Abstract
Large scale optical inhomogeneities were analysed in different Si(sub 3)N(sub 4) ceramics containing different additive ratios and densified by hot pressing, gas pressure sintering and spark plasma sintering.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 Si(sub 3)N(sub 4) skeleton, or the volume change during crystallisation of the grain boundary phase.