The thermodynamic stability of amorphous oxide overgrowths on metals

Abstract

On a thermodynamic foundation, in particular for interface energetics, the initial oxide overgrowth on a bare metal surface can be predicted to be either amorphous or (semi-)coherent crystalline, as function of the oxidation temperature and substrate orientation. Model calculations were performed for a range of metal/oxide systems (oxidation of Al, Ni, Cu, Cr, Fe, Mg, Zr and Ti) on the basis of the sum of the surface, interfacial and bulk energy differences between the competing amorphous and crystalline oxide overgrowths on the same metal substrate. It follows that an amorphous state for the initial oxide overgrowth can be thermodynamically (instead of kinetically) preferred as long as the higher bulk Gibbs energy of the amorphous oxide overgrowth is overcompensated for by its low sum of surface and interface energies. The dominating factors, which thermodynamically favour the formation of an amorphous oxide overgrowth, are exposed and discussed.