Effects of reducing environments on aging behavior and performance of air plasma-sprayed yttria-stabilized zirconia thermal barrier coatings

In advanced hydrogen-fueled gas turbines, incomplete combustion of hydrogen creates a reducing environment, which introduces additional potential failure modes for existing thermal barrier coatings (TBCs). Therefore, this study investigated the discrepancies in failure modes of air plasma-sprayed yttria-stabilized zirconia (YSZ) coatings after heat treatment at 1100 °C in reducing (Ar + 5% H 2 ) and oxidizing (air) environments. Isothermal exposure in an oxidizing environment for 500 h caused a high degree of sintering of the YSZ top coat, and the hardness and Young’s modulus increased by 54.41% and 61.96%, respectively. Conversely, in a reducing environment, sintering of the top coat was inhibited, and the hardness and Young’s modulus only increased by 13.69% and 37.01%, respectively. In contrast to the stiffening-induced failure observed in the oxidizing environment, the weak intergranular and intersplat bonding were considered to be the main factors that reduced the toughness in the reducing environment. Moreover, Raman spectra confirmed that no phase transformations occurred in either environment. Therefore, this study showed that a reducing environment changed the potential failure modes of a YSZ coating, which provides insight into the long-term degradation behavior of TBCs in hydrogen-fueled gas turbines.