Synergy for Enhancing Strength and Toughness of Diamond through Polytypic Heterointerface

Hierarchical diamond nanocomposites, incorporating diverse coherently interfaced diamond polytypes, exhibit remarkable fracture toughness while maintaining exceptional hardness. However, the underlying mechanisms governing the strengthening and toughening of these polytypic heterointerfaces (PHIs) remain elusive. In this study, we employed first-principles approaches to derive the ideal strength and Peierls stress, conducting a comprehensive investigation into the influence of various PHIs on the plasticity of nanostructured diamond. A ubiquitous strengthening effect was observed across all PHI types under uniform shear deformation, as the introduction of PHIs invariably aligned a portion of the crystal in the hard shear direction, yielding strength comparable to that of the nanotwinned diamond. Surprisingly, graphitization and bond collapse were suppressed through a sequential transformation of stacking sequences, including an experimentally observed non-3C to 3C polytype transition. This phenomenon was attributed to the systematic bond realignment driven by continuous metallization confined to specific atomic layers. The heterointerface-mediated bonding reorganization effectively dissipated energy through phase transitions, thereby achieving supertoughness. Under localized deformation, all PHIs were found to enhance the barrier against parallel slip of 1/2 ⟨110⟩ shuffle-set full dislocations and 1/6 ⟨112⟩ glide-set partial dislocations, leading to a pronounced strengthening effect. These findings not only deepen our fundamental understanding of the synergistic strengthening and toughening of diamond through PHIs but also offer valuable insights for the design of other superhard materials and engineering ceramics via coherent heterointerfaces.