Effect of Packing Powder on Densification and Translucency of Pressureless-Sintered Si3N4 Ceramics

To clarify the effect of packing powder on densification and translucency of Si3N4 ceramics, a Si3N4 ceramic doped with Al2O3 and Y2O3 was fabricated by pressureless-sintering at 1650-1850°C under N2 atmosphere using a carbon heating-element furnace and a graphite crucible. Three different packing powders were used: BN+Al2O3, Si3N4+Al2O3 and (a raw powder for sintering)+BN. Density measurement, color of sintered body evaluation, optical absorption measurement from 200 to 2500nm of wavelength, X-ray diffraction analysis, oxygen content analysis, scanning electron misroscopy, and transmission electron microscopy coupled with energy dispersive X-ray spectroscopy were conducted. The specimen sintered with BN+Al2O3 packing powder at 1750°C kept its original oxygen content after sintering, and reaches almost full density. This specimen was more translucent than the others at its wavelength longer than 800nm. In the case of sintering with (a raw powder for sintering)+BN or without packing powder at 1750°C, the oxygen content in the sintered bodies decreased with the starting mixture and the color of sintered bodies changes to a more black one. The low translucency and black coloring could be attributed to pores and to a change in the microstructure or to Si, respectively. The reaction/decomposition of both Si3N4 and SiO2 was effectively suppressed by sintering using the BN+Al2O3 packing powder, without changing composition.