Plasticity Mechanisms in Nanostructured Cubic Boron Nitride: Internal Defects and Amorphous Layers

Nanostructured cubic boron nitride (NS-cBN) has attracted significant attention due to its high hardness and excellent thermal stability, yet a systematic strategy to balance strength and toughness through atomically structural design remains elusive. Here, we integrate plasticity theory with large-scale atomistic simulations to elucidate the size-dependent roles of internal defects, i.e., twin boundaries (TBs), stacking faults (SFs), and dislocation networks, and amorphous interfacial layers (AILs) in NS-cBN. In samples containing TBs and SFs, we demonstrate uniquely that the competition between hard slip modes (e.g., dislocation penetration) and soft slip modes (e.g., sliding parallel to defects), together with grain-boundary sliding, governs the scaling of strength and crack-initiation strain. Specially, a cross-slip of 1/2[110] screw dislocations emerges as the dominant plastic mechanism penetrating planar defects, while high-density SFs leverage stress concentration to activate the destacking fault mechanism, thereby improving crack-initiation strain with high strength. Introducing pre-existing dislocation networks shifts deformation from a grain-boundary-dominated to a dislocation-dominated regime, achieving a 76% increase in failure strain (up to 15% compressive strain) and a metal-like plastic plateau at a dislocation density of 0.115 nm^-2. Moreover, a 0.5 nm-thick AIL is found to simultaneously enhance strength and toughness by homogenizing stress, suppressing shear bands, and crack-initiation; further thickening of the AIL leads to softening, while increasing its density or bond strength amplifies its reinforcing effect. By synergistically tailoring internal defects and AILs, we achieve NS-cBN materials that combine high strength with high toughness, and thereby, we establish general design principles to guide the development of next-generation superhard materials.