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Thermomechanical and thermochemical stability of hafnon (HfSiO 4 ) was evaluated for application as an environmental barrier coating (EBC) candidate for SiC/ SiC CMCs. High-temperature X-ray diffraction (XRD) analysis showed that hafnon has an excellent coefficient of thermal expansion (CTE) match with SiC as well as minimal CTE anisotropy, which supports hafnon as an EBC candidate. Alternatively, high-velocity water vapor testing at 1,200-1,400°C showed large amounts of silica depletion from chemical reaction at all velocities and excessive material erosion at steam velocities greater than 190 m/s. HfSiO 4 is therefore not suitable as an EBC for turbine applications due to insufficient thermochemical stability in water vaporcontaining combustion environments.