Extreme Mechanics of Probing the Ultimate Strength of Nanotwinned Diamond

Recently synthesized nanotwinned diamond (NTD) exhibits unprecedented Vickers hardness exceeding 200 GPa [Q. Huang et al., Nature (London) 510, 250 (2014)]. This extraordinary finding challenges the prevailing understanding of material deformation and stress response under extreme loading conditions. Here we unveil by first-principles calculations a novel indenter-deformation generated stress confinement mechanism that suppresses the graphitization or bond collapse failure modes commonly known in strong covalent solids, leading to greatly enhanced peak stress and strain range in the indented diamond lattice. Moreover, the twin boundaries in NTD promote a strong stress concentration that drives preferential bond realignments, producing a giant indentation strain stiffening. These results explain the exceptional indentation strength of NTD and offer insights into the extreme mechanics of the intricate interplay of the indenter and indented crystal in probing ultrahard materials.