Mechanically activated combustion synthesis of molybdenum silicides and borosilicides for ultrahigh-temperature structural applications

Abstract

Molybdenum silicides and borosilicides are promising structural materials for gas-turbine 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 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 is considered as an attractive material. In the present paper, MoSi2−T1 composites and materials based on T2 phase are obtained by mechanically activated SHS. Use of SHS compaction (quasi-isostatic pressing) significantly improves oxidation resistance of the obtained MoSi2−T1 composites. Combustion of Mo−Si−B mixtures for the formation of T2 phase becomes possible if the composition is designed for the addition of more exothermic reactions leading to the formation of molybdenum boride. These mixtures exhibit spin combustion, the characteristics of which are in good agreement with the spin combustion theory. Oxidation resistance of the obtained Mo−Si−B materials is independent on the concentration of Mo phase in the products so that the materials with a higher Mo content are preferable because of better mechanical properties.