Improvement in corrosion resistance of NiTi by anodization in acetic acid

NiTi was galvanostatically anodized in acetic acid to increase the oxide film thickness for improving corrosion resistance. The galvanostatic anodization behavior of NiTi in acetic acid differed markedly from that of Ti. In particular, the anode potential reached was much lower for NiTi, and thus only thin oxide films could be obtained. With a suitable choice of anodizing conditions, the anodic oxide film formed had a thickness of 20–25 nm, as determined by profilometric measurement. Atomic force microscopy (AFM) revealed that the surface roughness was increased after anodization. Analysis by X-ray photoelectron spectroscopy (XPS) showed a low Ni/Ti ratio of 0.04 at the anodic oxide surface versus a value of 0.30 for bare NiTi. Electrochemical impedance measurements of the anodized sample in Hanks' solution at 37 °C recorded a 9-fold increase in polarization resistance, and cyclic polarization tests also recorded a matching reduction in the passive current density. These observations indicate that anodization of NiTi can serve as a simple low-temperature method to enhance the corrosion resistance of NiTi when used as an implant material.     

The location of atomic hydrogen in NiTi alloy: A first principles study

A first principles density functional theory study to investigate the H defect in NiTi alloy is presented. We have determined the interstitial H atom position in bulk B2 phase NiTi alloy. H positions on both the Ti and Ni terminated NiTi surfaces are calculated. Surface adsorptions of H atom on Ni/Ti terminated surfaces are calculated for a low surface coverage of 1.96 × 10¹⁴ cm^?2. We have also calculated the penetration barrier energy for an H atom from the surface site to the bulk lattice site.     

Mechanical properties of additive laser-welded NiTi alloy

Laser welding would be a suitable joining technique for NiTi shape memory alloy if the mechanical properties of laser weld were improved. With this purpose, effects of additive on mechanical properties of laser-welded NiTi alloy have been experimentally studied. Welding specimens used in this study were 2 mm thick hot-rolled plates with a chemical composition of Ni50.9Ti49.1. (Ni50.9Ti49.1)-Ce2 (at.%) alloy foil or Ni47Ti44Nb9 plate was used as filler metal to add Ce or Nb element into NiTi laser weld metal. Both tensile strength and the toughness of additive-welding specimens were improved significantly compared with non-additive-welding specimen. The mechanical property improvement was attributed to the fine solidification NiTi grains and good grain-linking in weld center. The microstructure control mechanisms of these two additive welds were discussed.     

Extremely high pitting resistance of NiTi shape memory alloy thin film in simulated body fluids

In this paper the corrosion behavior of NiTi thin films fabricated by sputtering from Ni and Ti targets has been studied by cyclic potentiodynamic polarization tests in Hank's and Ringer's solution at 310 K. For comparison, bulk NiTi Shape Memory Alloy (SMA) has also been studied to elucidate the different corrosion behavior of bulk and thin film material. The electrochemical experiments reveal that thin film NiTi SMA has comparable corrosion current density (i_corr), much higher pitting corrosion potentials and wider passive range than the bulk NiTi. We show that NiTi SMA vapour deposited thin films are less susceptible to pitting corrosion than the bulk.     

NiTi合金上沉积氮化碳薄膜的力学和血液相容性研究

采用磁控溅射法在生物医用NiTi合金基体表面制备了Ti/CN_x（x≈0.26）梯度薄膜,并制备了Ti/类金刚石(DLC)以及Ti/TiN薄膜进行对比研究. 利用显微硬度计、划痕仪比较分析了上述各薄膜的力学性能,通过表面接触角法研究了薄膜的亲水性. 着重测试并分析了溶血率和血小板粘附行为,进而对薄膜的血液相容性进行评估. 结果表明：Ti/CN_x梯度薄膜与NiTi合金基底的结合牢固,结合力达到63.6N. Ti/CN_x薄膜硬度为23.01GPa,和Ti/TiN薄膜硬度相当,略高于Ti/DLC薄膜. 溶血率和血小板粘附试验表明,Ti/CN_x梯度薄膜能有效改善NiTi合金基底的亲水性和血液相容性,与Ti/TiN和Ti/DLC薄膜相比,Ti/CN_x梯度薄膜具有最小的溶血率,仅为1.12％,并且无论在血小板的粘附数量还是在血小板变形程度都最少,因此具有良好的血液相容性.     

Superelasticity of free‐standing NiTi films depending on the oxygen impurity of the used targets

Off‐stoichiometric NiTi sputter targets were manufactured by ingot metallurgy in order to produce Ni‐rich NiTi thin films. Crystalline samples, which undergo a martensitic transformation, were fabricated by magnetron sputtering and subsequent thermal treatment. The sacrificial layer technique was applied to obtain free‐standing NiTi films. The stoichiometry of the sputtered film is different to the stoichiometry of the target material. Sputtering targets with a Ti content higher than 49.5 at.% have a typical Ti‐loss rate in the order of 4.3 at.%, whereas at more Ni‐rich targets a Ti loss rate of only 0.5–1.5 at.% is observed. Due to this Ti loss rate target composites of Ti‐53.1Ni‐46.9 at.% and Ti‐53.2Ni‐46.8 at.% were taken. The NiTi films obtained from these two targets were compared. The main difference between the two targets is the impurity content of oxygen and carbon. Auger electron spectroscopy measurements were performed to determine qualitatively the content of oxygen in the free‐standing NiTi films. Transformation temperatures and hysteresis properties of the martensitic transformation were characterised by differential scanning calorimetry. Free‐standing films obtained from the Ti‐53.1Ni‐46.9 at.% and Ti‐53.2Ni‐46.8 at.% sputtering targets revealed superelastic properties at 45 °C as demonstrated by tensile testing. Tensile tests were taken at various temperatures of 40 °C, 45 °C, 50 °C, 55 °C and 60 °C, revealing e.g. a maximum superelastic strain of approximately 5 % at 45 °C.     

Crystallization Kinetics of NiTi Films with Different Ni Contents

Three kinds of NiTi films with different Ni contents were prepared by DC magnetron sputtering.The crystallization kinetics of amorphous films was determined by using non-isothermal singlescan techniques. The results show that the activation energy of crystallization of Ni-rich NiTi     

Characterization and corrosion behavior of NiTi–Ti2Ni–Ni3Ti multiphase intermetallics produced by vacuum sintering

An in-situ synthesis method was employed to produce NiTi–Ti₂Ni–Ni₃Ti multiphase intermetallics. In this regard, the amorphous/nanocrystalline Ni–Ti powders were sintered at 1300 °C for 2 hrs to obtain Ni–Ti alloys with dense structure. Tafel polarization tests were employed to study corrosion behavior of multiphase materials in 3.5% NaCl and 0.1 M H₂SO₄ corrosive media. The results indicated that the microstructure of sintered samples consists of NiTi(Fe) and Ti₂Ni/Ti₄Ni₂O_x phases embedded in a Ni₃Ti matrix. The synthesized multiphase materials had microhardness up to 873 HV1 kg.Further investigations showed the corrosion performance of multiphase samples in 3.5% NaCl solution was inferior to that of wrought NiTi alloy. In contrast, the corrosion resistance of multiphase samples in 0.1 M H₂SO₄ solution was comparable to that of wrought NiTi alloy.     

Synthesis of carbon nanotubes using Ni95Ti5 nanocrystalline film as a catalyst

Carbon nanotubes (CNTs) were synthesized by low-pressure chemical vapour deposition (LPCVD) using N2:C2H2:H2 gas mixtures on nanocrystalline Ni95Ti5 film. This nanocrystalline film was deposited on silicon substrate using vapour condensation method. The growth temperature and growth time was kept at 800 degrees C and 30 mins, respectively and the pressure was maintained at 10 Torr. The growth mechanism of CNTs was investigated using FESEM, TEM, HRTEM, and Raman Spectroscopy. From FESEM image of Ni95Ti5 nanocrystalline film, it is clear that the particle size varies from 5-10 nm. EDX analysis suggests that Ni95Ti5 alloy contains Ni and Ti both. It is clear from TEM images that CNTs are multiwalled with the diameter varying from 10-30 nm and length of several micrometers. HRTEM image shows that the structure of these multi-walled nanotube (MWNTs) is bamboo-shaped and the catalyst exists at the tip of MWNTs. Fourier Transform Raman Spectroscopy confirmed that graphitic structure of as-prepared CNTs. Field emission measurements reveal that the carbon nanotubes grown for 30 mins showed a turn-on field of 7.2 V/microm, when the current density achieves 10 microA/cm2. The field enhancement factor was calculated to be 708.50 for carbon nanotubes grown for 30 mins.     

Enhanced electron emission from titanium coated multiwalled carbon nanotubes

Enhanced field emission properties and improved crystallinity of titanium (Ti) coated multiwalled carbon nanotubes (MWCNTs), prepared by microwave plasma enhanced chemical vapour deposition have been observed. Ti films of extremely low thicknesses (0.5 nm, 1.0 nm and 1.5 nm) were coated over carbon nanotubes (CNTs) and their field emission behaviour was investigated. The turn on field of Ti coated CNTs was found to be low (~ 0.8 V/μm) as compared to pristine CNTs (~ 1.8 V/μm). The field enhancement factor for Ti coated CNTs was quite large (~ 1.14 × 10⁴) as compared to pristine CNTs (~ 6 × 10³). This enhancement in electron emission is attributed to the passivation of defects and improved crystallinity of CNTs. Surface morphological and microstructural studies were carried out to investigate the growth of pristine and Ti coated CNTs. It was observed that Ti nanoclusters adsorb on the edges of MWCNTs and increase their crystallinity. This increase is directly correlated with the thickness of Ti film deposited. Micro Raman spectroscopy confirmed the improved crystallanity of Ti coated CNTs.     

Corrosion behavior of a laser-welded NiTi shape memory alloy

Corrosion behaviors of the laser-welded Ni–49.4 at.% Ti shape memory alloy and base metal in 0.9% NaCl solution were investigated by means of electrochemical techniques (the open circuit potential measurement, linear and potentiodynamic polarizations). The results showed that corrosion resistance of the laser-welded NiTi alloy is better than that of the base metal. Compared to the base metal, the laser-welded NiTi alloy exhibits higher open circuit potential, higher polarization resistance, a wider passive region and higher breakdown potential. The improvement of corrosion resistance of the laser-welded NiTi alloy is ascribed to a smoother, defect free surface and an absence of carbides.     

Effect of catalyst thickness and plasma pretreatment on the growth of carbon nanotubes and their field emission properties

We demonstrated that the diameter and the density of carbon nanotubes (CNTs) which had a close relation to electric-field-screening effect could be easily changed by the control of catalytic Ni thickness combined with NH3 plasma pretreatment. Since the diameter and the density of CNTs had a tremendous impact on the field-emission characteristics, optimized thickness of catalyst and application of plasma pretreatment greatly improved the emission efficiency of CNTs. In the field emission test using diode-type configuration, well-dispersed thinner CNTs exhibited lower turn-on voltage and higher field enhancement factor than the densely-packed CNTs. A CNT film grown using a plasma-pretreated 25 angstroms-thick Ni catalyst showed excellent field emission characteristics with a very low turn-on field of 1.1 V/microm @ 10 microA/cm2 and a high emission current density of 1.9 mA/cm2 @ 4.0 V/microm, respectively.     

Optimization of field emission properties of carbon nanotubes by Taguchi method

It is the purpose of this study to evaluate the field emission property of carbon nanotubes (CNTs) prepared by microwave plasma-enhanced chemical vapor deposition (MPCVD) method. Nickel layer of 5 nm in thickness on 20-nm thickness titanium nitride film was transformed into discrete islands after hydrogen plasma pretreatment. CNTs were then grown up on Ni-coated areas by MPCVD. Through the practice of Taguchi method, superior CNT films with very low emission onset electric field, about 0.7 V/μm (at J = 10 μA/cm²), are attained without post-deposition treatment. It is found that microwave power has the most important influence on the field emission characteristics of CNT films. The increase of methane flow ratio will downgrade the degree of graphitization of CNT and thus its field emission characteristics. Scanning electron microscope and transmission electron microscopy (TEM) observation and energy dispersive X-ray spectrometer analysis reveal that CNT growth by MPCVD is based on tip-growth mechanism. TEM micrographs validate the hollow, bamboo-like structure of the multi-walled CNTs.     

Improving the wear resistance of AA 6061 by laser surface alloying with NiTi

To improve the wear resistance of a popular aluminum alloy AA 6061, a 1.5 mm thick hard surface layer consisting of Ni–Al and Ti–Al intermetallic compounds was synthesized on the alloy by laser surface alloying technique. NiTi powder was preplaced on the aluminum alloy substrate and irradiated with a high-power CW Nd:YAG laser in an argon atmosphere. With optimized processing parameters, a modified surface layer free of cracks and pores was formed by reaction synthesis of Al with Ni and Ti. X-ray diffractometry (XRD) confirmed the main phases in the layer to be TiAl₃ and Ni₃Al. The surface hardness increased from below 100 HV for untreated AA 6061 to more than 350 HV for the laser-treated sample. Accompanying the increase in hardness, the wear resistance of the modified layer reached about 5.5 times that of the substrate.     

Laser oxidation of NiTi for improving corrosion resistance in Hanks' solution

NiTi plates were surface treated to form an oxide film with a pulsed Nd:YAG laser in ambient air using a carefully selected set of processing parameters. The oxidized samples had a yellowish interference color. The temperature rise of the sample bulk was only about 20 °C though the surface temperature reached could be much higher. Atomic force microscopy (AFM) recorded an average surface roughness of 12.5 nm for the oxidized surface. X-ray photoelectron spectroscopy (XPS) showed that the surface Ni/Ti atomic ratio was reduced from 0.30 for the mechanically polished samples to 0.17 for the oxidized samples, indicating improved biocompatibility. Measurements using electrochemical impedance spectroscopy (EIS) at open-circuit potential recorded a 15.8-fold increase in corrosion resistance in Hanks' solution. In cyclic polarization tests, the laser oxidized NiTi samples did not show pitting up to an applied potential of 1550 mV SCE, similar to bare NiTi, but the passive current density was an order of magnitude lower. The present study positively indicates the feasibility of employing laser oxidation for improving the corrosion resistance of NiTi.     

高场发射性能碳纳米管薄膜的沉积与表征

利用电泳法将碳纳米管（CNTs）沉积在表面镀覆了50～150 nm Ti薄膜的Si基底表面,900℃真空退火后形成了具有良好场发射性能的Ti-CNTs薄膜阴极.利用X射线衍射和扫描电子显微镜对制备的Ti-CNTs薄膜进行了表征.结果表明,高温退火过程中,CNTs的C原子和基底表面的Ti原子发生化学反应,在CNTs与基底之间形成了导电性钛碳化物,明显改善了CNTs与基底之间的电导性和附着力等界面接触性能;与Si基底表面直接电泳沉积的CNTs薄膜相比,制备的Ti-CNTs薄膜的开启电场从1.31 V/μm降低到1.19 V/μm;当电场强度为2.50 V/μm时,Ti-CNTs薄膜的场发射电流密度可达13.91 mA/cm^2;制备的Ti-CNTs薄膜显示出改善的发射稳定性.     

Mechanical properties and phase structure of (TiAlZr)N films deposited by multi arc ion plating

A series of Ti-Al-Zr alloy targets with the atomic ratio of (Al + Zr)/(Ti + Al + Zr) from 0.29 to 0.40 were used directly to prepare (Ti,Al,Zr)N multi component hard reactive films on high speed steel substrates by multi arc ion plating (MAIP) technology. The surface morphology, the cross-fracture microstructure, the surface compositions and the phase structure of the (Ti,Al,Zr)N films were investigated by scanning electronic microscope (SEM) and X-ray diffraction (XRD). The dense columnar microstructure was obtained in all of the (Ti,Al,Zr)N films, though micro-droplets evidently existed on the surface of the films. The XRD analysis revealed f.c.c. structure only existing in all of the (Ti,Al,Zr)N films. The lattice parameter was changed with varying the Al and Zr contents in alloy targets.The micro-hardness of film surface and the adhesive property of film/substrate were measured. All the (Ti,Al,Zr)N films displayed excellent mechanical properties. The adhesive strength, in terms of critical load, was larger than 100N and the hardness was bigger than 3000Kg(f)/mm². The (Ti,Al,Zr)N film displayed the highest micro-hardness as the atomic ratio of (Al + Zr)/(Ti + Al + Zr) in Ti-Al-Zr alloy target reached 0.40. The present results suggest the expectant substitution of (Ti,Al,Zr)N films for TiN, (Ti,Al)N and (Ti,Zr)N hard films in industrial application and the technical advantage of Ti-Al-Zr alloy targets in preparing (Ti,Al,Zr)N hard films by multi arc ion plating.     

Pore structures of high-porosity NiTi alloys made from elemental powders with NaCl temporary space-holders

The unusual pseudo-elasticity and shape memory effect make NiTi alloys promising energy absorption materials. In the present study, powders of Ti, Ni and NaCl particles were mixed and cold-pressed into green ingots and green ingots were then desalted and sintered in vacuum to form high-porosity NiTi alloy specimens with porosity up to 90%. Microstructure observation shows that two kinds of pores with sizes of 200-400 μm and 10-50 μm respectively are well-distributed in these high-porosity NiTi alloys. Characteristics of pores were studied and formation mechanism was discussed.     

医用NiTi合金表面溶胶-凝胶法制备TiO2-SiO2薄膜

采用溶胶－凝胶法在ＮｉＴｉ形状记忆合金表面制备了ＴｉＯ_２－ＳｉＯ_２复合薄膜,在提高医用ＮｉＴｉ合金的抗腐蚀性方面,收到了显著的效果．运用电化学方法对不同组成的ＴｉＯ_２－ＳｉＯ_２薄膜在模拟体液中的腐蚀行为进行了研究,结果表明,随薄膜中 Ｔｉ／Ｓｉ比的增加,ＴｉＯ_２－ＳｉＯ_２薄膜的抗腐蚀性增强．划痕试验表明 ＴｉＯ_２－ＳｉＯ_２（Ｔｉ／Ｓｉ＝４：１）膜与ＮｉＴｉ合金基体具有较高的界面结合强度．用原子力显微镜（ＡＦＭ）对ＴｉＯ_２－ＳｉＯ_２薄膜的表面形貌及表面粗糙度进行观察和分析,解释并讨论了ＴｉＯ_２－ＳｉＯ_２薄膜的配方组成与其抗腐蚀性的关系,ＳｉＯ_２含量较少时,薄膜结构致密,膜层均匀平滑,且膜基结合力好,作为医用ＮｉＴｉ合金的表面保护层,可以使其耐腐蚀性有显著提高．     

Growth and characterization of shape memory alloy thin films for Si microactuator technologies

NiTi thin film alloys have been grown on Si (1 0 0) substrates by sequential multilayer deposition of Ni and Ti layers followed by metal interdiffusion via annealing. Short-time (5 min) annealing of the deposited layers at 500 °C leads to alloying of Ni and Ti, provided that preferential oxidation of Ti is avoided. This has been achieved by capping the layers with AlN, which constitutes a very efficient barrier preventing oxygen contamination of the film. For uncapped films, annealing has to be performed under high vacuum conditions. The structure of the processed films strongly depends on the temperature and time of the thermal process, obtaining Ni- and Ti-rich phases when the films are annealed at higher temperatures and/or during longer times. As-grown and processed films are analyzed by X-ray diffraction, in-depth Auger electron spectroscopy and X-ray photoelectron spectroscopy techniques. Finally, resistivity measurements show the existence of a phase transition in the films annealed at 600 °C without the capping layers (vacuum furnace). This contrasts with the behavior observed for AlN capped films, where cracking of the film occurs during cooling, which has been attributed to generation of stress in the buried film during the phase transition.