Sliding wear map for the magnesium alloy Mg-9Al-0.9 Zn (AZ91)

Abstract

Dry sliding wear tests were performed on a Mg-9.0%, Al-0.9%, Zn (AZ91) alloy using a block on-ring (AISI 52100 steel) configuration. Wear rates were measured within a load range of 1–350 N and a sliding velocity range of 0.1–2.0 m/s Compositions, morphologies and microstructures of worn surfaces and wear debris were characterised by scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS) and X-ray diffractometer (XRD). A wear mapping approach, consisting of identification of micromechanisms of wear leading to the generation of loose debris and surface damage and determination of the range of applicability of each mechanism as a function of load and speed, has been adopted. Two main wear regimes, namely a mild wear regime and a severe wear regime have been observed. The role of contact surface temperature on the mild to severe wear transition was investigated in detail. It was shown that the transition from mild to severe wear was controlled by the contact surface temperature of the alloy and that the onset of the severe wear coincided with a surface temperature of 347 K. In the mild wear regime two different micromechanisms operated, i.e. two sub-wear regimes, namely an oxidational wear regime and a delamination wear regime, were identified. Similarly, in the severe wear regime two different wear mechanisms were responsible for surface damage and debris formation. These were classified as the severe plastic deformation induced wear and the melt wear regimes. The results of wear tests and metallographic investigations on worn surfaces have been summarised in a wear mechanism map. An empirical contact surface temperature model, which serves to predict the critical surface temperature at the onset of severe wear, has been developed.