The Role of Water in Modifying Friction within MoS2 Sliding Interfaces

To explore the environmental dependence of friction for solid lubricants containing molybdenum disulfide (MoS2), we have investigated friction on the basal plane of single-crystal MoS2 with atomic force microscopy (AFM) as a function of relative humidity (RH) and tip composition. For both a bare Si3N4 tip and a MoS2-coated tip, changes in interfacial friction are observed with increasing relative humidity, however, with markedly different behaviors. For sliding contacts involving bare Si3N4 tips, the friction coefficient is observed to increase with increasing RH, from 0% to the point of water saturation. For Si3N4 tips precoated with MoS2 particles, friction appears to be relatively insensitive to increasing RH in the range of 0-40%. However, above 40% RH, a drastic increase in friction is observed and is accompanied by evidence for interfacial wear provided in images of the basal plane following the friction measurements. A comparison to the tribological properties of the basal plane of highly oriented pyrolytic graphite (HOPG) using identical probe tips highlights the unique character of self-mated MoS2 interfaces.