Mechanical Properties of coated systems

Abstract

Coatings are designed either to protect structural materials from degradation owing to surface attack such as corrosion, erosion and wear or to reduce the temperature of materials. Therefore the optimization of their composition and structure is primarily directed towards these specific tasks. Other important properties of a component, i.e. bulk properties such as mechanical strength or integrity, are normally given by the relevant bulk property of the structural material. This property again is a result of optimization of composition and structure. Coating and structural materials may thus differ from each other not only in chemical composition and microstructure but also in important physical properties (i.e. thermal expansion and conductivity) and/or mechanical properties (i.e. Young's modulus, yield strength, creep and fatigue). This “mismatch” of properties may cause coating-substrate interactions which may influence the properties of a coated component and its lifetime. Therefore, mechanical properties of coated systems, as well as their predominant failure mechanisms, will be governed by the relevant critical property of the substrate or the coating or both, depending on the geometrical situation and load cases. In systems where coating and structural wall thicknesses are of the same order of magnitude, mixture rules can be applied to describe, for instance, the deformation behaviour as a response to external mechanical or thermal loading. When the coating thickness is small compared with the wall thickness of the structural component, the coating deformation in the first instance will follow the deformation of the base material. At high temperatures the chemical composition, the microstructure and the dependent mechanical and physical properties will change owing to interdiffusion and aging. A high temperature component such as a gas turbine blade is subjected to static and cyclic mechanical and thermal loading. It also exhibits residual stresses as a result of processing and its thermal history. In this paper the response of such a coated system to those loadings is described and the principles of deformation and damage mechanisms are derived. Creep deformation is determined by the load-carrying cross-section of a component since it is mainly a volume effect. In contrast, fatigue causes locally concentrated damage. In this latter case, brittle phases which developed during high temperature service owing to aging or interdiffusion, and surface attack owing to hot corrosion, gain importance as crack nuclei which may limit the lifetime of the component. A model will be developed which describes deformation and the development of damage in a coated high temperature component.