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 3N 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 4Ti 3. The results have also shown that the presence of an intermediate layer of Si 3N 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 3N 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 4Ti 3 precipitates that formed during the heat treatment.