**3.1. The base fluid of nanofluid**

Nanofluids are formulated by suspending nanoparticles in various types of fluid, which can be the water, vegetable oil, industrial oil and so on. The properties of the medium are considered an important parameter, which directly influences on the activity of nanoparticles. Along with the trend of sustainable machining all over the world, the cutting fluids used in nanofluids should not be contained different toxic ingredients, and therefore water and vegetable oils are motivated to use as the alternative solution. There are some types of vegetable oils, which are commonly utilized in MQL machining: soybean, peanut (groundnut), maize, rapeseed, palm, castor, and sunflower oils. The ingredients, molecular structure, viscosity, and friction coefficient of the base fluids are the key parameters for vegetable oils [37]. **Figure 24** shows the relationship between vegetable oils and their coefficient of friction, which strongly influences in the contact area during machining. Vegetable oil is mainly composed of fatty acid and triglyceride -COOH in the fatty acid molecules and -COOR in triglyceride both belong to polar groups, which gives them excellent lubrication property [38]. **Figure 25** illustrates the typical polar molecule of vegetable oil.

**Figure 24.** Friction coefficient of different vegetable oils [36].

**Figure 25.** Polar molecule of the typical vegetable oil [36].

**Table 2.** Ingredient of various vegetable oil (%) [36].

**Palmic acid Stearic acid Oleic acid Linoleic acid Linolenic acid**

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091 181

Peanut oil 6–9 3–6 55–71 13–25 0.5 Soybean oil 7–10 3–5 22–31 49–55 6–11 Maize 9–19 1–3 26–40 44–55 < 1 Castor oil — — 3–9 3–5 Trace Palm 35–48 4–6 38–41 8–12 Trace Rapeseed oil 2–4 1–2 40–60 19–20 7–8 Sunflower oil 4–19 3–6 14–35 50–75 0.1

**Oil types Ingredient**

**Table 2** lists the basic ingredients of fatty acids of seven vegetable oils. The lubrication properties of saturated fatty acids are better than those of unsaturated fatty acids [41]. Saturated fatty acids have a strong effect on decreasing friction and wear, especially stearic acid, which


**Table 1.** Technical properties of different types of nanoparticles.

**Figure 24.** Friction coefficient of different vegetable oils [36].

From **Table 1**, it can be seen that the technical properties of each type of nanoparticles are different, and so the effectiveness of various nanofluids on MQL cutting performance will be an

In this section, the authors will present the effects of six different types of nanoparticles on nanofluids mainly used in MQL machining in term of MQL base fluid, types of nanoparticles,

Nanofluids are formulated by suspending nanoparticles in various types of fluid, which can be the water, vegetable oil, industrial oil and so on. The properties of the medium are considered an important parameter, which directly influences on the activity of nanoparticles. Along with the trend of sustainable machining all over the world, the cutting fluids used in nanofluids should not be contained different toxic ingredients, and therefore water and vegetable oils are motivated to use as the alternative solution. There are some types of vegetable oils, which are commonly utilized in MQL machining: soybean, peanut (groundnut), maize, rapeseed, palm, castor, and sunflower oils. The ingredients, molecular structure, viscosity, and friction coefficient of the base fluids are the key parameters for vegetable oils [37]. **Figure 24** shows the relationship between vegetable oils and their coefficient of friction, which strongly influences in the contact area during machining. Vegetable oil is mainly composed of fatty acid and triglyceride -COOH in the fatty acid molecules and -COOR in triglyceride both belong to polar groups, which gives them excellent lubrication property [38]. **Figure 25** illustrates the typical polar molecule of vegetable oil.

**Table 2** lists the basic ingredients of fatty acids of seven vegetable oils. The lubrication properties of saturated fatty acids are better than those of unsaturated fatty acids [41]. Saturated fatty acids have a strong effect on decreasing friction and wear, especially stearic acid, which

**O3 SiO2 MoS2 ZrO2 CNT ND**

Ellipsoidal Mainly

) 3.97 2.4 4.8 5.89 ~2.1 3.05–3.30

40 7.6 138 < 2 3000 2000

2200 1600 1185 2715 3127 3727

spherical

Coaxial circular tubes

Spherical & flaky

size and morphology of nanoparticles, and nanoparticle concentration.

important investigated factor.

180 Microfluidics and Nanofluidics

**3.1. The base fluid of nanofluid**

**Al<sup>2</sup>**

spherical

**Table 1.** Technical properties of different types of nanoparticles.

Porous & nearly spherical

Purity (%) > 99 ~99.5 > 99 99.9 > 95 93-95 Color White White Black White Black Gray

Friction coefficient — — 0.03 ~0.05 — — —

Morphology Nearly

True density (g/cm<sup>3</sup>

Melting temperature

Thermal conductivity (W/m.K)

(°C)

**Types of nanoparticles technical properties**

**Figure 25.** Polar molecule of the typical vegetable oil [36].


**Table 2.** Ingredient of various vegetable oil (%) [36].

provides a more stable oil film in contact zone. Besides, they tightly contaminate to the molecular film and remain on metal surfaces, such as tiny magnets, to form lubricating film for anti-friction and anti-wear [39]. Hence, this characteristic gives vegetable oils good lubrication property and shows the excellent lubrication effects in the application of MQL fluids. The double bonds in unsaturated fatty acids are relatively unstable and easily generate chemical reactions, such as oxidization. Moreover, they can also weaken the acting force between molecules, which leads to poor lubrication properties [36]. Lubrication property of unsaturated fatty acids with a higher carbon atom number is stronger than those with a lower carbon atom number, and so the friction coefficient decreases as the chain length increases [42].

On the other hand, the selection of proper vegetable oil depends on the climate and soil texture of each country, but it contributes very little to total manufacturing cost because of very small amount of fluids used in MQL techniques. The small quantity lubrication (SQL) grinding process of Inconel 718 using silver and zinc oxide NPs mixed with DI water at cutting velocity (V) =18 m/s, table speed (Vw) = 6 m/min, depth of cut (ap) = 10 mm. The flow rates of MQL nanofluids are 50, 100, 150, 200, 250 ml/h [40]. Minimum tangential forces have been obtained in the case of SQL grinding with nanofluids (seen in **Figure 26**), which is essentially due to better cooling, and lubrication that helps in preserving the cutting ability of the grits over a longer period. Additionally, hard NPs under grinding pressure might convert sliding to rolling friction.

Vegetable oil evidently has better lubrication property than water-soluble fluid, but the lubrication properties of seven typical vegetable oils also differ. Among these vegetable nanofluids, castor oil has the best lubrication property, followed by palm oil. In addition, peanut, sunflower, soybean, and rapeseed oils also exhibit excellent lubrication properties.

The viscosity of vegetable oils also has a strong influence on the machining performance and strongly affects its cooling and lubricating properties. They have a high natural viscosity as the machining temperature increases and drops more slowly than that of mineral oils [36]. These statements have significant meanings in machining difficult-to-cut materials such as hardened steel, tool steel, and so on. The formulation of oil film containing nanoparticles in cutting zone plays a key role in reducing the friction, which leads to decrease cutting temperature and tool wear. This film forms and loses continually, and so the higher the viscosity of cutting fluids, the more stable the film on contact faces.

#### **3.2. The types and morphology of nanoparticles**

Recently, nanomaterials have attained much attention because of their unique properties and tremendous application potentials in a wide range of industries. Currently, more and more researchers have been devoted to enhance the lubricant properties by using nanoparticles as lubricant additives (also called as nanolubrication or nanofluids). There are numerous types of nanoparticles in markets, and they are used in a wide range of industries. For machining processes, some types of nanoparticles are mainly used as Al<sup>2</sup> O3 , MoS<sup>2</sup> , ZrO<sup>2</sup> , SiO<sup>2</sup> , CNT, and ND. From **Table 1**, it is clearly seen that the morphology of six different NPs is different, and so it causes the various effects on cutting performance. Owing to the high cost of nanoparticles, the appropriate selection of nanoparticle type to suspend in MQL fluid is so important.

Al<sup>2</sup> O3

resistance. The Al<sup>2</sup>

area [43]. When Al<sup>2</sup>

Furthermore, the Al<sup>2</sup>

O3

**Figure 26.** Grinding forces under different SQL nanofluids [36].

O3

O3

nanoparticles, one of hexagonal close-packed crystal materials, exhibit the good lubri-

NPs are hard phase (HR = 2700-3000), showing good abrasive resistance

nanoparticles are added into cutting fluid, they can easily move into the

nanoparticles demonstrate good resistance to high temperature. The

O3

are spherical (shown in **Figure 27**) with characteristics of high strength, hardness, and heat

during the friction process, and can carry some support to friction surface load between the

worn area under the compressive stress of nanocutting fluid, and then a self-laminating film can be formed, which results in micropolish and can self-mend the friction surface [44, 45].

O3

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091 183

nanoparticles are mostly

nanoparticles

cation performance, which is related to its structures and characteristics. Al<sup>2</sup>

melting point of oil film can reach 2200°C. The morphology of Al<sup>2</sup>

**Figure 26.** Grinding forces under different SQL nanofluids [36].

provides a more stable oil film in contact zone. Besides, they tightly contaminate to the molecular film and remain on metal surfaces, such as tiny magnets, to form lubricating film for anti-friction and anti-wear [39]. Hence, this characteristic gives vegetable oils good lubrication property and shows the excellent lubrication effects in the application of MQL fluids. The double bonds in unsaturated fatty acids are relatively unstable and easily generate chemical reactions, such as oxidization. Moreover, they can also weaken the acting force between molecules, which leads to poor lubrication properties [36]. Lubrication property of unsaturated fatty acids with a higher carbon atom number is stronger than those with a lower carbon atom

On the other hand, the selection of proper vegetable oil depends on the climate and soil texture of each country, but it contributes very little to total manufacturing cost because of very small amount of fluids used in MQL techniques. The small quantity lubrication (SQL) grinding process of Inconel 718 using silver and zinc oxide NPs mixed with DI water at cutting velocity (V) =18 m/s, table speed (Vw) = 6 m/min, depth of cut (ap) = 10 mm. The flow rates of MQL nanofluids are 50, 100, 150, 200, 250 ml/h [40]. Minimum tangential forces have been obtained in the case of SQL grinding with nanofluids (seen in **Figure 26**), which is essentially due to better cooling, and lubrication that helps in preserving the cutting ability of the grits over a longer period. Additionally, hard NPs under grinding pressure might convert sliding

Vegetable oil evidently has better lubrication property than water-soluble fluid, but the lubrication properties of seven typical vegetable oils also differ. Among these vegetable nanofluids, castor oil has the best lubrication property, followed by palm oil. In addition, peanut,

The viscosity of vegetable oils also has a strong influence on the machining performance and strongly affects its cooling and lubricating properties. They have a high natural viscosity as the machining temperature increases and drops more slowly than that of mineral oils [36]. These statements have significant meanings in machining difficult-to-cut materials such as hardened steel, tool steel, and so on. The formulation of oil film containing nanoparticles in cutting zone plays a key role in reducing the friction, which leads to decrease cutting temperature and tool wear. This film forms and loses continually, and so the higher the viscosity of

Recently, nanomaterials have attained much attention because of their unique properties and tremendous application potentials in a wide range of industries. Currently, more and more researchers have been devoted to enhance the lubricant properties by using nanoparticles as lubricant additives (also called as nanolubrication or nanofluids). There are numerous types of nanoparticles in markets, and they are used in a wide range of industries. For machining

ND. From **Table 1**, it is clearly seen that the morphology of six different NPs is different, and so it causes the various effects on cutting performance. Owing to the high cost of nanoparticles, the appropriate selection of nanoparticle type to suspend in MQL fluid is so important.

O3 , MoS<sup>2</sup>

, ZrO<sup>2</sup>

, SiO<sup>2</sup>

, CNT, and

sunflower, soybean, and rapeseed oils also exhibit excellent lubrication properties.

cutting fluids, the more stable the film on contact faces.

processes, some types of nanoparticles are mainly used as Al<sup>2</sup>

**3.2. The types and morphology of nanoparticles**

number, and so the friction coefficient decreases as the chain length increases [42].

to rolling friction.

182 Microfluidics and Nanofluidics

Al<sup>2</sup> O3 nanoparticles, one of hexagonal close-packed crystal materials, exhibit the good lubrication performance, which is related to its structures and characteristics. Al<sup>2</sup> O3 nanoparticles are spherical (shown in **Figure 27**) with characteristics of high strength, hardness, and heat resistance. The Al<sup>2</sup> O3 NPs are hard phase (HR = 2700-3000), showing good abrasive resistance during the friction process, and can carry some support to friction surface load between the area [43]. When Al<sup>2</sup> O3 nanoparticles are added into cutting fluid, they can easily move into the worn area under the compressive stress of nanocutting fluid, and then a self-laminating film can be formed, which results in micropolish and can self-mend the friction surface [44, 45]. Furthermore, the Al<sup>2</sup> O3 nanoparticles demonstrate good resistance to high temperature. The melting point of oil film can reach 2200°C. The morphology of Al<sup>2</sup> O3 nanoparticles are mostly

nanofilm can be formed by diffusing and penetrating into machined surface or sub-surface of

system combining Mo and S through a covalent bond, and the bond between them is short, but the spacing between sulfur atoms is large. Accordingly, the bond between two adjacent sulfur atom layers is weak. That is the best explanation why a plane, so called "an easy-toslide plane," will be generated from weak binding of sulfur atoms between molecular layers by shearing force caused by cutting processes. The numerous easy-to-slide planes make the contact faces sliding relatively to each other and they do not contact directly [43]. This unique

the crystal surface on the metal surface have a very strong adhesion to form a very solid film,

size becomes smaller, it is attached to the surface of the friction material and the coverage has increased significantly, and anti-wear friction properties have been significantly improved.

Carbon nanotubes or CNTs are coaxial circular tubes (shown in **Figure 30**) composed by layers and dozens of layers of carbon atoms in hexagonal arrangement. CNTs present a high modu-

CNTs will not be ground into hard film under high loads and pressure because of its high strength and hardness. As such, CNTs can reduce the friction force of the cutting area and improve the lubrication effect of nanofluids. Tool-workpiece interface thus provides efficient lubrication. Two types of CNTs include the single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). The performance between CNTs found by them revealed better performances of MWCNT over the SWCNT in terms of cutting temperature

nanoparticles (135 nm) (www.us-nano.com).

therefore lubrication is superior to other general-lubricating materials. When MoS<sup>2</sup>

tools and the workpiece, thus reducing the friction and achieving better surface quality.

it can provide low coefficient of friction up to 0.03–0.05 or even lower. The MoS<sup>2</sup>

ticles are ellipsoidal (shown in **Figure 29**). The layer structure of MoS<sup>2</sup>

lus and strength because carbon atoms in CNTs adopt the sp<sup>2</sup>

a higher proportion of S pathways than sp<sup>3</sup>

NPs [48]. This phenomenon prevents direct contact between cutting

has been considered to be the best solid lubricant material, because

good friction-reducing effect. Moreover, exposure sulfur atoms of

has been utilized in machining processes as solid lubricant

nanopar-

185

particle

is a hexagonal crystal

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091

hybridization, which follows

hybridization [49]. During machining processes,

the elements in some SiO<sup>2</sup>

for many years. MoS<sup>2</sup>

characteristic makes MoS<sup>2</sup>

**Figure 29.** The SEM image of MoS<sup>2</sup>

Molybdenum disulfide or MoS<sup>2</sup>

**Figure 27.** The SEM image of Al<sup>2</sup> O3 nanoparticles (30 nm) [57].

spherical; therefore, they can play the role of ball bearings that prevent the direct contact of friction pairs, and the sliding friction is changed to rolling friction in contact zone, which improves the cutting performance and the carrying capacity of lubricant [46].

The SiO<sup>2</sup> nanoparticles are spherical (shown in **Figure 28**), and the surface molecules exhibit a 3D network structure. Given the abundant unsaturated vacant bonds on the surface, SiO<sup>2</sup> nanoparticles exert high surface energy and activity, making them easy to sediment onto the workpiece friction surface [47]. For such machining processes having the intensive friction as grinding, hard turning, hard milling, and so on, the melting point of SiO<sup>2</sup> nanoparticles in contact faces decreases under local extremely high temperature and pressure. Therefore, they may be melted, semi-melted, or sintered to form the lubrication film. Furthermore, a ceramic-like

**Figure 28.** The SEM image of SiO<sup>2</sup> nanoparticles (8 nm) (www.us-nano.com).

nanofilm can be formed by diffusing and penetrating into machined surface or sub-surface of the elements in some SiO<sup>2</sup> NPs [48]. This phenomenon prevents direct contact between cutting tools and the workpiece, thus reducing the friction and achieving better surface quality.

Molybdenum disulfide or MoS<sup>2</sup> has been utilized in machining processes as solid lubricant for many years. MoS<sup>2</sup> has been considered to be the best solid lubricant material, because it can provide low coefficient of friction up to 0.03–0.05 or even lower. The MoS<sup>2</sup> nanoparticles are ellipsoidal (shown in **Figure 29**). The layer structure of MoS<sup>2</sup> is a hexagonal crystal system combining Mo and S through a covalent bond, and the bond between them is short, but the spacing between sulfur atoms is large. Accordingly, the bond between two adjacent sulfur atom layers is weak. That is the best explanation why a plane, so called "an easy-toslide plane," will be generated from weak binding of sulfur atoms between molecular layers by shearing force caused by cutting processes. The numerous easy-to-slide planes make the contact faces sliding relatively to each other and they do not contact directly [43]. This unique characteristic makes MoS<sup>2</sup> good friction-reducing effect. Moreover, exposure sulfur atoms of the crystal surface on the metal surface have a very strong adhesion to form a very solid film, therefore lubrication is superior to other general-lubricating materials. When MoS<sup>2</sup> particle size becomes smaller, it is attached to the surface of the friction material and the coverage has increased significantly, and anti-wear friction properties have been significantly improved.

Carbon nanotubes or CNTs are coaxial circular tubes (shown in **Figure 30**) composed by layers and dozens of layers of carbon atoms in hexagonal arrangement. CNTs present a high modulus and strength because carbon atoms in CNTs adopt the sp<sup>2</sup> hybridization, which follows a higher proportion of S pathways than sp<sup>3</sup> hybridization [49]. During machining processes, CNTs will not be ground into hard film under high loads and pressure because of its high strength and hardness. As such, CNTs can reduce the friction force of the cutting area and improve the lubrication effect of nanofluids. Tool-workpiece interface thus provides efficient lubrication. Two types of CNTs include the single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT). The performance between CNTs found by them revealed better performances of MWCNT over the SWCNT in terms of cutting temperature

**Figure 29.** The SEM image of MoS<sup>2</sup> nanoparticles (135 nm) (www.us-nano.com).

spherical; therefore, they can play the role of ball bearings that prevent the direct contact of friction pairs, and the sliding friction is changed to rolling friction in contact zone, which

a 3D network structure. Given the abundant unsaturated vacant bonds on the surface, SiO<sup>2</sup> nanoparticles exert high surface energy and activity, making them easy to sediment onto the workpiece friction surface [47]. For such machining processes having the intensive friction as

tact faces decreases under local extremely high temperature and pressure. Therefore, they may be melted, semi-melted, or sintered to form the lubrication film. Furthermore, a ceramic-like

nanoparticles (8 nm) (www.us-nano.com).

nanoparticles are spherical (shown in **Figure 28**), and the surface molecules exhibit

nanoparticles in con-

improves the cutting performance and the carrying capacity of lubricant [46].

nanoparticles (30 nm) [57].

grinding, hard turning, hard milling, and so on, the melting point of SiO<sup>2</sup>

The SiO<sup>2</sup>

**Figure 27.** The SEM image of Al<sup>2</sup>

184 Microfluidics and Nanofluidics

**Figure 28.** The SEM image of SiO<sup>2</sup>

O3

**Figure 30.** The SEM image of single-walled carbon nanotube (outside diameter: 1-2 nm; inside diameter: 0.8-1.6 nm) (www.us-nano.com).

and cutting force due to higher wettability although MWCNT had lower thermal conductivity than SWCNT [50]. However, they cannot produce effective rolling like other spherical nanoparticles due to their cylinder structures. Therefore, CNTs can only produce limited antifriction effect [43]. Furthermore, CNTs possess the highest coefficient of thermal conductivity among nanoparticles (**Table 1**), and so their application can be broadened by additionally dispersing other appropriate types of nanoparticles to form hybrid nanofluids [51, 52].

The ZrO<sup>2</sup> nanoparticles are mainly spherical (shown in **Figure 31**). They appear oblique crystal at low temperature and show tetragonal crystal formation at high temperature. ZrO<sup>2</sup> is soluble in sulfuric acid and hydrofluoric acid and has good thermal-chemical stability due to very high melting temperature. When at high temperature, they have good strength and toughness. ZrO<sup>2</sup> nanoparticles have the lowest coefficient of thermal conductivity among nanoparticles (**Table 1**), but their high surface energy and surface activity tend to be adsorbed onto the machined surface establishing a layer of self-healing lubrication film on the friction pair surface and achieving good lubrication effect [43].

friction coefficient, cutting forces, cutting temperature [53–56]. Interestingly, NDs can serve as abrasive grains and take part in cutting processes under high pressure like grinding process [43]; therefore, if the size of NDs becomes larger, they may deteriorate the surface quality.

nanoparticles (40 nm) (www.us-nano.com).

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091 187

The size of nanoparticles should not be avoided because it not only influences on the performance of nanofluids and cutting processes but also contributes significant amount of nanofluid cost. In markets, the smaller the grain size of nanoparticles, the higher their cost will be. **Figure 33** shows the relationship between the NP size and its cost, and it can be seen that the

**3.3. The size of nanoparticles**

**Figure 32.** The SEM image of ND nanoparticles (50 nm) (ndp.diamonds).

**Figure 31.** The SEM image of ZrO<sup>2</sup>

The nanodiamond or ND exhibits cubic structure, and the morphology of nanoparticles is spherical or flaky (shown in **Figure 32**). ND presents very high hardness (HV = 98 GPa) which is superior to that of workpiece materials. Moreover, the ND exhibits extremely large elasticity modulus (980 GPa) with a compressive strength of about 13 GPa. With a size less than 1 μm, ND has attracted remarkable scientific attention due to their excellent mechanical and optical properties, high surface areas, and tunable surface structures. Due to unique properties, the excellent performances will certainly influence the lubrication performance in term of reducing

**Figure 31.** The SEM image of ZrO<sup>2</sup> nanoparticles (40 nm) (www.us-nano.com).

**Figure 32.** The SEM image of ND nanoparticles (50 nm) (ndp.diamonds).

friction coefficient, cutting forces, cutting temperature [53–56]. Interestingly, NDs can serve as abrasive grains and take part in cutting processes under high pressure like grinding process [43]; therefore, if the size of NDs becomes larger, they may deteriorate the surface quality.

#### **3.3. The size of nanoparticles**

and cutting force due to higher wettability although MWCNT had lower thermal conductivity than SWCNT [50]. However, they cannot produce effective rolling like other spherical nanoparticles due to their cylinder structures. Therefore, CNTs can only produce limited antifriction effect [43]. Furthermore, CNTs possess the highest coefficient of thermal conductivity among nanoparticles (**Table 1**), and so their application can be broadened by additionally

**Figure 30.** The SEM image of single-walled carbon nanotube (outside diameter: 1-2 nm; inside diameter: 0.8-1.6 nm)

nanoparticles are mainly spherical (shown in **Figure 31**). They appear oblique crys-

nanoparticles have the lowest coefficient of thermal conductivity among

is

dispersing other appropriate types of nanoparticles to form hybrid nanofluids [51, 52].

tal at low temperature and show tetragonal crystal formation at high temperature. ZrO<sup>2</sup>

soluble in sulfuric acid and hydrofluoric acid and has good thermal-chemical stability due to very high melting temperature. When at high temperature, they have good strength and

nanoparticles (**Table 1**), but their high surface energy and surface activity tend to be adsorbed onto the machined surface establishing a layer of self-healing lubrication film on the friction

The nanodiamond or ND exhibits cubic structure, and the morphology of nanoparticles is spherical or flaky (shown in **Figure 32**). ND presents very high hardness (HV = 98 GPa) which is superior to that of workpiece materials. Moreover, the ND exhibits extremely large elasticity modulus (980 GPa) with a compressive strength of about 13 GPa. With a size less than 1 μm, ND has attracted remarkable scientific attention due to their excellent mechanical and optical properties, high surface areas, and tunable surface structures. Due to unique properties, the excellent performances will certainly influence the lubrication performance in term of reducing

pair surface and achieving good lubrication effect [43].

The ZrO<sup>2</sup>

toughness. ZrO<sup>2</sup>

(www.us-nano.com).

186 Microfluidics and Nanofluidics

The size of nanoparticles should not be avoided because it not only influences on the performance of nanofluids and cutting processes but also contributes significant amount of nanofluid cost. In markets, the smaller the grain size of nanoparticles, the higher their cost will be. **Figure 33** shows the relationship between the NP size and its cost, and it can be seen that the

The MQL grinding process of hardened AISI 52100 steel with Al<sup>2</sup>

NP size (**Figure 35**). When comparing the surface roughness of Al<sup>2</sup>

Another MQL grinding process with Al<sup>2</sup>

of nanofluids in machining practice.

**3.4. The nanoparticle concentration**

be made from **Figure 36** by the comparison of Al<sup>2</sup>

**Figure 35.** MQL grinding temperature of hardened AISI 52100 steel with Al<sup>2</sup>

the grinding depth 10 μm) [61].

and grinding forces.

investigate the effect of size of nanoparticles at grinding wheel speed =0.05 m/s and the grinding depth = 10 μm [61]. The grinding temperature was reduced with the nanofluid of smaller

of 40 nm and 80 nm, it is clearly seen that the better surface roughness can be achieved by using NFs with NP size of 40 nm, even in different nanoconcentration (shown in **Figure 36**).

tigate the effect of size of nanoparticles [62]. From **Figures 37** and **38**, MQL grinding process with nanofluids exhibits better surface roughness and reduces grinding forces when compared to those of dry and pure MQL grinding. Considered the NP size among nanofluids, the ND with smaller size 30 nm gives the best grinding performance in term of surface roughness

Overall, the nano/microparticle sizes have the strong effects on cutting performance. Nanofluids exhibit better machining performance than microfluids. The smaller the NP size is, the better surface quality will be. However, the cost of NPs rises with smaller size, and so the appropriate nanoconcentration in fluid will be the key parameter affecting the application

The nanoparticle concentration has attained a significant attention of many researchers because it influences on the performance of nanofluids and directly contributes a large fraction of the NF cost. The experimental study on nanolubricant of nanographite with different concentration 0.1% and 0.5% reveals that the lower friction coefficients and average temperature of lubricated surfaces of the specimens can be achieved in case of nanolubricant with larger volume of fraction 0.5% (shown in **Figures 39** and **40**) [60]. The similar observation can

3, and 5%. The value of surface roughness decreases as the nanoconcentration rises from 1 to

O3

O3

O3

O3

O3

nanofluids and nanodiamond was done to inves-

nanofluids with three concentrations 1,

nanofluids (grinding wheel speed =0.05m/s;

nanofluids was done to

189

Micro/Nanofluids in Sustainable Machining http://dx.doi.org/10.5772/intechopen.75091

nanofluids with NP size

**Figure 33.** The relationship between 500 g of MoS<sup>2</sup> nanoparticle size and its cost.

grain size of nanoparticles strongly influences the cost of NPs. Hence, the NP size is definitely needed to optimize while remaining the good performance of nanofluids and the reasonable manufacturing cost [58, 59]. The experimental study on nanolubricant of nanographite (0.1 vol%) was carried out with diffrent particle sizes 5 μm, 450 nm, and 55 nm [60].

From **Figure 34**, the friction coefficient of nanolubricant of the disc specimen as a function of the applied normal force exhibits the much lower values compared to microlubricant and raw mineral lubricant. In this test, the fluid with the smallest NP size 55 nm shows the lowest friction coefficient and reaches stable state when increasing the applied normal force. Moreover, the microfluid of the microparticle size 5 μm shows the highest friction coefficient, which is also higher than that of pure mineral lubricant.

**Figure 34.** Friction coefficients of the disc specimen as a function of the applied normal force at different particle sizes [60].

The MQL grinding process of hardened AISI 52100 steel with Al<sup>2</sup> O3 nanofluids was done to investigate the effect of size of nanoparticles at grinding wheel speed =0.05 m/s and the grinding depth = 10 μm [61]. The grinding temperature was reduced with the nanofluid of smaller NP size (**Figure 35**). When comparing the surface roughness of Al<sup>2</sup> O3 nanofluids with NP size of 40 nm and 80 nm, it is clearly seen that the better surface roughness can be achieved by using NFs with NP size of 40 nm, even in different nanoconcentration (shown in **Figure 36**).

Another MQL grinding process with Al<sup>2</sup> O3 nanofluids and nanodiamond was done to investigate the effect of size of nanoparticles [62]. From **Figures 37** and **38**, MQL grinding process with nanofluids exhibits better surface roughness and reduces grinding forces when compared to those of dry and pure MQL grinding. Considered the NP size among nanofluids, the ND with smaller size 30 nm gives the best grinding performance in term of surface roughness and grinding forces.

Overall, the nano/microparticle sizes have the strong effects on cutting performance. Nanofluids exhibit better machining performance than microfluids. The smaller the NP size is, the better surface quality will be. However, the cost of NPs rises with smaller size, and so the appropriate nanoconcentration in fluid will be the key parameter affecting the application of nanofluids in machining practice.
