Improvement of densification and microstructure of HfB ₂ ceramics by Ta/Ti substitution for Hf

Ultrafine HfB 2 powders were synthesized by the combination of borothermal reduction of HfO 2 and solid solution of 5 mol% TiB 2 or 5 mol% TaB 2 , prototypically, (Hf 0.95 Ti 0.05 )B 2 and (Hf 0.95 Ta 0.05 )B 2 . The influence of substitution on the particle growth, high‐temperature stability, densification, microstructure, and mechanical properties of HfB 2 was investigated. Results showed that the particle sizes of HfB 2 , (Hf 0.95 Ti 0.05 )B 2 and (Hf 0.95 Ta 0.05 )B 2 powders prepared by borothermal reduction at 1500°C were 1.73, 0.87, and 0.21 µm, respectively. The substitution of TaB 2 led to a greater decrease in particles size than TiB 2 . After heat treatment at 1800°C, the particle sizes of HfB 2 , (Hf 0.95 Ti 0.05 )B 2 and (Hf 0.95 Ta 0.05 )B 2 powders increased to 2.60, 1.59, and 0.32 µm, respectively, indicative of the good high‐temperature stability of TaB 2 ‐substituted HfB 2 . The relative densities of HfB 2 , (Hf 0.95 Ti 0.05 )B 2 and (Hf 0.95 Ta 0.05 )B 2 ceramics after spark plasma sintering at 2000°C were 76.1%, 85.2% and 99.8%, respectively. The fully dense (Hf 0.95 Ta 0.05 )B 2 ceramics with fine microstructure showed comparably high Vickers hardness of 21.1 GPa combined with flexural strength of 521.2 MPa. It was proved that the solid solution of TaB 2 could effectively inhibit the grain growth of HfB 2 powders, and improve the densification, microstructure, and mechanical properties of HfB 2 ceramics.