Fabrication, microstructure and mechanical properties of (Me_1/3Hf_1/3Nb_1/3)B₂(Me=Ti, Zr, Ta) ceramics

With (Me_1/3Hf_1/3Nb_1/3)B₂ (Me=Ti, Zr, Ta) powders self-synthesized via boron/carbothermal reduction, (Me_1/3Hf_1/3Nb_1/3)B₂ ceramics were prepared by spark plasma sintering (SPS), and their phase composition, relative density, microstructure and mechanical properties were investigated systematically. The result reveals that (Ti_1/3Hf_1/3Nb_1/3)B₂ and (Zr_1/3Hf_1/3Nb_1/3)B₂ possess the high relative density (~98%), while (Ta_1/3Hf_1/3Nb_1/3)B₂ has the lowest relative density (~93%). (Ti_1/3Hf_1/3Nb_1/3)B₂ possesses relatively coarse grains, while (Zr_1/3Hf_1/3Nb_1/3)B₂ and (Ta_1/3Hf_1/3Nb_1/3)B₂ possess relatively fine grains and Nb segregation is found in (Ta_1/3Hf_1/3Nb_1/3)B₂. (Ti_1/3Hf_1/3Nb_1/3)B₂ shows the most excellent mechanical properties with the Vickers hardness and fracture toughness of (20.08±0.81) GPa and (2.52±0.15) MPa·m^1/2, respectively, whereas the mechanical properties of (Zr_1/3Hf_1/3Nb_1/3)B₂ are inferior, with hardness and fracture toughness of (17.21±0.63) GPa and (1.50±0.18) MPa·m^1/2, respectively.