Unidirectionally Solidified Eutectic Ceramic Composites for Ultra-High Efficiency Gas Turbine Systems

Abstract

To help solve environmental problems, it is vital to develop a material for use in energy conservation technologies and for curbing the emission of pollutants such as CO2. In the advanced power generation field, studies all over the world are seeking to develop ultrahigh temperature structural materials that will improve thermal efficiency in aircraft engines and high-efficiency gas turbines. For example, to improve the thermal efficiency of gas turbines, operating temperatures must be increased and to achieve this, the development of ultra-high temperature resistant structural materials is necessary. Currently Ni-base superalloys are the main thrust in this field, but these have melting points of less than 1673 K, and their strength deteriorates sharply near 1200 K. In order to overcome the high temperature limitations of metals, the development of turbine technology using advanced ceramic matrix composites, has been vigorously pursued in recent years. It has been reported that a unidirectionally solidified Al2O3/YAG eutectic composite has superior flexural strength, thermal stability and creep resistance at high temperature (Mah & Parthasarathy, 1990; Parthasarathy et al., 1990; Parthasarathy et al.,1993) and is a candidate for high-temperature structural materials. However, since the eutectic composite consists of many eutectic colonies, a fairly strong influence of colony boundaries may be predicted (Stubican et al., 1096). The recently developed MGCs are a new class of ceramic matrix composites made by melting and unidirectional solidification of raw material oxides using a eutectic reaction to precisely control the crystal growth. The MGCs have excellent high temperature characteristics due to their unique microstructure without eutectic colony boundaries (Waku et al., 1996; Waku et al., 1997; Waku et al., 2001). In this paper, the fundamental concept of the MGC is explained and compared with sintered polycrystalline monolithic ceramics and single crystal ceramics. High temperature strength, tensile creep properties, compressive deformation, and oxidation resistance and thermal stability as typical high temperature characteristics of the MGCs are introduced. MGC ultra-high efficiency gas turbine systems and the manufacturing process for near-net-shaped MGC component castings are also discussed.