An in situ mechanism for self-replenishing powder transfer films: Experiments and modeling

Pellets were formed by compacting MoS2 powder. A series of tests were conducted on a tribometer that consisted of simultaneous pellet-on disk and pad-on disk sliding contacts. The purpose of the tests was to intentionally transfer MoS2 third-body particles to a disk where its lubrication characteristics could be studied. This work also showed that the MoS2 pellet actually acted as a self-repairing, self-replenishing, oil-free lubrication mechanism. In the experiment, a pellet is sheared against the disk surface while the loaded slider rides on the lubricated surface and depletes the deposited powder film. A control-volume fractional coverage modeling approach was employed to predict both (1) the friction coefficient at the pad/disk interface and (2) the wear factor for the lubricated pellet/disk sliding contact. The fractional coverage varies with time and is a useful modeling parameter for quantifying the amount of third body film covering the disk asperities. In the model, the wear rate of a pellet and pad friction coefficient can be determined as a function of the pellet load, slider pad load, disk speed, and material properties. Results from the model qualitatively and quantitatively predict the tribological behavior of the experimental sliding contacts reasonably well.