Curved graphite precursors enable cubic-hexagonal diamond heterostructures with unprecedented toughness-hardness synergy.

The intrinsic trade-off between hardness and toughness presents a long-standing challenge for diamond-based materials, limiting their use in extreme environments. Here, we report a bioinspired strategy to overcome this limitation by engineering graphite precursors with mimosa-like microscale curvature. Under high-pressure and high-temperature conditions (15 gigapascals and 2300 kelvin), these precursors concentrate local stress, promoting nucleation of hexagonal diamond within a cubic diamond matrix and forming cubic-hexagonal heterostructures. The resulting composites exhibit exceptional hardness (169 gigapascals) and toughness (15.7 megapascals multiplied by square root of meter), representing 36 and 104% improvements over single-phase nanopolycrystalline diamond, respectively. This dual enhancement arises from stacking fault interlocking and semi/coherent boundaries that resist deformation, coupled with phase transformation and crack deflection that dissipate fracture energy. Our results demonstrate a microstructural design paradigm for mitigating the property trade-off in superhard materials and offer a scalable strategy for engineering robust diamond-based systems.