Study On Process And High Temperature Corrosion Behavior Of NiCr/YSZ Coating On ?-TiAl By Multi-Arc Ion Plating

The intermetallics of gamma-TiAl have the characteristics of low density,high specific strength and modulus.Their excellent mechanical properties make them ideal lightweight and high temperature structural materials and have wide application prospects.However,the deficiencies of high temperature oxidation resistance and thermal corrosion resistance of gamma-TiAl limit its application.Based on the above problems,NiCr/YSZ coating was prepared on the surface of gamma-TiAl by multi-arc ion plating technology in order to improve its high-temperature performance and explore its high-temperature oxidation behavior and hot corrosion mechanism.The structure of NiCrYSZ coating was optimized by finite element method.The optimum combination of coating thickness was found to be 5 micron for NiCr layer and 10 micron for YSZ layer.The optimum parameters of multi-arc ion plating process are as follows:bias voltage-200V,arc current 80A,working pressure 0.7Pa,chamber temperature 400? and plating time 90 min.The surface of NiCr/YSZ coating prepared under the optimum technological parameters is flat and compact,with a small amount of large droplets of metal and a thickness of about 15 micron,which binds well with gamma-TiAl.The average microhardness of NiCr/YSZ coating is 853.4 HV_0.1,and that of NiCr/YSZ coating is about 2.77 times of that of gamma-TiAl.The indentation depth of NiCr/YSZ coating is 0.76um,the nano-hardness is 9.826GPa and the elastic modulus is 244GPa.Thermal shock test shows that the crack propagation is very fast and the edge of YSZ coating begins to fall off when thermal shock occurs 60 times.The thermal shock resistance of the coating is better and the bonding strength of the coating is higher.The high temperature oxidation resistance of gamma-TiAl matrix and NiCr/YSZ coating at 750?,850?and 950?was investigated by comparison of isothermal oxidation tests.The oxidation weight gain of NiCr/YSZ coated specimens at 750?,850? and 950? for 100 h was 7.187 mg/cm²,10.572mg/cm² and 17.371 mg/cm²,respectively.Compared with the matrix of gamma-TiAl,the oxidation rate of NiCr/YSZ coated specimens at high temperature was reduced by about 40%.The surface oxides of NiCr/YSZ coated specimens are mainly ZrO₂ with t phase after 100 hours at 750?,850? and 950?.Under the stability of YO₃,there is no t?m phase transformation in ZrO₂.ZrO₂ stabilized by Y₂O₃ on the surface effectively prevents the inward diffusion of oxygen,and the thermal growth Cr₂O₃ formed by the bonding layer is continuous and dense,which delays the diffusion rate of oxygen.The high temperature oxidation resistance of the matrix is significantly improved by the whole composite coating.The hot corrosion properties of gamma-TiAl matrix and NiCr/YSZ coating at 850?and 950?were investigated by comparison of hot corrosion tests at constant temperature.The results show that the weight gain of hot corrosion of NiCr/YSZ coated specimens at 100 h at two temperatures is 70.1mg/cm² and 118.2 mg/cm²,respectively.The surface sulphide increases and the porous surface is porous.t-ZrO₂ is the main oxidation product after 100 hours of high temperature corrosion at 850?.The reaction of Y₂O₃ with Na₂SO₄,which is the main stabilizer in YSZ at 950?,causes Y₂O₃ to be consumed continuously.A great deal of t-ZrO₂ is transformed into m-ZrO₂.The change of volume of ceramic layer results in internal stress,crack propagation and a little spalling.The continuous and dense internal TGO effectively prevents the diffusion of molten salts and oxygen,and greatly improves the thermal corrosion resistance of thermal barrier coatings.