Diamond and cubic boron nitride (cBN) are the dominant ultrahard materials used for industrial applications. Diamond, with exceptional physical characteristics such as hardness, wear resistance, thermal sensitivity, thermal conductivity, and semiconductivity, is the hardest natural substance and well known as the “king of hardness”. Diamond has a wide range of applications, such as handicrafts and industrial cutting tools. cBN, as a novel ultrahard material emerging after synthetic diamond, can be synthesized catalytically from hexagonal boron nitride under high temperature-high pressure (HTHP). cBN has excellent performances such as high hardness, thermal stability, chemical inertness, good infrared transmittance, and large bandgap. cBN is the second hardest material after diamond, but has much higher thermal stability than that of diamond, and is chemically more stable to ferrous metals. With very excellent grinding and cutting performance, cBN abrasives are not only capable of processing difficult-to-grind materials to improve productivity, but also capable of effectively improving the cutting quality of work pieces. Utilization of cBN is a great contribution to metal processing, resulting in a revolutionary change of grinding and cutting industry. To realize a wider range of industrial applications, diamond or cBN is usually sintered into polycrystalline diamond or cBN under HTHP by using suitable binders.
However, particle sizes of diamond or cBN used in industry are generally very small. Although polycrystalline diamond or cBN can be processed into bulk materials with larger size, the strength of such polycrystals is much lowered due to the presence of hinders. To further improve the performance of ultrahard materials. Japanese researchers, Tetsuo Irifune et al., transformed graphite directly into ultrahard polycrystalline diamond under 12-25 GPa and 2300-2500° C. by employing the HTHP technique. The produced polycrystalline diamonds are light yellow transparent bulks with a size up to 7.5 mm and Knoop micro-indentation hardness up to 140 GPa. For commercially available polycrystalline cBN, the Vickers hardness is usually 33-45 GPa. To improve the performance of polycrystalline cBN, Japanese researchers, Takashi et al., synthesized sintered cBN bulk from amorphous BN powders as raw materials under 7.7 GPa and 2200° C. The Vickers hardness is improved to 51 GPa for this bulk. Recently, Scilozhenho et al. synthesized nanostructured cBN with Vickers hardness up to 85 GPa by using graphite like BN under 20 GPa and 1770 K.
However, these cBN materials have a variety of shortcomings, especially the relatively low hardness not satisfying the practical necessity. Moreover, the particle sizes of the currently available cBN materials are too small for direct applications. Therefore in industry, there exists a persistent demand for cBN materials with higher hardness and larger bulk size.