Recent Developments in the Field of Thermal Barrier Coatings
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obert Vassen, Alexandra Stuke, and Detlev Sto¨ver (Submitted October 30, 2008; in revised form December 18, 2008) Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat-isolative topcoat. Several recent research activities are concentrating on developing improved bondcoat or topcoat materials; for the topcoat especially, those with reduced thermal conductivity are investigated. Using advanced topcoat materials, the ceramic coating can be further divided into layers with different functions. One example is the double-layer system in which conventional yttria-stabilized zirconia (YSZ) is used as bottom and new materials such as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. In addition, new functions are introduced within the TBCs. These can be sensorial properties that can be used for an improved temperature control or even for monitoring remaining lifetime. Further increased application temperatures will also lead to efforts for a further improvement of the reflectivity of the coatings to reduce the radiative heat transfer through the TBC.
Keywords
ceramic top coats, gas turbines, multifunctional coatings, plasma spraying, thermal barrier coating
1. Introduction Thermal barrier coating systems (TBCs) are frequently used in gas turbines. The isolative layer can provide a reduction of the temperature of the metallic substrate that results in an improved component durability. Alternatively, an increase of fuel efficiency can be achieved by allowing an increase of the turbine inlet temperatures (Ref 1). Thermal barrier coating systems consist typically of two layers, a so-called bondcoat layer and an isolative, ceramic topcoat. The bondcoat is often a metal and has two major functions. It improves the bonding between the substrate and the topcoat, and it protects the substrate from corrosion and oxidation. Two types of bondcoats are frequently used, a (platinum-) aluminide based one and a so-called MCrAlY with M being Ni or Co. The choice of the adequate bondcoat depends on the deposition technique used for the topcoat. Electron beam physical vapor deposition (EB-PVD) and atmospheric plasma spraying (APS) are the most frequently used techniques. The development of ceramic thermal barrier coatings for components of gas turbines started in the 1940s and 1950s (Ref 2). In the 1960s zirconia was identified as a promising candidate material. Pure zirconia is not suitable for the application as it undergoes a different phase transition (Ref 3). The martensitic monoclinic-tetragonal at about Robert Vassen, Alexandra Stuke, and Detlev Sto¨ver, Institute of Energy Research (IEF-1), Forschungszentrum Ju¨lich GmbH, 52428 Ju¨lich, Germany. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
1000 °C is accompanied by a large volume change (3 to 9%, Ref 4) and, hence, by the risk of cracking of the coating. Therefore, different doping additio
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