On the oxidation of the silicon bond coat in environmental barrier coatings

Deal and Grove’s formulation is applied to model the oxidation of the silicon bond coat in environmental barrier coatings. The approach is similar to the author’s previous study, except now the oxidant is assumed to flow through the coating as ions via vacancy diffusion. The use of an “effective permeability” of the oxidant in the coating is required under the vacancy diffusion assumption. It is found that the linear-parabolic growth equation from Deal and Grove, xox2+Axox=Bt, is still applicable for the oxidation of the silicon bond coat, but the equation for the parameter A must be modified to account for the coating. The expression for B is the same as that obtained by Deal and Grove and thus is unaffected by the presence of a coating. The modified expression for A is a linear function of the coating thickness and includes the ratio of the oxidant permeability in the oxide to its effective permeability in the coating.