Unraveling β-NiAl degradation in aluminide coatings: A comparative study of isothermal oxidation and vacuum heat treatment

Aluminide coatings are essential for protecting gas turbine hot-section components, but their longevity is often limited by the degradation of the protective β-NiAl phase. This study aims to elucidate the fundamental mechanisms driving β-NiAl degradation by comparing its behavior in aluminide coatings deposited on a nickel-based superalloy under isothermal oxidation (1100°C, air) and vacuum heat treatment (1100°C, vacuum) conditions. The results demonstrate conclusively that β-NiAl degradation is primarily caused by coating-substrate interdiffusion, which leads to Ni enrichment within the coating, rather than by Al depletion resulting from the formation of the surface oxide scale. Degradation was observed to initiate preferentially at the coating surface, the interface between coating sublayers, and the coating-substrate interface. Furthermore, thinner coatings exhibited accelerated β-NiAl degradation due to enhanced Ni enrichment resulting from shorter diffusion distances. These findings highlight the importance of managing diffusion processes to enhance coating performance and durability in high-temperature environments.