Microstructural evaluation of ZrB₂/ZrO₂ ceramic powders prepared by milling-assisted magnesiothermic reduction of oxide raw materials

This study reports on the synthesis and microstructural evaluation of ZrB₂/ZrO₂ ceramic powders prepared by milling-assisted magnesiothermic reduction of oxide raw materials. Powder blends containing ZrO₂, B₂O₃ and Mg reactants were milled in different type of mills at different durations and were subsequently annealed in a tube furnace under Ar atmosphere. An additional purification step (HCl leaching) was conducted on the milled and annealed samples to obtain only Zr-based products. FactSage™ thermochemical software was used in order to show a preliminary route for the experiments. The effects of milling duration (up to 100 h), milling type (a SPEX™ 8000D Mixer/Mill and a planetary ball mill), excess Mg amount (5–20 wt.%) and annealing duration (6 and 12 h) on the formation and microstructure of the products were examined. The milled, annealed and leached products were characterized using an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM) and differential scanning calorimeter (DSC). Pure ZrB₂/ZrO₂ ceramic powders having particles in size range of 200 nm–1 μm and containing two different crystal structures of ZrO₂ phase (monoclinic and tetragonal) were obtained after milling in the SPEX™ 8000D Mixer/Mill for 30 h, annealing at 1200 °C for 12 h nad el aching with 5 M HCl.