The oxygen shielding properties of polycrystalline Al4+2x
(mullite) films applied as environmental barrier coatings (EBCs) on SiC fiber-reinforced SiC matrix composites (SiC/SiC) are determined by the grain boundary (GB) diffusion of oxide ions in the films, from the higher oxygen partial pressure (PO₂
) surface to the lower PO₂
surface, with simultaneous GB diffusion of Al ions in the opposite direction. Herein, strategies to improve the oxygen shielding and phase stability of these films when applied to SiC/SiC substrates through bond coats are proposed, based on oxygen permeation data for mullite at high temperatures. The validity of these strategies is verified using experimental trials at 1673 K with bilayer specimens consisting of mullite films and bond coat substrates, serving as model EBCs. The data show that employing a bond coat made of β’-SiAlON rather than Si provides a source of Al for the overlying mullite film that greatly improves the phase stability of the film in the vicinity of the junction interface. Because the minimum equilibrium PO₂
values required to form SiO2
due to oxidation of the β’-SiAlON on a thermodynamic basis are significantly larger than those for oxidation of Si, the inward GB diffusion of oxide ions is effectively retarded, resulting in excellent oxygen shielding characteristics.
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