Effect of sintering parameters on microstructure and properties of nanopolycrystalline diamond bulks synthesized from onion-like carbon

Additive-free nanopolycrystalline diamond (NPD) bulks were sintered from the onion-like carbon (OLC14) under industrial conditions of 4–6 GPa/1000–1400 °C/10–30 min. OLC14 was fabricated by annealing detonation nanodiamond (ND) at the conditions of 1400 °C/2 h/1 Pa (furnace cooling). The diamond content, average ND grain size, relative density and Vickers hardness of NPD bulk reached the maximum values of 81.56 wt%, 12 nm, 94% and 42 GPa sintered in the conditions of 5 GPa/1300 °C/20 min. Lots of nanotwins were formed in the NPD bulks due to the wrinkles and defects of OLC14 precursors. Because the carbon atom space and position of OLC14 spherical graphite layer was close to those of diamond, OLC14 could easily be transformed into ND, which grew larger and bonded with each other to form NPD bulks. The transformation of OLC14 into ND grain was martensitic transition. The sp3 carbon core of OLC14 as crystal seed had effect on this transformation. OLC14 with smaller grain size could be preferentially transformed into ND. [Display omitted] •Additive-free nanopolycrystalline diamond (NPD) with Vickers hardness of 42 GPa was sintered from onion-like carbon (OLC14) under industrial conditions of 4-6 GPa/1000-1400 °C/10-30 min.•Nanotwins were formed in NPD due to the wrinkles and defects of OLC14.•The transformation of OLC14 into nanodiamond was martensitic transformation of solid state, while annealing temperature had much effect on the OLC conversion.•The sp3 carbon core acted as crystal nucleus to reduce the HPHT sintering conditions.•Smaller OLC14 was transformed into nanodiamond preferentially.