Synthesis, microstructure, and mechanical properties of novel high entropy carbonitrides

A series of novel (Hf0.25Ta0.25Zr0.25Nb0.25)CxN1–x (x= 1, 0.9, 0.8, 0.7, 0.6, and 0.5) high entropy ceramics were successfully synthesised from transition metal monocarbides and mononitrides using spark plasma sintering. The effect of the [N]/([C]+[N]) ratio on the microstructure, phase compositions, bonding characteristics, and mechanical properties of high entropy carbonitrides was investigated for the first time. The lattice parameter of the solid solution phases gradually decreased and the theoretical density gradually increased with increasing nitrogen ratio, while the grain size showed no noticeable change in the oxide-free materials. There was no obvious change in the chemical bonding with increasing nitrogen ratio as determined from the XPS data. The hardness and modulus of the HECxN1–x ceramics both decreased by about 12% when the [N]/([C]+[N]) ratio increased from 0 to 0.5. Compared with the rule of mixture values of the monocarbides/mononitrides for each composition, the HECxN1–x ceramics showed an increase of 30-37% in hardness and 7-11% in indentation modulus. Both the electronic state and bonding characteristic of the individual components and the solid solutions play an important role in the mechanical properties of multi-component ceramics.