Friction Characteristics of Nanoscale Sliding Contacts between Multi-Asperity Tips and Textured Surfaces

Nanoscale sliding contacts of smooth surfaces or between a single asperity and a smooth surface have been widely investigated by molecular dynamics simulations, while there are few studies on the sliding contacts between two rough surfaces. Actually, the friction of two rough surfaces considering interactions between more asperities should be more realistic. By using multiscale method, friction characteristics of two dimensional nanoscale sliding contacts between rigid multi-asperity tips and elastic textured surfaces are investigated. Four nanoscale textured surfaces with different texture shapes are designed, and six multi-asperity tips composed of cylindrical asperities with different radii are used to slide on the textured surfaces. Friction forces are compared for different tips, and effects of the asperity radii on the friction characteristics are investigated. Average friction forces for all the cases are listed and compared, and effects of texture shapes of the textured surfaces are discussed. The results show that textured surface II has a better structure to reduce friction forces. The multi-asperity tips composed of asperities with R=20r0 （r0=0.227 7 nm） or R=30r0 get higher friction forces compared with other cases, and more atoms of the textured surfaces are taken away by these two tips, which are harmful to reduce friction or wear. For the case of R=10ro, friction forces are also high due to large contact areas, but the sliding processes are stable and few atoms are taken away by the tip. The proposed research considers interactions between more asperities to make the model approach to the real sliding contact problems. The results will help to vary or even control friction characteristics by textured surfaces, or provide references to the design of textured surfaces.

Two Dimensional Nanoscale Reciprocating Sliding Contacts of Textured Surfaces

Detailed behaviors of nanoscale textured surfaces during the reciprocating sliding contacts are still unknown although they are widely used in mechanical components to improve tribological characteristics. The current research of sliding contacts of textured surfaces mainly focuses on the experimental studies, while the cost is too high. Molecular dynamics（MD） simulation is widely used in the studies of nanoscale single-pass sliding contacts, but the CPU cost of MD simulation is also too high to simulate the reciprocating sliding contacts. In this paper, employing multiscale method which couples molecular dynamics simulation and finite element method, two dimensional nanoscale reciprocating sliding contacts of textured surfaces are investigated. Four textured surfaces with different texture shapes are designed, and a rigid cylindrical tip is used to slide on these textured surfaces. For different textured surfaces, average potential energies and average friction forces of the corresponding sliding processes are analyzed. The analyzing results show that ＂running-in＂ stages are different for each texture, and steady friction processes are discovered for textured surfaces II, III and IV. Texture shape and sliding direction play important roles in reciprocating sliding contacts, which influence average friction forces greatly. This research can help to design textured surfaces to improve tribological behaviors in nanoscale reciprocating sliding contacts.

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 = 60rand 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.

A Local Region Molecular Dynamics Simulation Method for Nanoscale Sliding Contacts

Computational e ciency and accuracy always conflict with each other in molecular dynamics(MD) simulations. How to enhance the computational e ciency and keep accuracy at the same time is concerned by each corresponding researcher. However, most of the current studies focus on MD algorithms, and if the scale of MD model could be reduced, the algorithms would be more meaningful. A local region molecular dynamics(LRMD) simulation method which can meet these two factors concurrently in nanoscale sliding contacts is developed in this paper. Full MD simulation is used to simulate indentation process before sliding. A criterion called contribution of displacement is presented, which is used to determine the e ective local region in the MD model after indentation. By using the local region, nanoscale sliding contact between a rigid cylindrical tip and an elastic substrate is investigated. Two two?dimensional MD models are presented, and the friction forces from LRMD simulations agree well with that from full MD simulations, which testifies the e ectiveness of the LRMD simulation method for two?dimensional cases. A three?dimensional MD model for sliding contacts is developed then to show the validity of the LRMD simulation method further. Finally, a discussion is carried out by the principles of tribology. In the discussion, two two?dimensional full MD models are used to simulate the nanoscale sliding contact problems. The results indicate that original smaller model will induce higher equivalent scratching depth, and then results in higher friction forces, which will help to explain the mechanism how the LRMD simulation method works. This method can be used to reduce the scale of MD model in large scale simulations, and it will enhance the computational e ciency without losing accuracy during the simula?tion of nanoscale sliding contacts.

Molecular Dynamics Simulation on Friction and Thermal Properties of FCC Copper in Nanoscale Sliding Contacts

In nanoscale sliding contact,adhesion effects and adhesive force are predominant,and high friction force will be produced.Friction energy is mainly converted into heat,and the heat will make nanomaterials become soft to affect friction behaviors,so it is important to investigate the friction and thermal properties of the nanoscale sliding contacts.A model of a nanoscale sliding contact between a rigid cylindrical tip and an FCC copper substrate is developed by molecular dynamics simulation.The thermal properties of the substrate and the friction behaviors are studied at different sliding velocities and different tip radii.The results show that at a low sliding velocity,the friction force fluctuation is mainly caused by material melting⁃solidification,while at a high sliding velocity the material melting is a main factor for the friction reduction.The average friction forces increase at initial phase and then decrease with increasing sliding velocity,and the average temperature of the substrate increases as sliding velocity increases.Increasing tip radius significantly increases the temperature,while the coupled effects of tip radius and temperature rise make friction force increase slightly.

Development and tests of sliding contact line-powered track transporter

In order to solve the problems of complexity of control systems and the limited power supply of traditional fuelpowered and battery-driven transporters operating in mountainous orchards,a sliding contact line-powered track transporter was designed and manufactured based on theoretical calculations.Key components of the transporter were developed such as a PLC-based(programmable logic controller)control system,a sliding contact power supply,and transmission system,and a position limit device.The functions and performance of designed transporter were tested.The test results showed that the transporter exhibited a high stability of operation with an average operation velocity of 0.70 m/s,the maximum working slope of 48°,the maximum load of 400 kg,and the maximum remote control distance reaching 1482 m.When the power supply circuit of sliding contact line was 108.8 m in length,the maximum voltage drop was 2.4 V,and the maximum power loss was 174.72 W,which were close to the theoretical calculation values.With a single power supply cabinet,the transporter can operate normally for a maximum track distance of 175.69 m.All the technical indicators of the transporter met the design requirements,and the above-mentioned problems such as complexity of the control system and limited energy supply of the traditional mountain orchard transporter were well solved.This study can provide reference for the design and optimization of mountain orchard transporter.

Effects of magnetic ionic liquid as a lubricant on the friction and wear behavior of a steel-steel sliding contact under elevated temperatures

A magnetic ionic liquid(abridged as MIL)[C_(6)mim]_(5)[Dy(SCN)_(8)]was prepared and used as the magnetic lubricant of a steel-steel sliding pair.The tribological properties of the as-prepared MIL were evaluated with a commercially obtained magnetic fluid lubricant(abridged as MF;the mixture of dioctyl sebacate and Fe_(3)O_(4),denoted as DIOS-Fe_3O_4)as a control.The lubrication mechanisms of the two types of magnetic lubricants were discussed in relation to worn surface analyses by SEM-EDS,XPS,and profilometry,as well as measurement of the electric contact resistance of the rubbed steel surfaces.The results revealed that the MIL exhibits better friction-reducing and antiwear performances than the as-received MF under varying test temperatures and loads.This is because the MIL participates in tribochemical reactions during the sliding process,and forms a boundary lubrication film composed of Dy_(2)O_(3),FeS,FeSO_(4),nitrogen-containing organics,and thioether on the rubbed disk surface,thereby reducing the friction and wear of the frictional pair.However,the MF is unable to form a lubricating film on the surface of the rubbed steel at 25°C,though it can form a boundary film consisting of Fe_(3)O_(4) and a small amount of organics under high temperature.Furthermore,the excessive Fe_(3)O_(4) particulates that accumulate in the sliding zone may lead to enhanced abrasive wear of the sliding pair.

Numerical Analysis of a Sliding Frictional Contact Problem with Normal Compliance and Unilateral Contact

This paper represents a continuation of [1] and [2]. Here, we consider the numerical analysis of a non-trivial frictional contact problem in a form of a system of evolution nonlinear partial differential equations. The model describes the equilibrium of a viscoelastic body in sliding contact with a moving foundation. The contact is modeled with a multivalued normal compliance condition with memory term restricted by a unilateral constraint and is associated with a sliding version of Coulomb’s law of dry friction. After a description of the model and some assumptions, we derive a variational formulation of the problem, which consists of a system coupling a variational inequality for the displacement field and a nonlinear equation for the stress field. Then, we introduce a fully discrete scheme for the numerical approximation of the sliding contact problem. Under certain solution regularity assumptions, we derive an optimal order error estimate and we provide numerical validation of this result by considering some numerical simulations in the study of a two-dimensional problem.

An analytical method for assessing the initiation and interaction of cracks in fused silica subjected to contact sliding

Understanding the fracture behavior of fused silica in contact sliding is important to the fabrication of damage-free optics.This study develops an analytical method to characterize the stress field in fused silica under contact sliding by extending the embedded center of dilation(ECD)model and considering the depth of yield region.The effects of densification on the stress fields were considered by scratch volume analysis and finite element analysis.Key mechanisms,such as crack initiation and morphology evolution were comprehensively investigated by analyzing the predicted stress fields and principal stress trajectories.The predictions were validated by Berkovich scratching experiment.It was found that partial conical,median and lateral cracks could emerge in the loading stage of the contact sliding,but radial and lateral cracks could be initiated during unloading.It was also found that the partial conical crack had the lowest initiation load.The intersection of long lateral cracks makes the material removal greater.

Research on dynamic characteristics of arc under electrode relative motion

The relative motion of the electrodes is a typical feature of sliding electrical contact systems.The system fault caused by the arc is the key problem that restricts the service life of the sliding electrical contact system.In this paper,an arcing experimental platform that can accurately control the relative speed and distance of electrodes is built,and the influence of different electrode speeds and electrode distances on arc motion characteristics is explored.It is found that there are three different modes of arc root motion:single arc root motion mode,single and double arc roots alternating motion mode,and multiple arc roots motion mode.The physical process and influence mechanism of different arc root motion modes are further studied,and the corresponding relationship between arc root motion modes and electrode speed is revealed.In addition,to further explore the distribution characteristics of arc temperature and its influencing factors,an arc magnetohydrodynamic model under the relative motion of electrodes is established,and the variation law of arc temperature under the effect of different electrode speeds and electrode distances is summarized.Finally,the influence mechanism of electrode speed and electrode distance on arc temperature,arc root distance,and arc root speed is clarified.The research results enrich the research system of arc dynamic characteristics in the field of sliding electrical contact,and provide theoretical support for restraining arc erosion and improving the service life of the sliding electrical contact system.

Microstress cycle and contact fatigue of spiral bevel gears by rolling-sliding of asperity contact

The rolling contact fatigue(RCF)model is commonly used to predict the contact fatigue life when the sliding is insignificant in contact surfaces.However,many studies reveal that the sliding,compared to the rolling state,can lead to a considerable reduction of the fatigue life and an excessive increase of the pitting area,which result from the microscopic stress cycle growth caused by the sliding of the asperity contact.This suggests that fatigue life in the rolling-sliding condition can be overestimated based only on the RCF model.The rubbing surfaces of spiral bevel gears are subject to typical rolling-sliding motion.This paper aims to study the mechanism of the micro stress cycle along the meshing path and provide a reasonable method for predicting the fatigue life in spiral bevel gears.The microscopic stress cycle equation is derived with the consideration of gear meshing parameters.The combination of the RCF model and asperity stress cycle is developed to calculate the fatigue life in spiral bevel gears.We find that the contact fatigue life decreases significantly compared with that obtained from the RCF model.There is strong evidence that the microscopic stress cycle is remarkably increased by the rolling-sliding motion of the asperity contact,which is consistent with the experimental data in previous literature.In addition,the fatigue life under different assembling misalignments are investigated and the results demonstrate the important role of misalignments on fatigue life.

ANALYSIS OF TRANSMISSION CHARACTERISTICS OF COSINE GEAR DRIVE

Based on the mathematical model of a novel cosine gear drive, a few characteristics, such as the contact ratio, the sliding coefficient, and the contact and bending stresses, of this drive are analyzed. A comparison study of these characteristics with the involute gear drive is also carried out. The influences of design parameters including the number of teeth and the pressure angle on the contact and bending stresses are studied. The following conclusions are achieved： the contact ratio of the cosine gear drive is about 1.2 to 1.3, which is reduced by about 20% in comparison with that of the involute gear drive. The sliding coefficient of the cosine gear drive is smaller than that of the involute gear drive. The contact and bending stresses of the cosine gear drive are lower than those of the involute gear drive. The contact and bending stresses decrease with the growth of the number of teeth and the pressure angle.

Debris effect on the surface wear and damage evolution of counterpart materials subjected to contact sliding

This paper aims to explore the debris effect on surface wear and damage evolution of counterpart materials during contact sliding.A cylinder-on-flat testing configuration is used to investigate the wear behaviours of the contact pair.To explore the roles of wear debris,compressed air is applied to remove the debris in sliding zones.The comparative study demonstrates that the influence of debris removal is related to the surface properties of contact pairs.When substantial wear debris accumulates on the tool surface,debris removal can considerably alter surface damage evolution,resulting in different friction transitions,distinct surface morphology of contact pair,as well as different rates of material removal.It has been found that the surface damage evolution will not reach a stable stage unless the increase of wear particle number ceases or the average size of wear particles becomes lower than a specific threshold.However,the influence of debris removal reduces when the adhesion between the contact pair materials gets smaller.

Tribological performance and microstructural evolution ofα-brass alloys as a function of zinc concentration

Tailoring a material's properties for low friction and little wear in a strategic fashion is a long-standing goal of materials tribology.Plastic deformation plays a major role when metals are employed in a sliding contact;therefore,the effects of stacking fault energy and mode of dislocation glide need to be elucidated.Here,we investigated how a decrease in the stacking fault energy affects friction,wear,and the ensuing sub-surface microstructure evolution.Brass samples with increasing zinc concentrations of 5,15,and 36 wt%were tested in non-lubricated sphere-on-plate contacts with a reciprocating linear tribometer against Si3N4 spheres.Increasing the sliding distance from 0.5(single trace)to 5,000 reciprocating cycles covered different stages in the lifetime of a sliding contact.Comparing the results among the three alloys revealed a profound effect of the zinc concentration on the tribological behavior.CuZn15 and CuZn36 showed similar friction and wear results,whereas CuZn5 had a roughly 60%higher friction coefficient(COF)than the other two alloys.CuZn15 and CuZn36 had a much smaller wear rate than CuZn5.Wavy dislocation motion in CuZn5 and CuZn15 allowed for dislocation self-organization into a horizontal line about 150 nm beneath the contact after a single trace of the sphere.This feature was absent in CuZn36 where owing to planar dislocation slip band-like features under a 45°angle to the surface were identified.These results hold the promise to help guide the future development of alloys tailored for specific tribological applications.

An improved algorithm for McDowell＇s analytical model of residual stress

The analytical elastoplastic rolling/sliding model for two-dimensional contact proposed by McDowell is an important tool for predicting residual stress in rolling/sliding processes. In application of the model, a problem of low predicting precision near the surface layer of the component is found. According to the volume- constancy of plastic deformation, an improved algorithm for McDowell＇s model is proposed in order to improve its predicting accuracy of the surface residual stress. In the algorithm, a relationship between three normal stresses perpendicular to each other at any point within the component is derived, and the relationship is applied to McDowell＇s model. Meanwhile, an unnecessary hypothesis proposed by McDowell can be eliminated to make the model more reasonable. The simulation results show that the surface residual stress predicted by modified method is much closer to the FEM results than the results predicted by McDowell＇s model under the same simulation conditions.