High-temperature strength of a thermally conductive silicon carbide ceramic sintered with yttria and scandia

The high-temperature strength of a thermally conductive SiC ceramic sintered with 1 vol% equimolar Y2O3–Sc2O3 additives (thermal conductivity = 234 W (m K)−1) was investigated at temperatures up to 1800 °C. Observation of the ceramic using high resolution transmission electron microscopy (HRTEM) exhibited both clean and crystallized SiC/SiC boundaries, as well as clean SiC/junction phase boundaries with a fully crystallized junction phase. No microstructural or polytype changes after flexural testing at 1800 °C were observed using scanning electron microscopy and phase analysis with the Rietveld method. The ceramic maintained 93% of its room temperature (RT) strength up to 1600 °C, and showed rapid degradation at 1700 °C and 1800 °C. Degradation at temperatures above 1600 °C was due to softening of the grain boundary phase, as evidenced by the nonlinear behavior of load-displacement curves. Flexural strengths of the highly thermally conductive SiC ceramic at RT and 1800 °C were 536 MPa and 345 MPa, respectively.