Formation of Mo5Si3/Mo3Si–MgAl2O4 Composites via Self-Propagating High-Temperature Synthesis

Abstract

In situ formation of intermetallic/ceramic composites composed of molybdenum silicides (Mo5Si3 and Mo3Si) and magnesium aluminate spinel (MgAl2O4) was conducted by combustion synthesis with reducing stages in the mode of self-propagating high-temperature synthesis (SHS). The SHS process combined intermetallic combustion between Mo and Si with metallothermic reduction of MoO3 by Al in the presence of MgO. Experimental evidence showed that combustion velocity and temperature decreased with increasing molar content of Mo5Si3 and Mo3Si, and therefore, the flammability limit determined for the reaction at Mo5Si3 or Mo3Si/MgAl2O4 = 2.0. Based upon combustion wave kinetics, the activation energies, Ea = 68.8 and 63.8 kJ/mol, were deduced for the solid-state SHS reactions producing Mo5Si3– and Mo3Si–MgAl2O4 composites, respectively. Phase conversion was almost complete after combustion, with the exception of trivial unreacted Mo existing in both composites and a minor amount of Mo3Si in the Mo5Si3–MgAl2O4 composite. Both composites display a dense morphology formed by connecting MgAl2O4 crystals, within which micro-sized molybdenum silicide grains were embedded. For equimolar Mo5Si3– and Mo3Si–MgAl2O4 composites, the hardness and fracture toughness are 14.6 GPa and 6.28 MPa m1/2, and 13.9 GPa and 5.98 MPa m1/2, respectively.