Turbine blade protected

by 7YSZ TBC

During EB-PVD, samples are periodically removed

from the vapor flux and allowed to cool from

1000ºC to 750ºC. Each interruption

of the vapor deposition and

cooling produces a new

distinct interface in the

7YSZ coating.

Reducing Thermal Conductivity: Multilayer Approach

The traditional TBC system is composed of a nickel-based super-alloy

(often with internal cooling passages) coated with a metallic bond coating

of either platinum-nickel-aluminide or MCrAlY (where M is either Ni, Co,

Fe, or mixed combination), followed by a TBC of which 7YSZ is the most

common. Between the metallic bond coating and ceramic TBC, a

thermally grown oxide (TGO) is produced during coating deposition

(initially leading to better coating adhesion, but will act as the primary

failure mechanism if the thickness grows beyond 5-7um during engine

operation). The main deposition methods for applying the PtAl-based

bond coatings are plating and CVD, whereas MCrAlY coatings are

generally applied by low-pressure plasma spray (LPPS) and HVOF,

cathodic arc. TBC is generally applied by either thermal spray or EB-PVD,

with the growing trend towards the latter. 7YSZ is an ideal candidate for

thermal barrier coatings as it has good thermal shock resistance, high

thermal stability, low density, and low thermal conductivity.

Hemispherical reflectivity of EB-PVD 7YSZ TBC produced by the (a) “in and out” and (b) “shutter” methods after 960ºC

exposure for 20 hours and (c) showing the percent improvement for each multilayer method. Reflectivity increases in both

methods with increasing number of layers, thus reducing radiative (photon) heat transfer in the TBC. This phenomena is

attributed to an increase in strain fields and stable micro-porousity.

Traditional TBC Design

Industrial prototype Sciaky, inc., EB-PVD commonly used for TBC

application. Electron-beams (1-6) vaporized material from the ingots (A-C)

allowing deposition on to the substrate material.

TBC’s undergo large and frequent temperature changes during the

operation of gas turbine engines which creates internal residual

stress. If stress caused by thermal cycling becomes too large,

premature failure can occur from coating delamination. To ensure

that the “in and out” and “shutter” applied TBC’s endure thermal

cycling as long as or longer than standard single-layer TBC’s,

samples were thermally cycled between 1175ºC and room

temperature to simulate the operation of a gas turbine engine.

Thermal Cycling Testing

“In and out”

TBC: Failure

“Shutter” TBC:

SuccessThe average thermal

cyclic life of 10- and

40-layer TBC applied

on CoNiCrAlY in this

method was 20 and

79 cycles, less than

the standard single

layer 7YSZ. Because

the “in and out”

method allowed for a

temperature drop at

each layer deposition,

interfaces became

too distinct with a

higher density

coating. This leads to

a higher residual

stress between the

layers during heating

and acts as the

primary cause of

coating delamination.

TBC Number of “shutter” layers

Percent Improvement

OEM 1 x-baseline PSU 1 21% PSU 5 31% PSU 10 91% PSU 20 118%

Cyclic life significantly improved for

“shutter” deposition on both PtAl and CoNi-

CrAlY coatings. Compressive residual

stress was reduced with increasing total

number of shutter layers.Table 1: 8YSZ deposited on PtAl-coated MARM-247

Diagram showing (a) typical standard vapor phase

columnar microstructure and (b) modified columnar

microstructure with multiple interfaces. The additional

interfaces interrupt the formation of a large grained

columnar crystallographic structure.

Zone I

Zone II(b)(a)

During EB-PVD, a “shutter” mechanism periodically pre-

vents vapor flux deposition on the surface. It

differs from the “in and out” method

in that there is little tempe-

rature change result-

ing in non-distinct

interfaces

Multiple Interfaces

(a) 1-layer (b) 1-layer (c) 1-layer

(g) 10-layer (h) 10-layer (i) 10-layer

(j) 20-layer (k) 20-layer (l) 20-layer

(d) 5-layer (e) 5-layer (f) 5-layer

The periodic interruption of the vapor cloud

created stable strain fields leading to a reduction

in thermal conductivity by 20-30% and an

increase in hemispherical reflectance.

(a) (b)

SEM micrographs showing the fracture surface of (a) 10-layer

and (b) 40-layer 8YSZ TBC deposited by EB-PVD using the “in

and out” method. The distinct lattice mismatch caused by the

interruption of deposition and temperature drop can be seen.

SEM micrographs showing the surface morphology, fracture

surface, and polished cross-section (left to right) of the (a-c) 1-

layer, (d-f) 10-layer, and (g-i) 40-layer EB-PVD deposited 8YSZ

TBC by the “in and out” method on CoNiCrAlY-coated

MARM247 alloy.

(a) 1-layer

(e) 10-layer

(b) 1-layer

(g) 40 layer

(d) 10-layer

(i) 40 layer

(f) 10-layer

(c) 1-layer

(h) 40 layer

a b

40-layer

10-layer

1-layer

a b

Thermal conductivity (Wm-k) at 1316ºC

(a) for the standard single layer, 10-

layer “shutter”, and 10-layer “in and

out” EB-PVD 7YSZ TBC as a function of

time, and (b) as a function of total

number of layers. The standard single

layer thermal conductivity of ~1.8 W/m-

K was reduced (increased phonon

scattering) to ~1.6 W/m-K in the

“shutter method” and reduced even

lower for the “in and out” method.

Increasing total number of layers

reduced conductivity by 20-30%.

Thermal Conductivity

Hemispherical Reflectivityc

SEM micrographs showing the surface morphology, fracture

surface, and polished cross-section (left to right) of the (a-c) 1-

layer, (d-f) 5-layer, (g-i) 10-layer, and (j-l) 20-layer EB-PVD 8YSZ

deposited by the “shutter” on CoNiCrAlY-coated MARM247 alloy.

Tailored Microstructure of EB-PVD 7YSZ

Thermal Barrier Coatings

“Shutter”“In and Out”

Failed turbine blade due

to thermal stress

The phonon mean free path (lp) is dependent on the mean free paths of

vacancies (lv), intersitials (li), grain boundareis (lgb), and strain (ls)

1/lp = 1/lv + 1/li + 1/lgb + 1/ls

Reducing the component free paths is one method of

reducing phonon thermal conductivity.

Kp = 1/3 ∫Cvρlp

Heat is transferred in the TBC through phonons (vibrations) and photons (radiation).

Microstructural voids, porosity, and grain boundaries cause scattering of these phonons

and photons, leading to the desired reduction of thermal conductivity.

Time (hrs)

0 5 10 15 20 25 30 35 40 45

Th

erm

al

con

du

ctiv

ity

(W

/m-K

)

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4ZrO

2-8wt.%Y

2O

3 (standard-single layer)

ZrO2-8wt.%Y

2O

3 (10-layer "shutter" method)

ZrO2-8wt.%Y

2O

3 (10-layer "in and out" method)

Time (hrs)

Th

erm

al c

on

du

cti

vit

y (

W/m

-K)

Total number of layers

0 5 10 15 20 25

Th

erm

al

con

du

ctiv

ity (

W/m

-K)

1.0

1.2

1.4

1.6

1.8

2.0

Ko (initial)

K2 (after 2hrs)

K5 (after 5 hrs)

Total number of layers

Th

erm

al c

on

du

cti

vit

y (

W/m

-K)

Wavelength (m)

1.0 1.5 2.0 2.5 3.0

Refl

ecti

vit

y (

%)

0

20

40

60

80

YSZ (1-layer) referenceYSZ (10-layer) "in & out"

YSZ (40-layer) "in & out"

Wavelength (um)

Refl

ec

tivit

y (

%)

Wavelength (m)

0.5 1.0 1.5 2.0 2.5 3.0

Rel

fect

an

ce (

%)

10

20

30

40

50

60

YSZ (1-layer) reference

YSZ (5-layer) by "shutter"

YSZ (10-layer) by "shutter"

YSZ (20-layer) by "shutter"

20-layer10-layer

1-layer

5-layer

Wavelength (um)

Refl

ec

tivit

y (

%)

Number of layers

0 10 20 30 40

% I

mp

rovem

ent

in R

elfe

ctiv

ity

-10

0

10

20

30

40

50

60

In and out method

shutter method

Number of layers%

Im

pro

ve

me

nt

in R

efl

ec

tivit

y

Percent Improvement

• Thermal conductivity lowered

by 25-30%

• Thermal reflectance increased

by 28-56%

• Strain tolerance improved

through stress reduction of

~15-20% by incorporation of

stable periodic strain fields.

• Increased thermal cyclic life

up to 118%

Periodic vapor flux interruption

by the “shutter” method

successfully improved TBC

performance:

“Shutter” EB-PVD enhancements

benefit TBC use in turbine

operation by extending

component life under increased

inlet and combustion

temperatures, leading to less fuel

consumption and fewer by-

product emissions.Total number of layers (shutter method)

0 5 10 15 20 25

Str

ess

(MP

a)

-250

-200

-150

-100

-50

0residual

stress of

7YSZ TBC

deposited by

EB-PVD as a

function of

total number

of “shutter”

layers.

Conclusions

Efficiency of gas turbine engines can be

improved by increasing the inlet and op-

eration temperatures, thus leading to lower

fuel consumption and fewer byproducts.

Hot section components, however, have

limited tolerance to high temperature dam-

age, making developments in thermal

protection critical to further advance-

ment (reduction of metallic temperature by 30-60ºC can increase

component life by two-fold). Thermal barrier coatings (TBC’s) are often

used for insulating hot section materials. Achieving further elevation of

temperatures with minimal loss of component life requires tailoring of the

microstructure and/or composition of TBC’s for better thermal protection

Demand for TBC Innovation

• Reduce fuel consumption

• Reduce environmentally

unfriendly NOx and COx

emissions

• Increase turbine inlet

temperature

• Increase operating

temperature (1300ºC to

1500ºC)

• Increase component life

Goals of the

Turbine Industry

Modifying the EB-PVD process with the “in and out” and “shutter” methods have been

tested as means of altering the TBC microstructure for reduced thermal conductivity.

Distance

YSZ

Top-Coat

100-400 µm

Te

mp

.

Bond-Coat

~50-100 µm

Superalloy

SubstrateCooling

Air Film

TGO

Al2O3

1-10µm

Ho

t G

as

es

Co

oli

ng

Air

Douglas E. Wolfe*1, Jogender Singh1, Robert A. Miller2, Jeff I. Eldridge2, Dong-Ming Zhu2

1The Applied Research Laboratory, The Penn State University, 2NASA-GRC, *dew125@psu.edu; 814.865-0316