Reformulation of Oxide Growth Equations for Oxidation of Silicon Bond Coat in Environmental Barrier Coating Systems

The formulation of Deal and Grove for the oxidation of silicon and the growth of a silica layer is revisited to include the effects of a protective coating layer. The linear-parabolic equation of the form x_o^2+Ax_o=Bt is still applicable, but the expressions for the constants A and B must be modified to account for the coating. The modified expression for A is a linear function of the coating thickness. The expression for B is modified by the relative solubility of the outer coating layer. The modified expressions are applied to assess recently published experimental results for the oxidation of a silicon bond coat in an environmental barrier coating (EBC). Conclusions are drawn regarding the diffusivity and solubility of oxidants in the EBC top coating.