Inorganic Functional Materials

Superelastic Shape Memory Films and Applications

The SFB (Sonderforschungsbereich) 459 is an initiative coordinated by the Ruhr University of Bochum where an interdisciplinary group of engineers, scientists and medics work together aiming the development of fundamentals, applications and processing of shape memory alloy.
Within the third period of SFB 459 (2006-2008) our main aim is the development of NiTi tubes which are fabricated within the clean room environment by magnetron sputtering technique and the process technology for microstructuring those NiTi tubes. Those tubular structured devices are basic prototypes which one can use as a medical application, e.g. as stents for neurovascular implants or embolic filters. Magnetron sputtering technique is based on acceleration of gas ions towards a target of the material to be deposited. In the case of dc sputtering a negative potential (U) is applied to the target. To enable the plasma ignition Ar gas can be used. The gas is fed in the high vacuum chamber with a pressure of 0.5 up to 12 Pa. The interaction of ions within the target sputters the material, as well as generates secondary electrons, which will be responsible for further ionization of gas. A magnet ring is used above the target to increase the ionization rate by further increasing of the emitted secondary electron rate. Figure 1.1 shows schematically a magnetron-sputtering device.

superelastic1

Fig. 1.1: Schematic of magnetron sputtering device

A planar NiTi thin film deposited by magnetron sputtering technique has shown a remarkable superelastic strain up to 6.5%[1]. Based on those results a NiTi non-planar geometry is being sputtered using a device that provide to the substrate rotational movement during the sputtering process. This device is shown in the figure 1.2.

superelastic2_sc

 
Fig. 1.2: Magnetron sputtering with rotational device installed

 

The first challenge of this new concept is to understand the plasma motion during sputtering and the influence of the thin film growth in the mechanical properties of a non-planar film. To achieve such superelasticity some parameters will be adjusted, e.g.: target stoichiometry, substrate temperature, substrate material, sacrificial layer material, substrate rotation per minute, shutter geometry, distance between target and sample.

 

[1] H. Rumpf, V. Wipperfürth, Christiane Zamponi, E. Quandt. Fabrication of superelastic thin film based net - like structures - Actuator 2006

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