| ?-TiAl alloy is a kind of intermetallic compound with low density,high specific strength,excellent high temperature strength and creep resistance,which becomes the emerging material in the fields of aerospace,engine and so on.Howerer,the application of ?-TiAl alloy is restricted severely because of its high-temperature oxidation is so insufficient that it can't meet the use temperature of parts.In order to solve this problem,NiCoCrAlY/ZrO2 composite coating was prepared on the surface of ?-TiAl alloy by Radio Frequency sputtering and reactive sputtering technique in this article,expecting the improvement of high-temperature oxidation resistance.Firstly,NiCoCrAlY coating was prepared on the surface of ?-TiAl alloy by Radio Frequency sputtering technique.Analyzed the effects of radio frequency power,work pressure,pole spacing and holding time by orthogonal method and then got the best process parameters;Secondly,ZrO2 coating was prepared on the surface of NiCoCrAlY coating by reactive sputtering technique;Finally,the composite coating was treated by Vacuum diffusion annealing technique.The surface and cross-section morphology,composition and phase structure of NiCoCrAlY/ZrO2 composite coating were analyzed by SEM,EDS and XRD.The hardness test,adhesion test and Nano-indentation test were also carried out.High-temperature oxidation behaviors of NiCoCrAlY/ZrO2 composite coating were studied at different temperatures and time,then analyzed oxidation mechanism.Thermal-structural finite element analysis of NiCoCrAlY/ZrO2 composite coating was conducted,and obtained the results of temperature field and thermal stress distribution.The results showed that:?1?The optimum process parameters of NiCoCrAlY coating are: frequency power 320 W,work pressure 0.7Pa,pole spacing 20 mm,holding time 4h;the process parameters of ZrO2 coating are: frequency power 230 W,work pressure 0.5Pa,pole spacing 20 mm,holding time 3h,flow ratio of Ar and O2 5:1;the process parameters of Vacuum diffusion annealing are: temperature 1000?,holding time 3h.?2?NiCoCrAlY/ZrO2 composite coating is compact and uniform;the total thickness is nearly 60?m,consisted of a ZrO2 outer coating with 8?m,a NiCoCrAlY deposited coating of 38?m and a NiCoCrAlY diffused coating with 14?m.?3?The surface hardness of NiCoCrAlY/ZrO2 composite coating reaches 1176.7 HV0.1,which is improved by 4 times compared with the substrate;the binding force of the composite coating is 57.5N,and the adhesion strength is between 38.98 MPa and 40.11MPa;the nano-hardness and elastic modulus of the composite coating are 13850 MPa and 247 GPa,which are 1.8 and 1.3 times larger than the substrate respectively.?4?During oxidation test at 750??850? and 950?,NiCoCrAlY/ZrO2 composite coating exhibits excellent high-temperature oxidation resistance without the appearance of cracks and spalling;During oxidation test at 1050?,the composite coating produces cracks with the increase of time,which reduce high-temperature oxidation resistance.?5?NiCoCrAlY/ZrO2 composite coating plays a good thermal insulation effect,because it withstands the maximum temperature and reduce the temperature of the substrate effectively accord to the result of temperature field distribution;The thermal tensile stress of the interface between the composite coating and substrate is increasing with the rise of temperature.The thermal tensile stress of the interface is close to the adhesion strength at 1050?,and the thermal stress leads to the crack of the composite coating easily under high-temperature oxidation,destroying the high-temperature oxidation resistance. |
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