**3.1. Rheological properties**

**Figure 3** displays the kinematic viscosity values and the calculated viscosity index (VI). The kinematic viscosity of the reference oil (SE) at 40 and 100°C are 21.5 and 5.6 mm<sup>2</sup> /s, respectively. It can be seen that MRPO + hBN0.05% shows the highest kinematic viscosity values of 22.2 mm<sup>2</sup> /s at 40°C and 6.35 mm<sup>2</sup> /s at 100°C. The addition of hBN particles in MRPO-based oil improves the viscosity values due to lower thermal expansion coefficient of hBN particles (1 × 10−6/°C), thus enhanced the thermal stability [17]. Meanwhile, MRPO + PIL1% recorded the kinematic viscosity values of 22.21 mm<sup>2</sup> /s at 40°C and 6.25 mm<sup>2</sup> /s at 100°C. Both MRPO-based oils demonstrated the highest kinematic viscosity value compared to MJO-based oils and SE due to the high saturation of fatty acids in the MRPO-based oil. MRPO-based oil contains high composition of saturated fatty acid (palmitic acid, C15H31COOH) at 50–70% [44]. Meanwhile, both MJO-based oils had the lowest kinematic viscosity values at both temperatures. This can be explained by the presence of unsaturated fatty acids (oleic acid, C17H33COOH and linoleic acid, C17H31COOH) in MJO-based oil [45]. Moreover, MRPO + PIL1% demonstrates the highest VI value of 259 which was 17% higher than SE. The high VI is desirable as it indicates little changes in viscosity across a wide range of operating temperature. Both MJO-based oils had the lowest VI values which had 5% reduction compared to SE.

Tribological Interaction of Bio-Based Metalworking Fluids in Machining Process http://dx.doi.org/10.5772/intechopen.72511 53

**Figure 3.** The kinematic viscosity values at 40 and 100°C and the calculated viscosity index value.

#### **3.2. Tapping torque performance**

Finally, the sliding region on the tool insert's rake face was analyzed under an optical microscope and the tool-chip contact length was measured for data analysis. Scanning electron microscope (SEM) was used to further analyze the surface morphology of the sliding regions.

**Figure 3** displays the kinematic viscosity values and the calculated viscosity index (VI). The kine-

can be seen that MRPO + hBN0.05% shows the highest kinematic viscosity values of 22.2 mm<sup>2</sup>

the viscosity values due to lower thermal expansion coefficient of hBN particles (1 × 10−6/°C), thus enhanced the thermal stability [17]. Meanwhile, MRPO + PIL1% recorded the kinematic

strated the highest kinematic viscosity value compared to MJO-based oils and SE due to the high saturation of fatty acids in the MRPO-based oil. MRPO-based oil contains high composition of saturated fatty acid (palmitic acid, C15H31COOH) at 50–70% [44]. Meanwhile, both MJO-based oils had the lowest kinematic viscosity values at both temperatures. This can be explained by the presence of unsaturated fatty acids (oleic acid, C17H33COOH and linoleic acid, C17H31COOH) in MJO-based oil [45]. Moreover, MRPO + PIL1% demonstrates the highest VI value of 259 which was 17% higher than SE. The high VI is desirable as it indicates little changes in viscosity across a wide range of operating temperature. Both MJO-based oils had the lowest VI values which had

/s at 100°C. The addition of hBN particles in MRPO-based oil improves

/s, respectively. It

/s at 100°C. Both MRPO-based oils demon-

/s

matic viscosity of the reference oil (SE) at 40 and 100°C are 21.5 and 5.6 mm<sup>2</sup>

/s at 40°C and 6.25 mm<sup>2</sup>

**3. Results and discussions**

**Figure 2.** Orthogonal lathe cutting set-up.

52 Lubrication - Tribology, Lubricants and Additives

**3.1. Rheological properties**

at 40°C and 6.35 mm<sup>2</sup>

viscosity values of 22.21 mm<sup>2</sup>

5% reduction compared to SE.

**Figure 4** shows the tapping torque and efficiency for all lubricant samples. The reference oil (SE) had the highest tapping torque at 129 Nm. The results reveal that the tapping torque for all modified vegetable oils exceeded the tapping torque of SE. MJO + PIL1% had the lowest tapping torque of 104 Nm correlated with the highest tapping torque efficiency of 124%. The presence of PIL as an additive improves the tapping torque performance. This is because of the addition of PIL in MJO-based oil, which is thermally more stable than SE. The alkyl chain length and hydrogen bonding between the cation and the anion seem to influence the tribofilm formation of PIL [29]. Meanwhile, MJO + hBN0.05% recorded tapping torque of 117 Nm with the tapping torque efficiency of 110%. The presence of hBN particles provided a thin lubrication film that allows the particles to change from sliding friction to the rolling friction [18]. Moreover, the presence of long carbon chain length of MJO-based oil and MRPObased oil which is between 16 and 18 carbon number had enhanced the adsorption ability of the fatty acids on the metal surfaces, thus exhibited better tapping torque performance. MRPO + hBN0.05% and MRPO + PIL1% had tapping torque efficiency of 107 and 106%. It can be seen that the addition of PIL and hBN particles as the additive in MRPO-based oil did not significantly affect the tapping torque performance compared to the MJO-based oils. This scenario is due to the weak tribo-chemical reactions of additives with the MRPO-based oil, thus reduced the adsorption ability of the lubricant molecules on the metal surface [46].

#### **3.3. Orthogonal cutting performance**

The orthogonal lathe cutting operations were conducted at a constant speed and feed. The cutting force, cutting temperature, chip thickness, specific cutting energy, and tool-chip contact length are the main outputs of this experimental section analysis. The results for each lubricant mixture were compared with the conventional cutting fluid, synthetic ester (SE).

give similar improvement effect as shown by the results of MRPO base oil. MRPO + PIL1% and MRPO + hBN0.05% show a reduction in both cutting force and cutting temperature when

Tribological Interaction of Bio-Based Metalworking Fluids in Machining Process

http://dx.doi.org/10.5772/intechopen.72511

55

The addition of 1 wt. % PIL has enhanced the antifriction and antiwear properties of the base oil by reducing the scuffing effect and the abrasive wear mechanism [25]. The polarity of the phosphonium-based IL additive has resulted in the increased adsorption rate of the additive molecules on the metal surface. The result is also corroborated by the tapping torque and efficiency values reported in the previous section. The tribofilm formed helps lower the frictional torque of the base oil corresponding to the reduction of friction coefficient. It acts as a separation layer between the metal asperities and kept them apart from direct contact, thus

The addition of 0.05 wt. % hBN solid particles in the base oil reduces the average cutting force as well as the heat generated within the cutting zone by separating the metal asperities contacts during the sliding processes. However, the ability of the particles in reducing the frictional force and the heat generation is greatly affected by the particles filling rate in the asperity valleys which enabled them to align in parallel to the relative sliding motion, thus reducing the stress concentration on the contact surfaces [31]. Therefore, the high polarity of the PIL and the ability of its anionic moieties that can quickly adsorb on the sliding metal surfaces via strong electrostatic interactions even at high temperature and load working conditions has become the most attractive contribution of the PIL additive toward the formation of tenacious lubricant films on the metal surface that greatly reduces friction and wear [20]. This type of lubricant additive has successfully improved the tribological performance of the polar oil of MJO- and MRPO-based lubricant samples during the machining of the plain medium carbon steel of AISI 1045 [47].

The average chip thickness after the machining processes is exhibited in **Figure 6**. During the material removal process, the chip is formed due to the elastic, elastic-plastic, and plastic deformation processes of the workpiece material. It is mainly influenced by the heat generation under high stresses and temperature arisen due to the high deformation resistance between the cutting insert and the workpiece material being cut [48]. The chip thickness is one of the parameters that affected the chip formation mechanisms with the shearing angle between the uncut chip thickness and the cutting forces required during the material removal process [39]. An effective surface lubrication on the cutting zone has helped reduce the chip thickness produced after the machining operations by reducing the thermal stresses that occurred on the sliding surfaces. As presented in the previous subsection, the reduced friction due to high lubrication effect of MJO + PIL1% compared to the SE has successfully decreased 20% of the chip size which indicates the reduced tensile strain on the outer surface of the chip during bending. The specific thermal effects were reduced due to the adequate lubricant being sprayed and penetrated the sliding interfaces [49]. In addition, the fast and strong electrostatic interactions between the lubricant and additive molecules with the metal substrates had formed the tenacious lubricant film that reduced the contact area at the shear zone, thus resulting in the reduction of frictional force [20, 30, 38]. Furthermore, this phenomenon also contributed to thinner chips being cut with large shear angle and low cutting energy. These results were

compared to the SE in a range between 0.4 and 8% decrement respectively.

reduces the cutting force and generates less heat.

*3.3.2. Chip thickness analysis*

**Figure 4.** Tapping torque and torque efficiency value of all lubricant samples.

#### *3.3.1. Cutting force and temperature analysis*

**Figure 5** shows the results of cutting force, *Fc* measured in the Z-axis and the maximum cutting temperature results after the orthogonal cutting operations. It is shown that SE produced the highest cutting force and cutting temperature at ca. 612 N and 308°C respectively. SE generated poor lubrication condition on the cutting zone as compared to the other lubricant samples. MJO + PIL1% produces the greatest reduction of cutting force (2% reduction) as well as the cutting temperature (10% reduction) compared to the SE which corresponds to the good lubrication ability of the PIL additive contained in the base oil, MJO. The addition of 0.05 wt. % hBN solid particles also improved the lubrication ability of the MJO base oil. It is anticipated that the different type of lubricant used with the addition of the same additive did

**Figure 5.** Cutting force and temperature results.

give similar improvement effect as shown by the results of MRPO base oil. MRPO + PIL1% and MRPO + hBN0.05% show a reduction in both cutting force and cutting temperature when compared to the SE in a range between 0.4 and 8% decrement respectively.

The addition of 1 wt. % PIL has enhanced the antifriction and antiwear properties of the base oil by reducing the scuffing effect and the abrasive wear mechanism [25]. The polarity of the phosphonium-based IL additive has resulted in the increased adsorption rate of the additive molecules on the metal surface. The result is also corroborated by the tapping torque and efficiency values reported in the previous section. The tribofilm formed helps lower the frictional torque of the base oil corresponding to the reduction of friction coefficient. It acts as a separation layer between the metal asperities and kept them apart from direct contact, thus reduces the cutting force and generates less heat.

The addition of 0.05 wt. % hBN solid particles in the base oil reduces the average cutting force as well as the heat generated within the cutting zone by separating the metal asperities contacts during the sliding processes. However, the ability of the particles in reducing the frictional force and the heat generation is greatly affected by the particles filling rate in the asperity valleys which enabled them to align in parallel to the relative sliding motion, thus reducing the stress concentration on the contact surfaces [31]. Therefore, the high polarity of the PIL and the ability of its anionic moieties that can quickly adsorb on the sliding metal surfaces via strong electrostatic interactions even at high temperature and load working conditions has become the most attractive contribution of the PIL additive toward the formation of tenacious lubricant films on the metal surface that greatly reduces friction and wear [20]. This type of lubricant additive has successfully improved the tribological performance of the polar oil of MJO- and MRPO-based lubricant samples during the machining of the plain medium carbon steel of AISI 1045 [47].
