Microstructure in silicon nitride containing beta-phase seeding: Part I

A proper powder preparation technique was used to develop elongated beta-Si 3N4, particles as seeds from raw materials. The phase transformation and de velopment of microstructure in Si3N4 ceramics containing beta-Si3N4 seeds w ere investigated. The specimens of seeded silicon nitride had higher phase transformation rates than the specimens without seed. A core/shell structur e was observed by transmission electron microscopy in seeded specimens owin g to the difference in aluminum concentration in Si3N4 grains. The misfit b etween the core and the shell was accommodated by interfacial dislocation t hat has a rotation character. The growth mode was epitaxial, although there was some compositional difference between the core and the shell. A relati vely larger grain size and wider,grain size distribution in seeded Si3N4 sp ecimens was due to the amount of beta-phase seeds that could act as the nuc lei, and the final modification of the microstructure was due to the coales cence process. The crack wake process characterized the mechanism of toughe ning.