The densification behaviors of two silicon nitride nanopowder mixtures based respectively on α-Si3N4 and ß-Si3N4 as the major phase constituent were studied by spark plasma sintering. Sintering conditions were established where a low viscous liquid not in equilibrium with the main crystalline constituent(s) stimulated the grain sliding yet did not activate the reprecipitation mechanism that unavoidably yields grain growth. By this way of dynamic grain sliding full densification of silicon nitride nanoceramics was achieved with no noticeable involvement of α- to β-Si3N4 phase transformation and grain growth. This processing principle opens the way toward flexible and precise tailoring of the microstructures and properties of Si3N4 ceramics. The obtained silicon nitride nanoceramics showed improved wear resistance, particularly under higher Hertzian stresses.

Nano Si3N4 composites with tailored microstructure were developed using fine beta-Si3N4 powders. Their wear behaviour was investigated. Whereas pure Si3N4 Composites showed improved wear behaviour under dry rolling conditions with slip, TiN-reinforced nano Si3N4 composites generate a self-lubricating behaviour under dry sliding conditions. After chemical treatment with hydrogen sulphide, the friction coefficient and wear rate was found to be significantly decreased under dry sliding conditions. Additionally, the new composites possess higher fracture toughness than the pure nano Si3N4 materials. Copyright (C) 2009 John Wiley & Sons, Ltd.

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.