Abstract
Super-hard nano-polycrystalline diamond up to 5 mm in diameter has been successfully synthesized by direct conversion from high-purity graphite under static pressures above 15 GPa and temperatures above 2573 K. The polycrystalline diamond has a very fine texture consisting of diamond grains of several tens of nanometer in size, and has an extremely high hardness, which is equivalent to or even higher than that of single crystal diamonds. It is presumed that the microstructural features (very fine mixed structure consisting of granular and lamellar grains, no secondary phases) lead to the extremely high hardness. The very fine microstructure and extremely high hardness of the polycrystalline diamond promise well its applications as high-precision and high-efficiency cutting tools of the next generation.
