Friction between a Steel Ball and a Steel Flat Lubricated by MoS 2 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 (MoS 2 ) 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 MoS 2 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 MoS 2 /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-MoS 2 particles have edge sites and dangling bonds, they oxidize to MoO 3 at temperatures above 350 °C. The high modulus (240 GPa) and low density (4.8 g/cm 3 ) of molybdenum disulfide (MoS 2 ) nanoparticles make it a candidate for use in polymers as a filler, 9 to enable support when applied stresses are high. MoS 2 is also used widely as a lattice-layered solid lubricant. 1 The material has a hexagonal layered structure. When deformed, the MoS 2 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 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 MoS 2 particles are dispersed in oil. The mechanism of MoS 2 transfer film formation has been studied extensively. 3,11,12 Nanoparticles of MoS 2 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, nano- particles 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 MoS 2 at temperatures in the range 25-200 °C are few and far between. 17 In dry lubrication of substrates coated densely with MoS 2 , it has been observed that the friction decreases with reduced environmental contami- nation. 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 MoS 2 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 MoS 2 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 works 18-20 on the role of environmental moisture on the friction of MoS 2 , the present study explores the Received: May 1, 2012 Revised: September 1, 2012 Accepted: September 5, 2012 Published: September 5, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 12321 dx.doi.org/10.1021/ie3011337 | Ind. Eng. Chem. Res. 2012, 51, 12321-12328