Influence of sulfur on the adhesion of the nickel/alumina interface

The effects of sulfur on the adhesion of Ni/Al₂O₃ interfaces are determined from first principles. The interfacial bonding is analyzed in terms of its ionic, covalent, and metallic components as well as its local electron orbital symmetries. The results reveal that S segregates to intact Ni/Al₂O₃ interfaces unless those interfaces are Al rich. In all cases, the segregated S weakens interfacial bonds. The effect of S on adhesion arises from a competition between interfacial strain and new S-containing bonds formed across the interface. Results of this competition depend on whether the S is substitutional or interstitial as well as on interfacial stoichiometry. The propensity for segregation and weakening depends on the interfacial stoichiometry: governed by whether the interface is prepared by diffusion bonding or is thermally grown.