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

In all analysed materials showing these optical inhomogeneities (“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 boundary phase.