Abstract
High-purity and super-hard nano-polycrystalline diamond has been successfully synthesized by direct conversion from high-purity graphite under static pressures above 15 GPa and temperatures above 2573 K. TEM analysis revealed that the polycrystalline diamond has a mixed texture of a homogeneous fine structure (particle size: 10-30 nm, formed in a diffusion process) and a lamellar structure (formed in a martensitic process). Differences in the direction of maximum compression force on graphite particles create different conversion processes to cubic diamond, leading to the mixed texture. Results of indentation hardness tests using super-hard synthetic diamond Knoop indenter showed the polycrystalline diamond has very high Knoop hardness of 120-145 GPa. On the contrary, the polycrystalline diamonds synthesized from the non-graphitic carbons at 15-18 GPa and 1873-2273 K have a single texture consisting of a very fine homogeneous structure (5-10 nm, formed in a diffusion process) without a lamellar structure and containing no hexagonal diamond phase. The hardness values of such single-nano polycrystalline diamonds from non-graphitic carbons (70-90 GPa) are significantly lower than that of polycrystalline diamond from graphite. The excessively small size of diamond particles and the absence of the lamella structure seem to lower the hardness of the polycrystalline diamond.
