Article pubs.acs.org/IECR
Friction between a Steel Ball and a Steel Flat Lubricated by MoS2 Particles Suspended in Hexadecane at 150 °C Manimunda Praveena,† Vikram Jayaram,† and Sanjay K. Biswas*,‡ †
Ceramic Lab, Department of Materials Engineering, Indian Institute of Science, Bangalore -560 012, India Nanotribology Lab, Department of Mechanical Engineering, Indian Institute of Science, Bangalore -560 012, India
‡
ABSTRACT: A steel ball was slid on a steel flat lubricated by molybdenum disulfide (MoS2) particles suspended in hexadecane oil at 150 °C. The friction data is compared with that obtained when the ball was slid on the flat sprayed apriori with nominally dry MoS2 particles. The friction in the dry experiment was found to increase with temperature while the friction in wet condition was found to decrease with increasing temperature. Micro-Raman and Fourier transform IR spectroscopy are used to explore the roles of environmental moisture and chemical degradation of oil on the formation of antifriction film on the steel substrate.
1. INTRODUCTION It has been known now for nearly 100 years that layer-lattice solids such as graphite, molybdenum disulfide, and tungsten disulfide shear easily.1 Solid surfaces have been modified by dry coatings of these layered materials to yield low friction.2 The configuration is particularly useful in moving machineries transmitting power such as internal combustion engines and bearings as well as in metal working processes such as rolling where the rolls may be surface modified to control the torque. The layered particles are weak in the direction normal to the basal plane as much as they cleave easily on the basal plane;3 they offer unique opportunity to operate the machinery under high contact pressure. If they are in suspension, they provide load-bearing capacity under boundary lubrication as the suspending liquid carries the particles to the active zone of power transmission.4−6 In engines and gears, the layered particles are often in grease or oil suspension where the temperature of the suspension is limited by the decomposition temperature of the liquid or the semiliquid phase. In internal combustion engines, the operating temperature may be in the 150−360 °C range. In metal cutting and disk brake applications, the temperatures generated are more than 150 °C.7 Arslon et al.8 studied the high temperature wear behavior of MoS2/Nb coatings and observed a low friction of 0.014 at 100 °C without any failure of the coating. At temperatures above 300 °C, rapid oxidation leading to failure of the coating was observed. Since the 2H-MoS2 particles have edge sites and dangling bonds, they oxidize to MoO3 at temperatures above 350 °C. The high modulus (240 GPa) and low density (4.8 g/cm3) of molybdenum disulfide (MoS2) nanoparticles make it a candidate for use in polymers as a filler,9 to enable support when applied stresses are high. MoS2 is also used widely as a lattice-layered solid lubricant.1 The material has a hexagonal layered structure. When deformed, the MoS2 layers shear easily, and exfoliated sheets transfer to the mating bodies to form a transfer film that internally shear to give low friction and wear.2 Formation of the protective transfer film at sliding contact is a key phenomenon that controls the tribology of the particles.10 Traditional antiwear additives such as zincdialkyldithiophospate © 2012 American Chemical Society
decompose in lubricated tribology to form a reactive (decomposed material react with the substrate) spongy tribofilm.4 In contrast, a nonreactive transfer film forms when MoS2 particles are dispersed in oil. The mechanism of MoS2 transfer film formation has been studied extensively.3,11,12 Nanoparticles of MoS2 of different morphologies (2H, IF, hollow tube) have been used as antifriction additives in oils. The morphology of the nanoparticles however does not appear to have a dramatic effect on friction characteristics and wear behavior, in sliding contact most of them deform to layered morphology in forming transfer films.13 For example, nanoparticles of IF structures undergo deformation in opening of the ring structure leading to the transfer of exfoliated layers. Tribological behavior of such particles is also influenced by operating temperatures. The main body of work on the tribology of these particles, whether under dry condition or suspended in liquid, has been done at room temperature.14−16 Published data of friction of MoS2 at temperatures in the range 25−200 °C are few and far between.17 In dry lubrication of substrates coated densely with MoS2, it has been observed that the friction decreases with reduced environmental contamination.17−20 Extending the rationale to where temperature is a parameter, one may therefore expect the friction coefficient to decrease with increasing temperature as the adsorbed water evaporates. The present study investigates the frictional behavior of the MoS2 suspended in n-hexadecane between a steel ball and a steel flat during sliding interaction in the temperature range 25−150 °C. A known volume of MoS2 particles is dispersed in oil, and the suspension is used to lubricate the steel on steel sliding contact. The data is benchmarked against the data collected when the same volume of the solid particles is sprayed on the nominally dry steel substrate. In considering the interest shown in previous works18−20 on the role of environmental moisture on the friction of MoS2, the present study explores the Received: Revised: Accepted: Published: 12321
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3. RESULTS AND DISCUSSION 3.1. Role of Environmental Moisture on Dry MoS2 Lubrication. Environmental moisture influences the frictional resistance offered by third body MoS2 particles at steel on steel contact.19 Figure 1a shows representative friction traces
role of moisture when the operating temperature is increased to 150 °C. Micro-Raman spectroscopy and ex situ Fourier transform (FT)IR spectroscopy are used to study the combined roles of oil decomposition and water evaporation at this temperature, as they affect the frictional behavior of the lubricant.
2. EXPERIMENTAL DETAILS 2.1. Materials. Stainless steel disk (SS304) (Hardness: 5.4 GPa; roughness: 25 nm) and EN9 ball (diameter: 6 mm; hardness: 6.8 GPa; roughness: 80 nm) are used in the tribological experiments. The MoS2 particles (average particle size: 90 nm) were procured from M. K. Impex, Canada. The model lubricant used in all the experiments was n-hexadecane (>99%), supplied by Sigma−Aldrich, Inc. 2.2. Sample Preparation. A suspension of MoS2 particles was prepared by mixing 2 mg of particles in 10 mL of nhexadecane solution and sonicated for 10 min. For dry sliding experiments, a suspension of MoS2 particles (2 mg) in hexane (10 mL) was prepared, and a drop of the solution was sprayed on the steel disk. The hexane solution evaporates leaving dry MoS2 particles on the steel substrate. 2.3. Tribology. Tribological experiments were performed using a pin on disk microtribometer (CSM instruments, Switzerland) at a fixed load of 1 N and 0.9 cm/s sliding speed. In the lubricated sliding test, both the pin and the disk were immersed inside an oil bath, which contained a fixed volume (per oil bath volume) of MoS2 particles. The oil bath was heated using a resistive heating stage (CSM instruments, Switzerland). For high temperature dry sliding experiments, the disk was heated using a resistive heating stage .The temperature rise of the disk surface due to sliding was estimated (see Appendix A) to be less than 1 °C for all the experimental configuration. Experimental parameters are presented in Table 1.
Figure 1. (a) Representative friction characteristics of MoS2 lubricated contact, at room temperature (ambient and nitrogen atmospheres). (b) Optical profilometer images of the wear track formed when dry MoS2 lubricated steel is slid against steel ball in ambient and dry nitrogen atmospheres. Two-dimensional (2D) profiles are those taken along the red lines, drawn transverse to the wear track.
Table 1. Experimental Parameters parameter
value
temp (°C) normal load (N) sliding velocity (cm/s) humidity
25, 50, 100, 150 1 0.9 35% RH (relative humidity), nitrogen atmosphere
generated by sliding a steel ball on a steel flat using nominally dry MoS2 powder as a solid lubricant sprayed a priori on the flat. The friction generated in a dry nitrogen environment is consistently (except at sliding time, t < 2 min) higher than that generated in the ambient. The minimum difference between the two traces (2 < t < 40 min) is about 0.08 ± 0.02, from three repeated experiments. When the experiment was conducted in the ambient, the track showed MoS2 films and no significant wear of the steel surface. When dry nitrogen was used as the environment, the track (Figure 1b) showed significant wear. A micrograph (Figure 2a) of the track generated in the ambient and recorded by an optical microscope shows well-formed bluish-gray smooth strips: uniform, long, and wide (20−40 μm). The smooth patches on the disk break up (Figure 2b, c) when the experimental temperature is increased. The strips are replaced by fragmented small particles (colored white) and some small islands (∼2−5 μm). The pin surface at 50 °C (Figure 3a) shows a bluish-gray deposit at the contact entrance, the thickness of which decreases rapidly along the sliding direction. The Raman spectra (Figure 3b) confirm the presence of MoS2 on the pin and disk surfaces generated at 50 and 150 °C.
2.4. Wear Track Analysis. The imaging of the wear track was done using optical and scanning electron microscopy (SEM) and optical profilometry. To understand the chemical composition of the films formed on the wear track, resonance Raman spectra were recorded using a micro-Raman system (Renishaw, UK). An excitation wavelength of 785 nm was used, and to prevent sample damage, different optical filters were used; this enabled the laser power to be decreased to 30 mW. Raman spectrum of bulk crystalline MoS2 gives prominent dual peaks at 383 and 408 cm−1. 2.5. High-Temperature (ex Situ) IR Experiments. A drop of n-hexadecane-MoS2 suspension was placed on top of the steel substrate, and by using a sample heating accessory (Harrick Scientific Products, Inc., USA), substrate was heated to 170 °C. By using a FTIR spectrometer (PerkinElmer, USA), the FTIR spectra were recorded for every 10 °C rise in temperature. 12322
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surfaces is a rare event and the sliding interaction is between loose MoS2 particles and fragments. The appearance (Figure 3b) of the longitudinal acoustic mode, LA (M) peak at 240 cm−1 (characteristic of MoS2 nanostructures) at 150 °C confirms the presence of MoS2 particles and fragments on both the pin and disk surfaces. One of the possible reasons for this may be the fact that, at these temperatures, the particles are never anchored to the substrate to allow them to be sheared extrudes to form sheets. The friction coefficient at the commencement of an experiment with dry MoS2 was high (Figure 4a). Generally, after 30 min of running-in, the average value stabilized with periodic fluctuations in time. Most of the data reported here pertains to this stabilized value that may be designated as the “steady-state” value. Figure 4b shows that (when the experiment was done with 2 mg of sprayed MoS2) the coefficient of friction increased steadily with increase in substrate temperature. The trend of coefficient of friction with temperature that is observed in the dry test is contrary to what has been reported for MoS2 coatings.21,22 We may suggest that this is due to the fact that the volume of MoS2 particles sprayed a priori on the disk is low. The volume had to be low to ensure that the volume of the MoS2 sprayed is the same as that dispersed in oil in the lubricated test (20 vol %; vol % = (vol of MoS2 /vol of oil) × 100). When the volume of the sprayed particle was increased (8 mg), the steady-state friction coefficient decreased from a value of 0.15 (± 0.02) to 0.08 (± 0.02) at 100 °C (Figure 4b), which was a trend very similar to that reported for MoS2 coatings.2 In a similar vein, the “low-volume” trend that the friction of sprayed particles increases when an ambient environment is replaced by a dry N2 environment (Figure 1a) is contrary to that reported for MoS2 coating.17 We do not have a comprehensive explanation for this apparent disagreement with earlier works8,17 at this stage. We may however speculate that, when a relatively small amount of MoS2 particles is available, a mixture of such particles and environmental water are entrained in the contact zone. The particles, probably anchored to the substrate by water capillarity forces, are sheared repeatedly in the presence of bulk water to form patches of a transfer film. Steel surfaces are easily wet by water (contact angle 66°). MoS2 is reported to have a contact angle of 107° and a low interfacial energy (20 mJ/m2) when interacting with water.23 In the part of contact zone where there is a dearth of particles (inadequate reservoir), there is metal-to-
Figure 2. Optical micrograph of the wear track showing patches of MoS2 film formed during dry MoS2 lubricated steel−steel sliding.
Figure 3a (50 °C) shows the extrusion of a MoS2 sheetlike structure (5−20 μm width) on the pin close to the contact entrance. The same figure also shows that outside the entrance zone; the wide worn flats on the pin are denuded of MoS2, and only some of the wear grooves carry MoS2 particles. Now, if we take Figure 2a and Figure 3a (50 °C) together, we may infer that strips of MoS2 of Figure 2a have originated on the pin at the near entry zone and were transferred to the disk where they are “ironed” into the disk surface by the bare pin, bereft of MoS2 deposit. Thus, a substantial MoS2 film forms on the disk surface, and a sliding interaction between a nearly bare pin and this film yields low friction (μ = 0.12, Figure 4a). At 150 °C, the pin shows (Figure 3a, 150 °C) a uniform spread of patchy deposits and particles on top of the wear grooves. It is clear that, at 100 and 150 °C, formation of large coherent strips and patches of MoS2 on the pin and the disk
Figure 3. (a) Optical micrograph of the pin surface (dry MoS2 lubricated). (b) Raman spectra of the pin, disk surface. Peaks characteristic of MoS2 are identified by arrow marks. 12323
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Figure 4. (a) Friction traces at different temperatures (dry MoS2 lubricated steel−steel contact). (b) Effect of temperature and MoS2 volume sprayed on friction coefficient.
recondenses at the particle−substrate interface when the experiment was done on the same (pin and disk) tracks (generated in step 1) but at room temperature.
metal contact, and scratches are made (Figure 2a). It is possible that the recorded friction is an average of the high friction experienced at metal-to-metal contact and the low friction that corresponds to the shearing in an area of limited but coherent transfer film. When the temperature is raised, the anchoring water evaporates. This also would disallow bonding between particles to form a coherent film. The individual particles in the contact zone in this case reside in the scratched grooves (Figure 2b, c). By contrast, if a large reservoir of MoS2 particles is available in the contact zone, the particles are pressed into forming a thick transfer film over the entire contact zone. The interlayer adsorbed water comes into play as the shear takes place within the film itself and the friction decreases with increasing temperature. This trend with temperature has been observed by others, when the film is a coating of densely packed particles.17 We suggest that the friction in the low-volume case is controlled by the presence of bulk water at the MoS2 steel interface that is needed to keep the MoS2 anchored to the steel surface. When the volume is high, it is the adsorbed interlayer water molecules that control the friction behavior of the particles. To explore this contention further, an experiment was done where the following sequence was implemented: (1) the sprayed particles (MoS2 20 vol %) were slid in dry tribology at 150 °C; (2) the pin and the disk were cooled to room temperature (in the ambient); and (3) a sliding experiment was done at room temperature without disturbing the pin and disk of step 1. Figure 5 shows that the value of coefficient of friction of 0.21 obtained in step 1 is brought down to 0.17 in step 3.We suggest that this happens because water expelled at 150 °C
Figure 5. Effect of moisture on dry MoS2 lubrication. The heater was switched off after 25 min of running, and the pin and disk were cooled to room temperature. The experiment continued in room temperature (ambient) showed a lower friction coefficient than that recorded at 150 °C.
3.2. High-Temperature Tribology of MoS2−Hexadecane Suspension. Figure 6a shows that, when MoS2 particles are put in suspension in hexadecane at room temperature, the presence of these particles reduces the coefficient of friction from more than 0.4 (pure hexadecane) to about 0.14 over 5 min, after which the friction is invariant with time. The coefficient of friction as a function of temperature is given in 12324
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Figure 6. (a) Interrupted friction test, showing the effect of MoS2 addition to hexadecane after 15 min of sliding in track. (b)Friction coefficient of steel−steel contact lubricated by MoS2−hexadecane suspension and dry MoS2 (20 vol %, 2 mg of MoS2) as a function of test temperature.
Figure 6b. The average friction coefficient (Figure 6b) of the hexadecane−MoS2 suspension unlike in the case of the dry experiments reduces with increasing temperature. At 150 °C, the coefficient of friction is 0.09. We have suggested here that, in dry MoS2 sliding experiments, water retains the MoS2 particles bound to the substrate. The following scenario is a possibility in the liquid lubrication experiment at 50 °C. The hydrophobic hexadecane expels moisture from the suspension at 50 °C. MoS2 particles without being anchored to the substrate move to the edge of the contact. The optical micrograph in Figure 7a (50 °C) shows MoS2 film only at the edge of the disk track and wide iron flats in the middle of the track with some MoS2 particles on the flat. The Raman spectra (Figure 7b, 50 °C) of the pin and disk show a very broad MoS2 peak. The optical micrograph (Figure 7a) of the pin surface (50 °C) shows pale blue streaks signifying the presence of a very thin MoS2 film. The broad MoS2 peak (383, 408 cm−1) with steel background (Figure 7b) confirms the existence of a very thin MoS2 film on the pin surface. Thus, the pin with some traces of this streaky MoS2 film rubs on a bare steel flat as it pushes MoS2 into an edge pileup. At 50 °C, some metal-to-metal contact is very likely to happen, giving a relatively high friction (Figure 6). The high (150 °C) temperature track, (Figure 7a) shows thick films of MoS2 distributed throughout the track. The optical micrograph (Figure 7a) and Raman spectra (Figure 7b, strong and sharp MoS2 signal) confirm the presence of MoS2 film24 and iron oxide deposits on the high temperature (150 °C) disk tracks. The optical micrograph of the pin (150 °C) shows dark patches covering the track area. The Raman spectrum of the pin track confirms the presence of MoS2 particles on the pin and iron oxide at the edges of the track (brown-colored region). The pin develops a comprehensive thick MoS2 film at 150 °C (Figure 7a). Figure 8 shows smooth grooves on the disk track. The grooves, the flats, and the fractured edge of the grooves have smooth MoS2 films (not broken fragments or individual particles as in the case of the track generated at 50 o C). A pin carrying a film of MoS2 thus slides on a disk track thickly deposited with MoS2 and iron oxide. The resulting coefficient of friction is low. Although we do not have any additional supporting evidence, we believe that there are substantial chemically bound tribofilms on both the pin and the disk at 150 °C. We believe that this is made possible by chemical reactions that happen between chemically decomposing hydrocarbon and the MoS2 at 150 °C. It seems
Figure 7. (a) Optical micrograph of the disk and pin surfaces (steel− steel sliding lubricated by hexadecane−MoS2). (b) Raman spectra of the pin and disk surfaces. Peaks characteristic of MoS2 are identified by arrow marks. At 150 °C, iron oxide (667 cm−1) is detected on disk track.
likely that the presence of a smooth MoS2 tribofilm on the 150 °C track accords a relatively low friction tribology. We have argued that, in the case of dry tribology of sprayed MoS2 at 150 °C, the friction is high because of the expulsion of the condensed water vapor at the particle−substrate interface at this temperature. Given that, in the 100−150 °C range of temperature, moisture is not available, we suggest below a mechanism that attempts to explain low friction in liquid lubrication tribology of MoS2 at high (150 o C) temperature. 12325
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has been suggested that MoS2 bonds with FeO (see Figure 9) using one of the two possible reaction routes.12
Figure 9. Schematic showing the possible binding mechanism of MoS2 on the steel surface.
The scheme is tested here indirectly by heating a suspension of MoS2 in hexadecane on a steel substrate. The FTIR spectra of the suspension (Figure 10) shows that the carbonyl
Figure 10. FTIR spectra of a hexadecane−MoS2 suspension on the steel surface as a function of temperature. The zoomed in portion compares the carbonyl peak seen in the cases of hexadecane and hexadecane−MoS2 suspension at 150 °C.
compounds appear only at temperatures above 130 °C. A closer examination of the spectrum shows that there is a broad peak at 1760 cm−1. This indicates the presence of a mixture of compounds including cyclic ester. The Raman (Figure 7b) and EDAX (Figure 8) analyses show the presence of MoS2 and iron oxide on the wear track. The above scheme thus provides a possible rationale for the binding of MoS2 on the steel surface at 150 °C and for the generation of a low friction tribofilm.
Figure 8. (Top) SEM image and (bottom) EDAX spectra from the disk wear track formed when hexadecane−MoS2-lubricated steel−steel contact slid at 150 °C. Regions: (1) outside the track; (2) on a groove; (3) on a flat; (4) fractured edge of a groove.
3.3. Oxidation of Hexadecane. We have shown in an earlier work25 that hexadecane decomposes readily (especially above 100 °C) when used to lubricate a steel-on-steel sliding contact. The second stage reaction products are alcohol, aldehyde, and carboxylic acid.26−30 We suggest that these secondary reaction products recombine to give esters (see Appendix B for a suggested reaction scheme) releasing water that preferentially oxidizes iron debris. As no iron oxide was detected on wear track generated in the ambient (Figure 7a), we may infer that the iron oxide detected at 150 °C (Figure 7b and 8) is a product of nascent iron debris reacting with the decomposition product of the oil (available only at 150 °C). It
4. CONCLUSIONS When a relatively low volume of nominally dry MoS2 particles are sprayed on a steel plate and rubbed by a steel ball, the friction coefficient increases with temperature. We suggest that this happens because a lubricating bulk environmental water film condensed/adsorbed on the substrate evaporates with increasing temperature promoting metal-to-metal contact and increasing friction. When the sliding contact takes place at 150 °C in hexadecane oil carrying the same volume of MoS2, the 12326
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(4) Gansheimer, J.; Holinski, R. A study of solid lubricants in oils and greases under boundary conditions. Wear 1972, 19, 439−449. (5) Sahoo, R. R.; Biswas, S. K. Deformation and friction of MoS2 particles in liquid suspensions used to lubricate sliding contact. Thin Solid Films 2010, 518, 5995−6005. (6) Arbabi, H.; Eyre, T. S. Investigation in to the lubricating effectiveness of Molybdenum disulfide dispersion in a fully formulated oil. Tribol. Int. 1986, 19, 87−91. (7) Cho, M. H.; Ju, J.; Jin, K. S.; Ho, J. Tribological properties of solid lubricants (graphite, Sb2S3, MoS2) for automobile brake friction materials. Wear 2006, 260, 855−860. (8) Arslan, E.; Totik, Y.; Bayrack, O.; Efeoglu, I.; Celik, A. High temperature friction and wear behaviour of MoS2/Nb coating in ambient air. J. Coat. Technol. Res. 2010, 7 (1), 131−137. (9) Zhu, P.; Wang, X.; Wang, X. D.; Huang, P.; Shi, J. Tribology performance of molybdenum disulfide reinforced thermoplastic polyimide under dry and water lubrication condition. Ind. Lubr. Tribol. 2006, 58/4, 195−201. (10) Wahl, K. J.; Singer, I. L. Quantification of a lubricant transfer process that enhances the sliding life of a MoS2 coating. Tribol. Lett. 1995, 1, 59−66. (11) Moser, J.; Lavy, F. MoS2−x lubricating films: structure and wear mechanisms investigated by cross-sectional transmission electron microscopy. Thin Solid Films 1993, 228, 257−260. (12) Tannous, J.; Dassenoy, F; Lahouij, I; Le Mogne, T; Vacher, B; Bruhacs, A .; Tremel, W. Understanding the tribochemical mechanisms of IF-MoS2 nanoparticles under boundary lubrication. Tribol. Lett. 2011, 41, 55−64. (13) Hu, K. H.; Hu, X. G.; Xu, Y. F.; Huang, F.; Liu, J. S. The effect of morphology on the tribological properties of MoS2 in liquid paraffin. Tribol. Lett. 2010, 40, 155−165. (14) Chhowalla, M.; Amaratunga, G. A. Thin films of fullerene like MoS2 nanoparticles with ultra low friction and wear. Nature 2000, 407, 164−167. (15) Huang, H. D.; Tu, J. P.; Zou, T. Z.; Zhang, L. L.; He, D. N. Friction and wear properties of IF−MoS2 as additive in paraffin oil. Tribol. Lett. 2005, 20, 247−250. (16) Rapoport, L.; Feldman, Y.; Homyonfer, M.; Cohen, H.; Sloan, J.; Hutchinson, J. L.; Tenne, R. Inorganic fullerene like materials as additives to lubricants: structure−function relationship. Wear 1999, 225−229, 975−982. (17) Sliney, H. E. Solid lubricant materials for high temperaturesa review. Tribol. Int. 1982, 15, 303−315. (18) Haltner, A. J. An evaluation of role of vapor lubrication mechanisms in MoS2. Wear 1964, 7, 102−117. (19) Lancaster, J. K. A review of the influence of environmental humidity and water on friction, lubrication and wear. Tribol. Int. 1990, 23, 371−389. (20) Uemura, M.; Saito, K.; Nakao, K. A mechanism of vapor effect on friction coefficient of molybdenum disulfide. Tribol. Trans. 1990, 33, 551−556. (21) Dangsheng, X. Lubrication behaviour of Ni-Cr based alloys containing MoS2 at high temperature. Wear 2001, 251, 1094−1099. (22) Kubart, T.; Polcar, T.; Kopecky, L.; Novak, R.; Novakova, D. Temperature dependence of tribological properties of MoS2 and MoSe2 coatings. Surf. Sci. Coat. Technol. 2005, 193, 230−233. (23) Huogui, Y. U.; Ping, H. U.; Ting, S.; Ming, J.; Qian, C. Phase representation and property determination of raw materials of solid lubricant. J. Wuhan Univ. Technol., Mater. Sci. Ed. 2008, 23, 130−133. (24) Frey, G. L.; Tenne, R.; Matthews, M. J.; Dresselhaus, M. S.; Dresselhaus, G. Raman and resonance Raman investigation of MoS2 nanoparticles. Phys. Rev. B 1999, 60, 2883−2892. (25) Singh, A.; Gandra, R. T.; Schneider, E. W.; Biswas, S. K. Lubricant degradation and related wear of a steel pin in lubricated sliding against a steel disc. ACS Appl. Mater. Interfaces 2011, 3 (7), 2512−2521. (26) Jensen, R. K.; Korcek, S.; Mahoney, L. R.; Zinbo, M. Liquidphase autoxidation of organic compounds at elevated temperatures. 2.
bulk water evaporates, but the oil decomposes to release water and form FeO. We argue that it is possible that the FeO anchors MoS2 to allow particles to shear and form a low friction (0.1) tribofilm. Such a film is not formed when the lubricated experiment is done at room temperature; the experiment there records a friction coefficient of 0.17.
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APPENDIX A The temperature rise due to sliding is given by31 ΔT =
μNV 4Ja(K1 + K 2)
where μ is the coefficient of friction and equals 0.12, N is the normal load and equals 1 N, V is the sliding velocity and equals 0.009 m/s, J is the mechanical equivalent of heat and equals 1, K1 and K2 are the thermal conductivities and equal 16 and 0.5 W/(m·K), respectively, a is the contact radius and equals 60 μm, and ΔT is the calculated temperature rise and equals 0.273 K.
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APPENDIX B The possible reaction mechanism leading to oxidation of the base oil hexadecane is shown below.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +918022932589. Fax: +918023600648. E-mail: skbis@ mecheng.iisc.ernet.in. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to the Hindustan Petroleum Corporation Ltd (HPCL) and Defence Research and Development Organisation (DRDO) for providing the financial support for this work. We acknowledge the help given by Dr. Sudhakara Aralihalli, Ms. Arathi Chikorde, and Mr. H. S. Shama Sunder of the IISc, in carrying out this work. We gratefully acknowledge our discussions related to this work with Prof Nicholas. D. Spencer of ETH Zurich.
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REFERENCES
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