Microstructural evolution of the interface between NiCrAlY coating and superalloy during isothermal oxidation

• Interfacial oxide layer comprises α-Al2O3 and Cr2O3 sub-layers.
• Pure metal Al precipitates at the coating/substrate interface.
• AlN and TiN form in the substrate and AlN transforms into α-Al2O3 later.

NiCrAlY coating was plasma-sprayed on a Ni-based single crystal superalloy and then subjected to isothermal oxidation at 1100 °C in air up to 50 h. Using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we investigate interfacial microstructure evolution of the coating induced by internal oxidation and nitridation. After 5 h oxidation, α-Al2O3 and Cr2O3 sub-layers form at the coating/substrate interface. Interestingly, after 10 h oxidation, pure Al phase is found at the coating/substrate interface owing to decomposition of substrate phase γ′-Ni3Al. Although, after 25 h oxidation, hexagonal AlN and cubic TiN form in the substrate close to the coating/substrate interface, after 50 h oxidation, AlN transforms into α-Al2O3 owing to internal oxidation while TiN remains in the substrate because of its high stability. The findings of this work provide solid experimental evidence of microstructural evolution at the coating/substrate interface, especially demonstrate the conjecture that pure metal phases and nitrides could precipitate at coating/substrate interface after high temperature oxidation.

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