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This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.

Snakes can climb a range of surfaces, from tree trunks to brick walls, using a hitherto poorly understood mechanism. The bellies of snakes are covered in a series of flexible scales that can be activated by the snake to prevent sliding. It is previously shown that conscious snakes can use this ability to double their friction coefficient relative to unconscious snakes. In this combined experimental and theoretical study, we give further evidence that snakes actuate their belly scales. We perform experiments where we slide snakes backwards atop an array of pillars. Our theoretical model suggests that snakes that do not apply an opening moment to their scales should have quite short contact with these pillars. In our experiments, snakes slide their ventral scales down the pillars, prolonging contact. Our modeling suggests that this phenomenon can only occur if snakes apply a moment at the scale base. We hope this result encourages further research in actively-generated friction.

In this work, a three-dimensional model for liquid-mediated adhesion between two rough surfaces is presented. The approach is based on a spectral (multi-scale) representation of compressive rough surface deformation along with the capillary equations governing the tensile deformation. An iterative numerical algorithm is designed to solve the equations of elasticity and capillarity simultaneously. It is shown that, under certain conditions, a contact instability occurs leading to unbounded rates of change of tensile force, average gap and wetted radius. The effects of liquid volume, liquid surface tension, surface topography, nominal contact area, and external load on the stability of contact interface are studied. Key dimensionless ratios are identified that govern the equilibrium state and onset of instability.

The frictional interactions of two bodies sliding pass each other is modeled using a modification of the Olami, Feder, Christensen (OFC) spring-block model. The major modification is based on relaxing the assumption of global driving, whereby one can simulate cases of partial contact. The effect of real contact area on several statistical measures of the sliding process is investigated. Scale-independent behavior is observed in the slip sequence size distribution, similar to OFC results, but with a different functional form. Additionally, it is found that the mean and standard deviation of the friction force may be used to determine the number of contact regions as well as a local (asperity-level) friction coefficient.

A liquid film can flow between two solid surfaces in close proximity due to capillary effects. Such flow occurs in natural processes such as the wetting of soils, drainage through rocks, water rise in plants and trees, as well as in engineering applications such as liquid flow in nanofluidic systems and the development of liquid bridges within small-scale devices. In this work, a numerical model is formulated to describe the radial capillary-driven flow between two contacting, elastic, annular rough surfaces. A mixed lubrication equation with capillary-pressure boundary conditions is solved for the pressure within the liquid film and both macro- and micro-contact models are employed to account for solid–solid contact pressures and interfacial deformation. Measurements of interfacial spreading rate are performed for liquids of varying viscosity flowing between an optical flat and a metallic counter surface. Good agreement is found between modeling and experiment. A semi-analytical relation is developed for the capillary flow between the two contacting surfaces.

This study considers the approaches of Blok and Jaeger for the estimation of maximum and average temperatures in sliding elliptical contacts with both uniform and semi-ellipsoidal (Hertzian) heat distributions. The accuracy of each of these methods, which are based on single-point temperature matching between contacting bodies, is assessed relative to a numerical solution of the heat partition problem developed in a previous work, which imposed temperature matching at all nodal points. Comparisons are made for a wide range of Peclet numbers, as well as for moderate ranges of thermal conductivity ratio and elliptical ratio. It is found that the application of Blok's hypothesis yields remarkably accurate predictions of the maximum interfacial temperature, with typical errors less than 3%, whereas the hypothesis of Jaeger leads to predictions of the average interfacial temperature that have typical errors of around 6%. The authors also assess the accuracy of approximate formula developed by Tian and Kennedy to predict the maximum temperature at the interface for the case of sliding circular contacts and find the error to be no more than 2.6% for the full range of Peclet number. Further, the authors of the current study use fundamental heat source solutions developed by Tian and Kennedy to arrive at formulae for average temperature rise for circular contacts that are analogous to the Tian and Kennedy maximum temperature rise formulae. It is found that the formulae for computing the average interfacial temperature rise are also quite accurate, but have slightly more error than the maximum temperature rise formulae. Finally, in the present work, extensions are suggested to the maximum and average temperature rise formulae of Tian and Kennedy to include the effects of elliptical contact geometry. It is found that these formulae are at least 91% accurate for elliptical ratios between 0.4 and 5.0.

Aluminum is widely used in electrical contacts due to its electrical properties and inexpensiveness when compared to copper. In this study, we investigate the influence of operating conditions like contact load (pressure), sliding speed, current, and surface roughness on the electrical and tribological behavior of the interface. The tests are conducted on a linear, pin-on-flat tribo-simulator specially designed to investigate electrical contacts under high contact pressures and high current densities. Control parameters include sliding speed, load, current, and surface roughness. The response of the interface is evaluated in the light of coefficient of friction, contact resistance, contact voltage, mass loss of pins, and interfacial temperature rise. As compared to sliding speed, load, and roughness, current is found to have the greatest influence on the various measured parameters. Under certain test conditions, the interface operates in a “voltage saturation” regime, wherein increase in current do not result in any increase in contact voltage. Within the voltage saturation regime the coefficient of friction tends to be lower, a result that is attributed to the higher temperatures associated with the higher voltage (and resulting material softening). Higher interfacial temperatures also appear to be responsible for the higher wear rates observed at higher current levels as well as lower coefficients of friction for smoother surfaces in the presence of current.

Electrical contact resistance affects the performance of electrical switches and other current-carrying interfaces. This study investigates the behavior of electrical contact resistance for copper–copper and aluminum–aluminum sphere-on-flat contact as a function of current through the interface. It is observed that the contact resistance may either increase or decrease with increasing current, depending on the current level as well as the current history. At low current levels the voltage drop across the interface increases initially with increasing current until it saturates. The voltage level remains nearly constant even if the current is increased beyond the value corresponding to saturation. Hereafter any subsequent decrease in current yields a corresponding decrease in voltage, so that the associated current cycle shows substantial hysteresis. However, subsequent cycles of current are reversible so long as the voltage remains below the saturation point. Analyses of the results suggest that the mechanism of viscoplastic creep is responsible for the voltage saturation phenomenon.

The issue of thermal management of two components sliding against each other plays an important role in the proper functioning of the machine components. Jaeger, using his and Blok's hypotheses, presented the curves to compute the average and the maximum temperature rise in square and band shaped interfaces. It is often observed that designers refer to those curves even in scenarios when the contact geometry is different. In many cases, a more accurate temperature distribution is needed and using a constant heat partition to evaluate the temperature rise at the interface can lead to over-design or under-design of the components. In this study the authors apply a recently developed methodology of obtaining the temperature distribution at the interface of two sliding semi-infinite bodies by partitioning the heat between them in such a way that the temperature at every point in the interface is same for both the bodies. Elliptical contacts with a semi-ellipsoidal heat distribution are considered. In order to aid designers, curve fit equations are presented to calculate maximum temperature and average temperature rise in non-dimensional form over a wide range of Peclet numbers, thermal conductivity ratios and ellipticity ratios.

A computational analysis is conducted on a solid–liquid–solid interface: a rough, elastic surface is brought into contact with rigid flat while being influenced by an intervening liquid film of fixed volume. The problem is solved (iteratively) by satisfying the equations of elasticity and capillarity, while conserving liquid volume and honoring the constraints of no surface interpenetration and no negative pressures at dry contact points. As the liquid volume is increased from low values, a critical condition is reached whereby there are sudden increases in the values of certain output parameters, such as tensile force and wetted length. For the smaller values of surface roughness, the critical point is associated with a near complete collapse of the surface topography.

Small-scale devices are particularly vulnerable to adverse effects of adhesion because of large surface area to volume ratios. Additionally, small gaps can be easily bridged at high humidity or when there are other contaminant liquids present. The bridging of a portion of an interface by a liquid droplet of given volume tends to pull surfaces in closer proximity due to the sub-ambient pressures that arise. In turn, the area spanned by the bridge will increase and lead to a greater adhesive force. A previous study considering a liquid bridge between two elastic half-spaces initially separated by a uniform gap demonstrated that, under certain conditions, an instability will arise whereby the surfaces come into full contact. Owing to the regularity of the geometry considered, the problem was amenable to a semi-analytical treatment and the stability condition was expressible analytically in terms of a single dimensionless group. In the present work, we develop a model to include the effects of surface roughness. The influence of asperities on the surface is treated by means of a recently developed multi-scale model that considers the full range of wavelengths comprising the surface profile. In the simulations, two nominally flat rough surfaces are brought together under a prescribed load and a liquid bridge of given volume is introduced in the contact, the initial areal coverage being determined by the initial mean surface separation. The interface configuration is then iterated until a configuration is found that satisfies the equations of elasticity and capillarity for the given liquid volume. As a result of the simulation, critical values are found for combinations of parameters that delineate stable and unstable conditions.

The friction-induced temperature rise at the interface of two sliding bodies can cause a number of effects, including yield strength reduction, accelerated oxidation wear and thermoelastic instability, all of which may lead to degradation of performance or even the failure of the associated components. It is of interest, therefore, to accurately determine the temperature field resulting from sliding contact. In this study, the authors present a method for obtaining the temperature rise at the interface of two sliding semi-infinite bodies by partitioning the heat between them in such a way that the temperature at every grid point in the interface is same for both the bodies. A polynomial form is assumed for the unknown heat partition function and then a linear regression is performed to find the coefficients that optimize the temperature matching at the interface. The results of the analysis, when compared to the predictions of Blok and Jaeger, who each calculated a constant heat partition factor based on a single temperature match, indicate that their more simple analyses provide good estimates of either the maximum temperature rise (Blok) or the average temperature rise (Jaeger). The method of the current study, which yields the complete temperature and heat partition distributions within the interface, may be used in conducting a detailed thermal analysis of sliding bodies.

With the present-day electromagnetic launchers (EMLs), the choice of copper for the rail and aluminum for the armature is the most popular. There have been few published studies investigating the behavior of the copper-aluminum pair under high contact pressures and high current densities. We consider the interface between a stationary copper flat (C-110) and translating aluminum (Al-6061) pins with hemispherical and flat tips. Tests are conducted on a tribo-tester designed specially to study wear and friction of sliding electrical contacts. Electrical current to the interface is supplied by a programmable 8 V/350 A power supply. Operating conditions of the experiments are designed to yield the contact pressures of ~250-375 MPa and electrical current densities (~1-14 GA/m2) representative of a typical electromagnetic launcher, albeit at much lower sliding speeds (~0.15 m/s). The wear of the pins is quantified by measuring change in their mass and using the optical microscopy and scanning electron microscopy. Energy dispersion spectroscopy (EDS) is used to study the material transfer onto the pins. In-situ measurements during the experiments show that during the sliding process the voltage drop across the contact is negatively correlated with the friction force. The measurements of the wear patch on the pins show that as the current is increased through the contact the wear diameter also increases.

In this work, a numerical method is developed to study the liquid-mediated adhesion between 3D rough surfaces. The presence of a wetting liquid between two rough surfaces generates a negative pressure inside the liquid film, which in turn induces tensile stresses in the contact region. Opposing these tensile stresses are the compressive stresses generated at solid–solid contact spots. An iterative deterministic contact model based on influence coefficients is employed to solve the equations of elasticity and capillarity simultaneously. Results for tensile (adhesive) force, wetted radius, average gap, and contact area between the rough surfaces are obtained. The results show that sudden changes in tensile force, average gap, and contact area occur with an increase in certain parameters, suggesting an elasto-capillary instability and surface collapse.

The interaction between two elastic spheres with an intervening liquid film of given volume is studied theoretically. Using an energy minimization approach, equilibrium contact configurations are determined through numerical computation. Several dimensionless groups are identified that govern the character of the solution. Curve fits are performed to reveal analytical relationships among the dimensionless groups. At extreme values of particular parameters, the curve fits are found to recover the analytical results of the well-known Hertzian and Johnson–Kendall–Roberts elastic (dry contact) models, as well as the force of a liquid bridge between rigid spheres. Qualitative agreement is found between the current model and some published experiments.

An approximate analytical model is proposed to describe the process of separating a sphere from a flat in the presence of a Newtonian liquid, under conditions of constant force. The model is based on consideration of the effects of sphere inertia in characterizing the critical point of separation—namely, that there is a sudden onset of significant sphere acceleration. Good agreement is found between the predictions of the analytical model with those of a full numerical simulation.

The interaction between two elastic half-spaces coupled by a small liquid bridge is investigated theoretically. A non-dimensional number, Nc, is found to govern the interface stability. When Nc is greater than or equal to 1, the two surfaces jump into solid–solid contact. Once this occurs, the contact area will continue to expand until the two surfaces are in full contact.

A recent study showed that for a liquid bridge of volume Vo interposed between two half-spaces and originally separated by uniform gap H, the interface stability was determined by a single dimensionless parameter. When the parameter exceeds a certain value, the interface loses stability and the solid surfaces come into contact. Further, once contact is made, the contact zone will continue to spread without bound. In the present work, the interaction between two curved elastic bodies coupled via a small liquid bridge is investigated. The equilibrium configurations are determined by minimizing the total free energy stored in the interface among all provisional equilibrium configurations; i.e., those configurations consistent with the equations of elasticity and capillarity. In the case of curved bodies, the stability of the interface is governed by the above parameter as well as by a dimensionless ratio involving the liquid volume (Vo), the composite radius (R) and the original (undeformed) minimum sphere–sphere separation (H). When the interface becomes unstable, solid–solid separation cannot be sustained and contact ensues. The resulting solid–solid contact radius is found to depend on the above parameters as well as on the ratio of solid–liquid and liquid–air surface energies. Based on the numerical calculations, simple relations are found between the contact radius and the other relevant parameters.

This paper reports on a feasibility study in the detection of nanometer thick lubricated regions on a hard disk by a novel use of a Kelvin probe. A nonvibrating Kelvin probe was constructed and used to measure the voltages between the copper surface of the probe and a hard disk partially-lubricated with a perfluoropolyether. The probe, constructed of a shielded 1.6 mm diameter gold-coated copper wire, was fixed above the hard disk and the probe voltage was obtained as a function of the speed of the disk and lubricant thickness. The probe generates an electrical signal as the interface between the lubricated and unlubricated regions is crossed. Results show that the probe can distinguish between regions on the hard disk that contain 3 and 10 nm thick lubricant films.

Previous friction measurements in the magnetic head-disk interface (HDI) indicated some anomalous friction force behavior with low sliding speeds (.25 mm/s to 0.25 m/s) and lubricant film thicknesses in the range of 20–80 nm, showing decreasing friction force with increasing apparent shear rate. The physical explanation for such behavior has not yet been established. Possible explanations for such behavior have included dramatic shear thinning, interfacial slip and partial loss of contact with the lubricant film due to slider tipping. In the present study, we investigate the possibility of slider tipping as an explanation for the decreasing friction force with increasing sliding speed. Measurements with an optical probe indicate that slider tipping does not occur for the conditions tested. Numerical analysis of slider equilibrium also supports this conclusion.

Further increase in magnetic recording density requires a reduction in slider flying height. The current study employs the short-bearing approximation to determine analytically the static equilibrium configuration of a slider supported by a starved liquid bearing that operates between ideally smooth surfaces. The solution incorporates rheological behavior based on previously acquired data. The accuracy of the short-bearing approximation is assessed by determining how well the resulting solution satisfies the Reynolds equation. The analysis suggests a means of designing a slider to achieve head/medium spacings in the neighborhood of 20 nm.

The transition from static friction to kinetic friction results from the attainment of a point of instability, whereby interfacial slip becomes more energetically favorable than sticking. Such an instability is explored in this work via a plane-strain elastostatic analysis. A rigid pin of prescribed geometry is placed in contact with an elastic slab and translated horizontally under conditions of constant load. An intrinsic static coefficient of friction is prescribed, which limits the ratio of shear stress to contact pressure at each location within the interface. Additionally, the surface of the elastic slab is given a desired undulation to simulate the effects of surface roughness. As the pin is translated horizontally, a lateral reaction force (i.e., friction force) is developed and is observed to grow nearly linearly with increasing lateral displacement. At a critical point, a substantial portion of the interface experiences slip, leading to a large decrease in the friction force and thereby revealing a stick-slip behavior. It is found that the overall (macroscopic) static friction coefficient can be significantly less than the intrinsic friction coefficient and that the presence of even a small amount of roughness can have a large effect on the friction force.Copyright © 2012 by ASME

In 2011, investigators at the Georgia Institute of Technology performed a series of experiments and analyses to provide a further understanding of railgun performance as a function of the armature-to-rail interface. In this paper, results of this effort are presented offering insight with respect to interface wear generated using a lab-scale railgun and a gas-gun.

Lubricant rheology governs the friction in any lubricated contact. In hydrodynamic lubrication, the genesis of friction is well-understood. However, when asperity contacts occur, the situation becomes more complex. In this “mixed” lubrication regime, it is known that lubricants can deviate dramatically from Newtonian behavior, but the source of this effect has not been identified. In particular, the question arises as to whether the non-Newtonian behavior of the lubricant is due to the extreme thinness of the film or to the very large shear rates to which it is subjected. In the current work, we analyze friction force measurements in the magnetic slider/disk interface to help resolve this question. Because of its precision geometry, the slider/disk interface is ideal for such an investigation. Results of the analysis indicate (1) the lubricant retains its bulk viscosity in films as thin as 11–12 molecular diameters; (2) the rheological state of the lubricant is determined by a parameter we introduce here as the “Newtonian” shear stress, and (3) the rheology of the lubricant at high Newtonian shear stress may indicate a newly discovered property of liquids.

The well-known problem of stiction in a magnetic disk drive largely depends on the forces induced by the presence of a thin liquid film. It is commonly recognized that both adhesive and viscous effects contribute to the magnitude of the stiction force, but is is not known what relative roles the two effects have in a lubricated contact. In the present work, the nature of adhesive and viscous effects is investigated for the slider/disk interface under conditions of constant-speed sliding. Friction measurements are conducted over a range of sliding speeds, 0.25-250 mm/s, with eight perfluoropolyether (PFPE) lubricants applied in various thicknesses, 0-6.6 nm, to carbon-coated magnetic thin-film disks. The lubricants were selected to cover a broad range of viscosities. For several sliding speeds and lubricant film thicknesses, the friction force is found to decrease significantly with increasing sliding speed for all lubricants. In several instances, large friction forces are observed at the lowest sliding s...

One of the key obstacles precluding the maturation and commercialization of planar solid oxide fuel cells has been the absence of a robust sealant. A computational model has been developed in conjunction with leakage experiments at Oak Ridge National Laboratory. The aforementioned model consists of three components: a macroscopic model, a microscopic model, and a mixed lubrication model. The macroscopic model is a finite element representation of a preloaded metal-metal seal interface, which is used to ascertain macroscopic stresses and deformations. The microscale contact mechanics model accounts for the role of surface roughness in determining the mean interfacial gap at the sealing interface. In particular, a new multiscale fast Fourier transform-based model is used to determine the gap. An averaged Reynolds equation derived from mixed lubrication theory is then applied to approximate the leakage flow across the rough annular interface. The composite model is applied as a predictive tool for assessing how certain physical parameters (i.e., seal material composition, compressive applied stress, surface finish, and elastic thermophysical properties) affect seal leakage rates. The leakage results predicted by the aforementioned computational leakage model are then compared with experimental results.

This letter reports on the detection of nanometer-scale perfluoropolyether (PFPE) lubricants with thickness between 2–40 nm on a magnetic disk surface using a nonvibrating Kelvin probe. Magnetic disks were partially dipped in a PFPE lubricant to create lubricated and nonlubricated regions on the disk surface. The nonvibrating Kelvin probe measures an electrical signal proportional to the surface charge gradient at the interfaces between the lubricated and nonlubricated regions, and is linearly related to the scanning velocity. The surface charge is related to the difference in surface potentials between the lubricated and nonlubricated regions. The surface potential at the nonlubricated/lubricated transition ranges from 7–30 mV. The surface potential is found to be related to the corresponding lubricant thickness through an exponential relationship.

In many tribological investigations, the measurement of friction is complicated by the vibrational characteristics of the transducer. In particular, when a mechanical force transducer is excited, the static calibration factor no longer provides the friction force in the interface. In a recent study (Streator and Bogy, 1992), the problem of determining the friction force from a measurement of the periodic bending strain in a cantilever-mass transducer was solved using the equations of Euler-Bernoulli beam theory. The method showed that modelling the transducer as a single degree-of-freedom (DOF) system could lead to inaccuracies depending on the frequency content of the friction force, and depending on whether a measurement of beam strain or mass displacement was available. Employing the Euler-Bernoulli analysis, however, requires substantially greater analytical and computational effort. In the present work a two DOF model is considered which represents a compromise between the simplicity of the single DOF model and the accuracy of the more thorough analysis. Force computations are made for a simulated slider/disk tribological contact at different frequencies of disk rotation. One set of computations is performed based on knowledge of the beam strain and the other based on knowledge of the mass displacement. It is found that the friction force computed from the two DOF model compares well with that of the Euler-Bernoulli analysis for an extended range of frequencies as against the single DOF model, but loses accuracy at high frequencies. Nevertheless, if used in a careful manner, the two DOF model offers an improved method of determining dynamic friction forces for certain applications.

One of the outstanding issues in the analysis of contacting surfaces regards properly accounting for the multi-scale nature of the surface topography. Many treatments of surface contact, particularly those that derive from the well known Greenwood Williamson (GW) model, proceed from the basis that one can determine a representative curvature of the surface peaks (or summits) by means of computing the appropriate numerical derivative of the sampled profile data. However, as has been demonstrated over the last 20 years or so, numerical derivatives of surface data are sensitive to the lateral spacing between sampled surface points. In view of the potential ambiguity associated with such a determination of peak curvature, a number of models have been put forth that account for the fact that topographical features exist at many scales of observation. In the present work, predictions of a recently developed spectral-based model of elastic (rough) surface contact are compared to those of a deterministic numerical computation in which the equations of elasticity are solved at every nodal point of a 3D surface.Copyright © 2009 by ASME

It is well known that surface topography is composed of multiple length scales. Yet it remains a challenge to ascertain how the various length scales affect the contact area and contact stresses. Several micro-contact models exist in the literature that seek to account for the multi-scale nature of surface topography through fractal or spectral descriptions. A limitation of most such micro-contact models is that they neglect the interaction between asperity contacts. In the current work, a deterministic numerical simulation of contact is performed on the contact between a rigid flat and a rough, elastic surface whose heights vary in a single dimension (i.e., y(x)). A simulated surface profile is created and then “sampled” at several resolutions to create a set of related profiles of various levels of refinement. Contact is investigated with each of the simulated surfaces to gain insight as to the role of various length scales.Copyright © 2008 by ASME

Sliding electrical contacts are subject to surface damage and wear, which can be enhanced by the heating at the interface arising from electrical contact resistance. For example, in electromagnetic launcher (EML) technology, thermally assisted wear processes can result in unacceptable levels of material loss at the armature-rail interface. The control of the interface tribology in sliding electrical contacts requires an understanding of the Joule heating in the vicinity of the interface. In the current study, a multiphysics numerical simulation is conducted of transient heat conduction in both a stationary and a sliding electrical contact. The interface under investigation consists of a flat-ended aluminum cylindrical pin sliding against an aluminum rail. Electrical contact resistance is modeled by introducing a thin layer of high resistivity between the pin and the rail. Results show that shortly after sliding has commenced, (1) the maximum temperature rise occurs in the bulk of the pin rather than at the interface, (2) the bulk of the Joule heat goes into the rail, and (3) that sliding can have a significant effect on the temperature field, even when the speed is quite low.Copyright © 2008 by ASME

This research seeks to characterize the gas leakage of a mica-based compressive seal assembly in planar solid-oxide fuel cells through modeling and experiment. In particular, it is of interest to assess how certain physical parameters (i.e., seal material composition, compressive applied stress, and surface finish) affect leakage rates. Finite element analysis is used to determine the macroscopic stresses and deformations in the sealing interface, while a microscale contact mechanics analysis is employed to model the role of surface roughness on the mean interfacial gap at the interface. An averaged Reynolds equation from mixed lubrication theory is applied to model the leakage flow across the sealed interface, which is of nanometer to micrometer dimensions in the vertical direction. In conjunction with the mathematical modeling, leakage results are reported. For these tests, an annular Inconel tube was pressed against a stainless steel substrate, creating an annular sealing zone. The inside of the tube is pressurized with a test gas, the mass of which is monitored during the leakage experiment. Test results are compared to model predictions.

The current work considers the multiscale nature of surface roughness in a new model that predicts the real area of contact and surface separation, all as a function of load. By summing the distance between the two surfaces at all scales, a model of surface separation as a function of dimensionless load is also obtained. The model is also able to make predictions for thermal (and electrical) contact resistance. In striving for a more realistic model, the multi-scale model accounts for the effects of a rough surface geometry ranging from macro down to the nano scale. A previous rough surface contact model was based on stacked elasto-plastic spheres. This work uses stacked 3-D sinusoids to represent the asperities in contact at each scale of the surface. The results are also compared to several other existing rough surface contact models and experimental results.

Small-scale devices are particularly vulnerable to adverse effects of adhesion because of large surface-area-to-volume ratios. Additionally, small gaps can be easily bridged at high humidity or when there are other contaminant liquids present. The bridging of a portion of the interface by a liquid droplet of given volume, tends to pull surfaces in closer proximity due to the sub-ambient pressures that arise. In turn, regions spanned by the bridge will increase in size and lead to a greater adhesive force. In the present work we develop a model for these effects in the presence of surface roughness. The influence of asperities on the surface is treated by means of a recently-developed multi-scale model that considers the full range of wavelengths comprising the surface profile. In the simulations, two nominally flat rough surfaces with profiles that vary only in one direction are brought together under a prescribed load. A liquid bridge of given volume (per unit depth) is then introduced into the contact, assuming an initial areal coverage. The interface configuration is then iterated until one is found that satisfies the equations of elasticity and capillarity for a given liquid volume. As a result of the simulation, critical values are found for combinations of parameters that delineate stable and unstable conditions.Copyright © 2007 by ASME

Wear and friction in sliding electrical contacts is affected by a multitude of factors such as contact load, current density, sliding speed, sliding distance and materials in contact [1], [2]. Very limited studies have been done to investigate the effect of current polarity on wear and coefficient of friction. In the present study, the authors investigate the effect of current polarity on the frictional response and wear of copper-aluminum interface. Copper wire mounted on a fixture is slid on aluminum flat under different current levels. Large scale melting at the interface was observed when slider (copper) was maintained as anode. Observations of the slider and flat surface made under microscope reveal material transfer of aluminum on copper even at low current levels of 60 Amperes. The results of the study can be used in the light of applying different coatings or surface design for anode and cathode in order to minimize their wear or degradation.Copyright © 2007 by ASME

A critical factor in the successful operation of an electromagnetic launcher (EML) is the maintenance of good electrical contact between the armature and rail. While there have been numerous experimental and theoretical investigations of armature-rail interfaces, a well-accepted model of the contact conditions has not yet been established. In the current study, we consider the interface between a stationary metallic flat and a translating pin with a hemispherical tip. Operating conditions of the experiments are designed to yield the contact pressures (∼500 MPa) and electrical current densities (∼5 GA/m2 ) existing in typical EMLs, albeit with much lower sliding speeds (∼3 m/s). Electrical current to the interface is supplied by a programmable 8V/350A power supply. The test apparatus is configured so that the pin can translate continuously along a 1.5 m rail, its carriage being powered by a direct drive linear motor. Loading of the pin against the flat is provided by means of a pneumatic actuator. Current, voltage, load, friction, surface temperature and position are monitored throughout the sliding event. Changes in surface topography and morphology are assessed via optical and stylus profilometry as well as optical microscopy. Arcing and phase change events are observed.Copyright © 2006 by ASME

Fuel cells represent a promising energy alternative to the traditional combustion of fossil fuels. In particular, solid oxide fuel cells (SOFCs) have been of interest due to their high energy densities and potential for stationary power applications. One of the key obstacles precluding the maturation and commercialization of planar SOFCs has been the lack of a robust sealant. This paper presents a computational model of leakage with the utilization of mica-based compressive seals. A finite element model is developed to ascertain the macroscopic stresses and deformations in the interface. In conjunction with the finite element model is a microscale contact mechanics model that accounts for the role of surface roughness in determining the mean interfacial gap at the interface. An averaged Reynolds equation derived from mixed lubrication theory is applied to model the leakage flow across the rough, annular interface. The composite model is applied as a predictive tool for assessing how certain physical parameters (i.e., seal material composition, compressive applied stress, surface finish, and interfacial conformity) affect seal leakage rates.Copyright © 2006 by ASME

This work describes a non-statistical multi-scale model of the normal contact between rough surfaces. The model produces predictions for contact area as a function of contact load, and is compared to the traditional Greenwood and Williamson (GW) and Majumdar and Bhushan (MB) rough surface contact models, which represent single-scale statistical and fractal analyses, respectively. The current model incorporates the effect of asperity deformations at multiple scales into a simple framework for modeling the contact between nominally flat rough surfaces. Similar to the “protuberance upon protuberance” theory proposed by Archard, the model considers the effect of having smaller asperities located on top of larger asperities in repeated fashion with increasing detail down to the limits of current measurement techniques. The parameters describing the surface topography (areal asperity density and asperity radius) are calculated from an FFT performed of the surface profile. Thus, the model considers multi-scale effects, which fractal methods have addressed, while attempting to more accurately incorporate the deformation mechanics into the solution. After the FFT of a real surface is calculated, the computational resources needed for the method are very small. Perhaps surprisingly, the trends produced by this non-statistical multi-scale model are quite similar to those arising from the GW and MB models, but seem largely unaffected by the sampling resolution at the employed surface data.

A simulation is undertaken to consider the effects of the scale of surface roughness on elastic contact and friction. In the simulation, a cylindrical rigid indenter of a prescribed profile is brought into contact with an elastic half-space. Interaction between the indenter and the half-space occurs through an assumed surface potential derived from the 6-12 Lennard-Jones intermolecular potential. The hypothesis under investigation is that by assuming the frictional stress is proportional to the magnitude of the normal stress, irrespective of sign, one can account for the enhanced frictional stress at the nano-scale vs. the micro-scale that has been observed experimentally. Results of the simulations indicate qualitative agreement with recent experimental findings.Copyright © 2003 by ASME

A numerical simulation is developed to investigate the role of adhesive forces during the normal impact of a rigid body against an elastic surface. The model consists of a rigid, 2D translating body and a surface that is composed of a flexible linear array of particles, coupled to a fixed base via linear springs. Adhesive effects are incorporated in the model by ascribing a Lennard-Jones type interaction potential between the surface and the body. Dynamical equations of motion for the interface are integrated numerically during an impact event. It is found that the coefficient of restitution decreases with increasing interaction strength and decreasing approach velocity. Above a certain interaction strength for given velocity and below a certain velocity for given interaction strength, the body is found not to rebound, but sticks to the surface. The simulation results are found to be in qualitative agreement with the impact model of Dahneke (1973, 1975). Quantitative discrepancies are explained by the effects of surface “plucking.”