Room-temperature mechanochemical synthesis and consolidation of nanocrystalline HfB₂-HfO₂ composite powders

This study reports on the in-situ preparation of HfB₂-HfO₂ composite powders at room temperature by means of mechanochemical synthesis (MCS) from HfCl₄-B₂O₃-Mg powder blends. The effects of milling duration and excess amounts of B₂O₃ and Mg reactants (20 and 30 wt%) on the HfB₂ formation mechanism were investigated. After MCS and purification, HfB₂, HfO₂ and Mg₂Hf₅O₁₂ phases were obtained. The Mg₂Hf₅O₁₂ phase decomposed during the annealing treatment conducted at 1000 °C under Ar flow. The as-synthesized, purified, annealed and subsequently leached powders were characterized with an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM), transmission electron microscope (TEM) and particle size analyzer (PSA). The HfB₂-HfO₂ composite powders with an average particle size of 140 nm and predominantly rounded morphology were consolidated with cold pressing/pressureless sintering (PS) and spark plasma sintering (SPS) techniques. The relative density values of the HfB₂-HfO₂ composites obtained by means of PS (with 2 wt% Co) and SPS techniques were 91.82 % and 93.79 %, respectively. A relatively high densification rate for the HfB₂-HfO₂ ceramic was achieved by means of Co addition, which was considered a promising sintering aid for HfB₂-based ceramics. The HfB₂-HfO₂ composite sample consolidated with SPS exhibited hardness, wear volume loss amount and friction coefficient values of 18.45 GPa, 4.30 mm³ and 0.60, respectively.