Effect of Ta, Hf, and Si on the High Humidity Oxidation Resistance of MCrAlY Bond Coat Materials

Abstract

The continued focus to include high hydrogen fuels such as Syngas in aircraft operation to reduce emissions and increase engine efficiency has led to an ongoing investigation into bond coat materials capable of withstanding unfavourable oxidation in high temperature humid environments. The increased presence of water in the engine exhaust leads to increased oxygen activity in the hot section of the engine. In this work, four commercially available MCrAlY bond coat materials were oxidized in high temperature environments with various humidities to understand the behaviours of different reactive element inclusions in resisting high temperature oxidation. Oxidation tests were done at 0%, 18%, and 33% water by volume at 1100C in a 1atm environment to simulate conditions expected in engines using high hydrogen fuels. Oxidation was done for 2h and 20h to observe transient oxide formation behaviour. The surfaces and cross sections of the specimens were examined using SEM and EDS analysis, along with XRD analysis. The progression of surface oxides, TGO thickness, and element depletion zones were observed. Two opposing mechanisms are observed: the upward diffusion of metal cations to the free surface and the inward diffusion of oxygen to the alloy. The presence of water is shown to increase internal oxidation of the bond coat alloy and delay the formation of a protective alumina TGO. Tantalum inclusion in the alloy composition is shown to produce the most stable alumina TGO with the least internal oxidation after 20h exposure in 33% H2O (%vol); the most hostile oxidation environment tested.