Preparation, microstructure and toughening mechanism of superhard ultrafine-grained boron carbide ceramics with outstanding fracture toughness

Dense ultrafine-grained B4C ceramics with superhigh hardness and outstanding fracture toughness were fabricated via spark plasma sintering at 1700 °C and 80 MPa. This process used the ultrafine B4C powder prepared by high-energy ball milling as starting powder. During sintering, densification began earlier at 1050 °C and the maximum densification rate appeared earlier at 1500 °C under the influence of high pressure and high sintering activity of the powder. The B4C ceramics possessed ultrafine grains, with average grain size of 370 nm, which contributed to the superhigh hardness. Many small B4C grains with size of less than 200 nm were individually dispersed among large B4C grains or accumulated to form congregated areas. The small grains induced crack deflection, particle pullout and crack bridging, thereby enhancing the toughness of the ceramics. The relative density, Vickers hardness and fracture toughness of the B4C ceramics reached 99.4 ± 0.49%, 41.1 ± 0.9 GPa and 4.57 ± 0.12 MPa m1/2, respectively.