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
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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
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.
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