Synthesis, mechanical properties and thermal conductivity of high-entropy (TiTaNbZrMox)(CN) ceramics

Novel high-entropy (TiTaNbZrMoX)(CN) (x = 0, 5, 10, 15, and 20 at%) carbonitride ceramics were synthesised using the carbothermal reduction-nitridation and spark plasma sintering methods. The effects of Mo doping on the microstructure, mechanical properties, and thermal characteristics of these high-entropy ceramics were investigated. The increase in Mo contents caused increased grain size, uniform distribution of elements, improved Vickers hardness and fracture toughness, and reduced thermal conductivity at the same temperature. High-entropy (TiTaNbZrMo0.2)(CN) ceramic displayed superior Vickers hardness of 21.41 GPa and fracture toughness of 4.34 MPa.m(1/2), compared with 20.51 GPa and 3.94 MPa.m(1/2) for (TiTaNbZr)(CN) ceramic, respectively. Density functional theory calculations indicated that adding Mo atoms promoted the hybridisation of metal and carbon elements, thereby preferentially enhancing electron filling in the bonding orbital due to high valence electrons. This, in turn, improved the mechanical properties of (TiTaNbZrMox)(CN) ceramics. Additionally, the increased Mo contents strengthened covalent bonding and induced lattice distortion, reducing the thermal conductivity of high-entropy ceramics.