Al-Si and Al-Si-Y coatings deposited by HS-PVD for the oxidation protection of γ-TiAl

Abstract

Many attempts have been made to extend the service temperatures of lightweight materials based on γ-TiAl alloys due to their insufficient oxidation resistance at T>850°C for turbine applications. Among them, deposition of high-temperature oxidation protective coatings has been proved to be a promising approach. The key purpose hereby is the formation of protective oxide scales such as Al2O3, Cr2O3 and SiO2, which offer a barrier against diffusion of oxygen into the bulk material. Besides, a dense microstructure for a suppressed interdiffusion and an adequate coating-substrate adhesion in terms of the chemical compatibility and the similarity of the coefficients of thermal expansion are indispensable.In the present work, a total of five coating systems containing Al-Si and Al-Si-Y with different chemical compositions were deposited on γ-TiAl substrate by high speed physical vapor deposition (HS-PVD), which offers high deposition rates owing to hollow cathode discharge (HCD) and gas flow sputtering (GFS). The as-deposited coatings were characterized with regards to their coating thickness, morphology and phase composition. Subsequently, thermal cyclic oxidation tests were carried out at T=950°C in air. The results confirmed the high oxidation resistance of the Al-Si and Al-Si-Y coatings compared to the uncoated γ-TiAl substrates. Both Si and Y were found beneficial for the enhancement of oxidation resistance of the aluminide coatings by promoting the formation of α-Al2O3, YAlO3 and a continuous intermetallic scale of TiSi. Significantly suppressed inward oxidation and outward diffusion of Ti could be obtained by the sample coated with Al-Si-Y during cyclic oxidation test. The results of the conducted research reveal a high potential of the HS-PVD Al-Si-Y coating system for the oxidation protection of γ-TiAl at T>850°C in turbine applications.