Performance of thermal barrier coatings on γ-TiAl

Abstract

The current development of new generation gamma titanium aluminides is expected to result in alloy chemistries and microstructures capable of operating at 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 and aerospace applications. Thermal barrier coatings (TBCs), typically applied to nickel-based alloys, offer the potential to increase the service temperature of components by lowering the metal surface temperature in combination with cooling systems. The paper is focussed on development of thermal barrier coatings for gamma titanium aluminides. Different coatings were used for oxidation protection and bond coat application. Substrate specimens were either pre-oxidized or coated with PVD-Al 2 O 3 , TiAl-CrYN, or diffusion aluminides. Yttria-stabilized zirconia TBCs were deposited applying electron-beam physical vapour deposition. Cyclic and quasi-isothermal oxidation tests were carried out at 900 °C in air. Post-oxidation analysis of the coating systems was performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy. Zirconia top coats offer a promising thermal protection concept to be applied on γ-TiAl components. However, high oxidation resistance has to be supplied by protective coatings. Diffusion layers of the TiAl 3 aluminide provided excellent environmental protection because of the formation of a continuous alumina scale. No spallation of the thermal barrier coatings was observed on aluminized specimens during 1000 1-h cycles and 3000 h of cyclic and isothermal oxidation testing, respectively.