Improvement of densification and microstructure of HfB2 ceramics by Ta/Ti substitution for Hf

Ultrafine HfB₂ powders were synthesized by the combination of borothermal reduction of HfO₂ and solid solution of 5 mol% TiB₂ or 5 mol% TaB₂, prototypically, (Hf_0.95Ti_0.05)B₂ and (Hf_0.95Ta_0.05)B₂. The influence of substitution on the particle growth, high-temperature stability, densification, microstructure, and mechanical properties of HfB₂ was investigated. Results showed that the particle sizes of HfB₂, (Hf_0.95Ti_0.05)B₂ and (Hf_0.95Ta_0.05)B₂ powders prepared by borothermal reduction at 1500°C were 1.73, 0.87, and 0.21 µm, respectively. The substitution of TaB₂ led to a greater decrease in particles size than TiB₂. After heat treatment at 1800°C, the particle sizes of HfB₂, (Hf_0.95Ti_0.05)B₂ and (Hf_0.95Ta_0.05)B₂ powders increased to 2.60, 1.59, and 0.32 µm, respectively, indicative of the good high-temperature stability of TaB₂-substituted HfB₂. The relative densities of HfB₂, (Hf_0.95Ti_0.05)B₂ and (Hf_0.95Ta_0.05)B₂ ceramics after spark plasma sintering at 2000°C were 76.1%, 85.2% and 99.8%, respectively. The fully dense (Hf_0.95Ta_0.05)B₂ 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 TaB₂ could effectively inhibit the grain growth of HfB₂ powders, and improve the densification, microstructure, and mechanical properties of HfB₂ ceramics.