Asperity Contact Regions Research Articles

On the Electric Contact Resistance

When an electric current flow across an interface between two contacting solids a change in the electric potential occur over the interface. One part of the electric contact resistance is due to the constrictions at asperity contact regions. Barber has shown that the constriction resistance can be related to the interfacial contact stiffness. I discuss the relation between the classical (approximate) Holm theory for the constriction resistance and the (exact) theory involving the contact stiffness.Graphical

A New Deterministic Model for Mixed Lubricated Point Contact With High Accuracy

Abstract Mixed lubrication is a major lubrication regime in the presence of surface roughness. A deterministic model is established to solve mixed lubricated point contact in this paper, using a new method to solve the asperity contact region in mixed lubrication. Treatment of pressure boundary conditions between elastohydrodynamic lubrication region and asperity contact region is discussed. The new model is capable of calculating the typical Stribeck curve and analyzing the transition of lubrication regime, from full film lubrication to boundary lubrication. Moreover, the final result of the model is independent of pressure initialization. High performance in accuracy and convergence has been achieved, which is of great importance for further lubrication modeling with consideration of nano-scale roughness, intermolecular, and surface forces.

Design and optimization of textures on the surface of crankpin bearing to improve lubrication efficiency and friction power loss of engine

The study proposes a design and optimization of textures on the surface of crankpin bearing to improve the lubrication efficiency and friction power loss (LE-FPL). A hydrodynamic lubrication model of crankpin bearing considering the impact of the external dynamic load and micro asperity contact is established. Based on the established model, the lubrication textures designed on the bearing surface are then simulated and optimized through the algorithms developed in Matlab environment and multi-objective optimization method. Increasing the oil film pressure and reducing the contact force ( Wac) in the asperity contact region, friction force ( Ff), and friction coefficient ( µ) of crankpin bearing are the objective functions to evaluate the LE-FPL. The study results indicate that the lubrication textures designed on the bearing surface have an obvious effect on improving the LE-FPL. Especially, with the optimized textures, the maximum oil film pressure is greatly increased by 44.8% while the maximum values of Wac and Ff are significantly reduced by 22% and 25%. Consequently, the lubrication textures added on the surface of crankpin bearing can greatly improve the LE-FPL.

Optimization of crankpin bearing lubrication under dynamic loading considering effect of micro asperity contact

Purpose To improve the lubrication and tribology performance (LTP) of the crankpin bearing, this paper aims to propose the optimization of the crankpin bearing parameters considering effect of the high-speed dynamic load and micro asperity contact. Design/methodology/approach A numerical simulation method combined by the slider-crank-mechanism dynamic and lubrication models is applied to solve the dynamic and lubrication equations of crankpin bearing. These equations are then computed via an algorithm program written in Matlab software. The contact force (Wac) in the asperity contact region, friction force (Ff) and friction coefficient (μ) of crankpin bearing are chosen as objective functions. The original parameters and experimental data of the engine are used for the simulation to enhance the reliability of the research results. The parameters are then optimized to obtain the minimum values of Wac, Ff and μ. Findings The research results show that the LTP is significantly improved with optimized parameters. Particularly, the maximum values of Wac and Ff are greatly decreased by 27 and 32%, respectively. Originality/value Reducing the width, radius and surface roughness and increasing the radial clearance of crankpin bearing can improve better the LTP. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-02-2020-0072/

Lubricated sliding friction: Role of interfacial fluid slip and surface roughness

.We derive approximate mean field equations for the fluid flow between elastic solids with randomly rough surfaces including interfacial fluid slip and shear thinning. We present numerical results for the fluid flow and friction factors for realistic systems, in particular, we consider the case of an elastic cylinder with random surface roughness in relative sliding contact with a flat rigid (low-energy) counter-surface. We present experimental data for the sliding friction between rubber stoppers and glass barrels lubricated with baked-on silicone oil. We find that the frictional shear stress acting in the rubber asperity contact regions is nearly velocity independent for velocities in the 10-1000μm/s range, and very small tau_{{rm f}} approx 0.04 MPa, while for bare glass in silicone oil tau_{{rm f}} is much larger and velocity dependent.Graphical abstract

Rubber friction: The contribution from the area of real contact.

There are two contributions to the friction force when a rubber block is sliding on a hard and rough substrate surface, namely, a contribution Fad = τf A from the area of real contact A and a viscoelastic contribution Fvisc from the pulsating forces exerted by the substrate asperities on the rubber block. Here we present experimental results obtained at different sliding speeds and temperatures, and we show that the temperature dependency of the shear stress τf, for temperatures above the rubber glass transition temperature Tg, is weaker than that of the bulk viscoelastic modulus. The physical origin of τf for T > Tg is discussed, and we propose that its temperature dependency is determined by the rubber molecule segment mobility at the sliding interface, which is higher than in the bulk because of increased free-volume effect due to the short-wavelength surface roughness. This is consistent with the often observed reduction in the glass transition temperature in nanometer-thick surface layers of glassy polymers. For temperatures T < Tg, the shear stress τf is nearly velocity independent and of similar magnitude as observed for glassy polymers such as PMMA or polyethylene. In this case, the rubber undergoes plastic deformations in the asperity contact regions and the contact area is determined by the rubber penetration hardness. For this case, we propose that the frictional shear stress is due to slip at the interface between the rubber and a transfer film adsorbed on the concrete surface.

Numerical Prediction of Surface Wear and Roughness Parameters During Running-In for Line Contacts Under Mixed Lubrication

A stochastic model for predicting the evolutions of wear profile and surface height probability density function (PDF) of initial line contacts during running-in under mixed lubrication condition is presented. A numerical approach was developed on the basis of stochastic solution of mixed lubrication, which combined the Patir and Cheng's average flow model for calculation of the hydrodynamic pressure and the Kogut and Etsion's (KE) rough surface contact model for calculation of the asperity contact pressure. The total friction force was assumed to be the sum of the boundary friction at the contact asperities and the integration of viscous shear stress in the hydrodynamic region. The wear depth on the contact region was estimated according to the modified Archard's wear model using the asperity contact pressure. Sugimura's wear model was modified and used to link the wear particle size distribution and the variation of surface height PDF during wear. In the wear process, the variations of profile and surface height PDF of initial line contacts were calculated step by step in time, and the pressure distribution, friction coefficient, and wear rate were updated consequently. The effect of size distribution of wear particles on the wear process was numerically investigated, and the simulation results showed that the lubrication condition in which small wear particles are generated from the asperity contact region is beneficial to reduce friction coefficient and wear rate, and leads to a better steady mixed lubrication condition.

On the load dependence of friction: Role of the long-range elastic coupling

Abstract When an elastic block is squeezed in contact with a substrate the contact will in general consist of many asperity contact regions. During sliding the long-range elastic coupling between the contact regions have a big influence on the sliding dynamics: when the loading force increases the density of contact regions increases, which affects the lateral coupling between the contact region, and introduces a dependence of the friction force on the loading force. Here we present a full three-dimensional model study of the role of the elastic coupling on the friction force. The theory is applied to rubber friction, but the mechanism for the dependence of the friction coefficient on the load is relevant for non-rubber materials as well.

Crack propagation in finite-sized viscoelastic solids with application to adhesion

I develop a theory for crack propagation in finite-sized, viscoelastic solids. The theory is applied to interfacial crack propagation, and found to be in good agreement with experimental adhesion data. I also discuss the contribution to rubber friction on hard, randomly rough surfaces, from the opening crack propagation in the asperity contact regions.

Static or breakloose friction for lubricated contacts: the role of surface roughness and dewetting

We present experimental data for the static or breakloose friction for lubricated elastomer contacts, as a function of the time of stationary contact. Due to fluid squeeze-out from the asperity contact regions, the breakloose friction force increases continuously with the time of stationary contact. We consider three different cases: (a) PDMS rubber balls against flat smooth glass surfaces, (b) PDMS cylinder ribs against different substrates (glass, smooth and rough PMMA and an inert polymer) and (c) application to syringes. Due to differences in the surface roughness and contact pressures the three systems exhibit very different time dependences of the breakloose friction. In case (a) for rough surfaces the dry contact area A is a small fraction of the nominal contact area A0, and the fluid squeeze-out is fast. In case (b) the dry contact area is close to the nominal contact area, A/A0 ≈ 1, and fluid squeeze-out is very slow due to percolation of the contact area. In this case, remarkably, different fluids with very different viscosities, ranging from 0.005 Pa s (water–glycerol mixture) to 1.48 Pa s (glycerol), give very similar breakloose friction forces as a function of the time of stationary contact. In case (c) the contact pressure and the surface roughness are larger than in case (b), and the squeeze-out is very slow so that even after a very long time the area of real contact is below the percolation threshold. For all cases (a)–(c), the increase in the breakloose friction is mainly due to the increase in the area of real contact with increasing time, because of the fluid squeeze-out and dewetting.

On the Validity of the Method of Reduction of Dimensionality: Area of Contact, Average Interfacial Separation and Contact Stiffness

It has recently been suggested that many contact mechanics problems between solids can be accurately studied by mapping the problem on an effective one dimensional (1D) elastic foundation model. Using this 1D mapping we calculate the contact area and the average interfacial separation between elastic solids with nominally flat but randomly rough surfaces. We show, by comparison to exact numerical results, that the 1D mapping method fails even qualitatively. We also calculate the normal interfacial stiffness $K$ and compare it with the result of an analytical study. We attribute the failure of the elastic foundation model to the neglect of the long-range elastic coupling between the asperity contact regions.

A mixed lubrication model with consideration of starvation and interasperity cavitations

This article presents a mixed lubrication model for point contacts with considering the inlet starvation and interasperity cavitations. The pressure distribution on the entire computational domain is obtained through solving a unified Reynolds equation system without identifying hydrodynamic or asperity contact regions. The Jacobsson–Floberg–Olsson mass-conservative boundary conditions are incorporated in the lubrication analysis based on Elrod’s algorithm. The inlet oil supply condition which may bring about starvation and local cavitation phenomenon is considered by defining the inlet lubricant thickness distribution. The fast Fourier transform based technique is employed to speed up the surface deformation calculation. A unique solution scheme is constructed to solve full film region, cavitation region, and contact region simultaneously. The model has been applied to the lubrication problems of textured surface under starved conditions. Numerical examples are analyzed and the results show that the present model can be used to investigate the lubrication performance under starved conditions, and that the solution methods are computationally efficient and robust.

Development of algorithm for 3D mixed Elasto-Hydrodynamic lubrication analysis

In this study, new 3-dimensional mixed lubrication algorithm is developed. For a converged solution of the mixed lubrication problem, the asperity contact pressure is calculated for the amount of asperity contact occurred due to insufficient lubricant pressure, then the procedure adding the asperity contact pressure to the hydrodynamic pressure is continually iterated until the further asperity contact amount does not occur. This algorithm has advantage that the pressure and the film shape can be calculated from Reynolds equation for all mixed lubrication region without any specific boundary condition for edge of asperity contact region. In order to investigate usefulness of this algorithm, we conducted the mixed lubrication analysis for the spherical contact with a single asperity and a rough surface and showed that the developed algorithm gives a stable convergent solution even in which severe contact takes place due to asperity height being considerably large.

Asperity Creep Measured by the Reflection of Ultrasound at Rough Surface Contact

The asperity contact regions in static contact are subjected to very high stress. Deformation is plastic and the material can suffer localized creep that is not usually observed at conventional stress levels. Creep of the asperity contacts causes an increase in contact area and hence an increase in the adhesive component of friction. The strains are so small compared with the bulk deflections that they are hard to measure by displacement of strain transducers. However, one measurement approach is to use the reflection of an ultrasonic pulse since this depends only on the interface behavior, specifically its stiffness. In this study, ultrasound was used to investigate the increase in interfacial stiffness with time. A power law relationship between stiffness and hold time was observed for both steel and aluminum surfaces pressed together. An analytical model that assumes a simple geometry for the contact has been developed. A single asperity was considered to determine the geometry of the whole interface by superposition. The stiffness predicted by the model was compared with experimental data and was used to determine the creep rate exponent for the material.

A Computer Thermal Model of Mixed Lubrication in Point Contacts

This paper presents a transient thermal model for mixed lubrication problems in point contacts. The model deterministically calculates pressure and surface temperature by simultaneously solving a system of equations that govern the lubrication, contact and thermal behaviors of a point contact interface. The pressure distribution on the entire computation domain is obtained through solving a unified Reynolds equation system without identifying hydrodynamic or asperity contact regions. The point heat source integration method is applied to determine the temperature distributions on contact surfaces. The interactions between pressure and temperature are considered through incorporating viscosity-temperature and density-temperature relations in the Reynolds equation, then solving the equation system iteratively. With the successful implementation of an FFT-based algorithm (DC-FFT) for calculation of surface deformation and temperature rise, the numerical analysis of lubricated contact problems, which used to involve a great deal of computation, can be performed in acceptable time. The model enables us to simulate various lubrication conditions, from full film elastohydrodynamic lubrication (EHL) to boundary lubrication, for a better understanding of the effect of surface roughness. Numerical examples are analyzed and the results show that the present model can be used to predict pressure and surface temperature over a wide range of lubrication conditions, and that the solution methods are computationally efficient and robust.

Nano-Scale Fatigue Wear of Carbon Nitride Coatings: Part II—Wear Mechanisms

This is the second part of two companion papers, the first of which reported the empirical data on wear properties in carbon nitride coatings by a spherical diamond counter-face in repeated sliding contacts through in situ examination, with an emphasis on the effect of friction cycles and normal load. The second part will concentrate on wear mechanisms for the transition from “No observable wear particles” to “Wear particle generation.” The relationship between the critical number of friction cycles, Nc, and the representative plastic strain, Δεp, at asperity contact region was confirmed to follow the Manson-Coffin equation with two empirical constants, β and C. The observed generation of wear particles in carbon nitride coatings is therefore concluded to be a low cycle fatigue wear by surface flow and surface delamination in the ploughing mode. For further predicting lifespan, a simplified theoretical expression, combining the Manson-Coffin equation with the analytical solution of a proposed elastic perfectly-plastic indentation model, gives the relation between the critical number of friction cycles, Nc, and the coating thickness h, with respect to the contact pressure P, and the radius R of the asperity on the tip of the diamond pin.

A note on contact stress and closure in models of rock joints and faults

We have re‐examined asperity deformation predicted by joint closure models based on Greenwood and Williamson [1966] which use a statistical representation of loaded, rough surfaces. Although such models assume small elastic strains within contacting asperities (Hertzian contact) and well predict the observed dependence of closure on normal stress, large elastic normal strains measured in experiments violate the model assumptions. This inconsistency between observations and models can be resolved. The model dependence of closure on macroscopic normal stress results primarily from the statistics of the surface topography, and the functional dependence of closure on normal stress can be independent of assumed contact‐scale elastic interactions. Thus, a joint model of the Greenwood and Williamson kind, modified to allow a portion of the elastic deformation to occur outside of the asperity contact region, predicts macroscopic behavior consistent with Hertzian models. Contact stresses derived from previously published models of this kind may be in error.

A computer model of mixed lubrication in point contacts

A numerical model of mixed lubrication is presented in this paper. The idea introduced here is that asperity contact may be viewed as a result of a continuous decrease in film thickness, so that the transition between contact and non-contact is continuous and the same mathematical model should work for both regions. The pressure over the thin films is assumed to obey the Reynolds equation, and the solution of the equation, under the condition of h→0, is expected to be the same as that predicted by the theory of elasticity. To achieve convergent and stable solutions, the left-hand side terms of the Reynolds equation are switched off when the local film thickness approaches zero, leading to a reduced Reynolds equation. Pressure distributions over the entire computation domain are thus obtained through solving a unified equation system without identifying hydrodynamic or asperity contact regions. Computations were conducted for several example cases and results show that convergent solutions are achievable on different types of roughness, over a wide range of λ ratios (0.01 to infinity), and for different slide-to-roll ratios (0.0–2.0).

The use of solid ceramic and ceramic hard-coated components to prolong the performance of perfluoropolyalkylether lubricants

The performance of perfluoropolyalkylether (PFPE) oils under boundary lubrication conditions was tested using 440C stainless steel, silicon nitride/440C hybrid, and titanium nitride hard-coated ball-bearing components. By limiting the chemical interactions between the lubricant and iron, the performance was extended by a factor of 5–10 using the ceramic materials instead of 440C components. The maximum performance enhancememt for the PFPE oils under our test conditions was limited by thermal degradation at the high temperatures generated in the asperity contact region.