Wet Contacts Research Articles

Numerical investigation of squeeze film lubrication on bioinspired hexagonal patterned surface

PurposeThis paper aims to investigate the squeeze film lubrication properties of hexagonal patterned surface inspired by the epidermis structure of tree frog’s toe pad and numerically explore the working mechanism of hexagonal micropillar during the acquisition process of high adhesive and friction for wet contacts.Design/methodology/approachA two-dimensional elastohydrodynamic numerical model is employed for the squeezing contacts. The pressure distribution, load carrying capacity and liquid flow rate of the squeeze film are obtained through a simultaneous solution of the two-dimensional Reynolds equation and elasticity deformation equations.FindingsHigher pressure is found to be longitudinally distributed across individual hexagonal pillar, with pressure peak emerging at the center of hexagonal pillar. Expanding the area density and shrinking the channel depth or initial film thickness will improve the magnitude of squeezing pressure. Relatively lower pressure is generated inside interconnected channels, which reduces the load carrying capacity of the squeeze film. Meanwhile, the introduction of microchannel is revealed to downscale the total mass flow rate of squeezing contacts.Originality/valueThis paper provides a good proof for the working mechanism of surface microstructures during the acquisition process of high adhesive and friction for wet contacts.

Design of Disc on Disc Wear Test Equipment Using VDI 2221 Method

This study aims to design the build of a wear test tool or a tribometer that intends to measure the coefficient of friction and wear rate in contact materials in form of disc-on-disc. This tribometer test tool can be used in dry contact (without any intermediate material between contacts) or wet contacts (by using intermediate material between contacts such as lubricant). The application of this disc-on-disc type of tribometer is the contact that occurs between roller elements (solid cylinders) and inner rings on cylindrical rolling bearings. In this paper is used the design method of VDI 2221. The steps are to clarify the task, determine the function of structure, look for the principle of the solution and its structure, describe the variants that can be realized and give shape to the model and detail the manufacture and used. This tool is used to test components consisting of two discs where the material can be varied. The design of the surface contact side mechanism is done in a radial direction. So, the disc can rotate (rolling contact) and this test tool can be varied load and rotation. The final result of this study is an engineering document in the form of layout drawings and assembly images as well as detailed images of each component and bills of materials.

Wear behaviour of cast aluminium silicon (Al-Si) alloy in Dot 4 brake fluid

The frictional sliding behaviour of cast Al-Si alloy was investigated with and without DOT 4 hydraulic brake fluid lubrication/environments. Cast Al-Si specimen was produced from wrought alloy and the specimen surface was metallurgically prepared. The as-cast product was characterized with aid of micro-hardness tester, the chemical composition was analysed by X-ray fluorescence (XRF), while X-ray diffraction (XRD) was used to identify phases present, microstructures and surface examinations were done on a high resolution metallurgical Optical Microscope. The properties of the polished and worn sample surfaces were examined by Scanning electron microscope (SEM) equipped with Energy dispersive X-ray (EDS) facilities. A 6mm diameter alumina ball under varying normal loads (3-10 N) was rubbed against the surface of the cast Al-Si coupon specimen. The dry and wet sliding wear behaviour was assessed based on the CoF-sliding time behaviour. From the results, the average CoF values of 0.9064 for dry and 0.2038 for wet contacts were obtained. Comparatively, 1.03E-06 and 4.21E-07 minimum wear intensities; and 467290 and 132170 wear resistances were obtained for dry and wet contacts respectively. Behaviour of cast Al-Si in DOT4 were compared with its performance in DOT3 regarding the hydraulic brake system application.

Transitions between nanomechanical and continuum mechanical contacts: new insights from liquid structure.

The use of continuum mechanics to describe contacts involving nanoscale and atomic interactions has been one of the key controversies in nanoscience, tribology, and petrophysical and geological studies. By applying a novel nonequilibrium molecular dynamics scheme to wet quartz contacts, this study revealed the key transitions between continuum electrostatic, nanomechanical and Hertzian contact behaviors at around one nm of surface separation, which results in critical contact pressure fluctuations between -30 and 100 MPa. Using a novel liquid-structure analysis scheme based on the spatial distribution of water molecules, the nanomechanical behavior was found to originate from the collapse and localization of layers of water molecules. Moreover, the role of surface curvature on this effect was also quantified and explained based on a new topological descriptor. The findings of this study enrich our understanding of wet contacts and have a wide range of applications from the nanoscale to macroscale.

Mechanical strength of wet particle agglomerates

Using particle dynamics simulations, we investigate the strength and microstructure of agglomerates of wet frictional particles subjected to axial compression. The numerical model accounts for the cohesive and viscous effects of the binding liquid up to a debonding distance with the liquid assumed to be distributed homogeneously inside the agglomerate. We show that wet agglomerates undergo plastic deformation due to the rearrangements of primary particles during compression. The compressive strength is thus characterized by the plastic threshold before the onset of failure by the irreversible loss of wet contacts between primary particles. We find that the agglomerate plastic threshold is proportional to the characteristic cohesive stress defined from the liquid-vapor surface tension and the mean diameter of primary particles, with a prefactor that is a nearly linear function of the debonding distance and increases with size span. We analyze the agglomerate microstructure and, considering only the cohesive capillary forces at all bonds between primary particles, we propose an expression of the plastic strength as a function of the texture parameters such as the wet coordination number and packing fraction. This expression is shown to be consistent with our simulations up to a multiplicative factor reflecting the distribution of the capillary bridges.

A comparative study of wet etching and contacts on [formula omitted] and (010) oriented β-Ga2O3

We report on the effect of β-Ga2O3 crystal orientation on wet etching and Ohmic contact formation. The photochemical etching rate in KOH solutions of (2¯01) oriented, n-type bulk single crystals grown by the edge-defined film-fed growth method is ∼3–4 times higher than for the (010) planes. The activation energy for etching was 0.498 eV and 0.424 eV for (2¯01) and (010) orientations, respectively, suggesting the etching is reaction-limited with the same rate-limiting step. Ti (200 Å)/Au (1500 Å) metallization deposited on the two different orientations and annealed at 450 °C showed Ohmic current-voltage (I-V) behavior for (2¯01) but rectifying characteristics for (010). For (010) Ga2O3, there exists 2 types of surfaces having Ga and O atomic densities of 0.58 and 0.87 × 1015 cm−2, respectively. By contrast, for (2¯01) Ga2O3 surfaces, there exist 2 types of surface, with each type terminated with only Ga or O. If the surface is terminated with O, the dangling bond densities of O are 1.78 and 2.68 × 1015 cm−2, respectively. We found that (2¯01)–oriented Ga2O3 is etched at higher rates and is easier to form Ohmic contacts than (010) Ga2O3. The higher density of oxygen dangling bonds on the (2¯01) plane correlates with the faster etch rates and pronounced Ohmic behavior from deposited metals.

Influence of water on the tribological properties of zinc dialkyl-dithiophosphate and over-based calcium sulphonate additives in wet clutch contacts

Usually the wet clutch lubricant properties vary with different formulations of base oil types and additives. The aim of this paper was to evaluate the effect of water on the performance of additives in ATF. Simplified lubricants, ZDDP and over-based Ca-sulfonates detergent additives in an API Group I mineral base oil, were employed to compare with the commercial fully-formulated automatic transmission fluid (DEXRON®VI) during water-contamination. A full-scale wet clutch test rig was used to evaluate the frictional response due to water contamination of the lubricants. Fourier Transform Infrared Spectroscopy was utilized to evaluate the variation in the solubility of these polar organic additives in the water phase and Karl-Fischer titration was utilized to evaluate the post-test water content for different formulations.

Electrical resistivity measurement of unsaturated concrete samples

Electrical conduction in concrete primarily occurs via the liquid in pores. Since concrete resistivity is sensitive to moisture content, resistivity can be used to infer other moisture-dependent concrete properties. However, quantitative assessment of the relationship between concrete resistivity and the pore liquid content can be a difficult task due to contact issues between electrodes and the surface of unsaturated concrete. Resistivity is usually measured using wet sponges to assess contact with a concrete surface. This is a very effective procedure for saturated concrete but implies minor alterations in the condition of unsaturated concrete. This paper deals with a contact method to measure the resistivity of unsaturated concrete samples. Resistivity measurements dispensing with wet contacts are performed on thin samples of conventional concrete. A dry contact is achieved by means of painted electrodes on the concrete surface. Dry and wet contact methods are compared when applied to saturated concrete, and resistivity values of unsaturated concrete are related to concrete moisture content and the relative humidity of the surrounding environment. As a result, resistivity is related to the volumetric fraction of conducting water in pores and its hygroscopic equilibrium with the surrounding environment.

Towards dense, realistic granular media in 2D

The development of an applicable theory for granular matter—with both qualitative and quantitative value—is a challenging prospect, given the multitude of states, phases and (industrial) situations it has to cover. Given the general balance equations for mass, momentum and energy, the limiting case of dilute and almost elastic granular gases, where kinetic theory works perfectly well, is the starting point.In most systems, low density co-exists with very high density, where the latter is an open problem for kinetic theory. Furthermore, many additional nonlinear phenomena and material properties are important in realistic granular media, involving, e.g.:multi-particle interactions and elasticitystrong dissipation,friction,long-range forces and wet contacts,wide particle size distributions andvarious particle shapes.Note that, while some of these issues are more relevant for high density, others are important for both low and high densities; some of them can be dealt with by means of kinetic theory, some cannot.This paper is a review of recent progress towards more realistic models for dense granular media in 2D, even though most of the observations, conclusions and corrections given are qualitatively true also in 3D.Starting from an elastic, frictionless and monodisperse hard sphere gas, the (continuum) balance equations of mass, momentum and energy are given. The equation of state, the (Navier–Stokes level) transport coefficients and the energy-density dissipation rate are considered. Several corrections are applied to those constitutive material laws—one by one—in order to account for the realistic physical effects and properties listed above.

Rolling–sliding laboratory tests of friction modifiers in dry and wet wheel–rail contacts

Friction management has been carried out extensively in the majority of railway networks in the last few years. A popular practice is the application of friction modifiers to increase the adhesion level in contaminated wheel–rail contacts. Two friction modifiers have particularly been used or tested on several railway networks as adhesion enhancers to facilitate the traction and braking operation under poor adhesion conditions. However, for assessment of the performance the railway operators and infrastructure managers mostly rely on practical observations that do not elucidate completely the effectiveness and side effects of these adhesion enhancers. In this paper, the constituents of the two friction modifiers are identified and the solid components are analyzed. A twin-disk roller rig has been used to study their performance in dry and wet contacts under closely controlled laboratory conditions. The adhesion characteristics of both friction modifiers are examined for different slip ratios. Furthermore, the wheel and rail disks are examined after a series of dry tests to analyze the mass loss, the surface damage, the change in surface hardness and roughness, and the subsurface deformation caused by the friction modifiers compared to dry clean contacts.

Competition between wear and rolling contact fatigue at the wheel—rail interface: some experimental evidence on rail steel

Recent studies of the rail—wheel interface have shown that wear and rolling contact fatigue (RCF) are competitive phenomena. Wear, by removing the material from the contact surface, tends to limit the propagation of cracks formed by cyclic contact stresses. Several parameters influence these phenomena. This article studies the effects of various working conditions on the surface damage of railway steels. The competition between wear and RCF is particularly strong in dry rolling—sliding contacts, in which the damage severity of our test cases can be adequately predicted using the shakedown map. In wet contacts, RCF is prevalent: cracks rapidly propagate into the subsurface layer in response to hydraulic pressure penetration and then branch towards the surface causing severe damage (macroscopic pitting). The severity of this phenomenon depends strongly on the applied load and cannot be inhibited by wear due to the low friction. On the basis of these results, a general procedure is proposed to the structural integrity of rails.

Effect of morphology and extent of infiltration on the cohesivity and dispersion mechanisms of particle agglomerates

A unified approach to predict the tendency for dispersion of particle agglomerates, inclusive of a wide range of particle and agglomerate properties, is presented. This framework is applied to analyze the behavior of three prototypical materials (fumed silica, calcium carbonate and titanium dioxide) across a range of agglomerate packing densities. Simulations of dispersion phenomena, which employ our previously developed solution for liquid–bridge interactions for wet interparticle contacts and the Rumpf model for the tensile strength of the dry and wet portions of the agglomerate, have been performed. Various mechanisms of dispersion are predicted for various conditions of agglomerate density and extent of fluid infiltration. These range from an adhesive mechanism at the wet–dry interface for sparse materials to a cohesive mechanism by erosion as agglomerate density increases. The results correspond well with the results of earlier experimental studies involving the same materials.

Mineralogical characterization of the Tournemire argillite after in situ interaction with concretes

This study aims to investigate, through mineralogical characterization (SEM, XRD) and mass-balance calculations, the effects of contact time, concrete types and presence of free water on the Tournemire argillite under in situ conditions. Three sampling zones from Excavation Disturbed Zone (EDZ) areas have been chosen: (1) dry contacts, collected at the tunnel masonry/argillite interface (contact time – 125 years), (2) wet contacts, taken close to drained areas below the tunnel roadbed in contact with the canal draining the Cernon fault water (contact time – 15 and 7 years) and at the tunnel masonry/argillite interface over 70 m from the Cernon fault (contact time – 125 years). This study shows that: • in the absence of water, no significant modification of argillite is observed after 125 years, except for pyrite dissolution and gypsum precipitation. • in the presence of water, precipitation of gypsum, recrystallization of mixed-layer clays, neoformation of zeolites and K-feldspars overgrowths are observed. At the concrete/argillite interface near the Cernon fault, important dolomite neoformation and leaching of chlorite and kaolinite occur. These processes are enhanced with contact time, low flow rate and the nature of the concrete (compound cement Portland CEM II 32.5). Evidence for oxidation processes linked to the EDZ (pyrite oxidation, Fe-oxy-hydroxide and gypsum precipitation) is observed.

Particle-surface capillary forces with disjoining pressure

A new, atomic force microscopy (AFM) based experimental setup for the continuous acquisition of friction force data as a function of humidity has been developed. The current model of interactions between wet contacts under the influence of capillary effects, has been amended to include a vertical component due to the disjoining pressure and takes into account the influence of liquid films adsorbed on the surface. This is a 'switching' model, i.e. the contact between nanometer-sized sphere and a flat surface can exist in two distinct states due to capillary bridge formation/destruction as the humidity is varied. The model has been qualitatively verified on samples of differing wettability produced by UV-ozone treatment of polystyrene (PS). Results of AFM analysis of the friction vs. vapor pressure curves collected from the surface are presented. Correlation between important surface properties such as wettability, adsorption, and contact angle and friction force under varying humidity were found and discussed.

The effect of extrusion parameters on the fretting wear resistance of Al-based composites produced via powder metallurgy

The work reported in this paper is aimed at establishing the relationship between processing and the wear resistance of the metal matrix composites (MMCs) based on a novel alloy, Al-20Si-5Fe-3Cu-1Mg. The MMCs were processed via a commercially viable powder metallurgy (PM) route, i.e. through mixing the atomized matrix alloy powder with 10 vol % SiC or Al2O3 particles, cold isostatic pressing, degassing and hot extrusion. It has been found that the extrusion window of the MMCs is greatly narrowed due to their increased deformation resistance on one hand and incipient melting of their matrix on the other. For a sound MMC extrudate, a reduction ratio over a critical value must be applied. However, a further rise of this ratio leads to deterioration of local interfacial cohesion between the ceramic phase and the matrix dispersed with a high volume fraction of silicon crystals and intermetallic dispersoids, thus degrading the MMCs in tensile properties. Furthermore, fretting wear tests at room and elevated temperatures and with dry and wet contacts show that the MMCs extruded at a higher reduction ratio has a higher mass loss and an increased friction coefficient. The work points to the direction of further research, i.e. on MMCs containing spherical reinforcement instead of commonly used angular particles.

Tribology of Solid Contacts in Magnetic Recording Devices

The demand for close proximity of magnetic head to the medium during operation of magnetic recording devices, high surface smoothness, and high reliability requires that friction and wear of the head-medium interface be minimized. Friction and wear are minimized by suitable selection of interface materials and lubricants, optimization of surface roughness, and operating conditions. This invited paper presents the state of the art of the characterization of surface roughness and modelling of dry and wet solid contacts which are critical to the design of the modern magnetic storage devices.

Dependence of Friction at Wet Contacts upon Interfacial Potential

An analysis is made of the forces contributing to the coefficient of friction between two solids, at least one of which is an electronic conductor, through a film of liquid electrolytes. It is shown that the friction coefficient should vary parabolically with potential. The derived expression, without calibrated constants, fits experimental measurements of the dependence of friction coefficients as a function of applied interfacial potential.