Mechanical stability limits of bi-layer thermal barrier ...

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Thermal Barrier Coatings IV Proceedings

Summer 6-26-2014

Mechanical stability limits of bi-layer thermalbarrier coatingsMario RudolphiDECHEMA-Forschungsinstitut, [email protected]

Mathias GaletzDECHEMA-Forschungsinstitut

Michael SchutzeDECHEMA-Forschungsinstitut

Martin FrommherzIfW, Technische Universität Darmstadt

Alfred ScholzIfW, Technische Universität Darmstadt

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Recommended Citation[1] R.A. Miller, J.L. Smialek, and R.G. Garlick, in Science and Technology of Zirconia, A. H. Heuer and L. W. Hobbs, Eds., Columbus,OH, USA: The American Ceramic Society, (1981) 241-253. [2] R. Subramanian, A. Burns, and W. Stamm, in Proceedings of ASMETurbo Expo 2008: Power for Land, Sea and Air, ASME, (2008). [3] R. Vaßen, F. Traeger, and D. Stöver, International Journal ofApplied Ceramic Technology, vol. 1, no. 4 (2004) 351-361

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AuthorsMario Rudolphi, Mathias Galetz, Michael Schutze, Martin Frommherz, Alfred Scholz, Mathias Oechsner,Emine Bakan, Robert Vassen, and Werner Stamn

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MaterialsChemical Engineering

Biotechnology

Research for Sustainable Technologies

M. Rudolphi1, M.C. Galetz1, M. Schütze1,M. Frommherz2, A. Scholz2, M. Oechsner2, E. Bakan3, R. Vaßen3, W. Stamm4

Mechanical Stability Limits of Bi‐Layer Thermal Barrier Coatings

Thermal Barrier Coatings IV, Irsee, 26.06.2014

1DECHEMA-Forschungsinstitut, Theodor-Heuss-Allee 25, 60486 Frankfurt am Main, Germany2Fachgebiet und Institut für Werkstoffkunde, Technische Universität Darmstadt, Grafenstr. 2, 64283 Darmstadt, Germany3Forschungszentrum Jülich GmbH, IEK-1, 52425 Jülich, Germany 4Siemens Power Generation, Mellinghofer Str. 55, 45473 Mülheim an der Ruhr, Germany

2

Acknowledgement

High Temperature Materials research group of theDECHEMA Research Institute

M. Frommherz,A. Scholz,M. Oechsner

E. BakanR. Vaßen

Project Partners:


Funding:

Financial support:

3

Motivation


Ongoing effort to increase operating temperature / efficiency

~ 1200-1250 °C

[after: Marini and Morrison; Proc. 7th Int. Charles Parsons Turbine Conf. /2007 ]

However, the temperature limit of 7YSZ is around 1250°Cdue to phase transformations above this temperature [1] Search for new materials / new TBC solutions

[1] W. Pan et al., MRS BULLETIN , Vol. 37 (2012)

4

Approach – Bi‐Layer TBC


Bi‐Layer Concept:• surface temperatures > 1250 °C

• crack resistance to TGO growth induced stresses

• avoiding unwanted reactions between GZO and TGO

GZO: Gd2Zr2O7(APS)

APS YSZ

bond coatTGO

• Optimization of spray process

• Sample manufacturing

• Oxidation testing• Mechanical testing

(Charalambides test, Gic)• TGMF testing

• Oxidation testing• Mechanical testing

(4‐point bending test, c)• Lifetime modeling

Ni‐base substrate

400µm

100µm

250µm

5

4‐pt. Bending with Acoustic Emission Measurement


4‐Point Bending

50kNUniversal TestingMachine

F F

FF

sensor sensor

wave guide(Pt‐wire)

TBC damage(e.g. cracks)

acoustic emission system

coatingsubstrate

Testing was performed at RT

6

4‐Point Bend Testing – TBC in Tension


1. Segmentation

Mode I failureTBC outer fiber strain

Mode II failureTBC/BC interface strain

2. Delamination

7

4‐Point Bend Testing – TBC in Compression


1. Delamination

2. Shear cracking

Mode I failureTBC/BC interface strain

Mode II failureTBC outer fiber strain

not always observed, strong interface

8

Experimental Results


Compressive Loading of TBC,Bi‐Layer System

9

4‐PB Results ‐ Compression


0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.00

100

200

300

400-1.70 -2.02

-1.97

F591_051S2 - LPas sprayed

AE

Eve

nts

Outer fiber strain [%]

-1.68

0

-100

-200

-300

-400

-500

-600

-700

-800

-900

-1000

-1100

Stre

ss [M

Pa]

0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.00

500100015002000250030003500400045005000550060006500700075008000

F591_051S2 - LPas sprayed

AE

Ene

rgy


-1.70 -2.02

-1.97-1.68

0

-100

-200

-300

-400

-500

-600

-700

-800

-900

-1000

-1100

Stre

ss [M

Pa]

1. 2.

Two distinct peaks can be identified in the acoustic emission signal under compressive loading!

What are the individual peaks?

as sprayed as sprayed

10

4‐PB Results ‐ Compression


0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.00

2000

4000

6000

8000

10000

12000

14000F591_054S2 - LP500h @ 1050°C

AE

Ene

rgy


-0.83

-1.61

0

-100

-200

-300

-400

-500

-600

-700

-800

-900

-1000

Stre

ss [M

Pa]

1. 2.

2.

1. 2.

1.

GZO shear failure YSZ shear failure

4‐PB in compression500h1050°C

11 Thermal Barrier Coatings IV, Irsee, 26.06.2014

Tensile Loading of TBC,Bi‐Layer System

12

4‐PB Results ‐ Tension


0.0 0.5 1.0 1.5 2.0 2.5 3.00

5000

10000

15000

20000

25000

30000F591_055S2 - LP500h @ 1050°C

AE

Ene

rgy


0.76

1.09

0.33

0

200

400

600

800

1000

Stre

ss [M

Pa]

0.0 0.5 1.0 1.5 2.0 2.5 3.00

200

400

600

800

1000

1200

1400F591_055S2 - LP500h @ 1050°C

AE

Eve

nts


0.76 1.09

0.31

0

200

400

600

800

1000

Stre

ss [M

Pa]

500h1050°C

0.0 0.5 1.0 1.5 2.0 2.5 3.00

200

400

600

800

1000

1200

1400F591_055S2 - LP500h @ 1050°C

AE

Eve

nts


0.76 1.09

0.31

0

200

400

600

800

1000

Stre

ss [M

Pa]

0.0 0.5 1.0 1.5 2.0 2.5 3.00

100

200

300

400

500

600

700

800F591_053S2 - LP100h @ 1050°C

AE

Eve

nts


0

200

400

600

800

1000

Stre

ss [M

Pa]

1.78

1.36

0.47

1.59

13



1. 2.

What are the individual peaks?

• Tensile geometry does not lead to well separated peaks• Some samples show gradually increasing AE signal at the beginning

However, maybe 3 signals can be identified:

3. 1. 2. 3.

14



Macroscopic images do not provide sufficient insight. Only final failure can be observed.

1. Segmentation failure of GZO‐layer2. Delamination of GZO along GZO/YSZ interface3. Segmentation failure of YSZ layer

15

Critical Strain Values


max. tolerable strain at TBC/BC interfaceMay be used in similar manner as SN‐curves for lifetime assessment

0 100 200 300 400 500 600-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

YSZ segmentation failure

YSZ shear failure

GZO delamination failure

GZO shear failure

Crit

ical

Str

ain

(TB

C/B

C In

terf

ace)

(%)

Oxidation Time (h)

Bi-Layer TBCIsothermal Oxidation 1050°C

GZO segmentation failure

16

Fracture Mechanics Approach

Griffith‐Criterion:

cKc

c

cEfK

TBC

Icc

E

Material Constant(But: Measurements maybe influenced by sample history)

Geometry Factor(Defect geometry)

Damage Parametersc – defect sizeE – Young‘s modulus

Critical Strain:

Possible Values:1.12 surface defect of inifinite lentgh1.0 burried defect0.64 semi-circular surface defect


17

Possible Failure Modes in 4‐Point Bending

cEfK

TBC

IIcshc

2

TBC

dr

Icdc Ecf

K2

)1()1(

Compression Delamination

CompressionShear cracking

Delamination

Delamination Through

TensionSegmentation

TensionDelamination

cEfK

TBC

Icsc

cEfK

TBC

IIcdc

2

Segmentation

Delamination

M. Schütze, Protective Oxide Scales and their Breakdown, John Wiley, (1997)

Griffith:

cKIc

c


εc is strain in the coating!

18

Strain gradient across the TBC‐thickness under pure bending

neutral axis

Em

EBCM M

EGZO=EYSZ<Em


~30% difference in strain betweenTBC/BC interface and outer fiber for 500µm TBC Failure position has to be considered!

EBC=ETBCEYSZ

EGZO

19

Microstructure has an influence on Kc‐values


crack path mostlythrough spray flatsKIc(path1)e.g. tensile segmentation

crack path along spray flat boundariesKIc(path2)e.g. compressivedelamination

KIc(path1) > KIc(path2)

20

Chosing failure mode and critical strain position


Mode I failureTBC/BC interface strain

not observed strong interface

Mode II failureTBC outer fiber strain

Mode I failureTBC outer fiber strain

Mode II failureTBC/BC interface strain

0 500 1000 1500 20000

100200300400500600700800900

1000

Def

ect S

ize

(µm

)

Time (h)0 500 1000 1500 2000

0100200300400500600700800900

1000

Def

ect S

ize

(µm

)

Time (h)

0 200 400 600 800 1000 1200 14000

102030405060708090

100

YSZ

Stiff

ness

(GPa

)

Time (h)

21

Modeling input values


cEfK

TBC

cc

YSZ GZOno exp. data yet,assumption:

same trend as YSZ

f=1.0(buried defect)

KIc=5.3 MPa m1/2

KIIc=10.6 MPa m1/2

KIc=2.3 MPa m1/2

KIIc=2.8 MPa m1/2

Kc currently used as fittingparameter!

0 200 400 600 800 1000 1200 14000

102030405060708090

100

GZO

Stif

fnes

s [G

Pa]

Time [h]

~49GPa~25GPa

22

Bi‐Layer System – GZO Failure


0 100 200 300 400 500 600-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

delamination failure

shear failureCrit

ical

Str

ain

(%)

Oxidation Time (h)

GZO Failure (Bi-Layer TBC)Isothermal Oxidation 1050°C

segmentation failure

0 100 200 300 400 500 600-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5 YSZ, segmentation YSZ, shear YSZ Segmentation YSZ Shear Cracking


YSZ shear failure

Crit

ical

Str

ain

(%)

Oxidation Time (h)

YSZ Failure (Bi-Layer TBC)Isothermal Oxidation 1050°C

23

Bi‐Layer System – YSZ Failure


offset

offset


single‐layerYSZ TBC(500µm)

0 100 200 300 400 500 600-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0



YSZ shear failure

Crit

ical

Str

ain

(%)

Oxidation Time (h)

YSZ Failure (Bi-Layer TBC)Isothermal Oxidation 1050°C

Identical values for values for E, Kc or c!

25

Bi‐Layer System – YSZ Failure


0 100 200 300 400 500 600-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0



YSZ shear failure

Crit

ical

Str

ain

(%)

Oxidation Time (h)

YSZ Failure (S2-LP)Isothermal Oxidation 1050°C

offset +0.5%

offset +0.5%

Possible explanation for offset:Residual stresses in the TBC are relieved by GZOfailure prior to measurement of YSZ failure....currently under investgation!

0 100 200 300 400 500 600-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

safe operation


YSZ shear failure

Crit

ical

Str

ain

(%)

Oxidation Time (h)

YSZ FailureIsothermal Oxidation 1050°C

0 100 200 300 400 500 600-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

safe operation

GZO delamination failure

GZO shear failureCrit

ical

Str

ain

(%)

Oxidation Time (h)

GZO FailureIsothermal Oxidation 1050°C

GZO segmentation failure

26

Mechanical Stability Diagrams


GZO Failure YSZ Failure

27

Summary


• Mechanical 4‐point bending with in‐situ acousticemission measurement is a valuable tool to assessdamage processes in bi‐layer TBCs

• A modeling approach for bi‐layer TBCs has been developed to delineate areas of safe operation from areas where failure is imminent ‐> mechanical stability diagram


Thank you for your attention!

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