Correlative trends between tribological and electronic properties of dry lubricants: Influence of humidity and dopants

Ambient gases and water vapor are known to impact, either positively or negatively, the coefficient of sliding friction (COF) of many solid lubricants. However, the underlying mechanism remains poorly understood. This study reports on the analysis of prior results on the tribological and electronic properties of several classes of dry lubricants. The results show a direct correlation between these two properties of the sliding surfaces, with the ambient humidity strongly influencing both through the surface transfer doping process that occurs in the nanoscale adsorbed water film. The resulting space charge layer (hole accumulation versus electron depletion) strongly influences the COF, with low values achieved when a high density of charge carriers are present on both the sliding surfaces.