Preparation and pressureless sintering of chemical vapour deposited SiC-B composite powder

SiC-B composite powder was prepared by chemical vapour deposition (CVD) using (CH₃)₂SiCl₂+ B₂H₆+ H₂ as source gases at 1673 K. The powder was β-type polycrystalline silicon carbide containing several per cents of boron and carbon. The boron content increased from 0 to 7.7 mass % as the B₂H₆ gas concentration increased from 0 to 0.7 mol %. Boron and carbon in amorphous form dispersed homogeneously in the β-SiC polycrystalline particles. The particles were spherical, non-agglomerated and uniform in size with an average particle size of about 50 nm. Sintering tests were performed with the resulting composite powder without applying pressure. Powder containing 1 mass % boron and 2 mass % carbon was sintered to a density of 3.16 × 10³ kg m^-3 at 2273 K, and the Vickers hardness of the sintered body was 30 GPa. When the sintering temperature was higher than 2323 K, significant grain growth due to the phase transformation from β to α form occurred, which decreased bulk density and Vickers hardness.