Dense high-entropy boride ceramics with ultra-high hardness

Two‐step synthesis process for high‐entropy diboride powders

“…Heating to 2000°C also further reduced the oxygen content to 0.3 wt%, while the carbon content increased slightly to 0.3 wt% (Table ), presumably because diffusion of carbon from the graphite crucible and furnace became active at higher temperatures. The presence of oxide phases has been reported for HEB powders in previous studies. In the present study, (Hf,Zr,Ti,Ta,Nb)B 2 powders with higher purity were produced by the optimization of batch composition and reaction temperature resulting in lower oxygen (0.3 wt%) and carbon (0.3 wt%) contents than for previously reported processes.…”

“…Several process for the synthesis of HEB powders have been reported . Tallarita et al synthesized the precursors of (Hf,Mo,Ta,Nb,Ti)B 2 by self‐propagating high‐temperature synthesis (SHS) using elemental metal and B powders as the starting materials.…”

“…1-3 ZrB 2 and HfB 2 are the most studied boride UHTCs due to their melting temperatures in excess of 3000°C, thermal conductivities that can be over 100 W/m K at 25°C, superior mechanical properties, and good chemical stability. [4][5][6] Recently, high-entropy ultra-high temperature ceramics (HE-UHTCs), including high-entropy boride (HEB), [7][8][9][10][11][12][13] high-entropy carbide (HEC), [14][15][16][17][18] and high-entropy nitride (HEN) 19 ceramics have been reported. HEB materials are solid solutions of individual transition metal diborides that have attracted increasing interest due to the high thermal and electrical conductivities, and chemical stability.…”

“…Several process for the synthesis of HEB powders have been reported. [8][9][10][11][12] 4 C, and C. Oxide phases remained in the synthesized powders due to the loss of B during BCTR and the oxides were persistent through the densification of the HEB ceramics. Based on the previous studies, the presence of residual oxide impurities or excess B in HEB powders or precursors has been common among reported synthesis methods.…”

A step forward from high-entropy ceramics to compositionally complex ceramics: a new perspective

“…1). Although the initial work on high-entropy borides resulted in relatively low densities (~ 92%) due to significant oxide contamination from high-entropy ball milling, several subsequent studies quickly improved powder synthesis and fabrication methods that enhanced relative densities and properties [46,47,[49][50][51][52][53][54][55][56]. In 2018 and 2019, several groups [21,22,[57][58][59][60][61][62][63][64][65][66] also independently reported the fabrications of high-entropy carbides as another subclass of high-entropy UHTCs.…”