1. Introduction-

Thermal barrier coatings are highly advanced material systems usually applied to

metallic surfaces, such as gas turbine or aero-engine parts, operating at elevated

temperatures, as a form of exhaust heat management. These coatings serve to insulate

components from large and prolonged heat loads by utilizing thermally insulating

materials which can sustain an appreciable temperature difference between the load-

bearing alloys and the coating surface. In doing so, these coatings can allow for higher

operating temperatures while limiting the thermal exposure of structural components,

extending part life by reducing oxidation and thermal fatigue. In conjunction with active

film cooling, Thermal barrier coatings permit working fluid temperatures higher than the

melting point of the metal airfoil in some turbine applications.

Fig. 1

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2. General anatomy-

Thermal barrier coatings typically consist of four layers-

1. Ceramic topcoat.

2. Thermally grown oxide.

3. Metallic bond coat.

4. Super alloy substrate

the metal substrate, metallic bond coat, thermally grown oxide, and ceramic topcoat. The

ceramic topcoat is typically composed of yttria-stabilized zirconia (YSZ) which is

desirable for having very low conductivity while remaining stable at nominal operating

temperatures typically seen in applications. Recent advancements in finding an

alternative for yttria-stabilized zirconia ceramic topcoat identified many novel ceramics

(rare earth zirconates) having superior performance at temperatures above 1200 °C,

however with inferior fracture toughness compared to that of yttria-stabilized zirconia.

This ceramic layer creates the largest thermal gradient of the TBC and keeps the lower

layers at a lower temperature than the surface.

Thermal barrier coatings fail through

various degradation modes that include mechanical rumpling of bond coat during thermal

cyclic exposure, especially, coatings in aircraft engines; accelerated oxidation, hot

corrosion, molten deposit degradation. There are issues with oxidation (areas of the TBC

getting stripped off) of the TBC also, which reduces the life of the metal drastically,

which leads to thermal fatigue.

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3. Uses-

3.1 Automotive-

Thermal barrier ceramic-coatings are becoming more common in automotive

applications. They are specifically designed to reduce heat loss from engine exhaust

system components including exhaust manifolds, turbocharger casings, exhaust headers,

downpipes and tailpipes. This process is also known as "exhaust heat management".

When used under-bonnet, these have the positive effect of reducing engine bay

temperatures, therefore lessening the intake temperature. Although most ceramic-coatings

are applied to metallic parts directly related to the engine exhaust system, some new

technology has been introduced that allows thermal barrier coatings to applied via plasma

spray onto composite materials. This is now commonplace to find on high-performance

automobiles and in various race series such as in Formula 1. As well as providing thermal

protection, these coatings are also used to prevent physical degradation of the composite

due to frictional processes. This is possible because the ceramic material bonds with the

composite (instead of merely sticking on the surface with paint), therefore forming a

tough coating that doesn't chip or flake easily.

Although thermal barrier coatings have

been applied to the inside of exhaust systems, this has encountered problems due to the

inability to prepare the internal surface prior to coating.

One of the most common use of thermal barrier coatings is in the combustion chamber of

aircraft turbine engines.  With the demand for fuel economy and increased power,

combustion temperatures are approaching the design limits of the metal alloys from

which turbine components are made.  The use of thermal barrier coatings in this and other

application enables the use of the alloys at higher temperatures, by reducing the

temperature to which the parts are exposed.  With the ability of thermal spray to apply an

almost limitless number of materials, well-engineered thermal barrier coatings can be

produced to solve even some of the most complex thermal barrier problems.

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When engineering a thermal barrier coatings system, Thermal Spray Technologies uses

its strong expertise in materials engineering and its strength of understanding of the

processes of thermal spray.  The combination of this knowledge provides application

specific solutions to thermal management problems.

Fig.2

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3.2 Industrial-

In industrial applications, where space is at a premium, thermal barrier coatings are

commonly used to protect from heat loss or gain.

In general the thermal barrier coatings not let the heat to come into the system or let it go

out of the system as required by or applied for the particular applications and needs.

Some of the machines and parts of industries or factories such as turbines, heat engines

heat pumps, boilers etc. use thermal barrier coating technology. Also Thermal barrier

coating can be used to insulate some of the delicate machine parts. It is also used for the

prevention of the parts which are very heat sensible and tend to wear and tear on

exposure to high temperature.

Use of TBCs is proven to be very profitable to the

industries as it gives long life to the machines as well as their individual parts giving

more profits and even production. Also we have seen that in industries there is a very

strong need for the management of heat or the energy being produced and being used.

Heat losses greatly affect the production by its machines. And in case of industries it is at

a very large scale, So use of thermal barrier coatings has also given a great contribution

towards the heat or energy management.

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4. Processing-

In industry, thermal barrier coatings are produced in a number of ways some the general

methods are discussed below :

4.1 Electron Beam Physical Vapor Deposition (EBPVD)-

Electron Beam Physical Vapor Deposition or EBPVD is a form of physical vapor

deposition in which a target anode is bombarded with an electron beam given off by a

charged tungsten filament under high vacuum. The electron beam causes atoms from the

target to transform into the gaseous phase. These atoms then precipitate into solid form,

coating everything in the vacuum chamber (within line of sight) with a thin layer of the

anode material.

4.2 Air Plasma Spray (APS)-

In plasma spraying process, the material to be deposited (feedstock) typically as

a powder, sometimes as a liquid, suspension  or wire is introduced into the plasma jet,

emanating from a plasma torch. In the jet, where the temperature is on the order of

10,000 K, the material is melted and propelled towards a substrate. There, the molten

droplets flatten, rapidly solidify and form a deposit. Commonly, the deposits remain

adherent to the substrate as coatings; free-standing parts can also be produced by

removing the substrate. There are a large number of technological parameters that

influence the interaction of the particles with the plasma jet and the substrate and

therefore the deposit properties. These parameters include feedstock type, plasma gas

composition and flow rate, energy input, torch offset distance, substrate cooling, etc

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4.3 High Velocity Oxygen Fuel spraying (HVOF)-

This class of thermal spray processes called high velocity oxy-fuel spraying was

developed in the 1980’s. A mixture of gaseous or liquid fuel and oxygen is fed into

a combustion chamber, where they are ignited and combusted continuously. The resultant

hot gas at a pressure close to 1 MPa emanates through a converging–diverging nozzle and

travels through a straight section. The fuels can be gases (hydrogen,

methane, propane, propylene, acetylene, natural gas, etc.) or liquids (kerosene, etc.). The

jet velocity at the exit of the barrel (>1000 m/s) exceeds the speed of sound. A powder

feed stock is injected into the gas stream, which accelerates the powder up to 800 m/s.

The stream of hot gas and powder is directed towards the surface to be coated. The

powder partially melts in the stream, and deposits upon the substrate. The resulting

coating has low porosity and high bond strength.

4.4 Electrostatic Spray Assisted Vapor Deposition (ESAVD)-

Electrostatic spray assisted vapour deposition (ESAVD) is a technique to deposit both

thin and thick layers of a coating onto various substrates. In simple terms chemical

precursors are sprayed across an electrostatic field towards a heated substrate, the

chemicals undergo a controlled chemical reaction and are deposited on the substrate as

the required coating. 

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5. Property profiles-

TBC coating systems must possess a combination of properties to be effective. These

include a low thermal conductivity, high resistance to spallation, good erosion resistance,

Phase stability and pore morphological stability. For aircraft turbine applications the

spallation resistance and the thermal conductivity of the coating system are the most

critical to performance. The thermal conductivity is strongly dependent on the volume

fraction and morphology of the porosity found in this layer. The spallation resistance,

however, is dependent on the mechanical properties of all three layers. For example the

TBC top layer must have a high in-plane compliance to minimize the coefficient of

thermal expansion (CTE) mismatch stress between the top TBC layer and the underlying

superalloy substrate.

Even when highly compliant TBC top layers are deposited,

spallation failure can still occur. Such failures have been observed to initiate either within

the TBC layer, at the TBC/TGO interface or at the TGO/bond coat interface. One

contributing factor is the development of large stresses in the TGO layer. Clarke and

Christensen have measured ambient temperature residual compressive stresses of 3 to 4

GPa in the TGO layer of TBC systems. This stress has been linked to the CTE mismatch

between the TGO layer and the substrate/bond coat and to growth stresses in the TGO.

Evans et al. have analyzed the thermomechanical stresses in these systems and shown

that they can lead to the initiation of cracks at the TGO/bond coat interface. Out-of-plane

tensile stresses resulting from undulations or morphological defects that form on an

otherwise smooth surface, ratcheting effects caused by cyclic plasticity in the substrate,

TGO/bond coat interface embrittlement (due to sulphur impurities) and sintering induced

increases in the TBC in-plane compliance are all thought to play a role in the spallation

failure of TBC systems. Recent work also suggests that the TGO undulations can result in

the formation of cracks the TBC layer. Control of these thermally induced failure

mechanisms is clearly a critical issue for the development of more durable TBC systems.

Increasing the thermal resistance of the TBC layer is expected to reduce the growth rate

of the TGO layer and slow the rate of ratcheting by reducing the temperature below it.

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Fig 3

Now, Fig.3 explains about the wear resistance of a machine part with thermal barrier

coatings and without it. So it can be clearly observed that the parts with Thermal barrier

coatings are more resistive to wearing.

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6. 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.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. Investigation of the current state-of-

the-art in TBC technology has indicated that opportunities exist to significantly improve

upon modern TBC systems.

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7. References-

Wikipedia.

 F.Yu and T.D.Bennett (2005). "A nondestructive technique for

determining thermal properties of thermal barrier coatings".

Research articles by ‘University of Virginia’, Material science and

engineering department.

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