Abstract
ABSTRACTCold isostatic pressed (CIP) and slip cast Si3N4 composite materials containing up to 20 vol.% SiC-whiskers were densif led by pressureless sintering and post-HIPing. The slips were characterized by viscosimetry and showed no influence of whisker loading on the rheology of the highly concentrated aqueous suspensions up to 15 vol.! whiskers. During casting the whiskers were preferentially aligned parallel to the mould surface and the CIPed samples showed a 3-dimensional random whisker distribution. The influence of the whisker length and orientation on the densification behaviour was studied in a high-temperature dilatometer. While the CIPed samples exhibit an isotropic shrinkage behaviour, the slip cast samples showed a strong shrinkage anisotropy with a maximum linear shrinkage of 21% perpendicular and only 7% parallel to the whisker plane for the samples with 20 vol.% whiskers. With increased whisker loading, the sintered densities decreased from 0.99 to 0.88 fractional density. The poor sinterability of the composites was attributed to the sintering stresses around the non-sintering SiC- whiskers and the formation of a whisker network which inhibits further shrinkage in the final stage of sintering. Thermodynamic calculations show the optimum fabrication conditions for SIC-whisker reinforced Si3N4-composites.
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