Surface Temperature and Its Relation to Periodic Changes in Sliding Conditions between Unlubricated Steel Surfaces

The authors have previously observed periodic changes in the frictional force and wear rate in unlubricated sliding between SAE1113 steel surfaces. Also the steady-state value of the coefficient of friction, f, and the periodic time of surface breakdown, T, were observed to be continuous functions of (normal load, W)½ × (sliding speed, U). Theoretical considerations predict the increase in temperature, θF, at the contacting asperities to vary with f and to be a function of (W½U). Temperatures have been measured using both microhardness and coaxial thermocouple techniques. In the steady-state it has been found that the bulk surface temperatures, θB is a continuous function of (W½U) and hence f and T are expressible as functions of θB. Attempts to relate θB and θF are discussed. It is observed that during surface breakdowns, θB rises rapidly due to a sudden increase in f. It is concluded that this temperature increase promotes the formation of a new oxide surface layer and the return to steady-state conditions.