NASA/TM--2001-210824 C7-027-01

Thermal Conductivity and Thermal Gradient

Cyclic Behavior of Refractory Silicate Coatings

on SiC/SiC Ceramic Matrix Composites

Dongrning Zhu

Ohio Aerospace Institute, Brook Park, Ohio

Kang N. Lee

Cleveland State University, Cleveland, Ohio

Robert A. Miller

Glenn Research Center, Cleveland, Ohio

April 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. English°

language 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--2001-210824 C7-027-01

Thermal Conductivity and Thermal Gradient

Cyclic Behavior of Refractory Silicate Coatings

on SiC/SiC Ceramic Matrix Composites

Dongming Zhu

Ohio Aerospace Institute, Brook Park, Ohio

Kang N. Lee

Cleveland State University, Cleveland, Ohio

Robert A. Miller

Glenn Research Center, Cleveland, Ohio

Prepared for the

25th Annual International Conference on Advanced Ceramics and Structures

sponsored by the Engineering Ceramics Division of the American Ceramic Society

Cocoa Beach, Florida, January 21-26, 2001

National Aeronautics and

Space Administration

Glenn Research Center

April 2001
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 !Ltt_:d / gltrs.grc.nasa.gov/GLTRS
THERMAL CONDUCTIVITY AND THERMAL GRADIENT CYCLIC BEHAVIOR

OF REFRACTORY SILICATE COATINGS ON SIC/SIC

CERAMIC MATRIX COMPOSITES

Dongming Zhu, Kang N. Lee and Robert A. Miller

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

ABSTRACT

Plasma-sprayed mullite and BSAS coatings have been developed to protect SiC/SiC

ceramic matrix composites from high temperature environmental attack. In this study, thermal

conductivity and thermal barrier functions of these coating systems are evaluated using a laser

high-heat-flux test rig. The effects of water vapor on coating thermal conductivity and durability

are studied by using alternating furnace and laser thermal gradient cyclic tests. The influence of

laser high-thermal-gradient cycling on coating failure modes is also investigated.

INTRODUCTION

Environmental barrier coatings (EBCs) have been developed to protect SiC-based

ceramic components in gas turbine engines from high temperature environmental attack [1-3].

With continuously increasing demands for significantly higher engine operating temperature, fuel

efficiency and better engine reliability, future EBC systems must be designed for both thermal and

environmental protections of the engine components in gas turbine combustion gas environment.

In particular, thermal barrier functions of EBCs become a necessity for reducing the engine

component thermal loads and chemical reaction rates, thus maintaining required mechanical

properties and durability of these components. The advances in EBC development will directly

impact the successful use of ceramic components in advanced engine systems.

Plasma-sprayed BSAS and mullite EBC coatings have been successfully used as

protective coatings for SiC/SiC ceramic composite systems. In this study, thermal conductivity of

the two EBC coating systems is evaluated using a laser high-heat-flux test rig. The effects of water

vapor on coating thermal conductivity and durability are studied by using alternating furnace and

laser thermal gradient cyclic tests. The influence of laser high-thermal-gradient cycling on coating

failure modes is also discussed.

EXPERIMENTAL MATERIALS AND METHODS

Three EBC systems, namely, BSAS, mullite-20wt%BSAS, and BSAS/mullite20wt%BSAS two-layer coatings, were air plasma-sprayed (APS) onto the 25.4 mm-diameter and

2.3 mm-thick MI SiC/SiC ceramic composites substrates. The coating thicknesses of the BSAS

and mullite-20wt%BSAS coating systems were approximately 254 run. For the BSAS/mullite20wt%BSAS two-layer coating system, the thickness of each layer was about 127 tam. A high

power CO2 laser was used to test the EBC specimens under a high thermal gradient with 60 min

heating and 5 min cooling temperature cycles. A uniform laser heat flux was obtained over the

23.9 mm diameter aperture region of the specimen surface by using an integrating ZnSe lens

NASA/TM--2001-210824 1
combinedwiththespecimenrotation.Theuniformlydistributedlaserbeamprovidedthespecimen

surfaceheating. The required specimen temperatures and thermal gradients for this study were

achieved by controlling the delivered laser heat flux and backside air-cooling. Thermal

conductivity of the ceramic coating systems was also measured in-situ as a function of the laser

cycle number using the steady-state laser heat flux test approach [4-6]. During the laser thermal

cycling test, the ceramic surface temperature was measured by an 8 run infrared pyrometer, and

the backside CMC surface was measured by a two-color pyrometer. The laser test surface

temperatures were set at 1482°C and 1371°C, respectively, while the interface temperature was

approximately controlled at 1300°C. Besides the pure laser thermal cyclic testing, a set of

specimens were also tested using alternating furnace cycling in a 90% water vapor environment

and the laser thermal gradient cyclic testing in air, to study the possible water vapor effect. The

furnace test temperature was at 1300°C with 60 min hot time temperature cycles.

EXPERIMENTAL RESULTS AND DISCUSSION

Thermal Conductivity of Environmental Barrier Coatings

Fig. 1 shows the thermal conductivity test results of the EBC coatings on SiC/SiC CMC

substrate using the laser heat flux technique. For the as-sprayed EBC coatings, average initial

coating thermal conductivity varies from 1.4 to 2.2 W/m-K in the temperature range of 900 to

1350°C. Conductivity increases were observed during the heating/cooling cycles due to the

ceramic sintering. The BSAS coating showed the largest increase in the conductivity. After the

first test cycle, the conductivity values were approximately 2.2 W/m-K for BSAS coating, and 1.7-

1.8 W/m-K for mullite-20wt%BSAS and the BSAS/mullite-20wt%BSAS two-layer coatings.

Thermal conductivity of the EBCs further increased in the first 10 hour furnace water

vapor cyclic testing. As shown in Fig. 2, the conductivity of the EBCs increased by as high as 30%

after the furnace test. The increase in the conductivity is attributed to the coating sintering. The

subsequent 10 hr laser cyclic testing, however, reduced the conductivity of the coatings. This may

be due to coating cracking and micro-delaminations under the high temperature, high thermal

gradient cyclic testing.

Fig. 3 shows thermal conductivity changes of the BSAS coatings as a function of cycle

number. In the pure laser test case shown in Fig. 3 (a), the conductivity had a minimum value of

1.5 W/m-K at about 20 cycles. The conductivity then increased, and maintained at about 2.5-

2.6 W/m-K with further thermal cycling. In the combined furnace and laser thermal cycling case,

significant thermal conductivity increase was also observed. As shown in Fig. 3 (b), the measured

conductivity increased to above 3.0 W/m-K. SubstantiaTgiass phase formation was observed at

both coating surfaces after the high temperature cyclic testing, as shown in Fig. 4.

Fig. 5 shows the thermal conductivity changes of the BSAS/mullite-20wt%BSAS twolayer coating systems as a function of the cycle number. For both pure laser tested and combined

furnace and laser tested specimens, initial thermal conductivity increased due to the coating

sintering. However, the later coating conductivity decreased due to the coating cracking and

delaminations under the further testing. From the tests, it can be seen that both the coating

sintering and coating delaminations occurred much faster for the combined furnace water vapor

and laser cyclic test, as compared to the pure laser test. The preliminary test results indicated that

the water vapor has a detrimental effect on coating durability, especially under the thermal

gradient cyclic test conditions. The coating failure can be greatly accelerated, by the interface pore

formation in the water vapor environments, and by subsequent coating cracking and delaminations

under the severe laser thermal transient and thermal gradient loadings. Typical EBC failure modes

and cracking morphologies of the BSAS and BSAS/mullite-20wt%BSAS two-layer coatings

under the combined furnace and water vapor cycles exposure are shown in Fig. 6.

NASA/TM--2001-210824 2
!

o 10 mil BSAS

3.5

30

'_" cooling

2.5 ,_ _---

1.5 _. _ y_.--.--_r "- heating

1.0

0.5

600 700 800 900 I000 II00 1200 1300 1400

Surface temperature, °C

(a)

:g

I

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

.... I'''_1 .... I''' I .... I .... I .... i ....

10 mil mullite+20%BSAS

._ cooling

-_, . tap.

/3_,,

heating

600 700 800 900 1000 1100 1200 1300 1400

Surface temperature, °C

(b)

!

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

10 mil BSAS+lmunite+20mol%Bi

o

o cooling

heating

600 700 800 900 1000 1100 1200 1300 1400

Surface temperature, °C

(c)

;AS)

i

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

10 rail BSAS+(mullite+20mol%BS

'_ cooling

heating

600 700 800 900 1000 1100 1200 1300 1400

Surface temperature, °C

(d)

AS)

Fig. 1 Thermal conductivity of EBCs on SiC/SiC CMC substrate as a function of coating

surface test temperature. Conductivity increases were observed during the

heating/cooling cycles due to ceramic sintering. (a) BSAS coating, the first

heating/cooling cycle; (b) mullite-20wt%BSAS coating, the first heating/cooling cycle;

(c) and (d) BSAS/mullite-20wt%BSAS two-layer coating, the first, and the second

heating/cooling cycles, respectively.

NASA/TM--2001-210824 3
3.0

2.5

2.0

1.5

1.0

["'7 Laser 1482°C, 1hr cyclic test

Laser 1371_C, lhr cyclic test

As received After 10hr 1300°C

fi.ma_e cycles

(a)

subsecltlenl

1fl_tr laser cycles

8

3.0

2.5

2.0

1.5

1.0

f""] Laser 1482°C, lhr cyclic test

As received

,.._..._ i

I

After ]Oh" 1300°C

fumace cycles

(b)

subsequenl

10hr laser cycles

Fig. 2

3.0

2.5

2.0

1.5

1.0

[-'] Laser 1482°C, llar cyclic test

As received After 10In" 1300°C AfterMsubsequent

furnace cycles I 0hr laser cycles

(c)

Thermal conductivity of EBCs after the first 10 hr furnace cyclic testing, and a

subsequent 10 hr laser cyclic testing. (a) BSAS coating; (b) mullite-20wt%BSAS coating;

0

(c) BSAS/mullite-20wt SBSAS two-layer coating.

NASA/TM--2001-210824 4
Fig.3

>

O

[-

3.0

2.5

2.0

1.5

1.0

0.5

0.0

4.0

3.5

3.0

i_ 2.5

._ 2.0

1.5

1.0

q)

0.5

0.0

70

• , , I ,

0 20 40 60 80 100 120 140

Cycle number

(a)

......... kcera Tsurface

---o---- Tinterface

Tback

.... ,.... ,.... I .... ,.... ,.... ,.... 2000

jj

1000

75 80 85 90 95 100 105

Cycle number

(b)

1800

1600

1400

1200

1-

Thermal conductivity changes of the BSAS coatings as a function of cycle number.

(a) Laser thermal cycling testing (surface temperature 1482°C and interface 1300°C);

(b) Combined furnace water vapor and laser thermal cyclic testing during the 70-104

cycles.

NASA/TM--2001-210824 5
(a)

(b)

Fig. 4 Surface morphologies of the glass phase formed on the BSAS coating surface. (a) Laser

tested specimen after 136 cycles; (b) Combined furnace water vapor and laser thermal

cyclic tested specimen after 120 cycles.

NASA/TM--2001-210824 6
_Z

).

O

n_

o

o

ID

3.0

2.5

2.0

1.5

1.0

0.5

0.0

l ' I I I

Total 166 1-hr cycles

o o

©

0 50 100 150

Cycle number

(a)

i

200

Fig. 5

O

O

O

3.0 * ' I I I ' ' ' I '

/Furnace cycling 2.5f _ i_ \

i-i

2.0 I _

i

Total 120 1-hr cycles

l o

l o

:_0 0

5I

l.O

J

0.5

0.0 , i

0

i

i

I

20 40 60 80 100 120

Cycle number

(b)

Thermal conductivity changes of the BSAS/mullite-20wt%BSAS two-layer coating

systems as a function of the cycle number. (a) Laser thermal cycling testing (surface

temperature 1482°C and interface 1300°C); (b) Combined furnace water vapor and laser

thermal cyclic testing.

NASA/TM--2001-210824 7
Fig.6

(a) (b)

Micrographsof cross-sections of BSASandBSAS/mullite-20wt%BSAS two-layer

coatingcoatingsafterthecombinedlaserandfurnacewatervaporcycletesting,showing

thecoatinginterfaceporestructuresandcoatingcrackinganddelaminations aftertesting.

(a)BSASCoating;(b)BSAS/mullite-20wt%BSAS two-layercoating.

ThermalBarrierConsiderations ofEnvironmental BarrierCoatings

Thermalbarrierfunctionsof theEBCsdependonthecoatingthermalconductivity,

thicknessandheatfluxconditionsencounteredinanengine.Fig.7(a)and(b)showscalculated

andexperimentallymeasuredtemperaturedistributionsacrossacoatingsystemasafunctionof

theheatfluxandcoatingthickness. Forthegivenconductivityvaluesofthecoatingandsubstrate,

inorderto maintaintheinterfacetemperatureof approximately1300°Cunderthefixedsurface

temperature1450°C,eitherahigherheatflux(I00W/cm2)orathickercoating(0.508mmthick)

isrequired.TheeffectofEBCconductivityonthecoatingtemperaturereductionisillustratedin

Fig.8(a)and(b),fortheheatfluxesof50W/cm2 and 100 W/cm 2, respectively.

La

e

Distaace from the surface, nail

20 40 60 80 100

1500 . . . , . . . , . . . , . . . , . . , . •

1450 - _intefface k_ =1.8W/m-K

1400 _ kcMc=21'0 W:m-K

1350 _:'tw.,_ "-'" """

1300

1250 _ 25 W,'cm-', Ct 254 _"_ _ _ _

1200 .-- "50 W:cm 2,0,254 mm

1150 -- -- "100 W/cm2, 0'254 mm

"........50 W:cm 2,0.508 mm

II00 .... J .... ' .... _ .... _ .... ' ....

0.0 0.5

1400

' 2700

2600 1200

o

2500 i i 10_

241)0

2300 800

2200

6OO

2100

1.0 1,5 2.0 2.5

Distance from the surface, mm

(a)

3.0

4O0

Coa7 t_7 11 rail _:

............__

60

1000

P

s0o

1-

6oo

0

80 1(30 120 140

Pass-thru heal flux, W/crn -_

(b)

Fig. 7 Calculated (a) and experimentally measured (b) temperature distributions across a coating

system as a function of heat flux.

NASAJTM--2001-210824 8
o

O

O.2

02

_D

[-

600 .... I .... I .... i .... I .... i ....

Heat flux 50 W/cm_

500 \

400 ,,\

300 \ •

200 _ \ k\ "- 1.8 W/m-K

100

0

0.0

Coating thickness

5 mil

_ l0 rail

-- -- - 15 mil

- - 20 mil

"_ " _ 300F AT

......... .... .Xz......_.. _..'_._,-. -- _ ................

] I ] _ 1 .... ] .... I i , • ; I .... I ....

0.5 !.0 1.5 2.0 2.5

Coating thermal conductivity, W/m-K

(a)

1000

800 ._

600 ,o.

400

200

3.0

=-

@

O

O

1000

800

600

400

200

0

0.0

' ' '" _ .... I .... I .... Heat I .... I .... flux 100 W/cm 2

I

\ \ Coating_5milthickness

-- - 10 rail

\ \ I 5 mil

\ _ - - 20 mil

•. \ _,, " _ 1.8 Wtm-K

--\ " ." _ 300F AT

.... I .... t .... I , • , ; i , , , , I ....

0.5 1.0 1.5 2.0 2.5 3.0

Coating thermal conductivity, W/m-K

(b)

1600

1200

800

400

i

[-

Fig. 8 The effect of EBC conductivity and thickness on the coating temperature reduction. (a)

50 W/cm2; (b) 100 W/cm 2.

NASA/TM--2001-210824 9
CONCLUSIONS

A laser steady-state heat flux rig has been used to investigate thermal conductivity of

several environmental barrier coating systems. Thermal conductivity values of the current EBC

coatings were in the range of 1.4-3.0 W/m-K. Significant conductivity increase was observed for

the coating systems after the laser and furnace water vapor cyclic testing. The preliminary test

results showed that the coating failure can be greatly accelerated by enhanced interface pore

formation in water vapor environments and the promoted coating cracking and delaminations

under the severe laser thermal transient and thermal gradient loadings.

REFERENCES

[1] K.N. Lee and R. A. Miller, "Development and Environmental Durability of Mullite and

Mullite/YSZ Dual Layer Coatings for SiC and Si3N4 Ceramics," Surface and Coatings

Technology, vol. 86-87, pp. 142-148, 1996.

[2] K.N. Lee, "Key Durability Issues with Mullite-Based Environmental Barrier Coatings

for Si-Base Ceramics," Transactions of the ASME, vol. 122, pp. 632-636, 2000.

[3] K.N. Lee, "Current Status of Environmental Barrier Coatings for Si-Based Ceramics,"

Surface and Coatings Technology, vol. 133-134, pp. 1-7, 2000.

[4] 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. Also in Journal of Thermal Spray

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

[5] D. Zhu, R. A. Miller, B. A. Nagaraj, and R. W. Bruce, "Thermal Conductivity of EBPVI) Thermal Barrier Coatings Evaluated by a Steady-State Laser Heat Flux Technique,"

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

Surface and Coatings Technology, vol. 138, pp. 1-8, 2001.

[6] D. Zhu and R. A. Miller, "Thermal Conductivity Change Kinetics of Ceramic Thermal

Barrier Coatings Determined by the Steady-State Laser Heat Flux Technique," NASA

Glenn Research Center, Cleveland, Ohio, Research and Technology 1999, NASA TM209639, March 2000.

NASA/TM--2001-210824 10

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,

gatherfng and maintaining the data needed, and completing and re'clewing the collection of information. Send comments regarding this burden estimate or any other aspect of 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 I204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

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

April 2001 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Thermal Conductivity and Thermal Gradient Cyclic Behavior of Refractory

Silicate Coatings on SiC/SiC Ceramic Matrix Composites

6. AUTHOR(S)

Dongming Zhu, Kang N. Lee, 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

WU-714---04-30-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E- 12742

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-210824

C7-027-01

11. SUPPLEMENTARY NOTES

Prepared for the 25th Annual International Conference on Advanced Ceramics and Structures sponsored by the Engineering Ceramics Division of the American Ceramic Society, Cocoa Beach, Florida, January 21-26, 2001. Dongming Zhu,

Ohio Aerospace Institute, 22800 Cedar Point Road, Brook Park, Ohio 44142, Kang N. Lee, Cleveland State University,

1983 E. 24th St., Cleveland, Ohio 44115, and Robert A. Miller, NASA Glenn Research Center. Reponsible person,

Dongmin_ Zhu, organization code 5160, 216-433-5422.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Unclassified - Unlimited

Subject Categories: 24 and 27 Distribution: Nonstandard

Available electronically at ht _tp://glt_._c.nasa.gov/GLTRS

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

13. ABSTRACT (Maximum 200 words)

Plasma-sprayed mullite and BSAS coatings have been developed to protect SiC/SiC ceramic matrix composites from

high temperature environmental attack. In this study, thermal conductivity and thermal barrier functions of these coating

systems are evaluated using a laser high-heat-flux test rig. The effects of water vapor on coating thermal conductivity and

durability are studied by using alternating furnace and laser thermal gradient cyclic tests. The influence of laser highthermal-gradient cycling on coating failure modes is also investigated.

14. SUBJECT TERMS

Environmental barrier coatings; Thermal gradient cyclic testing

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

19. SECURITY CLASSIFICATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

16

16. PRICE CODE

20. LIMITATION OF ABSTRACT

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

Prescribed by ANSI Std. Z39-18

298-102