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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.

Experimental evidence for plowing under dry and lubricated sliding conditions is presented and analytical expressions for the coefficient of friction due to plowing are obtained. The theoretical friction coefficient was found to be a function of the sharpness of the hard asperities, the interfacial “friction” conditions and the shape of the plastic zone. The agreement between theoretical and experimental friction coefficients from lubricated sliding and cutting experiments was remarkably good. The discrepancy between theory and experiment in the case of dry sliding between like metals was shown to be due to plastic deformation of the asperities. Consequently, a different model for plowing was proposed for the case of dry sliding between like metals which produced estimates for the coefficient of friction in fair agreement with the experimental results.

Lubricated and dry experiments on titanium and steel surfaces with and without TiN sputtered coatings of various thicknesses have been conducted. The significance of the layer thickness, interfacial “friction”, magnitudes of normal and tangential surface tractions, and the mechanical properties of the layer and of the substrate (e.g., elastic modulus and hardness) are critically examined. The conditions under which the deformation mode at the solid-solid contacts is elastic or plastic are addressed in light of the experimental evidence and a finite element analysis. It is shown that surfaces with very low friction, especially for unlubricated sliding, and practically zero wear rates can be obtained in both lubricated and dry sliding by coating the surfaces with sufficiently thick TiN layers. Removal of the protective TiN layer resulted in plowing, severe damage, and delamination.

The primary friction mechanism between boundary-lubricated sliding surfaces was investigated. Experiments were performed on well-polished aluminum, copper, and chromium using mineral oil lubricant. It was found that the prevailing boundary lubrication model, which is based on the adhesion between asperities and shearing of the lubricant film, cannot account for the formation of plowing grooves on polished surfaces. Scanning electron micrographs of the worn surfaces and surface profiles have shown that plowing is the dominant mechanism of friction in boundary lubrication. Theoretical analysis has shown that the coefficient of friction depends on the sharpness and the size of the entrapped wear debris or the surface asperities, and the interfacial “frictional” conditions. Reasonable agreement was obtained between theoretical and experimental friction coefficients.