Phase Formation, Microstructure and Mechanical Properties of (V,Nb,Ti,Ta,Zr)C High-Entropy Ceramics Under Spark Plasma Sintering: Thermal Decoupling of Densification and Cation Homogenization

In this paper, the relationship between the temperature regimes of spark plasma sintering (SPS), the evolution of the phase composition, the local atomic structure, and the mechanical properties of high-entropy ceramics of equiatomic composition (V,Nb,Ti,Ta,Zr)C has been established, and the temperature interval of complete cation homogenization has been determined. A series of samples was obtained by SPS in the temperature range of 1700–2100 °C; the evolution of the structure and properties was traced by a set of complementary methods—XRD with Rietveld refinement, XPS, EXAFS, NEXAFS, SEM-EDS, dilatometry, and hardness measurements. It is shown that cation homogenization proceeds non-monotonically and does not coincide in temperature with the densification of the powder body. The shrinkage kinetics indicate that the liquid-phase densification mechanism, caused by the presence of residual metallic niobium in the initial NbC powder, is realized at 1470–1550 °C and is completed already by 1760–1790 °C, whereas further heating up to 2100 °C serves exclusively for the diffusion mixing of cations. According to EXAFS, the nearest carbide environment of niobium stabilizes already at 1800 °C and does not change substantially with a further increase in the sintering temperature. At 2100 °C, according to XRD, a single cubic carbide phase of the NaCl type forms. Changes in the XPS chemical shifts and in the parameters of the local environment of Nb according to EXAFS are consistent with a rearrangement of the cation sublattice but do not allow one to unambiguously establish a completely random distribution of cations. At 1900 °C, a local maximum of microstrains of the NbC-like phase and the largest positive shift in the carbide component of Ta 4d5/2 are observed. The competition of two strengthening mechanisms has been revealed: solid-solution strengthening increases monotonically with temperature, reaching a maximum microhardness of HV0.2 = 2896 at 2100 °C, whereas the maximum hardness HV30 = 2930 was recorded at 1800 °C due to the contribution of interphase boundaries in the still multiphase structure; the fracture toughness in all regimes remains low (K1C < 1 MPa·m1/2), and the fracture character changes from radial Palmqvist cracks to local chipping as the interphase boundaries disappear. The results obtained show that densification and cation homogenization in high-entropy carbides are kinetically and temperature-separated processes, and the choice of the SPS technological regime should be determined not by the maximum achievable temperature, but by the temperature-time resource sufficient for the completion of cation diffusion after the end of shrinkage.