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 (m) and electrical contact resistance (Rc) measurements were recorded at ~2mm 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. Rc measurements, both averaged and spatially resolved, were more sensitive to coating damage and loss than m measurements. In the averaged data, fluctuations in Rc were observed before fluctuations in µ. Spatially resolved data showed that local drops in Rc could be detected as early as 20 % of life. Additionally, recovery of both high µ and low Rc regions, interpreted as healing of damage in the contact, occurs. Friction coefficient data were insensitive to changes in MoS2 coating thickness; conversely, Rc followed wear track thickness and consequently may provide an in situ method of monitoring coating wear