Synthesis of carbonado-like polycrystalline diamond in the B4C-graphite system

Carbonado-like polycrystalline diamond was synthesized in the non-metallic B4C-graphite system at a high pressure of 8-9 GPa and very high temperatures, about 2600-2800 K. The Vickers hardness of the polycrystalline diamond containing less than 5 mass% of B4C was 70-120 GPa at the indenter load 4.9 N. The experimental evidence suggests that the graphite-to-diamond transformation starts in the temperature range between the B4C-graphite-liquid and B4C-diamond-liquid eutectic temperatures. The presence of a volatile component, boric acid, in the starting B4C-graphite system breaks the polycrystalline microstructure of the diamond material leading to the formation of separate diamond microcrystals. In addition, we demonstrate that the toroid-type high-pressure chamber can be used for experiments at the temperature up to 2800-3100 K.