E. Quandt, University of Kiel, Kiel, Germany; C. Zamponi, Research Center Caesar, Bonn, Germany

Summary: Shape memory alloys (SMA) are able to provide high work output when they undergo the martensitic transformation. Therefore, they are a promising candidate for an actuation mechanism in microsystems, e.g. in microvalves, switches or microgrippers. Sputter deposited thin SMA films are already in use as free-standing films or as composites in combination with other materials. In the case of composites, stresses between the different layers can be generated which lead to a two-way behavior of the whole composite. TiNi-based thin film composites have been developed which cover a wide range of transformation temperatures. Since it is also possible to deposit and structure the SMA composites on Si substrates by photolithographic steps, the fabrication process is compatible to MEMS and therefore most favorable for an number of applications. By means of additional polymer layers, actuators can be created exhibiting a bistable behavior. In this case no energy is necessary for holding the actuator position, only for changing the position. The essential precondition for the bistability is that the glass transition temperature of the polymer is placed within the hysteresis of the SMA. It is shown, that the combination of the SMA composite (Ti,Hf)Ni/Mo with a broad hysteresis and the PMMA polymer Lucryl provides this bistable behavior. Superelastic shape memory materials are of special interest in medical applications due to the large obtainable strains, the constant stress level and their biocompatibility. Superelastic TiNi thin films have been fabricated by magnetron sputtering using extremely pure cast melted targets. Special heat treatment were performed for the adjustment of the superelastic properties and the transformation temperatures. A superelastic strain of up to 6.5% at 37°C was obtained. The talk will discuss the properties of TiNi-based films using their two-way behavior or superelasticity in view of different applications. Funding of the DFG through the SFB 459 is gratefully acknowledged