Asperity Tips Research Articles

Contact mechanism of a rock fracture subjected to normal loading and its impact on fast closure behavior during initial stage of fluid flow experiment

SummaryFast closure of rock fractures has been commonly observed in the initial stage of fluid flow experiments at environmental temperatures under low or moderate normal stresses. To fully understand the mechanisms that drive this fast closure, the evolution of local stresses acting on contacting asperities on the fracture surfaces prior to fluid flow tests needs to be evaluated. In this study, we modeled numerically the asperity deformation and failure processes during initial normal loading, by adopting both elastic and elastic–plastic deformation models for the asperities on a real rock fracture with measured surface topography data, and estimated their impact on initial conditions for fluid flow test performed under laboratory conditions. Compared with the previous models that simulate the normal contact of a fracture as the approach of two rigid surfaces without deformations, our models of deformable asperities yielded smaller contact areas and higher stresses on contacting asperities at a given normal stress or normal displacement. The results show that the calculated local stresses were concentrated on the contacts of a few major asperities, resulting in crushing of asperity tips. With these higher contact stresses, however, the predicted closure rates by pressure solution are still several orders of magnitude lower than that of the experimental measurements at the initial stage of fluid flow test. This indicates that single pressure solution may not likely to be the principal compaction mechanism for this fast closure, and that the damages on contacting asperities that occur during the initial normal loading stage may play an important role. Copyright © 2015 John Wiley & Sons, Ltd.

Quantifying aggregate microtexture with respect to wear—Case of New Zealand aggregates

Quantifying road surface microtexture (wavelengths shorter than 0.5mm) and relating it to surface friction help to better understand mechanisms involved in the evolution of skid resistance. In this study, a three-step methodology was developed to systematically measure and analyse aggregate profiles and compare the evolution of surface friction and roughness parameters. Criteria used for the three steps (selection of the areas to be measured, filtering of the profiles to keep only the relevant part for analysis, selection of roughness parameters) are presented and justified by considering on the one hand polishing and abrasion mechanisms and, on the other hand, the contact between a viscoelastic solid and a rough surface. The methodology is applied to three different New Zealand geologically sourced aggregates and polished by using the Wehner/Schulze device. Three-dimensional images of the samples were taken periodically during the test to record the microtexture changes due to polishing. Results show that roughness parameters expressing local shape, such as curvature of asperity tips, can reasonably explain the surface friction variation; height parameters such as the standardised profile root-mean-square are also explanatory but to a lesser extent. Relevance of the studied roughness parameters is discussed and interpretation in terms of physical change of surface texture is provided.

In-cycle and life-time friction transience in piston ring–liner conjunction under mixed regime of lubrication

The piston ring/cylinder liner conjunction can experience various regimes of lubrication during piston strokes inside the engine cylinder. In the current engines, the nature of lubrication usually remains hydrodynamic at mid-stroke, while a mixed regime of lubrication may be experienced at and near reversals. The direct contact between the tips of some of the asperities of opposing surfaces leads to mixed (partial) regime of lubrication. A model proposed by Greenwood and Tripp can be used to predict asperity-level contribution to the total piston friction. At the same time, Reynolds equation can be employed to predict the portion of load carried by the lubricant trapped between the asperities. Friction between the asperity tips is usually proportional to the load that they support, stated in terms of a proportionality factor, that is, coefficient of friction. The surfaces are usually furnished with hard wear-resistant coatings and in parts by solid lubricants. Both the piston rings and cylinder liner surfaces are usually coated. These coatings change the friction characteristics of the counterfaces because of their surface topography as well as material mechanical properties. Atomic force microscope is used to obtain surface topographical parameters in contact tapping mode. The corresponding surface topographical parameters are obtained from representative regional areas of the contacting solid surfaces, using a Talysurf. The combination of topography and coating characteristics are used to develop the necessary parameters for a boundary friction model. A numerical model of the top compression ring to cylinder liner is developed based on mixed-hydrodynamic regime of lubrication. The results for friction and the effect of coating on the power loss and wear of the conjunction are discussed in this article.

Micromechanics of quartz sand breakage in a fractal context

From a Quaternary science perspective, sand-sized quartz as well as silt-sized quartz is often acknowledged as final products of glacial abrasion through different evolution mechanisms. This view challenges the existence of any universal comminution process, which may relate the formation of detrital quartz sand and silt. The contribution of grain size, energy input, and crystalline integrity in the scale of quartz crushability has long been the matter of much debate. The present empirical work examines the micromechanics of sand-to-silt size reduction in the quartz material. A series of grinding experiments was performed on Leighton Buzzard Lower Greensand using a high-energy agate disc mill. Analogous conditions to glacial abrasion are provided due to the combined abrasion between grains' asperity tips, and also between grains and rotating smooth tungsten carbide pestle. Simulation of discontinuous grain breakage allowed the examination of grains' crystalline defects. To enable an objective assessment of micromechanics of size reduction, measurements of particle and mode size distribution, fractal indexes and micro-morphological signatures were made. The crushing approach was probed through varied grinding times at a constant energy input, as well as varied energy inputs at constant grinding time. Breakage pathway was inspected via laser diffraction spectroscopy and transmission light microscopy. Results suggested that the grain breakdown is not necessarily an energy-dependent process. Non-crystallographically pure (amorphous) quartz sand and silt are inherently breakable materials through a fractal breakdown process. Results also revealed that the internal defects in quartz are independent from size and energy input.

Impact of Joule heating, roughness, and contaminants on the relative hardness of polycrystalline gold

Asperities play a central role in the mechanical and electrical properties of contacting surfaces. Changes in trends of uniaxial compression of an asperity tip in contact with a polycrystalline substrate as a function of substrate geometry, compressive stress and applied voltage are investigated here by implementation of a coupled continuum and atomistic approach. Surprisingly, an unmodified Au polycrystalline substrate is found to be softer than one containing a void for conditions of high stress and an applied voltage of 0.2 V. This is explained in terms of the temperature distribution and weakening of Au as a function of temperature. The findings in this communication are important to the design of materials for electrical contacts because applied conditions may play a role in reversing relative hardness of the materials for conditions experienced during operation.

Tyre/road noise: Influence of multi-asperity road surface properties on tyre-road contact stresses

This paper deals with the relation between properties of road surfaces and measured tyre/road noise. A number of artificial road surfaces composed of academic asperities of simple shapes (spherical, conical or cylindrical) are generated. The dynamical contact forces are calculated over several loops for a slick tyre rolling at different speeds. A multi-asperity contact model developed by the authors is used. The spectra for a periodic road surface clearly show that the excitation frequency of the tyre within the contact area is directly due to the speed and the distance between asperity tips in the rolling direction. For randomly distributed artificial surfaces, broad band spectra are obtained with a cut off frequency linked to distance between successive contacts in the rolling direction while the mean relative height has few influence. The results are discussed and compared with spectra of contact stresses for real road surfaces within the framework of tyre/road noise.

Measurement of Surface Roughness of Tablets Made From Polyethylene Glycol Powders of Various Molecular Weight

Laser profilometry has been used to study the possibility of surface damage to tablets as a result of a friability test. Polyethylene glycol powders of various molecular weight were used as model substances. The surface roughness of the tablets before testing was found to be a linear function of the brittleness of the materials expressed as the critical stress intensity factor. The type of surface damage was, however, more related to the softness of the surfaces formed. Surfaces of softer materials were damaged by a knocking-off of asperity tips, while harder, more brittle materials were damaged by breaking of the asperities at their points of connection to the surface. In general it was found that surfaces of tablets made from powders with a higher resistance to crack propagation were less liable to surface damage during a friability test.

Physics-based modeling for fretting behavior of nominally flat rough surfaces

A physics-based modeling approach for fretting behavior of nominally flat rough contact is proposed. This approach employs physics-based models for partial slip of spherical contacts to formulate the contact forces at asperity tips. The individual asperity forces are added by a statistical method to obtain the fretting response of a flat rough contact. This approach suggests the plasticity index as an important parameter for studying the surface roughness effects on fretting. Fretting responses obtained by one of the models favorably compare with experimental results obtained from bolted steel lap joints. Tangential stiffness and energy loss per cycle obtained from the experiments and the model predictions deviate at higher preloads. This discrepancy is due to limitations of the modeling approach in accounting for plastic response to tangential loading.

Faults smooth gradually as a function of slip

Geometry and roughness of fault surfaces plays a central role in the dynamics and kinematics of faulting. Faults smooth with increasing slip, but the degree of the smoothing has not previously been well-constrained for natural faults. We measure the roughness as a function of displacement for a suite of 16 faults with cumulative offsets ranging from 0.1m to approximately 500m. We find that slip parallel roughness evolves gradually with slip. For instance, for segments of length 0.5m, H≈2×10−3D−0.1 where H is the RMS roughness and D is the displacement on the fault strand with both quantities measured in meters. The gradual nature of the smoothing is robust to varying lithology and erosion. The weak function implies a decrease in the rate of gouge formation for a model with increasing slip for a model in which gouge is generated by abrading an asperity tip. The relatively gradual evolution of roughness could be explained by lubrication by the accumulated gouge that mitigates the abrasional smoothing that occurs during slip and/or re-roughening processes.

Surface Roughness Effects on Energy Dissipation in Fretting Contact of Nominally Flat Surfaces

The effect of roughness on the frictional energy dissipation in fretting contact of nominally flat rough surfaces is studied. The contact is modeled as the statistical sum of asperity tip junctions. A mathematical analysis with a probability distribution of asperity heights in the form of a delta sequence is conducted to analytically show that a rougher surface dissipates more energy than a smoother surface. Numerical simulations with three typical measured surface roughness profiles are presented, validating the analytical finding that rougher surfaces dissipate more energy than smoother surfaces in fretting contact. The proposed statistical approach is compared with so called “direct” calculation methods, which analytically model discrete asperity contacts, and the differences regarding the energy dissipation in fretting are discussed.

A Generalised Asperity-Based Friction Model

Despite considerable research effort, the use of physics-based modelling to predict frictional behaviour is still a debatable question in modern tribological research. This article presents a dry-friction model, based on physical phenomena such as adhesion, elastic–plastic contact and deformation. This contribution offers a means to simulate all kinds of frictional behaviour that is observed in experimental research. The contact of two bodies through their surfaces is transformed into the contact of a body that is provided with asperities and containing material and geometrical information of both of the mating surfaces, and a counter profile, holding solely geometrical information. The local adhesion between the asperity tips and the counter profile, together with the elastic–plastic behaviour of the asperities themselves, form the basis for this model. The simulation results show qualitatively good agreement with experimental study. Friction and contact phenomena such as normal creep, increasing static coefficient of friction with increasing dwell time, pre-sliding hysteresis with nonlocal memory, Stribeck and viscous effect, frictional lag, stick–slip and dynamical oscillations are revealed by this model. Furthermore, future improvement and refinement of the model is possible (and ongoing) so as to incorporate lubrication and asperity wear.

Experimental Analysis of Pad Wear Response Effect on Removal Rate Variations in Tungsten Chemical Mechanical Polishing

We investigated the effects of the pad surface response during polishing and conditioning on the removal rate variation in a tungsten chemical mechanical polishing (CMP) process. We examined the coefficient of friction (COF) to identify the tribology of the pad conditioning. The variation in the removal rate was dependent on the COF during conditioning. We also evaluated the effects of diamond edge degradation of the pad conditioner disk on its cutting ability from the viewpoint of mechanical wear. The results showed that the decline in pad wear rate was caused by a decrease in the cutting ability as the diamond sharp edges rounded off, while the diamond contact area with the pad surface depended little on the diamond abrasive degradation. Furthermore, we demonstrated the usefulness of measuring the height ratio of the asperity called “top surface area (TSA) ratio” as an appropriate topographical parameter to study the dependence of COF during conditioning on the decay of the removal rate. We present a clear correlation between the pad wear rate degradation and the TSA ratio after CMP to elucidate an increase in the conditioning COF related to truncation of the asperity tip by wear and plastic deformation.

Effect of Mechanical and Chemical Wear on Consistency of Conditioning in Diamonds

AbstractWe evaluated the effects of the edge degradation of diamonds in terms of their cutting ability from the viewpoint of mechanical, chemical and chemical-mechanical wear in chemical-mechanical polishing (CMP) systems for high-volume manufacturing. The results showed that both micro-chipping on the cutting edges of diamonds caused by a mechanical shock test and the rounding off of the sharp edges of diamonds in a marathon wear test degraded the pad cut rate, while no adverse effects on the cutting ability were observed during an acidic slurry immersion test. We also examined in situ coefficient of friction (COF) monitoring for use in identifying the tribology of the pad conditioning. Furthermore, we demonstrated the usefulness of measuring the height ratio of the asperity called “top surface area (TSA) ratio” as an appropriate topographical parameter to study the dependence of COF during conditioning on the decay of the tungsten removal rate. We presented a clear correlation between the pad cut rate degradation and the TSA ratio after CMP in order to elucidate an increase in the conditioning COF related to truncation of the asperity tip by wear and plastic deformation.

Elastic–plastic contact analysis of an ellipsoid and a rigid flat

The work presents a finite element model (FEM) of the equivalent von-Mises stress and displacements that are formed for the different ellipticity contact of an ellipsoid with a rigid flat. The material is modeled as elastic perfectly plastic and follows the von-Mises yield criterion. The smaller the ellipticity of the ellipsoid is, the larger the depth of the first yield point from the ellipsoid tip happens. The FEM produces contours for the normalized normal and radial displacement as functions of the different interference depths. The evolution of plastic region in the asperity tip for a sphere (ke=1) and an ellipsoid with different ellipticities (ke=12and15) is shown with increasing interferences. It is interesting to note the behavior of the evolution of the plastic region in the ellipsoid tip for different ellipticities, ke, is different. The developments of the plastic region on the contact surface are shown in more details in Fig. 7. When the dimensionless contact pressure is up to 2.5, the uniform contact pressure distribution is almost prevailing in the entire contact area. It can be observed clearly that the normalized contact pressure ascends slowly from the center to the edge of the contact area for a sphere (ke=1), almost has uniform distribution prevailing the entire contact area for an ellipsoid (ke=12), and descends slowly from the center to the edge of the contact area for an ellipsoid (ke=15).

Material and temperature dependent behaviour of single macro-asperity tip when crushed against flat platen

Thermo-mechanical forming of metals is of special concern due to its complex nature which involves temperature, load and particularly tribological parameters at elevated temperatures. Previous studies have given much emphasis to the deformation behaviour of surface asperities at micro level which in turn determines quality and surface finish of a component produced by thermo-mechanical forming. In this work, special attention is given to the deformation behaviour in the case when a single macro-asperity tip is crushed by a flat platen, dependent on the constitutive behaviour of the asperity material and the asperity temperature. Different cone-shaped single asperity tips with two different cone angles, as well as with top-flat facets, were studied bearing in mind that a surface may contain these forms of asperities. Experiments were conducted on the Al-alloy AA6082 at room temperature (RT) and FEM analyses were run to study the behaviour at high temperature (450°C). This work gives deep insight into the deformation behaviour of the macro-asperity tips, with regard to for instance lateral spread of the tip, and its ability to provide contact pressure against the crushing platen. The analysis also reveals how high and low friction against the platen affects the deformation behaviour of the asperity tips.

Effects of Surface Roughening on Asperity Flattening

A series of experiments have been conducted using aluminum strips, which manifest a strong tendency toward roughening in forming processes, to investigate the evolvement of surface roughness and its resistance to flattening. The strips are stretched to specific bulk strains and are then compressed by a smooth glass at various pressures. It is found that except for very high bulk strain, the free surface on the transverse specimen roughens more seriously and irregularly than the longitudinal one due to the fierce rotations of grains, which make the grains parallel to the straining direction and eventually mutate the topography into an isotropic one. As the roughened surface is compressed, the transverse specimen may roughen further in the case of large bulk strain. According to the finite element method (FEM) analysis using serrated surfaces, the sharper asperity undergoes more serious straining on its peak and basically such a strain increment is not affected by the degree of flattening. The ability to resist flattening, defined as the “asperity hardness,” varies with asperity angle as well as the enhanced shear strength on the asperity tip, rather than the substrate. A new quantitative description of the asperity hardness is suggested, and the new calculated contact area ratios are in good agreement with the experiments except for very high pressure where additional mechanics such as adiabatic recrystallization might happen. The concept of the additional hardening on asperity peaks can be employed to refine the prediction of asperity contact in the analysis of metal forming.

Impact dynamics of rough and surface protected MEMS gears

The paper provides an analysis of dynamics of micro-gear pairs, typically used in an assortment of micro-electromechanical system (MEMS) devices. It includes a mathematical hierarchical model of the impact dynamics of meshing gear teeth. It comprises the nanoscopic effect of asperity tips’ adhesion for relatively rough surfaces on a microscopic level (overall contact domain). The analysis is extended to the depletion of long chain molecule self-assembled molecules (SAM) in impact behaviour of meshing gear-teeth pairs. The analyses show that for the usual high operating speeds of MEMS gears, due to high impact velocities, the role of asperity tips’ adhesion is quite insignificant. However, the same is not true for lower impact velocities, which would occur under start-up, run-up to normal operating speeds or during deccelerative motions. The paper proposes a novel spectral-based approach to predict the degradation of the protective SAM layer between meshing teeth, while the mechanism is in continual relative motion.

Pressure Dependence of Shear Strengths of Thin Films on Metal Surfaces Measured in Ultrahigh Vacuum

The friction coefficient is measured for systems consisting of a thin potassium chloride film deposited onto a variety of clean, flat metal substrates, namely Pb, Sn, Au, Ag, Cu, Pd, Fe, Ta, and two types of steel, which are rubbed by a tungsten carbide pin in an ultrahigh vacuum. The friction coefficients are plotted versus 1/H S, the inverse of the substrate hardness, where two regimes are found. In the first regime, where deformation at the asperity tips is suggested to be plastic, the observed variation in friction coefficient with substrate hardness is rationalized by assuming that the shear strength S for sliding on a KCl film varies with contact pressure P as S = S 0 + aP, yielding values for a of 0.14 ± 0.02 and S 0 of ~60–70 MPa. In the second regime, it is proposed that the softer, film-covered Pb and Sn substrates are closer to being in conformal contact with the rough tribopin. These values of S 0 and a, along with the measured surface asperity height distribution of the tribopin and the value of the friction coefficient for a KCl monolayer on the metal, are used to rationalize the observed increase in friction coefficient with increasing film thickness.

Tribology of compression ring-to-cylinder contact at reversal

Piston ring-pack-to-cylinder contact accounts for one of the major sources of frictional losses in internal combustion engines. The regime of lubrication alters during the piston cycle because of the transient nature of applied load and kinematic contact conditions. Ring geometry, surface topography, and lubricant rheology also play an important role. The aim is to attain full fluid film lubrication, thus reducing friction because of boundary interactions. Therefore, accurate prediction of lubricant film thickness and pressure distribution constitutes the first step in a proper analysis of piston ring—cylinder conjunction. The creation of a gap through elastic deformation is sought in order to inhibit asperity tip interactions. The generated contact pressures in the lubricant film are due to combined entraining motion and squeeze film effect. The integrated pressure distribution balances the elastic force due to ring tension and the applied combustion pressure acting behind the ring. The article highlights a detailed analysis, which forms the basis for its future expansion to include the study of mixed regime of lubrication, which may be prevalent in some real engines.

Effect of a hard artificial asperity on the crack closure behavior in an annealed SAE 1015 steel

The load–crack opening displacement (COD) curves and deformation characteristics in the vicinity of a hard artificial asperity in an annealed SAE 1015 steel were studied. The artificial asperity was found to have a significant effect on the trend of the load–COD curves. The lower portion of the load–COD curves in the unloading phase exhibited a convex shape without the asperity, but a concave shape with the asperity. The concave shape, signifying the acceleration in the COD decrease, was further verified by varying the size of the asperity, conducting special compression tests and elastic–plastic load–COD tests. The plastic deformation in the vicinity of both asperity and crack tip was studied via microhardness tests, etching techniques, and finite element analysis. Based on the experimental observations, a modified crack closure process model was proposed, where three stages of the unloading curve was defined: (i) the asperity does not contact the upper crack face, (ii) a process where both the asperity and the specimen material deform elastically, and the elastic-wedge model is applicable, and (iii) the plastic deformation of the specimen material adjacent to the asperity occurs, thus resulting in the concavely shaped load–COD curves. An equation was proposed to estimate the COD values, in which the plastic deformation both at the crack tip and at the asperity was considered. The residual COD calculated from the proposed equation was found to be consistent with the experimental results.