Room-temperature synthesis of refractory borides: A case study on mechanochemistry and characterization of Mo-borides and W-borides

Mo-boride and W-boride powders were produced from native boron oxide, magnesium, and related metal oxide starting materials by mechanochemical synthesis (MCS) followed by an purification treatment. The reaction formation mechanisms and the products were predicted with the FactSage™ thermochemical simulation program. Different conditions were tested to determine the optimum synthesis parameters. MCS was conducted at stoichiometric ratios and different milling durations, using excess reactant amounts over the determined optimum time. After MCS, unwanted phases were removed by HCl acid leaching. Detailed phase analyses of the final powders were obtained by X-ray diffractometer (XRD), whereas detailed microstructure characterization was conducted by scanning electron microscope/energy dispersion spectrometer (SEM/EDS), transmission electron microscope (TEM) and particle size analyzer. Among the utilized parameters, the ideal composition chosen for Mo boride synthesis was 6 h milled and leached MoO₃-100 wt% B₂O₃-50 wt% Mg (1.41 μm), including α-MoB, β-MoB, MoB₂, Mo₂B, Mo₂B₅, and Mo phases. For the synthesis of W boride, the proper composition was found as WO₃-100 wt% B₂O₃-50 wt% Mg (0.37 μm) containing W₂B₅, WB, β-WB, WB₄, W₂B, and W phases after milling for 20 h and leaching. Besides, as a result of the oxidation resistance measurements at 700 and 800 °C, phases belonging to MoO₂ and WO₂ were found along with boride phases.