Microstructure evolution and mechanical strength of silicon nitride ceramics

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H. Takahashi, K. Ishikawa, N. Shinohara, and M. Okumiya Asahi Glass Co., Hazawa, Kanagawaku, Yokohama, Japan

T. Yamada Ube Industries, Ube, Japan

H. Kamiya Department of Chemical Engineering, Nagoya University, Nagoya, Japan K. Uematsua> Department of Chemistry, Nagaoka University of Technology, Nagaoka, Japan (Received 10 October 1993; accepted 12 January 1994)

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.

I. INTRODUCTION Defects are present in virtually all ceramics and govern their various properties, such as fracture strength.1 Reduction in the concentration and size of defects is very important for the improvement of strength and reliability of high performance ceramics.2 To achieve this, it is necessary to fully understand the behavior of microstructure evolution during densification with particular focus on the behavior of large processing defects. Behavior of large processing pores during densification has been a subject of both theoretical and experimental studies. Thermodynamic argument showed that pores much larger than the surrounding grain are stable and grow during densification.3'4 This argument was recently confirmed in alumina ceramics that were densified through the solid-state sintering process.5"7 Although the same argument is expected to hold for the liquid phase sintering, experimental confirmation has been absent. It should be very important to directly examine the behavior of processing pores in this sintering process. Eventually all important high strength ceramics a) Address

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all correspondence to this author. J. Mater. Res., Vol. 9, No. 5, May 1994

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as well as functional ceramics are densified through the liquid phase sintering process.8 This paper presents a direct observation on the behavior of processing pores during the liquid phase sintering in silicon nitride ceramics. This is representative of high performance ceramics, and the formation mechanism of fracture origin is of great practical interest. Following previous studies,9"12 the liquid immersion method was applied for characterizing pores in both green and partially sintered bodies. With this method, the specimens were made transparent with the immersion