| TiAl-based alloys are new type high temperature structural materials applied for aerospace and aircraft industry because of their low density,high strength and good creep resistance.The problem of poor ductility of these materials at room temperature has been mostly solved by alloying and microstructure controlling.Thus the problem of further improving high temperature oxidation resistance has become one of the most important research subjects.A pack silinconizing method and a Sol-Gel spin coating method were adopted separately in order to obtain the two different protective composite coatings on TiAl-based alloy.The cyclic oxidation resistance was tested at 900℃in static air, and that of the composite silinconized coating was tested at 900℃in strong flowing air.XRD and SEM/EDS were used to analyze the two different protective coatings before and after oxidation.The formation mechanism,structure and oxidation resistance of the two coatings were investigated.It was found that Ti5Si3+Al2O3 composite coatings were formed on the surface of the specimens after silinconization embedding in the powder of 15％Si+85％Al2O3 in a vacuum furnace at 1250℃for 2h.The Ti5Si3-based layer is about 7～8μm thick, lying in the inner. And the Al2O3 layer is about 1μm, lying in the outer. Ti5Si3+Al2O3 composite coating on TiAl alloy shows excellent resistance of oxidation at 900℃. The weight gain of Ti5Si3+Al2O3 composite coating at 900℃in static air for 240h is 0.44mg/cm2 and 0.55mg/cm2 in flowing air. After long-term oxidation in flowing air, an oxide layer mainly composed of SiO2 and TiO2 is formed on the surface owing to a little decompose of Ti5Si3. This layer has good adhesion to Al2O3 layer. After coated by spin coating for two times and subsequently heated at 1000℃for 6h, it formed SiO2+Al2O3+TixSiy composite coating on the surface of the specimens. From outer to inner were SiO2,Al2O3,TixSiy.The SiO2 layer is thin,while Al2O3 layer is about 1～2μm thick.The composite coating of SiO2+Al2O3+TixSiy on the surface of TiAl alloy shows excellent oxidation resistance at 900℃.The weight gain of SiO2+Al2O3+TixSiy composite coating is 0.54mg/cm2 at 900℃in static air for 300h.After long-term oxidation in static air, TiO2 formed on local area on the surface of TiAl alloy.The reasons for the two coatings' excellent resistance at high temperature as followings. First, each layer which composes of multi-layer is effective in protecting oxidation. Second, the two composite coatings are both gradient with suitable framework. From outer toward inner, the siliconized coatings are Al2O3→Ti5Si3 and the Sol-Gel spin coatings are SiO2→Al2O3→TixSiy. With grade thermal expansion coefficient, the composite gradient coating is hard to spall from TiAI alloy in the impact of stress.Third,the combination between the composite coating and the substrate is firm.The transitional layer was formed by diffusion through heat treatment.The combination between layer and layer or layer and substrate is so firm that the layer is hard to spall from the substrate in the impact of stress which is generated during oxidation.
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