Researchof The Properties Of Composite Coating On TC4 Titanium Alloy Surface

TC4 has been widely used because of their high specific strength and easy machining.But the application of TC4 was limited by poor wear resistance and low operating temperature.Surface treatment can improve the surface properties while maintaining the mechanical properties of the TC4 titanium alloy substrate.The requirement of the performance of material is becoming higher,so a single means of treatment has been unable to meet the need.In this paper,the composite coating was prepared on the surface of TC4 alloy by hot-dipping aluminum and micro-arc oxidation.The properties of the coating were also disscused.TC4 titanium alloy was hot-dipped in aluminum at 750℃for 3 min,and a layer of pure aluminum coating was observed on the surface of titanium alloy.After diffusion annealing at 800℃for 3h,the pure Al layer disappeared and transformed into Ti Al₃.After micro-arc oxidation,an Al₂Ti O₅ ceramic layer was formed on the surface of TAl₃layer.The thickness of the ceramic layer increases with the addition of Na Al O₂,and the surface quality becomes worse when the content exceeds 4g/L.Both the duty cycle and the forward voltage contribute to the growth of the ceramic layer.Both the TC4 hot-dip aluminum treatment and untreated were cyclic oxidized at850℃and 900℃.The result shows that the high-temperature oxidation of TC4 was improved by hot-dip aluminum.After high temperature oxidation at 850℃,a protective layer formed by Al₂O₃ was appeared on the surface of sample.After high temperature oxidation at 900℃,a thicker oxidation layer composited by Al₂O₃ and Ti O₂ appeared on the surface.Both the sample treated in the electrolytic solution 1（Na₂Si O₃ 8 g/L,KOH2 g/L,Na Al O₂ 4 g/L）and 2(Na₂Si O₃ 8 g/L,KOH 2 g/L)were cyclic oxidized at850℃、900℃and 950℃.The experimental results show that the composite treated at850℃high temperature oxidation performance is better than hot-dip aluminum samples.Due to the different thermal expansion coefficient between Al₂Ti O₅ and Ti Al₃,cracks of ceramic layer appearing.The ceramic layer was peeled off after a period of oxidation.The ceramic layer which was prepared in solution 2 released earlier.The samples prepared in the solution 2 were cyclic oxidized at 1000℃.The result show that the Al₂O₃ and Ti O₂ decomposition by Al₂Ti O₅,bridging a part of the crack of ceramic layer,slowing down the loss of the ceramic layer.A new diffusion layer appears between the diffusion layer and the substrate.A part of the Ti Al₃ phase is transformed into Ti Al₂ during the diffusion process.