Asperity Interaction Research Articles

Vibrational asperity interaction in slowly sliding contacts

AbstractWhen two elastic bodies slide against each other, contacting asperities carry tangential load. In the case of slow sliding, each asperity can show a stick‐slip‐like movement that can be modelled as an integrate‐and‐fire (IF) oscillator. Asperities influence each other by an elastic coupling which allows a globally synchronized state instead of local events. For two neighboured asperities, the dynamics of coupled IF oscillators is investigated in view of synchronization modes, energy dissipation and their significance for the tribological contact. Additionally, the influence of a relaxating coefficient of stiction, i.e. a time‐dependent firing threshold, is discussed. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

An earthquake model with interacting asperities

SUMMARY A model is presented that treats an earthquake as the failure of asperities in a manner consistent with modern concepts of sliding friction. The mathematical description of the model includes results for elliptical and circular asperities, oblique tectonic slip, static and dynamic solutions for slip on the fault, stress intensity factors, strain energy and second-order moment tensor. The equations that control interaction of asperities are derived and solved both in a quasi-static tectonic mode when none of the asperities are in the process of failing and a dynamic failure mode when asperities are failing and sending out slip pulses that can trigger failure of additional asperities. The model produces moment rate functions for each asperity failure so that, given an appropriate Green function, the radiation of elastic waves is a straightforward calculation. The model explains an observed scaling relationship between repeat time and seismic moment for repeating seismic events and is consistent with the properties of pseudo-tachylites treated as fossil asperities. Properties of the model are explored with simulations of seismic activity that results when a section of the fault containing a spatial distribution of asperities is subjected to tectonic slip. The simulations show that the failure of a group of strongly interacting asperities satisfies the same scaling relationship as the failure of individual asperities, and that realistic distributions of asperities on a fault plane lead to seismic activity consistent with probability estimates for the interaction of asperities and predicted values of the Gutenberg–Richter a and b values. General features of the model are the exterior crack solution as a theoretical foundation, a heterogeneous state of stress and strength on the fault, dynamic effects controlled by propagating slip pulses and radiated elastic waves with a broad frequency band.

Rock-Joint Micromechanics: Relationship of Roughness to Closure and Wave Propagation

The closure behavior of rock joints is intimately related to joint roughness. Here we utilize a micromechanical approach that explicitly considers asperity interactions on joint surfaces to study the rock-joint closure and wave propagation behavior. Elastic deformations and inelastic frictional sliding are considered at inclined asperity contacts. Rock-joint roughness is modeled through distributions of asperity heights and asperity contact orientations. The micromechanical approach developed in this paper establishes the link between the rock-joint closure behavior, the initial overlap of the joints, the asperity height distribution parameters, and the average asperity slope. The model is verified by comparison with experimental measurements. Subsequently a parametric study is performed. The results show that rock joints with the same roughness can exhibit a range of closure behavior depending upon initial overlap and rock intrinsic friction. Therefore, unique descriptions of rock-joint closure behavior ...

Effect of surface texturing on rolling contact fatigue within mixed lubricated non-conformal rolling/sliding contacts

The rolling contact fatigue (RCF) life of highly loaded machine components is significantly influenced by the surface roughness features so that there is a continuous effort to design the topography of rubbing surfaces to enhance lubrication efficiency and prolong the operation of machine components. It can be suggested from the recent experimental results that lubricant emitted from shallow micro-dents could effectively lift off the real roughness features and reduce the asperities interactions within rolling/sliding mixed lubricated contacts. Thereby the additional supply of lubricant from surface features could help to reduce the risk of surface damage through the reduction of the interaction of rubbing surfaces during start-up or starvation. However, the introduction of such roughness features into the rubbing surfaces of highly loaded non-conformal contacts should consider not only the effects on lubrication film thickness but also on RCF. That is why this study is focused on the effects of surface texturing on RCF within non-conformal rolling/sliding contacts operated under mixed lubrication conditions. The principal task has been whether possible beneficial effect on film thickness is not accompanied by the reduction in RCF life. Textures with various sizes of micro-dents and their arrangement within the contacts have been considered. It has been found that results obtained with textured surfaces have exhibited no obvious reduction in RCF. Conversely, some increase in RCF using textured surfaces was observed that could be attributed to the positive contribution of micro-dents working as lubricant micro-reservoirs that reduce asperities interactions. Nevertheless, further experiments are necessary to confirm this possible beneficial contribution of surface texturing on RCF.

Effect of asperity interactions on rough surface elastic contact behavior: Hard film on soft substrate

An improved elastic micro-contact model of rough surfaces accounting for asperity interactions is proposed. The contact behavior of a single asperity system is composed of a stiffer hemi-spherical asperity deformation and bellowing softer substrate deformation, which is then extended to rough surface contact including asperity interactions. Using the solution of substrate deformation, normal positions of individual asperities are adjusted during quasi-static contact, from which surface interactive forces are obtained. Analytical simulations are performed using the proposed rough surface contact model, whose results are compared to Greenwood–Williamson-based models and with experimental measurements.

Effect of surface topography on mixed lubrication film formation during start up under rolling/sliding conditions

The start up operation of mixed lubricated contacts represents one of the transient conditions that bring the risk of the surface damage because of asperities interactions. This paper focuses on the effects of both artificially produced and real roughness features on mixed lubrication film formation during start up of non-conformal contacts operated under rolling/sliding conditions. Chromatic interferograms captured during start up enabled the detailed changes in lubrication film caused by surface features to be observed. The observation of the effects of surface dents artificially produced on the ball surface helped to better understand the behaviour of real surface topography. It was found that the presence of shallow surface features can help to separate mixed lubricated rubbing surfaces more efficiently than it could be suggested from the results obtained with smooth surfaces. Lubricant emitted from surface features appears to spread more easily within the mixed lubricated contact because of local decrease of pressure in the vicinity of real surface features. Conversely, high pressure within smooth contacts restricts the migration of lubricant emitted from surface pits much more. It can be suggested from the results obtained that the properly designed topography of the rubbing surfaces can help to reduce the asperities interactions under transient operational conditions.

Friction in ultra-thin conjunction of valve seals of pressurised metered dose inhalers

In many drug dispensing devices, such as syringes and inhalers, an elatomeric gasket is used to prevent the formulation from leaking from the chamber. During device actuation, the seal is subjected to friction, which in turn causes its deformation and can cause unintentional leakage, thus dose variability. Additionally, friction of seal is responsible for a host of potential problems such undue effort required for actuation and potential wear. The mechanism of friction generation in the seal conjunction is complex, arising from adhesion of rubber in contact with the moving interface, viscous action of a thin film of fluid and deformation of seal asperities. Therefore, the first step in understanding the conjunctional behaviour of rubber seals is a fundamental study of mechanisms of friction generation. A developed model can then be validated against measurements and used at a later stage to predict product performance, robustness and variability due to manufacturing tolerances. A friction model, based on the aforementioned mechanisms, for prediction of seal friction has been developed and validated against measured friction tests performed at both nano and component level scales. Pressure changes in the metered chamber have been taken into account in the model. Friction data are presented for nitrile rubber, using a silicon nitride AFM tip for nano-scale interactions and polybutylene terephthalate (PBT) for asperity interactions at a component level, where a traditional friction test apparatus is utilised. Reasonable agreement is found between measurements and model predictions for the nano-scale coefficient of friction of rubber against silicon nitride. Similarly, good agreement has been obtained for the mean coefficient of friction of rubber against PBT. It is found that dominant mechanism of friction is adhesion. For the typical asperity geometry of such seals and the usual low sliding speeds the other friction contributions, viscous shear and asperity deformation are negligible. However, this is not the case in many other seals or gaskets in other applications such as in hydraulic actuators.

A Transient Mixed Elastohydrodynamic Lubrication Model for Spur Gear Pairs

In this study, a transient, non-Newtonian, mixed elastohydrodynamic lubrication (EHL) model of involute spur gear tooth contacts is proposed. Unlike the contact between two cylindrical rollers, spur gear contacts experience a number of time-varying contact parameters including the normal load, radii of curvature, surface velocities, and slide-to-roll ratio. The proposed EHL model is designed to continuously follow the contact of a tooth pair from the root to the tip to capture the transient characteristics of lubricated spur gear contacts due to these parameter variations, instead of analyzing the contact at discrete positions assuming time-invariant parameters. The normal tooth force along the line of action is predicted by using a gear load distribution formulation and the contact radii and tangential surface velocities are computed from the kinematics and geometry of involute profiles. A unified numerical approach is adapted for handling asperity interaction in mixed EHL conditions. The differences between the transient and discrete EHL analyses are shown for a spur gear pair having smooth surfaces and different tooth profile modifications. The transient behavior predicted by the proposed model is found to be mainly due to the squeezing and pumping effects caused by sudden load changes. The lubrication behavior under rough conditions is also investigated at different operating conditions.

Effects of surface topography on lubrication film formation within elastohydrodynamic and mixed lubricated non-conformal contacts

This article focuses on the effects of surface topography modifications on lubrication film thickness within non-conformal lubricated contact. An optical test rig is used to study lubricant film behaviour between the flat surface of a chromium-coated glass disc and a steel ball. An array of micro-dents was produced on the ball surface to be able to demonstrate the effect of surface topography on lubrication film formation. Experiments were carried out under elastohydrodynamic and mixed lubrication using steady-state and transient operational conditions (start-up and reversal motion). The obtained results showed that surface texturing could represent a method of increasing the lubrication efficiency of rolling/sliding non-conformal contacts under operational conditions when the lubrication film is not able to separate rubbing surfaces. Lubricant emitted from the micro-dent helps in reducing asperities interaction. Moreover, the effect of surface micro-cavities has been observed within starved non-conformal contact under pure rolling conditions. An increase in film thickness within severe starved contact was localized upstream of the micro-dents, and was recently predicted numerically. This effect was further amplified as lubricant released from micro-dents remains on the ball surface and again enters into contact after the ball turns round.

Steady friction state and contact models of asperity interaction

One of the important problems in tribology is the evaluation of contact and wear modes under different contact conditions. A deformation predicted by contact models should correspond to an actual wear mode. The aim of this work is to compare the deformation of metallic materials in a steady friction state using contact models and a description of the dislocation structure. In order to analyze the behavior of materials under repeated sliding, the steady friction state was characterized. It was shown that the steady friction state is characterized by reaching a saturated state where a balance between hardening and relaxation of dislocations in high-deformed layers and by an equilibrium between deformation and fracture due to wear particle detachment, is exhibited. Based on an analysis of the dislocation structure of high-deformed layers in steady friction state, it was found out that the asperity contacts enable elastic deformation in macroscopic scale, whereas plastic deformation of wall-cell structure and the accommodation of substrate grain layers is reached in microscopic and mesoscopic scales. It is suggested that the hardness or yield strength and the radius of hemispherical asperity of soft metallic materials before the sliding test used in the contact models, do not reflect the deformation of asperity contact in the steady friction state. It was concluded that the development of the dislocation dynamics of surface layers under repeated sliding while taking into account the real asperity contact in steady friction state, can advance the analysis of the transition from one contact/wear mode to another.

Friction mechanisms at the micro-scale

Contact between macroscopic surfaces occurs on asperities and local nano-/micro-scale asperity interactions govern macroscopic tribological behavior. Tribological phenomena like friction and wear are highly scale dependent. Friction measurement carried out in the range of micro-/milli-Newton loads are gaining prominence in the tribological research as they are bridging the gap between classical macro-scale tests and high precision nano-scale experiments. In this paper, recent advancements in micro-scale domain are discussed and compared with the existing macro- and nano-scale friction research. There exists a striking similarity between micro- and nano-scale frictional behavior of various materials and dependence of friction on capillary forces, surface topography, and phase composition of mated materials. Micro-tribological experiments not only offer precision and sensitivity of nano-scale (or LFM) measurements, but also give reliable information due to its multiple asperity contact configuration.

Mathematical slip-line field solutions for ploughing a hard particle over a softer material

Wear mechanisms and friction in metals can be investigated by the analysis of the unit event represented by the interaction of a hard particle or asperity with a softer surface. Effective friction is the result of the interaction of many such asperities which constitute the roughness of the harder of the solid surfaces. Three types of plastic deformation at the metal surface can be identified: ploughing, edge formation and chip formation. Each mode of plastic deformation can be analysed using the slip-line field plasticity theory which requires as inputs the geometry of the hard particle and some information on the interface between the harder and the softer surfaces. The classical and the recent chord solution by Oxley assumes a sharp edge sliding against a metal surface but does not consider a curved roughness profile. However, the profiles of real asperities are more like waves with rounded summits. In the present work a new model for the asperities interaction is shown, using the slip-line field theory to calculate the friction forces, depth of sheared layer, average contact pressure and friction coefficient for a cylindrical hard particle sliding over a softer surface. The theoretical results are presented as friction graphs and maps in which the regions of elastic deformations are shown using the Hertz theory while the region of plastic strains is obtained from the present analysis. Present model results are in good agreement with experimental data obtained by Busquet et al. and are quite different from the Oxley chord model for sliding a circular particle.

Application of the Amplitude Reduction Technique Within Probabilistic Rough EHL Models

Over the years, the deterministic elastohydrodynamic lubrication (EHL) approach has been widely used. This technique is very powerful in capturing details of asperity deformation and interaction. The probabilistic EHL methodology is still used when the main interest of the engineer is directed toward computations of bulk properties. During recent years, the results of many deterministic analyses have been published. The reduction of the waviness amplitude in EHL contacts under rolling-sliding was systematically studied and it was shown that the amplitude reduction is completely described by a single parameter that includes relative wavelength and the operating conditions. This approach, usually referred to as the amplitude reduction technique, has opened the way for developing improved probabilistic EHL models by incorporating the effects of fluid-induced roughness deformation, which is calculated using the fast fourier transform. In this paper we provide a review of the latest developments in the amplitude reduction technique and we present a probabilistic EHL algorithm for the computation of the load supported by the fluid, the elastically deformed asperities and the plastically deformed asperities, in a mixed EHL contact with either isotropic on nonisotropic roughness. The fluid-induced roughness deformation is incorporated into the probabilistic model via the use of the amplitude reduction technique.

Tribology of the ring—bore conjunction subject to a mixed regime of lubrication

This paper provides a detailed analysis of the compression ring—bore/liner conjunction. The analysis includes ring—bore conformability and global in-plane deformation of ring fitted in situ. The analysis for fitted ring in an out-of-round bore shows very good agreement with precise measurements, using a coordinate measuring machine. The analysis also includes the lubricated conjunction under a transient regime of lubrication, taking into account combined elastohydrodynamics and asperity interactions. The transient nature of the tribological conjunction has been demonstrated, particularly the prevalent mixed/boundary regime of lubrication at the top and bottom dead centres. The analysis is applied to a high performance motorbike engine subjected to very high impact loads and engine speeds of the order of 13 000 r/min. Furthermore, the predictions of the model show good conformance to the measurements of friction reported by other research workers.

Tribology of rough ultra-film contacts in drug delivery devices

Elastomeric seals are extensively used in an assortment of drug delivery devices, such as syringes and pressurized metered dose inhalers. Although tribology of rubber seals and o-rings is reasonably well understood in engineering applications, the drug mixtures and formulations do not enjoy the required rheology to ensure coherent hydrodynamic action. In fact formation of uninterrupted hydrodynamic films is not actually sought in drug delivery devices, which often contain mixtures that are volatile when exposed to the environment. Furthermore, while engineering devices are often driven to overcome friction, many drug delivery systems are actuated manually and frequently by frail individuals. Therefore, the tribological problem is quite complex with many biological and environmental constraints. This paper highlights a parametric friction model for combined adhesive friction due to asperity interactions and non-Newtonian viscous action of the formulation. The model predicts the hysteretic behaviour of elastomeric seal contacts and conforms reasonably well to the experimental measurements of the same through actuation and release of inhaler valves.

Thermoelastohydrodynamic Analysis of Spur Gears with Consideration of Surface Roughness

Thermoelastohydrodynamic lubrication (TEHL) analysis for spur gears with consideration of surface roughness is presented. The model is based on Johnson’s load sharing concept where a portion of load is carried by fluid film and the rest by asperities. The solution algorithm consists of two parts. In the first part, the scaling factors and film thickness with consideration of thermal effect are determined. Then, simplified energy equation is solved to predict the surfaces and film temperature. Once the film temperature is known, the viscosity of the lubricant and therefore friction coefficient are calculated. The predicted results for the friction coefficient based on this algorithm are in agreement with published experimental data as well as those of EHL simulations for rough line contact. First point of contact is the point where the asperities carry a large portion of load and the lubricant has the highest temperature and the lowest thickness. Also, according to experimental investigations, the largest amount of wear in spur gears happens in the first point of contact. Effect of speed on film temperature and friction coefficient has been studied. As speed increases, more heat is generated and therefore film temperature will rise. Film temperature rise will result in reduction of lubricant viscosity and consequently decrease in friction coefficient. Surface roughness effect on friction coefficient is also studied. An increase in surface roughness will increase the asperities interaction and therefore friction coefficient will rise.

Micropitting Generation Mechanism for Gears

To determine the mechanism behind micropitting, we measured micropit shape occurring in surface durability test, based on the real contact area size formed by asperity interaction in surface roughness. Individual micropitting within surface roughness asperity does not exceed asperity size. Micropitting occurs due to contact stress increased by a high friction coefficient due to metallic contact. Stress analysis showed that maximum stress causes micropitting.

Interaction of Asperities on Opposing Surfaces in Thin Film, Mixed Elastohydrodynamic Lubrication

A novel experimental method has been developed to investigate how model asperities, on opposing surfaces in an elastohydrodynamic (EHD) contact, interact to influence the lubricant film distribution. This technique allows direct measurements of lubricant film thickness during asperity-asperity collision. A surface having a single transverse ridge asperity was rubbed against a second surface having three different roughness features, a transverse ridge, multiple transverse ridges, and an array of hemispherical bumps to study the resultant micro-EHD films. This work reveals how the film thickness is greatly reduced when the peaks of opposing asperities coincide, and how asperities can combine to cause a larger volume of lubricant to be entrapped at their leading edges. The new technique described shows considerable promise for the study of mixed lubrication.

Lubrication improvement and friction reduction of DLC‐coated sliding surfaces by in‐process structuring and post polishing

AbstractThe paper describes an experimental investigation on the influence of the surface roughness of diamond‐like hard carbon (DLC) coatings on the friction and wear in sliding contact against steel under conditions of starving lubrication. Experimental studies employed a reciprocating ball‐on‐flat tribometer in two different modes of operation. The study showed a micro‐lubrication effect in terms of lubrication assistance achieved by microscopic cavities in the topography of a DLC coating and good sliding behaviour even in the case of starving oil lubrication. Post‐process polishing of the coated surfaces was adopted for eliminating unnecessary friction losses due to asperity interactions and micro‐abrasive wear. Copyright © 2008 John Wiley & Sons, Ltd.

Inclusion of “interaction” in the Greenwood and Williamson contact theory

Recent direct implementation of asperity theories is reinterpreted here to formulate an improved version of the Greenwood and Williamson (GW) theory with the inclusion of interaction between asperities. This is achieved by treating the contact pressures as uniformly distributed over the apparent contact area and the resulting deformation as uniform. The correction is equivalent to an increase of the effective separation of the mean planes by a quantity proportional to the nominal pressure, resulting in a reduction of the “real” area of contact and of total load for a given separation. However, the area–load relationship is unchanged. The correction effectively depends on the ratio between the nominal pressure and the elastic modulus multiplied by the ratio between the size of the nominal contact area and standard deviation of the asperity heights. For contacts much larger than the size of roughness, uniform interaction effects would be dominant at relatively modest pressures (particularly for soft materials). This also means that the effect of interaction is unlimited. However, the only significant change is in the prediction of gas-tightness, it is harder to seal a large area than a small one. The modification of the theory has a significant effect on stiffness and conductance. Indeed, a parallel is drawn between this correction and the “clustering” terms of resistance in the Holm–Greenwood formulae for a cluster of circular spots. Finally, numerical contact simulations using Weierstrass–Mandelbrot (WM) surfaces show a general agreement with the improved theory but also significant scatter for low load levels. Taking into account the effect of asperity interaction, the improved GW theory is now able to predict the numerically obtained contact response for intermediate load levels.