Weak bonding of alumina coatings on Ni(1 1 1)

We examine the structure and stability of an ultrathin ceramic film coating a metal substrate, specifically, an α-alumina, Al₂O₃, film grown on fcc nickel. This metal-ceramic interface may play a role in materials failure of current combustion engine thermal barrier coatings (TBC's). We characterize the (0 0 0 1) surface of α-Al₂O₃ and study the effect of increasing alumina film thickness on the alumina/nickel interface using periodic slab density functional theory within the generalized gradient approximation. Since Ni forms stable alloys with Al, it is not obvious whether the bonds between Ni and alumina will be Ni-Al, Ni-O, or both. Interestingly, our calculations indicate that the preferred bonding mode depends on the thickness of the alumina film. Namely, for one monolayer of alumina, the alumina appears amorphous and both Ni-O and Ni-Al interactions take place, while for two and three monolayers, Ni-O interactions decrease and Ni-Al bonds become more pronounced. By studying the effect of increasing alumina thickness on the Ni substrate, we observe a marked decrease in the work of adhesion for thicker alumina coatings. This provides a new atomic-scale explanation for the observed increase in spallation with increasing thickness of oxide layer (alumina) that forms during preparation and operational cycling of TBC's. The thickest alumina layers energetically prefer intraceramic bonding over Al₂O₃/Ni interface formation. Connections to metal catalyst-oxide support interfaces are also discussed.