Silicon nitride has been researched intensively, largely in response to the challenge to develop internal combustion engines with hot-zone components made entirely from ceramics. The ceramic engine programs have had only partial success, but this research effort has succeeded in generating a degree of understanding of silicon nitride and of its processing and properties, which in many respects is more advanced than of more widely used technical ceramics. This review examines from the historical standpoint the development of silicon nitride and of its processing into a range of high-grade ceramic materials. The development of understanding of microstructure–property relationships in the silicon nitride materials is also surveyed. Because silicon nitride has close relationships with the SiAlON group of materials, it is impossible to discuss the one without some reference to the other, and a brief mention of the development of the SiAlONs is included for completeness.

Silicon Nitride and Related Materials

Structural ceramic nanocomposites are reviewed with emphasis on the Al 2 O 3 SiC and Si 3 N 4 SiC systems. The incorporation of a nanosized second phase, such as SiC, into a ceramic matrix can lead to an improvement in mechanical properties. It is still unclear, however, whether those improvements can directly be related to an intrinsic ‘nanocompossite effect’ or to other factors. This review is divided into three parts. First, basic processing routes for nanocomposites, namely conventional powder processing, sol-gel processing and polymer pyrolysis are presented in detail. Second, the mechanical properties of different nanocomposites are compared. Finally, models which attempt to explain the improvements in these properties are explored. It will be shown that the strength increase can best be related to a reduction in processing defect size. For applications the most interesting properties of nanocomposites are their wear, creep and high temperature performance.

Structural ceramic nanocomposites

The theme of the review is the construction of a model embracing the mechanism of formation of reaction-bonded silicon nitride, the development of microstructure and mechanical properties. Possible nitridation reactions are discussed, with emphasis on kinetics and on phase composition and microstructure of the reaction product. The influence of Fe, a common impurity in silicon powders, and of H2, as an additive to the nitriding atmosphere, is considered in some detail. The optical, electrical and thermal properties are briefly discussed and areas for further research and development studies identified.

Reaction-bonded silicon nitride: its formation and properties

An attempt is made to rationalize various reaction mechanisms for the formation of Si3N4 from gaseous nitrogen and solid silicon It is suggested that a fundamental difference betweenα- andβ-Si3N4 is that the former results from silicon complexing with molecular nitrogen and the latter results from complexing with essentially atomic nitrogen Well established mechanisms (as well as controversial ones) are analysed in the light of this difference and are shown to be operative under various reaction conditions The influence of some common reaction variables are interpreted with respect to their influence on either silicon or nitrogen or both

On reactions between silicon and nitrogen

Silicon nitride (Si 3 N 4 ) based ceramics are gaining more and more attention due to their promising high-temperature thermal and mechanical properties. They have been expected to be the main candidates for applications such as turbocharger rotors and gas turbine engine components which can withstand severe conditions of temperature and heavy loads. Although a big number of studies on silicon nitride are published, a continuous progress in monolithic Si 3 N 4 as well as Si 3 N 4 /Si 3 N 4 composites (seeded materials) leads to new scientific and technological data providing new insight that should be reviewed taking into account their excellent properties at high temperatures. Silicon nitride possesses a bunch variety of interesting properties that can be specifically designed to produce a given behavior profile. That is why the room temperature and high-temperature properties are discussed and described in more detail.

Mechanical properties of silicon nitride-based ceramics and its use in structural applications at high temperatures

The literature on the α/β silicon nitride transformation is reviewed briefly. Data are presented on the kinetics of the tranformation of 1600° C on low and high purity silicon nitride powders. The addition of magnesia increased the rate of transformation while the addition of yttria had no effect. Scanning electron photomicrographs show clearly the morphology changes that accompany the transformation. It is concluded that the transformation occurs via a solution-precipitation mechanism and that α and β are probably low and high temperature forms of silicon nitride.

D. R. Messier

Papers

The α/β silicon nitride phase transformation