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    Requirements for and Expected Benefits from the Application of Coatings to High-Temperature Components of a Jet Engine

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    Materials are still the key to the improvement of aircraft engines. Because of the high temperature, the corrosive atmosphere, and in some components the high stresses, materials in current engines are required to operate near their damage points. Further increase in engine performance through increase in operating temperature requires improvement in materials. It is the purpose in this discussion to show how coatings can be used to help alleviate the materials problem. We shall describe a typical engine, discuss the conditions under which the components operate, show the contribution that can be made by applying coatings to these materials, and discuss the requirements that the coatings must meet. Considering the characteristics of ceramics, ceramic coatings might be expected to serve several functions. Since they are essentially oxides, it would be expected that the most important function would be protection against oxidation and perhaps corrosion. Because of their low conductivity, they would also be expected to serve as insulation against high temperatures. If coatings can serve these functions of protection against oxidation and corrosion and insulation against high temperatures, appreciable strides may be made toward the major jet-engine development goals of increased service life, reduced use of strategic materials, and increased operating temperatures. The important engines today are the turbojet, the turboprop, the ramjet, and the rocket. For this discussion, we shall consider the application of coatings to the major production engine, the turbojet, shown schematically in Fig. 1. The turbojet is a typical heat engine in which air is compressed, heated, and expanded. Air enters the front of the engine and is compressed, in this case by an axial flow compressor. The axial flow compressor is made up of alternate stages of stationary and rotating blades and moves the air through the turbine much in the same manner as the house-hold fan. Compression is indicated by the difference in area of the duct at the inlet and the exit. The amount of compression may be of the order of 4 to 1. The compressed air then enters the combustion chamber where fuel is injected and burned. This greatly expands the volume of the air under conditions of constant pressure, and the high-velocity hot gases are directed by the stationary turbine inlet guide vanes against the rotating turbine blades. The turbine and compressor are on a common shaft and the entire output of the turbine is used to drive the compressor. The hot gases then expand out the rear, and forward thrust is achieved by the reaction of the jet.

    Author Information:

    Francisco, A. C.
    National Advisory Committee for Aeronautics, Cleveland, Ohio

    Ault, G. M.
    National Advisory Committee for Aeronautics, Cleveland, Ohio

    Committee/Subcommittee: B08.08

    DOI: 10.1520/STP47969S