Thermodynamic stability of amorphous oxide films on metals: Application toaluminum oxide films on aluminum substrates

It has been shown on a thermodynamic basis that an amorphous structure for an oxide film on its metal substrate can be more stable than the crystallin e structure. The thermodynamic stability of a thin amorphous metal-oxide fi lm on top of its single-crystal metal substrate has been modeled as a funct ion of growth temperature, oxide-film thickness, and crystallographic orien tation of the metal substrate. To this end, expressions have been derived f or the estimation of the energies of the metal-substrate amorphous-oxide fi lm interface and the metal-substrate crystalline-oxide film interface as a function of growth temperature, and crystallographic orientation of the sub strate (including the effect of strain due to the lattice mismatch). It fol lows that, up to a certain critical thickness of the amorphous oxide film, the higher bulk Gibbs free energy of the amorphous oxide film, as compared to the corresponding crystalline oxide film, can be compensated for by the lower sum of the surface and interfacial energies. The predicted occurrence of an amorphous aluminum-oxide film on various crystallographic faces of a luminum agrees well with previous transmission electron microscopy observat ions.