Synthesis of zirconia/magnesia composite coating on AZ91 magnesium alloy

Magnesium is the lightest structural metal commonly used in automotive, aerospace and communication fields due to its good properties such as low density, high strength to weight ratio, excellent dimensional stability, good electromagnetic shielding, superior damping capacity, good machinability and recycling ability. However, the poor surface properties such as corrosion and wear resistances and the very low hardness of magnesium and its alloys restrict their widespread use in industry. Micro arc oxidation is an attractive and effective technique for surface protection of magnesium alloys through formation of an oxide coating on their surface. The present study is focused on the incorporation of zirconia (ZrO₂) in magnesia-based oxide coatings on AZ91 magnesium alloy by micro arc oxidation under DC conditions in an alkaline electrolyte. The thickness, the phase composition and the surface/cross-sectional morphologies of the synthesized coatings are analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The dominant phases formed on AZ91 magnesium samples are found to be a mixture of Mg₂SiO₄ (forsterite), MgO (periclase), monoclinic and tetragonal ZrO₂. The coating characteristics are found to be mainly altered with varying the frequency; hence, the results are comparatively evaluated on the basis of pulse frequency during micro arc oxidation.Keywords: micro arc oxidation, zirconia, magnesia, oxide coating, microstructure.