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Microstructural Design and Control of Silicon Nitride Ceramics Mamoru Mitomo, Naoto Hirosaki, and Hideki Hirotsuru Introduction The improvement of mechanical properties by microstructural control has been one of the main topics of interest in the development of silicon nitride ceramics.M Toughening, by developing an in situ composite or self-reinforced microstructure, has attracted particular attention.*~7 Microstructural design is a key factor in the optimization of processing parameters. The microstructures of sintered materials are composed of silicon nitride grains and grain boundaries, which can be either crystalline, amorphous, or partially crystalline, depending on the composition, amount of sintering additives, and processing parameters. Silicon nitride ceramics have been fabricated with an addition of metal oxides and rare-earth oxides that form a liquid phase during sintering and accelerate grain boundary diffusion. The effect of composition of the glassy phase on the mechanical properties of ceramics is presented by Becher et al. and Hoffmann elsewhere in this issue. This article focuses specifically on the design and control of grain size. As it is well recognized, many processing parameters affect grain growth behavior and the resulting microstructure. During sintering, the a- to /3-phase transformation leads to a self-reinforcing microstructure on account of the anisotropic grain growth of the stable hexagonal /8Si3N4 phase. Therefore, a-rich powders are widely used for starting materials. Phase transformation accelerates anisotropic grain growth, resulting in an increase in the fracture toughness of Si3N4 ceramics.8'9 Kang and Han discuss the effect of phase transformation on nucleation and grain growth in an article in this is-

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sue. The effect of the grain-size distribution on microstructural development is described in this article, based on studies conducted mostly with /3-Si3N4 powders.

Microstructural Design Uniform Microstructures As is well-known, abnormal grain growth must be prevented in order to fabricate high-density ceramics. Abnormal grain growth usually accompanies pore growth around large grains. During sintering, additives accelerate the densification rate relative to grain growth rate.10 Therefore, sintering science and techniques have largely focused on the development of fine and uniform microstructures. Silicon nitride ceramics generally fracture due to crack growth at grain boundaries. Materials with uniform microstructures have a low and constant fracture toughness over a wide range of crack sizes. Material strength is given by the Griffith's equation:

•y

(1)

where Kic, Y, and c are fracture toughness, a numerical constant, and flaw size, respectively. The strength of ceramics is inversely proportional to the square root of the flaw size. This equation shows an important characteristic of brittle fracture: that strength is very sensitive to the flaw size or crack growth. Ceramics governed by this equation are called Griffith materials. Griffith materials with grain size