High-temperature oxidation protection of γ-based TiAl by sputtered AlOF films

The implementation of γ-based titanium aluminides in various components of aircraft engines has been motivated by the need to optimize their thrust-to-weight ratio and limit greenhouse gas emissions. The interesting mechanical properties of TiAl alloys make them suitable for use in coldest stages of low-pressure turbines, but their sensitivity to oxidation at temperatures greater than 750 °C limits their use in the hotter sections of aircraft engines. The halogen effect is known to efficiently protect TiAl against high-temperature oxidation and is a promising solution toward increased implementation of TiAl in higher-temperature parts of the engines. Amongst all halogens, fluorine offers the best protection in a wide set of operating conditions. This work demonstrates the possibility of using reactive magnetron sputtering of Al in an Ar, O2, and CF4 atmosphere to fluorinate the surface of γ-based Ti-48Al-2Cr-2Nb and promote the alumina-forming characteristics of the alloys. The chemistry of the reactive plasma is first investigated using mass spectrometry and is then linked to the coating composition, measured by Rutherford backscattering spectrometry. Finally, an array of coating compositions is tested in isothermal oxidation at temperatures up to 875 °C. The coatings are protective, generating an alumina oxygen barrier upon oxidation.