Effect of Oxide Film on the Friction of 52100 Steel in Vacuum

Abstract

To solve the anomalous behaviors of friction coefficient in reciprocated slidings of 52100 steel in ultrahigh vacuum, studies were made if the wear track, friction level, and number of traverses for a 52100 steel test ball sliding on a 52100 steel flat over a pressure range of 760 to 3 × 10⁻⁹ torr. Plots of the friction coefficient versus number of traverses show a maximum (f = 1.0–1.2) followed by a drop (f = 0.5–0.6). Both the friction force-time curves and microscopic friction area change from smooth to rough at the point of the drop in friction coefficient. The shape of the friction plot is ascribed to a build-up and rupture of the oxide film that forms on the surface of the 52100 steel specimen in atmosphere and is affected by applied load, direction of finishing marks and pressure level of the environment. Further, above a critical pressure of 10⁻⁶ torr the drop was not experienced. It was also found that higher friction does not always mean that the sliding surfaces are clear or more oxide-free.