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CONTENTS Introduction History Material requirements Structure of thermal barrier coating Composition and role of additions Processing Applications Advantages and Disadvantages Conclusion


INTRODUCTION Thermal Barrier Coatings are highly advanced material systems applied to metallic surfaces, such as gas turbines or aero-engine parts, operating at elevated temperatures. A thousandth of an inch of TBC can contain temperature in the range of 15 ° F to 25 ° F, considerably improving the material life. Thermal barrier coatings are typically ceramic composites based on zirconia , alumina and titanium. TBCs provide good resistance against the corrosive , high temperature environment of air craft engines as well.


HISTORY Economical and, today , environmental concerns continue to provide impetus (moving forces) for operating the engines at ever increasing temperatures, there by improving the thermodynamic efficiency and reducing pollutant emissions. Coatings in gas turbines serve a variety of purposes, whether in jet engines , land-based power generation turbines or marine engines. Typical coatings for high-temperature applications involve an oxidation resistant coating and a thermal barrier coating (TBC). The oxidant resistant coating is also called bond coat because it provides a layer on which the ceramic TBC can adhere. Higher temperatures in gas turbine engines is one of the major driving forces for the development of insulating ceramic coatings, called thermal barrier coatings (TBCs).


MATERIAL REQUIREMENTS Thermal Barrier Coatings are typically ceramic composites based on zirconia , alumina and titanium. Some of the important characteristics necessary for a TBC are: * high melting point * low density * high thermal shock resistance * resistance to oxidation and chemical environment * high surface emissivity * low vapor pressure * resistance to mechanical erosion * low thermal conductivity * high coefficient of thermal expansion


MATERIAL REQUIREMENTS Zirconia stabilized with 6-8 wt% Y2O3 [YSZ] is used, which consists completely of non-transformable tetragonal t phase. The most commonly applied TBC material is yittria stabilized zirconia (YSZ) which exhibits resistance to thermal shock and thermal fatigue upto 1150 ° C. Yittria-stabilized zirconia is applied by low temperature (1292 ° F [700 ° C]) chemical vapor deposition to create a cost-effective, robust thermal barrier coating that can be applied to complex-shaped components.


STRUCTURE OF THERMAL BARRIER COATINGS Thermal Barrier Coating consists of two layers. The first layer, a metallic one, is called bond coat, whose function is to protect the basic material against oxidation and corrosion. The second layer is an oxide ceramic layer, which is glued or attached by a metallic bond coat to the super alloy. The oxide that is commonly used is zirconia oxide(ZrO2) and yttrium oxide(Y2O3). The metallic bond coat is an oxidant or hot corrosion resistant layer. The bond coat is empherically represented as MCrAlY alloy where M- Metals like Ni, Co or Fe Y- Reactive metals like Yittrium CrAl- base metal MCrAlY also typically contain 1 wt% yttrium , which enhances adherence of the oxide layer.

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Figure 3. Schematic of the structure of a two layer thermal barrier coating on a turbine blade surface together with a temperature profile.

Structure of Thermal Barrier Coatings:

Structure of Thermal Barrier Coatings In the first and second stage of gas turbine, metal temperatures may exceed 850 ° C. The thermal barrier is made up of plasma sprayed ceramic layer. Plasma sprayed zirconia compositions have been investigated and the most suitable composition was found to be ZrO2-6 to 8 wt% Y2O3, which formed an adherent layer with the Ni, Cr, Al, Y, bond coat. BOND COATS: Aluminides: In modern applications involving very high temperatures or severe hot corrosion problems, aluminide coatings provide relatively limited protection. MCrAlY: Overlay coatings as opposed to diffusion coatings, provide more independence from the substrate alloy, but also more flexibility in design as compositions can be modified depending on the degradation mechanisms expected to prevail.

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Schematic illustration of aluminide coating obtained by low activity pack cementation. Schematic illustration of MCrAlY microstructure.

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Schematic microstructure of a thermal barrier coating (TBC) obtained by air plasma spray (APS). Schematic microstructure of a thermal barrier coating (TBC) obtained by electron beam physical vapour deposition (EBPVD). The columnar microstructure considerably enhances the strain resistance and therefore thermal cycling life.


PROCESSING In industry , thermal barrier coatings are produced in a number of ways: * Electron Beam Physical Vapor Deposition: EBPVD * Air Plasma Spray: APS * Electrostatic Spray Assisted Vapor Deposition: ESAVD * Direct Vapor Deposition The thermal barrier coating is the development of systems for higher application temperatures and with a longer lifetime . For that purpose , works are being carried out in four basic areas. * Thermal spraying * Modeling * Thermal cycling * New thermal barrier coating


THERMAL SPRAYING Thermal spraying is being used to coat mechanical elements , particularly for wear protection and for thermal insulation in highly loaded components of energy systems. There are three types of thermal sprayings: * Flame spraying : This is the basic form of thermal spraying and often involves an oxy-acetylene burner as the heat source. * Plasma spraying : It uses an electric arc as the heat source which is much hotter than the temperature produced by flame spraying. * High Velocity Oxy Fuel (HVOF) : This system is a refined oxy fuel burner which uses advanced nozzle design technology to accelerate the gas/particle stream to achieve particle velocities in excess of 600m/s.

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Modeling of thermal plasma spraying: During the manufacturing of thermal barrier coatings by means of thermal plasma spraying powder particles are being melted by the plasma and deposited on the substrate. Modeling of plasma is carried out in two steps: * In the first step, for a given gas composition and energy supply from plasma torch , the distribution of plasma velocity and temperature is simulated. * In the second step, when the plasma has reached a stationary state, the trajectories of the injected particles are calculated, whereas the main force accelerating the particles is the drag force due to the plasma velocity. Schematic of Thermal Spraying Schematic structure of thermal spray coating ,showing only a few layers of particles .

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NEW TBC: Today, the only material used as thermal barrier coating (TBC) in turbine construction is YSZ (Yittria partially stabilized Zirconia), which has proved very good behavior up to 1200°C. New TBC materials should maintain good properties of YSZ like low thermal conductivity and high thermal coefficient of expansion. Double layered TBC

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Turbine Blade Applications: * TBCs help protect turbine blades from the harsh environment of the aircraft engine. * Temperature generally exceed 2600 ° F in engines . The TBC allow a 300 ° F improvement in the temperature capability of the engine parts. * The low thermal conductivity of ceramics provides thermal combustion gases present in the engine. Cutting Tool Applications: * The addition of a TBC is credited for increasing cutting speeds of tools and for providing deeper cuts. * In particular, TBCs provide excellent wear resistance. * Multi-layer coatings increase performance as the combination exhibits the best qualities of each coating.

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ADVANTAGES: * Low temperature deposition for super alloy turbine engine components. * Low thermal conductivity. * Increase considerably the creep life of the blade while maintaining level of blade cooling. * The TBCs produced with EB-PVD process as it reduces stress build-up caused by thermal expansion mismatch. DISADVANTAGES: * TBCs produced with plasma spray lack reproducibility as porosity is introduced when the molten particles splat down . * TBCs deposited by electron beam evaporation are reported to generally fail on cooling by spalling and separation occurs at the interface with the bond coat.


CONCLUSION The thermal barrier coating performs the function of insulating components. Due to the good corrosion resistance and wear resistance these will improve the mechanical behavior . The TBCs are suitable for low temperature deposition for super alloy turbine engine components due to their low thermal conductivity. TBCs exhibits resistance to thermal shock , thermal fatigue upto 1150 ° C. Because of these properties the thermal barrier coatings are widely used in jet engines , turbine blades , aero engine parts , cans , gas turbines.

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