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Lalgudi V. Ramanathan / IPEN
Abstract High temperature (HT) material degradation manifests itself mainly as oxidation, although solid particle erosion, phase transformation, hot corrosion, spalling of surface oxides, volatilization are also encountered. Materials exposed to industrial atmospheres must be resistant to attack by the environment, and if the component is load bearing, also to deformation. For many such components it is possible to choose a material with a suitable combination of properties. However, as mechanical loads and severity of attack increase, the range of materials with an acceptable combination of properties becomes more limited. Coatings are widely used to protect metallic components and are ideally specified to ensure the component is capable of operating efficiently throughout its design life. Oxidation resistant coatings are generally either metallic or ceramic and should: (a) be resistant to further extensive oxidation, if metallic; (b) not undergo phase transformation; (c) form a spall resistant oxide; (d) have a low coefficient of thermal expansion (CTE) mismatch with the substrate at the desired operating temperature; (e) not compromise the coating/substrate integrity caused by inter-diffusion or solid state reactions. The dominant coating processes are diffusion and overlay. Diffusion coatings have been used for many years to protect metallic components by enriching the surface in Cr, Al or Si, and thereby aiding the formation of chromia, alumina or oxide layers. Formation and modification of the oxide that grows naturally on alloys or metallic coatings is also considered as a diffusion process in which oxygen is the active external element. Overlay coatings are produced by the deposition of a corrosion resistant alloy or a ceramic.
This presentation will include: (a) HT environmental degradation of metallic materials; (b) HT coatings (metallic, ceramic, composite and nanostructured) and techniques to coat; (d) 'reactive element effect' and its advantages in coating design; (e) future technologies – new coating techniques, nano/gradient materials, systems approach for lifetime prediction.
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