Surface Engineering at High Temperature: Thermal Cycling and Corrosion Resistance

Surface Engineering at High Temperature: Thermal Cycling and Corrosion Resistance

John Henao (CIATEQ, Mexico) and Oscar Sotelo (Universidad Nacional Autónoma de México, Mexico)
DOI: 10.4018/978-1-5225-4194-3.ch006

Abstract

Thermal and chemical properties are key aspects that determines whether a coating material is useful for high temperature applications. The progress in the application of coatings as high performance and functional materials is currently driven by their reliability under changing environmental conditions. Particularly, for high temperature applications, new gas turbines, propulsion systems, and cutting tools are outstanding examples for the development of enhanced coating systems. With the increase in the complexity of applications of coatings at high temperature, it became necessary that engineers and designers look inside into various aspects of material properties. In that order of ideas, this chapter focuses on reviewing the main physical and chemical concepts related with the performance of engineering coatings at high temperature. Especially, the subjects for reviewing here are thermal shock and hot corrosion.
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Thermal Shock

Thermal shock is a concept associated with the thermal stresses that occurs in a material as a result of its exposure to a gradient of temperature between the surface and the interior and/or among various regions of the material. Thermal stresses generated from temperature gradients during cooling are defined according to the following equation:

(1) where E is the elastic modulus, is the temperature difference, v is the poisson’s ratio and is the coefficient of thermal expansion. This equation is valid for bulk materials having spheres, cylinders and slabs shapes, and indicates that thermal stresses increase as the thermal expansion coefficient and elastic modulus of the material increase and also when the gradient of the temperature increases. In bulk materials, thermal stresses can give rise to crack initiation and propagation (brittle materials) and/or permanent deformations (ductile materials). With the aim of quantifying the thermal shock resistance, engineers have defined the thermal shock parameter R, which is commonly expressed in various forms as summarized in Table 1.

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