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ABSTRACT Efforts to increase efficiency of energy conversion devices have required their operation at ever higher temperatures. This will force the substitution of highertemperature structural ceramics for lower temperature materials, largely metals. Yet, many of these ceramics will require protection from high temperature corrosion caused by combustion gases, atmospheric contaminants, or the operating medium. This paper discusses examples of the initial development of such coatings and materials for potential application in combustion, aluminum smelting, and other harsh environments. INTRODUCTION Existing power conversion systems can obtain increased energy efficiency either through the addition of topping and bottoming cycles or through increased operating temperatures. Typically, operating temperatures are limited by materials capabilities, which in practice means the metallic alloy temperature limitation. Thus, any thrust toward higher operating temperatures involves the use of new materials for that application, with ceramic materials generally selected because of their superior high temperature stability and corrosion resistance. The Si-based ceramics are of particular interest for energy conversion because of

their excellent mechanical and physical properties. New Si3N4 ceramics have high toughness and excellent thermomechanical stability. They are also stable in purely oxidizing environments. Silicon carbide has been an attractive material because of its high thermal conductivity, and when in the form of a continuous fiber ceramic composite (CFCC) it is tough, thermal shock resistant, and thermomechanically stable. The use of silicon-based ceramics has been somewhat limited because of their high temperature corrosion behavior in the presence of impurities and steam.1' 2' 3 A particularly severe example is seen in the rapid, catastrophic failure of a SiC CFCC tube in a coal slag environment (Fig. 1). The issue in most combustion systems is the attack by alkali metal compounds and/or steam. Alkali's can react with the native silica surface scale, which protects the Si 3N 4 or SiC in an oxidizing environment, to form a lowmelting, soda-silica glass that allows active corrosion of the ceramic. In static steam environments the protective silica cracks and becomes non-protective, and under high gas velocities there is even significant recession through the formation of volatile hydroxides from the protective scale. The current approach to allowing the use of Si-based ceramics in these environments is to protect them with oxide ceramic overcoat systems. Another important area for chemically stable, high-temperature materials is in the electrochemical smelting of aluminum. Structural materials, including thermocouples and other probe sheaths, must be stable in molten aluminum or cryolite. These are particularly reactive systems, and thus, there has been little progress in developing usable 109 Mat. Res. Soc. Symp. Proc. Vol. 555 ©1999Materials Research Society

Fig. 1. Silicon carbide composite corroded by imping