Hardness and deformation microstructures of nano-polycrystalline diamonds synthesized from various carbons under high pressure and high temperature

Mechanical properties of high-purity nano-polycrystalline diamonds synthesized by direct conversion from graphite and various non-graphitic carbons under static high pressures and high temperatures were investigated by microindentation testing with a Knoop indenter and observation of microstructures around the indentations. Results of indentation hardness tests using a superhard synthetic diamond Knoop indenter showed that the polycrystalline diamond synthesized from graphite at >= 15 GPa and 2300-2500 degrees C (consisting of fine grains 10-30 urn in size and layered crystals) has very high Knoop hardness (Hk >= 110 GPa), whereas the hardness of polycrystalline diamonds synthesized from non-graphitic carbons at >= 15 GPa and below 2000 degrees C (consisting only of single-nano grains 5-10 nm in size) are significantly lower (Hk = 70 to 90 GPa). Microstructure observations beneath the indentations of these nano-polycrystalline diamonds suggest that the existence of a lamellar structure and the bonding, strength of the grain boundary play important roles in controlling the hardness of the polycrystalline diamond.