Thermodynamic and Microstructural Studies of Coloration and Decoloration Processes During Gas-Pressure Sintering of Silicon Nitride Ceramics

The interaction between the gas atmosphere and silicon nitride ceramics during gas-pressure sintering leads to an inhomogeneous coloration of the area near the surface. The coloration and decoloration mechanisms, responsible for the optical inhomogeneity, can be revealed through idealized tempering experiments. By varying the holding time and the tempering pressure, the coloration process which leads to the dark-grey-colored near-surface area can be explained thermodynamically. By adding a holding time to the cooling period of a tempering experiment, a coupled reaction-diffusion mechanism, which offers a good explanation for the outer bright-grey-colored area, can be identified. By means of intensive TEM studies a physical explanation for the dark coloration can be given.