Multi-scale Synergistic Effects Enhanced Hierarchically Structured Polycrystalline Diamond for Exceptional Hardness and Toughness

The unrivalled hardness of diamond is offset by an intrinsic brittleness that severely limits its technological applications. This long-standing trade-off can be resolved by architecting hierarchically microstructures as confirmed in metals, but this routine is normally forbidden in diamond due to the strong covalent structure. Starting from a rationally designed mixture of onion-like carbon and graphite, we exploit the precursor-directed high-pressure and high-temperature (HPHT) transformation to nucleate hierarchically interpenetrating network of nano/micron-crystalline diamond. Simultaneously, high stress caused by asynchronous phase transformation triggers high density of dislocations array in micrometer grains. The resulting hierarchical superhard polycrystalline diamond (HSPD) show exceptional Vickers hardness of 146.0 GPa along with a high fracture toughness of 14.1 MPa·m ^0.5 , which is triple that of single-crystal diamond. The hierarchically microstructure allows multi-scales plastic deformation mechanisms to be activated concurrently, in which Hall-Petch strengthening from nanoscale domains restricts dislocation motion, whereas dislocations network together with coarse sub-micron grains activate crack deflection, branching and bridging to dissipates fracture energy. This multi-scale strategy, achieved through precursor-directed transformation, provides a general pathway to customize microstructure in covalent materials for fabricating damage-tolerant, ultrahard diamond and hard ceramics without sacrificing hardness.