Effects of carbon nanotubes and sintering parameters on microstructure and properties of PCD

Polycrystalline diamond (PCD) was sintered from onion-like carbon (OLC) mixed with multi-walled carbon nanotube (MWCNT) at 5.5 GPa/1100–1300 °C/5–20 min holding time, in which OLC was generated by vacuum annealing of nano-diamond. Structures, morphologies, and properties of composites were characterized by X-ray diffraction, field emission scanning electron microscope, high-resolution transmission electron microscopy, and hardness measurement. The purposes of this paper were to lower the temperature of OLC conversion into PCD and optimize the performance of the PCD. The influence of sintering parameters on the microstructure and properties of PCD and the change of MWCNT and OLC structure in high-pressure and high-temperature (HPHT) were studied. Finally, when the mass fraction of carbon nanotubes is 10 %, the optimal process conditions were as follows: sintering pressure 5.5 GPa, temperature 1150 °C, holding time 20 min, sintering hardness was 18.84 GPa, and conversion rate of OLC to diamond was 25.96 %. Under the influence of HPHT and surrounding OLC, the MWCNT tends to be transformed into spheroids, while the structure of the OLC/MWCNT binding zone is graphite. This work not only provided a theoretical basis for the study of carbon nanotubes but also provided reasonable parameters for the synthesis of PCD. [Display omitted] •The formed additive-free polycrystalline diamond had a Vickers hardness of 18.84 GPa.•The length of carbon nanotubes decreases, the diameter increases, and gradually becomes spherical at HPHT.•The OLC around MWCNTs does not change into the diamond phase, but into the graphite phase.•The reaction of MWCNT with OLC occurs sequentially from the open end to the closed end of MWCNT.