Structural and compositional design of a double-layer NiCoCrAlY bond coat: HfSi co-doping in the top layer for enhanced oxidation resistance in water-vapor/oxygen atmospheres

To address the oxidation and spallation risks of NiCoCrAlY bond coats under hydrogen-fueled service conditions, this study proposes a structural–compositional co-design strategy by constructing a dual-layer architecture consisting of a dense HVOF-deposited inner layer and an APS-sprayed outer layer. The top layer was modified through Hf/Si co-doping to enhance interface stability and oxidation resistance. Comparative isothermal oxidation tests at 1100 °C under 3 atm (Ar + 50 %H₂O, Ar + 50 %O₂/50 %H₂O, and 50 %H₂O + 50 %O₂) demonstrated that the coexistence of water vapor and oxygen causes the most severe oxidation and spallation. Quantitative evaluation of TGO thickening, spallation area, and residual stress, combined with microstructural and phase analyses, revealed that Hf/Si co-doping effectively suppressed rapid oxide growth and interfacial failure. Mechanistically, segregation of Hf/Si to interfaces and grain boundaries strengthens interfacial bonding and attenuates fast grain-boundary diffusion pathways, yielding a synergistic enhancement between compositional modification and structural design. This work elucidates the synergistic water–oxygen acceleration effect and validates the efficacy of Hf/Si modification, providing a basis for coordinated composition–architecture optimization of bond coats and their engineering application under hydrogen-fueled service conditions.