Search

In the present study, influences of friction conditions on friction and wear behaviors, as well as characteristics of brake pad surfaces were investigated. Low metallic (LM), semi metallic (SM) and non-asbestos organic (NAO) brake pads sliding against an iron disc were tested using a pin-on-disc tribometer. Results show that the friction coefficient and specific wear rate decrease with increasing contact pressure and sliding velocity. For the morphology of brake pad surface, friction layers are large with few cracks on surfaces of SM and LM brake pads. While for NAO brake pad, friction layers have a comparatively slender and irregular shape. From fractal analysis, the fractal dimension of brake pad surface is in the range of 2.38–2.84 for all brake pads.

Al2O3 fiber was introduced into copper-based brake pads in order to improve the braking performance under high speed and heavy load. Two fabricated samples containing Al2O3 fiber and no fiber additive were tested on a reduced-scale dynamometer using a special designed braking testing procedure. The results indicated Al2O3 fiber additive mainly contributes to promote the increase of friction coefficient at relative low braking speeds and pressures and helps to stabilize the friction coefficient at high braking speed and heavy load. The addition of Al2O3 fiber can also greatly reduce the wear loss of brake pads. Moreover, the sample with Al2O3 fiber exhibits the lower maximum temperature during braking. Further, the relationship between braking properties and tribo-film is clearly revealed.

The development of alternative compositions instead of hazard asbestos based materials, in brake pad application, is crucial. In the present study, the asbestos free friction materials were prepared using natural waste ingredients. A variety of banana peel and Bagasse fiber/particle content as a functional/friction additives and nano-alumina particles as an abrasive were used to achieve an optimized friction materials composition. Then, the prepared samples were investigated about microstructure, density, hardness, wear and friction behavior. The results showed, an increase in natural wastes addition lead to a decrease in composites’ density. The dynamometery wear/friction test results revealed the conventional feature of wear surface, including 1st and 2nd plateaue and wear debris. Also, the specific wear coefficients of samples with natural waste ingredients are in order of 10−7 and the friction coefficients are about 0.3–0.4. The composite sample with 5 wt % of bagasse fiber/particle and 5 wt % of nanoalumina particle had the optimized condition among investigating samples with specific wear coefficient of 1.18 *10−7 and friction coefficient of 0.369.

This study explores the potential of high-performance Zylon fibers (ZF) for the first time in brake-pads. It also reports on the influence of the aspect ratio of these fibers on the performance properties of the eco-friendly brake-pads first time, enabling the right selection of the fiber length for the best performance. In addition, Aramid short fibers (AF) and pulp (AP) were also selected as theme ingredients, which would-be competitors to the Zylon fibers (ZF) in brake-pads. A series of six types of brake-pads was developed (theme ingredients- 5 vol.%) by keeping parent ingredients fixed. A comparative performance assessment for different properties was carried out. Tribological studies on a full-scale brake dynamometer showed that almost all performance properties such as resistance to wear and fade (pressure, speed, and temperature) improved due to Zylon fibers. The average coefficient of friction slightly decreased for ZF-based pads compared to Aramid pads. Overall, ZF with 3 mm length proved the best one based on the multiple objective optimization by ratio analysis (MOORA) method. Worn surfaces of pads were analyzed using scanning electron microscopy (SEM).

Nowadays, the usage of Copper in brake friction materials (FMs)/pads is a major concern in the brake industry. Despite some of the earlier attempts for the Cu substitution, an issue with high wear of FMs due to their low adhesion of metal particles with the matrix (i.e., phenolic resin) is still largely unresolved. In the current study, the surface energy (SE) of stainless steels (SS316, SS410) and Cu particles was augmented using the right kind of plasma in optimized dose to improve the wettability/adhesion of metal particles with the matrix. The SE of treated particles increased significantly due to the reduction of oxides from the surfaces. A series of seven types of brake-pads was developed containing 3 vol.% of SS and Cu (untreated and treated) particles as the theme ingredients keeping parent composition fixed. A reference pad without metal particles was also developed for benchmarking. The developed pads were characterized for physical properties and then evaluated for the tribological and noise-vibration (NV) performance on a brake-dynamometer following global test standards. The results revealed that the pads with treated SS and Cu particles showed a higher fade resistance (~ 2- 3%) and reduced wear by ~ 10% compared to the pads with untreated particles. Overall, based on optimized technique, treated SS316 particles proved the best. The underlying mechanisms were studied through worn surface analysis using a field emission scanning electron microscope attached (SEM) with an energy dispersive X-ray spectroscopy (EDAX).

In this study, low-steel composite friction materials containing different amount of h-BN were developed for railway vehicles. Addition of h-BN has no considerable effect on mechanical properties of the matrix structure. However, h-BN addition significantly affects friction and wear properties of the matrix material. While h-BN increases coefficient of friction of samples at low disk surface temperatures, it has a reverse effect after the temperature reaches to a critical level of 250–300 °C. Also, addition of 1.5 wt% h-BN to the matrix brings about more stable changes in friction coefficient during the braking periods under the extreme conditions as compared to the h-BN-free sample. Furthermore, low h-BN addition into the matrix results in better lubricity properties at relatively high temperatures.

The present paper provides a statistical analysis of the error obtained in the measurement of density (Dcp), size (Scp) and area fraction (Acp) of the contact plateaus existing on the surface of brake friction materials (BFM). Two different analysis were performed: (1) random: micrographs were taken in random locations of the BFMs' surfaces; (2) deterministic: micrographs were taken in 3 arrangements, that are border (B), center (C) and distributed (D). Results of the random analysis revealed that a micrograph area of 43.6 mm2 (12 images) provides an error lower than 5% for all the 3 morphological parameters studied. Deterministic analysis showed that arrangement D provides the lowest error in Acp, while B and C are prone to generate biased errors.

The effect of combining various reinforcement materials, such as potassium titanate, Cu fiber, and steel fiber on the frictional and morphological properties of brake pads was investigated. The pad types were denoted as PT (with only potassium titanate), PC (with potassium titanate and Cu fiber), and PF (with potassium titanate and steel fiber). The PC pad exhibited the most stable coefficient of friction during repetitive braking across all the test temperatures. The PF pad exhibited a stable coefficient of friction below 250 °C, with unstable behavior at 300 °C. The surfaces of all three pads contained contact plateaus. The PC and PF pads showed primary and secondary contact plateaus, while the PT pad showed only primary contact plateaus.

As one of the most important components in high-speed trains, demands are improved on the mechanical and tribological properties of materials for brake pads. In this study, a newly developed copper metal matrix composite (Cu-MMC) for the aforementioned brake pads was fabricated by powder metallurgy route. The microstructure and mechanical properties of Cu-MMC were investigated. Cu-MMC was tribo-evaluated by a full-scale dynamometer, and special attention was paid on the braking performances during emergency stop-braking at initial speeds from 300 to 380 km/h. Examination and analysis of the worn surface and subsurface corroborated the wear mechanism. The results indicate that Cu-MMC exhibits excellent properties and can meet the technical requirements, so it holds great promise for applications in high-speed trains.

The role of metallic fibers, present in brake pad materials, have been investigated with particular attention to the formation of the friction layer. The aim of the research was to establish an effective approach for the development of less polluting copper-free friction materials. Starting from a reference material, two more compositions were prepared: one obtained just by removing copper; in another, the quota of the removed copper fibers was replaced by steel fibers. The samples were wear tested and the results compared with those obtained with the reference material. The worn surfaces exhibit specific features, like cracks and compacted wear debris, that provide useful indications for interpreting the main wear phenomena and for a further development of novel friction materials.

Copper (Cu) is almost an essential constituent of friction materials (FMs) since it shows multi-functionality in performance. However, it is being banned due to its adverse effects on aquatic life. In the present research work, iron-aluminum (FA25) alloy particles were explored for Cu replacement. With the help of fixed parent composition, five types of brake-pads were manufactured with variations in FA25 alloy content as 0, 5, 10, 15, and 20 wt%. One more kind of pad was developed containing Cu in 10 wt% for the sake of comparison. The brake-pads were characterized for physical, mechanical, thermal, tribological properties and noise-vibration (NV) performance. The 10 wt% alloy proved to be the optimum amount to replace the Cu with superior performance if fade resistance and counterface friendliness can be compromised slightly.

Particulate matter (PM) emitted during brake application is investigated, focusing on the size effect of zircon particles in brake pads. The results show that the concentration, size distribution, and composition of PM are strongly influenced by the size of the zircon particles. The iron content in ultrafine particles was higher when small zircon particles were used in the pad, whereas coarse zircon particles produced high iron content in the larger wear particles, suggesting that the amount and chemistry of the brake-induced PM are governed by different wear mechanisms at the sliding interface, which depend on the abrasive size.

Running-in process of low metallic brake pads and cast iron discs are investigated using full scale inertia brake dynamometer designed for particle emission studies. The airborne particles are measured using ELPI+ and collected on filters. The pads and disc contact surfaces are studied using microscopy techniques. It is observed that the particle emissions from the new pads and discs are significantly higher compared with the used ones and indicates importance of proper running-in of the pads and disc for wear particle emission tests. The results also indicate that pads and disc pairs which are able to stabilize friction behavior faster will produce less particle emissions which could influence the strategies of brake material formulations or steps during their production. QC 20170630

The disc brake squeal is a very annoying sound which affects the customer comfort. The generation of this noise is considered as a structural dynamics and/or tribological problems of the pad-disc system. Unlike drum brake design, this system is exposed to external environmental sand particles, wear debris and water spray. Their presence into the disc and pad interface may create dynamic and physics phenomena induced by friction surface changes which lead to brake noise and vibration issues. Thus, this paper aims to investigate the effect of environmental sand particles (silica sand and road sand particles) with a range size of 100–150, 150–200 and 300–400 µm on the brake squeal noise occurrences. The sensitivity of the sand particles to produce noise is correlated with the wear scar, friction coefficient and variation of brake operational parameters. It was concluded that the primary reason for the reduced squeal noise generation was contributed to the reduced COF with the particle embedment. The analysis of the particle size influence on the squeal occurrence indicated that a small size of silica sand particle produces less squeal occurrence than that of road sand particles. However, the biggest silica and sand particles seriously damage the pad surface and reduce consequently the real contact surface, inducing a decrease on the friction coefficient.

In the present investigation, optical microscopy, confocal laser scanning microscopy (CLSM), microindentation and image segmentation were employed to investigate the different structures existing on the worn surface of a non asbestos organic (NAO) and a low metallic (LM) brake pad. The following structures were identified on the surfaces of the studied pads: (i) deformable and non-deformable primary plateaus, (ii) secondary contact plateaus formed with and without the support from the structural components of the pad (the primary plateaus), and (iii) the “elastic highlands”. This last structure stays in a high position on the pad's surface topography and responded very flexible to applied loads. Finally, correlation between friction and the structures found on the pads’ surfaces is discussed throughout this paper.

Brake emissions during high-temperature brake applications were investigated using four different brake discs—FC170 and FC250 Gy iron discs, oxynitride-coated gray iron discs, and ceramic discs. The brake emission tests were performed with a low-steel brake pad using a 1/5 scale brake dynamometer with a high resolution electrical low-pressure impactor. The highest number concentration was observed using ceramic discs, whereas the gray iron discs produced higher mass concentrations than others. The effect of the disc material on brake emissions at elevated temperatures was primarily determined by the thermal conductivity of the disc and the time- and temperature-dependent variations of contact plateaus on the pad surface.

Powder metallurgy (PM) brake pads with stable friction behavior are one of the ideal choices for the counterparts of C/C–SiC brake disc that are expected to be applied to the future high-speed trains. In this study, iron- and copper-based PM pads were used to mating with the C/C–SiC brake disc. After testing the brake pairs using a lab-scale dynamometer, the C/C–SiC disc generated almost no wear loss. The coefficient of friction of the carbon/ceramic disc-PM pads brake pair varied nearly between 0.35 and 0.6 under different braking conditions. The analysis of friction surface indicated that the wear mechanism of Fe-based PM pads was mainly dominated by abrasion, while that of Cu-based pair mainly relied on adhesion and fatigue peeling.

Rising environmental concerns and implementing restrictions on the use of copper in friction materials (FM) make it mandatory for manufacturers to look for a reliable replacement. Two commercial pads: 1) low metallic FM contains 7 wt.% copper fibers and 2) low metallic FM contains thermal graphite (TG), and cellulose fibers as replacements for copper were studied. The two FMs showed almost identical thermal behavior. However, Cu-contained FM exhibited a bit more thermal stability. The fade ratio and recovery ratio in the first fade and recovery stages showed almost the same values. Moreover, in the second fade, change from 86% to 78% and second recovery change from 90% to 77%, respectively, for Cu-contained and Cu-Free FMs.

Cu-based brake pads for high-speed train were tested in full-scale at braking speeds as high as 380 km/h. The microstructure, composition and properties of the original, worn and lateral surfaces were investigated. The results show that a ~2 μm thick nanostructured tribolayer composed of CuO and Fe2O3 was formed on the worn surface, owing to the plastic deformation, mechanical mixing, oxidation and sintering processes. The tribolayer enables significantly higher friction coefficient and wear resistance than original surface, which can be attributed to oxide-based composition and high hardness. The graphite particles in the worn and lateral surfaces were burned and the copper matrix was oxidized. The tribological property of brake pad highly depends on the composition and microstructure of the tribolayer.

In recent few years, the environmental issues especially related to the aquatic life are pushing the Friction Industry to develop NAO FMs (non-asbestos organic friction materials) without Cu. Stainless-steel swarf (SSS) having good corrosion resistance was tried as one of the replacing ingredients for Cu in FMs in Authors’ laboratory in earlier work. It proved to have a great potential for Cu replacement almost in all the performance properties but at the cost of slightly lower wear resistance and hence the theme of current research was to enhance the wear resistance capability of SSS. Hence, in the present study SSS were treated with air-plasma and used in brake-pads. Thus, two types of brake-pads with the identical composition but differing in type of SSS 10 wt % (treated and untreated) were tribo-evaluated on a full-scale brake inertia dynamometer and treated SSS containing pads exhibited higher wear resistance (approx. 10%) confirming success of the treatment, first time explored for the friction materials. Enhanced adhesion with resin was the main cause for the improved tribo-performance.

In this work, the braking performance of full-carbon/ceramic pairs that are suitable for high-speed trains and automobiles was examined. After braking, the morphologies of the friction surface were observed via SEM and the sub-surface of the brake pads were characterized by FIB and TEM. According to the results, introducing a ductile phase like FeSi2 into the brake pads was beneficial for decreasing the coefficient of friction (COF) to an acceptable range and effectively improved the wear resistance. When braking under high pressure, the friction surface of the FeSi2 modified C/C–SiC pads were maintained while that of the virgin C/C–SiC pads were totally destroyed, the wear resistance of the former was up to an order of magnitude higher than the latter.

A pad-on-disc-type brake apparatus was constructed based on a theoretical principle for suppressing frictional vibration. The pad for this apparatus was supported by parallel leaf springs with a unidirectional compliance. Braking tests were conducted using the apparatus under a constant normal load. It has been found that a yaw angle misalignment between the directions of the pad and disc motions provides a positive damping to suppress the frictional vibration and brake noise. In addition, it has been also found that when an appropriate misalignment angle is selected, a low-noise performance and a good braking performance can be achieved simultaneously.

This contribution aims at introducing tribological contact properties into the stability analysis of macroscopic structural systems with focus on brake-squeal applications. First, the surfaces of several brake pads at different states of wear are analyzed experimentally. Based on these data a statistical contact model as well as a numerical simulation are presented. Using these models two constitutive contact laws are derived, which link contact pressure and stiffness. Second, a simple brake model incorporating these contact models is developed. Results of stability analyses are presented and the influence of the contact models on the steady state stability is discussed.

Wear particles originating from disc brakes contribute to particulate concentration in the urban atmosphere. In this work novel nanoporous composite-based and conventional brake materials were test ...

Copper is a major ingredient in friction materials used for automotive braking. The purpose of this study was to find out how copper contributes to good brake performance properties in addition to providing good thermal conductivity. Microstructural investigations of copper chips at the surfaces of brake pads revealed a zone of severe plastic deformation which provides high hardness, but there is also evidence of recrystallized copper nano-particles which are incorporated into friction layers as soft ingredient once detached from the pad surface. Thus copper seems to play a dual role, firstly as reinforcing element of the brake pad providing primary contact sites, and secondly as solid lubricant by contributing to the formation of a layer of granular material providing velocity accommodation between the rotating disc and fixed pad. Confirmation for this hypothesis was obtained by modelling contact sites on the nanometre scale with the method of movable cellular automata. Results show both, the similarity of steel fibres and copper macro-particles in respect to providing primary contact sites, as well as similar sliding behaviours of friction layers containing either copper or graphite as soft inclusions. Furthermore, it is shown that not only material properties, but also the concentration of solid lubricant particles in the friction layers, determine conditions for friction force stabilization and smooth sliding behaviour.

Sensitivity of μ (coefficient of friction) of friction composites towards load and speed is composition specific. For an ideal friction material, it should be zero. It was of interest to examine the influence of increasing amount of three commercially popular metallic fillers (steel fiber, brass fiber and copper powder) on the tribological performance of friction composites including sensitivity of μ towards load and speed. Three series of non-asbestos organic (NAO) friction composites comprising of seven composites in the form of brake pads were developed in the laboratory using three metallic fillers as a single variant. All composites were characterized for physical, chemical and mechanical properties. These were further tribo-tested on reduced scale prototype (RSP) for friction, wear and sensitivity of friction coefficient (μ) towards load and speed characteristics. It was concluded that inclusion of metal contents led to enhancement in friction performance of the composites but at the cost of wear resistance, in general. From μ sensitivity point of view, composites with higher metallic contents and hence thermal conductivity (TC) showed better performance. Overall it was observed that copper powder based composite (with 10%) proved as the best performer from both friction and wear point of view.

The frictional behavior at local contacts in an automotive brake system was analysed on the basis of computer simulation by movable cellular automata method. The boundary conditions of the model were adjusted to experimental observations obtained by TEM. The model proved to be adequate for simulating mechanical mixing and velocity accommodation at the pad-disc interface. Dynamics of particle interaction were visualized by showing rotation angles and velocity vectors. The model provided information on the development of plastic deformation for metal-on-metal contacts and on crack formation at graphite lamellae of cast iron disc. Results are in agreement with conventional friction theories.

The friction dynamics in the boundary layer of a brake system is closely linked with the growth and the destruction of typical structures on the brake pad. These processes are caused by an interaction of friction and wear and determine the (as well measured) time-dependence of global parameters, such as the friction coefficient μ or the temperature. Based on the simulation with a set of differential equations, this equilibrium of flow has also already been described with a cellular-automaton-discretization. This paper will show new results and fundamental conclusions with respect to the surface topography dynamics and the friction behaviour of a brake system.

Tribological behavior of stir-cast $Al-Si/SiC_p$ composites against automobile brake pad material was studied using Pin-on-Disc tribo-tester. The Al-metal matrix composite (Al-MMC) material was used as disc, whereas the brake pad material forms the pin. It has been found that both wear rate and friction coefficient vary with both applied normal load and sliding speed. With increase in the applied normal load, the wear rate was observed to increase whereas the friction coefficient decreases. However, both the wear rate and friction coefficients were observed to vary proportionally with the sliding speed. During the wear tests, formation of a tribo-layer was observed, presence of which can affect the wear behavior, apart from acting as a source of wear debris. Tribo-layer formed over the worn disc surfaces was found to be heterogeneous in nature. Morphology and topography of worn surfaces and debris were studied using scanning electron microscope (SEM). Chemical composition of different wear products was obtained using electron probe micro analyzer (EPMA) and X-ray diffraction (XRD) techniques. Possible wear mechanisms operative in Al-MMC-brake pad tribo-couple have been discussed.

In brake systems the surface topography of the brake pad is characterized by an equilibrium of flow of hard patches, which are built up and destroyed, depending on the load history. Since these patches carry the main friction power, the friction coefficient itself depends on the load history, too. So the friction coefficient has to be described by differential equations instead of algebraic equations, connecting intimately the friction- and wear process. With these dynamic dependencies we are able to build up a new model, using the technique of Cellular Automata, which gives a dynamic and detailed insight into the brake pad's surface topography during the friction process.

The surfaces of a brake pad and rotor were investigated after a run-in period during which a stable coefficient of friction had developed. The Focused Ion Beam Technique (FIB) was used to reveal tribologically induced surface films and for cross-sectional preparation of superficial layers. Additional information was obtained by TEM/EDS of thin lamellae prepared with the FIB and by surface analytical methods (GDOS, XPS and RS). Microscopic contact areas of the pad showed bright contrast in Scanning Ion Microscopy (SIM). This was attributed to severe plastic deformation finally leading to a nanocrystalline microstructure. Metallic particles of the pad, the so-called primary contact areas, were mostly covered with a smooth oxide layer of less than 1 μm thickness. Above this layer a thin (100 nm) partly amorphous film was often observed. The film was not only restricted to the metal particles, but also spread over adjacent regions, suggesting that secondary plateaus had formed. Similar layers and films, although with slightly different composition and structure, were also observed at the surface of the rotor.

Friction materials for car and truck disc brake pads are typically complex composites containing metals, ceramics, and polymers that are designed to provide stable frictional performance over a range of vehicle operating conditions, exhibit acceptable wear life, and produce minimal noise and vibration. Despite the fact that brakes must operate under a variety of environmental conditions, most standardized and developmental laboratory tests for brake materials are conducted under dry sliding conditions. Studies of wet braking behavior are rare in the tribology literature. The present work investigated the frictional behavior of commercial truck brake pad materials sliding on gray cast iron simulated brake discs in the presence of water films. An instrumented sub-scale testing machine equipped with a water spray was used. There was a significant effect of water films on reducing pad/disc friction compared with dry tests on the same materials and at the same sliding speeds. The friction coefficient under water spray conditions was inversely proportional to the square of the sliding speed. The reduction of friction with increasing speed was principally due to hydrodynamic effects, although the water spray also reduced frictional heating and changed the nature of the transfer film formation on the cast iron contact surface. Frictional recovery occurred within seconds after the water spray was removed.

Despite the enormous amount of testing and development of automotive brakes and brake pad materials, rather little is known about their tribological contact situation on a microscopic level. The sliding of an organic brake pad against a grey cast iron rotor is very different from most tribological systems. When worn against the rotor, the complex structure and very inhomogeneous composition of the pads results in a particular surface structure, with large contact plateaus rising a few micrometers above the rest of the surface [Wear 232 (1999) 163; Wear 2000 (submitted for publication); Proceedings of the Nordtrib 2000, vol. 2 (2000), 358]. The present investigation involves a more comprehensive study of the formation, mechanical properties and composition of the tribological surfaces of such pads, using high resolution scanning electron microscopy, nanoindentation, energy dispersive X-ray analysis and three-dimensional profilometry using white light optical interferometry. The observations are discussed and correlated to observed friction phenomena.

This paper describes a new type of electromagnetic multiple-disc brake for controlling braking torque. The brake consists of a rare-earth permanent magnet, an electromagnet and multiple friction discs. The friction torque is controlled by the electric current in a coil of the electromagnet. In the analysis the design formulae for the brake are first derived from electromagnetic theory and tribological considerations, then the transient braking torque under the electric current control is derived and vibrations of the disc considered. To verify the present system and the analysis, experimental tests were made on the brake.

Cu is required to be abated in brake pads due to its toxicity. There are on the market several Cu-free brake pads. These Cu-free brake pads are only evaluated regarding their friction and wear performance, whereas, their airborne particle emissions are not considered. A pin-on-disc tribometer is used to evaluate the friction, wear and airborne particle emission from two Cu-free commercial brake pads used in the Europe. Moreover, a commercial brake pad containing Cu is evaluated as a reference. The results indicate that Cu-free brake pads yield comparable coefficient of friction as the Cu-contained brake pad. All three brake materials result in similar wear to the mating brake rotor. Cu-free brake pads generate more airborne particles than Cu-contained brake pad.