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NANOSTRUCTURED SURFACES THAT CAN KILL BACTERIA Engineers at Purdue University, West Lafayette, Ind., have developed a laser treatment method with the potential to transform metal surfaces into bacteria killers. The [...]

Nonadiabatic coupling of nuclear motion to electronic excitations at metal surfaces is believed to influence a host of important chemical processes and has generated a great deal of experimental and theoretical interest. We applied a recently developed theoretical framework to examine the nature and importance of nonadiabatic behavior in a system that has been extensively studied experimentally: the scattering of vibrationally excited nitric oxide molecules from a Au(111) surface. We conclude that the nonadiabatic transition rate depends strongly on both the N-O internuclear separation and the molecular orientation and, furthermore, that molecule-surface forces can steer the molecule into strong-coupling configurations. This mechanism elucidates key features of the experiments and provides several testable predictions regarding the dependence of vibrational energy transfer on the initial vibrational energy, molecular orientation, and incident angle. 10.1126/science.1179240

A chemicurrent is a flux of fast (kinetic energy [??] 0.5-1.3 eV) metal electrons caused by moderately exothermic (1-3 eV) chemical reactions over high work function (4-6 eV) metal surfaces. In this report, the relation between chemicurrent and surface chemistry is elucidated with a combination of top-down phenomenology and bottom-up atomic-scale modeling. Examination of catalytic CO oxidation, an example which exhibits a chemicurrent, reveals 3 constituents of this relation: The localization of some conduction electrons to the surface via a reduction reaction, 0.5 [O.sub.2] + [delta][e.sup.-] [right arrow] [O.sup.[delta]-] (Red); the delocalization of some surface electrons into a conduction band in an oxidation reaction, [O.sup.[delta]-] + CO [right arrow] C[O.sup.[delta]-.sub.2] [right arrow] C[O.sub.2] + [[delta]e.sup.-] (Ox); and relaxation without charge transfer (Rel). Juxtaposition of Red, Ox, and Rel produces a daunting variety of metal electronic excitations, but only those that originate from C[O.sub.2] reactive desorption are long-range and fast enough to dominate the chemicurrent. The chemicurrent yield depends on the universality class of the desorption process and the distribution of the desorption thresholds. This analysis implies a power-law relation with exponent 2.66 between the chemicurrent and the heat of adsorption, which is consistent with experimental findings for a range of systems. This picture also applies to other oxidation-reduction reactions over high work function metal surfaces. heterogeneous catalysis | hot electrons | metal surfaces | surface science | transition metals

Hemming is a manufacturing process of folding a panel onto itself or another sheet. Quality of hemming is characterized by geometry and formability. This paper presents a response surface study of three-dimensional (3D) curved-surface-curved-edge hemming of an aluminum alloy, AA6111-T4, using finite-element ( FE) analysis. Solid elements and explicit FE solver are used for simulations of flanging, pre- and final hemming, and shell elements with implicit solver are deployed for springback prediction. A novel procedure called 'solid-to-shell mapping' is developed to bridge the solid elements with the shell elements. Verified to be accurate and efficient, the model is utilized in a central composite design to quantitatively explore the relationships between certain key process variables and the hem dimensional quality and formability. The most significant variables are identified as: (i) prehemming angle on roll-in/roll-out; (ii) nominal surface curvature on sheet springback; and (iii) initial sheet strain and flanging die radius on the maximum hemline surface strain of the produced hem. These results provide insights for process parameter selections in designing and optimizing 3D hems under material formability constraints. [DOI: 10.1115/1.2515430] Keywords: aluminum hemming, dimensional quality, formability, solid-to-shell mapping, central composite design

In the present paper the applicability of state and rate dependent friction laws in describing the phenomena of high speed slip at metal-on-metal interfaces is investigated. For the purpose of model validation, results of plate-impact pressure-shear friction experiments were conducted by Irfan in 1998 and Irfan and Prakash in 2000 using a Ti6AI4V and Carpenter Hampden tool-steel tribo pair are employed. In these experiments high normal pressures (1-3 GPa) and slip speeds of approximately 50 m/s were attained during the high-speed slip event. Moreover, these experiments were designed to investigate the evolution of friction stress in response to step changes in normal pressure and also in the applied shear stress during the high-speed slip event. A step drop in normal pressure is observed to result in an exponential decay of the friction stress to a new steady-state characteristic of the current normal pressure and the current slip velocity. A step drop in applied shear stress is observed to lead to an initial drop in friction stress, which later increases toward a new steady-state friction stress level. In response to the step drop in applied shear stress the slip velocity initially increases and then decreases to a new steady-state level consistent with the new friction stress level. A modified rate and state dependent friction model that employs both velocity and normal stress dependent state variables is used to simulate the experimental results. A good correlation is found between the experimental results and the predictions of the proposed state and rate dependent friction model. [DOI: 10.1115/1.2401217] Keywords: mathematical modeling, high speed frictional slip, plate impact friction experiments, state and rate dependent friction

Scuffing or scoring is an important form of damage leading to component replacements in lubricated mechanical systems such as power drives, gears, bearings, cams and followers, piston rings etc. Since scuffing necessarily involves localized welding of asperities, suitable surface modifications or coating can impart a good resistance to scuffing. A new class of low temperature salt bath nitriding process provides good resistance to scuffing. The current work is on evaluation of the scuffing resistance of AISI 4340 steels imparted with two such treatments. Experimental investigations were conducted on Sursulf and Arcor treated AISI 4340 steel specimen using a pin on ring test system under boundary lubrication condition. Continuous monitoring of friction records and (near surface) bulk temperature were done under step loading. Limiting loads and load velocity relations were evaluated and using the data generated and a thermo-mechanical wear model, performance is indexed. Off line studies on surface finish, hardness variations and surface transformations were also accomplished. Some salient aspects of the investigation and data generated are presented here. Onset of scuffing was observed to be far delayed with pin specimen imparted with these treatments compared to hardening. [DOI: 10.1115/1.1504089]

One of the failings of the pressure die casting process is that variable surface quality is a common result. This paper provides evidence that die vibrant can be used to polish castings as they form during solidification. Evidence is obtained from experiments performed on a purpose built rig that is designed to be representative of the pressure die casting process. The rig's design is such that part of the casting face can be subjected to tangential vibration at prespecified frequencies and amplitudes. The castings made are viewed under a microscope and micrographs produced. These combined with Talysurf readings show that die vibration can be used to generate casting surfaces with a quality significantly superior to that of the die. Supporting evidence is also provided by thermocouple readings and the solution of the inverse parabolic heat equation. The expected increase in the interfacial heat transfer coefficient is obtained with improved casting surface quality. Established in the paper is the effect of vibration frequency and amplitude on the Ra value of the casting surface. It is shown that threshold values for frequency and amplitude exist, below which surface improvement is diminished. [DOI: 10.115/1459085]

The dissociative chemisorption of hydrogen on metal surfaces is examined in the light of quantum mechanics using density functional theory for interpreting molecule-surface interaction and quantum dynamics for simulating hydrogen atom motion.

Spontaneous polymerization (S-Poly) is a novel process to form protective polymer coatings on aluminum and other metals. Here we discuss polymerization and coating properties when two fluorinated monomers are introduced into the S-Poly system. The resulting coatings were characterized using attenuated total reflectance fourier transform infrared (ATR-FTIR), differential scanning calorimetry (DSC), optical microscope and other measurements. Fluorinated coatings showed excellent dielectric properties and strong hydrophobicity. Their dry adhesion strength was a little lower than the coatings in previous systems, but changed very little after immersion in hot water for several days. Some of the fluorinated coatings exhibited significantly improved corrosion resistance in most corrosion tests., Zheng, Haipeng (*) Guy D. Davis (+) James P. Bell (*) INTRODUCTION Coatings on metal surfaces are widely used in applications ranging from macro equipment to microelectronics devices. Among many [...]

The Mode I stress intensity factor of a sector crack in a three-dimensional Voronoi polycrystal is computed by the body force technique. Microstructural stresses arising from the elastic anisotropy of grains (cubic and hexagonal) and the random grain orientations are estimated using the Eshelby procedure and incorporated in the stress intensity factor calculations. For metallic polycrystals, it is shown that the stress intensity factor depends significantly on the elastic anisotropy ratio, the grain orientations, the remote stress state, and the microstructural stresses. [S0021-8936(00)03401-2]

A theoretical model of the gap nucleation process during pure metal solidification on a deformable mold of finite thickness is presented. Both surfaces of the mold follow a sinusoidal lay for which the ratio of the amplitude to the wavelength, or aspect ratio, is much less than one. This makes the aspect ratio a convenient perturbation parameter for the thermal and mechanical problems since it is indicative of the spatial variation in the surfaces. The thermal and mechanical fields are coupled along the upper surface of the mold through a pressure-dependent thermal contact resistance. The main goal of the model is to develop a means for examining the contact pressure along the mold-shell interface and how variation of the mold surface wavelength affects the time and location of gap nucleation. Gaps, which result from irregular distortion of the shell due to the modest variation of the mold surface geometry, are assumed to nucleate when the contact pressure locally falls to zero. The model leads to two coupled differential equations for the shell thickness and contact pressure perturbations which are solved with a numerical scheme. Using a series solution methodology, it is shown that the contact pressure perturbation predicted by the present model reduces to that for a rigid, perfectly conducting mold (which was considered in another work) in the limit of zero mold thickness. In the companion paper, we specifically examine various combinations of pure materials acting either as the shell or the mold material. The concept of a critical wavelength, which separates those wavelengths that lead to gap nucleation at the crests, from those that lead to gap nucleation at the troughs, is introduced. The potential for development of design criteria for mold surface topographies using the present theoretical model as a limiting solution for finite element models of more complex casting processes is discussed. [S0021-8936(00)03201-3]

Flattening of random rough surfaces on a workpiece undergoing bulk deformation has been analyzed using a model of the surface consisting of just two wavelength components. Asperities are flattened at a rate which depends on the ratio of the initial r.m.s, amplitudes of the long and short wavelength components. The flattening behavior of the long wavelength asperities only becomes important when the amplitude of the long wavelength asperities is much greater than that of the shorter wavelength asperities. The surface modification was investigated experimentally by cold rolling of aluminum strips. The power spectral density of the roughness was used to extract appropriate amplitudes for the short and long wavelength components of roughness. The change in roughness amplitudes showed excellent agreement with theory.

The influence of harmonic normal loads on sliding friction is investigated through analysis of contacts consisting of conical and spherical sliders of hard materials on softer metal surfaces. Friction for such contacts is assumed to result from a plowing component and a shearing component. Calculations and experiments show that the coefficient of friction is essentially independent of normal load for contacts with conical sliders. However, for spherical sliders the relation between the coefficient of friction and normal load is highly nonlinear. In the presence of harmonic variations in normal load, this non-linearity causes a shift in the average coefficient of friction. For ideal lubricated contacts, the shearing component of friction is very small and for this case, it is shown that the maximum average reduction in the coefficient of friction is ten percent. When the shearing component is more significant, as with dry contacts, the shift is less. For example, when the shear strength is one-sixth the hardness of the softer material, the maximum average reduction in the coefficient of friction is five percent.

Research is conducted to examine the applicability of an existing model created for coated metallic surfaces for use over the range of parameters utilized in an engine block. Experimental data is compared to a thermal contact conductance model created for plastic asperity deformation of uncoated surfaces. Results indicate that material composition, anisotropic surface roughness and interface porosity will cause the measured thermal contact conductance values to deviate from the values predicted by the two models.

The velocity, normal pressure, and slider size dependence of the coefficient of dry friction of metals in the range of high sliding velocities (V [greater than or equal to] 1 m/s) is investigated theoretically. Failure of the adhesive junctions by adiabatic shear banding is considered as the underlying process. The concept of asperity shearing by the adiabatic shear banding mechanism represents a new approach to unlubricated high velocity friction. Analytical solutions of a coupled thermomechanical problem are given for two constitutive relations. Numerical solutions for steel-on-steel friction showing a decrease of the coefficient of friction with the sliding velocity for different normal pressures are presented. The model is considered to be adequate in the velocity range of 1-10 m/s where friction enhanced oxidation or surface melting are believed not to interfere with the asperity shearing process.

Fretting is the tribological phenomenon observed in nominally-clamped components which experience vibratory loads or oscillations. Associated with fretting contacts are regions of small-amplitude relative motion or microslip that occurs at the edges of contact. A newly-available infrared technology capable of resolving temperatures fields finely, both spatially and temporally, is used to characterize the near-surface conditions associated with fretting contact between an aluminum alloy cylinder and fiat. Both frictional heating due to interfacial slip and the coupled-thermoelastic effect arising from strains in the material induce these temperatures. The experimental results provide insight into not only the magnitude and distribution of near-surface temperatures, but also the nature of the contact stress field and the mechanics of partial slip fretting contacts. Comparisons of the measured temperature fields are made with those predicted by considering both conduction of the frictional heat flux and coupled-thermoelastic theory.

Micropatterning as a method of controlling surface properties has applications in many fields, including electronics and cell biology. A new and simple way to pattern microscopic surface structures uses self-assembled monolayers deposited on topographic islands on a metal surface. The study shows that disorder is greater at the step edges, creating fine-scale surface-energy inhomogenities that can be selectively etched.

A technique for measuring temperature close to the primary cutting edge in turning has been developed. The cutting temperatures of a 0.16 percent carbon bright drawn mild steel, have been measured for a range of cutting speeds and feedrates at a constant depth of cut. Tool nose radius was also varied. The correlations for the workpiece temperature of cutting speed and feedrate have been developed. The results show that the temperature correlates well with cutting speed and feedrate but the nose radius has little effect. Cutting forces were measured by a dynamometer and these were used to find the non zero forces at zero feedrate. These forces have been related to the deformation of the work material near the cutting edge of the tool and a method for calculating the cutting temperatures from these has been proposed.

In many instances machined parts experience sliding motion while in contact with a counter surface. The performance of the contacting parts depends on their friction and wear characteristics. In this work the effects of the machining condition on the tribological performance of steel, brass, and duralumin are investigated. For steel, it is shown that the friction coefficient value remains about the same whereas the rate of wear varies as the cutting condition is altered. The friction coefficients for brass and duralumin were lower than that of steel and their tribological properties seem to be less sensitive to the machining condition. In conclusion, it is emphasized that the machining condition directly influences the surface integrity which in turn affects the tribological performance of the machined parts, and therefore, the machining condition for parts that experience contact sliding motion should be optimized for best tribological performance in addition to dimensional accuracy and surface roughness.

ABSTRACT: This paper discusses the transmitting capacity of the so-called surface or metal contact joints by which the compressive stresses are transmitted through the directly contacting surface of steel plates. The imperfections of the contacted surface are modeled by the cylindrical and periodical sinusoidal form, and the stress distribution up to the yield state is simulated using the finite element method as a contact problem. The yielded portion of the plate spreads in the relatively small portion close to the surface, and the longitudinal displacements are not so large in this state. Therefore, the contact joint can transmit compressive loads of almost the full plastic capacity. The stress sharing with the splice plate is also evaluated by a simple spring model associated with the roughness of the contact joint, and is closely related to the bolt pitch. Therefore, the maximum tolerance for the gaps should be determined by the tolerable deformation of the contact joints and the requirements for the combined bolted joints as a total system.

Auger electron spectroscopic studies of the concentrations of sulfur in OFHC copper show that at concentrations around 10 a/o, the contact resistance values increase. Exposure of the sample to air leads to disappearance of sulfur and formation of an oxide film that enhances the contact resistance. Depending on the quantity of cold work done on the specimen, the thermal kinetics of sulfur segregation varies.

A finite element method simulates the flame-cutting process of two 50 millimeter thick steel plates to estimate the risk of crack occurrence in the plate due to flame-cutting under different conditions. The parameters considered in the simulation include temperature history effects. The samples are cut in air after preheating, which reduces residual stresses, and by inference, which reduces crack occurrence.

The formation of a surface metal-metal bond can produce large perturbations in the electronic, chemical, and catalytic properties of a metal. Recent studies indicate that charge transfer is an important component in surface metal-metal bonds that involve dissimilar elements. The larger the charge transfer, the stronger the cohesive energy of the bimetallic bond. On a surface, the formation of a heteronuclear metal-metal bond induces a flow of electron density toward the element with the larger fraction of empty states in its valence band. This behavior is completely contrary to that observed in bulk alloys, indicating that the nature of a heteronuclear metal-metal bond depends strongly on the structural geometry of the bimetallic system., In general, metals are elements characterized by the presence of a large number of valence orbitals that can donate or accept electrons. Isolated metal atoms are unstable, very reactive, and [...]

Author(s): Bogdan Diaconescu [1]; Karsten Pohl (corresponding author) [1]; Luca Vattuone [2]; Letizia Savio [2]; Philip Hofmann [3]; Vyacheslav M. Silkin [4]; Jose M. Pitarke [5]; Eugene V. Chulkov [4]; [...]

Interaction of high-power laser light with materials often causes irreversible damage of the near-surface region. It is shown that copper single-crystal surfaces can be patterned by laser light. Irradiation with green light produced adatoms and vacancies, which self-organized into nanoscale pyramids. This restructuring can be removed by annealing. In contrast to green light, infrared laser irradiation at equivalent absorbed energy density did not produce any structural change. This, for metallic systems, unforeseen spectral difference in laser light action points to a concerted process as the source for structural modification, which involves long-lived primary excitation of localized d-electrons through interband transition together with phonon excitation., The creation and dissipation of electronic excitation are fundamental for a variety of physical and chemical processes. Lasers, as a source of primary excitation, play an expanding role in material [...]

Time-resolved two-photon photoemission in combination with the coherent excitation of several quantum states was used to study the ultrafast electron dynamics of image-potential states on metal surfaces. For a (100) surface of copper, the spectroscopy of quantum beats made previously unresolved high-order states (quantum number n [is greater than or equal to] 4) experimentally accessible. By exciting electrons close to the vacuum level, electron wave packets could be created and detected that described the quasi-classical periodic motion of weakly bound electrons. They traveled more than 200 A away from the surface and oscillated back and forth with a period of 800 femtoseconds., Photoelectron spectroscopy has developed into one of the most versatile and successful tools for surface studies. Particularly attractive features of this technique are the high surface sensitivity associated with the [...]

Scanning tunneling microscopy is used to investigate adsorbate interactions and reactions on metal surfaces. The manners in which adsorbates perturb the surface electronic structure in their vicinity are discussed.

Research describes controlled synthesis of carbon nanotubes by chemical vapor deposition over catalyst arrays to achieve surface specificity in nanotubes, such as large wafers. The process is controllable in terms of van der Waals self-assembly forces and electric fields. The use of this controlled technology to materials deployed in diverse fields of application is discussed.

Many processes at surfaces, from chemical reactions to electron transfer, involve the creation and decay of electronically excited states. At metal surfaces, the density of electrons is high, leading to [...]

A flick of a switch lights up the room, powers up the television, and turns on a host of other gadgets large and small. Now a group of chemists and [...]

Queen's University (Ontario, Canada) researchers developed a new process that allows organic compounds to bind to metal surfaces. 'Imagine pouring vegetable oil onto a metal surface and expecting it to [...]

The interaction of water with solid surfaces plays a crucial role in catalysis and weathering. The structure and properties of the first water layers on well-defined metal surfaces have been [...]

Atomic Arrangements at Metal Surfaces THE TERMINATION OF A SOLID CREATES A SURFACE. THIS statement is so obvious that many of its consequences are often overlooked. For example, the corrosion [...]

NMR Studies of Simple Molecules on Metal Surfaces ONE OF THE MOST ACTIVE AREAS OF RESEARCH TODAY IS the study of metal surfaces. While a broad range of scientific questions [...]

Until only a few years ago, the idea that a traditional organic or metal-organic substance could exhibit the electrical, optical, and magnetic properties of a metal seemed a complete contradiction [...]

Small metal clusters and related molecules have been the focus of increasing attention in recent years. This interest stems, in part, from the broader interest in catalysis and from the [...]

Researchers at Universitat Jaume I de Castellon, Spain, are working on a new type of coating that provides higher corrosion protection for metals due to improved corrosion resistance and adherence [...]

Electrons are normally not supposed to wander very far from metal surfaces. Yet on page 1480 of this issue, Hofer et al.[1] report an interdisciplinary investigation that catches surface electrons [...]

A recently designed single-crystal surface calorimeter has been deployed to measure the energy difference between two solid surface structures. The clean Pt{100) surface is reconstructed to a stable phase in which the surface layer of platinum atoms has a quasi-hexagonal structure. By comparison of the heats of adsorption of CO and of [C.sub.2][H.sub.4] on this stable Pt{100}-hex phase with those on a metastable Pt{100}-(1x1) surface, the energy difference between the two clean phases was measured as 20 ± 3 and 25 ± 3 kilojoules per mole of surface platinum atoms., Phase transitions in bulk systems can be readily characterized by their enthalpy change by using calorimetric measurements. Although surface phase transitions are well known, their energetics have been inferred indirectly, [...]

Harvard University (Cambridge, Mass.) physicists demonstrated a new way to customize the color of metal surfaces by exploiting an optical phenomenon. They discovered that even very 'lossy' thin films, if [...]

Aluminum exposed to air possess a natural protective oxide coating, and if further protection is required, several methods can be used. To withstand harsh conditions, the durability of aluminum is increased by generating a thicker oxide film by anodizing. For a thicker film coating, hard anodizing at low temperature is necessary. The aluminum surface can protected by painting the surface and taking the proper pretreatments. An even coating is possible by electroless nickel plating of aluminium. A high-strength aluminum alloy sheet having a zinc-phosphate coating can be used for cars.