Adhesion between Al slabs and mechanical properties

The interaction energy between two Al slabs is found as a function of separation, d. For d greater than about 6 au the van der Waals interaction predominates, given accurately by the Lifschitz formula. For d less than 1 au pseudopotential perturbation theory can be used to find the interaction, and it turns out that there is a weak maximum coming from the Friedel oscillations in the ionic screening. At intermediate d the results of Ferrante and Smith (1973) are used in which the charge densities of the separate slabs are superimposed, and the interaction energy found using a local density approximation. Using these results, the relative contributions of the short-range forces and the long-range van der Waals forces to mechanical adhesion are studied, and it is found that in general the van der Waals forces make a negligible contribution. The consequences of these interaction energy results for strength properties of materials are also considered, and a simple model for the grain boundary energy discussed.