PREPARATION AND PRESSURELESS SINTERING OF CHEMICAL-VAPOR-DEPOSITED SIC-B COMPOSITE POWDER

SiC-B composite powder was prepared by chemical vapour deposition (CVD ) using (CH3)(2)SiCl2 + B2H6 + H-2 as source gases at 1673 K. The powd er was beta-type polycrystalline silicon carbide containing several pe r 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 %. B oron and carbon in amorphous form dispersed homogeneously in the beta- SiC polycrystalline particles. The particles were spherical, non-agglo merated 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 x 10(3) kg m(-3) at 2273 K, and the Vickers hardness of the sintered body was 30 GPa. When the si ntering temperature was higher than 2323 K, significant grain growth d ue to the phase transformation from beta to alpha form occurred, which decreased bulk density and Vickers hardness.