Investigation of Mechanical Properties of Oxide Films on the Base Nb, Ta and Zr

Electro-chemo-mechanical properties of anodic oxide (passive) films formed on metals have been reviewed focusing on the results of stress variations caused by anodic oxidation of Cu, Ni, and Fe thin film electrodes in deaerated pH 8.4 borate buffer solution at 25 °C. The surface stress varies toward compressive direction due to adsorption of OH on Cu from aqueous solution as well as adsorption of oxygen on metals from gas phase. The stresses are generated with the growth of three-dimensional anodic oxide films on metals. The magnitude and sign (tensile or compressive) of the intrinsic film stress were determined by taking the residual stress of the substrate and the dielectrostriction into consideration. The tensile or compressive intrinsic film stress depends on p-type or n-type semiconductive properties of the anodic oxide films, which is explained in terms of the void formation or oxide formation in the metal side at the metal/film interface. Furthermore, the stress variation toward compressive direction during cathodic reduction of the anodic oxide films is explained in terms of the volume expansion due to the formation of intermediate species.

Semiconductive properties of titanium anodic oxide films in McIlvaine buffer solution

The properties of the growth of the 6061‐T6 aluminum alloy oxide were studied using sulfuric acid anodization. The parameters for the manufacturing process include electrolyte categories, electrolyte concentration, and operating voltages. The results showed that the aluminum oxides obtained by anodization process are mainly amorphous structure and the anodic current density is an important factor affecting the rate of response for oxygen and aluminum ions in barrier. In this experiment, polish process is very important to stable the anodic aluminum oxide film and then it will get the better properties of anodic film. Besides, when using sulfuric acid as the electrolyte, the increase of anodic voltage also increases the rate of reaction which increases the mechanical and electrical properties of anodic oxide film, but too large applied anodic voltage will reduce the mechanical and electrical properties of film because of the crack of the anodic oxide film.

Sputter-deposited titanium substrates were anodically treated in sulfuric acid solution both in potentiostatic and potential-sweep modes. The morphology, crystallization, chemical compositions and electrochemical properties of anodic titanium oxide films were detected by AFM, SE, Raman spectra, XPS and EIS. The formed anodic films are smooth and homogeneously crystallized, and that the potentiostatically grown film is slightly thicker and less crystalline than the potentiodynamically formed film. Moreover, a comparison of the structure and properties of the anodic oxides films formed on the mechanical-chemical polished bulk titanium and the sputter-deposited titanium substrates is also presented. The titanium substrates can largely influence the properties of the formed anodic films. A more smooth and compact titanium oxide film could grow on the sputter-deposited titanium substrate, which is unfavorable to the ionic migration through the film and delays the film growth and crystallization.

We reported previously in this journal1) on the effect of Al ion in aluminium anodic oxidation by 15% sulphuric acid.That report discussed the effects of dissolved aluminium ion on specific electric conductivity of electrolyte, current density and properties of anodic film.In this report, we studied the effects of Al ion dissolved in electrolyte on properties of oxide film of Al anodized in oxalic acid at our production shop and our laboratory.We studied the relation between the increase of dissolved Al ion and current quantity (A. H.) at our production shop, and the effects of Al ion on the specific electric conductivity, current density and properties of anodic oxide film, at our laboratory.The results are as follows.(1) The increase of dissolved Al ion is proportional to current quantity (A. H), and the dissolved Al ion increases at the rate of 0.0085gr/A. H.(2) Concentration of oxalic acid is proportional to specific electric conductivity, and this relation is shown by following experimental formula.K=0.0060+0.0120x x=concentration of H2C2O4% 0.5<x<3.Concentration of Al ion has little effect on specific electric conductiviy. As it has, however, the property which prevents the current in anodizing, the increase of dissolved Al ion lowers the current density.(3) The effects of Al ion on the properties of anodic oxide film are summarized as follows.When the electrolitic voltage is constant (AC 80V, AC 70V, + DC35V), current density decrease by the effect of Al ion. And then film thickness becomes thinner.The corrosion resistance of anodic oxide film lowers til the dissolved Al ion increases up to 2g/l, but when the content of Al ion increases above 2g/l, it becomes rather better.The abrasion resistance of anodic oxide film is rapidly lower as the dissolved Al ion increases. According to the results of the constant electric power anodizing keeping the oxide film at constant thickness, the effects of Al ion on corrosion resistance and abrasion resistance have the same tendency as above mentioned, although the effect of the film thickness has been eliminated.

The influence of fluoride (and its concentration) on the electrochemical and semiconducting properties of anodic oxide films formed on titanium surfaces was investigated by performing electrochemical measurements (potentiodynamic/pontiostatic polarization, open circuit potential (OCP), and capacitance measurements) for a titanium/oxide film/solution interface system in fluoride-containing 1.0 M HClO(4) solution. On the basis of the Mott-Schottky analysis, and with taking into account both the surface reactions (or, say, the specifically chemical adsorption) of fluoride ions at the oxide film surface and the migration/intercalation of fluoride ions into the oxide film, the changes in the electrochemical behavior of titanium measured in this work (e.g., the blocked anodic oxygen evolution, the increased anodic steady-state current density, the positively shifted flat band potential, and the positively shifted film breakdown potential) were interpreted by the changes in the surface and the bulk physicochemical properties (e.g., the surface charges, surface state density, doping concentration, and the interfacial potential drops) of the anodic films grown on titanium. The fluoride concentrations tested in this work can be divided into three groups according to their effect on the electrochemical behavior of the oxide films: < or =0.001 M, 0.001-0.01 M, and >0.01 M. By tracing the changes of the OCP of the passivated titanium in fluoride-containing solutions, the deleterious/depassive effect of fluoride ions on the titanium oxide films was examined and evaluated with the parameter of the film breakdown time. It was also shown that the films anodically formed on titanium at higher potentials (>2.5 V) exhibited significantly higher stability against the fluoride attack than that either formed at lower potentials (<2.5 V) or formed natively in the air.

The influence of the growth rate on the semiconductive properties of titanium anodic oxide films

Zr–Al alloys containing up to 26 at.% aluminum, prepared by magnetron sputtering, have been anodized in 0.1 mol dm−3 ammonium pentaborate electrolyte, and the structure and dielectric properties of the resultant anodic oxide films have been examined by grazing incidence X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and AC impedance spectroscopy. The anodic oxide film formed on zirconium consists of monoclinic and tetragonal ZrO2 with the former being a major phase. Two-layered anodic oxide films, comprising an outer thin amorphous layer and an inner main layer of crystalline tetragonal ZrO2 phase, are formed on the Zr–Al alloys containing 5 to 16 at.% aluminum. Further increase in the aluminum content to 26 at.% results in the formation of amorphous oxide layer throughout the thickness. The anodic oxide films become thin with increasing aluminum content, while the relative permittivity of anodic oxide shows a maximum at the aluminum content of 11 at.%. Due to major contribution of permittivity enhancement, the maximum capacitance of the anodic oxide films is obtained on the Zr–11 at.% Al alloy, being 1.7 times than on zirconium at the formation voltage of 100 V.

The inter–relationships of alloy composition, film composition and ionic transport for formation of amorphous anodic oxide films are addressed quantitatively through systematic study of sputter–deposited Al–Ta alloys containing up to 39 at.% Ta. The work reveals the dependence of electric field, ionic transport number, incorporation of species into the anodic film at the alloy–film interface and mobility and distribution of species within the anodic film on alloy composition. Anodic oxidation, at high current efficiency, of alloys containing 2.8, 15, 32 and 39 at.% tantalum results in formation of two–layered anodic films by migration of cations outwards and by migration of anions inwards: an outer layer, 20% or less of the total film thickness, composed of relatively pure alumina and an inner layer containing units of Al2O3 and Ta2O5 distributed relatively homogeneously. Two–layered films develop due to the slower migration rate of Ta5+ ions relative to A13+ ions in the inner layer of the growing anodic films, which changes progressively from about 0.6 for dilute alloys to about 0.9 for Al–39 at. per cent Ta. The average nm V−1 ratios, total transport numbers of cations and average Pilling–Bedworth ratios for the films change almost linearly with alloy composition between the values for anodic alumina and anodic tantala. A tantalum–enriched layer, about 1 nm thick, is formed in the Al–2.8 at.% Ta and Al–15 at.% Ta alloys just beneath the anodic film, indicating prior oxidation of aluminium in the initial stages of anodizing. In contrast, aluminium and tantalum in the alloys containing more than 30 at.% tantalum are immediately incorporated into anodic films in their alloy proportions, without development of a tantalum–enriched layer, at the available resolution. Boron species, incorporated from the electrolyte into the outer parts of the films, are immobile in films on alloys up to 15 at.% Ta but migrate outwards in other films, possibly due to the increased Lorentz field. Though the inter–relationships between film parameters and alloy composition are established for Al–Ta alloys specifically, the findings are considered to be equally relevant to amorphous anodic oxides formed on alloys and semiconductors generally.

Effect of aluminum annealing on the galvanoluminescence properties of anodic oxide films formed in organic electrolytes

The semiconductor properties of anodic oxide film formed on commercial pure aluminum were analyzed using Mott-Schottky theory and point defect model (PDM). The donor density, oxygen vacancy diffusion coefficient and flat-band potential were measured for the oxide films sealed by boiling water and K2Cr2O7, respectively. The results indicated that the anodic oxide films showed the n-type semiconductor property and the donor density decreased exponentially with the voltage elevating. The value of oxygen vacancy diffusion coefficient is about (1.12-5.53)×10-14 cm-2·s-1. The flat-band potential of anodic oxide film declined after sealing.

Mechanical properties, such as fracture strength and Young’s modulus, of the anodic oxide films formed on aluminum specimens in sulfuric acid, oxalic acid and chromic acid baths were measured by means of a tensile testing machine. Experimental results show that the shapes of stress-strain curves of aluminum specimens with the anodic oxide films varied according to the kinds of the anodizing baths used for the formation of the films. Based on theoretical analysis of the above curves of the specimens, Young’s modulus and the fracture strength of the oxide films obtained by various baths were calculated. These results revealed that the values of fracture strength of the anodic oxide films formed in the above baths were between 140-220 MPa, while the values of Young’s modulus of the anodic oxide films were more different according to the baths used for anodizing. The values were around 36 GPa for the film formed in oxalic acid bath, around 28 GPa in the sulfuric acid bath, and around 15 GPa in the chromic acid bath. Elongation of the oxide film formed in the chromic acid bath was greater than those of the films formed in the other baths. The formation of the anodic oxide films on aluminum alloys resulted in a great decrease of the fatigue strength of aluminum specimens.

Anodic luminescence, structural, photoluminescent, and photocatalytic properties of anodic oxide films grown on niobium in phosphoric acid

Photoelectrochemical properties of niobium oxide film formed by anodic oxidation in a saturated boric acid solution have been studied. The oxide film is considered as an n-type semiconductor from the polarization curves measured in 0.5 N sulfuric acid solution. Anodic photocurrent is observed when the oxide film is illuminated by an ultraviolet light while it is anodically polarized. From the spectrum of the photocurrent of the film, the band gap energy of the oxide is determined to be about 3.3 eV. When the specimen is polarized at a cathodic potential before the photocurrent measurements, the film shows an additional photoresponse for the light with longer wavelength than that corresponds to the band gap energy. This can be explained by the formation of new electron levels in the band gap by cathodic hydrogen charging

A brief review is given of mechanical property measurements on oxide films. This review is followed by a detailed discussion of the mechanical and fracture properties of anodic aluminum oxide films as observed in the author's laboratory. Extensive measurement of Young's modulus, ,and fracture strain, , for separated films 3000Aå thick is reported as a function of environmental water vapor pressure. The fracture of these unsupported films is shown to occur by a brittle mechanism. Mechanical properties of adhering aluminum oxide films are given as a function of their thickness. These oxides were observed to fracture either at slip steps, or at right angles to the tensile axis in a regularly spaced fashion. A theory of adhering oxide fracture is discussed which accounts well for the observations. An equation which describes the spacing of regular oxide fracture cracks as a function of substrate strain ε is given in the form , where () are the initial conditions for regular fracture, is the oxide thickness, and is a constant.