Mechanically Activated Combustion Synthesis of Molybdenum Silicides and Borosilicides
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Mechanically Activated Combustion Synthesis of Molybdenum Silicides and Borosilicides
Mohammad S. Alam, Alan A. Esparza, and Evgeny Shafirovich Department of Mechanical Engineering, The University of Texas at El Paso, 500 W. University Ave., El Paso, TX 79968, USA
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 Morich silicides such as Mo5Si3 (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, Mo5SiB2 (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 Mo5SiB2 phase and an alloy consisting of α-Mo, Mo5SiB2, and Mo3Si phases. INTRODUCTION The thermal efficiency of gas-turbine power plants could be increased by the development of new structural materials based on molybdenum silicides and borosilicides, which can operate at temperatures higher than 1300 °C with no need for cooling [1-4]. Molybdenum disilicide (MoSi2) has a high melting point (2030°C) and excellent high-temperature oxidation resistance, but a single MoSi2 phase has low fracture toughness at room temperature and low strength at elevated temperatures [5]. The mechanical properties of Mo-richer silicides such as Mo5Si3 (called T1) phase and Mo3Si are better, but they have lower oxidation resistance [4,5]. Composites of MoSi2 with these materials could provide both acceptable mechanical properties and sufficiently good oxidation resistance.
Another approach focuses on adding boron to Mo-rich silicides [6]. The addition of boron improves oxidation resistance because of the formation of a borosilicate surface layer [1,6]. Currently, Mo5SiB2 (called T2) phase is considered as a
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