抄録
Single-phase (binderless) super-hard polycrystalline cBN and polycrystalline diamond have been successfully synthesized by direct conversion sintering from high-purity hBN and graphite, respectively, under static high pressure and at high temperature. The polycrystalline cBN synthesized at ≥7.7GPa and 2200-2400°C consists of homogeneous fine grains of <0.5μm. The polycrystalline diamond synthesized at ≥15GPa and 2200-2400°C reveals a homogeneous fine structure comprised of very small diamond grains of several tens of nanometers. The polycrystalline cBN and polycrystalline diamond have considerably higher hardnesses than those of their single crystals. The transverse rupture strengths (TRSs) are much higher than those of conventional polycrystalline cBN and diamond containing binder and show a positive temperature dependence above 800-1000°C where the TRSs of conventional polycrystals are rather deteriorated. The fine microstructure without any secondary phases and excellent mechanical properties of the polycrystalline cBN and diamond thus fabricated are promising for next-generation high-precision and high-efficiency cutting tools.
