Synthesis of ultrafine nano-polycrystalline cubic boron nitride by direct transformation under ultrahigh pressure

Using a turbostratic pyrolytic boron nitride as a starting material, we synthesized a variety of ultrahard polycrystalline cubic boron nitride (PcBN) as a function of the heating duration changing from 1 to 60 min under a constant temperature and pressure conditions (1950 °C and 25 GPa) using a multi-anvil apparatus. When the heating duration was less than 13 min, ultrafine nano-polycrystalline cBN (U-NPcBN) with the mean grain size of <50 nm was produced. Among these U-NPcBNs those synthesized with 11-13 min were found to have a uniform texture composed purely of cBN (i.e. with no wurzite BN residue) and a Knoop hardness of >53 GPa, which is 20% higher than that of the hardest conventional binderless PcBN in practical use. Furthermore, the PcBNs synthesized with 18-20 min showed a unique nanocrystalline texture composed of relatively coarse grains dispersed in a fine grained matrix and even higher Knoop hardness (54.5-55.2 GPa).