TEM analysis and tribological properties of Plasma Electrolytic Oxidation (PEO) coatings on a magnesium engine AJ62 alloy

In order to reduce the fuel consumption and pollution, automotive companies are developing magnesium-intensive components. However, due to the low wear resistance of the magnesium (Mg) alloys, Mg cylinder bores are vulnerable to the sliding wear attack. In this paper, a Plasma Electrolytic Oxidation (PEO) process was used to produce oxide coatings on a Mg alloy AJ62 (MgAl6Mn0.34Sr2), developed for Mg engine block, to battle against the wear attack. The surface morphology, coating thickness and tribological properties were tailored by adjusting the PEO process parameters. TEM analysis demonstrated that the PEO coatings had a nanocrystalline structure in the inner dense layer next to the substrate. The PEO coatings exhibited a much better wear resistance and a smaller friction coefficient than the uncoated AJ62 substrate. The tribological performance of the PEO-coated Mg alloy was even better than that of a hypereutectic Al-Si alloy currently used for engine applications under a high contact load.