Interference colors of thin oxide layers on titanium

This work deals with the characterization of the color properties of different titanium oxide films, obtained by means of anodic oxidation. The color of these oxides varies with film thickness, since it is due to light interference phenomena taking place at the metal‐oxide‐air interfaces. Color measurements were carried out by using spectrophotometry techniques: the values obtained belong to the colorimetric space CIELAB, which is defined as standard colorimetric space. The results of these analyses were related to the oxide structure, analyzed with X‐ray diffraction techniques, which was determined to be either amorphous or semicrystalline. Also the oxide thickness was taken into account. This feature was derived both from ellipsometric data and from reflectance spectra: the two data are shown to be in good adherence. Both commercial purity titanium (grade 2 ASTM) and titanium alloy Ti‐6Al‐4V substrates were investigated. © 2008 Wiley Periodicals, Inc. Col Res Appl, 33, 221–228, 2008     

ESCA study of oxidized wood surfaces

Results of a preliminary study of the surface of wood exposed to outdoor weathering as well as to UV irradiation showed that ESCA provides valuable information and insight into the manifestation of weathering and photooxidation. From the ESCA spectra, the increase in signal intensities of carbon–oxygen bonds and oxygen–carbon–oxygen bonds (or unsaturated carbon oxygen bond) and oxygen-to-carbon ratio, and the decrease in carbon–carbon and carbon–hydrogen bonds of weathered and UV-irradiated wood surfaces suggested that wood surface was oxidized. Nevertheless, it was a superficial effect. Only a slow oxidation was observed at 100 μm under the exposed wood surfaces. From the oxygen-to-carbon ratio data, it revealed that weathered wood surface was rich in cellulose, poor in lignin. The leached-away degradation products from weathered wood surface accounted for the discrepancy between the ESCA line shapes of UV-irradiated and weathered wood surfaces.     

Determination of tocopherol in oxidized fats. Interference from heat-formed reducing substances in highly oxidized fats

Summary and Conclusions By application of a heating method to determine tocopherol in oxidized fats it was shown that polymeric reducing substances were produced when fats have a peroxide value exceeding 100. By comparison with a chromatographic method to remove peroxides it was shown that the interference in determining tocopherol from these heat-produced reducing substances was negligible at peroxide levels lower than 100. It is concluded that the heating method is satisfactory within the peroxide range (0–100) most important in oxidative and flavor-stability studies of edible oils. The chromatographic method for removing peroxides can be relied upon for determining tocopherol in more highly oxidized fats as well as in methyl esters of fatty acids which are distillable under conditions of the heating method. The appearance of reducing substances in heated oxidized fats is related to the peroxides present in the fats prior to heating. It is accompanied with an increase in browning, viscosity, acid, and carbonyl values of the fats and with a decrease in iodine values. The polymeric material from heated-oxidized methyl esters of unsaturated fatty acids has been separated by vacuum distillation and chromatography on silicic acid. It is believed to be principally dimeric in nature.     

X-ray photoelectron spectroscopy study of anodically oxidized SIMFUEL surfaces

The surface composition of anodically oxidized SIMFUEL (doped uranium dioxide) has been determined as a function of applied potential over the range −500 to +500 mV (versus SCE). Cathodically cleaned UO₂ specimens were anodically oxidized for 1 h and subsequently analyzed by XPS. Using published binding energies, the U (4f_7/2) and O (1s) peaks were resolved into contributions from U^IV, U^V, U^VI, O^II, OH⁻ and H₂O. It was shown that over the potential range −500 to approximately +50 mV a thin surface layer of U^IV/U^V oxide (UO_2+x) formed. At more positive potentials, a U^VI hydrated oxide (UO₃·yH₂O) was deposited on the electrode surface. At very positive potentials (≥400 mV) the rapid anodic formation and hydrolysis of UO₂²⁺ led to local acidification in pores in the deposited UO₃·yH₂O layer and its subsequent re-dissolution.     

Nano-mechano-electrochemistry of passive metal surfaces

In situ nanoindentation tests were performed to evaluate the mechanical properties of the iron (100) and (110) surfaces passivated potentiostatically for 1 h in pH 8.4 borate solution after or without immersion in 5×10⁻² M K₂Cr₂O₇ solution for 24 h. It was found from the measured load–depth curves at a maximum load of 400 μN that the hardness (3.2–3.3 GPa) of the passivated iron (110) surfaces was larger by 10% than that (2.9–3.1 GPa) of the passivated iron (100) surfaces. Moreover, the hardness of the passivated iron (100) and (110) surfaces increased slightly with increasing formation potential of passive film, which was ascribed to the presence of passive film. The dichromate treatment increased the hardness of the iron (100) and (110) surfaces to some extent. The loading discontinuity appeared frequently at a load of about 90 μN in the load–depth curves for the passivated iron (110) surfaces as compared with the passivated iron (100) surfaces. The loading discontinuity in the load–depth curves for the iron (110) surface, however, disappeared completely due to the dichromate treatment.     

Thermal desorption spectra of the oxidized surfaces of diamond powders

Thermal desorption mass spectrometry was carried out in the range from room temperature to 960°C for the diamond powders oxidized in 10^?2Torr O₂ at temperatures from 25 to 554°C. The spectra show two desorption peaks (α, β) and the major peak α seems to consist of at least four types of adsorption states (α₁?α₄). The first state α₁, may arise from “labile” carbon atoms created during degassing, and disappears with high temperature oxidation changing into the more stable states (α₂-α₄). IR absorption spectroscopy shows that α₂?α₄ states include carbonyl and ether structures. The minor peak β shows little change with oxidation temperature. The amount of oxygen chemisorbed at 420°C (1.22 × 10¹⁵ atom/cm²) is in good agreement with the estimated value of full coverage.     

Fabrication of stable superhydrophilic surfaces on titanium substrates

We report a method of producing superhydrophilic surfaces on titanium substrates via sandblasting and dip-coating with colloidal silica nanoparticles. The surface exhibits a high level of hydrophilic stability, as it stays superhydrophilic for an excess of 40?days and through multiple wetting–dewetting cycles. The combination of microscale roughness from the sandblasting and nanoscale roughness from the silica particles results in a micro-nano binary structure, which greatly enhances the hydrophilicity of the titanium samples. Due to the simplicity and ease of implementation of this method, such a surface is suitable for potential use in a variety of applications, such as prosthetic dentistry and other biomedical fields.     

Contact angle measurements on oxidized polymer surfaces containing water-soluble species

Advancing and receding contact angle measurements on polymer surfaces can be performed using a number of different methods. Ballistic deposition is a new method for both rapidly and accurately measuring the receding contact angle of water. In the ballistic deposition method, a pulsed stream of 0.15-μL water droplets is impinged upon a surface. The water spreads across the surface and then coalesces into a single 1.8-μL drop. High-speed video imaging shows that, on most surfaces, the water retracts from previously wetted material, thereby forming receding contact angles that agree with the receding angles measured by the Wilhelmy plate technique. The ballistic deposition method measures the receding angle within one second after the water first contacts the surface. This rapid measurement enables the investigation of polymer surface properties that are not easily probed by other wettability measurement methods. For example, meaningful contact angles of water can be obtained on the water-soluble low-molecular-weight oxidized materials (LMWOM) formed by the corona and flame treatment of polypropylene (PP) films. Use of the ballistic deposition method allows for a characterization of the wetting properties and an estimation of the surface energy components of LMWOM itself. Both corona- and flame-generated LMWOM have significant contact angle hysteresis, almost all of which is accounted for by the non-dispersive (polar) component of the surface rather than by the dispersive component. Surface heterogeneity is thus associated primarily with the oxidized functionalities added to the PP by the corona and flame treatments.     

UV-resistant amorphous fluorinated coating for anodized titanium surfaces

In this work, a high molecular weight copolymer of tetrafluoroethylene and perfluoro-4-trifluoromethoxy-1,3-dioxole (HYFLON^® AD60) and two perfluoropolyethers (PFPEs) containing ammonium phosphate or triethoxysilane functionalities, FLUOROLINK^® F10 and FLUOROLINK^® S10 respectively, have been evaluated as protective coatings that can be easily applied on anodized titanium surfaces. Water and n-dodecane contact angle measurements have been recorded by using the sessile drop method for bare and coated surfaces in order to determine the hydrophobic and oleophobic properties of the coatings. The UV-stability of coatings have been studied by Fourier transform infrared spectroscopy (FT-IR) analyses and by observing the variation of water contact angles on coated substrates before and after UV irradiation at regular time intervals. The thickness of the fluorinated films has been measured by ellipsometry and by weight evaluation. Preliminary tests of the adhesion between films and substrate have been conducted.     

The interaction of bacteria and metal surfaces

This review discusses different examples for the interaction of bacteria and metal surfaces based on work reported previously by various authors and work performed by the author with colleagues at other institutions and with his graduate students at CEEL. Traditionally it has been assumed that the interaction of bacteria with metal surfaces always causes increased corrosion rates (“microbiologically influenced corrosion” (MIC)). However, more recently it has been observed that many bacteria can reduce corrosion rates of different metals and alloys in many corrosive environments. For example, it has been found that certain strains of Shewanella can prevent pitting of Al 2024 in artificial seawater, tarnishing of brass and rusting of mild steel. It has been observed that corrosion started again when the biofilm was killed by adding antibiotics. The mechanism of corrosion protection seems to be different for different bacteria since it has been found that the corrosion potential E_corr became more negative in the presence of Shewanella ana and algae, but more positive in the presence of Bacillus subtilis. These findings have been used in an initial study of the bacterial battery in which Shewanella oneidensis MR-1 was added to a cell containing Al 2024 and Cu in a growth medium. It was found that the power output of this cell continuously increased with time. In the microbial fuel cell (MFC) bacteria oxidize the fuel and transfer electrons directly to the anode. In initial studies EIS has been used to characterize the anode, cathode and membrane properties for different operating conditions of a MFC that contained Shewanella oneidensis MR-1. Cell voltage (V)—current density (i) curves were obtained using potentiodynamic sweeps. The current output of a MFC has been monitored for different experimental conditions.     

Coagulation of polymer emulsions on metal surfaces

Emulsions of polyethylacrylate using sodium dodecylsulfate, cetylpyridinium chloride or nonylphenol ethoxylate as anionic, cationic and nonionic emulsifiers were prepared and the polymer deposition on metal panels in presence of an acid and an oxidizing agent was studied. Homogeneous films are formed within limited concentration ranges of the reactants. Diluted hydrofluoric acid and diluted hydrogen peroxide are useful components. Instead of a chemical oxidant a potential of 250 to 500 mV or more may be applied. The iron ion content of the film was determined by atomic absorption spectroscopy. A ratio of iron to emulsifier of 4.1 : 1 was found. The distribution of iron in film cross-cuts by scanning electron microscopy using the energy dispersive technique shows a high iron concentration near the metal interface and a sharp drop towards the film surface. In the outer third of the film the ratio iron to emulsifier is nearly 1 : 1. By adding iron salt solutions to polyethylacrylate emulsions the coagulation point was determined and a ratio iron ion to emulsifier of 1 : 1.2 was found in the coagulate. Steel panels were coated as well as phosphated steel or zinc but no film formation appeared on aluminum.     

Characterization of the gonioapparent character of colored anodized titanium surfaces

We investigate the gonioapparent character of colored anodized titanium surfaces. In appropriate experimental conditions, the anodizing process generates a homogeneous oxide layer at the surface of the metal, with a thickness comparable with the visible wavelengths of light. The anodized metallic surfaces thus exhibit interferential colors, which depend on the thickness of the oxide layer and have the particularity to vary according to the illumination and observation directions. They are said to be gonioapparent. This article describes an experimental method to characterize these variations precisely. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 483–490, 2015     

Discoloration at anatase titanium dioxide surfaces by organic contamination

A major cause of vacuum discoloration of some anatase titanium dioxide powders at 5 × 10^?2 torr has been identified as contamination by rotary pump oil. The discoloration is stable in dry air but fades on exposure to moist air.     

In vitro assessment of zinc apatite coatings on titanium surfaces

In this paper, coatings of hydroxyapatite partially substituted with zinc (ZnHA) were produced on titanium substrates by a two-step hydrothermal process using a precursor solution rich in calcium, phosphate and zinc. Activation of titanium surfaces was performed by oxidation with an acidic HF/HNO₃ solution. The coated substrates were then converted into HA by immersion in an alkali 0.1 M NaOH solution. The ZnHA samples were characterized by several techniques and their in vitro behavior was studied in comparison to hydroxyapatite (HA) and titanium (Ti-control) samples. A uniform and homogeneous calcium-deficient carbonate apatite coating was obtained for all samples, both doped and undoped with zinc. The percentage of zinc incorporated in the coatings is 7 at%, and the Ca/P ratio is 1.61(±0.01) for both types of samples, suggesting that Zn is incorporated substitutionally, replacing Ca atoms into the HA structure. The incorporation of Zn in the HA structure changed the crystals morphology, reduced crystals sizes and decreased the deposition rate showing that zinc is an inhibitor of the growth of HA crystal. X-ray diffraction showed that HA is the single crystalline phase present after alkali treatment. The coating adhesion strength was evaluated in terms of the critical load (Lc) obtained from scratch tests and no significant difference was found between the two tested groups, indicating the good adhesion of ZnHA to Ti substrates. The in vitro response of human osteoblasts (HOB) exposed to the surfaces of HA and ZnHA coatings was evaluated. The results of Live/Dead tests showed cell viability for all samples surfaces, but the adhesion and proliferation tests showed that ZnHA samples presented better adhered and spread cells compared with HA. ZnHA coatings presented cells with elongated or polygonal shapes and clearly more spread than HA. Quantitative analysis showed that there was a significantly higher number of cells adhered to ZnHA coatings compared to HA, indicating the zinc incorporation stimulates osteoblast proliferation.     

Electrochemistry of freshly-generated titanium surfaces—I. Scraped-rotating-disk experiments

Experiments were conducted in which a rotating disk of titanium was scraped with a sapphire cutter in HCl solution. The amount of titanium going into solution determined chemically and by oxidation of Ti³⁺ to Ti(IV) at a gold ring accounted for a large fraction of the anodic current to the scraped surface. The data are consistent with a revised potential-pH diagram taking into account the simultaneous formation of Ti³⁺ and a metastable passivating oxide. Oxidation of Ti³⁺ to Ti(IV) on gold was highly irreversible even in 6 M HCl.