Mechanical activation-assisted autoclave processing and sintering of HfB₂-HfO₂ ceramic powders

This study reports on the synthesis and consolidation of HfB₂-HfO₂ ceramic powders via mechanical activation-assisted autoclave processing followed by pressureless sintering (PS) or spark plasma sintering (SPS). HfCl₄, B₂O₃ and Mg starting powders were mechanically activated for 5 min to obtain homogeneously blended precursors with active particle surfaces. Autoclave synthesis was carried out at a relatively low temperature at 500 °C for 6 or 12 h. As-synthesized powders were purified from reaction by-products such as MgO and MgCl₂ by washing and acid leaching treatments. The characterization investigations of the as-synthesized and purified powders were performed by using an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM) and particle size analyzer (PSA). The purified powders with an average particle size of about 190 nm comprised the HfB₂ phase with an amount of 79.6 wt% in addition to the HfO₂ phase and a very small amount of Mg₂Hf₅O₁₂ phase after mechanical activation for 5 min and autoclave processing for 12 h. They were consolidated at 1700 °C both by PS for 6 h and SPS for 15 min. The Mg₂Hf₅O₁₂ phase decomposed during sintering and bulk samples only had the HfB₂ and HfO₂ phases. The bulk properties of the sintered samples were characterized in terms of microstructure, density, microhardness and wear characteristics. The HfB₂-HfO₂ ceramics consolidated by PS exhibited poor densification rates. A considerable improvement was obtained in the relative density (~91%), microhardness (~16 GPa) and relative wear resistance (2.5) values of the HfB₂-HfO₂ ceramics consolidated by SPS.