Synthesis of nano-polycrystalline diamond in proximity to industrial conditions

Nano-polycrystalline diamond (NPD) can be obtained by direct conversion of various carbons at high temperature and high pressure (HPHT). However, the ultra-high synthesis condition is an obstacle for large-scale production of NPD. This work aimed to reduce the synthesis condition. Annealed precursors fabricated by annealing detonation nano-diamond in vacuum were used for synthesizing NPD at HPHT conditions of 1600-1800 °C, 10-15 GPa and holding time of 1-10 min. High-purity NPD with high Vickers hardness of 147 ± 17 GPa was synthesized at 1800 °C and 10 GPa for 10 min. The synthesis conditions were much lower than that using graphite precursors; in particular, the pressure was dramatically reduced to 10 GPa from 15 GPa. X-ray diffraction and transmission electron microscopy analyses suggested that most carbon particles in the annealed precursors contain a diamond core and the residual diamond core is the key for the reduction in synthesis pressure. Meanwhile, the onion-like carbon became metastable at HPHT because of the compression from the surrounding carbon particles and then transformed to diamond or graphite. Our findings proved that pure NPD with high hardness can be synthesized in proximity to industrial conditions.