Improvement of oxidation resistance of γ-TiAl At 800 and 900 °C in air by TiAl2 coatings

In the present work, γ-TiAl and TiAl2 coatings with the thickness of about 20 μm were made on γ-TiAl substrates by using a close-field magnetron sputter deposition (CFMSD) technique. The coatings showed good quality with no micro-cracks being observed in the as-deposited TiAl2 coating cross-sections. Oxidation exposure tests at 800 and 900 °C in air were carried out to assess the oxidation resistance of the coatings and the cast γ-TiAl. The γ-coating and cast TiAl showed similar oxidation performance. Both of them had suffered great oxide losses with the formation of Al2O3 + TiO2 mixtures and no Al2O3-rich layer was found in either case. By contrast, only external Al2O3 layers with a θ-phase structure was formed on the TiAl2 coatings at 800 and 900 °C, and the θ-Al2O3 scales showed high protective ability with low mass-gains and good scale adherence. After the oxidation exposures, the TiAl2 coating remained to be almost free of micro-clacks, and an inter-diffusion region of about 5 μm was observed between the TiAl2 coating and the γ-substrate, which could act as a metallurgical bonding between the coating and substrate.