Abstract:A composite ceramic coating containing Y₂O₃-ZrO₂-MgO (YSZ-MgO) was prepared on AZ91D magnesium alloy, which was immersed in Y(NO₃)₃ aqueous solution as pretreatment, by micro-arc oxidation (MAO) process. The morphology, elemental and phase compositions, corrosion behavior and thermal stability of the coatings were studied by SEM, EDX, XRD, electrochemical corrosion test, high temperature oxidation and thermal shock test. The results show that the coating mainly consists of ZrO₂, Y₂O₃, MgO, Mg₂SiO₄, and MgF₂. Among these compounds, Y₂O₃ accounts for 26.7% of (Y₂O₃ + ZrO₂). The thickness of YSZ-MgO coating is smaller than that of ZrO₂-MgO coating, but its compactness and surface roughness are better than those of ZrO₂-MgO coating. YSZ-MgO coating has a good corrosion resistance, and its corrosion rate in 5% NaCl aqueous solution is lower than that of ZrO₂-MgO and only about 8.5% of that of AZ91D magnesium alloy. After oxidation at 410 °C, the mass gain of AZ91D magnesium alloy presents a linear increase with the oxidation time. The YSZ-MgO coating and ZrO₂-MgO coating can remarkably decrease the oxidation mass gain. The oxidation mass gain of YSZ-MgO coating is lower than that of ZrO₂-MgO coating, especially during a long oxidation period. The thermal shock resistance of YSZ-MgO coating is superior to ZrO₂-MgO coating.

Key words: AZ91D magnesium alloy; micro-arc oxidation (MAO); Y₂O₃-ZrO₂-MgO composite coating; corrosion behavior; thermal stability