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Engineering ceramics based on silicon nitride are well known as low-density materials with high strength and toughness. With the combination of these properties, silicon nitride ceramics are an ideal candidate for several structural and functional applications. However, because of the relatively high brittleness of ceramics there is a continual need to improve their mechanical characteristics. At the same time, intensive research is being performed to improve the thermal and electrical properties of ceramics. In general, there are two ways to improve the mechanical properties of ceramics: controlling the microstructure, and preparation of the composite.

In connection to the microstructure–property relationship, new observations have been performed on structural and morphological development of silicon nitride ceramics. The development of an interlocking microstructure of elongated grains was found to be vital to ensure that this family of ceramics has good damage tolerance. A fast (within minutes) in situ formation of a tough microstructure has been observed by a so-called dynamic Ostwald ripening process resulting from the rapid heating rate. In this way, through formation of a tough interlocking microstructure, mechanical properties may be improved.

On the other hand, physical and mechanical properties of ceramics can be improved through nanocomposite processing. To increase the fracture toughness, various energy-dissipating components have been incorporated into ceramic matrices. These secondary constituents can be introduced in whisker, platelet, particle or fiber forms. Depending on processing route, micro/nano or nano/nano type microstructures can be synthesized, and silicon nitride–silicon carbide nanocomposites have been developed, which retained high strength and good oxidation resistance up to 1400 C. Moreover, through nanocomposite processing not only the high-temperature properties may be improved, but high-performance structural materials can be synthesized. From the literature, several methods are known for porous ceramic preparation, such as partial sintering or using sintering additives with high melting points to hinder the sintering process. It has been reported that by the addition of small amounts of carbon, high-porosity silicon nitride composites with low shrinkage have been realized. This process involved the formation of SiC particles at the grain boundaries, which inhibited the rearrangement of Si₃N₄ particles during sintering, assuring high porosity.

Sanjay K. Mazumdar, Ph.D. COMPOSITES MANUFACTURING

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