Thermodynamics of Zr/Hf-mixed silicates as a potential for environmental barrier coatings for Tyranno-hex materials

The synthesis and characterization of Zr/Hf-mixed silicates, which are promising as an environmental barrier coating (EBC) on Tyranno-hex materials, have been carried out. The coefficient of thermal expansion (CTE), the corrosion resistance against high- temperature water vapor, and the drops of strength and elastic modulus after exposure for high-temperature water vapor were investigated. Furthermore, the chemical stability of the Zr/Hf-mixed silicates under high-temperature water vapor and oxygen gas was thermodynamically investigated. The CTE value can change to (3.6-5.1) x 10(-6) K-1 when the Zr/Hf ratio fluctuates. The weight losses of the mixed silicate samples after exposure for high-temperature water vapor were very small. The contents of the main component elements, such as Zr, Hf, and Si in the samples, were unchanged within the error limits by the exposure. The flexural strength and bending modulus of the samples decreased by 5-21 and 6-29% due to the exposure, respectively. The chemical stability of the Zr/Hf-mixed silicates under the experimental conditions was discussed. (c) 2009 Elsevier B.V. All rights reserved