Quantum confinement effect in nanotwinned diamond

The success in enhancing diamond by introducing nanotwins opens a new frontier in the development of superhard materials. However, the underlying hardening mechanism of nanotwinned diamond (nt-diamond) remains elusive and a persistent research focus. In this study, we employ first-principles calculations to unveil the performance enhancement in nt-diamond mediated by quantum confinement effect. This effect is characterized by the non-uniform valence charge density of C-C bonds near the twin boundary, leading to incomplete bond breakage at the onset of elastic instability and identified as the key factor in delaying cracking. These findings not only contribute to establishing the theory of hardness in superhard materials, but also suggest new avenues for enhancing their mechanical performance through the introduction of heterogeneous structures and dopant atoms aligned with the principle of quantum confinement effect.