NASA / TM--1998-208505

//7 _y

/--.-/3..L/ Y _'-

Effect of Layer-Graded Bond Coats on

Edge Stress Concentration and Oxidation

Behavior of Thermal Barrier Coatings

Dongming Zhu

Ohio Aerospace Institute, Cleveland, Ohio

Louis J. Ghosn

Case Western Reserve University, Cleveland, Ohio

Robert A. Miller

Lewis Research Center, Cleveland, Ohio

November 1998

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N ASA/TM--1998-208505

Effect of Layer-Graded Bond Coats on

Edge Stress Concentration and Oxidation

Behavior of Thermal Barrier Coatings

Dongming Zhu

Ohio Aerospace Institute, Cleveland, Ohio

Louis J. Ghosn

Case Western Reserve University, Cleveland, Ohio

Robert A. Miller

Lewis Research Center, Cleveland, Ohio

Prepared for the

193rd Meeting of the Electrochemical Society,

Symposium on High Temperature Corrosion and Materials Chemistry

sponsored by the Electrochemical Society

San Diego, California, May 3-8, 1998

National Aeronautics and

Space Administration

Lewis Research Center

November 1998
Acknowledgments

The authors are grateful to G.W. Leissler and D.L. Humphrey for their assistance

in the preparation of TBC coatings and TGA experiments, respectively.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

Available from

National Technical Information Service

5287 Port Royal Road

Springfield, VA 22100

Price Code: A03

ili I:_
EFFECT OF LAYER-GRADED BOND COATS ON EDGE

STRESS CONCENTRATION AND OXIDATION BEHAVIOR

OF THERMAL BARRIER COATINGS

Dongming Zhu *, Louis J. Ghosn, Robert A. Miller

NASA Lewis Research Center

21000 Brookpark Road, Cleveland, OH 44135

ABSTRACT

Thermal barrier coating (TBC) durability is closely related to design,

processing and microstructure of the coating systems. Two important

issues that must be considered during the design of a thermal barrier coating

are thermal expansion and modulus mismatch between the substrate and the

ceramic layer, and substrate oxidation. In many cases, both of these issues

may be best addressed through the selection of an appropriate bond coat

system. In this study, a low thermal expansion and layer-graded bond coat

system, that consists of plasma-sprayed FeCoNiCrAI and FeCrAIY

coatings and a high velocity oxyfuel (HVOF) sprayed FeCrAIY coating, is

developed to minimize the thermal stresses and provide oxidation

resistance. The thermal expansion and oxidation behavior of the coating

system are also characterized, and the strain isolation effect of the bond coat

system is analyzed using the finite element method (FEM). Experiments

and finite element results show that the layer-graded bond coat system

possesses lower interfacial stresses, better strain isolation and excellent

oxidation resistance, thus significantly improving the coating performance

and durability.

I. INTRODUCTION

Ceramic thermal barrier coatings (TBC) have been developed for advanced gas

turbine [j' 21 and diesel engine applications [3-51 to improve engine durability and fuel

efficiency. However, durability issues of these thermal barrier coatings under high

temperature cyclic conditions are still of major concern. The coating delamination failure is

closely related to thermal stresses in the coating systems, and oxidation of bond coats and

substrate I6-91 . Coating shrinkage and through-thickness cracking resulting from ceramic

sintering and creep at high temperatures [I01 , will further accelerate the coating failure

process due to edge stress concentration effects along the coating interfaces. The coating

reliability can be greatly improved through the development of an appropriate bond coat

* Research Scientist, Ohio Aerospace Institute at NASA Lewis Research Center.

Keywords: Thermal barrier coatings, High temperature oxidation, Laser thermal gradient cycling,

Edge effect, Finite element meth_)d

NASA/TM--1998-208505 I
system. The bond coat system should not only be oxidation resistant, but also be tailored to

grade the thermal expansion differences (provide strain isolation for the ceramic coating)

and protect the substrate from oxidation. Thus, the stress arising from thermal expansion

can be minimized and interfacial strength can be maintained. The strain isolation provided

by a graded bond coat is especially beneficial for coating edges that exist due to component

geometry, or due to through-thickness cracking resulting from ceramic sintering and creep

at high temperatures. This layer-graded bond coat concept was proposed in earlier

work where thermal fatigue testing of graded bond coat systems showed improved

performance I1ll. In the present study, the thermal expansion and oxidation behavior of a

low thermal expansion, three-layer-graded bond coat system are investigated. This coating

system is comprised of plasma-sprayed FeCoNiCrAl, and FeCrAIY coatings and a high

velocity oxyfuel (HVOF) sprayed FeCrAIY coating, as shown in Figure 1. The strain

isolation effect of the bond coat system and the interfacial elastic stresses are analyzed

using dilatometry and the finite element method (FEM). The influence of bond coat

thickness on coating interface delaminations is evaluated based on thermal expansion

behavior of the TBC system and the ceramic/metal interface microstructure characterization

after thermal cycling experiments.

E

e'_

ZrO2-8wt%Y203

Low expansion FeCoNiCrAI

Plasma sprayed FeCrAIY

_Dense HVOF FeCrAIY_

...................................... z_

.................... _ _-f4140 Steel

Fig. 1 Schematic diagram showing a layer-graded thermal barrier coating system that

consists of a plasma-sprayed ZrO2-8%Y203 top coat, and a plasma-sprayed

FeCoNiCrAI, FeCrA1Y and thin HVOF sprayed FeCrAIY bond coat system.

II. EXPERIMENTAL METHODS AND MATERIALS

The oxidation kinetics of air plasma-sprayed Fe-28Co-24Ni-5Cr-4Al and Fe-25Cr5A1-0.5Y bond coats were determined by thermogravimetric analysis (TGA) in flowing

air, using the free-standing bond coat coupons (specimen dimension 25.4 x12.7 x l ram).

The oxidation kinetics of 4140 steel were determined by TGA for the uncoated 4140

specimens, and by measurements of the oxide scale thicknesses from the cross-sections of

the coated diameter 12.7 mm disk specimens, respectively.

NASA/TM--1998-208505

II 1 ?_
Thermalexpansionexperimentswere carriedout on free-standingceramic and

bondcoatmaterialsin air usinga dilatometersystem.Thermalexpansionresponseof the

TBC systemwasconductedontheceramiccoatingsattachedto thesubstratein high purity

argonusingdilatometery,asshownin Figure2. Thespecimenlengthwas25.4mm, and a

platinumstandardspecimenwasusedasreference.Thethicknessof theceramiccoating

was 1.5ram, andthicknessesof thebondcoatswerechosenas 0.1-0.5mm. The 4140

steelsubstratewas 12.7mm in thicknessto ensureno significantbendingduring the

experiments.

Seramic coating

/// Thermocouple

[ _/£ __ _ / _=__,_asoutlet

I t_lk_ a------__ P_ush Rods \ _---7",_"aWater inlet

.. _Gas inlet

II --

Fig. 2 The dilatometer system used for the measurement of thermal expansion behavior

of thermal barrier coating systems.

In order to investigate the bond coat type on edge crack initiation and propagation,

segmented thermal barrier coating systems (1.5 mm thick ceramic coating on

25.4 x !2.7 x 12.7 mm steel substrate with various bond coat thicknesses) were tested up to

50 cycles (5 minute heating and 3 minute cooling) under laser thermal gradient cyclic

conditions. Before each test, the ceramic coating was segmented with a diamond saw, so

through-thickness, parallel notches were formed in the ceramic coating. The coating

segment length (the spacing between notches) and the notch width were approximately 5

mm and 0.28 ram, respectively, as shown in Figure 3 (a). These notch edges near the

ceramic/bond coat interface were used to simulate the coating sintering segmentation

induced edges. The segmented TBC specimens were then thermally cycled using a rotating

laser rig, with the uniform laser beam heating the entire ceramic coating surface. The

substrate backside air cooling was used to establish the temperature gradients across the

coating system. During these tests, the maximum ceramic surface and back side metal

temperatures were maintained at about 850°C and 400°C, respectively. The typical heating

and cooling temperature profiles are illustrated in Figure 3 (b).

NASA/TM-- 1998-208505 3
laser

5 _ -; ..................

Substrate

/AI,2

•'_ 25.4 mm •

(a)

¢

A

1.5 mm

12.7 mm

Fig. 3

r,,.

E

Y.

1200.0

1000.0

800.0

600.0

400.0

200.0

0.0

0.0

Tceramic surface

Wmetal backside

I I

I __ I

• a

10.0

[ , i __ i

20.0 30.0

Time, minutes

40.0 50.0

(b)

Laser thermal gradient cycling experiments of thermal barrier coatings. (a)

Segmented ceramic coating specimen configuration for Iaser testing; (b) Typical

heating and cooling cycles for the thermal barrier coating systems under laser

thermal cycling conditions•

NASA/TM--1998-208505
III. STRESS ANALYSIS PROCEDURE

The analytical procedure implemented in this study is the Finite Element Methods

(FEM). Two-dimensional FEM meshes were generated for various specimen lengths and

various bond coat systems. A typical FEM mesh is shown in Figure 4 (a) for a half TBC

specimen length of 12.7 ram. The metal substrate is 12.7 mm high and the ceramic coating

is around 1.5 mm. Two bond coat layers are sandwiched in between. The bond coat layer

thicknesses were in the range of 0.1 to 0.5 mm. The mesh consisted of a total of 1024

eight-noded quadrilateral elements with 3201 nodes. The element sizes decreased

approaching the free edge (the right hand-side) as well as the sandwiched bond coats to

capture the sharp stress gradients at these locations. A total of 32 unequal elements span the

length of the specimen with a higher element density at the free edge. While the steel

substrate is meshed by 16x32 layers, the ceramic coating is layer by 8x32 elements since

the ceramic coating is much smaller than the steel substrate. The two bondcoat layers have

4x32 elements each. The use of only four layers for each bond coat is deemed appropriate

given the parabolic variation in displacement and stresses formulated in the eight-noded

element used.

The numerical analyses were implemented in the ABAQUS general purpose finite

code [121 . Only elastic stress analyses were considered in this study. The material

properties assumed in the analyses are given in Table 1. In this analysis, the stress free

temperature is assumed to be at room temperature. The effect of the residual stress built up

from the plasma spraying of the bond coat systems is ignored. For constant temperature

simulations only a stress analysis is required, but for the through thickness simulations, a

steady state heat transfer analysis is required to determine the nodal temperature followed

by the elastic stress analysis. A typical mesh of the segmented TBC specimen is shown in

Figure 4 (b).

Table 1 Physical and mechanical properties of the thermal barrier coating systems used in

, the FEM stress calculations 1__1

Materials Properties

Thermal Conductivity

k, W/m K

Thermal Expansion

Coefficient a , m/m K

Density p, kg/m 3

Heat Capacity c, J/kg K

Young's modulus E, GPa

Plasma Sprayed

ZrO2-8wt%Y 203

0.9

10.8x10 -6

5236

582

27.6

4 !40 steel

46.7

14.2x10 -6

7850

456.4

207

Plasma

Sprayed

FeCrA1Y

11

12.5x10 -6

6500

Plasma

Sprayed

FeCoNiCrA1

11

11.0× 10-6

6500

575 575

137.9 100

NASA/TM--1998-208505 5
ceramic

bond coat

substrate

Y

edge

fITIIIlill_

llllllffJh

HJt

(a) (b)

Fig. 4 Typical FEM meshes used in the stress analysis. (a) A FEM mesh for a half TBC

specimen length of 12.7 mm. (b) A FEM mesh for a half segment TBC specimen length

of 2.5 mm.

NASA]I'M-- 1998-208505
IV. RESULTS AND DISCUSSION

The air plasma-sprayed bond coats exhibited a complicated transient oxidation

behavior due to its relatively high porosity, as has been reported previously [71 . Figure 5

shows the Arrhenius plots of apparent oxidation parabolic rate constants of the plasmasprayed FeCoCrNiA1 and FeCrAIY bond coats. As compared to the commonly used Ni36Cr-5A1-Y bond coat, the FeCrA1Y bond coat showed a faster oxidation rate mainly at

the lower temperatures. The FeCoCrNiAI showed the fastest oxidation rate. X-ray

diffraction results suggested that the plasma sprayed FeCrA1Y and FeCoCrNiA1 bond

coats sometimes can form non-protective Fe-containing oxides at low oxidation

temperatures. Air plasma-sprayed FeCrAIY bond coat could not effectively prevent the

4140 steel substrates from oxidation due to its relatively high porosity in the coating [71

However, the denser HVOF sprayed FeCrAIY bond coat provided a higher bond strength,

and showed better oxidation protection for the substrate. Figure 6 shows typical

microstructures of the plasma-sprayed FeCrA1Y bond coat after oxidation at 800°C for 50

hours.

Figure 7 shows the experimentally measured thermal expansion behavior of the

free-standing ceramic coating, bond coat and steel substrate materials used in the TBC

systems. As have been determined in an early study [71 , the 4140 steel and ZrO2-8%Y203

ceramic possess the highest and the lowest thermal expansion coefficients, respectively,

while the air-plasma-sprayed FeCrAIY bond coat shows an intermediate thermal

expansion coefficient. The low thermal expansion bond coat and FeCrNiCoAI has a similar

coefficient of thermal expansion to the ceramic coating at temperatures below 600°C.

Due to the thermal expansion mismatch in the ceramic coating and substrate,

thermal stresses are generated in the coating system under thermal cycling conditions.

Stress concentrations near the coating edges due to the temperature change can lead to

coating cracking and delamination along the ceramic/metal interfaces. Figure 8 shows

typical stress distributions predicted by FEM in thermal barrier coating systems with the

finite length specimen geometries under the condition of uniform temperature variation

from 25 to 700°C. The stress distributions near the coating edges in this uniform heating

case are characterized by the essentially very low normal in-plane stress component o-,;_.,

the high normal tensile stress component cr_;yperpendicular to the interfaces and shear

stress component cra_, along the ceramic/bond coat and the bond coat/substrate interfaces.

The tensile strength of the ceramic coatings varies from 30 to 50 MPa, the large tensile

stress component o-_;v developed during the heating cycle near the coating edges can be an

important factor for coating delamination. The shear strength of the ceramic coating parallel

to the bond coat is approximately 10 to 14 MPa [13], delamination cracks can also be easily

initiated at coating edges near the ceramic/bond coat interface where the highest shear stress

cr___,is expected. However, as will be discussed later, the coating shear delamination

becomes a predominant mechanism under thermal gradient conditions, because the tensile

stress component O-y_,near the coating edge can be significantly lowered or even be slightly

compressive for the non-uniform heating case. Oxidation of the bond coat and substrate

will complicate this process, and in general facilitate the coating delamination.

NASA/TM--1998-208505 7
6

_3

.,q.

E

E

¢L

Fig. 5

-8.00

-9.00

-10.00

- I ! .00

-12.00

-13.00

-14.00 ....

8.0

-25

-27

6

-29

E

e-

-33

-35

I

Temperature, °C

900 800 700 600 500

- _ _ I I I I I I

"'-A. j/FcCoCrNiAI: In Kp = -1.0595 - 9068.1 (l/T)

" >. t_ :. FcCrAIY: In Kp = -3.4919 - 7047 (I/T)

/

.... a .... In Kp, FeCoNiCrAI " - .. "...

+ In Kp, FeCrAIY --. -

- -_ - - In Kp, NiCrAIY " _

, , [ ,__L__-_ I .... I , _ _1 ....

9.0 10.0 II.0 12.0 13,0

1/T.104, K-I

(a)

Temperature, °C

600 550 500 450 40_

i _ I I 4140 without _oating (TGA): 1

Air plasma FeCrA1Y coated 4140:

lnkp'(cm2/sec.) = - 14.604 - 86083/RT

HVOF FeCrAIY coated 4140:

= -5.9019 - 177928/RT

.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 1

I/T. 104, K -1

(b)

-10

-11

-12

E

-13 _

-14

;.0

Arrhenius plots of the parabolic rate constants of the plasma sprayed bond coats

and the 4140 steel substrate. (a) In kp (in mg2/cm4.sec) - 1FF relations for the free

standing FeCoNiCrAI, FeCrA1Y and NiCrAIY specimens by TGA

measurements; (b) In k'_,(in cm2/sec) - 1/T relation for 4140 steel specimens with

and without FeCrA1Y bond coat by scale thickness measurements.

NASA/TM-- 1998-208505

![[IT_
(a)

Fig. 6

(b)

Typical microstructures of the plasma-sprayed FeCrA1Y bond coat after

oxidation at 800°C. (a) Optical micograph of the coating section parallel to the

ceramic/bond coat interface; (b) Transmission electron micrograph of the coating

showing the A1203 scales grown on the FeCrAlY grains.

NASA/TM--1998-208505 9
d

O

E

1.00

0.80

0.60

0.40

0.20

0.00

0

.... I .... I .... I .... i .... I .... I .... I ....

4140 /'_

-- o- - NiCrAIY J 60_

---.o- - FeCrAIY j / j_

- -_- - - FeCoNiCrAl .g/" _ "/_-_cP

.... _---- ZrO -Y,O_ J_¢. _ P.--a

. of_g"

_,1,,_,,I .... I .... I .... I .... I .... I ....

100 200 300 400 500 600 700 800

Temperature, °C

Fig. 7 Experimentally measured thermal expansion behavior of the flee-standing

ceramic coating, bond coat and substrate materials.

Oxidation resistant and thermal expansion-graded bond coat systems can help

minimize shear crack initiation and propagation by improving the stress distributions near

the interfaces and the interface adhesion. The FEM calculation results shown in Figure 8

demonstrate that the two layer-graded bond coat system can reduce the both shear and

tensile stress concentrations near the ceramic/metal interface, and shift the peak stress away

from the edge, compared with no bond coat or only the FeCrA1Y bond coat case. In

addition, with the two layer low expansion bond coat system, the overall stress levels are

lowered and the high stress region is shifted to the bond coat/substrate interface.

The stress concentrations in TBC systems are also closely related to the bond coat

thickness and specimen size. Figure 9 illustrates the influence of the bond coat thickness

and the specimen length on shear stress distribution. For specimens of various lengths, the

edge stress concentration increases with decreasing total bond coat thickness. In addition,

the high shear stress regions occupy a larger portion of the ceramic/metal interface area for

a shorter segment length, thus increasing the trend to delaminate the coating by the shear

mechanism near the interface.

NASA/TM--1998-208505 10
86.13

71 12 -..ii

=

5611

4I 10

2609

1108

_3929

-1894

-3395

-4B g6

-63 97

-7898

-g399

-1090

-1240

-139 0

No bond coats

O'xx

3g09

3085

2262

392.7

3655

3384

3112

6696

39 81

1267

-14.47

Fig. 8

(a)

FEM stress distributions in thermal barrier coating systems under the condition of

uniform temperature variation from 25 to 700°C. (a) Normal and shear stress

distributions of a coating system without bond coats; (b) Normal and shear stress

distributions of a coating system with two layer bond coats; (c) Influence of the

bond coat type on edge shear stress distributions in the ceramic coating at the

ceramic/bond coat interface.

NASA/TM--1998-208505 I1
2430

216.3 .......

1B9.7

163.o

lO9.7

83,05

56.39

2973

3.075

-23, 58

-50,24

-7690

-1036

-130.2

-I56.9

Fig. 8

350.1

325 4

ajE¢

16.01

9.642

3 274

-3,094

-9.461

-15.83 _

53 63

28.92

4 222

-2048

Two bond Coats,0.254/0.254mm

(b)

-2220

-28 56

-3493

-41.30

-47.67

-54,03

-60.40

-66 77

-73.14

-79.51

X

(Continued) FEM stress distributions in thermal barrier coating systems under

the condition of uniform temperature variation from 25 to 700°C. (a) Normal and

shear stress distributions of a coating system without bond coats; (b) Normal and

shear stress distributions of a coating system with two layer bond coats; (c)

Influence of the bond coat type on edge shear stress distributions in the ceramic

coating at the ceramic/bond coat interface.

NASA/TM--1998-208505 12
Fig. 8

¢_

r_

r,¢3

¢_

¢D

60.00

. ' I I I I I I [ i I

li"" - ZrO JFeCoNiCrAI/FeCrA1Y/4 1401

40.00 I-i_ "", ..... ZrOJFeCrAIY/4140

I_: ,,___, ,_........ ZrO?4140

¢¢, " ".

20.00 /

0.00

-20.00 .. J

-40.00 ",,

Total coating thickness 1.0 mm ",

,,

-60.00 _ .... l__, J , L ,_ ..... a_ , J , ! , __

-2.5-2.0-1.5-I.0-0.5 0.0 0.5 1.0 1.5 2.0 2.5

Distance from specimen center, mm

(c)

(Continued) FEM stress distributions in thermal barrier coating systems under

the condition of uniform temperature variation from 25 to 700°C. (a) Normal and

shear stress distributions of a coating system without bond coats; (b) Normal and

shear stress distributions of a coating system with two layer bond coats; (c)

Influence of the bond coat type on edge shear stress distributions in the ceramic

coating at the ceramic/bond coat interface.

Fig. 9

t_

t_

¢D

r.¢3

40.0 .... , .... _ .... , .... , .... , ....

\ _ h .... 0.25mm FeCoNiCrAI:

30.0 \ I _/ \ + 0.25mm FeCrA1Y-

:'k,,, I.:'}f "_ [_ ........ 0.5mm FeCoNiCrA1

20.0 "', +0.5mm FeCrAIY

I0.0

0.0

-I0.0

-20.0

-30.0

-40.0 .... i .... I .... t ......... ] .... _ ....

-15.00 -10.00 -5.00 _ 0.00 5.00 10.00 I5.00

Distance from specimen center, mm

Influence of bond coat thickness and specimen length on edge shear stress

distributions in the ceramic coating at the ceramic/bond coat interface under the

condition of uniform temperature variation from 25 to 700°C.

NASA/FM--1998-208505 13
It has been realized that sintering-segmentation-enhanced delamination can be an

important failure mechanism for a thermal barrier coating system during thermal cycling [91

• The through-thickness cracks generated can create numerous edges in the ceramic coating

systems. The modulus of the ceramic coating can also be significantly increased due to the

sintering densification process. Figure 10 shows the shear stress distributions at the

ceramic/bond coat and the bond coat/substrate interfaces for two effective ceramic modulus

values (27.6 GPa and 100 GPa) to simulate the densification effect. It can be seen that the

shear stresses at both interfaces increase with increasing the modulus of the ceramic

coating. The increased stress concentrations due to sintering and oxidation of the bond coat

and substrate can further accelerate the coating delamination process. The densification and

oxidation processes in the coating systems increase the driving force for crack initiation and

propagation, and reduce the coating adhesion• The graded bond coat systems proposed in

this study can have beneficial effects in improving the coating performance.

In order to evaluate the stress concentrations in sintering segmentation ceramic

coatings under thermal gradient heating conditions, FEM has been used to obtain the stress

distributions of the segmented, through-thickness notch specimens described above. The

stress distributions for a ceramic, two layer bond coat and 4140 substrate TBC system are

illustrated in Figure 11. Since the notch width is wide enough, an edge stress concentration

effect similar to the free edge case described above are expected near the ceramic/bond coat

and the bond coat/substrate interfaces. However, under the present thermal gradient heating

and low interfacial temperature conditions, a compressive (instead of a tensile) stress

component ay._, is developed. Therefore, in this case, shear induced cracking at the

interfaces becomes a major mechanism for delamination crack initiation.

Experimental work has been performed to confirm several aspects of the coating

edge effects predicted by the finite element analysis. The strain isolation of the bond coat

system was studied by measuring the displacements in the ceramic coating attached to the

substrate with various bond coat systems under the condition of uniform temperature

variation from 25 to 700°C, and compared with the FEM calculations• The results are

shown in Figure 12. Figure 12 (a) shows the effect of bond coat type on strain isolation. A

lower thermal expansion bond coat gives a better strain isolation effect. It should be

mentioned that although the one-layer FeCoCrNiAI bond coat appears to have the best

strain isolation for the ceramic coating, it can generate significantly higher stresses at the

bond coat/steel substrate interface as compared with the other bond coat systems. Figure 12

(b) shows the effect of bond coat thickness on strain isolation. The improved strain

isolation effect by a thicker, multilayer, graded bond coat system suggest a reduced stress

concentration at the ceramic/bond coat interface. Although discrepancies are observed

between the experimental data and the FEM calculations, the strain isolation effects of the

various bond coats obtained by both methods are similar and consistent• It should be noted

that the FEM solutions did not consider any interracial sliding and substrate plasticity or the

possibility of low coating modulus near the interface regions.

NASA/TM--1998-208505 14
100.00 .... I .... I .... I .... i .... I ....

f Ceramic modulus 100 GPa ......... 27.6 GPa

_i

 000i

0.00

&

-5o.oo

Total bond coat thickness 1.0 mm

-100.00 ......... L...... i _ , ,_l_ ...... 1....

-15.00 -10.00 -5.00 0.00 5.00 10.00 15.00

Distance from specimen center, mm

(a)

I00.00 .... _ .... I .... I .... I .... I ....

uoo p

modulus 27.6 GP_

,-,

50.00

-.

o.oo --

e.-,

-50.00

Total bond coat thickness 1.0 mn x.)

- 100.00 ..... __A ............ i .... _ .... I ....

-15.00 -10.00 -5.00 0.00 5.00 10.00 15.00

Distance from specimen center, mm

(b)

Fig. 10 Influence of increased ceramic elastic modulus on shear stress distributions in the

ceramic/FeCoNiCrAIY/FeCrAlY/4140 system under the condition of uniform

temperature variation from 25 to 700°C. (a) At the ceramic/bond coat interface;

(b) At the bond coat/substrate interface.

NASA/TM-- 1998-208505 15
139.6

121.6

103.5

8553

tr+;

67.51

49.50

31 48

13.47

,.4544

-22 56

-40.57

-5859 Gx'¢

-76.60

-9462

-112.6

-130.6

: . . .... .L.?__" ....

44,7 ;_--_:_i_'_ _-_-'_-'_ _

=_.

40, 55

-2901

36.94

33.32

29.70 ._,_-.

26 08

22 46 __

188_

15.2,_

11 6(

7.98E

4.36;

.748_

-2.670

-6.468

O".lg/ -1011

-46.77

-55.65

-64.53

-73.41

-82.29

-91.17

-100.0

-108.9

-117.8

Y

lz x

Two Bond Coats, 0.254/0.254mm

Fig. 11 FEM stress distributions near a notch edge for a ceramic, two layer bond coat and

4140 substrate TBC system under thermal gradient conditions. Ceramic surface

temperature and 4140 substrate backside temperature are 850°C and 400°C,

respectively.

NASA/TM--1998-208505 16
_ 0"0 K,_' • ......... Total bond coat thickness 0°5 mm:-" - ql_ _,_i

[_ _ _ specimen length 25.4 mm

" li

_-! _ 2.0_- ........ 4140 _ _ -] |

"3 _ ._ f+ ZrO2/4140,FEM & _ ] 4

IT _ 3 oL+ ZrO2/FeCrAIYI4140,FEM }a _ _ l

_10.0 H_- __ " ]-----<z_ F---'n_ZrO2/FeC°CrNiA1/zl40'FEM ZrO2/FeCoNiCrAI/FcCrAIY/4140,FEM_ /_lI ]_ |1

_ 4.0[- . ZrO2/FeCrAlY/4140,measured _ --1

_ _ [ • ZrO2/FeCoCrNiA1/FeCrA1Y/4140,measured ]

_ A_.r()._eCoNiCrAl(4140 measurt_d ] i_ i 5.0 ........... " , , _.

._ _ 0.30 0.35 0.40 0.45 0.50

[l Thermal strain AL/2L, %

_15.0__ ......

-0.50 0.00 0.50

Thermal strain +_AL/2L, %

(a)

0.0

E

E

5.0

. ,....q

¢,P3

_10.0

4....0

15.0

i ..... ' ' u&:!J

! _ o.o

1.0

2.0

E

3.0

E

0

4.0

_D

_=

i5 5.0.40

ZrO2/FeCoNiCrAI/FeCrA1Y/4140

Specimen length 25.4 mm

' ' I ' ' ' I ' ' ' I I

• _ 5°C

Fotal Bond coat thicknes_ta_ _ _ "_,

......... 4140 -"=_"N "_

-+- 0.25ram,FEM "a\5,

---o-- 0.5 mm, FEM "_'_

_ 0.76 mm, FEM "_\ !

 .Omm,FEM , i

• 0.5 mm, measured ';,\\.\ !

• /.0 mm, measured, _'_'_ i,

0.42 0.44 0.46 0.48 0.50

Thermal strain AL/2L,%

n i a n n n n _ i l q * i b 1 ! i * i _

-0.50 0.00 0.50

Thermal strain,%

(b)

Fig. 12 Determination of strain isolation effect by dilatometry and FEM for thermal

barrier coating systems under the condition of uniform temperature variation

from 25 to 700°C (specimen center as the reference point). (a) The effect of bond

coat type on strain isolation; (b) The effect of bond coat thickness on strain

isolation.

NASA/TM--1998-208505 17
Laser thermal cycling experiments of segmented thermal barrier systems have also

demonstrated that cracks are always initiated at the coating edges. As shown in Figure 13

(a), for a thin single layer FeCrAIY bond coat, the crack initiated and propagated in the

ceramic coatings near the ceramic/bond coat interface. However, as shown in Figure 13

(b), for a thick single layer FeCrA1Y coating, delaminations were observed at both the

ceramic/bond coat and the bond coat/substrate interfaces. This is because the increase in

FeCrA1Y bond coat thickness can effectively shift the high stress concentration region

from the ceramic/bond coat interface to the bond coat/substrate interface. For the

segmented specimens, the long delamination cracks were observed near the throughthickness machined notch edges for the single layer bond coat systems, as shown in Figure

14 (a). The two-layer graded bond coat system in Figure 14 (b) showed a much better

performance in resisting edge crack initiation and propagation under thermal cycling

conditions at both the ceramic/bond coat and the bond coat/substrate interfaces, due to the

improved bond coat strain isolation at both interfaces.

V. CONCLUDING REMARKS

A low thermal expansion and layer-graded bond coat system, that consists of

plasma-sprayed FeCoNiCrAI and FeCrA1Y coatings and a high velocity oxyfuel (HVOF)

sprayed FeCrAIY coating, was'developed for minimizing the thermal stresses and

providing excellent oxidation resistance. Experimental results and finite element analysis

show that the layer-graded bond coat system possesses lower interfacial stresses, better

strain isolation and oxidation resistance in the thermal barrier coating system, which will

lead to improved coating performance and durability.

NASA/TM--1998-208505 18

[I1_ _li_
"crack

lOOpm

(a)

lO0._m

Fig. 13

(b)

Edge crack initiation and propagation in single layer FeCrA1Y bond coat thermal

barrier coating systems after 50 laser thermal cycles. (a) Bond coat thickness

0.127 mm; (b) Bond coat thickness 0.5 mm.

NASA/TM--1998-208505 19
lO0_m'

(a)

lO01Jm

Fig. 14

(b)

Delamination crack initiation and propagation near the vertical machined notch

edges for the thermal barrier coating systems after 50 laser thermal gradient

cycles. (a) Single layer FeCrA1Y bond coat thickness 0.127 mm; (b) Two-layer

FeCoNiCrAI- FeCrA1Y bond coat total bond coat thickness 0.5 mm (each layer

0.25 mm).

NASA/TM--1998-208505 2O

IFI_il ii
REFERENCES

[ 1] Miller, R. A., Surface and Coatings Technology, 30, 1 (1987).

[2] Miller, R. A., Journal of Thermal Spray Technology, 6, 35 (1995).

[3] Miller, R. A., NASA TM-103130, DOE/NASA21794-1, May 1990.

[4] Yonushonis, T. M., NASA CR-187111, 1991.

[5] Beardsley, M. B. and Larson, H. J., DOENASA0332-1, NASA CR 190759,

1992.

[6] Miller, R. A., Journal of the American Ceramic Society, 67, 517 (1984).

[7] Zhu, D. and Miller, R. A., in Fundamental Aspects of High Temperature

Corrosion (eds. Shores, D. A., Rapp, R. A. and Hou, P. Y.), PV 96-26, 289

(1997). NASA TM-I07360, Army Research Laboratory Technical Report ARLTR-1254, November 1996.

[8] Zhu, D. and Miller, R. A., Surface and Coatings Technology, 94-95, 94 (1997).

[9] Zhu, D. and Miller, R. A., Materials Science and Engineering, A245, 212 (1998).

Also NASA TM-206633, February 1998.

[10] Zhu, D. and Miller, R. A., Journal of Materials Research, in press.

[11] Miller, R. A., Zhu, D., Goedjen, J. G. and Cruse, T. A., NASA TM, to be published

(1998).

[12] "MSC/ABAQUS User's Manuel, Version 5.5", The MacNeal-Schwendler

Corporation, Los Angeles, California (1996).

[13] Cruse, T. A., Dommarco, R. C. and Bastias, P. C., Mechanical Testing Program

for Thermal Barrier Coating Development, Final Report (Vanderbilt University),

NASA Cooperative Agreement NCC3-187 (1996).

NASA/TM-- 1998-208505 21
REPORT DOCUMENTATION PAGE I Form Approved

I

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,

gathering and maintaining the data needed, and completing and reviewing 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 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503,

1. AGENCYUSE ONLY(Leaveblank) 2. REPORTDATE 3. REPORTTYPEAND DATESCOVERED

November 1998 .... Technical Memorandum

4. TITLEANDSUBTITLE 5. FUNDINGNUMBERS

Effect of Layer-Graded Bond Coats on Edge Stress Concentration and

Oxidation Behavior of Thermal Barrier Coatings

6. AUTHOR(S)

Dongming Zhu, Louis J. Ghosn, and Robert A. Miller

7. PERFORMINGORGANIZATIONNAME(S)ANDADDRESS(ES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORINGAGENCYNAME(S)ANDADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-523-23-2U-00

8. PERFORMINGORGANIZATION

REPORTNUMBER

E-I1331

10. SPONSORING/MONITORING

AGENCYREPORTNUMBER

NASA TM--1998-208505

11. SUPPLEMENTARYNOTES

Prepared for the 193rd Meeting of the Electrochemical Society, Symposium on High Temperature Corrosion and Materials ChemistD' sponsored by the Electrochemical Society, San Diego, California, May 3-8, 1998. Dongming Zhu, Ohio

Aerospace Institute, Cleveland, Ohio; Louis J. Ghosn, Case Western Reserve University, Cleveland, Ohio; and Robert A.

Miller, NASA Lewis Research Center. Responsible person, Dongming Zhu, organization code 5160, (216) 433-5422.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 23, 26, and 27 Distribution: Nonstandard

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

12b. DISTRIBUTIONCODE

13. ABSTRACT(Maximum200 words)

Thermal barrier coating (TBC) durability is closely related to design, processing and microstructure of the coating

systems. Two important issues that must be considered during the design of a thermal barrier coating are thermal expansion and modulus mismatch between the substrate and the ceramic layer, and substrate oxidation. In many cases, both of

these issues may be best addressed through the selection of an appropriate bond coat system. In this study, a low thermal

expansion and layer-graded bond coat system, that consists of plasma-sprayed FeCoNiCrAI and FeCrAIY coatings and a

high velocity oxyfucl (HVOF) sprayed FeCrA1Y coating, is developed to minimize the thermal stresses and provide

oxidation resistance. The thermal expansion and oxidation behavior of the coating system are also characterized, and the

strain isolation effect of the bond coat system is analyzed using the finite element method (FEM). Experiments and finite

element results show that the layer-graded bond coat system possesses lower interfacial stresses, better strain isolation and

excellent oxidation resistance, thus significantly improving the coating performance and durability.

14. SUBJECTTERMS

Thermal batTier coatings; High temperature oxidation; Laser thermal gradient cycling;

Edge effect; Finite element method

17. SECURITYCLASSIFICATION

OF REPORT

Unclassified

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Unclassified

[15. NUMBEROF PAGES

27

16. PRICECODE

A03

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I',[ !I