Fabrication and Characterization of Freestanding NiTi Based Thin Film Materials for Shape Memory Micro-actuator Applications

Thin film shape memory actuators fabricated by micro-system technology processes are of particular interest in micro-actuator applications due to their high work output, large displacements, and a comparably easy and compact system setup. Micro-system technology processes allow to deposit and structure ultra-precise parts from a wide range of alloys, among them alloys that are difficult to process with conventional fabrication techniques. However, sputtered NiTi based thin films are seldom deposited with thicknesses > 10 µm, which limits their range of application as well as makes the handling of freestanding films difficult. In this work, freestanding NiTi, NiTiCu, and NiTiHf shape memory structures were fabricated by means of sputtering, lithography, and wet etching, with a process that allows for thicknesses up to 80 µm. Their high cycle actuation behavior and microstructure were characterized. NiTiCu actuators show an excellent cyclic stability, resulting in high fatigue lives of > 10⁸ cycles at 1.5% strain, even for stresses as high as 550 MPa. A distinct martensite/austenite interface is observed during transformation, in contrast to NiTi exhibiting a rather homogeneous transformation during heating and cooling throughout the sample volume. Sputtered NiTiHf actuators tested in air can reach fatigue lives as high as 1.5 × 10⁶ cycles at 1% strain.