MORPHOLOGICAL INSTABILITY OF A PLANAR OXIDE-ALLOY INTERFACE FOR INWARD OXIDE-GROWTH

The morphological stability of an oxide layer during selective oxidation of a binary alloy is investigated theoretically for the case of inward oxygen diffusion. According to the result of a linear stability analysis, a roughening of a planar oxide-metal interface can occur if interdiffusion in the alloy is rate determining. The wavelength of the most unstable mode is evaluated taking into account interface smoothing via volume and interface diffusion. For parabolic scale growth, the wavelength increases with time according to the power law t(1/4) for volume diffusion and t(1/6) for interface diffusion. The theoretical results are compared with recent experiments on the oxidation of gamma'-Ni3Al at low oxygen partial pressure where protrusions of the oxide into the metal were found in the initial stage of oxidation. The observed smoothing of the interface in the course of further oxidation is interpreted as a transition from metal- to oxygen-diffusion controlled growth.