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Tribological improvement of moving microparts by application of thin films and micropatterning

: Bandorf, R.; Paulkowski, D.M.; Schiffmann, K.I.; Küster, R.L.A.


Journal of Physics. Condensed Matter 20 (2008), No.35, Art. 354018, 9 pp.
ISSN: 0953-8984
Journal Article
Fraunhofer IST ()

The relevance of active microsystems continually increases and with the expansion of applications also aspects of reliability and durability of the moving microparts. In particular, the microtribological improvement of active MEMS therefore was investigated and improved. On the one hand, the influence of different thin films was investigated; on the other hand, improvement by micropatterning of the tribological surfaces in combination with coatings was studied. Most promising results were gained using thin carbon- based films. The typical film thickness of the coatings was in a range of several tens up to a few hundred nanometres. The films were deposited by plasma processes varying different process parameters, mainly the applied bias voltage. Different doping elements ( Si, Au and W) were added to the diamond- like carbon ( DLC) films ( a- C, a- C: H, a- C: H: Si, a- C: H: Au, and a- C: H: W). Different methods were used for characterizing the thin films. Under single asperity contact indentation and scratch tests were performed to determine microhardness, microwear and microfriction. An influence of the applied bias voltage on the micromechanical and microtribological properties was found. At low constant loads of 100 mu N the more brittle a- C films showed an initially higher microwear volume than the more elastic a- C: H films. With increasing load the microwear of the a- C films showed less increase of microwear compared to a- C: H films. At low load in the range from 50 mu N to a few 100 mu N the friction coefficient in single- asperity contact decreased with increasing load. Reaching a critical load the behaviour changed and due to inelastic effects, the friction coefficient increased with further increasing load. For investigation of microabrasive wear under multi- asperity contact a specifically developed tester was used. Besides a ranking of different materials regarding their abrasive microwear resistance the influence of the substrate material on the resulting wear behaviour was investigated for tungsten- doped a- C: H films on silicon and polymer substrates. Using a softer substrate material the abrasive wear resistance for constant normal loading was doubled. Furthermore the friction coefficients of the films were determined by a pin- on- disc test and a specifically developed oscillating friction and wear tester working at flat- to- flat microcontact. It turned out that the friction coefficient was strongly dependent on the resulting contact pressure. Therefore the contact area was modified by micropatterning. The resulting friction coefficient of the patterned samples was significantly lower compared to results for full area contact, depending on the structures used and resulting local pressure. Different geometric patterns were investigated. A reduction of the friction coefficient from 0.09 to 0.04 was measured for a- C- coated silicon on increasing the local pressure from 12.5 to nearly 600 kPa by micropatterning.