NiTi Layers Research Articles

Surface modification of NiTi/PZT heterostructure thin films using various protective layers for potential MEMS applications

The present study explored the in-situ deposition of hard and adherent nanocrystalline protective coatings on NiTi/PZT/TiOx thin film heterostructure prepared by dc/rf magnetron sputtering. Protective layers (AlN, CrN and TiCrN) of approximate thickness (~200nm) were used to improve the surface, mechanical and corrosion properties of NiTi/PZT/TiOx heterostructure without sacrificing the shape memory effect and ferroelectricity of the NiTi and PZT layers, respectively. The influence of the protective layer on structural, electrical and mechanical properties of NiTi/PZT/TiOx heterostructure was systematically investigated and the results were compared. Nanoindentation studies were performed at room temperature to determine the hardness and reduced modulus. The surface modified NiTi/PZT/TiOx heterostructures were found to exhibit high hardness, high elastic modulus and thereby better wear resistance as compared to pure NiTi/PZT/TiOx films. From the results of potentiodynamic polarization test conducted in 1M NaCl solution, the CrTiN coated NiTi/PZT/TiOx heterostructure showed the best corrosion resistance with the lowest corrosion current density (1.52×10−8Acm−2) and the highest protective efficiency (96.8%). The results presented here prove the potential of a surface modified NiTi/PZT/TiOx heterostructure to be used in various microelectromechanical (MEMS) applications.

Influence of Oxide and Alloy Formation on the Electrochemistry of Ti Deposition from the NaCl ­ KCl ­ NaF ­ K 2TiF6 Melt Reduced by Metallic Ti

The redox reactions in melts reduced by titanium metal have been studied by cyclic voltammetry and chronopotentiommetry. At platinum and nickel electrodes waves due to alloy formation were seen preceding the metal deposition wave. The presence of oxide species could be detected as waves on voltammograms performed with platinum electrodes. Deposits on nickel plates in general consisted of multiphase Ni-Ti layers. By varying the deposition potential outer layers with compositions ranging from pure titanium over NiTi to nickel-rich alloys such as could be obtained. Pulse plating produced the best titanium coating. With constant potential electrolyses it was also possible to deposit titanium diboride coatings of high quality from the mixed chloride-fluoride melts. © 2004 The Electrochemical Society. All rights reserved.

ＮｉＴｉプラズマ肉盛層形成によるチタンの摩耗特性改善

In order to improve wear resistance of titanium, fabrication of NiTi intermetallic compounds as a surface layer by plasma transferred arc surfacing process (PTA process) was investigated. Powder of unalloyed nickel was fed into the plasma during the PTA process and NiTi intermetallic layers were successfully synthesized. The surface layers have no cracks and blowholes with optimized conditions of the PTA process. The surface layers that largely consist of NiTi intermetallic phase could be achieved, while the microstructures of them were significantly influenced by the conditions such as the arc current. The NiTi layers exhibited high resistance to sliding wear under non-lubricated conditions and were much more resistant than titanium.

Biocompatibility Studies of the Nitinol Thin Films

AbstractPulsed Laser Deposition method (PLD) was used to grow nitinol (NiTi) thin films with goal of investigating their biocompatibility. High purity Ni and Ti targets were alternatively ablated in vacuum with a laser beam (λ=355 nm, 10 Hz) and the material was collected on room temperature Ti substrates. X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy and atomic force microscopy analyses have been performed to investigate the chemical composition, crystalline structure and surface morphology of the NiTi films. The nitinol layers biocompatibility has been tested using as a metric the extent to whichthe cells adhereduring the culture period on the surface of NiTi layers deposited on Ti substrates. Vero and fibroblast cell lines dispersed into MEM (Eagle) solution containing 8% fetal bovine serum, at 37° C, were used for tests. Preliminary studies indicate that the interaction at the interface is specifically controlled by the surface morphology, (especially by surface roughness), and by the chemical state of the surface. Cell behavior after contact with NiTi/Ti structure for different intervals (18, 22 and 25 days for the Vero cells, and after 10 and 25 days for fibroblasts) supports the conclusion that NiTi is a very good candidate as a biocompatible material.

Vacuum plasma spraying of NiTi protection layers

A promising application for NiTi intermetallics is as a protection layer against corrosion or wear. In the present investigation, NiTi layers were produced by vacuum plasma spraying (VPS) using elemental powder mixtures as the starting material with a composition of 50.8 at.% Ni. This paper reports on studies of the microstructure and the phase formation of samples produced by different spraying parameters.

A new fabrication process for Ni–Ti shape memory thin films

Abstract A new fabrication method for Ni–Ti thin films showing martensitic phase transformation is presented. A multilayer of up to 100 alternating pure Ni and pure Ti layers is deposited in an automated DC magnetron sputter system optimized for homogeneous deposition on large substrates. The single layer thickness is in the range of 10–20 nm. The multilayer is subjected to different heat treatments in order to allow for interdiffusion of Ni and Ti and subsequent recrystallization as a Ni–Ti intermetallic compound. The martensitic and austenite phase in the resulting films could be identified by means of X-ray diffraction. Differential scanning calorimetry (DSC) measurements monitor the transformation behavior of films with different composition and indicate the existence of an R-phase. The microstructure of the annealed films is investigated by transmission electron microscopy (TEM). The composition and transformation temperatures can be adjusted by varying the thickness ratio of the initial Ni and Ti layers. These films show a well pronounced shape memory effect for certain annealing procedures.

Cycling Times of Thin-Film NiTi on Si

AbstractThe thermo-mechanical properties of NiTi are well known for bulk material although its deposition and utilization as a thin film are still in their earliest stages. The deposition of thin-films of Shape Memory Effect NiTi onto Si(100) wafers offers several advantages over bulk NiTi, including fast response times and comparitively large transformation forces, and so is a promising candidate as a micro-actuator for MicroElectroMechanical (MEMS) systems as well as in strain measurements. The response time for a variety of NiTi layers were modelled under different boundary conditions and show response times similar to the acoustic velocities for one micron thick NiTi.

A comparison by means of calorimetry of solid state interdiffusion reactions in Ni/Zr and Ni/Ti composites

We have investigated solid state amorphization reactions in mechanically deformed composites in both the Ni-Ti system and the Ni-Zr system. The growth of amorphous material in our Ni/Ti composites is apparently facilitated by the relatively large degree of disorder induced in the metal layers by the mechanical deformation process. The growth of amorphous material is slower in Ni/Ti composites than in Ni/Zr composites, while we found similar kinetic constraints on the formation of equilibrium compounds in both systems. Thus the maximum thickness of amorphous Ni-Ti layers was an order of magnitude less than the 1000-Å layers grown in the Ni-Zr system.

On the validity of a thermal spike mixing model for low-Z metals

Low temperature ion beam mixing rates for Cu-Ti, Ni-Ti, and Fe-Ti layers have been found to be significantly lower than predicted by a popular semi-empirical thermal spike model. It has been proposed that the unavoidable hydrogen contamination of the as-deposited Ti films may have reduced the mixing rates, but the measurement of even lower mixing rates for Fe-V and Fe-Co bilayers shows the discrepancy to be more fundamental. Still, a systematic dependence on heat of mixing suggests that some sort of diffusional (thermal spike?) mechanism is involved.