Preparation and Densification of Nitrogen Ceramics from Oxides

Abstract

A method of preparation of silicon nitride from silica by carbothermal reduction in nitrogen gas was first published in 1925 but received little further attention until the upsurge of interest in nitrogen ceramics in the 1970s. Several methods of formation of silicon nitride and sialons from oxides have now been patented and the carbothermal reduction process is a viable commercial alternative for the manufacture of a wide range of powders of different grades of purity. It is particularly suited to the production of low-grade sialons from clays and other naturally occurring raw materials.

The present paper describes the principles of the reduction and nitriding reactions and the thermodynamic basis for prediction of phase stabilities. Examples are given of the role of impurities on the reactions to form nitrides from oxides and of their effect on carbide formation. Methods of preparation of β’-sialon and of silicon oxynitride are described and the assessment of extent of reaction by measurement of carbon monoxide evolution is discussed.

Densification of nitrogen ceramics by reaction-sintering of the component compounds is well established but the densification of sialon powders prepared by carbothermal reduction has not been investigated. An example is given of the sintering of silicon oxynitride and the role of impurities inherited from the powder preparation is described.