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Magnetic fraction isolated from steel furnace slag was tested as a component of Cu-free friction composites. The friction–wear performance and production of wear particles during their testing using a pin-on-disc tester against a cast iron disc were evaluated. To compare the effect of the magnetic fraction on the parameters studied, the composite with alumina and the composite with original steel furnace slag were also prepared and tested. All composites showed a comparable friction coefficient. The composite with original steel furnace slag, and the composite with a magnetic fraction showed higher wear resistance compared to the composite containing alumina. The positive effect of the magnetic fraction on the extent of the emission of wear particles was observed and explained by the decreased aggressiveness of this composite to the cast iron disc. The influence of the phase composition of the steel furnace slag and the magnetic fraction on the friction film formation was also indicated, and its effect on the production of wear particles was proposed.

This study aims to investigate the outcomes of carbonaceous products, derived from the decomposition of the components of vehicular brake materials, under high-temperature wear tests. Pin-on-disc (PoD) wear tests were conducted by using cast iron discs against pins made of commercially available low-steel friction material. Tests were carried out at different temperatures: 155 °C, 200 °C, 250 °C, and 300 °C. The characterization of the sliding plateaus on worn pin surfaces was based on X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. It was noted that at temperatures above 200 °C, the thermal degradation of the inorganic resin, used as a material binder, occurs. An interesting observation was recorded at 300 °C; the brake pin material’s friction curve showed higher stability despite having an excessive wear rate. However, the brake pin’s specific wear coefficient was higher at this temperature than was observed in the other friction tests. A detailed study of the friction plateaus on the worn-out pins at 300 °C revealed that the decomposed carbon resin product, i.e., the distorted graphite, was widespread over the surface of the pin. Lubricant stabilization can be expected, as established by the observed values of the coefficient of friction (CoF), retaining values within the 0.4–0.6 range, even at high temperatures. Other friction material components may have contributed to the formation of this ubiquitous carbonaceous interface film.

Abstract Keeping recycling and the circular economy in mind, this study explores the reduction in emission of a highly optimized, commercially employed friction material formulation through the addition of metallurgical slags from a basic oxygen furnace in varying quantities from 6 to 38 wt%. The various compositions were paired with a pearlitic grey cast iron counterface and tested on a pin on disc tribometer. The friction coefficient and pin wear increased with the slag addition but were still within the permissible limit when compared to the original formulation. Specimens with higher slag content observed extremely compacted and extended secondary contact plateaus, which also recorded significant slag presence. The extended plateaus detached in the form of chunks from the mating surfaces, which settled on the equipment hardware and restricted the production of airborne particles. From an industrial symbiosis perspective, the addition of metallurgical slags proved to be a promising way of obtaining green friction materials with reduced emissions.

With sustainability dominating the industry, recycling the generated waste from different processes is becoming increasingly important. This study focuses on recycling waste generated during aluminum anodizing waste (AAW) in friction material formulations for automotive braking applications. However, before utilization, the waste needs to be pre-treated, which mainly involves drying. Hence, four different industrial drying methods were studied to dry the AAW, and the corresponding characteristics were observed by evaluating its residual humidity and crushability index. The waste powders were further characterized using FT-IR and SEM/EDXS to understand their constituents. The initial analysis showed that the waste subjected to the drying process P2 and P1 with the lowest final humidity fetched the most desirable results, with P1 having the simpler drying procedure. The AAW powders were added in a commercial friction material formulation at 6 and 12 wt.% and subjected to friction, wear, and non-exhaust particulate matter analysis. The worn surfaces were analyzed using SEM/EDXS evaluation to understand the extension and composition of the deposited secondary contact plateaus. It was seen that the 12 wt.% addition of waste processed using the P1 technique provided the most satisfactory friction, wear, and emission characteristics, along with expansive secondary contact plateaus with a good contribution of the waste in its formation. This study showed a good relationship between the processing method and a formulation’s tribological and emission characteristics, thereby paving the way for using this drying method for other waste requiring pre-treatment.

Abstract The need to reduce non-exhaust particulate matter emissions is of paramount importance as they pose repercussions on human lives and the environment. In this study, a novel way to limit emissions is proposed based on the minimization of the vibration of the mating bodies. Two model friction material formulations were tested in the form of pins and paired with a pearlitic grey cast iron disc counterface in a laboratory pin on disc apparatus. To reduce the vibrations, a damping tape was wrapped around the pins. With the damping of vibration, a significant drop in the emissions was recorded, and this was correlated with the friction layer establishment during sliding, which observed low disruption. It is believed that the use of this method for reducing emissions can accompany the optimization phase of the brake squeal noise of friction materials, thereby, providing new design perspectives.

In this study, the potential of Fe3Al coating material as an environmentally friendly alternative to coatings containing critical elements for brake discs was investigated. A buffer layer of Cr–Mo steel (Ferro 55) that was about 500 µm thick was applied on a gray cast iron disc to enhance the coating quality and prevent the formation of hot cracks during solidification. The microstructural analysis of the cross-section of the coating showed that the buffer layer diffused into the Fe3Al coating, forming a combination of Fe3Al, Fe, and Fe3AlC0.5 phases. The tribological properties of the Fe3Al-coated disc were evaluated using pin-on-disc tests against two different copper-free friction materials extracted from commercial brake pads. The wear results show a coefficient of friction comparable to that of an uncoated disc (≈0.55), but with a reduction in particulate matter (PM) emissions, which decreased from 600 to 476 #/cm3. The last issue is an interesting aspect that is gaining increasing importance in view of the upcoming international standards.

With recycling and reuse in mind, this study explores the possibility of the utilization of waste generated during the plasma cutting operations to produce aluminum products in a commercial friction material formulation for car braking applications. The as-received waste, analyzed in the form of a powder, was subjected to various characterization techniques to obtain its composition and phase constituents. The friction, wear, and emission analyses were organized in two ways: the first part focused on the selection of the most suitable waste content in the friction material formulation—10, 15, and 20 wt.%—tested in the form of pins, using a pin-on-disc tribometer. The second part evaluated the most suitable content in the formulation in the form of coupons on a subscale dynamometer to comprehend its friction and emission characteristics in a more complex scenario. Through the pin-on-disc analysis, it was seen that the 10 wt.% of waste in the formulation provided permissible friction and wear behavior, and similar emission magnitude and secondary plateau characteristics were seen, as compared to the reference composition. Similar results were recorded on the subscale dynamometer at mild braking conditions. At ‘severe’ testing conditions, a different trend was observed. Through this analysis, the possibility of the addition of this kind of waste in friction material composition has been evaluated deeply, paving a path for further analysis and possible implementation.

The aim of our research was to indicate the suitability of metallurgical slags (two blast furnace slags and one steel furnace slag) as the components of Cu-free friction materials. The base mixture consisted of nine components including phenolic resin, graphite, tin sulphide, steel and aramid fibers, iron powder, a mixture of barite with calcite, and vermiculite. To this base mixture, the slags with a particle size below 0.1 mm were added individually in the amount of 20 wt.%. A base friction mixture with alumina in the amount of 20 wt.% represented the reference. Samples for the friction-wear tests were produced in the form of pins by hot press molding. The prepared pins were tested using a pin-on-disc tester in a drag mode at the pressure of 1 MPa and a constant sliding speed of 1.51 m/s for 90 min. The samples with slags exhibited slightly lower values of steady-state friction coefficient compared to the reference composite with alumina, and at the same time produced lower wear particle emissions. The particle concentration was reduced for the samples with slowly cooled blast furnace and steel furnace slag. The results obtained indicated steel furnace slag as a promising component of Cu-free friction composites.

In this paper, the refurbishing of worn-out brake discs by coating with powder via direct energy deposition (DED) is evaluated. A medium carbon steel powder was used to coat cast-iron discs. Deposition of this steel was carried out directly on the disc surface or, alternatively, after a previous deposition of a buffer layer made of stainless steel. It was seen that the use of a buffer layer ensured a good coating adhesion, despite the formation of cast microstructures at the interfaces between the disc and the two different coatings (buffer and outer layer). Coated discs were tested against two different Cu-free commercial friction materials to evaluate their tribological properties. Very similar friction coefficients, specific wear rates, and total emissions were measured for both friction materials sliding against the coated disc. These tribological data are slightly higher with respect to those obtained in the case of an uncoated disc, suggesting that improvement of the top coating composition and surface finishing is necessary in order to achieve better performances.

Dry sliding wear tests and corresponding particulate matter (PM) analysis were conducted on a newly developed SiC-graphite-based composite friction material, paired with two types of HVOF counterface/discs: WC-CoCr and WC-FeCrAlY coatings, with a conventional martensitic stainless steel counterface as a reference. The trials were conducted on a pin-on-disc testing equipment at room temperature and a constant sliding velocity and contact pressure of 7 m/s and 0.5 MPa, respectively. The coefficient of friction (CoF) curves with the uncoated disc exhibited considerable fluctuations. On the other hand, the coated discs featured an increase in the CoF at the beginning of the tests, followed by either a continuous reduction until the end of the testing duration or the attainment of a steady state regime. The pin wear and emissions with both coatings were appreciably lower when compared to the trials with the uncoated disc. The evaluation of the friction layer observed a significant contribution of the counterface for all the pairings. The PM analysis was conducted on the particles that were lying in the range of 10 μm and 2.5 μm on a scanning electron microscope (SEM), and particles from 2.5 μm and 1 μm on transmission electron microscope (TEM), with an emphasis on the particles that were detached from the pin surface and friction layer to explain the wear mechanisms for each pairing. Through this, the need for the proper selection of both friction material and counterface to avoid the emission of harmful compounds in the environment was highlighted.

This study compares the effect of the addition of two types of lubricants on the dry sliding behavior of a simplified Cu-free phenolic resin-based composite material. The lubricants were commercial graphite and exfoliated graphitic carbon nitride (codenamed: TEX6). The graphite particles were rounded and of ‘flaky’ character. The TEX6 particles were not only flaky, but also irregular in shape, and ‘fluffy’. Both lubricants were added individually in the basic formulation and subjected to dry sliding tests on pin-on-disc testing equipment in mild conditions and against a grey cast-iron counterface. The tests with TEX6 observed a stable steady state in the friction coefficient (CoF) with lower scatter and lower average friction coefficient and pin wear magnitude when compared to samples containing graphite. Additionally, the worn surfaces of the TEX6-containing samples had extremely smooth, compact, and continuous secondary plateau coverage when compared to the graphite-containing samples. The counterface paired with the TEX6-containing samples observed much lower abrasive action compared to the graphite-containing samples. Through the wear testing and further evaluation of the secondary plateaus, the possible addition of TEX6 as a lubricant in friction material composition was explained, making it a promising component for automotive braking applications.

In this study, graphitic carbon nitride (g-C3N4, labelled as gCN) was tested in the formulation of copper-free (Cu-free) friction mixtures, which are potentially interesting for brake pad manufacturing. Three formulations of friction composites were prepared starting from a common Cu-free master batch: (i) without graphite, (ii) with graphite and (iii) with gCN. The mixtures were pressed in the form of pins by hot-press moulding. The friction-wear performance of the prepared pins was investigated using a pin-on-disc (PoD) test at room temperature (RT), high temperature (HT) (400 °C) and, again, at room temperature (H-RT). The values of the friction coefficient (µ) for the composites with gCN (or graphite) were as follows: (i) RT test, µRT = 0.52 (0.47); (ii) HT test, µHT = 0.37 (0.37); (iii) RT after the HT tests, µH-RT = 0.49 (0.39). With respect to wear resistance, the samples with graphite performed better than the samples without this solid lubricant. To the best of our knowledge, this is the first report regarding the evaluation of the role of gCN in friction composites designed for automotive brake lining applications. The results indicate the main role of gCN as a soft abrasive.

The tribological behavior and the related airborne particles emission of three copper-free automotive friction materials are investigated. The tests were conducted using a pin-on-disc tribometer equipped with a specifically designed clean-enclosure chamber for the emission measurement. Particle number concentration from particle size 0.3 µm up to 10 µm and the mass of emitted particles between 1 µm to 10 µm were measured. Particular emphasis was given to the chemical composition of the bulk materials, the friction layers and the emissions, in order to explain the acting wear mechanisms, and the recorded emission of airborne particles. The results indicate that the recorded emissions do not correlate with the friction coefficient and the wear rates, since the wear mechanisms exert a different influence on the tribological and emission behavior of the materials under study.