Liquid phase sintering and microstructure–property relationships of silicon carbide ceramics with oxynitride additives

SiC ceramics incorporating sintering additives from the system AlN–Y2O3 and having variable α-SiC/β-SiC, AlN/Y2O3 ratios and additive contents were gas-pressure sintered to theoretical density. Post-sintering heat treatments were performed in order to induce phase transformations from β-SiC to α-SiC and anisotropic grain growth, leading to platelet-strengthened microstructures. The indentation fracture toughness of the platelet materials reaches values in excess of 6 MPa m1/2 after annealing at 1950°C. Four-point bending strengths were measured at room temperature and at high temperatures, and were correlated with microstructural data obtained by scanning electron microscopy and X-ray powder diffraction. Both the kinetics of the phase transformation/platelet formation and the strength retention at high temperature were found to exhibit maxima as a function of the AlN/Y2O3 ratio in the sintering additive.