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

Boron carbide-alumina (B 4 C–Al 2 O 3) composite ceramics were fabricated using boron carbide (B 4 C) as the matrix and alumina (Al 2 O 3) as the second phase by hot-press sintering at 1950 °C. The phase composition, microstructure, relative density, mechanical properties, and toughening mechanism of the composite ceramics were evaluated. When the content of Al 2 O 3 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 B 4 C-30 wt%Al 2 O 3 composite ceramics reach up to 98.8 %, 24.7 GPa, 477 MPa, and 4.64 MPa m1/2, respectively. The introduction of Al 2 O 3 as the second phase can significantly improve the fracture toughness of the B 4 C ceramics. Firstly, when cracks extend to the Al 2 O 3 grains, the cleavage fracture of the Al 2 O 3 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 B 4 C and Al 2 O 3 , 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 B 4 C grains can inhibit crack extension. Thirdly, submicron-sized Al 2 O 3 grains wrapped within the B 4 C 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 B 4 C–Al 2 O 3 composite ceramics. [ABSTRACT FROM AUTHOR]