Curved Graphite Precursors Enable Cubic–Hexagonal Diamond Heterostructures with Unprecedented Toughness–Hardness Synergy

Tough and Superhard Diamond Spheres

Diamond is the hardest known material but suffers from brittleness that limits its structural applications. We report a centimeter-scale diamond sphere with an ultrahigh Vickers hardness of ≈200 GPa and exceptional toughness. The spheres were synthesized by direct phase transformation of fullerene C₆₀ under high-pressure and high-temperature (HPHT) conditions, forming nanoscale twins (~5 nm) within an ultrafine-grained diamond matrix (~3–10 nm).

The combination of nanograins and dense twin boundaries effectively blocks dislocation motion and crack propagation, resulting in simultaneous enhancement of strength, hardness, and fracture toughness. The material exhibits near-isotropic mechanical response, maintaining high hardness across different crystal orientations.

Microstructural analysis via TEM and synchrotron X-ray diffraction reveals a defect-free, twin-dominated lattice with high elastic strain accommodation. Finite-element modeling confirms that the twin boundary network efficiently dissipates stress concentrations.

This synthesis strategy demonstrates a pathway for designing superhard yet tough covalent materials, bridging the traditional trade-off between hardness and fracture resistance, and enabling large-scale fabrication of high-performance diamond components for extreme mechanical and thermal environments.