High-pressure synthesis of high-performance submicron-sized polycrystalline β-Si₃N₄ bulk without additives

Dense and submicron-sized polycrystalline β-Si₃N₄ bulk samples with average grain size of 263 ± 68 nm have been synthesized using micron-sized α-Si₃N₄ powder (2 μm) as the starting material by simultaneously tuning the pressure and temperature conditions. The measured Vickers hardness H_V of the submicron-sized polycrystalline β-Si₃N₄ could reach up to about 20 GPa, approaching the upper limit of the single crystal β-Si₃N₄. The measured fracture toughness K_IC of the polycrystalline β-Si₃N₄ has been enhanced from 3.0 to 4.1 MPa m^1/2, as much as 36%, while without sacrificing the hardness. This study demonstrates that using micron-sized powder as the starting material to synthesize submicron-sized polycrystalline bulks by phase transformation under extreme pressure conditions is an efficient pathway to break through the old paradigm—the higher the fracture toughness of a material, the lower the hardness.