Capacitive thin-films sensor systems for face seals

In order to determine the gap height as a function of process parameters in face seals made by EagleBurgmann Germany GmbH & Co. KG, a sensorized thin-film system has been developed at the Fraunhofer IST which is applied directly to the surface of the face seal. Being able for the first time to measure these variables without interrupting operation is a major step in optimizing and verifying simulations and operating modes.

The capacitive coating system

In the first coating process an electrically isolating coating consisting of a silicon- and oxygen-modified hydrocarbon layer, also known as SiCON®, is homogeneously deposited on ceramic seal rings made by EagleBurgmann Germany GmbH & Co. KG. A laser system is used to prepare contour masks which are adhesive on one side and these are applied manually to the coated surface at defined distances apart. A coating of chromium only 200 nm thick is then homogeneously applied by the PVD process to the masked surface of the ring. At the end of this lift-off process the masks are removed and the structures are left on the surface. Each individual structure can be regarded as a capacitor plate. What is special here is that the contacting areas are not on the top face but on the sloping inner face since it is only here that it is possible to locate soldered connections. Finally a coating of SiCON® is once again laid down, this time as the top coating.

The principle behind capacitive measurement

A capacitor plate is created in the slide face of the stationary seal ring with the aid of a thin-film sensor system integrated into the surface. Several capacitor plates are accommodated on the seal ring. The other capacitor plate is the rotating counterring. The measured value of the capacity of a capacitor is a direct measurement of the plate distance. This in turn is directly related to the gap height which is found when the seal is in operation. This means that it is possible to determine experimentally the gap width which results as a function of operating parameters such as speed and operating pressure. A very good correlation was achieved between experiment and numerical calculations.


The development of the capacitive thin-film sensor system for face seals manufactured by EagleBurgmann Germany GmbH & Co. KG was an objective of the INTELLA (a German acronym for ‚intelligent, lightweight bearings and seals for automotive and mechanical engineering‘) funded by the Federal Ministry of Education and Research and supervised by the project sponsors Jülich. Other project partners were Schaeffler AG, Cerobear GmbH, KSB AG, ScienLab Electronic Systems GmbH and the Fraunhofer Institute for the Mechanics of Materials IWM.