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 grains and enhanced hardness are obtained through chromium (Cr)-metal-assisted spark plasma sintering (SPS) at a temperature as low as 1600 °C. The results show that the addition of 5 vol% Cr promotes the formation of highly densified single HEC phase ceramics with a high relative density (98.4%) and an ultrafine-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 °C (3.03 μm) or via traditional low-temperature liquid-phase sintering (1.3–1.5 μm). Importantly, the addition of 5 vol% Cr substantially increased the hardness of the ceramics, with a remarkable increase from 23.57 to 28.16 GPa compared to that of the pure (Ti,Zr,Nb,Ta,Mo)C ceramics, owing to the fine-grain strengthening and solid-solution strengthening mechanisms. 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 for improving the properties of HEC materials for further development in the near future.