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To obtain thermodynamically stable single-phase high-entropy monoborides (HEMBs), the formation abilities of two-compound monoborides were evaluated in advance by first principles calculations, which were constructed from two cation combinations chosen from 11 transition metals including Ti, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, and Ni. And this method was extended to HEMBs on predicting the relative feasibility of synthesis and the tendency of phase segregation from the mean mixing enthalpy (μ local) and the standard deviation (σ local). Results showed that the contribution of entropy to free energy for entropy-stable HEMBs is large, and HEMBs with small σ local are easy to obtain the higher single-phase formation ability. In addition, four HEMBs ceramic with Vickers hardness (41–45 GPa) were synthesized as superhard materials. This work provides a realistic theoretical guidance for the design and performance research of HEMBs, and will effectively promote the application of superhard HEMBs ceramics. [Display omitted] [ABSTRACT FROM AUTHOR]

The effects of high-energy ball milling (HEBM) on the sintering ability and microstructure of hafnium diboride (HfB 2) ultrahigh-temperature ceramics (UHTCs) were investigated. After HEBM, the average particle size range of the HfB 2 ceramics decreased to 10–20 nm with a slight alteration in the crystal structure. The microscopic grains of the sintered samples were homogeneous and fine in size. WC removed oxide impurities on the surface of the HfB 2 particles by reacting with the oxide impurities, which further improved the sintering ability of the HfB 2 ceramics. The densification mechanism of the HfB 2 ceramics was also investigated, and nearly fully dense HfB 2 (99.1%) was fabricated by reactive spark plasma sintering (R–SPS) at 2100 °C for 35 min under a pressure of 40 MPa. HEBM increased the surface energy of the fine particles, which provided a sufficient driving force to improve the sintering ability and significantly increase the densification of HfB 2. • HfB 2 ceramics with nearly full density (99.1% relative density) has been successfully prepared. • The high energy ball mill slightly causes lattice distortion and improves sintering property. • Study of the mechanism of densification and grain coarsening of HfB 2 during SPS. [ABSTRACT FROM AUTHOR]