| Magnesium and its alloy have many excellent physical and mechanical properties,which make it valuable in a number of applications including aeronautics and astronautics,automobile manufacture, electronic communications, military affairs and nuclear power, etal. However, their corrosion resistance, heat resistance and wear resistance are rather poor,which seriously hinder their further applications. Microarc oxidation (MAO) is a novelsurface treatment technique developed from the common anodic oxidation. By use of thistechnique, a ceramic coating can be in situ formed on Al, Mg, Ti and their alloys. Thus,this method has been generally recognized as the most promising surface treatmenttechnique for magnesium alloy. At present, the researches on MAO for Mg alloy aremainly focused on the improvement of corrosion resistance, and the electrolytes used aremainly silicate, phosphate and aluminate, the coating components are single and thecomprehensive properties of the coating are not good enough. In addition, there are nosystematic and in-depth researches on the fundamental theory of MAO process, theformation mechanism of the coating, as well as the internal relations between the coatingcompositon, structure and its properties. Therefore, the development of new electrolytesystem for improve coating composition and structure, the further study of the MAOtreatment process and the coating formation mechanism, the systematic study of therelations between the coating composition, structure and its integrated performaces, willhave a very important significance for the further development and application of MAOtechnique.Based on the comprehensive analysis of the electrolyte characteristics at home andabroad, two novel electrolyte systems (Si-Zr-Y system and Al-Zr-Y system) have beendeveloped and the ZrO2-Y2O3 composite ceramic coatings have been firstly prepared on AZ91D magnesium alloy in the alkaline electrolyte. For comparison, a silicate-phosphatecomposite electrolyte system (Si-P system) has been developed. This work systematicallystudied the thickness, surface roughness, composition and microstructure of the threedifferent coatings, as well as the relations between them and the corrosion resistacne, heatresistance and wear resistance, et al. A good foundation for the development of novelMAO coatings and further improvement of the integrated performances of AZ91Dmagnesium alloy was established by this research.Effect of (NaPO3)6 concentrations on composition, microstructure and corrosionresistance of MAO coatings formed on AZ91D Mg alloy in silicate electrolyte was studied.The results showed that with the increase of (NaPO3)6 concentration, the contents of MgOand Mg2SiO4 was increased in the concentratin range of 0～3 g·L-1 and 0～7 g·L-1, and thendecreased in the range of 3～10 g·L-1 and 7～10 g·L-1, respectively. But the content ofMgAl2O4 was decreased gradually. After adding (NaPO3)6 to the electrolyte, theelectrochemical corrosion resistance of Si-P films had been significantly improved. Themain cause was that (NaPO3)6 increased oxygen vacancies on the Mg substrate surfaceand the content of PO43-, which speeded up the formation of magnesium compounds at theinterface between metal and coating, increased the thickness and compactness of thecoatings, and then improved their corrosion resistance.The gowth process, microstructure and chemical compositon of Si-Zr-Y film andAl-Zr-Y film have been systematically studied. The results showed that the coatingthickness incresed, but the gowth rate was decreased as the oxidation time increased. Thetwo coatings both had double-layer structures, namely the outer porous layer and the innerdense layer. The Si-Zr-Y film was mainly composed of t-ZrO2, Y2O3, SiO2, MgO, MgF2and Mg2SiO4, the ceramic particales of the coating were relatively large and distributedunevenly. It has a high surface roughness (～3.495μm) and a low porosity (～0.9%).However, the Al-Zr-Y film was mainly composed of c-ZrO2, t-ZrO2, Y2O3, Al2O3, MgO,MgF2 and MgAl2O4, the ceramic particales of the coating were numerous and fine and distributed evenly. It had a low surface roughness (～2.128μm) and a high porosity(～39.4%).The corrosion resistances of three coatings were studied. The results showed that thedropping corrosion resistance of the Si-P film and Si-Zr-Y film were better, but that of theAl-Zr-Y film was poor. However, they all were better than untreated AZ91D magnesiumalloy. The three coatings showed a good corrosion resistance in 3.5 wt. % NaCl solution,the corrosion resistances of which were improved by 3 orders of magnititude comparedwith that of untreated AZ91D alloy. The analysis results showed that the corrosionresistance of coating was mainly determined by thickness and compactness of the innerlayer in the long-time electrochemical reaction process, but in the short-time reactionprocess, the corrosion resistance of the coating was also influnced by coating surfaceporosity and hole size.The results of high temperature oxidation and thermal shock test showed that the threeMAO coatings had better high temperature oxidation resistance than that of untreatedAZ91D magnesium alloy. The thermal shock resistance of Si-Zr-Y film and Al-Zr-Y filmwere better than that of Si-P film, which was mainly attributed that the coatings containedthe high-temperature stable phsases of Y2O3 stablized ZrO2, Al2O3 and so on. During theheating process, the coatings had little change in the phase composition and less sensitvieto temperature change. In addition, the inner layer of the coating was thick and compact,and with a high bonding strength, which made the formation of crackes and abhesion ofthe coating difficult during the thermal shock process.The hardness, wear resistance and tensile properties of three coatings weresysmatically studied. The results showed that the coating hardness was significantlyimproved after MAO treatment. This was mainly attributed that the main phases of thecoating were ceramic phases, the hardness of which were higher than that of Mg, whichmade the hardness of Mg substrate improved. Due to the presence of MAO coating, thetribological mechanism of Mg alloy was changed, the adhesive wear and abrasive wear were reduced, and then the wear resistance was significantly improved. During the tensiletest, a mixed fracture including quasi-cleavage fracture and dimple fracture had takenplace in AZ91D magnesium alloy. But quasi-cleavage fracture was the main fracture form.However, there was no obvious layer seperation occurring in the interface of coating andsubstrate, and there was no large and perfoliate crack in the interface. Therefore, the MAOcoatings have very good anti-tensile properties.
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