Densification and mechanical properties of high-entropy diboride ceramics sintered using Ni activation

High-entropy diboride ceramics (HEBs) represent a novel and promising category within the realm of ultra-high temperature ceramics (UHTCs). Due to their exceptional performance in extreme service conditions, HEBs have emerged as the preferred choice for thermal structural components and thermal protection systems. However, the robust nature of covalent bonding, coupled with a low self-diffusion coefficient, presents significant challenges for the densification of HEBs. In addition, the inferior fracture toughness of HEBs severely hinders their promotion in large-scale applications as thermal structures and thermal protection materials. Currently, research on HEBs both domestically and internationally is still insufficient. Therefore, there is an urgent need for in-depth studies aimed at improving the densification and fracture toughness of HEBs. In this study, dense (Hf_0.2Zr_0.2Ti_0.2Ta_0.2Cr_0.2)B₂ and (Hf_0.2Zr_0.2Ti_0.2Ta_0.2Mo_0.2)B₂ ceramics were successfully prepared with Ni metal as an additive. It was demonstrated that the incorporation of 10 wt% Ni led to an enhancement in the relative densities of the two HEBs, achieving values of 97.24 % and 96.83 %, respectively. This improvement is likely attributable to the development of a Ni-B based liquid phase during the sintering process, which facilitated the densification of the ceramic by effectively wetting the grain boundaries. The fracture toughness of the two materials also reached maximum values of 6.76 MPa m^1/2 and 6.43 MPa m^1/2 when the Ni addition was 10 wt%. This enhancement in fracture toughness is attributed to the pinning effect exerted by the dispersed Ni particles within the ceramic matrix, which inhibits crack propagation.