Sliding friction of copper

Abstract

The sliding friction between copper specimens was measured under atmospheric conditions and loads of 0.1–20 g. With very clean surfaces, the coefficient of friction was 1.0–1.1 for the entire load range. With less clean surfaces, the coefficient of friction obtained was about 0.4. Since the degree of cleanliness cannot be controlled quantitatively, the friction-load curve of sliding copper pairs in air exhibits a bifurcation characteristic. The higher friction value may be satisfactorily explained by adhesion theory. No sign of adhesion, however, was detectable when the friction coefficient was 0.4. All observations to date indicate that plastic deformation exists during the sliding process. Using published data on the total expended work in plastic deformation, the coefficient of friction between copper pairs was estimated to be about 0.2. When adhesion is negligible, based on the plastic deformation mechanism, one may deduce that (1) the friction coefficient depends largely on the properties of the softer material of the two specimens, and that (2) upon repeated sliding, a slight drop in friction may occur. Both of these phenomena have been observed experimentally.