Colour of gas-pressure-sintered silicon nitride ceramicsPart II. Thermodynamic considerations

Gas-pressure sintering is the most widely used process for manufacturing high-performance silicon nitride ceramics. Generally gas-pressure sintering of silicon nitride is conducted under reducing atmospheric conditions due to the use of graphite heaters. The interaction between the sintering atmosphere and the silicon nitride ceramics is often featured by an inhomogeneous grey coloration of the near surface area and a grey coloration of the bulk material. In the first part of this paper [Herrmann, M. and Goeb, O., Colour of gas-pressure-sintered silicon nitride ceramics Part I. Experimental data J. Eur. Ceram. Soc., 2001, 21(3), 303–314] the dependence of the colour of the material on the sintering parameters was investigated experimentally. It was shown that silicon inclusions, having sizes ranging from a few nm up to several μm, cause the coloration of the material. Gas pressure, temperature and sintering time influence the formation of the inhomogeneous grey coloration. In this second part, the dependence of the coloration in silicon nitride ceramics on the sintering parameters will be explained on the bases of thermodynamic considerations. The model developed explains the possibilities of governing the colour formation of silicon nitride materials.