Thin thermal barrier coatings (TBCs) for protecting aircraft turbine section airfoils are examined. The discussion focuses on those advances that led first to TBC use for component life extension and more re-cently as an integral part of airfoil design. Development has been driven by laboratory rig and furnace testing, corroborated by engine testing and engine field experience. The technology has also been sup-ported by performance modeling to demonstrate benefits and life modeling for mission analysis. Factors that have led to the selection of current state-of-the-art plasma-sprayed and physical-vapor-deposited zirconia-yttria/MCrAlX TBCs are emphasized, as are observations fundamentally related to their behav-ior. Current directions in research into TBCs and recent progress at NASA are also noted.

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Thermal barrier coatings for aircraft engines: History and directions

The science and technology of thermal barrier coatings has advanced considerably since reports of the first test on turbine blades in a research engine in 1976. Today thermal barrier coatings are flying in revenue service in a low risk location within the turbine section of certain gas turbine engines. The state-of-the-art coating system for gas turbine applications is currently a plasma-sprayed ZrO 2 -(6%–8%) Y 2 O 3 ceramic layer over an MCrAlY (M ≡ Ni, Co or NiCo) bond coat layer plasma sprayed at low pressure. Although the potential for meeting current and short-term goals is high, longer-range goals may not be attainable with current coating concepts. These longer-range goals will involve high risk designs where coating loss could lead directly to component loss. Several steps must be taken to help meet these goals. Improved understanding of coating failure mechanisms is required. Models are needed to predict lifetimes. Process automation and quality control procedures must be instituted. Finally, new concepts in plasma-sprayed coatings must be developed and alternatives to the plasma- spraying process may be required. The current status of thermal barrier coatings and prospects for future progress in the above areas are summarized.

Current status of thermal barrier coatings — An overview

Thermal barrier coatings (TBCs) have been used for almost three decades to extend the life of combustors and augmentors and, more recently, stationary turbine components Plasma-sprayed yttria-stabilized zirconia TBC currently is bill-of-material on many commercial jet engine parts A more durable electron beam-physical vapor deposited (EB-PVD) ceramic coating recently has been developed for more demanding rotating as well as stationary turbine components This ceramic EB-PVD is bill-of-material on turbine blades and vanes in current high thrust engine models and is being considered for newer developmental engines as well To take maximum advantage of potential TBC benefits, the thermal effect of the TBC ceramic layer must become an integral element of the hot section component design system To do this with acceptable reliability requires a suitable analytical life prediction model calibrated to engine experience The latest efforts in thermal barrier coatings are directed toward correlating such models to measured engine performance

The Evolution of Thermal Barrier Coatings in Gas Turbine Engine Applications

A wide range of coatings and coating processes is now available for protecting components in gas turbines operating in a variety of conditions. Coated aerofoils are less likely to fragment, and they keep their original shape – and hence their aerodynamic efficiency – for longer. Aluminide diffusion coatings perform well in relatively clean environments, especially on alloy bases which have a moderate resistance to the many forms of hot corrosion, and with additions of chromium or platinum they provide resistance to sulphate–induced hot corrosion. In hotter or more aggressive corrosive environments, overlay coatings based on MCrAlY (M = Fe,Co, Ni), CoCr, and MCrSi(Al), applied by electron beam evaporation or plasma spraying, are suitable for a wide variety of applications. It is not yet possible to design coatings from first principles for particular applications. Development still proceeds by repeated selection and testing, but modern technology has made this approach more effective than it was in...

Protective coatings – purpose, role, and design

This report assesses the current status and future potential for the application of ceramic coatings in advanced diesel and turbine engines A previous report describes the authors findings in greater detail Ceramics, in contrast with metals, generally exhibit better structural and thermal capabilities at high temperatures In general, ceramics are more resistant to creep, oxidation, corrosion, erosion, and wear, as well as being better thermal insulators The benefits of thermal barrier coatings on the hotter side of diesel or turbine engine components are well-documented, and thermal barrier coatings are one of several applications of ceramics that are being pursued actively for heat engines Ceramic coatings are being considered for diesel engine cylinder liners, piston caps, valve faces and seats, piston rings, and other parts and for turbine components such as combustors, blades, stators, seals, and bearings

Ceramic coatings for advanced heat engines - A review and projection

Thermal barrier ceramic coatings of stabilized zirconia over a bond coat of Ni Cr Al Y were tested for durability on air cooled turbine rotor blades in a research turbojet engine. Zirconia stabilized with either yttria, magnesia, or calcia was investigated. On the basis of durability and processing cost, the yttria stabilized zirconia was considered the best of the three coatings investigated.

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R. E. Jacobs

Papers

Durability of zirconia thermal-barrier ceramic coatings on air-cooled turbine blades in cyclic jet engine operation