Metal / ceramic adhesion: a first principles study of MgO/Al and MgO/Ag

Abstract

The energetics of adhesion and the electronic structure are determined for MgO/Ag(100) and MgO/Al(100) interfaces via local density-functional calculations. At the interface, both Ag and Al atoms energetically favour the site directly above the O atom, which is consistent with recent high-resolution transmission electron microscopy experiments on MgO/Ag(100). For that site, electron density distributions in the metal surface regions are reminiscent of a charge array imaging only the surface Mg and O ions. This implies a substantial ionic component to the adhesive bond. The screening charge distributions appear smoother in the A1 case than in the Ag case, suggesting a direct role of the Ag d electrons in the screening. Despite the ionic contribution to the bonding, the adhesive energy versus interfacial separation curves obey the same, universal form originally discovered for bimetallic adhesion. This could be explained by a significant metallic and/or covalent contribution to the metal/ceramic bonds. Further evidence for a metallic/covalent contribution is found in the density of states plots, including a contribution from the (filled) Ag d-band. These plots also suggest that the surface MgO layer is metallized by wave function overlap from the contiguous metal layers. The MgO/Ag(100) interfaces have larger ideal adhesive energies and ideal peak interfacial stresses than the MgO/Al(100) interfaces have. This is apparently due to a significant contribution by Ag d electrons to the adhesion, consistent with their aforementioned role in screening, the densities of states, and with the difference between MgO/Ag and MgO/Al ideal adhesive energies being comparable to the difference in Ag and Al surface energies. Finally, using the dislocation network model of Trampert et al. for MgO/Ag(100), our computed work of adhesion and contact angle are shown to be in good agreement with experimental values.