Mechanically Activated Combustion Synthesis of Molybdenum Silicides and Borosilicides

摘要

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 (MoSi_2) has excellent oxidation resistance and poor mechanical properties, while Mo-rich silicides such as Mo_5Si_3 (called T_1) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi_2-T_1 composites that combine high oxidation resistance of MoSi_2 and good mechanical properties of T_1. 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, Mo_5SiB_2 (called T_2) phase and alloys based on this phase are promising materials. In the present paper, MoSi_2-T_1 composites and materials based on T_2 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 T_2 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 Mo_5SiB_2 phase and an alloy consisting of α-Mo, Mo_5SiB_2, and Mo_3Si phases.