Ti-Al-Cr based Coatings for High Temperature Oxidation Protection of γ-TiAl

Abstract

To extend the oxidation resistance of gamma titanium aluminides for applications above 750°C the formation of non-protective titanium oxide has to be prevented. The primary objective to improve the resistance of titanium aluminides against oxidation is the use of alumina forming coatings. The paper is focussed on the development of oxidation resistant intermetallic coatings on the basis of Ti-Al-Cr. Ti-Al-Cr and Ti-Al-Cr-X coatings with X=Ag,Hf,Y,Si,Zr and W were produced by magnetron sputtering techniques. The coated specimens were exposed to air under cyclic conditions at 950°C and 1000°C for 1000 cycles or until failure, respectively. All coatings formed a thin alumina layer on top providing a diffusion barrier for oxygen at the beginning. But interdiffusion between substrate and Ti-Al-Cr coating leads to a depletion of Laves phase which promotes the formation of alumina. The effect of interdiffusion between coating and substrate for the various coating chemistries and the significance of each quaternary element will be discussed on the basis of post-oxidation SEM/EDX and oxidation kinetics analysis.