Study of the sintering properties of plasma synthesized ultrafine SiC powders

A study on the sintering of ultrafine SiC powders synthesized from elemental Si and CH4 using radio frequency (r.f.) induction plasma technology is reported. The powder had a particle size in the range of 40 to 80 nm and was composed of a mixture of alpha and beta-SiC. It was subjected to pressureless sintering in an induction furnace in the presence of different sintering aids. With the addition of B4C (2.0 wt% B) by mechanical mixing, the powders could only be partially densified, with the highest value of 84.5% of theoretical density being achieved at 2170 degrees C for 30 min. Through the use of ''in-flight'' boron doping of the powder during the plasma synthesis step (1.65 wt% B), the ultrafine powder obtained could be densified to above 90% of its theoretical density at 2050 degrees C for 30 min. The addition of oxide sintering aids (7.0 wt % Al2O3 + 3.0 Wt % Y2O3) by mechanical mixing produced sintered pellets of 95% of theoretical density at 2000 degrees C for 75 min. The Vicker's microhardness of the sintered pellets in this case was as high as 31.2 GPa. In order to improve our understanding of the basic phenomena involved, extensive microstructural (scanning electron energy microscopy: SEM), physical (shrinkage, weight loss, porosity, hardness) as well as chemical analysis (prompt gamma neutron activation analysis (PGNAA), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA)) was carried out. This helped establish a relationship between the properties of the as-synthesized powder and their sintering properties. The influences of sintering temperature, sintering time, additive concentration, and powder purity on the densification behaviour of the plasma-synthesized powders was investigated. The results were compared with data obtained using commercial powder.