Asperity Tips Research Articles

How does heat generation affect the cut and chip wear of rubber?

AbstractTire wear is a fracture process that has a decisive impact on tire life and on the environment. When a tire rolls, a heating process occurs due to friction caused by the viscoelastic rubber sliding over uneven road. This process occurs globally in the contact patch area and locally around the asperity tips, heating the tread and transferring heat to the surrounding material. On roads with a good quality pavement, the stress, and therefore the heat, is evenly distributed throughout the rubber material of the tire, which has a direct effect on fatigue wear. In contrast, unevenly distributed stress, and therefore heat, is generated in the tread when the tire rolls and slides over sharp asperities in rough terrain. This leads to very pronounced, unstable fracture processes that cause unevenly distributed wear, known as cut and chip (CC). The extent of heat generation or temperature evolution and its effect on CC wear have not yet been investigated and described in scientific publications. Therefore, this study firstly presents the detailed characterization of the influence of temperature development on the CC wear of a styrene–butadiene rubber, which commonly is used in treads of consumer tires. The investigations were carried out using the unique instrumented cut and chip technique in combination with a high-speed thermography.

Rough surface rolling contact fatigue crack stress intensity factor calculation for modern rail steels

For premium grades (and rail surface treatments such as ‘laser cladding’) there is little plastic damage expected during typical operating conditions, and smooth surface contact models predict a long life. However, damage still develops, including rolling contact fatigue, affecting premium steels from multiple suppliers. A model is described predicting stress intensity factors and indicative crack growth rates for cracks driven by surface roughness stresses. Accompanying field data quantify the asperity separation and tip radii for rails from metro, mixed traffic and a freight line in ground or unground conditions. The results predict that in rough surface contact small surface cracks (50–100 μm, exceeding microstructural dimensions) can grow faster than crack removal by wear, even when smooth surface models predict no growth. The model has application in identifying how rail grinding marks or wheel roughness may be an important factor in determining rail life through raised stresses at the rail surface (leading to wear), and increased stress intensities and growth rates for incipient cracks.

Self-Adaptive Macroscale Superlubricity Based on the Tribocatalytic Properties of Partially Oxidized Black Phosphorus.

The high-flash heat generated by direct contact at asperity tips under high contact stress and shear significantly promotes the tribocatalytic reaction between a lubricating medium and a friction interface. Macroscale superlubricity can be achieved by using additives with good lubrication properties to promote the decomposition and transformation of a lubricating medium to form an ultralow shear interface during the friction process. This paper proposed a way to achieve self-adaptive oil-based macroscale superlubricity on different tribopairs, including steel-steel and steel-DLC (diamond-like carbon), which is based on the excellent lubricating performance of black phosphorus with active oxidation and the catalytic cleavage behavior of oil molecules on the surface of oBP. This work potentially expands the industrial application of superlubricity.

Interfacial voids and microstructure evolution, bonding behavior and deformation mechanism of TC4 diffusion bonded joints

The interfacial voids closure and related microstructure evaluation of diffusion bonding process was studied by the method of combining simulation and experiment on TC4 joints. It was found that the interfacial microvoids of joints transformed narrow oval to rounded circle with the increase of bonding time. Meanwhile, the distance of two adjacent interfacial microvoids increased, indicating the improvement of the bonded ratio. At the tip of microscopic convex region, the initial boundary was replaced by recrystallized grains. At the voids neck region, the initial bonding interfaces were replaced by straight grain boundary for the inadequate deformation, resulting from the incomplete driven force of recrystallization and grain boundary migration. The bonded interface developed from straight grain boundary to grains across the original interface. The evolution of Mises equivalent stress fields and the local strain fields of the interfacial region were by Finite Element Analysis (FEA) method. The results indicated that the stress at the asperity tip region was always higher than that at the neck of the void. The distribution of dislocations on the bonding interface region demonstrated that the grain boundary migration was controlled by the multiple slip of dislocations, which controlled by Power-Law Creep mechanism.

Micromechanics of material detachment during adhesive wear: A numerical assessment of Archard's wear model

Adhesive wear is a direct result of material removal from surface asperities in contact due to adhesive forces. While macroscopic experiments confirmed the existence of a linear wear relation, referred to as Archard's wear relation (i.e. wear volume varies linearly with the normal load and sliding distance), recent small-scale wear experiments and simulations have questioned the validity of Archard's wear relation at the asperity level. We perform systematic long-timescale asperity-level wear simulations using two recently developed coarse-grained model potentials with extremely brittle and ductile behavior. A linear wear relation can be recovered from our simulations only when the material removal progresses by plastic deformation at the asperity tip, confirming the long-standing theoretical hypothesis made by Archard. However, this linearity breaks down when cleavage cracking dominates the material removal. As a result, we hypothesize that the breakdown of Archard's wear relation observed in previous numerical simulations and AFM experiments can be associated with the activation of other removal mechanisms than plastic deformation (e.g. asperity cracking or atomic attrition) and/or an inaccurate measurement of the normal load carried by the junction and the effective sliding distance (i.e. contact time). Supporting basic theoretical assumptions made by Archard, future studies should focus on up-scaling the mechanics of single-asperity to a multi-asperity contact to calculate the wear coefficient as the probability of material removal from asperity encounters.

Tribological, Thermal, Kinetic, and Pad Micro-Textural Studies Using Polyphenylene Sulfide Retaining Rings in Interlayer Dielectric Chemical Mechanical Planarization

We compared the long-term wear performance of 2 different PPS retaining ring materials (E-PPS and O-PPS) provided by 2 different suppliers. After 4 h of wear, O-PPS resulted in higher values of mean pad summit height as compared to E-PPS (26.4 ± 1 vs 23.9 ± 1 μm, respectively). We believed this was due to excessive pad fragment generation (confirmed by its associated higher level of pore obscuration) which caused the asperities to artificially appear taller. Regarding mean summit curvature, O-PPS did not sharpen the asperity tips beyond what had already been achieved through conditioning (341 ± 50 vs 352 ± 50 per μm2, respectively). On the other hand, E-PPS caused the tips to sharpen significantly (508 ± 50 per μm2). The pad-slurry-ring coefficient of friction (COF) was higher for E-PPS (0.69 vs 0.65). This was consistent with the higher observed pad surface temperature for E-PPS and its associated sharper pad asperities. Both rings showed similar tribological behaviors, and the contact mechanism was one of “boundary lubrication.” Both rings caused the pad to wear at comparable rates, but regarding ring wear rate, E-PPS seemed to wear about 30 percent faster than O-PPS. This was deemed to be inconsequential in IC manufacturing due to the estimated excessive life of the ring (regardless of which type of PPS material was used) compared to the useful life of the pad and other consumables such as filters and discs.

Interfacial plasticity controls material removal rate during adhesive sliding contact

Adhesive wear is a result of material exchange due to localized adhesive bonding between surface asperities at small scales. It has been recently debated whether a linear wear relation can be observed at the single-asperity level. Using large-scale atomistic simulations, we show that the wear relation is a direct result of the material removal mechanism at the asperity level. In the presence of weak adhesion, sliding is dominated by frictional slipping (i.e., dislocations glide in the contact plane), where occasional atomic cluster detachments and a sublinear wear relation are expected. Alternatively, a linear relation between the volume of detached material and the frictional work can be obtained when bulk plasticity dominants material removal at the asperity tip. Under this condition, high-shear stresses at the contact trigger the migration of misfit dislocations into the asperity bulk, causing severe plastic deformation via dislocation-mediated interface migration. This result highlights that the state of stresses at contact governs the process of material removal and wear relation at the asperity level.

Anisotropic Wetting Characteristics of Biomimetic Rice Leaf Surface with Asymmetric Asperities.

Rice leaf surface has known as having functional performances such as self cleaning and antifouling as well as directional flowing due to a unique micro structure with groove. In this study, we investigated the effects of asymmetrical cone protrusions on the surface of droplet flow through the contact angle and contact angle hysteresis of the droplet. First, static and dynamic contact angles of droplet on the rice leaf are measured. We found that the rice leaf surface has a directional flow characteristic through the difference of the contact angle hysteresis with flow directions. We also fabricated the rice leaf-like surfaces with asymmetric asperities along microgrooves using rapid prototyping technique and evaluated anisotropic wettability properties for the produced biomimetic surfaces. The experimental results show that the direction of the micro asperity tip relative to the droplet flow and its inclined angle has a very important influence on the anisotropic flow. This research can help to clarify the anisotropic wettability by the surface structure.

Friction in Cold Ice Within Outer Solar System Satellites With Reference to Thermal Weakening at High Sliding Velocities

AbstractThe icy shells of Enceladus and Europa consist from top down of cold (~100 K) ice at low (<0.1 MPa) pressure, cold ice at high pressure up to ~10 MPa, and warm ice near 273 K the base. The pressure ~10 MPa and temperature near 273 K of basal ice within Enceladus and Europa are similar to that within terrestrial glaciers that are known to have seismicity (icequakes). Warm ice easily melts during sliding so its icequakes are qualitatively explained and expected on these satellites if the macroscopic strain rates are comparable to those within terrestrial glaciers subjected to oceanic tides. However, cold ice at high pressures does not readily macroscopically melt during sliding. A dynamic weakening mechanism for crustal faults in rock may be applicable to Enceladus and Europa. Micron‐scale real contacts support ~0.4‐GPa shear tractions and normal tractions on rapidly sliding ice faults. At sliding velocities above ~0.1 m/s, the asperity tips of the contacts become hot and weak in ice. The macroscopic friction depends on the average strength of the asperity tips during the lifetimes of contact. The strength of the asperity tips self‐organizes so that frictional heating balances the heat lost from the asperity tip by conduction. The macroscopic coefficient of friction at coseismic sliding velocities decreases to a modest fraction of the low‐velocity coefficient of friction, but does not approach zero. This velocity‐weakening mechanism likely allows major icequakes within the cold interiors of Europa and Enceladus.

Development of Empirical Asperity Contact Model for Wet Friction Material

<div class="section abstract"><div class="htmlview paragraph">A wet clutch couples or decouples gear elements to alter torque paths in an automatic transmission system. During the gear shifting event, the clutch torque is directly transmitted to the output shaft. Hence, clutch torque heavily influences the dynamics of the transmission. In order to evaluate the behavior of the transmission early and efficiently, the development process increasingly relies on high-fidelity transmission system simulations with added complexity. However, a wet clutch continues to be modeled using Coulomb’s friction in a typical shift simulation. Its linear framework does not physically represent non-linear hydrodynamic effects due to the presence of oil layer during clutch engagement. To make up the lack of physics, Coulomb’s clutch model often requires extensive tuning to match actual shift behaviors. Alternatively, a squeeze film based clutch model, coupled with an asperity contact model, can be employed to represent hydrodynamic behaviors and enable the broader use of dynamic simulation models in transmission development. However, while the squeeze film model has been extensively studied over the years, the asperity contact model remains largely unexamined. In this research, the contact behaviors of the asperities are empirically characterized for a wet clutch friction material. The results are compared against the base theory of Greenwood-Williamson asperity contact model (GW model) which is commonly accepted in wet clutch modeling. The analysis shows that the key assumptions of GW model, specifically the elastic deformation of spherical asperity tip and Gaussian distribution of their heights, do not hold for clutch friction materials. A new empirical asperity contact model is developed for wet friction material based on asperity roughness characterization and microscopic contact area measurements. The empirical model provides an accurate representation of asperity behaviors in wet clutch modeling, as an alternative to the conventional GW model, for high-fidelity transmission system simulations. The modeling framework is also applicable to a broad range of friction materials used in dry clutches, brakes and other applications that are characterized with hard constituents embedded in an elastic matrix.</div></div>

A new method for predicting the three-dimensional surface texture transfer in the skin pass rolling of metal strips

Revealing the mechanisms of texture transfer in the skin pass of metal rolling is challenging due to the complex contact of randomly distributed surface asperities in both the roll and strip surfaces. This paper presents a novel three-dimensional characterisation method to overcome such difficulties. In this method, the plastic deformation of the random asperities was treated by the material redistribution at asperity tips. A time increment scheme was used to draw the rough surfaces of work roll and strip into the rolling bite. Within each time increment, an iterative technique was developed to predict the surface topography of the strip with the integration of the randomly-rough surface of the work roll. A systematic comparison with the finite element analysis (FEA) confirmed that the method developed can reliably predict the surface texture transfer. In addition, it was found that the surface topography of a metal strip can be significantly affected by the texture transfer from the work roll, that a larger reduction ratio results in a higher texture transfer ratio, and that the work roll surface topography is critical to the formation of desired strip surfaces.

Thermal Weakening of Asperity Tips on Fault Planes at High Sliding Velocities

AbstractReal contacts on the scale of ~10 μm support gigapascal shear tractions and normal tractions on rapidly sliding faults. At sliding velocities above ~0.1 m/s, the asperity tips of the contacts become hot and weak. The macroscopic friction depends on the average strength of the asperity tips during their lifetimes of contact. The strength of the asperity tips does not go essentially to zero once they become weak because frictional heating would then cease and the tips would cool and strengthen. Rather, the contact retains finite strength so that frictional heating balances the heat lost from the asperity tip by conduction. Crudely, the macroscopic coefficient of friction at high sliding velocities decreases with the inverse of the square root of velocity rather than the inverse of the velocity in the widely used model of Rice (2006, https://doi.org/10.1029/2005/JB004006). Numerical thermal‐mechanical models support this inference. The finite strength of the asperity tips retards lateral extrusion of weakened material from the tips. Otherwise, extrusion would allow the sliding surfaces to converge establishing new contacts, which would renew the strength of the surface. The macroscopic coefficient of friction with somewhat weakened asperity tips remains high enough that the fault surface becomes hot on a millimeter scale in large crustal earthquakes. Fluid pressurization and eventually millimeter‐scale melting then reduce the macroscopic shear traction to lower values than does asperity tip weakening acting alone.

Insights into Tungsten Chemical Mechanical Planarization: Part I. Surface Micro-Texture Evolution during Pad Break-In

The effect of conditioner types and downforces during pad break-in on pad surface micro-texture evolution is investigated. Two substantially different discs are employed (i.e. conventional vs CVD-coated), each at two different downforces. Pad samples are extracted throughout the break-in process and their surface micro-topography and pad-wafer contact characteristics are analyzed using confocal microscopy. The two conditioning discs result in different evolution paths during break-in. In general, the conventional disc produces more pad “fragments” that get counted as taller “artificial” asperities as compared to the CVD-coated disc. In contrast, the gentle shaving action of the CVD-coated disc promotes eventual flattening of the asperity tips. Regardless of the disc type, the mean summit heights decrease and reach stable values as break-in progresses. Compared to the CVD-coated disc, the conventional disc results in higher mean summit curvature indicating sharper asperities. This work underscores the need for optimum conditioner design for attaining a steady pad surface micro-texture at a given downforce and within a reasonable break-in time.

Friction and wear behavior of PI and PTFE composites for ultrasonic motors

High‐performance polymer‐based frictional materials have become increasingly important to improve the mechanical output properties of ultrasonic motors. This study discussed the friction and wear behavior of 2 dominating frictional materials of polymer composites for ultrasonic motors, polyimide (PI), and polytetrafluoroethylene (PTFE) filled by aramid fibers (AF) and molybdenum disulfide (MoS2). To explore the wear mechanisms, the tribo‐pair contact stress was theoretically characterized, and the interface temperature rise was numerically predicted. The predictions showed that the flash temperature on asperity tips could reach the glass transition temperature of the polymer materials. The experimental results indicated that the contact stress and sliding speed have a small effect on the friction of the PI composite but influence considerably the friction of the PTFE composite. A higher contact stress brings about a higher specific wear rate, but a higher sliding speed reduces the wear rate. Compared with AF/MoS2/PTFE, the AF/MoS2/PI has much better tribological performance under high loads and speeds.

Bioavailability Enhancement of Partially Water Soluble Solid Medicament in Traditional System of Medicine

Liquid state medicaments like Kwatha (decoction), sneha kalpa (oleaginous preparation), asavarista (biomedical fermentation) are supposed to have higher bioavailability due to higher absorption whereas solid state medicaments such as herbal churna (powder), bhasma and herbomineral formulations has theoretical low bioavailability, this may be due to poor water solubility. It has been claimed that bhasma is very effective at low dose. Thus the question arises ‘how Bhasma and others will be effective in very low dose despite lower water solubility?’ To find the answer of this question, we scrutinize ancient and contemporary literature regarding pharmaceutical aspects. It was found that particle reduction technology improving solubility, enhancing bioavailability and numerous particle size reduction technologies such as bhavan, bhasmikaran etc. were in practice of Ayurveda, which might correspond to liquid assisted grinding and tribochemistry. Particle size below 1 mm causes disruption of solute-solute interaction and increases bioavailability. Tribochemistry may produce nanoparticle by two sets of reactions, mechanically and thermally induced chemistry at the asperity tips due to flash temperatures and one affects the other. The reaction of solid state components by liquid assisted grinding also is an important method responsible for the conversion of inert precursors, such as metal oxides, into hybrid inorganic-organic materials. Thus particle reduction technology enhances the bioavailability of poor water soluble medicament, apart from that use, liquid media in process supposed to helps in particle reduction technology and potentiation. Thus, with the help of suitable particle size reduction technology and synergetic action of liquid medicament, bioavailability of poor water soluble medicaments can be enhanced.

Instantaneous stress release in fault surface asperities during mining-induced fault-slip

Fault-slip taking place in underground mines occasionally causes severe damage to mine openings as a result of strong ground motion induced by seismic waves arising from fault-slip. It is indicated from previous studies that intense seismic waves could be generated with the shock unloading of fault surface asperities during fault-slip. This study investigates the shock unloading with numerical simulation. A three-dimensional (3D) numerical model with idealized asperities is constructed with the help of discrete element code 3DEC. The idealization is conducted to particularly focus on simulating the shock unloading that previous numerical models, which replicate asperity degradation and crack development during the shear behavior of a joint surface in previous studies, fail to capture and simulate. With the numerical model, static and dynamic analyses are carried out to simulate unloading of asperities in the course of fault-slip. The results obtained from the dynamic analysis show that gradual stress release takes place around the center of the asperity tip at a rate of 45 MPa/ms for the base case, while an instantaneous stress release greater than 80 MPa occurs near the periphery of the asperity tip when the contact between the upper and lower asperities is lost. The instantaneous stress release becomes more intense in the vicinity of the asperity tip, causing tensile stress more than 20 MPa. It is deduced that the tensile stress could further increase if the numerical model is discretized more densely and analysis is carried out under stress conditions at a great depth. A model parametric study shows that in-situ stress state has a significant influence on the magnitude of the generated tensile stress. The results imply that the rapid stress release generating extremely high tensile stress on the asperity tip can cause intense seismic waves when it occurs at a great depth.

Running-in and micropitting behaviour of steel surfaces under mixed lubrication conditions

The paper investigates the running-in of hardened steel surfaces under mixed lubrication conditions. Pairs of surfaces of both equal and differing hardness were loaded together under rolling/sliding conditions in a twin-disk rig, and the evolution of surface topography was investigated using in-situ profilometry. Evaluation of roughness parameters, height distributions and profile relocation showed that the running-in of these surfaces is a rapid process where the most prominent asperity tips undergo plastic deformation during the initial loading cycles. Finally, the pair of equal hardness disks, following further running in a separate series of experiments, was found to suffer from micro-pitting. This micropitting predominantly occurred along the tips of prominent asperities, and the potential link between running-in and surface failure is discussed.

Adhesion effect in sliding of a periodic surface and an individual indenter upon a viscoelastic base

Solutions of two three-dimensional contact problems for a linearly viscoelastic foundation in sliding contact with a separate indenter and a periodic wavy surface taking into account adhesion are presented. Based on these solutions, the influence of the complete surface geometry, that is, shape of both tips of asperities and valleys between them, on the contact characteristics and deformation friction force is analyzed for various values of the normal load, sliding velocity, and adhesion parameter.

Nanoscale friction as a function of activation energies

Understanding the scale-dependence of friction is increasingly viewed as a critical quest. With progressively thinner films, mixed and boundary regimes of lubrication have become commonplace. Therefore, at the micro-scale a greater need for mitigating friction is desired in order to improve operational efficiency of many machines and mechanisms. Furthermore, there is a growing tendency to use low friction hard wear-resistant advanced coatings to guard against wear. In parallel, there has been much attention paid to lubricant rheology and formulation. However, only in recent times there has been an emerging view of lubricant–surface combination as a system. In this perspective it is essential to relate the observed and measured friction at component level to the underlying interactions in micro/nano-scales. This is the approach in this paper. Observed phenomenon at micro-scale are related back to the activation energies of lubricant–surface system, providing in particular results for surface modified Ni–SiC coated specimen in combination with formulated lubricants, the combination of which represent the lubricant–surface system of choice in cylinders of high performance race engine. The nano-scale conjunction of an AFM tip with lubricated surface-engineered specimen, subjected to various conjunctional loading and sliding kinematics is investigated. It is shown that the measured frictional characteristics can be adequately described in terms of activation energies in line with the Eyring’s thermal activation model for cases of fairly smooth asperity tip contact conjunctions.

Topographical orientation effects on friction and wear in sliding DLC and steel contacts, part 1: Experimental

The effect of surface roughness and topographical orientation on friction and wear has been investigated for diamond like carbon (DLC) coated and uncoated steel surfaces with three levels of surface roughness in the range of 0.004–0.11μm Ra value and with topographical orientations at 0°, 45° and 90° angles from grinding marks. In this first part we report the experimental observations that form the basis for future computational modelling of the tribological effects and mechanisms. The surfaces were characterised by the scanning electron microscopy (SEM) and focused ion beam (FIB) method and mechanical properties were measured. In the topographical characterisation measurements included the fractal signatures, the texture aspect ratio signatures and the texture direction signatures were measured and calculated by the variance orientation transform (VOT) method. The friction and wear were measured and observed in scratch testing, micro tribological testing and linear reciprocating testing in three directions of topographical orientation, as well as in rotational pin-on-disc testing. The topographical orientation had considerable effect on both friction and wear in DLC vs DLC contacts while the effect was minor and sometimes not even observable in steel vs steel contacts. A surface strengthening effect which is higher for smooth DLC surfaces and micro-cracking and micro-delamination on asperity tips at low loads for rougher surfaces is reported. The 45° orientation resulted in higher friction and considerably higher ball wear in linear reciprocating pin-on-plate testing of DLC surfaces compared with the 0° and 90° orientations.