Investigations of water vapor enhanced oxidation on γ-TiAl based alloys: evaluation of protective coating systems

Abstract

Currently, intermetallic γ-TiAl alloys are being used as material for turbine blades in the low-pressure turbine to replace the heavier Ni-based superalloys due to their due to their comparatively half the density. Their limitation to service temperatures below 800 °C is due to strength and creep resistance at elevated temperatures, as well as a reduced oxidation resistance. The latter can be surpassed with the aid of remarkably effective oxidation-protective coatings. These coatings result in the formation of a dense thermally grown oxide (TGO) scale, Al2O3. However, new turbine engine concepts, such as the Water Enhanced Turbofan (WET) engine concept or the use of hydrogen-based fuels in jet engines, introduce higher amounts of water vapor. Now, not only the γ-TiAl alloys need to be evaluated under more severe conditions, but also the highly studied protective coatings need to be tested in this harsher environment. The mechanism by which water vapor content and temperature may be affecting uncoated and coated γ-TiAl alloys needs to be understood. For a further enhancement of the protection, an environmental barrier coating (EBC) concept should be considered. Protective EBC system on SiC/SiC CMCs forming SiO2 as TGO, mostly containing Yb or Y-silicates due to their low oxygen diffusion and matching thermal expansion coefficient (CTE). A protective EBC system for γ-TiAl alloys with Al2O3 forming bond coats could be completed by Yb-aluminates, which also show low oxygen diffusion and matching with the CTE. In the present work, γ-TiAl alloys were tested isothermally under water vapor enhanced oxidation up to 30 wt.% water in a tube furnace. The growth of a thick Al2O3/TiO2 mixed oxide scale was analyzed on uncoated TiAl alloys. Different coating concepts such as Ti-Al-Cr, Al-Si and Ti-Al-C deposited by DC magnetron sputtering could already improve the resistance of TiAl alloys in dry oxidation conditions. In water vapor enhanced oxidation processes, the growth of the protective Al2O3 oxide scale is increased. In order to assess the need for an EBC system, the oxide growth as a function of temperature and different water contents will be evaluated. Therefore, different analyses like SEM, EDX, XRD and thermogravimetric analyses have been performed. Finally, first concepts of Yb-aluminates as a protective ceramic top layer by using the previously established layers as bond coating are introduced in terms of reactive sputter deposition, phase stability and improvement of the coating system.