High temperature mechanical behavior of silicon nitride ceramics

The temperature dependences of Young's modulus (E) and fracture toughness ( K-1c), as well as the flow behavior, including stress relaxation, creep and superplasticity of several silicon nitride-based materials, monoliths and composites, are reviewed. A transition range between a low softening rate a nd a higher one, which coincides with the onset of creep ductility, is obse rved between 1080 and 1150 degreesC on the E(T) curves and is attributed to the behavior of the secondary glassy phases. The higher the Y/Al ratio or the SiC content, the higher the transition temperature. The K-1c(T) curves exhibit four different stages which were discussed and interpreted through a theoretical analysis, warning against the frequent confusion between the intrinsic and the apparent (experimentally accessible) toughness. Reliable creep resistant- or inversely superplastic-ceramics are now available. Howe ver, the high temperature deformation mechanisms are not well understood ye t. Non-Newtonian flow regimes and the pronounced tension/compression flow a symmetry are still intriguing. Today, it can be anticipated that with the d evelopment of materials containing low amounts of highly refractory grain b oundary phases, ceramists are facing a situation that places them closer to metallurgists, and which should allow them to derive more benefit from the exceptional intrinsic properties of covalent crystals such as Si3N4, Conse quently, grains should play a more and more important role on the high temp erature mechanical behavior.