Unlocking superior mechanical properties: the synergistic enhancement of hardness and fracture toughness in nanopolycrystalline tantalum diboride

Achieving optimal mechanical properties, including hardness and fracture toughness, by controlling grain size is a fundamental and long-standing objective in the development of hard and superhard transition metal borides (TMBs) ceramics. It is expected that the mechanical performance of TMBs will be substantially enhanced in nano-crystalline ceramics. However, the fabrication of dense, nano-scale TMBs compacts presents challenges due to poor sintering behavior and pronounced grain growth at high temperature. Here, thanks to the pressure reduced activation energy effect, nano-polycrystalline tantalum diboride (NP-TaB 2 ) monoliths were fabricated under high pressure and moderate temperature conditions. These NP-TaB 2 bulks achieve dense microstructure with an average grain size as fine as 36 nm, due to the high nucleation rates and minimal grain growth induced by high pressure. With the decreasing of grain size, the hardness of NP-TaB 2 reaches up to 27.5 GPa by the Hall-Petch effect, making it nearly 45% harder than dense, micron-scale grain specimens. Additionally, the fracture toughness of NP-TaB 2 is enhanced by 70% at the same time in the nano scale specimens, attributed to the effective energy dissipation by nano grains through crack deflection, branching, and bridging, which enhances fracture toughness with synergistic hardness improvement. This discovery demonstrates that correlating grain size and microstructure with mechanical properties offers valuable insights for enhancing the mechanical properties of TMBs, and potentially benefiting the manufacturing of scientific and industrial tools. Controlling grain size to optimize mechanical properties has been a fundamental and long-sought goal for hard/superhard transition metal borides ceramics. Highly dense nano-polycrystalline tantalum diboride is synthesized due to high nucleation rate and low grain growth under high pressure and high temperature method. The hardness and fracture toughness are greatly enhanced by the nanopolycrystalline microstructure with Hall-Petch effect.