Observations of frictional behaviour of MoS2 and dependence on surface roughness orientation

This paper outlines some of the results obtained from the study of a solid lubricant added to a grease and its interactions with the topography of metallic contacting surfaces. The application of particular relevance is the lubrication of constant velocity joints (CVJs). Tests were conducted on a unique rig in which an elliptical contact undergoes reciprocating motion under variable loads, temperatures, speeds, slide-roll ratios and using different lubricants. The friction coefficient was recorded real-time and captured using a PC. Using a base oil and a simple grease as suspension media with molybdenum disulphide (MoS2) as the solid lubricant, the behaviour of the solid lubricant was recorded under different contact conditions. The results indicated a very strong dependence on lay orientation. Reductions of up to 40% in friction coefficient have been measured when lay orientation has been changed from "across" to "in-line" (relative to the direction of reciprocation of the contact). There was no obvious trend at 45 degree lay. Results using combinations of particle size, surface finish and physical test conditions suggest tailored surface topography could produce significant reductions in friction and wear, directly beneficial to CVJ operation. Keywords: CVJ, Lay dependence, Grease, MoS2, Boundary lubrication.