A paper abstract to be submitted to

Symposium on Advanced Ceramic Coatings for Structural, Environmental and Functional

Applications

The 27th Annual International Conference on Advanced Ceramics and Composites, January 26-31, 2003,

Cocoa Beach, Florida

Development of Advanced Thermal and Environmental Barrier Coatings Using A High­

Heat-Flux Testing Approach

Dongming Zhu* and Robert A. Miller**

* U.S. Army Research Laboratory, NASA Glenn Research Center

**NASA Glenn Research Center

Cleveland, Ohio 44135

The development of low conductivity, robust thermal and environmental barrier coatings requires advanced testing techniques that can accurately and effectively evaluate coating thermal conductivity and cyclic resistance at very high surface temperatures (up to 1700°C) under large thermal gradients. In this study, a laser high-heat-flux test approach is established for evaluating advanced low conductivity, high temperature capability thermal and environmental barrier coatings under the NASA Ultra Efficient Engine Technology (UEET) program.

The test approach emphasizes the real-time monitoring and assessment of the coating thermal conductivity, which initially rises under the steady-state high temperature thermal gradient test due to coating sintering, and later drops under the cyclic thermal gradient test due to coating cracking/delamination. The coating system is then evaluated based on damage accumulation and failure after the combined steady-state and cyclic thermal gradient tests.

The lattice and radiation thermal conductivity of advanced ceramic coatings can also be evaluated using laser heat-flux techniques. The external radiation resistance of the coating is assessed based on the measured specimen temperature response under a laser­heated intense radiation-flux source. The coating internal radiation contribution is investigated based on the measured apparent coating conductivity increases with the coating surface test temperature under large thermal gradient test conditions. Since an increased radiation contribution is observed at these very high surface test temperatures, by varying the laser heat-flux and coating average test temperature, the complex relation between the lattice and radiation conductivity as a function of surface and interface test temperature may be derived.

, ." ...

Development of Advanced Thermal and Environmental Barrier Coatings Using A High­

Heat-Flux Testing Approach

Dongming Zhu * and Robert A. Miller • +

N~SA ". " 'J,'

NASA John H. Glenn Research Center . Cleveland, Ohio 44135, USA

* Also with Vehicle Technology Directorate, U.S. Army Research Laboratory, NASA Glenn Research Center

The 27th Annual International Conference on Advanced Ceramics and Composites January 26-31,2002, Cocoa Beach, Florida

.: -."

Thermal and Environmental Barrier Coatings (T/EBCs) are Critical to Future Advanced Engine Systems

Advanced TEBCs can increase engine gas temperatures, reduce cooling requirements, lower emission and improve engine efficiency and reliability The coating can provide thermal and environmental protections for engine components, thus improve engine reliability and durability Low thermal conductivity and long-term high temperature stability are important issues for developing advanced coating systems

Metal substrate

(a) without TBe

Cera m ic coating

~- l IrRl k

coat substrate co at

(b) with a TBe system (c) with an advanced TBe

Higher surface temperatures and thermal gradients are expected in advanced thermal barrier coating systems

Temperature Reductions by Ceramic Coatings will Increase for Future Advanced High Performance and Low Emission Engine Applications

800

700 u 0

600 ~

en

§ 500 .....

t)

] 400 ~ 300 ~ & 200 5

E-c 100

0 Current TBCs/TEBCs Advanced TBCs/TEBCs

Engine Level High Heat Flux Thermal Gradient Testing is Crucial to Advanced Coatings Development

- High heat-flux and large thermal gradients are expected in advanced thermal/environmental barrier coatings

- Development of advanced T/EBCs requires a better understanding of the coating behavior under very high temperatures and large thermal gradient conditions

- Advanced high temperature coatings can only be tested under thermal gradients because of the substrate temperature limits

Objectives • Laser heat flux based technique for advanced high

temperature coatings development: A comprehensive thermal gradient test approach for evaluating

advanced thermal and environmental barrier coating systems

High power laser heat flux based thermal gradient test approach and test facilities at NASA Glenn Research Center Coatings property evaluation and development based on laser high heat flux techniques:

• Ceramic coating thermal conductivity at very high temperatures and coating sintering evaluation

• Thermal conductivity changes due to coating sintering and delamination

• Test approach for advanced coatings development - durability assessment

• Typical coating failure modes under thermal gradients • Radiation performance evaluations

High Power Laser Heat Flux Facilities for Advanced Ther_~ Barrier Coatings (TBes and EBCs) Development at

?

- Simulated engine high temperature thermal gradient testing is emphasized

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I: _IC ••• ,'f' •• C . I.)CI

CDDI •• '0 a .. J .ID~.

1."11:, •• ~.- u" .. : I •• d (. DID IIC --------

Steady-State Laser Heat Flux Approach for Ceramic Coating Thermal Conductivity Measurements

- A uniform laser beam (wavelength 10.6 micron) power distribution achieved by using an integrating lens with specimen or lens rotation to provide coating surface heating

- The ceramic surface and metal substrate temperatures measured by 8 micron and two­color pyrometers and/or by an additional embedded miniature thermocouple

- The thermal conductivity change kinetics measured at 5 second intervals and real time - Thermal cycling can be incorporated

o laser beam! r--I

integrating lens ~ =

000 000 r- 3. 0 KW CO 2 High Power Laser

platinum flat coils

ceramic coating

I '-,.:tJ::tr!i';:,~,~id··';' :,-m"~i·~·-·--"··I ___ b'ond coat

'----i�~1 ..

miniature thermocouple

TBC coated back aluminum

plate edge

aluminum back plate

cooling air tube /L.....J ~i-base s:perallOy or CMC substrate

~

Ceramic Coating Thermal Conductivity can be obtained from Measured Surface and Backside Temperatures Combined

with a One-Dimensional Heat Transfer Analysis

kceramic (t) = qthru . 1 ceramic / I1Tceramic (t) Whe;e rbond qthru· dl rSubstrate qthru·

qthru = qdelivered - qrejlected - qradiated and IlTceramic(t) = Tceramic-surajce - Tmetal-back -.b k (T) - .b k (T)

7.9 J.1m pyrometer

for T ceramic-surface

Two-color and 7.9 J.1m pyrometers for

T substrate-back

qrejlected ,

qthru

bond substrate

~ q,adlm,d

Il.Tceramic = Il.Tmeasured

- Il.Tsubstrate - Il.Tbond

Optional miniature thermocouple for additional

heat flux calibration

Sintering Behavior and Thermal Conductivity Increases can be Evaluated in-situ at Very High Temperatures

3.0 3.0 ~ 1600 1500 1400 1300 1200 ""I ~ ---<r--- k(1280/1137) ~ kat 20 hour I --0- k(l3361l200) I

~ 2.5 S --<>- k(1375/1 JOO) ~ 2.5

k(l45311325)

0 k(136011200) >. ..... .... 2.0 lSI k(1470/1352 .... > > ....

~ k(1565/1293) .... ..... .....

(,) (,) 2.0 ~ ~ "0 "0 \::

1.5 \::

0 0 (,) (,)

~ Activation energy 96 kJ/mol ....... ....... ~ ~

S S 1.5 I-< I-< 0 1.0 0

...c: ...c: E-< Plasma-sprayed 8YSZ E-< ~ Plasma-sprayed 8YSZ

0.5 1.0

0 5 10 15 20 25 30 5.0 5.5 6.0 6.5 7.0

Time, hours Iff .104 K-1 , average

Thermal Conductivity of Zr02-8wt% Y 203/mullite+BSAS/Si System under Steady-State Laser Heat-Flux Testing

~ I

-€ ~ ~ .... . ~ > .~ .... u ::s

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...... ~

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Conductivity initially increased due to ceramic coating sintering Conductivity later decreased due to coating delamination cracking resulting from the large sintering shrinkage

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0

....... _---_.-..... _._ .. -_. __ .. _ ..... --_ .. _ ..

~ Tsurface= 1482 °C T interface=1250°C

--- Measured thermal conductivity ........ - Predicted thermal conductivity

5 10 15

Time, hours

20

Conductivity reduction due to sintering cracking induced delamination cracking

D. Zhu, S. R. Choi, J. I. Eldridge, K. N. Lee and R. A. Miller, The 104th Annual Meeting & Exposition of The American Ceramic Society, April 28-May 1,2002

Surface cracking and delamination of ,{EBC Systems under Laser Heat-Flux Thermal Gradien{Cyclic Testing

Zirconia based

Coating

BSAS+Mullite

Si

SiC/SiC

(a) after 3000°F testing in air (b) after 2700°F testing in air and furnace water vapor exposure

Coating Failure Mechanisms under Thermal Gradients Surface cracking and resulting coating delamination due to sintering/creep and CTE mismatch are identified a major failure mechanism Thermal gradient conditions encountered in the coating system significantly increase the sintering and thermal stresses for coating failure Interface degradation due to coating surface cracking, and phase instability can further accelerate coating spallation under thermal gradient conditions

-14

u -IS

~

u

E -16

co c ·c u -17

! ..5

-18

Baseline 8YSZ Zirconia based oxide -19

1.2xlO·7 , lIsee

5.0 5 .5

High sintering rate is a leading mechanism for coating severe cracking

6.0 6 .5

!total

7 .0

Thermal expansion mismatch induced stresses increase significantly with increasing thermal gradients

liT , K-1

Surface cracking and delamination facilitated by a thicker thermal barrier coating

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0 .014

0 .012 c:: .; b 0 .010 II)

c:; E

0 .008 .... ., .c: E--

0 .006

<),004

0 .002 0 .0 0 .2 0.4

Coating temperature,OC

--0-- 1000°C .......[]-1200 °C ~ 1400 ° C

9E 1600 Q C

increasing thermal gradient

0 .6 0.8

T IT eM C su bstrate ceram ic coat in g

~

~ tIl N

N-----.~

uniform \j

+ heating §

5 ~ 0

1.0

0 6

5 r

4 I-

Crack length a/TBC thickness t Ibe

1 234 5 ._ I I • , I I I I • I I I I I I •• ".

'.~ through thickness cracking I~ 4

•• I '. delamination ! ""';?- I ~ 2

._ I -.... - . .~.-- .

-~ ... --3 r ,," I 0 .... 2 r

o [ 0 .0

through thickness cracking •••• ""- .-~ ...... deIam ination ..... :.. ~ ---...... ..... - -

I I

0 .2 0.4 0 .6 0 .8 1.0

TBC thickness t Itotal coating thickness t ~ w~

~ll

tIl­N --ll N;:; \j

+

,s "=:::' o

;;

5 -5

Coating Delamination can be Evaluated from the Conductivity Decreases under the Laser Heat-Flux Testing

Conductivity initially increased due to ceramic coating sintering Conductivity later decreased due to coating delamination cracking resulting from the large sintering shrinkage

q

t t t t t t t t i1T(t)

q / A = 1 / k + II he + lsub / ksub e mtaet

f gap

h = _1_ ( Aadherent 2kint aetk sub + Adebond k . J e 19ap A kintaet + ksub A air

fsub

Aadherent ,...., 1 gap k debond ,....,

A 21 c kint act - k debond

Advanced Coatings Development Sintering and Cyclic Response of Advanced Hf02 Coating Systems on SiC

Substrate Tested at 3000°F - Initial 20 hr sintering testing and then thermal cyclic testing at 3000°F - The advanced coating system showed excellent performance

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......-4 c!j

§ .B '"d

(1) N

. ~

m ~ o Z

2.0

1.5

1.0

0.5

o

Hf02 based oxide/mullite/Si/SiC systems

Steady­State

20 40 60 80 Time, hours

100

Coating Radiation Performance Evaluation and Radiation Barrier Coatings Development

Coating internal and external radiation performance can be evaluated using the laser heat flux approach at up to 1760°C (3200°F)

Laser heat flux

! ! !

(a) Internal radiation

Laser heat flux High emissivity layer

(b) Combined internal & external radiation

Laser heat flux Radiation emitter

(c) External radiation

Evaluation of Lattice and Radiation Thermal Conductivity of lEBC Systems at High Temperatures

- Zr02-8wt% Y 203/BSAS/mullite+20wt%BSAS/Si coating on MI SiC/SiC CMC substrate

- Conductivity determined by steady-state laser heat-flux technique

- Coating surface radiation can contribute 5-150/0 total heat transfer

.:. Ts -=- Tb2cl ~Tb2cl --<>-- Laser power

~ 1.40 E ...... ~ >. 1.30

.<;:! > ~ Co)

~ '1:j c o Co)

-; E ... Q)

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'1:j Q) N

-; E ... o

1.20

1.10

1.00

Z 0.90

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...... ~ ....... : ..... .. 11: .. .. 0..40 ...... ~ .. ... .. ~ ....... ; ..•.. . : O.4~

:::::: :::::::::::::::":::::: 9.:;~ ::::· . .

- - - ~ - - ., - - - - - - - , - - - - - - -., - - - - - - - r - - - - -

5 10 15 20 25 30 35 Time, min

Laser power 1600

1400 ... Q)

1200 ~ 0 c.. ...

1000 Q) ~

!:

800 u 'li ...

600 ~ ~

~ ... Q)

400 c.. E Q)

E-< 200

0

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~ 0

1200 c.. ...

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u 800 'li ...

~ ~

1.10 ~ ...

600 Q)

c.. E Q)

400 E-< 1.00

0.90 200

1350 1400 1450 1500 1550 1600 1650 Surface temperature, C

•

Evaluation of Lattice and Radiation Thermal Conductivity of a Plama-Sprayed Zr02-8wt% Y 203

Coatings at High Temperatures - Radiation component becomes increasingly important above 1500°C even for the plasma-sprayed coatings

~ I

~ ~ ~ ..... ;>

. .0 u ~ = 0 u -C\S

a Q)

..c:= ~

ZrO -8wt% Y 0 plasma-sprayed porous coating 2 2 3

3.0 E I I I I I I I I I I ,

2.5

2.0

1.5

1.0

0.5

o kmeasured --- k fit due to lattice conduction-radiation

sintering induced conductivity rise

0.0 IV I

200 400 600 800 1000 1200 1400 1600 1800 Surface temperature, °C

•

Sintering and Cyclic Response of Advanced Hf02 Coating Systems on SiC Substrate Tested at 3000°F

Plasma-sprayed coatings showed significantly radiation shielding as compared to the dense materials due to the micro-porosity

cu .g ~

~-"-"cu

u .~

~-' ..... e

~ Dense materials-ll00°C -0-- Dense materials-1200°C -[] - Dense materials-l 300°C

- -'\} - - Dense materials-1400°C

~ Coatings-1550°C • .- Coatings-16()()OC .... - Coatings-1650°C • ... - Coatings-170(Y'C

1.2 :::a i __ i i~

1.0 " , .,

, if!' ., ., " ., .,

~ 0.8 $f"...D

.""",.-."." Dense materials ~g: '-"

i g o a o u c: o ...... ~ ...... ~ 0:::

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[

_.",IJ - -"""'" -

O -~

.4 ~-------------------------------~1 0.2

0.0 I " h ...... ·,...,-T I .... , ... _, .. ,...,~, "...... Plasma-sprayed o 200 400 600 800 1000 1200 coatings

Thermal gradient, Klmm

Conclusions

• A laser based heat flux thermal gradient test approach has been established for advanced T/EBC coatings development.

• Quantitative coating properties can be obtained at very high temperatures under thermal gradients.

• Coating durability assessment has been demonstrated.

• Coating radiation resistance performance at high temperature is being investigated.

,--.I

Acknowledgements

This work was supported by NASA Ultra-Efficient Engine Technology (UEET) Program.

The authors wish to thank Narottam P. Bansal, Jeffrey I. Eldridge and Kang N. Lee for helpful discussions.

The authors are also grateful to George W. Leissler at aSS/NASA Glenn Research Center for his assistance in the preparation of plasma­sprayed T/EBC coatings.