Toughening of reaction-bonded silicon nitride ceramics through microstructural design

摘要

The mechanical properties of silicon nitride ceramics (Si{sub}3N{sub}4) 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 β-Si{sub}3N{sub}4 seed particles of varying morphologies, was carried out at 1370°C 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 β-Si{sub}3N{sub}4 seed particles served as a nuclei for the epitaxial growth of large and elongated grains whilst another generation of β-Si{sub}3N{sub}4 with relatively small grains developed from the α-β 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 β-Si{sub}3N{sub}4 seed particles gave rise to larger elongated grains. In both cases, the grains showed a core- rim structure. Samples containing Si{sub}3N{sub}4 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{sub}(IC)) of about 8.5 Mpa·m{sup}(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 ～1000 MPa was obtained.