Some factors affecting the properties of sputter deposited NiTi-based shape memory alloy thin films

Abstract

Several factors affecting the surface morphology, transformation characteristics and shape recovery of co-sputtered NiTi-based shape memory alloy thin films were investigated. It is found that the Ar gas pressure, substrate orientation, deposition temperature and annealing condition all affect the surface morphology. In particular, when under high Argon gas pressure, the deposited film contains surface cracks, while the film is smooth and dense if a 2.3mTorr Argon gas pressure was used during deposition. The film deposited at 450 o C on a (100) Si substrate has a rough surface associated with a large distribution of island sizes. Post-annealing treatment leads to a more homogeneous distribution of island dimensions and a smoother surface. On the other hand, the films deposited at 450 o C on a (111) Si substrate or on a SiO 2 buffer layer have more homogeneous island sizes. It is also found that both the substrate orientation and the SiO 2 buffer layer dramatically affect the transformation behavior and shape recovery magnitude. Film deposited on (100) Si has a higher recovery strain than that deposited on (111) Si under the same stress. The oxygen in SiO 2 buffer layer may have deteriorated the deposited film, which results in a very low shape recovery strain. Interfacial stress between the substrate and the thin films is found to lower the transformation hysteresis.

Composition–Structure–Function Diagrams of Ti–Ni–Au Thin Film Shape Memory Alloys

“…Phase transformation hysteresis values (Δ T = A f – M s or R s or O s ) as a function of composition are plotted in Figure c. The compositions undergoing B2–R transformation exhibited a very small Δ T < 3 K. For some compositions they exhibited zero hysteresis behavior, which is associated with the physical-mechanical constraint (such as the substrate or large amount of precipitates) effect on a phase transforming material. ,− The compositions undergoing a two-stage transformation from B2 to R and successively to B19′ martensite, exhibited higher hysteresis values (Δ T > 30 K) that is consistent with the wide Δ T observed typically for transformation to B19′ martensite. Compositions undergoing the transformation from B2 to B19 martensite have intermediate Δ T values from 20 to 45 K, which is also consistent with values reported for bulk Ti–Ni–Au SMA…”