Equiatomic 9-cation high-entropy carbide ceramics of the IVB, VB, and VIB groups and thermodynamic analysis of the sintering process

The preparation of high-entropy (HE) ceramics with designed composition is essential for verifying the formability models and evaluating the properties of the ceramics. However, inevitable oxygen contamination in non-oxide ceramics will result in the formation of metal oxide impurity phases remaining in the specimen or even escaping from the specimen during the sintering process, making the elemental compositions of the HE phase deviated from the designed ones. In this work, the preparation and thermodynamic analysis during the processing of equiatomic 9-cation HE carbide (HEC9) ceramics of the IVB, VB, and VIB groups were studied focusing on the removing of the inevitable oxygen impurity existed in the starting carbide powders and the oxygen contamination during the powder mixing processing. The results demonstrate that densification by spark plasma sintering (SPS) by directly using the mixed powders of the corresponding single-component carbides will inhibit the oxygen-removing carbothermal reduction reactions, and most of the oxide impurities will remain in the sample as (Zr,Hf)O₂ phase. Pretreatment of the mixed powders at high temperatures in vacuum will remove most part of the oxygen impurity but result in a remarkable escape of gaseous Cr owing to the oxygen-removing reaction between Cr₃C₂ and various oxide impurities. It is found that graphite addition enhances the oxygen-removing effect and simultaneously prevents the escape of gaseous Cr. On the other hand, although WC, VC, and Mo₂C can also act as oxygen-removing agents, there is no metal-containing gaseous substance formation in the temperature range of this study. By using the heat-treated powders with added graphite, equiatomic HEC9 ceramics were successfully prepared by SPS.