Decrease Of Friction Force Research Articles

TRIBOLOGICAL PROPERTIES OF A RUBBER SEAL UNDER OPERATION WITH OIL CONTAINING MoS2 NANOPARTICLES

The issues associated with sealing operation are of vital significance due to their function in mechanics. The most universal, simple, and cheap solution of seals are O-rings. They are most commonly used for all seals. The article evaluates the operation characteristics of an O-ring seals under operation with oils containing MoS2 nanoparticles. The measurements were made on the sealing node model in a hydraulic cylinder where the round cross section seal cooperated with the piston rod. The changes of the friction force during rod movement resulting from the friction between the seal and the rod were analysed. The investigations presented in this paper allow one to conclude that, for the assumed research conditions, the average values of the friction force during instroke are higher for PAO oil with MoS2 additive in comparison to pure PAO base oil. In addition, the boundary velocity of the rod above in which the decrease of friction force was observed is higher for pure PAO oil.

Electro-plastic metal cutting

In the process of cutting of steels, high strength and heat-resistant alloys a strong warming-up of the cutting instrument takes place, necessitating its cooling by special emulsions and resulting in quick wear and increase of products cost. It was determined by experiment, that during a metal with current cutting, an electro-plastic effect arises. During the lector-plastic cutting, the plastic deformation of a metal under pulse current effect becomes easier, making the friction force between the metal chips and the cutting instrument front edge lower. The electro-plastic metal cutting method accounting the current polarity, current density vector directions, as well as pulse current parameters, can considerably improve the cut surface microstructure and increase the instrument service life. At that, the thermal regime of the cutting can be lowered due to cutting force lowering and heating zone shifting inside the piece or the instrument due to Thomson effect. It was shown, that during the electro-plastic metal cutting the friction force can decrease by 25–30% at the favorable current density vector orientation, as it takes place during electro-static metal drawing and rolling. The current plasticizing action results in decreasing friction force and the chips twisting radius, which can be confluent even for cast iron. At the example of metal drilling with the pulse current, the important current effects on the cutting mechanical parameters revealed. The conditions of metal electro-plastic cutting stated. Results of the experiment study of metal electro-plastic cutting quoted for the processes of steel and cast iron drilling.

FE simulation study of deep drawing process of SUS304 cups having no delayed cracks under enhanced blank holding force

In the experiment, delayed cracks in deep drawing processes of metastable stainless steel SUS304 cylindrical cups were prevented using elevated blank holding force aided by nanolubrication. Besides tensile residual hoop stresses, the elimination of the cracks was also attributed to the change in wall thickening profile along the wavy cup edges. The wall thickening is a result of the high circumferential stress acting in the flange, leading to the high concentration of deformation-induced martensite and high risk of cracks. The amount of increase in wall thickness in the valleys along the edge during the deep drawing process was higher than the peaks at low blank holding force range due to shorter heights. Therefore, the portions of blank equivalent to the valleys were subject to higher holding force during the process, resulting in decrease in degree of wall thickening with increase in height for blank holding force up to 25 kN. However, the wall thickening and the height increased at blank holding force of 28 kN due to the same amount of increase in wall thickness in both valleys and peaks, resulting in a larger contacting area and lower holding force. Therefore, the wall thickness in the valleys sharply increased, and the formation of the cracks persists. Within the crack-free range, that is, from 29 to 31 kN, both the heights and wall thickening decreased. The decrease in frictional force by means of the nanolubrication has facilitated the flow of material into the die, resulting in lower cup height. It also facilitated the flow of materials away from the thick valley regions under the high pressure, resulting in significant decrease in degree of wall thickening. The cracks were prevented. The amount of compression at blank holding force of 32 kN was insufficient to suppress the increase in wall thickening in valleys, resulting in the formation of the cracks again.

High-efficiency laser fabrication of drag reducing riblet surfaces on pre-heated Teflon

Bio-inspired surfaces are able to decrease friction with fluids and gases. The most recognizable are shark-skin-like riblet surface structures. Such bio-inspired surfaces can be formed by the laser ablation technique. In this work, bio-inspired riblet surfaces with grooves were formed using picosecond ultraviolet laser ablation on pre-heated polytetrafluoroethylene (PTFE) at various sample temperatures. The ablation of hot PTFE was found to be 30% more efficient than the conventional laser structuring at the room temperature. The friction of structured PTFE surfaces with the flowing air was investigated by using drag a measurement setup. Results show the decrease of friction force by 6% with dimensionless riblet spacing around 14–20.

Parameter Optimization of a Vertical Spring‐Viscous Damper‐Coulomb Friction System

A vertical spring‐viscous damper‐concave Coulomb friction isolation system was firstly proposed, and their parameters were firstly optimized to achieve the best performance under earthquakes. An incremental dynamic analysis method (IDA) and a performance‐based assessment framework were used to calculate the system and assess its seismic vulnerability, respectively. Results show that both the friction force and the horizontal component of spring force gradually increase when an earthquake enforce the isolator to slide from its central location. Although other papers propose that an increase of spring force and a decrease of friction force can reduce the structural residual displacement, this paper can minimize the residual displacement value to be 0 by using a super lubrication in the middle of contact surface and a variable increment ratio of concave friction distribution. The reason is that the horizontal component of spring force is always greater than the friction force within any relative displacement between the structure and the ground in this paper. As for the peak relative displacement and peak acceleration of structure, one is reduced while the other is increased when selecting the optimal isolation parameters. If the structure is very sensitive to the acceleration response, a low friction parameter, a small spring constant, and a small and even zero damping constant could be adopted to yield a small peak acceleration of system. The tightness of vertical spring can be adjusted to be appropriately loose to continuously reduce the structural acceleration response.

Effect of velocity on polytetrafluoroethylene friction coefficient using molecular dynamics simulaiton

<sec> Velocity is an important factor affecting the friction coefficient of polymers. Polytetrafluoroethylene (PTFE), as a typical self-lubricating polymer, has attracted extensive attention because of its low friction coefficient. Currently, the friction coefficient of PTFE is investigated usually by using experimental method. The experimental study which is limited by the functionality and precision of the apparatus is inaccessible to the exploration of the microscopic tribological mechanism of PTFE. Therefore, the coarse-grained molecular dynamics simulation method is adopted in this study. In the coarse-grained model, one PTFE molecule is simplified into ten beads, including two end beads and eight backbone beads. The non-bonding and bonding interactions between beads are described by using Lennard-Jones (L-J) and multi-centered Gaussian-based potential. In order to investigate the effect of velocity on the friction coefficient of PTFE at an atomic level, we build a two-layer PTFE friction model by using the coarse-grained molecular dynamics simulation method. To directly compare the experimental results with the simulation results, we set the value of the externally applied load and the range of the velocities that match each other as closely as possible. The mechanism of how the velocity affects PTFE friction coefficient is obtained at an atomic level through analyzing the bond length distribution, bond angle distribution, the deformation of the bottom PTFE molecules within the contact area, and the friction force and normal force as a function of simulation time. </sec><sec> The simulation results show that the bond length and bond angle decrease, the deformation of the bottom PTFE molecules along the <i>x</i>-direction and the friction force increase with velocity increasing. This is because the bounce back caused by the deformed PTFE molecules enhances the friction force. The severer the deformation, the larger the friction force will be. However, when the velocity exceeds a critical velocity, the bond length and bond angle increase, the deformation of the bottom PTFE molecule and the friction force decrease with velocity increasing. This is most likely due to the fact that the bottom PTFE molecules within the contact area tend to tilt along the moving direction of the upper PTFE layer, thereby reducing the angle between the upper and the bottom PTFE molecules to an angle close to the angle of parallel sliding, finally resulting in the decrease of the friction force. The deformations of PTFE molecules along the <i>z</i>-direction are nearly invariable under different velocities. This corresponds to the variation of the normal force. Therefore, for a constant externally applied load, the friction coefficient first increases then decreases with velocity increasing. In addition, the critical velocity is 1.2 m/s, which is in line with the published experimental result. </sec>

Force Model for Ultrasonic Needle Insertion

A better understanding of needle-tissue interaction forces when needle is inserted into tissue during injection and minimally invasive surgery can lead to more accurate needle placement, better needle path planning, and better surgical simulation. In this study, a vibratory needle insertion force model is experimentally developed to explore the effect of ultrasonic amplitude on the insertion force. The maximum insertion force is comprised of three forces: tearing force, spreading force, and friction force. Ultrasonic vibration needle insertion experiments are performed on a tissue simulant to measure how vibratory parameters influence friction force, dynamic fracture toughness, and crack length. Results show that with the vibration amplitude changing from 0 to 517.2 μm, the maximum puncture force reduces up to 60.8%, the friction force reduces up to 80.7%, the dynamic fracture toughness of the phantom decreases up to 76.8%, and the crack length increases up to 325.8%. The increase in crack length created by the high amplitude vibration allows for the substantial decrease in frictional force.

Unexpected friction behaviours due to capillary and adhesion effects

In this present work, we show that pouring water into the interface of SiC sliding against the same SiC material makes the friction coefficient far exceed that governing initial dry friction, and a subsequent load increase causes a dramatic abnormal friction force decrease. With the load further increasing to a critical value, friction force sharply increases again. The small amounts of water in the interface generates a large capillary force, the capillary bridges break to cause micro-separation of interface increase to allow bulk water to enter the friction interface, and a squeezing out of the water film to form a strong adhesion force between SiC-SiC materials, are considered to be responsible for the three stages, respectively.

Improvement of glass formability in ultrasonic vibration assisted molding process

Micro optical elements with complex shapes are needed increasingly in optical, optoelectronic and biomedical industries. Since glass molding process is an effective approach to fabricate microstructures, and its surface quality strongly depends on the filling capacity of glass at high temperature. To improve the formability and reduce adhesion between the glass and the mold at high temperature, ultrasonic vibration is applied to improve the formability in the molding process. Fundamental experiments are carried out to test the effectiveness of ultrasonic vibration on friction force decrease and a bonding model on the glass-mold interface at elevated temperature is proposed. Finite element method (FEM) simulation and glass molding experiments are conducted to evaluate the improvements of material formability brought about by ultrasonic vibration. The results show that the ultrasonic vibration can significantly lower the friction force and increase the glass formability.

Investigation of geometrical effects in the carbon allotropes manipulation based on AFM: multiscale approach

Carbon allotropes are used as nanocarriers for drug and cell delivery. To obtain an accurate result in the nanoscale, it is important to use a precise model. In this paper, a multiscale approach is presented to investigate the manipulation process of carbon allotropes based on atomic force microscopy (AFM). For this purpose, the AFM setup is separated into two parts with different sizes as macro field (MF) and nano field (NF). Using Kirchhoff’s plate model, the cantilever (the main part of MF) is modeled. The molecular dynamics method is applied to model the NF part, and then the MF and NF are coupled with the multiscale algorithm. With this model, by considering the effect of size and shape, the manipulation of carbon allotropes is carried out. The manipulations of armchair CNTs and fullerenes are performed to study the diameter changing effects. The result shows that the manipulation and friction force increases by increasing the diameter. The result of the indentation depth for the armchair CNTs indicates that decreasing the diameter causes the indentation depth to reduce. Moreover, the manipulations of four kinds of carbon allotropes with the same number of atoms have been studied to investigate the geometrical effects. The shapes of these nanoparticles change from sphere to cylinder. The results illustrate that the manipulation and the friction force decrease as the nanoparticle shape varies from sphere to cylinder. The Von-Mises results demonstrate that by changing the nanoparticle shape from the spherical to the cylindrical form, the stress increases, although the manipulation force reduces.

Lubrication of Individual Microcontacts by a Self-Assembled Alkyl Phosphonic Acid Monolayer on α-Al2O3(0001).

We report on the tribological behavior of a self-assembled alkyl phosphonic acid monolayer on the microscale using the colloidal probe technique. Friction-load data and adhesion forces were measured with borosilicate glass particles on uncoated and octadecylphosphonic acid (ODPA) coated α-Al2O3(0001) surfaces. A significant decrease in friction force was observed after surface coating, while the adhesion force was only moderately reduced. We assume the lubrication effect of the ODPA self-assembled monolayer (SAM) to be close to the maximum obtainable of alkyl phosphonic acids in the studied system due to the high molecular order which was confirmed by vibrational sum-frequency generation. At small loads, a nonlinear dependence of friction force to load was maintained after surface coating. However, a shift from a contact behavior well described by the DMT model toward the JKR model occurred that is possibly related to the altered elastic properties of the coated surface. With increasing load, a linear friction-load behavior was observed on the coated samples. Molecular plowing and adhesive interactions were identified as responsible mechanisms. In all friction experiments, we could not detect any wear neither of the colloidal probes nor at the surfaces of uncoated and coated samples. This proves the high wear resistivity of the studied ODPA SAM.

THE EFFECTS OF SURFACE ROUGHNESS ON FRICTION REDUCTION AS THE RESULT OF SURFACE TEXTURING BY ABRASIVE JET MACHINING

Various machining methods are currently used to obtain the best co-action of sliding surfaces. Application of two-process surfaces led to a decrease of frictional resistance. Textured surfaces after abrasive jest machining are practical examples of two-process topographies. The results of the application of textured discs with the same array of oil pockets of similar sizes, but with different roughness in areas free of dimples, are presented in this paper. It was determined that after this type of machining, the effect of the friction force decrease was caused by surface texturing.

Frictional Contact Finite Element Analysis of Pneumatic Cylinders

Friction is present in most mechanical systems. The aim of this paper is to analyse friction between a pneumatic cylinder and its piston seal using the finite element method (FEM). The high-fidelity numerical model used real geometry and material characteristics of a double acting pneumatic cylinder. The simulations required fine tuning: the piston seal was created with a hyperelastic, nearly incompressible material model utilizing mixed u-p elements. Three friction characteristics were analysed with slow velocity motion of the piston: Coulomb, an extended Coulomb and the Benson-Hallquist model. The result of the Coulomb model has given a constant friction force after an oscillation at first. The extended Coulomb friction model is used to show the effect of cohesion between two surfaces. After that, the Benson-Hallquist model was simulated by FE and then experimental and FE results were compared. As a result, an exponentially decreasing friction force was obtained. It was shown that the friction behaviors can be simulated with FE and this method will allow studying the effects of different types of piston seal and cylinder barrel on the friction force.

A Computational Chemistry Approach for Investigation of Low Friction Mechanisms Based on FEP Film with Functionalized SiO<sub>2</sub> Nanoparticles

To improve the fuel efficiency of automobile internal combustion engines, we investigated the fundamental mechanism of friction reduction within engine moving parts. A new coating was designed by introducing SiO2 nanoparticles in FEP film. The SiO2 nanoparticles were functionalized with hydrophobic fluoroalkyl units on their surface to create additional low friction property. Universal Surface Tester friction measurements revealed a significant reduction of the friction coefficient with increasing number of hydrophobic fluoroalkyl units for SiO2 surface functionalization. To clarify the friction reduction mechanisms by the functionalization of SiO2 nanoparticles, a quantum chemical calculation was carried out. The result indicates that an attractive force occurs between nanoparticle Si atoms and polymer F atoms, while by adding fluoroalkyl units on the SiO2 nanoparticle surface, this force changes to repulsive. By performing a molecular dynamics simulation of a shear model between FEP film and SiO2 nanoparticles, we observed a decrease of friction force with increasing fluoroalkyl units which lead smooth rolling motion of nanoparticles, thus confirming the repulsive effect of nanoparticle functionalization. We conclude that fluoroalkyl units on the SiO2 surface play an important role in creating a repulsive force between nanoparticle and FEP film which lead to low friction coefficient.

Numerical investigation of seismic behavior of short-core all-steel buckling restrained braces

Conventional buckling restrained braces used in concentrically braced frames are expected to yield in both tension and compression without significant degradation of capacity under severe seismic ground motions. On the other hand, a new short core buckling restrained brace system could be introduced as an alternative for a conventional full core BRB. In a short core BRB (SCBRB), the core element is built shorter than usual. Therefore, for a given story drift, the core accepts bigger axial strains compared to a conventional (full core) BRB. A short core BRB seems to be easily fabricated, inspected, and replaced after a severe earthquake. The purpose of this study is to show how this type of buckling restrained braces is feasible. Reducing the core length in a buckling restrained brace may result in a shorter encasing member, decrease in frictional forces acting at the core and buckling restraining mechanism interface, and, as a consequence, reduction of the compression strength adjustment factor in the brace. This paper numerically investigates the seismic behavior of short core buckling restrained braced frames. The minimum core length of BRB is determined by considering the low cycle fatigue life of the core plate and the maximum anticipated strain demand under standard loading protocol. Nonlinear time history analyses were also performed on four and ten story prototype buildings equipped with full core (conventional BRBs) and short core BRBs and the story drifts were compared. The results showed that the SCBRB system is partially able to reduce the story and residual story drifts in the braced frames. In addition, SCBRBs sustain large plastic deformations without crossing the low cycle fatigue life borders or instability of the encasing system. However, the economic and practical aspects of using SCBRB seem to be more distinct in comparison to its mechanical characteristics.

Nano‐adhesion and friction of multi‐asperity contact: a molecular dynamics simulation study

In our study, the contact and sliding processes between a flat plate and a substrate with multiple asperities are studied by molecular dynamics (MD) simulations, and how the number of asperities and asperity height influence the adhesion force and friction force are investigated thoroughly. The normal force versus the separation distance curve during contact processes is analyzed completely and from which the van der Waals (vdW) force (FvdW) and the adhesion force (Fadh) are obtained and compared with the Katainen model. The adhesion force and the friction force increase linearly as the increase of the number of asperities (i.e. real contact area) with same asperity height. With the identical number of asperities, the adhesion force and the friction force decrease with the increase of the asperity height at first. However the reductions of the adhesion force and the friction force become less obvious, when the asperity height is larger than a critical value (20 Å for our simulation parameters). Copyright © 2015 John Wiley & Sons, Ltd.

MICROSTRUCTURE AND SLIDING WEAR BEHAVIOUR OF STIRCAST TI-GR AND TI –FE METAL MATRIX COMPOSITES

The type and weight percentage of reinforcement play important role in microstructure and sliding wear resistance of the composite. The low melting point nonferrous metal matrix composites Tin- Graphite and Tin-Iron metal matrix composites are cast by the liquid stir casting technique with up to 5 and 15 weight percentages of carbon and iron respectively as reinforcements. The sliding wear characteristics are important if the specimens are used as bearing materials for light duty applications. The specimens are polished and etched for microstructure examination to verify the homogeneous dispersion of reinforcements in the matrix. The microstructure examination reveals the uniform dispersion of reinforcements in the matrix. The specimens are also subjected to dry sliding wear on Pin –on –disc wear testing equipment and the weight loss method technique was used to find specific wear rate at the respective sliding distance. The cumulative specific wear rates and friction coefficients are plotted against sliding distance. The specific wear rate curves show the necessity of an optimum weight percentage of reinforcements in the composite to reduce the wear rate. The frictional coefficient versus sliding distance curves show the decrease in frictional force at higher sliding distances as the weight percentage of particulates increases. At higher sliding distances, in case of the Tin –Graphite composite the surface of the specimen observed to be smooth which makes the graphite particles to expose as flakes and act as solid lubricant to reduce wear. The scanning electron microscope image of the surface which is normal to the sliding surface also studied to verify the flake formation during wear. Both mild and severe wear regions are observed.

Nanoscale frictional behavior of graphene on SiO2 and Ni(111) substrates

Friction characteristics of graphene deposited on different substrates have been studied by atomic force microscopy (AFM). In particular, we compared mechanically exfoliated graphene transferred over Si/SiO2 with respect to monolayer (ML) graphene grown in our laboratory by low temperature chemical vapor deposition on Ni(111) single crystal. Friction force measurements by AFM have been carried out as function of load under different environment conditions, namely vacuum (10−5 Torr), nitrogen and air. The typical decrease of friction force with increasing number of layers has been observed on graphene over Si/SiO2 in all environment including vacuum. Continuum mechanical approximation has been used to analyze the friction versus load curves of ML graphene on Ni(111). Analysis shows that Derjaguin–Mueller–Toporov model is in good agreement with our experimental data indicating that overall behavior of the interface graphene–Ni(111) is relatively rigid respect to out of plane deformations. This result is consistent with the structural characteristics of the interface since graphene grows in registry with Ni(111) surface with covalent bonding character. Finally, the shear strength and the work of adhesion of the two systems with respect to AFM tip in vacuum have been compared. The result of this procedure indicates that shear strength and work of adhesion measured on graphene–Si/SiO2 interface are always greater than those on graphene–Ni(111) interface.

1P1-R02 金属の電着による摩耗修復機構の提案

We proposed repairing method for the self-repairable joint of robot body. We adopted the copper-electroplating from three proposed methods (plating, self-assembled monolayers, and electro polymerization). We conducted two tests; evaluation test and abrasion test. In evaluation test, we established 3 criteria (adhesion, smoothing the surface, and decrease of friction force) to evaluate repair performance of the plating. In abrasion test, we made a system that supposed the joint, and tested abrasion and plating simultaneously. As a result of evaluation test, we confirmed that plating can repair the worn down surface. As a result of abrasion test, we confirmed that plating can repair the surface in the sliding portion.

S1620103 ナノ厚さ潤滑膜を介した固体二面間摺動時の接触状態と凝着・摩擦特性に関する研究

To achieve ultra-high recording density magnetic disk drives, it is essential to understand contact conditions and adhesion and friction properties between two sliding solid surfaces mediated by nanometer-thick lubricant films. With the aim of gaining such an understanding, we designed and developed a coaxial vertical illumination type microscope and integrated it with a previously developed pin-on-disk type tribotester. Measurements were performed with a semi-spherical glass sliding pin loaded on a magnetic disk covered by a 1.4-nm-thick perfluoropolyether Z03 film as a function of sliding speed. At sliding speeds lower than 2.09 mm/s, we observed that a liquid bridge formed between the sliding pin and disk surfaces, and it grew with time particularly in the direction of disk rotation. With increasing sliding speed, adhesion force decreased monotonically, whereas friction force decreased initially and then increased. We suggest that the sharp decrease of friction force in low sliding speed regime is associated with the sharp decrease of adhesion force, which is one component of the normal loading force, and the sharp decrease of adhesion force is due to the decrease of meniscus force caused by the increased difficulty in forming liquid bridge with increasing sliding speed.