Exploring Nanoscale Lubrication Mechanisms of Multilayer MoS₂ During Sliding: The Effect of Humidity

Solid lubricants have received substantial attention due to their excellent frictional properties. Among others, molybdenum disulfide (MoS₂) is one of the most studied lubricants. Humidity results in a deterioration of the frictional properties of MoS₂. The actual mechanism at the nanoscale is still under debate, although there are indications that chemical reactions are not likely to occur in defect-free structures. In this study, we performed nonequilibrium molecular dynamics simulations to study the frictional properties of multilayer MoS₂ during sliding in the presence of water. Moreover, we also investigated the effect of sliding speed and normal load. We confirmed earlier results that a thin layer of water organizes as a solidified, ice-like network of hydrogen bonds as a result of being confined in a two-dimensional fashion between MoS₂. Moreover, we found that there exists an energy-driven, rotational dependence of the water network atop/beneath MoS₂. This orientational anisotropy is directly related to the dissipative character of MoS₂ during sliding. Finally, three distinct frictional regimes were identified, two for a thin layer of water and one for bulk water. In the case of a thin layer and low coverage, water represents a solid-like contaminant, causing high energy dissipation. For a thin layer and high coverage, water starts to act as a solid-like lubricant, reducing dissipation during sliding. Finally, a regime where water acts as a liquid lubricant, characterized by a clear velocity dependence was found.