Friction Maps Research Articles

Large-eddy simulation of turbulent boundary layer separation from a rounded step

Highly resolved large-eddy simulation (LES) is used to investigate the characteristics of a canonical boundary layer separating from a curved step in a channel of height 8.5 times that of the step. The flow is treated as statistically spanwise homogeneous, in line with the conditions of a related experimental study in a large aspect ratio channel, undertaken within a companion research programme. Primary attention focuses on the details of the separation process and the properties of the separated region, including reattachment. Results are reported and analysed, from a flow physical perspective, for a wide variety of properties, including wall pressure and skin friction, mean velocity, Reynolds stresses and related anisotropy maps, two-point-correlation functions, unsteadiness indicators, budgets of the Reynolds stresses and length scales characterising the turbulence, and mean strain fields. The study highlights a range of distinctive features of separation from gently curved surfaces: the separation process is highly unsteady in time and space; turbulence is highly non-local in character; the mean reverse-flow region is thin and highly elongated; no part of the flow is reversed at all times; the level of production is extremely high following separation, resulting in massive departures from turbulence energy equilibrium, very high anisotropy and a trend towards one-component turbulence in the separated shear layer. The result, apart from offering insight into the physics of separation, constitutes a valuable data set for benchmarking model solutions and investigating statistical turbulence-closure proposals.

Elastohydrodynamic lubrication friction mapping – the influence of lubricant, roughness, speed, and slide-to-roll ratio

A friction test is conducted in a Wedeven Associates Machine ball-on-disc test rig. The output from the test, friction coefficient versus entrainment speed and slide-to-roll ratio (SRR), is presented as a three-dimensional friction map. A number of parameters are varied while studying the friction coefficient; surface roughness, base oil viscosity, base oil type, and extreme pressure (EP) additive package. Entrainment speed, SRR, and oil temperature are also varied. The results show that the mapping is efficient in showing the different types of friction that may occur in an elasto-hydrodynamic lubrication contact. The results also show that the friction behaviour can be strongly influenced by changing surface roughness as well as base oil viscosity, base oil type, EP additive content, and operating temperature.

Role of Plastic Deformation on Elevated Temperature Tribological Behavior of an Al-Mg Alloy (AA5083): A Friction Mapping Approach

Friction maps have been developed to explain the behavior of aluminum alloys under dynamic tribological conditions generated by the simultaneous effects of temperature and strain rate. A specially designed tribometer was used to measure the coefficient of friction (COF) of AA5083 strips subjected to sliding with a simultaneous application of tensile strain in the temperature range of 693 K to 818 K (420 °C to 545 °C) and strain rates between 5 × 10−3 s−1 and 4 × 10−2 s−1. The mechanisms of plastic deformation, namely, diffusional flow, grain boundary sliding (GBS), and solute drag (SD), and their operation ranges were identified. Relationships between the bulk deformation mechanism and COF were represented in a unified map by superimposing the regions of dominant deformation mechanisms on the COF map. The change in COF (from 1.0 at 693 K (420 °C) and 1 × 10−2 s−1 to 2.1 at 818 K (545 °C) and 4 × 10−2 s−1) was found to be largest in the temperature–strain rate region, where GBS was the dominant deformation mechanism, as a result of increased surface roughness. The role of bulk deformation mechanisms on the evolution of the surface oxide layer damage was also examined.

Influence of crystalline diamond nanoparticles on diamond-like carbon friction behavior

Crystalline diamond (CD) particles have been incorporated in diamond-like carbon (DLC) film structure in order to improve DLC electrochemical corrosion resistance. This paper shows the investigation of CD-DLC friction behavior according to the CD average sizes and concentration. The films were growth over 304 stainless steel using plasma enhanced chemical vapor deposition. The response surface methodology was used to develop a mathematical modeling of friction for these films, using the experimental results, in order to identify parameters that control friction and construct tribological maps according to the CD average sizes. The presence of bigger CD particles (250 and 500 nm) increased the film roughness. Films with CD particles of 4 nm presented the most homogeneous friction map, with minor variation in friction coefficient with the increase/decrease of load and sliding speed even when the CD concentration increase. This result suggests that in CD-DLC films containing CD particles of 4 nm average size, the nanoparticles are better incorporated in DLC structure due to its average size (4 nm) that is near than DLC grain size and could occupy the nanospaces between DLC grains.

Lateral and in-depth distribution of functional groups on diamond-like carbon after oxygen plasma treatments

A diamond like carbon material has been exposed to a low pressure microwave and atmospheric pressure plasma of oxygen to enhance its hydrophilicity and surface energy. For comparison, data are also reported after activation with a beam of neutral atoms of oxygen. The surface incorporation of oxygenated functional groups and the determination of the in-depth distribution of this element have been analysed by means of the X ray photoemission spectroscopy (XPS). Atomic force microscopy (AFM) has been used to get information of the surface topography and, by recording friction maps of the surface, the lateral distribution of oxygenated functional groups formed after the different activation treatments. Differences in surface composition, topography and in-depth and lateral distribution of oxygen have been correlated with the intrinsic characteristics of the activation plasma processes.

THE FRICTION AND WEAR CHARACTERISTICS OF SI3N4 CERAMIC AGAINST SI3N4 CERAMIC ZRO2 CERAMIC AND CHILLED CAST IRON AND AL2O3 CERAMIC UNDER DRY FRICTION

The friction and wear of silicon nitride ( Si 3 N 4) against silicon nitride ( Si 3 N 4) and zirconia (Y–TZP) and chilled cast iron and Alumina sliding under dry friction at room temperature conditions were investigated with pin-on-disk tribometer at sliding speed of 0.56ms-1 and normal load of 50N, 80N, respectively. Based on the variety regulation of the wear maps, the wear mechanisms of the two couples were analyzed. Get the result of friction coefficient and maps of wear Rate of the Pin and the Disk. The results of comparing this couple is Si 3 N 4/ chilled cast iron < Si 3 N 4/ ZrO 2< Si 3 N 4/ Si 3 N 4< Si 3 N 4/ Al 2 O 3.

Synthesis and morphological characterization of block copolymers for improved biomaterials

Biocompatible polymers are known to act as scaffolds for the regeneration and growth of bone. Block copolymers are of interest as scaffold materials because a number of the blocks are biocompatible, and their nanostructure is easily tunable with synthetic techniques. In this paper, we report the synthesis of a novel class of biomaterials from block copolymers containing a hydrophobic block of methyl methacrylate and a hydrophilic block of either acrylic acid, dimethyl acrylamide, or 2-hydroxyethyl methacrylate. The block copolymers were synthesized using a combination of reversible addition–fragmentation chain transfer (RAFT) polymerization and click chemistry. Since the surface morphology is critical for successful cell growth, atomic force microscopy (AFM) studies were conducted for selected block copolymers. The topography, phase angle and friction maps were obtained in dry and physiological buffer environments to study the morphology. Results of AFM imaging identified the presence of polymer domains corresponding to the copolymer components. The distribution of nanoscale features in these block copolymers is comparable to those found on other surfaces that exhibit favorable cell adhesion and growth. In physiological buffer medium, the hydrophilic component of the block copolymer (acrylic acid or hydroxyethyl methacrylate) appears to be present in greater amounts on the surface as a consequence of water absorption and swelling.

Interface roughness effect on friction map under fretting contact conditions

In many industrial applications where fretting damage is observed in the contact (e.g. rotor/blade, electrical contacts, assembly joint, axe/wheel, clutch) the external loadings or geometry design cannot be changed. Therefore, the surface preparation and finishing process become essential to control and reduce the damage caused by fretting. In this paper, the authors present the experimental study of the initial surface roughness and machining process influence on fretting conditions in both partial and full sliding regimes. Surfaces prepared by milling and smooth abrasive polishing processes have been analysed. The influence of roughness on sliding behaviour and analysis of friction have been reported. Also, the contact pressure influence and qualitative analysis of fretting wear scar have been presented.

Friction and wear maps of titanium alloy against a-C:H20% (DLC) film

Understanding the mechanism that control friction and wear is a complex and sometimes difficult task. There is a general recognition that these parameters are strongly dependent on load, sliding speed, temperature, time, contact geometry, surface roughness, ambient atmosphere and material surface compositions. Friction and wear maps may be a way to estimate friction and wear behavior over wide load and speed ranges. Additionally, they can be used as material selection guides, as well as design guides for different engineering applications. Despite the many attractive properties of DLC (diamond-like carbon) films, there are no maps available for DLC films. To date, on the web scientific search site there are only articles with dimensional wear maps for ceramic materials. This paper presents the wear rate map for DLC film with 20% hydrogen content (a-C:H20%) using a titanium alloy (Ti6Al4V) ball as counter body. Hardness, Raman scattering, and stress measurements were used to prove DLC film quality. Then, by using the results from the friction and wear tests it presents friction and ball wear volume maps as two new evaluations tools which better project the tribological relations between material behavior, load and speed. These maps successfully highlighted abrupt shifts in DLC material behavior, thus by-passing many hours of tests. Once more DLC maps are available, DLC films will become more economically viable for use in industry and space.

Friction and Wear-Mode Comparison in Copper Electrochemical Mechanical Polishing

This paper investigates friction and wear mechanisms of copper electrochemical mechanical polishing (ECMP) in pH and potential parameters as illustrated in the Pourbaix diagram. Three approaches were used in this study. First, a unique experimental setup was designed to investigate the effects of pH and electropotential on planarization. Second, the friction was measured in situ during polishing and used as a monitoring process. Third, the worn areas correlated with the frictional behavior of post-chemical mechanical polishing samples were characterized to study ECMP removal mechanisms. Results showed that friction maps can be generated in conjunction with the equilibrium Pourbaix diagram. This map is helpful in monitoring the removal rate and wear during copper ECMP. These findings can be used as guidance in ECMP process design and optimization.

Morphology of ice wear from rubber–ice friction tests and its dependence on temperature and sliding velocity

Friction processes for ice on carbon-black filled styrene butadiene rubber have been investigated by observing wear and debris morphology on wear surfaces using low temperature scanning electron microscopy (LT-SEM) and correlating the features with sliding velocity and temperature. Friction experiments were carried out over a velocity range of 0.003 m s −1 to 2.6 m s −1 and a temperature range of −33 °C to −0.5 °C and the results used to form a friction map. Observation of worn surfaces at high velocity showed melt films that result from frictional heating. At low velocities and low temperatures morphologies indicative of much drier contact were seen. At higher temperatures and low velocity there is evidence of liquid films that we propose arise from the salt content of the ice. High friction ( μ ∼ 1) at low velocity and at temperatures around −25 °C was caused by the internal friction associated with the rubber glass transition.

Amphibian mortality levels on Spanish country roads: descriptive and spatial analysis

AbstractRoad-kills are the greatest source of direct human-induced wildlife mortality, especially in amphibians. Country roads could act as the most important source of mortality when main roads act as strong barriers hampering the migration movements of some species. Mortality patterns of amphibians on country roads (1380 km) were studied in Salamanca (Spain) in order to quantify the mortality levels, to test the effects of sex and age factors on road-kills, to determine the spatial distribution patterns of road-kills, and to identify routes of migration through a friction map and hotspots of road-kills. From a total of 819 records of amphibians, 38.1% were road-killed and 61.9% were live. Fourteen amphibian species were recorded during the surveys (10 anurans and four urodeles). The species more affected by road-kills were the anurans Bufo calamita, Pelobates cultripes and B. bufo (38.5, 23.4 and 11.9%, respectively). Females had higher incidence of road-kills than males, due to the differential activity patterns of both sexes during the reproductive period. Adults were the most common age period and also the most road-killed. The spatial distribution patterns of live and road-killed records were clustered. On the sampled roads, there were 0.23 road-kills per kilometre and 52 hotspots of road-kills. The friction map showed that most of the road-killed and live specimens were located on migration routes crossing suitable habitats. Conservation measures should be implemented in these areas, as these mortality patterns may be causing significant negative impacts at the population level.

Nanoscale friction and wear maps

Friction and wear are part and parcel of all walks of life, and for interfaces that are in close or near contact, tribology and mechanics are supremely important. They can critically influence the efficient functioning of devices and components. Nanoscale friction force follows a complex nonlinear dependence on multiple, often interdependent, interfacial and material properties. Various studies indicate that nanoscale devices may behave in ways that cannot be predicted from their larger counterparts. Nanoscale friction and wear mapping can help identify some 'sweet spots' that would give ultralow friction and near-zero wear. Mapping nanoscale friction and wear as a function of operating conditions and interface properties is a valuable tool and has the potential to impact the very way in which we design and select materials for nanotechnology applications.

Quantitative Friction Map on Surface Composed of β-Cyclodextrin Monolayer

Our developing method “quantitative friction map” enables us to compare the spatial distribution of friction forces on the surface among separately scanned samples sensitively. We applied this technique to evaluate the friction acting between mono-[6-deoxy-6-[(mercaptoethyl)amino]]-β-cyclodextrin (β-CD-EA-SH) and OH-terminated C11 alkanethiol (OH-C11-SH) self-assembled monolayers (SAMs) in terms of symmetric and asymmetric features. The friction acting between symmetric β-CD-EA-SH SAMs is significantly larger than that acting between asymmetric β-CD-EA-SH SAM vs OH-C11-SH SAM and symmetric OH-C11-SH SAMs, although all the surfaces are commonly composed of hydroxyl terminated groups. The different frictional forces on these surfaces are closely correlated with the different adhesion forces, in which symmetric β-CD-EA-SH SAMs exhibited 1.5- and 1.3-fold larger pull-off force than β-CD-EA-SH SAM vs OH-C11-SH SAM and symmetric OH-C11-SH SAM vs OH-C11-SH SAM, respectively. Both force data can be explained by the “specific interactions” between glucose units on cyclodextrin heads rather than the deformation of the SAM by cantilever scanning.

Assessing functional landscape connectivity for disturbance propagation on regional scales—A cost-surface model approach applied to surface fire spread

Leaning on concepts from landscape ecology and functional landscape connectivity, we formulated and developed an operational definition of regional connectivity using a cost-surface modelling approach to assess fire connectivity whereby its structural and spatial components are explicitly isolated. Once the model is calibrated, it allows comparing different scenarios of vegetation composition and moisture contents. The use of commonly available input data and an easy to implement method to code resistance to fire propagation for a given landscape facilitates the application of this approach to other areas of interest. Functional landscape connectivity with regard to fire propagation is expressed through cost surfaces that are computed from a fire friction map and a random set of ignition points. The spatial complexity of the cost surfaces is assumed to be proportional to the landscape connectivity, and its fractal properties are used to measure and describe such spatial complexity. The fractal dimension of a cost-surface serves to assess the regional connectivity in terms of the spatial structure of frictions to fire spread, while the mean value of a cost-surface describes the overall resistance to fire propagation across the landscape in a lumped, non-spatial form. The fire friction map is derived using objective and empirically confirmed techniques enabling to account for the major factors of general fire behaviour. Furthermore, an easy to implement and repeatable method is presented to select the optimum size of random sets of ignition points, implicitly fine-tuned to the spatial variation of the input data. The model was tested on running a number of landscape scenarios based on a NFFL fuel model map. An initial series of runs served to select an optimum number of ignition points and to assess the model sensitivity to fuel moisture. Then, a set of three scenarios of vegetation cover change was devised by replacing a fast fuel model by slower fuels, and the existing network of fuelbreaks was also overlaid. The model performed as expected by quantifying the differential resistance to fire spread implicit to such scenarios. As an overall result, our model indicates that reducing the length scale of the landscape texture has a greater effect preventing fire connectivity than creating large, homogeneous patches of fire resistant vegetation.

Discussion on: “Adaptive Estimation of the Engine Friction Torque”

The paper by A. Stotsky introduces an algorithm for real-time estimation of the engine friction torque, capable to achieve a dynamic adaptation of the static maps widely used in the engine control functionality. This adaptation of the friction map allows a better vehicle driveability performance and an enhanced control by the driver of the vehicle longitudinal transients. There are several interesting features of the approach proposed by the Author, while the discussion will focus on four issues that may be seen as suggestions for further development: (i) analysis of cold start condition, (ii) accessories torque; (iii) working condition other than idling; (iv) effects of engine wear. As first, since the method aims to improve the vehicle comfort and driver’s control, the possibility to get friction torque assessment during cold start with consequent map and look-up table adaption is given by the theoretical models and experimental outcomes in [1,2]. The conclusions of these papers underline that, for some tribological pairs, the friction mean effective pressure (fmep ¼ 4� Tf,avr/V; V is the engine displacement) during cold transient is up to three or four times higher than fmep at warmed up condition. The piston assembly and the crankshaft main bearings contribute significantly to the total engine fmep and the changes in the oil viscosity produced by changes in film temperature are the most substantial influence on friction during cold start and cold running at constant speed. The use of theoretical models for fmep evolution as function of engine average temperature may improve the controller performance during cold start phases. The ECU friction map could be updated by acquiring some points (e.g., 2 � 4 points as proposed by the Author) during the cold start. The second point deals with accuracy of the instantaneous accessories torque estimate, since some devices can account for up to 40% of the whole Tloss þ Tacs torque, in some conditions [3]. Therefore, the uncertainty on the Tacs has momentous influence on Tloss (and its Tf share calculation); this effect is clearly shown taking into account the relationship for the friction torque deviation evaluation (see Eq (4)):

Finite strain model for contact interface in forming processes

AbstractA phenomenological rate and state‐variable friction law for the continuous sliding under lubricated and dry conditions, as it occurs in metal forming processes, is presented in this paper. The contact model is developed within the framework of continuum thermodynamics of irreversible processes with internal variables and for large strains assuming the contact area as a material surface. The model reproduces the experimental boundary friction map obtained by Luengo et al [1]. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reciprocating sliding behaviour of human skin in vivo at lower number of cycles

Interfacial rub phenomena between human skin and other external surfaces is a prevalent problem in every day life. The improper skin friction would induce skin trauma. However, there are few publications on the friction mechanism of human skin and subsequent trauma to date. In this paper, the reciprocating friction testing of human volar forearm skin under different normal force and displacement amplitude have been performed. The normal force ranged from 0.1 to 12 N while the imposed displacement amplitude ranged from 2.5 to 17.5 mm. Tests lasting up to 1800 cycles with a frequency of 0.5 Hz were conducted. The tangential force (Ft) was recorded as a function of the displacement (D) during each cycle of the whole testing. The results showed that there were three kinds of Ft−D curve: the quasi-closed, elliptic and parallelepipedic cycle. A friction map comprising three friction regimes has been constituted according to the different kinds of Ft−D curve. The concept of friction sensation was introduced to qualitatively describe the pain, drag and heat of the tested skin at the different friction regimes. The most discomfort sensation has been obtained at the friction regime from sticking to gross relative sliding regime.

포토리소그라피를 이용한 마이크로 딤플의 밀도에 따른 마찰 특성

Surface texturing of tribological application is another attractive technology of friction reducing. Also, reduction of friction is therefore considered to be a necessary requirement for improved efficiency of machine. In this paper attempts to investigate the effect of density for micro-scale dimple pattern using photolithography on bearing steel flat mated with pin-on-disk. We demonstrated the lubrication mechanism for a Stribeck curve, which has a relationship between the friction coefficient and a dimensionless parameter for lubrication condition. It is found that friction coefficient is depended on the density of surface pattern. It was thus verified that micro-scale dimple could affect the friction reduction considerably under mixed and hydrodynamic lubrication conditions from based on friction map. Lubrication condition regime has an influence on the friction coefficient induced the density of micro dimple.

Ice friction, wear features and their dependence on sliding velocity and temperature

AbstractFriction processes for ice samples sliding on steel have been determined by examining wear and debris morphology with low-temperature scanning electron microscopy and relating the processes to the velocity and temperature of formation. Friction experiments were carried out over a temperature range of −27 to −0.5°C and velocity range of 0.008–0.37 m s−1. Data were used to develop a friction map. Low friction (µ < 0.1) at high temperature (–3.4ºC)–low velocity (0.02 ms−1), and low temperature (–25.1ºC)–high velocity (0.30 ms−1) is due to the presence of liquid water which lubricates the sliding interface. Diagnostic morphologies for lubricated sliding include the presence of residual liquid in wear grooves and the development of a consolidated mass of debris on the trailing side of the wear surface with distinct grain boundaries and spheroidal air bubbles. High friction (µ > 0.15) at low temperature (−24.5ºC)–low velocity (0.03 m s−1) results from insufficient lubrication at the sliding interface, leading to plastic deformation. Diagnostic morphologies of plastic deformation include scuffing features on the wear surface and the accumulation of sheets of unconsolidated debris on the trailing edge of the wear surface.