Formation of Mo(5)Si(3)/Mo(3)Si–MgAl(2)O(4) Composites via Self-Propagating High-Temperature Synthesis

In situ formation of intermetallic/ceramic composites composed of molybdenum silicides (Mo(5)Si(3) and Mo(3)Si) and magnesium aluminate spinel (MgAl(2)O(4)) 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 MoO(3) by Al in the presence of MgO. Experimental evidence showed that combustion velocity and temperature decreased with increasing molar content of Mo(5)Si(3) and Mo(3)Si, and therefore, the flammability limit determined for the reaction at Mo(5)Si(3) or Mo(3)Si/MgAl(2)O(4) = 2.0. Based upon combustion wave kinetics, the activation energies, E(a) = 68.8 and 63.8 kJ/mol, were deduced for the solid-state SHS reactions producing Mo(5)Si(3)– and Mo(3)Si–MgAl(2)O(4) 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 Mo(3)Si in the Mo(5)Si(3)–MgAl(2)O(4) composite. Both composites display a dense morphology formed by connecting MgAl(2)O(4) crystals, within which micro-sized molybdenum silicide grains were embedded. For equimolar Mo(5)Si(3)– and Mo(3)Si–MgAl(2)O(4) composites, the hardness and fracture toughness are 14.6 GPa and 6.28 MPa m(1/2), and 13.9 GPa and 5.98 MPa m(1/2), respectively.