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 β-Si3N4 and 7 wt.% cordierite, at 1600 °C under a constant load of 40 MPa and subsequent annealing at 1750 °C for 30 min. The resulting composites featured a microstructure of elongated Si2N2O grains (∼0.64 μm in diameter and ∼5.5 in aspect ratio) dispersed in a fine-grained β-Si3N4 matrix (∼ 0.30μm in diameter and ∼3.5 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 β-Si3N4. Fracture toughness, however, was still low (∼5.2 MPa m1/2) for these composites in comparison to self-reinforced silicon nitrides, resulted from the strong Si2N2O-matrix interfacial bond and nearly equiaxed β-Si3N4 with a small grain size. Anticipated property anisotropies were clearly observed as a result of the textured microstructure.