Evolution of microstructure and thermal conductivity of multifunctional environmental barrier coating systems

Environmental barrier coating (EBC) systems are applied to the surface of silicon-based composites exposed to high temperature combustion gas flow paths in gas turbine engines. They reduce the rate of composite oxidation, its volatilization by reactions with water vapor, and the temperature of the composite as their thermal conductivity decreases. Current EBC systems consist of a silicon bond coat covered by an ytterbium disilicate (Yb2O3⋅2SiO2: YbDS) permeation resistant and low silica activity barrier. When applied by atmospheric-plasma spray deposition, this outer layer contains 10–15% of a secondary ytterbium monosilicate (Yb2O3⋅3SiO2: YbMS) phase. YbDS has a coefficient of thermal expansion (CTE) of 4–5×10−6 °C−1, similar to those of the silicon and the silicon-based composite, but a relatively high thermal conductivity of 5–7 W m−1 K−1. YbMS has a higher steam volatility resistance than YbDS, and it has a much lower thermal conductivity (∼2–2.5 W m−1 K−1) at ambient temperature compared to YbDS, but its higher, highly anisotropic CTE (3–11×10−6 °C−1) results in channel cracking which reduces environmental protection. Here, we use a combination of scanning electron beam and laser-based thermoreflectance methods to spatially map the distribution of the silicate phases and thermal conductivity at ambient temperature in a Si-ytterbium disilicate EBC system exposed to thermal cycling in water vapor. We show that during thermal cycling, diffusion of silica from the thermally grown oxide on the Si bond coat surface to nearby YbMS regions transforms this phase to YbDS, thereby reducing the risk of thermomechanical coating failure but decreasing its effective thermal resistance. We also show that as silica is volatilized at the water vapor–ytterbium silicate interface, YbDS is transformed to YbMS, restoring some of the thermal protection of the coating system lost by its reduction in thickness and the YbMS to YbDS transformation near the bond coat.