Characterization and properties of the MgF2/ZrO2 composite coatings on magnesium prepared by micro-arc oxidation

Through micro-arc oxidation, the MgF2/ZrO2 composite coatings were prepared on magnesium at the different applied voltages (in the range of 400–550 V) in a zirconate electrolytic solution. The morphologies, phase components, microhardness, bond strengths, and corrosion resistances of the composite coatings were investigated. The effect of the applied voltages on the characteristics and properties of the composite coatings and the basic formation mechanism of the coatings were also discussed. The results indicate that the composite coatings are relatively dense and uniform in thickness, and predominantly composed of MgF2, tetragonal ZrO2 (t-ZrO2) and monoclinic ZrO2 (m-ZrO2). The composite coatings exhibit a gradient distribution in phase component from the surface to the inner part. It is found that the applied voltage plays an important role in the characteristics and properties of the composite coatings. With the increase of the applied voltage, the thickness and the t-ZrO2 content of the composite coatings increase, while the m-ZrO2 content decreases and no significant variation is observed in the MgF2 content. Moreover, the surface microhardness and bond strength of the coatings increases with the applied voltage increasing. The microhardness values display a gradient distribution in the cross sections of the coatings, and the maximum microhardness value and its corresponding position in the cross sections are related to the applied voltage. In addition, the corrosion resistances of the composite coatings on magnesium surface are obviously superior to the magnesium substrate in the NaCl solutions, and the effect is more remarkable at higher voltage.