Surface Friction Coefficient Research Articles

Modeling the micro-scale static friction coefficient of the MEMS silicon surfaces affected by Ag and Au deposition using the thermal evaporation method

In different macro-scale applications, regarding what should be done, many of the prepared surfaces are immediately used after machining with no limitation. Since the micro-electromechanical applications have different conditions, their different properties, such as friction and adherence, must be checked. In the present study, using the thermal evaporation method, the silver and gold layers with different thicknesses were deposited on the silicon surfaces widely used in micromechanical systems (e.g. micro-assembly). Thereafter, the effects of the deposition on the geometrical properties of the surfaces were studied. Also, the effects of the deposition on the adherence and friction properties of the surfaces were examined by modeling the geometrical condition. To this end, the height function distribution was used to exploit the surface friction model in conjunction with the DMT surface adhesion model. The results show the extent to which gold affects the adhesion and friction properties of the coated surfaces.

Enhanced surface hydrophilicity of polylactic acid sutures treated by lipase and chitosan

Polylactic acid (PLA) surgical sutures are a new type of absorbable sutures that can be degraded and absorbed in the body. However, there is high hydrophobicity for the surface of PLA sutures, which leads to poor biocompatibility and cellular affinity. In order to increase the hydrophilicity, the PLA sutures were etched by lipase firstly, and then grafted with chitosan. The results indicate that the optimal conditions of treating PLA sutures by lipase were as follows: 45℃ reaction temperature, 4.5 g/L concentration of lipase and 8 h reaction time. The sutures were etched by lipase and then formed some grooves and a number of hydroxyl (-OH) bonds, which led to increased surface area and hydrophilicity, but a drop in mass and strength. The optimal conditions of grafting chitosan onto PLA sutures were as follows: 4 h reaction time and 3 g/L concentration of chitosan. The chitosan grafted and loaded on the surface of PLA sutures, and in some areas of the sutures the chitosan reunited, which led to a rough surface and large friction coefficient. Finally, the hydrophilicity of the PLA sutures, treated by lipase and then grafted with chitosan, was greatly improved.

Numerical Investigation of Transitional Flow over a Flat Plate under Constant Heat Fluxes

In this study, a boundary layer flow over a flat plate is investigated numerically at constant inlet freestream velocity and turbulence intensity. After intensive mesh refinements, an adequate computational domain is determined. Four turbulence models (k-epsilon, k-omega, k-omega SST, Transition SST) are used to analyze the boundary layer flow. Local surface friction coefficient distribution is obtained and compared to each other to assess the most convenient turbulence model. The Computational Fluid Dynamics (CFD) results show that the Transition SST turbulence model demonstrates the most realistic surface friction coefficient (Cf) distribution in agreement with the experimental data. Additionally; the effects of constant heat fluxes on Cf values are investigated and it is found that the heating process moves transition backward compared to isothermal case. Moreover, it is fount that Cf values in the turbulent region decrease compared to isothermal case.

Improvement of friction characteristics of cast aluminum-silicon alloy by laser shock peening

Piston friction is the primary cause of power loss in automotive engines and thus reducing the friction on piston surface is crucial for the improvement of engine performance. In this work, the effects of laser shock peening on friction characteristics of JIS-AC8A aluminum-silicon alloy, a piston material for automotive engines, were investigated experimentally. Laser shock peening was carried out using a Nd:YAG laser (wavelength = 532 nm, pulse width = 8 ns) at the conditions of laser intensity 4 GW/cm2, overlapping ratio 50% and spot diameter 2.06 mm with no protective coating. When laser shock peening was applied, surface hardness increased by 22% and significant enhancement of compressive residual stress was achieved. Friction coefficients of the laser peened surface decreased by 19%, 41% and 45% from those of unpeened surface at the 50, 100 and 150 N normal load conditions, respectively, demonstrating the effectiveness of laser shock peening in reducing surface friction. Also, the mass loss by wear decreased by 94%.

Relevant factors affecting the direction of crack propagation in complete contact problems under fretting fatigue

Abstract In fatigue problems, an accurate estimation of the propagation direction is important for life prediction. We identify the most relevant factors that affect the crack orientation during the propagation stage of fretting fatigue cracks, arising from complete contacts. Contrary to what initially expected, parameters such as normal load, cyclic bulk load, etc. do not have a noticeable influence on the orientation. However the relative Young's moduli of indenter/specimen materials, the indenter width and the surface coefficient of friction are the most influencing factors. Analyses are performed through the extended finite element method (X-FEM) and an orientation criterion for non-proportional loading proposed by the authors. Experimental fretting fatigue tests confirm the predicted trends. An explanation of this behaviour is also given.

Numerical study of the effects of surface roughness on the mixed convection heat transfer of a laminar flow inside a horizontal curved dimpled tube

The effects of surface roughness on the hydro-thermal performance of laminar mixed convection heat transfer of water have been investigated numerically. The geometry was a semicircular curved horizontal dimples tube with a fixed dimensionless radius of curvature (2R/D = 6.62) and dimensionless roughness height of e/D = 0.1. Under low values of Re (100,200,400) and Gr numbers (20,000.100000,400,000), a constant heat flux was imposed on the tube walls. The three-dimensional governing equations were discretized, using a finite volume method. Dimensionless temperature, convective heat transfer coefficient, axial dimensionless velocity contours, secondary flow vectors, and surface friction coefficient are presented and discussed. The results showed that for identical conditions, the dimpled tube has better thermal performance compared to a smooth tube, especially in low Re values. So that in the low Re values, the dimpled tube has 58.2% higher heat transfer coefficient in comparison with the smooth ones. However, this predominance in thermal characteristics is mitigated by increasing Re number. Additionally, it has been shown that the skin friction coefficient of a curved dimpled tube is less than that of a smooth one in low Reynolds values. Using Colburn j factor and friction coefficient (f), and presenting j/f versus Re shows that the dimpled tube is more efficient at Re < 200.

Proposal of the Tire Longitudinal Characteristics Real-Time Estimation Method

For vehicle dynamics control and Autonomous Driving (AD) system, it is important to know the friction coefficient μ of the road surface accurately. It is because the lateral and the longitudinal force characteristics of the tire depend on the road surface condition largely. However, currently, it is difficult to detect tire performance degradation before the deterioration of vehicle dynamics in real time because tire force estimation is usually conducted by comparing the observed vehicle motion with the onboard reference vehicle-model motion. Such conventional estimators do not perform well if there is a significant difference between the vehicle and the model behavior. In this paper, a new tire state estimation method based on this tire longitudinal characteristic is proposed. In addition, the estimator for tire-road surface friction coefficient μ is proposed by using this geometric relationship. Using this method, the friction coefficient value for a real road can be determined from relatively simple calculations. Also, the advantage of this method is that it can be estimated in a small slip region before the tire loses its grip. In addition, this paper explain how to apply and the effect on the actual vehicle.

Microstructure of High-Performance Aluminum Alloy Surface Processed by the Single-Excitation Same-Frequency Longitudinal⁻Torsional Coupled Ultrasonic Vibration Milling.

The high performance of parts is determined by the microstructure of the machined surface to some extent. Different processing methods have been used to construct different microstructures on machined surfaces; the effective improvement of the serviceability of parts has been the focus of research in the field of precision and ultra-precision machining. In the presented work, a microscratch was formed on the machined surface in ultrasonic assisted machining, and the surface microstructure of high-performance aluminum alloy processed by single-excitation rotational longitudinal–torsional coupled ultrasonic vibration (LTCUV) milling was investigated. First, the motion paths model of the cutting edge in the LTCUV milling were established; then, the single-excitation LTCUV milling system has been set up, and the acoustic performance of the LTCUV system was examined. The surface microstructure of aluminum alloy was processed by different machining techniques, and the effect of processing parameters on the surface microstructure and performance were investigated by the orthogonal design of experiment (DOE). The surface roughness was found to be proportional to the ultrasonic cutting speed and feeding rate. The surface roughness was mainly controlled by the ultrasonic amplitude, and the optimal surface quality corresponded to the ultrasonic amplitude of 4 μm. The cutting speed contributes greatly to the surface roughness. The water contact angle of surfaces obtained by ultrasonic processing was larger than that of surfaces achieved by the conventional processing, while the surface water contact angle was negatively related to the ultrasonic amplitude. Once the rotation speed exceeded a critical level, the ultrasonic amplitude exerted a negligible effect on the surface water contact angle. The cutting speed contributes the most to the water contact angle. The friction coefficients of surfaces treated by ultrasonic processing were lower than those obtained by conventional processing at constant processing parameters, while the friction coefficient was minimized at the ultrasonic amplitude of 4 μm. In the case of grease lubrication friction, the surface wear decreased with the ultrasonic amplitude, indicating the improved wear resistance of the processed surfaces. Similarly, the ultrasonic amplitude has the highest contribution rate to friction and wear.

Design and dynamic analysis of landing gear system in vertical takeoff and vertical landing reusable launch vehicle

Landing gear system is a key part of the implementation of reusable vertical takeoff and vertical landing launch vehicle, where its buffing performance is directly related to the vehicle whether it can land safely or stably. According to the reusable launch vehicle general scheme, outrigger landing legs are designed, and the hydraulic absorber is used for the landing gear system. Meanwhile, a scaling principle prototype of landing gear system is developed, and the landing impact test is carried out. A dynamic simulation model of the landing vehicle has been set up, researching the influence of parameters, such as the horizontal velocity, initial inclination, surface friction coefficient, and pitch angular velocity on the landing performance. Four kinds of extreme conditions are identified, and dynamic response characteristics of landing system under each extreme condition are conducted. The simulation results are in good agreement with the experimental data. The buffing performance of the vehicle meets the design requirements, which provides a reference for the design of landing gear system of the vehicle.

Conversion of organic films into fluorine-containing onion carbon as anti-adhesion solid lubricants

Coating a thin-layer solid lubricant with low adhesion and friction is desired for silica-based micro/nano electromechanical systems (MEMS/NEMS). Onion carbon is a promising one, especially fluorine-containing onion carbon with low surface adhesion. Herein we report a new route to synthesize fluorine-containing onion carbon (FOC) through carbonizing electrophoresis-deposited organic films. A little fluorine (1.65–3.64 at.%) could manyfold decrease the surface adhesion. The optimized FOC integrates the merits of fluorine doping and onion carbon, presenting ultralow surface adhesion (2.72 nN) and friction coefficient (0.03). These results suggest that this strategy is a promising approach to fabricate the desired solid lubricants for MEMS/NEMS.

Enormously Low Frictional Surface on Tough Hydrogels Simply Created by Laser-Cutting Process

We measured the friction forces and calculated the friction coefficients of non-processed and laser-processed surfaces of a double network hydrogel (DN gel), which is one of the more famous high-strength gels. The results indicate that laser processing has the ability to reduce the friction coefficients of the gel surfaces. The observation of gel surfaces suggests that the cause of friction reduction is a change in the roughness of the gel surfaces due to laser processing. This finding is expected to lead us to further understanding of the physicochemical properties of hydrogels.

Friction Properties and Surface Observation By Surface Processing on High Strength Gels

We measured the friction forces and calculated the friction coefficients of non-processed and laser-processed High Strength gel surfaces. The results indicate that laser processing (finishing) has an ability to reduce the friction coefficients. The observation of gel surfaces suggests that the cause of friction reduction is change in roughness of gel surfaces due to laser processing. This finding is expected to lead us to further understanding of physicochemical properties of hydrogels.We showed the low-friction processing technique. We demonstrated that the laser processing has an ability to reduce the friction of hydrogel surfaces. The results of microscopic observations suggest that the cause of friction reduction is decrease in apparent contact area between hydrogel surface and glass ball. This technique may lead us to next step in the fields of physics, physical chemistry, and medical applications.

Application of Model Kinetic Equations to Calculations of Super- and Hypersonic Molecular Gas Flows

For the purpose of taking the internal degrees of freedom into account, threetemperature approximating model equations, which are a generalization of the R- and ES–BGKmodels, are proposed for a diatomic gas. The surface pressure, friction, and heat transfer coefficients are compared with the direct simulation Monte Carlo (DSMC) solution in the problem of flow past a cylinder in the super- and hypersonic flow regimes. The dependence of the surface coefficients on the rotational collision number is analyzed.

Detection and Characterization of Randomly Distributed Micro-cracks in Elastic Solids by One-Way Collinear Mixing Method

Numerical simulations on the propagation of bulk waves in elastic solids with randomly distributed micro-cracks by the one-way collinear mixing method are performed to investigate the behavior of the resonant wave. The results show that the resonant wave can be generated due to the effect of micro-cracks, and the position and the size of micro-crack zone can be detected by the time signal of the resonant wave. Based on statistical analysis from numerous results of random micro-crack models, we find that the acoustic nonlinear parameter increases linearly with the micro-crack density and the size of micro-crack zone, which is also related to the frequency of resonant wave and friction coefficient of micro-crack surfaces. This study numerically reveals that the one way mixing method can be employed to quantitatively identify and characterize micro-cracks in elastic solids.

Numerical modelling of lockbolted lap connections for aluminium alloy plates

A numerical modelling method for lockbolted lap connections of aluminium alloy plates is studied based on experimental tests. The mechanical properties and constitutive models of aluminium alloy 6061-T6 and stainless steel 304HC were intensively determined by tensile tests. A tensile test of a single-lockbolted lap connection was conducted to obtain its elongation and strain responses. A basic numerical model that considers the lockbolt preload, interface contact and friction, and gaps between the lockbolt and bolt hole was established to simulate the tensile behaviours of this connection. This model was updated to minimize errors between the numerical and test results, and values for the friction coefficients of different contact surfaces were suggested. To improve the accuracy and efficiency of the numerical analysis, an incompatible mode solid element was adopted in the numerical model. It was determined to be better than refining the mesh or accepting a higher-precision element type. This numerical modelling method was investigated by two types of double-lockbolted connections, and its validity was verified by comparing the displacement and strain responses between the numerical and test models. The modelling method suggested in this paper is applicable for simulating the mechanical behaviours of complicated joints with lockbolted lap connections.

Effect of Workpiece Surface Topography on Friction in Cold Forging Using Environmentally-Friendly Lubricant

An optimal surface topography for the use of an environmentally friendly lubricant was elucidated, by making a wide combination of surface roughnesses Ra and RSm representing height and frequency of surface asperity, using the wet blast treatment. A forward conical can - backward straight can extruded friction test method proposed by the present authors was used. Coulomb friction coefficient was evaluated using a calibration curve representing the relationship between the surface friction coefficient of the conical punch and the front/back extrusion height of the workpiece. It was found that the surface topography of workpiece largely affects the lubricating property when the friction coefficient is large. Especially, the surface treatment with larger Ra and smaller RSm is effective for improving the friction. Furthermore, the correlation between the friction coefficient and the proposed parameter for the surface topography Ra/RSm was shown to be better than the Ra.

Impact of drilling fluids on friction coefficient of brittle gas shale

The stability of the horizontal long section of a wellbore is the main restriction of efficient development of shale gas. The use of oil-based drilling fluids in the initial stage of Longmaxi shale gas exploration in the Southern Sichuan Basin, China still causes wellbore collapses for reasons not well understood. Based on the geological characteristics and engineering conditions of brittle shale, a significant factor influencing shale wellbore stability-friction coefficient is proposed. Due to the developed facture networks of brittle shale in this region, frictional sliding along the fracture surface is the main source of movement within the Longmaxi Formation. Here, we determined the effects of drilling fluids on the Longmaxi Formation shale in Southern Sichuan Basin. Natural fractures slide due to compression shear slip, and the friction coefficient of the fracture surfaces is a controllable factor of shear and slip in natural fractures in shales. The typical experimental apparatus of rock friction sliding was modified to accommodate for drilling fluids and typical in-situ temperatures. Results show that the friction coefficient decreases, induced by infiltration of drilling fluids filtrate, and the friction coefficient is more sensitive to oil-based drilling fluid filtrate than water-based. The high-pH oil-based drilling fluid filtrate shortens the length of time taken by static friction stage, and smooths the time vs friction coefficient curve, which creates an easier situation for frictional sliding. High viscosity and good wettability lead to the strong lubricity of oil-based drilling fluid. The surface properties of the shale samples changed after the high alkalinity fluid immersion, such as fracture surface planarization, and floc-like particle generation, causing an obvious reduction of friction coefficient between fracture surfaces. As a result of rock strength decreasing after immersion into drilling fluids filtrate, asperities on fracture surface were prone to shear failure in the process of frictional sliding, and their failure products migrated with the fracture surface movement. This not only decreased the roughness shale surface but also acted as a friction reduction agent between the sliding surfaces. This study provides theoretical guidance for drilling fluid selection and optimization during brittle shale well drilling.

Effect of Electropulsing-Assisted Ultrasonic Nanocrystalline Surface Modification on the Surface Mechanical Properties and Microstructure of Ti-6Al-4V Alloy

The effect of electropulsing-assisted ultrasonic nanocrystalline surface modification (EP-UNSM) on surface mechanical properties and microstructure of Ti-6Al-4V alloy is investigated. Compared to conventional ultrasonic nanocrystalline surface modification (UNSM), EP-UNSM can effectively facilitate surface roughness and morphology, leading to excellent surface roughness (reduced from Ra 0.918 to Ra 0.028 μm by UNSM and Ra 0.019 μm by EP-UNSM) and smoother morphology with less cracks and defects. Surface friction coefficients are enhanced, resulting in lower and smoother friction coefficients. In addition, the surface-strengthened layer and ultra-refined grains are significantly enhanced with more severe plastic deformation and a greater surface hardness (a maximum hardness value of 407 HV and an effective depth of 550 μm, in comparison with the maximum hardness value of 364 HV and effective depth of 300 μm obtained by conventional UNSM). Remarkable enhancement of surface mechanical properties can be attributed to the refined gradient microstructure and the enhanced severe plastic deformation layer induced by coupling the effects of UNSM and electropulsing. The accelerated dislocation mobility and atom diffusion caused by the thermal and athermal effects of electropulsing treatment may be the primary intrinsic reasons for these improvements.

A new approach to improve the surface properties of H13 steel for metal forming applications by applying the TiAlN multi-layer coating

This work is an attempt to improve the mechanical and tribological properties of nanostructured TiAlN coatings through the plasma nitriding/TiN/TiAlN/Ti(2)N process for metal forming applictions. The coatings were deposited using the pulsed DC plasma-assisted chemical vapor deposition (PACVD) method at the constant working conditions including the frequency of 10 kHz, the deposition temperature of 470 °C, and the duty cycle of 33%. H2, Ar, and N2 gases and TiCl4 and AlCl3 precursors were fed to the chamber as the depositing agents. The results indicated that the three-layered coatings (TiN/TiAlN/Ti(2)N) have the enhaced adhesion strength and wear resistance than the single-layered ones (TiN or TiAlN). This is also showed that the surface morphology, grain size, and friction coefficient of the TiN coating are respectively smoother, smaller, and lower than those of the TiAlN coating. Accordingly, it was concluded that the Ti(2)N coating as a top coat is an appropriate material to improve the tribological and mechanical properties of nanostructured TiAlN coatings.

Improving surface wettability and lubrication of polyetheretherketone (PEEK) by combining with polyvinyl alcohol (PVA) hydrogel.

Poor surface wettability and relative high friction coefficients of pristine polyetheretherketone (PEEK) have limited its application in orthopedic implants. In this study, inspired by the structure of natural articular cartilage, we presented a novel method to fabricate a "soft-on-hard" structure on the surface of pristine PEEK specimens, which combined a soft polyvinyl alcohol (PVA) hydrogel layer and a three-dimensional porous layer with PEEK substrates. A variety of analytical methods were used to evaluate their properties, our results demonstrated that the hydrogel layer could be seamlessly connected with substrate, and the hydrogel-covered PEEK owned a highly hydrophilic surface, a very low water contact angle of 7° could be obtained. The friction coefficients of untreated and hydrogel-covered PEEK surfaces were measured using a tribometer under water lubrication, due to the presence of the top hydrogel layer and the hard substrate could provide excellent aqueous lubrication and bearing capacity, respectively, the friction coefficient could be reduced from 0.292 to 0.021. In addition, the porous layer under PVA hydrogel layer could work as gel reservoirs, the reserved hydrogel would be released after the surface layer was sheared off, and a regenerable lubrication status was obtained. This work provides a new route for the design of improving the surface wettability and tribological properties of PEEK.