Toughening of reaction-bonded silicon nitride ceramics through microstructural design

The mechanical properties of silicon nitride ceramics (Si3N4) depend significantly on their microstructures. In this work, nitridation of silicon compacts (with 20 vol% SiC added as a second phase), containing 0, 1, 3, and 5 vol% respectively of an in-house prepared beta-Si3N4 seed particles of varying morphologies, was carried out at 1370degreesC for 20 hours. The nitridation product, a porous reaction-bonded silicon nitride-silicon carbide composite, was post-sintered in a gas pressure sintering furnace. Analysis of the post-sintered samples showed a bi-modal microstructure. The added beta-Si3N4 seed particles served as a nuclei for the epitaxial growth of large and elongated grains whilst another generation of beta-Si3N4 with relatively small grains developed from the alpha-beta transformation of silicon nitride during the liquid phase sintering process. Further microstructural analysis indicated that equiaxed seed particles gave rise to larger equiaxed grains whilst elongated beta-Si3N4 seed particles gave rise to larger elongated grains. In both cases, the grains showed a core- rim structure. Samples containing Si3N4 seed particles with the largest aspect ratio gave the best nitridation results. Evaluation of mechanical properties of post-sintered samples showed that those containing 3 vol% seed material had better properties than those that contained 1 or 5 vol%. Fracture toughness (K-IC) of about 8.5 MPa.m(1/2), an improvement of nearly 35% above the value for the sample without seed material used as the datum level for comparing results, was measured for samples that contained 3 vol% seed material. In the same samples, flexural strength of similar to 1000 MPa was obtained.