Friction Maps Research Articles

Conversion of images of 3D friction maps to study the coupling between coefficient of friction, velocity and contact temperature of the disc brake

The coefficient of friction during braking is not constant. Determining its variability requires measurements in real conditions. This paper presents a methodology of conversion of the measured time profiles of the sliding velocity Veq, contact temperature T, and coefficient of friction f, into analytical formulation of two variables z = f(x,y). This was executed in the following steps: 1) creating a three-column table Veq, T, f from experimental measurements; 2) interpolation using Kriging; 3) creating a contour map of the coefficient of friction dependent on the sliding velocity and temperature, 4) import of a grey scale image of the friction map into the finite element (FE) software, and 5) definition of a function z = f(x,y). The studies were carried out for five composite organic friction materials and cast-iron brake disc of a railway vehicle. The developed five friction maps for each friction pair were based the data from the full-scale bench tests during braking from the initial vehicle velocity of 80 kmh−1, 120 kmh−1, 140 kmh−1, 160 kmh−1, 200 kmh−1 to a stop. In the second part of the study the initial-value problem for the equation of motion combined with the heat conduction problem were formulated and solved numerically using special functionalities of the FE software, e.g. Deformed Geometry, Mathematics. Changes in the vehicle velocity and time profiles of the coefficient of friction were determined from the solution of the coupled thermal problem. The computational FE model was verified by comparing the measured and calculated changes in the vehicle velocity, coefficient of friction and evolutions of the brake disc temperature during single braking. The maximum calculated temperature 1 mm under the contact surface of the brake disc, during braking from initial vehicle velocity of 80 kmh−1, was obtained for the base friction material and was equal to T1,3,5Num.= 72 °C. The corresponding experimental value was T1-6Exp.= 66.1 °C. When braking from 140 kmh−1, for the same material it was T1,3,5Num.= 129.7 °C and T1-6Exp.= 130.7 °C. The obtained differences in braking times calculated and measured did not exceed 1.7 %. The corresponding temperature differences of the disc at the stopping time were lower than 5.5 %.

Approach to Determine the Limiting Shear Stress of Lubricants at High Pressures Based on Traction Mapping

Typical lubricants behave in a non-Newtonian manner under conditions of high shear and high pressure, as is commonly observed in lubricated rolling/sliding contacts. To optimize and predict the friction therein, knowledge of the high-pressure rheological behaviors of lubricants and limiting shear stress (LSS) is essential. This study developed an approach for determining the LSS of lubricants based on friction mapping of rolling/sliding contacts, using a ball-on-disc traction machine. The main contribution lies in the introduction of a practical approach for the selection of a proper entrainment velocity for determining the LSS, with reduced thermal influences and near isothermal conditions. The proposed approach enables full film lubrication, while keeping the film as thin as possible to prevent excessive shear heating and, thus, thermal effects. The LSS of two lubricants, PAO40 and complex ester, has been measured at pressures ranging from 1.2 GPa to 1.7 GPa. A bilinear model has been used to describe the variation of LSS with pressure. The impact of entrainment velocity selection on the measurement of LSS is also discussed.

Boxes-Based Representation and Data Sharing of Road Surface Friction for CAVs

Vehicles can easily lose control unexpectedly when encountering unforeseen hazardous road friction conditions. With automation and connectivity increasingly available to assist drivers, vehicle performance can significantly benefit from a road friction preview map, particularly to identify where and how friction ahead of a vehicle may be suddenly decreasing. Although many techniques enable the vehicle to measure the local friction as driving upon a surface, these encounters limit the ability of a vehicle to slow down before a low-friction surface is already encountered. Using the connectivity of connected and autonomous vehicles (CAVs), a global road friction map can be created by aggregating information from vehicles. A challenge in the creation of these global friction maps is the very large quantity of data involved, and that the measurements populating the map are generated by vehicle trajectories that do not uniformly cover the grid. This paper presents a road friction map generation strategy that aggregates the measured road-tire friction coefficients along the individual trajectories of CAVs into a road surface grid. In addition, through clustering the friction grids further, an insight of this work is that the friction map can be represented compactly by rectangular boxes defined by a pair of corner coordinates in space, a friction value, and a confidence interval within the box. To demonstrate the method, a simulation is presented that integrates traffic simulations, vehicle dynamics and on-vehicle friction estimators, and a highway road surface, where friction is changing in space, particularly over a bridge segment. The experimental results indicate that the road friction distribution can be measured effectively by collecting and aggregating the friction data from CAVs.

Winter Road Friction Estimations via Multi-Source Road Weather Data—A Case Study of Alberta, Canada

Road friction has long been recognized as one of the most effective winter road maintenance (WRM) performance measures. It allows WRM personnel to make more informed decisions to improve their services and helps road users make trip-related decisions. In this paper, a machine-learning-based methodological framework was developed to model road friction using inputs from mobile road weather information systems (RWIS) that collect spatially continuous road weather data and road grip. This study also attempts to estimate friction using data from stationary RWIS that are installed far from each other, thereby leaving large areas unmonitored. To fill in the spatial gaps, a kriging interpolator was developed to create a continuous friction map. Slippery road risk levels were classified to provide an overview of road conditions via a risk warning map. The proposed method was evaluated with a selected highway segment in Alberta, Canada. Results show that the models developed herein are highly accurate (93.3%) in estimating friction and identifying dangerous road segments via a color-coded risk map. Given its high performance, the developed model has the potential for large-scale implementation to facilitate more efficient WRM services while also improving the safety and mobility of the traveling public.

Reverse Thinking: The Logical System Research Method of Urban Thermal Safety Pattern Construction, Evaluation, and Optimization

The acceleration of urbanization has significantly impacted the changing regional thermal environment, leading to a series of ecological and environment-related problems. A scientific evaluation of the urban thermal security pattern (TSPurban) strongly benefits the planning and layout of sustainable development and the construction of comfortable human settlements. To analyze the characteristics of the TSPurban under cross-regional differences and provide targeted solutions to mitigate the urban heat island effect in later stages, the logical system research framework of the TSPurban based on the “construction–evaluation–optimization” model was explored using reverse thinking. This study selected the Wuhan metropolitan area in China as the research object. First, a morphological spatial pattern analysis (MSPA) model was used to extract the top 30 core heat island patches, and Conefor 2.6 software was used for connection analysis to evaluate their importance. Second, based on the characteristics of various land cover types, the friction (cost) map of surface urban heat island (SUHI) diffusion was simulated. The spatial attributes of the heat island resistance surface were examined using a standard deviation ellipse and hot spot analysis. Finally, this paper used circuit theory to find 56 low-cost heat island links (corridors) and circuit scape software to find widely distributed vital nodes. The optimization of the TSPurban network was then investigated using a reverse thinking process. Heat island patches, corridors, and vital nodes are among the crucial components of the TSPurban. By obstructing corridor links and disturbing important nodes, it is possible to appropriately and proficiently reduce the TSPurban network’s connection efficiency and stability, which will have a positive influence on regional climate mitigation and the heat island effect.

High energy braking behaviors and tribo-map constructions of Cu metal matrix composites with different Cr volume contents

Cr, a promising abrasion component, has been introduced into Cu metal matrix composites (Cu-MMCs) to improve their high-energy braking performance. The Cu–Cr interfacial characteristics, worn surface/subsurface morphologies, and effect of Cr on the high energy braking behaviors of Cu-MMCs were measured in this study. The results indicate that an increase in Cr content prevented severe surface plastic deformation (0–5 vol. % Cr) and promoted the formation of mechanical mixed layer (MML) (15–20 vol % Cr), facilitated the improvement of wear resistance and the coefficient of friction, and reduced the thermal fade of composites. The main wear mechanism of the Cu-MMCs changed from adhesion (0–10 vol % Cr) or abrasion (10–20 vol % Cr) to delamination or mechanical mixing with an increase in braking energy density (BED). A series of corresponding friction, wear, and wear mechanism maps were established, which revealed the variation in the wear mechanism with the Cr content and BED.

The relationship between surface structure and super-lubrication performance based on 2D MOFs

Metal-Organic Frameworks (MOFs) have the potential to be super-lubricative materials because their advantages of inorganic-organic hybridization and molecular-level regulation. MOFs constructed from Cu2+ and meso-Tetra(4-carboxyphenyl)porphine (TCPP) can act as an excellent candidate in previous predictions. In this work, the CuTCPP with various surface structure were designed to investigate the specific influence of surface microstructure on the micro-friction performance. The experimental results showed that the lubrication performance is determined by the surface structure. When the surface is changed from the undefiled state to the modified state with surfactants, the friction coefficient quadrupled (5.6*10−4 to 2.1*10−3) and the adhesion doubled (4.4 to 8.7 nN). We innovatively observed the surface structure through the atomic friction maps and figured out their influencing mechanisms on the friction performance. The anchoring effect is the main source of friction on the undefiled CuTCPP surface, while the entanglement, viscous hysteresis or persistent viscous state become the dominant factor for the CuTCPP surface decorated with SDS, CTAB or PVP molecules respectively. In addition, the positive effect of surface porosity on friction reduction was confirmed. Therefore, the relationship between the tribological properties and the surface structure was successfully established that can guide the future research on super-lubricative materials.

A step towards SDMs: A “couple‐and‐weigh” framework based on accessible data for biodiversity conservation and landscape planning

AbstractAimThe ongoing global change makes landscape planning and management of ecological corridors crucial to preserve biodiversity. We propose a workflow optimizing the use of different data sources to convert ecological niche models (ENMs) into landscape‐focused species distribution models (SDMs), using these latter to compute ecological corridors. We infer corridors connecting present occurrence localities to future climatic refugia as well as to localities where extinct populations occurred. Also, a continuous connectivity change index is proposed to assess current–future differences. Finally, we discuss possible applications of our workflow to conservation, assessing the capability of established protected areas to preserve ecological corridors.LocationEurope.MethodsAs case study to illustrate our framework, we use a database comprising occurrence localities of Vipera ursinii, one of the most endangered European reptiles. We obtain weighted SDMs for each of the four V. ursinii subspecies by coupling climate‐based ENMs with standardized occurrence frequencies along land use and altitude gradients through weighted averaging in GIS. We calculate current and future landscape connectivity for each subspecies based on the corresponding weighted SDM. We compare predictive performance of “traditional” ENMs, including climate, land use and topography as predictors and weighted SDMs.ResultsWeighted SDMs outperform ENMs, according to Boyce index. SDMs are used to infer connectivity, predicted to decrease in all future scenarios for V. ursinii, and assess where connections may favour movements of individuals to, for example, future suitable areas. Generally, protected areas are predicted to cover low‐connectivity territories.Main conclusionsThe proposed “couple‐and‐weigh” approach could represent a helpful tool to investigate biogeography, conservation and landscape planning topics, as it permits to capitalize on occurrence records and accessible environmental predictors by narrowing the target species’ potential distribution, estimated within “traditional” ENMs, to the realized one through post‐modelling GIS analyses, which in turn improves estimation of friction maps used to infer connectivity.

Superlow Friction of a-C:H Coatings in Vacuum: Passivation Regimes and Structural Characterization of the Sliding Interfaces

A combination of atomistic simulations and vacuum tribometry allows atomic-scale insights into the chemical structure of superlubricious hydrogenated diamond-like carbon (a-C:H) interfaces in vacuum. Quantum molecular dynamics shearing simulations provide a structure-property map of the friction regimes that characterize the dry sliding of a-C:H. Shear stresses and structural properties at the sliding interfaces are crucially determined by the hydrogen content CH in the shear zone of the a-C:H coating. Extremely small CH (below 3 at.%) cause cold welding, mechanical mixing and high friction. At intermediate CH (ranging approximately from 3 to 20 at.%), cold welding in combination with mechanical mixing remains the dominant sliding mode, but some a-C:H samples undergo aromatization, resulting in a superlubricious sliding interface. A further increase in CH (above 20 at.%) prevents cold welding completely and changes the superlubricity mechanism from aromatic to hydrogen passivation. The hydrogen-passivated surfaces are composed of short hydrocarbon chains hinting at a tribo-induced oligomerization reaction. In the absence of cold welding, friction strongly correlates with nanoscale roughness, measured by the overlap of colliding protrusions at the sliding interface. Finally, the atomistic friction map is related to reciprocating friction experiments in ultrahigh vacuum. Accompanying X-ray photoelectron and Auger electron spectroscopy (XPS, XAES) analyses elucidate structural changes during vacuum sliding of a hydrogen-rich a-C:H with 36 at.% hydrogen. Initially, the a-C:H is covered by a nanometer-thick hydrogen-depleted surface layer. After a short running-in phase that results in hydrogen accumulation, superlubricity is established. XPS and XAES indicate a non-aromatic 1–2-nm-thick surface layer with polyethylene-like composition in agreement with our simulations.

Friction maps and wear maps of Ag/MoS2/WS2 nanocomposite with different sliding speed and normal force

The friction maps and wear maps for Ag/MoS2/WS2 nanocomposite under different sliding speeds and normal forces were constructed using the Response Surface Classification Study Method. The investigation found that the lower normal force and the lower sliding speed led to the ploughing wear, and the higher normal force and the higher sliding speed resulted in the oxidation delamination. The lower normal force and the higher sliding speed facilitated the formation of the transfer film. However, the sliding speed of 1.0 m/s and the normal force of 0.25 N showed a stable low friction coefficient and wear rate. The wear mechanism, friction coefficient and wear rate of Ag/MoS2/WS2 nanocomposite were predicted by the wear maps with different sliding speeds and normal forces.

How to build a heat network to alleviate surface heat island effect?

Numerous studies have proposed cooling measures to mitigate surface urban heat island (SUHI) effect from the perspective of landscape pattern. However, rare studies have considered to alleviate the SUHI from a network perspective, which was demonstrated by first building a SUHI network to identify the key nodes and links of a SUHI network and subsequently breaking this network to effectively mitigate SUHI. Here, a new approach to build SUHI network is proposed, which integrates morphological spatial pattern analysis and circuit theory. It includes: i) identification of stable and high-risk SUHI patches, ii) extraction of the “core” type, iii) evaluation of “core” importance, iv) construction of a friction map, and v) generation of links and pinch-points. Dongguan city was selected as case and the results showed: 1) 42 links were identified with 31 pinch points located in these links; 2) Most of these links and pinch-points were distributed in the southwestern and northwestern regions; 3) Cooling measures (patch-based) should be implemented in these specific areas to avoid connections between the links and nodes that could seriously aggravate the thermal environment of entire region. The method and reverse thinking process adopted could provide new insights for climate adaption planning and urban sustainability.

Implementation of zoning to guide management of foot and mouth disease in Tunisia.

The World Organisation for Animal Health advocates the zoning approach for the surveillance and monitoring of foot and mouth disease (FMD), a highly contagious animal disease. Our purpose is to implement the zoning approach in Tunisia by identifying existing natural and artificial barriers to the movement of live animals. A Geographic Information System (GIS)-based MultiCriteria Evaluation approach was developed. Eight national and international experts were asked to identify the barriers and prioritize them, characterized by a percentage weight between 0 and 100. These barriers were mapped and combined, taking into account their relative importance, to create a friction map that makes it possible to visualize areas where animal movements are restricted. Uncertainty analysis was performed to assess the robustness of the model. The results showed that the selected barriers were in order of decreasing importance: maritime borders with a weight of 33.5%, rivers (13.8%), slopes equal to or greater than 10% (13.8%), wetlands (13.3%), forests (7.7%), land borders (7.7%), railway networks (5%) and main roads (4.9%). The Cap Bon zone is the only favourable zoning area for the control of FMD in Tunisia. A regional approach resulting from this work could be a major asset in identifying regions suitable for zoning in North Africa.

Mechanical properties of the surface of gastric epithelial cells in patients with oncologic pathology

Objective:to study the efficiency of the application of atomic force microscopy for the identification of differences in the structural and mechanical properties of gastric epithelial cells in normal conditions and in gastric cancer.Material and methods.The structural and mechanical properties of the membrane of the epithelial cells of the gastric mucosa in 7 patients with gastric cancer were assessed using atomic force microscopy in contact mode.Results.The comparison of the size and shape of gastric mucous cells (GLC), their healthy and tumor areas has showed that in both the cases they have a slightly elongated rounded shape and similar dimensions: length × width × height of the cell is norm (in μm): 7.5 × 6.8 × 0.6, and in case of oncologic pathology (in μm): 7.2 × 6.9 × 1.0. Tumor cells are characterized by a 13% increase in their surface roughness compared to normal epithelial cells. The lateral force microscopy method allows of obtaining data on the structure of the submembrane cortical layer of the cell cytoskeleton. According to experimental data, a 2.48-fold decrease in the value of the friction force (Fl) in epithelial cells from healthy areas of the coolant indicates increased density and integrity of the cortical layer of the cytoskeleton in normal conditions compared to those of tumor cells.Conclusion.The analysis of the experimental data has found that tumor cells from the gastric mucosa differ from healthy cells in terms of their nanomechanical properties: they are characterized by a rougher surface, which consists of fewer structural elements of a larger size. The work has showed the interrelation of the indicators of the frictional properties of the cell surface: the values of the friction forces and the roughness of the friction map.

A friction perspective for negotiating renewable energy targets: the Israeli case

Policy design studies have addressed the role of political and institutional limitations in formulating effective climate policies including renewable energy targets (RETs). However, it is still not entirely clear how and why these limitations result in policy designs that are incapable of staying on track to meet the overall goals of the Paris Agreement on climate change. In order to deepen our understanding, this study introduces a friction perspective—one of the core components of the punctuated equilibrium theory used in policy change literature—and adopts it to the policy design process of energy transitions. This study argues that in cases where governments struggle to design stringent RETs, the level of friction between the elastic sub-coalitions (comprising bureaucrats and politicians) can shed light on policy design choices. By using a causal mechanism approach, the study developed several friction mechanisms to test how friction has been built and often dissolved, resulting in inadequate policy outcomes. The design process for setting Israel’s national RETs negotiated between 2015 and 2017 was used as a longitudinal case study to illustrate the role of friction and assess its impact. Unraveling how friction operates within policy design was found to be a good litmus test for the political feasibility of policy design choices. In other words, this study gives us a rudimentary blueprint of a “friction map” that, by tracing sequences of conflict and sequences of resolution, shows which particular design choices may generate more tension than others.

Friction and temperature mapping of environmentally acceptable gear oils

In recent years, environmental issues have raised the demand to protect the environment against the pollution caused by the uncontrolled spillage of lubricating oils. One solution is using Environmentally Acceptable Lubricants (EALs), however, these oils are more expensive than the common mineral oils. The consumers require to test the oil performance using test machines but testing in real machines is costly and time-consuming. Small test machines like ball-on-disc have been previously used for friction mapping and ranking gear oils. In this paper, the friction maps are measured from 0.65 GPa to 1.25 GPa, and temperature maps are devised to experimentally simulate the gear contact along the line of action. Results illustrate that EALs can provide up to 60 % better frictional efficiency that leads to 20 oC cooler oil temperature in high-pressure contacts operating under elastohydrodynamic lubrication (EHL) regime.

Wear Study on SOL–GEL-Coated Ti-6Al-4V Alloy

By using SOL–GEL Technique, novel nanocomposites made of alumina/hydroxyapatite/graphene were deposited on Ti-6Al-4V Substrates. The wear performances of the deposited coatings and coating thickness were assessed by scanning electron microscopy and EDS. The crystallography and the topography of the coated surface were examined by X-ray Diffraction and Atomic Force Microscope with porosity analysis, whereas incorporation of graphene was verified by Raman spectroscopy. The wear test was conducted using the pin-on-disc, with varying parameters exhibiting superior wear resistance and coefficient of friction and wear mechanism maps were constructed. The hardness of coated surface is increased by 70% approximately which subsequently increased the wear performance of the coated Ti-6Al-4V.

The potential of Ti3C2TX nano-sheets (MXenes) for nanoscale solid lubrication revealed by friction force microscopy

Ti3C2TX nano-sheets (MXenes) are an emerging class of two-dimensional materials with outstanding potential to be employed in energy storage, catalysis, and triboelectric applications, on length scales ranging from the nano- to macroscopic. Despite rapidly accelerating interest in this new class of materials, their nanoscale frictional properties and in particular, their potential for solid lubrication on the nanoscale, have not been explored in detail yet. In this short communication, we present the first results on the nanoscale frictional characteristics of MXenes, in the form of a friction map obtained on an isolated Ti3C2Tx nano-sheet deposited on a silicon dioxide substrate via friction force microscopy. Our experiments reveal that few-layer Ti3C2Tx nano-sheets indeed act as solid lubricants on the nanoscale, reducing friction on the silicon dioxide substrates, although not as effectively as few-layer graphene. The results reported here pave the way for further studies focusing on nanoscale solid lubrication achieved by Ti3C2TX (MXene) nano-sheets.

Effects of oxide and water on friction of rail steel – new test method and friction mapping

ABSTRACT A novel tribo-testing method using a pin-on-plate tribometer was developed to test and visualize the coefficient of friction over surfaces with oxides present to examine low adhesion issues. Tribo-test data was recorded each pass over test surfaces showing how friction changes with mechanical action. Trackside environmental monitoring and railhead swabbing was performed to investigate the physical and chemical environment of the railhead. Iron oxides were both synthesized on and deposited on rail steel substrates to simulate ‘wet-rail’ conditions. Powdered oxide layers of magnetite, haematite, goethite and lepidocrocite were deposited on rail steel substrates to investigate individual oxides. Composition was analysed using X-ray diffraction and scanning electron microscopy before testing. Magnetite, Haematite and Lepidocrocite were formed when water alone was applied to the surfaces. Low friction was observed on oxidized sample surfaces only outside high roughness, oxide pitted, areas, but these conditions were shown to be difficult to achieve and transient.

Friction titration measurements of electrochemically generated mixed alkylthiol monolayers on polycrystalline gold

An electrochemical procedure was used to generate a heterogeneous mixed monolayer composed of 1-undecanethiol (UDT) and 11-mercaptoundecanoic acid (MUA) on polycrystalline gold and the interface was characterized over a range of solution pH using Lateral Force Microscopy (LFM). To generate the heterogeneous mixture, a single component layer of UDT was formed on the gold electrode and portions were removed by reductively desorbing molecules selectively from the Au(111) facets. The voids created by this method were backfilled with MUA using either short-term passive incubation or electrochemically assisted deposition and LFM was used to gather friction maps of the surface in pH adjusted KClO4. From pH 3 to 8, the friction of both UDT and MUA zones in the mixed layer were relatively constant but above this range, the friction of the MUA regions increased significantly. The elevated friction was attributed to the ionization of the MUA groups since pH induced frictional differences were not observed when backfilling with the non-ionizable thiol 11-mercapto-1-undecanol (MUD). The relative difference in friction between the MUA and UDT zones were fit to a modified Henderson-Hasselbach equation to extract the apparent surface pKa (or pK1/2) of MUA bound to the Au(111) facets. The results were similar for both passive and electrochemically backfilled MUA and rendered a pK1/2 of 10.3 ± 0.8. The elevated pK1/2 was attributed to hydrogen bonding of the COOH groups when tightly packed on Au(111) as well as the presence of UDT, which created a local hydrophobic environment near the MUA molecules.

Time-optimal trajectory planning for a race car considering variable tyre-road friction coefficients

This paper shows the planning of time-optimal trajectories, which allows an autonomous race car to drive at the handling limits, taking into account locally changing road friction values. For this purpose, the minimum lap time problem is described as an optimal control problem, converted to a nonlinear programme using direct orthogonal Gauss-Legendre collocation and then solved by the interior-point method IPOPT. Reduced computing times are achieved using a curvilinear abscissa approach for track description, algorithmic differentiation using the software framework CasADi, and a smoothing of the track input data by approximate spline regression. The vehicle's behaviour is approximated as a single track and double track model with quasi-steady state tyre load simplification and nonlinear tyre model. The results are used to evaluate which vehicle physics are important for the calculation of the time-optimal trajectory. The novelty of this work is the consideration of wheel-specific tyre-road friction coefficients along the racetrack using a track friction map. It is shown that variable friction coefficients have a significant impact on the trajectory, and therefore significantly improve lap times on inhomogenous racetracks. The proposed trajectory planning has proven its practical suitability in first tests on an autonomous race car and will be used in the coming racing season in the Roborace competition.