Dongming Zhu

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Low Conductivity Thermal Barrier Coatings

NASA/TM—2005-213857

September 2005
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Dongming Zhu

U.S. Army Research Laboratory, Glenn Research Center, Cleveland, Ohio

Low Conductivity Thermal Barrier Coatings

NASA/TM—2005-213857

September 2005

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

School of Materials Engineering Seminar

sponsored by Purdue University

West Lafayette, Indiana, February 28, 2005
Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov
Low Conductivity Thermal Barrier Coatings 

Dongming Zhu 

U.S. Army Research Laboratory 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Thermal barrier coatings will be more aggressively designed to protect gas turbine engine hot-section 

components in order to meet future engine higher fuel efficiency and lower emission goals. A 

fundamental understanding of sintering and thermal cycling degradation of thermal barrier coating 

systems under engine high-heat-flux conditions will provide insights into how to further maximize the 

coating capabilities. In this presentation, thermal barrier coating development considerations and 

requirements will be discussed. An experimental approach is established to monitor in real time the 

thermal conductivity of the coating systems subjected to high-heat-flux, steady-state and cyclic 

temperature gradients. It is demonstrated that the increasing and decreasing trends in thermal conductivity 

can be closely related to the coating sintering and subsequent delaminations. Advanced low conductivity 

thermal barrier coatings have also been developed using a multi-component defect clustering approach, 

and shown to have significantly improved thermal stability due to nano-sized and low mobility defect 

clusters associated with the paired rare earth dopant additions. The durability and erosion resistance of 

low conductivity thermal barrier coatings have been improved utilizing advanced coating architecture 

design, composition optimization, in conjunction with more sophisticated modeling and design tools. 

NASA/TM—2005-213857 iii

Motivation

— Thermal and environmental barrier coatings (T/EBCs) can significantly 

increase gas temperatures, reduce cooling requirements, and improve 

engine fuel efficiency and reliability

(a) Current TBCs (b) Advanced TBCs

Ceramic 

coating

Ceramic 

coating

Bond Substrate Substrate

coat

Bond 

coat

Tgas

Tsurface

Tgas

Tsurface

Tback

Tback

Ceramic 

coating

Ceramic 

coating

Bond Substrate Substrate

coat

Bond 

coat

Tgas

Tsurface

Tgas

Tsurface

Tback

Tback

Ceramic 

coating

Ceramic 

coating

Bond Substrate Substrate

coat

Bond 

coat

Tgas

Tsurface

Tgas

Tsurface

Tback

Tback

Piston head

Combustor Vane Turbine blade

Turbine

Valve

Pulse detonation engine 

Diesel

Revolutionary Ceramic Coatings Greatly 

Impact Gas Turbine Engine Technology

— Ceramic coatings are critical to future engine efficiency, power density, 

and compactness goals

NASA UEET Goals

• 70% NOx reduction

• 8-15% increase in efficiency

• 8-15% reduction in CO2

2400 °F (1316 °C)

3000 °F+ (1650 °C+)

Gen I

Temperature 

Capability (T/EBC) surface

Gen II – Current commercial Gen III Gen. IV

Increase in ∆T 

across T/EBC

Single Crystal Superalloy

Year

Ceramic Matrix Composite

Gen I

Temperature 

Capability (T/EBC) surface

Gen II – Current commercial Gen III Gen. IV

Increase in ∆T 

across T/EBC

Single Crystal Superalloy

Year

Ceramic Matrix Composite

Si3N4 and coating systems

2700 °F (1482 °C)

2000 °F (1093 °C)

NASA/TM—2005-213857 1
OBJECTIVES

• Thermal barrier coating high-heat-flux testing

– Conductivity measurements and coating degradation evaluation

– Sintering and failure mechanisms

• Low conductivity thermal barrier coating development

– Requirements and design considerations 

– Advanced oxide defect cluster coatings

• The 3000 °F (1650 °C) thermal and environmental barrier coatings for 

SiC/SiC ceramic matrix composites

– Coating concept

– Radiation conductivity evaluation

– Advanced 3000 °F (1650 °C) coating durability evaluation

 Thermal barrier coating high-heat-flux testing

• Low conductivity thermal barrier coating development

• The 3000 °F (1650 °C) thermal and environmental barrier coatings for 

SiC/SiC ceramic matrix composites

NASA/TM—2005-213857 2
Conventional 0.3 Mach Burner Rig

― Thermal barrier coating burner testing: relatively low heat flux ~20 W/cm2

Burner rig bar specimen testing Hex bar specimen testing

High Heat-Flux Test Approaches

– Oxy-fuel torch (GEAE), plasma torch (Westinghouse) and CO2 laser 

(NASA, Purdue) high heat flux rigs are being used to assess coating 

failure mechanisms and durability

GEAE Oxy-Fuel (JETS) Rig Westinghouse Plasma Torch Rig NASA-GRC CO2 Laser Rig

NASA/TM—2005-213857 3
NASA CO2 Laser High-Heat-Flux Test 

Approach

 A uniform laser (wavelength 10.6 μm) power distribution achieved using an 

integrating lens

 Real time conductivity measurements by monitoring the ceramic surface and 

substrate temperatures at given heat flux

camera

pyrometer

cooling air

thermocouple

specimen

3. 0 KW CO 2 High Power Laser

ceramic coating

bond coat

CMSX4 substrate

laser beam/ 

integrating lens

slip ring

300 RPM aluminum laser 

aperture plate

platinum flat coils

aluminum back plate

air gap

cooling air tube

TBC coated 

back aluminum plate edge

miniature 

thermocouple

Ni-base superalloy or CMC substrate 

reflectometer

Thermal Conductivity Measurement by a Laser HighHeat-Flux Approach

Two-color and 8 μm 

pyrometers for 

Tsubstrate-back

8 μm pyrometer 

for Tceramic-surface

k (t) q l / T (t) Where ceramic thru ceramic ∆ ceramic = ⋅ qthru = qdelivered − qreflected − qradiated ∫ ∫ ⋅ − ⋅ ∆ = − − − −

bond substrate l

substrate

thru l

bond

thru ceramic ceramic surafce metal back k T

q dl

k T

q dl T t T T 0 0 ( ) ( ) and ( )

qreflected qradiated

Optional miniature 

thermocouple for additional 

thru heat-flux calibration q

thru q

delivered q

ceramic coating

bond coat

substrate

substrate bond

ceramic measured

T T

T T

− ∆ − ∆

∆ = ∆

∆Ttc

NASA/TM—2005-213857 4
Laser Heat Flux Testing in Water Vapor Environments 

for Si-Based Ceramics/Coatings

– Laser heat flux “steam” rig

- Precise control of heat fluxes and temperatures

- High temperature and high heat flux testing capabilities

- Innovative “micro-steam environment” concept

- Real-time specimen health monitoring capability

- Steam injected at up to 5m/sec

- Testing temperature >1700 °C

Laser High Heat Flux Combustor and 

Turbine Airfoil Rigs

– Emphasize realistic temperatures, heat flux and stresses 

Laser heat flux combustor 

TBC test facility

Coated Mini-segment

video camera 

with 

UV-pass filter quartz lamp

laser beam tube

with -13” lens 8 micron 

pyrometer

cooling air line

4-point bend fixture

load cell load frame

thermocouple

video camera 

with 

UV-pass filter quartz lamp

laser beam tube

with -13” lens 8 micron 

pyrometer

cooling air line

4-point bend fixture

load cell load frame

thermocouple

Turbine high heat flux (up to 315W/cm2) rig Advanced cooling design Selected test articles

NASA/TM—2005-213857 5
Baseline ZrO2-(7-8) wt%Y2O3

Thermal Barrier Coating Systems

 Relatively low intrinsic thermal conductivity ~2.5 W/m-K

 High thermal expansion to better match superalloy substrates

 Good high temperature stability and mechanical properties

 Additional conductivity reduction by micro-porosity

100 μm

Ceramic coating

Bond coat

(a) Plasma-sprayed coating

25 μm

Ceramic coating

Bond coat

(b) EB-PVD coating

Thermal Conductivity Increase Kinetics of 

Plasma-Sprayed ZrO2-8wt%Y2O3 Coatings

— The conductivity reduction by microcracks and micro-porosity can not 

persist under high temperatures due to coating sintering

— The coating durability can be affected by sintering

⎭

⎬

⎫

⎩

⎨

⎧

⎥

⎦

⎤ ⎢

⎣

⎡ ⎟ − − ⎠

⎞ ⎜

⎝

⎛ = ⋅ − −

−

τ

t

k k RT

k k

c c

c c 1 exp 68228 102.2 exp inf 0

0

⎟

⎠

⎞ ⎜

⎝

⎛ = ⋅ RT

41710 τ 572.5 exp

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

15000 20000 25000 30000 35000

lnk at 990°C

lnk at 1100°C

lnk at 1320°C

lnk, W/m K

L-M=Tave·[lnt+10]

lnk= -0.560 + 2.9326·10-5 L-M

NASA/TM—2005-213857 6
Elastic Modulus of Free-Standing PlasmaSprayed ZrO2-8wt%Y2O3

— The coating “elastic modulus” also increases significantly with 

annealing and sintering

ANNEALING TIME, t [h]

0 100 200 300 400 500 600

ELASTIC MODULUS, E [GPa]

0

20

40

60

80

100

120

140

By straingaging

By impulse excitation

Impulse excitation

0

100

200

300

400

500

600

700

800

900

1000

0 500 1000 1500 2000

Strain (microstrain)

Load (N)

500 hr 100 hr 20 hr 5 hr As sprayed

Constitutive relation

compression

1316 °C in air

Thermal Conductivity Response of Plasma-Sprayed ZrO2-

8wt%Y2O3 Coatings under Thermal Gradients

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

0 10 20 30 40 50

Thermal conductivity, W/m-K

Time, hours

steady-state testing cyclic 

testing Tsurface=1316 °C

Tinterface=1100 °C

20min heating/3min cooling

Initial 

conductivity 

reduction

ZrO2-8wt%Y2O3

— Sintered coating delamination under thermal cycling

NASA/TM—2005-213857 7
ZrO2-8wt%Y2O3/Mullite+BSAS/Si System 

under Steady-State Heat-Flux Testing

— Plasma-sprayed ZrO2-8wt%Y2O3/mullite+BSAS TEBC system on SiC/SiC 

tested at 1482 °C (2700 °F)

— Coating delaminates at temperature due to sintering/creep

Tsurface=1482°C

Tinterface=1250°C

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 5 10 15 20

Measured thermal conductivity

Thermal conductivity, W/m-K

Predicted thermal conductivity

Time, hours

Conductivity reduction due to 

sintering cracking induced 

delamination cracking

500 mm

Sintering cracks

After 20h testing

ZrO2-Y2O3

Mullite+BSAS

Si

Sintering Cracks and Delaminations

— High heat flux surface sintering cracking and resulting coating 

delaminations

Kokini and Takeuchi, Mat. Sci. and Eng., 189 (1994), pp. 301-309. 

Zhu and Miller, J. Mater. Res. 14(1999). Mater. Sci. Eng. A245(1998), 212-223.

Tsurface=1280°C

Tinterface=1095°C

Thickness=130 μm

Zhu et al., Surf. Coat. Tech., 138 (2001), 1-8

surface vertical cracks

Delamination cracks

50 μm

NASA/TM—2005-213857 8
Increased Surface Sintering under Pulsed 

Heat Flux

— More severe surface sintering expected under pulsed heat flux

1.6 mm diesel thick TBC

Surface Sintering Cracks and Delamination 

Under Pulsed Heat Flux Conditions

— Surface sintering, cracking and delaminations under pulsed heat flux

0.5

1.0

1.5

200

400

600

800

1000

1200

1400

0 10 20 30 40 50 60

kcera

Tsurface

Tinterface

Tback

Thermal conductivity, W/m-K

Temperature, °C

Time, hours

50Hz, qave=120 W/cm2

200 μm

NASA/TM—2005-213857 9
• Thermal barrier coating high-heat-flux testing

 Low conductivity thermal barrier coating development

• The 3000 °F (1650 °C) thermal and environmental barrier coatings for 

SiC/SiC ceramic matrix composites

Low Conductivity Thermal Barrier Coating 

Design Requirements 

— Low conductivity (“1/2” of the baseline) retained under thermal 

gradient

— Improved sintering resistance and phase stability

— Better durability and mechanical properties

• Cyclic life

• Erosion/impact resistance

— Preferably use existing infrastructure and easy processing 

systems

— Other design considerations

• Favorable optical properties

• Suitable for various metal and ceramic components

• Affordable and safe

NASA/TM—2005-213857 10
Low Conductivity Thermal Barrier Coating 

Design Approaches 

 Many efforts on modifying coating microstructures and porosity, 

composite TBCs, or alternative oxide compounds

 Emphasize ZrO2- or HfO2-based alloy systems – defect cluster approach

 Advantages of defect cluster approach

•Advanced design approach: design of the defect clustering at the 

molecular level

•Better thermal stability: point defects are thermodynamically stable

•Improved sintering resistance: effective defect concentration reduced and 

activation energies increased by clustering

•Easy to fabricate: plasma-sprayed or EB-PVD processes

Toughness Consideration

Bast and Schumann, Ceram. Eng. Sci. Proc. 23(2002), 525-532 

 The alloy based systems have higher fracture toughness than 

oxide compounds such as perovskite and pyrochlore oxides

Normalized

Toughness

NASA/TM—2005-213857 11
Development of Advanced Defect Cluster Low 

Conductivity Thermal Barrier Coatings

— Multi-component oxide defect clustering approach (Zhu and Miller, US 

Patent No. 6,812,176)

— Defect clusters associated with dopant segregation

— The 5 to 100 nm size defect clusters for significantly reduced thermal 

conductivity and improved stability

EELS elemental maps of EB-PVD ZrO2-

14mol%(Y, Gd,Yb)2O3

Plasma-sprayed ZrO2-

13.5mol%(Y, Nd,Yb)2O3

EB-PVD ZrO2-12mol%

(Y, Nd,Yb)2O3

e.g.: ZrO2-Y2O3-Nd2O3(Gd2O3,Sm2O3)-Yb2O3(Sc2O3) systems

Primary stabilizer

Oxide cluster dopants with distinctive ionic sizes

Defect Clusters in a Plasma-Sprayed Y2O3, Nd2O3 and 

Yb2O3 Co-Doped ZrO2-Thermal Barrier Coating

— Yb, Nd rich regions consisting of small clusters with size of

5 to 20 nm

Energy (keV)

Counts

0 5 10 15 20

1000

800

600

400

200

0

Cu

Cu

Cu

O

Al

Mo

Mo

Zr

Zr

Y

Y

Yb

Yb

Yb

Yb

Yb Nd

Nd

Nd

Nd

5 nm

Yb rich region clusters

Energy (keV)

Counts

0 5 10 15

1200

1000

800

600

400

200

0

Mo

Mo

Cu

Cu

Cu

Zr

Zr

Zr

Zr Zr

Nd

Nd

Nd

Nd

Nd

Y

Y

Y

Y

Sc Sc

Yb

Yb

Yb

Yb

Yb

O

Overall EDS

Yb rich region EDS

~40Å

(111)

~11.4Å

9.07Å

NASA/TM—2005-213857 12
Low Conductivity Defect Cluster Coatings 

Demonstrated Improved Thermal Stability

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 5 10 15 20 25

Thermal conductivity, W/m-K

Time, hours

ZrO2-4.55mol%Y2O3 (ZrO2-8wt%Y2O3 )

ZrO2-13.5mol%(Y,Nd,Yb)2O3

rate increase: 2.65×10-6 W/m-K-s

rate increase: 2.9×10-7 W/m-K-s

Plasma-sprayed coatings 1316°C

— Thermal conductivity significantly reduced at high temperatures for the 

low conductivity thermal barrier coatings

— Phase stability also improved

Thermal Conductivity of Dense Monolithic Specimens

1.0

1.5

2.0

2.5

3.0

200 400 600 800 1000 1200 1400

N1-6-1

N1-6-2

N1-6-3

N1-11-1

N1-11-2,3

N2-5-1

N2-5-2

N2-8-1

N2-8-2,3

N1

Thermal conductivity, W/m-K

N2

Temperature, °C

Baseline ZrO2-4.5mol%Y2O3

ZrO2-10mol%(Y,Gd.Yb)2O3

— Lower conductivity observed for the N1 composition that has only

slightly higher Gd, Yb concentrations

m1

NASA/TM—2005-213857 13
Advantages of Four-Component Defect Cluster Low 

Conductivity Coating Approach Demonstrated

― The coatings showed significantly lower thermal conductivity compared 

to two- or three-component systems under higher temperatures

Total dopant, mole %

Relative k 

20 hrs at

2400 °F

1

4 6 8 10 12 14 16 30

1/2

1/3

2 or 3 component (open symbols)

4 component

(preferred ratio red) 

Baseline zirconia-yttria

(1.4 W/m-K at 2400 °F)

4 6 8 10 12 14 16 18

Plasma Sprayed Coatings EB-PVD Coatings

Baseline zirconia-yttria

(1.7 W/m-K at 2400 °F)

Thermal Conductivity of Oxide Defect Cluster 

Coatings Tested at Higher Temperatures

― Cubic and t’ phase coatings showed lower thermal conductivity than 

baseline ZrO2-8wt%Y2O3

― Both composition regions are important for various applications

t’ phase region Cubic phase region

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2 4 6 8 10 12 14

8YSZ k0 (2500F)

8YSZ k20 (2500F)

8YSZ k0 (2600F)

8YSZ k20 (2600F)

Refractron k0 (~2500F)

Refractron k20 (~2500F)

Refractron k0 (~2700F)

Refractron k20 (~2700F)

Praxair k0 (2500F)

Praxair k20 (2500F)

NASA k0 (2500F)

NASA k20 (2500F)

Thermal conductivity, W/m-K

Total dopant concentration, mol%

Plasma-sprayed 

coatings

Advanced

ZrO2-

based 

coatings

k0

k20

k0

k20

k0

k20

k0

k20

t’ phase region Cubic phase region

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2 4 6 8 10 12 14

8YSZ k0 (2500F)

8YSZ k20 (2500F)

8YSZ k0 (2600F)

8YSZ k20 (2600F)

Refractron k0 (~2500F)

Refractron k20 (~2500F)

Refractron k0 (~2700F)

Refractron k20 (~2700F)

Praxair k0 (2500F)

Praxair k20 (2500F)

NASA k0 (2500F)

NASA k20 (2500F)

Thermal conductivity, W/m-K

Total dopant concentration, mol%

Plasma-sprayed 

coatings

Advanced

ZrO2-

based 

coatings

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2 4 6 8 10 12 14

8YSZ k0 (2500F)

8YSZ k20 (2500F)

8YSZ k0 (2600F)

8YSZ k20 (2600F)

Refractron k0 (~2500F)

Refractron k20 (~2500F)

Refractron k0 (~2700F)

Refractron k20 (~2700F)

Praxair k0 (2500F)

Praxair k20 (2500F)

NASA k0 (2500F)

NASA k20 (2500F)

Thermal conductivity, W/m-K

Total dopant concentration, mol%

Plasma-sprayed 

coatings

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2 4 6 8 10 12 14

8YSZ k0 (2500F)

8YSZ k20 (2500F)

8YSZ k0 (2600F)

8YSZ k20 (2600F)

Refractron k0 (~2500F)

Refractron k20 (~2500F)

Refractron k0 (~2700F)

Refractron k20 (~2700F)

Praxair k0 (2500F)

Praxair k20 (2500F)

NASA k0 (2500F)

NASA k20 (2500F)

Thermal conductivity, W/m-K

Total dopant concentration, mol%

Plasma-sprayed 

coatings

Advanced

ZrO2-

based 

coatings

k0

k20

k0

k20

k0

k20

k0

k20

NASA/TM—2005-213857 14
Furnace Cyclic Behavior of ZrO2-(Y,Gd,Yb)2O3

Thermal Barrier Coatings 

― The cubic-phase ZrO2-based low conductivity TBC durability can be further 

improved by an 8YSZ or low k t’-phase interlayer

― The t’-phase low conductivity TBCs achieved at least baseline 8YSZ life

0

200

400

600

800

1000

4 5 6 7 8 9 10 11 12

Cubic phase low k TBC

Tetrogonal t' phase low k TBCs

8YSZ

Cycles to failure

Total dopant concentration, mol%

2075°F (1135°C)

Low-k t’-phase 

region Low-k cubic 

phase region

With the 

interlayer

1135 °C

Without 

interlayer

EB-PVD low k coatings

― The low conductivity turbine airfoil thermal barrier coatings successfully 

tested under simulated engine thermal gradient cyclic conditions

Advanced Low Conductivity Coatings Showed 

Excellent High Temperature Cyclic Durability

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 50 100 150 200 250 300 350

0 500 1000 1500 2000 2500

Thermal conductivity, W/m-K

Time, hours

Cycle number

Tsurface=2480 °F (1360 °C)

Tinterface=2020 °F(1104 °C)

6 min heating, 2 min cooling cycles

Low conductivity EB-PVD turbine 

airfoil coating

NASA/TM—2005-213857 15
― The low conductivity combustor coatings showed better performance 

than baseline coating at the 2700 to 2800 °F

― The 3000 °F (1650 °C) capable ZrO2 and HfO2 based combustor thermal 

barrier coatings also successfully developed

Advanced 2800 to 3000 °F Low Conductivity Coatings 

Developed for Metallic Combustor Applications

0.0

0.5

1.0

1.5

2.0

2.5

0 20 40 60 80 100 120 140

Thermal conductivity, W/m-K

Time, hours

spallation

Low k 256:Tsurface 2800°F/Tinterface 2030°F

200, 30 min cyclic after 20hr steady-state sintering test

7YSZ: Tsurface 2700°F/Tinterface 2030°F

30 min cyclic after 20 hr steady-state sintering test

Development of Advanced Erosion Resistant Turbine 

Blade Thermal Barrier Coatings

― High toughness, multi-component defect cluster erosion resistant low 

conductivity thermal barrier coatings also under development

0

50

100

150

200

250

300

350

Erosion

Impact

Erosion and impact resistance

(specific erodent weight required to penetrate coating), 

mg/mil coating thickness

Coating type

ZrO2

-7wt%Y2O3

Cubic-phase

multi-component coating

High toughness, tetragonal

phase multi-component coating

EB-PVD coatings

Baseline coating Low conductivity coating

Burner rig erosion and impact test 

results at 2200 °F

Test specimen

NASA/TM—2005-213857 16
Advanced Impact/Erosion Resistant Thermal 

Barrier Coatings

― Improved toughness and strain tolerance

Burner rig erosion and impact test results at 2200 °F

• Thermal barrier coating high-heat-flux testing

• Low conductivity thermal barrier coating development

 The 3000 °F (1650 °C) thermal and environmental barrier coatings 

for SiC/SiC ceramic matrix composites

NASA/TM—2005-213857 17
Advanced 3000 °F (1650 °C) Coatings For 

SiC/SiC CMCs

High temperature capability 

thermal and radiation barrier

Energy dissipation and chemical 

barrier interlayer

Secondary radiation barrier, thermal 

control with chemical barrier interlayer

Environmental barrier

Ceramic matrix composite (CMC)

— Low thermal conductivity

— High temperature stability

— Excellent thermal stress resistance

— Enhanced radiative flux resistance and radiation cooling

— Improved environmental protection

— Designed functional capability

Coating Radiation Performance Evaluation and 

Radiation Barrier Coatings Development

— Radiation conductivity evaluated using the laser heat flux approach

— Significant conductivity increase due to increased radiation at high 

temperatures especially under thermal gradients

0.0

0.5

1.0

1.5

2.0

2.5

3.0

200 400 600 800 1000 1200 1400 1600 1800

ZrO2-8wt%Y2O3 plasma-sprayed porous coating

k measured

k fit due to lattice conduction-radiation

Thermal conductivity, W/m-K

Surface temperature, °C

lattice conduction

radiation

sintering induced

conductivity rise

0

1

2

3

4

5

200 400 600 800 1000 1200 1400 1600

La2Zr2O7 sol-gel hot-press

La2Zr2O7 sol-gel hot-press

La2Zr2O7 hot-press

Thermal conductivity, W/m-K

Surface temperature, °C

increasing porosity

La2Zr2O7

NASA/TM—2005-213857 18
Evaluation of Radiation Thermal Conductivity of 

T/EBC Systems at High Temperatures

— Advanced HfO2 coatings demonstrated improved radiation resistance 

compared to the baseline ZrO2-8wt%Y2O3 coating

0.1

1.0

0.00 0.02 0.04 0.06 0.08 0.10 0.12

ZrO2-8wt%Y2O3

HfO2-Y2O3

HfO2-(Y,Nd,Yb)2O3

HFO2-(Y,Gd,Yb)2O3+NiO+Al2O3

HfO2-(Y,Nd,Yb)2O3+NiO-Al2O3

0 200 400 600 800 1000 1200

Radiation penetration qrad/qrad0

Coating thickness, cm

Coating thickness, microns

Laser heat flux

Radiation emitter

Ceramic coating

qrad0

qrad=hc(Tback-Tcooling air)

Advanced 3000 °F (1649 °C) Coatings Development for 

SiC/SiC Combustor Liner and Vane Applications

— The hafnia (zirconia) top coating/modified mullite intermediate layer 

systems demonstrated excellent cyclic durability and radiation resistance 

at 1650 °C (3000 °F)

— Advanced high temperature ceramic bond coats also developed

Modified mullite nanocomposite interlayer coating

HfO2-Y2O3 coatings

Multi-component HfO2

coating system

SteadyState 30 min cyclic

Tsurface=1650 °C, Tinterface=1316 °C

0.5

1.0

1.5

2.0

0 20 40 60 80 100

Normalized thermal conductivity k/k0

Time, hours

Sintering and cyclic durability evaluations 

15YSHf

5YSHf

15(YGdYb)Hf

3100°F coatings

NASA/TM—2005-213857 19
Summary and Conclusions

• High-heat-flux testing approaches established for low 

conductivity thermal barrier coating development

• Real-time monitoring of coating thermal conductivity 

demonstrated as an effective technique to assess coating 

performance

• The low conductivity thermal barrier coatings demonstrated 

improved thermal stability and cyclic durability

• High toughness, erosion resistant turbine airfoil thermal barrier 

coating development showed significant progress

• Advanced 1650 °C (3000 °F) thermal/environmental systems 

developed for Si-based ceramics

NASA/TM—2005-213857 20
This publication is available from the NASA Center for AeroSpace Information, 301–621–0390.

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September 2005

NASA TM—2005-213857

E–15227

27

Low Conductivity Thermal Barrier Coatings

Dongming Zhu

Thermal barrier coatings; Environmental barrier coatings; Thermal conductivity; Erosion;

Thermal radiation

Unclassified -Unlimited

Subject Categories: 23, 24, and 27

WBS–22–714–20–09

Viewgraphs prepared for the School of Materials Engineering Seminar sponsored by Purdue University, West

Lafayette, Indiana, February 28, 2005. Responsible person Dongming Zhu, organization code RMD, 216–433–5422.

Thermal barrier coatings will be more aggressively designed to protect gas turbine engine hot-section components in

order to meet future engine higher fuel efficiency and lower emission goals. In this presentation, thermal barrier coating

development considerations and requirements will be discussed. An experimental approach is established to monitor in

real time the thermal conductivity of the coating systems subjected to high-heat-flux, steady-state and cyclic temperature

gradients. Advanced low conductivity thermal barrier coatings have also been developed using a multi-component defect

clustering approach, and shown to have improved thermal stability. The durability and erosion resistance of low

conductivity thermal barrier coatings have been improved utilizing advanced coating architecture design, composition

optimization, in conjunction with more sophisticated modeling and design tools.