Cr-assisted low-temperature densification of (Ti,Zr,Nb,Ta,Mo)C high-entropy carbides with ultrafine grain and enhanced hardness

While the use of low-melting-point metals as sintering aids for high-entropy carbide (HEC) ceramics has been well-established, their existence can compromise hardness due to residual metallic inclusions. This study demonstrates an innovative strategy to meet this challenge, where (Ti, Zr, Nb, Ta, Mo)C high-entropy carbide ceramics with ultrafine grain and enhanced hardness are obtained through Cr-metal-assisted spark plasma sintering at a temperature as low as 1600 ℃. Result shows that the addition of 5 vol.% Cr metal promotes the formation of highly densified single HEC phase ceramics with a high relative density (98.4 %) and an ultra-fine grained microstructure (0.17 μm). This low-temperature densification mechanism can be attributed to Cr's solid-solution effect within the matrix and the increased carbon vacancies generated during sintering. The grain size of the (Ti, Zr, Nb, Ta, Mo)C ceramics with 5 vol.% Cr metal addition is significantly smaller than that of Cr-free (Ti, Zr, Nb, Ta, Mo)C ceramics sintered at 2000 ℃ (3.03 μm) or via traditional low-temperature liquid-phase sintering (1.3-1.5 μm). Importantly, the addition of 5 vol.% Cr metal substantially increases the hardness of ceramics, with a remarkable increase from 23.57 GPa to 28.16 GPa as compared to pure (Ti, Zr, Nb, Ta, Mo)C ceramics, owing to fine-grain strengthening and solid-solution strengthening mechanism. This work highlights the uniqueness of Cr metal as a sintering aid in achieving densification and hardness improvements in (Ti, Zr, Nb, Ta, Mo)C ceramics, offering a promising strategy of property improvement for further development of HEC materials in the near future.