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 discussed. The characterization method involves an immersion liquid for making green and partially sintered bodies transparent, and a subsequent direct optical microscopic examination. Granules for compaction process were prepared with the spray-drying process and were found to contain pores or deep dimples. Green bodies formed by CIP with these granules contain regularly arrayed pores at the center of granules and also crack-like voids at the boundaries of granules. These pores were preserved in the sintering process and resulted in large pores in the sintered body. They behave as fracture origin in ceramics and reduce the fracture strength. The Weibull modulus was high due to the presence of uniformly distributed pores.