Mechanical properties and toughening mechanism of B4C-Al2O3 composite ceramics prepared by hot-press sintering

Boron carbide-alumina (B4C-Al2O3) composite ceramics were fabricated using boron carbide (B4C) as the matrix and alumina (Al2O3) as the second phase by hot-press sintering at 1950 degrees C. The phase composition, microstructure, relative density, mechanical properties, and toughening mechanism of the composite ceramics were evaluated. When the content of Al2O3 is 30 wt%, the composite ceramics show the best comprehensive mechanical properties. The relative density, Vickers hardness, flexural strength, and fracture toughness of the obtained B4C-30 wt%Al2O3 composite ceramics reach up to 98.8 %, 24.7 GPa, 477 MPa, and 4.64 MPa m1/2, respectively. The introduction of Al2O3 as the second phase can significantly improve the fracture toughness of the B4C ceramics. Firstly, when cracks extend to the Al2O3 grains, the cleavage fracture of the Al2O3 grains resulting from its crystal structure causes the crack deflection along the cleavage plane. Secondly, due to the mismatch of the coefficient of thermal expansion between B4C and Al2O3, residual stress is generated at the grain boundary of the two phases and within the grains. The tensile stress at the grain boundary can cause some cracks to extend along the grain boundary, making cracks deflected, and the compressive stress within the B4C grains can inhibit crack extension. Thirdly, submicron-sized Al2O3 grains wrapped within the B4C grains form a subboundary structure, which can lead to crack extension along the subboundary. All of these factors consume the crack extension energy, which can contribute to increasing the fracture toughness of B4C-Al2O3 composite ceramics.