Oxide growth and stress evolution underneath cracked environmental barrier coatings

Abstract

SiC composites at elevated temperatures are prone to oxidation, prompting the use of environmental barrier coatings to limit or prevent the ingress of oxidants. Cracks in such coatings can provide fast diffusion pathways that can lead to accelerated local oxide growth. The localized oxide confined at the tip of such features generates significant stress due to the large molar volume change associated with converting SiC to SiO2. In this work, a multi-physics computational framework is used to simulate the evolution of such internal oxide domains; the framework integrates analyses of oxidant transport, oxide growth (domain evolution) in dry air environments and stress evolution mitigated by creep. Significant insights regarding oxidation-driven damage are obtained from a broad parametric study involving the crack opening, the effective diffusivity of the coating, the crack spacing and the magnitude of applied loads. (i) Oxide growth at the tip of the crack is comparable to that of a bare surface; even very narrow open channels lead to rapid, localized oxidation. (ii) Coatings with low diffusivity produce highly confined oxide ‘bubbles’ at the tip of the crack; confinement produces high compressive stresses in the oxide and significant tensile stresses in the surrounding material. Stresses along the coating/substrate interface and in the substrate beneath the bubble can be 2–7 times those arising from mechanical loading alone, promoting coating decohesion and substrate cracking. (iii) Significant applied stress reduces high hydrostatic stresses in the oxide and mitigate the stress elevation associated with local oxidation. The paper concludes with a brief discussion of the implications of these observations for damage mechanisms occurring at elevated temperature, and indicate the importance of further study of the impact of wet environments and different oxide compositions.

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