Processing and Properties of Nanograin Silicon Carbide


  • R. W. Rice—contributing editor


The manufacture of fine-grained SiC ceramics by using nanophase SiC powders with particle sizes of about 20 nm is described. Conventional sintering of these powders led to extreme grain growth and hence the nanophase microstructure was destroyed. Pressure-assisted sintering was applied to reduce the sintering temperature and also grain growth. Hot isostatic pressing (HIPing) of samples with about 1 wt% carbon and 1 wt% boron addition at temperatures below 1700°C and with pressures up to 350 MPa resulted in densities of more than 95% of the theoretical density (TD) and grain sizes of 150 nm. A further reduction of grain size became possible by an optimized high-temperature heat treatment of the samples prior to HIP. This procedure removed at least partially the oxygen layer on the surface of the nanophase SiC particles. Samples with densities of more than 97% TD and grain sizes below 80 nm were produced. Grain sizes were measured with scanning and transmission electron microscopes; both methods gave similar values. Grain sizes determined from the peak broadening of X-ray diffraction peaks showed in some cases lower values. This was attributed to the relatively high amount of twinning boundaries and stacking faults produced during crystal growth. The influence of the grain size on different mechanical and thermal properties was investigated. Vickers hardness and indentation fracture toughness were measured for samples with different densities and grain sizes. The results revealed that besides grain size features like density and/or oxygen content can strongly influence hardness and fracture toughness. Similar results have been found for the wear resistance of fine-grained materials. Results of pin-on-disk type experiments showed the importance of a high density of the samples for high wear resistance. Thermal diffusivity measurements were performed for samples with different grain sizes up to 1400°C. A large decrease with decreasing grain size was found at room temperature. At higher temperatures the difference in the thermal diffusivity of fine- and large-grained materials was reduced.