Pulsed electric current sintering of silicon nitride

Pulsed electric current sintering (PECS) has been used to density alpha-(SiN4)-N-3 powder doped with oxide additives of Y2O3 and Al2O3. A full density (>99%) was achieved with virtually no transformation to beta-phase, resulting in a microstructure with fine equiaxed grains. With further holding at the sintering temperature, the alpha-to-beta phase transformation took place, concurrent with an exaggerated grain growth of a limited number of elongated beta-grains in a fine-grained matrix, leading to a distinct bimodal grain size distribution. The average grain size was found to obey a cubic growth law, indicating that the growth is diffusion-controlled. In contrast, the densification by hot pressing was accompanied by a significant degree of the phase transformation, and the subsequent grain growth gave a broad normal size distribution. The apparent activation energy for the phase transformation was as high as 1000 kJ/mol for PECS, almost twice the value for hot pressing (similar to500 kJ/mol), thereby causing the retention of alpha-phase during the densification by PECS.