Freestanding NiTi-based thin film actuator materials

Besides binary NiTi, many ternary and quaternary NiTi-based alloys exist that reveal extraordinary properties, such as reduced or increased hysteresis, excellent functional stability during superelastic or thermal cycling, high transformation temperatures, etc. Some of these alloys with specifically tuned properties are difficult to process into thin wires or sheets, e.g. due their brittleness. High purity thin films of these alloys can be fabricated and patterned by the combination of several microsystem technology processes, i.e. magnetron sputtering, UV-lithography and wet chemical etching.

In this work, freestanding dog bone shaped thin film actuators of 20-25µm thickness were fabricated and their functional behavior, fatigue life and microstructure investigated by means of DSC, XRD, and a self-built setup for strain detection during thermal cycling under constant load. Different NiTi-based actuator alloys were investigated. Results indicate a tremendous fatigue life for NiTiCu_12.5 films (e.g. 10⁸ cycles at 350 MPa and ~1.5% strain, tested in tempered silicone oil), which correlates with the good crystallographic compatibility of the martensitic and the austenitic phase.
The transformation temperature A_f of the sputtered high temperature alloy NiTiHf₁₉ varies with heat treatment, between 215°C and 260°C, and shows a constant hysteresis of ~100K. First experiments in air show promising results (>10⁶ cycles at 300 MPa and ~1%strain), and even higher fatigue life is expected when surface oxidation during thermal cycling is avoided.

With regard to the above results, the fabrication route using microsystem technology processes is a promising alternative for the development and fabrication of new high performance shape memory actuators.