Revisiting the dwell effect on friction behavior of molybdenum disulfide

Solid lubricants, like molybdenum disulfide (MoS2) are often used in space applications, and subject to prolonged periods where components sit stationary before use (i.e., during storage, transport or in between duty cycles). When sliding is resumed after dormancy, the friction behavior of MoS2 can vary due to a variety of factors such as accrued adsorbates and oxidation. This phenomenon, referred to as the dwell or stop-time effect, was first investigated over 50 years ago, and is characterized by an increase in the coefficient of friction and a prolonged run-in back to steady-state friction. After nearly five decades, the fundamental driving mechanism for the dwell effect is still not well understood. In this work, the dwell-effect for MoS2 coatings is studied through intermittent sliding experiments with dwell times ranging from 30 s to 48 h (172,800 s) at pressures from 7 x 10-9 to 2 x 10-1 torr. Vacuum pressure was varied to investigate the role of surface contaminants (i.e., water). Results suggest that the change in the coefficient of friction is driven by interactions of water with the sliding interface. The role of microstructure on the dwell effect was investigated using nanocrystalline sputter-deposited and highly oriented spray-deposited MoS2 coatings. Results show that the shear-modified surfaces of sputter- deposited coatings have a-2 x smaller dwell effect than spray-deposited surfaces due to the reduction of po- tential edge-sites that can interact with contaminants. Additionally, intermittent sliding experiments after vac- uum annealing show that the contribution of latent water can be minimized by driving intercalated water from the coating.