MASS TRANSFER MECHANISM IN Yb2Si2O7 UNDER OXYGEN POTENTIAL GRADIENTS AT HIGH TEMPERATURES

摘要

The oxygen permeability of polycrystalline Yb2Si2O7 wafers having a thickness of several hundred microns and serving as model environmental barrier coating (EBC) layers were evaluated under steep oxygen potential gradients (dμo) at high temperatures. These dμo were produced by exposing the upper and lower surfaces of wafers to atmospheres with different oxygen partial pressures (P02). The resulting data were compared with literature data for mullite and AI2O3. It was found that the oxygen permeation of Yb2Si2O7 is controlled by grain boundary (GB) diffusion of oxygen from the high P02 surface to the low P02 surface, with simultaneous GB diffusion of ytterbium in the opposite direction. Oxygen permeation associated with the GB diffusion of silicon was negligibly small compared to that generated by ytterbium GB diffusion. The concurrent GB diffusion of oxygen and single cations proceeds without acceleration or inhibition due to interdiffusion, and can be described by the Gibbs-Duhem equation, as can the mullite and AI2O3 data. The oxygen permeability constant for Yb2Si2O7, normalized by the GB density, was significantly larger than those for mullite and AI2O3.