Thermal Properties of Rare‐Earth Monosilicates for EBC on Si‐Based Ceramic Composites

Rare‐earth ( RE ) monosilicates are promising candidates as environmental barrier coating ( EBC ) materials for ceramic matrix composites for aerospace applications. Five rare‐earth monosilicate materials have been investigated: Y 2 SiO 5 , Gd 2 SiO 5 , Er 2 SiO 5 , Yb 2 SiO 5 , and Lu 2 SiO 5 produced from RE oxides and silica starting materials pressed and sintered at 1580°C under flowing air. Relative densities above 94% were obtained for all samples and ceramics were made containing 85–100 wt% of the RE monosilicate according to X‐ray diffraction ( XRD ) with RE disilicates as the second phase in the Gd, Yb, and Lu silicate systems. Microstructures were characterized using scanning electron microscopy and XRD , and thermal properties measured including specific heat, thermal expansion, and thermal diffusivity. For the first time, specific heat capacity values are reported for the monosilicates [0.45–0.69 J·(g·K) −1 ]. Thermal expansion coefficients ( TEC s) of the dense samples ranged between 5.9 and 10.3 × 10 −6 K −1 measured for 473 to 1473 K. All EBC s have low thermal conductivities [1.8 W·(m·K) −1 or less] making them excellent EBC insulators.