Low-temperature densification of high entropy diboride based composites with fine grains and excellent mechanical properties

Abstract

High entropy diborides require high temperature (>2000 °C) for densification, leading to grain coarsening and deteriorated mechanical properties. Herein, fully dense and fine-grained (Hf, Zr, Ta, Nb, Ti)B2 ceramics were successfully fabricated by spark plasma sintering at 1500 °C using metallic cobalt as additives. The result showed that the relative density of (Hf, Zr, Ta, Nb, Ti)B2 increased tremendously from 49.3% to 99.5% as 5 vol% cobalt (HEB5C-15) were introduced. The Co–B based liquid phase was reactively formed during sintering, accelerating the densification by wetting grain boundaries and resulting the presence of Co segregation. Due to the significantly reduced sintering temperature, sluggish diffusion characteristic of the high entropy matrix and the segregation drag effect, the resulting (Hf, Zr, Ta, Nb, Ti)B2 grain size was as small as 0.5 μm. Vickers hardness of HEB5C-15 densified at 1500 °C was 24.90 ± 1.39 GPa, at least 20% higher than the corresponding value in the cobalt free counterpart densified at 2000 °C.