A comparative overview of molybdenum disilicide composites
At higher temperatures, between 1000 and 1600 °C, primary candidates are silicon-based ceramic materials. At operating temperatures greater than 1000°C, severe requirements for oxidation-resistant materials become important. Although the silicon-base materials (and their composites) have excellent oxidation resistance and lower density, their development is considered to be higher risk because of their brittleness over the entire temperature range. In addition, they are at present not cost effective, since the cost of the starting material, fabrication and machining are significant. As a result, alternative candidate materials are under investigation. These are based on aluminide (NiAI, NbA13, TaA13 ) and silicide (MoSi2, TisSi 3 ) matrix compositions. Their lower densities, higher melting points and high thermal conductivities make them attractive for high temperature engine applications. The aluminides are brittle at room temperature, have low strengths (and creep) at the required high temperatures, and lack long-time oxidation resistance above 1200 °C. Recently, however, a new class of silicide matrix composite (SMC) materials has been identified, designed around MoSi 2 which provides an alternative to structural ceramics as shown in Fig. 3, a plot showing the operating temperature vs. strength-to-weight ratio of various generic classes of materials . The use of these higher temperature materials can mimimize turbine air cooling (commonly required for the superalloys) and also minimize the number of engine parts, thus simplifying the design to reduce the engine weight. In addition, with higher operating temperature capabilities, one can expect efficient fuel combustion that can result in beneficial environmental effects. Details of the materials selection and design analysis are given by Stephens  and Smith .
A. K. Vasudfvan
J. J. Petrovic