Real Contact Area Research Articles

A Closure Contact Model of Self-Affine Rough Surfaces Considering Small-, Meso-, and Large-Scale Stage Without Adhesive

Contact interface is essential for the dynamic response of the bolted structures. To accurately predict the dynamic characteristics of bolted joint structures, a fractal extension of the segmented scale model, i.e., the JK model, is proposed in this paper to comprehensively analyze the dynamic contact performance of engineering surfaces and revisit the multi-scale model based on the concept of asperities. The influence of asperity geometry, dimensionless material properties, and the elastic, elastoplastic, and full plastic mechanical models of a single asperity is established considering the asperity–substrate interaction. Then, a segmented scale contact model of rough surfaces is proposed based on the island distribution function in a strict sense. The mechanical contact process of determining rough surfaces is divided into small-scale, medium-scale, and large-scale stages. Moreover, cross-scale boundary conditions, i.e., al1′, al2′, and al3′, are provided through strict mathematical deduction. The results show that the real contact area and contact stiffness are positively correlated with fractal dimension and negatively correlated with fractal roughness. On a small scale, the contact damping decreases with an increase in load. In meso-scale and large-scale stages, the contact damping increases with the load. Finally, the reliability of the proposed model is verified by setting up three groups of modal vibration experiments.

Enhancing the anti-wear performance of graphene sheets embedded carbon films through interface nanotexture fabricated on the substrate

The effect and mechanism of surface texture on friction have been extensively explored, such as storing the abrasive and reducing the real contact area. However, studying the impact of the interface texture remains crucial, especially as the texture size approaches the nanoscale. In the present work, the silicon substrate with different parameters nanotexture was fabricated using a metal-assisted chemical etching process, and the graphene sheets embedded carbon (GSEC) film was deposited on the nanotextured substrate via a low-energy electron irradiation process to produce the interface nanotextured carbon films. After the addition of the interface nanotexture, the tribological performance of carbon films exhibited significant enhancement, with the degree of improvement being influenced by the parameters of the interface nanotexture. A super-low wear rate was achieved, with a value of 6.54 ± 0.70 × 10−8 mm3/N m. The improved tribological performance is due to the enhancement of film-substrate adhesion strength facilitated by interface nanotexture, and its role in promoting the rapid formation of a stable transfer film on the counter-grinding ball. The carbon film with nanotexture displays a small size of the abrasive particles, which contributes to the stabilization of the friction process.

Effects of chemical etching on surface structure and tribological behavior of silicate substrates

Abstract This study investigated the effect of chemical etching on the surface structure and tribological behavior of silicate substrates. Silicate surfaces were etched using a mixture of nitric acid (HNO3) and ammonium bifluoride (NH4HF2) for durations ranging from 1 to 60 min. The etched surfaces were characterized using optical microscopy, scanning electron microscopy, surface profilometry, water contact angle measurements, and UV–vis spectroscopy to evaluate the changes in surface morphology, roughness, wettability, and optical properties. Tribological performance was assessed using reciprocating ball-on-plate friction tests. The results showed that increasing the etching time resulted in the formation of microscale surface features, increased surface roughness, enhanced hydrophilicity, and reduced optical transmittance. The average friction coefficient decreased with an increase in the etching time up to 30 min, beyond which a slight increase was observed. The 1-minute etched specimen exhibited the best wear resistance with the narrowest wear track and the least material removal. The improved tribological performance was attributed to the formation of a stable transfer film, reduced real contact area, and entrapment of wear debris. This study highlights the potential of chemical etching as a technique to tailor the surface structure and tribological properties of silicate materials for various applications.

Fractal contact and asperities coalescence of rock joints under normal loading: Insights from pressure-sensitive film measurement

Direct measurement of the real contact area of rock joints under normal loading is crucial for comprehending the subsurface geological processes. However, measuring this phenomenon quantitatively at site-scale or laboratory-scale is challenging. Here, we investigate the evolution mechanism of the real contact area in rock joints by conducting closure tests on artificial and saw-cut sandstone joints under normal stresses up to 50 MPa. Geometrical shapes of contact patches are quantified by the pressure-sensitive film using the adaptive threshold method. An extensive range of contact stress within contact patches is innovatively measured by integrating the results from multi-type pressure-sensitive films. Experimental results demonstrate that the real contact area increases with the increasing normal stress hyperbolically. Such a nonlinear contact evolution behavior can be attributed to the coalescence of adjacent contact patches. The fractal dimension of composite surface governs the geometrical shapes of contact patches and the distribution of contact stress. The relationship between patch areas and bearing loads follows the Hertzian theory when the patches are small, while it gradually becomes linear with the increasing patch size. A power model with exponential cut-off is proposed to predict the size distribution of contact patches. This work can provide new insights for estimating the patch-dependent seismic nucleation length and slip stability of subsurface joints.

Multiscale Parametrization Of a Friction Model For Metal Cutting Using Contact Mechanics, Atomistic Simulations, And Experiments

In this study, we developed and parametrized a friction model for finite element (FE) cutting simulations of AISI4140 steel, combining experimental data and numerical simulations at various scales. Given the severe thermomechanical loads during cutting, parametrization of friction models based on analogous experiments has been proven difficult, such that the cutting process itself is often used for calibration. Instead, our model is based on the real area of contact between rough surfaces and the stress required to shear adhesive micro contacts. We utilized microtextured cutting tools and their negative imprint on chips to orient chip and tool surfaces, enabling the determination of a combined surface roughness. This effective roughness was then applied in contact mechanics calculations using a penetration hardness model informed by indentation hardness measurements. Consistent with Bowden and Tabor theory, we observed that the fractional contact area increased linearly with the applied normal load, and the effective roughness remained insensitive to cutting fluid application. Additionally, we calculated the required shear stress as a function of normal load using DFT-based molecular dynamics simulations for a tribofilm formed at the interface, with its composition inferred from ex-situ XPS depth profiling of the cutting tools. Our friction model demonstrated good agreement with experimental results in two-dimensional FE chip forming simulations of orthogonal cutting processes, evaluated by means of cutting force, passive force, and contact length prediction. This work presents a proof of concept for a physics-based approach to calibrate constitutive models in metal cutting, potentially advancing the use of multiscale and multiphysical simulations in machining.Graphical abstract

Flexible printed circuit-based triboelectric film sensor for integrity monitoring of tensile bolted joints

This study presents an improved design of a triboelectric film sensor for integrity monitoring of tensile bolted joints, which is designed to capture the micro-scale relative motion due to the bolt’s looseness by utilizing the triboelectric effect of the polymer layer of the sensor in contact with the metal surface of the fastened objects. The key idea is twofold: First, we use the triboelectric effect between the polymer layer and the fastened object itself, instead of the triboelectric effect between two polymer layers. This allows the sensor to be a single sheet configuration instead of two-piece. The second idea is to make the sensor design fabricable as a standard flexible printed circuit. This makes it possible to produce sensors accurately and inexpensively. Experimental tests incorporating the proposed sensor into a tensile bolted joint have demonstrated that the sensor’s voltage output is inversely related to the bolt’s tightness. Additionally, a modeling study adopting Persson’s contact theory has been conducted to refine the understanding of the real contact area, triboelectric charging, and separation dynamics between the polymer and metal layers, which is crucial for the accurate modeling of sensor outputs under dynamic loading conditions. It has been concluded that the integrated mechanical and triboelectric model successfully aligns with the experimental findings, indicating the sensor’s potential for practical applications in bolt integrity monitoring.

Friction Reduction Effect Caused by Microcontact and Load Dispersion on the Moth‐Eye Structure

Friction reduction is important from the viewpoint of energy problems and other issues. Frictional forces are known to vary depending on the material property, surface texture, and measurement scale. However, the effect of submicron‐sized moth‐eye structures prepared of robust plastic deformation materials on dry friction under high‐load conditions has not been investigated in detail. To investigate this, a copper moth‐eye structure is fabricated via electroforming for experimental measurements. Results from the friction tests reveal that real contact area increase is suppressed, as the friction coefficient of the moth‐eye structure decreases exponentially with increasing load. Further friction simulation demonstrates nanoscale contact between the structure's tip and indenter, indicating that the real contact area increase requires deformation of the moth‐eye structure itself (microcontact). However, the contact pressure on the surface is reduced by dispersing the load to the sides and bottom of the moth‐eye structure. Therefore, the suppression of real contact area increase can be attributed to the deformation suppression facilitated by load dispersion. These findings expand the possibilities for friction design with surface textures because they reveal the role of robustness due to submicron‐scale surface microstructure in reducing friction between plastic deformation materials.

Experimental investigation of junction growth of rough contacts using X-ray computed tomography

The real contact area (RCA) of randomly rough contacts has received a great deal of attention because it correlates strongly with friction, lubrication, sealing, and conductivity. Simulations have revealed that the RCA associated with deterministic normal squeezing loads increases when tangential loads are also applied, in a phenomenon called junction growth. However, experimental investigations of the junction growth of randomly rough contacts are rare. Here, we used X-ray computed tomography (CT) to measure junction growth when two aluminum alloy surfaces were in contact. A high-resolution experimental setup was used to apply loads and observe contact behaviors at a resolution of 4 µm. The RCA and average contact gaps were computed using a three-dimensional (3D) geometric model constructed from gray CT images using the Otsu thresholding method. The results showed that the RCA increased as the normal load increased. The RCA increased by 22.67% after a tangential load was applied (junction growth), and the average gap decreased by 14.01% after a tangential load was applied. Thus, X-ray CT accurately measured the junction growth as a novel quantitative method.

Static and dynamic behaviours of a self-expanding nitinol stent real contact area within a PTFE catheter: A multiscale approach

Current research on self-expanding super-elastic nitinol stents is mainly focused on designing geometries suited to the human venous and arterial systems. This study specifically considers a patented one-piece double cross-sectional stent designed to address venous stenosis affecting the vena cava, iliac veins, and their bifurcations. Before its placement within the vein, numerous tribological challenges arise as the stent slides along the guide-wire (so-called catheter). These are mainly connected to the lack of knowledge related to (i) its frictional behaviour and (ii) the level of contact pressure linked to the unknown real contact area between the compressed stent and the polytetrafluoroethylene (PTFE) catheter.This paper describes an original multi-scale approach, based on the Persson’s contact theory, that allows determination of (i) the contact pressure, (ii) the evolution of the real contact area, and (iii) the shear stresses at the interface during the stent sliding. A topographical optimization criterion will finally be provided, enabling control of both friction and stick–slip phenomena at the stent-catheter interface.

Synergistic Effect of Microconvex Texture and Particle Medium on the Tribological Property of the Rubber Sliding Interface.

In this study, we examined how surface topography and particle medium interact to affect the tribological performance of rubber sliding interfaces, uncovering the mechanisms of particle lubrication under various conditions. We found that microtextured surfaces, created using a mold transfer method, modestly reduced the friction coefficient of rubber under both dry and lubricated states, primarily by altering the real contact area. Additionally, the presence of different microconvex textures on the surface topography significantly influenced rubber's tribological properties. Our three-dimensional morphological analysis revealed that microtextured rubber surfaces with higher Sa, Sku, and Sal and lower Str values consistently showed lower friction coefficients during sliding. The friction mechanism was attributed to the combined effects of the material properties, surface topography, and contact area. With the addition of a particle medium, the dry friction coefficient of the rubber interface decreased but exhibited an initial increase, followed by a decrease with increasing particle diameter. When particles were mixed with a water-based cutting fluid, the concentration, diameter, and wettability of the particles significantly impacted the tribological properties due to the synergistic effects of surface topography and particle lubrication. This work enhances our understanding of tribological control for viscoelastic materials through surface design, providing a theoretical basis for the tribological optimization of rubber surfaces.

Prediction of formation abrasiveness under the action of a PDC bit based on the fractal dimension of a rock surface

During rock drilling, a drill bit will wear as it breaks the rock. However, there is no uniform grading standard for rock abrasiveness. To solve this problem, the wear mechanisms of a polycrystalline diamond compact (PDC) bit and the formation it is drilling into are analyzed in depth, and an abrasiveness evaluation method based on the fractal dimension of the rock surface topography is established. Initially, a three-dimensional digital model is generated from a scanning electron microscope image of the rock after drilling; next, an evaluation of the irregularities on the rock surface is performed using an adapted Weierstrass–Mandelbrot (W-M) function to ascertain the fractal dimensionality. Then, the microcontact characteristics of the contact surface between the formation and the PDC bit are analyzed, and the distribution of the microconvex contact points of the two-body friction pair in a region is obtained. Because the sliding friction between the drill bit and the rock produces a large amount of heat, according to the contact area formula of the friction surface and heat conduction theory, the temperature rise and overall temperature distribution of the formation and PDC bit under the condition of sliding friction are revealed, and the real contact area between the formation and the drill bit within a certain temperature range is obtained. Finally, the evaluation index of rock abrasiveness under sliding conditions is established by adopting the wear weight loss of the rock cutting tool per unit volume as the index of rock abrasiveness, and the model is verified by a microdrilling experiment. The research in this paper is highly important for improving the rock-breaking efficiency and bit service life during drilling.

The effect of age on the attachment ability of stick insects (Phasmatodea).

Many insect species have found their way into ageing research as small and easy-to-keep model organisms. A major sign of ageing is the loss of locomotory functions due to neuronal disorders or tissue wear. Soft and pliable attachment pads on the tarsi of insects adapt to the substrate texture to maximize their real contact area and, thereby, generate attachment during locomotion. In the majority of stick insects, adhesive microstructures covering those pads support attachment. Stick insects do not molt again after reaching the imaginal stage; hence, the cuticle of their pads is subject to continuous ageing. This study aims to quantify how attachment ability changes with age in the stick insect Sungaya aeta Hennemann, 2023 and elucidate the age effects on the material and microstructure of the attachment apparatus. Attachment performance (adhesion and friction forces) on substrates with different roughnesses was compared between two different age groups, and the change of attachment performance was monitored extending over a larger time frame. Ageing effects on the morphology of the attachment pads and the autofluorescence of the cuticle were documented using light, scanning electron, and confocal laser scanning microscopy. The results show that both adhesion and friction forces decline with age. Deflation of the pads, scarring of the cuticle, and alteration of the autofluorescence, likely indicating stiffening of the cuticle, were observed to accumulate over time. This would reduce the attachment ability of the insect, as pads lose their pliant properties and cannot properly maintain sufficient contact area with the substrate.

Effect of orientation and hardness of rubber tread block on the friction coefficient under glycerol lubrication and its underlying mechanisms

This study examines the influence of orientation and hardness of rubber tread blocks on friction under glycerol lubrication, focusing on negative fluid pressure. Measurements of fluid pressure, film thickness, and friction coefficient were conducted at the contact interface. Results showed that when tread blocks aligned parallel to the sliding direction, the friction coefficient increased by 57–191 %, attributed to either reduced film thickness or enlarged real contact area from greater negative fluid pressure. Conversely, with a perpendicular orientation, decreased hardness led to increased friction, due to thinner fluid films from enhanced negative pressure.

A general contact model for rough surfaces based on the incremental concept

It is now commonly accepted that real contact area rises linearly with increasing normal load for the contact of rough solids. However, the coefficient of proportionality is distinct in various models and simulations. This study advances a general method to determine the area-load proportionality based on the incremental contact concept. A pair of loose bounds on the coefficient of proportionality for general Gaussian random rough surfaces and a tight one working in the thermodynamic limit are derived. The resultant predictions are in accord with numerical results presented in literature. The current study might offer some deep insight into the role of surface morphologies in contact of rough elastic solids.

Controlling the Friction Coefficient and Adhesive Properties of a Contact by Varying the Indenter Geometry

In the present paper, we describe a series of laboratory experiments on the friction between rigid indenters with different geometrical forms and an elastic sheet of elastomer as a function of the normal load. We show that the law of friction can be controlled by the shape of the surface profile. Since the formulation of the adhesive theory of friction by Bowden and Tabor, it is widely accepted and confirmed by experimental evidence that the friction force is roughly proportional to the real contact area. This means that producing surfaces with a desired dependence of the real contact area on the normal force will allow to “design the law of friction”. However, the real contact area in question is that during sliding and differs from that at the pure normal contact. Our experimental studies show that for indenters having a power law profile f(r) = cnrn with an index n < 1, the system exhibits a constant friction coefficient, which, however, is different for different values of n. This opens possibilities for creating surfaces with a predefined coefficient of friction.

Electrical detachment of triboelectrically charged bumpy particles from rough surfaces

The detachment of bumpy particles from smooth and rough surfaces through electrostatic fields was studied. The Johnson–Kendall–Roberts (JKR) adhesion model for elastic interface deformations was used to account for the individual bump adhesion, including the electrostatic forces. It was assumed that the elastic deformation of asperities determines the real contact area, and the effect of topographic properties of substrates was included in the analysis. For a fixed charge per unit mass, the Coulomb, the image, the dielectrophoretic, and the polarization forces acting on the particle in the presence of an imposed electric field were evaluated. The electrostatic detachment of bumpy toner particles for various applied electrostatic fields was studied. The triboelectric charges were concentrated on the hemispherical bumps, and the critical electric field strength needed to detach charged particles from surfaces with varying roughness levels was evaluated. It was shown that the particle charge, surface roughness, and the number of bumps significantly affect the removal of particles from surfaces. For higher charges (∼5 × 10−15 C ), decreasing the number of bumps from 22 to 14 and the roughness parameter from 0.83 to 0.77 approximately doubled the detachment field needed to remove 8.3 μ m polystyrene (PSL) particles from the aluminum oxide substrate. The model predictions are compared with the available experimental data, and good agreement was observed. The study’s findings predicting the electric field needed for charged particle removal could lead to more efficient copiers and electrostatic cleaning devices.

Investigation of lubricant viscosity and third-particle contribution to contact behavior in dry and lubricated three-body contact conditions

The generation of third particles and change in viscosity lead to the gradual degradation of the performance of the machine interface. The generation of third particles may come from wear debris or environmental particles, which form a three-body contact system at the contact interface. The viscosity of the lubricant will also change with the long-term operation of the components. This paper uses a three-body lubrication model to study the influence and interaction of lubricant viscosity change and the presence of third particles on the contact characteristics, including the real contact area, the particle contact area ratio, the solid load percentage, the film thickness, and the evolution of the lubrication regime. The results show that when the interface is in a three-body mixed lubrication regime, the dimensionless total real contact area increases with the increase in particle size and density at the same lubricant viscosity, while the trend is the opposite in dry contact and boundary lubrication interfaces. When viscosity decreases, a three-body contact interface is more prone to entering boundary lubrication than a two-body contact interface, resulting in surface damage. Regardless of surface roughness, particle size, and dry or lubricated contact conditions, the turning point of the contact area (TPCA) phenomenon is usually when the ratio of particle size to surface roughness is 0.8–1.3. Under the same ratio of particle size to surface roughness, the critical load of the TPCA phenomenon increases with the increase in third-particle size and surface roughness, but decreases with the increase in lubricant viscosity and particle density.

Molecular Probing of the Microscopic Pressure at Contact Interfaces.

Obtaining insights into friction at the nanoscopic level and being able to translate these into macroscopic friction behavior in real-world systems is of paramount importance in many contexts, ranging from transportation to high-precision technology and seismology. Since friction is controlled by the local pressure at the contact it is important to be able to detect both the real contact area and the nanoscopic local pressure distribution simultaneously. In this paper, we present a method that uses planarizable molecular probes in combination with fluorescence microscopy to achieve this goal. These probes, inherently twisted in their ground states, undergo planarization under the influence of pressure, leading to bathochromic and hyperchromic shifts of their UV-vis absorption band. This allows us to map the local pressure in mechanical contact from fluorescence by exciting the emission in the long-wavelength region of the absorption band. We demonstrate a linear relationship between fluorescence intensity and (simulated) pressure at the submicron scale. This relationship enables us to experimentally depict the pressure distribution in multiasperity contacts. The method presented here offers a new way of bridging friction studies of the nanoscale model systems and practical situations for which surface roughness plays a crucial role.

Electrical measurement method of static friction force on rough surface

Friction plays a key role in the assessment of the safety and stability of mechanical systems (such as superconducting magnet quench explosion, aerospace vehicle bearing wear, etc.). Due to the closeness of the interfaces in engineering structures and the randomness of the contact surfaces, existing methods for measuring static friction force are unable to measure it at the contact interfaces of engineering structures under service conditions. In this paper, a new method for measuring the static friction force at the interface based on electrical signals is proposed. This method enables the measurement of the static friction force at interfaces of complex engineering structures under service conditions solely through electrical signals. The results indicate that the contact resistance gradually decreases with the increase in tangential load during the static friction stage until a monotonic behavior of macroscopic sliding occurs. The evolution of contact resistance is linked to the evolution of the real contact area, and this monotonic behavior can be explained as the deformation form of contact points. The accuracy of the proposed electrical measurement method is verified by comparison with experimental results (with an error of less than 9%). The indirect measurement method of friction force proposed in this paper can effectively measure the static friction force at the interfaces of engineering structures under service conditions, and it is expected to be applied to the detection of friction performance at engineering structure interfaces in extreme service environments.

Simulation on metal-rubber contact behavior based on a reconstructed 3D topography of rough surfaces

A contact model with refined surface topography is important to the frictional wear study of rubber seal. But the traditional modeling method relies on the measurement of surface characteristics of rubber material, which increases the cost due to high-precision instruments and complex data processing. In the paper, a new modeling method on reconstructing the 3D surface topography is proposed. The coordinate cloud of the surface is generated based on the random distribution of asperity height, which is established according to the initiate parameters of the surface roughness. The metal-rubber assembly model with 3D topography of rough surfaces are built. The simulation on metal-rubber contact behavior is conducted in ANSYS. The result shows a power-exponent relation is between the real contact area and the external pressure, and three contact states of no contact, adhesion and slippage are defined to describe the dynamic distribution of real contact area. The authors proved the reliability of the topography reconstruction method by an indentation experiment on rubber material. The area ratio reflects not only the change rule of contact area under an increasing pressure, but also the difference of contact behavior of the contact pairs under different surface roughness.