| TiNi shape memory alloys are suitable for MEMS microactuator applications due to their high work densities, high recoverable stresses, and high recoverable strains. Near-equiatomic TiNi shape memory alloy thin films with high transformation temperatures, low residual stresses, and high recoverable stresses have been sputter-deposited successfully. The correlation between deposition parameters (target, Ar pressure, substrate temperature, substrate rotation speed, and target power and voltage) and material properties (transformation temperature, film stress, and resistivity) has been characterized. It was found that near-equiatomic TiNi films with residual austenite and R-phase at RT are more suitable than equiatomic TiNi films with 100% martensite for MEMS microactuators, because of higher recoverable stresses. Beside film composition, film stress also plays a critical role on the shape memory effect of TiNi films. The intrinsic and thermal stress evolutions of TiNi films have been studied.; The phase content and transformation behaviors in the TiNi films have been studied using high temperature X-ray diffraction, resistivity measurement, and transmission electron microscopy. The phase fraction was obtained using the Rietveld analysis. R-phase was observed in all TiNi films; however the R-phase related transformations do not have an apparent effect on stress-temperature hysteresis curves. Instead, the stress-temperature hysteresis curve is closely related to the phase fraction evolution of martensite. Grazing-incidence angle XRD reveals a greater phase fraction of martensite near the film surface, indicating that phase fraction distribution is not uniform through the film thickness. This phenomenon plays an important role on the transformation temperatures, transformation slope, and temperature hysteresis of stress-temperature hysteresis curves. Below 0.5 μm, film thickness has a significant effect on the shape memory effect because of the presence of surface and interface reaction layers. R-phase, martensite, and austenite in the same grain was observed using TEM. However, the transformation sequence between these three phases within a single grain still needs more studies. |
