The Effect of Carbon on the Sintering of Boron Carbide

Two kinds of boron carbide powders, S-1 and S-2, were prepared by the reaction of boric oxide with carbon black in magnesium (2B2O3+6Mg+C=B4C+6MgO). Powder S-1 was prepared by heat-treating for 10min at 1500°C immediately after the reaction was completed, while S-2 was prepared by heat-treating at 1650°C. After purifying the powders, the chemical composition of the samples was analyzed. The boron contents in S-1 and S-2 were found to be 78.2±0.5at% and 76.2±0.5at%, respectively. A trace amount of Al, Mg, Si, Fe, Mn, and Cu was observed, but total amounts of these impurities were estimated to be less than 0.1%, and the residual portion was regarded as solely consisted of carbon. Therefore the amounts of carbon were derived as 21.8 and 23.8at%, respectively. Some boron and carbon were further added to the powder prepared, and the powder compacts were subjected to sintering for 1h at either 2200° or 2250°C in pure helium. It was found that boron carbide pellet with high density (>90%TD) could be obtained by using powders that contained C by 25 to 30at%, which corresponds to the region of the eutectic point of B (or B4C)-C system. In particular, boron carbide pellet with 93.7% TD was obtained by sintering the powder, which contained C by 27.7at%.
It was concluded that excess carbon on grain boundaries in the B4C+C phase would inhibit grain growth during sintering. Simultaneously, due to the presence of carbon on grain boundaries, the melting point of the interface of the B4C-C phase at the grain boundaries would decrease to that of the eutectic point; thereby, materials transport was enhanced. As a result of this effect, a high-density pellet was obtained in the carbon content region by 25 to 30at%.