Grain Growth in Si 3 N 4 -Based Materials
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MRS
BULLETIN/FEBRUARY 1995
In this article, the grain-growth problems in Si3N4-based materials are basically viewed as the growth of faceted grains in a liquid matrix and divided into three parts. First, the formation and growth of an individual Si3N4 grain are described by considering the nucleation site of the grain and the variation of chemical composition observed within each grain. The kinetics and mechanism of normal grain growth are then examined in view of the
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Figure 1. Two types of microstructures observed during sintering of {S-SiAION: (a) unimodal and (b) bimodal structure.7
growth behavior of faceted crystals in a matrix. Finally, the causes for the development of a duplex structure are examined on the basis of the experimental conditions for abnormal grain growth of faceted crystals.
Nucleation and Grain Growth During the sintering of a powder compact of Si3N4 and additives, an oxynitride liquid forms during heating to sintering temperature. When additives are soluble in Si3N4,a9 the nuclei of a new phase containing solute elements, such as /3-SiAlON or a-SiAlON, which may be thermodynamically in equilibrium with the oxynitride liquid, must form. In particular, when a//3-phase transformation occurs during sintering, nuclei of a new phase must first form and then grow. The nucleation site of a newly formed phase has been a major subject of investigations because it must considerably affect the growth of grains and overall microstructure. The possible nucleation sites are preexisting Si3N4 particles and bulk oxynitride liquid matrix. In early investigations,10'" all nuclei of a new phase were simply believed to form within a liquid matrix. In recent investigations,1"8 however, core structures—/3-Si3N4 particles within /3-SiAlON grains, a-Si3N4 particles within a-SiAlON grains, and a-Si3N4 particles within /3-SiAlON grains—have been observed. These core structures suggest that the preferential nucleation sites can be the preexisting solid particles. The nucleation of /3- or a-SiAlON on a- or /3Si3N4, respectively, appears prevalent, most likely because of a similar crystalline structure of the nucleating phase with that of preexisting particles. When the chemical composition of newly formed SiAlON grains differed little from that of Si3N4 particles (for example, z < 0.5 for pSi6-ZA12O2N8.2 on /3-Si3N4), few defects were observed at the interface.14 In case of a large difference in chemical composition between the two phases, however, many dislocations were observed near the interface.12'16'17 In contrast to prevalent observations of core structures of a similar crystalline phase, the observation of core structures of a dissimilar phase is uncommon. Two recent transmission electron microscopy (TEM) works showed a-Si3N4 cores within jS-SiAlON grains;17J8 however, the examples were limited to a few in a large number of grains. If the preexisting particles had been usual nucleation sites of a dissimilar phase, many grains would have contained cores of a dissimilar phase, in contrast to the observati
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