Investigations of Some Hard Alloys in the Boron-Silicon-Carbon System
The compounds and alloys of the boron-silicon-carbon system are of interest due to their high melting points, hardness, and abrasive properties. Formation of these alloys by melting met with some difficulty due to partial boron and silicon volatilization. In the literature, some methods of obtaining such alloys by sintering or by hot-pressing from mixtures of B4C + SiC powders have been described, but, in these cases, partial volatilization of boron and silicon was also observed at high temperatures during sintering. We carried out the synthesis of boron-silicon-carbon alloys from fine elemental powder mixtures (B + Si + C) by stepwise increase in pressure and temperature (up to 2000°C) in the hot-pressing process. This contributed to diffusion formation of high-melting phases with low vapor pressure. Thus, the compositions of the final nonporous alloys were similar to compositions of the initial mixture. The solid solutions of silicon and carbon in boron carbide were found to have a very high microhardness of about 7000 kg/mm2 (in comparison to 5000 kg/mm2 for B4C); the solid solutions of boron and carbon in silicon carbide were found to have a microhardness of about 5000 kg/mm2 (instead of 3500 kg/mm2 for SiC). At 900°C, the B4C-base solid solutions retained a microhardness of about 3000 kg/mm2, and the SiC-base solid solutions retained about 1350 kg/mm2. A continuous series of solid solutions in B4C-B4Si pseudobinary section with maximum microhardness of about 7000 kg/mm2 was determined. On the basis of the pseudobinary section SiC-BC, studied by deposition from gaseous phases, the solubility of boron and carbon in monatomic relation in silicon carbide up to 40 mol. % BC was found. From the point of view of crystallographic chemistry, it is of scientific interest that in the boron-silicon-carbon system there exist covalent phases of variable composition both with carbon and silicon substitution (B4C-B4Si) and with the partial substitution of silicon by boron (SiC-BC) and with the partial carbon-boron substitution (B4C-base homogeneous region).
KeywordsSilicon Carbide Hard Alloy Boron Carbide Carbon System Metal Powder Industry Federation
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