Investigations on the Lifetime of Alumina-forming Ti-Al-Ag Coatings

摘要

One important hindrance to the use of γ-TiAl alloys at high temperatures is their relatively poor oxidation resistance and their sensitivity towards environmentally induced embrittlement. The latter effect is also of major concern during elevated temperature application of Ti-based alloys. These problems can principally be avoided by the application of oxidation resistant, alumina-forming coatings. In the present paper the possibility of using a Ag-containing TiAl alloy as a coating material for Ti and high-strength γ-TiAl-alloys is being studied. First, oxidation tests in different environments were carried out, to compare the isothermal and cyclic oxidation resistance of a selected Ag-containing coating alloy with that of two, new generation, high strength γ-titanium aluminides. Subsequently, magnetron sputtering was used to apply the TiAl-Ag coating to a γ-TiAl alloy (Ti—15Al-8Nb-0.2C) and to pure titanium. The coated materials were tested at 800℃ in case of the γ-TiAl substrate, and at 600℃ in case of the Ti substrate. The results illustrate, that the coatings in all the cases studied formed a protective alumina surface scale even up to the longest studied test times of 1000 h. The interdiffusion processes between coating and base material, which eventually determine the coating life, were studied by SEM/EDX investigation after different exposure times.