NASA/TM--2001-210816

r Thermal Fatigue and Fracture Behavior

of Ceramic Thermal Barrier Coatings

Dongming Zhu and Sung R. Choi

Ohio Aerospace Institute, Brook Park, Ohio

Robert A. Miller

Glenn Research Center, Cleveland, Ohio

June 2001
The NASA STI Program Office... in Profile

Since its founding, NASA has been dedicated to

the advancement of aeronautics and space

science. The NASA Scientific and Technical

Information (STI) Program Office plays a key part

in helping NASA maintain this important role.

The NASA STI Program Office is operated by

Langley Research Center, the Lead Center for

NASA's scientific and technical information. The

NASA STI Program Office provides access to the

NASA STI Database, the largest collection of

aeronautical and space science STI in the world.

The Program Office is also NASA's institutional

mechanism for disseminating the results of its

research and development activities. These results

are published by NASA in the NASA STI Report

Series, which includes the following report types:

TECHNICAL PUBLICATION. Reports of

completed research or a major significant

phase of research that present the results of

NASA programs and include extensive data

or theoretical analysis. Includes compilations

of significant scientific and technical data and

information deemed to be of continuing

reference value. NASA's counterpart of peerreviewed formal professional papers but

has less stringent limitations on manuscript

length and extent of graphic presentations.

TECHNICAL MEMORANDUM. Scientific

and technical findings that are preliminary or

of specialized interest, e.g., quick release

reports, working papers, and bibliographies

that contain minimal annotation. Does not

contain extensive analysis.

CONTRACTOR REPORT. Scientific and

technical findings by NASA-sponsored

contractors and grantees.

CONFERENCE PUBLICATION. Collected

papers from scientific and technical

conferences, symposia, seminars, or other

meetings sponsored or cosponsored by

NASA.

SPECIAL PUBLICATION. Scientific,

technical, or historical information from

NASA programs, projects, and missions,

often concerned with subjects having

substantial public interest.

TECHNICAL TRANSLATION. Englishlanguage translations of foreign scientific

and technical material pertinent to NASA's

mission.

Specialized services that complement the STI

Program Office's diverse offerings include

creating custom thesauri, building customized

data bases, organizing and publishing research

results.., even providing videos.

For more information about the NASA STI

Program Office, see the following:

• Access the NASA STI Program Home Page

at http:/[www.sti.nasa.gov

• E-mail your question via the Internet to

help@sti.nasa.gov

• Fax your question to the NASA Access

Help Desk at 301-621-0134

• Telephone the NASA Access Help Desk at

301-621-0390

Write to:

NASA Access Help Desk

NASA Center for AeroSpace Information

7121 Standard Drive

Hanover, MD 21076
NASA/TM--2001-210816

Thermal Fatigue and Fracture Behavior

of Ceramic Thermal Barrier Coatings

Dongming Zhu and Sung R. Choi

Ohio Aerospace Institute, Brook Park, Ohio

Robert A. Miller

Glenn Research Center, Cleveland, Ohio

Prepared for the

25th Annual International Conference on Advanced Ceramics and Structures

sponsored by the American Ceramic Society

Cocoa Beach, Florida, January 21-26, 2001

National Aeronautics and

Space Administration

Glenn Research Center

June 2001
Acknowledgments

This work was supported by NASA Ultra-Efficient Engine Technology (UEET) Program. The authors are grateful to

George W. Leissler and Sandra L. Leissler of Dynacs Engineering, Inc. atNASA Glenn Research Center for their

assistance in the preparations of plasma-sprayed ZrO2-8wt%Y203 coatings and the metallographic specimens.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov/GLTRS
THERMALFATIGUEANDFRACTUREBEHAVIOROFCERAMIC THERMAL

BARRIER COATINGS

Dongming Zhu and Sung R. Choi

Ohio Aerospace Institute

Brook Park, Ohio 44142

Robert A. Miller

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

ABSTRACT

Thermal fatigue and fracture behavior of plasma-sprayed ceramic thermal barrier

coatings has been investigated under high heat flux and thermal cyclic conditions. The coating

crack propagation is studied under laser heat flux cyclic thermal loading, and is correlated with

dynamic fatigue and strength test results. The coating stress response and inelasticity, fatigue and

creep interactions, and interface damage mechanisms during dynamic thermal fatigue processes

are emphasized.

INTRODUCTION

Ceramic thermal barrier coatings (TBCs) have received increasing attention for advanced

gas turbine engines. The future TBC systems will be more aggressively designed for the thermal

protection of engine hot section components, thus allowing significant increase in engine

operating temperatures, fuel efficiency and engine reliability. However, the increases in engine

temperature and heat flux can raise considerable coating durability issues. The development of

next generation advanced TBCs will greatly rely on the better understanding of the coating

behavior and failure modes under the high temperature, high thermal gradient cyclic conditions.

Critical coating properties affecting the TBC life, such as ceramic sintering and creep,

and thermal conductivity change kinetics at high temperatures, have been previously addressed

[1-5]. Fundamental TBC materials properties including deformation, cyclic fatigue, creep and

fracture behavior have also recently been characterized [6-12]. This present paper is primarily

focused on thermal fatigue properties of plasma-sprayed TBC systems under temperature and

stress gradients that can be encountered in advanced engine systems. In particular, quantitative

crack propagation in a TBC system is investigated under laser heat flux thermal cyclic loading.

The coating life prediction methodologies can be established based on the materials response and

failure mechanisms observed under the high-heat-flux tests.

EXPERIMENTAL MATERIALS AND METHODS

Air plasma-sprayed (APS) ZrO2-8wt%YzO3 thermal barrier coatings were used for the

coating thermal fatigue-crack propagation study. In this work, 25.4 mm-diameter and 3.2 mmthick nickel base superalloy specimens were used as substrates. This button specimen

configuration, which was previously used for coating thermal conductivity studies [5,13], is ideal

NASA/TM--2001-210816 1
forinvestigatingthecoatingdelaminationcrackpropagationbecausethecoatingconductivity

changeissensitivetothedelaminationcracks.Thetestcoatingsystemconsistsof anairplasmasprayedZrO2-Cwt%Y203 topcoating(typicalthicknessrangeabout127to 200_tm)andan

intermediatelowpressureplasma-sprayed NiCrAIYbondcoat(thicknessabout120/am),plasmasprayedonthebuttonspecimensubstrates. A through-thickness hole, with either I mm- or 2 mmdiameter, was drilled in the center of the substrate by the electro-discharge machining (EDM). The

central substrate hole thus prepared was able to successfully initiate a controlled coating

delamination crack during initial laser thermal cyclic loadings.

A high power CO2 laser was used to test the TBC specimens under high temperature,

high thermal gradient cyclic conditions, thus providing the necessary thermal fatigue loading for

advancing the pre-initiated delamination cracks within the ceramic coatings near the

ceramic/metal interface. A uniform laser power distribution was achieved over the specimen

coating surface by using an integrating ZnSe lens combined with the specimen rotation. A

description of the laser test rig system and the general approaches to testing the TBCs have been

described elsewhere [4,5]. For this work, low to medium heat fluxes of 60 to 100 W/cm 2 were

used depending on the specimen coating thickness. Initial ceramic surface temperature was set at

approximately 1287 °C. Either 10 or 20 rain heating/cooling cycles were used for the laser thermal

gradient cyclic tests. During the test, the ceramic surface temperature was measured by an 8 /am

infrared pyrometer. The back surface of the metal substrate was measured by both a two-color

infrared pyrometer and a calibrated 8 /am pyrometer. The spot size (diameter) of the 8 /am

pyrometer was 8 to 10 mm. The simultaneous measurements of the surface and backside

temperaiures _d-etraris_hi I:ie-a_g-iemperaiUre iffforn-aation,-wl_6_fi ]S esp_ci_alY3_- imporian(for

the cracked Coaffngspecimens. The spatially_eighted-average temperature information at Coating

surface in the presence of a delamination crack was calibrated in-situ with a high sensitivity

camera under the given crack sizes, and ex-situ with a blackbody calibration furnace with various

aperture sizes.

EXPERIMENTAL RESULTS AND DISCUSSION

Effective Thermal Conductivity and Delamination Crack Propagation

The laser thermal fatigue and crack propagation study is based on a modified thermal

conductivity test. In this test, the surface temperature and metal backside temperature of a precracked TBC specimen are continuously measured during the laser cyclic thermal fatigue test. The

nominal, or effective thermal conductivity of the ceramic coating can thus be determined as a

function of the laser cycle number. The effective thermal conductivity at any given cycle contains

valuable information about the advancing delamination crack in the coating in laser thermal cyclic

loading. Fig. 1 shows typical test results for a 127/am-thick, pre-cracked TBC specimen under the

laser cyclic loading. It can be seen that the surface temperature increased continuously due to the

crack initiation and propagation. The metal backside temperature and the predicted metal/ceramic

interface temperature, however, remained relatively constant or a slight decreasing trend. The

measured ceramic coating conductivity initially increased and decreased with further increasing

the testing time and cycle number. The initial rise in the measured conductivity is attributed to the

ceramic sintering effect. It is also noted that a sudden drop in thermal conductivity that is

corresponding to a large surface temperature jump, was observed at about 30 hrs, suggesting a

coating spallation event occurred at that time.

The measured ceramic thermal conductivity k ....... d for a pre-cracked TBC specimen

can be expressed as

kmeasured = ](intact + k cracking (1)

NASA]TM---2001-210816 2
wherekin ,act is the thermal conductivity of non-cracked TBC specimen, which typically increases

with time or cycle number due to the ceramic sintering effect, and k,,racking is the thermal

conductivity reduction due to the coating cracking effect. The k_.ta_, can be estimated from the

previously reported coating property data under the current test conditions [3,14]. From the

k_,,a_,and measured k.... ured data, the kcr_c_i,g can be obtained, as shown in Fig. 1. The kc,.om,,s

showed a monotonic decreasing trend that corresponds to the delamination crack growth in the

TBC system during laser thermal cycling. Fig. 2 shows similar test results of a TBC specimen

with a 2 mm hole in the substrate. Again, the change in the specimen effective thermal

conductivity clearly quantified the evolution of the crack propagation and coating spallation

behavior. The coating propagation process has also been confirmed independently by a high

sensitivity video camera [15].

Fig. 1

._,,¢

O

O

O

_D

e_

• ksintering increase o Tceramic surface

• kceramic measured o Tinterface

• kcracking effect m Tmetal

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

1400

5 10 15 20 25 30 35 40

Time, hours

1200

o

1000

E

©

800

6O0

Typical laser thermal fatigue test results of a 127 _tm-thick, precracked TBC specimen

showing the coating temperature and thermal conductivity changes as a function of

cycle number under 10 min heating and 2 min cooling laser cycling. The ceramic

surface temperature increases and the metal backside temperature slightly decreases as

the delamination crack is initiated and propagated. The effective ceramic coating

conductivity shows an initial increase due to the coating sintering, and then a decrease

due to the crack propagation. A large conductivity drop during the test indicates the

coating spallation.

NASAfrM---2001-210816 3
Fig.2

_J 1000

©

_ 800

b- 600

©

400

2oo

E

Cycle number

0 50 100 150 200 250

.... J 1 , , 10

I

w

AT

C¢l,'arrd(, ll]e a_l]r_,d

crack length

6

0 0

0 20 40 60 80 100

Time, hours

i

_a

_=-E

_E

The laser thermal fatigue test results of a 0.2 mm-thick TBC specimen with a 2 mm

hole in the substrate when exposed to 20 min heating and 4 min cooling laser cycling.

A close relationship between the coating thermal conductivity and delamination crack

length is demonstrated. The changes in the coating's effective thermal conductivity can

be used to quantitatively describe the evolution of the delamination crack propagation

and coating spallation behavior.

TBC Buckling, Spallation and Critical Failure Strains

Under the relatively low heat flux laser thermal cycling of thin coatings, the coating

failure mode was found to be primarily coating delamination crack growth with eventual buckling

spallation. Figure 3 shows crack lengths and the corresponding crack propagation rates of several

laser tested TBC specimens as a function of cycle number. For the specimen tested, the initial

average crack propagation rate was in the range of 3 to 8 _tm/cycle. However, the crack

propagation increased significantly with further laser cycling. At the later stage near the coating

spallation, the crack propagation rates increased to as high as 30 to 40 _tm/cycle. A mixed coating

delamination crack propagation path (either in the ceramic near the ceramic/bond coat interface or

in the thermally grown oxide scales) was observed for the tested specimens. The accelerated crack

growth under laser heat flux cyclic conditions is attributed to the increased driving force (the

increased thermal stresses) for the crack propagation. A critical crack size can be reached to

initiate catastrophic coating spallation from the subcritical crack propagation during laser thermal

cyclic loading exposure.

The critical buckling-spallation crack size, 2a (the diameter of an approximate pennyshaped delamination crack), can be determined from the crack size plots (Fig. 3) at the coating

spallation point. The critical crack size for initiating spallation was found to be in the range of 3 to

5 mm. However, the final spallation size for the coating ranged from 5 to 12 mm after the laser

cyclic testing, due to the fast, unstable crack propagation after the initial spallation. The critical

spallation size as a function of coating thickness was plotted in Fig. 4. It can be seen that a larger

critical crack size is needed to spall a thicker coating. For simplicity purpose and without

considering the complex nature of thermal visco-plasticity behavior of the TBC system, the critical

coating buckling spallation strain can be estimated from the classical elastic theory for a clamped

circular plate [ 16]

am,,/E = [tc/12(I-v2 )_t / a) 2 (2)

NASA/TM---2001-210816 4
whereo',p,,, is the critical buckling/spallation stress, E and v are Young's modulus and Poisson's

ratio of the coating, respectively, r is a constant about 14.68, t is the coating thickness, and a is

the crack radius at the spallation. The compressive, critical spallation strain is about 0.8 percent for

the plasma-sprayed thermal barrier coatings under the laser test conditions. This failure strain can

correspond to a compressive spallation stress possibly ranging from 240 to 480 MPa, depending

on the actual elastic modulus of the coatings. The laser test results are in good agreement with

uniaxial compression test results [6,11].

E

E

rJ

I0 f .... d _ i

........... ..%,

4 f _- ,//".

0

4O

35 _

2s _

20 ._

I0

0

50 100 150 200

Cycle number

(a)

E

8 _._ da.;dN.r. // 40 {:n_

6 / rr' ,z !3o

/ 1.o

" I I

-- 2a _:

2 10 "_

o

0 _' _ _ "_-'_-__ _J 0

0 20 40 60 80 100 120

Cycle number

(b)

Fig. 3

:!............ , 403s _,

---_-- d;vdN _.,,.- 30 _:_g

[ -Y / .__

0 20 40 60 80 100 120

Cycle number

(c)

10 .... ,, . , .... _ _ i40

0 ......... L J _ ..... 0

0 50 100 150 200 250

Cycle number

(d)

Crack lengths and the corresponding crack propagation rates of laser tested TBC

specimens as a function of cycle number. The crack propagation rates increase with the

cycle number. The accelerated crack growth under laser heat flux cyclic conditions is

attributed to the increased driving force for the crack propagation. (a) 127 lam-thick

coating; (b) 176 p.m-thick coating; (c) 185 _tm-thick coating; (d) 200 I.tm-thick coating.

NASA/TM---2001-210816 5
Fig.4

d

eq

_D

e9

6.0

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0 •

0.12

I t I I

°

° •c'° ° °_

°°.o°_*

oOOOB o°°t

°oB °

°$._o o°°°

0.14 0.16 0.18 0.20 0.22

Coating thickness, mm

The critical spallation size of the ZrO2-Swt%Y203 coatings as a function of coating

thickness under laser heat flux testing.

Fatigue Crack Subcritical Growth

The delamination crack propagation rate da / dN under the laser thermal cyclic loads can be

generally expressed as

da / dN = da / dNh_,,ans + da / dN cooti,_s= CtAKh_,7,i,__ + C2AK ,.'oo#,_g (3)

where da/dNhean,, s and da/dNcooti,,s are the crack propagation rates under laser thermal transient

heating and cooling, respectively, AKhe_,i_s and aKcoot_g are the stress intensity factor amplitudes

associated with the heating and cooling thermal loads, C_, C2 and m are constants. Under the

laser heat flux test conditions, due to the relatively large temperature difference (AT) across the

ceramic coating, the residual compressive stresses for driving the coating delamination crack

propagation are greatly reduced as compared to a uniform heating-cooling case. On the other

hand, the dynamic transient thermal stresses, induced during the laser surface rapid heating of the

partially delaminated coating may become a predominant driving force for the crack propagation,

primarily because of the large thermal compressive stress magnitudes and the fast dynamic

loading characteristics near the crack tip. Previous dynamic fatigue tests of the plasma-sprayed

ZrOz-8wt%Y203 have shown that the coating fracture stress is significantly reduced with

increasing the loading stress rate. This phenomenon has been attributed to the reduced crack tip

visco-plasticity induced blunting under the high stress rates [11].

In this study, in order to assess the effect of the thermal stress intensity factor amplitude

AK on coating crack propagation, a coating delamination and buckling spalling model was used

[17]. Based on the stress amplitude derived from the measured transient temperature profiles of

the coating system, the subcritical crack propagation driving force zkK can be estimated

AK = [(11 + V)]I/z [1- (O"c / AO't)2 ]]/2 AO',tz/2 (4)

NASA/TM--2001 - 210816 6
wherea is a constant and a --0.399 if taking v =0.25, Act, is the transient stress associated stress

amplitude, t7_ is the post-buckling stress in the coating. In general, t7_ << Act, under the laser high

temperature test conditions. Note that the stress intensity factor amplitude, ___, increases with

laser thermal cycle number, because of the increased temperature difference At7, with increasing

coating crack length 2a. Figure 5 illustrates the relationship between the delamination crack

propagation rate da/dN and the laser thermal transient stress associated stress intensity factor

amplitude _, during the laser thermal fatigue testing. The exponent, m, for the delamination

fatigue crack growth under the laser thermal cycle driving force AK, was found to be about 5.

Lower m values were observed under larger AK conditions, which may be related to the

complexity of the coating delamination-buckeling mechanism under the laser thermal gradient

testing. Fig. 6 shows micrographs of laser thermal fatigue tested TBC specimens. As shown in

Fig. 6(a), severe fatigue damages are observed near the earty crack propagation wake surfaces

with strong coating asperity/debris interactions and coating multiple delaminations under the laser

thermal cyclic loading. However, the later crack paths show relatively smooth surfaces, which

corresponds to the faster crack propagation regions under the increased crack propagation driving

force. A coating spallation morphology after the laser thermal fatigue test is shown in Fig. 6(b).

Fig. 5

v

The relationship between the delamination crack propagation rate da/dN and the laser

thermal stress associated stress intensity factor amplitude zXK.

NASA/TM---2001-210816 7
(a)

-=_- ;::_:;::.i

Fig. 6

(b)

Micrographs of laser thermal fatigue tested TBCs showing the coating delamination

crack propagation and coating spallation. (a) Severe fatigue damages are observed near

the early crack propagation wake surfaces with strong coating asperity/debris

interactions and coating multiple delaminations under the laser thermal cyclic loading.

The later crack paths show relatively smooth surfaces, which corresponds to the faster

crack propagation regions under the increased crack propagation driving force. (b)

Coating spallation morphology after the laser thermal fatigue test.

CONCLUSIONS

A laser thermal fatigue test technique has been used to study the delamination crack

propagation of thermal barrier coatings under the heat flux heating and thermal cycfic loading. For

the ZrO2-8wt%Y203 coating specimen tested, the initial average crack propagation rate was in the

range of 3 to 8 _tm/cycle. The crack propagation rates increased to 30 to 40 gm/cycte at the later

stage of the tests. The coating spallation failure mode was observed, and the critical spalling crack

size ranged from 3 to 5 mm for the TBC coatings. The accelerated crack growth is attributed to the

increased driving force for the crack propagation under the laser heat flux cyclic test conditions.

[1]

[21

[3]

[4]

REFERENCES

D. Zhu and R.A. Miller, "Sintering and Creep Behavior of Plasma-Sprayed Zirconia and

Hafnia:Based Thermal Barrier Coatings," Surface and Coatings Technology, vol. 108-

109. pp. 114-120, 1998.

D. Zhu and R.A. Miller, "Determination of Creep Behavior of Thermal Barrier Coatings

under Laser Imposed High Thermal and Stress Gradient Conditions," Journal of

Materials Research, vol. 14, pp. 146-161, 1999.

D. Zhu and R.A. Miller, "Thermal Barrier Coatings for Advanced Gas-Turbine Engines,"

MRS Bulletin, vol. 27, pp. 43-47, 2000.

D. Zhu, R.A. Miller, B.A. Nagaraj, and R.W. Bruce, "Thermal Conductivity of EB-PVD

Thermal Barrier Coatings Evaluated by a Steady-State Laser Heat Flux Technique,"

NASA Glenn Research Center, Cleveland, NASA/TM---2000-210238, July 2000.

NASA/TM---2001-210816 8
[5]

[6]

[7]

[8]

[9]

[lO]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

D. Zhu and R.A. Miller, "Thermal Conductivity and Elastic Modulus Evolution of

Thernml Barrier Coatings Under High Heat Flux Conditions," Journal of The_wtal Spray

Technology, vol. 9, pp. 175-180, 2000.

E.F. Rejda, D.F. Socie, and M.B. Beardsley, "Fatigue Behavior of A Plasma-Sprayed 8%

YzO3-ZrO2 Thermal Barrier Coating," Fatigue Fract. Eng. Mater. Struct., vol. 20,

pp. 1043-1050, 1997.

T.A. Cruse, B.P. Johnsen, and A. Nagy, "Mechanical Properties Testing and Results for

Thermal Barrier Coatings," Journal of The_Tnal Spray Technology, vol. 6, pp. 57-66,

1997.

G. Thurn, G.A. Schneider, and F. Aldingger, "High Temperature Deformation of PlasmaSprayed ZrOz-Thermal Barrier Coatings," Mater. Sci. Eng., vol. A233, pp. 176, 1997.

S.R. Choi, D. Zhu, and R.A. Miller, "High-Temperature Slow Crack Growth, Fracture

Toughness and Room-Temperature Deformation Behavior of Plasma-Sprayed ZrO2-

8wt%Y203," Ceramic Eng. Sci. Proc., vol. 19, pp. 293-301, 1998.

C.A. Johnson, J.A. Ruud, R. Bruce, and D. Wortman, "Relationships between Residual

Stress, Microstructure and Mechanical Properties of Electron Beam-Physical Vapor

Deposition Thermal Barrier Coatings," Surface and Coatings Technology, vol. 108-109,

pp. 80-85, 1998.

S.R. Choi, D. Zhu, and R.A. Miller, "Flexural and Compressive Strength, and RoomTemperature Creep/Relaxation Properties of Plasma-Sprayed ZrO2-8wt%Y:O3," Ceram.

Eng. Sci. Proc., vol. 20, pp. 365-372, 1999.

E.F. Rejda, D.F. Socie, and T. Itoh, "Deformation Behavior of Plasma-Sprayed Thick

Thermal Barrier Coatings," Surface and Coatings Technology, vol. 113, pp. 218-226,

1999.

D. Zhu and R.A. Miller, "Thermal Conductivity and Elastic Modulus Evolution of

Thermal Barrier Coatings Under High Heat Flux Conditions," NASA Glenn Research

Center, Cleveland, Ohio, NASA TM-209069, 1999.

D. Zhu and R.A. Miller, "Thermophysical and Thermomechanical Properties of Thermal

Barrier Coating Systems," Ceram. Eng. Sci. Proc., vol. 21, pp. 623-633, 2000.

D. Zhu, A.M. Calomino, and R.A. Miller, "Real-Time Thermal Conductivity and

Delamination Mechanisms of Plasma-Sprayed Thermal Barrier Coatings under HighHeat-Flux Conditions," in International Conference on Metallurgical Coatings and Thin

Films, San Diego, in press, 2001.

S. Timoshenko and J.M. Gere, Theory of Elastic Stability, 2nd Edition ed. New York:

McGraw Hill, 1961.

A.G. Evans and J.W. Hutchinson, "On the Mechanics of Delamination and Spalling in

Compressed Films," International Journal of Solids and Structures, vol. 20, pp. 455--466,

1984.

NASAfIqk'I_2001- 210816 9
REPORT DOCUMENTATION PAGE FormApproved

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources.

gathedng and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect ot this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services. Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, PapenNork Reduction Project (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave b/ank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

June 2001 Technical Memorandum

4. TITLE AND SUBTITLE

Thermal Fatigue and Fracture Behavior of Ceramic Thermal Barrier Coatings

6. AUTHOR(S)

Dongming Zhu, Sung R. Choi, and Robert A. Miller

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

5. FUNDING NUMBERS

WU-714-04-20-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-12731

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-210816

11. SUPPLEMENTARY NOTES

Prepared for the 25th Annual International Conference on Advanced Ceramics and Structures sponsored by the American

Ceramic Society, Cocoa Beach, Florida, January 21-26, 2001. Dongming Zhu and Sung R. Choi, Ohio Aerospace

Institute, 22800 Cedar Point Road, Brook Park, Ohio 44142, and Robert A. Miller, NASA Glenn Research Center.

Responsible person, Robert A. Miller, organization code 5160, 216-433-3298.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Categories: 24 and 27 Distribution: Nonstandard

Available electronically at http:l/gltrs._¢,ne_sa.gov/GLTRS

This publication is available from the NASA Center for AeroSpace Information, 301-621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

Thermal fatigue and fracture behavior of plasma-sprayed ceramic thermal barrier coatings has been investigated under

high heat flux and thermal cyclic conditions. The coating crack propagation is studied under laser heat flux cyclic

thermal loading, and is correlated with dynamic fatigue and strength test results. The coating stress response and

inelasticity, fatigue and creep interactions, and interface damage mechanisms during dynamic thermal fatigue processes

are emphasized.

14. SUBJECT TERMS

Thermal barrier coatings; Thermal fatigue; Thermal conductivity; Coating delaminations

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

19. SECURITYCLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

I5

16. PRICE CODE

20. LIMITATION OF ABSTRACT

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribedby ANSI Std.Z39-18

298-102