Research For Synthesis Of Nano-polycrystalline Diamond And Phase Transformation Mechanism Of Carbon Onions Under High Pressure And High Temperature

In 2003,nano-polycrystalline diamond（NPD）with binder-free was successfully synthesized from polycrystalline graphite at high pressure and high temperature（HPHT）by Irifune et al.,which is payed comprehensive attention because of its excellent mechanical properties.NPD potentially replace as an ideal material for cutting tool.However,for NPD,two fundamental issues should be figured out.One is how to reduce the ultrahigh synthesis conditions of NPD,which allows industrial production.The other is how to further improve the properties.In 2014,Huang et al.reported the synthesis of nanotwinned diamond（nt-diamond）with remarkable hardness,fracture toughness and thermostability.Nevertheless,to date,the formation mechanism from carbon onions precursors to nt-diamond has not been fully addressed.In this study,carbon onions was annealed from high-disperse nano-diamond in vacuum and was used as the precursors for synthesizing NPD at HPHT.The influences of annealing temperature,heating rate and heat preservation time on the synthesis of carbon onions was studied.Carbon onions with different structures can be synthesized by controlling the annealing process.At annealing temperature between 1100°C and 1300°C,the obtained carbon onions have a large amount of diamond cores which can be eliminated completely at higher 1600°C.Carbon onions obtained at different annealing temperature was used as precursors for synthesis of NPD at a relative low pressure（≤6GPa）.Because of the existence of diamond cores in carbon onions,NPD with high strength can be synthesized at 4-6 GPa and1000-1300°C by using the carbon onions annealed from 1100-1300°C.Under 6 GPa and1250°C,the NPD obtained from carbon onions-1150°C have highest hardness of⁵0GPa.The dense and high-purty NPD with high Vickers hardness of 147±17 GPa was synthesized at 10 GPa and 1800°C by using multi-anvil apparatus.At this condition,NPD with large bulk size of 10 mm was synthesized and showed high Vicker hardness over 100 GPa and unprecedented fracture toughness of¹7 MPa m^1/2.The synthesis conditions are much lower than that using graphite precursors（≥15 GPa,2300°C）,in particular,the pressure was dramatically reduced to 10 GPa from 15 GPa.Our experimental results suggest 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.In order to study phase transformation behavior of carbon onions under HPHT and reveal formation mechanism of nanotwinned diamond,the defect-free carbon onions with few diamond cores annealed at 1500°C and holding time of 30 min was used as precursors.Carbon onions without a high concentration of stacking faults and diamond cores prefers to transform to metastable graphite rather than diamond at 10 GPa and1800°C.With increasing the pressure up to 15 GPa,carbon onions can transform to diamond,however,the nanotwinned diamond just was synthesized at higher pressure of25 GPa.The synthesized NPD an exceptionally high Vickers hardness of²15 GPa,which is due to the existence of high-density nanotwins.Our results demonstrate that the transformation from carbon onions to diamond is a nondiffusion transition so-called martensitic process.Specifically,the appearance of{111}nanotwinned structure and stacking faults is determined by the characteristics of the onion shells.