Novel Mg₂Zr₅O₁₂/Mg₂Zr ₅O₁₂-ZrO₂-MgF₂ gradient layer coating on magnesium formed by microarc oxidation

A novel Mg₂Zr₅O₁₂-based coating on magnesium was formed by microarc oxidation (MAO) in a K₂ZrF ₆-containing electrolyte. The structure of the coating was examined by X-ray diffraction using the grazing angle method, scanning electron microscopy, and transmission electron microscopy. The friction and wear properties of the MAO-coated and -uncoated Mg samples were evaluated in a ball-on-disk testing system. The corrosion resistance of the coating in a 3.5% NaCl solution was investigated by the potentiodynamic polarization test. The coating is relatively dense and composed of a Mg₂Zr₅O ₁₂-ZrO₂-MgF₂ inner layer and a nanocrystalline Mg₂Zr₅O₁₂ outer layer with a maximum hardness of 1240 Hv. The friction coefficient of the coating against Si₃N ₄ is 0.35 under a dry-sliding condition. The corrosion resistance of the magnesium substrate is improved considerably by MAO treatment. The corrosion potential of the Mg₂Zr₅O₁₂-coated sample is -1.43 V with a current density as low as 7.06 × 10^-8 A/cm ². It is expected that the coating can considerably protect magnesium from wear and corrosion.