Room-Temperature Synthesis of Refractory Borides: a Case Study on Mechanochemistry and Characterization of Mo-Borides and W-Borides

Abstract

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 FactSageTM 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 mu m), including alpha-MoB, beta-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 mu m) containing W2B5, WB, beta-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 degrees C, phases belonging to MoO2 and WO2 were found along with boride phases.