Breakdown Of Oxide Film Research Articles

Correlation between dislocation density and nanomechanical response during nanoindentation

The crucial role of dislocations in the nanomechanical response of high-purity aluminum was studied. The dislocation density in cold-worked aluminum is characterized by means of electron channeling contrast and post-image processing. Further in situ heat treatment inside the chamber of a scanning electron microscope was performed to reduce the dislocation density through controlled heat treatment while continuously observing the structure evolution. The effect of dislocation density on both the pure elastic regime before pop-in as well as elastoplastic deformation after the pop-in were examined. Increasing the dislocation density and tip radius, i.e. the region with maximum shear stress below the tip, resulted in a reduction in the pop-in probability. Since the oxide film does not change with dislocation density, it is therefore clear that pop-ins in aluminum are due to the onset of plasticity by homogeneous dislocation nucleation and not oxide film breakdown. Hertzian contact and the indentation size effect based on geometrically necessary dislocations are used to model the load–displacement curves of nanoindentation and to predict the behavior of the material as a function of the statistically stored and geometrically necessary dislocation density.

Structure of Chlorine K-Edge XANES Spectra During the Breakdown of Passive Oxide Films on Aluminum

We generated molecular simulations of Cl– interacting at different sites in aluminum oxide models and carried out FEFF8 calculations to obtain the local l-projected density of states (LDOS) spectra. These are compared to our earlier experimental X-ray absorption near-edge structure (XANES) data in order to study the interactions of chloride ions with the passive oxide film on aluminum as a function of electrochemical potential at the Cl K edge. This led to a number of new insights in the mechanism of the breakdown of the passive film in chloride solutions. Importantly, we show the chloride first attacks the hydroxyl components of the aluminum oxide, penetrates the oxide film, and finally attacks the metal surface.

Enhanced corrosion resistance of interstitially hardened stainless steel: Implications of a critical passive layer thickness for breakdown

Immense interstitial hardening of 316L austenitic stainless steel via low-temperature paraequilibrium carburization also leads to greatly improved corrosion resistance. Both the hardening and the improved corrosion resistance owe their origin to a “colossal” supersaturation of interstitial carbon. The corrosion resistance of stainless steel involves a Cr 2O 3-rich passive film, and the composition and thickness of the passive film developed during anodic polarization at various potentials were determined for both carburized and non-treated steels using grazing incidence X-ray photoelectron spectroscopy. Passive oxide film breakdown is a necessary step in pitting corrosion, and appears to occur in these steels at a critical film thickness of ≈3 nm. We suggest that this breakdown is of chemomechanical origin. Long wavelength thickness perturbations occur during film growth to reduce the strain energy density in the passive film arising from intrinsic and electric field-induced stresses. At the critical thickness, the localized thinning is sufficient to lead to dielectric breakdown and nucleation of pitting corrosion. The improved corrosion resistance for the carburized material results from thinner passive films at a given potential and hence a delay in the detrimental effect of the thickness perturbations.

Synthesis of niobium oxide nanopowders by field-crystallization-assisted anodization

Herein, for the first time, we demonstrate that a large quantity of niobium oxide nanopowders consisting of amorphous round shape nanoparticles and highly crystalline needle shape nanoparticles can be prepared by anodization of a niobium foil in ethylene glycol containing NH 4F at a high voltage. FE-SEM, TEM, XRD measurements were performed to characterize the anodically prepared nanopowders and to reveal the formation mechanism. We ascribed the formation of niobium oxide nanopowders on the foil to field-crystallization induced breakdown of oxide film and chemical dissolution.

Dielectric breakdown and healing of anodic oxide films on aluminium under single pulse anodizing

Single pulse anodizing of aluminium micro-electrode has been employed to study the behaviour of dielectric breakdown and subsequent oxide formation on aluminium in alkaline silicate and pentaborate electrolytes. Current transients during applying pulse voltage have been measured, and surface has been observed by scanning electron microscopy. Two types of current transients are observed, depending on the electrolyte and applied voltage. There is a good correlation between the current transient behaviour and the shape of discharge channels. In alkaline silicate electrolyte, circular open pores are healed by increasing the pulse width, but such healing is not obvious in pentaborate electrolyte.

Electrical breakdown as an insulator-to-metal transition

The effect of dielectric strengthening under electrical breakdown in thin oxide films and other materials is discussed. The breakdown phenomenon is considered as an insulator-to-metal phase transition. The effect of strengthening is thus associated with the fact that no phase transition seems to be possible when the system size is decreased below a certain characteristic length d c (it is a so-called ‘tachyon instability’). This dimension is estimated to be d c ∼ ξ, where ξ is the correlation length for metal–insulator transition.

A theoretical analysis of the breakdown of electrostrictive oxide film on metal

Oxide films that form to protect (passivate) metal substrates from corrosive environments can be severely damaged when they are subjected to sufficient levels of electric potential. A continuum mechanics model is presented that captures the intimate electromechanical coupling of the environment and the film responsible for either growth or dissolution of the oxide. Analytical solutions, obtained for a finite-thick film experiencing a uniform electric field, illustrate the existence of a critical combination of electric field strength, initial film thickness and shape, beyond which the passivating oxide can become thin enough to undergo dielectric breakdown, or the substrate can become exposed to the corrosive environment. An experimental procedure is proposed to measure combinations of material properties required by the theoretical model to predict the lifetime of the oxide or to avoid the critical state. Illustrative numerical examples are provided to describe the morphological evolution of oxide films with a periodically wavy surface.

Mechanistic Studies on Room Temperature Photoexcitation Effects on Passivity Breakdown of Ultrathin Surface Oxide Films Formed on Ternary Al−5%Cu−5%Ni Alloys

How the composition of nanoscale surface oxides formed on complex metal alloys affects their passive state is a problem of great interest in physical chemistry as well as in technological applications. In this work, we report on experimental studies regarding the problem concerning the role of room-temperature photoexcitation on altering composition of ultrathin oxides formed on ternary Al−Cu−Ni thin film alloy surface and its influence in passivity breakdown in aqueous media. Extensive studies have been carried out on pure copper, Al−5%Cu, and Al−5%Cu−5%Ni alloy thin films to investigate the mechanistic role of alloying elements as well as their oxide formation characteristics on the passive state. The differences in nanoscale oxide composition formed have a remarkable influence on the passivity breakdown of the oxide films as verified from electrochemical measurements, and possible mechanisms leading to the observations are discussed in detail.

Corrosion analysis of Al–Zn–In–Mg–Ti–Mn sacrificial anode alloy

The corrosion behaviour of Al–5Zn–0.02In–1Mg–0.05Ti–0.5Mn (wt%) alloy was investigated by SEM, EDX and TEM. The results show that there exist different corrosion types of the alloy in 3.5% NaCl solution with immersion time. At the initial stage of immersion, pitting predominate the corrosion around precipitates. The major precipitates of the alloy are MgZn 2 and Al 6Mn particles. The corrosion potential of the bulk MgZn 2 alloy is negative with respect to that of the α-Al and the bulk Al 6Mn alloy, so the MgZn 2 precipitate can act as activation centre, make the aluminum oxide film breakdown and cause the pitting corrosion. In the late stage, it turns out to be a uniform corrosion due to the control of dissolution precipitation of In and Zn ions.

Current Understanding of Ti Anodisation: Functional, Morphological, Chemical and Mechanical Aspects

This paper provides an overview of the current understanding of the Ti anodisation process. As compared to other valve-metals, Ti exhibits several significant differences regarding its anodisation behaviour, the most important one being the marked semiconducting character of its anodic oxide. In the first part of this paper, a general introduction to anodisation processes is given and theoretical models describing the growth kinetics, breakdown and internal stress development in anodic oxide films are presented. In the second part, the peculiarities of Ti regarding the anodisation behaviour are discussed extensively. In particular, the influence of the main experimental parameters (metal substrate, electrolyte and growth rate) on both the anodisation process and the functional, morphological, chemical and mechanical characteristics of the anodic films is reviewed.

High temperature corrosion monitoring by electrochemical noise techniques

Co-firing biomass with coal in pulverized fuel combustion plants can lead to significant corrosion problems due to the release of aggressive species from some types of biomass. The resulting deposits on heat exchangers in the power plant hot gas path can contribute to breakdown of protective oxide films and lead to accelerated fireside corrosion. The electrochemical noise (ECN) technique has been investigated as a method of monitoring such corrosion where molten salts are present. Three identical sensors manufactured from the same material as the heat exchangers (either superheater/reheater or waterwall) were assessed. Sensor designs were developed as uncooled prototypes, representing geometries suitable for use in probes for waterwall or superheater/reheater locations. Simulated deposits were used to determine the sensitivity of the sensors and monitoring technique to changes in deposit composition and temperature. Further tests investigated the effect of different concentrations of SOX in the simulated combustion gas. Analysis of ECN data using statistical and frequency domain treatments has been combined with real-time observations of raw data. Results have shown that appropriately calculated values of resistance can be realistic indicators of the corrosion rate of alloys tested where general, uniform corrosion is taking place. Attempts to identify localised corrosion mechanisms by statistical data treatment will also be presented.

Atomistic Modeling of Voiding Mechanisms at Oxide/Alloy Interfaces

For the first time, voiding mechanisms resulting from the condensation of atomic vacancies injected at oxide/alloy interfaces by the growth of oxide layers have been studied by means of periodic density functional theory (DFT) calculations. Several interfaces were built by superimposing ultrathin films of alumina on the γ-TiAl(111) surface, and their relative stabilities were compared by calculating the interface energy variation. The formation energy of single Ti or Al vacancies and clustered and dispersed ensembles of 2Ti + 1Al or 2Al + 1Ti vacancies injected into the alloy were calculated. The results show that it is easier to inject the vacancies into the oxide/alloy interface than into the bare alloy surface and into the bulk alloy. The injected vacancies, trapped at the oxide/alloy interface, condense in the topmost plane of the alloy to form 2D clusters. The minimization of the coordination number of the vacancies with metal atoms of the alloy and O atoms of the overlaying oxide favors vacancy condensation and interfacial voiding. The data are relevant for a detailed understanding of the adherence and breakdown of protective oxide films and the lifetime of metallic materials.

Corrosion mechanisms of candidate structural materials for supercritical water-cooled reactor

Nickel-based alloys, austenitic stainless steel, ferritic/martensitic heat-resistant steels, and oxide dispersion strengthened steel are presently considered to be the candidate structural or fuel-cladding materials for supercritical water-cooled reactor (SCWR), one of the promising generation IV reactor for large-scale electric power production. However, corrosion and stress corrosion cracking of these candidate alloys still remain to be a major problem in the selection of nuclear fuel cladding and other structural materials, such as water rod. Survey of literature and experimental results reveal that the general corrosion mechanism of those candidate materials exhibits quite complicated mechanism in high-temperature and high-pressure supercritical water. Formation of a stable protective oxide film is the key to the best corrosionresistant alloys. This paper focuses on the mechanism of corrosion oxide film breakdown for SCWR candidate materials.

Synthesis of Ti-Based Glassy Alloy/Hydroxyapatite Composite by Spark Plasma Sintering

The spark plasma sintering process has been used to fabricate Ti40Zr10Cu36Pd14 glassy alloy/hydroxyapatite composites. XRD, DSC, SEM and compression testes were used to examine the microstructure and properties of the composites. No crystallization was observed in the glassy alloy matrix. The glass transition temperature (Tg) is independent of the HA addition and the onset temperature of crystallization (Tx) slightly decreases with increasing HA content. The sintered composite exhibited reduced value of Young's moduli as compared with the as-cast Ti40Zr10Cu36Pd14 bulk glassy alloy. (doi:10.2320/matertrans.MBW200716) Since bulk glassy alloys were synthesized for the first time by a copper mold casting process in the late 1980's, 1) a number of bulk glassy alloys in Mg-, La-, Zr-, Pd-, Ti, Fe-, Co-, Ni- and Cu-based systems have been developed. 2) Unique properties of bulk glassy alloys make them extremely attractive for biomedical applications. Recently, bulk glassy alloy composites have attracted much attention due to their improved characteristics. Most of researches for bulk glassy alloy composites have been focused on the improvement of mechanical properties, especially for ductility. 3-5) Hydroxyapatite (HA), which is a main mineral constituent of teeth and bone, has an excellent biocompatibility with hard tissue, skin and muscle tissue. Moreover, HA does not exhibit any cytoxic effects and can directly bond to human bone. 6-8) A number of studies have been reported on HA coating on Ti metal and Ti-alloys for improving the surface bioactivity. 9-12) However, the HA coatings often flake off as a result of poor ceramic/metal interface bonding. 13) The problems mentioned above may be solved by fabrication of metal/HA composites. Some papers have been published on the preparing of Ti/HA composite materials. 14-16) As a novel rapid sintering technique, the use of a spark plasma sintering (SPS) technique enables the sintering of high quality materials within a short time by controlling the spacing between powder particles, electrical energy and sintering pressure. The SPS includes various phenomena: 17) (a) electrical breakdown of surface oxide film and removal of contaminated layer on particle surface by spark generation and sputtering effect; (b) destruction of surface oxide film and neck formation between powder particles; (c) focused current and Joule heat at the neck; and (d) enhanced migration of atom or ion by the difference in temperature between neck and particle core as well as by electrical field, and neck growth. The phenomena provide advantages which cannot be obtained using the conventional sintering process. The sintering can be carried out at a lower temperature and in a shorter time as compared with the conventional sintering process. Consequently, the SPS can be applied to materials which are required to suppress crystallization and grain growth, such as glassy alloys. 17)

Formation of coatings on rectifying metals and alloys by microarc oxidizing in electrolytes with a capacitive energy control

Coating formation on titanium and aluminum samples in aqueous electrolytes at breakdown potentials (under conditions of microarc oxidizing) is described. The oxidizing was performed with either no current restriction during the oxide film breakdown or with an additional reactive resistance in the external circuit allowing to control the energy input intensity to the breakdown channels. It is shown that the external reactive resistance increases the coating formation potentials with no visible additional breakdown areas; the final coating formation currents decrease by order of magnitude; the energy evolved in the electrolyte decreases by a factor of 5 to 10. The variable external reactive resistance allows forming coatings with novel properties.

Surface Segregation of Indium by Heat Treatment of Aluminium

High temperature heat treatment of aluminium alloys causes surface enrichment of the trace elements in Group IIIA - VA, specifically the low melting point elements Pb, Bi, In and Sn. The phenomenon has practical significance in promoting certain types of localised corrosion, such as galvanic and filiform corrosion, while mitigating other types, such as pitting corrosion of the bare surface. The purpose of this paper is to investigate the surface enrichment and microstructure of indium relative to the available data for Pb. Model binary AlIn alloys, containing 20-1000 ppm of In, were used after heat treatment at various temperatures. In addition to electrochemical investigations, the microstructures were characterised by field emission scanning electron microscopy (FEG SEM) and field emission transmission electron microscopy (FEG TEM). Heat treatment at temperatures as low as 300°C gave significant segregation of In as opposed to 600°C for Pb. As a result of this and yet unresolved oxide film breakdown mechanism on aluminium, In was significantly more effective than Pb in anodically activating aluminium. These results suggest the possibility that significant activation earlier observed on certain commercial alloys as a result of low temperature heat treatment may be due to the trace elements In.

Temperature stability of thin anodic oxide films in metal/insulator/metal structures: A comparison between tantalum and aluminium oxide

The dielectric breakdown of thin ( d = 3–4 nm) aluminium and tantalum oxide films was investigated by means of current voltage plots in metal/insulator/metal systems. Dielectric breakdown field strengths, E DB, of 0.6 GV m − 1 were found for both oxide types at room temperature. Differences appear in the temperature dependence of E DB. Tantalum oxide films show an unchanged breakdown behaviour for temperatures up to 420 K while aluminium oxide films lose already 80% of their E DB value in the same temperature range. Time-resolved investigations of the electric breakdown revealed intermediate states of both oxide types which were stable for several ms being characterized by an enhanced tunnel current. The breakdown voltage clearly scales with the oxide thickness for both oxide types.

Breakdown of ultrathin native oxide films at contact interfaces of electromechanically stressed silicon microdevices

Breakdown of oxide films at contact interfaces can result in significant current fluctuations with severe consequences on the long-term performance of electromechanical microdevices. Contrary to metal-oxide-semiconductor technology, the physical phenomena leading to breakdown of ultrathin native oxide films at silicon contact interfaces due to electromechanical stresses remain elusive. The objective of this study was to bridge this gap by performing experiments with special silicon microdevices subjected to different electrical actuation voltages and dwell contact times. Abrupt excursions in the current flow across the contact interface indicated the occurrence of breakdown events in the electromechanically stressed oxide films at the contact interface. The post-breakdown current–voltage response remained non-Ohmic, which is characteristic of the conduction paths produced during the breakdown events. The relatively long duration (a few seconds) of the electrical transient response is not indicative of contact bouncing, which is on the order of a few microseconds. The abrupt changes in the current density across the contact interface are associated with the formation of defects in the silicon oxide film.

Cathodic breakdown of anodic oxide film on Al and Al–Sn alloys in NaCl solution

The effect of cathodic polarisation on stability of defined oxide films on Al and Al–Sn alloys (with up to 0.40% Sn) has been investigated in a 0.5 M NaCl solution using the potentiostatic pulse method. The dependence of the cathodic current on time (in the period of 1, 10 and 100 s) was recorded on Al and Al–Sn alloys when subjected to a potential pulse from E OCP to different negative values (up to −2.0 V). Anodic current responses to the return to the E OCP were also recorded at three different time scales (1, 10 and 100 s). It has been established that the cathodic polarisation of passivated Al and Al–Sn alloys in a chloride solution is characterized by two regions of potentials with distinctly different phenomena: the range of low and high cathodic potentials (LCP and HCP). In the LCP range, the oxide film retains its properties, while in the HCP range cathodic breakdown and hydration of the oxide take place. The boundary between these two potential ranges shifts towards more negative potential values when the percentage of Sn in the alloy increases. The longer the duration of the cathodic pulse, the more positive the potentials at which the oxide film breakdown takes place. This shift is more marked with alloys containing higher percentage of Sn. Cathodic polarisation (duration of 100 s) activates alloys with 0.20% and 0.40% Sn for anodic dissolution.

Scaling laws of femtosecond laser pulse induced breakdown in oxide films

The scaling of the single-pulse laser threshold fluence for dielectric breakdown with respect to pulse duration and material band gap energy was investigated in the subpicosecond pulse regime using oxide films (${\mathrm{TiO}}_{2}$, ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$, ${\mathrm{HfO}}_{2}$, ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, and ${\mathrm{SiO}}_{2}$). A phenomenological model attributes the pulse duration dependence to the interplay of multiphoton ionization, impact ionization, and subpicosecond electron decay out of the conduction band. The observed linear scaling of the breakdown fluence with band gap energy can be explained within the framework of this model by invoking the band gap dependence of the multiphoton absorption coefficient from Keldysh photoionization theory. The power exponent $\ensuremath{\kappa}$ of the observed dependence of the breakdown threshold fluence ${F}_{\mathit{th}}$ on pulse duration ${\ensuremath{\tau}}_{p}$, ${F}_{\mathit{th}}\ensuremath{\propto}{\ensuremath{\tau}}_{p}^{\ensuremath{\kappa}}$, is independent of the material and is attributed to photoionization seeded avalanche ionization.