The role of latent water and transfer films on the dwell-time friction of MoS₂

Molybdenum disulfide (MoS₂) solid lubricant films exhibit a tribological behavior known as the “dwell-effect”, defined by an increase in friction coefficient for a sliding contact that had remained dormant for a prolonged time. In this paper, we explored the dwell-time friction of MoS₂ films as a function of film thickness, porosity, and annealing to decouple the contributions of latent and surface water. Additionally, the role of contact area and transfer film formation was investigated by varying the tip radius across two orders of magnitude. In more detail, the multiscale dwell-time tests were conducted on “dense, thin”, “dense, thick”, and “porous, thick” films using sapphire tips with radii R_tip from 50 μm to 1.6 mm to quantify steady-state μ_ss and dwell-time Δμ friction as a function of dwell time t_d. For thinner films with limited latent water, annealing and hydrating had minimal effects on μ_ss and Δμ and dwell-time increases were suppressed, whereas for thicker films with more latent water, annealing (hydrating) decreased (increased) μ_ss and Δμ and suppressed (enhanced) dwell-time effects. For larger R_tip, steady-state friction was constant at μ_ss = 0.08, while for smaller R_tip, increased to μ_ss = 0.12. Complementary electron microscopy measurements of the transfer films and wear tracks showed microstructural changes of MoS₂ crystallites at the sliding interface, with reorientation over nm to 100s of nm and (0002) basal plane alignment. The electron microscopy data also revealed a strong correlation between transfer films and dwell-time effects; as the amount of transfer film decreased, μ_ss increased and Δμ decreased. Altogether, the results confirmed that latent water and transfer films play a significant role in dwell-time friction of MoS₂, which has implications in low-cycle and single actuation mechanisms for space applications.