High-temperature effects in the fracture mechanical behaviour of silicon carbide liquid-phase sintered with AlN-Y2O3 additives

It has been shown that pressureless sintering of SiC to theoretical density is possible with sintering additives from the system AlN-Y(2)O(3). While commonly a combination of oxides is used such as Al(2)O(3)-Y(2)O(3) (-SiO(2)), the oxynitride additives offer the advantage that only a nitrogen atmosphere is required instead of a powder bed for thermochemical stabilisation at the sintering temperature. The thermal decomposition of AIN is suppressed quits effectively when a moderate nitrogen overpressure is applied. resulting in very small mass loss during densification which only depends on the oxygen content of the SIC starting pou der. By varying the mass ratio of beta -SiC to alpha -SiC and applying dedicated post-densification heat treatments. a platelet-strengthened microstructure is obtained which shows enhanced fracture toughness. The platelet formation is attributed to a solution / precipitation process with simultaneous phase transformation from beta -SiC to alpha -SiC. followed by anisotropic grain growth of alpha -SiC. In the present work. recent progress in the mechanical properties of these materials is reported. By means of a simple surface treatment-annealing in air - it is possible to obtain four-point bending strengths in excess of 1 GPa in liquid phase sintered SiC. The strength retention at temperatures around 1200 degreesC is significantly improved. (C) 2001 Elsevier Science Ltd. All rights reserved.