TiNiCu Films Research Articles

TiNiCuAg shape memory alloy films for biomedical applications

TiNi-based shape memory alloys (SMAs) have been largely utilized in stents, orthopedic endo-prostheses and orthodontic implants. While bacterial infection in medical implants is the most common complication, development of antibacterial TiNi-based SMAs is essential. In the present study, a novel antibacterial TiNiCuAg shape memory alloy film (SMAF) was successfully developed by adding silver into TiNiCu matrix. The structural and biological properties of TiNiCuAg SMAF, including crystal structures, phase transformation temperatures, shape memory effect, antibacterial ability and cell cytotoxicity, were systematically investigated. The results showed that the phase transformation temperature gradually decreased and silver started to precipitate with the increasing silver content. The single-stage B2 ↔ B19′ transformation occurred in TiNiCu and Ti51Ni43.5Cu4.5Ag1 films, while the two-stage B2 ↔ R ↔ B19′ transformation occurred in Ti49.6Ni42Cu6.9Ag1.5, Ti47.4Ni41.2Cu6.4Ag5.6 and Ti47.7Ni41.2Cu4.1Ag7 films. Moreover, compared to TiNiCu films, TiNiCuAg films performed the better shape memory effect with the increasing silver content. For antibacterial tests, the TiNiCuAg films with the Ag content of 5.6 at.% had the best antibacterial ability against S. aureus and E. coli compared to those of TiNi and TiNiCu films. For cytotoxicity assay, all films maintained good biocompatibility and revealed no cell toxicity for L929 cells. As a result, excellent antibacterial ability and good biocompatibility suggested that TiNiCuAg SMAFs had potential applications for medical implant to avoid bacterial infection.

Ultra-Low Fatigue Quaternary TiNi-Based Films for Elastocaloric Cooling

Elastocaloric applications require superelastic shape memory materials which show high fatigue resistance, adjustable transformation temperatures, short heat transfer times, and large elastocaloric effect sizes. Ti-rich TiNiCu films are known for their high functional and structural stability of several million cycles without any degradation, accompanied by a small hysteresis caused by the good crystallographic compatibility of austenite and martensite phase. Still, for the application of TiNiCu as an elastocaloric cooling agent, transformation temperature adjustment is necessary. Quaternary Co and Fe alloying is found to reduce the transformation temperature by 42 and 22 K at.%−1, respectively, while maintaining high transformation enthalpies of 7.9 J g−1 for Ti54.7Ni30.7Cu12.3Co2.3 films. Furthermore, this specific alloy shows a 25 % larger coefficient of performance compared to binary TiNi films. Combined with the high fatigue resistance, the small transformation strain of 1.6 %, and the operational temperature range of ~50 K, this material is a very attractive candidate for elastocaloric cooling applications.

High cyclic stability of the elastocaloric effect in sputtered TiNiCu shape memory films

The elastocaloric effect that occurs during the stress-induced martensitic transformation in shape memory alloys is a promising mechanism in view of solid state cooling applications. It also allows for downscaling to feature sizes in the μm range, thus, being attractive for micro-cooling applications using thin film materials. In this study, elastocaloric properties of TiNi and TiNiCu films and their relation to functional fatigue were investigated. Both materials show similar effect sizes, their fatigue behavior is however different. While the temperature change in TiNi degrades by a factor of two within 150 cycles, no significant elastocaloric fatigue was found in TiNiCu.

Magnetron Sputtering Applied in Shape Memory Alloys Preparation

Shape memory alloys (SMAs) thin films have attracted much attention in recent years as intelligent and functional materials because of their unique characteristics. The sputtering thin films have been attracted great interest as powerful actuators in microelectromechanical systems (MEMS) such as microvalves, microfluid pumps, and microgrippers. Amongst SMAs, the equiatomic TiNi compound is the most widely used. In this paper, TiNi films prepared by magnetron sputtering are reviewed. The influence of parameters including thickness, target, substrate temperature and work pressure on the microstructure and properties of TiNi films are analysised. In additionally, several TiNi based ternary alloys films such as TiNiCu films, TiNiPd films and TiNiPt films are introduced. At last, the research direction of SMAs thin films is presented.

Stress induced texture and shape memory trench in TiNiCu films

Film texture and surface morphology of a crystallized TiNiCu film (deposited at 350 °C and post annealed at 450 °C in vacuum on a four inch wafer) have been characterized. X-ray diffraction analysis showed a preferred (002) martensite orientation at room temperature and a strong (110) austenite texture at temperature above 80 °C. Surface trenches were observed on the crystallized film surface at room temperature and disappeared when the film was heated to a high temperature above 80 °C. The formation of the strong texture and shape memory trenches is attributed to a significant stress evolution during post-annealing process. The variation of the trench morphology on the surface of the TiNiCu film surface over a four inch wafer after annealing has also been studied.

Effect of composition on surface relief morphology in TiNiCu thin films

TiNiCu films were deposited on a (100) type Si substrates by co-sputtering of TiNi and Cu targets at room temperature, and then post-annealed at 450 °C for crystallization. Significant surface relief due to martensitic transformation occurred on the film surface after cooling, and variations in surface relief morphology (the size and density of martensite crystals) have been quantified with atomic force microscopy. Energy dispersive X-ray analysis showed that the surface relief morphological difference in the TiNiCu films is due to the change in composition. It was proposed that the crystallization of the amorphous thin film during annealing behaved differently with chemical composition, which in turn induced fluctuations in phase transformation temperatures at various regions, as confirmed by differential scanning calorimeter measurements.

A TiNiCu Thin Film Micropump Made by Magnetron Co-Sputtered Method

Thin film SMAs have the potential to became a primary actuating mechanism for micropumps. In this study, a micropump driven by TiNiCu shape memory thin film is designed and fabricated. The micropump is composed of a TiNiCu/Si bimorph driving membrane, a pump chamber and two inlet and outlet check valves. The property of TiNiCu films and driving capacity of TiNiCu/Si bimorph driving membrane are

Crystal structure of orthorhombic martensite in TiNi-Cu and TiNi-Pd intermetallics

The crystal structure of orthorhombic martensite in ternary TiNi-based compounds has been several times examined, although, in all cases, the structure was not completely solved as the positions of atoms in the unit cell remained unknown. In the present work, the X-ray patterns of TiNi-Cu and TiNi-Pd films were refined by the Rietveld method revealing important regularities in the location of Ti and Ni atoms. In particular, the (010) atomic planes in the orthorhombic martensite structure are always distorted and loose the planar centre of symmetry. This observation was confirmed by ab-initio calculations, where the atomic positions were varied in order to minimise the energy of electron bands.

Deposition and characterization of Ti 1− x(Ni,Cu) x shape memory alloy thin films

TiNiCu films with different Cu contents (up to 15 at.%) were prepared by co-sputtering of TiNi and Cu targets using a magnetron sputtering system. Film crystalline structure, phase transformation and shape memory behaviors were characterized by X-ray diffraction, differential scanning calorimeters (DSCs) and curvature method. The substitution of Ni by Cu in TiNi based films resulted in a dramatic change in martensite structure, film orientation and phase transformation behavior. With the increase of Cu content in the films, both the transformation temperatures and hysteresis decreased significantly. From DSC and curvature measurement results, the specific heat and the maximum recovery stress (corresponding to actuation force) generated during martensite transformation decreased with Cu contents, indicating the weak performance of phase transformation, especially at high Cu contents above 9 at.%. There was a maximum value for the stress increase rate (corresponding to actuation speed) at a Cu content of 9 at.%.

Constrained martensitic transformations in TiNiCu films

TiNiCu shape memory alloy films were deposited at different temperatures on heated (1 0 0) Si cantilever-type substrates. The films are crystalline when deposited above 200 °C. The stress relief observed in the TiNiCu/Si bimorphs depends on the composition and the deposition temperature. For lower Cu contents a B2→R-phase→B19′ sequence of transformations was observed for films that start to transform under a low thermoelastic stress and a B2→B19′ sequence when the stress at the onset of the transformation is high. For higher Cu content the sequence of transformation observed was B2→B19→B19′ and B2→B19′ for low and high stress, respectively.

RF magnetron sputtered TiNiCu shape memory alloy thin film

Shape memory alloys (SMAs) offer a unique combination of novel properties, such as shape memory effect, super-elasticity, biocompatibility and high damping capacity, and thin film SMAs have the potential to become a primary actuating mechanism for micro-actuators. In this study, TiNiCu films were successfully prepared by mix sputtering of a Ti 55Ni 45 target with a separated Cu target. Crystalline structure, residual stress and phase transformation properties of the TiNiCu films were investigated using X-ray diffraction (XRD), differential scanning calorimeter (DSC), and curvature measurement methods. Effects of the processing parameters on the film composition, phase transformation and shape-memory effects were analyzed. Results showed that films prepared at a high Ar gas pressure exhibited a columnar structure, while films deposited at a low Ar gas pressure showed smooth and featureless structure. Chemical composition of TiNiCu thin films was dependent on the DC power of copper target. DSC, XRD and curvature measurement revealed clearly the martensitic transformation of the deposited TiNiCu films. When the free-standing film was heated and cooled, a ‘two-way’ shape-memory effect can be clearly observed.

Wear of titanium carbide base hardmetals: J.Kübarsepp et al. (Tallinn Technical University, Tallinn, Estonia.)

TiNiCu films were deposited on a (100) type Si substrates by co-sputtering of TiNi and Cu targets at room temperature, and then post-annealed at 450 °C for crystallization. Significant surface relief due to martensitic transformation occurred on the film surface after cooling, and variations in surface relief morphology (the size and density of martensite crystals) have been quantified with atomic force microscopy. Energy dispersive X-ray analysis showed that the surface relief morphological difference in the TiNiCu films is due to the change in composition. It was proposed that the crystallization of the amorphous thin film during annealing behaved differently with chemical composition, which in turn induced fluctuations in phase transformation temperatures at various regions, as confirmed by differential scanning calorimeter measurements.