Effects of segregating elements on the adhesive strength and structure of the α-Al₂O₃/β-NiAl interface

We investigate with first-principles density functional theory (DFT) the adhesion of the Al₂O₃(0 0 0 1)/NiAl(1 1 0) interface as a model for the thermally grown oxide/bond coat alloy interface in thermal barrier coatings. We find that the clean interface has an ideal work of adhesion of 0.66 J m^-2. We predict that S impurities reduce interfacial adhesion significantly, due to a reduction in cross-interface bonds. The presence of Pt alters the interface adhesion only slightly, while Hf dopants dramatically increase adhesion via formation of strong Hf-O bonds, as expected from Hf's open-shell character. We discuss the implications of these predictions, which are consistent with experimental observations of the effects of S, Pt, and Hf on the lifetime of thermal barrier coatings.