Showing papers on "Contact area published in 2010"
TL;DR: In this article, a review of the literature describing the packing structure and effective thermal conductivity of randomly packed beds consisting of mono-sized particles is presented, with particular attention paid to the packing structures (porosity, coordination number, and contact angles) and heat transfer.
332 citations
TL;DR: This work shows how frictional strength evolves from the short times and rapid slip velocities at the onset of motion to ageing at the long times following slip arrest, and shows how the singular logarithmic behaviour generally associated with ageing is cut off at short times.
Abstract: The behaviour of systems as diverse as earthquakes and hard drives is influenced by frictional motion and its strength. What at first glance appears to be a continuous sliding process between touching surfaces is in fact a product of a series of 'slip' and 'stick' events on the microscopic scale. The mechanism of evolution of frictional strength at this level, though, is still unclear. Ben-David et al. have studied the evolution of the local contact area between two sliding bodies (PMMA plastic blocks) and the motion of their interface, and find that it involves four distinct phases. Within microseconds, all the contact area reduction has occurred. This is followed by a rapid slip phase, then a sharp transition to much slower slippage culminating in a 'stick' phase when motion is arrested. After several hundred microseconds the contact area begins to increase again. These results provide a basis for a better understanding of this kind of motion in many technologically important contexts. From earthquakes to hard drives, frictional motion and its strength are involved in a wide range of phenomena. The strength of an interface that divides two sliding bodies is determined by both the real contact area and the contacts' shear strength. By continuous measurements of the concurrent local evolution of the real contact area and the corresponding interface motion from the first microseconds when contact detachment occurs, frictional strength is now characterized from short to long timescales. The evolution of frictional strength has great fundamental and practical importance. Applications range from earthquake dynamics1,2,3,4 to hard-drive read/write cycles5. Frictional strength is governed by the resistance to shear of the large ensemble of discrete contacts that forms the interface that separates two sliding bodies. An interface’s overall strength is determined by both the real contact area and the contacts’ shear strength6,7. Whereas the average motion of large, slowly sliding bodies is well-described by empirical friction laws3,8,9,10, interface strength is a dynamic entity that is inherently related to both fast processes such as detachment/re-attachment11,12,13,14 and the slow process of contact area rejuvenation6,7,13,15,16. Here we show how frictional strength evolves from extremely short to long timescales, by continuous measurements of the concurrent local evolution of the real contact area and the corresponding interface motion (slip) from the first microseconds when contact detachment occurs to large (100-second) timescales. We identify four distinct and inter-related phases of evolution. First, all of the local contact area reduction occurs within a few microseconds, on the passage of a crack-like front. This is followed by the onset of rapid slip over a characteristic time, the value of which suggests a fracture-induced reduction of contact strength before any slip occurs. This rapid slip phase culminates with a sharp transition to slip at velocities an order of magnitude slower. At slip arrest, ‘ageing’ immediately commences as contact area increases on a characteristic timescale determined by the system’s local memory of its effective contact time before slip arrest. We show how the singular logarithmic behaviour generally associated with ageing is cut off at short times16. These results provide a comprehensive picture of how frictional strength evolves from the short times and rapid slip velocities at the onset of motion to ageing at the long times following slip arrest.
248 citations
TL;DR: The batch fabrication of ultrasharp diamond-like carbon tips that contain significant amounts of silicon on silicon microcantilevers for use in atomic force microscopy are demonstrated and it is found that the classical wear law of Archard does not hold at the nanoscale.
Abstract: Understanding friction1,2,3,4 and wear5,6,7,8,9,10,11 at the nanoscale is important for many applications that involve nanoscale components sliding on a surface, such as nanolithography, nanometrology and nanomanufacturing Defects, cracks and other phenomena that influence material strength and wear at macroscopic scales are less important at the nanoscale, which is why nanowires can, for example, show higher strengths than bulk samples12 The contact area between the materials must also be described differently at the nanoscale13 Diamond-like carbon is routinely used as a surface coating in applications that require low friction and wear because it is resistant to wear at the macroscale14,15,16,17,18,19,20, but there has been considerable debate about the wear mechanisms of diamond-like carbon at the nanoscale because it is difficult to fabricate diamond-like carbon structures with nanoscale fidelity Here, we demonstrate the batch fabrication of ultrasharp diamond-like carbon tips that contain significant amounts of silicon on silicon microcantilevers for use in atomic force microscopy This material is known to possess low friction in humid conditions, and we find that, at the nanoscale, it is three orders of magnitude more wear-resistant than silicon under ambient conditions A wear rate of one atom per micrometre of sliding on SiO2 is demonstrated We find that the classical wear law of Archard21 does not hold at the nanoscale; instead, atom-by-atom attrition7,8 dominates the wear mechanisms at these length scales We estimate that the effective energy barrier for the removal of a single atom is ∼1 eV, with an effective activation volume of ∼1 × 10−28 m3 Ultrasharp scanning probe microscope tips made of diamond-like carbon that contains silicon and oxygen demonstrate very high levels of wear resistance
215 citations
TL;DR: Using friction force microscopy, Wang et al. as discussed by the authors have investigated the frictional behavior of graphene deposited on various substrates as well as over micro-fabricated wells.
Abstract: Using friction force microscopy, we have investigated the frictional behavior of graphene deposited on various substrates as well as over micro-fabricated wells. Both graphene on SiO 2 / Si substrates and graphene freely suspended over the wells showed a trend of increasing friction with decreasing number of atomic layers of graphene. However, this trend with thickness was absent for graphene deposited on mica, where the graphene is strongly bonded to the substrate. Measurements together with a mechanics model suggest that mechanical confinement to the substrate plays an important role in the frictional behavior of these atomically thin graphite sheets. Loosely bound or suspended graphene sheets can pucker in the out-of-plane direction due to tip-graphene adhesion. This increases contact area, and also allows further defonnation of the graphene when sliding, leading to higher friction. Since thinner samples have lower bending stiffness, the puckering effect and frictional resistance are greater. However, if the graphene is strongly bound to the substrate, the puckering effect will be suppressed and no thickness dependence should be observed. The results can provide potentially useful guidelines in the rational design and use of graphene for nano-mechanical applications, including nano-lubricants and components in micro- and nanodevices.
215 citations
TL;DR: The results demonstrate that simplifications to the geometry of the bone-cartilage interface, cartilage surface and bone material properties can have a dramatic effect on the predicted magnitude and distribution of cartilage contact pressures in the hip joint.
187 citations
TL;DR: In this article, the experimental measurements of contact thermal resistance between individual carbon nanotubes were performed and it was shown that the contact thermal conductance can increase by nearly two orders of magnitude (from 10−8 to 10−6 W/K) as the contact area increases from a cross contact to an aligned contact.
Abstract: We report on experimental measurements of contact thermal resistance between individual carbon nanotubes. Results indicate that the contact thermal conductance can increase by nearly two orders of magnitude (from 10−8 to 10−6 W/K) as the contact area increases from a cross contact to an aligned contact. Normalization with respect to the contact area leads to normalized contact thermal resistance on the order of 10−9 m2 K/W at room temperature, one order of magnitude lower than that from a molecular dynamics simulation in literature. These results should have important implications in the design of carbon nanotube-polymer composites for tunable thermal properties.
158 citations
TL;DR: In this paper, the effect of galvanic coupling between areas without and covered with surface films, on the open-circuit potential is analyzed in terms of Hertzian contact area, and a new approach of the concept of synergism in tribocorrosion is presented and validated for AISI 316 immersed in 0.5 M H2SO4 under sliding against zirconia.
113 citations
TL;DR: The mechanics of wet adhesion between a water strider's legs and a water surface were studied and it was showed that the nanoscale to microscale hierarchical surface structure on striders' legs is crucial to the stable water-repellent properties of the legs.
Abstract: The mechanics of wet adhesion between a water strider's legs and a water surface was studied. First, we showed that the nanoscale to microscale hierarchical surface structure on striders' legs is crucial to the stable water-repellent properties of the legs. The smallest structure is made for the sake of a stable Cassie state even under harsh environment conditions, which sets an upper limit for the dimension of the smallest structure. The maximum stress and the maximum deformation of the surface structures at the contact line are size-dependent because of the asymmetric surface tension, which sets a lower limit for the dimension of the smallest structure. The surface hierarchy can largely reduce the adhesion between the water and the legs by stabilizing the Cassie state, increasing the apparent contact angle, and reducing the contact area and the length of the contact line. Second, the processes of the legs pressing on and detaching from the water surface were analyzed with a 2D model. We found that the superhydrophobicity of the legs' surface is critically important to reducing the detaching force and detaching energy. Finally, the dynamic process of the legs striking the water surface, mimicking the maneuvering of water striders, was analyzed. We found that the large length of the legs not only reduces the energy dissipation in the quasi-static pressing and pulling processes but also enhances the efficiency of energy transfer from bioenergy to kinetic energy in the dynamic process during the maneuvering of the water striders. The mechanical principles found in this study may provide useful guidelines for the design of superior water-repellent surfaces and novel aquatic robots.
100 citations
TL;DR: In this article, a systematic examination of the oblique impacting of copper particles in cold spraying was conducted by using the smoothed particle hydrodynamics (SPH) method compared to the Lagrangian method.
99 citations
Patent•
TSMC1
TL;DR: In this paper, a phase change memory structure and method for forming the same, including providing a substrate comprising a conductive area, forming a spacer having a partially exposed sidewall region at an upper portion of the spacer defining a phase-change memory element contact area.
Abstract: A phase change memory structure and method for forming the same, the method including providing a substrate comprising a conductive area; forming a spacer having a partially exposed sidewall region at an upper portion of the spacer defining a phase change memory element contact area; and, wherein the spacer bottom portion partially overlaps the conductive area. Both these two methods can reduce active area of a phase change memory element, therefore, reducing a required phase changing electrical current.
99 citations
TL;DR: In this paper, the in-vivo time-dependent contact behavior of tibiofemoral cartilage of human subjects during the first 300s after applying a constant full body weight loading and determine whether there are differences in cartilage contact responses between the medial and lateral compartments.
TL;DR: In this article, a model for the combined adhesive and tangential loading of a rigid sphere on a flat half-space was developed for the mixed-mode contact of biological materials or similar soft elastomers.
Abstract: Tangential loading in the presence of adhesion is highly relevant to biological locomotion, but mixed-mode contact of biological materials or similar soft elastomers remains to be well understood. To better capture the effects of dissipation in such contact problems owing to viscoelasticity or irreversible interfacial adhesive processes, a model is developed for the combined adhesive and tangential loading of a rigid sphere on a flat half-space which incorporates a phenomenological model of energy dissipation in the form of increased effective work of adhesion with increasing degree of mode mixity. To verify the model, contact experiments are performed on polydimethylsiloxane (PDMS) samples using a custom-built microtribometer. Measurements of contact area during mixed normal/tangential loading indicate that the strong dependence of the effective work of adhesion upon mode mixity can be captured effectively by the phenomenological model in the regime where the contact area stayed circular and the slip was negligible. Rate effects were seen to be described by a power-law dependence upon the crack front velocity, similar to observations of rate-dependent contact seen for pure normal loading.
TL;DR: In this article, the authors used stacked 3D sinusoids to represent the asperities in contact at each scale of the surface and used the distance between the two surfaces at all scales to predict the surface separation as a function of dimensionless load.
TL;DR: In this paper, the authors used a stacked asperity model to predict the contact area as a function of load, which was first introduced by Archard in the early 1990s.
Abstract: Due to the existence of multiple scales of features on surfaces, during the past two decades models have been developed that employ a fractal description of the rough surface profile. Most of these models use a truncation calculation to predict the contact area. The load corresponding to the truncated area of the fractal surface is then calculated using elastic and elastic–plastic asperity contact models. The current work instead uses a stacked asperity model that was first introduced by Archard to predict contact area as a function of load. From this the elastic contact of multiscale rough surfaces can be solved analytically, providing a closed-form solution for certain cases of surface profiles. The results of the model are then compared to a two-dimensional (2D) deterministic model of rough surface contact. Finally, the analytical relationships are used to create models for three-dimensional (3D) elastic and elastic–plastic contact of fractal/multiscale rough surfaces.
Patent•
05 Apr 2010TL;DR: In this article, a display control apparatus is provided including a detection unit for detecting contact of an operation tool with a display surface of the display unit, a contact determination unit for determining contact state of the operation tools with the display surface based on a detection result, and a contact mark display processing unit.
Abstract: A display control apparatus is provided including a detection unit for detecting contact of an operation tool with a display surface of the display unit, a contact determination unit for determining contact state of the operation tool with the display surface based on a detection result, a contact mark display processing unit for displaying on the display unit a contact mark indicating a form of a contact area of the operation tool when the operation tool is determined to be in contact with the display surface, and a screen display processing unit for causing an object, the contact mark, and the pointer displayed in a predetermined display area to be displayed near the contact area of the operation tool when a predetermined state to start a change of display of the display unit is detected in the state in which the operation tool is in contact with the display surface
TL;DR: In this article, a simple test method was applied to obtain the frictional properties of the most popular lubricants in the Taiwan area, which were used for jacking force estimation and numerical analysis of soil-pipe interaction for linear and curved pipe-jacking.
TL;DR: In this paper, the roles of temperature, adhesive interactions and atomic structure are studied for simple geometries, where an elastic, crystalline substrate contacts a rigid, atomically flat surface or a spherical tip.
Abstract: Molecular dynamics simulations are used to study different definitions of contact at the atomic scale. The roles of temperature, adhesive interactions and atomic structure are studied for simple geometries. An elastic, crystalline substrate contacts a rigid, atomically flat surface or a spherical tip. The rigid surface is formed from a commensurate or incommensurate crystal or an amorphous solid. Spherical tips are made by bending crystalline planes or removing material outside a sphere. In continuum theory the fraction of atomically flat surfaces that is in contact rises sharply from zero to unity when a load is applied. This simple behavior is surprisingly difficult to reproduce with atomic scale definitions of contact. Due to thermal fluctuations, the number of atoms making contact at any instant rises linearly with load over a wide range of loads. Pressures comparable to the ideal hardness are needed to achieve full contact at typical temperatures. A simple harmonic mean-field theory provides a quantitative description of this behavior and explains why the instantaneous forces on atoms have a universal exponential form. Contact areas are also obtained by counting the number of atoms with a time-averaged repulsive force. For adhesive interactions, the resulting area is nearly independent of temperature and averaging interval, but usually rises from zero to unity over a range of pressures that is comparable to the ideal hardness. The only exception is the case of two identical commensurate surfaces. For nonadhesive surfaces, the mean pressure is repulsive if there is any contact during the averaging interval $\Delta t$. The associated area is very sensitive to $\Delta t$ and grows monotonically. Similar complications are encountered in defining contact areas for spherical tips.
TL;DR: In this article, the authors compared the analytical models for the determination of the contact radius between a spherical indenter and an elastic-plastic material with the experimental results obtained for different spheroidized steels and those obtained numerically and with various analytical models.
TL;DR: In this article, an elastomeric colloidal probe is used to measure adhesion energies and to study other contact phenomena of surfaces, which can be accessed by using the Johnson-Kendall-Roberts (JKR) approach.
Abstract: We present a method based on colloidal probe atomic force microscopy (AFM) to measure adhesion energies and to study other contact phenomena of surfaces. The method employs an elastomeric colloidal probe, rendering the contact area between probe and sample much larger as compared to standard atomic force microscopy techniques. The technique allows us to determine the contact area via microinterferometry and measure the applied forces at the same time. The adhesion properties can then be accessed by using the Johnson–Kendall–Roberts (JKR) approach, i.e. measuring (a) the contact area as a function of applied load, and (b) the elastic parameters and the thermodynamic work of adhesion. We test this method in ambient conditions as well as in aqueous media on well-known surface chemistries, and can clearly characterize the contributions of capillary in air, hydration forces and hydrophobic interactions in water. This novel method provides a means to study the contact behavior of soft colloids and enhanced sensitivity for adhesion measurements.
TL;DR: In this paper, the authors used a harmonic mean-field theory to describe the relationship between the instantaneous and time-averaged forces on flat and spherical surfaces with different atomic geometries, adhesion, and temperatures.
Abstract: Contact area plays a central role in continuum theories of friction and adhesion, and there is growing interest in calculating it with atomic resolution. Molecular dynamics simulations are used to study definitions of contact area based on instantaneous and time-averaged forces or separations between atoms. Flat and spherical surfaces with different atomic geometries, adhesion, and temperatures are examined. In continuum theory, the fraction of two flat surfaces that is in contact rises sharply from zero to unity when a load is applied. This simple behavior is surprisingly difficult to reproduce with atomic scale definitions of contact. At typical temperatures, nonadhesive surfaces are held apart by a small fraction of atoms with large thermal fluctuations until the normal pressure is comparable to the ideal hardness. The contact area associated with atoms interacting at any instant rises linearly with load. Time averaging produces a monotonic increase in area with time interval that only converges to the sharp rise in continuum models for the special case of identical crystal surfaces. Except in this special case, the time-averaged contact area between adhesive surfaces also rises to full contact over a range of pressures comparable to the ideal hardness. Similar complications are encountered in defining contact areas for spherical tips. The fraction of atoms in contact rises linearly with local pressure, and the contact area based on time-averaged forces does not fit continuum theory. A simple harmonic mean-field theory provides a quantitative description of the simulation results and explains why the instantaneous forces on atoms are observed to have a universal exponential form. The results imply that continuum models of contact only apply to forces averaged over areas containing many atoms.
01 Apr 2010
TL;DR: The time-dependent response of cartilage contact behavior under constant full body weight loading was significantly different in the medial and lateral tibiofemoral compartments, with greater peakcartilage contact deformation on the lateral side and greater contact area on the medial side.
Abstract: Summary Objective The purpose of this study was to investigate the in-vivo time-dependent contact behavior of tibiofemoral cartilage of human subjects during the first 300s after applying a constant full body weight loading and determine whether there are differences in cartilage contact responses between the medial and lateral compartments. Design Six healthy knees were investigated in this study. Each knee joint was subjected to full body weight loading and the in-vivo positions of the knee were captured by two orthogonal fluoroscopes during the first 300s after applying the load. Three-dimensional models of the knee were created from MR images and used to reproduce the in-vivo knee positions recorded by the fluoroscopes. The time-dependent contact behavior of the cartilage was represented using the peak cartilage contact deformation and the cartilage contact area as functions of time under the constant full body weight. Results Both medial and lateral compartments showed a rapid increase in contact deformation and contact area during the first 20s of loading. After 50s of loading, the peak contact deformation values were 10.5±0.8% (medial) and 12.6±3.4% (lateral), and the contact areas were 223.9±14.8mm 2 (medial) and 123.0±22.8mm 2 (lateral). Thereafter, the peak cartilage contact deformation and contact area remained relatively constant. The respective changing rates of cartilage contact deformation were 1.4±0.9%/s (medial) and 3.1±2.5%/s (lateral); and of contact areas were 40.6±20.8mm 2 /s (medial) and 24.0±11.4mm 2 /s (lateral), at the first second of loading. Beyond 50s, both changing rates approached zero. Conclusions The peak cartilage contact deformation increased rapidly within the first 20s of loading and remained relatively constant after ∼50s of loading. The time-dependent response of cartilage contact behavior under constant full body weight loading was significantly different in the medial and lateral tibiofemoral compartments, with greater peak cartilage contact deformation on the lateral side and greater contact area on the medial side. These data can provide insight into normal in-vivo cartilage function and provide guidelines for the improvement of ex-vivo cartilage experiments and the validation of computational models that simulate human knee joint contact.
TL;DR: In this paper, a coupled experimental and computational investigation of carbon nano-onions was carried out to better elucidate the mechanisms by which these processes occur, and it was found that lubricious iron oxide nanoparticles are generated in the core of the steel contact through mechanisms that are not yet known.
Abstract: The carbon nano-onion can be considered as a new kind of interesting lubricating nanoparticle. Used as lubricant additives, carbon nano-onions lead to a strong reduction of both friction and wear, even at low temperature. To better elucidate the mechanisms by which these processes occur, coupled experimental and computational investigations are carried out. In addition, it is found that lubricious iron oxide nanoparticles are generated in the core of the steel contact through mechanisms that are not yet known. The molecular dynamics simulations of carbon onions placed between sliding diamond-like carbon surfaces at high contact pressure indicate that the lubrication mechanism of the onions is based on a coupled process of rolling and sliding inside the contact area. We conclude that most of carbon onions seem to remain intact under friction processes and do not generate graphitic planes, which is in contrast to the previously determined behavior of MoS2 fullerenes that are mainly exfoliated inside the contact area and liberate lubricating lamellar sheets of h-MoS2.
TL;DR: In this paper, the authors studied the large deformation mechanics of contact and adhesion between an inflated hyperelastic membrane and a rigid substrate and derived an exact expression for the energy release rate in terms of local variables at the contact edge, thus linking adhesion to the contact angle.
Abstract: We study the large deformation mechanics of contact and adhesion between an inflated hyperelastic membrane and a rigid substrate. The initial configuration of the membrane is flat and circular and is clamped at the edge. Two types of friction conditions between the membrane and the substrate are considered: frictionless and no-slip contact. We derive an exact expression for the energy release rate in terms of local variables at the contact edge, thus linking adhesion to the contact angle. Our model can account for the effects of fluid pressure for experiments performed in solution. We also extend our formulation to include surface tension. Numerical simulations for a neo-Hookean membrane are carried out to study the relation between applied pressure and contact area.
TL;DR: In this paper, the complete process of linear friction welding of Ti6Al2Sn4Cr6Mo in two conditions (β- and α+β-forged) was simulated using the Finite Element Method.
TL;DR: In this article, a new uniform friction model is proposed based on the assumption of open-close lubricant pockets and a function of real contact area (RAC) α RC and normal press p / σ 0 is established by introducing size/scale factor λ to describe the influence of size effects.
TL;DR: Molecular dynamics simulations are used to study contact between a rigid, nonadhesive, and spherical tip with radius of order 30 nm and a flat elastic substrate covered with a fluid monolayer of adsorbed chain molecules.
Abstract: Molecular dynamics simulations are used to study contact between a rigid, nonadhesive, and spherical tip with radius of order 30 nm and a flat elastic substrate covered with a fluid monolayer of adsorbed chain molecules. Previous studies of bare surfaces showed that the atomic scale deviations from a sphere that are present on any tip constructed from discrete atoms lead to significant deviations from continuum theory and dramatic variability in friction forces. Introducing an adsorbed monolayer leads to larger deviations from continuum theory but decreases the variations between tips with different atomic structure. Although the film is fluid, it remains in the contact and behaves qualitatively like a thin elastic coating except for certain tips at high loads. Measures of the contact area based on the moments or outer limits of the pressure distribution and on counting contacting atoms are compared. The number of tip atoms making contact during a time interval $\ensuremath{\Delta}t$ grows as a power of $\ensuremath{\Delta}t$ when the film is present and as the logarithm of $\ensuremath{\Delta}t$ for bare surfaces. Friction is measured by displacing the tip at a constant velocity or pulling the tip with a spring. Both static and kinetic friction rise linearly with load at small loads. Transitions in the state of the film lead to nonlinear behavior at large loads. The friction is less clearly correlated with contact area than load.
TL;DR: In this paper, a macroscopic finger friction device consisting of a piezoelectric force sensor was evaluated on 21 printing papers of different paper grades and grammage and found that rougher papers have a lower friction coefficient than smoother papers.
Abstract: A macroscopic finger friction device consisting of a piezoelectric force sensor was evaluated on 21 printing papers of different paper grades and grammage. Friction between a human finger and the 21 papers was measured and showed that measurements with the device can be used to discriminate a set of similar surfaces in terms of finger friction. When comparing the friction coefficients, the papers group according to paper grade and the emerging trend is that the rougher papers have a lower friction coefficient than smoother papers. This is interpreted in terms of a larger contact area in the latter case. Furthermore, a decrease in friction coefficient is noted for all papers on repeated stroking (15 cycles back and forth with the finger). Complementary experiments indicate that both mechanical and chemical modifications of the surface are responsible for this decrease: (1) X-ray photoelectron spectroscopy measurements show that lipid material is transferred from the finger to the paper surface, (2) repeated finger friction measurements on the same paper sample reveal that only partial recovery of the frictional behaviour occurs and (3) profilometry measurements before and after stroking indicate small topographical changes associated with repeated frictional contacts.
TL;DR: In this article, a generalized adhesive wear analysis that takes into account the effect of interfacial adhesion on the total load was developed for three-dimensional fractal surfaces in normal contact, and a wear criterion based on the critical contact area for fully-plastic deformation of the asperity contacts was used to model the removal of material from the contact interface.
TL;DR: In this article, the authors calculate contact and coefficient of friction in fully plastic contact conditions based on a good description of the shapes of contact patches instead of individual summits, and show that as the nominal contact pressure increases, the effect of surface topography becomes less significant while the impact of interfacial shear strength remains important.
TL;DR: It is shown that hysteresis can exist in the absence of these effects, and that its magnitude depends on surface roughness, and a theoretical model is developed which can be used to estimate material properties from contact experiments even when the measured P–h curves are not unique.
Abstract: In experiments that involve contact with adhesion between two surfaces, as found in atomic force microscopy or nanoindentation, two distinct contact force (P) vs. indentation-depth (h) curves are often measured depending on whether the indenter moves towards or away from the sample. The origin of this hysteresis is not well understood and is often attributed to moisture, plasticity or viscoelasticity. Here we report experiments that show that hysteresis can exist in the absence of these effects, and that its magnitude depends on surface roughness. We develop a theoretical model in which the hysteresis appears as the result of a series of surface instabilities, in which the contact area grows or recedes by a finite amount. The model can be used to estimate material properties from contact experiments even when the measured P-h curves are not unique.