Mechanically Activated Combustion Synthesis of Molybdenum Silicides and Borosilicides. Issue 1760 (2nd January 2015) - Alam, Mohammad S.
ABSTRACT: Molybdenum silicides and borosilicides are promising structural materials for advanced power plants. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. For example, molybdenum disilicide (MoSi2 ) has excellent oxidation resistance and poor mechanical properties, while Mo-rich silicides such as Mo5 Si3 (called T1 ) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi2 −T1 composites that combine high oxidation resistance of MoSi2 and good mechanical properties of T1 . Another approach involves the addition of boron to Mo-rich silicides for improving their oxidation resistance through the formation of a borosilicate surface layer. In particular, Mo5 SiB2 (called T2 ) phase and alloys based on this phase are promising materials. In the present paper, MoSi2 −T1 composites and materials based on T2 phase are obtained by mechanically activated self-propagating high-temperature synthesis (MASHS). To obtain denser products, the so-called SHS compaction (quasi-isostatic pressing of hot combustion products) has been employed. Thermal analysis has shown that SHS compaction significantly improves the oxidation resistance. Self-sustained combustion of Mo/Si/B mixtures for the formation of T2 phase becomes possible if the composition is designed for adding a more exothermic reaction of MoB formation. These mixtures exhibit spin combustion. Oxidation resistance of the obtained multi-phase Mo−Si−B materials is independent on the concentration of Mo phase in the products. The "chemical oven" technique has been used to obtain a single Mo5 SiB2 phase and an alloy consisting of α-Mo, Mo5 SiB2, and Mo3 Si phases.