Theory of metallic adhesion

Abstract

The self-consistent electronic structure and adhesive energies are computed for the following contacts: Al(111)-Al(111); Mg(0001)-Mg(0001); Zn(0001)-Zn(0001); Na(110)-Na(110). Electronic charge densities and potentials are quite sensitive to small separations in the contacts. Friedel oscillation amplitudes increase with decreasing average bulk electron densities and with increasing separation in the contact. The inclusion of nonlocal effects increases adhesive binding energies but has little effect on the shape of the adhesive energy versus separation plots. The importance of bulk equilibrium is investigated. Binding energies agree well with experimental surface energies and very good agreement is found between computed elastic constants and experiment. Both the binding energies and the elastic constants increase with increasing average bulk-electron number density. The variation of the components of the adhesive energy with separation reveal a rather close analogy between the characteristics of the metallic adhesive bond and those of molecular bonds in simple molcules. The kinetic energy initiates the bond, while the electrostatic and particularly the exchange energy lead to the strong adhesive bond. The range of the strong bonding is about 0.2 nm. Adhesive energies and charge densities saturate much faster with contact separation than do electronic potentials.