Preparation of molybdenum borides by combustion synthesis involving solid-phase displacement reactions

Preparation of molybdenum borides of five different phases in the Mo–B binary system (including Mo2B, MoB, MoB2, Mo2B5, and MoB4) was performed by self-propagating high-temperature synthesis (SHS) with two kinds of the reactant samples. When elemental powder compacts with an exact stoichiometry corresponding to the boride phase were employed, self-sustaining reaction was only achieved in the sample with Mo:B = 1:1 and nearly single-phase MoB was yielded. Therefore, the other four boride compounds were prepared from the reactant compacts composed of MoO3, Mo, and B powders, within which the displacement reaction of MoO3 with boron was involved in combustion synthesis. Experimental evidence shows that the extent of displacement reaction in the overall reaction has a significant impact on sustainability of the synthesis reaction, combustion temperature, reaction front velocity, and composition of the end product. An increase in the solid-phase displacement reaction taking place during the SHS process contributes more heat flux to the synthesis reaction, thus resulting in the increase of combustion temperature and enhancement of the reaction front velocity. Based upon the XRD analysis, formation of Mo2B, MoB2, and Mo2B5 as the dominant boride phase in the end product was successful through the SHS reaction with powder compacts under appropriate stoichiometries between MoO3, Mo, and B. However, a poor conversion was observed in the synthesis of MoB4. The powder compact prepared for the production of MoB4 yielded mostly Mo2B5.