Snow flake structures in silicon nitride ceramics - Reasons for large scale optical inhomogeneities

Large scale optical inhomogeneities were analysed in different Si3N4 ceramics containing different additive ratios and densified by hot pressing, gas pressure sintering and spark plasma sintering. In all analysed materials showing these optical inhomogencities (snow flakes) non-filled triple junctions were locally found which could be correlated with the snow flake structure. The internal tensile stress in the amorphous grain boundary phase is the most likely reason for this phenomenon. These stresses are caused by thermal mismatch between the grain boundary phase and the Si3N4 skeleton, or the volume change during crystallisation of the grain phase. (c) 2007 Elsevier Ltd. All rights reserved.