Colour of gas-pressure-sintered silicon nitride ceramics. Part 2 : Thermodynamic considerations

Gas-pressure sintering is the most widely used process for manufg. high-performance silicon nitride ceramics. Generally gas-pressure sintering of silicon nitride is conducted under reducing atm. conditions due to the use of graphite heaters. The interaction between the sintering atm. and the silicon nitride ceramics is often featured by an inhomogeneous gray coloration of the near surface area and a gray coloration of the bulk material. In the first part of this paper the dependence of the color of the material on the sintering parameters was investigated exptl. It was shown that silicon inclusions, having sizes ranging from a few nm up to several .mu.m, cause the coloration of the material. Gas pressure, temp. and sintering time influence the formation of the inhomogeneous gray 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 thermodn. considerations. The model developed explains the possibilities of controlling the color formation of silicon nitride materials.