Synthesis of high-purity (Ti,V,Ta,Nb,Hf)(C,N) powder via two-step method and its densification

A two-step method including low-temperature pretreatment and high-temperature homogenization was developed to prepare high-purity (Ti 0.2 V 0.2 Ta 0.2 Nb 0.2 Hf 0.2 )(C 0.75 N 0.25 ) powder. In the first step, the reaction mixture was heated to 1450 ℃ to form intermediate carbonitride phases through carbothermal reduction-nitridation, and the second step at 1700 ℃ produced a single-phase high-entropy carbonitride. The powder prepared by single-step treatment at 1750 ℃ contained 2.28 wt% nitrogen and 0.18 wt% oxygen. In contrast, the two-step powder exhibited higher nitrogen content (2.45 wt%) and lower oxygen content (0.09 wt%), and the formation of a complete single-phase solid solution enabled accurate calculation of its composition (Ti 0.2 V 0.2 Ta 0.2 Nb 0.2 Hf 0.2 )(C 0.78 N 0.21 O 0.01 ). The ceramic prepared from the two-step powder achieved 99.8% relative density, with an average grain size of 4.58 ± 0.32 μm, a Vickers hardness of 22.73 ± 0.51 GPa, and a fracture toughness of 3.16 ± 0.30 MPa·m 1/2 , whereas the single-step ceramic reached only 86.5% relative density.