Development of sputtered Shape Memory Alloy (SMA) Ni–Ti films for actuation in ice cooled environments

Abstract Due to the high sensitivity of Ni–Ti films to environmental changes, e.g. thermal, and/or to stress, they are ideal materials for applications on micro-sensors. It was aimed to obtain Ni–Ti films exhibiting the beginning of the B2 ⇔ R-phase transformation between room temperature (RT) and 0 °C. Thus, films with a slightly Ni-rich composition were prepared by sputtering, without intentional heating of the substrate. The Ni–Ti films were deposited on an Si 3 N 4 intermediate layer previously deposited on naturally oxidized Si(100). The crystallization behaviour of the samples (at a constant temperature of 430 °C) was studied by X-ray diffraction in grazing incidence geometry off-plane (GIXD) at a synchrotron-radiation beamline. The GIXD patterns obtained during the annealing process of the Ni–Ti polycrystalline films revealed mainly an austenitic structure (B2 phase) and the precipitation of Ni 4 Ti 3 . The results have also shown that the presence of an intermediate layer of Si 3 N 4 enhances the crystallization process of the Ni–Ti sputtered films when compared to the films deposited directly on single-crystal Si (with native oxide). The phase transformation behaviour of the Ni–Ti film on Si 3 N 4 was evaluated by XRD in off-plane Bragg–Brentano geometry during cooling (RT → −40 °C) and heating (−40 °C → RT). It has been observed that a high fraction of the Ni–Ti film is already transformed to R-phase at 9 °C (austenitic at RT), as well as a very small temperature hysteresis for the B2 ⇔ R-phase transformation. After the characterization described above, the film was removed from the substrate. The free-standing film showed a pronounced “two-way” shape memory effect (SME). In the austenitic state the film presents a flat shape. During cooling, by reducing its distance from ice cubes (i.e., decreasing the surrounding temperature), the film starts bending exhibiting a final curled shape (yet without touching the ice). On heating it recovers its flat shape. The authors attribute the nature of this “two-way” SME to the Ni 4 Ti 3 precipitates that formed during the heat treatment.