Degradation behavior of yttria-stabilized zirconia in thermal barrier coatings under reducing environments after short-term heat treatment

The gradual transition of hydrogen as a fuel for land-based gas turbines has resulted in direct changes to the combustion environment. The inadequate combustion of hydrogen fuel can lead to a transition from an oxidizing environment to a partially reducing environment, and further introduces a new potential failure mode for existing thermal barrier coating materials. In this study, tests were conducted on thermal barrier coating samples at 1000 °C in Ar + 5 % O₂ and Ar + 5 % H₂ environments, in addition to samples subjected to heat-treatment in pure argon and air environments to provide a comparison against low oxygen partial pressure and conventional failure modes. The results demonstrated that the degree of sintering of the top coat decreased gradually with a decreasing oxygen partial pressure, and was significantly inhibited in a reducing environment. Faster cooling rates led to the expansion of vertical cracks in the top coat toward the interface, which was accompanied by the generation of numerous transverse cracks in the reducing environment. In contrast, the structure of the top coat remained intact in the other three environments. Furthermore, effective methods for improving the coating durability in reducing environments are discussed. This study therefore contributes to a comprehensive understanding of the failure behavior of thermal barrier coatings in reducing environments, providing new insights into enhancing the stability under such conditions.