Nanotwinned diamond synthesized from multi-core onion carbon

TL;DR

This study demonstrates the synthesis of high-hardness nanotwinned diamond from onion carbon precursors under high-pressure high-temperature conditions, highlighting the role of stacking faults and twin boundaries in enhancing hardness.

Contribution

It introduces a novel method to produce nanotwinned diamond with exceptional hardness using onion carbon precursors and elucidates the microstructural factors influencing hardness.

Findings

High-quality NtD with 6.8 nm twin size synthesized

Vickers hardness of NtD reaches 180 GPa

Stacking faults are crucial for twin boundary formation

Abstract

Nano-polycrystalline diamond (NPD) and nanotwinned diamond (NtD) were successfully synthesized in multi-anvil high pressure apparatus at high pressure and high temperature (HPHT) conditions using precursors of onion carbons. We found that the choices of distinct onion carbons with hollow or multi-cored microstructures lead to the synthesis of different diamond products of NPD or NtD ones. High quality NtD with an average twin size of 6.8 nm has been synthesized whose Vickers hardness reaches as high as 180 GPa as measured by indentation hardness experiment. The existence of stacking faults other than various defects in the onion carbon is found to be crucial to form twin boundary in the product. The origin of the extraordinarily high Vickers hardness in the NtD sample is attributable to the high concentration of twin boundary. Our work gave a direct support on the argument that pursuit of nanotwinned microstructure is an effective strategy to harden materials, in good coincidence with the well-known Hall-Petch effect.