Search

The aim of this study was to identify the areal surface parameters that correlated with lowering of sliding friction. Different ground surfaces were created on stainless steel and the lubricated sliding friction generated at the contact interface with a flat-faced aluminum pin was studied. The frictional force encountered is an order of magnitude lower for a P1200-finished surface than the other ground surfaces. Using 3D surface profilometry, a unique surface parameter ratio "Spk/Sk" was found to predict the frictional performance of these surfaces. When this surface parameter ratio was less than 1, average sliding friction was close to 0.1. When this ratio was greater than 1, the coefficient was an order of magnitude lower. Using energy dispersive spectrometry, such surfaces after wear showed the presence of a uniform dispersed layer of iron oxide on the surface of the pin. This was absent on the surfaces having high friction, indicating the role of the steel counter surface in building this beneficial transfer layer. Scanning electron microscopy provided topography images to visualize the surface wear. The motivation for the authors was to use a commercially scaled process like precision grinding for the surface modifications on stainless steel. [ABSTRACT FROM AUTHOR]

Aluminium alloy sheets cause many problems in sheet metal forming processes owing to their tendency to gall the surface of the tool. The paper presents a method for the determination of the kinematic friction coefficient of friction pairs. The determination of coefficient of friction (COF) in sheet metal forming requires specialised devices that 'simulate' friction conditions in specific areas of the formed sheet. In this article, the friction behaviour of aluminium alloy sheets was determined using the strip drawing test. The 1-mm-thick 6082 aluminium alloy sheets in T6 temper were used as test material. Different values for nominal pressures (4.38, 6.53, 8.13, 9.47, 10.63, and 11.69 MPa) and different sliding speeds (10 and 20 mm/min.) were considered. The change of friction conditions was also realised with several typical oils (hydraulic oil LHL 32, machine oil LAN 46 and engine oil SAE 5W-40 C3) commonly used in sheet metal forming operations. Friction tests were conducted at room temperature (24 °C). The main tribological mechanisms accompanying friction (adhesion, flattening, ploughing) were identified using a scanning electron microscope (SEM). The influence of the parameters of the friction process on the value of the COF was determined using artificial neural networks. The lowest value of the COF was recorded when lubricating the sheet metal surface with SAE 5W40 C3 engine oil, which is characterised as the most viscous of all tested lubricants. In dry friction conditions, a decreasing trend of the COF with increasing contact pressure was observed. In the whole range of applied contact pressures (4.38–11.69 MPa), the value of the COF during lubrication with SAE 5W40 C3 engine oil was between 0.14 and 0.17 for a sliding speed of 10 mm/min and between 0.13 and 0.16 for a sliding speed of 20 mm/min. The value of the COF during dry friction was between 0.23 and 0.28 for a sliding speed of 10 mm/min and between 0.22 and 0.26 for a sliding speed of 20 mm/min. SEM micrographs revealed that the main friction mechanism of 6082-T6 aluminium alloys sheet in contact with cold-work tool steel flattens surface asperities. The sensitivity analysis of the input parameters on the value of COF revealed that oil viscosity has the greatest impact on the value of the COF, followed by contact pressure and sliding speed. [ABSTRACT FROM AUTHOR]

For the wear problem of the real rough surface during sliding friction, based on fractal theory and Hertz contact theory, a 3-D fractal rough surface with random characteristics is constructed, and the relationship between the wear deformation depth of the rough peak and its real contact area during the wear process is derived. Furthermore, considering the peak wear and pit scratch phenomena of rough surfaces in different contact states, the time-varying wear calculation model of the worn surface and the compensation wear calculation model of the unworn surface are established, respectively, and the relationship between the instantaneous wear amount and the dynamic change in the rough surface topography is comprehensively characterized. Combined with image digitization technology, the 3-D rough surface is converted into a 2-D discrete plane with 3-D information. According to the dynamic real-time update of the graph data, the iterative calculation of the wear cycle is completed, the time-varying wear calculation method for fractal rough surfaces of friction pairs is proposed, and the dynamic change in the wear amount and surface topography of the rough surface is simulated. The simulation results are experimentally verified and the influence of friction parameters on the surface topography is analyzed. The results show that after the wear simulation, the profile height of the rough surface is reduced, and the average wear depth is 0.02 mm. Increases in rotational speeds and external loads can exacerbate surface wear, surface topography tends to be flattened, and surface carrying capacity increases. This provides theoretical guidance for the development and manufacture of friction pairs. [ABSTRACT FROM AUTHOR]

The present paper introduces a tire/road friction prediction tool based on modelling the tire/road contact as dynamic, viscoelastic, rough, and lubricated. The tool takes into account a considerable part of influent parameters related to tire, road, contaminant, and contact operating conditions: For the tire, the tool takes into account its geometry and rubber material behaviour. For the road, the texture is taken into account via the surface topography. At the contact interface, dry or wet conditions are taken into account through the lubricant depth, viscosity, and density. The operating conditions are taken into account through the normal load, speed, and slip ratio of the tire. The real novelty of this tool lies in its ability to reproduce the complete curve of the tire/road friction coefficient as a function of the slip rate. The validation of the tool is initially done through parametric studies by analyzing the trends of the results, then by performing braking tests on a passenger car at various speeds on different wet roads with different textures. The tool correctly ranks the peak friction and the sliding friction on these various road surfaces. [ABSTRACT FROM AUTHOR]

The paper presents experimental analysis of relation between friction coefficient and contact pressure of MoS 2 film deposited on Ti 6 Al 4 V substrate in contact with sapphire ball during reciprocating sliding motion. It is shown that the value of friction coefficient decreases with increasing contact pressure. A microscale modeling approach is next developed to mimic the experimental observations. Representative volume element is defined based on the actual topography of outer surface of MoS 2 film. Assuming thermo-elastic material properties, the calculations on the asperity level are performed in two steps. Firstly, the mechanical contact between two surfaces is calculated. As a result, the relation between the global load and micro-stress distribution is obtained. Secondly, for a given stress load, thermal analysis is performed providing temperature fluctuation within simplified conical asperity. By assuming relation between friction coefficient and temperature on the microscale, it is possible to obtain macroscopic friction coefficient as a function of contact pressure. In the end, model results are compared with experimental data. The novel aspects of presented approach lie in the selection of three main factors on a micro-level defining macroscopic friction. They are actual surface topography, microscopic temperature and microscopic friction-temperature relation. [ABSTRACT FROM AUTHOR]

Purpose: During the production of cardboard food cans, the packaging bottom and the cylindrical wall are joined in the seaming process. In order to achieve a high-quality, crack-free surface of the cardboard seam, low friction between the seaming chuck and the cardboard must be ensured. The goal of this study was to minimise the friction between the seaming chuck and the cardboard can surface. Design/methodology/approach: Tribological properties of the seaming chuck were optimised by adjusting its material properties, surface topography and surface energy followed by measurements of the resulting friction response in sliding contact with a representative paper sample. Findings: A strong correlation between the surface free energies of the tribological samples and their measured coefficients of friction was observed, indicating that in tribological tests, adhesion was the dominating friction mechanism. Furthermore, the fact that the smoother samples yielded higher friction values than the rougher ones is most likely also correlated with the higher adhesion of the smoother samples originating from their larger contact area. Research limitations/implications: The existing results indicate that for tribological optimisation of paper and cardboard contacts primarily the adhesive friction component should be considered – by either reducing the surface free energy of the counter-body or optimising its surface topography. Practical implications: By applying the selected solution concept, a friction reduction of more than 50% as compared to the benchmark was achieved. Originality/value: The present study provides a guideline for tribological optimisation of paper and cardboard contacts. Peer review: The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-02-2020-0064/ [ABSTRACT FROM AUTHOR]

Purpose: Additive manufacturing (AM) is a promising alternative to the conventional production methods (i.e., machining), providing the developers with great geometrical and topological freedom during the design and immediate prototyping customizability. However, frictional characteristics of the AM surfaces are yet to be fully explored, making the control and manufacturing of precise assembly manufactured mechanisms (i.e., robots) challenging. The purpose of this paper is to understand the tribological behavior of fused deposition modeling (FDM) manufactured surfaces and test the accuracy of existing mathematical models such as Amontons–Coulomb, Tabor–Bowden, and variations of Hertz Contact model against empirical data. Design/methodology/approach: Conventional frictional models Amontons–Coulomb and Tabor–Bowden are developed for the parabolic surface topography of FDM surfaces using variations of Hertz contact models. Experiments are implemented to measure the friction between two flat FDM surfaces at different speeds, normal forces, and surface configuration, including the relative direction of printing stripes and sliding direction and the surface area. The global maximum measured force is considered as static friction, and the average of the local maxima during the stick-slip phase is assumed as kinematic friction. Spectral analysis has been used to inspect the relationship between the chaos of vertical wobbling versus sliding speed. Findings: It is observed that the friction between the two FDM planes is linearly proportional to the normal force. However, in contrast to the viscous frictional model (i.e., Stribeck), the friction reduces asymptotically at higher speeds, which can be attributed to the transition from harmonic to normal chaotic vibrations. The phase shift is investigated through spectral analysis; dominant frequencies are presented at different pulling speeds, normal forces, and surface areas. It is hypothesized that higher speeds lead to smaller dwell-time, reducing creep and adhesive friction consequently. Furthermore, no monotonic relationship between surface area and friction force is observed. Research limitations/implications: Due to the high number of experimental parameters, the research is implemented for a limited range of surface areas, which should be expanded in future research. Furthermore, the pulling position of the jaws is different from the sliding distance of the surfaces due to the compliance involved in the contact and the pulling cable. This issue could be alleviated using a non-contact position measurement method such as LASER or image processing. Another major issue of the experiments is the planar orientation of the pulling object with respect to the sliding direction and occasional swinging in the tangential plane. Practical implications: Given the results of this study, one can predict the frictional behavior of FDM manufactured surfaces at different normal forces, sliding speeds, and surface configurations. This will help to have better predictive and model-based control algorithms for fully AM manufactured mechanisms and optimization of the assembly manufactured systems. By adjusting the clearances and printing direction, one can reduce or moderate the frictional forces to minimize stick-slip or optimize energy efficiency in FDM manufactured joints. Knowing the harmonic to chaotic phase shift at higher sliding speeds, one can apply certain speed control algorithms to sustain optimal mechanical performance. Originality/value: In this study, theoretical tribological models are developed for the specific topography of the FDM manufactured surfaces. Experiments have been implemented for an extensive range of boundary conditions, including normal force, sliding speed, and contact configuration. Frictional behavior between flat square FDM surfaces is studied and measured using a Zwick tensile machine. Spectral analysis, auto-correlation, and other methods have been developed to study the oscillations during the stick-slip phase, finding local maxima (kinematic friction) and dominant periodicity of the friction force versus sliding distance. Precise static and kinematic frictional coefficients are provided for different contact configurations and sliding directions. [ABSTRACT FROM AUTHOR]

Tribological tests are adopted to estimate the kinetic friction coefficient (COF) of the material of interest according to the American Society for Testing and Materials (ASTM) standard. Typically, for the measurement process, several replicas, as well as a postprocessing data treatment, are necessary to take into account the observed casual variability of the measurand. This article describes a statistical approach aiming to highlight the running-in phase and the most significant time intervals during the steady-state for each test replica. A two-steps procedure based on the adoption of the bootstrap method allows the automatic detection of the running-in time interval and the outlier filtering of the steady-state. Experimental activity has been carried out by performing multiple tests through the ball-on-flat tribometer in order to verify the improvement allowed by the authors’ proposal about the dry friction COF measurement in terms of both accuracy and repeatability. [ABSTRACT FROM AUTHOR]

It has been found that empirical analysis provides a possibility to select the optimum concentration of the repair and restoring composition (RVS), addition of which to the transmission oil provides an increase in wear resistance of 42Cr4 steel under rolling and slip conditions. The structural adaptability of finely dispersed components of the friction geo-modifier RVS, boundary lubricating films and surface layers of metal under friction and under dynamic loading conditions contribute to reducing the wear of contact surfaces. It has been established that the addition of 1.5 % RVS to the lubricant provides a 17 % reduction in the wear of rolling friction pairs, provides hardening of the surface layers of the metal by 1.36 times, compared with the initial surface. The micro-hardness of the surface layers of the metal and its gradient depth distribution, the microgeometry of the surface layer formed during friction, and the formation of boundary lubricating layers on the friction-activated contact surfaces are key indicators that increase the wear resistance in friction pairs. [ABSTRACT FROM AUTHOR]

Wear is a common phenomenon which most mechanical components suffer from. Available studies are mainly focused on the wear performance after contact pairs operate for a period of time. The studies on evolution of surface topography and friction coefficient resulting from the wear process under mixed lubrication conditions are still limited. Further study of this issue may have a significant influence on wear reduction and lubricating performance improvement. In the present study, surface topography and friction coefficient evolution during sliding wear in a mixed lubricated rolling-sliding contact are investigated by the method based on the numerical approach developed by Zhu.et al. [1]. The numerical results are compared with experiment results to demonstrate the efficiency and accuracy of the present wear model. The evolutions of wear track, root mean square (RMS) roughness, and friction coefficient are observed systematically during wear process. The influences of load, slide-to-roll ratio and surface orientation on these characteristics are studied as well. The simulation results are in good agreement with experimental observations. The wear track is quite different from the one under dry contact, due to the effect of oil and the contact deformation. The friction coefficient drops rapidly at first and then it keeps stable with small fluctuations with respect to wear evolution. It indicates that load and slide-to-roll ratio have a remarkable impact on wear rate, though the effect of surface topography is limited. Further, the results show that the RMS roughness deceases slightly at first and then it increases gradually during the wear process. • A point wear model of real, engineering rough surfaces has been introduced in this paper. • In the early running-in process, the wear tracks on both sides are deeper than in the middle, which is different from that under dry contact. • In the running-in process, the friction coefficient drops rapidly first and then keeps stable with small fluctuations. • Increasing load and slide-to-roll ratio can increase the wear rate. [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]

A series of experimental tests were carried out using turned and ground mild steel surfaces rubbing against smooth and rough stainless steel 304 (SS304) conterfaces under different operating conditions on a pin-on-disc test rig. During testing, the friction coefficient and wear rate were recorded. The topography of worn surfaces was also observed with optical microscope. The average surface roughness of turned and ground mild steel were measured after friction test. The result displays that normal load, sliding speed and the surface conditions of sliding pairs have a distinct effect on the friction and wear behaviour of turned and ground mild steel rubbing against smooth and rough SS304 conterfaces. During this study, the effects of duration of rubbing on friction coefficient are also observed. Friction coefficient and wear rate of four types of disc-pin combinations such as ground-smooth, turned-smooth, ground-rough and turned-rough are observed under different normal loads and sliding velocities. Experimental results reveal that friction coefficient of mild steel for all types of disc-pin combinations decreases with the increase in normal load and sliding velocity. [ABSTRACT FROM AUTHOR]

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]

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]

The aim of the present work is to determine the influence of boron surface layers on the friction parameters in sliding pairs under lubricated friction conditions. Tribological evaluation included pack boronizing, laser boronizing, and TiB2 coating deposited on 46Cr2 steel (AISI 5045). The pack-borided surface layer had a characteristic denticular structure with a hardness of 2,000 HV and thickness of up to 40 μm. The laser-borided surface layer had a maximum hardness of 1,800 HV and the melting depth of 25 μm. The TiB2 coating was characterized by a hardness of about 3,000 HV and thickness reaching about 3 μm. Modified surface layers of ring specimens were matched under test conditions with a counterpart made from SAE-783 bearing alloy. Tested sliding pairs were lubricated with LOTOS 5W/40 synthetic engine oil. The modification technologies applied to the surface layer of steel allowed obtaining construction materials with predetermined tribological characteristics required for the elements of sliding pairs in lubricated contacts. The results showed differences in the wear of the bearing alloy due to the interaction between the cooperating surface layers and the physiochemical changes of their surfaces induced by external forces. The pack-borided surface layer reduced the friction coefficient, the moment during the start-up of the frictional pair, and the wear of the bearing material. Analysis of the surface layer indicated that the content of aluminum on pack-borided ring specimens reached 21%, and on the TiB2 coating it exceeded 8%. [ABSTRACT FROM AUTHOR]

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: A non-dimensional parameter descriptive of the plowing nature of surfaces is proposed for the case of sliding between a soft and a relatively hard metallic pair. From a set of potential parameters which can be descriptive of the phenomenon, dimensionless groups are formulated and the influence of each one of them is analyzed. A non-dimensional parameter involving the root-mean square deviation (Rq ) and the centroidal frequency (Fmean ) deducted from the power-spectrum is found to have a high degree of correlation (as high as 0.93) with the coefficient of friction obtained in sliding experiments under lubricated condition. [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]

New possibilities have emerged in recent years, with the development of high-precision fabrication techniques, to exploit microscale surface patterning to modify tribological properties of polymeric materials. However, the effect of surface topography, together with material mechanical parameters, needs to be fully understood to allow the design of surfaces with the desired characteristics. In this paper, we experimentally assess the effect of various types of micropatterned Polydimethylsiloxane surfaces, including anisotropic ones, on macroscopic substrate friction properties. We find that it is possible, through surface patterning, to modify both static and dynamic friction coefficients of the surfaces, demonstrating the possibility of achieving tunability. Additionally, we compare experimental observations with the numerical predictions of a 2D Spring-Block model, deriving the material parameters from tests on the corresponding flat surfaces. We find a good quantitative agreement between calculated and measured trends for various micropattern geometries, demonstrating that the proposed numerical approach can reliably describe patterned surfaces when appropriate material parameters are used. The presented results can further contribute to the description and understanding of the frictional effects of surface patterning, with the aim of achieving surfaces with extreme tunability of tribological properties. Unlabelled Image • The effect of patterning on static/dynamic friction of soft polymer surfaces is investigated experimentally and numerically • Micro-patterns modify macroscopic static friction coefficients between -57 and +20% and dynamic ones between -35 and 30% • Calculations using an in-house developed 2D Spring-Block model are in good agreement with experimental results [ABSTRACT FROM AUTHOR]