Fabrication of silicon nitride silicon oxynitride in-situ composites

Fine-grained beta-Si3N4 ceramics were fabricated fr om fine and uniform beta-powder by hot-pressing at 1700-1750 degrees C using different sintering aids (cordierite, Y2O3-MgO, and Y2O3-Al2O3) The microstructural 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 sintering aids, fine and uniform microstructures were maintained because of the low driving force for grain growth of beta-Si3N4 When cordierite was used as an aid, beta-Si3N4 reacted with the SiO2 in the additive and developed a bimodal microstructure with large Si2N2O grains in fine beta-Si3N4 matrix grains after annealing. Local segregation of beta-Si3N4 grains was observed after hot-pressing. Nucleation of Si2N2O occurred at a part of the segregation surface in the early stage of annealing. Si2N2O grains grew selectively with annealing time, resulting in improved fracture toughness. (C) 1998 Elsevier Science Limited. All rights reserved.