Uncertainties in Thermal Barrier Coating Life Prediction

by Karl A. Mentz

A Thesis Submitted to the GraduateFaculty of Rensselaer Polytechnic Institute

in Partial Fulfillment of theRequirements for the degree of

MASTER OF SCIENCE IN MECHANICAL ENGINEERING

Thermal Barrier Coating (TBC) Life

Overview: Thermal barrier coating life is usually predicted based on oxidation life of the bond coating. The effect of thermal fatigue on coating life has been studied but is generally not included in life predictions due to the complex crack dynamics and interactions with the coating and substrate. An attempt to quantify the effect of different thermal cycling on coating life is made using test data.

Method: Coating life data was obtained from multiple sources and tested in multiple test cells.Data analysis is first done to determine a coating life for the base test cycle (short cycle). This is to quantify the manufacturing and test variation and give an accurate life number for comparison.The data for the long cycle will then be compared to the base short cycle coating life to determine if a difference in coating life can be detected.

Sample Information

Material: Cobalt base superalloyBond Coat: NiCoCrAlY. Applied by low pressure plasma spray (LPPS)TBC: ZrO2 partially stabilized with 7wt % Y2O3. Applied by air plasma spray (APS)

Thermal Barrier Coating (TBC) Life

TBCs reduce substrate metal temperatures by being an insulating layer between the hot gas on the outside of a component and the base metal.

The coating consists of a ceramic coating to provide thermal protection and a metallic bond coat to provide oxidation protection and provide a bonding surface for the ceramic.

Temperature gradient in a TBC coated part [1]. Coating structure on a substrate material. [2].

Thermal Barrier Coating (TBC) Life

TBC failure is believed to be due to a combination of oxidation damage and crack damage from thermal cycling.

Failure ModesOxidation: A thermally grown oxide layer (TGO) forms at the interface between the bond coat and the ceramic coating. Spallation of the ceramic coating occurs when the TGO reaches a critical thickness where the internal stresses exceed the TGO properties and delamination of the ceramic occurs.

Thermal fatigue: Cracks propagate through the ceramic and TGO layers, driven by thermally induced cyclic stresses until the cracks link together and the coating spalls.

Test Cycles

Short cycle: Typical cycle used for testing aerospace components.Thermal fatigue damage should decrease life compared to pure oxidation failure mode.

Long cycle: Test cycle used for industrial gas turbine enginesOxidation failure mode should dominate. Expect longer life due to decreased cycling.

Cycle Temperature

Cycle Time(Min)

Flame Time(min)

Cooling Time(min)

Short cycle, high temp 1950 6 4 2Long cycle, high temp 1950 120 118 2Long cycle, low temp 1850 120 118 2

Test Method

Burner Rig Testing

Burner rig schematic [3]

Test bar geometry [3]

Short Cycle Test Data

Coatings produced at five different sources.

1) Secondary production source2) Development process at source 33) Main production source4) Main production source5) Development process at source 4

Testing Done in 4 different test cells.

Different coating properties

Coating life by source.

“Hot time” is the amount of time the test specimen spends at the maximum test temperature of 1950F. Hot time is used to compare life from different test cycles and can be directly used to calculate TGO growth.

Typical failed test specimen.

Failure defined as spallation of ~50% of the hot zone area. Hot zone area is the face of the test bar that is exposed to the burner rig [3].

Relative coating life by coating source

Data Fitting

Failure data fitted using Weibull distribution.• Can mimic behavior of normal, exponential, and other distributions• Will not predict negative lives• Functions from multiple sample sets can be combined to predict an overall

population life.• Can be used to fit small data samples.

Alpha: Scale parameterBeta: Shape parameterx: time

Weibull Plot. Source 4 Coating Life

0.01 0.1 1 10

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lati

ve

Pe

rce

nt

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cu

rre

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%

0.01

0.05

5

Hot Time

0.1

0.5

1

10

50

999590

63.2

1

13

35

83

20

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 1 2 3 4 5

Test Cell

Re

lati

ve

Lif

e

Source 3

Source 4

Relative life data by test cell and source.

Short Cycle Test Data

Coating Life by Test Cell.

Failure data was fit with a Weibull distribution.

Weibull parameters used to predict the average coating life for each source and test cell.

Predicted Life Cell 2 Cell 3 DeltaSource 3 1.03 1.08 0.05Source 4 0.87 0.92 0.05

Delta 0.16 0.16

Data shows source 3 has a higher life than source 4Test cell 3 produces higher coating life results than test cell 2.

Weibull Parameters Shape Scale St DevSource 3 Cell 2 3.61 1.14 0.33Source 3 Cell 3 2.06 1.29 0.45Source 4 Cell 2 5.24 0.93 0.18Source 4 Cell 3 2.82 1.05 0.37

Weibull Probability Plot

-5.0000

-4.0000

-3.0000

-2.0000

-1.0000

0.0000

1.0000

2.0000

-1.0000 -0.8000 -0.6000 -0.4000 -0.2000 0.0000 0.2000 0.4000

ln(x)

ln(l

n(1

/(1

-pi)

))

Series1

Linear (Series1)

Weibull plot of Source 4, Test cell 2 data

5.24 Alpha (Shape)0.93 Beta (Scale)

B.xx Life0.50 0.8707

Long Cycle Test Data

Long cycle test data fitted with Weibull distribution and average life predicted.

All data was from source 4.

Test cell 2 showed results that agreed with expected trends, the long cycle testing produced longer coating life.

Test cell 4 showed drastically reduced life for the long cycle testing.Sources of error in the data;1. Small amount of data available for long cycle tests (5 samples each)2. Little information on test cell 4 and no test data to determine the impact of the test cell on life

results.3. Extremely short life indicates an anomaly in coating specimens or test conditions for samples in

cell 4.

Relative life for long cycle testing by test cell.

Predicted Life Short Cycle Long Cycle DeltaTest CellSt Dev

Source 4, Cell 2 0.87 0.93 0.06 0.18Source 4, Cell 4 0.83 0.65 -0.18 0.24

Delta 0.04 0.28

Conclusions

Coating life is extremely variable.

Coating life may vary by a factor of 3x or more within coatings produced by the same source.

Test conditions can introduce significant variation in addition to the actual coating life variation by source.

Differences in coating life due to thermal cycle damage is small compared to production and testing differences.

Life predictions for thermal barrier coatings are limited by the consistency of production and testing.

References

1. Dynamic-Ceramic LTD.Crewe, England http://www.dynacer.com/coatings.htm

2. IMR Test Labs. Ithaca, NY. http://www.imrtest.com/what_we_do/Thermal_Spray_Coatings/

3. DeMasi, J.T., Sheffler, K.D., Ortiz, M. “Thermal Barrier Coating Life Prediction Model Life Development.” United Technologies Corporation, 1989. NASA Contract NAS3-23944