On friction-induced temperatures of rubbing metallic pairs with temperature-dependent thermal properties

Abstract

This paper investigates the temperature rises for dry sliding systems when the variation in the thermal conductivity with temperature is taken into account. For the purpose of the analysis, it has been assumed that the thermal conductivity of the rubbing materials vary linearly with temperature. Accordingly, materials are classified into three categories based on that variation: materials for which the conductivity drop with temperature elevation (class a): materials for which the conductivity increases with temperature elevation (class b): and materials for which the conductivity-temperature curve has an inflation point (class c). The variable conductivity temperatures are obtained by applying the so called ‘Kirchoff transformation’ to the fundamental solution of the heat equation. The results indicate that the behavior of the conductivity with temperature is significantly influential to the magnitude of the temperatures reached by the rubbing pair. For a variety of sliding pairs analyzed in this work, significant variation between the constant and the variable conductivity predictions were found. For example, the temperature rise for a mild steel (AISI 1020) rubbing pair, sliding at 6 m/s and 30 N nominal load, predicted by the variable conductivity solution is about 30% higher than that predicted using a constant conductivity solution. It is also shown that the estimates of the heat conducted through the surface may be in error (by about 30–40%) if based on a constant conductivity solution. Such behavior has direct effects on the thickness of the thermally affected subsurface layer (the so-called thermal skin), and the thermal distortion of the contact interface. The error introduced in the estimates of the temperature rises for class c materials is shown to be proportional to the ratio between the inflation to the melting temperatures of the moving solid.