Oxidation resistant coatings in combination with thermal barrier coatings on γ-TiAl alloys for high temperature applications

Abstract

Providing attractive properties such as low density, high specific stiffness, high specific yield strength and good creep resistance up to high temperatures, γ-TiAl alloys are considered for applications in automotive, aircraft, aerospace as well as in power generation industry with the potential to replace the presently used heavy steels and nickel-based superalloys. To extend the oxidation resistance granted up to 750°C of bare γ-TiAl, protective coatings are under development. A novel approach is the use of thermal barrier coatings (TBCs) on γ-TiAl, typically applied to nickel-based superalloys. This allows to reduce the metal surface temperatures in combination with component cooling for use at even higher temperatures. The paper is focussed on both development of oxidation resistant coatings as well as thermal barrier coatings for gamma titanium aluminides. Ti-Al-Cr-Y nitride and metallic overlay coatings based on Ti-Al-Cr were produced by magnetron sputtering techniques. Both coatings provide excellent oxidation protection for γ-TiAl alloys in air at 750°C. EB-PVD partially-yttria-stabilized zirconia TBCs were deposited onto Ti-45Al-8Nb, either pre-oxidized or coated with various "bond coats" mentioned above. The specimens were examined under cyclic conditions at 900°C and 950°C, respectively. In comparison with base alloy oxidation the nitride coating reveals poor oxidation resistance when exposed at 900°C for 420 cycles whereas the Ti-Al-Cr layer shows moderate oxidation behaviour under these conditions up to 1000 cycles. Furthermore, the effect of several TBC deposition temperatures, 850°C and 950°C, in combination with oxidation resistant coatings was examined.