Friction and Wear Properties of AZ91 Magnesium Alloy and Its Hybrid Composites Reinforced with Alumina Fiber and Graphite Particulates

The AZ91 magnesium-alloy-based composites reinforced with alumina short fiber and with hybrid alumina short fibre-graphite particulates were fabricated by squeeze-casting route.The friction and wear properties of the resulting AZ91 magnesium alloy and its composites sliding against SAE-52100 steel in unlubricated block-on-ring configuration were investigated on an MM-200 friction and wear tester.The morphologies of the original and worn surfaces of the composites were observed on a scanning electron microscope, while the elemental compositions thereon were determined by means of energy dispersive X-ray analysis. It was found that the incorporation of the alumina fiber or of the aluminar fiber-graphite particulates contributed to improve the friction and wear properties of the Mg-based alloy to different extents. The composites reinforced with the alumina short fiber alone showed the best wear resistance, while the inclusion of the graphite particulates together with the alumina fiber led to a decrease in the friction and wear properties of the Mg-based composites, though the composite reinforced with the aluminar fiber-graphite particulates still had better wear resistance than the alloy matrix. This could be because the graphite particulates were unable to form a solid lubricating layer on the worn surface of the soft Mg-based alloy. Moreover, the inclusion of the graphite particulates also caused differences in the wear mechanisms of the Mg-based alloy composites as well. Namely, the Mg-based alloy matrix and the composite reinforced with the alumina fiber alone was dominated by ploughing as they slid against the steel, while the ones reinforced with the alumina fiber-graphite particulates were characterized by less severe ploughing and delamination.