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Structural Stabilization of Mullite Films Exposed to Oxygen Potential Gradients at High Temperatures

Japan Fine Ceramics Center (JFCC), Nagoya 456-8587, Japan
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Coatings 2019, 9(10), 630; https://doi.org/10.3390/coatings9100630
Received: 23 August 2019 / Revised: 18 September 2019 / Accepted: 23 September 2019 / Published: 30 September 2019
(This article belongs to the Special Issue Environmental Barrier Coatings)
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. View Full-Text
Keywords: EBCs; mullite; SiAlON; diffusion; grain boundary; oxygen permeability EBCs; mullite; SiAlON; diffusion; grain boundary; oxygen permeability
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Kitaoka, S.; Matsudaira, T.; Kawashima, N.; Yokoe, D.; Kato, T.; Takata, M. Structural Stabilization of Mullite Films Exposed to Oxygen Potential Gradients at High Temperatures. Coatings 2019, 9, 630.

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