Abstract: We study the oxidation behavior and failure mechanisms of a NiCoCrAlY coating in high-temperature water vapor using experiments and modelling. Compared to air, water vapor promotes metastable γ-Al₂O₃ growth and delays its transformation to α-Al₂O₃ due to surface hydroxylation from OH⁻. Rapid γ-Al₂O₃ growth embeds base metals into the oxide, which later dissolve to form Al₂O₃ grains alloyed with Ni²⁺, Co²⁺, and Cr³⁺. The cation substitutions and interstitial protons lower formation energies of Al and O vacancies in γ/α-Al₂O₃, enhancing Al outward diffusion and accelerating oxide growth. This creates a counter vacancy flux, which drives the formation of interface pores that reduce metal-oxide contact and interface toughness, ultimately causing premature spallation. The study sheds light on water vapor-driven degradation in NiCoCrAlY and other Al₂O₃-forming coatings.

Key words: NiCoCrAlY coating, Water vapor, High-temperature oxidation, Al₂O₃ scale, Interface toughness