Response of ytterbium disilicate–silicon environmental barrier coatings to thermal cycling in water vapor

Abstract

A preliminary study of a promising bi-layer environmental barrier coating (EBC) designed to reduce the susceptibility of SiC composites to hot water vapor erosion is reported. The EBC system consisted of a silicon bond coat and a pore-free ytterbium disilicate (YbDS; Yb 2 Si 2 O 7 ) topcoat. Both layers were deposited on -SiC substrates using a recently optimized air plasma spray method. The two layers of the coating system had coefficients of thermal expansion (CTE) that were well matched to that of the substrate, while the YbDS has been reported to have a moderate resistance to silicon hydroxide vapor forming reactions in water vapor rich environments. Thermal cycling experiments were conducted between 110 °C and 1316 °C in a flowing 90 % H 2 O/10 % O 2 atmospheric pressure environment, and resulted in the formation of a thermally grown (silica) oxide (TGO) at the silicon-ytterbium disilicate interface. The TGO layer exhibited linear oxidation kinetics consistent with oxidizer diffusion through the ytterbium silicate layer controlling its thickening rate. The effective diffusion coefficient of the oxidizing species in the YbDS layer was estimated to be 2x10 -12 m 2 s -1 at 1316 o C. Slow steam volatilization of the YbDS topcoat resulted in the formation of a thin, partially protective, high CTE ytterbium monosilicate layer on the outside of the YbDS coating. Progressive edge delamination of the coating system was observed with steam exposure time, consistent with water vapor volatilization of the TGO edges that were directly exposed to the environment. This was aided by outward bending of the delaminated region to relax TGO and YbMS surface layer stresses developed during the cooling phase of each thermal cycle.

Yb2Si2O7 Environmental barrier coatings with reduced bond coat oxidation rates via chemical modifications for long life

“…Most of the current EBCs are variants of the second‐generation EBCs. Various aspects of current EBCs, including processing, volatility in water vapor, silica activity in Y 2 O 3 ‐SiO 2 and Yb 2 O 3 ‐SiO 2 systems, interactions with CMAS, stress analysis, fracture mechanism, impact damage, and oxidation, have been reported.…”

“…Since permeability = diffusivity x solubility, the permeability of H 2 O in SiO 2 is ~10 times larger than that of O 2 . In Si/Yb 2 Si 2 O 7 EBC water vapor permeates through Yb 2 Si 2 O 7 and reacts with the Si bond coat, forming a layer of SiO 2 scale, known as thermally grown oxide (TGO), at the Si/Yb 2 Si 2 O 7 interface . One of the most frequently observed EBC failure modes, identified in laboratory tests and confirmed in rig and engine tests, is spallation due to oxidation by water vapor (steam oxidation) .…”

“…The similar failure mode between the two coatings suggests that the strain‐energy induced TBC failure mechanism is likely applicable to EBC failure. Sources for strain energy due to the formation of SiO 2 TGO in EBCs include growth stress due to ~2.2x volume expansion during the conversion of Si to SiO 2 , phase transformation stress (in the case of crystalline SiO 2 TGO) due to β to α cristobalite transition at ~200°C which is accompanied by ~5% volume reduction, and thermal stress due to the mismatch in coefficient of thermal expansion (CTE) between α cristobalite (10.3 × 10 −6 /°C) and EBC (4~5 × 10 −6 /°C).…”

Crystallization kinetics for SiO2 formed during SiC fiber oxidation in steam

“…In fission power reactors, porous cristobalite scales that form on SiC cladding in high temperature steam degrade material performance . In SiC–SiC ceramic matrix composites (CMCs), the silicon bond coat for rare‐earth disilicate environmental barrier coatings (EBCs) oxidizes to cristobalite instead of SiO 2 glass at 1316°C in water vapor, causing delamination and EBC spallation . Crystallization of glass SiO 2 scales reduces SiC oxidation rates, but also causes tensile thermal stresses that degrade SiC fiber strength .…”

Stability of the Y2O3–SiO2 system in high‐temperature, high‐velocity water vapor

“…Environmental barrier coatings (EBCs) must be used to limit SiO 2 volatility, thereby increasing the life of CMCs . Rare earth (RE) disilicates (such as Y 2 Si 2 O 7 or Yb 2 Si 2 O 7 ) are proposed for EBCs . These disilicates are chemically compatible with the CMC, have a good coefficient of thermal expansion (CTE) match with SiC, and have lower volatility in combustion environments than the SiO 2 formed on SiC.…”