Designing ultrahard nanostructured diamond through internal defects and interface engineering at different length scales

Nanocrystalline diamonds (NCDs) are promising structural materials due to their extraordinary mechanical properties such as ultrahigh hardness and excellent toughness, however, a rational design rule in plasticity and fracture through controlling nanostructures at different length scales is far from being explored. By means of atomic simulations and plasticity theory in the present paper, we comprehensively explored the plastic deformation behaviors of a series of well-defined NCDs by varying amorphous interfacial layers (AILs) and internal defects, e.g., twin boundary, stacking fault, π-bonded interface, and fivefold twin. It was observed that the effect of internal defects on the mechanical response of NCD can be attributed to the competition between dislocation blocking process and interface sliding process. The introduction of AIL at grain boundary (GB) is found to provide an effective solution to decrease both dislocation nucleation and penetration at GBs. These findings provide not only a mechanistic insight into the unique strengthening and toughening in various NCDs, but a rational guidance in designing novel superhard carbon materials with superior performance by engineering internal defects and GB structures at different length scales.