Study Of Process, Microstructure And Properties Of Plasma Sprayed Composite And Gradient Coatings On Magnesium Alloy

Magnesium alloy has been widely used due to its excellent properties. How toimprove its corrosion resistance, hardness, wear resistance and thermal shock resistance isvery important. These problems can be solved by plasma sprayed ceramic coating. Theplasma sprayed80wt%Al₂O₃+20wt%Zr₂O（AZ20）,50wt%Al₂O₃+50wt%Zr₂O（AZ50）,20wt%Al₂O₃+80wt%Zr₂O（AZ80） composite coatings and NiCoCrAlY-AZ50gradientcoating were deposited on AZ91D magnesium alloy in this paper. NiCoCrAlY alloy wasused as bond coating. The microstructure and properties of these coatings were investigatedby means of SEM、EDS、XRD and so on. The best process parameters of AZ50compositecoating were optimized by orthogonal experimental design.The results show that the key parameters on bonding strength of AZ50compositecoating are ranked as the sequence: power, powder feeding rate, primary gas flow rate andspray distance. The optimized process parameters of bonding strength were: power40kw,powder feeding rate13g/min, spray distance70mm, primary gas flow rate55psi, thecorresponding bonding strength was as high as32.56MPa.The key parameters ondeposition efficiency of AZ50composite coating are ranked as the sequence: power, spraydistance, powder feeding rate and primary gas flow rate. The optimized process parametersof deposition efficiency were: power40kw, powder feeding rate13g/min, spray distance70mm, primary gas flow rate50psi. The bonding strength of Al₂O₃-Zr₂O（AZ） compositecoatings decreased with the increase of coating thickness and the content of Zr₂O. Thetensile fracture of AZ composite coatings occurred at the interface between ceramic coatingand bond coating.The AZ composite coatings （double layer structure） were characterized by a lamellarstructure with defects of few pores, microcracks and semi-molten particles. The mechanicalcombination was the primary bond form of magnesium alloy and coating. The ceramiccoatings consisted of α-Al₂O₃, γ-Al₂O₃, t-Zr₂O, t’-Zr₂Oand c-Zr₂O.The microcracks, poresand loose tissue of AZ composite coatings increased and the diffraction intensity ofα-Al₂O₃，γ-Al₂O₃became weaker with the increase of Zr₂Ocontent.The microhardness of AZ composite coating （double layer structure） showed a scatterdistribution and significantly conformed to Normal and Weibull distribution. The frictioncoefficient of AZ composite coatings increased with increase of linear speed, while decreased with the increase of load. The wear mechanism of AZ composite coatings wasadhesive wear. The thermal shock failure of AZ composite coatings includes three stages:crack forming, crack extension and coating spallation. The spallation of coating was relatedwith thermal stress and oxidation of bond coating. The bonding strength, hardness, wearresistance and corrosion resistance decreased while the thermal shock resistance increasedwith the increase of Zr₂Ocontent. The plasma sprayed AZ composite coatings couldeffectively improve the hardness, wear resistance and corrosion resistance of AZ91Dmagnesium alloy. The corrosion resistance of coating was better after sealing treatment.Compared to AZ50with double layer structure, the corrosion resistance, bondingstrength and thermal shock resistance of NiCoCrAlY-AZ50gradient coating were better.