Some aspects of processing, microstructure and properties of the various types of silicon nitride are discussed. Special emphasis is placed on the relationships between powder properties, process conditions, densification and microstructure, as well as the interdependence between microstructure and properties. After summarizing the areas of crystal structure and thermodynamic properties, and processing of the different types of Si3N4, the state-of-the-art of dense and reaction-bonded silicon nitride is given. For both types the formation mechanisms and microstructure, relationships between powder properties, additives (in the case of dense Si3N4), process conditions, and densification and microstructure, as well as data and microstructural effects of various mechanical, thermal and thermo-mechanical properties, are outlined. Advanced processing techniques, such as sintering, gas-pressure sintering, post-sintering, and the different routes of hot-isostatic pressing (starting with powder compacts, reaction-bonded Si3N4 or pre-sintered Si3N4 and the resulting properties, are discussed.

Relationships between processing, microstructure and properties of dense and reaction-bonded silicon nitride

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

Mechanical Properties of Sintered, In Situ-Reacted Mullite-Zirconia Composites

Silicon nitride ceramics is a generic term for a variety of alloys of Si3N4 with additional compounds necessary for a complete densification of the Si3N4 starting powder. They are heterogeneous, multicomponent materials characterised by the inherent properties of the crystalline modifications α and β of Si3N4 and the significant influence of the densification additives. With a view to ability of the α and β modification to form solid solutions α-Si3N4 (αss) and β-Si3N4 (βss) solid solutions can be distinguished. Each group contains engineered materials with interesting properties for special applications. Phase relations and micro-structures determine the properties decisively. Composition of the phases, the distribution of the grains, their aspect ratio and the grain boundary phase are pronounced microstructural features. The formation of the microstructure strongly depends on the one hand on the quality of the Si3N4 starting powders, which closely is related to the chemistry of the production process, and on the other on the liquid phase sintering as the most important step in the densification route. The interrelation between pure Si3N4, the densification of the powder including the role of sintering additives, microstructural engineering, physicochemical properties of the sintered Si3N4 ceramics (SSN, GPSN, HPSN, HIP-SSN, HIP-SN) are described in more detail and compared to reaction bonded Si3N4 ceramics (RBSN), which are produced by nitridation of silicon powders.

Silicon Nitride Ceramics

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

The kinetics of densification and the kinetics of theα-β phase transformation have been measured during the hot-pressing of silicon nitride ceramics using magnesia as additive. Two mechanisms of densification have been identified. The first is a very rapid particle rearrangement, liquid-enhanced above 1550° C, which operates up to relative densities of about 0.65. The kinetics of the much slower decelerating second stage obey the Coble hot-pressing equation and the rate of densification is found to be proportional to the amount of additive. The controlling mechanism is believed to be diffusion in a boundary second phase, and values for the diffusion coefficient,D
b, of the rate-controlling species in the boundary phase for temperatures above and below 1550° C are given. The kinetics of theα toβ transformation, the greater part of which occurs after densification is complete, are described by a first order reaction; the dependence of rate on the quantity of additive and on temperature is similar to that found for densification, and a similar controlling mechanism is believed to be responsible for the two processes.

Hot-pressing and the ?-? phase transformation in silicon nitride

The rates of densification and phase transformation undergone by α-Si3N4 during hot-pressing in the presence of Y2O3, Y2O3−2SiO2, and Li20−2Si02 as additives were studied. Although these systems behave less simply than MgO-doped Si3N4, the data can be interpreted during the early stages of hot-pressing as resulting from a solution-diffusion-reprecipitation mechanism, where the diffusion step is rate controlling and where the reprecipitation step invariably results in the formation of the β-Si3N4 phase.

Hot‐Pressing of Si3N4 with Y2O3 and Li2O as Additives

Measurements have been made of the concurrent rates of reaction and of densification during the hot-pressing of mixtures of alumina and zircon powders. The results can be interpreted in terms of a liquid-phase sintering model and evidence is presented for separable stages of particle rearrangement, diffusion-controlled densification and residual reaction subsequent to attainment of zero porosity.

T. G. Carruthers

Papers

Hot-pressing and the ?-? phase transformation in silicon nitride

Hot‐Pressing of Si3N4 with Y2O3 and Li2O as Additives

Reaction hot-pressing of zircon-alumina mixtures

Identification of Stages in Reactive Hot-Pressing

Development of mechanical strength in hot-pressed silicon nitride