Preparation of tungsten borides by combustion synthesis involving borothermic reduction of WO3

An experimental study on the preparation of two tungsten borides, WB and W2B5, was conducted by self-propagating high-temperature synthesis (SHS), during which borothermic reduction of WO3 and elemental interaction of W with boron proceeded concurrently. Powder mixtures with two series of molar proportions of WO3:B:W = 1:5.5:x (with x = 1.16–2.5) and 1:7.5:y (with y = 0.5–1.33) were adopted to fabricate WB and W2B5, respectively. The starting stoichiometry of the reactant compact substantially affected the combustion behavior and the phase composition of the final product. The increase of metallic tungsten and boron reduced the overall reaction exothermicity, leading to a decrease in both combustion temperature and reaction front velocity. The initial composition of the reactant compact was optimized for the synthesis of WB and W2B5. In addition to small amounts of W2B and W2B5, the powder compact of WO3 + 5.5B + 2 W produced WB dominantly. Optimum formation of W2B5 was observed in the sample of WO3 + 7.5B + 0.85W. Experimental evidence indicates that an excess amount of boron about 10–13% is favorable for the formation of WB and W2B5.