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

A Multilayered Thin Film Insulator for

Harsh Environments

NASA/TM—2002-211873 AIAA–2002–3731

September 2002
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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

A Multilayered Thin Film Insulator for

Harsh Environments

NASA/TM—2002-211873 AIAA–2002–3731

September 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

38th Joint Propulsion Conference and Exhibit

cosponsored by AIAA, ASME, SAE, and ASEE

Indianapolis, Indiana, July 7–10, 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.

Available from

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

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.

Available electronically at http://gltrs.grc.nasa.gov
NASA/TM—2002-211873 1 

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 8BJ, 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 μm, 

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.1,2

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 μm 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 μm 

thick NiCoCrAlY base coat on which alumina can be 

thermally grown and vapor-deposited.3

 Rolls-Royce 

thin film sensors use a reactively-sputtered layer of 

alumina to provide the electrical insulation. 
NASA/TM—2002-211873 2 

The application of alternative thin film dielectric 

insulators has been examined to further minimize the 

insulation thickness. However, ceramics lose their 

electrical insulation exponentially with increasing 

temperatures. Short circuit paths at grain boundaries 

and film defects can further reduce the insulation 

properties from the bulk values at high temperatures. 

To further this research, NASA Glenn Research Center 

and Rolls-Royce have pursued a joint investigation of 

utilizing multilayered thin film dielectrics as a reliable 

insulator for use in harsh environments. The use of a 

multilayered scheme is thought to be promising for the 

fabrication of electrically insulating thin films. A major 

cause of conduction in thin film dielectrics is the 

presence of defects, such as pinholes, that propagate 

clear through the film to the underlining substrate 

surface. By alternating the insulating material, a new 

growth pattern would eliminate the previous one. Thus, 

direct pathways for conduction to the substrate are 

eliminated. The requirements for such an insulator are 

driven by the environments of aerospace applications, 

which currently require insulation of 100 kΩ or better 

to 1100 °C in oxidizing conditions. 

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 1 μm 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 μm. 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 μm 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 MΩ. One 

cycle was performed to stabilize the sample, and the 

data from the second ramp was used for insulator 

evaluation. 

Test Shim Substrate

1 μm Platinum Base Coat

5 μm Multilayer Insulator

5 μm Pt

Sensor Pad

Figure 1.—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 μm 

alumina (Al2O3) 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 μm film. 

The next samples (run B) tested used an underlying 

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

an upper layer of 4 μm 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 3 

also showed a severe hysteresis in their resistance 

curves during the heat cycle. Despite the poor 

insulation properties, the run validated the concept of 

using a multilayer structure to eliminate conduction 

paths to the substrate that caused the short in run A 

samples. 

Samples in run C were composed of a bottom layer of 

2 μm alumina, followed by 1 μm of silica, and then 

covered with 2 μm of alumina sputter deposited on 

stainless steel shims. These samples showed properties 

similar to those of the run B samples on stainless steel, 

but with more severe cracking of the surface. Figure 2 

shows the cracking seen on a typical sample from run C 

on stainless steel. 

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

C (Al2O3-SiO2-Al2O3 Multilayer) after heat 

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

approximately 525 μm. 

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/H2O2 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 μm of sputtered chromium carbide followed by 4 μm 

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

yttria-stabilized zirconia followed by 4 μm of e-beamed 

alumina. The platinum base coat was not deposited for 

run R since the chromium carbide proved to be an 

excellent bond coat between chromium-containing 

metals and oxide ceramics. Figure 3 shows the pinholefree surface of a run R sample after heat-treating to 

800 °C on a steel substrate. The temperature limit of 

both insulators for 100 kΩ of resistance appears to be 

about 1000 °C for the 5 μm of material deposited. The 

zirconia-alumina insulator was deposited on a steel 

substrate in run T, but the alumina failed before any 

data could be gathered. 

The data from the temperature ramps of runs B, C, R, 

and S are plotted in figure 4. To give a quantitative 

comparison of the insulators, the data were fit to 

exponential decay curves. The results of the fittings are 

given in table 2, which gives the estimated 0°C values, 

decay constants, minimum use temperatures and 

uncertainties. 

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

(Cr3C2-Al2O3 Multilayer) after heat cycling 

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

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 4 

multilayer insulator sequence on metals, particularly 

when e-beam depositing the alumina at 800 °C. Even 

following a 500 °C annealing cycle, the silica cracking 

is sufficient to permit chromium and iron migration 

from the steel into the thin film insulation. In fact, 

under the 800 °C e-beam deposition temperatures of the 

alumina, the CTE mismatch between the steel and the 

silica is sufficient to cause cracking in the insulating 

film and adhesion failures. 

The slightly lower CTE of yttria-stabilized zirconia 

may account for the stresses observed in the alumina 

thin film when the layers were applied to the steel 

substrate in run T. It should be noted that since most 

engine components are “super-alloys” similar to 

Hastelloy or Inconel with CTE’s between 10 and 

13 ppm/°C,4

 the zirconia-alumina multilayer may be 

applicable for use on those materials. 

The success of run R is attributed to 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. 

References

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

Film Sensor Technologies for Engine Applications,”

NASA TM–107418, Turbo Expo ’97, Orlando, FL, 

June 2–5, 1997. 

2. L.C. Martin, J.D. Wrbanek, G.C. Fralick, “Thin 

Film Sensors for Surface Measurements,” NASA 

TM—2001–211149, 19th International Congress on 

Instrumentation in Aerospace Simulation Facilities, 

Cleveland, OH, August 27–30, 2001. 

3. 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. 

4. CRC Handbook of Chemistry and Physics, edited 

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

5. S.W Bradstreet, “Oxide Ceramics,” HighTemperature Materials and Technology, edited by 

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

Electrochemical Society Series, John Wiley & Sons, 

New York (1967), p. 235–303. 

6. 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. 
NASA/TM—2002-211873 5 

TABLE 1.—RUNS AND MULTILAYER SAMPLES GENERATED IN THIS EFFORT 

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

Run Substrates Preparation Multilayer Result 

A Alumina 

& SS Shims Silica Polish 5 μm Al2O3 

sputter deposited Short @ RT 

B Alumina & 

SS Shims Silica Polish

1 μm SiO2 – 

4 μm Al2O3 

sputter deposited 

Film Cracks after 500 °C Heat Treatment. 

Alumina Shim: 100 MΩ @ 720 °C, 1.78 MΩ @ 1000 °C 

SS Shim: 56 MΩ @ 41 °C, 1 kΩ @ 440 °C 

C SS Silica Polish

2 μm Al2O3 – 

1 μm SiO2 – 

2 μm Al2O3 

sputter deposited 

Film Cracks after 500 °C Heat Treatment 

100 MΩ @ 17°C, 2 kΩ @ 500 °C 

J SS Silica Polish 1 μm SiO2 

sputter deposited 

EDAX after 800 °C anneal indicates substantial Cr & Fe 

migrating from the SS into the cracks 

M & N SS Silica Polish

1 μm SiO2 – 

4 μm Al2O3 

e-beam deposited 

Short @ RT 

P & Q SS H2SO4/H2O2

Cleaning

1 μm SiO2 – 

4 μm Al2O3 

e-beam deposited 

Film Adhesion Failures 

Short @ RT 

R SS H2SO4/H2O2

Cleaning

1 μm CrC – 

4 μm Al2O3 

e-beam deposited 

Good Insulator 

84 MΩ @ 690 °C, 20 MΩ @ 750 °C 

S Alumina Silica Polish

1 μm ZrO2/Y2O3 – 

4 μm Al2O3 

e-beam deposited 

Good Insulator 

50 MΩ @ 690 °C, 17 MΩ @ 750 °C, 1.8 MΩ @ 900 °C 

T SS Silica Polish

1 μm ZrO2/Y2O3 – 

4 μm Al2O3 

e-beam deposited 

Film Adhesion Failure 

TABLE 2.—ESTIMATES OF THE THERMAL PROPERTIES OF SELECTED 

MULTILAYER INSULATORS 

 (R = R0*e-T/θ

 fit to Figure 4 data) 

Run Multilayer Substrate Resistance

@ 0°C (R0)

Decay 

Constant (θ) Max Use Temp ± 

B SiO2-Al2O3 Al2O3 3.85E+6 MΩ 65°C 1140 °C 42% 

B SiO2-Al2O3 SS 184 MΩ 34°C 260 °C 17% 

C Al2O3-SiO2-Al2O3 SS 164 MΩ 34°C 250 °C 1% 

R Cr3C2-Al2O3 SS 1.91E+9 MΩ 41°C 965 °C 1% 

S ZrO2-Al2O3 Al2O3 2.18E+6 MΩ 64°C 1080 °C 2% 

TABLE 3.—BULK PROPERTIES OF MATERIAL USED IN THIS STUDY 4,5,6

Material Density 

(g/cm3

) 

Elastic 

Modulus 

(106

 psi) 

Coefficient of Thermal 

Expansion (CTE) 

Resistivity (Ω-cm) 

@ 1100°C 

Stainless Steel (SS) 4 8.02 28 17 ppm/°C 70x10-6

Alumina (Al2O3) 5 3.65 56 8 ppm/°C 1x106

Silica (SiO2) 4,5 2.32 10 0.55 ppm/°C 0.5x106

Chromium Carbide (Cr3C2) 6 6.7 >50 10 ppm/°C 80x10-6

Yttria-stabilized Zirconia 

(ZrO2/Y2O3) 5 6.27 27 10 ppm/°C 50 
NASA/TM—2002-211873 6 

Temperature Dependance in Electrical Resistance

of 5 μm Multilayer Insulators (after heat treatment)

100

1,000

10,000

100,000

0 200 400 600 800 1000 1200

Temperature [°C]

Resistance [kOhms]

Run B: SiO2/Al2O3

Run B: SiO2/Al2O3 on SS

Run C: Al2O3/SiO2/Al2O3 on SS

Run R: Cr3C2/Al2O3 on SS

Run S: ZrO2/Al2O3

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. 
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12

A Multilayered Thin Film Insulator for Harsh Environments

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

and Valarie D. Thomas

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

Unclassified -Unlimited

Subject Category: 35 Distribution: Nonstandard

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 plc, Derby DE24 8BJ,

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

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

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.