Effects of carbon and silicon on microstructure and mechanical properties of pressureless sintered B₄C/TiB₂ composites

B₄C/TiB₂ ceramic composites were fabricated by reinforcing carbon and silicon (10 wt.% total mass) with the help of pressureless sintering at 2150 °C. The influence of C and Si additives on the phase composition, microstructures and mechanical properties of B₄C/TiB₂ ceramic composites were investigated through the characterizations of XRD, SEM, BSEM and EDS. Compared to B₄C/TiB₂ ceramic composites without C-Si additives, coarsely plate-like SiC grains were formed in composites at a C: Si mass ratio ∼0:10. By increasing the C:Si mass ratio, the size of the plate-like SiC grains decreases, while the amount of plate-like SiC grains increases. When the C:Si mass ratio was 7:3, multilamellar graphite appeared on the B₄C grain boundary. Moreover a large number of lamellar graphite grains appeared at a mass ratio of C:Si ∼10:0. Graphite reacted with B₂O₃ on the B₄C grain boundary, and the sintered body shrank quickly. Due to the presence of lamellar graphite, the relative density of the sintered body significantly increased from 89.2% to 98.9%. The mechanical properties such as flexural strength ∼336 MPa (∼26.79% increment) and fracture toughness ∼ 5.11 MPam^1/2 (∼21.19% increment) were significantly improved compared with the B₄C/TiB₂ ceramic composite.