The effect of substrate orientation on the kinetics and thermodynamics of initial oxide-film growth on metals

A thermodynamic model has been developed which predicts the growth of either an initial (semi-)coherent, strained crystalline oxide phase or an initial amorphous oxide phase (with a possible amorphous-to-crystalline transition) on the bare single-crystalline metal substrate as function of the metal-substrate orientation, the growth temperature and the oxide-film thickness. The model accounts for relaxation of residual stresses in a crystalline oxide overgrowth by plastic deformation (i.e., through the introduction of misfit dislocations at the metal/oxide interface). As an example, the microstructural evolution of the initial oxide film grown on the {111}, {110} and {100} crystallographic surfaces of a bare Cr substrate has been modelled as function of the growth temperature and the oxide-film thickness (< 5 nm). The initial oxide-film growth on the bare Cr{100} and Cr{111} substrates is predicted to proceed by the initial formation and growth of an amorphous oxide film up to a critical thickness of about 0.5 nm and 0.9 nm, respectively. On the other hand, the onset of oxidation on a bare Cr{110} substrate should proceed by the direct overgrowth of a semi-coherent, strained crystalline oxide film. These model predictions provide understanding for experimental observations reported in the literature.