Effect of Oxygen Potential Gradient on Mass Transfer in Polycrystalline α-Alumina at High Temperature

The oxygen permeability of polycrystalline α-alumina wafers, which served as model alumina scales formed on heat-resistant alloys, was evaluated at a temperature of 1873 K. Mass transfer along grain boundaries (GBs) in an alumina wafer exposed to a large oxygen potential gradient (dμO), where both oxygen and aluminum mutually diffuse along GBs, was analyzed using 18O2 and SIMS. 18O was concentrated at GB ridges on the high oxygen partial pressure (PO2(hi)) surface and along the GBs near the PO2(hi) surface. 18O adsorbed on the surface diffused almost immediately to surface GBs, resulting in the formation of new alumina by reaction with aluminum diffusing outward along the GBs. Oxygen GB diffusion coefficients in the vicinity of the PO2(hi) surface were determined from the 18O depth profile along each GB for the 18O map of the cross section of the exposed alumina wafer. The oxygen GB diffusion coefficients were comparable to the values calculated from the oxygen permeability constants assuming an electronic conductivity and were obviously lower than those of oxygen GB self-diffusion without an oxygen potential gradient.