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 ( h BN)-coated high entropy diboride powders in vacuum was developed, using metal salt Zr(NO 3 ) 4 ·5H 2 O, HfCl 4 , NbCl 5 , TaCl 5 , C 16 H 36 O 4 Ti, 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 h BN were finally obtained, under an optimal processing condition of C/ M  = 5.5 and synthesis temperature of 1400 °C. Parts of h BN 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, (Ti 0.2 Zr 0.2 Hf 0.2 Nb 0.2 Ta 0.2 )B 2 –11 vol% h BN ceramics were densified at 1900 °C under 50 MPa without holding, with a high relative density of 97.3%.