Environmental and thermal protection of γ-TiAl alloys

Kurzfassung

The current development of new generation gamma titanium aluminides is expected to result in alloy chemistries and microstructures capable of resisting temperatures in excess of 850°C. Under these conditions, environmental and thermal protection becomes a concern since oxidation might eventually limit the maximum service temperatures achievable. Therefore protective coatings are necessary to exploit the full potential of gamma titanium aluminides at moderately elevated temperatures; however, as yet no coating system tested has proven sufficient performance for long-term use in automotive, aerospace as well as power generation applications. Recently, promising coating systems have been developed that appear to have a fairly high potential for the targeted applications. 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. The paper is focussed on both development of oxidation resistant coatings as well as thermal barrier coatings for gamma titanium aluminides. TiAlCrY nitride and metallic overlay coatings based on TiAlCr were produced by magnetron sputtering techniques. Both coatings provide an excellent oxidation protection for γ-TiAl alloys in air at 750°C. Using the pack-cementation process, the surface region of γ-TiAl specimens was aluminized to TiAl3. Adopting state-of-the-art technology for nickel-base gas turbine components, EB-PVD yttria-stabilized zirconia TBCs were deposited on Ti-45Al-8Nb, either pre-oxidized or coated with various “bond coats” mentioned above. The oxidation behaviour of the specimens with environmental and thermal protective coatings was determined under cyclic oxidation conditions at 850 and 900°C in air up to 2000 h and compared to that of the γ-TiAl substrate material. Post-oxidation SEM/EDS analysis of the coating systems will be presented, highlighting the particular microstructure features which will be discussed with regard to oxidation behavior and lifetime of the coating systems