Densification and mechanical properties of boron carbide prepared via spark plasma sintering with cubic boron nitride as an additive

Hexagonal boron nitride (h-BN) can reinforce boron carbide (B4C) ceramics, but homogeneous dispersion of h-BN is difficult to achieve using conventional methods. Herein, B4C/h-BN composites were manufactured via the transformation of cubic (c-) BN during spark plasma sintering at 1800 °C. The effects of the c-BN content on the microstructure, densification, and mechanical properties of B4C/h-BN composites were evaluated. In situ synthesized h-BN platelets were homogeneously dispersed in the B4C matrix and the growth of B4C grains was effectively suppressed. Moreover, the c-BN to h-BN phase transformation improved the sinterability of B4C. The sample with 5 vol.% c-BN exhibited excellent integrated mechanical properties (hardness of 30.5 GPa, bending strength of 470 MPa, and fracture toughness of 3.84 MPa⋅ m1/2). Higher c-BN contents did not significantly affect the bending strength and fracture toughness but clearly decreased the hardness. The main toughening mechanisms were crack deflection, crack bridging, and pulling out of h-BN.