Interfacial Oxide Film Research Articles

Interfacial characteristics, interfacial oxide evolution and bonding mechanism of 2219 aluminum alloy joints manufactured by hot-compression bonding

The bonding joints of 2219 aluminum alloy with different deformations of 30%, 45% and 55% were fabricated using hot-compression bonding, and subsequent subjected to homogenization treatment. The interfacial microstructure and interfacial oxide evolution of the bonding joints were systematically investigated. The results showed that increasing deformation can effectively reduce the formation of interfacial voids and promote the interfacial grain boundary migration (IGBM). As the deformation increased from 30% to 55%, the flat bonding interface transformed into curved grain boundaries, and ultimately being occupied by dynamically recrystallized (DRXed) grains. The development of IGBM and interfacial DRXed grains is crucial for ensuring high-quality bonding joints. At 55% deformation, the interface bonding strength reached 156.2 MPa, corresponding to 98.1% of the matrix bonding strength. In addition, the residual interfacial oxide film Al2O3 reacted with MgO and transformed into MgAl2O4 spinel. The final evolution products at the bonding interface were the mixture of MgO, Al2Cu, Al2O3, and MgAl2O4. The bonding interface at 55% deformation successfully achieved relatively clean atomic bonding after homogenization. Based on interfacial void closure, interfacial oxide film fragmentation and evolution, and interfacial recrystallization, the bonding mechanism of 2219 aluminum alloy joints was proposed. These findings contribute to our understanding of the interfacial bonding process in metal solid bonding.

Corrosion Degradation Mechanism of Cr-Coated Zr-4 Alloy under Simulated Nuclear Conditions for Accident-Tolerant Fuel.

Cr coatings with a thickness of about 19 μm were synthesized on Zr-4 cladding using plasma-enhanced arc ion plating. A Zr-Cr micro-diffusion layer was formed via Cr ion cleaning before deposition to enhance the interface bonding strength. Cr coatings exhibit an obvious columnar crystal structure with an average grain size of 1.26 μm using SEM (scanning electron microscopy) and EBSD (electron backscatter diffraction) with a small amount of nanoscale pores on the surface. A long-term aqueous test at 420 ± 3 °C, 10.3 ± 0.7 MPa and isothermal oxidation tests at 900~1300 °C in air were conducted to evaluate the Cr-coated Zr-4 cladding. All the results showed that the Cr coatings had a significant protective effect to the Zr-4 alloy. However, the corrosion deterioration mechanism is different. A gradual thinning of the Cr coating was observed in a long-term aqueous test, but a cyclic corrosion mechanism of void initiation-propagation-cracking at the oxide film interface is the main corrosion characteristic of the Cr coating in isothermal oxidation. Different corrosion models are constructed to explain the corrosion mechanism.

Evaluation of the strength of FeCrAl alloy/surface Oxide film interface by micro double-notch shear test

The shear strength of the film formed by pre-oxidation treatment of FeCrAl(-ODS) alloys, likely to be used as pipework material for liquid LiPb breeder blankets, was evaluated by micro double-notch shear testing. Fracture generally occurred at the interface between the oxide film and the alloy substrate, but the pop- in the stress-strain curves was difficult to interpret, so all the test results in this study were compared to the full rupture strength. The rupture strength appears to be a function of the nature of the interface between the film and the alloy rather than the thickness of the film; for APMT and FeCrAl-ODS, the rupture strength was greater for FeCrAl-ODS.

Fabrication of yttria-stabilized zirconia/La2O3 composite coating by electrophoretic deposition to improve high-temperature oxidation resistance of stainless steel

Nanoscale yttria-stabilized zirconia (YSZ) and La2O3 were used to prepare a composite coating by electrophoretic deposition (EPD) and sintering to improve the high-temperature oxidation resistance of AISI430 stainless steel. SEM, XRD, and TEM were used to identify the microstructure of the YSZ/La2O3 composite coating and the oxide film formed at 900 °C. The effect of the EPD voltage on the coating thickness was studied. The microstructure properties and phase transition of the composite coatings during sintering were identified. The oxidation behavior of the composite coating at 900 °C was investigated by analyzing the oxidation kinetics curve, oxidation spallation, the formation of the interfacial oxide film, and the evolution of the coating microstructure. A model of the microstructure evolution of the YSZ/La2O3 composite coating during EPD, sintering, and high-temperature oxidation was proposed and discussed.

Interface microstructure and bonding mechanisms of 7050 aluminum alloy thick plates produced by vacuum roll cladding

Vacuum roll cladding (VRC) based on friction stir welding (FSW) is applied to prepare aluminum alloy thick plates, and the effect of rolling reduction ratios (60%, 70%, 80%) on the microstructure and mechanical properties at the bonding interface of aluminum alloy clad plates are investigated. The results show that voids at the interface are gradually closed and the original interface finally disappears with the increase of rolling reduction ratios. There are 3 kinds of recrystallization mechanisms at the interface during hot rolling. The contributions to the healing of the interface are in the following order: geometric dynamic recrystallization (GDRX) > discontinuous dynamic recrystallization (DDRX) > continuous dynamic recrystallization (CDRX). At a rolling reduction ratio of 80%, GDRX characterized by the “pinch-off” of original grains mainly occurs at the interface, and no aggregation of defects and impurity elements is detected. The tensile shear strength and fracture displacement at the interface both reach the level of the matrix, which indicates that a good metallurgical bonding has been achieved at the interface. Therefore, the key to improving the interfacial healing quality is to break the interfacial oxide film and further induce recrystallization at the interface through a large rolling reduction ratio.

Local Enhancement of Dopant Diffusion from Polycrystalline Silicon Passivating Contacts.

Passivating contacts consisting of heavily doped polycrystalline silicon (poly-Si) and ultrathin interfacial silicon oxide (SiOx) films enable the fabrication of high-efficiency Si solar cells. The electrical properties and working mechanism of such poly-Si passivating contacts depend on the distribution of dopants at their interface with the underlying Si substrate of solar cells. Therefore, this distribution, particularly in the vicinity of pinholes in the SiOx film, is investigated in this work. Technology computer-aided design (TCAD) simulations were performed to study the diffusion of dopants, both phosphorus (P) and boron (B), from the poly-Si film into the Si substrate during the annealing process typically applied to poly-Si passivating contacts. The simulated 2D doping profiles indicate enhanced diffusion under pinholes, yielding deeper semicircular regions of increased doping compared to regions far removed from the pinholes. Such regions with locally enhanced doping were also experimentally demonstrated using high-resolution (5-10 nm/pixel) scanning spreading resistance microscopy (SSRM) for the first time. The SSRM measurements were performed on a variety of poly-Si passivating contacts, fabricated using different approaches by multiple research institutes, and the regions of doping enhancement were detected on samples where the presence of pinholes had been reported in the related literature. These findings can contribute to a better understanding, more accurate modeling, and optimization of poly-Si passivating contacts, which are increasingly being introduced in the mass production of Si solar cells.

Interfacial two-dimensional oxide enhances photocatalytic activity of graphene/titania via electronic structure modification

The photocatalytic activity of titania nanoparticles deposited on epitaxial graphene is proven to be significantly affected by the substrate on which graphene is supported. In particular, it has been revealed that the addition of a two-dimensional TiO1.5 layer sandwiched between graphene and the supporting metal induces a p-doping of graphene itself and a consistent shift in the Ti d states. These modifications in the electronic structure are compatible with the reduction of the probability of charge carrier recombination and enhance the photocatalytic activity of the heterostructure. This is indicative of the capital role played by the interfacial thin oxide films in fine-tuning the properties of heterostructures based on graphene and pave the way to new combinations of graphene/oxides for photocatalysis-oriented applications.

Wetting of amorphous and partial crystalline Co73Si10B17 by Sn and Sn-based lead-free solders

Using wetting balance method, the wetting of Co73Si10B17 in amorphous and partial crystalline states by Sn and Sn-based lead-free solders (Sn-0.7Cu and Sn-0.3Ag-0.7Cu) were studied at 290–310 °C. The surface energy of the Co-based alloys was calculated by three methods (Zisman method, Fowkes and Owen theory). Although the calculated surface energy shows the presence of oxide film on the surface of all Co-based alloy, the amorphous Co73Si10B17 has a relatively higher surface energy. The wetting mechanism is the oxide film removal, which was caused by a series of complex multiple reactions. The final wettability of Co73Si10B17 was affected by the interfacial reaction products and surface oxide film, but was determined by the degree of the oxide film removal. In addition, RMA flux is not very effective for soldering of Co-based alloy whether in amorphous or partial crystalline state owing to its weak activity, especially it is not suitable for soldering of partial crystalline Co73Si10B17.

Effect of Reduction on Bonding Interface of Hot-rolled Wear-resistant Steel BTW1/Q345R Cladding Plate

Wear-resistant cladding plates consisting of a substrate (Q345R) and a clad layer (BTW1) were bonded through hot rolling at the temperature of 1 200 °C and a rolling speed of 0.5 m/s. The microhardness of the cladding plate was also tested after being heat treated. The microstructure evolution on the interface of BTW1/Q345R sheets under various reduction rates was investigated with a scanning electron microscope (SEM) and EBSD. It is found that the micro-cracks and oxide films on the interface disappear when the reduction is 80%, whereas the maximum uniform diffusion distance reaches 10 μm. As a result, a wide range of metallurgical bonding layers forms, which indicates an improved combination between the BTW1 and the Q345R. Additionally, it is discovered that the unbroken oxide films on the interface are composed of Mn, Si or Cr at the reductions of 50% and 65%. The SEM fractography of tensile specimen demonstrates that the BTW1 has significant dimple characteristics and possesses lower-sized dimples with the increment in reduction, suggesting that the toughness and bonding strength of the cladding plates would be improved by the increase of reduction. The results reveal that a high rolling reduction causes the interfacial oxide film broken and further forms a higher-sized composite metallurgical bonding interface. The peak microhardness is achieved near the interface.

Electrical Characterization of Graphite/InP Schottky Diodes by I–V–T and C–V Methods

A rectifying junction was prepared by casting a drop of colloidal graphite on the surface of an InP substrate. The electrophysical properties of graphite/InP junctions were investigated in a wide temperature range. Temperature-dependent I–V characteristics of the graphite/InP junctions are explained by the thermionic emission mechanism. The Schottky barrier height (SBH) and the ideality factor were found to be 0.9 eV and 1.47, respectively. The large value of the SBH and its weak temperature dependence are explained by lateral homogeneity of the junction, which is related to the structure of the graphite layer. The moderate disagreement between the current–voltage and capacitance–voltage measurements is attributed to the formation of interfacial native oxide film on the InP surface.

Introducing pinhole magnification by selective etching: application to poly-Si on ultra-thin silicon oxide films

Carrier selective junctions formed by polycrystalline silicon (poly-Si) on ultra-thin silicon oxide films are currently in the spotlight of silicon photovoltaics. We develop a simple method using selective etching and conventional optical microscopy to determine the pinhole density in interfacial oxide films of poly-Si on oxide (POLO)-junctions with excellent electrical properties. We characterize the selective etching of poly-Si versus ultra-thin silicon oxide. We use test structures with deliberately patterned openings and 3 nm thin oxide films to check the feasibility of magnification by undercutting the interfacial oxide. With the successful proof of our concept we introduce a new method to access the density of nanometer-size pinholes in POLO-junctions with excellent passivation properties.

A first insight on the bio-functionalization mechanisms of TiO2 nanotubes with calcium, phosphorous and zinc by reverse polarization anodization

The decoration of titanium (Ti) implant surfaces with TiO2 nanotubes has emerged as a promising strategy to improve osseointegration and avoid infection. Nevertheless, it has been reported that nanotubular films are prone to peeling off from the Ti substrate due to the poor interfacial adhesion. The knowledge on the interfacial properties of such interface, although not well explored, is crucial for understanding the mechanisms behind the poor adhesion problem of these films and to further achieve an easy and effective solution to solve it.This paper is focused on the bio-functionalization of TiO2 nanotubular films with zinc (Zn) as an antimicrobial and bone healing agent, together with two major components of bone matrix, namely calcium (Ca) and phosphorous (P). The main aim is, for the first time, the thorough characterization of the interface between TiO2 nanotubes and the Ti substrate, along with the better understanding of the bio-functionalization mechanisms of TiO2 nanotubes and their influence on the interfacial features of the films.TiO2 nanotubes were successfully synthesized by two-step anodization and their bio-functionalization with Ca, P and Zn was achieved by reverse polarization anodization treatments. The in-depth characterization of the morphological and chemical features of TiO2 nanotubes was carried out along their length by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS), before and after bio-functionalization treatments. STEM images showed that the interface between conventional TiO2 nanotubes and Ti is non-continuous due to the existence of a hollow space. However, bio-functionalized TiO2 nanotubes evidenced an interface with different features, due to the formation of an interfacial oxide film as a consequence of anodization, with a thickness comprised between 230 and 250nm.The results presented in this work may inspire the emergence of novel surface treatment strategies seeking the long-term performance of metallic-modified osseointegrated implants.

A simple method for pinhole detection in carrier selective POLO-junctions for high efficiency silicon solar cells

Polycrystalline silicon (poly-Si) layers on thin silicon oxide films have received strong research interest as they form excellent carrier selective junctions on crystalline silicon substrates after appropriate thermal processing. Recently, we presented a new method to determine the pinhole density in interfacial oxide films of poly-Si on oxide (POLO)-junctions with excellent electrical properties. The concept of magnification of nanometer-size pinholes in the interfacial oxide by selective etching of the underlying crystalline silicon is used to investigate the influence of annealing temperature on pinhole densities. Eventually, the pinholes are detected by optical microscopy and scanning electron microscopy. We present results on the pinhole density in POLO-junctions with J0 values as low as 1.4 fA/cm2. The stability of this method is demonstrated by proving that no new holes are introduced to the oxide during the etching procedure for a wide range of etching times. Finally, we show the applicability to multiple oxide types and thickness values, differently doped poly-Si layers as well as several types of wafer surface morphologies. For wet chemically grown oxides, we verified the existence of pinholes with an areal density of 2×107cm−2 even already after annealing at a temperature of 750°C (lower than the optimum annealing temperature for these junctions).

Effect of A122Si/ZL102 bimetal interface fabricated by extrusion at near-eutectic temperature

The A122Si/ZL102 bimetal was designed and prepared by extrusion at near-eutectic temperature. The properties and fracture behaviors of different surface treatments between oxide film and zinc coating were compared between the A122Si and ZL102 bimetal. The average bonding strength of bimetal with intermittent oxide film interface was about 89. 3 MPa, which is higher than that of the bimetal fabricated by zinc coating method (about 76.3 MPa). During the process of extrusion, the oxidation film was extruded to crush and the metal was extruded through the micro-cracks of the oxidation film, then the two surfaces were joined together. Altogether, the results showed that extrusion at near-eutectic temperature is favorable for achieving a high-quality metallurgical bonded interface.

Characterization of interfacial reactions and oxide films on 316L stainless steel in various simulated PWR primary water environments

The effect of water chemistry on the electrochemical and oxidizing behaviors of 316L SS was investigated in hydrogenated, deaerated and oxygenated PWR primary water at 310 °C. Water chemistry significantly influenced the electrochemical impedance spectroscopy parameters. The highest charge-transfer resistance and oxide-film resistance occurred in oxygenated water. The highest electric double-layer capacitance and constant phase element of the oxide film were in hydrogenated water. The oxide films formed in deaerated and hydrogenated environments were similar in composition but different in morphology. An oxide film with spinel outer particles and a compact and Cr-rich inner layer was formed in both hydrogenated and deaerated water. Larger and more loosely distributed outer oxide particles were formed in deaerated water. In oxygenated water, an oxide film with hematite outer particles and a porous and Ni-rich inner layer was formed. The reaction kinetics parameters obtained by electrochemical impedance spectroscopy measurements and oxidation film properties relating to the steady or quasi-steady state conditions in the time-period of measurements could provide fundamental information for understanding stress corrosion cracking processes and controlling parameters.

Ultrasonic effect on etching tunnel morphology and distribution of aluminum foil

Etching aluminum foil was prepared by electrochemical DC etching under ultrasonic superimposition. Specifically, the relationship of electrochemical behavior, interface behavior and mass transfer enhancement from ultrasound was investigated intensively by chronopotentiometry, potentiodynamic polarization, cyclic voltammetry and electrochemical impedance spectra. Meanwhile, the etching specimens were characterized by SEM, N2 adsorption and XRD patterns measurements. The results showed that ultrasonic agitation inhibited the growth of aluminum oxide film and facilitated pit initiation effectively via increasing the Cl− adsorption on electrolyte/aluminum oxide film interface, and strengthened the inward/outward migration of Cl− and AlCl3 within tunnels by thinning the thickness of diffusion layer and decreasing the electrolyte resistance. Moreover, the double layer capacitance Cdl, pit density, average pit size/tunnel length and its homogeneity are all enhanced under ultrasonic superimposition.

Ageing mechanisms of polyurethane adhesive/steel interfaces

For the achievement of a complete understanding of the ageing and corrosion of adhesively bonded steel substrates, both the ageing of a polyurethane reference adhesive and the simultaneous corrosion of a mild steel substrate were studied by combined spectroscopic, electrochemical and microscopic techniques. A corundum-blasted mild steel was used as metal substrate in combination with a cross-linked reference polyurethane adhesive free from additional additives or fillers. The ageing was performed under full immersion in a chloride-free aerated electrolyte at elevated temperature. A strong influence of the water saturated polymer phase on the change in the interfacial oxide film structure of the metal substrate was observed. The thickening and hydroxylation of the metal oxide film during the corrosive delamination of the polyurethane film indicated that the de-bonding mechanism of the polyurethane adhesive is mainly based on interfacial water enrichment and a thereby induced growth of the interfacial oxide. The measured electrochemical properties of this oxide indicate that it is highly conductive and allows for oxygen reduction as the counter reaction to metal oxidation.

Spectroelectrochemical investigation of the anodic and cathodic behaviour of zinc in 5.3 M KOH

A corrosion and electrodeposition study of a Zn electrode in 5.3 M KOH, used as a model system for the anode of a secondary Zn–air battery, was performed by means of electrochemical and spectroelectrochemical techniques. The formation of a zinc oxide passive film and its cathodic removal were monitored by electroreflectance spectroscopy, spectroellipsometry and optical second harmonic generation. A dynamic optical model of the growth pathway, morphology and failure modes of the interfacial Zn oxide films was proposed to rationalise the electrokinetics of the secondary battery anode. Some experiments were performed in the absence and in the presence of polyethylene glycol. In situ spectroscopy confirmed and placed on a molecular basis the well-known formation of more compact and uniform electrodeposits crucial for secondary batteries.

Inhibition of corrosion-driven organic coating delamination from a zinc surface using polyaniline pigments

In-coating dispersions of the para-toluene sulfonic acid emeraldine salt (ES) of polyaniline (PAni-pTS) are shown to be highly effective inhibitors of corrosion-driven organic coating delamination on zinc surfaces. However, this inhibition is achieved without the characteristic ennoblement of substrate potentials typically observed for ferrous metals. At high humidity, galvanic interaction of oxidizing PAni-pTS pigments with the underlying zinc causes the growth of an interfacial oxide film. The period of oxide growth coincides with the duration over which the Zn surface is temporarily ennobled by the PAni-pTS. The results of experiments designed to elucidate the inhibition mechanism are presented, where substrate potentials, along with delamination and oxide growth kinetics are compared with equivalent in-coating compositions of ES and emeraldine base (EB). It is demonstrated that the basic nature of the interfacial oxide layer causes PAni-pTS re-oxidation to the non-conducting EB form, accounting for the loss of ennoblement. Efficient inhibition is achieved by a combination of a PAni-pTS-induced oxide layer and a zinc para-toluene sulfonate phase, acting as a reservoir of underfilm Zn2+ ions, producing a suppression of cathodic oxygen reduction. Availability of Zn2+ ions at the organic coating–metal interface both reinforces and prevents the dissolution of the amphoteric zinc oxide surface layer.

Electrochromic Properties of Sol-Gel Synthesized Macroporous Tungsten Oxide Films Doped with Gold Nanoparticles

Macroporous tungsten oxide thin films were prepared by using a polystyrene microsphere template and the peroxitungstic acid precursor. Gold nanoparticles were introduced into the tungsten oxide film by two different methods, either in the polystyrene template, through a self-assembly process, or they were cast on the surface of the annealed film. Raman imaging experiments showed an enhancement effect of the intensity of the major tungsten oxide bands by the gold nanoislands. The electrochromic properties of gold-doped macroporous tungsten oxide are described and compared to those of the standard porous sol-gel film. The films with gold nanoparticles on the surface of the tungsten oxide film were found to have better electrochromic properties than the standard sol-gel tungsten oxide films and the ones doped with gold in the polystyrene template. The diffusion coefficients of protons in the gold-doped tungsten oxide films were determined from both Cyclic Voltammetry (CV) and Electrical Impedance Spectroscopy (EIS) measurements. The results indicate a faster diffusion of the proton in the porous film, compared with a compact tungsten oxide film. The model that evolves from the EIS data, involves fast charge transfer rates at the electrolyte/tungsten oxide film interface. The results demonstrated an enhancement of the coloration efficiency of the macroporous tungsten oxide films with Au nanoparticles on the surface of the film. The possible involvement of plasmon resonance of Au nanoparticles in the electrochromic process is discussed.