Interfacial thermodynamic regulation by HfC binder enables concurrent toughness and superhardness in PcBN composites

Achieving simultaneous enhancement of fracture toughness and intrinsic superhardness in polycrystalline cubic boron nitride (PcBN) remains an essential yet long-standing challenge in the development of cutting tool materials. Here, we propose a multiphase interfacial design strategy enabled by HfC binder to regulate interfacial thermodynamics and crack-resistance mechanisms in cBN/TiN/Al composites consolidated under moderate HPHT conditions (4.5–5.5 GPa, 1100–1350 °C). The optimized composition with 5 wt% HfC exhibits a flexural strength of 1675 MPa, a Vickers hardness of 3721 HV, and a relative density of 97.4%, revealing a robust toughness–hardness co-optimization. Microstructural characterization shows that HfC particles selectively distribute within the polycrystalline layers and at the interfaces, constructing metallic–ceramic hybrid interlocking bridges that effectively activate crack-deflection, bridging, and grain pull-out mechanisms. Dry turning tests on hardened Cr12 steel further verify the enhanced damage tolerance, reflected by significantly reduced flank-wear (VB) deterioration after 3000 m cutting distance. Complementary first-principles calculations demonstrate that HfC incorporation reduces the interfacial energy from 2.204 to 1.159 J·m⁻2 at the BN(111)/Co₃W₃C(110) interface, quantitatively confirming interfacial stabilization and explaining the suppressed binder migration. This work establishes a mechanistically driven interfacial engineering route toward achieving simultaneous toughness and superhardness in PcBN, providing design guidance for extreme-performance tool materials.