes and working conditions).

**Figure 14.** *Influence of sliding velocity, at F = 5 N, for PBT, PF5, PF10, PF15 [15].*

to reduce friction, for instance, a soften or molten layer of polymer, transfer films on harder surface etc. and the abrasive wear and deformation intensities decrease, 4 - stable evolution of friction. For polymer on steel or even on themselves, the authors will add a stage, 5 - slowly or sudden increase of COF meaning worsening the surface in contact due to severe wear, fatigue etc., in many times this increase

Too low load makes the friction coefficient to have higher oscillations as superficial layer of the polymer is not compresses and hard asperities will easier tear up micro-sheets or plaquettes. As the load increases, the tribolayer is compacting and the energy loss by tearing decreases. This phenomenon of oscillating the friction coefficient in dry contact of polymers have been notice also by Jones in 1971 [44]. Higher concentration of reinforcement increases the friction coefficient and makes its evolution wavy (high amplitude could mean an increase of the glass bead concentration in the tribolayer and low values could happen when the tribolayer is

Convergence of the curves for higher velocity (in **Figure 13**, for sliding speed of 0.5 m/s and 0.75 m/s) means that friction process is similar, very possible involving

*Influence of load at the same sliding velocity (GB10 - PBT +10% glass beads, GB20 - PBT +20% glass beads) [15].*

announcing the life end of at least one triboelement (**Figures 12** and **13**).

*Friction coefficient for three tests block-on-ring, with different sliding distances [15].*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

richer in polymer.

**Figure 12.**

**Figure 13.**

**76**

*Influence of sliding velocity under the same load [15].*

**Figure 11.**

**Figure 15.** *Influence of load, at v = 0,75 m/s, for PBT, PF5, PF10, PF15 [15].*

constant for v = 0.5 m/s and v = 1 m/s. As for the highest tested velocity, the plateau is zigzagged at almost regular time period. This could be explained by the polymer softening or even melting, followed by easier removal from the tribolayer, enrichment in glass beads of the tribolayer, with higher friction and thus, generating heat and rising the temperature. When the glass beads are embedded in the remaining

Another study [16] for emphasizing the importance of testing composites with polymer matrix has the results obtained on block-on-disk tribotester (**Figure 19**). The block is made of composite with 10% short aramid fibers (Twaron, grade, 225 μm in length see **Figure 5c**), with two different matrices: PA and PBT and the ring is the outer ring of a taped rolling bearing (the quality of rolling bearing ring keeps contact the influence of the counterbody in sliding). Analyzing **Figure 20**, the friction coefficient for PAX on steel has a steady evolution, in narrow ranges, for low loads (F = 5 N and F = 15 N) but for F = 30 N, for higher velocities, it increases and becomes steady at higher values, around 0.3. Temperature is steady for the same low loads, but it increases with different slopes for highest load. A too low load on polymer-based material - steel could rise COF and temperature in contact because the hard body does not contact the polymeric tribolayer enough and thus, the wear has a more intense abrasive component, tearing-off easier the polymer.

*Temperature evolution in time (a) for pin-on-disk tester, pin made of hard steel and disk made of PA + 10% grass beads + 1% black carbon, dry sliding for 10000 m and a thermal image during the test (b) (the rotation of*

matrix or removed, the temperature would reach a minimum.

*Evolution in time of temperature of polymeric surface in sliding contact [46].*

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

**Figure 17.**

**Figure 18.**

**79**

*the disk is clockwise) [11].*

#### **Figure 16.**

*Evolution of the friction coefficient for PBT and a hybrid composite (PBT + 10% glass beads+10% PTFE) [15].*

In some cases, material may be lost from both triboelements, or significant transfer of material may occur between the triboelements, and particular care is needed in both measuring the magnitude of wear and describing the damage it generates (material removals, abrasion, adhesion, transfer, plastic deformation, fragmentation and mixing the constituents of the tribolayer changes in the topography, the last one being investigating by the help of advanced non-contact profilometers [45].

The wear of polymeric material implies an aspect that is of interest only in pairs with a polymeric material: melting wear. A part of heat generate by friction is transferred to the polymeric materials and as thermal conductivity of polymers is low, a very thin layer could soften or even melt, the material latent heat of melting imposing a temperature limit in dry contacts. Stachowiack and Batchelor [46] described the scenario of temperature evolution in contact with a polymeric material (**Figure 17**). Similar observations are done by Briscoe and Sinha in [8], relating the polymer softening and its nature to transfer process on the harder counterface.

Experiment work validated this process of keeping constant the temperature in contact when a triboelement is made based on polymers. In order to support this conclusion, two studies are presented. First one is shortly presented in **Figure 18**. A cylindrical pin made of bearing steel is sliding against a disk made of composites PA + 10% wt glass beads +1% black carbon [11]. The thermo-image in left side presents the positions and their codes where the temperature was recorded with a thermo-camera. The temperature evolutions in time for these three points re given in the right. It is obvious the tendency of maintaining the temperature almost

*Tribological Behavior of Polymers and Polymer Composites DOI: http://dx.doi.org/10.5772/intechopen.94264*

**Figure 17.** *Evolution in time of temperature of polymeric surface in sliding contact [46].*

constant for v = 0.5 m/s and v = 1 m/s. As for the highest tested velocity, the plateau is zigzagged at almost regular time period. This could be explained by the polymer softening or even melting, followed by easier removal from the tribolayer, enrichment in glass beads of the tribolayer, with higher friction and thus, generating heat and rising the temperature. When the glass beads are embedded in the remaining matrix or removed, the temperature would reach a minimum.

Another study [16] for emphasizing the importance of testing composites with polymer matrix has the results obtained on block-on-disk tribotester (**Figure 19**). The block is made of composite with 10% short aramid fibers (Twaron, grade, 225 μm in length see **Figure 5c**), with two different matrices: PA and PBT and the ring is the outer ring of a taped rolling bearing (the quality of rolling bearing ring keeps contact the influence of the counterbody in sliding). Analyzing **Figure 20**, the friction coefficient for PAX on steel has a steady evolution, in narrow ranges, for low loads (F = 5 N and F = 15 N) but for F = 30 N, for higher velocities, it increases and becomes steady at higher values, around 0.3. Temperature is steady for the same low loads, but it increases with different slopes for highest load. A too low load on polymer-based material - steel could rise COF and temperature in contact because the hard body does not contact the polymeric tribolayer enough and thus, the wear has a more intense abrasive component, tearing-off easier the polymer.

#### **Figure 18.**

*Temperature evolution in time (a) for pin-on-disk tester, pin made of hard steel and disk made of PA + 10% grass beads + 1% black carbon, dry sliding for 10000 m and a thermal image during the test (b) (the rotation of the disk is clockwise) [11].*

In some cases, material may be lost from both triboelements, or significant transfer of material may occur between the triboelements, and particular care is needed in both measuring the magnitude of wear and describing the damage it generates (material removals, abrasion, adhesion, transfer, plastic deformation, fragmentation and mixing the constituents of the tribolayer changes in the topography, the last one being investigating by the help of advanced non-contact

*Evolution of the friction coefficient for PBT and a hybrid composite (PBT + 10% glass beads+10% PTFE) [15].*

The wear of polymeric material implies an aspect that is of interest only in pairs

Experiment work validated this process of keeping constant the temperature in contact when a triboelement is made based on polymers. In order to support this conclusion, two studies are presented. First one is shortly presented in **Figure 18**. A cylindrical pin made of bearing steel is sliding against a disk made of composites PA + 10% wt glass beads +1% black carbon [11]. The thermo-image in left side presents the positions and their codes where the temperature was recorded with a thermo-camera. The temperature evolutions in time for these three points re given in the right. It is obvious the tendency of maintaining the temperature almost

with a polymeric material: melting wear. A part of heat generate by friction is transferred to the polymeric materials and as thermal conductivity of polymers is low, a very thin layer could soften or even melt, the material latent heat of melting imposing a temperature limit in dry contacts. Stachowiack and Batchelor [46] described the scenario of temperature evolution in contact with a polymeric material (**Figure 17**). Similar observations are done by Briscoe and Sinha in [8], relating the polymer softening and its nature to transfer process on the harder

profilometers [45].

**Figure 16.**

**Figure 15.**

*Influence of load, at v = 0,75 m/s, for PBT, PF5, PF10, PF15 [15].*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

counterface.

**78**

• a too high load makes the temperature to have a slope, greater as velocity increases; a mild regime (thus, a favorable regime) will keep the temperature constant in contact as for tests under F = 15 N. The severe regime is marked by high oscillation of friction coefficient or even a constantly increased value and

Comparing curves in **Figure 20**, regimes with F = 30 N and high sliding velocity

(v = 0.5 … 0.75 m/s) could be considered as severe because they do not make tribological parameters as friction coefficient and temperature in contact, stable. The composite with PBT matrix with the same adding materials (10% short aramid fibers and 1% black carbon) has a similar evolution of COF, but temperature

The applications involving the friction couple polymeric material - metallic counterpart are preferred by mechanical requirements of the design solution and the better tribological behavior by monitoring and measuring a set of tribological characteristics (wear, friction, temperature in contact, changes in materials' structures etc.) as compared to sliding polymers against themselves (**Figure 21**). Wear process of polymeric materials are characterized by a transfer film, generated when sliding against a harder surface, strongly influencing on the tribological

A favorable transfer film should be continuous, very thin and regenerating without inducing troubles in the working systems. This is the ideal transfer film of a

*Evolution of friction coefficient and temperature at the contact edge in time, depending on load, sliding velocity,*

*for a sliding distance of L = 5000 m, block made of PBX (PBT +10% aramid fibers) [16].*

increases only for the extreme tested regime (F = 30 N, v = 0.75 m/s).

also by the same shape of the temperature curves.

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

behavior of the system [8].

**Figure 21.**

**81**

#### **Figure 19.**

*Images of thermal recordings of the temperature at the end of the test, for temperature at the contact edge, F = 30 v = 0,75 m/s (block made of PA- polyamide, PAX - polyamide +10% aramid fibers +1% black carbon, PBT polybythylene therephtalate, PBX - PBT +10% aramid fibers +1% black carbon) [16].*

#### **Figure 20.**

*Evolution of friction coefficient and temperature at the contact edge in time, depending on load, sliding velocity, for a sliding distance of L = 5000 m, block made of PBT +10% aramid fibers, L = 5000 m, block made of PAX (PA6 + 10% aramid fibers) [16].*

The combined analysis of two tribological parameters could reveal a qualitative change of the working regime. For instance, analyzing COF and temperature at the contact edge (**Figure 20**),


*Tribological Behavior of Polymers and Polymer Composites DOI: http://dx.doi.org/10.5772/intechopen.94264*

• a too high load makes the temperature to have a slope, greater as velocity increases; a mild regime (thus, a favorable regime) will keep the temperature constant in contact as for tests under F = 15 N. The severe regime is marked by high oscillation of friction coefficient or even a constantly increased value and also by the same shape of the temperature curves.

Comparing curves in **Figure 20**, regimes with F = 30 N and high sliding velocity (v = 0.5 … 0.75 m/s) could be considered as severe because they do not make tribological parameters as friction coefficient and temperature in contact, stable.

The composite with PBT matrix with the same adding materials (10% short aramid fibers and 1% black carbon) has a similar evolution of COF, but temperature increases only for the extreme tested regime (F = 30 N, v = 0.75 m/s).

The applications involving the friction couple polymeric material - metallic counterpart are preferred by mechanical requirements of the design solution and the better tribological behavior by monitoring and measuring a set of tribological characteristics (wear, friction, temperature in contact, changes in materials' structures etc.) as compared to sliding polymers against themselves (**Figure 21**).

Wear process of polymeric materials are characterized by a transfer film, generated when sliding against a harder surface, strongly influencing on the tribological behavior of the system [8].

A favorable transfer film should be continuous, very thin and regenerating without inducing troubles in the working systems. This is the ideal transfer film of a

#### **Figure 21.**

*Evolution of friction coefficient and temperature at the contact edge in time, depending on load, sliding velocity, for a sliding distance of L = 5000 m, block made of PBX (PBT +10% aramid fibers) [16].*

The combined analysis of two tribological parameters could reveal a qualitative change of the working regime. For instance, analyzing COF and temperature at the

*Evolution of friction coefficient and temperature at the contact edge in time, depending on load, sliding velocity, for a sliding distance of L = 5000 m, block made of PBT +10% aramid fibers, L = 5000 m, block made of PAX*

*Images of thermal recordings of the temperature at the end of the test, for temperature at the contact edge, F = 30 v = 0,75 m/s (block made of PA- polyamide, PAX - polyamide +10% aramid fibers +1% black carbon, PBT -*

*polybythylene therephtalate, PBX - PBT +10% aramid fibers +1% black carbon) [16].*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

• a too low load and sliding velocity make the temperature rising due to abrasive

• a higher speed makes the temperature curve higher for v = 0.75 m/s, but the

COF is kept low meaning a softening process happened,

contact edge (**Figure 20**),

*(PA6 + 10% aramid fibers) [16].*

**Figure 20.**

**80**

**Figure 19.**

wear (more intense under low load)

polymeric material but, actually, there are two types of polymers, those generating an almost continuously transfer film as high density polyethylene (HDPE) and ultra-high-molecular weight polyethylene (UHMWPE), and those that form lumps or islands, more or less regular. Transfer process is influenced by contact temperature and texture of the counterpart. Only few polymers have only a mechanical component of the transfer film (again, PTFE and UHMWPE have to be given as examples) and polymers that could chemically interact with the metallic surface.

Myshkin et al. [7] pointed out that the dependence of friction coefficient with velocity has different shapes depending on the polymer sliding on steel or on itself, and even for the same polymer, the curve depends on temperature of the environment. At low velocity (10�<sup>3</sup> … 10�<sup>2</sup> m/s), friction coefficient has an almost constant evolution, but at higher speed, its evolution could be with velocity could be parabolic, with minimum when the material is softening or has a thin melt layer, than it could increase. The conclusion of this work is that tests in the same conditions as the application are tremendously necessary for a reliable working of the tribosystem involving polymer-based materials in order to correct assess the power loss by friction and to prevent component failure by frictional heat.

The wear rate can then be defined as the rate of material removal or dimensional change per unit time, or per unit sliding distance. Because of the possibility of confusion, the term "wear rate" must always be defined, and its units stated. It is usually the mass or volume loss per unit time.

The Archard model of sliding wear [47] leads to the equation:

$$\mathbf{w} = \mathbf{K}\mathbf{W}/\mathbf{H},\tag{2}$$

where Δm is the mass loss of the block, L is the sliding distance and F is the

*Example of parameters monitored in actual time real on the tribotester UMT-2, block-on-ring test, block made of PBT, ring made of steel (100Cr6), F = 5 N (= Fz), v = 0,25 m/s, L = 7500 m, COF –friction coefficient, Fx – Resistant force (friction), AE – Acoustic emission, Z – Wear depth (linear wear) (linear change between ring*

Temperature in contact is very important in tribosystem with one or both elements made of polymeric materials as a jump in contact temperature of less amount as for metals (even 10°C) could change their mechanical and thermal properties, could even change the chemical organization of the molecular chains; the power dissipated in the contact is given by (μFv) where μ is the friction coefficient, F is the normal load and v is the sliding velocity. The local temperatures in the contact areas can therefore become much higher than the bulk temperatures. This factor needs to be considered when designing wear tests or interpreting test

In Botan's study [16], neat PBT had a very good tribological behavior (being analyzed, average values of COF during 5000 m of sliding on steel, low wear as compared to PA) but adding 10%wt short aramid fibers in PBT substantially improves wear resistance. Thermal monitoring of the contact edge allows for rank-

In study from 2012, Pei et al. [12] present the tribology of three polymers, considered as high-performance materials, introducing for evaluating the product pv (p being the average pressure in contact and v the sliding velocity). This parameter has to be used with precaution. Comparison should be done for the same tribosystem (dimensions and shapes) and under the same testing conditions. It is not recommended to extrapolate the results outside the investigated parameters. From **Figure 27**, one may notice that PPP grades exhibited low wear resistance as compared to PEEK and PBI had the lowest wear rate, due to its high value for heat resistance and very low decrease in mechanical characteristics under higher tem-

Obviously, in dry regime friction coefficient of a polymer on steel is lower than that for steel-on-steel and long and aligned carbon chain (as in PTFE and PE, even PA) will give lower dynamic friction coefficient, around 0.2 … 0.3, lower for PTFE, but polymers with higher mechanical characteristics as PPS and PEEK will have this parameter higher 0.3 … 0.5. Wear rate exhibits values that could not be deduced from the mechanical and structural characteristics. For instance, in **Figure 28**, the lowest wear rate among tested polymers under the same conditions was obtained for PA6, and wear rate increases from this to PI, PPS, PE-UHMW till PEEK, but

ing the tested materials having the temperature as criterion (**Figure 26**).

applied load in contact.

*and block), Fz – Normal load [15].*

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

results.

**Figure 22.**

peratures.

**83**

high values were obtained for POM and PTFE.

where w is the volume of material removed from the surface by wear per unit sliding distance, W is the normal load applied between the surfaces, and H is the indentation hardness of the softer surface. Many sliding systems do show a dependence of wear on sliding distance which is close to linear, and under some conditions also show wear rates which are roughly proportional to normal load. The constant K, usually termed the Archard wear coefficient, is dimensionless and always less than unity. The value of K provides a means of comparing the severities of different wear processes.

For the tribotester block-on-ring the wear parameter that reflects well the behavior of the materials could be the linear wear rate

$$\mathbf{W}\mathbf{l} = \Delta \mathbf{Z}/(\mathbf{F} \cdot \mathbf{L}) \left[ \mu \mathbf{m}/(\mathbf{N} \cdot \mathbf{km}) \right],\tag{3}$$

where ΔZ is the change in distance between ring and block at the end of the test, F is the normally applied load and L is the sliding distance. **Figure 22** presents test parameters, as recorded by the tribometer UMT-2, including friction coefficient (COF), wear depth (Z).

For pointing out wear parameters in a tribosystem with polymer-based material (s), the same two cases are analyzed (**Figure 23**).

A study has another objective [16]: to assess the tribological behavior of two polymer matrices, PA and PBT, with the same concentration of reinforcement, 10% wt short aramid fibers (Twaron, 225 microns as average length). There were measured several tribological parameters, average values of friction coefficient (COF, **Figure 24**), wear rate (**Figure 25**) and maximum value of the temperature at the contact edge (**Figure 26**). Wear rate in **Figure 25** was calculated as

$$\mathbf{W} = \Delta \mathbf{m} / (\mathbf{F} \cdot \mathbf{L}) \, [\, \text{mg} / (\mathbf{N} \cdot \text{km})],\tag{4}$$

*Tribological Behavior of Polymers and Polymer Composites DOI: http://dx.doi.org/10.5772/intechopen.94264*

**Figure 22.**

polymeric material but, actually, there are two types of polymers, those generating an almost continuously transfer film as high density polyethylene (HDPE) and ultra-high-molecular weight polyethylene (UHMWPE), and those that form lumps or islands, more or less regular. Transfer process is influenced by contact temperature and texture of the counterpart. Only few polymers have only a mechanical component of the transfer film (again, PTFE and UHMWPE have to be given as examples) and polymers that could chemically interact with the

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

Myshkin et al. [7] pointed out that the dependence of friction coefficient with velocity has different shapes depending on the polymer sliding on steel or on itself, and even for the same polymer, the curve depends on temperature of the environment. At low velocity (10�<sup>3</sup> … 10�<sup>2</sup> m/s), friction coefficient has an almost constant evolution, but at higher speed, its evolution could be with velocity could be parabolic, with minimum when the material is softening or has a thin melt layer, than it could increase. The conclusion of this work is that tests in the same conditions as the application are tremendously necessary for a reliable working of the tribosystem involving polymer-based materials in order to correct assess the power loss by

The wear rate can then be defined as the rate of material removal or dimensional

where w is the volume of material removed from the surface by wear per unit sliding distance, W is the normal load applied between the surfaces, and H is the indentation hardness of the softer surface. Many sliding systems do show a dependence of wear on sliding distance which is close to linear, and under some conditions also show wear rates which are roughly proportional to normal load. The constant K, usually termed the Archard wear coefficient, is dimensionless and always less than unity. The value of K provides a means of comparing the severities

For the tribotester block-on-ring the wear parameter that reflects well the

where ΔZ is the change in distance between ring and block at the end of the test, F is the normally applied load and L is the sliding distance. **Figure 22** presents test parameters, as recorded by the tribometer UMT-2, including friction coefficient

For pointing out wear parameters in a tribosystem with polymer-based material

A study has another objective [16]: to assess the tribological behavior of two polymer matrices, PA and PBT, with the same concentration of reinforcement, 10% wt short aramid fibers (Twaron, 225 microns as average length). There were measured several tribological parameters, average values of friction coefficient (COF, **Figure 24**), wear rate (**Figure 25**) and maximum value of the temperature at the

contact edge (**Figure 26**). Wear rate in **Figure 25** was calculated as

w ¼ KW*=*H, (2)

Wl ¼ ΔZ*=*ð Þ F � L ½ � μm*=*ð Þ N � km , (3)

W ¼ Δm*=*ð Þ F � L ½ � mg*=*ð Þ N � km , (4)

change per unit time, or per unit sliding distance. Because of the possibility of confusion, the term "wear rate" must always be defined, and its units stated. It is

The Archard model of sliding wear [47] leads to the equation:

friction and to prevent component failure by frictional heat.

usually the mass or volume loss per unit time.

behavior of the materials could be the linear wear rate

(s), the same two cases are analyzed (**Figure 23**).

of different wear processes.

(COF), wear depth (Z).

**82**

metallic surface.

*Example of parameters monitored in actual time real on the tribotester UMT-2, block-on-ring test, block made of PBT, ring made of steel (100Cr6), F = 5 N (= Fz), v = 0,25 m/s, L = 7500 m, COF –friction coefficient, Fx – Resistant force (friction), AE – Acoustic emission, Z – Wear depth (linear wear) (linear change between ring and block), Fz – Normal load [15].*

where Δm is the mass loss of the block, L is the sliding distance and F is the applied load in contact.

Temperature in contact is very important in tribosystem with one or both elements made of polymeric materials as a jump in contact temperature of less amount as for metals (even 10°C) could change their mechanical and thermal properties, could even change the chemical organization of the molecular chains; the power dissipated in the contact is given by (μFv) where μ is the friction coefficient, F is the normal load and v is the sliding velocity. The local temperatures in the contact areas can therefore become much higher than the bulk temperatures. This factor needs to be considered when designing wear tests or interpreting test results.

In Botan's study [16], neat PBT had a very good tribological behavior (being analyzed, average values of COF during 5000 m of sliding on steel, low wear as compared to PA) but adding 10%wt short aramid fibers in PBT substantially improves wear resistance. Thermal monitoring of the contact edge allows for ranking the tested materials having the temperature as criterion (**Figure 26**).

In study from 2012, Pei et al. [12] present the tribology of three polymers, considered as high-performance materials, introducing for evaluating the product pv (p being the average pressure in contact and v the sliding velocity). This parameter has to be used with precaution. Comparison should be done for the same tribosystem (dimensions and shapes) and under the same testing conditions. It is not recommended to extrapolate the results outside the investigated parameters. From **Figure 27**, one may notice that PPP grades exhibited low wear resistance as compared to PEEK and PBI had the lowest wear rate, due to its high value for heat resistance and very low decrease in mechanical characteristics under higher temperatures.

Obviously, in dry regime friction coefficient of a polymer on steel is lower than that for steel-on-steel and long and aligned carbon chain (as in PTFE and PE, even PA) will give lower dynamic friction coefficient, around 0.2 … 0.3, lower for PTFE, but polymers with higher mechanical characteristics as PPS and PEEK will have this parameter higher 0.3 … 0.5. Wear rate exhibits values that could not be deduced from the mechanical and structural characteristics. For instance, in **Figure 28**, the lowest wear rate among tested polymers under the same conditions was obtained for PA6, and wear rate increases from this to PI, PPS, PE-UHMW till PEEK, but high values were obtained for POM and PTFE.

• to compare features produced in a laboratory test with those observed in a

• (by studying debris) to identify the source of debris in a real-life application.

**Figure 29** presents two virtual images, reconstructed with SPIP The Scanning

*Average values for COF for 5000 m of dry sliding on steel (same scale for PA and PAX and PBT and PBX,*

*Wear rate of the block as a function of load (in N) and sliding speed (m/s), obtained on block-on-ring tester, dry regime, for blocks made of polymers (Polyamide 6 - PA and Polybuthyleneterphtalate - PBT) and their*

Probe Image Processor SPIPTM, Version 5.1.11/2012, from a study done by

practical application,

**Figure 24.**

**Figure 25.**

**85**

*composites with 10% short aramid fibers (PAX and PBX).*

*respectively) [16].*

• to identify mechanisms of wear,

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

*Tribological Behavior of Polymers and Polymer Composites*

#### **Figure 23.**

*Linear wear rate of the blocks made of polymer-based materials.*

Worn surfaces and the debris resulting from wear, may be examined for several reasons:

• to study the evolution of wear during an experiment, or during the life of a component in a practical application,


**Figure 29** presents two virtual images, reconstructed with SPIP The Scanning Probe Image Processor SPIPTM, Version 5.1.11/2012, from a study done by

#### **Figure 24.**

*Average values for COF for 5000 m of dry sliding on steel (same scale for PA and PAX and PBT and PBX, respectively) [16].*

#### **Figure 25.**

*Wear rate of the block as a function of load (in N) and sliding speed (m/s), obtained on block-on-ring tester, dry regime, for blocks made of polymers (Polyamide 6 - PA and Polybuthyleneterphtalate - PBT) and their composites with 10% short aramid fibers (PAX and PBX).*

Worn surfaces and the debris resulting from wear, may be examined for several

• to study the evolution of wear during an experiment, or during the life of a

component in a practical application,

*Linear wear rate of the blocks made of polymer-based materials.*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

reasons:

**84**

**Figure 23.**

#### *Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

Georgescu [15], pointing out initial surface (a) and traces as result of abrasive wear

*Virtual images of block surfaces made of PBT + 20% glass beads. (a) Initial surface. (b) Used surface (F = 5 N,*

After testing, the worn surface quality of the composite with only 10% glass beads was better, meaning a lower value for Sa, Sz (**Figure 30**). In tribological evaluation a ratio Sz/Sa, bringing together an averaging parameter with an extreme one (Sz) is important because singular or rare high peaks have a great influence on the tribological behavior, especially for composites with hard fillers. Adding micro glass beads in PBT increases the amplitude parameters (these are plotted for v = 0 m/s, in **Figure 30**). Ssk has high positive values for 20% glass beads in PBT, but the polymer and the composite with only 10% glass beads have lower values, oscillating between 1 and 1. If Ssk <0, it can be a bearing surface with holes and if Ssk > 0 it can be a flat surface with peaks. Values numerically greater than 1.0 may

*Roughness for worn surfaces of the block made of PBT, PBT + 10% glass beads (GB10) and PBT + 20% glass beads (GB20). (a) Sa- roughness average. (b) Sz - peak-peak height, the difference between the highest and lowest point in surface. (c) Surface skewness, Ssk, or the asymmetry of the height distribution histogram.*

*(d) Surface kurtosis, Sku, or the "peakedness" of the surface topography [15].*

on the composite.

*v = 0,75 m/s, L = 7500 m) [15].*

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

**Figure 29.**

**Figure 30.**

**87**

#### **Figure 26.**

*Maximum value of temperature at the contact edge, for all four tested materials in [16] (material codes as in previous figure).*

**Figure 27.** *Specific wear rate of polymer sliding on steel and counterpart temperature for [12].*

#### **Figure 28.**

*Two tribological parameters for polymer in dry sliding on steel [http://www.appstate.edu/clementsjs/polyme rproperties/\$p\$lastics\_\$f\$riction\$5f\$w\$ear.pdf]. (a) Friction coefficient. (b) Wear rate [48].*

**Figure 29.**

**Figure 26.**

**Figure 27.**

**Figure 28.**

**86**

*previous figure).*

*Maximum value of temperature at the contact edge, for all four tested materials in [16] (material codes as in*

*Tribology in Materials and Manufacturing - Wear, Friction and Lubrication*

*Two tribological parameters for polymer in dry sliding on steel [http://www.appstate.edu/clementsjs/polyme*

*rproperties/\$p\$lastics\_\$f\$riction\$5f\$w\$ear.pdf]. (a) Friction coefficient. (b) Wear rate [48].*

*Specific wear rate of polymer sliding on steel and counterpart temperature for [12].*

*Virtual images of block surfaces made of PBT + 20% glass beads. (a) Initial surface. (b) Used surface (F = 5 N, v = 0,75 m/s, L = 7500 m) [15].*

Georgescu [15], pointing out initial surface (a) and traces as result of abrasive wear on the composite.

After testing, the worn surface quality of the composite with only 10% glass beads was better, meaning a lower value for Sa, Sz (**Figure 30**). In tribological evaluation a ratio Sz/Sa, bringing together an averaging parameter with an extreme one (Sz) is important because singular or rare high peaks have a great influence on the tribological behavior, especially for composites with hard fillers. Adding micro glass beads in PBT increases the amplitude parameters (these are plotted for v = 0 m/s, in **Figure 30**). Ssk has high positive values for 20% glass beads in PBT, but the polymer and the composite with only 10% glass beads have lower values, oscillating between 1 and 1. If Ssk <0, it can be a bearing surface with holes and if Ssk > 0 it can be a flat surface with peaks. Values numerically greater than 1.0 may

#### **Figure 30.**

*Roughness for worn surfaces of the block made of PBT, PBT + 10% glass beads (GB10) and PBT + 20% glass beads (GB20). (a) Sa- roughness average. (b) Sz - peak-peak height, the difference between the highest and lowest point in surface. (c) Surface skewness, Ssk, or the asymmetry of the height distribution histogram. (d) Surface kurtosis, Sku, or the "peakedness" of the surface topography [15].*


the parameter of interest being the friction coefficient, wear or wear rate, temperature or durability till a particular value for wear temperature etc. are reached. The mathematical model for building the map surface is very important. For instance, maps in **Figure 23** are built with double spline curves, enforcing the obtained values from the tests to be on the surface. Sharp peaks or deep zones on the maps could indicate a qualitative change in tribological processes (change in wear process balance, tribochemical reactions induced by temperature threshold etc.)

The mapping technique is efficient for determining the overall behavior of a material or a tribosystem as it provides useful data about the position of transitions in wear behavior for a systematic test campaign. This comes at the expense of a reduction in the detailed knowledge of the variation of friction and wear with any one factor, but once the regime of interest is better defined through the use of maps,

**5. Characteristic mechanisms in the superficial layers of contacts**

blending with another polymer or reinforcements. A qualitative model of a

Initially, PTFE was simply used as bushes, seals, but its low mechanical characteristics make the researchers for materials to use it as matrix in composites [9, 39], adding material in other polymers, and even metallic sintered materials, more rigid

Burris and Sawyer studied the blend PEEK + PTFE [49]. PEEK has wear resistance, mechanical strength and a higher working temperature as compared to other polymers, but a high friction coefficient in dry regime μ = 0,4 and low thermal conductivity. PTFE has a high wear rate, and the fact that has the lowest friction coefficient in similar conditions does not recommend it to be used simple, without

Many researchers and producers of polymeric materials recommend only 5–20% PTFE [46, 50, 51], experiments done by Burris and Sawyer [49] obtained an optimum for the wear rate using the blend 30% PEEK +70% PTFE and, thus, underlined the necessity of testing new formulated materials for tribological applications.

Under 20% PEEK, wear has a sharp evolution, explained but not enough PEEK for creating a harder matrix for the soft polymer, thus the last one is easy to be deformed, abraded; the wear is supported by PTFE and not by the harder material (which has a higher wear resistance. The transfer process is more intense, and the wear debris have higher volumes. The authors suggest that preferentially lose of PTFE make the tribolayer grows rich in PEEK and the wear is reduced. At higher

*Contact surface 6,35 mm x 6,35 mm, F = 250 N and alternating sliding on 25.4 mm, v = 0.05 m/s, dry sliding on stainless steel AISI 304. (a) Model proposed by [49]. (b) Wear rate as a function of PEEK concentration.*

then a more detailed parametric study can be conducted.

*Tribological Behavior of Polymers and Polymer Composites*

*DOI: http://dx.doi.org/10.5772/intechopen.94264*

polymeric blend could be modeled as in **Figure 31a**.

**implying polymeric materials**

and less prone to wear.

**Figure 31.**

**89**

**Table 2.**

*List of important parameters that influence the tribological behavior [42].*

indicate extreme holes or peaks on the surface, as for the worn surfaces of composite PBT + 20% glass beads. For v = 0.50 m/s (**Figure 30**) and v = 0.75 m/s, Ssk < 0, reflecting the micro-plowing process. For Sku > 3, all worn surfaces indicated long and narrow valleys, with high peaks, the valley are dominated as result of tearingoff glass beads and maintaining the shape of the extracted beads. Smaller values of Ssk indicate broader height distributions but these polymeric materials have narrow height distribution as all values are above 3 (**Table 2**).

Components with high volume of polymeric material are less heat conductive and prone to have melt/soften contact. The solution given by research and practice: polymeric coatings, thick enough to reduce friction and to bear wear for a specified life and reliability.

During a test, many influencing factors have to be controlled. These can be grouped in


Some of them could be monitored during the test (as friction force), some only at before and after test. For polymers, investigations must be done just after the test as the specimens could age and thus, altering the information.

Researchers have to prioritize what factors are kept constant and what factors will vary on ranges of interest.

A full program of testing under all combinations of these factors would be timeconsuming and costly, and may not be required. Often a single factor can be identified as "key" to the material response, and in this case a good approach is to set all the other factors at constant values and vary the chosen factor in a controlled way in a series of tests. Test campaign must promote an objective, to establish variables (materials, working regime parameters, environment) and the most relevant results to be given, non-destructive investigation in order to understand and direction the damage processes during testing.

Tribologists is now using mapping technique when two (or more) factors are changed in a controlled way (normally more coarsely than in parametric studies),

#### *Tribological Behavior of Polymers and Polymer Composites DOI: http://dx.doi.org/10.5772/intechopen.94264*

the parameter of interest being the friction coefficient, wear or wear rate, temperature or durability till a particular value for wear temperature etc. are reached. The mathematical model for building the map surface is very important. For instance, maps in **Figure 23** are built with double spline curves, enforcing the obtained values from the tests to be on the surface. Sharp peaks or deep zones on the maps could indicate a qualitative change in tribological processes (change in wear process balance, tribochemical reactions induced by temperature threshold etc.)

The mapping technique is efficient for determining the overall behavior of a material or a tribosystem as it provides useful data about the position of transitions in wear behavior for a systematic test campaign. This comes at the expense of a reduction in the detailed knowledge of the variation of friction and wear with any one factor, but once the regime of interest is better defined through the use of maps, then a more detailed parametric study can be conducted.
