Surface Analytical Investigations Research Articles

Improved Tribological Performance of a Polybutylene Terephthalate Hybrid Composite by Adding a Siloxane-Based Internal Lubricant

To mitigate the environmental hazards aroused by fossil-based lubricants, the development of eco-friendly internal lubricants is imperative. Siloxane-based internal lubricants, widely applied as plasticizers in polymeric compounds, are a promising option. However, their impacts on the tribological properties of polymeric tribocomponents are still unclarified. Therefore, in the current study, a siloxane-based internal lubricant with the product name ‘EverGlide MB 1550 (EG)’ was dispersed into a polybutylene terephthalate (PBT)-based tribological composite to investigate whether the tribological properties of the composite can be optimized. A block-on-ring (BOR) test configuration was used for this purpose. It was found that the addition of EG to the composite significantly improved the tribological behavior; the improvement was particularly significant under lower load conditions (pv-product ≤ 2 MPa∙m/s). Compared to the reference PBT composite, the addition of EG reduced the friction coefficient (COF) by about 30% and the specific wear rate by about 14%. An accompanying surface analytical investigation using photoelectron spectroscopy to elucidate the effective mechanisms at the molecular level showed the availability of tribologically effective and free EG after its addition to the composite in the relevant tribocontact.

Influence of PTFE-Based Dry Lubricants on Friction and Wear Behavior in Dry Lubricated Steel–Bronze Contact

Abstract In this article, solid lubricants are investigated to examine their tribological performance in a dry lubricated steel–bronze contact. The examined solid lubricants are made of polyamide (PA) and irradiated polytetrafluoroethylene (PTFE), which are chemically bonded by reactive melt extrusion. For the tribological investigations, a block-twin-disc test rig, on the one hand, and a three-disc test rig, on the other hand, were used under ambient conditions, where the solid lubricant for lubricating the steel–bronze contact was released from a block or a disc. Results from the tribological investigations are presented here, showing the friction and wear behavior in a steel–bronze contact depending on the slide-to-roll ratio in the contact between the steel disc and the compound body. Furthermore, surface analytical investigations on the steel and bronze discs were carried out. These studies showed that the chemical bonding of 20 wt% of irradiated PTFE in PA12 improves the wear and friction behavior in steel–bronze contact significantly, due to the buildup of a transfer film of PTFE on the steel surface.

Natural based products for cleaning copper and copper alloys artefacts

Copper alloys objects can deteriorate their conservation state through irreversible corrosion. Since in the cultural heritage field every artefact is unique and any loss irreplaceable, solutions for conservation are needed. Hence, there is the necessity to stop the corrosion process with a suitable cleaning and conservation process to avoid further degradation processes without changing its morphological aspect. Chelating solutions are commonly used in chemical cleaning, mainly sodium salts of ethylenediaminetetraacetic acid (EDTA). However, it is resistant to water purification procedures and is not biodegradable. The goal of this study was to see if applying an ecologically friendly chelating agent as an alternative to EDTA cleaning procedures for cultural heritage was suitable. In this study were chosen six natural-based chelators that could be a new green non-toxic alternative to EDTA in corrosion-inhibiting properties. They were tested for cleaning copper artefacts exposed to atmospheric environment in polluted areas. The study considered four amino acids, a glucoheptonate (CSA) and an industrial green chelator (GLDA). The effectiveness was tested on corrosion copper compounds and on laboratory corroded copper sheets. Finally, the cleaning efficacy was tested on four Roman coins and a modern copper painting. To define the cleaning efficacy, surface analytical investigations have been carried out by means ICP-OES, UV-VIS, µ-Raman, spectro-colorimetry, XRD and FTIR. Among the amino acids, alanine was the most effective, showing an unaltered noble patina and a good effective copper recovery from corrosion patinas.

Natural passivation behavior and its influence on chloride-induced corrosion resistance of stainless steel in simulated concrete pore solution

This study presents a combined electrochemical and X-ray photoelectron spectroscopy (XPS) surface analytical investigation on the natural passivation process and its effect on Cl− induced corrosion resistance of traditional carbon steel, ferritic and austenitic stainless steels (SSs) (i.e. HRB400 steel, 430 and 304 SSs) in simulated concrete pore solution (SPS). It was found that a complete and more compact passive film with a stratified structure could be naturally formed on the tested steel surfaces after different immersion time. The chemical composition of the passive film and its corrosion resistance strongly depended on the alloying element and natural passivation time. For HRB400 steel, the outer layer of natural passive film was mainly composed of Fe3+ species and the inner layer was predominated by Fe2+ ones, while the outer layer of natural passive film formed on SSs surface consisted of Fe and Cr oxides and hydroxides, enriched in Fe3+ species, and an inner layer was made of mixed Fe2+ and Cr3+ oxides (enriched in Fe2+ species). The natural pre-passivation could effectively improve the Cl− induced corrosion resistance of three steels. Compared with HRB400 steel, both austenitic and ferritic SSs exhibited the better corrosion resistance due to the formation of Cr-enriched passive film, of which, 304 SS had the highest pitting corrosion resistance.

(Invited) Concepts of the Solid-Electrolyte Phase Boundary: Experimental Evidence and Resulting Devices

The original fundamental concepts at solid-electrolyte interfaces are reviewed including the traditional microcapacitor model of the Helmholtz layer1. Theoretical approaches that expand beyond the classical dielectric model for water and solvation shells are accounted for and recent advanced experiments will be presented that enable a direct profiling of the energetics at this junction by tender x-ray synchrotron radiation photoelectron spectroscopy (TSRPES) 2,3. Combined (photo)electrochemical and surface analytical investigations are presented. For the rectifying semiconductor - solution junction, the space charge behavior of semiconductors in concentrated electrolytes is examined and in dilute solutions, the Galvani potential profiles at metal - electrolyte junctions are followed by TSRPES. The role of truncated profiles for thinner water layers and dilute electrolytes on charge equilibration and build-up of contact potential differences is analyzed. Fig.1 shows an example of the resulting photoelectron spectroscopy tomography. Fig.1: (left): scheme for tomographic peak sampling; (right): influence of electrostaic potential on peak width Implications regarding charge transfer and the development of efficient devices for light - induced fuel generation are outlined and optimized structures which show unassisted water splitting are presented, obtained upon modifying the surfaces of III-V tandem absorbers. Application of (photo)electrochemically induced surface transformations 4,5 and of re-directed (photo)corrosion reactions allows passivation of surface and interface states and increased robustness. Details of these procedures will be given. The achieved efficiencies are compared to theoretically achievable limits of solar hydrogen evolution for the respective material combinations6. The influence of the optical properties of the combined catalyst - electrode - solution composite structures is described and routes to more advanced multi-electron and proton transfer reactions are outlined.

Surface Analytical Investigation of Inner and Outer Surfaces of Porous Components for Polymer Electrolyte Membrane Fuel Cells

Fuel cells allow the direct conversion of chemical energy (typically hydrogen and oxygen or air) to electrical energy and therefore are an important high efficiency option for future energy systems. The surface composition is related to the surface properties, which influence the catalytic and hydrophobic behavior. In polymer electrolyte membrane fuel cells, several components are porous, namely the gas diffusion layer consisting of carbon structure as support and a micro porous layer (MPL) and the catalyst layer (CL). The porous structure is important for the transport of the reactants (hydrogen and air) and products (water) as well as for the electrochemical reactivity at the interfaces. Caused by the porous structure not only the properties of the outer surfaces but also the properties of the inner surfaces are important. The combination of two different depth profiling techniques, ion etching and peel-off by an adhesive tape, and a surface sensitive analytic method, here X-ray photoelectron spectroscopy (XPS), allows to investigate the inner and outer surfaces. XPS allows to determine the chemical composition of the surfaces, which allows to derive information about the hydrophobic/hydrophilic properties. With the ion etching the structure of the surface layer will be investigated with a depth resolution in the range of nanometers in order to determine gradients in the surface compositions, e.g. thin films covering surface of the catalyst particles. The tape peel-off preparation allows to remove a film from the surface and gives the access to the inner surfaces. The surface compositions of the different surfaces prepared with and without tape peel-off of various porous fuel cell components were determined by XPS depth profiles with ion etching. The MPL and the CL consist of a fluorine rich polymer (PTFE or Nafion with a PTFE backbone) and carbon black. In the case of the catalyst layer the carbon black is used as support for the platinum catalyst and in case of MPL as porous conductive material. The fluorine rich compound is used to tailor the hydrophobicity of the components. XPS profiles measured by ion etching of the outer surface and an inner surface of a catalyst layer prepared by an inkjet printing process [1] will be presented. In the C 1s spectra two different carbon species can be distinguished, the signal from carbon black and the carbon signal from the carbon in a fluorine rich compound. The outer and inner surface show a very similar composition and depth profile. The concentrations of the fluorine rich carbon compound and of fluorine itself strongly decrease during the ion etching while the concentrations of the carbon black as well as of the platinum increase. This means that the outer surface as well as the inner surface have a higher concentration of polymer on the surfaces than in the bulk material, whereas the thin polymer film partially covers the Pt catalyst. This behavior is frequently observed for catalyst layers prepared from an ink mixed of the polymer, carbon supported catalyst and solvents [2]. Acknowledgement The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) for Fuel Cell and Hydrogen Joint Technology Initiative under Grant No. 303452 (IMPACT).

Interfacial Oxide Formation during Anodization of Hafnium/Aluminium Superimposed Layers

The anodization behaviour of Hf/Al stacked thin films, sequentially deposited from vapour phase without breaking vacuum, was investigated using scanning droplet cell microscopy. Surface microstructure and crystallographic analysis revealed the formation of highly textured and compact hexagonal Hf films for thicknesses above 7.5nm. Cyclic voltammetry suggested the formation of discrete anodic layers starting with pure HfO2 on the surface, a mixture of HfO2 and Al2O3 as an intermediate layer and pure Al2O3 in depth of the films. Coulometric investigations revealed oxide formation factors of 2.2nmV−1 for HfO2 while for pure Al2O3 a maximum of 1.4nmV−1 was measured. Electrochemical impedance spectroscopy performed for various Hf thicknesses and different final anodization potentials revealed almost ideal dielectrics formed from mixed HfO2/Al2O3, permittivities up to 26 being measured for the anodized Hf/Al films. Surface analytical investigations and depth profiling as well as high resolution transmission electron microscopy confirmed the discrete anodic layer growth and the mixing of both anodic oxides. The significant thickness values found for the mixed oxide layer must redefine the general expectations for the use of anodized superimposed layers in various applications.

Role of molybdenum oxide for organic electronics: Surface analytical studies

Extensive studies have been conducted on molybdenum oxide since it has outstanding properties as an insertion layer for efficient charge injection and extraction in organic semiconductor devices. Efficient charge transfer at semiconductor and electrode interface is one of the most crucial issues for the performance of organic electronic device. A lot of efforts have been spent to address this issue, but there are still many unclarified issues to understand the physical mechanisms. In this review, the authors summarize surface analytical investigations on the mechanisms that govern the effectiveness of the insertion layer. Measurement results on the electronic structure, composition, and morphology are presented. It is found that the high work function of MoOx is the dominant factor for the device performance improvement. Compromising environmental effects and methods to recover or prevent such effects are described. Finally, the criteria for MoOx insertion layer to be effective are provided by comparing the work function of MoOx and the ionization potential of organic semiconductors.

Surface Analytical Studies of Maxillofacial Implants

In the present work, surface analytical investigation of unimplanted as well as retrieved pyrolytic carbon-covered carbon/carbon composite implants and Ti osteosynthesis plates is reported. The Ti plates were covered by a 200-nm-thick, anodically and thermally formed TiO2 layer. Our results suggest that although the oxide layer on the Ti miniplates remained stable during the time spent in the human body, there is still material transport between the implant and the human body. In case of the carbon/carbon composite implants, damage of the carbon fibers constituting the material was found on one side of the sterile implant and attributed to the manufacturing process. The NaCl crystals originally present on the surface of the sterile material disappeared during the time spent in the human body. As a result of the interaction with the human body, a new surface layer (mainly constituted of carbon) appeared on the implant. The results indicate that both the time spent in the human organism and the preparation of the implants before operation can have detectable effects on the investigated surface properties. Surface analytical investigations could therefore provide information not only about the biocompatibility of these materials but also about the effect of their treatment before operation.

A surface analytical investigation of cerium‐based conversion coatings deposited onto an AA2024‐T3 aluminium alloy cladding layer

Cerium‐based conversion coatings are being investigated as alternatives to chromating treatments for the corrosion protection of aluminium and its alloys because of the environmentally unfriendly nature of the chromating process. This study investigates the surface film composition, structure and corrosion performance following a two‐step surface treatment for an AA2024‐T3 clad aluminium alloy. The two‐step treatment comprised of an initial cerium conversion process involving immersion in an aqueous solution containing Ce3+ ions at 75 °C followed by immersion in a propylene glycol solution at 75 °C. The coating surface morphology, composition and structure have been studied using SEM, XPS, Auger spectroscopy and Fourier transform infrared spectroscopy, while corrosion resistance was evaluated using electrochemical impedance spectroscopy. The coating formed by the two‐step treatment is an interconnecting fibrous (pseudo) boehmite layer with the incorporation of Ce3+ in the film. This two step treatment coating exhibits high impedance compared with the coatings formed through exposure to just stage 1 or stage 2 of the two‐step treatment and shows good potential for improved corrosion protection. Copyright © 2014 John Wiley & Sons, Ltd.

Electrochemical and surface analytical investigation of the effects of Zn concentrations on characteristics of oxide films on 304 stainless steel in borated and lithiated high temperature water

The characteristics of oxide films formed on 304 stainless steel (SS) in borated and lithiated high temperature water with Zn injection of 0ppb to100ppb were investigated using in-situ potentiodynamic polarization curves, electrochemical impedance spectra at 573.15K and ex-situ X-ray photoelectron spectroscopy (XPS). There was a high inhibition effect of Zn injection on the growth of oxide films in the testing solution. The lowest growth rate was corresponding to the highest Zn-injected level. The ≤50ppb Zn injection based on plant experience could significantly affect the formation of Zn-bearing oxides on the surfaces, while >50ppb Zn injection showed no obvious influence on the oxide films. A modified point defect model was proposed to discuss the effects of injected Zn concentrations on the oxide films on 304 SS in high temperature water.

Optical design and active optics methods in astronomy

Optical designs for astronomy involve implementation of active optics and adaptive optics from X-ray to the infrared. Developments and results of active optics methods for telescopes, spectrographs and coronagraph planet finders are presented. The high accuracy and remarkable smoothness of surfaces generated by active optics methods also allow elaborating new optical design types with high aspheric and/or non-axisymmetric surfaces. Depending on the goal and performance requested for a deformable optical surface analytical investigations are carried out with one of the various facets of elasticity theory: small deformation thin plate theory, large deformation thin plate theory, shallow spherical shell theory, weakly conical shell theory. The resulting thickness distribution and associated bending force boundaries can be refined further with finite element analysis. Keywords: active optics, optical design, elasticity theory, astronomical optics, diffractive optics, X-ray optics

Nickel-free manganese bearing stainless steel in alkaline media—Electrochemistry and surface chemistry

The use of austenitic nickel-containing stainless steels as concrete reinforcement offers excellent corrosion protection for concrete structures in harsh chloride bearing environments but is often limited due to the very high costs of these materials. Manganese bearing nickel-free stainless steels can be a cost-effective alternative for corrosion resistant reinforcements. Little, however, is known about the electrochemistry and even less on surface chemistry of these materials in alkaline media simulating concrete pore solutions. In this work a combined electrochemical ( ocp = open circuit potential) and XPS (X-ray photoelectron spectroscopy) surface analytical investigation on the austenitic manganese bearing DIN 1.4456 (X8CrMnMoN18-18-2) stainless steel immersed into 0.1 M NaOH and more complex alkaline concrete pore solutions was performed. The results show that the passive film composition changes with immersion time, being progressively enriched in chromium oxy-hydroxide becoming similar to the conventional nickel-containing stainless steels. The composition of the metal interface beneath the passive film is strongly depleted in manganese and enriched in iron; chromium has nearly the nominal composition. The results are discussed regarding the film growth mechanism (ageing) of the new nickel-free stainless steel in alkaline solutions compared to traditional austenitic steels. Combining the results from pitting potential measurements with the composition of the passive film and the underlying metal interface, it can be concluded that the resistance against localized corrosion of the new nickel-free stainless steel relies on the strong chromium(III) and molybdenum (VI) oxy-hydroxide enrichment in the passive film.

Influence of Structuring by Abrasive Machining on the Tribological Properties of Workpiece Surfaces

Surface structuring of workpieces has a significant influence on the tribological properties of contact partners and can therefore lead to a significant increase of the life time of components [1]. To identify an appropriate surface structure, analytical and experimental investigations were done at the Institute for Machine Tools and Factory Management (IWF) in order to improve the tribological properties in the contact area appreciably. To validate the theoretical analyses, technological investigations were carried out. 3D-surface structures were created on workpiece surfaces by varying process variables for a high speed 5-axes machine system to generate knowledge for developing defined surface structures by using kinematically modified grinding processes. Initial results show a positive influence of defined surface structures on the performance of contact partners.

Surface analysis of cylinder liners from tribological model experiments and internal combustion engines

AbstractHypereutectic aluminium‐silicon (AlSi) alloys have been increasingly used in the fabrication of internal combustion engines in recent years. They combine low weight with good mechanical properties and high wear resistance at the cylinder surface. Owing to the inhomogeneous structure of these alloys, the tribological processes taking place at the surface are highly complex. Therefore, interest in the tribological properties of the surfaces of these alloys has increased. Several tribological experiments with AlSi alloys have been reported in literature, as well as some surface analytical investigations. This work investigates the surfaces of AlSi alloy cylinder liners from an internal combustion engine and from a tribometer experiment. Surface hardness and elastic modulus were investigated with nanoindentation measurements, and the chemical composition of the surface was analyzed with XPS. The most striking difference between the samples from engine and tribometer was a layer of soot and residual lubricant on the Al matrix of the engine cylinder liner, whereas no such layer was found on the tribometer samples. This shows that the combustion process and its resulting products have a significant influence on the formation of tribolayers on the cylinder liner surface. Copyright © 2010 John Wiley & Sons, Ltd.

Electrochemical and surface analytical study of the corrosion behavior of mild steel with cathodically produced zinc phosphate coating

AbstractCorrosion protection of mild steel by zinc phosphate coatings produced by electrolytic phosphating under cathodic polarization was investigated by electrochemical and surface analytical techniques. The cathodically produced coating mainly consists of zinc phosphate as hopeite, with incorporation of a small amount of metallic zinc. Surface characterization by X‐ray diffraction, SEM and energy dispersive X‐ray as a function of exposure time in neutral and alkaline solutions gives information on solution‐induced changes of the protective layers. In neutral NaCl solution, the electrolytic zinc phosphate coating shows a very good corrosion protection, which even increases as a function of exposure time. Surface analytical investigations reveal that this is due to a formation of an underlayer of Zn(OH)2 at the metal/coating interface. This corrosion product layer is an additional barrier blocking the bare metal surface in the pores of the zinc phosphate coating. In alkaline solution, phosphate is preferentially dissolved from the coating, changing the chemical composition of the coating to ZnO. Copyright © 2009 John Wiley & Sons, Ltd.

Complex surface analytical investigations on hydrogen absorption and desorption processes of a TiMn2-based alloy

Metal hydrides are one of the most promising technologies in the field of hydrogen storage due to their high volumetric storage density. Important reaction steps take place at the very surface of the solid during hydrogen absorption. Since these reaction steps are drastically influenced by the properties and potential contamination of the solid, it is very important to understand the characteristics of the surface, and a variety of analytical methods are required to achieve this. In this work, a TiMn(2)-type metal hydride alloy is investigated by means of high-pressure activation measurements, X-ray photoelectron spectroscopy (XPS), secondary neutral mass spectrometry (SNMS) and thermal desorption mass spectrometry (TDMS). In particular, TDMS is an analytical tool that, in contrast to SIMS or SNMS, allows the hydrogen content in a metal to be quantified. Furthermore, it allows the activation energy for desorption to be determined from TDMS profiles; the method used to achieve this is presented here in detail. In the results section, it is shown that the oxide layer formed during manufacture and long-term storage prevents any hydrogen from being absorbed, and so an activation process is required. XPS measurements show the oxide states of the main alloy elements, and a layer 18 nm thick is determined via SNMS. Furthermore, defined oxide layers are produced and characterized in UHV using XPS. The influence of these thin oxide layers on the hydrogen sorption process is examined using TDMS. Finally, the activation energy of desorption is determined for the investigated alloy using the method presented here, and values of 46 kJ/mol for hydrogen sorbed in UHV and 103 kJ/mol for hydrogen originating from the manufacturing process are obtained.

Electrochemistry and surface chemistry of stainless steels in alkaline media simulating concrete pore solutions

Despite the increased use of stainless steel for concrete reinforcement in harsh chloride environments, comparatively little is known about the surface chemistry of these materials in alkaline media simulating concrete pore solutions. This work is concerned with a combined electrochemical and XPS surface analytical investigation on austenitic, ferritic and duplex stainless steels in simple NaOH and more complex alkaline concrete pore solutions. The results show that the passive films on these materials change with immersion time, the ferritic and duplex stainless steels becoming enriched in chromium oxy-hydroxide, the austenitic steel strongly enriched in nickel hydroxide. The composition of the metal layer beneath the surface film is strongly enriched in nickel and depleted in chromium and iron. The results are discussed with respect to the relation between the Fe(II) content in the films and the open circuit potential (OCP) during exposure, the film growth mechanism and localized corrosion resistance.

Surface analytical investigation of nearly-frictionless carbon films after tests in dry and humid nitrogen

In this paper, we attempt to elucidate the near-surface chemistry of the sliding contact interfaces of nearly-frictionless carbon (NFC) films subjected to sliding in dry and humid nitrogen environments. Using time-of-flight secondary ion mass spectrometry (ToF-SIMS), along with X-ray photoelectron spectroscopy (XPS), we determined the chemical state of the rubbing surfaces of NFC films and correlated these findings with the friction and wear test results. Tribological tests were performed in a ball-on-disk machine under a 5 N load and at sliding velocities of 0.2 to 0.3 m/s. An air-tight flexi-glass enclosure was used to create and maintain the dry and wet nitrogen environments. When tested in dry nitrogen, the NFC film provided a friction coefficient of ≈ 0.006 at steady state. However, when tests were performed in humid nitrogen, the friction coefficients increased by factors of 5 to 10 (depending on the humidity level). In both cases, the amount of wear was very low and hard to quantify, especially on the disk side. Imaging 3-D ToF-SIMS of the sliding contact surfaces of the NFC films revealed mostly C 2H and C 2H 2 within the wear tracks after testing in dry nitrogen. XPS confirmed the presence of mostly C–C and C–H bonded species within the same wear tracks. However, the surfaces that were rubbed in humid nitrogen contained large amounts of C–O and C O bonded species, in addition to C–C and C–H. We concluded that super-low friction of NFC in dry nitrogen was mainly associated with the hydrogen termination of its surface carbon atoms, while the much higher friction of NFC in humid nitrogen was perhaps due to oxidation and/or hydroxyl ion termination of its surface carbon atoms.

Observation of metastable self-organised structure during porous silicon formation

The initial processes leading from an atomically flat Si surface to porous silicon formation are not well known and detailed surface analytical investigations on the dissolution steps are still lacking. By performing electrochemical experiments in dilute ammonium fluoride solutions at moderate pH values, we are able to investigate the process of dissolution from first pit formation to the formation of transitory mesa-type terraces on atomically flat n -Si(111) by atomic force microscopy. Mesa-type structures with flat extended terraces surrounded by strongly corrugated areas are observed after different conditioning procedures. The mesas exhibit terrace edges aligned with respect to preferred surface orientations, which results in pentagonal, hexagonal or higher composed structures. The findings can be interpreted in terms of the formation of sidewalls exhibiting only a limited number of crystal orientations: {111}, {110} and {113}. Because these faces can be electrochemically H terminated in a 1×1 manner, we suggest that their stability accounts for the variety of observed structures.