thermal barrier coating

THERMAL BARRIER COATING

ABSTRACT

Thermal barrier coatings (TBC) are highly advanced material systems usually

applied to metallic surfaces, such as gas turbines 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,

TBCs permit working fluid temperatures higher than the melting point of the metal airfoil

in some turbine applications.

U.B.D.T.C.E mech dept. davangere


INTRODUCTION

Hundreds of different types of coatings are used to protect a variety of structural

engineering materials from corrosion, wear, and erosion, and to provide lubrication

and thermal insulation. Of all these, thermal barrier coatings (TBCs) have the most

complex structure and must operate in the most demanding high-

temperature environment of aircraft and industrial gas-turbine engines. Heat engines are

based on considering various factors such as durability, performance and efficiency with

the objective of minimizing the life cycle cost. Metallic coatings were introduced to sustain

the high temperatures. The trend for the most efficient gas turbines is to exploit more

recent advances in material and cooling technology by going to engine operating cycles

which employ a large fraction of the maximum turbine inlet temperature capability for the

entire operating cycle. Thermal Barrier Coatings (TBC) performs the important function of

insulating components such as gas turbine and aero engine parts operating at elevated

temperatures. Thermal barrier coatings (TBC) are layer systems deposited on thermally

highly loaded metallic components, as for instance in gas turbines. TBC’s are

characterized by their low thermal conductivity, the coating bearing a large temperature

gradient when exposed to heat flow. The most commonly used TBC material is Yttrium

Stabilized Zirconia (YSZ), which exhibits resistance to thermal shock and thermal fatigue

up to 1150oC. YSZ is generally deposited by plasma spraying and electron beam physical

vapour deposition (EBPVD) processes. It can also be deposited by high velocity oxy-fuel

(HVOF) spraying for applications such as blade tip wear prevention, where the wear

resistant properties of this material can also be used. The use of the TBC raises the process

temperature and thus increases the efficiency.



STRUCTURE OF THERMAL BARRIER COATINGS

Generally, TBC is a two layer’s system which incorporates about 250 μm thickness

layer of ceramic top coating applied to the outer surface of the substrate and about 150 μm

thickness underlying of metallic bond coating. The metallic bond coating performs two

functions:

(1) To provide oxidation resistance and

(2) To adhere the ceramic to the super alloy substrate physically and chemically.

The oxide that is commonly used is Zirconia oxide (ZrO2) and Yttrium oxide (Y2O3). The

metallic bond coat is an oxidation/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 Yttrium.

CrAl - base metal.

Fig. 1: Typical microstructure of TBC

By attaching an adherent layer of a low thermal conductivity material to the surface

of an internally cooled gas turbine blade, a temperature drop can be induced across the

thickness of the layer, Fig2. This results in a reduction in the metal temperature of the

component to which it is applied. Using this approach temperature drop of up to 170 oC at

the metal surface have been estimated for 150μm thick yttria stabilized zirconia coatings.

This temperature drop reduces the (thermally activated) oxidation rate of the bond coat

applied to metal components, and so delays failure by oxidation. Modern TBC’s are

required to not only limit heat transfer through the coating but to also protect engine



components from oxidation and hot corrosion. No single coating composition appears able

to satisfy these multifunctional requirements. As a result, a “coating system” has evolved.

Fig.2: A schematic illustration of a modern thermal barrier coating system

First, a thermally protective TBC layer with a low thermal conductivity is required

to maximize the thermal drop across the thickness of the coating. This coating is likely to

have a thermal expansion coefficient that differs from the component to which it is applied.

It must be able to retain its low thermal conductivity during prolonged environmental

exposure. A porous, columnar, 100-200 μm thick, yttria stabilized zirconia (YSZ) layer is

currently preferred for this function.

Second, an oxidation and hot corrosion resistant layer is required to protect the under

laying turbine blade from environmental degradation. This layer is required to remain

relatively stress free and stable during long term exposure and remain adherent to the

substrate to avoid premature failure of the TBC system. It is important that it also provide

an adherent surface for the TBC top coat. Normally, the thin (< 1μm), protective aluminum

rich oxide which is thermally grown upon the bond coat is utilized for this purpose. A

~50μm thick layer of either a low sulphur platinum aluminide or MCrAlY (where M is Ni

or Co) is utilized for this purpose.



COATING PROPERTIES

Depending on the following material properties we choose the coating materials.

Hardness:

As for all materials, the hardness of a coating is a measure of the resistance to plastic

deformation. It is widely recognized that the hardness increases with increasing density,

i.e. decreasing number of pores and micro-cracks. The hardness was determined from the

average length of the diagonals of each diamond shaped indentation.

Erosion resistance:

TBC degradation by erosion occurs mainly on turbine blades and vanes, especially when

the (aircraft) engine operates in a sandy environment (e.g. desert). High erosion resistance

is normally obtained by decreasing the porosity. However, high thermal shock resistance is

obtained by increasing the porosity.

Adhesion strength:

Coating thickness, bond coat pre oxidation and isothermal heating (1050C/5hr) affect the

adhesion strength. The chemical bond also affects the adhesion strength of TBC.

Thermal conductivity:

Thermal conductivity is one of the physical key properties of TBCs. Lowering TBCs

thermal conductivity would increase the engine performance by improving the combustion

efficiency, reduce the specific fuel consumption, allow a reduction of internal cooling,

reduce the metallic component temperature and extend their lifetime. Thus lowering

thermal conductivity of the ceramic layer can be achieved by engineering chemical

composition and/or coating microstructure.



COATING MATERIALS

The TBC material should have low thermal conductivity and a thermal expansion

coefficient. It should also have long term phase stability at whole service temperature

range and adequate corrosion resistance against the impurities present in the process.

Sufficient mechanical properties are also needed (bond strength, erosion resistance). Yttria

stabilized zirconia (YSZ) is the most used TBC material. The following are materials have

been proposed as possible alternative to YSZ.

Zirconates:

Zirconates have drawn the attention of several research groups as a promising alternative

to YSZ. The main advantages of zirconates are their low sintering activity, low thermal

conductivity, and good thermal cycling resistance. The main problem is the high thermal

expansion coefficient (TEC) which results in residual stress in the coating, and can cause

coating delamination. The total porosity in plasma sprayed zirconates is less than 10%

which is lower than the porosity in plasma sprayed PSZ. As the porosity has a vital role in

thermal insulation, process modification is required to increase the level.

Garnets:

Polycrystalline garnet ceramics are used in different applications due to their unique

properties. Particularly YAG (Y3Al5O12) is a good choice for many high-temperature

applications, due to its excellent high temperature properties and phase stability up to its

melting point (1970°C). Other advantages which make YAG a candidate as a TBC are

their low thermal conductivity and its low oxygen diffusivity.

Cordierite:

Cordierite (2MgO.2Al2O3.5SiO2) has a very low thermal expansion coefficient but, for

certain applications, could be an alternative for TBCs. However, after plasma spraying, the

cordierite deposition is amorphous. While heating, two phase transformation occur, at

830°C and 1000°C, which produce a volume change and cause cracking. Increasing the

temperature of the substrate to slow the cooling rate of the cordierite increases the

crystallinity, however the oxidation of metal substrates is a concern. Finally, another way

to increase the crystallinity of deposited cordierite is laser surface melting. Some studies

focused on zirconia-cordierite composites. But there are two major problems with this

system: the glassy nature of cordierite and its phase transformation is still unsolved. Also



yttria diffuses gradually into the cordierite destabilizing the ZrO2 to allow the destructive

tetragonal to monoclinic phase transformation to occur.

Forsterite:

The high thermal expansion coefficient of forsterite (2MgO.SiO2), 11x10-6 K-1, permits a

good match with the substrate. At thicknesses of some hundred microns, it shows a very

good thermal shock resistance.

Spinel:

Although spinel (MgO.Al2O3) has very good high temperature and chemical properties, its

thermal expansion coefficient, 7.68x10-6 K-1, prevents its usage as a reliable choice for

thermal expansion coefficient. There is only one report of applying spinel on bond coat

which failed after 719 thermal cycles.

Mullite:

Mullite is applied on SiC as an oxidation resistant layer to form an environmental barrier

coating (EBC). Its low oxygen diffusivity, low creep rate at high temperatures, high

thermo-mechanical fatigue resistance and close thermal expansion coefficient match with

SiC (4-5x10-6 K-1 vs. 5-6x10-6 K-1) makes it the ideal choice for this application.

In thin coatings (up to some hundred microns) on top of a metallic substrate, the durability

of mullite has been reported to be better than that of zirconia.

The low thermal expansion coefficient of mullite is an advantage relative to YSZ in high

thermal gradients and under thermal shock conditions. However, the large mismatch in

TEC with metallic substrate leads to poor adhesion.

Mullite has excellent creep resistance. Like cordierite, mullite is amorphous as deposited.

In service recrystallization occurs at 750 -1000°C which is accompanied by a volume

contraction, cracking and top coat debonding a solution is to heat the substrate to 1000°C.



THERMAL BARRIER COATING DEPOSITION

There are two different processes for the manufacturing of thermal barrier coatings:

(1) The plasma spraying (PS)

(2) The electron beam - physical vapour deposition (EB-PVD)

Plasma Spray Process:

Fig.3: Plasma Spray Process

It consists of tungsten cathode and water-cooled copper anode. The gas used is

Argon, Helium, Hydrogen and Nitrogen. Argon is usually chosen as the base gas due to its

ionization characteristics and chemical inertness. An electric arc is initiated between the

two electrodes using a high frequency discharge and then sustained using dc power. The

arc ionizes the gas, creating high-pressure gas plasma. The resulting increase in gas

temperature, which may exceed 30,000oC in turn, increases the volume of the gas and,

hence, its pressure and velocity as it exits the nozzle. Power levels in plasma spray torches

are usually in the range of 30 kW to 80 kW, but they can be as high as 120 kW.

The powder velocities usually range from 300 m/s to 550 m/s. Temperatures are

usually at or slightly above the melting point. Powder is usually introduced in to the gas



stream either just outside the torch or in the diverging exit region of the nozzle. Prior to

coating deposition by plasma spraying the substrate is degreased and roughened by grit

blasting to facilitate bonding with MCrAlY bond coat.

Some of the important factors to be considered in plasma spray process are:

The power settings of the plasma spray gun.

The gun to substrate distance.

The composition of plasma gas.

The motion and orientation of the gun relative to the item being coated.

The particle size distribution.

The thermal properties and temperature of the substrate.

Coating thickness usually ranges from about 0.05 mm to 0.5 mm but may be

thicker for some applications. In the as deposited condition, plasma sprayed coatings are

composed of many layer of splattered particles. MCrAlY bond coat alloys must have a

surface roughness of the order of 7 micron to produce a good mechanical bond with the

ceramic.

Electron Beam Physical Vapor Deposition:

EB-PVD is an evaporation process for applying ceramic thermal barrier coatings to

gas turbine engine parts. It has been the favored deposition process technique for TBCs

because of the increased durability of coating that is produced when compared to other

deposition processes. EB-PVD TBCs exhibit a columnar microstructure that provides

outstanding resistance against thermal shocks and mechanical strains.

The EB-PVD process takes place in a vacuum chamber consisting of a vacuum-

pumping system, horizontal manipulator, a water-cooled crucible containing a ceramic

ingot to be evaporated, an electron-beam gun, and the work piece being coated. The

electron beam gun produces electrons, which directly impinge on the top surface on the

ceramic coating, located in the crucible, and bring the surface to a temperature high

enough that vapor steam is produced. The vapor steam produces a vapor cloud, which

condenses on the substrate and thus forms a coating. The substrate is held in the middle of

vapor cloud by a horizontal manipulator that allows for height variation in the chamber.



Fig.4: EB-PVD Coating Chamber

During the coating process, oxygen or other gases may be bled into the vapor cloud in

order to promote a stoichiometric reaction of ceramic material. An “over source” heater or

an electron beam gun may be used for substrate heating, which keeps the substrate at a

desired temperature.

The primary control variables for the coating process are:

Ingot feed rate & ingot size

Gun current

Over source heater power

The feed rate and size controls how much mass is to be evaporated by the gun

current. The gun current controls how much power is transmitted to the melt pool, and thus

controls melt pool temperature for the evaporation rate and deposition rate. The maximum

gun current is limited by the stability of the ingot melt surface. Too high a current

overheats the ingot surface causing spattering, thus loosing mass evaporated. The melt

pool is a major source of heat energy for the substrate as well as substrate heating if used.

The substrate temperature is important because it controls many of the deposited coatings

material properties.

The coating operator must control the primary variables involved in the coating

process, as well as beam sweep frequency and rotation rate, which are set prior to the

coating process. Beam sweep frequency refers to the irregular scanning pattern the electron

beam gun makes on the ingot surface.



THE EFFECT OF TBC ON DIESEL ENGINE PERFORMANCE

Thermal Barrier Coatings developed primarily for turbine applications can also be

used in diesel engines benefits derived from using this technology includes

An improvement in the diesel engine fuel economy.

Increase in engine power density.

Reduction in the in-cylinder heat rejection.

Reduction in the ring groove temperatures.

Reduction in the metal piston temperatures.

Reduction in the lubricant temperatures

Experiments were conducted on a, single-cylinder, air-cooled, direct injection diesel

engine developing a power output of 5.2 kW at 1500 rev/min connected with a water

cooled eddy current dynamometer. The engine was operated at a constant speed of 1500

rpm and standard injection pressure of 220 bar.

Results and discussion:

Subsequently five fuel blends of dioxane with diesel ranging from 10 to 50% by

volume, namely (Dy10, Dy20, Dy30, Dy40 and Dy50) were prepared and tested. Readings

were taken, when the engine was operated at a constant speed of 1500 rpm for all loads.

Parameter like engine speed, fuel flow and the emission characteristic like NOx and smoke

were recorded. The performance of the engine was evaluated in terms of brake thermal

efficiency, brake power, brake specific fuel consumption from the above parameters. The

engine with a thin layer ZrO2 – Al2O3 coated piston, cylinder liner, head and bottom of

the valves.

As dioxane is having oxygen in its structure, more amount of fuel can be burnt in a

given amount of air and hence the BSFC decreases for the blends compared with baseline

fuel up to 10% dioxane addition. But, the heat value of the fuel mixture for higher blends

decreases as the heat value of dioxane is less than diesel. Hence in order to maintain the

same power, more fuels are consumed. As a result, BSFC will increase as the blended fuels

with high dioxane concentration are used. Fig.5 shows the BSFC for different dioxane

additions with and without thermal insulation.



Fig.5: Brake specific fuel consumption for different dioxane blends at full load

TBC engine improved the brake thermal efficiency of sole fuel by 3.8% when

compared to the standard engine due to the in cylinder heat transfer reduction and increase

in combustion duration. The presence of oxygen due to dioxane in the oxygenated fuel,

improve the combustion, especially diffusion combustion and hence increase the brake

thermal efficiency. Fig6. compares the effect of oxygenated fuel blend on the brake

thermal efficiency for the standard and TBC engine. The maximum Brake thermal

efficiency occurs for B10 blend ratio on the standard engine.

Fig.6: Brake thermal efficiency for different dioxane blends at full load



APPLICATIONS

Gas Turbine Engine Applications:

Thermal barrier coatings (TBCs) have the most complex structure and must operate

in the most demanding high temperature environment of industrial gas-turbine engines.

TBCs, which comprise metal and ceramic multilayers, insulate turbine and combustor

engine components from the hot gas stream, and improve the durability and energy

efficiency of these engines.

TBC’s For Jet Engine Turbine:

Thermal barrier coatings of jet engine turbines are critical for achieving powerful,

fuel efficient aircraft. Currently, engines operate under combustion temperatures well

beyond the melting point of the metal alloys that compose the engine parts.

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 which is

necessary in the harsh tool environment. Wear mechanisms of cutting tools, include crater,

attrition, flank, and abrasive wear. It has been shown that TBCs can limit the crater and

attrition wear processes.

Thermal barrier coatings can also be applied on furnace components, heat-treating

equipments, chemical processing equipments, heat exchangers rocket motor nozzles,

exhaust manifolds and nuclear power plant components. It has also been applied to

uncoated blades used to drive the high-pressure hydrogen turbo pump of the Space Shuttle

Main Engine. Here the coating is applied to provide thermal lag.



DISADVANTAGES

The main disadvantage is ceramic bond coat interface region which was determined

to be the weakest link in the ZrO2-12Y2O3 / NiCrAlY system. Adhesive plus cohesive

failure of the ceramic layer in this region occurred when the coating was subjected to a

normal tensile load at room temperature as well as when applied on bars subjected to axial

tension or compression at elevated temperature.

On prolonged high-temperature exposure in air, thermal barrier coatings (TBCs) on

bond-coated super alloys fail by spalling. Scientists focused on identifying the underlying

mechanisms of failure and sought to establish whether it is a direct consequence of the

failure of the underlying thermally grown oxide. They discovered a new TBC failure

mode, one in which failure is associated with moisture-enhanced sub-critical crack-growth

along the bond-coat/ thermally grown oxide interface.



CONCLUSION

1. The coating material has been applied successfully to the combustion chamber as the

thermal barrier coating of a diesel engine to prevent excessive heat loss during the

combustion.

2. Using insulation to reduce the heat loss to the cooling system of the engine causes the

cylinder walls to become hotter and increases the effective efficiency.

3. High temperatures on the combustion chamber wall surface due to insulation cause a

drop in volumetric efficiency although increased boost pressure from the turbocharger can

be used to overcome this problem.

4. Well-insulated engine show significant improvement in fuel consumption over a

conventionally cooled engine (standard engine).

5. It was found in tests performed on a naturally aspirated diesel engine with pistons

covered with TBCs, that specific fuel consumption was lower by 5-10%. Exhaust gas

temperature was found to be 200K higher than in the engine with metal pistons.

6. Processing improvement in the control and development of TBC are required. Further

study on, the mechanisms controlling coating adherence and degradation in clean and dirty

environments, the effects of coating composition and structure on coating properties and

correlation of models of engine tests are necessary to obtain thermal barrier coating that

have even better tolerance to high temperature and thermo mechanical stresses.



REFERENCE

C. Sundar Raj, S. Sendilvelan, S. Arul., June 2010, “Performance of a Thermally

Insulated Constant Speed Diesel Engine with Dioxane Blended Fuels”, Global

Journal of Researches in Engineering, Vol. 10.

Jeremy Bernier, 2001, “Evolution and Characterization of Partially Stabilized

Zirconia Thermal Barrier Coatings Deposited by Electron Beam Physical Vapor

Deposition”, Worcester Polytechnic Institute.

Miller, R.A., 1987, “Current Status of Thermal Barrier Coatings: an Overview”,

Surf. Coat. Technol., Vol. 30.

G. Moskal, 2009, “Thermal barrier coatings: characteristics of microstructure and

properties, generation and directions of development of bond”, Journal of

achievement in Materials & Manufacturing Engineering, Vol. 37.

H. Samadi, T.W.Coyle, “Alternative Thermal Barrier Coatings for Diesel Engines”,

Centre for Advanced Coating Technologies, University of Toronto, Toronto.

WEBSITES

http://dauskardt.stanford.edu/joanna_mroz/Thermal.html

http://en.wikipedia.org/wiki/Thermal_barrier_coating
