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Abstract: TiNi films were prepared by co-sputtering TiNi and Ti targets. X-ray photoelectron spectroscopy (XPS) was employed to study surface chemistry of the films and interfacial structure of Si/TiNi system. Exposure of the TiNi film to the ambient atmosphere (23°C and 80% relatively humidity) facilitated quick adsorption of oxygen and carbon on the surface. With time, carbon and oxygen content increased drastically at the surface, while oxygen diffused further into the layer. After a year, carbon content at the surface became as high as 65.57% and Ni dropped below the detection limit of XPS. Depth profiling revealed that significant inter-diffusion occurred between TiNi film and Si substrate with a layer of 90–100nm. The detailed bond changes of different elements with depth were obtained using XPS and the formation of titanium silicides at the interface were identified. [Copyright &y& Elsevier]

Abstract: TiNi films with different Ti/Ni ratios were deposited on Si substrates with and without silicon nitride interlayer. Near-equiatomic TiNi films were found to have the lowest residual stress and the highest recovery stress regardless of the existence of silicon nitride interlayer. The addition of silicon nitride interlayer between film and Si substrate did not cause much change in phase transformation behavior as well as adhesion properties. X-ray photoelectron spectroscopy (XPS) analysis revealed that there is significant interdiffusion of elements and formation of Ti–N and Si–Si bonds at TiNi film/silicon nitride interface. Scratch test results showed that adhesion between the TiNi film and substrate was slightly improved with the increase of Ti content in TiNi films. [Copyright &y& Elsevier]

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.%. [Copyright &y& Elsevier]

Shape memory alloy TiNi films were prepared on Si and Si/SiO2 by co-sputtering of TiNi target (Ti 50 at.% and Ni 50 at.%) and Ti target (99.999 at.%). When comparing Si/TiNi with Si/SiO2/TiNi, the addition of SiO2 sandwich layer did not result in significant change in the crystalline structures and shape memory properties. The SiO2 sandwich layer served as an effective diffusion barrier in preventing formation of interfacial titanium silicide at the expense of film adhesion. As revealed by X-ray photoelectron spectroscopy, under the same conditions, the interfacial diffusion zone in Si/TiNi system was approximately 120 nm while in Si/SiO2/TiNi system the diffusion zone was only approximately 30 nm—a four-fold reduction in inter diffusion. Meanwhile, the scratch adhesion testing registered a sharp drop in adhesion of the film from 170 mN in Si/TiNi system to 60 mN in Si/SiO2/TiNi system. [Copyright &y& Elsevier]

The presence of an adherent and hard TiN layer (200 nm) on TiNi-based shape memory alloy (SMA) film (3.5 μm) formed a passivation layer, improved hardness and tribological properties, without sacrificing the phase transformation and shape memory effect of the TiNi film. [Copyright &y& Elsevier]

TiNi based shape memory alloy (SMA) thin films have been widely used in biomedical and microelectromechanical systems. However, there are still some concerns for their wide applications because of their unsatisfactory mechanical and tribological performances, chemical resistance and biological reliability. This paper explored the deposition and effects of TiN protective layer on TiNi thin films to improve their surface properties while retaining the shape memory effects. Results indicated that the presence of an adherent and hard TiN layer (300 nm) on TiNi based SMA film (3.5 μm) formed a passivation layer, improved hardness and tribological properties, without sacrificing the phase transformation and shape memory effect of the TiNi film. [Copyright &y& Elsevier]

TiNi films were prepared by co-sputtering a Ti50Ni50 target and a Ti target under different process parameters. Stress in TiNi film on silicon substrate were evaluated using curvature method. A wide range of residual stress levels (either tensile or compressive) were found in the deposited films, depending significantly on Ti/Ni ratio, gas pressure, deposition temperature and/or annealing conditions. Stress evolution for as-deposited amorphous films during annealing process were systematically studied with the consideration of residual stress in as-deposited films; thermal stress; tensile stress due to densification crystallization; stress relaxation due to martensite transformation, etc. Upon heating, the crystallized TiNi films generated large tensile stresses when transforming from martensite to austenite, whereas during cooling, the stress relaxed significantly when austenite transformed back to the ductile martensite. The stress generation and relaxation behavior was significantly affected by film composition, deposition temperature and/or annealing temperature. [Copyright &y& Elsevier]

TiNi films with different Ni/Ti ratios were prepared by co-sputtering of a Ti50Ni50 (at.%) target with a separated Ti target at a temperature of 723 K. The stress values in the deposited films changed significantly with Ti contents and post-annealing temperatures due to the differences in phase transformation behaviors and intrinsic stress. For the film with Ti content of 51.3%, a two-step transformation was observed among martensite, R-phase and austenite, and residual stress was quite low at room temperature. After post-annealing the above film at 923 K, only one-stage transformation was observed. For the films with Ti contents of 47.3 and 53%, residual stress was quite high due to the high intrinsic stress and partial relaxation of stress caused by the R-phase transformation. When the films with Ti contents of 47.3 and 53% were annealed at 923 K, residual stress in films decreased significantly, because post-annealing could probably modify the film structure, reduce the intrinsic stress, increase the transformation temperatures, and cause martensite transformation above room temperature. For all the three types of films annealed at 1023 K for 1 h, the high thermal stress in the thin films could cause non-recoverable deformation during phase transformation, damage the shape memory effect, and result in the peeling-off of film from Si substrate. [Copyright &y& Elsevier]

Residual stress, relaxation and recovery of stress during martensite transformation of the sputtering-deposited TiNi films were studied in this paper. Results showed that a wide range of residual stress levels (both tensile and compressive) were obtained with the change of Ti contents in the deposited films. After film deposition, the thermal and intrinsic stress in the film could be very large, but due to the differences in phase transformation stress and transformation temperatures, the real stress values under room temperature, and stress evolution with temperature during phase transformation were quite different. For the film with a Ti content of 50.2%, due to the significant relaxation of stress caused by the perfect martensite transformation, the residual stress under room temperature was very low (−20 MPa) and recovery stress during heating was as high as 750 MPa. With both decrease and increase in Ti content in the deposited films, the tensile residual stress increased significantly. For the films with a Ti content of 51.3%, a two-step transformation was observed among martensite, R-phase and austenite. The recovery stress and average increase rate of stress during both heating and cooling were lower than those of film with 50.2% Ti. For the films with a Ti content of 47.3 and 53%, only partial relaxation of stress occurred due to the existence of R-phase transformation at room temperature. Also, because of non-equilibrium contents of Ni and Ti in these films, shape memory effect and the recovery of stress due to the phase transformation was not so significant. High residual stress in the thin films could cause non-recoverable deformation during phase transformation. [Copyright &y& Elsevier]