  Nano-polycrystalline diamond (NPD) and nanotwinned diamond (NtD) were
successfully synthesized in multi-anvil high pressure apparatus at high
pressure and high temperature (HPHT) conditions using precursors of onion
carbons. We found that the choices of distinct onion carbons with hollow or
multi-cored microstructures lead to the synthesis of different diamond products
of NPD or NtD ones. High quality NtD with an average twin size of 6.8 nm has
been synthesized whose Vickers hardness reaches as high as 180 GPa as measured
by indentation hardness experiment. The existence of stacking faults other than
various defects in the onion carbon is found to be crucial to form twin
boundary in the product. The origin of the extraordinarily high Vickers
hardness in the NtD sample is attributable to the high concentration of twin
boundary. Our work gave a direct support on the argument that pursuit of
nanotwinned microstructure is an effective strategy to harden materials, in
good coincidence with the well-known Hall-Petch effect.
