High-performance sputter strain gauges are developed at the Fraunhofer Institute for Surface Engineering and Thin Films IST, opening up various new fields of application – for instance in mechanical engineering, aviation and aerospace or weighing and medical technology.
The electrical resistance of strain gauges changes when they are stretched. This allows them to be used for measuring force, elongation, moments and pressure. Classic strain gauges consist of a constantan meander on a polymer carrier foil and are manually glued to the component. Today this gluing process is replaced by sputtered thin film strain gauges for various products. Pressure sensors are one example. There is a significant need for development regarding the measuring sensitivity and application to three-dimensional technical surfaces in order to further expand the application possibilities of sputter strain gauges. Fraunhofer IST offers solutions with great potential in this field.
Benefits of sputter strain gauges
Sputter strain gauges measure with high accuracy since neither adhesive nor foils are used, and because swelling or creeping depending on the ambient temperature and humidity is avoided. Due to the layer composition of sputter strain gauges with a thickness of just a few micrometers, the tolerances of a component usually remain unchanged. Exact positioning and direct bonding of the sensor film with the component surface also results in better measuring accuracy for the system.
Sputter strain gauges on complex components
An electrical insulating layer has to be applied to metallic component surfaces before applying the sputter strain gauge. A very smooth surface is required to realize defect-free insulation, which is applied for example as an Al2O3 layer with a thickness of just a few micrometres. An NiCr thin film for example is precipitated as the functional sensor layer. Usually the sensor layer is only a few hundred nanometres thick. The contact areas are gold-plated to prepare them for soldering or bonding. To realize a defined resistance, the sensor layer of the strain gauge is usually structured in a meander shape. This can be realized using photolithography or with a laser. Fraunhofer IST offers the entire process chain for the production of sputter strain gauges on three-dimensional technical surfaces.
New materials for sputter strain gauges
New materials for sputter strain gauges make it possible to increase the elongation sensitivity of the sensor layer (k-factor) and therefore improve the measuring accuracy. The goal of material development is to minimize the drift of the measuring signal under tough ambient conditions (temperature and humidity fluctuations). Fraunhofer IST is researching material combinations for high-temperature applications, for instance to take measurements on exhaust gas systems or in jet engines (including above 1000 °C). DLC (diamond-like carbon) is also being used successfully in pressure sensors since a few years. Using DLC layers containing metal (Me-DLC) enables elongation measurement that is largely independent of the ambient temperature. While most metals typically have a k-factor of 2, significantly higher elongation sensitivities can be realized with Ni-DLC (k‑factors > 10).
Industry-oriented production processes and test methods for the strain gauge layers are applied in the optimization of material properties. Important development objectives include short process times and a high stability of the sensor properties for fast industrial implementation.