Pressureless densification and properties of high-entropy boride ceramics with B4C additions

High entropy boride ceramics have great potential as structural materials serving in extreme environments. However, their applications are limited by the difficulty of sintering. In the present study, dense (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 ceramics with B4C additions were prepared through pressureless sintering at as low as 1900 ℃. Calculations based on the CALPHAD approach predict that (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 starts to melt at about 3315 ℃ whilst B4C additions reduce the temperature and broaden the temperature region where solid and liquid coexist. Results showed that the introduction of B4C could trigger the densification of (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 at a lower temperature and promote their densification significantly. The relative density of samples with 5 wt% of B4C additions sintered at 1900 and 2000 ℃ was 97.7% and 99.7%, respectively. While the sintering temperature was further increased to 2100 ℃, the liquid phase was reactively formed, leading to the rapid grain coarsening in samples with B4C additions. Strengthened by well-dispersed B4C grains, the sample with 5 wt% B4C sintered at 2000 ℃ exhibited excellent mechanical properties with the Vickers hardness, flexural strength, and fracture toughness of 21.07 ± 2.09 GPa, 547 ± 45 MPa, and 5.24 ± 0.14 MPa m1/2, which are comparable or even higher than counterparts sintered under pressure.