Effect of Graphene and MoS2 Flakes in Industrial Oils to Enhance Lubrication.

Tribological studies of the 2D nanoadditives such as MoS2 and graphene are mostly performed in base oils such as SN500, SN150, or paraffin. We have focused on their effect in lubrication properties of industrial oils (e.g., axle, transmission, and compressor oils) along with SN500 oil employing a four-ball tester. Two types of graphene powders (GpowA with fewer defects than GpowC), MoS2 powder, and their physical mixtures are chosen as nanoadditives. The tribology performance for 0.05 wt% of additives in various industrial oils is evaluated by monitoring the coefficient of friction (COF) during rubbing and wear scar diameter (WSD) of the steel balls after rubbing. Elemental analysis and electron microscopy have been performed on the wear surfaces for evidence of any tribofilm formation. GpowA favors antifriction for axle and transmission oils with 40% reduction in axle oil, whereas it improved antiwear properties in most of the oils. GpowC shows a COF decrement by 12% only for compressor oil, but contribute to wear reduction in all oils. The observed COF reduction is attributed to the compatibility of nonfunctionalized GpowA with nonpolar axle oil and functionalized GpowC with polar compressor oil. MoS2 shows a decrease in the COF and WSD in most industrial oils; the best being 60% COF and 7% WSD reduction in axle oil. For additives in oils that favor antiwear, flakes or particles are observed on the wear surface supported by the higher elemental contribution of the constituents from the wear region. The mixtures of GpowA or C with MoS2, however, does not seem to favor improvement in the COF or WSD in industrial oils. With assistance from oleylamine surfactants, the lubrication properties of most additives are improved, particularly for the mixtures with 12-15% COF reduction and 4-7% WSD reduction in compressor oil. The study indicates that a large sheet size of high-quality graphene aids antifriction and addition of surfactant molecules facilitates a co-operative effect between MoS2 and graphene for improved tribology.

Introduction

In the manufacturing world, productivity translates to smooth and durable functioning of machine parts for long periods of time thereby minimizing downtime needed for maintenance. Friction and wear are the primary reasons for the loss of energy and material in the machinery parts. The emergence of nanotechnology has influenced all disciplines of science[1,2] and tribology is no exception.[3] It has been well reported that the addition of nanoparticles has significantly altered the mechanical properties of lubricant oils.[3,4] Several mechanisms such as third-body effect, rolling/sliding mechanism, and protective film have been proposed to explain the improvement in properties upon the addition of the nanomaterials.[5,6] The properties of the lubricant can be modified by changing the concentration, shape, size, and crystallinity of these nanoparticles.[3,7]

Graphene, world’s first 2D material, has been a treasure trove for researchers[8] for exploring its novel electronic, mechanical, and chemical properties. Recently, transition metal chalcogenides such as molybdenum disulfide (MoS2), molybdenum diselenide, and tungsten sulfide have been included in the 2D family of the new nanomaterials. The most widely researched materials for lubrication purposes are 2D materials such as graphene,[9,10] analogs of graphene, and transition metal dichalcogenides such as MoS2.[11] Graphene and MoS2 have been extensively studied for their tribological properties, both in solid state[12−15] and as additives in fluid lubricants.[16−18] Both 2D nanomaterials, either as individual additives or physical and chemical mixtures, have been shown to improve lubrication or tribological properties.[19−21] In most cases, stable dispersions of nanoadditives in various hydrocarbon oils are obtained by employing suitable surfactants.[22] Because the presence of molecular surfactants can cause degradation under extreme tribological conditions, there have been efforts to prepare self-dispersible ultrafine additives.[23] The tribological properties of graphene, graphene oxide, and their composites have been largely studied in typical base oils such as paraffin, SN150, and SN500. Bai et al. have investigated the tribological behavior of graphene decorated with ceria particles in paraffin base oil and reported that the addition of 0.06 wt% nanoadditives in the base oil decreased the friction coefficient from 0.21 to 0.1 and the wear rate to 1.5% than those in the base oil.[7] Rajendhran et al. have studied the lubricating properties of the Ni-promoted MoS2 nanosheets in SN500 mineral oil and observed that 0.5 wt% nanoadditives in base oil improved the antiwear property by 40–50% and antifriction property by 15–20% compared to base oil.[16] The tribological properties of the reduced graphene oxide (rGO)/MoS2 heterostructures in paraffin oil were explored by Hou et al. and the nanoadditive showed friction coefficient as low as 0.09 and also improved wear resistance compared to the base oil.[24] Wu et al. analyzed the tribological properties of MoS2 on graphene as a lubricant additive in perfluoropolyether base oil wherein they observed that by addition of 1 wt% of an additive, the friction coefficient decreased to 0.06 and the wear rate reduced by 91%.[25]

Although the tribological properties of graphene and MoS2 dispersions have been studied in base oils (such as paraffin, SN150, and SN500), their application in the industry remains restricted because of the limited knowledge of their properties in industrially available oils. In the present study, we examine the tribological properties of various types of industrial lubricating oils doped with two types of graphene powders (e.g., nonfunctionalized—GpowA and functionalized—GpowC), MoS2, and their physical mixtures.

Results and Discussion

Morphology and Structure

Raman spectroscopy was employed to study and validate the structure of the nanoadditives. The two types of graphene additives, GpowA and GpowC, were obtained commercially, whereas MoS2 was obtained via hydrothermal synthesis in the laboratory. The details are given in the Experimental Section. Figure depicts the Raman spectra of GpowA, GpowC, and MoS2 powders acquired at 532 nm excitation. For GpowA, the two most intense features are a peak at 1580 cm–1 corresponding to the G band and another peak at ∼2716 cm–1 assigned as the 2D band.[26] For GpowC, an intense band is observed at 1340 cm–1 corresponding to the D band along with the G band at 1597 cm–1, and a weak 2D band at around 2885 cm–1 is observed. The D band arises due to defects indicative of disordered graphene systems.[27] The 2D band is the first overtone of the D peak, which is very sensitive to defects arising from the functional groups or microstructural reasons such as lattice vacancy and edge states. The negligible D band intensity and a sharp 2D band in the case of GpowA indicate a very ordered graphene layer similar to graphite lattice.[27] However, the weak intensity and broad shape of a 2D band reveals the presence of a multilayer in GpowA. GpowC exhibits a broad and intense D-band indicative of functionalized graphene with more edge states. The observed 2D band is weak and very broad, which shows the presence of exfoliated layers of defect-rich functionalized graphene similar to the case of rGO, usually obtained by chemical reduction.[28,29] The calculated ID/IG ratio of GpowC from Figure is 0.74, which is higher than that of GpowA (ID/IG is 0.20), confirming high density of defects in GpowC.[30,31] To further investigate the presence of functional groups in GpowA and GpowC samples, Fourier transform-infrared spectroscopy (FT-IR) was conducted (Figure S1). The GpowC spectra show bands at 3500 and 1160 cm–1 corresponding to −OH and C–O vibrational modes supporting the presence of functional groups on GpowC. The GpowA spectra show bands due to C=C and C–H vibrations and remain mostly featureless, resembling the spectrum for nonfunctionalized pristine graphene.[32]

Figure 1

Raman spectra of the various nanoadditives in a powder form acquired at 532 nm excitation.

In the case of MoS2 powder, the signature peaks corresponding to the in-plane (E2g1) and out-of-plane (A1g) vibrational modes of MoS2 are observed at 380 and 405 cm–1, respectively, in the Raman spectra (Figure ).[33] The spacing between the two Raman peaks is 25 cm–1, indicating that the MoS2 consists of more than 10 layers. The X-ray diffraction (XRD) patterns of MoS2, GpowA, and GpowC are displayed in Figure S2 and further complement the inferences from Raman spectra. GpowA shows a sharp intense peak at 26.5° corresponding to (002) of graphite (PCPDF card no. 89-7213) indicating graphitic nature of the material, whereas GpowC exhibits a broad peak at 25.2° similar to that reported for rGO.[28] For MoS2, the peaks at 29.2°, 33.6°, and 56.1° could be indexed to the (004), (101), and (106) planes of hexagonal MoS2 (PCPDF card no. 87-2416). The peaks at 16.5° and 20.4° can be associated with the presence of unreacted sulfur (PCPDF card no. 78-0793). Raman spectra of the prepared mixtures of graphene and MoS2 are also given in Figure . For characterization purpose, the mixtures were prepared by physically mixing 0.03 wt% of GpowA or C with 0.02 wt% of MoS2 in isopropyl alcohol and subjected to sonication. The powders were further dried and used for Raman, XRD, and electron microscope studies. Raman spectra of mixtures show the bands due to the presence of both the components, GpowA/C and MoS2; however, with no pronounced Raman shift. This indicates that there is no chemical interaction between graphene and MoS2 in the additive mixtures, but rather a physical mixing is prevalent.

Figure a–c show the field emission scanning electron microscopy (FESEM) images of the additives, GpowA, GpowC, and MoS2. From the FESEM, GpowA is shown to have exfoliated graphene sheets with large flake sizes (>10 μm). GpowC consists of aggregated sheets with smaller flake sizes (1–2 μm), however, few particles with larger sizes are also observed. Particle size of the as-prepared graphene powders was also analyzed after 5 min sonication of corresponding graphene in isopropyl alcohol using light scattering technique. The results demonstrate that the average particle size of GpowA after shearing is below 30 μm and that of GpowC is 5 μm. FESEM of the synthesized MoS2 powder shows that it contains MoS2 curls aggregated to form sheets and spherical nanoparticles of size 400–700 nm (Figure c). The nanosheet morphology of the synthesized MoS2 powder is more obvious from transmission electron microscopy (TEM) images which will be discussed later. The average sheet size of MoS2 powder is 3–4 μm, and the wrinkled features that are composed of nanosheets are typically 20 nm thick. The thickness and size of the flakes were estimated from TEM and FESEM images.

Figure 2

FESEM images of the nanoadditives (a) GpowA (b) GpowC, and (c) MoS2 powder.

The FESEM images in Figure a,b show the morphology of the physical mixtures of MoS2 with GpowA and GpowC, respectively. The circled regions show the MoS2 components in the two cases and the same can be confirmed by the composite energy dispersive spectroscopy (EDS) maps (Figure c,d). The EDS maps for the individual elements are shown in Figure S3a,b where the blue, red, and green colors represent the regions rich in C, Mo, and S, respectively. The yellow colored region in the composite EDS maps (Figure c,d) shows regions containing both molybdenum and sulfur elements (a combination of red color denoting Mo and green color denoting S appears as yellow), whereas the blue color represents the graphene region. The EDS maps show the presence of both the blue and yellow regions that appear together in various parts of the sample indicating the presence of mixed as well as segregated phases, which is typical of mechanically mixed components.

Figure 3

FESEM and composite EDS images of the additive mixtures (a,c) GpowA–MoS2 and (b,d) GpowC–MoS2.

TEM imaging and analysis were carried out to further study the Morphology and Structure in detail. For GpowA, thin and large area graphene sheets with well-defined edges can be observed under the low magnification TEM image shown in Figure a. The selected area electron diffraction (SAED) pattern shows diffraction spots indicating that GpowA flakes are highly crystalline graphene. The inset of Figure a shows the high-resolution TEM (HRTEM) image for these flakes. The intensity analysis of the fringes reveals a lattice constant of 0.235 nm, which is close to the expected value of 0.246 nm of the lattice parameter of graphene’s triangular lattice structure.[34] GpowC also shows graphene flakes, however, with some amount of aggregation and smaller flake size (Figure b). The HRTEM image presented in the inset shows the layer spacing of the sheets to be 0.38 nm, which is higher than that of graphite (0.337 nm) indicating an increase in the interlayer spacing. This is consistent with the presence of functional groups deduced from Raman spectra, which contributes to the larger interlayer spacing and exfoliation. Diffraction spots in the SAED pattern (inset of Figure b) confirm the crystallinity of the GpowC flakes. However, it can be noticed from HRTEM that the lattice order across the flakes is poor when compared to GpowA and further confirms the presence of higher amount of defects in GpowC. In Figure c, the wrinkled sheets distinctive of chemically synthesized MoS2 can be easily identified. Thin sheets can be identified near the periphery of the MoS2 curls, whereas thicker stacked regions can be seen in other regions. The HRTEM image shows the stacked MoS2 region where the interlayer spacing is 0.62 nm that matches well with the previous reports.[35,36] The diffused rings in the SAED pattern (inset of Figure c) indicates that the synthesized MoS2 is nanocrystalline with no long-range order.

Figure 4

Low magnification and high-resolution TEM images (left insets) with the SAED pattern (right insets) of various nanoadditives and their mixtures (a) GpowA, (b) GpowC, (c) MoS2 powder, (d) GpowA–MoS2, and (e) GpowC–MoS2 powder.

TEM images along with the SAED patterns (inset) of the as-prepared mixtures of MoS2 with GpowA and GpowC are shown in Figure d,e. The low magnification image of GpowA–MoS2 shows the presence of the wrinkled MoS2 structure along with the thin flat flakes of GpowA, confirming the presence of both the components. Figure e shows a region where the wrinkled MoS2 can be seen on the edges of the GpowC flake. Unlike GpowA–MoS2, the individual components in the GpowC–MoS2 mixture cannot be identified easily under TEM.

Lubricant Properties of the Additives

Four types of industrial oils, base axle oil, base transmission oil, compressor oil, and SN 500 oil, were used for the tribological studies. Base axle oil, transmission oil (without lubricants/additives), and SN500 are composed of mineral oils, whereas compressor oil is composed of polyalkylene glycol derivatives. To understand the functional groups present in the oils, FT-IR spectral analyses of as-supplied oils were carried out and are presented in Figure S4. It is found that in all the four oils, strong peaks observed at 2850 cm–1 and at around 2930–3000 cm–1 arise from the asymmetric C–H bending vibrations of the methyl and methylene group. Symmetric bending vibration of the CH2 group is seen at 1458 cm–1. A band at 1376 cm–1 arises due to symmetric CH3 bending vibration of the methyl group. A weak band at 720 cm–1 is attributed to out-of-plane stretching vibration of the C–H group. The FT-IR spectral profile is very similar for the base axle and transmission oils and consists mainly of hydrocarbon signatures.[37] In the case of SN500, a band at 1110 cm–1 is observed which indicates the presence of C–O group in addition to hydrocarbon signatures. It can be clearly seen that compressor oil has a very different spectral profile with strong peaks due to C–O (1110 cm–1) and −OH (3400 cm–1) due to glycol and indicates its polar nature. Axle and transmission oils are nonpolar, whereas SN500 is slightly polar and compressor oil is polar in nature.

The nanoadditives, 0.05 wt% of MoS2, GpowA, GpowC, and the mixtures of GpowA or C (0.03 wt%) with MoS2 (0.02 wt%), were dispersed in the above mentioned industrial oils by sonication. The photographs of the dispersions in various oils are shown in Figure S5. The stability of the dispersions was monitored by measuring UV–vis absorption after day 1, 7, and 14 and their absorption intensities were compared (Figure S6). GpowA and its composite showed the poorest stability with the absorbance reduced to <20% after two weeks in all oils. For most cases, MoS2, GpowC, and GpowC–MoS2 showed better stability with absorbance reduction not less than 60% after 2 weeks.

The antifriction and the antiwear properties for each additive in various lubricating oils were evaluated using the four-ball friction tester. The friction and wear tests were conducted at a constant load of 400 N and at a rotating speed of 1200 rpm for 60 min at 75 °C. The lubricating oils with additives were sonicated prior to the friction test. The details of sample preparations and tests are given in the Experimental Section. The plots of the percentage change in the coefficient of friction (COF) and wear scar diameter (WSD) of the steel balls after addition of the active materials in comparison with the as-supplied oils are shown in Figure a,b, respectively. The plots show a relative change in the COF and WSD values when additives are added to the as-obtained oils. A value lower than zero (negative value) signifies that the COF or WSD for the industrial oil with the additive is lower than that of just the as-supplied oil (with no additives), which is indicative of the antifriction or antiwear property of the additives. The COF scans of the as-supplied industrial oils are shown in Figure S7.

Figure 5

Plot of % change in (a) COF and (b) WSD in various industrial oils containing nanoadditives and their mixtures with respect to the values obtained from the corresponding as-supplied oils (load, 400 N; rotating speed, 1200 rpm; T, 75 °C; t, 60 min).

From Figure , it can be seen that GpowA sample showed a significant reduction in the COF when dispersed in axle oil (40% reduction), whereas the improvement in the COF for transmission oil is 10%. However, COF of GpowA did not show any improvement in compressor oil and slightly increased in SN500. WSD for GpowA reduced for all the oils with highest reduction observed for SN500 (50%). For GpowC, the COF decreased 12% in compressor oil and increased for all other oils. On the contrary, however, WSD for GpowC was reduced to 50% for SN500, 17% for transmission, 12% for compressor, and 1% for axle oils. MoS2 when dispersed in oils showed significant reduction in the COF for axle (60%), transmission (10%), and compressor (10%) oils except for SN500. In terms of WSD, MoS2 showed improvement of antiwear with SN500 (42%), axle (7%), and transmission (1%) oils except for compressor oil. The COF values increased for GpowA–MoS2 mixture in axle and transmission oils, whereas the change was negligible in SN500. However, there was a reduction of 9% in the COF values with the GpowA–MoS2 dispersions in compressor oil. The % reduction in the WSD for GpowA–MoS2 mixture was 55% in SN 500 oil. The same additive mixture led to a reduction of 12% WSD in transmission oil, whereas there was no significant change in axle oil and 15% increase of WSD was observed in compressor oil. GpowC–MoS2 mixture showed no reduction in COF in any of the industrial oils and it seemed to contribute increased friction in axle and transmission oil. WSD also showed no improvement in this case for axle, transmission, and compressor oils. The only improvement with GpowC–MoS2 mixture was observed for SN500 with a 35% lower WSD than that for base oil.

The evidence of reduction in the wear of the steel balls with the addition of the nanoadditives can also be confirmed by FESEM imaging of the worn steel surfaces after the tribology test. The FESEM images of the wear scars of various additives in different industrial oils are given in Figures S8–S13. We have also looked for evidence of any tribofilm formation in the wear scar region using FESEM and EDS. The obtained results (in atomic %) of the EDS of the wear surface lubricated with various nanoadditives in a few oils, SN500 and axle oil, are summarized in Table . In Table , Fe, Cr, and Si contributions shown are from steel ball itself, C contribution can be from oil and GpowA/C additives, Mo from MoS2 and O may be from steel ball surface or due to additive oxidation. The higher contributions to C and Mo on the wear surface when compared with that of base oil arising from the presence of additive particles on steel surface indicate that a tribofilm formation during rubbing is responsible for better lubricant performance.[12,16]

Table 1

EDS Analysis (Atomic % Values) of the Wear Surface of the Steel Balls after Tribology Experiments (Four Ball Tester, Load, 400 N; Rotating Speed, 1200 rpm; T, 75 °C; t, 60 min) Employing Axle Oil and SN500 Oil with Various Nanoadditives of Graphene and MoS2 and Also Using Compressor Oil Containing Nanoadditives Treated with Oleylamine as a Surfactant

The superior antifriction and antiwear performance of GpowA, particularly in axle and transmission oils can be associated with its compatibility with the hydrocarbon oils. GpowA being highly graphitic can become more dispersible in nonpolar, axle, and transmission oils with high hydrocarbon content during rubbing under a load aiding the formation of a better tribofilm. This is supported by the comparison of the morphology of the surfaces tested under base axle oil and with the additive given in Figure . Figure a,b shows that the surface is very rough and severe scuffing can be observed in the case of base axle oil. On comparison to the base oil, the diameter of the wear scar with the additive is smaller, which is indicative of the anti-wear property (Figure c), and furthermore, the surface covered with the axle oil containing GpowA is significantly smoother with less scuffing (Figure d). Additionally, larger flakes of GpowA can be seen on the wear surface clearly indicating the formation of a tribofilm (Figure d). The EDS also shows a higher amount of carbon content in the wear region in the case of GpowA in axle oil (Table ). No particular improvement in the COF for GpowA in a compressor and SN500 oils is observed may be because of its less dispersibility in polar oils. However, the EDS results show that the carbon content in the wear region is highly increased (Table ), and FESEM shows the presence of large flakes (Figure S12b) on the worn surface for GpowA in SN500. This is attributed to its good anti-wear property in SN500 though it does not improve the COF, which indicates the formation of a nonuniform tribofilm in polar oils.[38] GpowC shows improved antifriction only in the case of compressor oil (Figure ), which can be associated with its compatibility with highly polar nature due to −OH and −C–O functionalized GpowC. However, GpowC shows good antiwear property in most of the oils except for axle oil. EDS (Table ) also shows high carbon content on the wear surface for GpowC in SN500 oil, whereas less C content in the case of axle oil. FESEM images of the wear surface for GpowC in SN500 (Figures S11c and S12c) show less scuffing compared to base SN500 oil (Figures S11a and S12c) and GpowC in axle oil (Figures S8c and S9c). Better anti-wear but less anti-friction property of GpowC indicates that it forms unevenly distributed particles in the tribofilms.[38] MoS2 seems to favor antifriction and antiwear property in most of the oils (Figure ). However, it is noticeable that though it shows a 60% COF reduction in axle oil, the wear reduction is only 7% in the same oil. Accordingly, EDS shows the presence of very less Mo in the wear tracks of MoS2 in axle oil when compared to SN500 oil (where it showed higher wear reduction). This may indicate that the tribofilm deterioration is faster than the film formation rate in the case of MoS2 in axle oil.[39]

Figure 6

FESEM images of the rubbing surfaces in (a) base axle oil and (b) magnified image of (a), (c) GpowA in axle oil, and (d) magnified image of GpowA on the wear scar after the tribology test.

The COF performance can also be correlated with the flake size and crystallinity of the additives. From TEM, FESEM, and particle size analysis, it is noticed that GpowA has a very large flake size with few defects and MoS2 exists as extended films consisting of nanosheets with spherical aggregates. Such large layered sheets allow the contact surface to slide over each other thereby reducing the frictional force.[18] The defects of GpowC appear to be detrimental for friction, causing film deterioration. However, the small flake size of GpowC may allow it to embed in the surface microstructure thereby reducing wear. Earlier studies have shown that graphene with extended and ordered lattice offers superior tribological properties, whereas the presence of defects can cause faster deterioration under load.[40,41] In another recent report on the tribological properties of different forms of rGO, it is concluded that rGO with more lattice defects can lead to an increase in the COF due to aggregation.[42]

In the case of physical mixtures of GpowA–MoS2 and GpowC–MoS2, in general, the additives do not improve the COF in most of the oils. There is no considerable reduction in wear scars as well except in the case of SN500 oil (Figure ). Accordingly, EDS shows an increase in the carbon content on the wear surface only in the case of SN500 oil for both mixtures (Table ). The examination of wear surface also indicates the presence of debris in the case of GpowA–MoS2 (Figures S11e and S12e) and GpowC–MoS2 (Figure S12 f) additives in SN500 oil. Aggregated and rough debris of additives can be noticed in the case of GpowA–MoS2 in SN500, which is indicative of a very patchy tribofilm formation that aids in antiwear but not in antifriction property.[38] This indicates that a physical mixture was not favorable for smooth sliding of surfaces.[43,44] Hou et al. have shown that rGO/MoS2 heterostructures formed by chemical synthesis are suitable for improved lubrication rather than a physical mixture.[24] Previous studies have demonstrated that the presence of graphene allows for better retention of the tribofilm and passivates MoS2 from oxidation thereby allowing the formation of a thicker and more durable tribofilm.[25,45] However, in the present case, a physical mixture of graphene and MoS2 would not have helped the above cause.

The effect of using oleylamine, as a surfactant, on the lubrication properties was also studied for the nanoadditives dispersed in compressor oil. As there is a slight positive effect on the COF values for all nanoadditives in compressor oil, we have examined the further impact of adding surfactant-mixed additives to the compressor oil in this work. For this, the additives were first wetted with oleylamine (weight ratio 10:1; 10 parts of additive and 1 part of oleylamine) before dispersing in the oil using an ultrasonicator. Figure compares the COF and WSD of the compressor oil samples with and without oleylamine. It can be seen from the plot that the addition of oleylamine leads to a higher reduction in the COF in all cases except GpowC. With the addition of oleylamine, the reduction in the COF values increased by an extra 6, 25, 5 and 15% for GpowA, MoS2, GpowA–MoS2, and GpowC–MoS2, respectively (Figure a). The highest reduction was observed in the case of MoS2. Chen et al. reported a similar effect wherein the extreme pressure property of MoS2 was hugely enhanced in the presence of oleylamine.[46] Oleic acid-modified graphene is also reported to have improved tribological properties in the gear oil polyalphaolefin-9 since oleic acid improves both solubility and dispersibility of graphene in the oil.[47] The positively charged amine group from the oleylamine molecules tends to bind with the negatively charged surface of MoS2, whereas the nonpolar hydrocarbon end of oleylamine easily disperses in the oil, thereby resulting in better dispersions. The improved antifriction and antiwear properties with nanoadditives, particularly, MoS2 and its mixtures with GpowA and GpowC complement the fact that oleylamine binding to MoS2 improves the interaction with the graphene and aid to form a tribofilm. A similar trend of the effect of addition of oleylamine on the anti-wear properties can be observed in Figure b. Upon addition of oleylamine, the WSD reduces by an additional 15, 22, and 12% for MoS2, GpowA–MoS2, and GpowC–MoS2, respectively. There was no significant change in the WSD for GpowA, whereas it slightly increased for GpowC with the addition of oleylamine. EDS analysis of the wear surface shows that there is a considerable increase in the Mo and S percentage content in the case of MoS2, GpowA–MoS2, and GpowC–MoS2 in compressor oil with oleylamine (Table ). For GpowA or GpowC in compressor oil with olelylamine, no mentionable increase in the carbon content is observed on the wear surface (Table ), and accordingly visible signatures of the additive are not seen in FESEM images given in Figure b,c when compared to that of MoS2 and mixtures (Figure d–f). This indicates that oleylamine interacts with MoS2 rather than with graphene. EDS mapping of wear surface for GpowC–MoS2 in compressor oil with oleylamine shown in Figure confirms the presence of MoS2 on the wear tracks.

Figure 7

Plot comparing the % change in (a) COF and (b) WSD with additives in compressor oil in the absence or presence of oleylamine surfactant with respect to the values obtained from the corresponding as-supplied oils (load, 400 N; rotating speed, 1200 rpm; T, 75 °C; t, 60 min).

Figure 8

Magnified FESEM images of the wear scars of the surfaces in (a) as-supplied compressor oil, (b) GpowA, (c) GpowC, (d) MoS2, (e) GpowA–MoS2, and (f) GpowC–MoS2 in compressor oil with the oleylamine surfactant.

Figure 9

EDS map of GpowC–MoS2 in compressor oil with oleylamine (a) composite map of S K (red), Mo L (green), C K (blue) yellow color is seen due to the overlapping of red and green color, (b) individual map of S K, (c) Mo L, and (d) C K level.

Conclusions

Our studies on the tribological properties of synthesized powders of graphene, MoS2, and their mixtures in various industrial oils demonstrate that the antifriction and antiwear contribution of different nanoadditives largely depend on the type of oil. The illustration of reduction in friction and wear in universal base oils is only indicative and may not hold true for all the industrially used oils. The COF and WSD along with EDS and FESEM characterization of the wear surface are performed to understand the tribofilm formation. GpowA with high-quality graphene flakes show a 60% reduction in the COF and a 15% reduction in WSD in axle oil. Better antifriction is attributed to the compatibility of GpowA with nonpolar axle oil and a larger flake size of GpowA. The presence of GpowA flakes on wear surface supports the improved tribological property. GpowC shows a 12% reduction in the COF only in compressor oil but good antiwear in most of the oils with 12–50% reduction in WSD. The COF reduction can be attributed to the compatibility of functionalized GpowC with polar compressor oil. MoS2 shows good antifriction and antiwear in most of the oils with 60–10% COF reduction and 40–7% WSD reduction. Friction coefficient and wear scar analysis from this study reveal that highly crystalline graphene flakes and MoS2 sheets with a larger particle size are good for antifriction properties, whereas functionalized graphene with a smaller size is better for antiwear properties. The larger flakes reduce friction by efficiently sliding between the contact surfaces, whereas smaller aggregates embed in the microstructure reducing a progressive wear. The physical mixtures of MoS2 with both types of graphene are found to have poor antifriction and antiwear properties. The modification of MoS2 with the oleylamine surfactant not only promotes dispersion stability but also improves antifriction (12–15% COF reduction) and antiwear properties (4–7% WSD reduction) of mixtures of MoS2 with both graphene types and, particularly, of MoS2 with additional 25% COF and 15% WSD reduction than in as-supplied compressor oil. Wear surface analysis shows more contribution from MoS2 indicating that oleylamine interacts largely with MoS2, while improving the synergy between the constituents in additive mixtures for better lubrication.

Experimental Section

Materials and Methods

Four types of industrial oils, base axle oil, base transmission oil, compressor oil, and SN 500 oil were used for the tribological studies. Base axle oil (without lubricants/additives) and transmission oils (without lubricants/additives) are composed of mineral oils and compressor oil is composed of polyalkylene glycol derivatives. SN500 (Solvent Neutral 500), categorized to grade 1 by American Petroleum Institute, is refined from petroleum crude oil and contains more than 0.3% sulfur by weight. Two types of graphene powders, GpowA and GpowC, were provided by Graphene Center, Tata Steel Limited. MoS2 nanosheets were synthesized using a hydrothermal method. In a typical synthesis, 200 μmol of molybdenyl acetylacetonate powder was dissolved in 30 mL of toluene, and 400 μmol of sulfur powder was then added followed by the addition of 0.1 mL of hydrazine hydrate and stirred well. This mixture was then transferred to a Teflon autoclave which was sealed and kept at 200 °C for 24 h in a hot air oven. The obtained powder was collected, washed, and stored in a dry atmosphere.

FESEM, TESCAN MIRA 3 LM, and attached Bruker Nano XFlash 6130 were used to study surface morphology of nanosheets and perform EDS. Raman spectroscopy studies were carried out using a HORIBA XploRA PLUS spectrometer. An FEI Technai, HRTEM (T20 S-TWIN TEM, 200 kV) was used to acquire high-resolution images. Particle size measurement was performed using a HORIBA Laser Scattering Particle Size Distribution Analyzer LA-960.

Tribological Characterization

The nanoadditives such as MoS2, GpowA, GpowC, and the mixtures of graphene with MoS2, were all dispersed in four types of industrial oils (axle, transmission, compressor, and SN500) to study their effective tribological properties. The effect of oleylamine as a surfactant on the above additives in compressor oil was also studied. The lubricant was prepared by mixing 0.05 wt % of the given additives into the oils. The powders were directly added to those oils to disperse using a bath sonicator for 6 h (2 h continuous mode and 30 min gap to cool the bath temperature down). Five sets of dispersions containing GpowA (0.05 wt%), GpowC (0.05 wt%), MoS2 (0.05 wt%), and mixtures of graphene with MoS2; GpowA (0.03 wt%)–MoS2 (0.02 wt%), GpowC (0.03 wt%)–MoS2 (0.02 wt%), were prepared for each oil type. In the study of compressor oil with oleylamine, the additives were first wetted with oleylamine (weight ratio 10:1) before dispersing in the oil using an ultrasonicator.

The four-ball friction tester (ASTM standard D4172-04)[48] was employed to study the lubricant capabilities of the oil containing the above-mentioned nanoadditives. The lubricant oils were sonicated for 5 min just before conducting the friction experiment. The friction and wear tests were conducted at a constant load of 400 N and at a rotating speed of 1200 rpm for 60 min at 75 °C. The relative motion of the top ball with respect to the bottom ones results in the formation of a circular wear scar on the bottom balls (see Scheme ). The WSD of the three bottom balls was measured using FESEM and the average value was used to evaluate the wear properties. The friction coefficient values were also acquired during the ball rotation. The surface of the steel balls was observed under FESEM for a morphological study.

Scheme 1

Schematic of the Working of the 4-Ball Tester