High pressure synthesis of magnesium aluminate composites with MoSi2 and Mo5Si3 in a self-sustaining manner

Magnesium aluminate/MoSi2 and magnesium aluminate/Mo5Si3 composites were successfully prepared by combustion synthesis with a special emphasis on the thermodynamic of the reactions involved in the process. The process involves loading of a very weak exothermic formation reaction of MgAl2O4 onto a highly exothermic formation reaction of MoSi2 and Mo5Si3. The starting material was a blend of MoO3, SiO2, Al and MgO. The effect of Al grain size (−5 to −71 μm), MoO3 stoichiometric value (0.7–1.25x), MgO additions (15–25 wt%) and working pressure (50 bar) on the synthesis process were investigated. Microstructure of the combustion products was inspected by SEM. It was found that using −5 μm grain size Al was necessary for reaction completion. Higher stoichiometric value of MoO3 was found to be necessary to compensate for its volatilization from the reaction media. Addition of MgO as diluents reduced combustion temperature but unexpectedly it reacted with some amounts of SiO2 and formed Mg2SiO2 phase and Mo5Si3 instead of MoSi2. However, increasing the reaction pressure was found to be the most effective factor to suppress the MoO3 volatilization. Thermodynamic study of the process revealed the production of entire phases in molten states whereas the adiabatic temperatures of the formation reaction of MgAl2O4/MoSi2 and MgAl2O4/Mo5Si3 composites were 2408 and 3210 K, respectively.