Synergistic hardening and toughening of diamond through stacking-fault engineering

Pre-oriented stacking-fault networks are programmed into polycrystalline diamond through precursor inheritance from elongated graphite flakes, producing a brick-wall architecture composed of high-aspect-ratio lamellae and dense fault bundles. This defect architecture preserves ultrahigh hardness while increasing crack resistance and oxidation stability in air. The optimized sample exhibits a Knoop hardness of 140.1 ± 3.0 GPa, an indentation fracture toughness of 16.9 ± 4.2 MPa·m1/2, and an oxidation-onset temperature of 1295 K. Correlative post-fracture high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and electron energy-loss spectroscopy (EELS) analyses reveal a confined multiphase bridge near fracture origins, comprising strained cubic diamond, disordered stacking-fault-rich regions, and localized sp2-rich domains, consistent with stress-assisted local reconstruction. At larger length scales, the aligned stacking-fault architecture biases intragranular crack propagation and promotes crack deflection and crystalline bridging, thereby increasing crack-path tortuosity. These results suggest defect architecture engineering as an effective route to mitigating the hardness–toughness trade-off in diamond and point to a broader design strategy for damage-tolerant superhard covalent solids.