Structural stabilization of multilayered EBCs at high temperatures

Abstract

Abstract The optimum structure for multi-layered environmental barrier coatings (EBC) with enhanced environmental shielding and structural stability was proposed based on the mass-transfer mechanisms of the EBC constituent oxides such as polycrystalline Yb2Si2O7 (Y2S2) and Al4+2xSi2-2xO10-x (mullite), as determined using the oxygen permeation technique under oxygen-chemical-potential gradient (dµO) at high temperatures. The oxygen permeation proceeded via grain boundary (GB) diffusion of oxygen ions in the mullite layer from the higher oxygen-partial-pressure PO2 surface to the lower PO2 surface, concurrently with GB diffusion of aluminum ions in the opposite direction, while oxygen ions and ytterbium ions were interdiffused along GBs for the Y2S2 layer. Thus, control of the outward diffusion of cations is essential to improve the oxygen shielding and structural stability of the layers under a dµO. For example, when the Y2S2 layer is set on the mullite layer and has limited oxygen shielding, the aluminum flux in the mullite layer will be significantly reduced. Another approach is to build a layer with an aluminum reservoir function under the mullite layer. Thus, even if an outward diffusion of aluminum ions occurs in the mullite layer, the underlying layer containing aluminum can supply aluminum to the aluminum-deficient zone in the mullite layer