Surface Roughness Effects on Metallic Contact and Friction

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Abstract
A study was made of surface roughness effects on metallic contact and friction in the transition zone between hydrodynamic and boundary lubrication. The system used was one of pure sliding and relatively high contact stress, namely a fixed steel ball riding on a rotating steel cylinder. It was found that very smooth and very rough surfaces gave less metallic contact than surfaces of intermediate roughness; very smooth surfaces also gave less friction. Four different types of antiwear/antifriction additives (including tricresyl phosphate) were studied and although they were found to reduce metallic contact and friction, they had little effect in reducing surface roughness. Rather, the additives merely slowed down the wearing-in process of the base oil. Thus, the “chemical polishing” mechanism advanced for the antiwear behavior of tricresyl phosphate appears to be incorrect. With rough surfaces, the improvement in load-carrying capacity with increasing viscosity was less than that shown previously with smooth surfaces. Also, oils with a large pressure-viscosity coefficient did not show the expected beneficial effect with rougher surfaces.

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