Study Of Carbon Onions Sintered To Polycrystalline Diamond In High Temperature High Pressure Wihtout Any Additives

Man-made polycrystalline diamond (PCD) is a kind of sinter synthesized by smalldiamond particles in high temperature and high pressure. Because of its isotropicproperties, the application performance of PCD is much better than singlecrystallinediamond. At present, the sintering of PCD is need to add additives. Si, Ni, B and Ti aregenerally used as additive in domestic production, and catalyst metals including Co and Niin the international community. The additives are easy to form the residue in the PCD, thephysical and mechanical properties of additive residue and their compound formed aremuch lower than that of the diamond leading to the presence of weak phase in the PCD,which affects the use and processing properties of PCD.Additive-free sintering of PCD is a fundamental way to solve these problems. Theemergences of detonation nanodiamond and onion-like carbon (OLC) bring the first lightof morning for the researchers in the field. Studies have shown that nanodiamond hasdouble features of nanomaterials and diamond. The small size effect makes thenanodiamond has a large volume of dangling bands on its surface and high surface energy,which makes additive-free sintering of nanodiamond PCD be possible. The OLC is a kindof carbon structure similar to onion, which consists of sp~2structural graphite and little sp~3structural diamond. Under certain conditions, the two structures can be transformedmutually. During the transforming from OLC to nanodiamond, the outer carbon atoms canlink with others among OLC particles and then transform to nanodiamond, which makes itpossible that the OLC transforms into PCD in the sintering process without additive.The OLC was fabiricated by annealing nanodiamond at0.1Pa vacuum andtemperature range from900℃to1600℃. The products of annealing nanodiamond at900~1300℃were OLC containing diamond core, at1400℃and above temperaturewere complete form OLC. Keeping temperature during annealing could effectivelypromote the transformation of OLC containing diamond core to complete form OLC, butfew effect to complete form OLC. There were diamonds synthesized when OLC heated to500℃and kept temperature for2hours in air. The graphite layer of OLC particles ruptured and linked to ruptured particles nearbyto transform large diamond particles during high temperature and high pressure sintering.And particles were connected by D-D (Diamond-Diamond) bonds. The phenomenonshowed that it is possible to prepare PCD in high temperature and high pressure withoutadditives. Using the OLC annealed at1100℃sintered in the5.5GPa,1200℃and15minute conditions bore the best sintering effects. The average Vickers hardness of PCDproduced in above conditions could achieve32GPa.