The Significance of Oxide Layers in Boundary Lubrication

Abstract

Analytical and experimental studies were conducted on oxidized and nonoxidized pure aluminum, OFHC copper, and electroplated chromium to investigate the role of surface oxide layers in boundary lubrication. The effects of the thickness of the oxide layers, the elastic moduli of the oxide and the metal, and the normal surface traction have been addressed. In addition, several possible failure mechanisms of both thin and thick oxide films have been proposed. The experimental results have shown that low coefficients of friction, about 0.1 or less, and especially low wear can be obtained in boundary-lubricated sliding if the metal surfaces are protected from plastic deformation by sufficiently thick oxide layers. Scanning electron microscopy has shown that when the oxide layers are not ruptured, the wear of the surfaces is negligibly small. In this case, the oxide-oxide contacts deform primarily elastically and the predominant friction mechanism is the shear of the lubricant film. Based on this evidence, a theoretical model for friction was proposed and the agreement between theoretical and experimental coefficients of friction was reasonably good. Disruption of the oxide layers during sliding, however, was found to result in plastic deformation and plowing of the surfaces.