The effect of silicon nitride powder characteristics on SiAlON microstructures, densification and phase assemblage

The surface characteristics, particle size distribution and impurities of starting Si3N4 powders exert a very significant influence on the microstructure of sintered silicon nitride based ceramics. Even a change of the processing conditions such as milling liquid media (water or isopropyl alcohol) and milling time can have a substantial effect on particle surface groups, and hence on the microstructure of sintered samples. In this study, SEM, XRD, FTIR, BET, elemental analysis and laser diffraction techniques were used for the comprehensive characterization of Si3N4 powders which were produced by diimide, direct nitridation and combustion synthesis, in as received state, and after milling in different liquid media (aqueous or alcohol), for various milling durations. The correlation of the surface characteristics and properties of the Si3N4 powders with sintering behavior, and microstructural evolution, densification and phase assemblages of the resulting SiAlON ceramics were reported. The milling conditions affected the surface chemistry of Si3N4 powders and the subsequent microstructural evolution. The microstructures evolved from the coarser β-Si3N4 powders were coarser, but the fine β-Si3N4 powders yielded a bimodal microstructure. The critical particle diameter of the β-Si3N4 powder for the formation of needle like SiAlON grains was determined to be less than 0.5 µm.