FABRICATION OF SILICON-NITRIDE SILICON OXYNITRIDE IN-SITU COMPOSITES

Fine-grained beta-Si3N4 ceramics were fabricated fr om fine and unifor m beta-powder by hot-pressing at 1700-1750 degrees C using different s intering aids (cordierite, Y2O3-MgO, and Y2O3-Al2O3) The microstructur al development of hoc-pressed bodies was studied in annealing at 1750 degrees C for 0.5 to 8h. When Y2O3-MgO and Y2O3-Al2O3 were used as sin tering aids, fine and uniform microstructures were maintained because of the low driving force for grain growth of beta-Si3N4 When cordierit e was used as an aid, beta-Si3N4 reacted with the SiO2 in the additive and developed a bimodal microstructure with large Si2N2O grains in fi ne beta-Si3N4 matrix grains after annealing. Local segregation of beta -Si3N4 grains was observed after hot-pressing. Nucleation of Si2N2O oc curred at a part of the segregation surface in the early stage of anne aling. Si2N2O grains grew selectively with annealing time, resulting i n improved fracture toughness. (C) 1998 Elsevier Science Limited. All rights reserved.