Intermediate stage densification kinetics of high-entropy ceramics during spark plasma sintering

The intermediate stage densification kinetics of (Hf, Nb, Ta, Ti, Zr)B2 and (Hf, Nb, Ta, Ti, Zr)C were studied using a Coble creep rate model. (Hf, Nb, Ta, Ti, Zr)B2 was synthesized by boro/carbothermal reduction and (Hf, Nb, Ta, Ti, Zr)C was synthesized by carbothermal reduction. Both ceramics were densified by spark plasma sintering at 1700-1850 degrees C. The activation energies for densification were 575 f 53 kJ/mol for the high-entropy boride and 646 f 4 kJ/mol for the high-entropy carbide. The densification mechanisms were grain boundary diffusion for the high-entropy boride and lattice diffusion for the high-entropy carbide. Dissolution of Nb into the high- entropy phase was determined to be the rate limiting mechanism for densification of both the carbide and the boride.