F r a u n h o F e r I n s t I t u t e F o r s u r F a c e e n g I n e e r I n g a n d t h I n F I l m s I s t

1

Sputtered Strain gaugeS

At the Fraunhofer Institute for Surface

Engineering and Thin Films IST high-

performance sputtered strain gauges are

being developed which are opening up a

wider range of new fields of application,

such as, for example, in mechanical

engineering, aerospace, weighing systems

or medical technology.

The principle

Strain gauges change their electrical resis-

tance with applied strain. This means that

they can be used for measuring force,

elongation, moments or pressure. Classic

strain gauges consist of a constantan me-

ander on a polymer foil which is glued by

hand to the work piece. Today this mode

of attachment is already being replaced

in various products by sputtered thin film

strain gauges. One example are pressure

sensors. To extend further the possible

applications of sputtered strain gauges a

great deal of development is still neces-

sary with regard to measurement sensitiv-

ity and applicability to three-dimensional

technical surfaces. The Fraunhofer IST

can here offer solutions which have great

potential.

Advantages of sputtered strain gauges

Sputtered strain gauges measure with

a high degree of precision since neither

adhesives nor polymer foils are used

and swelling or creeping due to ambient

temperature and humidity is avoided.

Since the thin film system of sputtered

strain gauges is only a few micrometres

thick this leaves the tolerances of a work

piece in most cases unchanged. In addi-

tion since the gauges can be positioned

exactly and the sensor layer connected

directly to the work piece surface, the

system gains increased measurement

accuracy.

1 Bearing with sputtered strain

gauges.

Fraunhofer Institute for Surface

Engineering and Thin Films IST

Bienroder Weg 54 E

38108 Braunschweig

Contact

Dr. Ralf Bandorf

Phone +49 531 2155-602

ralf.bandorf@ist.fraunhofer.de

www.ist.fraunhofer.de

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1000 °C). In addition, DLC (diamond-like

carbon) has also been used successfully

in pressure sensors for a number of years

now. Using DLC coatings containing

metal (Me-DLC) makes it possible for strain

for the most part independently of the

ambient temperature. While most metals

typically have a gauge factor of 2, by using

Ni-DLC markedly higher strain sensitivities

(gauge factors > 10) can be achieved. The

gauge factors of different Me-DLC films are

plotted in the diagram below.

During optimization of material properties,

industry-oriented manufacturing processes

and test methods are used for the thin film

strain gauges. Major goals of development

are short process times and high stability

of the sensor properties to permit rapid

industrial implementation.

strain gauges on three-dimensional techni-

cal surfaces.

New materials for sputtered strain

gauges

New combinations of materials for

sputtered strain gauges have enabled an

increase in the strain sensitivity of the sen-

sor layer (gauge factor) and thus improved

measurement accuracy. The aim of material

development is to obtain the lowest pos-

sible drift in the measurement signal under

harsh environmental conditions (fluctua-

tions in temperature and humidity). The

Fraunhofer IST is therefore investigating

material combinations for high-temperature

applications, such as, for example, mea-

surements within the exhaust gas system or

in engines (for temperatures up to and over

1 View into a high-temperature

oven with measuring devices for

automated strain gauge charac-

terization.

2 Sputtered strain gauges on

complex metallic work pieces.

2

Sputtered strain gauges on complex

work pieces

Before the sputtered strain gauge can be

applied to a work piece with a metallic

surface an electrical insulating layer has

to be provided. For defect-free insulation

an extremely smooth surface is needed to

which, for example, an Al2O3 layer of only

a few micrometers is applied. A thin film

of NiCr, for example, is deposited as sensor

layer. The sensor layer is in most cases only

a few hundred nanometers thick. Contact

areas are gold-plated to enable soldering

or bonding at the surface. Typically the

sensor layer has a meander-like structure

to realize a defined resistance. This can

be done either by photolithography or by

using a laser. The Fraunhofer IST offers the

full process chain for producing sputtered

k fa

ctor

Me / (Me+C) [at %]

Ni-DLC Fe-DLC Cu-DLC

20 30 40 50 60 70 80 900

2

4

6

8

10

12

14

16

18

20

Strain sensitivities of different Me-DLC films (gauge factor) as a function of the metal

content of the DLC film.