Mechanical aspects of high temperature coatings

Abstract

High temperature coatings are used for two main functions, either to protect a base metal against corrosion or erosion or to minimize wear. A third function is to reduce the base metal temperature in the case of thermal barrier coatings; however, resistance to hot corrosion and oxidation is again mandatory. The main functions of such a coating can only be fulfilled when the coating has sufficient resistance to thermal strains and protects the base metal without deterioration. In this paper we describe the strains that arise during the application of a coating-mainly due to thermal expansion mismatch-and during service-due to diffusion, aging and chemical reactions with atmosphere, e.g. the formation of oxides. These strains cause cracks within a coating which reduce its ability to protect the base metal against a corrosion attack. In the second part of this paper the influence of the coating on the mechanical properties of the coated component is described. In general, a high temperature component is subjected to static loadings-the material undergoes creep-and to cyclic loadings-the component undergoes fatigue. In both cases a coating has some influence either through changes in the material properties due to a different heat treatment or through changes in the surface of a component due to the presence of the coating; hence a coating alters the conditions for crack initiation. From laboratory experience of high temperature coatings applied to gas turbine blades it can be shown that, depending on the type of coating and the method of application, the changes in materials might be positive, negative or negligible. For all applications it seems to be possible to select coatings without marked effects on the mechanical properties of the base metal. However, it is not always possible to choose the coatings best able to protect the base metal against corrosion or wear, because to achieve mechanical compatibility it is sometimes necessary to use coatings which are not stable for the required lifetimes. In-service examples from gas turbine experience are presented.