The multifunctional thin film system developed at the Fraunhofer IST consists of the following five layers, which are deposited one by one on the surface of the die:
- a piezoresistive sensor layer (DiaForce®) 6 µm thick
- individual force measurement areas consisting of chromium electrode structures produced by the lift-off process; d = 200 nm
- an insulating and wear-protection interlayer 1 µm thick made of SiCON®
- chromium meander structures for local temperature measurement made by photolithography; d = 200 nm
- a further insulating and wear-protection layer 3 µm thick made of SiCON®
The performance of the sensorized tool with the multifunctional thin film system was investigated in a deep-drawing machine at the Fraunhofer Institute for Machine Tools and Forming Technology IWU. An example of measurement results from the forming process is shown in the diagram to the right. During the deep-drawing process there is a considerable drop in resistance due to the compressive load applied to the piezoresistive thin film sensor system by the aluminum sheet being formed. With this process, local heating of the surface amounts to only 1 K. The maximum permitted load during the sheet-metal deep-drawing process can therefore be derived directly from precharacterization and is 430 MPa.
Advantages of multifunctional thin film sensors
The multifunctional thin film systems developed at the Fraunhofer IST offer a number of advantages over conventional measurement systems. For instance:
- Optimization of simulation
- Optimization and monitoring of production processes
- Improvement of the understanding of operating conditions
- Optimization of maintenance intervals
- Reduction in the reject rate
The results described were obtained within the SensoFut project (Sensorized Future – Sensing of temperature and pressure in harsh environments), on which the Fraunhofer IST worked together with the Fraunhofer Institute for Machine Tools and Forming Technology IWU and Sirris, the Belgian research association. SensoFut is funded in the 13th Cornet Call (Collective Research Networking) by the Federal Ministry of Economics and Technology (BMWI) and the German Federation of Industrial Research Associations (AiF) and ran from 1.1.2014 to 30.6.2015.