Life Prediction of Functionally Graded Thermal Barrier Coatings

7/23/2019 Life Prediction of Functionally Graded Thermal Barrier Coatings

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Life Prediction ofFunctionally GradedThermal BarrierCoatings

In gas turbine components, particularly

nozzle, turbine blades are subjected to highthermal fatigue loads which make

challenge for designing it to have longer

life. The life of these components could befurther improved considerably by thermal

barrier coatings (TBC) which consist of a

complex micro structure of particles andvoids. It has limited spallation life due to

mismatch material properties as it leads to

large thermal stresses during thermalfatigue cycles in an oxidation

environment. Thermal barrier coatings

with functionally graded concept canreduce the degree of mismatch in material

properties; thereby the spallation life will

be improved. Difference in operatingtemperature between substrate and top coat

will be enhanced by ~35% whenfunctionally graded concept is

incorporated in replacement ofconventional thermal barrier coatings. An

analytical model for estimating life time of

functionally graded based thermal barriercoatings (Yttrium Stabilized Zirconium) is

developed and the life of conventional

thermal barrier coating is compared for thesame geometrical and loading parameters.


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Life Prediction of Functionally Graded Thermal Barrier Coatings

About the Author

N. Govindarajan works with Tata

Consultancy Services and is based inChennai. Govindarajan has done his Masters

in Machine design from IIT Madras and has

6 years of experience which includes 4 yearsin ISRO and 1 year in GE before joining

TCS. He has also authored 4 journals and

presented his work in 10 conferences in thearea of fatigue, powder metallurgy,

Functionally Graded Materials (FGM) and

fracture mechanics.

Dr.Rajesh K Bhangale is a ResearchScholar at IIT Chennai and has recently

joined in Mahindra & Mahindra, R&D

center, Automotive sector. Rajesh has 25referred publications in the area of FGM,

vibrations, thermal buckling, Fluid Structure

Interactions (FSI), damping and smartmaterials.

Prof N.Ganesan is a Professor, IIT Chennai,

Department of Mechanical Engineering.

Ganesan has 250 referred journals in the area

of vibrations, damping, smart materials, FSI

and structural stability and has guided 35PhDs.


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Contents

Introduction4

Concept of FGM.5

Analytical Model for FTBC..6

Mechanical Property Estimation9

FEM Simulation of TBC and FTBC..10

Life Time Estimation10

Conclusion..11

Acknowledgments.11

References..11

Future Work.12


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Introduction

An improvement in mechanical, creep and fatigue strength of super alloy at highoperating temperatures, especially at higher homologous temperature, will lead todecrease of its oxidation resistance. Hence the thermal barrier coating (TBC) isvery much essential to apply over microns level thickness to protect the structuralcomponents from corrosion, oxidation and mechanical property degradation whenit is subjected to high thermal load coupled with fatigue and/or creep phenomena.The main function of TBC is to provide sufficient surface reservoir of elementswhich will form a protective and adherent oxide layers near to the base structure.This will act as a protective layer for the underlying base structure from oxidation,corrosion attack and mechanical property degradation. The main characteristics

of TBC are, to provide low thermal conductivity layers (Yttrium Stabilized Zrconiaoxide-YSZ) and to give thermodynamically, chemically stable environment againstcorrosion and oxidation. Typical TBC consist of two coatings, the first one is bondcoat- MCrAlY, where M could be replaced with Ni or Co or Fe depends onmaterial composition of base structure, the second coat is YSZ which will providegood thermal resistance. Since the YSZ coating is very porous in nature, it allowshot gas which penetrates thro it and gets react with the bond coat material. Thiswill lead formation of a layer called thermally grown oxide layer (TGO). Figure 1and shows the typical TBC applied on a turbine blade which is used in aircraftengine/gas turbine. When it exposes to high thermal loading, (at operatingcondition), the bond coat builds up TGO as seen in figure 2.

Figure 1: Details of conventional thermal barrier coatings applied over a turbineblade.[1]

TGO consists mainly of alumina and the formation of TGO plays a critical role inthe failure pattern of TBC. The general failure pattern of TBC will be spallation ator near to the formed TGO interfaces with metallic bond coat or YSZ. The microcrack formations near to the TGO region on both sides of bond coat and ceramiccoat will lead to spallation failure. It is very much crucial to predict the spallationlife of TBC under high thermal loading in order to improve the life of thecomponents. Major concern in TBC is, it induces large thermal strains during thethermal cycles in an oxidation environment due to the thermal expansion


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mismatch between ceramic coat and metallic bond coat. Present study involvesFunctionally Graded Material (FGM) concept in the application of TBC with focusof getting more spallation life under thermo mechanical loading. Finite elementanalysis has been performed for the same geometrical and loading parameters tohave detailed comparison study in spallation life.

Figure 2: Microstructure of Conventional Thermal Barrier Coating system, (Bondcoat NiCrAlY, Zeramic coat YSZ, TGO-Thermally grown oxide). [2]

Concept of FGM

Ceramic-metal FGM have been attracting a great deal of attention as thermalbarrier coatings (TBC) for aerospace structures, gas turbines and aircraft engines,working under super high temperatures and thermal gradients. FGM is a relativelynew concept involving tailoring the internal microstructure of composite materials

to specific applications, producing a microstructure with continuously varyingthermal and mechanical properties at the continuum or bulk level. It hascontinuous variation of material properties from one surface to another and thusalleviates the stress discontinuities. Hence, they are ideal for applicationsinvolving severe thermo fatigue loadings.

Functionally graded thermal barrier coating (FTBC) introduces more reliability andreduces interfacial thermal stress between metallic and ceramic layers. FTBCprovides less inter-layer thermal stress since the gradient will vary smoothlyacross the coating thickness as shown in figure 3. It also effectively reduces thediscontinuities in thermal expansion coefficients between the bond coat andsubstrate. Each FTBC layers will act as a TBC layers with various material

compositions thereby it gives more spallation life cycles than that of TBC layers ofsame thickness under the same loading. With this conceptual of graded coating,the bond strength will be increased by almost twice time per mm coatingthickness. The main sticky situation with this type coating is cohesive failurepattern within the structure and may take anywhere within the coatings where asin TBC, the failure mostly occurred at the interface layers (Thermally GrownOxide). The present study considers FTBC layers composed of bond coattypically used as NiCrAlY metal and Yttrium Stabilized Zirconia oxide (YSZ)ceramic with five layers of different compositions. The spallation life model is usedfor predicting life, with the available strain value obtained from finite elementanalysis.

Zeramic Coat

TGO

Bond Coat


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0

200

400

600

800

1000

12001400

1600

-11.6 -11.1 -10.6 -10.1

Radius (mm)

Temperature(degC TBC

FTBC

HotGas`

Figure 3: Details of the temperature difference between TBC and FTBC acrossthe thickness of the same substrate.

Analytical Model for FTBC

A cylinder of 10mm inner radius with 1 mm thickness is considered for theanalysis with coating thickness of 450m. The thickness of bond coating is 150mand ceramic coating is around 300m for TBC. For the same coating thickness,five layers of uniform thickness of 90m are considered for FTBC. Thefunctionally grading in TBC is accounted across the thickness of the shell (in theradial direction). Figure 4 gives the geometrical features of model used for thisanalysis.

The present approach adopts the smooth and continuous variation of the volumefraction of either ceramic or metal based on the power law index. It is assumedthat there is no pore and makes use of only the smooth and continuous variationof the volume fraction of ceramic or metal based on the power law index. Thesimple power law index for the volume fraction of ceramic (Vc) across the radialdirection of shell is defined as follows [3]:

n

oi

oy

crr

rrV

=

-- (1)where ry is the radius at Y direction, ro is outer radius (100% ceramic coat), ri isinner radius (100% Bond coat) and n is power law index. Figure 5 gives the detailof variation in volume fraction with respect to power law index and radius. Power

law index of 1 is chosen for uniform variation of FGM layers (Table 1).


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Figure 4: Detail of geometric features used for TBC and FTBC model.

11.09

11.19

11.29

11.39

0 0.2 0.4 0.6 0.8 1Volume Fraction

Radius(

mm)

n=1 n=0.1n=2 n=0n=10 n=0.5

Figure 5: Distribution of power law index for different volume fraction and radius.(n=1 is chosen for this study).


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TBC FTBC LayersRadius(mm)

Composition Volume Fraction Vf(%)

Radius(mm)

Composition Volume FractioVf(%)

11.15 100% Bond 0 11.09 0% Ceramic Coat 0

Coat 11.18 25% Ceramic Coat 0.25

11.45 100% Ceramic 1 11.27 50% Ceramic Coat 0.5

Coat 11.36 75% Ceramic Coat 0.75

11.45 100% Ceramic Coat 1

Table 1: Details of Coating layers for TBC and FTBC


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Mechanical property estimation:

For conventional TBC, the material properties are available with varioustemperatures, but for FTBC layers. FTBC are typically made from the samematerials which are used in TBC at different layer coatings. Table 1 describes thedetails of FTBC and TBC layers. Based on simple mixer rule, the total volumefractions of ceramic and metal will be unity. The mechanical properties of YSZand NiCrAlY with various temperatures are available [4,5]. Hence the effectivematerial property will be computed as follows:

[ ] [ ] [ ] finnercoutereff VMPVMPMP += -- (2)where MPeff is the effective material property, Vc and Vf are volume fraction forceramic and metal. The effective mechanical properties for each layer with itsvolume fraction and power law index of 1 are calculated as:

( ) [ ] inncinoteff EVEEE += -- (3)

40

60

80100

120

140

160

180

200

220

240

1 2 3 4 5Layers

Young'sModulus(GPa).

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

ThermalConductivity

(W/mm/K)

Youngs Modulus-200degYoungs Modulus-500degYoung's Modulus-900degTherm Condu-200degTherm Cond-500degTherm Cond-900deg

Figure 6: Effect of temperature on mechanical properties of FTBC layers

Similar way for all the properties like, poisons ratio, thermal conductivity,expansion coefficient, creep constants are computed for all the FGM layers.Figure 6 gives the detail of mechanical and thermal properties of each layer withvariation of temperature.

.


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FEM Simulation of TBC and FTBC:

The Finite element analysis has been carried out by using a general purposecode ANSYS by plane strain formulations. Thermal contacts between the bondcoat and ceramic coat layers has been simulated to capture the realistic effect ofinterfacial layers. Transient thermal and structural finite element analysis iscarried out for the material properties given in figure6 with half symmetry model.Since the creep strain rate is the main parameter which decided the spallation life,Nortons creep law is used with thermal cycle of 950oC for the heating periods of2hours. This analysis is repeated for FTBC layers also so as to compare thecreep strain rate with respect to spallation life for the same rate of loading.

1.E-061.E-051.E-041.E-031.E-021.E-011.E+001.E+011.E+021.E+03

1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 1.E+05

Time (Sec)

CreepStrainRate

TBC

FTBC

Figure 7: Creep strain rate of the FTBC and TBC layers for transient thermalloading.

Even though the strain rate for both TBC and FTBC layers are same, the strainrate at the initial stage of loading quite differs which means that the initial damagewill be lesser for the FTBC layers compared to the conventional TBC (Figure 7).The analysis predicts that the higher stresses near to the edges of top coat inwhich failure usually occurs.

Life Time Estimation

Since the parameters involved in the damage mechanism of the coating are

irresolute, development of fatigue life model for coating is a complex one. Thereare two main parameters shear deformation due to thermal mismatch of thelayers and multiaxiality nature which leads the failure mode geometry dependent.Large shear strain gradient at the interface between the layers will play a majorrole in life estimation model. The combination of normal and shear strain givesgood correlation in predicting the damage [6]. With the available damage model,the number of cycles to creep failure is computed for the interfacial strain values.

058.2

SN

121.0

4.0D

+=

-- (4)Where N and S are normal strain and shear strain values respectively. Theinverse of creep damage will give number of cycles to failure. This model


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considers that the shear strain is the main parameter for failure induced by edges.Table 2 shows the estimated creep life for the TBC and FTBC layers based on FEresults. It is clear that the creep life of FTBC layers are much more than that ofthe life of TBC for same loading.

Table 2: Life comparison of TBC and FTBC layers for same rate of loadings (Atinterface)

Conclusion

Concept of functionally graded layers is introduced in conventional thermal barriercoatings so as to reduce the interfacial failures. For the same load, material andgeometric features, simulation of both TBC and FTBC is performed. The number

of cycles to failure for thermal cycle loading for creep damage has beencompared for both TBC and FTBC. As expected, the life for FTBC is much morethan conventional TBC.

Acknowledgments

The corresponding author wishes to acknowledge his colleagues in TCS Chennaifor their encouragements and support.

References

1. Padture. N.P., Gell. M., Jordan E.H., 2002, Thermal Barriercoatings for Gas Turbine Engine applications, Science, 296, p. 280-284.

2. Busso E.P. Wright.L, Evans. H.E., McCartney. L.N., Saunders.S.R.J., Osgerby. S. Nunn. J., 2007, A Physics-Based LifePrediction Methodology for Thermal Barrier Coating Systems, ActaMaterialia, 55, p. 1491-1503.

3. Dumir, P.C., Dube G.P and Mullick. A., 2003, Axisymmetric staticand dynamic buckling of laminated thick truncated conical cap, Int.J. Non-Linear Mechanics, 37, 903-910.

4. DeMasi, J.T., Sheffler K.D. and Ortiz. M., 1989, Thermal BarrierCoating Life Prediction Model Development, Phase-I, Final Report

Details Normal Shear Damage Life

Strain Strain

(No. of

Repeats)

TBC BC 0.0118 0.0265 - -

CC 0.0092 0.0209 0.023795 42.0251

FTBC F5 0.0248 0.0417 0.172984 5.780886F4 0.0052 0.0090 0.007286 137.2519

F3 0.0051 0.0088 0.006948 143.9202

F2 0.0048 0.0084 0.00629 158.9822

F1 0.0036 0.0063 0.003507 285.1558


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NASA-182230

5. Kerkhoff, G., Vaben, R., and Stover, D. 1999, NumericallyCalculated Oxidation Induced Stresses in Thermal Barrier Coatingson Cylindrical Substrates, Proc. Of an EFC Workshop,Frankfurt/Main, Edited by Schutze, M., and Quadakkers, W. J.,Woodhead publishing.

6. Hillery R. V. Pilsner B.H., McKnight R. L., Cook T.S., and HartleM.S., 1988, Thermal Barrier Coating Life Prediction ModelDevelopment, Final Report, NASA-180807.

Future Work

Porosity effect of the ceramic coat and sintering effect at hightemperature exposure are not considered in the analytical model.This will give variations in strain rate which directly affects thespallation life. The spallation life model with all this effect willproduce extensive research outcome.

Structural loads also along with the thermal load could beconsidered with realistic load history.

Analytical methods for progressively growing thermally grown oxidelayers on bond coat should be included for exact life prediction.

Fracture mechanics based life prediction model can be developed.


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All content / information present here is the exclusive property of Tata Consultancy Services Limited (TCS). The content /information contained here is correct at the time of publishing.No material from here may be copied, modified, reproduced, republished, uploaded, transmitted, posted or distributed in anyform without prior written permission from TCS. Unauthorized use of the content / information appearing here may violatecopyright, trademark and other applicable laws, and could result in criminal or civil penalties.Copyright 2007 Tata Consultancy Services Limited

Life Prediction of Functionally Graded Thermal Barrier Coatings

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