Friction Materials Research Articles

T–01M TESTER ADAPTATION FOR WEAR DEBRIS COLLECTIONAND ANALYSIS

The article describes the need to collect and analyse data on stereological features of car tire and brakesystem friction material wear products, as required under new European Union regulations and by the projectto expand the existing T–01M tester with a separation chamber and equipment enabling their capture. Thestructure of the chamber is described and the conditions for measuring the average diameter of particlesemitted by the tested contacts are presented. The basic task of the additional chamber is to separate thesurroundings of the tested friction node from environmental influences and to enable the capture of wearproducts to measure stereological features and analyse their chemical composition. A unidirectional filteredair flow towards the friction node is provided in the separation chamber in order to limit the deposition of wearproducts on the tester components. Mounting the separation chamber does not require any interference in thestructure of the T–01M tester, because it is mounted on a sealed base frame screwed to the station plate. Thechamber is equipped with holders for mounting a replaceable filter at the air inlet and a removable cycloneparticles separator with a container for wear debris collected.

Actuation strategy characterizations of a vibro-impact crawling robot driven by dielectric elastomer actuators

Abstract Vibro-impact crawling robots driven by the emerging dielectric elastomer actuators (DEAs) feature the advantages of reduced system complexity and bidirectional locomotion capability. However, due to the lack of systematic dynamic models and in-depth investigations, the fundamental locomotion principles of the robots are still unclear and therefore their applications in real-world scenarios remain hindered. In this paper, a comprehensive dynamic model of this robot is developed by considering the complex interactions between the electro-mechanical coupling, impact mechanism, and multiple nonlinear friction characteristics. By incorporating extensive modelling and experimental studies, we explain the fundamental principles that lead to the bidirectional locomotion of the robot. The actuation strategies for both forward and backward locomotion are characterized in-depth, which include the actuation frequencies, relative phases, and waveforms. Three typical contact surface cases (i.e. rigid & dry, rigid & viscous, compliant & viscous) are considered in this paper, where we report the changes in the optimal actuation strategies for bidirectional locomotion with the contact surfaces and reveal the influences of key factors of friction coefficient, viscosity, and material compliance. The key findings reported in this work can build foundations for developing a highly robust and efficient DEA-driven vibro-impact crawling robot for broad applications.

Design and experiment of an innovative rotational two-level friction damper

In this paper, a novel rotational two-level friction damper (RTFD) was proposed and its cyclic behavior was experimentally validated. The RTFD realizes the two-level working behavior through the gap mechanism between the rotary plates and the inner plates, which is optimized to mitigate the structural vibrations in various magnitudes of excitations. In particular, the RTFD amplifies the bearing force and deformation of the friction materials through the lever mechanism, achieving higher energy consumption efficiency. The working mechanism of the RTFD is discussed and the analytical design principles are presented to quantify the performance of the developed dampers. Moreover, a full-size prototype specimen was designed, manufactured, and tested with a repeated general loading scheme and cyclic loading scheme with a constant amplitude to investigate the seismic performance of the proposed damper. The experimental phenomena indicate the feasibility of the two-level working mechanism of the proposed RTFD and the ignorable wear of the friction subs. The experimental results confirm that the innovation damper is characterized by stable hysteresis behavior and favorable two-level energy dissipation capacity. Moreover, the remarkable recoverability of the developed RTFD is demonstrated by the narrow variation of 10.51% in the bearing force and 7.93% in the accumulative dissipated energy between repeated loadings.

The Tribological Properties of Plasma Electrolytic Oxidation Layers Synthesized on Arc Spray Coatings on Aluminum Alloys in Contact with Various Friction Materials

Oxide layers on the surface of the aluminum alloys D16 and AMg6 and on arc coatings sprayed with electrode wires made of the alloys D16 and AMg6 were synthesized using plasma electrolytic oxidation (PEO). The microstructure, phase composition and micro-hardness of the PEO layers were studied. In addition to the two main phases (α-Al2O3 and γ-Al2O3), a small amount of a metastable crystalized Al2.427O3.64 phase was found in their structure. A comparison was made of the wear resistance and friction coefficients of the synthesized PEO layers during friction tests in pairs with other PEO layers, a galvanic chromium coating, cast iron, steels and bronze of the BrC30 type. The results of the friction tests for the various PEO layers on aluminum alloys in tribo-contact with high-hardness elements made of hardened or chrome-plated steel justify the possibility of their use in a friction pair. We experimentally showed the influence of glycerin additive in motor oil 15W30 on the change in the friction coefficient of the PEO layers synthesized on the aluminum alloys and on the arc-sprayed coatings on their surfaces in tribocouples with hardened steel.

Braking performance of copper-based friction materials reinforced by AlCoCrFeNi high-entropy alloy

It is essential to improve the braking performance of friction materials for the high-speed train. Copper-based friction materials reinforced with AlCoCrFeNi high-entropy alloy contents of 0-40 wt% were prepared by powder metallurgy, and the braking performance of these materials at braking speeds of 40, 80, 120 and 160 km/h was investigated. The composition of AlCoCrFeNi HEA particles changes due to the elemental diffusion and alloying between AlCoCrFeNi HEA particles and the copper matrix during sintering. With the AlCoCrFeNi content increasing, the amount of interfaces between AlCoCrFeNi particles and Cu matrix increase and the bending strength of the materials decreases, owing to the formation of oxides. The friction coefficient, braking time and absorbing power of copper-based friction materials can be significantly improved by adding AlCoCrFeNi particles. However, the improvement in their wear resistance is not significant. The spalling pits and oxides are detected on the worn surfaces. The dominant wear mechanisms of copper-based friction materials are spalling and oxidation.

Research on damage optimization of copper-carbon gradient composite materials for current-carrying friction

To meet the demand for damage optimization of current-carrying friction materials and based on the uneven distribution characteristics of damage, copper-graphite gradient composite materials were prepared using spark plasma sintering technology. These materials were paired with H62 brass, conducting damage optimization studies on copper-graphite gradient composite materials using a self-made current-carrying friction test machine. The results indicate that the copper-graphite gradient composite materials possess excellent current-carrying performance. With an increase of graphite content, the degree of mechanical damage and arc damage on the worn surface is optimized. The friction coefficient and wear rate of the material significantly decrease. When the content of graphite in the material is 5 wt% − 10 wt%, The material's damage optimization reaches the best and the wear rate is 9.7 × 10−4 mg·m−1. At this point, the predominant form of damage to the material is plastic deformation.

All porous Ecoflex and SEBS-based stretchable high-performance triboelectric nanogenerator for self-powered human activity monitoring

Achieving a high output performance by expanding the surface area of triboelectric materials is key to developing triboelectric nanogenerators (TENGs) for future versatile applications. Herein, highly porous Ecoflex and styrene-ethylene-butylene-styrene (SEBS) using chloroform are introduced to improve TENG performance. The triboelectric output increases significantly with increasing porosity, which can be attributed to the increase in the contact area, dielectric constant, and electrostatic induction in the porous structure, thereby resulting in additional surface charges on the porous surface and increased surface charge density of the friction materials. The TENG can stretch by up to 550 % with the designed fabric electrode configuration. Facilitated by the increased contact area of 3D printed microstructure and porous Ecoflex substrate, the fabricated TENG demonstrates a high voltage of 2280 V, corresponding to a power density of 18.8 W m−2, which is enough to power 460 light-emitting diodes and stopwatches. Demonstration of monitoring the walking steps of patients with Parkinson’s and blind people was also performed, which revealed outstanding sensing capabilities. In addition, the TENG-based self-powered sensor demonstrated an identification accuracy of 99.3 % and a motion detection accuracy of 98.2 % based on a deep learning model. Furthermore, by exploiting the outstanding ultrahigh sensitivity, the TENG was demonstrated for low-frequency football player motion monitoring. Therefore, the results of this study provide new insights into investigating porous materials and an efficient method to construct high-performance TENGs that can promote their practical application in the fields of smart healthcare and sports.

Mechanical properties and current-carrying tribological behaviors of sliver-titanium nitride composite ceramic coating

Current-carrying friction materials with excellent electrical conductivity and anti-wear ability are in constant demand in both academia and industry. In this work, the sliver-titanium nitride (Ag-TiN) composite ceramic coating was fabricated by an arc ion plating method. The microstructure, chemical composition, crystal structure, mechanical properties and current-carrying tribological behaviors of the Ag-TiN composite coating were systematically studied and the worn surfaces were also thoroughly investigated to analyze the possible tribological mechanisms in detail. The results showed that the Ag-TiN composite ceramic coating achieved uniformly distributed co-deposition of Ag and TiN compositions and exhibited excellent mechanical properties and electrical conductivity. The current-carrying tribological tests also revealed that the Ag-TiN composite coating exhibited exceptional friction reducing, anti-wear and conductive properties during the friction process. The analysis of the worn surface showed that the excellent current-carrying tribological behaviors of the Ag-TiN composite ceramic coating were related to the excellent electrical conductivity, the appropriate mechanical properties and the tribo-layer film formed on the worn surface.

Prediction and comparative analysis of friction material properties using a GA-SVM optimization model

PurposeWith the rapid advancement in the automotive industry, the friction coefficient (FC), wear rate (WR) and weight loss (WL) have emerged as crucial parameters to measure the performance of automotive braking systems, so the FC, WR and WL of friction material are predicted and analyzed in this work, with an aim of achieving accurate prediction of friction material properties.Design/methodology/approachGenetic algorithm support vector machine (GA-SVM) model is obtained by applying GA to optimize the SVM in this work, thus establishing a prediction model for friction material properties and achieving the predictive and comparative analysis of friction material properties. The process parameters are analyzed by using response surface methodology (RSM) and GA-RSM to determine them for optimal friction performance.FindingsThe results indicate that the GA-SVM prediction model has the smallest error for FC, WR and WL, showing that it owns excellent prediction accuracy. The predicted values obtained by response surface analysis are closed to those of GA-SVM model, providing further evidence of the validity and the rationality of the established prediction model.Originality/valueThe relevant results can serve as a valuable theoretical foundation for the preparation of friction material in engineering practice.

Chemical Activity of Nascent Surfaces for Tribochemical Reactions of Lubricant Components.

Boundary lubrication characteristics are strongly dependent on tribochemical reactions at the contact interface. There are multiple factors that contribute to the activity of tribochemical reactions and the complexity of this phenomenon. Here, we focused on nascent surfaces created by friction and aimed to clarify their chemical properties. To achieve this, we first developed a method based on a mass spectrometer that can sensitively detect molecular adsorption and reactions on a nascent surface. We used various organic compounds as adsorbates and friction materials such as steel, aluminum, gold, and ceramics and examined adsorption of the organic compounds on the nascent surfaces. The adsorption properties of the additives differed between steel metal oxide and nascent steel surfaces. For example, polar organic phosphates, which are extreme-pressure additives, were more easily adsorbed on oxide surfaces, whereas nonpolar organic sulfur compounds were more easily adsorbed on nascent steel surfaces. Alcohols and ethers also adsorbed on nascent aluminum surfaces, whereas olefins and benzene did not. Nascent gold surfaces showed high catalytic activity. Saturated hydrocarbons and fluorinated compounds, which are chemically inert, were also adsorbed on nascent ceramic surfaces, showing high activity. Hydrocarbon oils decomposed through the influence of frictional heat on a nascent steel surface to generate hydrogen and methane. We applied reactive molecular dynamics to simulate the adsorption phenomenon on iron surfaces. As a result, the adsorption energy obtained by simulation and the experimentally determined adsorption activity correlated well. Thus, adsorption and reaction behaviors on nascent surfaces inferred by our method were theoretically supported. On the basis of the chemical properties of the nascent surfaces obtained here, it is possible to explain boundary lubrication phenomena.

Frictional Slip and Incremental Dynamic Analysis of Plate-rubber Bearing Continuous Girder Bridge by Ambient Temperature in Cold Region

Special mechanical environment, the environmental temperature or stress transformation easily to the final mechanical properties of the bridge components performance changes. The impact of the cold zone environment on the plate rubber-bearing beams is the object of study, Jining Road refined mechanics finite element analysis of the structural dynamic response study under seismic action. The results show that low temperature leads to bearing friction slip and material parameter changes, which affects the self-oscillation frequency and seismic susceptibility of the bridge. Due to the temperature-induced changes in the material properties and mechanical properties of the bearings, the first principal period of the bridge increases by 3% at high temperature for the EH and decreases by 19% at low temperature for the EL when compared to the first principal period under normal temperature conditions. Under different extreme temperature conditions, the fundamental period of the bridge is longitudinal, and the effective mode vibration participation mass is more than 90%. The maximum crossover frequency VALmax reaches 75.6 dB. Compared with room temperature, the bearing stress increased by 27.6% to 45.5%. The effect of EL stress change should be considered in the design of bridges in the alpine region.

Atomistic simulations of the frictional properties of 2D materials: a review

Abstract The two-dimensional (2D) materials are regarded as the ideal solid lubricants at micro- and nano-scale. Besides the experiments and analytical models, the atomistic simulations are important tools to investigate the frictional properties of 2D materials. This review will focus the recent atomistic simulation studies on frictional properties 2D materials with a particular emphasis on the density functional theory (DFT) calculations and molecular dynamics (MD) simulations. Starting from the proper calculation of long range dispersion forces, the correlations between the physical characteristics (e.g., electronic charge redistribution, interfacial commensurability, chemical modification, moiré superlattice, layer effect, atomic contact quality, defect, external fields, humidity and temperature) and frictional properties of 2D materials are reviewed for both the interlayer and surface sliding. At last, some shortcomings in current simulation techniques are summarized and it is suggested that the atomistic simulations combined with machine learning (ML) will be a more powerful strategy to investigate the frictional properties of 2D materials.

Experimental Comparison of Manufacturing Parameters in Automotive Friction Materials

In this study, a fixed automotive friction material content was determined and the mechani-cal and tribological effects of manufacturing parameters on friction materials were investigat-ed. Parameters; pre-forming time (1-3-5 min) and pre-forming pressure (8-10-12 MPa), hot pressing time (5-10-15 min) hot pressing pressure (8-10-12 MPa) and hot pressing temperature (125-150-175 °C), curing time (4-8-12 h) and curing temperature (120-150-180 °C) were de-termined. The friction test of the produced samples was carried out under 0.551 MPa pressure and 7 m/s rotation speed for 90 min. In addition, the average COF, friction stability, specific wear rate, density and hardness values of the samples were calculated. According to the results obtained, the average COF value increased as the pre-forming time and pressure increased. With the increase in hot pressing time, the tribological properties of friction materials improved. Although there was an increase in the average COF value with increasing hot pressing pressure and temperature, the specific wear rates increased in these parameters. The specific wear rate increased with the increase in curing time and temperature. The highest specific wear rate among all samples was obtained in the sample cured at 180 °C. Finally, it has been suggested that 3 min for pre-forming time, 12 MPa for pre-forming pressure; 15 min for hot pressing time, 12 MPa for hot pressing pressure, and 150°C for hot pressing temperature; and a curing time of 8 h and curing temperature of 150 °C may be sufficient.

Tribological behavior and mechanism of Mo2Ti2C3/polyimide composite films under dry friction condition

AbstractPolyimide (PI) has good mechanical properties and thermal stability, while high coefficient (COF) and wear rate limit its wide application. In order to endow PI with better friction and wear resistance, Mo2Ti2C3 MXene was used as solid lubricant to prepare Mo2Ti2C3/PI nanocomposites with different content of Mo2Ti2C3. The effect of the amount of Mo2Ti2C3 on the tribological behavior of PI was explored and the tribological mechanism was also analyzed Tribological tests showed that at 1.2 wt% of Mo2Ti2C3 addition, the composite material demonstrated superior friction and wear resistance, exhibiting a 19.9% decrease in the COF and a 79.7% decrease in the wear rate compared to single PI due to the good self‐lubricating properties of Mo2Ti2C3 MXene. Moreover, Mo2Ti2C3 can also effectively assist PI to form sleek transfer films without interruption or gaps on the steel balls, resulting in a reduced friction COF and wear rate of the material. This work demonstrates that Mo2Ti2C3 MXene is an aussichtsreich additive for polymers to improve the tribological properties and broadens the applications of Mo2Ti2C3/PI composites in friction material area.Highlights Mo2Ti2C3 significantly improves the tribological performance of polyimide. The thermal properties of Mo2Ti2C3/polyimide are enhanced by the introduction of Mo2Ti2C3. The minimum coefficient of Mo2Ti2C3/PI composites was only 0.246 at the 1.2 wt% content of Mo2Ti2C3. The wear rate of Mo2Ti2C3/PI composites was decreased by 79.7% at the 1.2 wt% addition of Mo2Ti2C3. Mo2Ti2C3 promotes the establishment of tribo‐film to protect the Mo2Ti2C3/PI composites from friction.

Emission and properties of airborne wear particles from train brake friction materials based on novolac phenolic resins and butadiene rubbers

The emission of airborne particulate matter from a train brake depends on the formulation of its friction material. This study investigates the emission and properties of wear particles from train brake friction materials based on straight or resorcinol-modified novolac phenolic resin and nitrile or styrene butadiene rubber used as binding ingredients. The wear particles are generated by a pin-on-disc tribomachine inside an aerodynamic chamber, counted and collected using aerosol measurement techniques, subjected to microscopic and elemental analysis. It has been found that the modification of novolac phenolic resin with resorcinol has no considerable effects on the emission of wear particles and their properties. By contrast, replacing styrene butadiene rubber with nitrile butadiene rubber leads to a significant decrease in the emission of 0.1–10 μm wear particles.

Assessment of benzoxazine resins as brake pad friction material binder: Tribological properties and PM emission

This study explores benzoxazine resins as alternative binders for brake pad friction materials, comparing them to the established phenolic resins. Benzoxazine resins feature great thermal properties, alongside industrially-attractive properties such as forgiving storage conditions and extended shelf life. Tribological Pin-on-Disc tests were conducted at different temperatures, with airborne emission monitoring. The characterization of the samples and their worn surfaces was carried out through TGA, SEM, and EDXS analyses. The results revealed promising tribological performance for benzoxazine-bound materials, suggesting potential environmental benefits, especially in high-temperature conditions. This research builds upon a preliminary study focusing on the processing aspects of benzoxazine resins.

Investigation on tribological behaviour of copper-free composite friction material under dry sliding at high-temperature

ABSTRACT This research uses a rotary pin-on-disc tribometer to explore the friction and wear behaviour of a copper-free semi-metallic friction composite over various operating temperatures, 25 °C, 150 °C, 250 °C, 350 °C, and 450 °C. The tribological performance of copper-free semi-metallic specimens is significantly affected by heat degradation. It was found that at 450 °C, severe wear of the pin specimen occurred. The friction layer coverage gradually decreased at ≥ 250 °C. The composite pin’s wear rate was found to increase 13.26 times from 450 °C to 25 °C, whereas the disc wear rate at 450 °C was 0.86 times the wear rate at 25 °C for the loading condition of 60 N load, 2.09 m/s sliding velocity and 6283 m sliding distance. Micro-ploughing was the primary wear process below and at 150 °C, whereas groove development, delamination, and severe adhesion dominated above 150 °C.

A Review of Friction Dissipative Beam-to-Column Connections for the Seismic Design of MRFs

The use of friction-based beam-to-column connections (BCCs) for earthquake-resistant moment-resistant frames (MRFs), aimed at eliminating damage to beam end sections due to the development of plastic hinges, has been prevalent since the early 1980s. Different technical solutions have been proposed for steel structures, and some have been designed for timber structures, while a few recent studies concern friction joints employed in reinforced concrete structures. Research aimed at characterizing the behavior of joints has focused on the evaluation of the tribological properties of the friction materials, coefficient of friction, shape and stability of the hysteresis cycles, influence of the temperature, speed of load application, effects of the application method, stability of preload, the influence of seismic excitation characteristics on the structural response, statistical characterization of amplitude, and frequency of the slip excursion during seismic excitation. Studies aimed at identifying the design parameters capable of optimizing performance have focused attention mainly on the slip threshold, device stiffness, and deformation capacity. This review compiles the main and most recent solutions developed for MRFs. Furthermore, the pros and cons for each solution are highlighted, focusing on the dissipative capacity, shape, and stability of hysteresis loops. In addition, the common issues affecting all friction connections, namely the characteristics of friction shims and the role of bolt preload, are discussed. Based on the above considerations, guidelines can be outlined that can be used to help to choose the most appropriate solutions for BCCs for MRFs.

Sustainable paper-based triboelectric nanogenerator with improved output performance through a general strategy of interface fusion

The paper-based triboelectric nanogenerators (PB-TENGs) satisfied the requirement of sustainability and aroused great research interests, however how to improve their outputs still remains as a huge challenge. Most of the reported strategies to enhance the performance of PB-TENG focused on the chemical modification of the friction layers (FL), which may be restricted by the high-cost and cumbersome protocols being required. In this work, a general strategy of interface engineering to boost the outputs of the PB-TENG was proposed and validated by employing graphene composite paper (GC-paper) as electrodes. At the beginning, the hydroxyethyl cellulose (HEC) and Ecoflex were cured as separated films and used as friction layers, which were adhered with copper or GC-paper electrodes. The PB-TENG device can be obtained and possesses acceptable performance. Subsequently, we were surprised to find that the outputs of the PB-TENG largely increased by coating the FLs on the GC-paper rather than adhering. The maximum open-circuit voltage jumped from 300 V to 734 V, and the maximum short-circuit current boosted from 3.4 µA to 8.7 µA. This phenomenon may be ascribed to an interface effect between the FLs and electrodes, which enhanced the electrostatic induction. The strategy of this interface engineering was also proved to be general for other PB-TENG devices with various friction materials. Therefore, it may pave a way for the development of sustainable TENG devices with high performance.

Role of copper and zinc additives in the stiction phenomenon of automotive braking systems

AbstractThe braking system of a motor vehicle is a multi‐material system, subjected to various aggressive conditions. Corrosion of the brake disc during stationary periods can determine the onset of a high adhesion force (stiction) capable of compromising the reliability of the braking system during vehicle motion. The purpose of this work is to study the effect of the introduction of Cu and Zn in the friction material composition. This effect was investigated through electrochemical measurements (electrochemical impedance spectroscopy, potentiodynamic polarization, and stiction tests), conducted using an electrochemical cell simulating the parking brake, complemented by the examination of the brake disc and pad surfaces and water absorption tests. The results suggest that porous components, like vermiculite, in the composite friction material led to high contact force. Moreover, 10 wt% of Cu in the friction material does not significantly affect its stiction behavior in our testing configuration. In contrast, 10 wt% Zn in the friction material significantly reduces the stiction propensity by acting with a complex synergistic mechanism combining physical and chemical shielding effects.