Fabrication of Mo5SiB2-based composites by combustion synthesis involving aluminothermic reduction of MoO3

Abstract

Fabrication of Mo5SiB2–Al2O3 composites with a broad range of the Mo5SiB2/Al2O3 ratio was conducted by thermite-based combustion synthesis in the SHS mode. Thermite reagents of MoO3 + 2Al were introduced into two Mo‒Si–B ternary systems adopting amorphous boron and MoB as their respective boron source. The boron-containing samples were more energetic and were applied to produce composites with Mo5SiB2/Al2O3 from 1.25 to 2.5, beyond which combustion was extinct. The composites with Mo5SiB2/Al2O3 from 0.8 to 1.3 were prepared from less exothermic MoB-based samples. Besides causing a decrease in combustion velocity and reaction temperature, the increase of the Mo5SiB2/Al2O3 proportion led to a transformation in combustion wave propagation from a steady to pulsating mode. For the samples featuring a pulsating combustion wave, the reaction time at peak combustion temperatures was extended and then the evolution of Mo5SiB2 from intermediate phases (Mo3Si and MoB) was significantly improved. The deduced activation energy suggests a lower kinetic barrier for thermite-based combustion synthesis to fabricate Mo5SiB2-based composites. The microstructure of synthesized composites indicates that quadrangular Mo5SiB2 grains with an average size of 10–15 µm are tightly packed and irregular Al2O3 grains are randomly dispersed.