Surface changes due to sliding

Abstract

Metallic bearing surfaces on sliding become coated with an amorphous Beilby layer formed by the smearing out of molten projections. The heating has been held to be due to the overcoming of friction between the metals themselves. However, there are many examples of the formation of Beilby layers where there is no evidence of the occurrence of direct intermetallic contact and where the frictional forces must have been other than between metals. In practice a metallic bearing surface is invariably covered with an amorphous oxide layer. The load-carrying capacity of this layer depends largely on the rigidity of the underlying metal. It is, therefore, a particular virtue of the Beilby layer that in addition to its smoothness it is, in general, much harder than the underlying metal. The bearing metals in general use are more fusible than their oxides. The heat generated in the sliding of the oxide-covered metal pair leads first to the softening of the more fusible metal and the consequent rupturing of its oxide film. Intermetallic contact is, however, still prevented by the oxide film which remains intact on the more refractory, and consequently still rigid surface. It is well known that Beilby layers are formed on both surfaces of a sliding element, even when the metals have widely different melting-points. This is due to the high temperature developed at the asperities of the rougher surface as these slide in long tracks over the smoother surface.