Process Optimization, Microstructure And Property Of Plasma Sprayed NiCoCrAlY/Al₂O₃-TiO₂Ceramic Composite Coatings On Magnesium Alloy

The poor corrosion resistance and wear resistance of magnesium alloys have beengreatly restricting their further applications. Ceramic coating fabricated on the surface ofmagnesium alloys can solve these problems. Owing to the excellent molten effect toceramic materials, plasma spraying technology has been a promising surface treatmentmethod. A lot of research has been done on principle of plasma spraying, microstructrueand performances of the coatings. However, very little research is based on magnesiumalloys. By means of plasma spraying, Al₂O₃-13%TiO₂ceramic coating was fabricated onAZ91D alloy with NiCoCrAlY as bond coating in this study. The plasma spraying processparameters of NiCoCrAlY bond coating and Al₂O₃-13%TiO₂ceramic coating wereoptimized by orthogonal experiment. The microstructure characteristics of the coatingswere investigated using scanning electron microscopy （SEM）, energy dispersivespectroscopy （EDS） and X-ray diffractometry （XRD）. The hardness, bonding strength andthermal shock properties of the coatings were evaluated by microhardness, tensile andthermal shock tests. Dry sliding friction and wear behavior of NiCoCrAlY/Al₂O₃-13%TiO₂composite coatings were investigated under different loads and linear speeds. Corrosionresistance of sealed and unsealed coatings were compared through immersion test andelectrochemical method.The results indicate that NiCoCrAlY/Al₂O₃-13%TiO₂composite coating is optimizedwhen the process parameters of NiCoCrAlY bond coating and Al₂O₃-13%TiO₂ceramiccoating are （24kW,105mm,25g/min,60psi） and （25kW,110mm,65psi,23g/min）,respectively.The optimized composite coating shows that denser interface will form betweenNiCoCrAlY and AZ91D substrate, NiCoCrAlY bond coating can interpenetrate into, crossand gear with Al₂O₃-13%TiO₂ceramic coating to form metallurgical and mechanicalbonding, which is beneficial to improve the bonding strength. NiCoCrAlY coating ismainly composed of Ni3Al and γ-Ni based solid solution while Al₂O₃-13%TiO₂coatingmainly consists of γ-Al₂O₃, α-Al₂O₃and few Al2TiO5.NiCoCrAlY/Al₂O₃-13%TiO₂composite coating reveals good thermal shock resistanceat20℃³50℃, since NiCoCrAlY bond coating has prolonged its service life in thermalshock environment significantly. The essence of the failure of NiCoCrAlY/Al₂O₃-13%TiO₂composite coating is the fatigue under oxidative stress and thermal cyclingstress. After plasma sprayed with NiCoCrAlY/Al₂O₃-13%TiO₂composite coating, thesurface hardness of AZ91D alloy will increase evidently. The average microhardness of thecentral region of Al₂O₃-13%TiO₂ceramic coating reaches975.83HV. Friction coefficient ofthe composite coating increases with the increase of load and linear speed. But frictioncoefficient decreased dramatically when load and linear speed increases to30N and100mm/s, respectively. The predominant wear mechanism of the coating is protuberanceabrasive wear under lower load and linear speed, but it turns to adherence wear with theincrease of load and linear speed.The weight loss of NiCoCrAlY/Al₂O₃-13%TiO₂composite coating sample is less thanthat of AZ91D alloy in3.5%NaCl solution, and it will show a much less weight loss of thesealed coating sample, especially sealed with paraffin. Corrosion rate of paraffin sealedcoating is only0.13g/（m2·h）. Electrochemical experiment indicates that corrosion currentof sealed and unsealed NiCoCrAlY/Al₂O₃-13%TiO₂composite coating dropped no lessthan4magnitudes compared to magnesium alloy substrate.