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 alpha -SiC/beta -SiC, AlN/Y2O3 ratios and additive content s were gas-pressure sintered to theoretical density. Post-sintering heat tr eatments were performed in order to induce phase transformations from beta -SiC to alpha -SiC and anisotropic grain growth, leading to platelet-streng thened microstructures. The indentation fracture toughness of the platelet materials reaches values in excess of 6 MPa m(1/2) after annealing at 1950 degreesC. Four-point bending strengths were measured at room temperature an d at high temperatures, and were correlated with microstructural data obtai ned by scanning electron microscopy and X-ray powder diffraction. Both the kinetics of the phase transformation/platelet formation and the strength re tention at high temperature were found to exhibit maxima as a function of t he AlN/Y2O3 ratio in the sintering additive. (C) 2001 Elsevier Science B.V. All rights reserved.