| A complete study on sintering and grain growth has been conducted in the two silicon nitride systems: SiAlON-YAG (U;Excellent sinterability has been demonstrated in both SiAlON-YAG and SiAlON-cordierite systems, especially in the former system. Full densification can be attained at 1500;Phase transformation, which is always completed behind densification, is not entirely required to achieve full density. The lag between the two reactions depends strongly on the liquid content and the liquid viscosity in the system. Because of the solution reprecipitation characteristics, phase transformation can lead microstructural refinement in the early stage of grain growth. The extent of refinement, however, is very sensitive to the ;A growth exponent, n, of 3 has been observed for the growth in (001) (length) and (210) (width) directions of the elongated grains. The growth rate constants measured in the length direction is about two order of magnitude higher than that in the width direction. This is the first result which quantifies the growth anisotropy. The growth anisotropy is believed to be introduced by the significant differences in structure and solute diffusivity at the growth front in the two directions.;The aspect ratio of ;Solute diffusion in the liquid is the rate controlling mechanism during sintering of silicon nitride ceramics. The conclusion is supported by the kinetic characteristics, the measured activation energies (586-753 KJ/mol), and repeated consistent correlation with the model prediction based on diffusion mechanism. (Abstract shortened with permission of author.)... |
