Synthesis and characterization of nPCD sintered from OLC and Microdiamond

Additive-free nano-polycrystalline diamond (nPCD) composite was sintered by using a mixture of micro-diamond (MD) and onion-like carbon (OLC) as raw materials at 4–6 GPa/1000–1400 °C/5–30-min holding time. The composition, morphology, microstructure, density, and hardness of the nPCD composite were measured by XRD, FESEM, HRTEM, Archimedes principle, and Vickers hardness tester. Molecular dynamics of OLC with MD addition phase transition to nPCD at HPHT was calculated. The effect of MD and high-pressure sintering parameters on the OLC transformation process and the composition, microstructure, properties of nPCD composite were studied. The results showed that the MD was uniformly distributed in the nPCD composite. MD acted as the nucleus to promote the conversion of OLC to diamond. The increase in sintering pressure enhanced the density and hardness of the nPCD composite. When the sintering pressure was raised from 4 to 6 GPa, the density of the nPCD composite was increased from 2.72 to 3.02 g/cm 3 , and the hardness was increased from 41.85 to 56 GPa. The increase in sintering temperature and holding time made the density and hardness of the nPCD composite increase first and then decrease. Nanotwinned diamond with 10-nm size appeared in the nPCD region in the nPCD composite. The presence of nanotwinned diamonds increased the hardness of the nPCD composite.