Microstructure, mechanical, and thermal properties of compositionally complex (Hf,Zr,Nb,Ti)B₂‒LaB₆ ceramics

Novel compositionally complex borides, (Hf,Zr,Nb,Ti)B₂ and (Hf,Zr,Nb,Ti)B₂‒LaB₆, were fabricated using spark plasma sintering process. (Hf,Zr,Nb,Ti)B₂‒LaB₆ exhibits a dual-phase microstructure, in which (Hf,Zr,Nb,Ti)B₂ is a primary phase with the hexagonal structure and LaB₆ is a secondary phase with a cubic structure. The mechanical properties of both (Hf,Zr,Nb,Ti)B₂ and (Hf,Zr,Nb,Ti)B₂‒LaB₆ are comparable, with a combination of high hardness and moderate fracture toughness. Thermal diffusivity and conductivity of (Hf,Zr,Nb,Ti)B₂ are much lower than the individual transition metal borides but are significantly increased by the addition of LaB₆. Herein, it is implied that the thermal properties of boride ceramics can be controlled through the appropriate design of principal metal element compositions.