Research On Surface Coating And Oxidation Resistance Of TiAl Alloy

TiAl alloy has become one of the most competitive materials for heat-resistant parts of high-power diesel engine,owing to its light weight,high specific strength and elastic modulus.However,the insufficient oxidation resistance of TiAl alloy above 800℃has limited its further application.Surface coating technology is one of the main methods to improve the oxidation resistance of TiAl alloy.Based on the above background,a pure Al coating was deposited on the surface of TiAl alloy by arc spraying technology,and Ni/Al composite coating was deposited on the surface of TiAl alloy by plasma spraying and arc spraying technology.The high temperature oxidation resistance and thermal shock resistance of Al coating and Ni/Al coating were systematically investigated by analyzing the microstructure,chemical composition,elemental distribution and phase composition of the coating before and after oxidation.The results are as follow:The results show that:the pure Al coating is dense,uniform,crack-free and spall-free.After oxidation in air at 900℃ for 100h,a dense Al₂O₃ oxide film was formed on the surface of the coating,the sub-surface layer is TiAl₃ phase and the diffusion layer is TiAl₂ phase.After oxidation in air at 950℃for 100h,the surface oxidation layer consists of Al₂O₃ and a little of TiO₂,the sub-surface layer is TiAl₃ phase,and the diffusion layer is TiAl₂ phase.After oxidation in air at 1000℃for 100h,the surface oxidation layer consists of Al₂O₃ and TiO₂,the sub-surface layer is TiAl₃ phase,and the diffusion layer is TiAl₂ phase.With the increase of oxidation temperature,the oxide particles on the surface of the oxide film gradually become longer.Al₂O₃ oxide film can effectively inhibit the invasion of O.TiAl₂ and TiAl₃ as aluminum-rich phases can provide sufficient Al for the surface to form Al₂O₃ oxide film with O in the air.According to the oxidation kinetics curves,the oxidation rate of the Al coating is significantly lower than the TiAl alloy without coating,and the oxidation rate slows down with the extension of time,indicating that the deposition of Al coating can improve the high temperature oxidation resistance of TiAl alloy.The Ni/Al coating is uniform and dense,and the interface between Al layer and Ni layer is clear.After isothermal oxidation tests at 900℃,950℃ and 1000℃,the composition of the oxidized coating from surface to interior is Al₂O₃oxide film,Ni Al intermetallic compound layer,Ni layer,TiAl₃ diffusion layer and Ti₂Al diffusion layer.The Al₂O₃ oxide film effectively inhibits the internal diffusion of O,and the Ni layer inhibits the external diffusion of Ti.The thickness of TiAl₃ and Ti₂Al layers gradually increases with the increase of oxidation temperature.According to the oxidation kinetics curve,the oxidation rate of TiAl alloy decreases with the deposition of Ni/Al coating,and the oxidation rate decreases with the extension of time,indicating that the deposition of Ni/Al coating can improve the high temperature oxidation resistance of TiAl alloy,and the effect is better than that of Al coating.The Al coating specimen appeared significant peeling after 83thermal shock cycles.The Ni/Al coating specimen appeared significant peeling after 104thermal shock cycles.The Ni layer in Ni/Al coating effectively inhibits the external diffusion of Ti,so that the thermal shock resistance of Ni/Al coating is better than that of Al coating.