Upper Temperature Limit of Environmental Barrier Coatings Based on Mullite and BSAS

TLDR

Environmental barrier coatings for Si‑based ceramics typically comprise a silicon bond coat, an intermediate mullite or mullite‑plus‑BSAS layer, and a BSAS top coat, with long‑term durability concerns including environmental resistance, chemical compatibility, volatility, phase stability, and thermal conductivity. The study applied these EBC variants to monolithic SiC and SiC/SiC composites, noting that BSAS reacts with silica to form a low‑melting glass (~1300 °C) that can spall the coating, that it recedes by volatilization above ~1400 °C in water‑vapor atmospheres, and that its sluggish hexagonal‑to‑monoclinic celsian transformation may further affect performance. Both the glass‑forming reaction and the volatilization of BSAS are identified as life‑limiting factors, while a rapid rise in thermal conductivity beginning at ~1300 °C signals early sintering of the coating.

Abstract

Current state‐of‐the‐art environmental barrier coatings (EBCs) for Si‐based ceramics consist of three layers: a silicon bond coat, an intermediate mullite (3Al 2 O 3 ·2SiO 2 ) or mullite + BSAS ((1− x )BaO· x SrO·Al 2 O 3 ·2SiO 2 , 0 ≤ x ≤ 1) layer, and a BSAS top coat. Areas of concern for long‐term durability are environmental durability, chemical compatibility, volatility, phase stability, and thermal conductivity. Variants of this family of EBC were applied onto monolithic SiC and melt‐infiltrated SiC/SiC composites. Reaction between BSAS and silica results in a low‐melting (∼1300°C) glass, which can cause the spallation of the EBC. At temperatures greater than ∼1400°C BSAS suffers significant recession via volatilization in water‐vapor‐containing atmospheres. Both reactions can be EBC life‐limiting factors. BSAS undergoes a very sluggish phase transformation (hexagonal celsian to monoclinic celsian), the implications of which are not fully understood at this point. Initial rapid increase in thermal conductivity at temperatures as low as 1300°C indicates the sintering of EBC.