Mechanisms for low temperature densification and enhanced mechanical properties of (Ti, Zr, Hf, Nb, Ta)(C, N) using CrSi2 as an additive: formation of (Ti, Zr, Nb)2Cr4Si5 and grain boundary strengthening
High-entropy carbonitride ultra-high temperature ceramics (HECN-UHTCs) typically require high densification temperatures, leading to grain coarsening and degraded mechanical properties. This study introduces CrSi2 as a sintering additive for (Ti, Zr, Hf, Nb, Ta)(C, N), effectively reducing the densification temperature by 200 °C. During sintering, interdiffusion and cation exchange result in the formation of an orthorhombic (Ti, Zr, Nb)2Cr4Si5 phase within the ceramic matrix. The resulting dual-phase ceramic exhibits a hardness of 24.65 ± 0.23 GPa and a fracture toughness of 6.03 ± 0.48 MPa m1/2, significantly surpassing most reported HECN-UHTCs. Enhanced mechanical properties are attributed to crack deflection, increased localized lattice strain, and Cr grain boundary segregation. This liquid phase-assisted low-temperature sintering strategy offers a promising pathway for densifying other ultra-high temperature ceramics.