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MECHANICAL TESTING OF ADVANCED

COATING SYSTEM

VOLUME 1

By

T. A. Cruse

A. Nagy

C. F. Popelar

FINAL REPORT

SwRI Project No. 06-1778-001

Cooperative Agreement No. NCC 3-89

Prepared for

National Aeronautics and Space AdministrationLewis Research Center

Cleveland, Ohio

June 1990

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C':CL. _ _.C

SOUTHWEST

SAN ANTONIO

WASHINGTON, DC

RESEARCH INSTITUTE

HOUSTON DETROIT

DALLAS/FT. WORTH

https://ntrs.nasa.gov/search.jsp?R=19910004221 2018-08-23T18:46:36+00:00Z

MECHANICAL TESTING OF ADVANCED

COATING SYSTEM

VOLUME 1

By

T. A. Cruse

A. Nagy

C. F. Popelar

FINAL REPORT

SwRI Project No. 06-1778-001

Cooperative Agreement No. NCC 3-89

Prepared for

National Aeronautics and Space AdministrationLewis Research Center

Cleveland, Ohio

June 1990

APPROVED: _

Ulric S. Lindholm, Vice President

Engineering and Materials Sciences

TABLE OF CONTENTS

3

4

5

6

7

8

9

10

List of Tables

List of Figures

Acknowledgments

EBPVD Coating Constitutive Tests

2.1 Specimen Design

2.2 Test Setup

2.2.1

2.2.2

2.2.3

2.2.4

Compression Setup

Tension SetupTest Matrix

Correlation of the Strain Measurements

2.3 Data Reduction

2.3.1

2.3.2

2.3.3

2.3.4

Composite Specimen ModelData Reduction Procedure

Compression Data

Tension Data

2.4 Data Interpretation

2.4.1 Compression Data2.4.2 Tension Data

2.4.3 Creep Data

References

EBPVD Tables and Figures

Appendix A: Elastic Bimaterial Model

Appendix B: Failed Specimen Photographs

Appendix C: Computer Plots of Compression Tests

Appendix D: Computer Plots of Tension Tests

Appendix E: Computer Plots of Creep Tests

Appendix F: MS-DOS ASCII Data Files (Floppy Disks)

e.ag °°,

Ul

iv

1

2

2

2

2

3

3

4

4

4

7

8

8

8

8

10

12

13

14

5O

54

66

87

103

107

TBC-FIN1.DOC -ii-

LIST OF TABLES

Table

I

IT

HI

IV

V

Compression Test Matrix

Tensile Test Matrix

Ceramic Modulus in Compression versus Temperature for Unexposed

and Exposed TBC

Ceramic Modulus in Tension versus Temperature for Unexposed TBC

Creep Rate Data Compression Tests

15

16

17

18

19

TBC-HN1.DOC -iii-

LIST OF FIGURES

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

Compression Specimen

Tensile Specimen

Biaxial Transducer

Compression Test Setup

Tension Test Setup

Schematic Compression Test Response

Schematic Tension Test Response

Compression Test Specimen

Schematic of Test Model

Linear Elastic Model Flow Chart

Substrate Modulus vs Temperature as Determined at NASA and SwRI

Deduced Ceramic Response m Compression for Unexposed Specimen18A at 75°F

Deduced Ceramic Response m Compression for Unexposed Specimen 19

at 1400°F

Deduced Ceramic Response m Compression for Unexposed Specimen21D at 1400°F


at 1800°F


at 1800°F

Deduced Ceramic Response m Compression for Exposed Specimen 30 at1000°F



Deduced Ceramic Modulus as a Function of the Test Temperature

eae,

20

21

22

23

24

25

25

26

26

27

28

29

30

31

32

33

34

35

36

37

TBC-FtNI.DOC -iv-

LIST OF FIGURES (CONCLUDED)

22

23

24

25

26

27

28

29

30

31

32

Deduced Ceramic Response in Tension for Unexposed Specimen 652801

at 72°F

Deduced Ceramic Response in Tension for Unexposed Specimen 652805at 72°F

Deduced Ceramic Response m Tension for Unexposed Specimen 653502

at 1400°F





Deduced Ceramic Response in Tension for Unexposed Specimen 653603

at 2000oF

Deduced Ceramic Response in Tension for Unexposed Specimen 652803at 2200oF



Deduced Ceramic Modulus as a Function of the Test Temperature

39

40

41

42

43

44

45

46

47

48

49

TBC-FIN1.DOC -v-

1 Acknowledgments

The authors appreciate the technical support of Dr. Robert A. Miller at NASA Lewis Research

Center, and also that of Pratt & Whitney Aircraft in supplying the test specimens. Mr. David Nissley,

Ms. Sue Manning, and Mr. Keith Sheffler at Pratt & Whimey are acknowledged for their technical

support and interaction on the test matrix.

TBC-F_I.DOC Page -1-

2 EBPVD Coating Constitutive Tests

2.1 Specimen Design

The EB-PVD coating material has ahighly columnar microstructure, and as a result it was expected

to have very low tensile strength. To be able to fabricate the required compression and tensile specimens,

a substrate was required to provide structural integrity for the specimens. Substmte and coating

dimensions were adjusted, based on data previously generated under NASA Contract NAS3-23944 for

plasma sprayed thermal barrier coating, to provide sufficient sensitivity to resolve the projected loads

carried by the EB-PVD coating.

The use of two distinctively different strain transducer systems, for tension and compression

loadings, mandated two vastly different specimen geometries. Compression specimens were of a short

tubular configuration, as shown in Figure 1_, with 0.705 or 0.660-inch inside diameters, 0.022 or

0.045-inch substrate wall thicknesses, and 0.085 or 0.075-inch coating thicknesses, respectively.

Specimen length for all compression specimens was 0.874 inch.

The tensile specimen geometry used in the program is shown in Figure 2. The long, tubular

configuration was required to accommodate internal mounting of the biaxial, capacitive transducer

shown mounted in a cutaway biaxial specimen in Figure 3. Offset of the gage section from the center

of the specimen was necessary to provide appropriate space for the sensing element (body) of the

transducer, as it should not be exposed to elevated temperatures. Substrate wall thickness for all tensile

specimens was in the 0.0050 to 0.0067-inch range with an EB-PVD coating thickness range of 0.050

to 0.105 inch. Although thinner substrates would be desirable, machining and coating practices did not

permit further reduction.

2.2 Test Setup

2.2.1 Compression Setup

Testing of the tubular compression specimens was performed using the setup shown in Figure 4.

The compression load wain consisted of two loading anvil assemblies mounted on the stationary load

cell and the hydraulic actuator shaft of a 22 KIP capacity MTS, servo hydraulic test machine. The

loading anvil assemblies were composed of 1-inch diameter SiC and AD995 rods mounted in tandem

with the SiC rods positioned in the hot zone, at the specimen-anvil interface. The AD995 rods were

supported in water-cooled, flanged collets. Parallelism between the SiC anvil faces was adjusted tobetter than 0.0005 inch.

Heating of the test specimens was accomplished using an Instron Model 3118-008 high

temperature short furnace with a 1.13-inch diameter by 3 1/2-inch long cylindrical working hot zone.

The specimen was sandwiched between the two SiC anvils, and the assembly was centered in the hot

section. Temperature deviation from the nominal within the specimen gage section was measured to

be less than _'F.

Strain measurements were made using an Instron Model 3118-151 capacitive strain transducer

with a working range of 0.080 inch. The transducer employs two, small diameter, alumina reachrods

to transfer the generated displacement to the sensing elements. The reachrods penetrate the high

temperature furnace through small, strategically located ports, creating minimal thermal disturbance.

1Figures and tables are given in Section 4.

TBC-FINI.DOC Page -2-

Contact by the reachrods to the load train was made on the loading anvils immediately next to the

specimen/load anvil interfaces, thereby minimizing measurement of extraneous strains. In addition to

the use of the extensometer, the room temperature tests were instrumented with strain gages to obtain

additional data on coating behavior.

Loading of the specimens took place under appropriate crosshead displacement control, which

produced an average strain rate of 2x10 _ sec 1.

2.2.2 Tension Setup

Testing of the tubular tensile specimens was performed using the test setup shown in Figure 5.

The setup utilized the 22 KIP MTS servo hydraulic test machine used in the compression tests. The

specimens were coupled to the test machine through plug type, pinned, superalloy extension rods, which

in turn were attached to pinned couplings at both ends of the specimen, as shown. Use of the pinned

couplings facilitated final alignment of the specimens.

Heating of the tensile specimens was again accomplished with the Instron high temperature short

furnace used in the compression test setup. The specimen gage section was centered in the hot zone of

the furnace, and the shanks of the specimens extended to the outside of the furnace, where they were

coupled to the extension rods. Additional insulation was provided around the extension rods to reduce

heat loss through the rods and thereby enhance generation of a more uniform temperature profile within

the gage section of the specimen. The maximum observed deviation from the nominal test temperature

within the gage section was + 1 I'F. Water cooling of the upper extension rod at its connection to the

pinned coupling was required to prevent heat conduction to the load cell of the test machine.

Strain measurements were made using a high temperature, biaxial, capacitive strain transducer

developed by SwRI. The transducer consists of two sets of capacitive sensing elements housed in the

body of the transducer. These elements are capable of measuring axial and torsional strains

independently. In these tests the transducer was used in the axial mode only. The transducer is mounted

inside the tubular test specimen with two sets of three mounting arms equipped with knife edge type

contact points, as shown in Figure 3. The arms are internally water cooled, permitting their use at high

temperatures. The arms exert sufficient contact force to support the transducer inside the specimen in

a stable, vertically suspended position. Initial positioning of the supporting knife edge contacts

establishes the starting gage length for strain measurements. The small water cooling lines and the

minim electrical cables of the transducer are routed through appropriately sized holes in the lower

shank of the specimen located above the pinned, extension rods. The biaxial transducer was used on

all tensile test specimens for strain measurements. On two of the tensile specimens, supplemental strain

measurements were made using the reachrod type Instron extensometer utilized in the compression test

setup. In addition, some of the room temperature tests were instrumented with strain gages to obtain

additional information regarding the behavior of the EB-PVD coating. This attempt appeared to be

unsuccessful because of premature debonding and possible localized reinforcing effect of the bonding

agent used in the installation of the strain gages.

2.2.3 Test Matrix

The test program was comprised of 15 compression and 18 tensile tests. All the tests were

conducted at an average strain rate of 2x10 4 sec "1. The test temperature range was between 75 and

2200"F. Identification numbers of the specimens tested, the type of specimen construction, such as

substrate, coated, etc., test modes and test temperatures, and relevant dimensional data for the

compression and tension specimens are summarized in Tables I and II, respectively.

"mC-Fn';r1.OOC Page -3-

As evidenced by the test matrices, baseline (substrate) data, in both compression and tension

modes, were generated for nearly all test temperatures, as permitted by specimen availability. Testing

of the coated specimens was divided between standard, monotonic compression and tension tests and

optional tests. Optional tests covered modulus probing, stress relaxation behavior, creep, and multiple

unloadingand reloadingpaths,asshown inTablesIand II.

2.2.4 Correlation of the Strain Measurements

To verifytheaccuracy ofstrainmeasurements usingthehigh temperatureInstronextensometer,

two of the compression specimens,No. 20 (thinwall substrate),and No. 18 (thinwall coated),were

instmrnentedwith foiltype straingages ina halfbridgeconfiguration.Straindataobtainedwith the

two measurement methods on the substratespecimen were ingood agreement. The coatedspecimen

exhibiteda slightlydivergingtrenduntilthe substratereached plasticvalues. Subsequent readings

trackedingood agreement.

During performance of the tensile tests, attempts were made to measure the relative strain behavior

of the substrate and the EB-PVD coating. Two tensile specimens, 652801 and 652805, were instrumented

with foil type strain gages. On specimen 653801, the swain gages were mounted with a light coat of

M-Bond 200 adhesive. The strain gages prematurely debonded during the test, and no useful data wereobtained.

On specimen 652805, the intended sites for the strain gages were prepared by filling the voids

between the columnar structure of the coating with AE-10 bonding agent until an even area was

established for gage mounting. The strain gages were again mounted with M-Bond 200. Relatively

early debonding was again observed during the loading process.

Strain values generated by the strain gages were approximately a factor of two lower than those

obtained by the extensometers. This difference in amplitudes may imply a localized reinforcing effect

by the additional bonding agent applied.

A custom built "spider" like clip gage was also mounted on the coated surface of the specimen.

This extensometer was selected over the high temperature Instron extensometer used on the compression

specimens because of its ability to measure larger strains. Readings between the biaxial extensometer,

mounted on the substrate, and the "spider" gage were in good agreement below the 0.005 in/in strain

amplitude. At approximately 0.005 in/in strain amplitude, the clip gage trace shows a sudden change

in continuity, implying a possible partial debonding of the coating.

The high temperature Instron extensometer was used on specimens 652803 and 653704 to monitor

strain behavior of the EB-PVD coating. As in the case of the strain gage measurements on specimen

652805, the strain values obtained on the coatings were approximately a factor of two lower than thosemeasured on the substrate.

2.3 Data Reduction

2.3.1 Composite Specimen Model

In order to deduce the mechanical response of the ceramic from the test data of the

ceramic/substrate birnaterial system response, an analytical procedure was developed. Typically, the

response of the substrate may be considered as elastic - perfectly plastic. In compression and in tension,

the bimaterial response is generally bilinear. In the compression tests, the onset of the bilinear response

is assumed to occur when the substrate becomes plastic, as shown schematically in Figure 6. However,


in thetensiontests,thebilinearresponseis attributedto crackinganddebondingof theceramic,afterwhichtheloadis assumedto becarriedby thesubstrate.A schematicof thetensiontestbehaviorisshownin Figure7.

Thededucedceramicresponsewhenthesubstrateis perfectlyplasticor theceramicdebondsistrivial. Staticequilibriumissufficienttodeterminetheloadsinbothcomponents.Whenbothmaterialsarelinearly elastic,however,the systemis staticallyindeterminate.Thus,it is necessaryto applycompatibilityconditions,inadditiontostaticequilibrium,to solvefor theindividualloadsin theceramicandsubstrate.Asthissolutionisnon-trivial,thepurposeofthissubsectionis todescribethedevelopmentoftheformulanecessarytodeducetheloadsandhencetheresponseoftheceramic.A detaileddescriptionof this developmentis givenin AppendixA.

Theassumptionsarethat(1)bothmaterialsarelinearlyelastic,(2)theaxialstrainsin theceramicandsubstrateareequal,(3)nodebondingoccurs,(4) thespecimendoesnotbuckle,and(5)themeasuredloadsandstrainsarenot influencedbyendeffects.Notethatin thefollowinganalysesanddiscussions,all loadsaretakenastensileandallpressuresareconsideredtobepositive.Themathematicalsymbolsusedin thederivationsaredefinedin AppendixA.

Thetestspecimen,shownin Figure8, is subjectedto anaxial load,Pr, and the corresponding

axial strain, _,, is measured. To deduce the individual stress state in each material, the ceramic andsubstrate were considered to be closed-ended, thick-walled cylinders [1,2] 2 whose state of stress is

described by

Pc pb 2 (la)o_ = _(c__ b_)+-(c__ b_)

c P b2 P c2b2 (lb)O_'-c__b2 r2(c2_b 2)

c P b2 P c2b2 (lc)t_aa- c2__ 2 + r2(c 2_ b 2)

and

_, _ Ps pb 2 (2a)_t(b2- a 2) + (b 2- a2--------_

• pb2a 2Oa,,- b 2-P_ba2+ r2(b 2 _ a 2)

(2b)

_ pb2a2 (2c)ors" b2-Pb_2 r2(b2-a 2)

for the ceramic and substrate, respectively.

2 References are given in Section 3.

TBC-FINI.IXX_ Page -5-

The elastic modulus and Poisson's ratio of the ceramic, denoted by Ec and Vo are generally

different from those of the substrate, denoted by Es and Vs. The resuk of these differences in the

mechanical properties is an "interfacial" pressure, p, which comes about with the application of an axial

load, shown schematically in Figure 9. During a compression (tension) test, the substrate tries to expand

(contract) radially more than the ceramic. Thus, there is a pressure (suction) at the ceramic/substrata

interface. This interracial pressure is related to the material properties (see Appendix A) through

vcEsAsPc - VsEcAcPs

P - x{EsAs[(1 - 2vc)b2 + (1 +Vc)C 2]+EcAc[(1-2vs)b2+ (1 +Vs)a2] }(3)

The pressure results in a triaxial state of stress for the ceramic and substrate, as is seen from Equations

(1) and (2).

Because the system is statically indeterminate when both materials are linearly elastic, it is

necessary to apply compatibility conditions, in addition to the equilibrium condition, to solve for theindividual loads in the ceramic and substrate. While the details of this work are given in Appendix A,

it can be shown that the load in the ceramic is related to the total applied load through

Pc = K(Ec)Pr (4)

where, for convenience, K(Ec) is defined as

K(Ec)b 2[EcAc(1 - 2vs) + EsA s(1 - 2Vc )]V,EcAc + D EcAc

DEsAs + b2[EcAc(1- 2Vs) + EsAs(1 - 2Vc)] (vsEcAc + vcEsAs) + DEcAc(5)

where

D = EsAs[(1 - 2vc)b 2+ (1 + Vc)C21+ EcAc[(1 - 2Vs)b 2 + (1 + Vs)a 2] (6)

It is possible (see Appendix A) to relate the ceramic modulus used in Equation (4) to the measured

axial strain and applied load by

_.__ Pr fK . _ VcEsAsK(Ec) (1 (7)

Although Equation (7) can be reduced to directly solve for the ceramic modulus, it was more convenient

to solve for the modulus by iterating on the ceramic modulus until the right-hand side of Equation (7)

agreed with the corresponding axial strain.

Thus, for a given axial strain and applied load, Equation (7) can be used to determine the ceramic

modulus, knowing the substrate modulus, Poisson's ratios for both the ceramic and the substrate, and

the specimen geometry. The ceramic modulus can then be used to calculate the axial load in the ceramic

by Equation (4). Equation (1) is used to calculate the triaxial stress state in the ceramic.

The technique described above is only valid when both materials remain linearly elastic. During

the compression tests, the substrate becomes plastic (taken to be perfect-plasticity). Conversely in the

TBC-FINI.IX:)C Page -6-

tensiontests,the substrateremainselastic,but theceramicundergoessevere damage in the form of

cracking and/or debonding. In either situation, the linear elastic approach is not valid and an additional

technique is required to analyze the test data.

For the compression tests, the substrate is assumed to become perfectly plastic for strains greater

than a critical strain, _,, after which the substrate is assumed to carry no further load than the load,

P_,,, corresponding to flow in the substrate. The system is then statically determinate. From static

equih'brium, the axial load in the ceramic is simply the difference between the total applied load and

the load in the substrate, P/_. Thus,

Pc=Pr-P_,,, (8)

Further, the interfacial pressure is assumed to be nonexistent. As a resuk, the state of stress becomes

uniaxial, as seen from Equations (1) and (2).

As was noted previously, cracking in and/or debonding of the ceramic occurs in the tension testing.

Because damage is not accounted for in the analysis, the linear elastic analysis is valid only to the onset

of severe damage to the ceramic. Presently, no technique is used to analyze the bimaterial response

beyond this point.

2.3.2 Data Reduction Procedure

The previous discussions described the development of the analytical technique used to deduce

the ceramic response from the ceramic/substrate bimaterial response. The following discussion describes

how to exercise the analytical technique to systematically reduce the experimental data (i.e., applied

load and corresponding axial strain data). A flow chart of this analytical procedure is shown in Figure10.

ha order to reduce the bimaterial response, specimen geometry and substrate material property

data are required. The substrate modulii in tension and compression were determined as a function of

temperature from tests conducted at SwRI. The data are shown in Figure 11. An "eyeball" fit to these

data is used to interpolate to intermediate temperatures. The substrate modulii are compared to apparent

modulii data derived from dynamic modulii data [3]. Poisson's ratios for the ceramic and substrate

were taken as 0.25 and 0.4, respectively. It is noted that Poisson's ratio was found to have only asecond-order effect on the deduced rest_onse.

The applied load and corresponding axial strain are also required inputs. If the axial strain is less

than the critical strain denoting the onset of plastic behavior in the substrate, an initial estimate of the

ceramic modulus is made. From this estimate, the right-hand side of Equation (7) is used to compute

an axial strain. If the computed axial strain does not reasonably agree with the measured strain, the

ceramic modulus is adjusted, and this iterative process is repeated until the strains agree. Once the

ceramic modulus is determined, the triaxial stress state is computed using Equation (1).

Should the axial strain exceed the critical strain denoting the onset of plasticity in the substrate,

the axial load in the ceramic is simply calculated by taking the difference between the applied load and

the load corresponding to plastic flow of the substrate. The uniaxial stress state is computed by ignoring

the interfacial pressure, as previously discussed.

TBC-HNI.DOC Page -7-

Hooke's law

1(9)

is used to relate the stress state of the ceramic coating to the axial strain. As it is convenient to plot

Hooke's law so that the slope of the plot is identically equal to the elastic modulus, o c -Vc(O c + o c) is

plotted against the axial strain in the ceramic.

2.3.3 Compression Data

The results of the compression testing of the exposed and unexposed ceramic/substrate bimaterial

specimens are shown in Figures 12-19. These datainclude the assumed and measured substrate response,the measured bimaterial response, and the deduced ceramic response. The substrate and bimaterial

responses are for uniaxial compression along the tube axis. It should be noted that the small dip in the

deduced ceramic response comes about from the difference between assumed and actual substrate

responses near the onset of yielding in the substrate.

The compressive ceramic modulus of the exposed and unexposed bimaterial specimens is given

as a function of temperature in Table FIT. Figure 20 shows these data as a function of test temperature.

2.3.4 Tension Data

The results of the tension testing for the unexposed ceramic/substrate bimaterial specimens are

shown in Figures 21-31. These data include the measured bimaterial response and the deduced ceramic

response. The bimaterial response is for uniaxial tension along the tube axis. As was previously noted,

the ceramic begins to crack and/or debond prior to yielding of the substrate. Thus, both materials are

assumed to be linear elastic throughout these calculations. The computations were carried only to the

point where the ceramic was thought to undergo severe damage.

The ceramic modulus in tension of the unexposed bimaterial specimens is given as a function of

temperature in Table IV. Figure 32 shows these data as a function of test temperature.

2.4 Data Interpretation

2.4.1 Compression Data

The substrate modulus test data are plotted in Figure 11, along with Pratt & Whitney-supplied

dynamic modulus data. Static data for the tension and compression tests were in good agreement with

each other at the various test temperatures, and slightly higher than the dynamic data. The test values

for the compression modulus at 1000"F are quite low, indicating a possible misalignment of the single

crystal axis relative to the test direction.

The ceramic modulus is inferred from the test records plotted in Figures 12-19 by taking a single

point fit to the bimaterial test curve at a point in the perceived linear regime. The analysis method of

Section 1.3 was then applied to derive the ceramic reponse below the yield strain of the substrate. The

resulting linear value of modulus is shown against each of the ceramic curves. As will be discussed for

each case, this approach was not always successful due to nonlinear behavior of the composite specimen

at low strains. Figure 20 plots the resulting ceramic modulus results versus temperature. Shifting linesfor some of the data occur as a result of applying engineering judgment to some of the test results. These

judgments will be presented below, for each test case.

"mC-FIN1.DOC Page -8-

Theapparentceramicmodulusat roomtemperatureseemsexceptionallyhigh comparedto thetrendline throughtherestof the data.TheRT testrecordhasananomaly,seenin Figure 12,whencomparedwith theothertestrecords.Thebimaterialstress-straincurveappearsto betrilinear, ratherthantheexpectedbilinearform. Theyieldpointof thesubstrateis setin this figureto thesecondbreakpointin thebimaterialresponsecurve.Theresultusingthefirst breakpointwasvisibly lesssatisfactoryin termsof thededucedceramicresponse,resultingin anincreasein themodulusof theceramicafterthebreak.The modulus fit used the two segments below the knee, as indicated. The resulting stress-strain

curve for the ceramic indicates a continued linear response, rather than a softening response. The second

linear region seems implausible. Most likely, the substrate modulus is far off, and the ceramic response

should be linear over all three of the linear portions of the test record. Use of only the last segment of

the ceramic stress-strain curve indicates a modulus much more consistent with the extrapolated value

from Figure 20, as shown by the shifting arrow on the data point.

This RT data interpretation is more consistent with the ceramic response at 1400°F in Figure 13,

where the tangent to the ceramic curve does not show apronounced bilinear break from elastic to inelastic

response. If a straight line is fit to the deduced ceramic response below the cusp, this slope is still valid

above the cusp, for a limited amount of further straining. The 1400"F data in Figure 14 contains a much

longer cusp effect, and it seems that the initial low slope is not valid. Superimposing the two 1400"F

curves gives a better feel for the gradual loss in linear elastic response starting at about 1% strain.

The test data at 1800"F (Figure 15) are linear above the cusp, although the response is offset in

the stress direction, due to the data interpretation system. This is best seen by parallel shifting a

straight-edge below to above the cusp. The linear behavior appears to extend to about 2% strain. The

1800*F data in Figure 16 indicate that the ceramic did not take up load until after about 0.8% strain. If

the two curves are superimposed but shifted in the strain direction, one can again see a reasonable sense

of an initial linear portion of the curve, to about 2% strain, before the ceramic shows nonlinear softening.

The data in Figure 20 for 1800*F have been corrected by taking the fitting point for the data reduction

model to be above the indicated cusp level.

The exposed specimen at 1000"F in Figure 17 is fitted by a high initial elastic slope (see Figure

20). However, if we use the trend from the earlier figures, we might take the modulus to be the fit from

about 0.5% strain out to about 0.1% strain. This value of slope is more consistent with the trend line

drawn in Figure 20, again shown by a shift in the original interpreted test point.

The initial modulus of the exposed 1800°F data (Figure 18) appears low, compared with the trend

line. However, again consistent with the other 1800"F test data, if we take the elastic response from

about 0.9% strain for fitting a linear response up to about 2% strain, the modulus agrees with the

unexposed data. The exposed specimen response at 1800"F (Figure 18) may be very easily superimposed

on the unexposed data (Figure 16). The negligible difference between these two is a strong

contraindication of any effect of exposure, at least at the test temperature of 1800*F.

The 2000"F test data were limited to a very small strain range. The test record was linear. If the

bimaterial response model is applied, the data point shown in Figure 20 predicts a very low modulus

for the ceramic. However, if we assume, consistent with other high temperature test results, that the

ceramic is not active over this range, the test record modulus is reasonably close to the substrate modulus

at 2000*F data. Thus, we recommend using the trend line for this test condition.

TaC-Fn_rl.DOC Page -9-

The2200"Fdata(Figure19)alsoindicateaconsiderablestrainin thebimaterialspecimenbeforeanyeffectof theceramiccanbeseen.After about0.8%strain,theresponseis nearlyelasticwith aslighteffectof softening,dueto creepin thebimaterialsystem(bothconstituentscreeping).If thedataafter 0.8%strainareusedto fit a modulusresult, the datapoint appearsquite consistentwith theextrapolationof thetrendline in Figure20.

Theceramicresponsein compressionis judgedto belinearlyelasticup to strainlevelsof about1-3%strainwith anonsetof strainsofteningatthatpoint,up to 1800"F.Beyondthattemperature,thelinearstrainlimit dropsagainto about2%,apossibleindicationof rateeffectsdueto creep.Theactual,detailedresultsshowconsiderablescatteratlow strainlevels,andengineeringjudgmentisrequiredtogetconsistentmodulusresults.

Theonsetof strainsofteningis seento increasewith strainlevel,from about1%strainat RT toabout3%strainat 1800"F,priorto theonsetof significantrateeffects.A possibleexplanationfor thiscouldbethereducednotchsensitivityof thematerialatthehighertemperatures(seeninplasmasprayedTBC), leadingto lessfracturesensitivityof themicrodefects,in compression.

No effectof residualstresscanbeseenin thecompressiondata. Thestrainat whichthebreakin thesubstrateresponseis used for the bilinear curve is shown as an arrow head on the strain scale.

Except for the RTtest, the break point is on the order of 0.7-0.75% strain. If we use the likely explanation

of the high breakpoint for the RT test, this breakpoint is consistent with the others.

No conclusion should be drawn regarding the existence of a residual stress for the actual

component condition. The compressive specimen is relatively short, and the substrate is quite thin

(flexible) compared with the component. We believe that the residual stress effect for these conditions

is likely to be too small to detect, given the level of crudeness necessary in this data reduction system.

2.4.2 Tension Data

The tension testing was qualitatively more difficult to interpret than the compression testing data.

The simple reason, of course, is the lack of any significant tensile strength in the ceramic. However,

unlike the expectation prior to test, the ceramic does have some apparent tensile strength. The following

discussion will attempt to provide some interpretation of the tension test resuks.

In general, the tensile behavior of the ceramic was seen to be elastic, up to a fracture strain. At

that point the ceramic ceased to have axial load carrying capability, but did act to reinforce the substrate.

In fact, the substrate single crystal material was found to exhibit absolutely simple, pure slip at RT

without any indication of slip locali7ation. The strain capacity of the substrate was enormous under

these conditions. While of little design importance, the phenomenon of reinforcement was very striking.

The reinforcement may affect the response of the substrate above the point of fracture of the ceramic.

The tensile testing procedure was changed from the original proposal to better evaluate some of

the unique phenomena of the ceramic's tensile behavior. Based on the compression test results, we

concluded that exposure had negligible, if any, effect on the ceramic response. We also deduced that

above 1400"F, the ceramic indicated an increasing creep rate. This behavior would also be expected in

tension, but is not judged to be of great importance, given the very limited tensile strength of the ceramic.

TBC-FIN1.DOC Page -10-

Thededucedceramicresponseis only meaningfulout to thepoint of maximumpredictedload.At thatpoint,theceramicdoesfracture(physicalevidenceduringtesting),andtheloadcarryingcapacityof theceramicdecreases,probablyquicklyasthestrainis furtherincreasedandmorecrackingoccurs.Thetestdesigndid succeedin minimizingtheeffectof thesubstrateonthetensilebarstiffness,andthebulk of theinitial stiffnessresponseis dueto thetensilestiffnessof theceramic.

TheRT testingresultedbothin pronounced, ideal slip in the bimaterial specimen data (Figure

21), and in sudden fracture of the substrate (Figure 22). The deduced RT tensile modulus was judged

to be higher than the trend line (shown for the compression specimen data fit in Figure 32). The ceramic

fracture strain in Figure 21 is about 0.2% strain. In the Figure 22 result, the substrate may have fractured

prior to achieving fracture of the ceramic. After fracture of the ceramic, the tensile bar load is all beingcarried in the metal substrate. Due to ceramic load shedding during fracture, the substrate quickly

acquires enough load to yield.

The first test result at 1400*F (Figure 23) shows linear substrate response following failure of the

ceramic at about 0.2% strain. The second test record (Figure 24) indicates about the same general level

of strain tolerance (0.2% strain), but considerably less stiffness. Vertical shifting of the test results for

superposition indicates significant variability in substrate elastic response. The variability in the test

results in tension, shown for these two specimens and the others, may be due both to thickness variations

in these very thin metal substrates and to the possible effect of the thinness on elastic response of the

single crystal.

The 1600*F data (Figure 25) clearly indicate linear substrate response with the fracture strain of

the ceramic at about 0.1% strain. However, the substrate modulus indicated in the extended elastic

response curve shows a value well below that expected for 1600*F. The low substrate modulus is

consistent with overpredicting the ceramic response.

The 1800"F data in Figure 26 indicate the point of ceramic fracture to be about 0.08% strain. The

second test result (Figure 27) probably has no ceramic strength in spite of what the data reduction system

shows. The modulus of the substrate is again probably too low, resulting in an inference that the ceramic

is carrying load. However, the linear response all the way to about 0.8% strain indicates that this is the

response of the substrate, not the ceramic.

The fracture strains of the ceramic at 2000°F and 2200"F are probably positive, but so small that

they are hard to record reliably (note that Figure 28 is plotted at a very high sensitivity). The data

interpretation problem is compounded by the increased creep effect in the substrate at these temperatures.The best estimate in the fracture strain comes from the 2200°F record, where the maximum tensile

ceramic strain seems to be about 0.025% strain. The creep in the substrate results in an apparent ceramic

hardening above the fracture strain as deduced by the data reduction model, but this resuk is not judged

to be real.

The reducing trend in the tensile fracture strain of the ceramic is probably an effect of the residual

stresses. As manufactured, the tensile specimens will lock in a tensile stress in the substrate and a

compressive stress in the ceramic. Due to the very thin substrate used, the load sharing between the

two is significant. If we take the stress-free temperature to be above 2200°F (assuming zero tensile

strength of the ceramic), the predicted residual strain in the ceramic is about 1.4%.


Thetensiletestingprogram included planned load reversals at various strain levels to evaluate

whether the ceramic fractures would close, causing a re-stiffening of the bimaterial specimen. The full

cyclic test records for these tests are shown in Appendix C. The conclusion from reviewing these test

records is affirmative, in that as the strain is reduced to the range of the apparent ceramic fracture strain,

the specimens exhibited higher stiffness, consistent with the stiffness prior to ceramic fracture. The

data also indicate, as expected, that creep in the substrate negates this closure effect unless one were to

drive the specimen into compression (this was not possible in the current loading arrangement). We

therefore conclude that the mechanism of ceramic failure is consistent with reducing the stiffness to

zero above a critical strain level, but requires a bilinear model to account for closure effects as strain is

reduced below the critical value. Creep growth of the substrate must be included to obtain the correctinteraction with ceramic crack closure.

The tensile modulus data have considerable scatter, owing to the extreme sensitivity of the data

acquisition system for the bimaterial specimen. However, the data indications are consistent with the

data obtained in compression. The failure mode in tension appears to be brittle fracture above a small

strain, and the fracture strain may be strongly dependent on the stress-free temperature effect and creepof the substrate.

2.4.3 Creep Data

Three compressive, sustained load, creep tests were performed on Specimen 21, at three

temperatures: 1400"F, 1600"F, and 1800"F. All three tests were performed at load levels below the

proportional limit of the compression tests performed at the respective temperature levels. Using the

fitted ceramic elastic modulus data from Figure 20 and the applied load value, the elastic stresses in the

ceramic were 11037 psi, 8880 psi, and 7611 psi, respectively.

A viscoplastic model for the composite system is recommended to obtain the correct flow stress

in each material. The reason is that the creep strain in each material is the same as the composite creep

strain. The stress in each material is then that stress which will produce the resulting strain rate and

satisfy equilibrium with the applied load. It can be expected that this stress will not be the same as the

elastic stress, given above. Therefore, before final use of the strain rate versus stress for the ceramic,

it is recommended that Pratt & Whitney apply the viscoplastic model for PWA 1480 at the strain rates

given in Table V, predict the PWA 1480 stress level, and determine a corrected ceramic stress that

satisfies equilibrium with the applied load.

The creep tests show a clear primary creep transition and approach a steady creep rate for each

temperature. The creep rate data in Table V were obtained by a simple, estimated linear fit to the final

portion of the strain versus time plot for these tests, for which the data are given in Appendix D. A

preliminary comparison of the EB-PVD creep rate at 1800"F with that for plasma sprayed coating

(PWA264) indicates that the current TBC is more creep resistant than the plasma sprayed coating.

TBC-F_N1.DOC Page -12-

3 References

1. Timoshenko, S. P., and Goodier, J. N., _ of _, 3rd ed., McGraw-Hill Book Co.,

New York, 1970, pp. 69-71.

2. Boresi, A. P., and Sidebottom, O. M., Advanced Mechanics of _, 4th ed., John Wiley

and Sons, New York, 1985, pp. 492-500.

3. Private Communication (FAX re Elastic Modulii for PWA 1480 SC), Mr. David Nissley, Pratt

& Whimey, 31 January 1990.


4 EBPVD Tables and Figures

"rBc-FINt.DOC Page - 14-

TABLE I

COMPRESSION TEST MATRIX

SpecimenNumber

17

18

19

20

21

24

25

26

27

28

30

31

32

33

34

Specimen

Type

Substrate

Coated

Coated

Substrate

Coated

Exp-CoatedCoated

Coated

Coated

Exp-Coated

Exp-Coated

Substrate

Substrate

Substrate

Substrate

Test

Mode

Compr

Compr

Compr

ComprStr Relax

Creep

Creep

Creep

Compr

Compr

Compr

ComprModulus

Modulus

Modulus

Modulus

Modulus

Modulus

Compr

Compr

Compr

Compr

Compr

Compr

Temp

(oF)

1400

RT

1400

RT

1400

1400

1600

1800

1400

1800

1800

1800

400

800

1000

1200

1600

2000

2200

1000

2200

1800

1000

1000

SpecimenInside

Diameter

(Inch)

0.7046

0.7060

0.7061

0.7040

0.7056

0.6598

0.6598

0.6598

0.6599

0.6597

0.7060

0.6596

0.6599

0.6590

0.7005

Ceramic

Thickness

(inch)

0.085

0.085

0.085

0.069

0.082

0.082

0.074

0.076

0.077

Substrate

Wall

Thickness

(inch)

0.022

0.022

0.022

0.022

0.022

0.045

0.045

0.045

0.045

0.045

0.022

0.045

0.045

0.045

0.022

TBC-bXNI.DOC Page -15-

TABLE II

TENSILE TEST MATRIX

SpecimenNumber

I

653601

653605

653701

653702

652801

652805

652902

653503

653504

653602

653603

653704

652802

652803

653502

Specimen

Type

Substrate

Substrate

Substrate

Substrate

Coated

Coated

Coated

Coated

Coated

Coated

Coated

Coated

Coated

Coated

Coated

Test

Modei ,

Standard

Standard

Standard

Standard

Standard

Standard

Standard

Standard

Standard

Standard

Standard

Standard

Optional

Optional

Optional

Temp

(°F)

RT

1400

1800

2200

RT

RT

1400

1800

2200

1400

2000

1600

1800

2200

1400

SpecimenInside

• Diameter

(Inch)

0.7506

0.7518

0.7515

0.7524

0.7510

0.7505

0.7507

0.7502

0.7517

0.7499

0.7527

0.7517

0.7525

0.7510

0.7530

Interface

Diameter

(Inch)

N/AN/A

N/A

N/A

0.7636

0.7637

0.7641

0.7630

0.7647

0.7633

0.7655

0.7641

0.7659

0.7636

0.7630

I P....

Outside

Diameter

(Inch)

0.7632

0.7652

0.7649

0.7652

0.8690

0.9620

0.9510

0.9535

0.9495

0.9530

0.9520

0.9422

0.9752

0.9525

0.8635


TABLE III

CERAMIC MODULUS IN COMPRESSION VERSUS TEMPERATURE

FOR UNEXPOSED AND EXPOSED TBC

Specimen Temperature Ceramic Modulus

ID (°F) (xl06 psi)I

Unexposed

18A 75 6.96

27A1 400 3.50

27B1 800 2.82i i i

27C2 1000 2.44

27D2 1200 2.52

19 1400 2.31[ II

21D 1400 2.50

21E 1400 5.40 <I)

27E2 1600 2.12

25 1800 2.48

26 1800 0.373i

27F2 2000 0.544

Exposed

30 1000 3.79

24 1800 0.957

28 2000 0.c2)

(1) Specimen was a second test of 21D.

(2) Ceramic modulus is very small.


TABLE IV

CERAMIC MODULUS IN TENSION VERSUS

TEMPERATURE FOR UNEXPOSED TBC

Specimen Temperature Ceramic ModulusID (°F) (xl06 psi)

Unexposed

652801 72

652805 72

653502 1400

6.15

6.12

4.92

653602 1400 0.770

652902 1400 0.874

653704 1600 4.10

653503 1800 4.28

652802 1800 0.607

652803 2200 2.08

653504 2200 2.03


TABLE V

CREEP RATE DATA

COMPRESSION TESTS

Temperature

(*F)Load

(Pounds)

6110

Strain Rate

(in/in/sec)

1400 2.4E-8

1600 5020 5.6E-8

1800 4250 8.3E-8


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"I'BC-Fn_I.DOC Page -25-

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5 Appendix A: Elastic Bimaterial Model

The following is a detailed derivation of the linear elastic model used to deduce the ceramic

response from the ceramic/substrate bimaterial system. Table A-1 defines the mathematical symbols

used in the derivation that follows.

Table A-1

Definition of Mathematical Symbols

Symbol Definition

a,b, c Inside, interface and outside radius, respectivelyk

Ac,As Cross-sectional area of the ceramic and substrate, respectively

uc, Us Radial displacement of the ceramic and substrate, respectively

e,c, _ Axial strain in the ceramic and substrate, respectively

Pr,_,, Total applied load and corresponding axial strain

Pc,Ps Axial load in the ceramic and substrate, respectively

Ec,Es Elastic modulus of the ceramic and substrate, respectively

Vc,Vs Poisson's ratio of the ceramic and substrate, respectively

o c, os,, Axial stress in the ceramic and substrate, respectively

oC_,oS,, Radial stress in the ceramic and substrate, respectively

o_,ffsoo Circumferential stress in the ceramic and substrate, respectively

_, P,o,, Critical strain denoting plastic substrate behavior and the corresponding

axial load

Figure 8 defines the cross-sectional area of the ceramic and substrate which are given by

A c = _c 2 - b 2) (A - la)

As = _(b 2 - a 2) (A - lb )

From the thick-walled cylinder solution [1,2], the radial displacements of the ceramic and substrate

are given by


us -rl-2vs)(-pb2)+(l+--v2s)b2a2(-p)-vs_1Es(b__a2 ) (1 r _ .J (A -2b)

The radial displacements at the ceramic/substrate interface are given from Equations (A-2a) and (A-2b)

by taking r = b. Thus,

b 1

b [ 1-2Vs)(-pb')+(1 +Vs)(-pa =)-_S1us =Es(b__a_)m ( s _ j

(A _ 3b)

Assuming no debonding at the ceramic/substrate interface, the radial displacements must be equal.

Equating Equations (A-3a) and (A-3b) and solving for the interfacial pressure, p, gives

VcEsAsPc - VsEcAcPs

P = _{gsAs[(1 - 2vc)b2+ (1 + Vc)C2] + EcAc[(1 - 2vs)b2 + (1 + Vs)a2] } (A - 4)

The axial strains from 3-D Hooke's law are given by

1 c= +o ,)I

x _v+(o_+_)1=gt4,

The normal stresses in the ceramic are given as

Pc pb 2

(_ = =(c'- b")+-(c_- O2)

c Pb 2 Pc 2b2

C,, = c2_ b2 r2(c 2_ b2 )

c Pb _- + PC 2b2o_- c__-_ :(c__b2)

Similarly, the normal stresses in the substrate are given as

(A -5a)

(,4-5b)

(A-6a)

(,4-6b)

(A -6c)


Ps pb 2

O_: - _(b2_ a2 ) + (b2_ a2) (A - 7a)

<"-,;":a'-"r',7;:"a"> (A -7b)

Substituting Equations (A-6) and (A-7) into Equations (A-5a) and (A-5b), respectively, gives

1e,c = [Pc + _1 - 2Vc)pb 2] (A - 8a)

= _ Its - _1 - 2Vs)pb 2] (a - 8b)

Noting that the axial strain in the ceramic must equal the axial strain in the substrate, equating

the axial strains given in Equations (A-8a) and (A-8b) and solving for the load in the ceramic yields

( E,:A_'_

2 s)p*J- 2 )pb

Substituting Equation (A-4) into (A-9) and simplifying gives

1

Pc - DEsAs{DEcAcPs + b2[EcAc( 1 - 2Vs)

where, for convenience, D is defined as

+ EsAs(l - 2vc)] (VsEcAcPs - VcEsAsPc)}

D = EsAs[(1 - 2Vc)b2+ (1 + Vc)C2]+ EcAc[(1 - 2vs)b2 + (1 + vs)a 2]

(A -9)

(A - 10)

(A- 11)

Pr= Pc + Ps (A - 12)

From equilibrium,

Or, equivalently

P_ =P_-P_

Substituting Equation (A-13) into (A-10), it can be shown that

Pc = K(Ec)Pr

where

b2[EcAc(1 - 2Vs) + EsAs(1 - 2Vc)]V,EcAc + D EcAc

(,4 -13)

(A- 14)

(,4 - 15)D EsAs + b 2[EcAc(1 - 2Vs) + EsAs (1 - 2Vc )] (vsEcAc + VcEsAs ) + D EcA c

K(Ec) -


Finally, substituting Equations (A-4), (A-13) and (A-14) into Equation (A-8a) yields

ec_ Pr tK(Ec)+b_VcEsasK(Ec)-VsEcac(a-K(Ec))](X-2Vc)}_cAc t D

(A - 16)

TBC-FI_I.EX_C Page -53-

6 Appendix B: Failed Specimen Photographs

"mC-FIN1.1:XXZ Page -54-

COMPRESSION SPECIMEN NO. 17, PHOTO NO. 40668


TBc-Fn_I.DOC Page-55-


#2O


TBC-_:NI.DOC Page -56-



TBC-F_tI.DOE Page-57-



TBC-IKN1.DOC Page -58-


TBC-FIN1.DOC


Page -59-

COMPRESSION SPECIMFM NO. 30, PHOTO NO. 40674

31


TaC-_I.tX3C Page -60-



TBC-F_.DOC Page -61-

COMPRESSIONSPECIMENNO. 34,PHOTO NO. 40677

TaC-F_I.DOC Page -62-

TENSIONSPECIMENNO.652801,VIEW NO. 1,PHOTONO.40731

TENSIONSPECIMENNO. 652801,VIEW NO.2, PHOTONO.40732

TBC-Fn_I.DOC Page-63-

TENSION SPECIMEN NO. 652801, VIEW NO. 3, PHOTO NO. 40733


I13C-FENLDOC Page-64-



"mC.FINI.IXXZ Page -65-

7 Appendix C: Computer Plots of Compression Tests

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