Humidity Dependence of Interfacial Friction between Mica Surfaces

Abstract

The dependence of frictional forces between mica surfaces on humidity, particularly in the low relative humidity range, has been studied with the surface force apparatus. The dynamic shear stress decreased with increasing humidity. When the humidity was lower than 40%, a large static frictional peak was observed before initial sliding. This peak did not reappear after stopping temporarily and resuming the motion but appeared again after the surfaces had been separated, if only for a few seconds, and brought into contact once more before sliding. To clarify the nature of this static frictional peak, frictional forces and adhesion during, before, and after sliding the mica surfaces were studied in detail. The dependence on humidity of shear stress and pull-off force suggests that layering of adsorbed water on and between the mica surfaces is important. The results of the measurements of kinetic shear stress as a function of sliding velocity at different humidities revealed the role of water condensed around the contact zone. A mechanism based on capillary condensation is proposed to explain the role of humidity on friction by adsorption of water layers on the free mica surfaces and at the contact.