MASS TRANSFER MECHANISM IN MULLITE UNDER OXYGEN POTENTIAL GRADIENTS AT HIGH TEMPERATURES.

The oxygen permeability of polycrystalline mullite wafers having a thickness of several hundred microns and serving as a model environmental barrier coating (EBC) layer were assessed under steep oxygen potential gradients (d⇔...) at temperatures above 1673 K. The d⇔O were produced by exposing the upper and lower surfaces of wafer to atmospheres with different oxygen partial pressures (PO₂). Exposure to a steep d⇔... at high temperatures was found to induce oxygen permeation via grain boundary (GB) diffusion of oxygen from the high PO₂ surface to the low PO₂ surface, with simultaneous GB diffusion of aluminum in the opposite direction. This concurrent GB diffusion of oxygen and aluminum proceeded without acceleration or inhibition due to interdiffusion, and could be described by the Gibbs-Duhem equation. Oxygen permeation associated with the GB diffusion of silicon was negligibly small compared to that generated by aluminum GB diffusion, resulting in decomposition of the mullite near the low-PO₂ surface. [ABSTRACT FROM AUTHOR]