Influence of Humidity on Friction Forces in Point-Contact under Small Loads

Abstract

The frictional force in point-contact between a Si₃N₄ probe and a polished SUS304 steel surface was investigated by lateral force microscope under different press forces and relative humidity (RH). Variations in the coefficient of friction due to adsorbed water and meniscus necking between the probe and the steel surface were analyzed. At 65% RH, the coefficient of friction between the probe and the hydrophilic surface sharply decreased from 4.0 to 1.0 as vertical nominal load increased from 6 nN to 35 nN, slowly decreasing to 0.7 when the load was increased to 66 nN. At a vertical load of 14 nN, the coefficient of friction for a hydrophilic surface increased with humidity, peaking at 3.5 at an RH of 65%, but decreasing when RH increased to 80%. The change was analyzed to be due to the amount of adhered water on the surface and the formation of meniscus necking. The coefficient of friction on a hydrophobic surface with 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDS) was much lower and independent of humidity, despite the much large adhesion force occurred between probe and PFDS molecular.