The influence of oxide and lubricant films on the friction and surface damage of metals

Abstract

It is shown that the protection against surface damage afforded by an oxide film during the sliding of two metal surfaces depends on the geometrical sliding arrangement used. In particular, the damage when sliding a hemisphere on a flat plate is greater than when sliding two crossed cylinders inclined at a small angle to each other. These differences in protection are attributed to the way in which the geometrical arrangement impedes or facilitates the growth of the initially small intermetallic junctions which form during sliding. The variation of the coefficient of friction with load has been studied, and for some metals deviations from Amontons’s first law have been found. In such cases there exists a breakdown load above which both friction and surface damage markedly increase, and, for a given metal, the magnitude of this load depends on the method of formation of the surface oxide film. Film thickness is of relatively minor importance; the rate of formation of the oxide is of more significance, the greatest protection being found with films formed slowly at room temperature. Similar deviations from Amontons’s law were also found under conditions of boundary lubrication, and the magnitude of the breakdown load is shown to depend on the combined properties of the boundary lubricant film and of the underlying oxide. The differences in protection given by differing oxide films on the same metal were often more noticeable in the presence of a lubricant, and an oxide film with poor protective properties can completely inhibit the potential protective action of the lubricant. Finally, it is shown that under lubricated conditions the work-hardened layer on an abraded metal may also act in conjunction with the oxide film and the lubricant in reducing the surface damage and friction during sliding.