John D. Wrbanek, Gustave C. Fralick, and Lisa C. Martin

Glenn Research Center, Cleveland, Ohio

Charles A. Blaha

Akima Corporation, Cleveland, Ohio

A Thin Film Multifunction Sensor for

Harsh Environments

NASA/TM—2001-211075

July 2001

AIAA–2001–3315
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NASA/TM—2001-211075

July 2001

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

37th Joint Propulsion Conference and Exhibit

cosponsored by the AIAA, ASME, SAE, and ASEE

Salt Lake City, Utah, July 8–11, 2001

AIAA–2001–3315

John D. Wrbanek, Gustave C. Fralick, and Lisa C. Martin

Glenn Research Center, Cleveland, Ohio

Charles A. Blaha

Akima Corporation, Cleveland, Ohio

A Thin Film Multifunction Sensor for

Harsh Environments
Acknowledgments

The authors would like to thank Dr. Jih-Fen Lei, now Chief of the Instrumentation and Controls Division at NASA

Glenn Research Center, and Dr. Herbert Will, now retired from the Instrumentation and Controls Division, for

their work on the original design of the multifunction sensor. Also, they would like to thank

Dr. Valarie Thomas of the Ohio Aerospace Institute for her help in reviewing this work.

Available from

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Available electronically at http://gltrs.grc.nasa.gov/GLTRS
NASA/TM—2001-211075 1 

A THIN FILM MULTIFUNCTION SENSOR FOR HARSH ENVIRONMENTS 

John D. Wrbanek, Gustave C. Fralick, and Lisa C. Martin 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Charles A. Blaha 

Akima Corporation 

Brook Park, Ohio 44142 

ABSTRACT

The status of work at NASA Glenn 

Research Center to develop a minimally intrusive 

integrated sensor to provide real-time measurement 

of strain, heat flux and flow in high temperature 

environments is presented in this paper. The sensor 

can be beneficial as a single package to characterize 

multiple stress and strain modes simultaneously on 

materials and components during engine 

development and validation. A major technical 

challenge is to take existing individual gauge designs 

and modify them into one integrated thin film sensor. 

Ultimately, the goal is to develop the ability to 

deposit the sensors directly onto internal engine parts 

or on a small thin substrate that can be attached to 

engine components. Several prototype sensors 

constructed of platinum, platinum-rhodium alloy, and 

alumina on constant-strain alumina beams have been 

built and bench-tested. The technical challenges of 

the design, construction, and testing are discussed. 

Data from the preliminary testing of the sensor array 

is presented. The future direction for the sensor 

development is discussed as well. 

GOALS

To meet the needs of aeronautic and 

aerospace research for measurement of high stress 

and heat loading on critical system components, 

NASA Glenn Research Center has been pursuing thin 

film physical sensor development. These thin film 

sensors are micro-scale in construction, and designed 

to be capable of measurements in the high 

temperature environments that engine components 

are exposed to. For example, these sensors are 

designed to withstand the presence of high 

temperature, high pressure, large stress/strain, large 

vibration as well as the severe thermal transient due 

to the cryogenic to combustion temperature change 

that occurs in high-pressure propulsion. Developed 

over a number of years, components of this 

technology have been demonstrated up to 1100°C in 

a variety of engine conditions.1,2 However, each 

sensor only provides information on one parameter at 

a time. In order to provide as much detailed 

information as possible from a single sensor unit, a 

multifunction sensor array approach has been 

pursued. 

This work further develops the various 

microfabricated thin film sensors needed for the next 

generation of engines and integrates them into a 

single array for real-time measurement of strain, heat 

flux, temperature, and flow. The complete physical 

sensor array produced in this work is designed such 

that multiple physical parameters (temperature, 

strain, vibration, or heat flux) can be obtained 

simultaneously from a single unit. The sensing 

elements are initially fabricated of high temperature 

materials such as platinum (Pt) and platinumrhodium (PtRh) alloys. This sensor technology can 

be applied to characterize propulsion components 

during their design and characterization development 

phase as well as during the operational phase. 

The technology has applications to RocketBased Combined Cycle (RBCC) and Turbine-Based 

Combined Cycle (TBCC) systems as well as to future 

launch vehicles, space vehicles, and ground systems. 

The resulting data can be used to verify the cooling 

system integrity including surface temperature, heat 

flux, and deformation. The increased understanding 

of the thermal and aerodynamic loads on system 

components will lead to an increased understanding 

of operating conditions and capabilities. 

Simultaneous measurement of temperature, strain, 

heat flux, and vibration will greatly aid in validating 

design codes. This will have a direct positive impact 

on improving system safety and reducing system 

design and development cost through a more 

thorough understanding of system operating 

conditions and capabilities. This capability is 

fundamental since there are engine environments in 

which optical access is not possible. 
NASA/TM—2001-211075 2 

These MicroElectroMechanical Systems 

(MEMS) based thin film sensors are being fabricated 

in a class 1000 clean room at NASA Glenn Research 

Center using physical vapor deposition and 

photolithography technologies. Surface analytical 

tools will be used to characterize these thin film 

sensors. The sensor response to strain, heat flux, 

temperature, and flow will be characterized as well. 

DESIGN

To meet the challenge of designing a single 

package sensor that combines the capabilities of 

several different sensors, and still be minimally 

intrusive in a hostile engine environment, the 

development of a MEMS-based sensor using high 

temperature metals is pursued. The sensor is 

conceptualized as having two major sub-gauges: 

1) the strain and flow gauge, and 2) the heat flux and 

temperature gauge. 

For the strain magnitude and direction, as 

well as for the flow magnitude and direction, a 

microfabricated triangular strain gauge design is 

used. The patented design, developed at NASA 

Glenn Research Center,3

 modified a rosette pattern 

strain gauge by linking the three legs of the rosette 

together and powering them under a single constant 

current source. A schematic of the concept for the 

triangular gauge is shown in Figure 1. 

θ 

εM

G1

G2

G3

60°

60°

60°

I+ 

IV1

V2

V3

V4

Figure 1: Concept for the Microfabricated 

Triangular Strain Gauge. 

The strain (εM) applied to the triangular 

gauge is sensed as individual strains by each of the 

three legs (εG1, εG2, εG3) of the rosette. These strains 

are determined by measuring the increase or decrease 

of the voltage drop along each leg due to the strain 

from the voltage drop measured with no strain (i.e., 

∆VG1 = V12(strain) – V12(no strain), ∆VG2 = 

V23(strain) - V23(no strain), ∆VG3 = V34(strain) - 

V34(no strain) ). The measured fractional change in 

the voltage drop is proportional to the strain seen on 

the leg by a gauge factor (Gf): 

GfV

VG

G ∗

∆ =

12

1

1 ε (1a) 

GfV

VG

G ∗

∆ =

23

2

2 ε (1b) 

GfV

VG

G ∗

∆ =

34

3

3 ε (1c) 

The rosette geometry provides the ability to 

measure the strain angle and magnitude from these 

measurements: 

( )

















−−

− = −

321

1 32

2

3

tan

2

1

GGG

GG

ε ε ε

ε ε θ (2) 





 



 −− = θ

ε ε ε

ε

cos 2

2

3

2 GGG 321

M (3) 

From the apparent strain (εapp) and the 

temperature coefficient of resistance (α) for the 

material used to make the rosette, the temperature 

change (∆T) of the sensor can be found as well: 

23

GGG 321 M

app

ε ε ε ε

ε − ++ = (4) 

α

ε Gf T app ∗ =∆ (5) 

These equations for rosette gauges is utilized 

to give strain magnitude and direction, but with half 

of the lead wires required for a standard rosette 

gauge. Also, by measuring the resistance change in 

the gauge under high current conditions, the sensor 

can be calibrated for flow measurement as an 

anemometer. 

A palladium-13%chromium alloy (Pd13Cr) 

was originally chosen as a strain gauge material due 

to the extensive work with this alloy that was pursued 

at NASA Glenn Research Center.4-7 However, this 

alloy is limited to temperatures under 1100°C for 

strain applications.8

 To meet the goal for the sensor 

to exceed 1000°C, the strain gauge metal was 

changed to platinum, which is not as susceptible to 

oxidation and physical changes at high temperatures. 
NASA/TM—2001-211075 3 

The determination of heat flux and 

temperature is accomplished by utilizing a 

microfabricated thin film heat flux gauge developed 

at NASA Glenn Research Center.1,9 The thin film 

heat flux gauge is a thermopile type, circular with 

two thickness of ceramic insulator, as shown in 

Figure 2. 

1 T2 T

K 

Temperature 

Sensors 

K 

Substrate 

X1

X2

Q

X2 = 5 μ

Ceramic 

X1 = 1 μ

Ceramic 

T1 – T2

Thermopile 

Ceramic 

Thermal 

Insulator 

Figure 2: Concept for the Microfabricated Thin 

Film Heat Flux Gauge used on the 

Multifunction Sensor. 

The heat flux (QÚ LV GHWHUPLQHG E\

measuring the temperature variation (T1-T2) under 

two thicknesses of insulator (x2-x1) by the equation: 

)(

)(

12

21

xx

TT KQ −

− =  (6) 

where K is the thermal conductivity. Assuming the 

thermopile is small enough such that the flux is 

uniform over its width, the voltage output is 

proportional to the heat flux passing into the gauge. 

Since the gauge is to be operated in a high 

temperature harsh environment, the thermopile 

consists of pairs of Type R platinum-13%rhodium vs. 

platinum (Pt13Rh/Pt) thermocouples. To amplify the 

small signal from this thermocouple type, 40 pairs of 

thermocouples are used. A thermocouple on the 

gauge provides the surface temperature of the system. 

The resulting design in integrating the two 

gauges is shown in Figure 3. The triangular strain 

gauge wraps around the heat flux gauge which is 

made up of Pt13Rh/Pt thermocouples. The entire 

gauge is enclosed in a triangle approximately 1.5 cm 

on a side and having element line widths of 50 μm. 

Triangular 

Strain 

Gauge 

Heat Flux 

Gauge 

Thermocouple 

Figure 3: Design of the Multifunction Sensor. 

FABRICATION

The sensors for this work were fabricated in 

the Microsystems Fabrication Lab, a class 1000 clean 

room facility in the Instrument Research Laboratory 

at NASA Glenn Research Center. To date, three 

sensors for this effort have been produced, as shown 

in Figures 4 & 5. The first sensor was a proof-ofconcept triangular strain gauge designed to 

demonstrate the validity of the modifications to the 

rosette pattern in thin films. This was produced by 

sputter depositing 5 microns of platinum on a 

constant-strain alumina beam patterned by using a 

metal shadow mask to make the 100-micron wide 

elements for the gauge. Since the only purpose of 

this sensor was to demonstrate the validity of the 

strain gauge concept, the heat flux gauge was not 

integrated. 

The second and third sensors were prototype 

designs on constant-strain alumina beams. They 

incorporated the strain gauge and heat flux gauge 

patterns on one sensor. Since the strain gauge 

material was identical to the material used as the 
NASA/TM—2001-211075 4 

negative side of the thermopile, a two metal process 

was developed for depositing the thin film sensor. 

The first prototype sensor was deposited 

with 1 micron of platinum and the second with 

1.2 microns of platinum. The sensors were patterned 

by using a lift-off process involving a copper 

sacrificial layer to attain precise line widths in 

platinum.10 A diagram of the procedure is shown in 

Figure 6. 

The first prototype sensor did not have the 

Pt13Rh patterned, and was used to demonstrate the 

triangular strain gauge as designed to be incorporated 

into the multifunction gauge. The second prototype 

was deposited with 1 micron of Pt13Rh to complete 

the heat flux sensor. The second prototype was then 

coated with 0.3 micron of alumina, with another 5 

microns deposited on the center of the heat flux 

gauge. 

Figure 4: The three sensors bench-tested on constant-strain alumina beams 

 (a) (b) (c) 

Figure 5: Close-up of the three sensors bench-tested: a) The proof-of-concept sensor; b) the first prototype 

sensor; and c) the second prototype sensor. 
NASA/TM—2001-211075 5 

1) Copper Layer on Alumina Base

2) Spin and Pattern Photoresist

3) Nitric Acid Etch Exposed Copper

4) Deposit Platinum

5) Lift-Off Platinum with Photoresist

6) Nitric Acid Etch Copper Layer

7) Spin and Pattern Photoresist for Alloy

8) Deposit Alloy

9) Lift-Off Alloy with Photoresist

Photoresist Copper 

Alumina Platinum

Alloy

Color Codes

10) Mask & Deposit 1st Alumina Layer

11) Mask & Deposit 2nd Alumina Layer

Figure 6: Lift-off Process used in constructing the Multifunction Sensor (Patent Pending to Akima Corp.) 

TESTING

The three sensors were tested for response to 

strain, flow and heat at the Instrument Research 

Laboratory at NASA Glenn Research Center to 

validate the multifunction gauge design before testing 

in an engine-type environment. The proof-of-concept 

sensor was tested for strain response, the first 

prototype sensor was tested for strain and flow 

response, and the second prototype sensor was tested 

for strain and heat response. The environment was 

limited to ambient bench-top conditions in these 

tests. 

Strain

The sensors were tested for strain response 

on a Dynamic Load Test Rig, as shown in Figure 7. 

A constant current source supplied the excitation 

current to the sensors, and the signals were filtered 

and then amplified before being collected by a 

computer. The maximum strain on the sensors was 

measured to be 230 microstrain (με) based on 

displacement measurements on a beam in the rig and 

measurements using a commercial strain gauge 

mounted on a similar constant-strain alumina beam. 

Measurements of both the static and dynamic 

response of the sensors were made and the results are 

shown in table 1. The dynamic measurements were 

made with the motor rotating at 5 Hz, 10 Hz, and 

15 Hz. The responses of the strains were found to be 

consistent for each individual sensor with a variance 

of ±10%. The results of the tests are shown in 

Table 1. 

Figure 7: Dynamic Loading Test Rig 

Table 1: Results of Strain Bench-Testing 

Sensor Mode Current ∆R 

max. Total R Gauge 

Factor 

Strain 

Angle 

Proof-ofConcept Dynamic 109 mA 1.21 mΩ 3.63 Ω 4.4 -1° 

First Static 7.2 mA 255 mΩ 1200 Ω 2.8 -30° 

Prototype Dynamic 7.2 mA 284 mΩ 1200 Ω 3.1 -29° 

Second Static 6.0 mA 132 mΩ 480 Ω 3.6 -32° 

Prototype Dynamic 6.0 mA 136 mΩ 480 Ω 3.7 -32° 
NASA/TM—2001-211075 6 

Considering that the gauge factor for 

platinum wire of the size of our traces is 4.00,11 the 

sensors responded well to our bench testing. The 

proof-of-concept gauge gave a gauge factor close to 

this wire value. The gauge factor of the first 

prototype sensor was between 2.8-3.1, considerably 

lower than the expected value. The 1200 Ω

resistance also was considerably different than the 

360 Ω expected, which may suggest impurities in the 

material for that sensor. The second prototype 

sensor’s 480 Ω resistance was as expected, and the 

gauge factor of between 3.6-3.7 was very close to the 

wire value. These deviations of the gauge factor may 

be explained by noting that our film is fixed to the 

surface of the testing beam along its entire length, 

unlike a wire, which will effect the deformation of 

the film when under strain. 

The angle determined from the three-leg 

rosette strain gauge algorithm was also reasonable. 

The proof-of-concept sensor was oriented to give a 

strain angle of 0°, and the first and second prototype 

sensors were oriented at -30°. From these results, the 

orientation is accurate to ±1°, and the measurements 

are accurate to ±1°. For these bench tests, a total 

accuracy better than ±3° is acceptable. 

Flow

The flow response was investigated using 

the first prototype sensor and a cool air gun. The 

response to changes in the voltage across one leg was 

observed while air was blown transversely and 

longitudinally across the leg. Currents between 

10 milliamps and 50 milliamps were used. The 

results are plotted in Figure 8. 

Fractional Resistance Change for Test Air Flows over 

First Prototype Multifunction Sensor (2nd leg)

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0 10 20 30 40 50 60

Gauge Current (mA)

dR/R

Transverse Flow

Longitudinal Flow

Figure 8: Response of one leg of the first prototype 

sensor to airflow 

The response to longitudinal airflow was 

linear with the current, with the transverse flow 

response weaker for currents exceeding 20 milliamps. 

From the trends shown in Figure 8, it is estimated 

that a 170 milliamp (35 Watt) current source would 

be needed to achieve longitudinal responses 5 times 

as great as the transverse response. Though this 

indicates that the multifunction gauge could be used 

to determine airflow, the exact incorporation using 

this high current with a low excitation current strain 

gauge remains unclear, as well as the resolution of 

transverse and longitudinal flows. 

Heat

The second prototype sensor had a heat flux 

gauge integrated with a triangular strain gauge. The 

sensor on the alumina substrate was clamped to a 

metal shim, and then exposed to a heat gun blowing 

perpendicular to the surface. The resulting signal is 

shown in Figure 9. Though the signal was not 

amplified or filtered, it was consistent with heat flux 

responses seen in gauges of the same design.1

 The 

signal reached a level shortly after the heat was 

applied. When the heat gun is turned off, the heat 

flux became negative as the sensor is cooled from the 

ambient air currents above the sensor. 

Heat Flux Response on 

Second Prototype Sensor

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0 100 200 300 400

Time (seconds)

Figure 9: Heat flux response on the second 

prototype sensor. 

FUTURE TESTING AND CONCLUSIONS

The design of the multifunction sensor has 

been bench-tested on three test sensors. The next 

level of testing involves the fabrication and testing of 

the sensor on an engine component. The first 
NASA/TM—2001-211075 7 

component that has been chosen for testing is a 

silicon carbide ceramic matrix composite watercooled thruster housing. 

By testing the sensor in an engine 

environment, the architecture and material limits can 

be better understood in developing an improved 

sensor design. Further tests will need to be 

accomplished to verify the design in harsher 

environments in order to push these limits. As these 

limits are met, alternate materials will be explored for 

the sensor. Ceramic thermocouple and strain gauge 

materials show promise and may be used as an 

alternate to the metals in the sensor. An electronics 

package will be developed to allow the sensor to 

become “smart” without the use of PC-based 

software. Also, an attachable version of the sensor 

will be developed, in which the sensor would be 

fabricated on metal or ceramic shims for installation 

into systems that cannot have the sensor deposited 

directly onto components because of component size 

or geometry. 

The development of sensors to determine 

simultaneous operating conditions is fundamental in 

improving system safety and design and development 

costs. At NASA Glenn Research Center, we have 

begun testing an integrated sensor design that can 

measure strain, heat flux, temperature and flow in 

one smart sensor that is minimally intrusive and 

survivable in harsh environments. 

REFERENCES

1 H.A. Will, H. Bhatt, “Status of Thin Film Heat Flux 

Sensors,” Advanced Earth-to-Orbit Propulsion 

Technology 1994, NASA Marshall Space Flight 

Center, AL, May 17-19, 1994. 2 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. 3 J.F. Lei, G.C. Falick & M.J. Krasowski, 

“Microfabricated Multifunction StrainTemperature Gauge,” US Patent 5,979,243, 

November 9, 1999. 4 C.O. Hulse, R.S. Bailey, F.D. Lemkey, “High 

Temperature Static Strain Gage Alloy 

Development Program,” NASA CR-174833, 1985. 

5 J.F. Lei, W.D. Williams, “PdCr Based High 

Temperature Static Strain Gage,” AIAA 2nd

International Aerospace Planes Conference, 

Orlando, FL, October 29-31, 1990. 

6 W.D. Williams, “Strain Sensing Technology for 

High Temperature Applications,” NASA TM4498, AIAA 4th International Aerospace Planes 

Conference, Orlando, FL, December 1-4, 1992. 7 S.P. Wnuk, Jr., V.P.Wnuk, “The Development of a 

PdCr Integral Weldable Strain Measurement 

System Based on NASA Lewis PdCr/Pt Strain 

Sensor for User-Friendly Elevated Temperature 

Strain Measurements,” NASA CR-202316, 

February 1997. 8 J.F. Lei, L.C. Greer III, L.G. Oberle, “Evaluation of 

Pd-Cr Wires for Strain Gage Application,” NASA 

TM-106857, February 1995. 

9 H. Bhatt, M.V. Zeller & H.A. Will, “Novel ThinFilm Heat Flux Sensors,” National Aerospace 

Planes Conference, Orlando, FL, Dec 1-4, 1992. 10 C.A. Blaha, “Liftoff Process of Difficult and NonEtchable Material using Photolithography,” to be 

published in NASA Tech Briefs.s 11 G.C. Kuczynski, “Effect of Elastic Strain on the 

Electrical Resistance of Metals,” Physical Review, 

Vol. 94, No. 1, 1954, pp. 61-64. 
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NASA TM—2001-211075

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Thin film sensor; Strain; Heat flux; Temperature; Instrumentation; Harsh environment

Unclassified -Unlimited

Subject Category: 35 Distribution: Nonstandard

Prepared for the 37th Joint Propulsion Conference and Exhibit cosponsored by the AIAA, ASME, SAE, and ASEE,

Salt Lake City, Utah, July 8–11, 2001. John D. Wrbanek, Gustave C. Fralick, and Lisa C. Martin, NASA Glenn Research

Center; Charles A. Blaha, Akima Corporation, 22021 Brookpark Road, Cleveland, Ohio 44126. Responsible person, John

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

John D. Wrbanek, Gustave C. Fralick, Lisa C. Martin, and Charles A. Blaha

A Thin Film Multifunction Sensor for Harsh Environments

The status of work at NASA Glenn Research Center to develop a minimally intrusive integrated sensor to provide realtime measurement of strain, heat flux and flow in high temperature environments is presented in this paper. The sensor

can be beneficial as a single package to characterize multiple stress and strain modes simultaneously on materials and

components during engine development and validation. A major technical challenge is to take existing individual gauge

designs and modify them into one integrated thin film sensor. Ultimately, the goal is to develop the ability to deposit the

sensors directly onto internal engine parts or on a small thin substrate that can be attached to engine components.

Several prototype sensors constructed of platinum, platinum-rhodium alloy, and alumina on constant-strain alumina

beams have been built and bench-tested. The technical challenges of the design, construction, and testing are discussed.

Data from the preliminary testing of the sensor array is presented. The future direction for the sensor development is

discussed as well.