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In order to reduce the sintering temperature and improve the mechanical properties of B₄C ceramics, ZrB₂ was formed in situ using the SPS sintering method with ZrO₂ and B₄C as raw materials. Thermodynamic calculations revealed that CO pressure affected the formation of ZrB₂ at temperatures from 814 °C to 1100 °C. The experimental results showed that the ZrB₂ grain size was <5 µm and that the grains were uniformly distributed within the B₄C ceramics. With an increase in ZrO₂ content, the Vickers hardness and flexural strength of the B₄C ceramics first increased and then decreased, while the fracture toughness continuously increased. When the content of ZrO₂ was 15 wt%, the Vickers hardness, fracture toughness and flexural strength of B₄C ceramics were 35.5 ± 0.63 GPa, 3.6 ± 0.24 MPa·m^1/2 and 403 ± 10 MPa, respectively. These results suggest that ZrB₂ inhibits B₄C grain growth, eliminates crack tip stress, and provides fine grain to strengthen and toughen B₄C ceramics.