Ultrafine‐grained B6O–diamond composites with superb mechanical properties.

We report here the high‐pressure and high‐temperature (HPHT) synthesis of well‐sintered B6O–diamond composites from B6O and carbon black nanopowders. The carbon black was transformed into diamond nanograins at HPHT conditions, and simultaneously formed high‐strength B6O–diamond interfaces. The ultrafine B6O and diamond nanograins and the high‐strength B6O–diamond interfaces synergistically construct excellent mechanical properties for the synthesized composites. The B6O–diamond composites possess a hardness (avg. 52 GPa) comparable to that of polycrystalline diamond (40–60 GPa), whereas the fracture toughness (avg. 7.2 MPa m1/2) is increased several times compared to previously synthesized polycrystalline B6O ceramics (1.7–3.1 MPa m1/2) and B6O‐based composites (3–4 MPa m1/2). Fracture behavior analysis demonstrates that the main toughening mechanisms in this B6O–diamond composite are nanotwin toughening, crack deflection, and crack bridging. [ABSTRACT FROM AUTHOR]