Showing papers on "Contact area published in 2007"
TL;DR: It is argued using experimental data that contact lines and not contact areas are important in determining wettability and that Wenzel's and Cassie's equations are valid only to the extent that the structure of the contact area reflects the ground state energies of contact Lines and the transition states between them.
Abstract: We argue using experimental data that contact lines and not contact areas are important in determining wettability. Three types of two-component surfaces were prepared that contain "spots" in a surrounding field: a hydrophilic spot in a hydrophobic field, a rough spot in a smooth field, and a smooth spot in a rough field. Water contact angles were measured within the spots and with the spot confined to within the contact line of the sessile drop. Spot diameter and contact line diameter were varied. All of the data indicate that contact angle behavior (advancing, receding, and hysteresis) is determined by interactions of the liquid and the solid at the three-phase contact line alone and that the interfacial area within the contact perimeter is irrelevant. The point is made that Wenzel's and Cassie's equations are valid only to the extent that the structure of the contact area reflects the ground state energies of contact lines and the transition states between them.
725 citations
Patent•
18 Sep 2007
TL;DR: In this paper, a method and an apparatus for manufacturing a memory cell having a nonvolatile resistive memory element with a limited size active area is presented, which comprises a first step of providing a dielectric volume and forming a plug opening within the dielectrics volume.
Abstract: A method and an apparatus for manufacturing a memory cell having a non-volatile resistive memory element with a limited size active area. The method comprises a first step of providing a dielectric volume and forming a plug opening within the dielectric volume. A recessed plug of a conductive material is then formed within a lower portion of the opening and a dielectric spacer is formed along the sidewalls of an upper portion of the opening. The spacer is cylindrical and has a central hole. A contact plug is subsequently formed within the central hole, the contact plug electrically coupled to the recessed plug. The contact plug can include a memory element or an additional memory element can be applied over the contact plug.
373 citations
TL;DR: The ability of the interface to support high pressure requires high asperity density and size, so it is in conflict with the requirement of low fractional solid-liquid contact area for low CAH and slip length.
248 citations
TL;DR: In this paper, the physical processes near a moving contact line are investigated systematically using molecular dynamics and continuum mechanics, and it is found that in the flow regime studied, the deviation of the wall contact angle from the equilibrium contact angle is proportional to the velocity of the contact line.
Abstract: The physical processes near a moving contact line are investigated systematically using molecular dynamics and continuum mechanics. Constitutive relations for the friction force in the contact line region, the fluid-fluid interfacial force, and the stresses in the fluid-solid interfacial region are studied. Verification of force balance demonstrates the importance of the normal stress jump across the contact line region. Effective boundary conditions are derived using force balance. It is found that in the flow regime studied, the deviation of the wall contact angle from the equilibrium contact angle is proportional to the velocity of the contact line. The effective continuum model is solved numerically and the behavior of the apparent contact angle and the wall contact angle is studied. It is found that the fluid-fluid interface near the wall exhibits a universal behavior. The onset of the nonlinear response for the contact line motion is studied within the framework of Blake’s molecular kinetic theory.
245 citations
TL;DR: In this article, the effects of cutting-edge radius on residual stresses (R.S) were investigated in orthogonal dry cutting austenitic stainless steel AISI 316L with continuous chip formation.
Abstract: Tool-edge geometry has significant effects on the cutting process, as it affects cutting forces, stresses, temperatures, deformation zone, and surface integrity. An Arbitrary-Lagrangian–Eulerian (A.L.E.) finite element model is presented here to simulate the effects of cutting-edge radius on residual stresses (R.S.) when orthogonal dry cutting austenitic stainless steel AISI 316L with continuous chip formation. Four radii were simulated starting with a sharp edge, with a finite radius, and up to a value equal to the uncut chip thickness. Residual stress profiles started with surface tensile stresses then turned to be compressive at about 140 μm from the surface; the same trend was found experimentally. Larger edge radius induced higher R.S. in both the tensile and compressive regions, while it had almost no effect on the thickness of tensile layer and pushed the maximum compressive stresses deeper into the workpiece. A stagnation zone was clearly observed when using non-sharp tools and its size increased with edge radius. The distance between the stagnation-zone tip and the machined surface increased with edge radius, which explained the increase in material plastic deformation, and compressive R.S. when using larger edge radius. Workpiece temperatures increased with edge radius; this is attributed to the increase in friction heat generation as the contact area between the tool edge and workpiece increases. Consequently, higher tensile R.S. were induced in the near-surface layer. The low thermal conductivity of AISI 316L restricted the effect of friction heat to the near-surface layer; therefore, the thickness of tensile layer was not affected.
206 citations
TL;DR: In this paper, a multi-layered model for sliding frictional contact analysis of functionally graded materials with arbitrarily varying shear modulus under plane strain-state deformation has been developed.
Abstract: A multi-layered model for sliding frictional contact analysis of functionally graded materials (FGMs) with arbitrarily varying shear modulus under plane strain-state deformation has been developed. Based on the fact that an arbitrary curve can be approached by a series of continuous but piecewise linear curves, the FGM is divided into several sub-layers and in each sub-layers the shear modulus is assumed to be a linear function while the Poisson's ratio is assumed to be a constant. In the contact area, it is assumed that the friction is one of Coulomb type. With this model the fundamental solutions for concentrated forces acting perpendicular and parallel to the FGMs layer surface are obtained. Then the sliding frictional contact problem of a functionally graded coated half-space is investigated. The transfer matrix method and Fourier integral transform technique are employed to cast the problem to a Cauchy singular integral equation. The contact stresses and contact area are calculated for various moving stamps by solving the equations numerically. The results show that appropriate gradual variation of the shear modulus can significantly alter the stresses in the contact zone.
188 citations
TL;DR: The spatial and temporal behavior of the true contact area A along a rough spatially extended interface between two blocks in frictional contact is measured to show that when frictional sliding occurs, the initially uniform contact area along the interface has already evolved to one that is highly nonuniform in space.
Abstract: We measure the spatial and temporal behavior of the true contact area A along a rough spatially extended interface between two blocks in frictional contact. Upon the application of shear the onset of motion is preceded by a discrete sequence of cracklike precursors, which are initiated at shear levels that are well below the threshold for static friction. These precursors arrest well before traversing the entire interface. They systematically increase in length with the applied shear force and significantly redistribute the true contact area along the interface. Thus, when frictional sliding occurs, the initially uniform contact area along the interface has already evolved to one that is highly nonuniform in space.
180 citations
TL;DR: In this paper, the effectiveness of polycrystalline diamond inserts (PCD) has been compared to that of uncoated tungsten carbide-cobalt inserts in machining titanium alloy Ti-6Al-4V, with respect to the applicable cutting speed ranges, metal removal per tool life and tool wear rates, tool wear morphology, surface finish, chip segmentation and chatter phenomena.
167 citations
TL;DR: In this paper, an approximate model for the contact problem of cylindrical joints with clearances is developed through modeling the pin as a rigid wedge and the elastic plate as a simple Winkler elastic foundation.
142 citations
TL;DR: In this article, the Fourier integral transform method is adopted to derive the general solution for the contact problem under pressure, and the results indicate some interesting characteristics in contact mechanics, which are distinctly different from those in classical elasticity theory.
Abstract: Based on the surface elasticity theory, we examined the effects of surface stresses on nanosized contact problems. The Fourier integral transform method is adopted to derive the general solution for the contact problem under pressure. As two examples, the deformations induced, respectively, by a uniform distributed pressure and a concentrated force are analyzed in detail. The results indicate some interesting characteristics in contact mechanics, which are distinctly different from those in classical elasticity theory. Both the contact normal stress and the deformation gradient on the deformed surface vary smoothly across the loading boundary as a result of surface stress. In addition, the indent depth and the maximum normal contact stress depend strongly on the surface stress for nanoindentation.
140 citations
TL;DR: In this paper, the authors investigate the assumption that knowing the height auto-correlation function of two solids in contact along with their materials properties is sufficient to predict the contact pressure distribution P(p).
Abstract: It is commonly assumed that knowing the height auto-correlation function of two solids in contact along with their materials properties is sufficient to predict the contact pressure distribution P(p). We investigate this assumption with contact mechanics calculations that are based on quickly converging Green's function molecular dynamics. In our simulations, elastically deformable solids are pressed against a rigid substrate. Their profile is either given by experimental data or produced with random numbers such that the artificially generated height spectra ressemble that of the real profiles. Randomly rough surfaces produce Gaussian tails in the P(p)'s, while they are exponential for experimentally determined topographies. This difference, however, does not affect significantly the true contact area, which, for the given real profile is about 20% larger than that of the random surface. Both surfaces obey Persson's contact mechanics theory reasonably well.
TL;DR: In this paper, a revised elastic-plastic contact model of a single fractal asperity has been proposed, showing that the critical contact area is scale dependent, contrary to the MB model prediction.
TL;DR: Both the AFM and MD results show that nanoscale tribological behavior deviates dramatically from the established macroscopic behavior of diamond, which is highly dependent on orientation.
Abstract: Atomic force microscopy (AFM) experiments and molecular dynamics (MD) simulations were conducted to examine single-asperity friction as a function of load, surface orientation, and sliding direction on individual crystalline grains of diamond in the wearless regime. Experimental and simulation conditions were designed to correspond as closely as state-of-the-art techniques allow. Both hydrogen-terminated diamond (111)(1 x 1)-H and the dimer row-reconstructed diamond (001)(2 x 1)-H surfaces were examined. The MD simulations used H-terminated diamond tips with both flat- and curved-end geometries, and the AFM experiments used two spherical, hydrogenated amorphous carbon tips. The AFM measurements showed higher adhesion and friction forces for (001) vs (111) surfaces. However, the increased friction forces can be entirely attributed to increased contact area induced by higher adhesion. Thus, no difference in the intrinsic resistance to friction (i.e., in the interfacial shear strength) is observed. Similarly, the MD results show no significant difference in friction between the two diamond surfaces, except for the specific case of sliding at high pressures along the dimer row direction on the (001) surface. The origin of this effect is discussed. The experimentally observed dependence of friction on load fits closely with the continuum Maugis-Dugdale model for contact area, consistent with the occurrence of single-asperity interfacial friction (friction proportional to contact area with a constant shear strength). In contrast, the simulations showed a nearly linear dependence of the friction on load. This difference may arise from the limits of applicability of continuum mechanics at small scales, because the contact areas in the MD simulations are significantly smaller than the AFM experiments. Regardless of scale, both the AFM and MD results show that nanoscale tribological behavior deviates dramatically from the established macroscopic behavior of diamond, which is highly dependent on orientation.
TL;DR: In this study, charge transfer due to a single impact of a particle against a stainless steel target was measured for alpha-lactose monohydrate, aspirin, sugar granules and ethylcellulose and it is found that the transferred charge is a linear function of the contact area.
TL;DR: In this paper, a thermocouple method with an exposed hot junction configuration was used for interface temperature measurement, and the magnitude and distribution of the friction interface temperature were investigated in a designed experiment approach, including the number of braking applications, sliding speed, braking load and type of friction material.
TL;DR: The detailed experiments and analyses have indicated that the introduction of the rugged nanoprotrusions on the surface of microstripes provided ideal 3D roughness, which could not only enhance the apparent contact angles close to 180 degrees by the "point" contact fashion but also make droplets easily roll off the surface without apparent CA hysteresis by regulating the triple-phase contact line (TCL) to become extremely discrete.
Abstract: A facile laser-etching method was used for the one-step creation of various controllable dimensions of anisotropic micropatterns consisting of an alternating arrangement of microgrooves and microstripes with rugged nanoprotrusions, which after modified with fluoroalkylsilane reagent, showed perfect isotropic superhydrophobicity without apparent CA hystereses, water adhesion, and drag resistance, other than the conventional view of anisotropic surface microstructures with anisotropic surface dewetting. The detailed experiments and analyses have indicated that the introduction of the rugged nanoprotrusions on the surface of microstripes provided ideal 3D roughness, which could not only enhance the apparent contact angles close to 180° by the “point” contact fashion to maximally reduce the liquid−solid contact area but, most importantly, make droplets easily roll off the surface without apparent CA hysteresis by regulating the triple-phase contact line (TCL) to become extremely discrete. These findings would...
TL;DR: In this article, the electro-mechanical response of radio frequency (RF) microelectromechanical (MEM) switches due to variations in surface roughness and finite asperity deformations was analyzed.
Abstract: Modeling predictions and experimental measurements were obtained to characterize the electro-mechanical response of radio frequency (RF) microelectromechanical (MEM) switches due to variations in surface roughness and finite asperity deformations. Three-dimensional surface roughness profiles were generated, based on a Weierstrass–Mandelbrot fractal representation, to match the measured roughness characteristics of contact bumps of manufactured RF MEMS switches. Contact asperity deformations due to applied contact pressures were then obtained by a creep constitutive formulation. The contact pressure is derived from the interrelated effects of roughness characteristics, material hardening and softening, temperature increases due to Joule heating and contact forces. This modeling framework was used to understand how contact resistance evolves due to changes in the real contact area, the number of asperities in contact, and the temperature and resistivity profiles at the contact points. The numerical predictions were qualitatively consistent with the experimental measurements and observations of how contact resistance evolves as a function of deformation time history. This study provides a framework that is based on integrated modeling and experimental measurements, which can be used in the design of reliable RF MEMS devices with extended life cycles.
TL;DR: In this paper, a validation study was conducted to determine the extent to which computational ankle contact finite element (FE) results agreed with experimentally measured tibio-talar contact stress.
TL;DR: Following the minimum free energy pathway associated with the imbibition process, a generalized expression for the apparent contact angle on a textured surface and the liquid-gas contact area within the groove that plays a key role is derived.
Abstract: Surface roughness is known to alter the wettability on a solid substrate. In general, either Wenzel or Cassie-Baxter theory is adopted to describe the apparent contact angle. Following the minimum free energy pathway associated with the imbibition process, we have derived a generalized expression for the apparent contact angle on a textured surface and the liquid-gas contact area within the groove that plays a key role. Depending on the geometrical characteristics of the grooves, the surface wetting falls into three regimes: (i) single stable state which is either Wenzel (completely wetted roughness) or Cassie-Baxter (completely nonwetted roughness) state, (ii) two stable states (Wenzel and Cassie-Baxter) separated by an energy barrier, and (iii) single stable state with partially wetted roughness. The sufficient condition for each regime is derived and several groove geometries are given to show the free energy path. Alteration in the geometric parameters may lead to the wetting crossover. We also show that the Cassie-Baxter can occur at a hydrophilic surface for particular pore shapes.
TL;DR: In this article, the effect of surface roughness on nanoindentation results was investigated instancing a series of CrN thin films deposited by unbalanced magnetron sputtering and was measured by atomic force microscopy.
Abstract: The effect of the surface roughness on nanoindentation results was investigated instancing a series of CrN thin films deposited by unbalanced magnetron sputtering. The arithmetic roughness (Ra) of the films ranged between 2 and 10 nm and was measured by atomic force microscopy. The measured surface topography was incorporated into a finite element model, which allowed simulating the indentation of an axisymmetric sample by a rigid spherical indenter. For the applied conditions it was found that plastic deformation could be neglected and thus purely elastic material behavior was assumed. For roughness values of Ra ≈ 2, 5, and 10 nm, 100 indents each were simulated. Subsequently, the software Elastica and the approach by Oliver and Pharr were used to evaluate Young's modulus of the CrN thin films from the simulated load-displacement curves. Under the applied conditions, the increasing roughness causes a reduction of the contact area and leads to an underestimation of Young's modulus. The mean Young's modulus of all simulated indents on the rough surfaces lies 5–14% below the Young's modulus determined for a perfectly smooth surface. This deviation seems to be independent of Ra, although the data scatter increases significantly with increasing roughness. Additionally, an influence of the lateral extension of the surface texture on the data scatter was observed which is not accounted for in roughness measures such as Ra.
TL;DR: In this paper, a non-adhesive, frictionless elastic/elastic-plastic contact between a rigid flat plane and a self-affine fractal rough surface using the commercial finite element package ANSYS is considered.
Abstract: The present study considers finite element analysis of non-adhesive, frictionless elastic/elastic–plastic contact between a rigid flat plane and a self-affine fractal rough surface using the commercial finite element package ANSYS. Three-dimensional rough surfaces are generated using a modified two-variable Weierstrass–Mandelbrot function with given fractal parameters. Parametric studies are done to consider the general relations between contact properties and key material and surface parameters. The present analysis is validated with available experimental results in the literature. Non-dimensional contact area and displacement are obtained as functions of non-dimensional load for varying fractal surface parameters in the case of elastic contact and for varying rates of strain hardening in the case of elastic–plastic contact of fractal surfaces.
TL;DR: In this article, a modified Young's equation is proposed to quantify the drop-sliding phenomenon, based also on the interfacial adhesion that develops across the contact area of the liquid/solid interface.
Abstract: Young's equation describes the wetting phenomenon in terms of the contact angle between a liquid and a solid surface. However, the contact angle is not the only parameter that defines liquid–solid interactions, an additional parameter related to the adhesion between the liquid drop and the solid surface is also of importance in cases where liquid sliding is involved. It is postulated that wetting which is related to the contact angle, and interfacial adhesion, which is related to the sliding angle, are interdependent phenomena and have to be considered simultaneously. A variety of models that relate the sliding angle to the forces developed along the contact periphery between a liquid drop and a solid surface have been proposed in the literature. Here, a modified model is proposed that quantifies the drop-sliding phenomenon, based also on the interfacial adhesion that develops across the contact area of the liquid/solid interface. Consequently, an interfacial adhesion strength parameter can be defined dep...
01 Aug 2007
TL;DR: In this article, the authors examined extant knowledge of how objects are gripped for manipulation, and the relationship, the coefficient of friction, between finger and object, has on various grip parameters.
Abstract: Gripping is an everyday task which is taken for granted by many. The current paper examines extant knowledge of howobjects are gripped for manipulation, and the relationship, the coefficient of friction, between finger and object, has on various grip parameters. It is found that friction is an essential part of the feedback and feedforward control system for grip. The friction of the finger-object contact can be measured in several different ways, using methods of either a probe moving on a finger or a finger moving on a flat surface. These friction measurements can then be used to gain information about the effect of normal force, sliding speed, contact area, and level of moisture present. They also can provide information about the changes between test subjects, for example, the effects of age, gender, and race. Knowing the effect of these parameters can help to improve the manoeuvrability of everyday items through inclusive design; designing products to be used by the whole population regardles...
TL;DR: Friction force measurements have been conducted with a colloid probe on mica and silica (both hydrophilic and hydrophobized) after long exposure to high humidity, leading to a large hysteresis between loading and unloading in the friction measurements with separation occurring at large negative applied loads.
Abstract: Friction force measurements have been conducted with a colloid probe on mica and silica (both hydrophilic and hydrophobized) after long (24 h) exposure to high-humidity air. Adhesion and friction measurements have also been performed on cellulose substrates. The long exposure to high humidity led to a large hysteresis between loading and unloading in the friction measurements with separation occurring at large negative applied loads. The large hysteresis in the friction-load relationship is attributed to a contact area hysteresis of the capillary condensate which built up during loading and did not evaporate during the unloading regime. The magnitude of the friction force varied dramatically between substrates and was lowest on the mica substrate and highest on the hydrophilic silica substrate, with the hydrophobized silica and cellulose being intermediate. The adhesion due to capillary forces on cellulose was small compared to that on the other substrates, due to the greater roughness of these surfaces.
TL;DR: In this article, the authors used the extended finite element method (XFEM) to model the presence of discontinuities caused by frictional contact, where the special functions are included in standard FEM to simulate discontinuity without considering the boundary conditions in meshing the domain.
TL;DR: In this article, a new design concept of tribologically active coatings aimed for low friction applications, have been explored through deposition of carbide and nanocomposite coatings by DC-magnetron sputtering.
Abstract: In this thesis a new design concept of tribologically active coatings aimed for low friction applications, have been explored. Materials modeled by ab initio DFT calculations were realized through deposition of carbide and nanocomposite coatings by DC-magnetron sputtering. The design concept employs destabilization of a carbide material by alloying with a weak carbide-forming element, which refines the structure into a nanocomposite. The destabilization creates a driving force for superficial ejection of carbon in a tribological contact, forming a lubricious graphitic carbon layer. The otherwise hard material limits the real contact area and the transformed layer accounts for low shear resistance. Hence, the ideal situation for low friction is provided by formation of an easily sheared thin surface layer on a hard material. TiAlC was chosen as a model system for the theoretical modeling as well as for the depositions. The elemental composition, microstructure and mechanical properties of the coatings were characterized to relate the inherent properties to the experimentally achieved tribological response. As predicted by theory, TiAlC coatings were shown to provide self-lubrication on the atomic scale by giving low friction through a tribologically induced surface restructuring. It was shown possible to reduce the friction coefficient from 0.35 for TiC to 0.05 by addition of Al. Alloying with Al also proved to be a potent method in tailoring residual stresses from high and often detrimental levels to acceptable levels, with no significant reduction in either hardness or Young’s modulus. The effect of adding Al into TiC on the oxidation resistance was also explored. The critical temperature for onset of oxidation proved to increase with the Al-content from about 350°C for TiC to about 450°C for TiAlC with about 7 at% Al. A further increase in Al content did not change the onset temperature further but reduced the oxidation rate.
TL;DR: In this paper, the effect of the initial roughness of a workpiece on the smoothing-burnishing process is considered and an approximation of the optimum burnishing force with respect to the geometry of the contact area between the tool and the workpiece is given.
Abstract: The effect of a smoothing-burnishing process strongly depends on the initial roughness of a workpiece. This factor has not been considered by existing classical models of the processes. In this paper, assuming a model of burnishing with a spherical tool, in the form of wedges of surface roughness deformed with a force normal to the base line, expressions describing the relation between burnishing force and displacement of the tops of surface asperities is derived. The expression includes the effect of mechanical properties of the workpiece material, geometry of contact of the tool with the workpiece and the roughness of the burnished surface. Using the derived expressions it is possible to determine an optimum burnishing force. This has been verified experimentally. The experiment made it possible to demonstrate that the optimum burnishing force of the ground 42CrMo4 steel samples was 11–15 daN and that the burnishing effect depends a lot not only on the mechanical properties of the machined workpiece and the geometry of the contact area between the tool and the workpiece but also on the initial surface roughness. The applied optimum burnishing force, calculated on the basis of the theoretical, assumed model-derived dependences, is 12–13 daN. The above proves the validity of the adopted assumptions and the formulas worked out.
TL;DR: In this article, friction and adhesion measurements between surfaces of cross-linked, stiff polymers of varying roughness against smooth, bare mica surfaces were carried out in dry air as well as in the presence of lubricating oil.
Abstract: Friction and adhesion measurements between surfaces of cross-linked, stiff polymers of varying roughness against smooth, bare mica surfaces were carried out in dry air as well as in the presence of lubricating oil. The nominal (macroscopic) contact area varies with the applied load according to the Johnson, Kendall and Roberts (JKR) theory, yet shows significant hysteresis due to the irreversibility arising from the loading/unloading curves of multiple asperities. Upon introducing the oil between the surfaces, the critical shear stress is reduced to zero due to the elimination of the adhesion force. However, the effect is less noticeable on the friction coefficient. Lastly, the effect of increasing the (RMS) roughness was greatest over the first few nanometers due to the diminution of the adhesion-controlled contribution to the friction, after which a further increase in roughness had less dramatic effects. A model is presented to account for the observed adhesion hysteresis during repeated loading/unloading cycles of purely elastically deforming rough surfaces.
TL;DR: In this paper, an experimental set-up, made of two coaxial rings in relative motion, a sapphire and steel, enabled temperature measurements on both sides of the third body at the friction interface.
TL;DR: This method of thresholding out the sensels with the lowest signal intensity values may not work for all surfaces and indenter shapes, but provides a new approach to improve the accuracy of contact area measurements collected with the Tekscan system.