Conditions and mechanism of formation of nano-polycrystalline diamonds on direct transformation from graphite and non-graphitic carbon at high pressure and temperature

The conditions and mechanism of formation of nano-polycrystalline diamonds directly from graphite and non-graphitic carbon (carbon black, glassy carbon, C60 and carbon nanotubes) at high pressure and high temperature have been investigated. The onset temperature for diamond formation at P≥q 15 GPa is 1500–1600 °C for all carbon materials, although the required temperature conditions for pure polycrystalline diamond are T≥q 2200 °C for graphite and T≥q 1600 °C for non-graphitic carbon. Polycrystalline diamond forms as a result of simultaneous diffusion and two-step martensitic processes from graphite, whereas it forms only due to diffusion without graphitization or formation of intermediate phases from non-graphitic carbon. Nano-polycrystalline diamonds consisting only of very fine particles (<10 nm in size) can be obtained from non-graphitic carbon at T∼ 1600–2000 °C under pressures≥q15 GPa.