MICROSTRUCTURE CONTROL OF SILICON-NITRIDE BY SEEDING WITH RODLIKE BETA-SILICON NITRIDE PARTICLES

The effect of seeding on microstructure development and mechanical properties of silicon nitride was investigated by the use of morphologically regulated rodlike beta-Si3N4 single-crystal particles with a diameter of 1 mum and a length of 4 mum as seed crystals. Silicon nitride with a bimodal microstructure was fabricated under a relatively low nitrogen gas pressure of 0.9 MPa owing to the epitaxial growth of beta-silicon nitride from the seed particles. Grain growth from seeds followed the empirical equation D(n) - D0n = kt, with growth exponents of 3 and 5 for the c-axis direction and the a-axis direction, respectively, being analogous to the kinetics of matrix grain growth. By seeding morphologically regulated particles, fracture toughness of silicon nitride was improved from 6.3 to 8.4-8.7 MPa . m1/2, retaining high strength levels of about 1 GPa.