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This work carried out a systematic analysis to help understanding how surface roughness and relative orientation between surface topography and direction of abrasive entrainment can affect particle dynamics during both pure microabrasion and microabrasion-corrosion. For that, tests were carried out in AISI 304 austenitic stainless steel test specimens using a recently developed microabrasion-corrosion tester. A 3D load cell ensures controlled application and measurement of forces during the tests. For the microabrasion tests, a slurry composed of small (ϕ=2.5 µm) silica particles dispersed in distilled water at a concentration of 20wt% was employed, whereas for the microabrasion corrosion tests the distilled water in the slurry was substituted by a corrosive solutions in distilled water (1 N H 2 SO 4). Potentiodynamic curves varying from cathodic potentials into the transpassive region were obtained within the course of duration of each microabrasion-corrosion test. The specimens were ground using two different SiC papers (#80 and #4000) to vary their surface topography and were tested with the grooves oriented perpendicular and parallel to the direction of rotation of the ball. Although the effect of surface topography was slight under pure microabrasion conditions, it was more significant under microabrasion-corrosion conditions. It was proposed that under microabrasion conditions, the formation of a low friction passive layer on the specimens made particle entrainment more difficult and therefore the relative orientation between surface topography and ball rotation had a stronger effect on the amount of abrasives that entered the contact, affecting wear rates. Grooves parallel to the rotation of the sphere increased wear rates under microabrasion-corrosion conditions when compared with grooves oriented perpendicular to the contact. Parallel grooves also resulted in higher passivation currents than perpendicular grooves, probably due to more severe damage to the passive layer caused by more abrasives into the contact [ABSTRACT FROM AUTHOR]

The effects of surface roughness and topographical orientation on surface stresses influencing wear have been investigated for diamond like carbon (DLC) coated steel surfaces with three levels of surface roughness in the range of 0.004–0.11 μm R a values, and with topographical groove orientations of 0°, 45° and 90°. A novel multiscale numerical finite element method (FEM) model was developed to integrate the layered and microstructural material features with the grooved topography. Fractal geometry and a surface voxelisation based approach were utilised to derive 3D surface topography. The surface texture representation includes: fractal signatures, which are sets of fractal dimensions calculated at individual scales in different directions, texture aspect ratio describing surface anisotropy, and texture direction signatures calculated by the variance orientation transform (VOT) method. The simulations show details of the main topographical orientation effects on local stresses affecting wear as they appear at a single scratch test with a spherical diamond ball and in a self-mated sliding situation of two rough surfaces. The 45° sliding direction in relation to grooves resulted in a mixed state of surface loading in the scratch test contact. In the complex state of stress-strain within the roughness peaks the overall tensile stress decreased, leading to greater surface resistance to cracking as compared to 0° and 90° directions. Model based calculations showed that the surface structure was about four times more rigid in the direction of grooves as compared to the more flexible behaviour in the perpendicular direction. This behaviour was empirically confirmed. The numerical calculations of rough vs rough sliding surface include real surface topographical features at various scales, material microstructural features down to nano-scale and topographical-microstructural interaction features. This approximation is thus more comprehensive than the classical approach. The real area of contact was 15–30% of the apparent contact area. The macro-topography dominated the tendency for surface cracking and plastic deformation, which is influencing on both wear and friction, while the micro-topographical features contributed to cracking and deformation by less than 40%. [ABSTRACT FROM AUTHOR]

Wear and friction properties depend on many parameters such as normal load and material hardness. However, coupled contributions of microstructure and abrasive particle size are less investigated. A contribution in this field is proposed with wear tests performed between a 35NCD16 steel and abrasive silicon carbide grains ranging from 35 µm to 200 µm under different normal loads (50–110 N). To vary hardness and microstructure, samples are subjected to water quenching and tempering at various temperatures (200–600 °C). These heat treatments result in a martensitic microstructure with various levels of carbon contents and carbide precipitations. For a given microstructure and hardness, the friction coefficient decreases with increasing the normal load and/or decreasing the abrasive particle size. However, the wear rate increases with increasing the normal load and/or the abrasive particle size. For high normal loads, the results also reveal that the friction coefficient decreases when the hardness and the martensite volume fraction increase. Nonetheless, for low normal loads, a transitional microstructure from martensite laths to carbides and equiaxed grains, for a tempering around 400 °C, impacts the friction coefficient. Whatever the normal load, this transitional microstructure also influences the wear rate. [ABSTRACT FROM AUTHOR]

The frictional capacity of rubber-tired metros decreases over time under the action of metro traffic. It is well known that the texture of rolling tracks contributes to evacuate water from the tire/track interface and generate friction forces. There is then a need to understand the evolution under traffic of the texture of rolling tracks to optimize their maintenance in terms of surface texturing. This paper investigates the friction and wear of steel rolling tracks subjected to laboratory polishing tests. Specimens are extracted from real metro tracks. Different initial textures obtained from different initial treatments of the surfaces are studied. Polishing and friction measurements are performed. During the polishing process, the surface topographies are measured at different polishing states and functional volumes and texture descriptors evaluated. Results show that the surface asperity's height and sharpness decrease with the number of polishing cycles. Also, the wear characterized by the difference in terms of material volumes between the initial and polished states, increases with the polishing duration. This evolution is composed of two phases corresponding respectively to transient and stationary wear. The evolution of wear volume during the stationary phase is in agreement with Archard's wear law. Tight correlation is found between friction and wear volume. [ABSTRACT FROM AUTHOR]

The objective of this research was to determine the influence of applying a hybrid treatment, consisting of fine particle bombardment (FPB) and powder impact plating (PIP), on the scuffing behavior of ductile cast iron. Ductile cast iron samples were processed by an FPB treatment, a PIP treatment, and a hybrid treatment of both FPB and PIP. Scuffing behavior of these samples was investigated using block-on-ring tests. A piston alloy of ductile cast iron was used for the block and a cast iron cylinder alloy was used for the ring. Results showed that scuffing resistance depends on surface hardness and friction coefficient for the test samples. FPB treatment of block samples increases their surface hardness by nanocrystallization and reduces the friction coefficient of the tribological pair by reducing the surface roughness and producing a micro-dimpled surface. Thus, the scuffing resistance of FPB treated block is improved. The scuffing resistance is also improved by PIP treatment, which reduces the friction coefficient by producing a soft Sn film on the contact surface. After a hybrid treatment of both FPB and PIP, the surface hardness of the ductile cast iron substrate is increased and the friction coefficient of the tribological pair is further reduced relative to either single treatment. Therefore, hybrid treatments such as the ones described here can improve the scuffing resistance of cast irons. [ABSTRACT FROM AUTHOR]

This paper aims at an improved understanding of the influence of different cooling and lubrication conditions on the tribological behavior at extremely high contact pressures. A novel tribometer was used to investigate the tribological behavior of the material pairing AISI 4140 Q + T and WC-Co carbides, which is common in cutting processes. Regarding the cooling and lubrication conditions, dry condition, liquid nitrogen (LN 2), carbon dioxide snow (CO 2), and two different types of sub-zero metalworking fluids (sub-zero MWF) were investigated. The tribological behavior is analyzed on the basis of the thermo-mechanical load in the tribological contact, the coefficient of friction, and the surface topography of the friction partners. Furthermore, the influence of the hydrodynamic lubrication gap height, resulting from different types of sub-zero metalworking fluids, on the lubricating effect is investigated. The analysis shows that the use of cryogenic media resulted in a substantial reduction of the thermal load. However, the coefficients of friction as well as the topography of the friction partners indicate that a lubricating effect can only be achieved up to a contact pressure of roughly 2100 MPa. In contrast, both types of sub-zero MWFs led to a high lubricating effect even at contact pressures of up to 3600 MPa. By using both types of sub-zero MWFs at a supply temperature of -30 °C a high cooling effect was also achieved. A higher hydrodynamic lubrication gap height can be beneficial for the lubricating effect depending on the type of sub-zero MWF. In this context, however, further friction-reducing effects such as adsorption films need to be considered. [Display omitted] • Novel tribometer is presented and integrated in a turning machine. • Transfer of the contact conditions during chip formation to tribological tests. • Investigation of several contact conditions as well as cooling and lubricating conditions that occur in cutting processes. • Analysis based on hydrodynamic lubrication theory in the context of cutting processes. • Identified ranges of contact pressures for which different cooling lubrication strategies lead to advantageous results. [ABSTRACT FROM AUTHOR]

In the last few years, top-of-rail friction modifiers have been designed and used in many railway systems all over the world. These adhesion enhancers are applied by either off-board or on-board system in order to achieve the intermediate level of friction and positive adhesion curve. Previous scientific effort was mainly focused on the effects of water-based friction modifier on adhesion, rolling contact fatigue, railway noise and corrugation formation. The objective of this study is to investigate the abilities of oil-based friction modifiers to control adhesion and reduce wear at wheel rail interface. For this purpose, two commercial oil-based friction modifiers were particularly utilised. A ball-on-disc tribometer was employed to investigate their traction and braking performance for various slip ratios under dry conditions. Furthermore, the effect of friction modifier amount has been studied. At the end of the performed tests, wear rate, surface damage and changes of surface topography were determined and compared to dry and oil-contaminated contact. The results indicate that oil-based friction modifiers are able to control adhesion in wheel-rail contact but it is strongly dependent on the applied amount of friction modifiers. Regarding to the friction behaviour and wear, the content of metal particles seems to be the crucial parameter. [ABSTRACT FROM AUTHOR]

Sliding electrical contact behavior of the brass fiber brush with 23% packing fraction was investigated as sliding against coin-silver and Au plating under vacuum and air conditions. The friction coefficient, contact resistance and electrical noise of the brushes were analyzed. The microstructure and worn surfaces of the brass fiber brush and counter discs were observed to discover the involved wear mechanisms. The friction coefficients in vacuum are much higher than those in air. The contact resistances and electrical noises of the brush against both discs in vacuum are extremely low. The contact resistance against coin-silver quickly increases as the sliding condition varies from vacuum to air, which is much higher than that against Au plating. Sliding speed has an insignificant effect on the contact resistance and electrical noise of fiber brush. Formation of oxide film and loose oxide particles on the worn surfaces of fibers and the coin-silver disc leads to the increase of contact resistance and electrical noise. The transfer of gold from Au plating to worn surfaces of brass fibers is used to prevent the oxidation of fibers and keep the contact resistance and electrical noise in a relatively low level. The multi-contact mechanism of brass fiber brush has been also discussed. [ABSTRACT FROM AUTHOR]

The correlation between the type of abrasive in the brake friction composite and the braking induced emission was studied. Brake pads with four different abrasives; silicon carbide, alumina, zircon, and magnesia, were evaluated using a reduced scale dynamometer with an airborne particle counter. The results indicated that the particle abrasivity, comprising aspects such as hardness, fracture toughness, and angularity, strongly affected the disc wear and airborne particle emission, whereas the pad wear was more influenced by the strength of the friction film that was changed by the iron moved from the grey iron disc. The fracture toughness of the abrasives had a significant effect on the disc wear rate and mass concentration of airborne wear particles emitted during brake application. In contrast, the number concentration was considerably influenced by the wear mode of the abrasives against the grey iron disc. • Type of abrasives in the friction composite affected wear and brake emission. • Pad wear increased by the iron transferred from the grey iron disc. • Fracture toughness of abrasives strongly affected the brake emission. • Abrasive wear mode causes a discrepancy between number and mass concentrations. [ABSTRACT FROM AUTHOR]

The aim of the present work was to investigate the influence of humidity and corrosion in a high-salinity environment on the friction and wear behaviour of three brake disc materials. Experiments were conducted using a commercially built disc-on-pin machine (SRV4, Optimal Instruments, Germany). Three different disc brake materials were used: cast iron (type FG25), chromium bearing steel (type 100Cr6) and an aluminium-based composite Al MMC (type A359/SiC p ). The pin in all tests was a commercial automotive brake pad material. The results showed that the friction coefficients and the wear rate for all tested materials were sensitive to increasing normal load and the sliding speed. At increased humidity level, a water film formed and behaved as a lubricant. The corrosion due to salt spray had a larger influence on the tribological behaviour of the cast iron and the chromium steel than it did on the Al MMC. The Al MMC A359/SiC p exhibited a better tribological behaviour than the other brake disc materials used in this investigation under the given test conditions. [ABSTRACT FROM AUTHOR]

This article presents the development of a method for manufacturing durable countersurfaces for testing the friction of rubber samples in a laboratory. The surface sample was designed to replicate the surface roughness characteristics of a predefined road surface. Four different types of samples were manufactured: two bitumen bound stone mastic asphalt samples, one concrete bound sample, and one epoxy bound sample. In addition, the surfaces were sandblasted to remove binder from the top of the surface. The similarity in surface roughness between the predefined target road surface and the laboratory surface samples was evaluated using optical 3D surface roughness measurements and surface roughness power spectrums. The durability of the surfaces was investigated by repeated dry rubber sliding friction tests. It was found that out of the studied surface samples the best one to both replicate the surface characteristics of the target surface and to produce a durable surface in terms of wear was the one using epoxy resin as a binder. By replacing bitumen with epoxy, more friction tests could be run without significant changes in the surface topography or rubber friction. In addition a reasonable correlation between the surface roughness power spectrums and the friction results was found, which supports the use of roughness power spectrums as a roughness metric for road surfaces in the context of rubber friction. [ABSTRACT FROM AUTHOR]

Clutch actuators in the automotive industry are used to convert the force applied by the driver to the clutch. A typical pneumatic actuator consists of an anodised aluminium cylinder in which a piston, with a rubber lip seal and a PTFE guiding ring, slides. The system is silicone grease lubricated. The aluminium cylinder is most often cast, e.g. permanent mould, sand, or high pressure die cast. An interesting alternative is extrusion. After anodising the cylinders display different surface topography due to, amongst other things, the fabrication method. In this study, the friction behaviour of the anodised surfaces from the four mentioned methods are investigated in a reciprocating sliding test. Two test setups were used, one simulating the real contact situation, and one using a simpler sphere on flat geometry. The extruded surface oxide cracked during testing, resulting in very fluctuating friction behaviour. The high pressure die cast surface showed rather good results, but never the best. However, it is considered not suitable for the application due to its poor oxide properties. The permanent mould cast surface displayed the lowest friction coefficient, while the sand cast surface often showed the highest friction. [ABSTRACT FROM AUTHOR]

Abstract: Numerical models of surface micro-topography find applications in the development of multiscale models for friction and wear between two interacting surfaces. Although much work has been dedicated to developing such multiscale models, they have been hampered by the strong dependence of surface statistics on the resolution of surface topography measurements. The objective of this study is to develop a systematic approach to reduce this dependence so that the resulting statistical parameters are suitable for developing micro-asperity based continuum friction models. The approach significantly reduces but does not eliminate the dependence of surface statistics on the measurement resolution. It is based on fitting a Gaussian function to numerically calculated autocorrelation functions for randomly selected profiles from a surface. The use of a Gaussian function filters out very small scale asperities that affect the statistical parameters but are not tribologically significant. The distributions of the resulting parameters allow us to calculate the spectral moments using Monte Carlo simulations. The approach is applied to numerically generated surfaces as well as micro-topography measurements of a high density polyethylene (HDPE) surface. Results show that the proposed approach is considerably less sensitive to the measurement resolution, especially in comparison to standard statistical sampling methods. We argue that the Gaussian autocorrelation function used in our work is a better choice compared to other forms for continuum-level applications. [Copyright &y& Elsevier]

Abstract: Titanium and its alloys generally display poor tribological properties in sliding contacts due to their high chemical activity and strong adhesion to the counter surface. The strong adhesion causes a high tendency to transfer and ultimately galling or build-up edge formation, resulting in severe surface damage. As a result, forming and machining of titanium and its alloys are generally associated with significant problems such as high friction, rapid tool wear and poor surface finish of the formed/machined surface. In the present study, in situ tests in a scanning electron microscope have been performed to increase the understanding of the mechanisms controlling the initial transfer of titanium (Grade 2) in sliding contact with tool surfaces. Tool materials included cover cold work tool steel, cemented carbide, CVD deposited Al2O3 and PVD deposited DLC. In these tests, a relatively sharp tip, representing the titanium work material, slides against a flat surface, representing the tool. The contact conditions result in plastic deformation of the work material against the tool surface, thereby simulating forming or machining. The limited and well-defined contact, along with the possibility to study the sliding in the SEM, makes it possible to correlate local surface variations to transfer of work material and frictional response. Post-test characterization of the contact surfaces was performed by high-resolution SEM, TEM, EDS and EELS. The initial friction was low and stable against all tested materials, but then gradually escalated against all surfaces except the DLC. The friction escalation was associated to increasing levels of transfer, while the DLC stayed virtually free from transfer. From these very initial sliding tests DLC is a promising tool coating in forming and machining of titanium. [Copyright &y& Elsevier]

Abstract: Effect of water absorption on friction and wear of polyamides, namely monolithic polyamide 66 (PA66) and PA66 composite with 30wt% of short glass fibers, was investigated using a block-on-ring-type tribotester. Monolithic PA66 showed high wear rates after water absorption because of softening by the absorbed water molecules and increased interfacial adhesion on the steel counter surface. Degraded wear characteristics of monolithic PA66 by chain scission of the amide functional group by water molecules were substantially improved by short glass-fiber reinforcement. The improved friction and wear characteristics were attributed to contact plateaus produced during sliding, which were held by condensed glass fibers, suggesting possible applications of PA66 in a humid environment by reinforcing it with short glass fibers. [Copyright &y& Elsevier]

In any attempt to theoretically calculate the real contact area for 3D engineering surfaces, a criterion is needed to identify the relevant asperity-peaks that carry the load in tribological contacts. In our recent work, we investigated how different, available 2D criteria affect the properties of the theoretically determined asperity-peaks in 2D surfaces. In this work, however, we focused on a 3D surface characterisation. The effect of different asperity-peak identification criteria on the properties of the asperity-peaks (numbers, radii and heights) is studied in the 3D domain. Several different criteria that take into account the number of neighbouring points, the distances between them (lateral resolution) and their heights were evaluated for real measured surfaces with five different surface roughnesses in the broad engineering range of arithmetic surface roughness from S a =0.005µm to S a =0.529µm. From the results it follows that all three chosen asperity-peak identification criteria (5PP-3D, 9PP-3D and 9PP-R-3D) result in reliable asperity-peak properties, and none of them can be favoured based on a theoretical evaluation only. There are, however, important differences between them. The data resolution in the x and y directions has a very important influence on the numbers, radii and heights of the asperity-peaks, and the results suggest that the data's lateral resolution, below 1µm, should be used for the relevant asperity-peak identification. [ABSTRACT FROM AUTHOR]

High-speed machining of austenitic stainless steel normally causes significant tool damage and generates reduced tool life. In this paper, five AlTiN PVD coatings with different Al/Ti atomic ratios (50/50, 60/40, 67/33, 70/30 and 73/27) which deposited on cemented carbide inserts were used to conduct high-speed of 370 m/min finish turning tests. The experiments were carried out under different cooling conditions (dry and wet) on SS304 to study the tribological behavior of the AlTiN coatings with different Al/Ti ratios and the effect of the coolant under such aggressive cutting conditions. During the experiments, tool life, cutting force, wear mechanism, friction condition and surface integrity of machined workpiece were investigated. Crater wear was found to be the predominant wear mode during the cutting test, while the complex combination of oxidation, abrasion/attrition, adhesion, and chipping contributed to the tool failure. Given the machining conditions proposed in this study, the results revealed that all coated inserts possessed an improved friction behavior in the wet cutting condition. Compared to the dry machining, all five coatings had exhibited 2–3 times longer tool life. The AlTiN coated insert (Al/Ti = 60/40), in particular, exhibited a cutting length of almost 7000 m, compared to 1000 m for the AlTiN coated insert (Al/Ti = 73/27). • Novel dry and wet high-speed turning (370 m/min) of austenitic stainless steel 304. • Wear performance and tribological characteristics of five different Al/Ti ratios in AlTiN PVD coating. • Chip cross-section characteristics including micro-hardness and microstructure topography. • Surface integrity of machined workpiece provides evidence of machinability of stainless steel 304. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an investigation on the correlation between surface topography and its mechanical properties measured by a novel multi-function tribological probe microscope (TPM) at the micro/nanometre level. Four surface functions of topography, friction, nano-hardness and Young's modulus are measured over a range of materials, on a point-by-point basis using the TPM. The capability of multi-function mapping of the TPM allows the cross correlation to be carried out, in order to investigate the influence of one function on another, especially the surface topography's effect on its mechanical properties. A general negative correlation between surface topography and hardness/Young's modulus has been found for most homogeneous surfaces, which indicates a modulation effect of surface geometrical features on its mechanical properties. [Copyright &y& Elsevier]

Abstract: Composites forming processes involve mechanical interactions on the ply, tow, and filament level. 0The deformations that occur during forming processes are governed by friction between tows and tooling material on the mesoscopic level and consequently between filaments and a counterface on the microscopic level. A thorough understanding of the frictional properties on the level of individual filaments is important to understand and predict the macroscopic deformations of a fabric during forming. The contact mechanics based friction model in this work confirms an experimentally observed decrease of frictional forces with an increasing roughness of the counterface. The developed model provides a qualitative understanding of the frictional behaviour of filaments on a cylindrical metal counterface. [Copyright &y& Elsevier]

Abstract: In this study, we investigate the running-in behavior of brass (i.e. 95% Copper and 5% Zinc) sliding against 100Cr6 under lubricated conditions. Prior to the tribological tests, the materials are characterized using microindention, nanoindentation, Focused Ion Beam analysis (FIB), X-Ray Photoelectron Spectroscopy (XPS), and White Light Interferometry (WLI) in order to quantify the mechanical properties, grain sizes, chemistry, and topography. The sliding experiments are performed using “real time” tribometry with a pin on plate tribometer with varying contact pressures (1–4MPa) and sliding velocities (10–20mm/s). We found that a significant reduction in friction during running-in occurs only at a small range of pressure (2.4–2.9MPa). We suggest that two main mechanisms are favorable for running-in. The first mechanism is the presence of ZnO in the near-surface region of the wear track, which acts as lubrication and reduces the friction. The second mechanism is the presence of C/CH X within the first 200nm of the pin which acts as a passivation layer and thus keeps the transfer film thin and stable. [Copyright &y& Elsevier]

Friction between articulating surfaces cause temperature rise in the acetabular cup and femoral head. This heating may influence the rate of wear, fatigue, creep and oxidative degradation of bearing materials. The objective of this study is to determine the effect of surface patterning on frictional temperature rise of the articulating surfaces of vitamin E blended ultra-high molecular weight polyethylene acetabular component paired with a cobalt–chromium (CoCrMo) femoral component. For this reason concave dimples were formed on inner surface of the acetabular cup with 0.5mm diameter and 0.5mm depth. Temperature rise between the bearing surfaces was measured under different loading conditions. Frictional measurements of the joints were carried out on a custom made hip joint friction experimental set up. The prostheses were of 28mm diameter. Applied static loads were changed from 200N to 1500N. In the flexion–extension plane, a simple harmonic oscillatory motion between ±24° was applied to the ultra high molecular weight polyethylene acetabular component. The frequency of motion was 1Hz and the tests were run up to 11,000 cycles. Temperature rise in acetabular and femoral component was recorded with embedded thermocouples. Lower temperature rise in patterned sample pairs were measured than unpatterned samples. So it can be said that the surface patterning contributes to reduction of frictional heating of sliding surfaces. [ABSTRACT FROM AUTHOR]

Abstract: In a ball-on-disc wear test, an alumina ceramic body sliding against a silicon nitride ceramic body in water achieved an ultra-low friction coefficient (ULFC) of 0.004. The profilometer and EDX measurements indicated that the ULFC regime in this unmated Al2O3–Si3N4 pair was achieved because of the formation of a flat and smooth interface of nanometric roughness, which favored the hydrodynamic lubrication. The triboreactions formed silicon and aluminum hydroxides which contributed to decrease roughness and shear stress at the contact interface. This behavior enables the development of low energy loss water-based tribological systems using oxide ceramics. [Copyright &y& Elsevier]

Abstract: Brake-induced stick–slip, which determines the propensity of a brake system to generate noise, was investigated by analyzing the friction oscillation at different loads, speeds, and surface topographies. The friction characteristics at the sliding interface were obtained using a pad-on-disk type tribometer and a 1/5-scale brake dynamometer. Results showed that the size and stiffness of the surface contact plateaus significantly affect the propensity of stick–slip and wear. The stick–slip amplitude increased with normal load and decreased with sliding speed. Contact stiffness of the friction material, which was modified by producing shallow slots on the rubbing surface, also strongly affected the stick–slip propensity, whereas Δμ(=μ s−μ k) was not changed by the surface modification. The strong effects from the contact stiffness suggest that the height distribution of the surface plateaus can change the stick–slip propensity of a brake friction material. [Copyright &y& Elsevier]

Abstract: This paper describes a robust methodology for inspection of the changing surface texture of road stones as they undergo simulated traffic polishing. Three-dimensional measurements of surface relief are made at five discrete levels of wear, corresponding to five levels of friction. The advantages of measurement in three dimensions are explored, observations are made about the change in surface appearance and the surface is characterised at each stage using a standard areal roughness parameter. [Copyright &y& Elsevier]

Abstract: Metal forming constitutes a group of industrially important processes to form metallic components to net shape. When forming aluminium and other materials that tend to stick to the tools, problems occur. The transferred work material increases the friction, which increases the forming forces. Additionally, the transferred work material becomes hardened and then scratches the softer work material in subsequent forming. This process, known as galling, compromises the surface finish of the next pieces to be formed. This paper employs a newly developed technique to investigate the initial stages of transfer at high resolution in situ in the SEM. We show that the complex microscale processes involved can be distinguished into three classes: primary transfer, secondary transfer and damage activated transfer. The damage activated transfer constitutes a new fundamental tribological phenomenon, involving the activation and healing of a soft metal in sliding contact with a harder surface. Damage activation leads to transfer onto surfaces such as the polished DLC in this investigation, which would otherwise not see any transfer. These processes are important when forming aluminium, but are expected to be of general tribological significance, in sliding involving non-perfect lubricant films, especially for soft metals with protective surface oxides. [Copyright &y& Elsevier]

Abstract: The tribological mechanisms behind the initiation of material transfer and build-up of transfer layers in aluminium forming have been studied in situ in the SEM where a tip of aluminium is put into contact with a tool steel surface under controlled sliding contact conditions. By combining in situ observations with post-test high resolution FEG-SEM studies of the contacting surfaces we have shown that aluminium is immediately transferred onto the fine polished tool steel. It was also confirmed that the initial transfer occurs on a very fine scale and is localised to the surface irregularities presented by the slightly protruding carbonitrides. In contrast, the less protruding M6C carbides, as well as the martensitic steel matrix exhibit very little initial transfer. Intentionally made scratches (roughly 5μm wide and 2μm deep) across the tool surface immediately result in larger scale transfer, which grows upon further passages of work material causing a high coefficient of friction. The study illuminates the extreme value of combining the in situ technique with high-resolution scanning electron microscopy using low acceleration voltage as a mean to detect the very thin initial transfer layers. With the higher acceleration voltages normally used, the transferred aluminium becomes transparent and can hardly be detected. [Copyright &y& Elsevier]

Abstract: Tip test is a friction measurement technique for forging process based on upsetting and backward extrusion with a specimen of which the diameter is larger than the punch and smaller than container. Due to the smaller diameter of the specimen, a sharp tip was formed during deformation and a unique linear relationship between the tip distance and maximum load measured was found. In the present paper, the effect of flow stress of the material of AL2024-O and 6061-O on sensitivity of the downsized tip test was investigated. Different friction conditions on the punch, counter punch, and container were introduced to determine their effects on calibration of the shear friction factor. A non-dimensional equation to determine the shear friction factor from the non-dimensional tip distance was derived based on tip test results under the various experimental conditions with different surface topographies of the counter punch, different order of deformation speeds, and different types of lubricant such as grease, VG100, VG32, and corn oil using the experimental data available in the literature. The linear relationships among the shear friction factor, measured tip distance, and maximum load were reconfirmed for all the experimental conditions in this study. It was found that the tip test was more sensitive and versatile in determining a representative shear friction factor in cold forging than other testing methods available so far. [Copyright &y& Elsevier]

Abstract: The dry rolling/sliding wear behaviour of Si alloyed carbide free bainitic steel austempered at different temperatures and sliding distances has been evaluated. 60SiCr7 spring steel samples were austempered in a salt bath maintained at 250, 300 and 350°C respectively for 1h. Rolling with 5% sliding wear tests were performed using self mated discs for three different test cycles, namely 6000, 18,000 and 30,000 cycles. The aim was to study the wear performance of the 60SiCr7 steel with a carbide-free microstructure containing different amounts of retained austenite. An in-depth microstructural characterization has been carried out before and after the wear tests in order to link the wear behaviour to the microstructure of each sample. The wear resistance has been expressed by means of the specific wear calculated from the mass loss after the tests. The worn surfaces were analysed by scanning electron microscopy and X-ray diffraction. Microhardness profiles were also obtained in order to analyse strain-hardening effects beneath the contact surfaces. The results indicate that the material with highest hardness—the one austempered at 250°C—exhibited the lowest wear rate in every case. It was also observed that the hardness increment and thickness of the hardened layer increases with increasing the austempering temperature and number of test cycles. Finally, the results appear to indicate that the initial roughness of the samples has no major effect in the wear rate of the samples above 2500 cycles. The higher wear performance of the sample austempered at 250°C has been attributed to its superior mechanical properties provided by its finer microstructure. It has been evidenced that all samples undergo the TRIP phenomenon since, after wear; no retained austenite could be detected by XRD. [Copyright &y& Elsevier]

Abstract: The tribological behavior of PMMA-grafted-MWCNT nanocomposite coatings that were prepared by applying the direct casting method to aluminum surfaces was investigated. Amino-functionalized MWCNTs were first covalently functionalized with PMMA to introduce the interaction force. The PMMA-g-MWCNT exhibited a higher dispersive capacity in the 6FDA-based polyimide matrix than pure MWCNT composite due to strong interfacial interactions in the nanocomposite. The maximum possible amount of PMMA-g-MWCNT was 5wt.% because excessive MWCNT in the nanocomposites makes them fragile and brittle. Sliding tests were performed in ball-on-disk configurations under dry conditions. The normal contact force was 9.7N against a 12.7-mm-diameter steel ball resulting in a Hertzian circular contact stress of 0.65GPa. The sliding speed was 0.02m/s and sliding tests were continued until the coefficient of friction stabilized. The nanocomposite coating reinforced with 3wt.% MWCNT was the most stable and had the lowest coefficient of friction in this study. This was due to the formation of a thin, uniform friction film on the wear track during sliding, and we concluded that the friction film plays an important role in facilitating the stable frictional behavior of the nanocomposite coatings. [Copyright &y& Elsevier]

Abstract: Main focus was on the correlation of the coefficient of friction (COF) with the geometrical parameters of the surface during early stage of the running-in period. TiN, TiAlN and AlTiN coatings were prepared by using rotating cathode arc evaporation unit. The roughness of the WC-Co substrate R a ranged from 0.05 to 0.2μm and the coating thickness was 2.3±0.2μm. Part of the samples were subjected to the surface modification by the drag grinding finishing. Geometrical parameters of the surface were evaluated by means of atomic force microscopy (AFM) and fretting tests were conducted with 10mm diameter alumina ball under 2N load, 100μm slip distance and the frequency of 10Hz. It was found that the distribution of macroparticles strongly influences the geometrical parameters of the surface and the number of macroparticles increases with increasing Al content in the coating. In the case of the positive skewness S sq of the coating surface, the kurtosis S ku provides a statistically significant effect on the COF, namely the COF depends inversely on the kurtosis S ku. The kurtosis value for TiN was the highest among as-deposited coatings, subsequently, the slowest increase rate in the COF was observed for the TiN coating. [Copyright &y& Elsevier]

Abstract: This paper describes future trends in surface metrology. Measurement techniques are briefly mentioned. A special attention was paid to tactile and optical methods. Selected problems of surface topography characterization are described. The effects of sampling and filtering on surface topography representation are analysed. Structured surfaces are becoming both technologically and economically critical. Therefore their description is a problem of a great practical importance. Multi-process textures are very important from functional point of view. Various methods of their description are compared. Surface texturing as a means for enhancing tribological properties of frictional pairs started to be extremely popular from for about last 10 years. The effects of surface texturing on improving tribological properties of sliding assemblies are analysed. The other influences of surface topography are mentioned in this paper. [Copyright &y& Elsevier]

Abstract: The objective of this work was to evaluate a set of standardized metrics proposed for characterizing a surface that has been scratched from a two-body abrasion test. This is achieved by defining a new abrasion region termed ‘Zone of Interaction’ (ZOI). The ZOI describes the full surface profile of all peaks and valleys, rather than just measuring a scratch width as currently defined by the ASTM G 171 Standard. The ZOI has been found to be at least twice the size of a standard width measurement, in some cases considerably greater, indicating that at least half of the disturbed surface area would be neglected without this insight. The ZOI is used to calculate a more robust data set of volume measurements that can be used to computationally reconstruct a resultant profile for detailed analysis. Documenting additional changes to various surface roughness parameters also allows key material attributes of importance to ultimate design applications to be quantified, such as depth of penetration and final abraded surface roughness. Data are presented to show that different combinations of scratch tips and abraded materials can actually yield the same scratch width, but result in different volume displacement or removal measurements; therefore, the ZOI method offers a more robust assessment than the current ASTM analysis method. Furthermore, by investigating the use of custom scratch tips for our specific needs, the usefulness of having an abrasion metric that can measure the displaced volume in this standardized manner, and not just by scratch width alone, is reinforced. This benefit is made apparent when a tip creates an intricate contour having multiple peaks and valleys within a single scratch. This work lays the foundation for updating scratch measurement standards to improve modeling and characterization of three-body abrasion test results. [Copyright &y& Elsevier]

Abstract: In fully plastic contact conditions where nominal contact pressure is relatively high, like in metal forming processes, individual summits join together to form contact patches. The geometry of these contact patches depends on the contact pressure, which results in load-dependent frictional behaviour. This paper describes a model that takes into account this effect, and calculates contact and coefficient of friction in such conditions based on a good description of the shapes of contact patches instead of individual summits. The results show that as the nominal contact pressure increases, the effect of surface topography becomes less significant while the effect of the interfacial shear strength remains important. [ABSTRACT FROM AUTHOR]

Abstract: Three-dimensional surface roughness of mating parts of engineering assemblies has a significant influence on their functional behaviour. Studies on load bearing capacity in elastohydrodynamic contacts, gap flow analysis in precision hydraulic assemblies using modeled 3D fluid continuum micro gap geometry, etc., have made it possible to quantify the effect of certain 3D surface roughness parameters on frictional behaviour of the assemblies. This set forth the need for artificially generated three-dimensional engineering surfaces having prescribed roughness values for a better understanding and prediction of tribological problems. In this paper, an algorithm is developed for the numerical generation of three-dimensional anisotropic surfaces with prescribed 3D surface roughness parameters. The procedures employ either the Non-linear Conjugate Gradient Method (NCGM) or 2-D Digital Filter method. The results show that both the methods can adequately produce surfaces at small correlation distances, while at higher correlation distances the NCGM yields better results. Comparison between model-simulated output and measurement results of three-dimensional surface roughness of engineering surface show a good match, supporting the validity of the model. [Copyright &y& Elsevier]

Abstract: The use of hardened high strength steel is found in applications where high wear resistance is required. The wear properties of high strength Boron steel are well known in applications with abrasive wear from stones, ore and other hard material. A unique concept of wear protection of rails is newly presented, a wear resistant cap made of hardened high strength Boron steel. Reducing the wear of rails and wheels and controlling the frictional behavior in the wheel/rail contact are two key issues for railway owners in order to reduce the increasing costs related to higher axle loads, higher speeds, more frequent traffic, etc. Therefore, the aim of this work has been to investigate and compare the tribological properties of Boron steel and UIC 1100 rail steel in contact with Blue Light wheel steel (AAR Class C (69-JDG-8)) under dry and water lubricated conditions in a two-disc tribometer. Advanced analytical instruments including 3D optical surface profiler, micro-hardness indenter, light microscope and SEM/EDS were used to analyze the results. Results from the experiments show that the friction coefficient in tests with Boron steel is more stable both in dry and water lubricated conditions than tests including UIC 1100 rail steel used in todays application. Surface damages seen from water lubricated tests on UIC 1100 rail steel are not seen on the surface of the Boron steel discs. In all tests, the wear decreased when water was added in the contact and friction was slightly decreased. [Copyright &y& Elsevier]

Abstract: Friction and wear are constant problems encountered in camshaft development. Among a wide amount of factors, surface topography is an important feature to improve in this complex system. Four parts of different shapes can be differentiated on an injection cam profile. Therefore the machining of such component is quite particular. Considering the complex shape of an injection cam, this paper is divided in two studies: 1. Variations of surface topography around freshly produced cams in order to emphasize the particular profile of cams which leads to a complex machining. 2. Evolution of surfaces during run-in in order to discover the effects of wear process on a group of roughness parameters. The results show a variation of density of peaks along the freshly produced cam surface and highlight also the importance of roller type on the evolution of topography. Further results show that summits of the surface are rounded off during run-in. Concerning the future, results of the study are encouraging to investigate more deeply the tribological behaviour (contact mechanic, oil film thickness) of the surfaces. [Copyright &y& Elsevier]

Fibre-reinforced thermosets and thermoplastics containing PTFE are commonly used in marine and hydropower applications due to their self-lubricating ability in both dry and water lubricated contacts. The constant development of such composite bearings over the last decades make them even more attractive in these fields of application, due to their low maintenance cost and long service life. Their tribological performance is usually estimated by extrapolation of short, accelerated tests, which may induce risks of inaccurate or even incorrect interpretation. In this work, reciprocating dry sliding behaviour of these materials against stainless steel is investigated under typical hydropower conditions. The aim is to study the wear and friction behaviour and the development of transfer layers during long sliding tests, corresponding to years of operation. In order to mimic operational shutdowns, the tests are stopped every 20 h and the counter surfaces are examined using 3D optical interferometry and SEM to study development of surface topography and transfer layers. Test results show that the wear rates of both materials decrease significantly with time. For the thermoplastic, COF decreases with time due to accelerated material transfer observed after 80 h. In contrast, much less material transfer is observed for the fibre-reinforced thermoset, which shows its highest transfer amount after 20 h. Surface analysis indicates severe abrasive wear of the counter surface caused by the wear debris from steel and reinforcements in the material and is accompanied by simultaneous COF increase for the thermoset. Cross-sectional analysis reveals thinner transfer layers and higher wear of the steel in the middle of the wear tracks, where the sliding speed and number of load cycles are highest. These findings contribute to a more accurate interpretation of results obtained than that of short time tests regarding the tribological performance of these materials. As a direct conclusion, the selection of counter surface material can be crucial. • Friction and wear results from long duration (160 h) tribological tests involving self-lubricating polymer composites. • The friction and wear mechanisms for the tested polymer bearing materials are transient processes over long durations. • Transfer layer formation involves wear debris accumulation in counter surface valleys and consolidation in to layers. • Abrasive wear is the dominant wear mechanism of the bearing materials as well as the stainless steel counter surface. [ABSTRACT FROM AUTHOR]

Abstract: Brake noise is an example of noise caused by vibrations induced by friction forces. During brake operation, the friction between the pad and the disc can induce a dynamic instability in the system. Brake squeal can occur in the frequency range between 1 and 20kHz, and more. Approaches to this problem are usually confined to the analysis of the system dynamics or to the tribological studies (analysis of the contact surfaces). In a previous dynamic analysis, the squeal events treated in this paper were identified to be dynamic instabilities of the brake system. This paper presents an experimental tribological analysis performed in parallel to the dynamic one and aimed to highlight the role of the contact problems in the squeal phenomena. The topography of the contact surface after braking phase with and without squeal is presented. The features of the third body and the surface topography were found to be completely different in the two cases. After squeal events the topography of the surface and the internal structure of the pad material close to the contact show several cracks and material exfoliations; on the contrary, after braking without squeal the contact surfaces were found to be smooth and compact. The analysis of the system vibrations during squeal and the numerical analysis of the contact stresses allow to relate these features to the fatigue excitation due to the local oscillation of the contact stresses at the contact surface at the squeal frequencies. The surface topography after squeal, by revealing several fatigue evidences, points out a further problem, i.e., the possible increasing of wear rates that are consequent to the squeal vibrations. [Copyright &y& Elsevier]

Abstract: The continued globalisation of the automotive industry, leading to increasing demands for competitiveness and escalating legislative requirements, is the main driving force of research activities of steel sheet surfaces. Recent studies on the stamping process have been carried out among others within AUTOsurf, a project funded by the European Community, and by Wihlborg and Crafoord. The purpose of this study was to evaluate the viability of the proposed parameters for friction prediction. Seventeen different surface topographies were investigated. The sheet materials were either, hot-dip galvanised, electrogalvanised or galvannealed, and electron beam or electric discharged textures. The frictional response was measured in a bending under tension (BUT) test under mixed lubricated conditions. This BUT test simulates the conditions of the die radius in a stamping tool. The laboratory test differs from the experimental work performed in AUTOsurf which simulated the conditions of the holding-down plate. In spite of the differences in test equipments in AUTOsurf, e.g. the rotational friction tester (RTF), on a comparison the correlation of frictional response was significant. But neither of the proposed parameters could predict the frictional response with sufficient accuracy in this study. In addition, the friction model in AUTOsurf describes peak lubrication as a dragging phenomenon on sliding surfaces. The movement eased friction in inverse proportion to the average peak area. However, the trend in this study showed the opposite, movement eased friction proportionally to the average peak area. The result indicates a switch of dominant friction mechanism when the sliding velocity is increased, i.e. from a dragging phenomenon at low velocities to micro-hydrodynamic wedge effects at high velocities. [Copyright &y& Elsevier]

According to the analysis made in this paper, squeal triggering, establishment or vanishing can be described in terms of a new index dependent on two topographic parameters; the mean radius of asperities and the standard deviation of the height distribution of asperities of the sliding surfaces. Experimental investigation of squeal noise generated from the friction sliding of a brake pad on a rotating plexiglass disc revealed that the pad surface at the beginning of squeal became glazed and that this pad abraded the disk. This means that plastic contact between asperities at the beginning of sliding caused the smoothening of the softer surface and roughening of the harder surface. Topographic analysis of the squealing surfaces has shown that, due to running in of a plastic surface (e.g. rough pad) with another smooth elastic surface (e.g. brand new disk), the mean radius of asperities of the sliding surfaces were largely increased which leads to a clear drop in the tangential stiffness of these asperities. It is believed that loading and unloading fatigue cycle of elastic asperities with weak stiffness in addition to their impact initiate cracks and lead to tearing away and separation of some asperities which causes the emission of squeal noise. Continuation of sliding process may reverse this trend due to wear debris, i.e. changing elastic contact condition to plastic one and consequently, squeal vanishes. [Copyright &y& Elsevier]

The friction behaviour of gear teeth in the context of tribology can have a strong effect on housing vibration, noise and efficiency. One of the parameters that greatly influences the friction under certain running conditions is surface roughness. In this work, rough friction was studied in lubricated sliding of roller surfaces, which were manufactured to simulate the real gear surfaces. By examining 3D surface topography of two mating bodies, both surface roughness and its effect on friction behaviour can be studied. In a previous study, a rough-friction test rig has been designed, constructed and initially verified. The types of surfaces involved in this study are ground, shot-peened, phosphated and electrochemically deburred. These rollers were subjected to the same friction testing procedures. Roller surfaces were then examined, and correlation between the topography and the frictional behaviour was analysed. Friction behaviour was interpreted in terms of Stribeck curves (friction coefficient as the function of Hersey parameter (ην/p)). The results showed that electrochemically deburred and certain phosphated surfaces provide lower friction coefficient values which are competitive to fine-ground surfaces in lubricated rolling/sliding contact. [Copyright &y& Elsevier]

Under reciprocating sliding there are at least four states available to distinguish whether squeal occurs or not and how squeal evolves if it is present: (1) no squeal is emitted in the whole reciprocating sliding as many as 3800 cycles or more; (2) squeal is not emitted when the friction coefficient is smaller in the beginning stage of reciprocating sliding; (3) squeal is emitted when the friction coefficient arrives at a larger (critical) value; (4) sometimes squeal disappears after more cycles. These four states are found to be to some extent in correlation with the topography characteristics of the wear scars. Acoustic and dynamic measuring techniques were applied to ascertain the area where squeal was present on a wear scar. By means of SEM observations of wear scars, a comparative investigation into the characteristics of scar surfaces was carried out under both silent and squealing conditions. The result shows that for flat–flat contact the area associated with squeal on a wear scar is characterized by uneven serious pit-like detachment of the specimen material. For flat–ball contact it is found that the area related to squeal is characterized by adhesively joined asperities. Moreover, the effect of a negative friction–velocity slope on squeal formation is examined. The result seems to demonstrate that not all negative friction–velocity slopes can cause squeal. A possible alternative mechanism of squeal formation is proposed and dependence of squeal generation on the friction coefficient are also discussed based on the detachment of the material or formation of adhesively joined asperities. [Copyright &y& Elsevier]

In the machining process, both the tool wear and the surface integrity of the machined workpiece are significantly influenced by the friction. For this reason, the reliable prediction of friction behavior is of great importance for simulation-based tool and process design. It is known from previous investigations that the relative speed is a significant factor influencing the friction behavior between metallic materials and cemented carbide. The exact relationship, particularly in relation to the topography of the tool surfaces, is mainly unexplored. In the context of this publication, results from a friction characterization under machining-similar conditions for AISI 1045 and differently prepared cemented carbide surfaces are presented. The focus of the paper is to develop models for the friction coefficient, based on these results, which take the relative speed and the surface topography into account. Within a Finite-Element chip formation simulation, these models are compared to conventional models that assume the friction coefficient to be constant. For the validation of the simulations, results are used, which were determined from orthogonal cutting tests with differently prepared rake faces. Within the scope of these cutting tests, knowledge was gained about the influence of the rake face topography and the cutting parameters on the friction in the secondary shear zone. [Display omitted] • Analysis of the friction behavior in the cutting zone depending on the rake face topography and the cutting values. • Characterization of the coefficient of friction for different relative speeds and cemented carbide surfaces.. • Improvement of the passive force prediction in a finite-element chip formation simulation. [ABSTRACT FROM AUTHOR]

Wear and friction characteristics are simulated on metal forming tools with tailored surfaces generated by micro-milling. Friction homogenisation is applied to study surface cut-outs on the meso-scale where the structures are resolved by means of finite element methods and where asperities are represented by a combined friction law appropriate for metal forming. Dissipation based and pressure based Archard wear relations are implemented in a postprocessor, and wear distributions as well as effective friction properties are investigated. Sinusoidal surface structures are able to provide anisotropic structural resistance throughout the progress of wear. A bionic surface structure shows quasi-isotropic structural resistance where sliding directions across the edge directions are benefitial with regard to the wear progress. Experimental measurements from a wear experiment give hints which support the dissipation based Archard relation while more experimental evidence is necessary. • Simulation of friction and wear of tailored surfaces produced by micro-milling operations. • Application of computational friction homogenisation to microstructured surfaces on the meso-scale. • Archard wear postprocessor with comparison of dissipation based and pressure based law considering wear distributions. • Qualitative experimental comparison with wear experiment under metal forming conditions. [ABSTRACT FROM AUTHOR]

In this study, AISI52100 steel surfaces were coated with bilayer Cr–N/TiB 2 thin films in total thickness of 65 and 120 nm by magnetron sputtering. The CrN/TiB 2 thin-films deposited under 10 and 20 sccm N 2 flow whereas no N 2 flow was used for a Cr/TiB 2 thin film. The friction and wear behaviors of these bilayer thin-films consisting of metallic (Cr)/ceramic (TiB 2) and ceramic(CrN)/ceramic(TiB 2) layers were investigated via the tribometer tests under unlubricated and lubricated conditions. Bilayer thin-films were characterized by nanoindentation, microscopic and spectroscopic analysis. Results showed that N 2 flow has significant effects on mechanical characteristics of bilayer coatings. Furthermore, the Cr/TiB 2 thin film with a high porosity showed good tribological performance under 2 GPa contact pressure whereas the CrN/TiB 2 thin film deposited at high N 2 flow with high thickness and hardness showed the best tribological behavior. • Magnetron sputtered bilayer Cr–N/TiB2 thin-films deposited on the AISI52100 steel surface. • The nitriding process had a significant effect on CrN layer thickness and TiB2 microstructure. • The pores filled with lubricant enhanced the lifetime of the bilayer Cr–N/TiB2 coatings. • The metal/ceramic layer with a functional ceramic layer showed different antiwear behaviors. [ABSTRACT FROM AUTHOR]