Wear in Al-Si alloys under dry sliding conditions

Abstract Al-Si alloys containing 3–22 wt.% silicon have been slid against a continuously rotating steel disc under dry sliding conditions in the ranges of bearing pressure and sliding speed of 6–195 kPa and 58–580 cm s −1 respectively. The steady state wear rates increase linearly with the bearing pressures in two distinct regions marked by a sharp transition point. This behaviour is indicative of an oxidative mechanism at low bearing pressure and a combined oxidative-metallic wear above the transition point. At all pressures above the transition point gross failure of metal takes place at the interface, preceded by severe plastic deformation. Evidence for the above process has been obtained in terms of debris structure and examination using scanning electron microscopy of the worn surface. The wear behaviour and wear rates have been found to be a function of silicon content and not dependent on the initial structure or the distribution of the silicon phase. This is due to the formation of a subsurface deformation layer in which the silicon phase is finely fragmented into spheroidal particles. The size of the fragments is nearly constant at 3–5 μm and is independent of all the parameters involved in the process and explains the non-dependency of the wear rate on the internal structure. However, the depth to which the silicon phase is fragmented, i.e. the subsurface damage, is critically dependent on all the experimental parameters. The wear rate in Al-Si alloys shows a steady decrease up to the eutectic composition and thereafter it decreases, and is explained on the basis of the structure of the subsurface region which is homogeneous. In high silicon alloys the subsurface region does not show any signs of plastic deformation having occurred in the direction of sliding. The results obtained have been used to propose a mechanism of metal removal during wear in Al-Si alloys.