Effect of sintering temperature on the mechanical properties and microstructures of pressureless-sintered B₄C/SiC ceramic composite with carbon additive

B₄C/SiC (15 wt%) ceramic composite with 2 wt% carbon additive was fabricated via solid-phase pressureless sintering under an argon atmosphere for 60 min between 2100 °C and 2200 °C. The effects of the sintering temperature on the microstructure and mechanical properties of the B₄C/SiC ceramic composites were studied in detail. The optimum values of the relative density, Vickers hardness, flexural strength, and fracture toughness of the B₄C/SiC ceramic composite sintered at 2150 °C were 95.3%, 25.5 GPa, 296 MPa, and 2.81 MPa m^1/2, respectively. The microstructural investigations revealed that when the sintering temperature increases from 2100 °C to 2150 °C, sintered necks begin to grow and the densification zone increases. With sintering temperatures above 2150 °C, the grains grow rapidly such that the pores are not eliminated, causing the formation of large closed pores and resulting in a decrease in the relative density and mechanical properties of the B₄C/SiC ceramic composite. The plate-like and irregular SiC grains are uniformly dispersed in the B₄C matrix to inhibit B₄C grain growth. The interface between SiC and B₄C is clean and narrow, and the clean boundary suggests strong binding, which contributes to the mechanical properties of the B₄C/SiC ceramic composite. At 2150 °C, the C additive undergoes significant crystallisation, forming lamellar structures with high aspect ratios, thereby promoting the pores in the ceramic composite to be discharged to the outside through volume diffusion to obtain a denser sintered body. The interface between B₄C and graphite is a semi-coherent interface that increases the flexural strength with a low defect concentration. The main toughening mechanisms of the B₄C/SiC ceramic composite are crack deflection and crack branching around the SiC grains and crack bridging by SiC grains in the wake of the crack tip.