Synergistic Lubricating Performance of Graphene Oxide and Modiﬁed Biodiesel Soot as Water Additives

: The tribological performance of graphene oxide (GO) nanosheets, modiﬁed biodiesel soot (MBS) nanoparticles, and their mixture (MBS–GO) nanoparticles as lubricant additives in water was evaluated using a reciprocating ball-on-plate tribometer. The effects of different mass ratios of GO to MBS, additive concentrations, and loads, as well as corresponding lubrication mechanisms, were studied. The tribological measurements showed that the water-containing 0.5 wt% additives at a mass ratio of 60:40 (GO to MBS) resulted in larger reductions in friction coefﬁcient (69.7%) and wear volume (60.5%) than water. Owing to the synergistic effect of GO nanosheets and MBS nanoparticles, the MBS–GO aqueous sample showed superior lubricating properties compared to water as well as GO and MBS aqueous samples. The good tribological properties of MBS–GO nanoparticles in water are attributed to the formation of a triboﬁlm of hybrid nanoparticles that effectively protects the friction interface. Moreover, the MBS nanoparticles can provide lubrication by acting as ball bearings.


Introduction
Friction, wear, and lubrication are constant problems in the various engineering fields of industrial production [1]. Lubricants are critical for reducing friction, wear, and energy consumption as well as prolonging the service life of mechanical equipment [2]. Conventional oil-based lubricants are still dominant in today's market; however, their environmental impacts cannot be ignored [3]. Recently, environment friendly water-based lubricants have been used in many fields, particularly in specialized engineering fields, such as underwater robots, ships, and biological systems [4,5]. As a result, water-based lubricants are an important development direction of lubricants. Good lubricating additives are the key to produce water-based lubricants with excellent antifriction properties and wear resistance [6].
Carbon nanomaterials have recently attracted extensive attention for use as lubricating additives in water [7]. Some carbon nanomaterials, such as carbon quantum dots, onionlike carbon, graphene, nanodiamonds, and carbon nanotubes, have been explored for the development of water-based lubricating additives owing to their unique physical and chemical properties [8][9][10][11]. Graphene oxide (GO) has a unique crystal structure and selflubricating properties [12,13]. Thus, it has become a research hotspot as a water-based lubricating additive. Su et al. [14] and Zhang et al. [15] found that GO as an additive in water show antifriction and antiwear effects as they can adsorb onto the friction plate to form a tribofilm. Gan et al. [16] prepared GO with a high degree of exfoliation (GO-EmimN(CN) 2 ) using EmimN(CN) 2 as an intercalator. The GO-EmimN(CN) 2 showed high lubrication due to the presence of multiple self-lubricating films including easily sheared

Tribological Tests
The tribological tests were performed on an HSR-2M reciprocating ball-on-plate tribometer (Lanzhou Zhongkekaihua Technology Development Co., Ltd., Lanzhou, China) at room-temperature. A schematic of the tribological test is shown in Figure 1. The upper ball (6-mm diameter) used for the tribological tests was made of GCr15 steel with a hardness of 647-861 HV. The lower plate was made of 304 austenitic stainless steel with dimensions of 60 mm × 30 mm × 5 mm and a hardness of 200-210 HV. The surface roughness was maintained at 0.02 µm for all upper balls and lower plates. Before the tribological tests, the upper balls and lower plates were cleaned ultrasonically with ethanol for 10 min. The friction tests were performed at an average sliding speed of 50 mm/s, a stroke length of 5 mm, an oscillating frequency of 5 Hz, and different loads of 10-40 N for 30 min. The friction coefficients were recorded using the software attached to the tribometer. The average friction coefficient was calculated from the average value of friction coefficient over the 30 min testing time. After the friction tests, the friction pairs were ultrasonically cleaned with ethanol and then dried using a hair dryer. The wear volume of the lower plate was measured and calculated using three-dimensional (3D) laser-scanning microscope (VK-X100, Keyence, Osaka, Japan). Experiments were repeated at least three times under each condition.
of GO to MBS, additive concentrations, and loads were discussed along with the corresponding mechanisms. We demonstrate by comparison that the addition of MBS-GO improves the friction reduction and wear resistance of water via its synergistic effect.

Materials
GO nanosheets were supplied by Nanjing XFNANO Materials Tech Co., Ltd. (Nanjing, China). From biodiesel soot, MBS nanoparticles were prepared using a nitric acid treatment method [28,32]. The details of the biodiesel soot collection and MBS nanoparticle preparation are presented in our previous report [39]. Ultrapure water was produced by a UPT-11-10T water purification system (Sichuan ULUPRE Technology Co., Ltd., Chengdu, China) and was used in all the experiments.
The detailed processes of MBS-GO aqueous sample preparation are as follows: GO and MBS with different mass ratios (100:0, 80:20, 60:40, 50:50, 40:60, 20:80, and 0:100) were mixed in water. The total additive concentration in water is 0.5 wt%. The mixtures were first stirred with a glass rod for 10 min and then magnetically stirred for 20 min. Finally, the mixtures were ultrasonicated for 30 min to achieve a homogeneous dispersion. After experimentally obtaining the best mass ratio, the MBS-GO aqueous samples with different additive concentrations of 1, 0.5, 0.25, and 0.125 wt% were prepared by gradient dilution. The same procedure was performed to disperse the additives in water.

Tribological Tests
The tribological tests were performed on an HSR-2M reciprocating ball-on-plate tribometer (Lanzhou Zhongkekaihua Technology Development Co., Ltd., Lanzhou, China) at room-temperature. A schematic of the tribological test is shown in Figure 1. The upper ball (6-mm diameter) used for the tribological tests was made of GCr15 steel with a hardness of 647-861 HV. The lower plate was made of 304 austenitic stainless steel with dimensions of 60 mm × 30 mm × 5 mm and a hardness of 200-210 HV. The surface roughness was maintained at 0.02 µm for all upper balls and lower plates. Before the tribological tests, the upper balls and lower plates were cleaned ultrasonically with ethanol for 10 min. The friction tests were performed at an average sliding speed of 50 mm/s, a stroke length of 5 mm, an oscillating frequency of 5 Hz, and different loads of 10-40 N for 30 min. The friction coefficients were recorded using the software attached to the tribometer. The average friction coefficient was calculated from the average value of friction coefficient over the 30 min testing time. After the friction tests, the friction pairs were ultrasonically cleaned with ethanol and then dried using a hair dryer. The wear volume of the lower plate was measured and calculated using three-dimensional (3D) laser-scanning microscope (VK-X100, Keyence, Osaka, Japan). Experiments were repeated at least three times under each condition.  The Hertz contact diameter (a) and contact stress (P) of the ball-on-plate contact were calculated using the Hertz's theory [40]: where W is the normal load (10, 20, and 40 N), R is the equivalent radius of curvature (3 × 10 −3 m), and E is the effective elastic modulus (2.33 × 10 11 Pa). Under different normal loads, the Hertz diameters were 8.82 × 10 −5 , 1.11 × 10 −4 , and 1.40 × 10 −4 m, respectively, and the contact stresses were 1.64 × 10 9 , 2.06 × 10 9 , and 2.60 × 10 9 Pa, respectively. The Lambda ratio (λ) was determined using the Dowson and Hamrock's minimum film thickness formula [41,42]: where R is the equivalent radius of curvature (3 mm), k is the elliptical parameter (~1), η 0 is the dynamic viscosity of water (1 × 10 −3 Ns/m 2 ), α is the pressure-viscosity coefficient of water (2.2 × 10 −8 m 2 /N), U is the sliding speed (0.05 m/s), and R a1 and R a2 are the surface roughness of the ball and plate, respectively (R a1 = R a2 = 0.02 µm). The maximum value of λ (for W = 10 N) obtained is 0.035, suggesting that the friction tests were performed in the boundary-lubrication regime.

Analysis Methods
The morphologies of the MBS nanoparticles, GO nanosheets, and MBS-GO nanoparticles were determined by a JEM-2100F field-emission transmission electron microscope (FETEM, JEOL, Tokyo, Japan). The structural information of the nanoparticles samples was characterized using a DXR2 micro-Raman spectroscope (ThermoFisher Scientific, Waltham, MA, USA). The particle size distributions in water were analyzed using a Nano-ZS90 zeta potentiometer (Malvern, Worcestershire, UK). Additionally, Figure 1 shows methods used for analyzing wear traces. Surface micrographs and elemental maps of wear traces of upper balls after tribological testing were determined using a SU8010 field-emission scanning electron microscope (FESEM, Hitachi, Tokyo, Japan) equipped with energy dispersive spectrometer (EDS). EDS applied an acquisition voltage of 20 kV. The 3D micrographs and profile curves of the wear traces of lower plates after tribological tests were obtained using a VK-X100 3D laser-scanning microscope (Keyence, Osaka, Japan). Raman mappings of the wear traces of lower plates after tribological tests were determined by micro-Raman spectroscopy.

Characterizations
The FETEM characterization results of the MBS nanoparticles, GO nanosheets, and MBS-GO nanoparticles are illustrated in Figure 2. As shown in Figure 2a, the MBS nanoparticles are spherical with diameters of approximately 40 nm. The MBS nanoparticles display an agglomeration phenomenon, wherein many primary particles formed chain-like conglomerates [43,44]. As shown in Figure 2b, the GO nanosheets have a typical layered structure with wrinkles. The representative FETEM images of the MBS-GO nanoparticles are shown in Figure 2c,d. The MBS nanoparticles adhered to the GO nanosheets, which reduced the agglomeration of MBS nanoparticles and could improve lubrication [21,45].
like conglomerates [43,44]. As shown in Figure 2b, the GO nanosheets have a typical layered structure with wrinkles. The representative FETEM images of the MBS-GO nanoparticles are shown in Figure 2c,d. The MBS nanoparticles adhered to the GO nanosheets, which reduced the agglomeration of MBS nanoparticles and could improve lubrication [21,45]. The particle size distributions of the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles in water are shown in Figure 3. The GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles in water comprise particles ranging from tens to hundreds of nanometers. The results show that the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles can be dispersed in water and that the preparation of MBS-GO aqueous samples has no effect on the particle size distribution and structure. The Raman spectra of the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles are shown in Figure 4. A typical Raman spectrum of graphite has intense G and D bands located at 1580 and 1350 cm −1 , respectively, ascribed to the first order of E2g-mode scattering and the vibration of sp 3 carbon atoms with dangling bonds in the termination plane of disordered graphite [46,47]. In the spectrum of the GO nanosheets, the peaks corresponding to the D and G bands are located at 1352 and 1590 cm −1 , respectively. The oxygenation of graphite causes a shift in the G band [14]. The spectrum of the MBS nanoparticles exhibits the D-and G-band peaks at 1350 and 1587 cm −1 . The large size of and defects in the MBS nanoparticles contribute to the upshift of the G band [48]. The spectrum The particle size distributions of the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles in water are shown in Figure 3. The GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles in water comprise particles ranging from tens to hundreds of nanometers. The results show that the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles can be dispersed in water and that the preparation of MBS-GO aqueous samples has no effect on the particle size distribution and structure.
like conglomerates [43,44]. As shown in Figure 2b, the GO nanosheets have a typical layered structure with wrinkles. The representative FETEM images of the MBS-GO nanoparticles are shown in Figure 2c,d. The MBS nanoparticles adhered to the GO nanosheets, which reduced the agglomeration of MBS nanoparticles and could improve lubrication [21,45]. The particle size distributions of the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles in water are shown in Figure 3. The GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles in water comprise particles ranging from tens to hundreds of nanometers. The results show that the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles can be dispersed in water and that the preparation of MBS-GO aqueous samples has no effect on the particle size distribution and structure. The Raman spectra of the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles are shown in Figure 4. A typical Raman spectrum of graphite has intense G and D bands located at 1580 and 1350 cm −1 , respectively, ascribed to the first order of E2g-mode scattering and the vibration of sp 3 carbon atoms with dangling bonds in the termination plane of disordered graphite [46,47]. In the spectrum of the GO nanosheets, the peaks corresponding to the D and G bands are located at 1352 and 1590 cm −1 , respectively. The oxygenation of graphite causes a shift in the G band [14]. The spectrum of the MBS nanoparticles exhibits the D-and G-band peaks at 1350 and 1587 cm −1 . The large size of and defects in the MBS nanoparticles contribute to the upshift of the G band [48]. The spectrum The Raman spectra of the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles are shown in Figure 4. A typical Raman spectrum of graphite has intense G and D bands located at 1580 and 1350 cm −1 , respectively, ascribed to the first order of E 2g -mode scattering and the vibration of sp 3 carbon atoms with dangling bonds in the termination plane of disordered graphite [46,47]. In the spectrum of the GO nanosheets, the peaks corresponding to the D and G bands are located at 1352 and 1590 cm −1 , respectively. The oxygenation of graphite causes a shift in the G band [14]. The spectrum of the MBS nanoparticles exhibits the D-and G-band peaks at 1350 and 1587 cm −1 . The large size of and defects in the MBS nanoparticles contribute to the upshift of the G band [48]. The spectrum of the MBS-GO nanoparticles shows two peaks centered at 1350 and 1588 cm −1 corresponding to the D and G bands, respectively. The shift in the G band might be related to the defects on the surfaces of the MBS-GO nanoparticles. These results show that no obvious changes occurred in the chemical structure of the MBS-GO aqueous sample during its preparation. This conclusion is consistent with the results of the particle size distributions.
of the MBS-GO nanoparticles shows two peaks centered at 1350 and 1588 cm −1 corresponding to the D and G bands, respectively. The shift in the G band might be related to the defects on the surfaces of the MBS-GO nanoparticles. These results show that no obvious changes occurred in the chemical structure of the MBS-GO aqueous sample during its preparation. This conclusion is consistent with the results of the particle size distributions.

Antiwear and Friction Reduction
The variations in the average friction coefficient and wear volume of the MBS-GO nanoparticles with different GO−to−MBS mass ratios (mass ratio: 100:0, 80:20, 60:40, 50:50, 40:60, 20:80, and 0:100; additive content = 0.5 wt%; testing time = 30 min; applied load = 10 N; and speed = 50 mm/s) are shown in Figure 6. The average friction coefficient and wear volume for water are 0.725 and 7.59 × 10 6 µm 3 , respectively; these are the highest values among the eight lubricant samples tested. When the GO nanosheets or MBS nanoparticles are added separately, the average friction coefficients and wear volumes of the both are lesser than those of water are. This shows that the introduction of the GO nanosheets or MBS nanoparticles to water effectively enhances the antifriction and antiwear properties of water [33,49]. Additionally, the average friction coefficient and wear volume after lubricating with the MBS-GO nanoparticles in water were lower than after lubricating with either GO nanosheets or MBS nanoparticles in water. For the MBS-GO nanoparticles, the average friction coefficient and wear volume decreased with decreasing GO content until  of the MBS-GO nanoparticles shows two peaks centered at 1350 and 1588 cm −1 corresponding to the D and G bands, respectively. The shift in the G band might be related to the defects on the surfaces of the MBS-GO nanoparticles. These results show that no obvious changes occurred in the chemical structure of the MBS-GO aqueous sample during its preparation. This conclusion is consistent with the results of the particle size distributions.

Antiwear and Friction Reduction
The variations in the average friction coefficient and wear volume of the MBS-GO nanoparticles with different GO−to−MBS mass ratios (mass ratio: 100:0, 80:20, 60:40, 50:50, 40:60, 20:80, and 0:100; additive content = 0.5 wt%; testing time = 30 min; applied load = 10 N; and speed = 50 mm/s) are shown in Figure 6. The average friction coefficient and wear volume for water are 0.725 and 7.59 × 10 6 µm 3 , respectively; these are the highest values among the eight lubricant samples tested. When the GO nanosheets or MBS nanoparticles are added separately, the average friction coefficients and wear volumes of the both are lesser than those of water are. This shows that the introduction of the GO nanosheets or MBS nanoparticles to water effectively enhances the antifriction and antiwear properties of water [33,49]. Additionally, the average friction coefficient and wear volume after lubricating with the MBS-GO nanoparticles in water were lower than after lubricating with either GO nanosheets or MBS nanoparticles in water. For the MBS-GO nanoparticles, the average friction coefficient and wear volume decreased with decreasing GO content until

Antiwear and Friction Reduction
The variations in the average friction coefficient and wear volume of the MBS-GO nanoparticles with different GO−to−MBS mass ratios (mass ratio: 100:0, 80:20, 60:40, 50:50, 40:60, 20:80, and 0:100; additive content = 0.5 wt%; testing time = 30 min; applied load = 10 N; and speed = 50 mm/s) are shown in Figure 6. The average friction coefficient and wear volume for water are 0.725 and 7.59 × 10 6 µm 3 , respectively; these are the highest values among the eight lubricant samples tested. When the GO nanosheets or MBS nanoparticles are added separately, the average friction coefficients and wear volumes of the both are lesser than those of water are. This shows that the introduction of the GO nanosheets or MBS nanoparticles to water effectively enhances the antifriction and antiwear properties of water [33,49]. Additionally, the average friction coefficient and wear volume after lubricating with the MBS-GO nanoparticles in water were lower than after lubricating with either GO nanosheets or MBS nanoparticles in water. For the MBS-GO nanoparticles, the average friction coefficient and wear volume decreased with decreasing GO content until a mass ratio of 60:40 (GO:MBS) was achieved. This may be attributed to the ability of the GO nanosheets to deposit on the friction interface as a carbon film. Meanwhile, due to the addition of the MBS nanoparticles, they also enter the friction region and provide lubrication. The lowest average friction coefficient and wear volume were found at a mass ratio of 60:40 (GO:MBS), which reduced the friction coefficient (0.220) and wear volume (3.00 × 10 6 µm 3 ) from those of water by 69.7% and 60.5%, respectively. With further decreases in the GO−to−MBS mass ratio, the average friction coefficient and wear volume increased to varying extents. Thus, the MBS-GO nanoparticles showed better lubricating performance than the constituting components (GO nanosheets or MBS nanoparticles), and the optimal GO−to−MBS mass ratio was 60:40.
a mass ratio of 60:40 (GO:MBS) was achieved. This may be attributed to the ability of the GO nanosheets to deposit on the friction interface as a carbon film. Meanwhile, due to the addition of the MBS nanoparticles, they also enter the friction region and provide lubrication. The lowest average friction coefficient and wear volume were found at a mass ratio of 60:40 (GO:MBS), which reduced the friction coefficient (0.220) and wear volume (3.00 × 10 6 µm 3 ) from those of water by 69.7% and 60.5%, respectively. With further decreases in the GO−to−MBS mass ratio, the average friction coefficient and wear volume increased to varying extents. Thus, the MBS-GO nanoparticles showed better lubricating performance than the constituting components (GO nanosheets or MBS nanoparticles), and the optimal GO−to−MBS mass ratio was 60:40.  Figure  7. The average friction coefficient and wear volume decreased with increasing concentration of additives when the concentration was <0.5 wt%, except for a slight increase in the wear volume of GO at 0.25 and 0.5 wt% concentrations. The results indicated that the additive concentration is important for enhancing the antifriction and antiwear properties of the aqueous samples. As the concentration was increased beyond 0.5 wt%, the average friction coefficient and wear volume increased with increasing additive concentrations because the agglomerated nanoparticles, originating from the high concentration of nanoparticles in water, caused abrasive wear [45]. The minimum average friction coefficient and wear volume were obtained using 0.5 wt% MBS-GO nanoparticles. Therefore, 0.5 wt% was chosen as the optimal concentration of these lubricant additives in water. Notably, the MBS-GO aqueous sample had the lowest average friction coefficient and wear volume among the three lubricant samples at all test concentrations. This result indicated that the MBS-GO aqueous sample had better lubrication properties than the other two lubricant samples, confirming the synergistic lubrication effects of the MBS nanoparticles and GO nanosheets dispersed in water.  Figure 7. The average friction coefficient and wear volume decreased with increasing concentration of additives when the concentration was <0.5 wt%, except for a slight increase in the wear volume of GO at 0.25 and 0.5 wt% concentrations. The results indicated that the additive concentration is important for enhancing the antifriction and antiwear properties of the aqueous samples. As the concentration was increased beyond 0.5 wt%, the average friction coefficient and wear volume increased with increasing additive concentrations because the agglomerated nanoparticles, originating from the high concentration of nanoparticles in water, caused abrasive wear [45]. The minimum average friction coefficient and wear volume were obtained using 0.5 wt% MBS-GO nanoparticles. Therefore, 0.5 wt% was chosen as the optimal concentration of these lubricant additives in water. Notably, the MBS-GO aqueous sample had the lowest average friction coefficient and wear volume among the three lubricant samples at all test concentrations. This result indicated that the MBS-GO aqueous sample had better lubrication properties than the other two lubricant samples, confirming the synergistic lubrication effects of the MBS nanoparticles and GO nanosheets dispersed in water.   The variations in the average friction coefficient and wear volume with applied load (mass ratio: 60:40; additive content: 0.5 wt%; testing time: 30 min; applied load: 10, 20, and 40 N; and speed: 50 mm/s) are shown in Figure 8. It was evident that the average friction coefficients of water as well as GO, MBS, and MBS-GO aqueous samples decreased with increasing tested load. Each of the three aqueous samples reduces the average friction coefficient of water under the tested load. The MBS-GO aqueous sample has a friction coefficient smaller than that of the GO and MBS aqueous samples as well as water. Interestingly, the wear volume of all four samples increased with increasing load, suggesting that all three aqueous samples could reduce the wear volume. The wear volume of the MBS-GO aqueous sample was lower than the GO and MBS aqueous samples as well as water. The results showed that the GO nanosheets, MBS nanoparticles, MBS-GO nanoparticles actively form a tribofilm under each load. Moreover, the MBS-GO aqueous sample showed the best tribological performance, which could be attributed to the GO nanosheets and MBS nanoparticles participating in the friction between the ball-plate friction pairs, creating a synergistic effect [20,50]. The variations in the average friction coefficient and wear volume with applied load (mass ratio: 60:40; additive content: 0.5 wt%; testing time: 30 min; applied load: 10, 20, and 40 N; and speed: 50 mm/s) are shown in Figure 8. It was evident that the average friction coefficients of water as well as GO, MBS, and MBS-GO aqueous samples decreased with increasing tested load. Each of the three aqueous samples reduces the average friction coefficient of water under the tested load. The MBS-GO aqueous sample has a friction coefficient smaller than that of the GO and MBS aqueous samples as well as water. Interestingly, the wear volume of all four samples increased with increasing load, suggesting that all three aqueous samples could reduce the wear volume. The wear volume of the MBS-GO aqueous sample was lower than the GO and MBS aqueous samples as well as water. The results showed that the GO nanosheets, MBS nanoparticles, MBS-GO nanoparticles actively form a tribofilm under each load. Moreover, the MBS-GO aqueous sample showed the best tribological performance, which could be attributed to the GO nanosheets and MBS nanoparticles participating in the friction between the ball-plate friction pairs, creating a synergistic effect [20,50].    Figure 9 shows the 3D micrographs and profile curves of the wear traces of the lower plates lubricated with water as well as GO, MBS, and MBS-GO aqueous samples at an additive content of 0.5 wt%, a mass ratio of 60:40, a load of 10 N, a speed of 50 mm/s, and a testing time of 30 min. The profile curves of and wear area on the lower plates were obtained using the VK Analyzer to calculate 200 cross sections from the middle of the wear traces. Based on the 3D micrographs and profile curves of the wear traces of the lower plates, those lubricated by water underwent the most severe wear, showing the poorest wear resistance, and had a wear area of 1658.05 µm 2 . It is clear that the addition of 0.5 wt% GO nanosheets or MBS nanoparticles decreased the wear area to 1028.17 and 765.96 µm 2 , respectively. This is because the GO nanosheets and MBS nanoparticles deposit on the sliding contact zone and generate a tribofilm, which plays an important role in improving the antiwear performance of water. As expected, the addition of the MBS-GO nanoparticles to water resulted in a significant reduction in deep and wide grooves, with a wear area of 457.28 µm 2 . The above results confirm that the MBS-GO nanoparticles could better improve the antiwear performance of water than the GO nanosheets or MBS nanoparticles alone [51].

Worn Surface Analysis
765.96 µm 2 , respectively. This is because the GO nanosheets and MBS nanoparticles deposit on the sliding contact zone and generate a tribofilm, which plays an important role in improving the antiwear performance of water. As expected, the addition of the MBS-GO nanoparticles to water resulted in a significant reduction in deep and wide grooves, with a wear area of 457.28 µm 2 . The above results confirm that the MBS-GO nanoparticles could better improve the antiwear performance of water than the GO nanosheets or MBS nanoparticles alone [51]. The surface micrographs and EDS elemental maps of the wear traces of upper balls lubricated with water as well as GO, MBS, and MBS-GO aqueous samples (additive content: 0.5 wt%, mass ratio = 60:40, load: 10 N, speed: 50 mm/s, and testing time: 30 min) are shown in Figure 10. The surface micrographs and EDS elemental maps are obtained from the center of the wear traces of the upper balls. Figure 10a shows severe wear of the surface lubricated with water, which has many wide grooves, suggesting abrasive wear [52,53]. There are some wide grooves on the worn surface lubricated with the GO aqueous sample (Figure 10b). Meanwhile, patches formed on the partially worn surface with smaller grooves than those on the surface lubricated with water are seen. This can be attributed to the protective graphene tribofilm present on the friction surface [54]. As shown in Figure 10c, the grooves were narrower and shallower on the worn surface lubricated MBS aqueous sample than with water, which can be attributed to the MBS serving as a ball bearing during sliding [55]. As shown in Figure 10d, the addition of the MBS-GO nanoparticles to the water yielded better antiwear performance with smaller grooves than those found on the surfaces lubricated with the individual aqueous sample additives. This may be due to the GO layers and spherical MBS nanoparticles forming hybrid nanoparticles that combined the advantages of both additives and effectively enhanced the wear resistance and reduced the friction [21,51]. These results are consistent with the results shown in Figure 9. Additionally, the C content on the worn surfaces lubricated with the  Figure 10a shows severe wear of the surface lubricated with water, which has many wide grooves, suggesting abrasive wear [52,53]. There are some wide grooves on the worn surface lubricated with the GO aqueous sample (Figure 10b). Meanwhile, patches formed on the partially worn surface with smaller grooves than those on the surface lubricated with water are seen. This can be attributed to the protective graphene tribofilm present on the friction surface [54]. As shown in Figure 10c, the grooves were narrower and shallower on the worn surface lubricated MBS aqueous sample than with water, which can be attributed to the MBS serving as a ball bearing during sliding [55]. As shown in Figure 10d, the addition of the MBS-GO nanoparticles to the water yielded better antiwear performance with smaller grooves than those found on the surfaces lubricated with the individual aqueous sample additives. This may be due to the GO layers and spherical MBS nanoparticles forming hybrid nanoparticles that combined the advantages of both additives and effectively enhanced the wear resistance and reduced the friction [21,51]. These results are consistent with the results shown in Figure 9. Additionally, the C content on the worn surfaces lubricated with the GO, MBS, and MBS-GO aqueous samples increased compared to that lubricated with water. This confirmed that carbon films are deposited on the surfaces of the friction pairs because of the nanomaterial additions.   Figure 11 shows the Raman mappings of the wear traces of the lower plates lubricated with water as well as GO, MBS, and MBS-GO aqueous samples (additive content = 0.5 wt%, mass ratio = 60:40, load = 10 N, speed = 50 mm/s, and testing time = 30 min). The change in color indicates the distribution and intensity of the G peak. Substantial wear traces, such as severe scratches, are shown in Figure 11a. The corresponding Raman mapping shows the absence of a G peak on the worn surface lubricated with water. As shown in Figure 11b, the wear traces formed on the lower plate lubricated with the GO aqueous sample are comparatively low. The corresponding Raman mapping of the worn surface detected a broad distribution of G peak, indicating the presence of graphene sheets along the wear trace surface [56]. As shown in Figure 11c, in comparison to Figure 11a, shallower and narrower scratches dominate the worn trace surface lubricated with the MBS aqueous sample. The corresponding Raman mapping of the worn surface showed that the G peak is partially distributed across the wear trace surface, showing that a tribofilm of the MBS nanoparticles has formed within the wear tracks [57]. In Figure 11d, shallower and narrower scratches are visible on the worn trace surface lubricated with the MBS-GO aqueous sample. The corresponding Raman mapping showed that the wide distribution of the G peak is observed throughout the wear surface, which could be attributed to the presence of hybrid nanoparticles in the formed tribofilm. The hybrid nanoparticle tribofilm prevented a direct contact between the two friction surfaces. In addition, the ball-bearing effect of the MBS nanoparticles in the hybrid could help reduce abrasive wear [34,58]. This is consistent with the results shown in Figures 9 and 10.  Figure 11 shows the Raman mappings of the wear traces of the lower plates lubricated with water as well as GO, MBS, and MBS-GO aqueous samples (additive content = 0.5 wt%, mass ratio = 60:40, load = 10 N, speed = 50 mm/s, and testing time = 30 min). The change in color indicates the distribution and intensity of the G peak. Substantial wear traces, such as severe scratches, are shown in Figure 11a. The corresponding Raman mapping shows the absence of a G peak on the worn surface lubricated with water. As shown in Figure 11b, the wear traces formed on the lower plate lubricated with the GO aqueous sample are comparatively low. The corresponding Raman mapping of the worn surface detected a broad distribution of G peak, indicating the presence of graphene sheets along the wear trace surface [56]. As shown in Figure 11c, in comparison to Figure 11a, shallower and narrower scratches dominate the worn trace surface lubricated with the MBS aqueous sample. The corresponding Raman mapping of the worn surface showed that the G peak is partially distributed across the wear trace surface, showing that a tribofilm of the MBS nanoparticles has formed within the wear tracks [57]. In Figure 11d, shallower and narrower scratches are visible on the worn trace surface lubricated with the MBS-GO aqueous sample. The corresponding Raman mapping showed that the wide distribution of the G peak is observed throughout the wear surface, which could be attributed to the presence of hybrid nanoparticles in the formed tribofilm. The hybrid nanoparticle tribofilm prevented a direct contact between the two friction surfaces. In addition, the ball-bearing effect of the MBS nanoparticles in the hybrid could help reduce abrasive wear [34,58]. This is consistent with the results shown in Figures 9 and 10. According to the obtained results, the addition of either the GO nanosheets or MBS nanoparticles improved the tribological performance of water. Additionally, when GO nanosheets and MBS nanoparticles were added simultaneously, the lubrication characteristics of water were significantly enhanced. The lubrication mechanisms of the waterbased lubricating additive can be summarized as GO nanosheets and MBS nanoparticles showing the characteristic lubricating action of the layered nanoparticles and spherical nanoparticles, respectively [59,60]. As shown in Figure 2, the MBS-GO nanoparticles are hybrid nanoparticles comprising MBS nanoparticles anchored on the GO surface. This type of nanomaterial can deposit on the friction interface, forming a protective tribofilm to effectively prevent a direct contact of the rubbing surfaces. Meanwhile, the MBS nanoparticles in the hybrid nanoparticles provide the lubricating effect by acting as ball bearings [61,62]. As a result, the MBS-GO nanoparticles provide synergistic lubrication effects in water. This suggests that MBS-GO nanoparticles could increase energy savings by improving the lubricating properties of water. In future studies, the wear and the corresponding deposited tribofilm of upper balls lubricated with water containing the MBS-GO nanoparticles should be investigated to further reveal the friction-reduction and antiwear mechanism. Furthermore, the impact of different steel materials of the friction pairs on MBS-GO nanoparticles friction and wear should be investigated systemically, which is crucial for the potential applications of MBS-GO nanoparticles.

Conclusions
In this study, the tribological performance of the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles as lubricating additives in water was investigated using a ball-on-plate tribometer. The following conclusions can be drawn: (1) GO nanosheets have a typical layered structure. When GO nanosheets are used as lubricating additives, a protective graphene tribofilm forms on the rubbing surfaces, improving the tribological properties of water. According to the obtained results, the addition of either the GO nanosheets or MBS nanoparticles improved the tribological performance of water. Additionally, when GO nanosheets and MBS nanoparticles were added simultaneously, the lubrication characteristics of water were significantly enhanced. The lubrication mechanisms of the water-based lubricating additive can be summarized as GO nanosheets and MBS nanoparticles showing the characteristic lubricating action of the layered nanoparticles and spherical nanoparticles, respectively [59,60]. As shown in Figure 2, the MBS-GO nanoparticles are hybrid nanoparticles comprising MBS nanoparticles anchored on the GO surface. This type of nanomaterial can deposit on the friction interface, forming a protective tribofilm to effectively prevent a direct contact of the rubbing surfaces. Meanwhile, the MBS nanoparticles in the hybrid nanoparticles provide the lubricating effect by acting as ball bearings [61,62]. As a result, the MBS-GO nanoparticles provide synergistic lubrication effects in water. This suggests that MBS-GO nanoparticles could increase energy savings by improving the lubricating properties of water. In future studies, the wear and the corresponding deposited tribofilm of upper balls lubricated with water containing the MBS-GO nanoparticles should be investigated to further reveal the friction-reduction and antiwear mechanism. Furthermore, the impact of different steel materials of the friction pairs on MBS-GO nanoparticles friction and wear should be investigated systemically, which is crucial for the potential applications of MBS-GO nanoparticles.

Conclusions
In this study, the tribological performance of the GO nanosheets, MBS nanoparticles, and MBS-GO nanoparticles as lubricating additives in water was investigated using a ball-on-plate tribometer. The following conclusions can be drawn: (1) GO nanosheets have a typical layered structure. When GO nanosheets are used as lubricating additives, a protective graphene tribofilm forms on the rubbing surfaces, improving the tribological properties of water.
(2) MBS nanoparticles have a typical spherical structure. The ball bearing-lubricating effect of the MBS nanoparticles enhances the friction-reduction and antiwear capabilities of water. (3) In comparison to the GO nanosheets and MBS nanoparticles, water-containing MBS-GO nanoparticles show better lubrication properties. When the additive content is 0.5 wt% and GO−to−MBS mass ratio is 60:40, the friction coefficient and wear volume are reduced by 69.7% and 60.5%, respectively, compared to water. The synergistic effect of the GO nanosheets and MBS nanoparticles during the friction process is primarily responsible for the significant improvement in the lubricating performance of water. Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.