Microstructure and mechanical properties of superplastically deformed silicon nitride-silicon oxynitride in situ composites

Silicon nitride-silicon oxynitride in situ composites were fabricated by plane-strain-compressing dense silicon nitrides, starting from 93 wt.% ultrafine beta-Si3N4 and 7 wt.% cordierite, at 1600 degreesC under a constant load of 40 MPa and subsequent annealing at 1750 degreesC for 30 min. The resulting composites featured a microstructure of elongated Si2N2O grains (similar to0.64 mum in diameter and similar to5.5 in aspect ratio) dispersed in a fine-grained beta-Si3N4 matrix (similar to0.30mum in diameter and similar to3.5 in aspect ratio), with the amount of Si2N2O, which had relatively strong textures, being strain-dependent. The mechanical properties were found to be improved due to the development of elongated Si2N2O grains, the texture formation, and the coarsening of beta-Si3N4. Fracture toughness, however, was still low (similar to5.2 MPa m(1/2)) for these composites in comparison to self-reinforced silicon nitrides, resulted from the strong Si2N2O-matrix interfacial bond and nearly equiaxed beta-Si3N4 with a small grain size. Anticipated property anisotropies were clearly observed as a result of the textured microstructure. (C) 2002 Elsevier Science Ltd. All rights reserved.