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Surface Modification of MoS2 Nanosheets as Effective Lubricant Additives for Reducing Friction and Wear in Polyalphaolefin Xinhu Wu, Kuiliang Gong, Gaiqing Zhao, Wenjing Lou, Xiaobo Wang, and Weimin Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00454 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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Industrial & Engineering Chemistry Research

Surface Modification of MoS2 Nanosheets as Effective Lubricant Additives for Reducing Friction and Wear in Polyalphaolefin Xinhu Wu,†,‡ Kuiliang Gong,†,§ Gaiqing Zhao,†,§ Wenjing Lou,*,†,§ Xiaobo Wang,*,†,§ and Weimin Liu† †

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese

Academy of Sciences, Lanzhou 730000, P. R. China. ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

§

Qingdao Center of Resource Chemistry & New Materials, Qingdao 266000, P. R. China.

ABSTRACT: Herein, MoS2 nanosheets (MoS2) densely modified with n-octadecyl mercaptan (NOM) are used as effective friction reducing and anti-wear additives. A facile and green strategy is employed to prepare the organic-inorganic hybrid nanosheets (MoS2-NOM) by mussel inspired chemistry combining with Michael addition reaction. When MoS2-NOM nanosheets are added in polyalphaolefin (PAO) 10, they form homogeneous and stable dispersion compared to the unmodified MoS2. Tribological measurements show that the dispersion of PAO 10 containing 1 wt % MoS2-NOM displays dramatic reductions in friction coefficient (~ 53%) and wear volume (~ 92%). The exceptional tribological behaviors of MoS2-NOM are ascribed to the formation of a boundary protection film during tribochemical reactions. Further investigations display that the

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addition of 1 wt% MoS2-NOM can efficiently improve the maximum load-carrying capacity, and the high-temperature lubricating property of PAO 10 base oil, which is likely due to the formation n-octadecyl thioether on the surface of MoS2 after the modification of MoS2polydopamine (PDA) with NOM via Michael addition. 1. INTRODUCTION Benefiting from the wide temperature performance range and excellent physical, chemical and thermo-oxidative stabilities, polyalphaolefin (PAO) is one of the most widely used synthetic base oils, and its most common applications are in aerospace, transmissions, hydraulic systems, automotive and marine.1 In addition, different kinds of lubricant additives such as friction reducers, anti-wear (AW) agents, and detergents are used to improve the performances of PAO base oils.2 Among these additives, friction reduction and AW additives have developed rapidly and drawn great attention in the last few years because of the increasing demand of improving fuel economy and reducing emission.3 Typically, zinc dialkyl dithiophosphates (ZDDP) and molybdenum dithiocarbamate (MoDTC) are the predominant agents used as friction reducing and AW additives in PAO base oils. In spite of their excellent tribological properties, gas emissions from ZDDP and MoDTC cause environment pollution, and the fast thermal degradation of these additives leads to a loss of their lubrication performances during their application,4 so it is very significant to find substitutes for these additives. Molybdenum disulfide (MoS2) is one of the most traditional solid lubricants. Recently, as nanoscience and nanotechnology advance, MoS2 nanoparticles have shown great potential for improving the friction reduction and AW properties of base oils. To realize tribological benefits, the dispersion stability in lubricating oils must be addressed. Therefore, several methods have been developed to promote dispersion of MoS2 nanosheets in lubricating oils or solvents. MoS2


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with different morphologies and sizes were fabricated and could be well dispersed in different kinds of oils.4-10 The nanocomposites of MoS2 with graphene also formed stable dispersion in base oils.11-13 Moreover, surface modification represents the most prominent method to promote MoS2 dispersion.14-17 Mussel inspired chemistry is an powerful method for surface modified various materials.18-24 In particular, Zeng et al. reported a simple and facile strategy to synthesize PEGylated MoS2 nanosheets via mussel-inspired chemistry combining with Michael addition reaction,25 and the resulting product shown stable dispersion in water for 24 hours. A very recent publication describes the synthesis of PEGylated MoS2 nanosheets by the same method and their use as friction reducing and AW additives in polyalkylene glycol (PAG) base oil.26 Another method is the decoration of MoS2 sheets with amine-terminated polymers that were linked to MoS2 surfaces through the interaction of Mo with NH2.27,28 Much less is known with respect to modify MoS2 nanosheets with oil-miscible organic components by a simple and facile method. N-Octadecyl mercaptan (NOM) is known to be soluble in PAO, and the combination of NOM with MoS2 nanosheets will exhibit improved dispersibility in PAO base oils. In this work, we modified MoS2 nanosheets with NOM via mussel inspired chemistry combining with Michael addition reaction.23,24 As illustrated in Scheme 1, MoS2 nanosheets were first covered with polydopamine (PDA) film in Tris-buffer solution, then the resulting product (MoS2-PAD) was linked with n-octadecyl mercaptan (NOM) by Michael addition reaction in strong alkaline solution. The obtained MoS2-NOM was used as friction reducing and AW additives in PAO 10 cSt base oil, and the tribological behaviors of the stable dispersion were evaluated by an Optimal-SRV-IV reciprocation friction tester with a ballon-disk configuration. 2. EXPERIMENT SECTION


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2.1. Materials. MoS2 nanosheets (MoS2) with particle size of ~ 90 nm were obtained from Nanjing XFNANO Materials Tech Co., Ltd (Nanjing, China). N-octadecyl mercaptan (NOM) was purchased from J&K Scientific Ltd., (Beijing, China). Dopamine hydrochloride (DA, > 98%) and tris-hydroxylmethylamiomethane (Tris, > 99.8%) were obtained from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China). Sodium hydroxide (NaOH, ≥ 96%) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Polyalphaolefin (PAO) 10 cSt was purchased from ExxonMobil Chemical. All the chemicals were used without further purification. 2.2. Preparation of MoS2-NOM. The functional MoS2 with polydopamine (PDA) coating were synthesized by mussel inspired chemistry. MoS2 nanosheets (500 mg) were dispersed in dopamine solution (160 mL, 5 mg/mL) by ultrasonic treatment for 5 min. Then the Tris buffer solution (1.6 mL, 1 M) was added into the dispersion, and reacted at room temperature for 6 h. After that, the solid product MoS2-PDA was separated by centrifuging (3000 rpm, 10 min), and washed several times by water and ethanol. The product was dried for further experiment. To obtain the MoS2-NOM, MoS2-PDA (100 mg) was dispersed in NaOH solution (60 mL, pH > 12) by ultrasonication for 5 min. Then NOM (300 mg) was added, and the mixture was stirred for 6 h at ambient temperature. The obtained MoS2-NOM was filled off and washed two times with ethanol and diethyl ether, respectively. Finally, the product was dried under vacuum for further characterization. 2.3. Physical Characterizations. High-resolution transmission electron microscopy (HRTEM) analyses were conducted on a FEI TECNAI F30 at 300 kV. Fourier transformation infrared (FTIR) spectra were acquired with a Nicolet iS10 FT-IR spectrometer. Specimens were prepared by a method of pressed KBr disk. X-ray photoelectron spectroscopy (XPS) spectrum was acquired


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using a PHI-5702 XPS spectrometer with a monochromatized Al Kα source. The C 1s binding energy of 284.8 eV was used as a reference. Thermogravimetric analysis (TGA) was performed with a STA 449 F3 Jupiter (NETZSCH) instrument in nitrogen atmosphere at a heating rate of 10 o

C/min from 25 to 800 oC. Raman measurements were performed on a LabRAM-HR (Horiba)

Raman microscope with a laser excitation wavelength of 514.5 nm. Rheological analysis was carried out on an Anton Paar MCR302 rheometer (Austria) using cone to plate geometry (24.958 mm diameter and 0.104 mm gap). Samples were tested at 25 oC and 0 oC, respectively, under air conditions. Moduli measurements were conducted in oscillation mode at a fixed frequency of 10 rad/s with change in shear strain from 0.1 to 100. Rheological tests were carried out in triplicate for each type of lubricants. 2.4. Tribological Measurements. The tribological tests were carried out on an Optimal-SRVIV tribometer. The schematic diagram of this tribometer and the detail of friction pair have been reported in our previous work.10 Lubricant (0.1-0.2 g) was added to the contact area before the tribological measurement. The friction coefficient was recorded automatically via a computer connected to the SRV tester. The corresponding wear volume of lower disks was acquired with a MicroXAM 3D surface profilometer. Surface analysis was performed using JOEL JSM-5600LV scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS) and XPS. Each experiment was repeated three times to confirm reproducibility of the data. 3. RESULT AND DISCUSSION 3.1. Characterization of MoS2-NOM. The morphology of MoS2 and MoS2-NOM could be clearly observed in the TEM images (Figure 1). Compared with MoS2 (Figure 1A), the size and thickness of MoS2-NOM were increased after MoS2 were coated with NOM (Figure 1B). Moreover, Figure 1B-D show that NOM is uniformly distributed on MoS2 surface, and the


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HRTEM (inset in Figure 1D) reveals the hexagonal lattice structure with a lattice spacing of 0.27 nm, corresponding to the distance between the (100) planes of MoS2 (Zoomed section of red contour from Figure 1D). These data provide evidence that NOM was attached to the MoS2 surface through mussel inspired chemistry combined with Michael addition reaction. The surface decorations of MoS2 with PDA and NOM were also evidenced by FT-IR. As shown in Figure 2A, a very broad peak located at about 3440 cm-1 corresponding to O-H and NH stretching vibration is observed in the spectra of MoS2-PDA, which is significantly decreased in the spectra of MoS2-NOM due to the interaction of MoS2-PDA with NOM. In the IR spectra of MoS2-NOM, the most prominent peaks in the 2790–2990 cm-1 region are attributed to the C-H stretching vibration. Further C-H bending vibration can be found at about 1470 cm-1. The peak at 721 cm-1 can be assigned to the S-C stretching vibration. In addition, the absorbance peak of thiol at 2561 cm-1 (data not shown) corresponding to S–H stretching vibration is not observed in the spectra of MoS2-NOM, an indication that no free thiol is present in the sample of MoS2NOM. These results demonstrated the successful modification of MoS2-PDA with NOM via Michael addition. The content of modified NOM was calculated from TGA data shown in Figure 2B, which also shown the TGA curves of pristine MoS2, MoS2-PDA and neat NOM for comparison purpose. The decomposition temperatures (Td) of neat NOM and MoS2-NOM are 232 and 307 oC, respectively, suggesting the excellent thermal stability of MoS2-NOM. Since the TGA curve of MoS2-NOM shows one step weight loss without a second weight loss step around 232 °C (typical for pure NOM), indicating a very high degree of modification on MoS2 surfaces, this is in accordance with the FT-IR result. Moreover, by comparing the residual masses in the TGA curves of pristine MoS2 (2.4 wt %), MoS2-PDA (12.2 wt %), and MoS2-NOM (78.2 wt %), the


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hybrid of MoS2-NOM contains 26.6 wt % MoS2 and 63.6 wt % modified NOM. The XPS measurements were also carried out to understand the chemical state and surface composition of MoS2-NOM. The XPS survey spectra of MoS2, MoS2-PDA and MoS2-NOM are similar to each other (Figure 3A), confirming the existence of MoS2 in MoS2-PDA and MoS2NOM. In the high-resolution spectrum of Mo 3d (Figure 3B), the XPS spectra of MoS2 and MoS2-NOM present three obvious peaks at 232.9, 229.7 and 226.9 eV, which could be assigned to Mo4+ 3d3/2, Mo4+ 3d5/2 and S 2s binding energies of MoS2,29 respectively. However, the XPS spectrum of MoS2-PDA show two weak peaks at 232.9 and 229.7 eV due to the fact that MoS2 are covered with a thin layer of PAD. This is coincident with the small peak centered at 162.7 eV in the S 2p XPS spectrum of MoS2-PDA (Figure 3C). Two peaks centered at 169.4 and 162.7 eV are observed in the S 2p XPS spectrum of MoS2 (Figure 3C). The peaks at 169.4 and 162.7 eV can be assigned to the S6+ and S2- states realized in SO42- and MoS2,29 respectively. In contrast, in the S 2p spectrum of MoS2-NOM, additional peak intensity is observed at binding energy of ~163.4−165.5 eV, corresponding to C-S bonds.30 These results indicate that MoS2-NOM has been successfully synthesized. Also, the S 2p, N 1s, O 1s and C 1s spectra of MoS2-PDA and MoS2-NOM demonstrate that a thick NOM film was coated on the surface of MoS2. 3.2. Dispersibility of MoS2-NOM in PAO 10. The dispersions of 1 wt% NOM, 1 wt% MoS2 and 1 wt% MoS2-NOM in PAO 10 were thoroughly mixed by ultrasonic oscillation for one hour at the temperature of ~ 50 oC. As illustrated in Figure 4, NOM is well soluble in PAO 10 at different temperatures, whereas pristine MoS2 shows obvious precipitation and stratification in the base oil after being kept at 25 oC for 7 days (Figure 4A,A`) and 0 oC for 30 days (Figure 4 B,B`), indicating poor dispersibility of MoS2 in the base oil. Compared with the unmodified MoS2, MoS2-NOM can be well dispersed in PAO 10 and the resulting dispersion stayed


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homogeneous without any sedimentation under the same conditions (Figure 4). In particular, liquid to solid-like gel can be facilely created when the addition of 1 wt% MoS2-NOM in PAO 10 after being kept below 0 oC (Figure 4B, B`). This point is confirmed by rheological measurements in Figure 5. From the results mentioned above, it is clearly manifested that surface modification of MoS2 with NOM has a significant effect on the dispersion stability of MoS2 nanosheets in PAO oils. 3.3. Rheological behaviors of MoS2-NOM in PAO 10 base oil. The shear strain evolution of storage (G`) and loss (G``) moduli for PAO 10 containing different additives were measured at varying temperatures. As shown in Figure 5, PAO 10, and PAO 10 additized with 1 wt% NOM and MoS2 exhibit higher loss (viscous) moduli than storage (solid) moduli (G``> G`) with an increase of shear strains from 0.01 to 100 at 25 oC (Figure 5A) and 0 oC (Figure 5B). This means that all of them can be regarded as viscous liquids. In addition, loss moduli of pure PAO 10 and PAO 10 with 1 wt% NOM and 1 wt% MoS2 are substantially unchanged, demonstrating that the addition of 1 wt% NOM or MoS2 has little effect on the viscosity of the base oil. When the measurements were carried out at 25 oC, the addition of 1 wt % MoS2-NOM in PAO 10 also shown the loss modulus dominates (G``> G`) at almost all shear strains (Figure 5A), but the G` and G`` are significantly higher than for the others, which means that PAO 10 with 1 wt % MoS2-NOM content is much thicker (more viscous) than the base oil and the oil with 1 wt % content of NOM and MoS2 at 25 oC. In contrast, for PAO 10 containing 1 wt% MoS2-NOM at 0 o

C (Figure 5B), the nanofluid shows a transition from G`- to G``-dominant (solid- to liquid-like)

behavior at strains in the linear viscoelastic (LVE) regime. Additionally, the G` of PAO 10 additized with 1 wt% MoS2-NOM is more than two orders of magnitude greater than G` of PAO 10 and PAO 10 with 1 wt% content of NOM and MoS2, showing that the material is in reality


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viscoelastic liquid at 0 oC. Meanwhile, it is observed from Figure 6 that the shear stress displays an obvious change of slope at strains, which is consistent with the solid-like to liquid-like transition behavior in Figure 5. 3.4. Tribology analysis of MoS2-NOM in PAO 10. The tribological behaviors of PAO 10 additized with varying contents of MoS2–NOM were investigated by SRV at a constant load of 100 N and a temperature of 50 oC. The oscillation frequency was 25 Hz with 1 mm stroke, and the duration was 30 min. The average friction coefficient and wear volumes of the steel disks lubricated by these lubricants are shown in Figure 7. It is seen that 1 wt % MoS2-NOM is the optimum concentration to significantly reduce friction coefficient and wear volume. To further study the tribological properties of MoS2-NOM, the friction and wear of PAO 10 and PAO 10 with 1 wt% content of neat NOM, MoS2 and MoS2-NOM were measured under the same conditions. As shown in Figure 8A, the addition of 1 wt% NOM, MoS2 and MoS2-NOM can dramatically shorten the running-in time with more stable and lower friction coefficient as compared to the base oil. In particular, the addition of 1 wt% MoS2-NOM can reduce the friction coefficient by about 53, 31 and 22%, compared to the base oil and the oil with 1 wt% content of NOM and MoS2, respectively, indicating the excellent friction reducing property of MoS2-NOM. Figure 8B gives the corresponding wear volume of steel disks lubricated by these lubricants. It can be found that PAO 10 additized with 1 wt % MoS2-NOM experiences the lowest wear loss during tribological tests. In fact, 1 wt % MoS2-NOM can reduce the wear by about 92%, 79%, and 51% with respect to the base oil, 1 wt % NOM and 1 wt % MoS2, respectively. These results show that MoS2-NOM as additive in PAO 10 not only leads to significant reductions in friction coefficient but also provides an exceptional AW protection. The AW performance of MoS2-NOM dispersed in PAO 10 was also confirmed with SEM and


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three-dimensional (3D) surface profilometer. It is apparent from Figure 9aa` that the worn steel surface lubricated by pure PAO 10 shows wide and deep wear scar, indicating significant wear. Although the worn surfaces lubricated by the addition of 1 wt % NOM (Figure 9bb`) and MoS2 (Figure 9cc`) are considerably reduced, both of wear scars are very rough, with a lot of small furrows on the friction surface. However, when 1 wt % MoS2-NOM is added in PAO 10, the worn surface obviously becomes narrower and the wear scratches also become more shallow and smoother (Figure 9dd`), indicating excellent AW property of MoS2-NOM. In addition, Figure 9A-D displays the 3D morphology of wear scars, this demonstrates the distinguishable AW properties of PAO 10 containing 1 wt% NOM, MoS2 and MoS2-NOM, and the result is also in agreement with the wear volumes in Figure 8B. In order to evaluate the load-carrying capacity of MoS2-NOM added in PAO 10, Figure 10 shows the variations of friction coefficient with time during a load ramp test from 50 to 500 N with 50 N increments for PAO 10 containing different additives at 50 oC. The test duration for each load was 5 min. It is seen that the maximum load-carrying capacity of pure PAO 10 is only 150 N, and the addition of 1 wt% NOM has little effect on the load-carrying property of base oil, which exclude the possibility that NOM is the cause of the superior lubricating property. In contrast, the addition of 1 wt % MoS2 and MoS2-NOM prevent scoring and seizure as the load increasing to 350 and 450 N, suggesting that the maximum load-carrying capacity of MoS2 and MoS2-NOM is 300 and 400 N, respectively. These results demonstrated that surface modification of MoS2 with NOM can efficiently improve the load carrying capacity of MoS2, likely owing to the formation n-octadecyl thioether on the surface of MoS2 after the modification of MoS2-PDA with NOM via Michael addition. The lubricating property of PAO 10 additized with 1 wt% NOM, 1 wt% MoS2 and 1 wt%


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MoS2-NOM were also investigated via a temperature ramp test from 25 oC up to 200 oC stepped by 25 oC, with 5 min test duration for each temperature. It is seen from Figure 11 that the pure PAO 10 exhibits a high friction coefficient with value of 0.20 ~ 0.35 when the temperature below 75 oC, from which the friction coefficient gradually decreases to 0.15 over the testing period. The addition of 1 wt% NOM, 1 wt% MoS2 and 1 wt% MoS2-NOM at low temperatures (25, 50 oC) have very similar effects on friction coefficient, with significantly lower value compared to the base oil. However, the lubricant containing MoS2-NOM distinguishes itself from other tested lubricants that it can withstand higher temperature than that of others. Obviously, the highest temperature with no seizure for PAO 10 additized with 1 wt% MoS2NOM is 125 oC, much higher than that of NOM (50 oC) and MoS2 (100 oC), indicating excellent friction reducing property of MoS2-NOM at elevated temperature. 3.5. Surface Analysis. The tribofilms on the wear scars lubricated with PAO 10 and PAO 10 containing 1 wt% MoS2-NOM were studied by SEM/EDS techniques (Figure 12). The SEM/EDS data obtained indicates that high concentrations of Mo and S in the tribofilm (inset of Figure 12B) suggest a contribution from the addition of 1 wt% MoS2-NOM, to film formation. However, little Mo and S were detected on the worn surface lubricated by pure PAO 10 (inset in Figure 12A). Additionally, the EDS element mapping in Figure 12C and D show that the tribofilm is rich in Mo and S, which is consistent with the result of SEM/EDS. The friction reducing and AW mechanism of MoS2-NOM was further investigated by XPS spectra. As shown in Figure 13A, the binding energy of Mo 3d can be deconvoluted into four peaks corresponding to S 2s (226.7 eV), MoS2 (229.5, 232.4 eV), and MoO3 (235.5 eV),13,30 respectively. The XPS spectra of S 2p appear at the binding energy of 162.3, 163.6, and 168.7


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eV (Figure 13B), which can be assigned to S2- and S6+ states realized in MoS2 and FeSO4/Fe2(SO4),13,30 respectively. The spectra of Fe 2p is deconvoluted into six peaks corresponding to FeS2 (709.0 eV), FeO (709.9 eV), Fe2O3 (710.7 eV), FeOOH (711.7 eV), FeS (712.7 eV), and FeSO4 (713.8 eV),13,30 respectively (Figure 13C). The peaks of O 1s also confirm the presence of FeO/Fe2O3 (530.1 eV), Fe3O4/FeOOH (531.7 eV), FeSO4/FeSO4 (532.2 eV), and C-O binding (533.3 eV) in the tribofilm which was formed during the sliding process (Figure 13D).30 The XPS results indicated that a stable tribofilm was generated on the contact surface lubricated by PAO 10 additized with MoS2-NOM. The protection film composed of MoS2, FeS2, FeS, FeO, Fe2O3, FeOOH, FeSO4/Fe2(SO4)3 and C-O binding species in the lubricated metal interface, which leads to the exceptional tribological behaviors of MoS2-NOM dispersed in PAO 10 base oil. 4. CONCLUSIONS In summary, the surfaces modification of MoS2 nanosheets (MoS2) with n-octadecyl mercaptan (NOM) was developed via mussel inspired chemistry combining with Michael addition reaction. The as-prepared products (MoS2-NOM) were characterized by spectral analysis (FT-IR, TEM, XPS) and TGA. These results indicated that MoS2-NOM with a high weight percentage of NOM were successfully synthesized, leading to the excellent dispersibility of MoS2-NOM in PAO 10 base oil. Moreover, the addition of 1 wt% MoS2-NOM can significantly improve the friction reducing and AW properties, the maximum load-carrying capacity, and the high-temperature lubricating property of PAO 10 base oil. The superior lubricating property of MoS2-NOM in PAO 10 can be explained by the fact that a tribofilm composed of MoS2, FeS2, FeS, FeO, Fe2O3, FeOOH, FeSO4/Fe2(SO4)3 and C-O binding species was formed on the steel rubbing surfaces, which contributed to the lower friction and wear of MoS2-NOM as additive in PAO 10 base oil.


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AUTHOR INFORMATION Corresponding author: E-mail: [email protected], and [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are thankful for financial support of this work by National Natural Science Foundation of China (NSFC 51475445 and 51775536). REFERENCES (1)

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FIGURES.

Scheme 1. Synthesis of MoS2-NOM by mussel inspired chemistry combining with Michael addition reaction.


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Figure 1. TEM micrographs of (A) MoS2 nanosheets (MoS2) and (B-D) MoS2-NOM sample. Insets of (D): The HRTEM micrographs of MoS2-NOM (zoomed from the red contour in D).


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Figure 2. (A) FT-IR spectra of MoS2 nanosheets (MoS2), MoS2-PDA and MoS2-NOM. (B) TGA curves of MoS2, MoS2-PDA, MoS2-NOM, and neat NOM.


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Figure 3. (A) XPS full scan spectra of MoS2, MoS2-PDA and MoS2-NOM. (B) Mo 3d, (C) S 2p, (D) N 1s (E) O 1s and (F) C 1s scan spectra of MoS2-NOM sample.


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Figure 4. Photos of PAO 10 containing 1 wt% NOM, 1 wt% MoS2, and 1 wt% MoS2-NOM after being kept at (A) 25 oC for 7 days and (B) 0 oC for 30 days. PAO 10 containing different additives shown in A and B were flipped, which are displayed in part A` and B`.


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Figure 5. Storage modulus (G`), and loss modulus (G``) versus strain (γ) of pure PAO 10, and PAO 10 additized with 1 wt% NOM, 1 wt% MoS2, and 1 wt% MoS2-NOM at (A) 25 oC and (B) 0 oC, respectively.


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Figure 6. Shear stress (τ) versus strain (γ) of pure PAO 10 and PAO 10 additized with 1 wt% NOM, 1 wt% MoS2, and 1 wt% MoS2-NOM at (A) 25 oC and (B) 0 oC, respectively.


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Figure 7. Friction coefficient (column) and wear volumes (line + symbol) of steel disks lubricated by PAO 10 containing various amounts of MoS2-NOM (SRV load, 100 N; temperature, 50 oC; stroke, 1 mm; frequency, 25 Hz).


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Figure 8. (A) Friction coefficient and (B) wear volumes of steel disks lubricated by PAO 10 and PAO 10 additized with 1 wt% NOM, 1 wt% MoS2, and 1 wt% MoS2-NOM (SRV load, 100 N; temperature, 50 oC; stroke, 1 mm; frequency, 25 Hz).


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Figure 9. (aa`-dd`) SEM and (A-D) 3D images of the wear scars lubricated by pure PAO 10, and PAO 10 with 1 wt% NOM, 1 wt% MoS2, 1 wt % MoS2-NOM at 50 oC (SRV load, 100 N; duration, 30 min; stroke, 1 mm; frequency, 25 Hz).


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Figure 10. Friction curves for PAO 10 and PAO 10 containing 1 wt% NOM, 1 wt% MoS2, 1 wt % MoS2-NOM. The tribological measurements were performed during a load ramp test from 50 to 500 N with 50 N increments, at 50 oC, a stroke of 1 mm, and a frequency of 25 Hz.


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Figure 11. Friction curves for PAO 10 and PAO 10 containing 1 wt% NOM, 1 wt% MoS2, 1 wt % MoS2-NOM. The tribological measurements were performed during a temperature ramp test from 25 to 200 oC with 25 oC increments, at a constant load of 50 N, a stroke of 1mm, and a frequency of 25 Hz.


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Figure 12. SEM micrographs of worn surfaces lubricated by (A) PAO 10 and (B) PAO 10 containing 1 wt % MoS2-NOM at 50 oC (SRV load, 100 N; duration, 30 min; stroke, 1 mm; frequency, 25 Hz). Insets of (A, B): the corresponding EDS spectra of tribofilms on the worn surfaces. (C, D) The EDS elemental mapping on the wear surface lubricated by PAO 10 containing 1 wt % MoS2-NOM.


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Figure 13. XPS spectra of the elements (A, Mo 3d; B, S 2p; C, Fe 2p and D, O 1s) on the wear scars lubricated by PAO 10 containing 1 wt %MoS2-NOM at 50 °C, 100 N.


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