MICROSTRUCTURE EVOLUTION AND MECHANICAL STRENGTH OF SILICON-NITRIDE CERAMICS

Microstructure evolution was studied in silicon nitride ceramics by a novel characterization method, and its relevance to the strength was d iscussed. The characterization method involves an immersion liquid for making green and partially sintered bodies transparent, and a subsequ ent direct optical microscopic examination. Granules for compaction pr ocess were prepared with the spray-drying process and were found to co ntain pores or deep dimples. Green bodies formed by CIP with these gra nules contain regularly arrayed pores at the center of granules and al so crack-like voids at the boundaries of granules. These pores were pr eserved in the sintering process and resulted in large pores in the si ntered body. They behave as fracture origin in ceramics and reduce the fracture strength. The Weibull modulus was high due to the presence o f uniformly distributed pores.