Abstract
The oxygen shielding properties of polycrystalline Al4+2xSi2−2xO10−x (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.
Highlights
Environmental barrier coatings (EBCs) can play a key role in enabling SiC fiber-reinforced SiC matrix (SiC/SiC) composites to be employed as advanced hot-section components in airplane engines, as a means of realizing exceptional fuel efficiency
As described in the Introduction, the oxidation of a Si-based bond coat proceeds via the inward grain boundary (GB) diffusion of oxide ions and the outward GB diffusion of Al ions through the exposed upper mullite film following the application of a large dμO at high temperatures
Coatings 2019, 9, x FOR PEER REVIEW. These results demonstrate that the oxygen permeation and the contribution of the diffusion of each ion through the mullite film are both significantly affected by the dμO value
Summary
Environmental barrier coatings (EBCs) can play a key role in enabling SiC fiber-reinforced SiC matrix (SiC/SiC) composites to be employed as advanced hot-section components in airplane engines, as a means of realizing exceptional fuel efficiency. EBCs must exhibit excellent oxygen/water vapor shielding characteristics and thermomechanical durability in severe combustion environments [1,2,3,4,5,6,7]. A multilayer structure is generally adopted, with each layer having its own unique properties. EBCs are exposed to a large oxygen potential gradient (dμO ) at elevated temperatures. This gradient results in the inward diffusion of oxide ions and outward diffusion of cations, according to the. Cation transport through an oxide layer typically induces decomposition of the oxide, and can cause the EBC structure to collapse.
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