Thermodynamics aided design of hBN-capsulated diboride powders from novel nitrate precursors for high entropy ceramics

Hexagonal BN-coated powders have been widely used in various engineering sectors, however, their productions are restricted by the complexity of gas-solid reactions. In this study, guided by thermodynamics, a novel approach to synthesize Layer-structured hexagonal BN (hBN)-coated high entropy diboride powders in vacuum was developed, using metal salt Zr(NO3)4 center dot 5H2O, HfCl4, NbCl5, TaCl5, C16H36O4Ti, boric acid, and sucrose as raw materials. By adjusting the ratio of carbon to metal source (C/M), powders only consisting of two boride solid solutions and hBN were finally obtained, under an optimal processing condition of C/M = 5.5 and synthesis temperature of 1400 degrees C. Parts of hBN were found to coat on high-entropy metal diborides ceramic (HEB) particles, corresponding formation mechanism for core-shell structured powders was investigated, together with the liquid precursor assisted boro/carbothermal reduction process. Starting from as-synthesized core-shell powders, (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 -11 vol% hBN ceramics were densified at 1900 degrees C under 50 MPa without holding, with a high relative density of 97.3%.(c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.