A triboscopic investigation of the wear and friction of MoS2 in a reciprocating sliding contact

Reciprocating sliding tests with ball-on-flat geometry were performed on a duplex coating at low speeds in moist air (RH = 60%). The coating, 55 nm MoS2 on 35 nm of TiN, was deposited by ion-beam assisted deposition onto a steel substrate. Friction coefficient (mu) and electrical contact resistance (R-c) measurements were recorded at similar to 2 mu m intervals along the track; these spatially resolved measurements were compared to the more commonly presented cycle-averaged values. The last-cycle tracks of several runs were also analyzed by a variety of microscopies and spectroscopies to identify compositions and determine thicknesses of films on the tracks and balls. R-c measurements, both averaged and spatially resolved, were more sensitive to coating damage and loss than mu measurements. In the averaged data, fluctuations in R-c were observed before fluctuations in mu. Spatially resolved data showed that local drops in R-c could be detected as early as 20% of life. Additionally, recovery of both high mu and low R-c regions, interpreted as healing of damage in the contact, occured. Friction coefficient data were insensitive to changes in MoS2 coating thickness; conversely, R-c followed wear track thickness and consequently may provide an in situ method of monitoring coating wear. (C) 1998 Published by Elsevier Science S.A.