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: Y2SiO5, Gd2SiO5, Er2SiO5, Yb2SiO5, and Lu2SiO5 produced from RE oxides and silica starting materials pressed and sintered at 1580 degrees 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(gK)(-1)]. Thermal expansion coefficients (TECs) of the dense samples ranged between 5.9 and 10.3 x 10(-6) K-1 measured for 473 to 1473 K. All EBCs have low thermal conductivities [1.8 W(mK)(-1) or less] making them excellent EBC insulators.