Densification of ultrafine SiC powders

Recent results on the densification behaviour of ultrafine SiC powders (below 20 nm) are presented and compared with results on the densification of ultrafine silicon-based ceramic powders given in the literature. A study of different powder processing routes and their influence on the pore-size distribution is given. Pressureless sintered green bodies having pore sizes of about 20 nm show extreme coarsening without significant densification. The results indicate a significant influence of green density on shrinkage. Encapsulated hot isostatic pressing (HIPing) led to a reduction of pore size and to considerable density increase at temperatures below 1600 degrees C. But even then full density without extensive grain growth was difficult to achieve. The applied method to determine grain sizes (X-ray diffraction measurements, XRD, using the Scherrer formula, scanning electron microscopy, SEM, and transmission electron microscopy, TEM) gave similar results for TEM and SEM but lower values for XRD. A possible explanation is presented. Density and grain growth both during pressureless sintering and HIPing showed significant differences between samples with and without sintering additives (B and C). Whether or not the use of sintering agents is favourable in reaching high densities and fine grain sizes, is discussed. HIP densification was modelled assuming diffusion to be the dominant mechanism. Grain growth according to a t(1/4) dependence and an activation energy of 6.8 eV was introduced into the model. Results on the properties (hardness, also at elevated temperatures, fracture toughness, bending and compression tests, thermal conductivity) of the hot isostatically pressed samples, are presented.