Cyclic oxidation behavior and TGO growth mechanism of Y‐ and Hf‐doped NiCoCrAlTaRe superalloy bond coatings in pure steam environment

Abstract This research presents a thorough assessment of the cyclic oxidation characteristics of Y‐ and Hf‐doped NiCoCrAlTaRe superalloy bond coatings in a pure steam atmosphere, emphasizing the distinct influences of reactive elements (Y and Hf) and refractory elements (Ta and Re) on the growth mechanisms of thermally grown oxide (TGO). The findings indicate that, in contrast to air conditions, elevated levels of water vapor significantly diminish the oxidation resistance of the bond coatings, leading to considerable porosity defects in both the central and lower regions of the TGO. Furthermore, this environment hinders the development of the “peg” structure at the TGO/metal interface, thereby accelerating the premature delamination of the coating. Additionally, the presence of doped elements such as Hf, Ta, and Y leads to their segregation at the Al 2 O 3 grain boundaries within the TGO, creating grain boundary structures characterized by a high density of defects. This defective architecture promotes the inward diffusion of water molecules at elevated temperatures, causing hydrogen atoms generated from oxidation and reduction reactions at the TGO/metal interface to become entrapped within the Al 2 O 3 lattice at the base of the TGO, rather than escaping efficiently. Ultimately, this phenomenon contributes to the formation of internal porosity defects during the oxidation of TGO in a steam environment.