NASA / TMm2002 -211873 A IAA-2002 -3731

A Multilayered Thin Film Insulator for

Harsh Environments

John D. Wrbanek and Gustave C. Fralick

Glenn Research Center, Cleveland, Ohio

Charles A. Blaha

Akima Corporation, Brook Park, Ohio

A. Rachel Busfield

Rolls-Royce plc, Derby, United Kingdom

Valarie D. Thomas

Ohio Aerospace Institute, Brook Park, Ohio

September 2002
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NASA / TM--2002-211873 AIAA-2002-3731

A Multilayered Thin Film Insulator for

Harsh Environments

John D. Wrbanek and Gustave C. Fralick

Glenn Research Center, Cleveland, Ohio

Charles A. Blaha

Akima Corporation, Brook Park, Ohio

A. Rachel Busfield

Rolls-Royce plc, Derby, United Kingdom

Valarie D. Thomas

Ohio Aerospace Institute, Brook Park, Ohio

Prepared for the

38th Joint Propulsion Conference and Exhibit

cosponsored by AIAA, ASME, SAE, and ASEE

Indianapolis, Indiana, July 7-10, 2002

National Aeronautics and

Space Administration

Glenn Research Center

September 2002
Acknowledgments

The authors would like to acknowledge the assistance of Drago Androjna (Akima Corporation) and Dorothy Lukco

(QSS Group, Inc.) for the EDAX and Auger analysis of the samples. This work was pursued under the Information

Rich Test Instrumentation (IRTI) project, part of NASA's Advanced Space Transportation Program (ASTP). The

authors would also like to acknowledge the support of the management of NASA Glenn Research Center,

together with Rolls-Royce and Allison Advanced Development Company, for arranging this collaboration.

Additional funding was provided by Rolls-Royce and the Royal Academy of Engineering.

This report is a formal draft or working

paper, intended to solicit comments and

ideas from a technical peer group.

This report contains preliminary

findings, subject to revision as

analysis proceeds.

The Aerospace Propulsion and Power Program at

NASA Glenn Research Center sponsored this work.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Available electronically at http://gltrs.grc.nasa.gov
A MULTILAYERED THIN FILM INSULATOR FOR HARSH ENVIRONMENTS

John D. Wrbanek and Gustave C. Fralick

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Charles A. Blaha

Akima Corporation

Brook Park, Ohio 44142

A. Rachel Busfield

Rolls-Royce plc,

Derby DE24 8B J, United Kingdom

Valarie D. Thomas

Ohio Aerospace Institute

Brook Park, Ohio 44142

Abstract

The status of work to develop a reliable high

temperature dielectric thin film for use with thin film

sensors is presented. The use of thin films to

electrically insulate thin film sensors on engine

components minimizes the intrusiveness of the sensor

and allows a more accurate measurement of the

environment. A variety of insulating films were

investigated for preventing electrical shorting caused by

insulator failure between the sensor and the component.

By alternating layers of sputtered high temperature

ceramics, a sequence of insulating layers was devised

that prevents pinholes from forming completely through

the insulator and maintains high electrical resistivity at

high temperatures. The major technical challenge

remaining is to optimise the fabrication of the insulator

with respect to composition to achieve a reliable high

temperature insulating film. Data from the testing of

various potentially insulating thin film systems is

presented and their application to thin film sensors is

also discussed.

Introduction

As part of NASA Glenn Research Center's mission to

develop critical technologies that enable safer, lighter,

quieter, and more fuel efficient vehicles for aeronautics

and space transportation, the Sensors and Electronics

Technology Branch has an effort to develop thin film

sensors for surface measurement in propulsion system

research. The sensors include those for strain,

temperature, heat flux and surface flow.

Rolls-Royce plc provides power systems for air and

marine propulsion and industrial applications. The

Instrumentation Engineering department is developing

thin film sensor technologies to support the company's

drive for designing and manufacturing systems having

increased safety and durability while minimizing noise

and emissions.

The use of thin film sensors has several advantages

over wire or foil sensors. The thin film sensors do not

require special machining of the components on which

they are mounted, and, with thickness less than 10 _tm,

they are considerably thinner than wire or foils. The

thin film sensors are thus much less disturbing to the

operating environment, and have a minimal impact on

the physical characteristics of the supporting

component. Several thin film sensors have been tested

up to 1100 °C in different engine conditions at NASA

Glenn Research Center. _'-"

Fabricating metal sensors directly on metal components

requires the use of an electrically insulating layer

between the component and the sensor. Flame-sprayed

insulators provide good insulating capability, but the

thickness of the coatings (300 pm and greater due to the

coating porosity) lessens the advantage of the thin film

sensor. NASA Glenn Research Center has been

examining the use of thin film insulators as an

alternative. One process developed utilizes a 125 lam

thick NiCoCrAIY base coat on which alumina can be

• "_

thermally grown and vapor-deposited. Rolls-Royce

thin film sensors use a reactively-sputtered layer of

alumina to provide the electrical insulation.

NASA/TM--2002-211873 I
The applicationof alternativethin film dielectric

insulatorshasbeenexaminedto furtherminimizethe

insulationthickness.However,ceramicslosetheir

electricalinsulationexponentiallywith increasing

temperatures. Shortcircuitpathsat grainboundaries

andfilm defectscanfurtherreducethe insulation

propertiesfromthebulkvaluesathightemperatures.

Tofurtherthisresearch, NASAGlennResearchCenter

andRolls-Roycehavepursuedajointinvestigationof

utilizingmultilayeredthinfilmdielectricsasareliable

insulatorforuseinharshenvironments. Theuseof a

multilayeredschemeisthoughttobepromisingforthe

fabricationofelectricallyinsulatingthinfilms.A major

causeof conductionin thin film dielectricsis the

presenceof defects,suchaspinholes,thatpropagate

clearthroughthe film to the underliningsubstrate

surface.By alternatingtheinsulatingmaterial,a new

growthpatternwouldeliminatethepreviousone.Thus,

directpathwaysfor conductionto thesubstrateare

eliminated.Therequirements forsuchaninsulatorare

drivenbytheenvironmentsof aerospaceapplications,

whichcurrentlyrequireinsulationof 100k_ orbetter

to1100°Cinoxidizingconditions.

Sample Preparation

The film depositions were conducted in the

Microsystems Fabrication Cleanroom facility at NASA

Glenn Research Center. Several samples of multilayered insulators were generated using alumina and

stainless steel shims as the test substrates. The alumina

substrates provide readily available test beds that

remain intact for the severe heating required of these

films, up to 1700 °C if necessary. The stainless steel

substrates can be annealed to 1000 °C, and the thermal

expansion of the steel should produce stresses similar to

those that the film would experience on an engine

component at that temperature.

After surfactant cleaning, the test shims were rinsed in

acetone and then methanol. A schematic of the typical

layer sequence of the test samples is illustrated in

Figure 1. A layer of platinum I /am thick was sputter

deposited over the shims. On the alumina substrates,

the platinum base coat insured the detection of a

conduction path if one were to exist in the insulator.

On the stainless steel substrates, the platinum base coat

assisted in the bonding of the dielectrics to the metal.

The dielectrics were then deposited with a total

thickness of 5 pare. On top of the dielectric, several

4.75 mm diameter platinum pads were sputter deposited

to simulate thin film sensors. The pads were deposited

5 pm thick to insure that any cracks or pinholes were

sufficiently filled to reveal any conduction paths. A

room temperature resistance check was then performed

to verify insulating capability of the sample.

Lead wires were connected to the platinum pads on the

insulating samples to afford high temperature testing in

an annealing oven. The platinum wires were parallelgap welded to the pads, which were then soldered

to leads outside of the test oven. The oven was ramped

at approximately 450 °C/hour while the resistance

between the pads and the temperature measured by a

type K thermocouple near the sample was monitored.

The maximum resistance readable was 100 MO. One

cycle was performed to stabilize the sample, and the

data from the second ramp was used for insulator

evaluation.

Test Shim Substrate

Figure l.--Schematic of Insulator Test Sample

Analysis

A lettering scheme was used to identify specific

samples of the study. The results of our sample tests

are shown in Table 1. The runs were lettered as they

were planned, and, as failures and successes dictated.

some runs were skipped. The first three runs of

samples set the baseline for the work.

The samples from run A had a single layer of 5 lam

alumina (A1203) sputter deposited at room temperature.

The samples indicated a short at room temperature,

verifying the need for a more complex insulating thin

film structure to achieve the required insulating

capabilities for a 5 _am film.

The next samples (run B) tested used an underlying

layer of 1 pm thick silica (SiO2) sputter deposited, with

an upper layer of 4 pm alumina sputter deposited at

room temperature. These samples showed insulating

properties at high temperature, with the sample on

stainless steel several orders of magnitude more

conducting than that on alumina. Both samples showed

significant cracking after heat treatment. The samples

NASA/TM--2002-211873
alsoshoweda severehysteresisn theirresistance

curvesduringthe heat cycle. Despitethe poor

insulationproperties, therunvalidatedtheconceptof

usinga multilayerstructureto eliminateconduction

pathsto thesubstratehatcausedtheshortin runA

samples.

Samplesin runCwerecomposedof abottomlayerof

2 lamalumina,followedby 1 pamof silica,andthen

coveredwith2 _amof aluminasputterdepositedon

stainlesssteelshims.Thesesamplesshowedproperties

similartothoseof therunBsamplesonstainlesssteel,

butwithmoreseverecrackingof thesurface.Figure2

showsthecrackingseenonatypicalsamplefromrunC

onstainlesssteel.

yttria-stabilized zirconiafollowedby4lamofe-beamed

alumina.Theplatinumbasecoatwasnotdepositedfor

runR sincethechromiumcarbideprovedto bean

excellentbondcoat betweenchromium-containing

metalsandoxideceramics.Figure3showsthepinholefreesurfaceof a runR sampleafterheat-treatingto

800°Cona steelsubstrate.Thetemperaturelimitof

bothinsulatorsfor 100kf_of resistanceappearstobe

about1000°Cforthe5pamofmaterialdeposited. The

zirconia-aluminainsulatorwasdepositedon a steel

substratein runT, butthealuminafailedbeforeany

datacouldbegathered.

ThedatafromthetemperaturerampsofrunsB,C, R,

andSareplottedinfigure4. To giveaquantitative

comparisonof the insulators,thedatawerefit to

exponential decaycurves.Theresultsofthefittingsare

givenintable2.whichgivestheestimatedO°Cvalues.

decayconstants,mlmmumuse temperaturesand

uncertainties.

i

Figure 2.--Cracking on the surface of sample from Run

C (A1203-SiO_,-AI203 Multilayer) after heat

cycling to 500 °C. The width of the image is

approximately 525 Fam.

Several samples were prepared to further investigate the

cause of the cracking. EDAX analysis of an annealed

silica sample revealed that chromium and iron from the

steel were migrating into the cracks formed in the films

when heated. The deposition of the alumina by e-beam

vaporization at 800 °C resulted in room temperature

shorts. Surface preparation was altered to smooth the

stainless steel by H2SO4/H20_, cleaning. However, the

poor film adhesion to the substrate surface and the high

conductivity of the resulting film terminated this path.

Two alternatives to using silica as an underlying layer

were tested and the results were promising. Run R used

1 lam of sputtered chromium carbide followed by 4 _am

of e-beamed alumina, and run S used 1 lam of sputtered

Figure 3.--Pinhole-free surface of sample from Run R

(CrsC2-AI20_ Multilayer) after heat cycling

to 800 °C. The width of the image is approximately 140 pam.

Conclusions

The results of this study indicate that the CTE

mismatch between silica and metals inhibits its use as

an effective thin film insulating layer or under layer for

alumina. The CTE's of various materials used in these

tests are given in the bulk properties listing in table 3.

It is clear from these tests that a silica under layer does

not expand freely enough to be used in a thin film

NASA/TM--2002-211873 3
multilayerinsulatorsequenceon metals,particularly

whene-beamdepositingthealuminat800°C. Even

followinga500°Cannealingcycle,thesilicacracking

is sufficientto permitchromiumandironmigration

fromthesteelintothethinfilm insulation.In fact,

underthe800°Ce-beamdepositiontemperatures ofthe

alumina,theCTEmismatchbetweenthesteelandthe

silicais sufficientto causecrackingin theinsulating

filmandadhesionfailures.

TheslightlylowerCTEof yttria-stabilizedzirconia

mayaccountforthestressesobservedin thealumina

thinfilm whenthelayerswereappliedto thesteel

substratein runT. It shouldbenotedthatsincemost

enginecomponentsare "super-alloys"similar to

Hastelloyor lnconelwith CTE'sbetween10 and

13ppm/°C, 4 thezirconia-aluminamultilayermaybe

applicableforuseonthosematerials.

Thesuccessof run R is attributedto the graded

interface between the chromium carbide and the

chromium-containing metal. The intimate contact is

believed to induce chemical-level bonding between the

two materials. Thus, adhesion is promoted even at high

temperatures. Further, the CTE of chromium carbide is

intermediate between the alumina and the steel, thus

grading the thermal stresses that would be produced

during heating. Systematic Auger analysis of the

chrome carbide chemistry is ongoing.

Ultimately, the goal is to use the multilayer insulator on

a real engine component. The next challenge in this

investigation is to optimize the chromium carbidealumina multilayer on nickel-based "'super-alloy" test

shims for use on these engine components. Testing of

the zirconia-alumina multilayer on these test shims may

also give more insight on the thin film chemistry of

these multilayer insulators.

2.

3.

4.

5.

.

References

J.F. Lei, L.C. Martin, H.A. Will, "Advances in Thin

Film Sensor Technologies for Engine Applications,"

NASA TM-107418, Turbo Erpo "97, Orlando, FL,

June 2-5, 1997.

L.C. Martin, J.D. Wrbanek, G.C. Fralick, "Thin

Film Sensors for Surface Measurements," NASA

TM--200 I-211149, 19_ International Congress on

Instrumentation in Aerospace Simulation Facilities,

Cleveland, OH, August 27-30, 2001.

L.C. Martin, G.C. Fralick, K.F. Taylor, "Advances

in Thin Film Thermocouple Durability Under High

Temperature and Pressure Testing Conditions,"

NASA TM-- 1999-208812, January 1999.

CRC Handbook of Chemistry and Physics, edited

by R.C. Weast and M.J. Astle, CRC Press (1982).

S.W Bradstreet, "Oxide Ceramics,"

Temperature Materials and Technology, edited by

I.E. Campbell and E.M. Sherwood, The

Electrochemical Societ), Series, John Wiley & Sons,

New York (1967), p. 235-303.

J. H. Westbrook and E.R. Stover, "Carbides for

High-Temperature Materials," High-Temperature

Materials and Technology, edited by I.E. Campbell

and E.M. Sherwood, The Electrochemical Society

Series, John Wiley & Sons, New York (1967),

p. 312-348.

NASAFFM--2002-211873
Run

A

C

J

M&N

P&Q

R

S

T

Substrates

Alumina

& SS Shims

TABLE I.--RUNS AND MULTILAYER SAMPLES GENERATED IN THIS EFFORT

)lete resistance data for runs B, C, R and S are plotted in Figure 4.

Prep_ration Multilayer Result

Silica Polish 5 pm AI203

sputter deposited

1 pm SiO2 -

4 pm AI203

sputter deposited

2 pm AI203 -

1 pm SiO2 -

2 pm AI203

Alumina &

Silica Polish SS Shims

SS Silica Polish

Short @ RT

Film Cracks after 500 °C Heat Treatment.

Alumina Shim: 100 M_ @ 720 °C, 1.78 M_z @ 1000 °C

SS Shim: 56 M_2 @ 41 °C, 1 I_ @ 440 °C

Film Cracks after 500 °C Heat Treatment

100 M_z @ 17°C, 2 k&2@ 500 °C

sputter deposited

1 pm SiO2 EDAX after 800 °C anneal indicates substantial Cr & Fe

SS Silica Polish sputter deposited migratin9 from the SS into the cracks

1 pm SiO2 -

SS Silica Polish 4 pm AI203 Short @ RT

e-beam deposited

H2SOdH202 1 pm SiO2 - Film Adhesion Failures

4 pm AI203 Short @ RT

SS Cleaning e-beam deposited

H:SOJH20: 1 pm CrC - Good Insulator

4 pm AI203 84 M_z @ 690 °C, 20 M_ @ 750 °C SS Cleaning e-beam deposited

1 pm ZrO2/Y203 - Good Insulator

Alumina Silica Polish 4 pm AI203 50 M_ @ 690 °C, 17 M_ @ 750 °C, 1.8 M_ @ 900 °C

e-beam deposited

1 pm ZrOJY203 -

SS Silica Polish 4 pm AI203 Film Adhesion Failure

e-beam deposited

TABLE 2.--ESTIMATES OF THE THERMAL PROPERTIES OF SELECTED

Ru_.._nn _ Substrate.

B SiO2-AI203 AI203

B SiO2-AI203 SS

C AI203-SiO2-AI203 SS

R Cr3C2-AI203 SS

S ZrO2-AI203 AI203

MULTILAYER INSULATORS

(R = Ro*e -T/° fit to Fi

Resistance

@ 0°C (R0)

3.85E+6 Mfz

;ure 4 data)

I_=av

Constant (e)

65°C

184 M_ 34°C

164 M_

1.91 E+9 M_

2.18E+6 M_

34oc

41oC

64°C

Max UseTemp ±

1140 °C 42%

260 °C 17%

250 °C 1%

965 °C 1%

1080 °C 2%

TABLE 3.--BULK PROPERTIES OF MATERIAL USED IN THIS STUDY 4.5.6

Density

Material (g/cm 3)

Stainless Steel (SS) 4 8.02

Alumina (AI20_) s 3.65

Silica (SiO2) 4.s 2.32

Chromium Carbide (Cr3C2) 6 6.7

Yttria-stabilized Zirconia

(ZrOjY203) s 6.27

Elastic

Modulus

(10 s psi)

28

Coefficient of Thermal

Expansion (CTE)

Resistivity (Q-cm)

@ 1100°C

17 pprn/°C 70xl 0-_

56 8 ppm/°C 1xl 06

10 0.55 ppm/°C 0.5xl 06

>50 10 ppm/°C 80xl 06

27 10 ppm/°C 50

NASA/TM--2002-211873 5
100,000

10,000

C

_o

1,ooo

100

[] I

I

0

[]

[]

0

Temperature Dependance in Electrical Resistance

of 5 lam Muitilayer Insulators (after heat treatment)

1

• Run B: SIO2/AI203 I

I

• Run B: SIO2/AI203 on SS

O Run C: AI203/SiO2/AI203 on SS

Run R: Cr3C2/AI203 on SS

× Run S: ZrO2/AI203

200 400 600 800

Temperature [°C]

Q

1000

' I

1200

Figure 4.--Temperature dependence on electrical resistance for samples from runs B, C, R, & S.

Note the large hysteresis shown by samples from Run B.

NASA/TM--2002-211873 6

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4. TITLE AND SUBTITLE

2. REPORT DATE

September 2002

A Multilayered Thin Film Insulator for Harsh Environments

3. REPORTTYPE AND DATES COVERED

Technical Memorandum

5. FUNDING NUMBERS

6. AUTHOR(S)

John D. Wrbanek, Gustave C. Fralick, Charles A. Blaha, A. Rachel Busfield,

and Valarie D. Thomas

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135 - 3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-708-73-51--00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-13555

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AGENCY REPORT NUMBER

NASA TM--2002-211873

AIAA-2002-3731

11. SUPPLEMENTARY NOTES

Prepared for the 38th Joint Propulsion Conference and Exhibit cosponsored by AIAA, ASME, SAE, and ASEE,

Indianapolis, Indiana, July 7-10, 2002. John D. Wrbanek and Gustave C. Fralick, NASA Glenn Research Center;

Charles A. Blaha, Akima Corporation, Brook Park, Ohio 44142; A. Rachel Busfield. Rolls-Royce pie, Derby DE24 8B J,

United Kingdom; and Valarie D. Thomas, Ohio Aerospace Institute, Brookpark, Ohio 44142. Responsible person, John D.

Wrbanek, organization code 5510, 216--433-2077.

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Available electronically at hUp://gltrs.grc.nasa.gov

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

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

The status of work to develop a reliable high temperature dielectric thin film for use with thin film sensors is presented.

The use of thin films to electrically insulate thin film sensors on engine components minimizes the intrusiveness of the

sensor and allows a more accurate measurement of the environment. A variety of insulating films were investigated for

preventing electrical shorting caused by insulator failure between the sensor and the component. By alternating layers of

sputtered high temperature ceramics, a sequence of insulating layers was devised that prevents pinholes from forming

completely through the insulator and maintains high electrical resistivity at high temperatures. The major technical

challenge remaining is to optimize the fabrication of the insulator with respect to composition to achieve a reliable high

temperature insulating film. Data from the testing of various potentially insulating thin film systems is presented and their

application to thin film sensors is also discussed.

14. SUBJECT TERMS

Insulator; High temperature; Thin film sensor; Instrumentation; Harsh environment

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