Average Friction Force Research Articles

Friction of two-dimensional colloidal particles with magnetic dipole and Lennard\u2013Jones interactions: A numerical study

We use Langevin simulations to study the sliding friction of two-dimensional colloidal particles on a substrate with randomly distributed point-like pinning centers. The colloidal particles are modeled to interact with each other through repulsive magnetic dipole and attractive Lennard–Jones potentials. The subsequent occurrence of superlubricity, wherein the average friction force equals to zero, is accompanied by the appearance of islands with clear boundaries in the microscopic colloidal structures for weak pinning substrates. Friction arises for strong pinning substrates, and the average friction force increases with the substrate pinning intensity, and further, the islands disperse into disordered plastic structures. Moreover, the average friction force decreases with the repulsion intensity between the colloidal particles, and superlubricity finally results when the repulsion becomes sufficiently strong. Superlubricity also occurs for sufficiently weak attraction between colloidal particles, with an increase in the attraction intensity between colloidal particles leading to a nonlinear increase in the average friction force. With increasing temperature, the average friction force firstly increases and subsequently decreases rapidly. The above results can provide a theoretical framework for biological self-organization via utilization of the friction properties of microscopic or mesoscopic colloidal systems.

Influence of crystal anisotropy on deformation behaviors in nanoscratching of AlN

Abstract In this paper molecular dynamics simulations are carried out to study the influence of crystal anisotropy on deformation behaviors in the nanoscratching process of AlN. We investigate the scratching process of AlN(0001), AlN(10 1 ¯ 0) and AlN( 1 ¯ 2 1 ¯ 0) surfaces, respectively. It is found that there exists significant anisotropy of the deformation behaviors of AlN for various crystal orientations in the scratching process. The defects evolution, surface topographies, scratching forces and pile-up in the scratching process are closely related to the crystal orientations of AlN workpiece. We observe that the dislocation activities in the scratching process have different characteristics for different crystal orientations of AlN. We also find that many atoms accumulate around the scratch groove evenly in scratching the AlN(0001) surface, while obvious chips are formed in front of the abrasive in scratching the AlN(10 1 ¯ 0) and AlN( 1 ¯ 2 1 ¯ 0) surfaces. Furthermore, the average friction force is the smallest for the AlN(0001) surface and the biggest for the AlN(10 1 ¯ 0)surface. However, the average normal force is the biggest for the AlN(0001) surface and the smallest for the AlN( 1 ¯ 2 1 ¯ 0) surface. In addition, the pile-up height is the largest in scratching AlN( 1 ¯ 2 1 ¯ 0) surface and the smallest in scratching AlN(0001) surface.

Sliding friction between two silicon-carbide surfaces

Sliding friction between two SiC surfaces is important due to its relevance to many practical applications. It is also important to study whether kinetic friction at the nanoscale follows Coulomb’s law. Since SiC exists both as an amorphous material and with a crystalline structure, the effect of surface roughness on the kinetic friction may also be significant. We report the results of an extensive molecular dynamics simulation of sliding friction between surfaces of the two types of SiC over a wide range of sliding velocities. The amorphous SiC was generated by the reactive force field ReaxFF, which was also used to represent the interaction potential for the simulation of sliding friction. As the sliding velocity increases, bond breaking occurs at the interface between the two surfaces, leading to their roughening and formation of excess free volume. They reduce the kinetic friction force, hence resulting in decreasing the difference between kinetic friction in the amorphous and crystalline surfaces. The average kinetic friction force depends nonlinearly on the sliding velocity V, implying that Coulomb’s law of friction is not satisfied by the surfaces that we study at the nanoscale. The average kinetic friction force Fk depends on V as Fk∝ln⁡V.

Influence of Nitride Hard Coating on Friction and Soldering on Surface of Aluminum High Pressure Die Casting Core

Wear and soldering are the two common failure types of high pressure die casting molds. Surface treatment solutions are often offered to limit these two forms of failure. In this study, the authors compared the average frictional force occurring when the die casting core was removed from the aluminum block to evaluate the effect of the chromium nitride and titanium nitride coating on the friction properties of the mold surface. Experience had shown that hard nitride coating significantly reduced the friction between the core and castings by aluminum alloys as well as significantly reduced the amount of adhesive applied on the mold surface. The experiment also showed that the coating of chromium nitride also reduced the level of damage to the core surface in real working conditions.

Coarse-grained molecular dynamics simulation on friction behaviors of textured ag-coating under vacuum and microgravity environments

Adhesive effects in vacuum and disturbance-induced impact in microgravity make the friction different from the traditional theory. In this paper, an impact sliding contact model is developed by coarse-grained molecular dynamics simulations under the vacuum and microgravity environments. Textured surfaces are designed, and the effects of texture parameters on friction forces are investigated. The results show that the friction forces could be underestimated if the impact is ignored. Compared with a smooth surface, the textured surfaces can improve friction behaviors during impact sliding contact process. The average friction forces are increased as the increase of the texture width. The textured surface with 0° orientation produces a lower average friction force than other orientations and textured surface with rectangular shape shows the optimum friction behaviors.

Molecular Dynamics Simulation of Nanoscale Abrasive Wear of Polycrystalline Silicon

In this work, molecular dynamics simulations of the nanoscratching of polycrystalline and singlecrystalline silicon substrates using a single-crystal diamond tool are conducted to investigate the grain size effect on the nanoscale wear process of polycrystalline silicon. We find that for a constant indentation depth, both the average normal force and friction force are much larger for single-crystalline silicon compared to polycrystalline silicon. It is also found that, for the polycrystalline substrates, both the average normal force and friction force increase with increasing grain size. However, the friction coefficient decreases with increasing grain size, and is the smallest for single-crystalline silicon. We also find that the quantity of wear atoms increases nonlinearly with the average normal load, inconsistent with Archard’s law. The quantity of wear atoms is smaller for polycrystalline substrates with a larger average grain size. The grain size effect in the nanoscale wear can be attributed to the fact that grain boundaries contribute to the plastic deformation of polycrystalline silicon.

Molecular dynamics simulation of friction and heat properties of Nano-texture GOLD film in space environment

In this paper, a molecular dynamics model is proposed to describe the collision friction of a hinge mechanism in the space environment. The EAM potential is used to describe the interaction between atoms. Based on the model, the collision friction process and thermal properties between the indenter and smooth gold film surface at different indenter amplitudes, frequencies and crystal orientations are studied. The temperature distribution and collision friction mechanism of the gold film are analyzed. The simulation results show that increasing the vibration frequency of the indenter in a certain frequency range can reduce the friction force. The amplitude of the indenter and crystal orientations at different frequencies shows different effects on the average friction force and the surface temperature of the substrate. In addition, textures are also made on the gold film and the effects of texture width and texture depth on the friction and thermal properties are investigated. The results show that the textured surface can significantly reduce the friction force. The average friction force and the surface temperature of the substrate decrease with the increase of texture depth, and increase with the increase of texture width. The average friction force and the surface temperature of the substrate are more sensitive to the texture depth.

Nanoscale Reciprocating Sliding Contacts of Textured Surfaces: Influence of Structure Parameters and Indentation Depth

Textured surfaces are widely used in engineering components as they can improve tribological properties of sliding contacts, while the detailed behaviors of nanoscale reciprocating sliding contacts of textured surfaces are still lack of study. By using multiscale method, two dimensional nanoscale reciprocating sliding contacts of textured surfaces are investigated. The influence of indentation depth, texture shape, texture spacing, and tip radius on the average friction forces and the running-in stages is studied. The results show that the lowest indentation depth can make all the four textured surfaces reach steady state. Surfaces with right-angled trapezoid textures on the right side are better for reducing the running-in stage, and surfaces with right-angled trapezoid textures on the left side are better to reduce wear. Compared with other textured surfaces, the total average friction forces can be reduced by 82.94%–91.49% for the case of the contact between the tip with radius R = 60r0 and the isosceles trapezoid textured surface. Besides, the total average friction forces increase with the tip radii due to that bigger tip will induce higher contact areas. This research proposes a detailed study on nanoscale reciprocating sliding contacts of textured surfaces, to contribute to design textured surfaces, reduce friction and wear.

On the Effect of Longitudinal Vibrations on Dry Friction: Modelling Aspects and Experimental Investigations

High-frequency vibrations have been shown to smooth the effective characteristics of dry friction and overcome undesired effects such as friction-induced vibrations. Although the effect of vibrational smoothing is subject of many theoretical studies and has been applied successfully to metal forming processes, only few publications are known presenting advanced modelling approaches or related experimental results. Using classical Coulomb friction, the agreement between experimental results and related calculations appears not to be sufficient. Therefore, the effect of contact compliance on vibrational smoothing has been discussed recently. Considering a class of dynamic friction models, the effect of high-frequency excitation on a simple 1-DoF friction oscillator has been investigated. The corresponding results show a mostly quantitative impact of compliant contacts on vibrational smoothing, while the qualitative results remain unchanged. Within this contribution, the recent analysis is extended using an elasto-plastic friction model and accounting for stiction and the transition to sliding motion. The corresponding results show similar characteristics, but contain new qualitative aspects and may thus serve as an explanation approach for insufficient agreement between experimental results and classical modelling. In order to validate the advanced approach, an experimental set-up is presented. Measurements are performed showing the hysteretic behaviour of the contact as well as the reduction of average friction forces in presence of vibrational excitation. A comparison of experimental and previously obtained numerical results yields good agreement and thus more insight into relevant aspects of vibrational smoothing.

Atomistic modeling of tribological properties of Pd and Al nanoparticles on a graphene surface

Background: The frictional properties of nanoparticles have been studied to gain insight into the fundamental origin of sliding friction.Results: Using molecular dynamics we investigate frictional properties of aluminum and palladium nanoparticles deposited on a graphene layer. We study the time evolution of the total momentum of the system, the total and potential energies, the temperature, the velocity and position of the center of mass, the dimensions of the nanoparticle, and the friction and substrate forces acting on the particle. We also study how the friction force depends on the nanoparticle–graphene contact area and the temperature.Conclusion: The tribological properties of nanoparticles strongly depend on the materials. The particles move in an irregular (saw-like) manner. The averaged friction force depends nearly linearly on the contact area and non-monotonously on temperature. We observe ordered crystalline domains of atoms at the bottom surface of the metal particles, but the peaks of radial distribution function are blurred indicating that the nanoparticles are amorphous or polycrystalline.

The Prandtl–Tomlinson model of friction with stochastic driving

We consider the classical Prandtl–Tomlinson model of a particle moving on a corrugated potential, pulled by a spring. In the usual situation in which pulling acts at constant velocity , the model displays an average friction force σ that relates to (for small as , where is a critical friction force. The possible values of β are well known in terms of the analytical properties of the corrugated potential. We study here the situation in which the pulling has, in addition to the constant velocity term, a stochastic term of mechanical origin. We analytically show how this term modifies the force-velocity dependence close to the critical force, and give the value of β in terms of the analytical properties of the corrugation potential and the scaling properties of the stochastic driving, encoded in the value of its Hurst exponent.

Lubrication adaptability to the variations of combustion modes by texturing cylinder liner in engines

The application of different combustion modes in engines has placed greater demands on the lubrication of cylinder liners and piston rings. In this study, a mixed lubrication model was built to investigate the tribological properties of a liner textured with micro-dimples. The simulation results indicate that surface texturing can improve the lubrication and friction properties of the ring/liner pair and adapt to different combustion modes in an engine. Due to the significant differences in external loads around top dead center of the compression stroke under pre-mixed charge combustion ignition, conventional combustion and retarded injection modes, when the liner remains untextured, the lubrication performance of the friction pair appears to be unstable in a mixed lubrication region. For the textured liner, micro-dimples enhance the hydrodynamic effects between the ring and liner, increasing the oil film thickness and its bearing capacity. As a result, the mixed lubrication interval shrinks and the micro asperity contact is reduced, thus improving the tribological properties of the friction pair. The numerical fluctuation ranges of parameters, the maximum asperity pressure, the average friction force and the frictional loss, are narrowed by 21.6%, 27.8% and 24.5%, respectively, under the three different combustion modes around top dead center of the compression stroke.

Numerical study on the lubrication performance of compression ring-cylinder liner system with spherical dimples.

The effects of surface texture on the lubrication performance of a compression ring-cylinder liner system are studied in this paper. By considering the surface roughness of the compression ring and cylinder liner, a mixed lubrication model is presented to investigate the tribological behaviors of a barrel-shaped compression ring-cylinder liner system with spherical dimples on the liner. In order to determine the rupture and reformulation positions of fluid film accurately, the Jacoboson-Floberg-Olsson (JFO) cavitation boundary condition is applied to the mixed lubrication model for ensuring the mass-conservative law. On this basis, the minimum oil film thickness and average friction forces in the compression ring-cylinder liner system are investigated under the engine-like conditions by changing the dimple area density, radius, and depth. The wear load, average friction forces, and power loss of the compression ring-cylinder liner system with and without dimples are also compared for different compression ring face profiles. The results show that the spherical dimples can produce a larger reduction of friction in mixed lubrication region, and reduce power loss significantly in the middle of the strokes. In addition, higher reduction percentages of average friction forces and wear are obtained for smaller crown height or larger axial width.

Relative statistic fluctuation of sliding friction force

The relative fluctuation of sliding friction force in a stationary friction regime has been estimated by the statistic theory of the fluctuation of adaptive physical variables. General mathematical equations and corresponding estimations of relative fluctuations have been given for various friction regimes, such as the regime of mixed and boundary friction, including friction with cladding metal nanoparticles in lubricant and friction with selective transfer. It has been found that relative fluctuations of friction forces are fairly high compared with the fluctuation of physical variables related to the volume of thermodynamic systems. Relative fluctuations for a mixed friction regime are particularly high and can reach 1–10% of the average friction force.

Reducing friction and miscibility studies of FEP dispersion/ PDMS fluid blends

To develop new polymer blends having reduced friction force of fluorinated ethylene propylene (FEP) dispersion and improved adhesion of polydimethylsiloxane (PDMS) fluid, FEP dispersion was blended with PDMS fluids at different viscosities of 20 cSt and 100 cSt by using solution mixing method. The FEP/PDMS blends were coated on short hollow tubes and examined by penetrating the tubes into the rubber stoppers. It was found that the tubes coated with the blends showed reduced penetration and friction forces and improved adhesion. The tubes coated with the 100 cSt-PDMS blend in the ratio of 5:1.5 demonstrated the penetration and average friction forces as low as 3828 mN and 1524 mN, respectively. The formation of physical blends was characterized and confirmed by FTIR and DSC analyses.

A Study on Force and Lubrication Characteristics of Ball Piston in Eccentric Ball Piston Pump

Ball piston is an essential design element in eccentric ball piston pump. The objective of this paper is to investigate the dynamic force and lubrication characteristics of the ball piston. In this study, a tribodynamic model of the ball piston is proposed. The model couples the elastohydrodynamic lubrication model of ball–ring pair and the elastohydrodynamic model of ball–cylinder pair with the dynamic model of the ball piston. The interaction between tribological behavior and dynamic performance and the effects of centrifugation and structural deformation of the ball piston are considered. The pressure distributions and film profiles of the ball–ring pair and ball–cylinder pair are calculated. The dynamic normal contact forces and friction forces are analyzed. The friction torque loss and mechanical efficiency of the whole pump are obtained by combing the tribodynamic model of the ball piston with a simplified friction torque model of cylinder-valve shaft pair. A test bench is established for validating the simulation results. The results show that the normal contact force of the ball–ring pair waves in each stroke, and in discharge stroke, the contact force is much bigger than that in suction stoke because of loading force, while the friction force changes very little for the ball sliding in the outer ring. Comparing with the average friction force of the ball–ring pair, the average friction force of the ball–cylinder pair is small, which reflects that the ball–ring pair is more prone to wear. For the ball–ring friction pair, when the load increases, the secondary contact pressure peak appears, the profile of film presents a horseshoe shape, and the classical shrinkage appears at the outlet of contact region. For the ball–cylinder pair, the film pits and the contact pressure jumps at the entrance of contact region for the heavy load and the high shear stain of film. In addition, the friction torque presents a nonlinear growth trend with the increasing working pressure

Tribological characteristics of piston rings in a single-piston hydraulic free-piston engine

PurposeA free-piston engine (FPE) is an unconventional engine that abandons the crank system. This paper aims to focus on a numerical simulation for the lubricating characteristics of piston rings in a single-piston hydraulic free-piston engine (HFPE).Design/methodology/approachA time-based numerical simulation program was built using Matlab to define the piston motion of the new engine. And a lubrication mode of piston rings was built which is based on the gas flow equation, hydrodynamic lubrication equation and the asperity contact equation. The piston motion and the lubrication model are coupled, and then the finite difference method is used to obtain the piston rings lubrication performances of the FPE. Meanwhile, the lubrication characteristics of the new engine were compared with those of a corresponding conventional crankshaft-driven engine.FindingsThe study results indicate that compared with the traditional engine, the expansion stroke of the HFPE is longer, and the compression stroke is shorter. Lubrication oil film of the new engine is thicker than the traditional engine during the initial stage of compression stroke and the final stage of the power stroke. The average friction force and power of the hydraulic free piston engine are slightly lower than those of the traditional engine, but the peak friction power of the FPE is significantly greater than that of the traditional engine. With an increase in load, the friction loss power and friction loss efficiency decrease, and with a decrease in equivalence ratio, the friction power loss reduces, but the friction loss efficiency decreases first and then increases.Research limitations/implicationsIn this paper, only qualitative analysis was performed on the tribological difference between conventional crankshaft engine and HFPE, instead of a quantitative one.Practical implicationsThis paper contributes to the tribological design method of HFPE.Social implicationsNo social implications are available now, as the HFPE is under the development phase. However, the authors are positive that their work will be commercialized in the near future.Originality/valueThe main originality of the paper can be introduced as follows: the lubrication and friction characteristics of the new engine (HFPE) were investigated and revealed, which have not been studied before; the effect of the HFPE’s special piston motion on the tribological characteristics was considered in the lubrication simulation. The results show that compared with the traditional crankshaft engine, the new engine shows a different lubrication performance because of its free piston motion.

The effect of longitudinal high-frequency in-plane vibrations on a 1-DoF friction oscillator with compliant contact

The effect of vibrational smoothing of dry friction has been studied intensively in the past. High-frequency vibrations have been shown to reduce average friction forces and thus smooth the effective characteristics of dry friction. Consequently, undesired friction-induced phenomena, such as friction-induced vibrations or stick-slip motion, can be avoided. While most publications focus on rigid contacts using classical Coulomb friction models, a class of dynamic friction models, namely the Dahl, LuGre and the elasto-plastic model proposed by Dupont et al., is considered here in order to account for contact compliance. Based on a simple 1-DoF friction oscillator, the effect of longitudinal high-frequency vibrations is investigated. An averaging procedure is applied in order to derive the effective friction–velocity characteristics of the system. Using a Galerkin procedure, analytical approximations can be derived for the Dahl and the LuGre friction model, while numerical calculations have to be performed for the elasto-plastic model. In order to validate the results, full timescale numerical simulations are performed. When accounting for contact compliance, the smoothing effect can still be observed. However, the calculated friction forces are higher compared to the classical results using Coulomb friction. Hence, the assumption of a rigid contact leads to overestimation of the smoothing effect.

Friction characteristics of piston rings in a free-piston engine generator

Free-piston engine generator is a new alternative to traditional reciprocating engine, which moves without mechanical restriction of crankshaft system. This article investigated numerically the friction characteristics of piston rings in a free-piston diesel engine generator by adopting coupled models of dynamic and friction. The development of the dynamic model and friction model was described, and an iterative calculation method was presented, giving insight into the coupled parameters of these two models. The detailed effects of the dynamic on friction and lubrication were investigated compared with a corresponding traditional crank engine. The friction characteristics of the free-piston engine generator were found to differ clearly from those of the traditional engine due to its special piston motion. Compared with the traditional engine, the minimum lubricant film thickness of piston rings in the free-piston engine generator is thicker and lasts shorter at the dead center regions, but it is generally thinner at other positions. The average friction force, friction power, and friction work of the piston rings in the free-piston engine generator are less than the traditional engine due to the better lubrication in endpoints region. Meanwhile, the friction power of the free-piston engine generator increases with the increase in fuel mass or decrease in load. The friction efficiency varies in correlation with the generator load; the optimum friction efficiency can be obtained by either increasing or decreasing from a certain generator load.

A Pin‐on‐Disk Experimental Set‐up for Vibrational Smoothing of Dry Friction

AbstractHigh‐frequency vibrations have been shown to smooth the effective characteristics of dry friction and may be used in order to quench undesired phenomena such as friction induced vibrations. Recent contributions have discussed the influence of contact compliance on vibrational smoothing. Within this contribution, an experimental set‐up is presented, which has been developed in order to validate the calculated results. After a brief description, some general properties of the system are investigated, including stationary and oscillatory behaviour. Without vibrational excitation, friction induced vibrations occur at low sliding velocities. The corresponding motion of the system is analysed and compared to the results of FE‐based simulations. Finally, a first attempt is presented reducing the average friction force by imposing appropriate vibrations. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)