Synthesis of bulk nanocrystalline HfB₂ from HfCl₄–NaBH₄–Mg ternary system

This study reports on the synthesis and consolidation of pure HfB₂ powders starting from HfCl₄–NaBH₄–Mg blends via autoclave processing, annealing and purification followed by pressureless sintering (PS, with 2 wt% Co aid) or spark plasma sintering (SPS). During autoclave reactions conducted at 500 °C for 12 h under autogenic pressure, excess amounts of NaBH₄ were utilized to investigate its effects on the reaction products and mechanism. A subsequent washing (with distilled water), annealing (at 750, 1000 and 1700 °C) and acid leaching (HCl) were applied on the as-synthesized products. Pure HfB₂ powders with an average particle size of 145 nm were obtained after autoclave synthesis in the presence of 200 wt% excess NaBH₄, washing, annealing at 1000 °C for 3 h and 6 M HCl leaching. SPS sample has higher relative density and microhardness values (94.18% and 20.99 GPa, respectively) than those of PS sample (90.14% and 14.85 GPa). Relative wear resistance was improved considerably (8.2 times) by employing SPS technique.