Technical and Economical Aspects of Current Thermal Barrier Coating Systems for Gas Turbine Engines by Thermal Spray and
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lbert Feuerstein, James Knapp, Thomas Taylor, Adil Ashary, Ann Bolcavage, and Neil Hitchman (Submitted June 19, 2007; in revised form August 17, 2007) The most advanced thermal barrier coating (TBC) systems for aircraft engine and power generation hot section components consist of electron beam physical vapor deposition (EBPVD) applied yttria-stabilized zirconia and platinum modified diffusion aluminide bond coating. Thermally sprayed ceramic and MCrAlY bond coatings, however, are still used extensively for combustors and power generation blades and vanes. This article highlights the key features of plasma spray and HVOF, diffusion aluminizing, and EBPVD coating processes. The coating characteristics of thermally sprayed MCrAlY bond coat as well as low density and dense vertically cracked (DVC) Zircoat TBC are described. Essential features of a typical EBPVD TBC coating system, consisting of a diffusion aluminide and a columnar TBC, are also presented. The major coating cost elements such as material, equipment and processing are explained for the different technologies, with a performance and cost comparison given for selected examples.
Keywords
advantages of TS, APS coatings, coatings for gas turbine components, TBC topcoats, thermal properties review
1. Introduction Thermal barrier coating systems (TBCs) are widely used in modern gas turbine engines to lower the metal surface temperature in combustor and turbine section hardware. Engines for both aero-jet propulsion and landbased industrial power generation have taken advantage of this technology to meet increasing demands for greater fuel efficiency, lower NOx emissions, and higher power and thrust. The engine components exposed to the most extreme temperatures are the combustor and the initial rotor blades and nozzle guide vanes of the high-pressure turbine. Metal temperature reductions of up to 165 °C are possible when TBCs are used in conjunction with external film cooling and internal component air cooling (Ref 1). In film cooling a protective blanket of cooling air is ejected onto the external surface of the turbine vane or blade, from internal passages within the airfoils, by means of holes or slots in the surface. Figure 1 is a schematic of the key elements of an aircraft gas turbine, with the pressure
Albert Feuerstein, James Knapp, Thomas Taylor, Adil Ashary, and Neil Hitchman, Praxair Surface Technologies, Inc., Indianapolis, IN USA; and Ann Bolcavage, Rolls Royce Corporation, Indianapolis, IN USA. Contact e-mail: albert_ [email protected].
Journal of Thermal Spray Technology
and temperature profiles present along the gas path from air intake to exhaust. A diagram of the relative temperature reduction achieved using both TBC and cooling air technologies on hot section hardware is shown in Fig. 2. A typical TBC consists of two key layers: an oxidation resistant bond coat such as diffusion aluminide or overlay MCrAlY bond coating, and a ceramic top layer, typically 7-8 wt.% Y2O3-stabilized ZrO2 (7YSZ), to reduce the heat flux into the component.
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