Molecular dynamics simulations of reactive wetting in metal-ceramic systems

We have simulated the wetting of sapphire by aluminum using the ES+ potential. This potential is unique among empirical potentials because it applies a combination of the embedded atom method (EAM) and Coulomb potentials on a system of atoms with defined electronegativities so that the atom charges as well as the interatomic forces and system energies found by typical empirical potentials can be determined. Because we do not have to specify atom bond character a priori, we can directly model reactive wetting where atoms from the metal droplet may replace cations in the ceramic substrate and undergo the appropriate change from metallic to ionic bonding. Also, by using this empirical potential, we can simulate large dynamic systems that cannot be simulated by first-principles methods, albeit with some loss in detail and accuracy. In our simulations of high-temperature wetting we observe the formation of an oxygen-deficient reaction layer between the liquid and the substrate. The driving force for the creation of this layer is the partial oxidation of the metallic aluminum, which results in partial reduction of the aluminum ions in the substrate and diffusion of oxygen from the substrate to the reaction layer. We believe that this change in stoichiometry at the metal-ceramic interface may influence diffusivities in the surface of the ceramic and may therefore have an important effect on the kinetics of the evolution of surface features observed experimentally during reactive wetting. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.