Since its discovery more than 2,700 years ago, natural diamond has been widely recognized as the hardest material in nature. However, Chinese scientists have successfully synthesized a new material that is twice as hard as natural diamond.

A research team led by Professor Tian Yongjun of Yanshan University, a Chinese materials scientist, in collaboration with Professor Ma Yanming of Jilin University and Professor Wang Yanbin of the University of Chicago in the United States, has successfully synthesized nanotwinning diamond bulk material under high temperature and high pressure, with a hardness twice that of natural diamond.

This research finding was published in the internationally renowned journal Nature on June 12. It represents another breakthrough following the synthesis of ultra-hard nanotwinned cubic boron nitride in 2013.

Last year, Professor Tian Yongjun’s research team successfully synthesized nanotwinning cubic boron nitride using an onion-like boron nitride precursor under high pressure. They reduced the characteristic size of the microstructure—specifically, the average twin thickness—to just 3.8 nanometers, achieving a Vickers hardness value as high as 108 GPa, surpassing that of synthetic single-crystal diamond. The successful synthesis of nanotwinning cubic boron nitride has opened up a new avenue for simultaneously enhancing the material’s hardness, toughness, and thermal stability.

To date, high-pressure phase transitions of carbon precursors such as graphite, amorphous carbon, glassy carbon, and C60 have not yet yielded nanotwinning diamond structures. To address this challenge, Professor Tian Yongjun’s research team and their collaborators have begun investigating the phase-transition process of onion-like carbon under high temperature and high pressure. At lower temperatures, onion-like carbon not only forms cubic diamond with a nanotwinning structure but also coexists with a monoclinic-phase diamond, which they term “M-diamond” in their paper. At higher temperatures, onion-like carbon transforms entirely into a single-phase diamond with a nanotwinning structure, where the average thickness of the twins is as low as 5 nanometers. This nanotwinning diamond exhibits unprecedented hardness and stability: its Vickers hardness is approximately twice that of natural diamond, reaching up to 200 GPa; and its onset oxidation temperature in air is more than 200 degrees Celsius higher than that of natural diamond.

Under the continued funding of the National Fund for Distinguished Young Scientists, key projects, and innovative research groups, Professor Tian Yongjun has devoted himself to theoretical and experimental studies on material hardness, ultimately achieving this breakthrough.