Preparation And Performances Of Y₂O₃-ZrO₂-MgO Composite Ceramic Coatings On AZ91D Magnesium Alloy By Micro-arc Oxidation

Micro-arc oxidation （MAO） has been generally recognized as the most promisingsurface treatment technique for magnesium alloy. A ceramic coating can be in situ formedon magnesium alloys, the corrosion resistance, wear resistance and heat resistance ofsubstrate are improved obviously. At present, the researches on MAO for Mg alloy aremainly focused on silicate, and the coating is mainly composed of MgO, Mg2SiO4andMgAl2O4. However, the oxide ceramic is brittle and its linear expansion coefficient isgreatly inconsistent with metal substrate, leading to cracks and sheds. As a result, theprotection for the substrate loses. The ceramic coating formed by MAO presents excellentflexibility and linear expansion coefficient being consistent with substrate. Therefore,research in this area is significant for the MAO process of magnesium alloys.In this paper, the oxide ceramic was prepared by two different ways: adding theY（NO3）3in the electrolyte and pretreatment of Y（NO3）3immersing before MAO.Y₂O₃-ZrO₂-MgO coating and YSZ-MgO coating were successfully prepared on thesubstrate respectively. The microstructure, components and phases composition of thecoatings were analyzed by scanning electron microscopy （SEM）, energy dispersive X-rayspectrometer（EDX） and X-ray diffractometer（XRD）. The errosion resistance and thermalstability of Y₂O₃-ZrO₂-MgO, YSZ-MgO coating, common MAO coating and MAOcoating with ZrO₂were investigated by potentiodynamic polarization curves,potentiodynamic polarization tests （3.5wt%NaCl）, high temperature oxidation test andthermal shock test.The Y₂O₃-ZrO₂-MgO coating was prepared in Y（NO3）3added NaAlO₂-K2ZrF6-KOHelectrolyte systems by MAO process. Compared with the general MAO coating preparedin silicate, this kind of coating presents regularly dispersing holes with small dimensionand excellent densification. The coating was mainly composed of MgO, MgAl2O4, ZrO₂, Y2O3and the mass percent of Y2O3in Y2O3+ZrO2was7.13%. According to the result of potentiodynamic polarization curves, the Y2O3-ZrO2-MgO coating showed smaller errosion current density than the general MAO coating and polarization resistance was one time higher. The errosion rate of Y203-ZrO2-MgO coating in5wt%NaCl was10%lower than that of the general MAO coating and it was only1%of the errosion rate of magnesium alloys. In the high temperature oxidation test, the mass increment of Y203-ZrO2-MgO coating was obviously lower than the magnesium alloys without MAO process and the general MAO coating. In the thermal shock test, Y203-Zr02-MgO coating showed better thermal stability than the general MAO coating.Y2O3crystal and Y（OH）3amorphous had be formed on the surface of magnesium alloys after the pretreatment of Y（NO3）3immersed. The Y203-ZrO2-MgO coating with26.78%Y2O3in （Y2O3+ZrO2）, namely, the YSZ-MgO coating, was prepared on the surface of magnesium alloys after pretreatment by MAO process in Na2SiO3-K2ZrF6-KOH electrolyte systems. Compared with the ZrO2-MgO coating without pretreatment, the YSZ-MgO coating presented better densification. Also, the holes on the surface of the YSZ-MgO coating were smaller and more regularly dispersed.The result of potentiodynamic polarization curves test showed that the YSZ-MgO presented smaller errosion current density, nobler open-circuit-potential and higher polarization resistance, compared with the ZrO2-MgO coating. After being immersed in the5%NaCl solution, the errosion rate of the YSZ-MgO coating was66%lower than that of ZrO2-MgO coating. Moreover, it was much lower than that of magnesium alloys without MAO process. The high temperature oxidation test in41℃showed that the YSZ-MgO coating holded better high temperature resistance than the ZrO2-MgO coating. Thermal shock test in500℃showed that the thermal shock resistance and spall resistance of the YSZ-MgO coating was superior to the ZrO2-MgO coating,. The DTA/TG analysis datas showed that, the change of mass and enthalpy of the YSZ-MgO coating little in1170℃. It demonstrated that ZrO2was stabilized by Y2O3in the coating, leading to improvement of oxidation resistance and thermal shock resistance of the coating.