Processing of dense high-entropy boride ceramics

Dense (Hf-0.2,Zr-0.2,Ti-0.2,Ta-0.2,Nb-0.2)B-2 high-entropy ceramics with high phase purity were produced by two-step spark plasma sintering of precursor powders synthesized by boro/carbothermal reduction of oxides. The reacted powders had low oxygen (0.404 wt%) and carbon (0.034 wt%) contents and a sub-micron average particle size (similar to 0.3 pm). Powders were synthesized by optimizing the excess B4C content of the reaction mixture and densified by a two-step spark plasma sintering process. The relative density increased from 98.9% to 99.9% as the final sintering temperature increased from 2000 degrees C to 2200 degrees C. The resulting ceramics were nominally singlephase (Hf,Zr,Ti,Ta,Nb)B-2 with oxygen contents as low as 0.004w% and carbon as low as 0.018w%. The average grain size increased from 2.3 +/- 1.2 mu m after densification at 2000 degrees C to 4.7 +/- 1.8 mu m after densification at 2100 degrees C, while significant grain growth occurred during sintering at 2200 degrees C. The high relative densities, low oxygen and carbon contents, and fine grain sizes achieved in the present study were attributed to the use of synthesized precursor powders with high purity and fine particle size, and the two-step synthesis-densification process. These are the first reported results for dense high-entropy boride ceramics with high purity and fine grain size.