Surface Friction Coefficient Research Articles

Nanochannel flow past permeable walls via molecular dynamics

The nanochannel flow past permeable walls with nanopores is investigated by molecular dynamics (MD) simulations, including the density distribution, velocity field, molecular penetration mechanism and surface friction coefficient. A low density distribution has been found at the gas-wall interface demonstrating the low pressure region. In addition, there exists a jump of the gas density on the permeable surface, which indicates the discontinuity of the density distribution across the permeable surface. On the other hand, the nanoscale vortices are observed in nanopores of the permeable wall, and the reduced mass flux of the flow in nanopores results in a shifted hydrodynamic boundary above the permeable surface. Particularly the slip length of the gas flow on the permeable surface is pronounced a non-linear function of the molecular mean free path, which produces a large value of the tangential momentum accommodation coefficient (TMAC) and a big portion of the diffusive refection. Moreover, the gas-gas interaction and multi-collision among gas molecules may take place in nanopores, which contribute to large values of TMAC. Consequently the boundary friction coefficient on the permeable surface is increased because of the energy dissipation consumed by the nanoscale vortices in nanopores. The molecular boundary condition provides us with a new picture of the nanochannel flow past the permeable wall with nanopores.

X-Ray Diffraction Analysis of the Sintered Y-TZP-AI2O3 Ceramics

The paper discusses the wear resistance, friction coefficient, and structure of friction surfaces of fine-grained (0.2 µm) Y-TZP-Al2O3 composite rubbed against a steel disk counterface at a pressure of 5 MPa in a range of sliding speeds from 1 to 20 m/sec. It is shown that, starting at 2 m/sec, the fricti on surface is subdivided by a crack network into separate regions within which local spalling occurs at the maximum wear rate and a sliding speed of 5 m/sec. X-ray diffraction reveals inversion (with respect to the initial state) of the peak intensities of the tetragonal phase with random crystalline grain orientation. The degree of this inversion increases with sliding speed. These results are discussed in terms of the effects exerted by the reorientation of martensite-free deformation twins in the tetragonal phase and the formation of a quasi-liquid film on the wear resistance of fine-grained Y-TZP-Al2O3.

Improving certain properties of wool fibers by applying chitosan nanoparticles and atmospheric plasma treatment

The nanochitosan particles were prepared by ionic gelation method using sodium tripolyphosphate (TPP) as anionic chemical agent. Structural and morphological properties of the nanoparticles were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR) analyses. The results showed that nanoparticles were spherical with a diameter range of 17-105 nm. After nanochitosan synthesis, the effects of chitosan and nanochitosan concentrations on the dyeability, fastness properties, shrink-proofing, tensile strengths, and surface friction coefficients of untreated and plasma treated wool fabrics were investigated. The studies revealed that nanochitosan treated wool fabric possesses better dyeing and shrink-proofing properties in comparison with conventional chitosan treated fabrics.

Rheology of non-Brownian suspensions of rough frictional particles under shear reversal: A numerical study

We perform particle scale simulations of suspensions submitted to shear reversal. The simulations are based on the Force Coupling method, adapted to account for short range lubrication interactions together with direct contact forces between particles, including surface roughness, contact elasticity, and solid friction. After shear reversal, three consecutive steps are identified in the viscosity transient: An instantaneous variation, followed by a rapid contact force relaxation, and finally a long time evolution. The separated contributions of hydrodynamics and contact forces to the viscosity are investigated during the transient, allowing a qualitative understanding of each step. In addition, the influence of the contact law parameters (surface roughness height and friction coefficient) on the transient is evaluated. Concerning the long time transient, the difference between the steady viscosity and minimum viscosity is shown to be proportional to the contact contribution to the steady viscosity, allowing in principle easy determination of the latter in experiments. The short time evolution is studied as well. After the shear reversal, the contact forces vanish over a strain that is very short compared to the typical strain of the long time transient, allowing to define an apparent step between the viscosity before shear reversal and after contact force relaxation. This step is shown to be an increasing function of the friction coefficient between particles. Two regimes are identified as a function of the volume fraction. At low volume fraction, the step is small compared to the steady contact viscosity, in agreement with a particle pair model. As the volume fraction increases, the value of the viscosity step increases faster than the steady contact viscosity, and, depending on the friction coefficient, may approach it.

Effect of atomic oxygen irradiation on the structural and tribological characteristics of polytetrafluoroethylene and its composites

Based on the ground-based simulation facility, the effects of atomic oxygen (AO) irradiation on the structural and tribological properties of pure polytetrafluoroethylene (PTFE) and carbon fiber and MoS2-filled PTFE composites were studied by scanning electron microscopy, X-ray photoelectron spectroscopy, and a ball-on-disc tribometer. The results shown that AO irradiation had significant effects on the structural and tribological properties of pure PTFE, in which the surface morphologies, mass loss, friction coefficient, and wear rate had been changed greatly after AO irradiation. However, it was noticeable that the addition of carbon fiber and MoS2 filler to PTFE could improve the AO resist capacity and tribological properties of PTFE composites significantly. Copyright © 2016 John Wiley & Sons, Ltd.

Quantifying the biotribological properties of forehead skin to enhance head impact simulations

Head injury severity is dependent on the loading and accelerations experienced during, and immediately after, impact. In England and Wales alone, for example, 700,000 head injury cases are reported annually within Emergency Departments; however, there remains a lack of data that quantifies the fundamental interaction of the head with an impacting surface. The consequence of impact depends upon a precise understanding of the head–surface interaction; hence, there is a need to appreciate the magnitude of, and variation in, frictional coefficient between the head and a range of possible surfaces. This study develops and validates a novel protocol for quantifying friction between the forehead skin and some potential surfaces. Thirteen participants were recruited and four materials tested, with the lowest (0.11) and highest (1.64) dynamic frictional coefficients measured between skin, and expanded polystyrene and laminate flooring, respectively. Preliminary computational simulation identified that a modest variation in head–surface frictional coefficient (0.6 & 0.7) increases rotational accelerations by 23–33%. Hence, this study highlights the significance of the head–surface interaction, whilst providing some data that will assist investigators evaluating head impacts within both a domestic and sporting environment.

Frictional characteristics of granular system under high pressure

In order to reveal the force transmission features of the granules in the solid granule medium forming (SGMF) technology, the frictional characteristics of the non-metallic granule medium (NGM) under high pressure were investigated by tests and simulations. And the relevant changing curves of the internal friction coefficient of the granular system under different normal pressures were obtained by self-designed shear test. By the granule volume compression test, the accurate discrete element simulation parameters were obtained, based on this, the discrete element method (DEM) was adopted to reveal the evolution law of the NGM granules movement in the sample shear process from the microscopic view. Based on the DEM, the influence of granule diameter, surface friction coefficient, normal pressure and shear velocity on the internal friction coefficient of the granular system were studied. And the parameters were conducted to be dimensionless by introducing the inertia coefficient. Finally, the expression showing power-law relationship of inertia coefficient, surface friction coefficient and internal friction coefficient is obtained.

Influence of Cutting Parameters on Frictional Coefficient of Micro-Textured Tool

Micro-textured technology which studies in the field of tribology shows that micro-textured surface can effectively improve the situation of friction between the friction interfaces. In this article, micro-textured surface technology has been applied to the cutting tools, in order to study the cutting mechanism of micro-textured carbide tools. The effect of cutting parameters on surface friction characteristics was studied when cutting 45 steel with YT15 cemented carbide tools. The results shown that all surface friction coefficient of the samples got the best value when the feed rate is 0.14mm/r; an appropriate increasing in feed rate can reduce the surface friction coefficient in the low-speed cutting. Throughout the course of the test, the average of the surface friction coefficient of the 4th and the 5th sample tools are better than other tools.

Tribological Properties of Laser Microtextured Surface Bonded With Composite Solid Lubricant at High Temperature.

A combination technology of the solid lubricant and the laser surface texturing (LST) can significantly improve the tribological properties of friction pairs. The plate sample was textured by fiber laser and composite lubricant of polyimide (PI) and molybdenum disulfide (MoS2) powders were filled in the microdimples. Sliding friction performances of micron-sized composite lubricant and nano-sized composite lubricant were investigated by ring-plate tribometer at temperatures ranging from room temperature (RT) to 400 °C. On the one hand, the results of the micron-sized composite lubricant show that the friction coefficient of the textured surface filled with composite lubricant (TS) exhibits the lowest level and the highest stability compared to a textured surface without solid lubrication, smooth surface without lubrication, smooth surface burnished with a layer of composite solid lubricant. The better dimple density range is 35-46%. The friction coefficients of the sample surface filled with micron-composite solid lubricant with the texture density of 35% are maintained at a low level (about 0.1) at temperatures ranging from RT to 300 °C. On the other hand, the results of the nano-sized composite lubricant show that these friction properties are better than those of MoS2-PI micron-sized composite. The friction coefficients of MoS2-PI-CNTs nano-sized composite solid lubricant are lower than those of the MoS2-PI composite lubricant at temperatures ranging from RT to 400 °C. In addition, the possible mechanisms involving the synergetic effect of the surface texture and the solid lubricant are discussed in the present work.

Effect of chemical additive on fixed abrasive pad self-conditioning in CMP

Self-conditioning performance of polishing pad is an important characteristic to influence processing efficiency and service life in chemical mechanical polishing (CMP). The slurry can react with the pad surface, which affects its self-conditioning performance in fixed abrasive polishing process. Wear ratio of wafer material removal rate (MRR) and pad wear rate is introduced to evaluate self-conditioning performance of fixed abrasive pad (FAP). To clear the effect of chemical additive on FAP self-conditioning, wear ratio, FAP surface topography, friction coefficient, and acoustic emission signal of polishing process were investigated in fixed abrasive polishing of quartz glass with ferric nitrate, ethylenediamine (EDA), and triethanolamine (TEA) slurry, respectively. Results indicate that TEA slurry can provide excellent self-conditioning of FAP in fixed abrasive polishing of quartz glass. MRR and wear ratio maintain high levels during the whole polishing process. Friction coefficient and acoustic emission signal are more stable than that of the other two chemical additives. An appropriate amount of TEA, which is beneficial to enhance MRR and extends service life of FAP, is added in the polishing slurry to improve FAP self-conditioning in fixed abrasive polishing process.

Investigation on cornering brake stability of a heavy-duty vehicle based on a nonlinear three-directional coupled model

A nonlinear three-directional coupled lumped parameters (TCLP) model is proposed for a heavy-duty vehicle to investigate the longitudinal, lateral and vertical dynamics of vehicles simultaneously. The nonlinear property of suspension damping is described by a fitted exponent model from experimental data. The nonlinear tire forces in different directions are modeled by the nonlinear Gim model and the vertical single dot contact model with square nonlinearity. The system responses are calculated by numerical integration and compared with the Functional Virtual Prototyping (FVP) model and the test data, so as to verify the validity of this new vehicle model. With Lateral-Load Transfer Ratio (LTR) and yaw rate as evaluation indexes, the influences of system parameters on cornering brake stability are analyzed. The critical vehicle speed table is also obtained. The study shows that high vehicle running speed, large front wheel steering angle, large tire cornering stiffness or small braking moment is harmful to yaw stability and roll stability. Road surface frictional coefficient mainly influences yaw stability, and vertical tire stiffness and road surface roughness mainly influence roll stability.

Aqueous lubrication of poly(N-hydroxyethyl acrylamide) brushes: a strategy for their enhanced load bearing capacity and wear resistance

Poly(N-hydroxylethyl acrylamide) brushes show a very low aqueous surface friction coefficient. Crosslinking of these brushes increases the surface friction coefficient, but can significantly enhance their load bearing capacity and wear resistance.

Mathematical Modeling of Heat Friction Contact Master Belt with the Gun Mount Barrel During the Process of High-Speed Motion

The friction in the gun mount barrel at sliding speeds of artillery projectile 500 - 700 m/s is not sufficiently studied. The main problem is to increase the efficiency of the master belt with a significant increase of the interaction parameters of the barrel with the projectile. To determine the effect of heating on the change of physical and mechanical properties of the surface layer and friction coefficient on the surface of the master belt were made mathematical modeling of heat transfer.

Surface Friction Behaviour of Anodized Commercially Pure Titanium Screw Assemblies

Dental implant abutment fixation by screw requires a prescribed preload (tensile screw load) for structural and implant performance purposes. This paper investigates the effect of surface modification on the apparent frictional behaviour of commercially pure Grade 4 titanium bolt assemblies by anodizing. 4mm screws are anodized at various anodizing voltages to control the oxide layer thickness. Fixation torque is measured along with the corresponding tensile load in the screw and used to determine the dynamic surface friction coefficient for fastening and the static surface friction coefficient for untightening. Surface hardness is measured and linked to the frictional behaviour. The reduction in the apparent frictional behaviour are presented and discussed.

Quantitative assessment of friction perception for fingertip touching with different roughness surface

Abstract There are many mechanical stimulation receptors and sensory nerve endings in human skin, which are the important tools in tactile perception. It is a complex process for human to perceive objects and friction relative motion plays an important role during this process. When human’s fingertips friction against objects, they will produce compression and tensile mechanical deformation, which can stimulate the mechanical stimulation receptors in fingertip skin to produce corresponding action potentials and impulses signals. The signals which contain object’s physical properties are transmitted to cerebral cortex by nervous system, thus the shape and surface texture of objects are perceived. Thus the friction between the fingertip and object is an important factor to perceive objects. There exist positive connection between friction and tactile perception. However, limited quantitative parameters can be used to evaluate the perception, and they have rarely been studied scientifically. In this paper, the friction perceptions of fingertip skin rubbing against different roughness sandpapers were studied by biofeedback of frictional signals, physiological and psychological responses. An UMT-II tribometer was used to measure tribological parameters of the fingertip, and corresponding physiological response of electroencephalogram (EEG) signals were monitored by using a Physiological Monitoring and Feedback Instrument (NeXus-10) with BioTrace+software. The psychological responses were scored according to the volunteer’s perception during friction tests. The correlation models among the perception of fingertip, friction coefficient and EEG signals were established by applying regression analysis method. Results showed that the friction coefficient, amplitudes of EEG signals and psychological responses increased with the roughness of sandpapers increasing. There existed a significant correlation among the friction perception of different surface roughness, friction coefficient and amplitudes of EEG signals. By using this method, the perception of fingertip skin for different roughness surface during friction can be evaluated quantitatively.

Numerical Study of the Effect of the Reynolds Numbers on Thermal and Hydrodynamic Parameters of Turbulent Flow Mixed Convection Heat Transfer in an Inclined Tube

A numerical investigation of the effect of the Reynolds number on the thermal and hydrodynamic parameters of mixed convection heat transfer of the water-Al2O3 nanofluid turbulent flow in an inclined circular channel is the subject of this article. The upper wall of the channel was under non-uniform heat flux and its lower part was isolated. The two-phase mixture model, the finite volume method, and the second-order upstream difference scheme were used to solve the governing equations. After reviewing the results, it was found that by increasing Reynolds number, the convective heat transfer coefficient and shear stress increase, but the surface friction coefficient decreases. In the case of Nusselt number and the surface friction coefficient, some equations were extracted.

Tribological performance of sub-100-nm femtosecond laser-induced periodic surface structures on titanium

Sub-100-nm laser-induced periodic surface structures (LIPSS) were processed on bulk titanium (Ti) surfaces by femtosecond laser pulse irradiation in air (30fs pulse duration, 790nm wavelength). The laser peak fluence, the spatial spot overlap, and the number of overscans were optimized in a sample-scanning geometry in order to obtain large surface areas (5mm×5mm) covered homogeneously by the LIPSS. The laser-processed regions were characterized by optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM). The friction coefficient of the nanostructured surfaces was tested during 1000 cycles under reciprocal sliding conditions (1Hz, 1.0N normal load) against a 10-mm diameter ball of hardened 100Cr6 steel, both in paraffin oil and in engine oil used as lubricants. Subsequently, the corresponding wear tracks were qualified by OM, SEM, and energy dispersive X-ray analyses (EDX). The results of the tribological tests are discussed and compared to that obtained for near wavelength-sized fs-LIPSS, processed under somewhat different irradiation conditions. Some constraints for a beneficial effect of LIPSS on the tribological performance are provided.

Optimization of scratch resistance and mechanical properties in wollastonite-reinforced polypropylene copolymers

The aim of this research was to establish a balance between scratch resistance and scratch damage visibility in the wollastonite-filled heterophasic polypropylene copolymers.The influences of various factors including the surface hardness, elasticity, friction coefficient, and combinations thereof on the scratch behavior (scratch resistance and scratch visibility) were elucidated. Using micro-scale and nano-scale scratch tests and image analysis techniques, the scratch hardness, scratch depth, and scratch visibility of the composites were characterized.It was found that the introduction of wollastonite in the polypropylene copolymer matrix contributes to ductile fracture behavior because of an induced crystallization alteration. Accordingly, the scratch resistance of reinforced composites revealed an increase as a result of higher stiffness of the wollastonite as well as contribution of new crystalline structure. The addition of siloxane to the composites improved the resistance to surface damage by lowering the surface friction coefficient originated from enhanced chain mobility. Simultaneous addition of high density polyethylene and siloxane induced a significant influence on the resistance to the scratch damage. Copyright © 2015 John Wiley & Sons, Ltd.

Turbulent Boundary Layer on a Finely Perforated Surface Under Conditions of Air Injection at the Expense of External Flow Resources

The characteristics of an incompressible turbulent boundary layer on a flat plate with air blown in though a finely perforated surface from an external confined flow through an input device, located on the "idle" side of the plate, have been investigated experimentally and numerically. A stable decrease in the local values of the coefficient of surface friction along the plate length that attains 85% at the end of the perforated portion is shown. The experimental and calculated data obtained point to the possibility of modeling, under earth conditions, the process of controlling a turbulent boundary layer with air injection by using the resources of an external confined flow.

Pulse electrodeposition of self-lubricating Ni–W/PTFE nanocomposite coatings on mild steel surface

Ni–W/PTFE nanocomposite coatings with various contents of PTFE (polytetafluoroethylene) particles were prepared by pulse current (PC) electrodeposition from the Ni–W plating bath containing self lubricant PTFE particles to be co-deposited. Co-deposited PTFE particulates were uniformly distributed in the Ni–W alloy matrix. The coatings were characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Analysis (EDAX), X-ray Diffractometry (XRD) and Vicker's micro hardness tester. Tafel Polarization and electrochemical Impedance methods were used to evaluate the corrosion resistance behaviour of the nanocomposite coatings in 3.5% NaCl solution. It was found that, the Ni–W/PTFE nanocomposite coating has better corrosion resistance than the Ni–W alloy coating. Surface roughness and friction coefficient of the coated samples were assessed by Mitutoyo Surftest SJ-310 (ISO1997) and Scratch tester TR-101-M4 respectively. The contact angle (CA) of a water droplet on the surface of nanocomposite coating was measured by Optical Contact Goniometry (OCA 35). These results indicated that, the addition of PTFE in the Ni–W alloy matrix has resulted moderate microhardness, smooth surface, less friction coefficient, excellent water repellency and enhanced corrosion resistance of the nanocomposite coatings.