NiTi Shape Memory Thin Films Research Articles

Synthesis, Structure and Optical Properties of NiTi Shape Memory Thin Films

NiTi Shape Memory Alloy (SMA) is now widely used in important applications due to its shape memory effect (SME) and biocompatibility. However, there is still a challenge to control the sizes and shapes of these nanoparticles. Here, we report on the use of pulsed laser deposition technique for the cultivation of NiTi nanoparticles on glass substrates. It discussed the implications of the case of the influence of depositing laser energy on the size of a grain of NiTi nanoparticle. The nominated effect on the optical properties of its thin-film was also discussed. X-ray diffraction (XRD) analysis revealed that the films prepared at 800[Formula: see text]mJ, 900[Formula: see text]mJ and 1000[Formula: see text]mJ were amorphous structures. The Atomic Force Microscopy (AFM) results confirmed that the thin films are made up of spherical particles that are evenly distributed (in terms of size). A blueshift in the bandgap was observed in the UV–Visible absorption spectra with increase in the depositing laser energy due to the quantum confinement effect of nanoparticle formation.

Surface microstructures and corrosion resistance of Ni-Ti-Nb shape memory thin films

Ni-Ti-Nb and Ni-Ti shape memory thin films were sputter-deposited onto silicon substrates and annealed at 600°C for crystallization. X-ray diffraction (XRD) measurements indicated that all of the annealed Ni-Ti-Nb films were composed of crystalline Ni-Ti (Nb) and Nb-rich grains. X-ray photoelectron spectroscopy (XPS) tests showed that the surfaces of Ni-Ti-Nb films were covered with Ti oxides, NiO and Nb2O5. The corrosion resistance of the Ni-Ti-Nb films in 3.5wt.% NaCl solution was investigated using electrochemical tests such as open-circuit potential (OCP) and potentio-dynamic polarization tests. Ni-Ti-Nb films showed higher OCPs, higher corrosion potentials (Ecorr) and lower corrosion current densities (icorr) than the binary Ni-Ti film, which indicated a better corrosion resistance. The reason may be that Nb additions modified the passive layer on the film surface. The OCPs of Ni-Ti-Nb films increased with further Nb additions, whereas no apparent difference of Ecorr and icorr was found among the Ni-Ti-Nb films.

Fatigue and adhesion properties of martensite and austenite phases of TiNi shape memory thin films deposited by magnetron sputtering

The application area of TiNi shape memory thin film is gradually increasing. Especially MEMS applications of TiNi film, such as micro-mirror, micro-pump and micro-actuator are widely used. Adhesion to the substrate and fatigue resistance of these films is particularly important for these applications. In this study, we investigated adhesion and fatigue properties of martensite and austenite phases of TiNi shape memory thin films. TiNi films were deposited on copper and Si wafers with magnetron sputtering. Structural properties of the films were determined via SEM, XRD and EDS analyses. Adhesion and fatigue properties of the films were investigated through progressive and multi-pass scratch tests. Phase transformation temperatures and hysteresis were observed with differential scanning calorimeter (DSC). Scratch test results showed that adhesion and fatigue resistance of austenite phase of TiNi film is higher than martensite phase.

A low temperature in-situ crystalline TiNi shape memory thin film deposited by magnetron sputtering

ABSTRACT TiNi films deposited by magnetron sputtering usually have amorphous structure and must be annealed at high temperature to obtain crystallization. We have synthesized an in-situ fully crystalline TiNi shape memory thin film at low temperature by dc magnetron sputtering. Application of pulsed direct current to the substrate is effective to obtain a crystalline TiNi film. Nine different conditions were used for deposition in silicon wafer and thin copper plate substrates. Structural properties and phase transformation temperatures of the TiNi films were investigated. To examine the structural properties of the films, XRD, SEM and EDS techniques were used. Austenitic and martensitic phase transformation temperatures were observed via DSC (differential scanning calorimeter) tests. TiNi (110) B2 austenite peaks were observed in the Run7 film. The crystalline Run7 TiNi film showed single-stage phase transformation (B19 to B2 on heating and B2 to B19 on cooling).

The martensitic transformation of Ti–Ni shape memory thin films under proton irradiation

The martensitic transformation behavior of a Ti–Ni alloy irradiated by proton with different doses has been investigated. It is found that the samples irradiated by 150keV protons have a two-step phase transformation during heating and only one-step transformation during cooling. The exothermic peak at higher temperature disappears during the following thermal cycling. A model based on the stress-assisted martensitic transformation was established by the Transport of Ions in Matter (TRIM) calculations in order to explain the transformation behavior.

X-ray diffraction study of the reverse martensitic transformation in NiTi shape memory thin films

The development of stresses, phase fractions and the microstructure of thin equiatomic NiTi substrate-bound films was investigated during the reverse transformation from martensite to austenite. Synchrotron X-ray diffraction (XRD) experiments were performed during the heating portion of thermal cycling applied to the thin films to capture, in particular, the reverse martensitic phase transformation (monoclinic martensite→cubic austenite). The phase fractions and microstructure, as a function of temperature and thermal cycling, were analyzed through the application of Rietveld refinement to the diffraction data. Further, using the XRD data, the overall macroscopic stress in the film (derived from the curvature of the film/substrate system determined by XRD rocking curve measurements) and the stress in the austenite phase (derived from the lattice strain) during the transformation were tracked as a function of the degree of the transformation. The state of the stress in the austenite was found to remain biaxially, rotationally symmetric, even in the two-phase (martensite and austenite) film. The developments of the total stress in the film and the stresses in each of the two phases are discussed in terms of the transformation-induced volume misfit and its accommodation by elastic deformation.

Characterization and Prediction of Texture in Laser Annealed NiTi Shape Memory Thin Films

Thin film shape memory alloys are a promising material for use in microscale devices for actuation and sensing due to their strong actuating force, substantial displacements, and large surface to volume ratios. NiTi, in particular, has been of great interest due to its biocompatibility and corrosion resistance. Effort has been directed toward adjusting the microstructure of as-deposited films in order to modify their shape memory properties for specific applications. The anisotropy of the shape memory and superelastic effects suggests that inducing preferred orientations could allow for optimization of shape memory properties. Limited work, however, has been performed on adjusting the crystallographic texture of these films. In this study, thin film NiTi samples are processed using excimer laser crystallization and the effect on the overall preferred orientation is analyzed through the use of electron backscatter diffraction and X-ray diffraction. A three-dimensional Monte Carlo grain growth model is developed to characterize textures formed though surface energy induced abnormal grain growth during solidification. Furthermore, a scaling factor between Monte Carlo steps and real time is determined to aid in the prediction of texture changes during laser crystallization in the partial melting regime.

Nanoindentation of NiTi shape memory thin films at elevated temperatures

The shape memory effect and nanoindentation response of various phases of sputtered NiTi shape memory thin films were investigated as a function of temperature. The phase transformation temperatures of NiTi films were observed to be sensitive to a compositional shift. The mechanical properties of NiTi thin films also presented a significant response to phase transformations. At the same load, the maximum indentation depth for austenite is smaller than for martensite, indicating that martensite is softer than austenite. A martensite thin film was converted to austenite via in situ heating nanoindentation and displayed the mechanical properties similar to the austenite film at room temperature. These results underscore the validity of elevated temperature nanoindentation methods as a means of interrogating the mechanical properties of materials that undergo thermally-induced phase transformations. The details of the load–displacement curves are also described.

The effect of film composition on the structure and mechanical properties of NiTi shape memory thin films

Shape memory NiTi-based thin films approximately 2 μm thick were deposited onto Si (100) substrates at room temperature by simultaneous DC magnetron sputter deposition from separate elemental Ni and Ti targets. The effect of composition on film structure, surface morphology, transformation temperature and mechanical behavior was studied using variable temperature X-ray diffraction, atomic force microscopy, electrical resistivity, and nanoindentation. The films showed the expected shape memory and superelasticity behavior corresponding to the different film compositions, comparable with bulk properties. The transformation from the low temperature martensitic phase to the high temperature parent phase takes place above room temperature in Ti-rich and near-equiatomic films, and below room temperature in Ni-rich films. Mechanical properties of films investigated at room temperature by a series of nanoindentations at mN loads (indentation depth < 200 nm) with a spherical indenter demonstrate superelasticity in Ni-rich material and martensitic deformation for Ti-rich and near-equiatomic compositions.

Ｔｉ‐Ｎｉ系形状記憶合金薄膜の微細組織と機械的挙動

Ｔｉ‐Ｎｉ系形状記憶合金薄膜の微細組織と機械的挙動

Shape memory surfaces

Temperature-controlled reversible surface protrusions can be made on NiTi shape memory alloys and thin films as a result of indentation induced two-way shape memory effect. First, spherical indents or scratches are made on the surface of a NiTi alloy in its martensite phase. Second, the indented or scratched surface is planarized to restore a flat surface. Reversible circular and line protrusions are produced by altering the temperature to drive the martensite to austenite phase transformation. This phenomenon can be exploited for a wide range of optical, tribological, and microelectromechanical device applications.

Characterization and MEMS application of low temperature TiNi(Cu) shape memory thin films

Shape memory thin films offer a unique combination of novel properties, which promise some exciting application potentials in micro-electromechanical systems (MEMS). In this study, Ti 48.9Ni 46.3Cu 4.8 (at.%) driving thin films were successfully prepared by radio-frequency magnetron sputtering. Crystallographic structure, surface morphology and phase transformation temperatures were investigated by X-ray diffraction, scanning electron microscopy, atomic force microscope and differential scanning calorimeter, respectively. Applying a MEMS process, a valve-less micropump of Ti 48.9Ni 46.3Cu 4.8 shape memory thin film for low temperature application is fabricated. The results show that the phase transformation between martensite and austenite is rather low, below 290 K, and the surface of the thin films is smooth. The displacement measurement clearly revealed the TiNi(Cu) diaphragm valve-less pump has superior performance such as high diaphragm displacement (up to 6 μm) and high working frequency (up to 85 Hz).

Fabrication and characterization of sputtered NiTi shape memory thin films

Sputtering conditions to produce nearly equiatomic NiTi films were studied. Two heat treatment methods, substrate heating (550 °C) and post-sputter heat treatments (solution heat treatment at 700 °C and precipitation heat treatment at 400 °C), were used to crystallize NiTi films. The NiTi films were analysed for their atomic composition, crystal structure, transformation temperatures, residual stress and Young's modulus. NiTi films produced by rf magnetron sputtering were slightly titanium rich. With appropriate post-sputter heat treatment, NiTi films with phase transformation temperatures within the human body temperature range (34–42 °C) were produced. The films also demonstrated two-way shape memory training capability.

Numerical investigation of CO2 laser heating for annealing Ni/Ti multilayer thin films

Annealing Ni/Ti multilayer thin films in order to achieve better composition control of theresultant NiTi shape memory thin films is proposed. To avoid furnace annealing, we numericallyinvestigate the feasibility of using a laser to heat the thin films. It is demonstrated that use of aCO2 laser can be an efficient approach for annealing Ni/Ti multilayer thin films. It is suggestedthat a Ti layer should be deposited first on a silicon substrate for better absorption of laserenergy and adhesion. As the high temperature area can be confined within the micronscale, it is fully compatible with the current IC/MEMS processing techniques.

Laser annealing of amorphous NiTi shape memory alloy thin films to locally induce shape memory properties

We present the results of a crystallization study on NiTi shape memory thin films in which amorphous films are annealed by a scanning laser. This technique has the advantage that shape memory properties can be spatially distributed as required by the application. A kinetics study shows that nucleation of the crystalline phase occurs homogenously in the films. Consequently, the laser annealing process produces polycrystalline films with a random crystallographic texture. The crystallized films have a uniform microstructure across the annealed areas. The material in the crystalline regions transforms reversibly to martensite on cooling from elevated temperature and stress measurements show that a significant recovery stress is achieved in the films upon transformation.

Micromirror structure actuated by TiNi shape memory thin films

TiNi films were deposited by co-sputtering TiNi and Ti targets. Results from differential scanning calorimetry and curvature measurement revealed martensitic transformation and shape memory effect upon heating and cooling. Two types of TiNi/Si micromirror structures with a Si mirror cap (40 µm thick) and TiNi/Si actuation beams were designed and fabricated. For the first design, a V-shaped cantilever based on the TiNi/Si bimorph structure was used as the actuation mechanism for the micromirror. In the second design, three elbow-shaped Si beams with TiNi electrodes were used as the arms to actuate the mirror. The TiNi/Si microbeams were flat at room temperature and bent up by applying voltage in the TiNi electrodes (due to phase transformation and shape memory effect), thus causing changes in angles of the micromirror.

Irradiation-induced phase transformation in undeformed and deformed NiTi shape memory thin films by high-energy ion beams

Irradiation-induced transformations in sputter-deposited Ti-rich NiTi thin films have been investigated with special attention given to the effects of prior deformation on the response of martensite phase. Irradiation at low fluence was carried out at room temperature with Au ions at 350 MeV, such that the projected ion range greatly exceeded the film thickness and lattice damage was attributable mainly to electronic stopping effects (43 keV nm−1). Ion-beam modified microstructures were investigated using transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry. Amorphous tracks were observed in both Ti2Ni precipitates and martensite. The track diameter and the amount of amorphization were larger in the pre-deformed sample as compared to the undeformed sample. In both cases the displacive transformation temperatures and transformation enthalpies were depressed by irradiation, but to a greater degree in the pre-deformed material. Similarly, the extent of austenitic and R-phase regions observed to surround individual ion tracks was substantially greater for the pre-deformed films. These observations are discussed in terms of differences between the deformed and undeformed cases with respect to microstructure.

The effect of crystallizing procedure on microstructure and characteristics of sputter-deposited TiNi shape memory thin films

Two kinds of Ti-rich TiNi shape memory thin films were deposited onto monocrystal silicon substrates at room temperature and high temperature by using the magnetron-sputtering technique. Their crystallizing procedures are different from each other. The first type of film was originally amorphous and post-crystallized at a higher annealing temperature (600 °C) after sputtering, but the second film type was crystallized in situ during sputtering at about 500 °C. It was found that there are clear differences of microstructure and characteristics between both kinds of film, such as grain size, growth texture, stress range, phase transformation behaviors etc. In order to improve and enhance film properties, the objective of the present work is to reveal the reason for these differences occurring and understand the relationship between the crystallizing procedures, microstructures and characteristics of the films.

Fabrication and characterization of Ti–Ni shape memory thin film using Ti/Ni multilayer technique

Alloying process and shape memory properties of a Ti/Ni multilayer thin film fabricated by a dual d.c. magnetron sputter–deposition method were investigated, and compared with those of a Ti—Ni amorphous thin film fabricated by an alloy target sputter–deposition method. The multilayer thin film was made by depositing Ti and Ni layers alternately on a SiO2/Si substrate. The Ti and Ni of the Ti/Ni multilayer thin film were crystalline after deposition. Alloying of the Ti/Ni multilayer thin film proceeded in multi steps. Amorphous phase was formed at the interfaces between the Ti and Ni layers by inter-diffusion of the Ti and Ni atoms during heating up to 640 K. Ni-rich Ti–Ni B2 phase was formed during heating up to 710 K. During further heating up to 750 K, the Ni-content of the Ti–Ni B2 phase decreased and Ti2Ni phase was formed. The Ti/Ni multilayer thin film exhibited shape memory effect after heat-treatment at 673 K where Ti–Ni amorphous thin films were not crystallized. The heat-treated Ti/Ni multilayer thin films exhibited the shape memory effect equivalent to that of the heat-treated Ti—Ni amorphous thin films when the heat-treatment temperature was above 873 K.

Characterization of sputtering deposited NiTi shape memory thin films using atemperature controllable atomic force microscope

NiTi shape memory thin films are potentially desirable for micro-electro-mechanical system(MEMS) actuators, because they have a much higher work output per volume and also asignificantly improved response speed due to a larger surface-to-volume ratio. A newtechnique using a temperature controllable atomic force microscope (AFM) is presentedin order to find the transformation temperatures of NiTi shape memory thinfilms of micrometer size, since traditional techniques, such as differential scanningcalorimetry (DSC) and the curvature method, have difficulty in dealing with samples ofsuch a scale as this. This technique is based on the surface relief phenomenon inshape memory alloys upon thermal cycling. The reliability of this technique isinvestigated and compared with the DSC result in terms of the transformation fraction(ξ). It appears that the new technique is nondestructive, in situ and capable of characterizingsputtering deposited very small NiTi shape memory thin films.