Wear mechanism maps for metal matrix composites

Abstract

Empirical wear transition maps have been constructed to delineate the load velocity conditions under which wear transitions occurred in an A356 AI alloy and an A356 Al20%SiC composite. Experiments were performed in dry sliding conditions, using a block-on-ring (SAE 52100) configuration, within a load range of 0.2–400 N and a sliding velocity range of 0.2–5.0 m s −1. Both materials displayed transitions from mild to severe wear at specific load and sliding velocity combinations. The mild wear regime for the composite was expanded to a higher range of sliding speeds and loads by comparison with unreinforced A356 Al. Within the mild regime, two sub-regimes existed for both materials, namely where mixing/oxidation occurred at low sliding speeds to produce a thermally insulating surface layer, and another at higher speeds where this layer was removed. In the composite an additional wear transition, i.e. from mild to ultra-mild regime, occurred at low loads and velocities where the wear rates of the composite were at least two orders of magnitude lower than unreinforced A356 Al, due to the load supporting effect of the particles at the contact surfaces. In both materials, the transition from mild to severe wear occurred at conditions where the surface (bulk) temperature exceeded a critical value of approximately 125 °C in the A356 Al and 338 °C for the composite. The temperature data were summarised in the form of surface temperature maps on log load versus log velocity axes. Wear mechanisms in each regime were determined using scanning electron microscopy, and energy dispersive spectroscopy techniques that were used to analyse morphologies, microstructures and chemical compositions of worm surfaces and wear debris. The dominant mechanisms in each regime were identified and correlated with wear rate and temperature map data to summarise the effects of reinforcement on wear rates and transitions.