Showing papers on "Contact area published in 1989"

Contact electrification using force microscopy.

Abstract: One of the oldest unresolved problems in physics is the mechanism of charge exchange between contacting surfaces when at least one of them is insulating. We describe a new technique, using force microscopy, for studying this problem with greater lateral resolution than has been previously possible. The force microscope is shown to have 0.2 \ensuremath{\mu}m lateral resolution and the sensitivity to detect 3 electronic charges. In contact-charging experiments between the microscope tip and polymethyl methacrylate, the charged region was much larger than the expected contact area and bipolar charge exchange was observed.

Measurements of and Relation Between the Adhesion and Friction of Two Surfaces Separated by Molecularly Thin Liquid Films

Abstract: A new technique is described for sliding (shearing) two molecularly smooth surfaces laterally past each other in liquids while monitoring their exact contact area, the normal and transverse forces, and the surface separation. First, we show that the elastic deformations of two initially curved surfaces in adhesive contact are the same under static and dynamic (i.e., sliding) conditions. Detailed results are then presented of how the shear properties of thin films of water and a simple nonpolar liquid are “quantized” with the number of layers. Results with water as the intervening liquid, as well as the effects of humidity on sliding in air, reveal that more complex mechanisms are operating than with simple liquids which appear to be related to the complex “hydration” forces between two surfaces in water or in aqueous salt solutions. The results suggest a close correlation between the static forces and shear properties of very thin liquid films, and the molecular structure of the liquids confined within such films.

The effect of contact area on the permeability of fractures

Abstract: Abstract The permeability of a rock fracture is controlled primarily by the geometry of its void space. One effect of void space geometry is to cause the fluid to follow a tortuous path to flow around the asperities, which are regions where the two faces of the fracture are in contact. To examine the tortuosity induced by the contact area, we consider an idealized fracture consisting of two parallel plates propped open by isolated asperities. Boundary-element calculations, analogue electrical conductivity measurements and an effective medium approximation are used to study the permeability of fractures with circular, elliptical and irregular asperity shapes. The permeability is seen to depend not only on the amount of contact area, but also on the shape of the asperities. For circular or elliptical asperities, very accurate estimates are found by using the effective medium theory proposed by Maxwell.

Contact angle measurements from the contact diameter of sessile drops by means of a modified axisymmetric drop shape analysis

Abstract: This paper presents a novel approach to measuring contact angles. The Axisymmetric Drop Shape Analysis-Contact Diameter (ADSA-CD) approach determines the contact angle by solving the Laplace equation of capillarity numerically, and using as input parameters the liquid surface tension, the drop volume, and the contact diameter of the drop. The contact diameter is the main experimental parameter to be determined as the liquid surface tension is known and the volume of the sessile drop is determined by means of a precision micrometer syringe. Various strategies for the determination of the contact diameter are possible, ranging from a simple measurement by means of a travelling microscope to digital image analysis determining the contact area and hence an effective contact diameter. An accuracy in contact angles of ±0.5° is readily obtainable for well-prepared surfaces. Indications are that the technique will be limited only by the quality of the solid substrate and hence the deviations of the contact circle from circularity. While the technique is applicable to any contact angle, in this paper, only contact angles of less than 90° were measured. The technique performs particularly well for small contact angles below 20°, where other techniques, such as the analysis of drop profiles, produce rather large errors. The method is therefore expected to be particularly useful for the study of usually very hydrophilic biological specimens.

On the mechanics of crack closing and bonding in linear viscoelastic media

Abstract: The mechanics of quasi-static crack closing and bonding of surfaces of the same or different linear viscoelastic materials is described. Included is a study of time-dependent joining of initially curved surfaces under the action of surface forces of attraction and external loading. Emphasis is on the use of continuum mechanics to develop equations for predicting crack length or contact size as a function of time for relatively general geometries; atomic and molecular processes associated with the healing or bonding process are taken into account using a crack tip idealization which is similar to that used in the Barenblatt method for fracture. Starting with a previously developed correspondence principle, an expression is derived for the rate of movement of the edge of the bonded area. The effects of material time-dependence and the stress intensity factor are quite different from those for crack growth. A comparison of intrinsic and apparent energies of fracture and bonding is made, and criteria are given for determining whether or not bonding can occur. Examples are given to illustrate use of the basic theory for predicting healing of cracks and growth of contact area of initially curved surfaces. Finally, the effect of bonding time on joint strength is estimated from the examples on contact area growth.

The effects of various load paths and different loads on the load transfer characteristics of the wrist.

Abstract: An experimental model that incorporated a static positioning frame, pressure-sensitive film, and a microcomputer-based videodigitizing system was used to analyze the effects of different loading pathways and various loads on the contact area and pressures within the wrist joint. There was no statistically significant difference in loading the wrist with comparable weights through the second and third metacarpals, through all five metacarpals, or through weights suspended from the wrist flexor and extensor tendons. A nonlinear relation was discovered between increasing loads and greater overall contact areas. The general distribution of the contact between the scaphoid and the lunate contact areas was consistent at all of the loads tested with 60% of the total contact area involving the scaphoid contact area and 40% involving the lunate contact area. Loads greater than 46 pounds were not found to significantly increase the overall contact areas implying that the cartilage of the wrist joint was maximally compressed at loads of this magnitude. At loads higher than 46 pounds it appears that average high pressures increase in a more direct correlation with the increase in weight. The overall contact area even at the highest loads tested were not more than 40% of the available joint surface. The contact areas were not concentric or symmetric as is characteristic of the incongruance of the radio/triangularfibrocartilage (ulna)/carpal joint.

Digitizer pad featuring spacial definition of a pressure contact area

Abstract: A digitizer pad apparatus includes at least one digitizer ply where each digitizer ply has a first and a second resistor strip, each with a resistance gradient along its length, oriented in spaced apart relationship. A plurality of conductor traces are interconnected along the length of each resistor strip to extend toward and be interleaved between each other to define a sensor pad region. Each sensor pad region defines a dimensional direction. A shunt ply is positioned to face the sensor pad region in normally non-conducting relationship so that when a selected area is pressed into contact with the conductor traces, conduction between adjacent conductor traces via the shunt ply will occur in the selected contact area. The selected contact area has a first edge and a second edge opposite the first edge along the defined dimensional direction. A voltage source is coupled across the first resistor strip and a switch is coupled across the ends of the second resistor strip for alternately coupling one end and then the other end of the second resistor strip to a utilization. The voltage and hence the location of the first edge and then the second edge of the contact area along the dimensional direction is thereby determined.

On the Nondimensionalization of Constriction Resistance for Semi-infinite Heat Flux Tubes

Abstract: The insulated semi-infinite cylinder with heat supplied uniformly through a coaxial contact area is an important unit cell in the theory of contact resistance. In the past, several investigators have examined the problem of the thermal constriction resistance of a circular contact area on an insulated semi-infinite, coaxial circular cylinder. Recently the case of a circular contact on a square cylinder has been investigated. By using a new approximate technique as well as double-infinite series solution to Laplace's equation in Cartesian coordinates, the case of a square contact area on a square cylinder can also be solved. Previously the nondimensionalization technique employed for the results has varied with different configurations and researchers. However, the similarity of these three configurations in terms of their integrated parameter, the thermal constriction resistance, will only be seen when nondimensionalization is made using a characteristic dimension that best describes these geometries.

Performance and failure mechanisms of TiN diffusion barrier layers in submicron devices

Abstract: Defects in reactively sputtered titanium nitride diffusion barriers in submicron devices were investigated. Four different failure mechanisms could be extracted, two originating from the production of the layers, two related to foregoing process steps. The latter are contact hole overetch and topology inside the contact hole, i.e., edges, created by deviating isotropic etch ratios of different dielectric layers in a post‐treatment after the contact etch. The insufficiencies related to the preparation of the TiN films are microcracks due to excessive stress incorporated and encroachment caused by a Ti‐rich titanium nitride on the contact hole walls, proved by an Auger electron spectroscopy analysis. Accidental accumulation of two or more of these defects in a production line can cause the barrier to fail. The process of contact degradation was investigated by transmission electron microscopy and energy‐dispersive x rays. It was found that after aluminum penetration the destruction of the contact area proceeds via a ternary AlxTiySiz reaction until all titanium is consumed, followed by the growth of a pure Al spike and the fatal breakdown of the contact.

Adhesion‐induced deformations of polymeric substrates: Particle size dependence of the contact area

Abstract: Adhesion‐induced deformations of a polyurethane substrate in contact with cross‐linked polystyrene spheres, having diameters ranging from less than 2 μm to approximately 12.5 μm were observed using scanning electron microscopy. The diameters of the contact areas were measured from the micrographs. It was found that the contact radius varied as the particle radius raised to the 0.75±0.05 power. Experimental results are compared to the predictions of various adhesion models. The results are also discussed in terms of the Dupre work of adhesion.

Process for making a self aligned vertical field effect transistor having an improved source contact

Abstract: An improved method for making a self-aligned vertical field effect transistor is provided wherein a nitride sidewall spacer is formed around a polysilicon gate, and an oxide sidewall spacer, which may be heavily doped with an n-type dopant, is formed covering the silicon nitride sidewall spacer. The silicon nitride sidewall spacer allows the oxide sidewall spacer of a conventional self-aligned vertical field effect transistor process to be removed partially or completely before making ohmic contact to the source thus increasing the contact area between the source and the source electrode and eliminating reliability problems related to n-type doped oxide in contact with aluminum electrodes.

Multi-pole momentary membrane switch

Abstract: A membrane keyboard is comprised of a dielectric substrate and an overlying deflectable membrane layer separated by a spacer. A plurality of contact poles are located on the dielectric substrate with an electrically conductive common contact area located on the flexible membrane. A portion of the flexible membrane is locally stiffened and, in a preferred embodiment, provided with a ridge extending away from a portion of the membrane towards the contact poles. Upon actuation of the switch, the stiffened portion of the membrane ensures simultaneity of contact between the common contact area on the membrane and the contact poles on the substrate. Preferred embodiments utilize a generally circular ridge in the deflectable membrane cooperating with interdigitated contact poles.

Some new results in rolling contact

Abstract: SUMMARY In the present paper three problems in the simplified theory of rolling contact are investigated. As to the first problem, three benchmark loadings, derived by Kalker in 1973 for Hertzian rollingcontact, are in existence. Each of these loadings gives rise to a value of the flexibility parameter of the simplified theory. These values are combined to a single, creepage dependent value of the flexibility, which appears to have an error of at most 10 to 15%. Secondly, the law of Coulomb is generalised by introducing two values of the coefficient of friction. The FASTSIM algorithm is adapted to that, and it is found that the traction, and hence the displacement, show a discontinuity inside the contact area. The discontinuity in the displacement is removed by introducing damping in the constitutive relations of the simplified theory. The damping constant is determined experimentally. When the damping coefficient decreases, the damped solution tends to the solution obtained directly without damping. This...

A dynamical model for receptor-mediated cell adhesion to surfaces in viscous shear flow.

Abstract: We present a dynamical model for receptor-mediated cell adhesion to surfaces in viscous shear flow when the surfaces are coated with ligand molecules complementary to receptors in the cell membrane. This model considers the contact area between the cell and the surface to be a small, homogeneous region that mediates the initial attachment of the cell to the surface. Using a phase plane analysis for a system of nonlinear ordinary differential equations that govern the changes in free receptor density and bond density within the contact area with time, we can predict the conditions for which adhesion between the cell and the surface will take place. Whether adhesion occurs depends on values of dimensionless quantities that characterize the interaction of the cell and its receptors with the surface and its ligand, such as the bond formation rate, the receptor-ligand affinity, the fluid mechanical force, the receptor mobility, and the contact area. A key result is that there are two regimes in which different chemical and physical forces dominate: a rate-controlled high affinity regime and an affinity-controlled low affinity regime. Many experimental observations, including the effects of temperature and receptor mobility on adhesiveness, can be explained by understanding which of these regimes is appropriate. We also provide simple approximate analytical solutions, relating adhesiveness to cell and surface properties as well as fluid forces, which allow convenient testing of model predictions by experiment.

Effect of tire pressure and type on response of flexible pavement

Abstract: Radial, bias, and wide-base radial single tires were tested in the laboratory under three levels of inflation pressures and axle loads. The measured characteristics of the tires were gross contact area, net contact area, tire deflection, and contact pressures. Distributions of the contact pressures were used in a mechanistic solution to predict various flexible pavement response parameters. A modified version of the BISAR computer program, which takes concentric circles of various pressures, was used to predict the response parameters. The response of various flexible pavement structures was investigated in terms of the tensile strains at the bottom of the asphalt layer, the compressive stress at the asphalt layer interface, and the surface deflection. The effects of tire inflation pressures and axle loads on the response parameters were evaluated for all tires and asphalt thickness combinations. The three tire types were also compared for underloaded, loaded, and overloaded cases.

Frequency-Dependent Creep Coefficients for Three-Dimensional Rolling Contact Problems

Abstract: SUMMARY Kalker's creep coefficients for linear rolling contact problems are only valid in the steady state case. A method for the extension of linear contact mechanics into the high frequency range is presented. Frequency dependent creep coefficients are obtained, which can be interpreted as harmonic responses of the creep forces for harmonically varying creepages. The influence of different constant longitudinal creepages and of different shapes of the contact area on the harmonic responses are discussed. The analysis is based on Kalker [5]

Measurements of Friction Coefficients of Snow blocks

Abstract: Snow blocks were slid down natural snow slopes and filmed with a video camera Friction coefficients were calculated from time-distance curves and the equation of motion Dry-friction coefficients ranged from 057 to 084, and could be separated into Coulomb friction and a friction component proportional to the contact area of the blocks (adhesion) These values are greater than the values usually used in avalanche dynamics, but are consistent with previous coefficients obtained for snow blocks sliding over snow When uniform ploughing occurred and a shear layer developed along the track the apparent friction coefficien ts increased with velocity, and could be modelled by considering the kinematic viscosity of the snow The values of kinematic viscosity ranged from 10-3 to 10-4 m2/s and agreed well with those values obtained by other researchers

Theory of rolling contact heat transfer

Abstract: This paper addresses the fundamentals of the phenomenon of steady heat transfer by rolling contact between two bodies at different temperatures. The contact region is modeled according to the classical Hertz theory, by which the bodies undergo elastic deformation and the contact area has the shape of an ellipse. In the first part of the study it is shown that when the two bodies make contact continuously over the elliptical area, the overall heat transfer rate is proportional to the square root of the Peclet number based on the ellipse semiaxis parallel to the tangential (rolling) velocity. In the same case the heat transfer rate increases as the square root of the normal force (F) between the two bodies. The second part of the study treats the case when the rolling contact is made through a large number of asperities (contact sites) distributed over the elliptical contact area. The heat transfer rate is again proportional to the square root of the Peclet number. When the asperities are distributed randomly, the heat transfer rate increases as F{sup 5/6}. In the case of regularly distributed asperities that undergo elastic deformation, the heat transfer rate is proporitonal to F{sup 13/18}. The high Pecletmore » number domain covered by this study is discussed in the closing section of the paper.« less

Film resistance and constriction effect of current in a contact interface

Abstract: A computer simulation based on the finite-element method was used to study the conductance of a contact interface, and the property of conductance was investigated using a global model of contact. The contact interface of the model consists of real contact part with the area fraction f and noncontact part (gap) with the area fraction 1-f. This conductance was computed against f. It was made clear that a great deal of current can flow through a small real contact area (current constriction effect) and hence the conductance does not decrease until f becomes close to zero. The increase of film resistance of the real conducting area is taken into account and the behavior of the contact conductance is studied. The decrease of constriction effect is quantitatively shown and discussed with respect to the increase of film resistance. >

Truck tire pavement contact pressure distribution characteristics for the bias goodyear 18-22.5, the radial michelin 275/80r/24.5, the radial michelin 255/70r/22.5, and the radial goodyear 11r24.5 tires. final report

Abstract: This report presents the results of an experimental investigation into the contact areas and tire contact pressure distributions produced by statically loaded truck tires. For this report, the bias Goodyear 18-22.5 LR-H tire, the radial Michelin 275/80R/24.5 LR-G tire, the radial Michelin 255/70R/22.5 LR-G tire, and the radial Goodyear 11R24.5 LR-G tire were tested. The testing consisted of making contact pressure and contact area prints at the interface between the tire and a steel plate at different wheel loads and tire inflation pressures. The pressure prints were produced using Fuji prescale film. The Fuji prescale film produces color variations, when pressure is applied to it, in such a way that darker pigmentation is produced in zones of higher pressure. The variations in color intensities of the Fuji film prints are related to contact pressure values produced for the film color calibration curve. Then, by digitizing the images and using computer software developed exclusively for this project, the tire contact pressure distributions were determined. The proportions of contact area covered by the various pressure ranges were computed and compared in order to observe the patterns and to estimate the significance of high contact pressures. The contact area prints were made by applying ink to the tire and pressing the tire over a white paper that covered the steel plate. The ink prints have only one color and were used for calculating the tire-plate contact areas. Also, the side tire movements were measured for the tires during testing to allow other researchers to relate subsequent theoretical studies to the experimental results. This report also proposes mathematical models for (1) estimating the tire contact area based on the relative area value (ratio of wheel load over inflation pressure) and (2) estimating the tire vertical stiffness based on the tire contact area.

The effect of contact geometry on the value of contact resistivity extracted from Kelvin structures

Abstract: A finite-element program that can model arbitrarily shaped contacts has been developed. It is used to evaluate the effects that changes in geometry, specific contact resistivity, sheet resistivity, and the modification of sheet resistivity under the contact have on contact systems. The Kelvin test structure is used to illustrate some of its capabilities. It is shown that the contact area of this test structure is of primary importance. The contact shape has little influence on the extracted value of contact resistance. >

Integrated circuit module connector assembly

Abstract: An electrical connector assembly for providing interconnections between a plurality of contact pads (22) on a daughter board (20) and contact areas (24) on a mother board (26). The assembly includes an elongated housing (28) with a daughter board receiving cavity (38) along its longitudinal axis (30) and a plurality of contact receiving cavities (36) which are substantially planar and parallel to each other arranged at a slight angle with respect to a normal to the axis (30). The contact receiving cavities (36) intersect the board receiving cavity (38) and within each of them there is disposed a contact member (40) having a contact portion (42) and a contact leg (44) on opposite sides of the board receiving cavity (38). The contact leg (44) extends through the bottom surface (34) of the housing (28) into contact with a contact area (24) on the mother board (26). The contact members (40) alternate within the contact receiving cavities ( 36) so that insertion of a daughter board (20) causes compressive stress to be applied to the housing (28) through the contact legs (44).

The line contact problem of elastohydrodynamic lubrication I. Asymptotic structure for low speeds

Abstract: This paper reports the first formal asymptotic solution to the line contact problem of elastohydrodynamic lubrication (EHL), a fundamental problem describing the elastic deformation of lubricated rolling elements such as roller bearings, gear teeth and other contacts of similar geometry. The asymptotic regime considered is that of small λ , a dimensionless parameter proportional to rolling speed, viscosity and the elastic modulus. The solution is shown to possess four regions: a zone where the lubricating film is both thin and slowly narrowing and which is closely related to the contact area that occurs in the absence of lubricant, an upstream inlet zone of low pressure, and two thin layers on either side of the contact zone. The solutions in the first two just-mentioned zones are given by simple analytical expressions. The solutions in the two thin layers are obtained from two universal functions obtained by Bissett & Spence ( Proc. R. Soc. Lond . A 424, 409 (1989)). Although these two functions, related to the local film thickness, are obtained by numerical techniques by Bissett & Spence, it should be emphasized that all cases in the asymptotic regime considered are hereby solved definitively without recourse to further computation. Although some features of this structure have been suggested by other solution approaches, generally, these are numerical or ad hoc approximations. See the texts by Johnson ( Contact Mechanics , pp. 328 (1985)) and Dowson & Higginson ( Elasto-hydrodynamic lubrication (1977)), this work provides a formal mathematical basis for understanding most of the principal features of EHL. The solution provides a simple formula for minimum film thickness and displays the sharp narrowing of the lubricating film in the thin layer near the exit. In the basic asymptotic solution provided here, the dimensionless pressure-viscosity coefficient, α , is assumed to be O (1), and in this parameter regime, no pressure spike will occur. By comparing with the work of Hooke ( J. mech. Engng Sci . 19(4), 149 (1977)), we can show that an incipient pressure spike occurs when α becomes as large as O ( λ -1/5 ). However, asymptotic solutions in this latter parameter regime require new numerical solutions for each case of interest and are not pursued here.

Optical observations of “junction growth” in asperities of copper, aluminium, PTFE and nylon under combined normal and tangential stresses

Abstract: An experimental investigation is described in which the phenomenon of “junction growth” in model conical and spherical asperities of copper, aluminium, PTFE and nylon has been examined optically. The test asperity was first normally pressed against a smooth flat surface of soda-lime glass and the contact observed through the glass surface. In the presence of the normal load the asperity deformed plastically. When a tangential force was applied, the size of the real area of contact between the two solids, as measured in situ, was found to increase for all the asperities used. Depending upon the geometry and material of the test asperity, the increase in the size of the contact area was up to 40%. The behaviour was compared with an analytical expression for junction growth in the case of a right circular cylindrical asperity. The closest agreement between the measurements and the theory was found for the 60° conical asperities of work-hardened copper and for nylon spheres. The sliding of the asperity on the glass plate caused transfer of the material of the former to the latter. This occurred for all the asperities used. Moreover, the sliding of the metallic asperities resulted in up to ∼5 μm deep grooves on them as well as on the glass plate. These observations have also been briefly discussed.

Mechanism of interaction between electric arc and breaking chamber in low voltage current limiting circuit breakers

Abstract: The authors survey different types of contact arrangements with simple, double, and quadruple arc interruption in current-limiting, low-voltage, protective switching devices. The influence of the arc commutation from moving contact arm to commutation bar is discussed. A rapid electromagnet with a rotating armature is introduced. The performance of the electromagnet has been investigated by the finite-element method. The same method is used to study the configuration of the arcing chamber and the influence of different parameters on the contact repulsion forces. The experiments have been carried out using a capacitor bank. It is shown that the motion of the contact arm and the magnetic blast of the arc, its elongation, and its commutation have to be coordinated in such a way that the device functions over the entire fault current region. A sophisticated breaking chamber is needed to obtain a high short-circuit interrupting capacity within a given geometry. A good arcing process is the one that displaces the 'thermal conflict' between the arc and the other elements of the chamber from the contact area to the deionization plate package. >

Bonding and Debonding Processes in Tack of Pressure-Sensitive Adhesives

Abstract: Rolling friction coefficient of two acrylic pressure sensitive adhesives (PSAs) are measured as a function of velocity at several temperatures. It is found that the time (or velocity)-temperature superposition procedure is applicable for one PSA, while it is not for the other. The authors came to the conclusion that the rate of increase of the true contact area is different for the two adhesives. The time-temperature superposition is possible only in the case where the activation energy of the bonding process is equal to that of the debonding process.