Paper 20: Stability of Self-Generated Oxide Films on Unlubricated En 1A Steel Surfaces

Abstract

Previous work has shown that within the range 20 < N1/2 U < 600 lbf1,2ft/s friction and wear between unlubricated En 1A steel surfaces is determined primarily by the condition of self-generated oxide films. The stability of these films has been examined using a contact resistance technique and metallurgical examination. At a sliding velocity of 100 ft/s, periodic changes have been observed to occur in the contact resistance between the surfaces. During a cycle the contact resistance increases rapidly at first and then more gradually until a critical value is attained when the film disintegrates. Simultaneous with the surface breakdown there is an increase in the coefficient of friction, pin wear rate, and bulk surface temperature. Conditions are such that rapid re-oxidation occurs, resulting in the formation of a new film A metallurgical examination has shown that even using pin specimens of cold-drawn low carbon steel, subsurface grain re-orientation can be responsible for pearlite-rich layers in the stratified structure produced; hence the hardenability of the substrate, and therefore the support provided for the oxide film, can undergo significant changes as wear proceeds. It is concluded that the unstable nature of self-generated oxide films on steel surfaces is responsible for the periodic increases in friction and wear which have been observed.