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 B(2)O3 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 degrees 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-Hf-O2 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-Hf-O2 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-Hf(O2 )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 mm(3) and 0.60, respectively.