Commercial Friction Materials Research Articles

Tribological Behavior of Friction Materials Containing Aluminum Anodizing Waste Obtained by Different Industrial Drying Processes

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.

A novel path towards limiting non-exhaust particulate matter emissions of a commercial friction material through the addition of metallurgical slag

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.

Effect of the Addition of Waste Generated during the Plasma Cutting of Aluminum Products on the Friction and Emission Properties of a Commercial Friction Material Formulation

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.

Study on the effect of the addition of bulk and exfoliated graphitic carbon nitride on the dry sliding behavior of a commercial friction material formulation through pin on disc and subscale dynamometer analysis

A novel analysis was conducted on the friction and wear properties of a commercial friction material formulation with the addition of bulk (named: gCN) and exfoliated (named: TEX6) graphitic carbon nitride in varying quantities: 4.5, 9, 13.5, and 18 wt%. The analysis was two-fold. In the first part, the samples were tested in the form of pins on a pin on disc testing equipment. The typical trend was that the friction coefficient magnitude increased with the addition of gCN. On the other hand, the friction coefficient decreased with the TEX6 addition. The pin wear reduced with the addition of gCN until 9 wt%. Beyond this amount, an increase in pin wear was observed. The TEX6 samples generally noted a decrease in pin wear with the increase in its content. An improvement in the characteristics of the secondary plateaus was seen with the increase in both gCN and TEX6 content. Through this study, the 13.5 wt% addition of both gCN and TEX6 was considered to display permissible friction and wear magnitude, and desirable extension and compaction of secondary contact plateaus. In the second part of the study, the formulation with 13.5 wt% of gCN and TEX6 was tested on a subscale dynamometer to replicate a real-life braking scenario. The friction and wear magnitudes were similar to the pin on disc study and the extension of the secondary contact plateaus was better than the virgin formulation. This study provides a complete analysis of the feasibility of the inclusion of g-C3N4 variations in automotive braking applications.

Mechanical and tribological properties of a WC-based HVOF spray coated brake disc

Abstract Thermally sprayed WC-based coating systems are generally used to increase engineering components’ wear resistance. This research presents the results of a comparative study conducted to examine the tribological characteristics of a new brake disc created with thermal sprayed WC-10Co4Cr coating. This study implemented braking tests on the WC-10Co4Cr coated brake disc against commercial friction material on a laboratory scale disc-pad tester and compared the findings with the reference disc. The coating system was created with a high velocity oxygen-fuel spraying on a cast iron disc. The braking tests followed the sections of the SAE-J2430 test procedure. The microstructure, phase composition and properties of the coating were characterized by SEM/EDS, XRD and Vickers microstructure. The effect of carbide coating on the friction and wear behaviour of the tribological system was examined. The WC-10Co4Cr coating has increased the hardness by 3 times with a thickness of 300 μm compared to the cast iron surface. The amount of wear in the coated disc (CD) was reduced by about 85% compared to the reference disc (BD). The coefficient of frictions of BD and CD was found to be in the range of 0.43–0.61 and 0.47–0.62, respectively. The inclusion of hard phases (WC and W2C) in the disc coating played an important role in improving the sliding wear resistance by maintaining the coefficient of friction of the brake disc at an acceptable level. In brief, carbide coating could be claimed to be promising for challenging braking implementations.

Microstructural and Tribological Evaluation of Brake Disc Refurbishing Using Fe-Based Coating via Directed Energy Deposition

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.

Research of friction materials applicable to the multi-disc brake concept

This manuscript is focused on the investigation of tribological and thermodynamic parameters of the coated C45 steel substrate. A metallic coating of the WC-Cu base was applied on the C45 steel substrate using electro-spark deposition (ESD) coating technology. Three types of experimental WC-Cu coatings with different ratio of chemical elements were deposited on the surface of the C45 steel substrate using WC-Cu electrodes produced by the powder metallurgy route. Each sample had a different ratio of WC-Cu chemical elements. Subsequently, the friction factor (f) and the total wear of the sample were examined in a tribological laboratory. The objectively most suitable sample in terms of friction factor and service life was subjected to a braking simulation in Matlab® software together with three other commercial friction materials. A mathematical model of Newton's equations of motion was used to simulate braking. The last phase of the research was focused on the thermodynamic parameter during automatically guided vehicle (AGV) braking. The tribological results of tested coatings showed that the most suitable combination of chemical elements in the WC-Cu type ESD coating is the WC50–Cu50 ratio. The simulation of non-stationary heat conduction process in the brake system revealed that the most suitable thickness of the steel brake disc should be at least 8.2 mm. Ultimately, this research will help the designer to choose the right procedures for designing a multi-disc brake system.

Pin-on-disc study of dry sliding behavior of Co-free HVOF-coated disc tested against different friction materials

The dry sliding behavior of three commercial friction materials (codenamed FM1, FM2, and FM3) tested against a Co-free cermet coating produced by high-velocity oxy-fuel (HVOF) on gray cast-iron discs is investigated. FM1 is a conventional low-metallic friction material, FM2 is developed for using against HVOF-coated discs, and FM3 is a Cu-free friction material with a low content of abrasives and a relatively high concentration of steel fibers. For the tribological evaluation, they are tested on a pin-on-disc (PoD) test rig against Co-free HVOF-coated discs, with particular attention to the running-in stage, which is fundamental for the establishment of a friction layer between the two mating surfaces, i.e., the pin and disc. The PoD tests are performed at room temperature (RT) and a high temperature (HT) of 300 °C. At RT, all materials exhibit a long running-in stage. At HT, no running-in is observed in FM1 and FM2, whereas a shorter running-in period, with respect to the RT case, is observed in FM3 followed by the attainment of a comparatively high coefficient of friction. At RT, the pin wear is mild in all cases but severe at HT. FM3 shows the lowest wear rate at both temperatures. Moreover, the coated disc shows no wear when sliding against the FM3 friction material. All the results are interpreted considering the microstructural characteristics of the friction layers formed on the sliding surfaces. The findings of the present study provide insights into reducing wear in braking system components and hence reducing environmental particulate matter emissions from their wear, through the use of disc coatings.

Friction elements based on phenolic resin and slate powder

Braking performance is dependent on friction materials of tribological components, generally metal and pad. The pad consists of a composite of thermosetting resin and reinforcement material that exhibits mechanical and wear resistant, heat conduction, lubricant, and others. Slate is a natural rock composed of extremely fine materials that brings interesting tribological properties, in addition, its industrial current activity generates a significant amount of mineral waste, which is a problem for the environment. The objective of this work was to propose a technological alternative as a new friction element based on slate particulate as tribological reinforcement in composite based on phenolic resin as a matrix, besides that, minimize the environmental impact due to the inadequate disposal of these slate tailings and add value to the mineral, classifying it as a residue to be used industrially and no longer as waste. In this sense, the effect of the amount of slate on the friction properties of the brake pads made was investigated. As the brake pad material, four different slate containing formulations were proposed, manufactured and analyzed, and as the brake wheel gray iron was chosen. The friction and wear characteristics were determined by Pin-on-Disk type tribological tests, the pair was composed by disk (pad formulations) and pin of gray iron, representing the brake wheel. The coefficients of friction of the composites were shown to be regular and stable, with an average of 0.44 between the samples. Among the results obtained, the formulation containing 40 % of slate and 35 % of phenolic resin, presented the most satisfactory parameters compared to commercial friction materials in current use.

Dry‐braking properties of resin based friction materials by different fibres

AbstractIn this study, the cashew nut shell oil modified phenolic resin based friction materials reinforced by aramid pulp/glass/carbon fibres (constant content 17 vol.%) are prepared by hot pressed sintering method, and the effect of different fibres on tribological properties of resin matrix materials is studied. The tribological properties of friction materials are carried out by a ring‐on‐block tribo‐tester sliding against AISI 51200 steel ring under different braking loads and speeds. The friction coefficient and wear rate increase with the increase of load and speed. The synergistic effect of aramid pulp fibre and carbon fibre improves the tribological properties of friction materials. However, the friction coefficient and wear rate increase as the glass fibre increases. The specimen with 9%/1%/7% of aramid pulp/glass/carbon fibres shows more reasonable friction coefficient (0.31–0.50) and lower wear rate (4.5–7.9 × 10−6 mm3 N−1 m−1) than that of commercial friction material.

The effect of contact area on velocity weakening of the friction coefficient and friction instability: A case study on brake friction materials

Newly found evidence on the negative slope of the friction-velocity curve is described in terms of the contact area on the surface of brake friction materials. The tribotests were performed using two commercial friction materials with and without steel fibers. The results showed that velocity weakening friction was pronounced in the friction materials containing steel fibers, which facilitated high pressure contact areas by the agglomeration of wear debris. The size of the high pressure contact area increased as the sliding velocity decreased, which supported the correlation among velocity weakening friction, friction instability, and high pressure contact areas. The pronounced velocity dependence of the friction coefficient due to transient contact area was highly relevant to noise propensity of the friction material.

A critical comparison of dynamometer data with pin-on-disc data for the same two friction material pairs – A case study

Friction and wear behavior of brake materials is often investigated using Pin-on-Disc (PoD) or dynamometer tests. Dynamometer tests reproduce the real braking conditions since they are carried out on real parts. whereas PoD tests are carried out using small pins, under constant sliding velocity and contact pressure. In the present technical note, the friction and wear behavior of two commercial friction materials, a low-met and a non-asbestos organic (NAO) material, dry sliding against a pearlitic cast iron counterface disc was investigated. The PoD tests were carried out at room temperature and at 300 °C; the dynamometer tests were conducted simulating urban braking conditions. The experimental data were compared and the main wear mechanisms were identified. The wear mechanisms observed with the PoD at room temperature turned out to be similar to those observed with the dynamometer tests. On the other hand, the PoD tests carried out at 300 °C revealed particular features in the friction layer that were not detected in the dynamometric tests. As far as the tribological data are concerned, the average friction coefficients recorded during the dynamometric tests were similar to the values recorded in the PoD tests, whereas the wear coefficients were one order of magnitude lower, most probably due to the limited area of contact of the pins.

Effect of roughness on the wear behavior of HVOF coatings dry sliding against a friction material

WC-CoCr coatings on cast iron discs were prepared with different surface roughness and submitted to dry sliding wear tests against a commercial friction material using a pin-on-disc configuration. The effect of roughness on the friction and wear of the tribological system was evaluated. The pin wear rate increases with the coating roughness and becomes very high for an arithmetic average roughness, Ra, in excess of 1 μm. In contrast, the friction coefficient was found to increase as roughness decreased. The discs wear was anyway negligible. The experimental results were explained considering the characteristics of the friction layer that forms during sliding at the interface of the mating bodies in dependence of the initial roughness of the coatings. It is shown that the friction layer plays an important role in determining the relative contribution of the abrasive and adhesive interactions and, thereby, the resulting friction and wear behavior of the tribological system.

Stiffness distributions of brake pad friction materials using static and dynamic measurement techniques

The elastic properties of a friction material play a significant role in the vibro-acoustic noise, vibration and harshness behaviour of braking systems on a structural scale. Common testing, such as pad compressibility testing and modal testing, allows identification of the global static and dynamic elastic properties respectively, which generally provide data for finite element numerical models. The present investigation presents static and dynamic testing methodology and focuses on the distribution of the elastic properties of friction materials on a pad scale. Besides trying to determine a correlation with bench testing, the added value in updating friction material models with distributed elastic properties for squeal simulation models is evaluated. For investigation of the distribution of the friction material properties of brake pads, quasi-square friction samples of friction materials were collected from several locations of the pad. The experimental meshing provides data on the inner radius and the outer radius of the brake pads as well as on the inlet and the outlet of the pad. For the purpose of this study, six different materials were tested. In addition to four known prototype friction material formulations obtained using the same production process, two commercial friction materials were investigated. The static stiffness is evaluated with a laboratory compression machine set-up whereas the dynamic stiffness is identified from laboratory ultrasonic investigations. The results and coherence of the static and dynamic stiffness distributions obtained by both approaches are compared. The trends with the noise, vibration and harshness bench testing results are also discussed for selected pads. From ultrasonic testing, it is found that the dynamic stiffness distribution can take several forms (flat, concave or convex) which can be different for the in-plane stiffness and the normal stiffness of the pad. It is mainly noticeable along the tangential direction of the pad, reaching a variation of up to 16% between middle of the pad and the edges. The stiffness can also vary by up to 8% along the radial direction of the pad. The measurements performed during this study confirm also that these stiffness distribution patterns are strongly dependent on the formulation friction of the material and on the production process itself. The above-mentioned stiffness distributions were implemented in finite element brake pad models to estimate the potential frequency shift arising from these considerations. The impact of taking into account such information was shown to be limited for the free–free pad modal properties. Nevertheless, the influence of such distributions on the complex eigenvalue analysis of complete finite element brake models was observed and is discussed. Even if the used pad models take into account to some extent the static and dynamic stiffness distributions for complex eigenvalue analysis, they still do not agree with the non-linear material models regarding the stress and the frequency, as shown experimentally. Even if an additional effort is made to determine these stiffness distributions which provide more detailed information for the brake pad finite element models, this physical degree of freedom could drive, to a certain extent, the noise, vibration and harshness responses of the brake systems. A more mature understanding of the influences of the formulations and the process on these distributions and their control at the pad design stage could add a complementary strategy for reduction in the noise, vibration and harshness issues in brake systems.

Friction-induced stick-slip intensified by corrosion of gray iron brake disc

The effect of brake disc corrosion on friction-induced stick-slip was studied to find the possible causes of friction instability in humid conditions. The friction and wear characteristics of gray iron discs and a commercial friction material were examined using a 1/5 scale dynamometer. Use of the corroded discs resulted in a higher friction coefficient, and a larger oscillation amplitude of the brake torque. Disc corrosion increased the critical velocity showing transition from steady sliding to stick-slip. This suggests a rapid increase of the initial static friction coefficient as a function of dwell time in a humid condition, which is supported by the increased hydrophilicity of the friction films. The contact stiffness of the friction material surface was increased after rubbing with corroded discs, due to densification of the friction films by iron oxide nanoparticles transferred from the disc surface. The wear rate of the friction material also increased with corrosion of the gray iron discs.

Friction-induced intermittent motion affected by surface roughness of brake friction materials

The effect of the surface roughness of the brake friction material on friction-induced instability was studied. Commercial friction materials with different surface roughness levels were tested using a scale brake dynamometer to find a correlation between the surface roughness and friction oscillation. The results showed that the normal contact stiffness was strongly affected by surface roughness and played a significant role in determining the friction oscillation pattern. The friction force oscillation changed from stick-slip to sinusoidal oscillation and steady sliding as the sliding velocity increased, and the velocity ranges for different oscillatory patterns were changed by the surface roughness. A smooth surface with glazed patches produced friction oscillation with larger amplitudes, and the oscillation was maintained over a wider range of sliding velocities due to the high contact stiffness. The correlation between the contact stiffness and surface roughness suggested that friction-induced instability might be avoided by designing the friction material to have low surface stiffness.

Reduction of Noise from Disc Brake Systems Using Composite Friction Materials Containing Thermoplastic Elastomers (TPEs)

Attempts have been made for the first time to prepare a friction material with the characteristic of thermal sensitive modulus, by the inclusion of thermoplastic elastomers (TPE) as viscoelastic polymeric materials into the formulation in order to the increase the damping behavior of the cured friction material. Styrene–butadiene–styrene (SBS), styrene–ethylene–butylene–styrene (SEBS) and nitrile rubber/polyvinyl chloride (NBR/PVC) blend system were used as TPE materials. In order to evaluate the viscoelastic parameters such as loss factor (tan δ) and storage modulus (E′) for the friction material, dynamic mechanical analyzer (DMA) were used. Natural frequencies and mode shapes of friction material and brake disc were determined by modal analysis. However, NBR/PVC and SEBS were found to be much more effective in damping behavior. The results from this comparative study suggest that the damping characteristics of commercial friction materials can be strongly affected by the TPE ingredients. This investigation also confirmed that the specimens with high TPE content had low noise propensity.

Tribological Properties of Polymer Composites Containing Barite (BaSO4) or Potassium Titanate (K2O · 6(TiO2))

Tribological properties of particulate barite (BaSO4) and potassium titanate (K2O·6(TiO2)) whiskers, the two major ingredients currently used for commercial brake friction materials, were investigated. A novolac resin was used as a binder for test specimens and a block-on-disk tribometer was used to assess friction characteristics of the two ingredients. Experimental results showed that the BaSO4-filled composite produced large frictional oscillations and created severe damage on the gray iron counter surface, while the composite filled with the same amount of K2O·6(TiO2) whiskers showed smooth sliding without large friction force fluctuation. The cause of the different frictional behavior was investigated by considering stick-slip and mechanical properties of the composites, which was largely based on the morphology of the two ingredients and their role in reinforcing the composite. The results from this comparative study suggest that the friction characteristics of commercial friction materials can be strongly affected by the two ingredients, which have been considered as minor constituents for brake performance.

Characteristics of wear particles produced during friction tests of conventional and unconventional disc brake materials

Recent attention to the improvement of line-haul truck brake effectiveness, to compensate for lowered aerodynamic drag and to increase road safety, has prompted the US Department of Energy to support several projects aimed at the development of lightweight, high-performance friction materials. Using a specially designed, sub-scale disc brake testing system, a series of experiments was conducted to study the friction, wear, and frictional heating characteristics of both conventional and unconventional candidate disc brake materials. The selected sliding speed (11.0 m/s) was comparable to that experienced by a commercial disc brake surface on a truck travelling 60 miles/h (96.6 km/h). Material combinations included a commercial friction material on a gray cast iron disc, a commercial friction material on an aluminum metal matrix composite disc, a commercial friction material on a cast iron aluminide (Fe 3Al) alloy disc, and a carbon/silicon carbide material pad on a disc of similar composition. An adhesive extraction method was used to remove the loose wear particles from the surfaces of the test discs. The characteristics of brake material wear particles differed significantly between the four sliding material combinations studied and are correlated with both surface roughness changes and wear modes.

Optimization of an Industrial Friction Material Formulation

A Golden Section optimization approach coupled with relational grade analysis was proposed to optimize friction material formulations. It was applied to enhance friction coefficient (μ)for a commercial friction material formulation (IFC) consisting of copper chips, steel wool, Al2O3, styrene-butadiene rubber (SBR), graphite, coke, CaCO3, ZrSiO4, BaSO4 and phenolic resin using the Friction Assessment and Screening Test (FAST). The IFC was used in a popular, commercially available car and it has an average μ of 0.372 and a total wear of 5.70 wt. %. All raw materials used were divided into a metals group, a non-metals group and a binder group. The optimization process was performed in three phases. In phase 1, the optimal proportion of the ingredients within the metals group and the non-metals group was searched. In phase 2, the optimal proportion between the metals group and the non-metals group was investigated and in phase 3, the optimal proportion between the binder group and the metals plus non-metals groups was determined. After 19 experiments, an alternative friction material formulation using identical constituents but optimized was obtained. This friction material exhibited an average μ of 0.535 and a total wear of 6.24 wt. %. Presented at the 57th Annual Meeting Houston, Texas May 19–23, 2002