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

• Published in: Frontiers in chemistry Vol. 9; p. 684441
• Main Authors: Claerbout, Victor E. P., Nicolini, Paolo, Polcar, Tomas
• Format: Journal Article
• Language: English
• Published: Frontiers Media S.A 24-06-2021
• Subjects: Chemistry; friction; hydrogen bond network; molecular dynamics simulations; molybdenum disulfide; tribology; water
• ISSN: 2296-2646; 2296-2646
• DOI: 10.3389/fchem.2021.684441

Solid lubricants have received substantial attention due to their excellent frictional properties. Among others, molybdenum disulfide (MoS
2
) is one of the most studied lubricants. Humidity results in a deterioration of the frictional properties of MoS
2
. 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 MoS
2
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 MoS
2
. Moreover, we found that there exists an energy-driven, rotational dependence of the water network atop/beneath MoS
2
. This orientational anisotropy is directly related to the dissipative character of MoS
2
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.