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Researchers have long been studying the effects of the modification of friction material compositions on their tribological properties. Predictive models have also been developed, but they are of limited use in the design of new compositions. Therefore, this research aims to investigate the tribological behaviour of single ingredients in friction materials to develop a tribological dataset. This dataset could then be used as a foundation for a cellular automaton (CA) predictive model, intended to be a tool for designing friction materials. Tribological samples were almost entirely composed of four distinct friction material ingredients, and one sample composed of their mixture was successfully produced. Pin-on-disc (PoD) tribometer testing and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDXS) analysis were used for the tribological characterization. Each material showed distinct tribological properties and evolution of the contact surface features, and the synergistic effect of their mutual interaction was also demonstrated by their mixture. [ABSTRACT FROM AUTHOR]

Severe friction on wheel/rail contact interface in the process of a train emergency braking can cause significant thermal and mechanical phenomena. Obvious friction heat and serious material damage will appear on the contact surface. In this article, the variation law of temperature and surface damage during a wheel sliding contact process are investigated. To achieve the research objective, a three-dimensional (3D) thermo-mechanical coupling contact finite element model (FEM) is established, and the temperature-dependent material parameters are used. The FEM is adopted to analyze the temperature distribution law on the contact surface. At the same time, a sliding contact testing machine is used to study the damage of materials in the contact area during the sliding contact. The study results indicate, the highest temperature of wheel and rail material is respectively 1014 ℃ and 461.8 ℃ during sliding contact. High temperatures are located at the subsurface and surface areas of contact region. When the distance to contact surface exceeds 1.4 mm, the temperature changes slightly. The types of damage on wheel surface are grooves and material peeling, while the flaky spalling, corrosive pitting, adhesion and grooves appear on the rail surface. Meanwhile, many cracks can be found on the contact surface, which is a major factor leading to material damage. The research results of this article are of great significance for understanding the thermal and mechanical damage of wheel/rail materials. [ABSTRACT FROM AUTHOR]

Abstract Severe friction on wheel/rail contact interface in the process of a train emergency braking can cause significant thermal and mechanical phenomena. Obvious friction heat and serious material damage will appear on the contact surface. In this article, the variation law of temperature and surface damage during a wheel sliding contact process are investigated. To achieve the research objective, a three-dimensional (3D) thermo-mechanical coupling contact finite element model (FEM) is established, and the temperature-dependent material parameters are used. The FEM is adopted to analyze the temperature distribution law on the contact surface. At the same time, a sliding contact testing machine is used to study the damage of materials in the contact area during the sliding contact. The study results indicate, the highest temperature of wheel and rail material is respectively 1014 ℃ and 461.8 ℃ during sliding contact. High temperatures are located at the subsurface and surface areas of contact region. When the distance to contact surface exceeds 1.4 mm, the temperature changes slightly. The types of damage on wheel surface are grooves and material peeling, while the flaky spalling, corrosive pitting, adhesion and grooves appear on the rail surface. Meanwhile, many cracks can be found on the contact surface, which is a major factor leading to material damage. The research results of this article are of great significance for understanding the thermal and mechanical damage of wheel/rail materials.

In this work, large-scale molecular dynamics (MD) computational simulations were performed in order to explore the sliding contact responses of rough surfaces with hexadecane lubricant and added nanoparticles. Simulation results revealed that the frictional state was dependent on the fluid, nanoparticle, and surface roughness. Three lubricating conditions were compared based on considerations of different amounts of fluid molecules. The lubricant was not able to separate the frictional contact surfaces if the quantity of lubricant molecules was insufficient. Particularly, there were no lubricating contributions when the amount of lubricant was too low, and the lubricant therefore only filled the pits in the surface roughness. Thus, the normal load was primarily supported by the contact between the two surfaces and nanoparticles, leading to significant surface morphology changes. In contrast, the frictional contact surfaces were able to be completely separated by the lubricant when there was a sufficient amount of fluid, and a very good lubricating effect could thus be achieved, resulting in a smaller friction force. In addition, the changes in surface morphology, contact area, and RMS are discussed in this paper, in order to reveal the dynamic frictional process. [ABSTRACT FROM AUTHOR]

The ongoing digitalization of processes and products in mechanical engineering is accompanied by an increasing demand for data. In order to provide this data, technical systems are being extended with sensory functions. To supply those sensory functions on rotating elements—such as shafts—with electrical energy, and to be able to transmit signals out of the system, sliding contacts can be used as a cost-effective and established solution. However, if electrical properties of machine elements are utilized for sensing purposes, such as condition monitoring of rolling element bearings by means of impedance measurement, sliding contacts are directly in the measurement path and can thus influence the measured impedance. The aim of this paper is to analyze the impedance of graphite and silver graphite carbon brushes under different rotational speeds, in different positions, and with different carrier frequencies. The material of the carbon brushes as well as the position have significant effects on the impedance behavior. Furthermore, carbon brushes show a significant running-in behavior. The results are discussed, and indications for use in impedance measurements are given. Silver graphite carbon brushes in axial positioning are particularly suitable for impedance measurements of sensory utilizable machine elements. Sufficient running-in time must be considered. [ABSTRACT FROM AUTHOR]

Aesthetic glass-ceramics are widely employed dental materials, both in bulk form and as veneers. As they are highly brittle, the durability of these materials is limited by fracture and wear processes originated from contacts with opposing dentition and/or third-body particles during mastication and bruxism. This work investigates the resistance to fracture of commercial dental glass-ceramics under sliding contact, simulated by means of scratch tests. It finds that materials with relatively larger crystals (feldspathic and leucite) require a lower stress to fracture upon sliding than lithium silicates containing smaller (but more elongated) crystals, due to their larger defects and lower toughness. Results are analyzed as a function of material microstructure within the framework of Weibull theory and fracture mechanics. Implications for materials selection and development in prosthetic dentistry are briefly discussed. [ABSTRACT FROM AUTHOR]

In this work, large-scale molecular dynamics (MD) computational simulations were performed in order to explore the sliding contact responses of rough surfaces with hexadecane lubricant and added nanoparticles. Simulation results revealed that the frictional state was dependent on the fluid, nanoparticle, and surface roughness. Three lubricating conditions were compared based on considerations of different amounts of fluid molecules. The lubricant was not able to separate the frictional contact surfaces if the quantity of lubricant molecules was insufficient. Particularly, there were no lubricating contributions when the amount of lubricant was too low, and the lubricant therefore only filled the pits in the surface roughness. Thus, the normal load was primarily supported by the contact between the two surfaces and nanoparticles, leading to significant surface morphology changes. In contrast, the frictional contact surfaces were able to be completely separated by the lubricant when there was a sufficient amount of fluid, and a very good lubricating effect could thus be achieved, resulting in a smaller friction force. In addition, the changes in surface morphology, contact area, and RMS are discussed in this paper, in order to reveal the dynamic frictional process.

The analysis of rail/wheel sliding friction heat is a very important research field. The numerical simulation is often adopted to calculate the friction heat, and experimental method is relatively few. In this paper, an experimental machine is designed to simulate the rail/wheel sliding contact. At the same time, a rail/wheel contact wear model and a friction heat transfer model are established. The characteristics of temperature on rail/wheel sliding contact are analyzed by experimental test and numerical calculation. The research results show that the temperature rise of wheel and rail is quick in the initial sliding contact stage, then gradually slows down. The temperature of wheel is higher than that of rail at the same depth from the contact surface. In the initial sliding stage, the wheel temperature rises faster than the rail temperature, which is related to the size of contact surface and the concentration degree of friction heat. Moreover, the results of this paper show that the temperature values of the rail/wheel obtained through experimental test and numerical calculation are in good agreement. The experimental and numerical calculation methods used in this article can be adopted to analyze the contact problems of other sliding friction pairs. [ABSTRACT FROM AUTHOR]

In advanced railway systems, electricity is supplied to train through sliding contact of pantograph and power-transmission catenary, which causes continuous wear of transmission wire in pantograph-catenary system (PCS). It is of crucial to monitor wear to ensure the safety and smooth operations of electric trains in terms of precision and efficiency in services. This work has developed a novel inverse procedure that uses measurable mechanical quantities to predict and monitor contact wire wear regardless of whether pantograph passes. The inverse technique is based on rigorous mechanics principle and verified by numerical simulation. Three-dimensional (3D) finite element models (FEM) of PCS is first established to simulate both static mechanics behavior and dynamic contact process during sliding. A useful relationship between wear-level and displacement or contact force has been established through static studies. Intensive nonlinear dynamic contact analysis of PCS is then performed and found that the wear-level relates strongly to displacement variation of collector head, so as to devise an inverse procedure between those two when the train is running. In addition, simple formulae are derived by curve-fitting to estimate wire-level based on either static or dynamic measurements, for easy real-time applications of the results from our 3D FEM study. [ABSTRACT FROM AUTHOR]

The ongoing digitalization of processes and products in mechanical engineering is accompanied by an increasing demand for data. In order to provide this data, technical systems are being extended with sensory functions. To supply those sensory functions on rotating elements—such as shafts—with electrical energy, and to be able to transmit signals out of the system, sliding contacts can be used as a cost-effective and established solution. However, if electrical properties of machine elements are utilized for sensing purposes, such as condition monitoring of rolling element bearings by means of impedance measurement, sliding contacts are directly in the measurement path and can thus influence the measured impedance. The aim of this paper is to analyze the impedance of graphite and silver graphite carbon brushes under different rotational speeds, in different positions, and with different carrier frequencies. The material of the carbon brushes as well as the position have significant effects on the impedance behavior. Furthermore, carbon brushes show a significant running-in behavior. The results are discussed, and indications for use in impedance measurements are given. Silver graphite carbon brushes in axial positioning are particularly suitable for impedance measurements of sensory utilizable machine elements. Sufficient running-in time must be considered.

Subsurface damage of fused silica optics is one of the major factors restricting the performance of optical systems. The densification-affected deformation and fracture in fused silica under a sliding contact are investigated in this study, via three-dimensional finite element analysis (FEA). The finite element models of scratching with 70.3° conical and Berkovich indenters are established. A refined elliptical constitutive model is used to consider the influence of densification. The finite element models are experimentally verified by elastic recovery, and theoretically verified by hardness ratio. Results of densification and plastic deformation distributions indicate that the accuracy of existent sliding stress field models may be improved if the spherical/cylindrical yield region is replaced by an ellipsoid/cylindroid, and the embedding of the yield region is considered. The initiation sequence, and the locations and stages of radial, median, and lateral cracks are discussed by analyzing the predicted sliding stress fields. Median and radial cracks along the sliding direction tend to be the first cracks that emerge in the sliding and unloading stages, respectively. They coalesce to form a big median–radial crack that penetrates through the entire yield region. The fracture behavior of fused silica revealed in this study is essential in the low-damage machining of fused silica optics. [ABSTRACT FROM AUTHOR]

A two-dimensional temperature prediction model of the multi-conical friction pair during a single engagement period is established. Energy governing equations and non-static thermal contact conductance are coupled, and the transient surface temperatures are obtained by using the finite difference method. The transient temperature field and the effects of contact pressure and relative speed are studied. The positive interaction effect of these two factors on the temperature rise is observed. The maximum temperatures of the friction plate and clutch disc appear in different positions, where the former is closer to the outermost contact surface. With the combination of high thermal conductivity, the friction plate exhibits a more significant radial temperature gradient. And the accompanying thermal stress may induce deformation and ablation. In addition, the temperature decreases axially from the top of the cone. This work provides a prerequisite for studying the friction, wear and torque transmission performance of clutch, offering a reference for the further control and optimization design of novel clutch systems. [ABSTRACT FROM AUTHOR]

Dynamic substructuring methods are initially developed for time-invariant systems to evaluate the dynamic behavior of a complex structure by coupling the component substructures. Sometimes, the component substructures change their position over time, affecting the dynamics of the entire structure. This family of problems can be tackled using substructuring techniques by isolating the time dependency in the coupling conditions among the time-invariant substructures. Mechanical systems, composed of subsystems in relative motion with a sliding interface, can be analyzed using this approach. In previous work, the authors proposed a solution method in the time and frequency domain using this approach under the assumption that the relative sliding motion at the contact interfaces is a-priori known, at least approximately. This assumption implies that the perturbation generated by the friction-induced vibration is neglected. In subsequent work, a more realistic contact assumption was considered to account also for the local vibration of the contact point and the geometric nonlinearity due to the elastic deformation. In this paper, a simplification with respect to the realistic contact assumption is introduced, which neglects the angular variation of the direction normal to the contact interface. The simplified approach is advantageous because it is equally able to highlight the occurrence of friction-induced instabilities, and it reduces the computational burden. The results of the substructuring methods using different contact assumptions are compared with those of a reference numerical method to show how the choice of the contact algorithm allows for tackling a wide range of operating conditions, from simple position-dependent problems up to complex friction-induced vibration phenomena. [ABSTRACT FROM AUTHOR]

Modern high-tech composite materials are widely used in various sectors of the economy, in particular, in railway transport. Among the areas of application of such innovative materials, the manufacturing of contact strips for pantographs of electric rolling stock should be mentioned. Innovation is primarily understood as the self-lubricating property of the working surface of the linings. The linings made of such materials differ from the traditional graphite (coal) ones by the increased content of metal additives, in particular copper, which limits the possibility of their use on electric rolling stock in the countries of the European Union. Regulatory restrictions on the content of copper (35% and 40%) are associated with possible damage to the contact wire, in the case of using overlays with a content of copper (metals) greater than these restrictions. On the railways of Ukraine (countries of the former USSR), there are also restrictions on the use of linings of different types according to the degree of wear of the contact wire, no more than 40 microns is allowed per 10 thousand passes of the locomotive pantograph along the contact wire. These standards are verified during operational and bench tests. The aim of the article is to present the types of tests and compare the obtained values with similar indicators in the case of using a traditional contact strip for pantographs of electric rolling stock. The results obtained confirm the possibility of using innovative linings on iron networks, since the wear of the contact wire during testing is much lower than that of the normative and knife than for traditional types of linings. In addition, during a set of tests, the fact of improving the quality of the working surface of the contact wire was established, which positively affects the extension of the service life of the contact wire. [ABSTRACT FROM AUTHOR]

The application of two-layer coatings is one of the most widely used ways to increase the performance capacity of heavily loaded frictional joints. Design of two-layer coatings for dry sliding friction surfaces and prediction of their efficiency involve the need to develop mathematical models of sliding contact allowing for wear and frictional heating. To achieve this goal, we consider a nonsteady quasi-static contact problem of uncoupled thermoelasticity of a rigid punch in the form of half-plane sliding with a constant velocity over the surface of a two-layer elastic coating bonded on its bottom face to a flat rigid substrate. During the sliding, the punch also penetrates into the two-layer coating in a direction normal to its surface. The punch friction against the coating surface is mathematically described by the Coulomb friction model. The heat flow generated by friction is directed into the coating depth. The frictional sliding contact gives rise to wear of the two-layer coating. The stated problem is solved using the Laplace integral transform. The basic characteristics of each layer, namely, displacements, temperature, and stresses, are presented in the form of the contour integrals of meromorphic functions. The analysis of the properties of the integrand poles in the complex plane of integration and determination of thermoelastic instability domains in the space of problem parameters are followed by evaluation of the contour integrals. The influence of the problem parameters on the sliding contact main characteristics, i.e., temperature, wear, and stresses, is studied. As an example, the wear of a two-layer coating with its upper layer made of titanium nitride and lower layer made of titanium (TiN/Ti) is considered. The numerical results show that by properly selecting the coating layer thicknesses, better wear resistance and lower contact temperature of the coating can be achieved. [ABSTRACT FROM AUTHOR]

Abstract 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.

Objective: The objectives of this study were: first, to compare solute uptake driven by sliding to cyclic uniaxial compression. And secondly, to evaluate the role of the superficial region on passive diffusion to determine if mechanical action is merely overcoming the low permeability of the superficial region or exceeding equilibrium capacity of the tissue.Design: Tests were performed on osteochondral plugs under two types of conditions: cyclic loading (sliding vs axial compression) and unloaded passive diffusion (intact vs superficial zone removed). The articular surfaces were exposed to a fluorescent bath and uptake was quantified from the surface to the subchondral bone using fluorescent microscopy. Primary outcome measures were total mass transfer, mass transfer rate, and surface partition factor.Results: Mass transfer was 2.1-fold higher at 0.5 h for sliding compared to uniaxial compression (p = 0.004). This increased to 4.4-fold at 2 h (p = 0.002). Solute transport for both loading conditions at 2 h had reached or exceeded intact passive diffusion at 12 h. Total mass transport and mass transport per hour was higher in samples without the superficial region compared to intact samples at equilibrium. Rate of mass transfer was not declining for samples subject to sliding indicating solute uptake induced by sliding would exceed passive tissue capacity.Conclusions: These results are the first to quantify solute uptake between two components of joint articulation. The study demonstrates that sliding is a larger driver of solute transport compared to cyclic uniaxial compression. This has implications for cell nutrition, tissue engineering and biochemical signaling. [ABSTRACT FROM AUTHOR]

The contact heat conduction problem between the rough sliding surfaces was studied through secondary development of finite element software ABAQUS.The thermal contact conductance was defined as a function of the sliding speed,the local contact pressure,material properties and the surface roughness, and the effeecs of three dimensionless parameters were also analyzed.The results reveal that reciprocating motion has a significant influence on contact heat conduction.When heat conduction between sliding surfaces reach a stable stape,the dimensionless mean heat flux per unit time increases with the rising dimensionless parameters η and ω the heat conduction decreases with the reduction of the contact pressure and the growth of the surface roughness.Besides,the variations of the dimensionless average temperature are consistent with those of the dimensionless average heat flux.

The existing level of application of hard coatings due to relatively high deposition temperatures is largely limited to the tool area. However, special interest deserves the possibility of applying contemporary PDV procedures for the conversion of constructional steels used for the manufacture of elements of the most varied tribomehanical sistems.Friction and wear behavior of different types of nodular cast irons in contact with carbon steel were studied. The lubricated sliding line contact was performed on the Pin and Disk tribometer. Value of wear were determined by PQ index. Pins were made of carbon steel without and with three types of coating (TiN, TiZrN, TiAIN) on the tool rake surface.

Competent design decisions for the optimum configuration of a coated contact require the prediction of the stress state developing in the coated body under contact load. Conventional methods of superposition of effects applied in the elasticity of homogenous bodies fail in case of multilayered materials as the response of the latters to point load has only been derived in the frequency domain. A technique for assessment of the elastic response of coated bodies to arbitrary loadings is advanced in this paper, by employing the convolution theorem in the calculation of the stresses and displacements resulting in a sliding contact process. The solution of the elastic frictional contact in full sliding is achieved iteratively, using a state-of-the-art method originally developed for the contact of rough homogenous bodies. Results presented in the literature for the sliding contact of coated bodied are replicated using the newly proposed algorithm. A parametric study is subsequently performed to assess the combined influence of the frictional coefficient, the coating thickness and the elastic modulus mismatch between the coating and the substrate, on the locations and intensity of the maximum von Mises equivalent stress.

Subsurface damage of fused silica optics is one of the major factors restricting the performance of optical systems. The densification-affected deformation and fracture in fused silica under a sliding contact are investigated in this study, via three-dimensional finite element analysis (FEA). The finite element models of scratching with 70.3° conical and Berkovich indenters are established. A refined elliptical constitutive model is used to consider the influence of densification. The finite element models are experimentally verified by elastic recovery, and theoretically verified by hardness ratio. Results of densification and plastic deformation distributions indicate that the accuracy of existent sliding stress field models may be improved if the spherical/cylindrical yield region is replaced by an ellipsoid/cylindroid, and the embedding of the yield region is considered. The initiation sequence, and the locations and stages of radial, median, and lateral cracks are discussed by analyzing the predicted sliding stress fields. Median and radial cracks along the sliding direction tend to be the first cracks that emerge in the sliding and unloading stages, respectively. They coalesce to form a big median–radial crack that penetrates through the entire yield region. The fracture behavior of fused silica revealed in this study is essential in the low-damage machining of fused silica optics.

The compression and shear of an elastic piecewise homogeneous strip with a semi-infinite crack at the interface are considered. The crack is completely closed, and its faces slide with friction. The analytical solution is obtained using the Wiener–Hopf method. The distributions of normal and tangential stresses on the crack line and its continuation are found, and the shear stress intensity factor is determined. It is shown that the crack faces lose contact in their middle when the ratio of shear and compressive intensities exceeds a certain critical value. [ABSTRACT FROM AUTHOR]

Dynamic substructuring methods are initially developed for time-invariant systems to evaluate the dynamic behavior of a complex structure by coupling the component substructures. Sometimes, the component substructures change their position over time, affecting the dynamics of the entire structure. This family of problems can be tackled using substructuring techniques by isolating the time dependency in the coupling conditions among the time-invariant substructures. Mechanical systems, composed of subsystems in relative motion with a sliding interface, can be analyzed using this approach. In previous work, the authors proposed a solution method in the time and frequency domain using this approach under the assumption that the relative sliding motion at the contact interfaces is a-priori known, at least approximately. This assumption implies that the perturbation generated by the friction-induced vibration is neglected. In subsequent work, a more realistic contact assumption was considered to account also for the local vibration of the contact point and the geometric nonlinearity due to the elastic deformation. In this paper, a simplification with respect to the realistic contact assumption is introduced, which neglects the angular variation of the direction normal to the contact interface. The simplified approach is advantageous because it is equally able to highlight the occurrence of friction-induced instabilities, and it reduces the computational burden. The results of the substructuring methods using different contact assumptions are compared with those of a reference numerical method to show how the choice of the contact algorithm allows for tackling a wide range of operating conditions, from simple position-dependent problems up to complex friction-induced vibration phenomena.

This paper presents an artificial intelligence (AI) method for the evolution prediction of surface scratching in sheet metals subjected to contact sliding. Ball-on-disk sliding was employed, and ball diameter, normal load, surface roughness, sliding cycles and the maximum scratching depth in the metal sheet were taken as the fuzzy variables to assess the contributions of individual variables to the surface damage. To improve the prediction accuracy, the quantum-behaved particle swarm optimisation (QPSO) algorithm was further developed and utilised to refine the fuzzy model by optimising the membership functions of the fuzzy variables. It was found that this AI technique, which integrates the fuzzy set theory with the improved QPSO algorithm, can accurately, reliably and efficiently predict the surface scratching evolution, which is otherwise impossible to be implemented. [ABSTRACT FROM AUTHOR]

In the quest for novel ceramics for tribological applications via bioinspired design, the differences in the fracture modes that arise upon scratching relevant locations of ceramic-like tooth enamel are investigated. It is found that fracture initiates from weak rod-sheath interfaces at relatively low loads, independent of the sliding direction. However, the geometry and propagation of the cracks depends on the orientation of the interfaces relative to the maximum tensile stress: scratching along the occlusal surface propagates approximately sinusoidal cracks, parallel to the sliding direction, while scratching along the cross-section produces straight cracks that propagate normal (scratch parallel to occlusal surface) or parallel (scratch perpendicular to occlusal surface) to the sliding direction. The formation of cracks is hindered in scratching near the enamel-dentine junction. Implications for the microstructural design of triboceramics (bulks and coatings) with improved durability are discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This article highlights the method of constructing a mathematical model of wear-resistance in potentiometer sensors. Depreciation process of working surfaces is so complex that by this time there are no reliable methods of its forecasting. Tribological processes taking place during potentiometer sensor performance are complicated by electric current influence that flows through the contact surface and accelerates the process of contact surfaces wear. As a result, the mathematical model is offered, that considers the simultaneous action of tribological elastic contact, plastic deformation, microcutting and electrical erosion with constant number of performance cycles. Wear-resistance is represented as the magnitude of change in volume of the worn material depending on the shape and material of contact surfaces, contact pressure and magnitude of electric current. The model is represented in the form of dependence, interpreted by the direct equation, greatly simplifies the forecasting of wear-resistance on the basis of experimental data. Mathematical model, obtained by adding bulk models of mechanical and current wear can be used to predict durability in the design and operation of potentiometer sensors, wire potentiometers and other devices that have a similar structure and work under the current.

Two-dimensional Cattaneo–Mindlin problem on a contact with slip and adhesion between two elastic bodies of the same material is considered in classical statement for circular cylinders as well as in more general statements: for non-circular cylinders with higher order of touch and cylindrical bodies with a wavy periodic surface. Stress distributions are found both in the contact area and inside the bodies. The question of how the friction affects a formation of adhesion zone in the contact area and distribution of maximum tangential stress within the bodies is investigated.

Multilayered and functionally graded coatings are extensively used for protection against wear of the working surfaces of mechanisms and machines subjected to sliding contact. The paper considers the problem of wear of a strip made of a functionally graded material, taking into account the heating of the sliding contact from friction. Wear is modeled by a moving strip along the surface of a hard abrasive in the form of a half-plane. With the help of the integral Laplace transform with respect to time, the solutions are constructed as convolutions from the law of the introduction of an abrasive into the strip and the original in the form of a contour integral of the inverse Laplace transform. The study of the integrands of contour quadratures in the complex plane allowed determination of the regions of stable solutions to the problem. Unstable solutions of the problem lead to the concept of thermoelastic instability of the contact with friction and formed regions of unstable solutions. The solutions obtained made it possible to determine a formula for the coefficient of functionally graded inhomogeneity of the coating material and to study its effect on the occurrence of thermoelastic instability of the contact taking friction into account, as well as on its main characteristics: temperature, displacement, stress and wear of the functionally graded material of the coating. The effects of the abrasive speed, contact stresses and temperature on wear of the coating with the functionally graded inhomogeneity of the material by the depth were investigated.

In this study, the frictional moving contact problem for an orthotropic layer bonded to an isotropic half plane under the action of a sliding rigid cylindrical punch is considered. Boundary conditions of the problem include the normal and tangential forces applied to the layer with a cylindrical punch moving on the surface of the layer in the lateral direction at a constant velocity V. It is assumed that the contact area is subjected to the sliding condition where Coulomb'⣙s law is used to relate the tangential traction to the normal traction. Using the Fourier integral transform technique and Galilean transformation, the plane contact problem is reduced to a singular integral equation in which the unknowns are the contact stress and the contact width. The singular integral equation is solved numerically using Gauss–Jacobi integration formulae. Numerical results for the contact widths and the contact stresses are given as a solution. [ABSTRACT FROM AUTHOR]

A theoretical frictional contact model of thermoelectric materials with nonlinear thermoelectric coupling loaded by a sliding punch is considered in which the punch is also assumed to be an electrical and thermal conductor. For the electric fields and the temperature fields, some analytical results are obtained by Fourier transform technique, such as the full field closed-form formulas of the electrochemical potential, the temperature, the electric current density and the energy flux. The distributions of normal electric current density and normal energy flux at subsurface are graphically given. For the elastic fields, the thermoelastic contact problem is reduced to a singular integral equation of the second kind in which thermoelectric effects are reflected in the right end of the resulting equation. The present results demonstrate that the normal electric current density, the normal energy flux and the contact pressure on the contact surface of thermoelectric materials exhibit high singularity in the vicinity of the edges of the punch. Furthermore, the effects of the friction coefficient and the thermoelectric loads on the critical stresses, the electric current density intensity factor, the energy flux intensity factor and the stress intensity factor are provided. The finding of the presented model is helpful for accessing the surface wear resistance of thermoelectric materials. [ABSTRACT FROM AUTHOR]

We study the problems of compression and shear of an elastic strip containing a semiinfinite crack on the boundary of the strip and a rigid wall. The crack is opened in the middle and closed at the ends, where its faces are under the conditions of friction sliding. By the Wiener–Hopf method, we reduce the system of integral equations of the problem to an infinite system of algebraic equations. The stress intensity factor and the crack opening displacements are determined. [ABSTRACT FROM AUTHOR]

• Temperature rises due to plastic and heat generation are inconsequential when sliding rates representative to galling are accounted for. • The strength of adhesion is not relevant to the deformation of rough surfaces in the context of single-phase austenitic 316L stainless steel during metal-on-metal contact. • Increasing temperatures show quantifiable effects on galling resistance but not to the level as found experimentally in the literature. • Galling appears to be controlled by mechanisms (e.g. phase transformation, surface coating degradation) other than the simple mechanisms considered here. The thermal effects of plastic and frictional heat generation and elevated temperature were examined along with the role of adhesion in the context of galling wear, using a representative crystal plasticity, normally loaded, sliding surface model. Galling frequency behaviour was predicted for 316L steel. Deformation of the surfaces was dominated by the surface geometry, with no significant effect due to variations in frictional models. Plastic and frictional heating were found to have a minimal effect on the deformation of the surface, with the rapid conduction of heat preventing any highly localised heating. There was no corresponding effect on the predicted galling frequency response. Isothermal, elevated temperature conditions caused a decrease in galling resistance, driven by the temperature sensitivity of the critical resolved shear stress. The extent of deformation, as quantified by the area of plastically deformed material and plastic reach, increased with temperature. Comparisons were made with literature results for several surface amplitude and wavelength conditions. Model results compared favourably with those in the literature. However, the reduction in predicted galling resistance with elevated temperature for a fixed surface was not as severe as observations in the literature, suggesting other mechanisms (e.g. phase transformations, surface coatings and oxides) are likely important. [ABSTRACT FROM AUTHOR]

Cold forming of aluminum alloys is frequently associated with problems related to severe adhesion and material transfer onto the forming tools which results in high friction forces and negatively affects the surface quality of the formed parts, a phenomenon frequently named galling. In the present study, well controlled laboratory tests using a scratch testing equipment have been performed to evaluate the friction characteristics and investigate the mechanisms controlling the initial transfer of aluminum in dry sliding contact with five different cemented carbide grades. In the tests, an aluminum pin (representing the work material) with a conical tip slides against a flat, fine-polished, cemented carbide surface (representing the tool). During sliding, the mechanical contact results in plastic deformation and flattening of the work material against the tool surface, thus simulating a metal forming contact. The small scale and well-defined tribo contact in combination with post-test surface characterization using optical surface profilometry, high resolution SEM and EDS makes it possible to evaluate the influence of material transfer on the friction characteristics. The results show that sub-μm surface irregularities in the cemented carbide surface trigger mechanical interaction with the softer aluminum surface which promotes aluminum transfer to the cemented carbide surface resulting in high friction. Common surface irregularities, promoting aluminum transfer, are sharp edges of slightly protruding carbide grains, surface steps in connection to binder phase pockets, surface steps in connection to surface pores, etc. It should be noted that even very small surface steps, < 20 nm in height, constitute efficient cutting edges able to effectively cut off the passing aluminum material and thus have a very strong impact on material transfer. In contrast, the effect of carbide composition, e.g. the presence of cubic carbides of different composition, seems to be of minor importance to reduce the adhesion and the tendency to material transfer. • Metal transfer on polished different cemented carbide grades has been evaluated. • Surface irregularities in the cemented carbide surface promote aluminum transfer. • A binderless grade displayed the lowest friction and tendency to aluminum transfer. [ABSTRACT FROM AUTHOR]

Sliding contact of a smooth indenter and a two-layered half-space is considered to take rheological properties of materials into account. A viscoelastic layer bonded with a rigid half-space is analyzed as well as the opposite viscoelastic half-space covered by a rigid layer. The problem is formulated as quasi-static. A numerical-analytical method of solution is based on a boundary element method and an iteration procedure. A new analytical solution is used to calculate the influence of coefficients for the computation procedure. Contact pressure, energy dissipation and internal stresses are analyzed depending on sliding velocity, layer thickness and Poisson's ratio.

Stress distribution of brake disks during braking is complicated due to sliding contact and frictional heat. To investigate the effect of sliding contact on the fatigue properties of low alloy steel, a new fatigue test method that enables the application of cyclic loading with sliding contact has been developed. Two main results have been obtained by this method. Firstly, the specimens with sliding contact had longer life than those without sliding contact (normal test) under a finite life regime. Secondly, the fatigue limit with sliding contact was 26% lower than that without sliding contact. Moreover, the results of normal test result showed no fatigue failure at more than 106 cycles, whereas the tests with sliding contact had fatigue failure even over 106 cycles. To clarify the fatigue limit reduction caused by sliding contact, fatigue tests applying sliding contact prior to cyclic loading (pre-wear) has been conducted. The results of pre-wear showed no fatigue failure at more than 106 cycles, which is similar to the normal test, but the fatigue limit is lower compared to that in the normal test. The effect of sliding contact on the fatigue properties is discussed from point of wear and surface roughness. Thickness of the test specimens was reduced by wear with an increase of sliding contact duration. This suggests that the sliding contact was possibly contributed to have longer life in the fatigue tests than the normal tests because of elimination of fatigue damaged surface layers. Surface roughness also increased due to sliding contact. This leads decreasing of fatigue limits. √area parameter calculated from surface roughness was used to predict the fatigue limits with sliding contact. The predicted fatigue limits corresponded well to experiment fatigue limits if surface roughness was considered as a single notch. Therefore, it is concluded that surface roughness variation due to sliding contact affects the fatigue strength of the tested low alloy steel.

In this paper, a sliding electrical contact with an electroconductive liquid is presented. We have proposed a sliding contact using a combination of an electroconductive liquid and a dipped probe electrode. The proposed connection can overcome the drawback of a conventional electrical connection, such as the suspension spring for microactuators, and enables the connection distance to be extended without increasing restriction. As a typical application of the sliding contact, a low-physical-restriction micropotentiometer is designed. Conventional potentiometers have physical restrictions or sliding friction caused by a sliding contact of a variable electrical resistance. It is not practical to apply conventional potentiometers having a high sliding friction to measure the motion of microactuators. The proposed micropotentiometer can significantly reduce the physical restrictions at the contact point of variable resistances. Moreover, the proposed micropotentiometer can detect displacements of 2 µm with good linearity. The measured restriction force is a few mN when the probe is moved in an electroconductive liquid at 5 µm/s, whereas the value for a commercial potentiometer is more than 100 mN. [ABSTRACT FROM AUTHOR]

The electrodynamic behavior of a medium caliber railgun launcher has been studied based on an electromechanical coupling model, which uses a unified modeling method for both electromagnetic and electromechanical field. This model adopts a material equivalent method to simulating the sliding contact behavior for the in-bore armature. An embedded node encoding program during field meshing operation provides good load transfer accuracy between electromagnetic field and mechanical field. Dynamic properties of in-bore magnetic field are firstly simulated and a subsequent dynamic analysis is conducted by mapping element results from electromagnetic analysis step to structure analysis step. The electromechanical-elastic response of the barrel is studied under the excitation current. The numerical model has shown reliability compared to experimental results. [ABSTRACT FROM AUTHOR]

• A transient thermal model is proposed for the study of the frictional sliding contact between rough surfaces. • The problem is solved with optimization techniques and the Fast Fourier Transform. • With this model, the rise of surface temperature and heat distribution are calculated considering surface roughness. • Several parametric analyses are performed with the aim to identify the most influent parameters on the thermal contact phenomena. Surface temperature in sliding contact systems has a considerable effect on friction and wear mechanisms, and thus the system performances. Hence its knowledge is crucial for the design of these systems. This is a complex challenge since real surfaces are rough and the contact area depends on several factors. Thus, the aim of this paper is to present an efficient thermal contact model allowing to study the transient rise of temperature and heat partition in sliding contact systems considering surface roughness. This model is based on the heat source theory. The produced heat is computed based on a contact mechanics model considering roughness. In addition to roughness, a thermal interface layer made of wear debris is considered within micro-contact zones which leads to a discontinuity of temperature at the scale of asperities. If the interface layer is considered, heat is generated within it. Otherwise, the generation of heat is at the top of surface asperities and the continuity of temperature is assumed. The numerical problem is solved using optimization techniques and the Fast Fourier Transform to accelerate calculations. A parametric study is presented with the aim to highlight the effects of material properties, roughness, velocity and the interface layer on the partition of heat and surface temperature. [ABSTRACT FROM AUTHOR]

In this study, contact problem for a homogeneous orthotropic layer loaded by a rigid cylindrical stamp is considered. The rigid cylindrical stamp slides over the contacting medium whose bottom surface is fixed to the ground in all directions. Using the integral transformation technique, the contact problem is formulated analytically into a singular integral equation. The resulting integral equation is converted to algebraic equations by using Gauss–Jacobi integration formulas and solved numerically. In addition to the analytical formulation, a finite element method (FEM) study is also conducted. The results that are obtained using FEM are compared with the results found using analytical formulation. It is found that the results obtained from analytical formulation and FEM study are in good agreement with each other. The primary intention of this paper is to demonstrate the effects of orthotropic material properties, geometrical properties and the coefficient of friction on the stresses generated due to the sliding motion of the rigid cylindrical stamp. The results of this study may provide benchmark results for engineers to be used in tribology applications involving friction and wear mechanisms. [ABSTRACT FROM AUTHOR]

Competent design decisions for the optimum configuration of a coated contact require the prediction of the stress state developing in the coated body under contact load. Conventional methods of superposition of effects applied in the elasticity of homogenous bodies fail in case of multilayered materials as the response of the latters to point load has only been derived in the frequency domain. A technique for assessment of the elastic response of coated bodies to arbitrary loadings is advanced in this paper, by employing the convolution theorem in the calculation of the stresses and displacements resulting in a sliding contact process. The solution of the elastic frictional contact in full sliding is achieved iteratively, using a state-of-the-art method originally developed for the contact of rough homogenous bodies. Results presented in the literature for the sliding contact of coated bodied are replicated using the newly proposed algorithm. A parametric study is subsequently performed to assess the combined influence of the frictional coefficient, the coating thickness and the elastic modulus mismatch between the coating and the substrate, on the locations and intensity of the maximum von Mises equivalent stress. [ABSTRACT FROM AUTHOR]

Sliding contact of electromagnetic railgun affects launch accuracy, launch efficiency, and rail life. The research on sliding contact of electromagnetic railgun is very important to the theory and engineering design. However, the whole process of electromagnetic launch is high speed and big current and high heat. The state of sliding contact is complicated. The whole process is also transient process. The stage division of sliding contact is vague, and there is no definite definition about stage division. In this paper, the theoretical analysis of sliding electric contact process is introduced, and the design test is carried out to verify it. The whole launch process of electromagnetic railgun is divided into three stages: dry friction contact, liquefied layer contact, and high-speed unstable contact. The line density 15 kA/mm is defined as the critical point between dry friction stage and liquefaction stage. The armature speed 1300 m/s is defined as critical point between the liquefied contact layer and the high-speed unstable contact stage, which provides reference for the study of sliding contact in the future. [ABSTRACT FROM AUTHOR]

Highlights • A thermal interface material for dry and nonflat interfaces under a low pressure. • A scalable fabrication method and electroless metal plating. • Thermal resistance outperforms direct metal-to-metal contact. • Compression cycling tests confirm the mechanical durability of the structure. • Insertion approach demonstrates use as an attachable thermal interface material. Abstract Thermal interface materials (TIMs), such as thermal pastes and pads, can successfully enhance contact thermal conductance by filling the gaps caused by the surface nonflatness and roughness. However, there is still an unaddressed demand for TIMs which can be applied to pluggable or reworkable interfaces in electronic systems, such as in opto-electronic transceiver modules. Reducing the contact thermal resistances at these interfaces has become increasingly important as device power density increases. These applications require dry contact interfaces that can offer the required thermal conductance under a low pressure and endure repeated mechanical compression and shear. We present a compliant metallized finned zig zag micro-spring array, as a low-cost dry TIM, that allows conformal interface contact at low pressures (∼10–100 s of kPa) by effectively accommodating surface nonflatness at a rate of a few µm per kPa. Experimental characterization of the mechanical compliance and thermal resistance confirm that this dry TIM can achieve conformal thermal contact between nonflat mating surfaces under low pressures. The total insertion thermal resistance of this dry TIM, even when mating to nonflat surfaces, is comparable to that of a polished and flat metal-to-metal contact. Mechanical compression and shear cycling tests are performed to assess the durability. [ABSTRACT FROM AUTHOR]

Abstract Different non-Fourier models were proposed to simulate temperatures in materials subjected to extremely fast thermal disturbances, when the speed of heat propagation should be concerned. The present study investigated temperature and heat balance at a microscopic sliding contact during a single frictional interaction based on the Cattaneo-Vernotte hyperbolic heat conduction equation. Two fundamental features of friction, namely, adhesion-deformational heat generation and wear, were taken into account. By applying the Laplace transform approach, non-stationary temperature expressions were derived for the hyperbolic and classical parabolic heat conduction equations. Parametric analysis was then done for parameter ranges typical of brake materials. It was found that the hyperbolic heat conduction generally results in a higher temperature at the sliding surface compared to the parabolic heat conduction. The influence of the heat propagation speed can be significant for thermal relaxation time of the order above microsecond. It becomes stronger with an increase in the contribution of the adhesive heat generation. Another finding obtained is that a considerable fraction of heat is removed from the contact zone along with wear debris, resulting in a lower temperature. This fraction is larger for the hyperbolic heat conduction. [ABSTRACT FROM AUTHOR]

Highlights • A new method for calculating friction coefficient is established based on the solid dynamics. • The new formula does not contain the material and thermal parameters of wheel/rail. • The sliding contact time is obtained through the strain time curve, and the contact time is given. • The friction coefficient is calculated by the new formula at different wheel temperature. • The morphology characteristics of wheel surface is analyzed. Abstract The main goal of this article is to study the friction coefficient of wheel/rail transient sliding thermal contact through a pendulum. A new method is firstly established to analyze the friction coefficient. The formula for calculating the friction coefficient is not dependent on material and thermal parameters of wheel/rail. The experiment measures the sliding contact time and friction coefficient at different wheel temperature. The results show that the sliding contact time is very short and the average sliding contact time is 8 ms. The friction coefficient slowly ascends before 500 °C and then gradually decreases from 500 °C to 1000 °C. From 1000 °C to 1200 °C, the friction coefficient quickly descends. The maximum friction coefficient is 0.388. With the increase of wheel temperature, the number of small pits gradually decreases, and the scratching on the wheel surface becomes more obvious. [ABSTRACT FROM AUTHOR]

The microstructural evolution in the near-surface regions of a dry sliding interface has considerable influence on its tribological behavior and is driven mainly by mechanical energy and heat. In this work, we use large-scale molecular dynamics simulations to study the effect of temperature on the deformation response of FCC CuNi alloys of several compositions under various normal pressures. The microstructural evolution below the surface, marked by mechanisms spanning grain refinement, grain coarsening, twinning, and shear layer formation, is discussed in depth. The observed results are complemented by a rigorous analysis of the dislocation activity near the sliding interface. Moreover, we define key quantities corresponding to deformation mechanisms and analyze the time-independent differences between 300 K and 600 K for all simulated compositions and normal pressures. Raising the Ni content or reducing the temperature increases the energy barrier to activate dislocation activity or promote plasticity overall, thus increasing the threshold stress required for the transition to the next deformation regime. Repeated distillation of our quantitative analysis and successive elimination of spatial and time dimensions from the data allows us to produce a 3D map of the dominating deformation mechanism regimes for CuNi alloys as a function of composition, normal pressure, and homologous temperature.

The theoretical study and evaluation of stress-strain condition of the friction pair’s surfaces of total hip joint prosthesis with partially regular texture on a ball head surface, depending on the values of its characteristic parameters, using finite element modeling are performed. It has allowed formulating the recommendations for dimple’s spacing rationing and the extent of a surface rough site with partially regular texture, aiming to increase the prosthesis’ lifespan. The problem is solved from the condition of dry or boundary lubrication. For the indicated range of nominal contact pressure in hip joint, the quantitative evaluation of allowable roughness parameters to which it is necessary to process the ball head surface for metal-on-metal and metal-on-polymer friction pairs are obtained, considering the dimple’s spacing and size on the surface.

The sliding contact of soft material surface due to a rigid indenter is different from metal and some other polymers. A stick-slip motion is more frequently obtained than a smooth motion. By modeling the soft material as low damping viscoelastic material, this study proposes an analytical model to identify the stick-slip behavior of sliding system. The sliding system is a fixed rigid indenter that slides against on a moving soft material surface. A stick-slip model is developed and the motion of the sliding system is assumed to be in a solely tangential direction. By implementing the simple coulomb friction law, an exact solution is presented in the case of no damping of the sliding system. Results show that the periodic displacement of the stick-slip model is strongly depending on the friction force, sliding velocity and material stiffness. Furthermore, the effect of a viscous damping and velocity-dependent friction on the behaviour of the sliding system are discussed.