Tribological Behaviour of Graphene Nanoplatelets as Additive in Pongamia Oil

This study investigated the tribological behaviour of Pongamia oil (PO) and 15W–40 mineral engine oil (MO) with and without the addition of graphene nanoplatelets (GNPs). The friction and wear characteristics were evaluated in four-ball anti-wear tests according to the ASTM D4172 standard. The morphology of worn surfaces and the lubrication mechanism of GNPs were investigated via SEM and EDS. This study also focuses on the tribological effect of GNP concentration at various concentrations. The addition of 0.05 wt % GNPs in PO and MO exhibits the lowest friction and wear with 17.5% and 12.24% friction reduction, respectively, and 11.96% and 5.14% wear reduction, respectively. Through SEM and EDS surface analysis, the surface enhancement on the worn surface by the polishing effect of GNPs was confirmed. The deposition of GNPs on the friction surface and the formation of a protective film prevent the interacting surfaces from rubbing, resulting in friction and wear reduction.


Introduction
Energy demand has been precipitously increasing due to the increase in population and rapid modernization and industrialization globally. Due to the rise in energy demand, the depletion of fossil fuel sources has become a critical issue for the nation. The desire to protect the environment has also triggered the search for new alternative sources. Global researchers have attempted to improve environmental friendliness, energy efficiency, dependability, and reliability in the automotive and transportation sector. Thus, the improvement of the lubrication properties of lubricants is essential. A good combination of base oils and additives to formulate the lubricants will play a critical role in this improvement. Generally, the conventional lubricants used for machinery or mechanical systems are mineral-oil-based lubricants.
However, the disposal of mineral oil causes pollution in aquatic and terrestrial ecosystems [1]. Vegetable oils are a promising alternative to mineral oil because they are renewable, non-toxic, and biodegradable. The properties of vegetable oils fulfil lubricant requirements such as high viscosity index, high flash point, low volatility, excellent lubricity, and these are excellent solvents for fluid additives [2,3]. In contrast, vegetable oils exhibit poor oxidation stability and low thermal stability due to their unsaturation. Still, these disadvantages can be overcome by chemical modification such as transesterification, selective hydrogenation, dimerization, or epoxidation [4]. As mentioned previously, the lubricant properties can be improved by additives. Usually, the anti-wear (AW) and extreme pressure (EP) additives are the main additives to lubricants. These additives are also crucial in several frictional conditions [5]. However, conventional EP and AW additives such as zinc dialkyl-dithiophosphate (ZDDP), tricresyl phosphate (TCP), and molybdenum dialkyl-dithiocarbamate (MoDTC) contain compounds such as sulphur, chlorine, and phosphorus, which are harmful to the environment. Therefore, new EP and AW additives with vastly enhanced tribological properties are urgently required. Nanoparticles (NPs) have been considered as potential candidates for conventional EP and AW additives because of their excellent tribological properties, such as functioning as EP/AW additives and also acting as friction modifiers [6,7]. Most NPs are eco-friendly, as they reduce the consumption of hazardous materials and additives, which benefits the environment. However, there are several NPs with toxic properties, so far, the evaluation carried out on some toxic effects of NPs has been conflicting and inconsistent [8], and the toxicity of NPs depends on shape, size, and other characteristics [9]. Additionally, the application of eco-friendly NPs may aid in the decrease of energy usage during manufacturing operations, hence lowering the carbon footprint [6,9].
Many studies have recently been carried out on nanoparticles' application as lubricant additives in the tribology field. Most of these studies reported that the addition of nanoparticles to base oils enhanced the tribological performance [10][11][12][13]. Additionally, the use of functionalized NPs not only improved the tribological properties but also achieved good dispersion stability in [14,15]. Several lubrication mechanisms of nanoparticles that enhance the tribological performance have been proposed [16], including the rolling effect [17,18], mending effect [19,20], protective film formation [12,[21][22][23], and polishing effect [24]. These mechanisms are classified into two groups: the first is the direct action of NPs on lubrication enhancement (rolling effect and protective film formation) and the second group is the surface enhancement by NPs (mending effect and polishing effect). For fullerene-like nanoparticles (IF-NPs), there are three main mechanisms reported [25]: rolling, sliding, and exfoliation. Rolling is when the NPs act like ball bearings between contact surfaces. For sliding, the NPs reduce friction and enhance shear by separating the contact surfaces. For the exfoliation mechanism, the exfoliated layer of the IF-NPs is deposited on the asperities of the interacting surface, which shears the surface and provides low friction and wear. Based on the literature, Jason et al. [26] mentioned four main parameters of nanoparticles that affect the tribological properties: (1) concentration, (2) size, (3) morphology, and (4) dispersion stability. This study will focus on the concentration effect of nanoparticles.
Despite the progress made in research, most researchers focus on investigating the nanoparticles as lubricant additives in mineral-oil-based or synthetic-oil-based lubricants, and only a few studies have been dedicated to vegetable-oil-based lubricants. According to Azman et al. [27], the addition of CuO in palm kernel oil reduces friction and wear by up to 56% and 48%, respectively, which represents a huge reduction. Wang et al. [28] reported that the addition of Cu, CNTs, Cu/CNTs, and Cu/CNTs/PDA improved the tribological performance in rapeseed oil, and the latter showed the greatest improvement. The formation of tribofilms was concluded to be the reason for the improvement. Rajubhai et al. [29] investigated the variation of Cu concentration in Pongamia oil. Minimum friction and wear were observed with the addition of 0.075% of Cu, and further increments of concentration showed a detrimental effect. Table 1 shows the previous research carried out on vegetable oil with added NPs for lubricant application.
The formulation of vegetable-oil-based nanolubricants could offer more significant benefits in terms of the environment. In the present study, Pongamia oil (PO) was selected as a base oil because it is inedible, and therefore its use does not affect the food supply. The annual Pongamia seed yield is 20 to 80 kg per tree; it can be easily obtained and adapted to marginal land, which reduces conflict with agriculture [30]. Thus, the present study investigates the tribological effect of graphene nanoplatelets (GNPs) in PO using a four-ball tester according to ASTM D4172 [31]. Furthermore, for comparison, the mineral oil SAE 15W40 (MO) was used as base oil. After a series of tribo-testing, worn surfaces were determined by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS).

Preparation of Graphene-Based Nanolubricant
In this study, PO and MO were selected as base oils without any modification. PO extracted by the cold pressing method was purchased from Ramamoorthy Exports, India. The physicochemical properties of these base oils are given in Table 2. Graphene nanoplatelets (GNPs) were purchased from Sigma-Aldrich (M) Sdn. Bhd, Malaysia. Figure 1 illustrates the morphology of the nanostructure and element composition of the graphene nanoplatelets. The particle size (<2 µm) was verified using SEM, and it matched the details provided by the supplier. The properties of nanoparticles provided by the supplier are listed in Table 3. Since the addition concentration of nanoparticles is one of the important parameters that affects the tribological properties, GNPs were added in the base oils with different concentrations of 0.01, 0.02, 0.05, 0.1 and 0.2 wt %. GNPs were dispersed in base oils using a hot plate with a magnetic stirrer, and this mixing method was proposed by previous researchers for stable suspension [35,36]. The agitation time was fixed at 30 min, the hot plate temperature was at 60 • C, and the agitation speed was between 350 and 500 rpm.

Tribological Experimental Test
The DUCOM TR30L-LAS four-ball tester (Ducom Instruments, Bengaluru, India) was used to investigate the friction and wear characteristics of lubricants with and with-

Tribological Experimental Test
The DUCOM TR30L-LAS four-ball tester (Ducom Instruments, Bengaluru, India) was used to investigate the friction and wear characteristics of lubricants with and without the addition of nanoparticles. Four balls were used to test the specimen in the operation of this equipment. The schematic diagram of the four-ball tester is shown in Figure 2. The steel balls were made of alloy steel AISI 52100 with a diameter of 12.7 mm and a hardness of 64-66 HRC. The steel balls and ball pot were thoroughly cleaned with acetone before and after each test. Three steel balls (stationary balls) were placed and fixed in a steel pot, a steel ball (rotating ball) was fixed at the chuck, and around 10 mL of oil specimen was poured into the steel pot to cover the steel balls at a depth of at least 3 mm. The experiment test parameters were regulated by ASTM standards D4172 [31], and the significant test condition is given in Table 4. Each series of experimental tests was performed three times to provide mean friction coefficient values. After every test, the stationary balls were taken out of the steel pot for wear scar diameter measurement. before and after each test. Three steel balls (stationary balls) were placed and fixed in a steel pot, a steel ball (rotating ball) was fixed at the chuck, and around 10 mL of oil specimen was poured into the steel pot to cover the steel balls at a depth of at least 3 mm. The experiment test parameters were regulated by ASTM standards D4172 [31], and the significant test condition is given in Table 4. Each series of experimental tests was performed three times to provide mean friction coefficient values. After every test, the stationary balls were taken out of the steel pot for wear scar diameter measurement.  The frictional torque (raw data) was obtained after the test utilizing Winducom 2010 software. The conversion of frictional torque to the coefficient of friction (COF) was calculated by Equation (1) [42]. The measurement of wear scar diameter (WSD) on the tested steel balls was measured by capturing images of the wear scars with an image acquisition system (optical microscope) connected to a computer equipped with image capture software.  Friction and Wear Measurement The frictional torque (raw data) was obtained after the test utilizing Winducom 2010 software. The conversion of frictional torque to the coefficient of friction (COF) was calculated by Equation (1) [42]. The measurement of wear scar diameter (WSD) on the tested steel balls was measured by capturing images of the wear scars with an image acquisition system (optical microscope) connected to a computer equipped with image capture software.

Coefficient of Friction
where T = friction torque (kg/mm), W = load (kg), r = the distance between the centre of the contact surfaces of the lower balls, and the rotation axis is 3.67 mm.

Surface Analysis
The characteristics and element distribution of the worn surface on the tested steel balls were analysed using Hitachi S-3400N (Hitachi High-Tech Corporation, Kawasaki, Japan) and Hitachi S-3700N (Hitachi High-Tech Corporation, Kawasaki, Japan) scanning electron microscopes (SEMs) equipped with energy-dispersive spectroscopy (EDS). The electron-accelerating voltage was 15 kV.

Results and Discussion
The tribological effects of GNPs as lubricant additives in PO and MO were evaluated using the four-ball test. Tests for the MO with and without GNPs were used as a reference to determine the influence of PO and PO/GNPs lubricants in terms of friction and wear behaviour.

Friction and Wear Behaviour
The tribological performance of lubricants with and without GNPs was evaluated by performing three runs for each series of experimental tests to obtain mean COF and WSD values. Figure 3 depicts the COF curves for PO-and MO-series oil samples as they varied with the testing time. In Figure 3a, the increase of COF for MO-series lubricants in the initial run-in stage (first 100-400 s) can be seen, as the downtrend sign of COF was between 300 s to 600 s and afterwards, the trend was almost linear. For MO + 0.2, the spike occurred at around 600 s and caused the COF to increase rapidly, though only in the short term. This can be attributed to NPs' suspension stability decreasing at high concentrations, which affects their anti-friction performance [43]. During the stable stage (900-3600 s), it could be observed that MO and MO + 0.2 maintained a slightly higher COF compared to the others, while MO + 0.05 clearly showed better anti-friction performance. In Figure 3b, it can be seen that the COF for PO and PO + 0.1 at the initial run-in stage (first 300 s) increased and then decreased linearly. PO + 0.01 and PO + 0.02 showed similar trends; it can be observed both COF curves were higher than PO. PO + 0.05 maintained a stable anti-friction performance from the beginning until the end of the test.

Surface Analysis
The characteristics and element distribution of the worn surface on the tested steel balls were analysed using Hitachi S-3400N (Hitachi High-Tech Corporation, Kawasaki, Japan) and Hitachi S-3700N (Hitachi High-Tech Corporation, Kawasaki, Japan) scanning electron microscopes (SEMs) equipped with energy-dispersive spectroscopy (EDS). The electron-accelerating voltage was 15 kV.

Results and Discussion
The tribological effects of GNPs as lubricant additives in PO and MO were evaluated using the four-ball test. Tests for the MO with and without GNPs were used as a reference to determine the influence of PO and PO/GNPs lubricants in terms of friction and wear behaviour.

Friction and Wear Behaviour
The tribological performance of lubricants with and without GNPs was evaluated by performing three runs for each series of experimental tests to obtain mean COF and WSD values. Figure 3 depicts the COF curves for PO-and MO-series oil samples as they varied with the testing time. In Figure 3a, the increase of COF for MO-series lubricants in the initial run-in stage (first 100-400 s) can be seen, as the downtrend sign of COF was between 300 s to 600 s and afterwards, the trend was almost linear. For MO + 0.2, the spike occurred at around 600 s and caused the COF to increase rapidly, though only in the short term. This can be attributed to NPs' suspension stability decreasing at high concentrations, which affects their anti-friction performance [43]. During the stable stage (900-3600 s), it could be observed that MO and MO + 0.2 maintained a slightly higher COF compared to the others, while MO + 0.05 clearly showed better anti-friction performance. In Figure 3b, it can be seen that the COF for PO and PO + 0.1 at the initial run-in stage (first 300 s) increased and then decreased linearly. PO + 0.01 and PO + 0.02 showed similar trends; it can be observed both COF curves were higher than PO. PO + 0.05 maintained a stable antifriction performance from the beginning until the end of the test. This can be attributed to the addition of NPs, which increased the viscosity of nanolubricant, thus a thick and stable tribofilm formed and provided better surface protection. An increase in the concentration of NPs increases the viscosity of nanolubricants. However, when a certain concentration is reached, the viscosity will decrease due to NP agglomeration [44,45]. PO + 0.2 showed an increasing COF trend from the initial run-in stage until 900 s, afterwards, the trend remained constant. This can be attributed to the agglomeration of NPs which decreased the viscosity, hence causing the highest friction. Additionally, Alves et al. [5] reported that the addition of 0.5 wt % CuO and ZnO in vegetable oil caused a small increment in the COF because the lubricant had low viscosity. The explanation of the phenomenon of the COF trend increase in the run-in-stage is suggested as follows: lubricious contaminants are removed from the contact surface [46] and the oxide layer on the contact surface is disrupted [47], thus increasing the metal-to-metal contact. Table 5 presents the value of mean COF, WSD, and the standard deviation. oil. The COF and WSD results for MO-series lubricants had similar trends. The COF results correspond to the WSD results. With the addition of GNPs, the COF and WSD decreased compared with MO. The friction and wear were reduced by the addition of GNPs from 0.01 to 0.05 wt %, but further addition resulted in a slight increment. The MO-enriched 0.2 wt % GNPs (MO + 0.2) showed the least improvement, and further concentration increases are expected to be detrimental. In contrast, MO containing 0.05 wt % GNPs (MO + 0.05) exhibited the highest friction and wear resistance (COF = 0.049 and WSD = 709.78 µm). A maximum improvement of 12.24% friction reduction and 5.14% wear reduction was observed in MO + 0.05 compared to MO.   For PO-series lubricants, the friction was enhanced after adding 0.01 and 0.02 wt % GNPs, and the COF values were higher by 7.84% and 0.68%, respectively, compared to bare Pongamia oil. This can be attributed to a lack of GNPs, which form an unstable tribofilm on the friction surface, resulting in a relatively high COF [48,49]. PO + 0.05 (COF = 0.040 and WSD = 831.4 µm) exhibited a maximum improvement by reducing friction by 17.5% and wear by 11.96% compared to PO (COF = 0.047 and WSD = 930.8 µm), while PO + 0.1 exhibited minimal improvement. The wear resistance was enhanced in PO consisting of 0.01 to 0.1 wt % GNPs. The addition of 0.2 wt % GNPs (PO + 0.2) resulted in the highest COF and WSD and increased friction by 12.96% and wear by 10.37% compared to PO.
In addition, the comparative study of the friction and wear behaviour for MO and PO is shown in Figures 4 and 5. PO showed better anti-friction performance, while MO exhibited better anti-wear performance. This phenomenon can be explained as follows: the linear fatty acid chain in vegetable oil promotes a smoother interaction of molecules during relative motion, which reduces friction. However, it is susceptible to metal-to-metal contact (less wear protection) [50], whereas the anti-wear performance of MO is treated by the existing additive package. Furthermore, a similar trend was observed with the addition of 0.05 wt % GNPs in PO and MO. Both exhibited the greatest improvement in tribological properties. However, a further increase in concentration caused the wear and friction to increase as a result of agglomeration caused by the high concentration of GNP aggregates that are unable to fit into surface asperities, causing the COF and WSD to increase [10,34]. Moreover, the results of the present study of PO with the addition of GNPs is consistent with those of previous studies. Azman et al. [34] reported that adding 0.01 and 0.03 wt % GNPs in polyalphaolefins (PAOs) blended with palm-oil trimethylolpropane (TMP) resulted in friction increase, and the optimum concentration was 0.05 wt %. The possible lubrication mechanisms in the presence of nanolubricants can be attributed to the direct action of nanoparticles and the surface enhancement by nanoparticles, which enhance the tribological performance [51,52]. Nonetheless, the possible lubrication mechanism of nanolubricants was further investigated and discussed in the surface analysis.

Worn Surface Analysis
To understand the anti-wear behaviour and the lubrication mechanism of the considered oil samples, the worn surfaces on the tested balls were analysed using SEM equipped with EDS. Figure 6 provides the SEM micrographs of the worn surfaces on the tested balls. Surface cracks and pits were observed on the worn surfaces after being lubricated by bare mineral oil, but the surface was slightly smoother for worn surfaces lubricated with MO + 0.05, and the surface defects were reduced. Additionally, the surface-mending-like mechanism can be seen on the micrographs, as referenced in previous work [10]. However, with the presence of 0.2 wt% GNPs in MO, there was only the slightest enhancement, and the surface cracks and pits remained. For the worn surface after being lubricated by bare Pongamia oil, deep grooves were found on the surface, evidencing the occurrence of abrasive wear. Slight surface enhancement was observed with the presence of 0.01 wt % GNPs in PO. Even though the friction was slightly higher than that of base PO, GNPs smoothed the surface, and the deep grooves were replaced by light and scratched grooves, as shown in Figure 6. In the SEM micrograph of PO + 0.05, the surface was further smoothened, it can be seen that the grooves were treated, and the deposition of GNPs (black marks) can be observed in the micrograph.
The results of the SEM analysis prove that GNPs were deposited on the worn surface by filling up the surface gaps and forming a protective film to prevent the interacting surface from rubbing, resulting in low friction and wear. Similar conclusions were reported in several previous works [11,13,53,54]. The surface deteriorated after lubrication by PO + 0.2; deep grooves and wear debris occurred on the surface, and the abrasive wear became serious compared to that in PO. This was due to GNP aggregation resulting in increased friction and wear, thus increasing the metal-to-metal contact, and the evidence was seen in the SEM micrograph, which was also mentioned in the study of Wang et al. [19]. In addition, through the observation in Figure 6, the wear mechanism involved for MO is suggested as fatigue wear (existence of cracks, pitting, delamination), while for PO it is suggested to be abrasive wear. Fatigue wear can be explained as repeated loading during the sliding process. Vegetable oil has poor oxidation stability compared to mineral oil. As a result, the abrasive wear that occurs after being lubricated by PO can be explained by the weakening of oxidation stability during the sliding process [55], causing the oil film to thicken and polymerize, resulting in abrasive wear. Similar wear mechanism findings for mineral oil and vegetable oil were also reported by Bahari et al. [50].

Worn Surface Analysis
To understand the anti-wear behaviour and the lubrication mechanism of the considered oil samples, the worn surfaces on the tested balls were analysed using SEM equipped with EDS. Figure 6 provides the SEM micrographs of the worn surfaces on the tested balls. Surface cracks and pits were observed on the worn surfaces after being lubricated by bare mineral oil, but the surface was slightly smoother for worn surfaces lubricated with MO + 0.05, and the surface defects were reduced. Additionally, the surfacemending-like mechanism can be seen on the micrographs, as referenced in previous work [10]. However, with the presence of 0.2 wt% GNPs in MO, there was only the slightest enhancement, and the surface cracks and pits remained. For the worn surface after being lubricated by bare Pongamia oil, deep grooves were found on the surface, evidencing the occurrence of abrasive wear. Slight surface enhancement was observed with the presence of 0.01 wt % GNPs in PO. Even though the friction was slightly higher than that of base PO, GNPs smoothed the surface, and the deep grooves were replaced by light and scratched grooves, as shown in Figure 6. In the SEM micrograph of PO + 0.05, the surface was further smoothened, it can be seen that the grooves were treated, and the deposition of GNPs (black marks) can be observed in the micrograph. The results of the SEM analysis prove that GNPs were deposited on the worn surface by filling up the surface gaps and forming a protective film to prevent the interacting surface from rubbing, resulting in low friction and wear. Similar conclusions were reported in several previous works [11,13,53,54]. The surface deteriorated after lubrication by PO + 0.2; deep grooves and wear debris occurred on the surface, and the abrasive wear became serious compared to that in PO. This was due to GNP aggregation resulting in increased friction and wear, thus increasing the metal-to-metal contact, and the evidence was seen in the SEM micrograph, which was also mentioned in the study of Wang et al. [19]. In addition, through the observation in Figure 6, the wear mechanism involved for MO is suggested as fatigue wear (existence of cracks, pitting, delamination), while for PO it is suggested to be abrasive wear. Fatigue wear can be explained as repeated loading during the sliding process. Vegetable oil has poor oxidation stability compared to mineral oil. As a result, the abrasive wear that occurs after being lubricated by PO can be explained by the weakening of oxidation stability during the sliding process [55], causing the oil film to thicken and polymerize, resulting in abrasive wear. Similar wear mechanism findings for mineral oil and vegetable oil were also reported by Bahari et al. [50]. Figure 7a-g demonstrates the EDS analysis of the worn surface on the tested balls, and this analysis was conducted corresponding to the SEM micrographs (2000×) shown in Figure 6. Based on Figure 7a-g, similar elements were found on all worn surfaces: iron (Fe), carbon (C), oxygen (O). Iron is the main element of AISI 52100 steel balls which contributed the highest value in the analysis. Oxygen can be related to the formation of oxide layers during the lubrication process, and it is a common element. The main element of GNPs, carbon (shown in Figure 1c), was detected in the analysis. Carbon was detected on MO worn surfaces; this can be attributed to the base oil containing the undefined carbonbased additive, and because carbon is the sub-element of steel balls. High carbon content (32.53 wt %) was found on the worn surface lubricated by MO + 0.05, indicating a surfacemending-like mechanism and GNPs' formation of a protective film on the surface. There was an absence of carbon on the worn surface lubricated by PO because it is bare oil without modification, as shown in Figure 7d. Based on Figure 7e-g, the increase in the concentration increased the carbon content, proving the deposition of GNPs on the surface. However, for PO + 0.01, even the lack of GNPs caused friction to increase, but the wear was reduced, whereas, for PO + 0.2, GNPs aggregated on the surface and presented 14.02 wt % of carbon.   Figure 6. Based on Figure 7a-g, similar elements were found on all worn surfaces: iron (Fe), carbon (C), oxygen (O). Iron is the main element of AISI 52100 steel balls which contributed the highest value in the analysis. Oxygen can be related to the formation of oxide layers during the lubrication process, and it is a common element. The main element of GNPs, carbon (shown in Figure 1c), was detected in the analysis. Carbon was detected on MO worn surfaces; this can be attributed to the base oil containing the undefined carbonbased additive, and because carbon is the sub-element of steel balls. High carbon content (32.53 wt %) was found on the worn surface lubricated by MO + 0.05, indicating a surfacemending-like mechanism and GNPs' formation of a protective film on the surface. There was an absence of carbon on the worn surface lubricated by PO because it is bare oil without modification, as shown in Figure 7d. Based on Figure 7e-g, the increase in the concentration increased the carbon content, proving the deposition of GNPs on the surface. However, for PO + 0.01, even the lack of GNPs caused friction to increase, but the wear was reduced, whereas, for PO + 0.2, GNPs aggregated on the surface and presented 14.02 wt % of carbon.

Conclusions
The tribological effects of GNPs on mineral oil and Pongamia oil were investigated by a four-ball test in accordance with ASTM D4172. The addition of GNPs as an additive in MO improved the tribological properties. MO + 0.05 (COF = 0.049 ± 0.0003 and WSD = 709.7 ± 23.8 µm) exhibited the greatest improvement, with 12.24% friction reduction and 5.14% wear reduction compared to MO (COF = 0.055 ± 0.0016 and WSD = 746.2 ± 19 µm). The presence of 0.01 wt % and 0.02 wt % GNPs in PO resulted in 7.84% and 0.68% higher friction than PO, and PO + 0.2 (COF = 0.054 ± 0.0029 and 1038 ± 13.9 µm) showed a detrimental effect and resulted in the highest COF and WSD in PO-series lubricants. PO + 0.05 (COF = 0.040 ± 0.0007 and WSD = 831.4 ± 51.2 µm) exhibited a maximum tribological improvement, reducing friction by 17.5% and wear by 11.96% compared to PO (COF = 0.047 ± 0.0007 and WSD = 930.8 ± 30.3 µm) in PO-series lubricants. Thus, GNPs had a similar optimum concentration (0.05 wt %) for MO and PO. Surface analysis via SEM and EDS confirmed the surface enhancement on the worn surface by the polishing effect of GNPs. The deposition of GNPs on the surface and protective film formation was observed; thus, the improvement in tribological properties is attributed to these lubrication mechanisms.