Effect of excess boron oxide on the formation of tungsten boride nanocomposites by mechanically induced self-sustaining reaction

The influence of the type of reducing agent, milling time and excess boron oxide on the formation of tungsten boride nanocomposites by mechanically induced self-sustaining reaction was studied. The powder mixtures were mechanically activated using a high-energy planetary ball mill under two distinct experimental conditions. In the first manner, a mixture of tungsten trioxide, boron oxide and graphite with the stoichiometric composition was milled for different times. In the second approach, a mixture of tungsten trioxide, boron oxide and elemental magnesium with a molar ratio of 2:x:y (with x=2.5-6.25 and y=13.5-24.75) was activated for 1 and 30 h. In the presence of 30-50 wt% excess boron oxide, WB was formed during milling. Further increasing the boron oxide content to 150 wt% led to a significant change in the mechanochemical behavior of the system so that WB and W2B5 became more dominant after 1 h of milling. During leaching in 18% HCl aqueous solution, MgO was completely removed and consequently tungsten boride nanocomposites with high phase purity were obtained. According to microscopic observations, the 30 h milled sample showed an average particle size of about 95 nm after the leaching process.