Synthesis of High-Purity Nano-Polycrystalline Diamond and Its Characterization

摘要

High-purity nano-polycrystalline diamond has been successfully synthesized by direct conversion from high-purity graphite under static pressures above 15 GPa and temperatures above 2300℃. The polycrystalline diamond has a very fine mixed texture of a homogeneous fine structure (particle size: 10-20 nm) and a lamellar structure. The results of electron diffraction analysis suggested that diamond particles in the homogeneous fine structure are transformed from graphite in a diffusion process while diamond layers in the lamellar structure are formed from graphite in a two-step martensitic process via the hexagonal diamond phase. The polycrystalline diamond is so hard that it is difficult to form indentations with regular polygon based diamond indenter. Measurable indentations can be formed using only Knoop indenter in a limited loading condition of around 4.9 N. The results of Knoop hardness at the load indicate that the nano-polycrystalline diamond has extremely high hardness, which is equivalent to or even higher than synthetic high purity (type Ⅱ a) diamond crystal and obviously higher than type Ⅰ diamond crystals. It is presumed that the microstructure features (very fine mixed structure, no secondary phases) lead to extremely high hardness. The very fine microstructure and extremely high hardness of the polycrystalline diamond promise well for its applications as high-precision and high-efficiency cutting tool for the next generation.