Nanotwinned diamond synthesized from multicore carbon onion

Nanopolycrystalline diamond (NPD) and nanotwinned diamond (NtD) were successfully synthesized in a multianvil, high-pressure apparatus at high temperatures by using the precursors of carbon onion. It was found that distinct carbon onions with hollow or multicore microstructures lead to the formation of different diamond products, namely NPD or NtD, respectively. The Vickers hardness of high-quality NtD with an average twin thickness of 6.8 nm reached as high as 180 GPa, which was measured by indentation hardness experiment. The existence of stacking faults other than various defects in the carbon onion was found to be crucial for the formation of twin boundaries in the product. The origin of the extraordinarily high Vickers hardness in the NtD sample is attributable to the high concentration of twin boundaries. Our work directly supports the argument that pursuit of nanotwinned microstructure is an effective strategy to harden materials, which is in good agreement with the well-known Hall-Petch effect.