Weak bonding of alumina coatings on Ni(111)

We examine the structure and stability of an ultrathin ceramic film coating a metal substrate, specifically, an alpha -alumina, Al2O3, film grown on f cc nickel. This metal-ceramic interface may play a role in materials failur e of current combustion engine thermal barrier coatings (TBC's). We charact erize the (0 0 0 1) surface of alpha -Al2O3 and study the effect of increas ing alumina film thickness on the alumina/nickel interface using periodic s lab 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 ca lculations indicate that the preferred bonding mode depends on the thicknes s of the alumina film. Namely, for one monolayer of alumina, the alumina ap pears 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 lay er (alumina) that forms during preparation and operational cycling of TBC's . The thickest alumina layers energetically prefer intra-ceramic bonding ov er Al2O3/Ni interface formation. Connections to metal catalyst-oxide suppor t interfaces are also discussed. (C) 2001 Elsevier Science B.V. All rights reserved.