The corrosion resistance of Nitinol alloy in simulated physiological solutions Part 2: The effect of surface treatment

The effect of surface treatment – boiling in water and thermal oxidation at temperatures up to 600 °C – on the corrosion behavior of Nitinol was investigated in simulated Hanks physiological solution using electrochemical polarization methods. Morphological and compositional properties of the modified surfaces were analyzed by scanning electron microscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy depth profiling. Surface preparation – grinding or polishing – is shown to have a decisive role in the degree of improvement of corrosion properties by surface treatments. Low temperature treatments like boiling in water and thermal oxidation at 100 °C resulted in the formation of oxide layers only a few nanometers thick, and composed mainly of TiO₂ and a small amount of NiO. These layers are well able to protect the underlying Nitinol substrate. Up to 500 °C, surface preparation directly determines the thickness of the oxide scale, as a 20-fold difference in thickness is observed between ground and polished samples. At higher temperatures, the oxide thickness was similar for the two samples. A multilayer structure is observed at all temperatures investigated. The outermost layer at the oxide/air interface is composed of TiO₂ and NiO, while the interior of the oxide scale is composed exclusively of TiO₂. Oxide layers formed by thermal oxidation at elevated temperatures also improve the corrosion characteristics of Nitinol, especially for polished substrates.     

Evaluation of wettability and surface energy of native Nitinol surfaces in relation to hemocompatibility

In the present study the dependence of Nitinol contact angles and surface energy on surface treatment is explored in order to better understand the material hemocompatibility that was evaluated in our previous studies. It is found that in the group of surfaces: (1) mechanically polished, (2) additionally heat treated, (3) chemically etched, and (4) additionally boiled in water, and (5) further heat treated, the contact angle could vary in the 50°–80° hydrophobic range and the total surface free energy in the 34–53 mN/m range. The polar surface energy, varying from 5 to 29 mN/m, constitutes a decisive contribution to the total energy change, and it seems to be a direct function of the Nitinol surface chemistry. Based on the complex analysis of surface energy together with the earlier results on electrochemistry and hemocompatibility it is concluded that the alteration of the polar component of surface energy and thrombogenicity is due to changes of the electron-acceptor/electron-donor character of native Nitinol surfaces during surface treatments.     

Hydrogen content in titanium and a titanium–zirconium alloy after acid etching

Dental implant alloys made from titanium and zirconium are known for their high mechanical strength, fracture toughness and corrosion resistance in comparison with commercially pure titanium. The aim of the study was to investigate possible differences in the surface chemistry and/or surface topography of titanium and titanium–zirconium surfaces after sand blasting and acid etching. The two surfaces were compared by X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, scanning electron microscopy and profilometry.The 1.9 times greater surface hydrogen concentration of titanium zirconium compared to titanium was found to be the major difference between the two materials. Zirconium appeared to enhance hydride formation on titanium alloys when etched in acid. Surface topography revealed significant differences on the micro and nanoscale. Surface roughness was increased significantly (p < 0.01) on the titanium–zirconium alloy. High-resolution images showed nanostructures only present on titanium zirconium.     

Improvement in the morphology of micro-arc oxidised titanium surfaces: A new process to increase osteoblast response

This study describes a method for combining sandblast-acid etching and micro-arc oxidation to optimise titanium implant surfaces, and examines the effects of these surfaces on osteoblast response. Titanium discs were grouped as: micro-arc oxidised (MAO), sandblast-acid etched and micro-arc oxidised (MAO-SA), micro-arc oxidised and heated (MAO-HT), and untreated smooth surface. The combination of sandblast-acid etching and micro-arc oxidation in the MAO-SA group created an average surface roughness of 2.02 ± 0.15 μm compared to the untreated machined surface of 0.31 ± 0.06 μm. Scanning electron microscopy observations of the surface structures showed that the irregularly ordered valleys created by sandblast-acid etching remained after micro-arc oxidation and that micropores had also formed. These microstructures provided a better place for osteoblasts to spread compared with the other surfaces. In addition, our results indicated that adherent osteoblasts expressed greater alkaline phosphatase (ALP) activity and osteocalcin (OC) production on MAO-SA surfaces compared with MAO, MAO-HT, and smooth surfaces. The overall results clearly indicate that combining sandblast-acid etching and micro-arc oxidation techniques improves the titanium surface morphology and increases the roughness, which provides an optimal surface for cell differentiation and osseointegration.     

In vivo synthesis of calcium oxalate whiskers on CoCrMo alloy surfaces via biomineralization

Surface treatments using bio-technology are valuable and fascinating in the sense that such treatments are natural and yield good biocompatibility. Calcium oxalate whiskers for biomedical applications were successfully synthesized on the CoCrMo alloy surfaces implanted in Aloe leaves which consist of many active bio-chemical elements. The effect of surface wettability and surface morphology on the formation of whiskers was investigated using four differently treated CoCrMo surfaces: (i) smoothly polished surface, (ii) electrochemical etched surface, (ii) textured surface with dimples, and (iv) parallel orientated-grooved surface. Results showed that the formed whiskers had a length ranging between 100 μm and 600 μm, and a diameter in the range of 2 μm to 5 μm. Electrochemically etched surfaces had better wettability and were favorably for growing whiskers. Surface morphology with (i) dimple textures or (ii) parallel grooves facilitated the effective control of the size and amount of the grown whiskers.     

In-vivo study of adhesion and bone growth around implanted laser groove/RGD-functionalized Ti-6Al-4V pins in rabbit femurs

Titanium surfaces were designed, produced, and evaluated for levels of osseointegration into the femurs of rabbits. A total of 36 Ti-6Al-4V pins (15 mm length, 1.64 mm diameter) were prepared into three experimental groups. These were designed to test the effects of osseointegration on laser grooved, RGD coated, and polished control surfaces, as well as combined effects. Circumferential laser grooves were introduced onto pin surfaces (40 μm spacing) using a UV laser (λ = 355 nm). The tripeptide sequence, Arginine-Glycine-Aspartic acid (RGD), was functionalized onto laser grooved surfaces. Of the prepared samples, surface morphology and chemistry were analyzed using scanning electron microscopy (SEM) and Immunoflourescence (IF) spectroscopy, respectively. The experimental pin surfaces were surgically implanted into rabbit femurs. The samples were then harvested and evaluated histologically. Sections of the sample were preserved in a methylmethacralate mold, sliced via a hard microtome, and polished systematically. In the case of the RGD coated and laser grooved surfaces, histological results showed accelerated bone growth into the implant, pull-out tests were also used to compare the adhesion between bone and the titanium pins with/without laser textures and/or RGD coatings.     

Antibacterial effects of silver-doped hydroxyapatite thin films sputter deposited on titanium

Since many orthopedic implants fail as a result of loosening, wear, and inflammation caused by repeated loading on the joints, coatings such as hydroxyapatite (HAp) on titanium with a unique topography have been shown to improve the interface between the implant and the natural tissue. Another serious problem with long-term or ideally permanent implants is infection. It is important to prevent initial bacterial colonization as existing colonies have the potential to become encased in an extracellular matrix polymer (biofilm) that is resistant to antibacterial agents. In this study, plasma-based ion implantation was used to examine the effects of pre-etching on plain titanium. Topographical changes to the titanium samples were examined and compared via scanning electron microscopy. Hydroxyapatite and silver-doped hydroxyapatite thin films were then sputter deposited on titanium substrates etched at ? 700 eV. For silver-doped films, two concentrations of silver (~ 0.5 wt.% and ~ 1.5 wt.%) were used. Silver concentrations in the film were determined using energy dispersive X-ray spectroscopy. Hydroxyapatite film thicknesses were determined by measuring the surface profile using contact profilometry. Staphylococcus epidermidis and Pseudomonas aeruginosa adhesion studies were performed on plain titanium, titanium coated with hydroxyapatite, titanium coated with ~ 0.5 wt.% silver-doped hydroxyapatite, and titanium coated with ~ 1.5 wt.% silver-doped hydroxyapatite. Results indicate that less bacteria adhered to surfaces containing hydroxyapatite and silver; further, as the hydroxyapatite films delaminated, silver ions were released which killed bacteria in suspension.     

Structure and corrosion resistance of nickel coatings containing tungsten and silicon powders

Ni + W + Si coatings were prepared by nickel deposition from a bath containing a suspension of tungsten and silicon powders. These coatings were obtained at galvanostatic conditions, at the current density of j_dep = − 0.100 A cm^− 2 and at the temperature of 338 K. For determination of the influence of phase composition and surface morphology of these coatings on changes in the corrosion resistance, these coatings were modified in an argon atmosphere by thermal treatment at 1373 K during 1 h. A scanning electron microscope was used for surface morphology characterization of the coatings. The chemical composition of the coatings was determined by EDS and phase composition investigations were conducted by X-ray diffraction. It was found that the as-deposited coatings consist of a three-phase structure, i.e., nickel, tungsten and silicon. The phase composition for the Ni + W + Si coatings after thermal treatment is markedly different. The main peaks corresponding to Ni and W coexist with the new phases: NiW, NiWSi and a solid solution of W in Ni.Electrochemical corrosion resistance investigations were carried out in 5 M KOH, using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. On the basis of these investigations it was found that the Ni + W + Si coatings after thermal treatment are more corrosion resistant in alkaline solution than the as-deposited coatings. The reasons for this are a reduction in the amount of free nickel and tungsten, the presence of new phases (in particular polymetallic silicides), and a decrease of the active surface area of the coatings after thermal treatment.     

Titanium–SiO2 nanocomposites and their scaffolds for dental applications

There have been a number of attempts to modify the properties of titanium implants to improve osseointegration. These modifications include alterations of the chemistry and roughness of the surface of the implant. In this work, Ti–10 wt.% SiO₂ nanocomposites and their scaffolds were synthesized using a combination of mechanical alloying and a “space-holder” sintering process. The phase and microstructure analysis was carried out using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and the properties were measured using hardness and corrosion testing equipment. An amorphous structure was obtained at 20 h of milling. The crystallization of the amorphous phase upon annealing led to the formation of a nanostructured Ti–10 wt.% SiO₂ composite with a grain size of approximately 40 nm. The Vickers hardness of the Ti–10 wt.% SiO₂ nanocomposites reached 670 HV_0.2. The in vitro cytocompatibility of these materials was evaluated and compared with conventional microcrystalline titanium, where normal human osteoblast (NHOst) cells from Cambrex (CC-2538) were cultured. The morphology of the cell cultures obtained on the bulk Ti–10 wt.% SiO₂ nanocomposite was similar to those obtained on the microcrystalline titanium. However, on the porous scaffold, the cells adhered to the insert that penetrated the porous structure with their entire surface, whereas on the polished surface, more spherical cells were observed with a smaller surface of adhesion. Porous Ti–10 wt.% SiO₂ scaffolds have been developed in order to promote bone ingrowth and to induce prosthesis stabilization.     

In vitro bioactivity evaluation of nano- and micro-crystalline anodic TiO2: HA formation,cellular affinity and organ culture

Anodization is an easily viable technique useful for producing TiO₂ coatings on titanium substrates. Nano-crystalline anodic TiO₂ structure was produced on titanium at 20 V using 1 M Na₂SO₄ and 0.5% NaF and consolidated by a further heat-treatment. Micro-crystalline anodic TiO₂ was produced on titanium by applying a galvanostatic current density of 70 A/m² in water medium. To assess the usefulness of these nano- and micro-oxides for bone implant stability, physical properties and bone in vitro bioactivity including HA formation, cellular affinity and mouse-tissue morphogenesis, were evaluated. Bioactivity of the different anodic surfaces was evaluated by treating them in a simulated body fluid (SBF) to form hydroxyapatite (HA) and the rates of HA formation were compared. Deposits of HA could be seen on the nano-oxide surface within 7 days, whereas HA was detected only after 14 days on the micro-oxide surface. In vitro cell culture tests done using mouse osteoblasts indicated that the nano-oxides showed statistically significant cell activity than the micro-oxides and the machined titanium. Branching morphogenesis test done for 72 h on these surfaces showed more branching on the micro- and nano-oxides as compared with titanium surface.     

Improved thrombogenicity on oxygen etched Ti6Al4V surfaces

Thrombus formation on blood contacting biomaterials continues to be a key factor in initiating a critical mode of failure in implantable devices, requiring immediate attention. In the interest of evaluating a solution for one of the most widely used biomaterials, titanium and its alloys, this study focuses on the use of a novel surface oxidation treatment to improve the blood compatibility. This study examines the possibility of using oblique angle ion etching to produce a high quality oxide layer that enhances blood compatibility on medical grade titanium alloy Ti6Al4V. An X-ray photoelectron spectroscopy (XPS) analysis of these oxygen-rich surfaces confirmed the presence of TiO₂ peaks and also indicated increased surface oxidation as well as a reduction in surface defects. After 2 h of contact with whole human plasma, the oxygen etched substrates demonstrated a reduction in both platelet adhesion and activation as compared to bare titanium substrates. The whole blood clotting behavior was evaluated for up to 45 min, showing a significant decrease in clot formation on oxygen etched substrates. Finally, a bicinchoninic acid (BCA) total protein assay and XPS were used to evaluate the degree of key blood serum protein (fibrinogen, albumin, immunoglobulin G) adsorption on the substrates. The results showed similar protein levels for both the oxygen etched and control substrates. These results indicate that oblique angle oxygen etching may be a promising method to increase the thrombogenicity of Ti6Al4V.     

Chemical etching of InP (1 0 0) wafer based on a volcanic mineral water

We have successfully demonstrated that a solution of spa water [Tamagawa Spa water (TaSW):H₂O₂ = 1:1] etches InP (1 0 0) wafer. The TaSW is a colorless acidic liquid of pH ∼1.1. It contains a considerable amount of positive ions, such as H⁺, Al³⁺, and Ca²⁺. The Cl⁻, HSO₄²⁻, and SO₄²⁻ ions are the main anions. The TaSW-etchant system provides shiny flat surfaces on the etched bottoms. The spa-etchant system has reproducible etching rates and does not erode photoresist masks. The etching kinetics is reaction-rate limited. The spa-etchant system is also found to etch GaAs (1 0 0) wafer, but the etched surface is considerably roughened.     

Electrophoretic bilayer deposition of zirconia and reinforced bioglass system on Ti6Al4V for implant applications: An in vitro investigation

The physical, chemical and biological properties of the bioglass reinforced yttria-stabilized composite layer on Ti6Al4V titanium substrates were investigated. The Ti6Al4V substrate was deposited with yttria stabilized zirconia — YSZ as the base layer of thickness ≈ 4–5 μm, to inhibit metal ion leach out from the substrate and bioglass zirconia reinforced composite as the second layer of thickness ≈ 15 μm, which would react with surrounding bone tissue to enhance bone formation and implant fixation. The deposition of these two layers on the substrate was carried out using the most viable electrophoretic deposition (EPD) technique. Biocompatible yttria-stabilized zirconia (YSZ) in the form of nano-particles and sol gel derived bioglass in the form of micro-particles were chosen as precursors for coating. The coatings were vacuum sintered at 900 °C for 3 h. The biocompatibility and corrosion resistance property were studied in osteoblast cell culture and in simulated body fluid (SBF) respectively. Analysis showed that the zirconia reinforced bioglass bilayer system promoted significant bioactivity, and it exhibited a better corrosion resistance property and elevated mechanical strength under load bearing conditions in comparison with the monolayer YSZ coating on Ti6Al4V implant surface.     

Fatigue life prediction of corrosion-damaged high-strength steel using an equivalent stress riser (ESR) model: Part I: Test development and results

The fatigue life of metallic aircraft structural components can be significantly reduced by environmentally induced corrosion. As part of a NAVAIR High Strength Steel Corrosion–Fatigue Assessment Program, methods were studied to predict the impact that corrosion-induced surface roughness has on the fatigue life of high-strength steel aircraft components. In order to adequately capture the corrosion damage features that cause fatigue cracking, a representative set of well-characterized corrosion–fatigue test results were generated to be used for model development. The test specimens fabricated for this program consisted of bare, unnotched AF1410 steel flat plates with a 25.4 mm diameter corrosion patch on one side. Two sets of test specimens were fabricated and tested, with one set abrasive blasted after heat treatment, and the other set hand polished after heat treatment. A method of growing corrosion in the laboratory was developed that consisted of filter paper soaked in a 3.5% NaCl solution and placed at the center of the test plate gage section, with a voltage applied across the filter paper to accelerate corrosion growth. High-resolution 3D surface topography data was collected from the corroded region on each test plate prior to fatigue testing using a commercial white-light interference microscope. Constant-amplitude fatigue tests were performed on corroded and uncorroded test plates at several different stress levels, for three different corrosion exposure levels. Post-test fractographic analysis of the corroded specimens indicate that all of the critical cracks originated from small corrosion notches on the order of 10–200 μm in width, 10–120 μm in height and 2–100 μm in depth. These notches were not considered to be pits in that the depth dimension was less than the surface dimensions. The repeatability of the fatigue initiating mechanism for corrosion damaged surfaces in this material indicates that it should be possible to develop a single modeling approach that reasonably captures the effects of corrosion notches in reducing fatigue life.     

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950 °C for 45 min under the compressive stress of 4 MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni₃Ti, NiTi and NiTi₂ intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ was observed in the Ni₃Ti intermetallic layer. At 950 °C processing temperature, black island of β-Ti phase has been observed in the NiTi₂ intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi₂ phase at the NiA–TiA interface when bonding was processed up to 900 °C, however, failure takes place through NiTi₂ and β-Ti phase mixture for the diffusion joints processed at 950 °C. Joint strength was evaluated and maximum tensile strength of ∼560 MPa and shear strength of ∼415 MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900 °C for 45 min.     

Hot pressing effect on the shear bond strength of dental porcelain to CoCrMoSi alloy substrates with different surface treatments

The purpose of this study was to evaluate the effect of hot pressing on the shear bond strength of a CoCrMoSi alloy to a low-fusing feldspathic porcelain, for two types of surface treatments: polished and grit-blasted. Moreover, the shear strength of hot pressed porcelain was also compared with that of conventional vacuum sintered porcelain. Bond strength of metal–porcelain composites were assessed by the means of a shear test performed in a universal test machine until fracture. Fracture surfaces and interfaces were investigated by optical microscope, stereomicroscope and SEM/EDS. Data was analyzed with Shapiro–Wilk test to test the assumption of normality. The 2-way ANOVA followed by Tukey HSD multiple comparison test was used to compare shear bond strength results and the t-test was used to compare the porcelain shear strength (p < 0.05). Hot pressed specimens exhibited significantly (p < 0.001) higher bond strength values than those obtained by conventional PFM technique. Significant differences (p < 0.001) were found in the shear bond strength between grit-blasted and polished specimens. Significant differences (p < 0.05) were also found between the shear strength of vacuum sintered and hot pressed porcelain. This study revealed that metal–ceramic bond strength is maximized for hot pressed porcelain onto rough metal substrates, with lower variability in results. Hot pressing technique was also shown to enhance the cohesion of porcelain.     

Investigation of UV-photon induced hydrophilicity of titanium ion-implanted soda-lime silicate glasses

The UV-induced wetting effect on titanium oxide surface is well-known; however, the UV-induced hydrophilicity of titanium implanted soda-lime silicate glass has not been investigated. Hence the contact angle of water droplet under the indoor fluorescent lights on titanium-ion implanted soda-lime silicate glasses was investigated. The silicate glasses were implanted by MEVVA ion implanter by 40 keV titanium ions with a fluence of 10¹⁵ ions cm^?2. The contact angle, the chemical bonding environment, and surface morphologies were examined. Results show the formation of TiO₂, the increase of surface roughness, and the reduction of the contact angle after the ion implantation. Further enhancement of hydrophilicity after the 254 nm pre-UV irradiation for 1 h on the implanted sample surface was observed. The enhancement of the wetting effect after ion implantation could be attributed to rougher TiO₂ content surface. However, according to the mechanisms of UV photo-induced hydrophilicity on TiO₂ proposed previously, the enhancement of hydrophilicity of titanium implanted surface with and without 254 nm pre-photon radiation can be attributed to not only the reduction of hydrocarbon on surface during the UV radiation but also to the oxygen vacancies produced by 254 nm UV photon irradiation.     

Thermal oxidation of CP Ti — An electrochemical and structural characterization

In the present study commercially pure titanium (CP Ti) samples were oxidized thermally at three different temperatures (500, 650 and 800 °C) for 24 h and evaluation of their morphological and structural characteristics, microhardness and corrosion resistance in Ringer's solution was done. The corrosion protective ability of thermally oxidized materials shows a strong dependence on the nature and thickness of the surface oxide layer. Based on the corrosion protective ability, the untreated and thermally oxidized samples can be ranked as follows: CP Ti (800 °C) > CP Ti (650 °C) > CP Ti (500 °C) > untreated CP Ti.     

Nanostructured severe plastic deformation processed titanium for orthodontic mini-implants

Titanium mini-implants have been successfully used as anchorage devices in Orthodontics. Commercially pure titanium (cpTi) was recently replaced by Ti-6Al-4 V alloy as the mini-implant material base due to the higher strength properties of the alloy. However, the lower corrosion resistance and the lower biocompatibility have been lowering the success rate of Ti-6Al-4 V mini-implants. Nanostructured titanium (nTi) is commercially pure titanium that was nanostructured by a specific technique of severe plastic deformation. It is bioinert, does not contain potentially toxic or allergic additives, and has higher specific strength properties than any other titanium applied in medical implants. The higher strength properties associated to the higher biocompatibility make nTi potentially useful for orthodontic mini-implant applications, theoretically overcoming cpTi and Ti-6Al-4 V mini-implants. The purposes of the this work were to process nTi, to mechanically compare cpTi, Ti-6Al-4 V, and nTi mini-implants by torque test, and to evaluate both the surface morphology and the fracture surface characteristics of them by SEM. Torque test results showed significant increase in the maximum torque resistance of nTi mini-implants when compared to cpTi mini-implants, and no statistical difference between Ti-6Al-4 V and nTi mini-implants. SEM analysis demonstrated smooth surface morphology and transgranular fracture aspect for nTi mini-implants. Since nanostructured titanium mini-implants have mechanical properties comparable to titanium alloy mini-implants, and biocompatibility comparable to commercially pure titanium mini-implants, it is suggestive that nanostructured titanium can replace Ti-6Al-4 V alloy as the material base for mini-implants.