Preparation of MoB and MoB–MoSi 2 composites by combustion synthesis in SHS mode

Abstract

Combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) was carried out in the Mo–B and Mo–B–Si systems for the preparation of molybdenum boride MoB and the composite of MoB–MoSi 2 from elemental powder compacts. Under a preheating temperature above 150 °C, the reaction of Mo with boron in the sample compact of Mo:B = 1:1 is characterized by a planar combustion front propagating in a self-sustaining and steady manner. As the preheating temperature or sample compaction density increased, combustion temperature was found to increase and the propagation rate of the combustion front was correspondingly enhanced. Moreover, the XRD analysis provides evidence of yielding nearly single-phase α-MoB from the Mo–B sample at equiatomic stoichiometry. In the synthesis of MoB–MoSi 2 composites, the starting stoichiometry of the Mo–B–Si powder compact was varied so as to produce the final composites containing 20–80 mol% MoB. It was also found the increase of flame-front velocity and combustion temperature with increasing MoB content formed in the composite. The composition analysis by XRD shows excellent conversion from the Mo–B–Si powder compact to the MoB–MoSi 2 composite through the SHS reaction; that is, in addition to a small amount of Mo 5Si 3, the as-synthesized composite is composed entirely of MoB and MoSi 2.