Effect of packing powder on densification and translucency of pressureless-sintered Si3N4 ceramics

To clarify the effect of packing powder on densification and translucency o f Si3N4 ceramics, a Si3N4 ceramic doped with Al2O3 and Y2O3 was fabricated by pressureless-sintering at 1650-1850 degreesC under Nz atmosphere using a carbon heating-element furnace and a graphite crucible. Three different pa cking powders were used: BN + Al2O3, Si3N4 + Al2O3 and (a raw powder for si ntering) + BN, Density measurement, color of sintered body evaluation, opti cal absorption measurement from 200 to 2500 nm of wavelength, X-rap diffrac tion analysis, oxygen content analysis, scanning electron microscopy, and t ransmission electron microscopy coupled with energy dispersive X-ray spectr oscopy were conducted, The specimen sintered with BN + Al2O3 packing powder at 1750 degreesC kept its original oxygen content after sintering, and rea ches almost full density. This specimen was more translucent than the other s at its wavelength longer than 800 nm, In the case of sintering with (a ra m powder for sintering) + BN or without packing powder at 1750 degreesC, th e 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 tran slucency and black coloring could be attributed to pores and to a change in the microstructure or to Si, respectively, The reaction/decomposition of b oth Si3N4 and SiO2 was effectively suppressed by sintering using the BN + A l2O3 packing powder, without changing composition.