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

Abstract

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 5min 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 12h. 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 190nm comprised the HfB2 phase with an amount of 79.6wt% in addition to the HfO2 phase and a very small amount of Mg2Hf5O12 phase after mechanical activation for 5min and autoclave processing for 12h. They were consolidated at 1700°C both by PS for 6h and SPS for 15min. 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 (~16GPa) and relative wear resistance (2.5) values of the HfB2-HfO2 ceramics consolidated by SPS.