Number Of Asperities In Contact Research Articles

Experimental Measurement of Roughness Data and Evaluation of Greenwood/Tripp Parameters for the Elastohydrodynamic Analysis of a Conrod Small-End/Piston Pin Coupling

<div class="section abstract"><div class="htmlview paragraph">For the investigation of the tribological behavior of lubricated contacts, the choice and the calibration of the adopted asperity contact model is fundamental, in order to properly mimic the mixed lubrication conditions. The Greenwood/Tripp model is extensively adopted by the commercial software commonly employed to simulate lubricated contacts. This model, based on a statistic evaluation of the number of asperities in contact and on the Hertzian contact theory, has the advantage of introducing a simple relationship between oil film thickness and asperity contact pressure, considerably reducing the simulation time. However, in order to calibrate the model, some non-standard roughness parameters are required, that are not available from commercial roughness measuring equipment. Standard values, based on some limited experiences, are typically used, and a limited literature can be found focusing on how to evaluate them, thus reducing the predictivity of the model. In this contribution, the roughness profile of the surfaces of the conrod small-end and the piston pin of a high performance motorbike engine are measured, adopting an optical measurement equipment. An algorithm is developed in order to properly evaluate the Greenwood/Tripp non-standard roughness parameters and set an elastohydrodynamic analysis of the lubricated coupling. A complementarity form of the Reynolds equation capable of handling the cavitation problem is coupled with both Greenwood/Tripp theory and with an alternative complementarity formulation of the asperity contact problem. The results obtained adopting the two different models are critically discussed, comparing them with empirical evidences provided by small-end/piston pin coupling failures, occurred in a severe running condition test at an early stage of the design process of the engine.</div></div>

Contact stiffness of regularly patterned multi-asperity interfaces

Contact stiffness is a fundamental mechanical index of solid surfaces and relevant to a wide range of applications. Although the correlation between contact stiffness, contact size and load has long been explored for single-asperity contacts, our understanding of the contact stiffness of rough interfaces is less clear. In this work, the contact stiffness of hexagonally patterned multi-asperity interfaces is studied using a discrete asperity model. We confirm that the elastic interaction among asperities is critical in determining the mechanical behavior of rough contact interfaces. More importantly, in contrast to the common wisdom that the interplay of asperities is solely dictated by the inter-asperity spacing, we show that the number of asperities in contact (or equivalently, the apparent size of contact) also plays an indispensable role. Based on the theoretical analysis, we propose a new parameter for gauging the closeness of asperities. Our theoretical model is validated by a set of experiments. To facilitate the application of the discrete asperity model, we present a general equation for contact stiffness estimation of regularly rough interfaces, which is further proved to be applicable for interfaces with single-scale random roughness.

Using the Multiasperity Models to Investigate the Effect of Cylindrical Micro/Nanoparticle Roughness on the Critical Manipulation Forces

In this paper, the process of moving rough cylindrical bacteria micro/nanoparticles on a smooth substrate by means of an atomic force microscope's (AFM) probe has been investigated. For this purpose, three common adhesion models of Rumpf, Rabinovich, and Cooper have been further developed in the form of multiasperity contact models and with regards to the cylindrical geometry of bacteria nanoparticles. The number of asperities in contact and also the areas of contact between nanoparticle/substrate and nanoparticle/AFM probe tip have been calculated. Then, a dynamic model for simulating rough cylindrical nanoparticles displacement on a smooth substrate has been presented and adhesive forces obtained from the aforementioned three models have been employed in this model to determine the critical force and time values for onset of rolling and sliding of particles on the substrate. Then, the effect of geometrical and roughness parameters on critical forces has been investigated. The simulation results have shown that critical forces required for pushing a rough particle are less than a smooth one. Also, the simulation results indicated that with increasing particle radius, the critical sliding force increases while the critical rolling force diminishes. For the results to be more realistic, simulations have been performed based on the roughness and geometrical parameters extracted from topographical images of bacteria micro/nanoparticles. For this purpose, two different bacteria samples were obtained and after preparing them, the topographic images of these samples were acquired by means of the AFM. Then, the roughness and geometrical parameters of these particles were extracted from the topographic images and using these parameters the manipulation process was simulated for these particles. The simulation results indicate that the maximum difference between Rumpf and Rabinovich models is 8%, so these model results are in a good agreement. Finally, the models developed in this research were validated by comparing the results with the findings of a research on gold nanoparticles.

Investigating the effect of surface roughness on the critical sliding and rolling forces of cylindrical nanoparticles based on the multi-asperity contact models

This paper has investigated the dynamic behavior of a cylindrically shaped DNA nanoparticle during its displacement on a rough substrate by an atomic force microscope. Due to the cylindrical geometry of the DNA nanoparticle, two multi-asperity models have been considered for the adhesion between nanoparticle and rough substrate. The two selected models for the contact between smooth and rough surfaces have been further developed for cylindrical geometries. One of these models is analytical and based on uniform asperities and the other one is based on random asperities. Also, in each of these models, the real area of contact between particle and rough substrate has been calculated based on the number of asperities in contact. Then, a 3D dynamic model for the manipulation of cylindrical nanoparticle on rough substrate has been developed and simulated. The maximum difference between the results obtained from the two multi-asperity models is <5 %, indicating a good agreement between the two models. The comparison of critical forces indicates that the critical force necessary for moving a particle is smaller for rough substrates than for smooth substrates and larger compared to the critical force obtained from the Rabinovich model, which is single-asperity model. Finally, the surface roughness parameters were estimated from the topographic images, and the manipulation process was simulated for these substrates by developing the relevant equations. The obtained results indicated that the critical force for a substrate with a higher root-mean-square roughness is smaller and particles can be moved easier on such a substrate.

Application of Indentation Size Effect in Finite Element Analysis for Friction Simulation

Contact simulation involving asperities was developed by assuming that the deformation by asperities is equivalent to the deformation by an indenter in a hardness test. Consequently, depth dependent flow stress curves were derived from the indentation size effect model from Abu Al-Rub and were used to simulate the influence of the number of asperities involved during contact on the distribution of contact pressure and the value of effective friction coefficient. Results from simulations suggested that multiplying the number of asperities in contact, when the size of the asperities is comparable to the size of the apparent contact, is not followed by proportional multiplication of the reaction forces. The competing phenomena observed in the simulation are then proposed as an explanation to friction size effect occurring in microforming.

Surface roughness, asperity contact and gold RF MEMS switch behavior

Modeling predictions and experimental measurements were obtained to characterize the electro-mechanical response of radio frequency (RF) microelectromechanical (MEM) switches due to variations in surface roughness and finite asperity deformations. Three-dimensional surface roughness profiles were generated, based on a Weierstrass–Mandelbrot fractal representation, to match the measured roughness characteristics of contact bumps of manufactured RF MEMS switches. Contact asperity deformations due to applied contact pressures were then obtained by a creep constitutive formulation. The contact pressure is derived from the interrelated effects of roughness characteristics, material hardening and softening, temperature increases due to Joule heating and contact forces. This modeling framework was used to understand how contact resistance evolves due to changes in the real contact area, the number of asperities in contact, and the temperature and resistivity profiles at the contact points. The numerical predictions were qualitatively consistent with the experimental measurements and observations of how contact resistance evolves as a function of deformation time history. This study provides a framework that is based on integrated modeling and experimental measurements, which can be used in the design of reliable RF MEMS devices with extended life cycles.

Load transmission by elastic, elasto-plastic or fully plastic deformation of rough interface asperities

The normal contact between two nominally flat, parallel rough surfaces is analysed on the microgeometric scale from a theoretical point of view. The microgeometry is described using a probabilistic model based on the quantification of the variability in the summit altitude and the asperity radius. This model takes for its basic data six standardised parameters for roughness and waviness defects measured on both surfaces in contact. For a given separation between two surfaces, this model allows us to determine the number of asperities in contact and their respective indentations. For each asperity in contact, realistic indentation laws are developed taking into account either elastic, elasto-plastic or plastic deformation. The continuous sum of the loads at each asperity gives the global load at the contact and the force transmitted through elastic, elasto-plastic or fully plastic deformed asperities. The theoretical analysis of classical surfaces possessing various mechanical characteristics confirms the combined influence of microgeometry (roughness and waviness parameters) and mechanical characteristics (Young's modulus, Poisson's ratio and yield stress) on load transmission. The association of these parameters into a classical index for plasticity confirms previous predictions. The last necessary parameter is the normal load applied through the contact. This parameter does not appear in previous theories for the plasticity index but can drastically change the degree of plasticity in the contact. The main contribution of our research consists in the quantification of the proportion of plastification in the contact at a given normal load.

Effects of slide-roll ratio and lubricant properties on elastohydrodynamic lubrication film thickness and traction

With tribology research aimed at decreasing energy consumption, two factors are inherently in focus: lubricant film thickness and traction. These factors are effectively decoupled and depend on lubricant properties which are sometimes contradictory-favourable for one factor and disadvantageous for the other. The film thickness ought to be maximized to reduce the number of asperities in contact and thus wear, whilst the traction should be minimized in order to reduce energy losses. In this experimental investigation the tested lubricants were investigated to see whether they possess beneficial properties for forming thick lubricant films under severe operating conditions while maintaining low traction forces. This is done by experimentally studying the film thickness reduction due to thermal and rheological effects for a fully flooded electrohydrodynamic lubrication (EHL) contact. The base oils tested were a naphthenic mineral VG150, a synthetic poly-α-olefin VG68 and a synthetic ester VG46. It was found that the synthetic ester maintained a relatively thicker lubricant film during sliding than the poly-α-olefin and mineral oil. The film thickness reduction for the mineral oil was greater than for the poly-α-olefin.