Synthesizing bulk polycrystalline diamond by method of direct phase transition

Pure-phase bulk polycrystalline diamonds are prepared through direct phase transition at ultrahigh pressure and high temperature. The phase composition and microstructure of the pure-phase bulk polycrystalline diamonds are studied by SEM, XRD and Raman spectrum. It is found that with different raw materials and different conditions, diamond samples have different colors and different grain sizes but they are all pure-phase polycrystalline diamonds.The average grain sizes for yellow-transparent and green samples are about 25 nm and 35 μm, respectively. The size of synthesized diamond blocks is φ6 mm×6 mm. The Vicker's hardness of yellow-transparent polycrystalline diamond, HV, is 130-270 GPa at the load of 3 N. According to China National Quality Supervision & Inspection Center for Abrasives, the abrasion ratio of yellow-transparent sample is 1.658×10⁶, which is 25 times higher than that of commercial Co-based PCD used on oil bits. TG/DSC shows that the thermal stability of the yellow-transparent sample is almost the same as that of carat-level single-crystal diamond.Contrast grinding test, using NPD to grind against carat-level single-crystal diamond, shows that the abrasion ratio for the nano-structured polycrystalline diamond is 2.5, which demonstrates that the abrasion performance of NPD is much better than that of carat-level single-crystal diamond.