KINETICS OF OXIDE CRYSTAL-GROWTH IN THE TRANSITION REGIME BETWEEN CABRERA-MOTT AND WAGNER THICKNESS REGIONS

We present a computational model for laser-induced Wagner oxidation in air of metals. We assume that oxidation is diffusion Limited, but tha t the diffusion constant is a function of temperature. We suppose that there are not temperature gradients in the metal, and that the ambien t temperature remains constant. Losses include convection and Stefan-B oltzmann terms; from the oxide-air surface at the focal spot these are modelled explicitly, and from the metal-oxide surfaces away from the focal spot they are approximated. We plot the rate of oxide layer grow th as a function of laser power and time. The parabolic rate law in th e case of a laser-induced oxidation process seems to be valid from the thickness of the oxide layer of about 0.5 mu m at long times and high powers, but it is not true for the very beginning of the process for the metals without native oxide layers. At lower powers the validity o f the Wagner parabolic law takes place at later times of irradiation a nd the mechanism of oxidation is governed by the temperature gradient in the oxide film arising during scale growth. Comparison with experim ent is made for the case of vanadium/vanadium oxide systems.