Microstructure and mechanical properties of fine-grained boron carbide ceramics fabricated by high-pressure hot pressing combined with high-energy ball milling

Dense and fine-grained boron carbide (B4C) ceramics were fabricated via high-pressure hot pressing (100 MPa) using powders, which are prepared by high-energy ball milling. These powders were sintered at a low temperature (1800 °C) without any sintering aid. The dense and fine-grained B4C ceramics demonstrate super high hardness, outstanding fracture toughness and modern flexure strength. The milled powders were characterised by disordered crystal structure and ultrafine particle size that ranges from a few nanometres to a few hundred nanometres. The combined contributions of high pressure and the characteristic of the milled powders guaranteed that the dense fine-grained microstructure was achieved at only 1800 °C. The grain size distribution of the ceramics was inhomogeneous and ranged from 70 nm to 1.6 µm. However, the average grain size was fine at only 430 nm, which partially contributed to the super high hardness of the B4C ceramics. The locally concentrated areas of the small grains changed the fracture mode of the B4C ceramics from the complete transgranular fracture to a mixture of transgranular and intergranular fractures, thereby enhancing the toughness of the B4C ceramics. The relative density, Vickers hardness, flexure strength and fracture toughness of the obtained B4C ceramics reached up to 99.5%, 41.3 GPa, 564 MPa and 4.41 MPa m1/2, respectively.