Size, Shape, and Elemental Composition of Airborne Wear Particles from Disc Brake Materials
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- Wahlström, J., Olander, L. & Olofsson, U. Tribol Lett (2010) 38: 15. doi:10.1007/s11249-009-9564-x
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During braking, both the rotor and pads experience wear, generating particles that may become airborne. In field tests, it is difficult to distinguish these particles from others in the surrounding environment, so it is preferable to use laboratory test stands to study the amount of airborne wear particles generated. The purpose of this work is to investigate the possibility of separate, capture, and analyze airborne wear particles generated by a disc brake in a disc brake assembly test stand. This test stand used allows the cleanliness of the air surrounding the test specimens to be controlled and thus the airborne portion of the wear particles to be studied separately. One pair each of low-metallic (LM) and non-asbestos organic (NAO) brake pads was tested against grey cast iron rotors. Before testing, the elemental contents of the brake materials were analyzed using glow discharge optical emission spectroscopy (GDOES). The concentration and size of airborne wear particles were measured online during testing. In addition, airborne wear particles were collected on filters during the tests and afterward analyzed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The analyzed wear particles contained elements such as iron, titanium, zinc, barium, manganese, and copper. Both the low-metallic and non-asbestos organic type of brake pads tested display a bimodal size distribution with peaks at 280 and 350 nm. Most of the airborne particles generated have a diameter smaller than 2.5 μm.
KeywordsWearAirborne particlesDisc brakeSEMEDXGDOES
Many studies have demonstrated an association between the adverse health effects and the concentration of airborne particles in the atmosphere [1–3]. Therefore, limit levels have been set for PM2.5 and PM10, i.e., particles with aerodynamic diameters less than 2.5 and 10 μm, respectively. These limits have been established by the European Commission in the European Union and by the Environmental Protection Agency in the United States. Particles with a diameter of D can be divided into a coarse fraction (2.5 μm < D < 10 μm), a fine fraction (0.1 μm < D < 2.5 μm), and an ultrafine fraction (D < 0.1 μm). Research suggests that particle size is an important factor influencing how particles are deposited in the respiratory tract and affect human health [2, 3]. Most coarse particles are deposited in the nose and throat, while fine and ultrafine particles can generally penetrate deep into the lung. One way particles affect health is believed to be by inflammation, while another is by forming so-called free radicals in the body’s cells. These free radicals are very reactive and can ultimately cause damage to DNA due to oxidative stress . Oberdörster et al.  recommend a number of key factors (i.e., size distribution, agglomeration state, shape, crystal structure, chemical composition, surface area, surface chemistry, surface charge, and porosity) that are important when investigating the toxicity of inhalable particles. The metal content of such particles has also been suggested to have a great influence on their toxicity level [6, 7].
In urban environments, airborne particles come from various sources and occur in all size intervals. Querol et al.  used data from European cities and demonstrated that exhaust and non-exhaust sources contributed approximately equally to total traffic-related particulate emissions. Gehrig et al.  measured PM1 and PM10 in the ambient air near busy roads and demonstrated that abrasion and resuspension processes represent a significant portion of the total primary PM10 emissions of road traffic. At sites with relatively undisturbed traffic flow, these sources are in the same range as exhaust pipe emissions. At sites with disturbed traffic flow due to traffic lights, emissions from abrasion/resuspension are even higher than exhaust pipe emissions. Abu-Allaban et al.  made PM measurements at roadside locations in the USA and concluded that resuspended road dust and tailpipe emissions were the dominant mechanisms contributing to PM10 and PM2.5; they also noted a contribution from brake wear. Hjortenkrans et al.  demonstrated that brake wear was one of the major sources of metal particulate emissions in Stockholm. Iijima et al.  also concluded that airborne wear particles originating from brake wear contribute considerably to levels of PM10. Furthermore, Furusjö et al.  identified brake wear as one of the major sources of PM10 during urban driving.
Most brake rotors used in passenger cars are made of gray cast iron. The brake pads can be made of many different material combinations, but are essentially constructed of four components: a binder, reinforcing fibers, fillers, and frictional additives . The main task of the binder, which is made of polymer-based resin, is to hold the components of the brake pad together. The main task of the reinforcing fibers, which can be metal, glass, carbon, and ceramic fibers, is to give mechanical strength to the brake pad. Fillers are used partly to reduce the cost and partly to alter the brake pad properties, for example, by reducing noise and improving thermal properties, they can be made of barium sulfate and mica. Frictional additives, such as graphite, metal sulfides, and metal oxides/silicates, are used to control the friction and wear.
Brake pads are grouped into three categories: non-asbestos organic (NAO), semi-metallic, and low-metallic (LM). According to Sanders et al. , NAO brake pads exhibit relatively low brake noise and low wear rates, but lose braking capacity at high temperatures. Semi-metallic brake pads have a high steel fiber and iron powder content and low wear, but are noisier than the other types. LM brake pads have a relatively high abrasive content, which results in high friction and good braking capacity at high temperatures. This work examines LM and NAO brake pads.
During braking, both the brake pads and rotors experience wear, generating wear particles. Some of these particles are deposited on the brake hardware, while others become airborne. When measuring airborne brake particles in field tests, it can be difficult to distinguish them from other traffic-generated aerosols. Therefore, it can be preferable to use laboratory tests that allow the cleanliness of the air surrounding the test samples to be controlled. Although several studies have focused on wear and friction at the pad–rotor interface, few studies [12, 15] have focused on airborne wear particles and none of these controls the cleanliness of the air surrounding the test specimens. Wahlström et al.  have developed a disc brake assembly test stand that allows this to be done, ensuring that the wear particles measured and collected on the filters are airborne and that they originate from the pad–rotor contact. Furthermore, few studies have examined the actual size, shape, and elemental composition [17, 18] of the generated wear particles using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). None of those studies has attempted to collect only the airborne portion of the wear particles on filters.
This work investigates the size, shape, and elemental composition of airborne wear particles collected on filters in tests conducted in a disc brake assembly test stand. This article presents the experimental setup and results of testing with LM and with NAO brake pads.
2 Experimental Setup
The electric motor is balanced by mounting bearings on both the ends. The torque applied on the motor is measured using a calibrated strain gauge force sensor, the output of which is multiplied by the distance from the motor center. The rotational speed of the disc is measured by a Hall-effect sensor built into the wheel bearing. A pneumatic system generates controlled low pressures of up to 0.4 MPa in the brake cylinder. The pressure level is measured by a calibrated piezoelectric pressure sensor mounted near the inlet of the brake cylinder. A type-K thersmocouple is mounted inside the pad on the piston side near the contact surface.
The front right brake assembly, the suspension device for the knuckle, and the drive shaft are contained in the sealed chamber used to control the cleanliness of the incoming air. A fan (B) takes the air from the room (A) and passes it into the chamber (G), through the air inlet opening (F) via a flow measurement system (C) and a filter (D). The fan and the measurement system, the measurement system and the filter, and the filter and the chamber are connected by flexible tubes (E). In the present test series, all connections from the measurement system to the chamber were sealed to prevent leakage. A leak would not disturb the tests, since the air pressure inside the tubes is higher than that outside; however, a leak would change the measured air flow rate, which would influence the particle measurements. The air in the chamber is well mixed (I) due to the complicated volume of the front right brake assembly and the very high air exchange rate. This mixing is also verified by the smooth concentrations measured during the tests. The air in the chamber transports the generated particles to the air outlet (J), which is a sampling point for the particle measurement instruments.
The main instrument used to measure particles is a GRIMM 1.109 aerosol spectrometer (Grimm Aerosol Technik, Ainring, Germany). This optical particle counter measures airborne particles from 0.25 to 32 μm in size in 31 size intervals, and concentrations from 1 particle/l to 2 × 106 particles/l at a sample flow rate of 72 l/h (0.02 l/s) . The particle concentration is recorded every 6 s. Since an optical particle counter is sensitive to the shape and refractive index of the particles, the measured particle sizes and thus the number distributions are approximate .
The second particle measurement instrument is a PTrak counter (TSI, Shoreview, NM, USA) . This condensation nucleus counter measures the number concentration of airborne particles between 0.02 and 1 μm in size. The 50% cutoff in counting is given for both limits, i.e., both limits are defined as the particle size at which the counting efficiency (counted numbers of particles in relation to actual numbers of particles) decreases to 50%. There is no size resolution between the upper and lower limits and the particle concentration is recorded once per second.
The third particle measurement instrument is a DustTrak aerosol monitor (TSI, Shoreview, NM, USA), which records the mass concentration in mg/m3. This instrument is also based on light scattering, and can measure particle concentrations corresponding to the respirable size, PM10, PM2.5, and PM1.0 size fractions. It is calibrated with solid particles having a density of 2,650 kg/m3. It was used in these experiments without any pre-precipitator to measure particle sizes between 0.1 and 10 μm. The mass concentration is recorded every 5 s. The instrument is calibrated with a standardized test dust having a different size distribution, density, and refractive index from those of the particles measured here. Although the output of this instrument can only be used as a relative measure, it is useful for detecting changes over time in the generated particle mass .
The flow rate measurement system consisted of a straight calibrated tube with separate connections for total and static pressure, which were measured using an ordinary U-tube manometer. The calibration was conducted for the 2–50 m3/h flow interval. The filter used to ascertain a particle-free inlet air was of class H13 (according to the EN 1822 standard) with a certified collection efficiency of 99.95% at maximum penetrating particle size (MPPS).
3 Test Plan
A pair of LM brake pads and a pair of NAO brake pads were used with a cast iron rotor and tested under constant load conditions (i.e., constant brake cylinder pressure and rotational speed of the rotor).
Rotors directly from production are treated with an antirust protection layer. To wear off this layer, the rotors and brake pads were worn in under a constant cylinder pressure of 0.1 MPa and at a rotational speed of 600 rpm for 12 min before the test. After this running-in period, most of the antirust protection layer on the rotors was worn off.
Both types of brake pads were subjected to a brake cylinder pressure of 0.22 MPa. For each test, the rotational speed was set to a constant level of 600 rpm, and a steady brake load was applied for 6 min. This corresponds to light braking from a vehicle speed of 55 km/h. All the tests started at room temperature.
The fan used to create a constant air flow through the test chamber was set to a flow rate of 33 m3/h, which gave an approximate air exchange rate of 144 m3/h during both tests. The air inside the sealed chamber was verified to be particle-free by measuring the particle concentrations in the chamber outlet before and after the tests. In both the cases, the measured particle concentrations were approximately zero. The measured torque includes the frictional losses in the transmission from the motor to the rotor. To obtain this background level, each test was run for 1 min with no contact between the pads and rotor before the pneumatic brake load was applied.
While the brake cylinder pressure was being applied, the rotational speed of the rotor, the braking torque, and the brake cylinder pressure were measured at a sampling frequency of 1,200 Hz. The finger-side pad temperature was measured at a sampling frequency of 3 Hz.
Unused brake materials were analyzed using glow discharge optical emission spectroscopy (GDOES) to gain an overview of the elemental content of the surfaces. GDOES provides a technique for element depth profiling by combining sputtering and atomic emission. Low-power argon plasma is generated in a chamber by the voltage between the anode and cathode (sample surface). Material is continually removed from the sample area by ionized argon atoms. The constituent atoms are then excited in an electrical field so that they glow. The light, which is characteristic of the emitting element, is detected and analyzed using a conventional optical emission spectrometer, and the elemental depth profile is obtained from the emission intensities as a function of sputtering time ; this technique is recognized in international standards . The diameter of the surface analyzed was 5 mm and the depth was as great as 900 nm.
During the tests, a pump was used to collect wear particles, airborne in the sealed chamber, on filters. A volume of approximately 6 dm3 of air was pumped through Nuclepore (polycarbonate) filters with a pore size of 0.4 μm (Whatman, Maidstone, UK). The airborne wear particles collected on filters were coated with gold and then analyzed using SEM and mapped using INCAEnergy software.
As can be seen in Fig. 3, the tests indicate a maximum in particle concentration at a particle diameter of approximately 350 nm. Mosleh et al.  presented size distributions of wear particles collected on filters during pin-on-disc tests of brake materials under different testing conditions, while Riediker et al.  tested different brake pad materials on different passenger cars using a chamber to seal off the brake from the environment. Wahlström et al.  presented a comparison of the number and volume distributions of airborne wear particles as measured online in field tests, in a disc brake assembly test stand, and in a pin-on-disc machine. All three noted a peak in the size distribution at 350 nm.
Sanders et al.  conducted dynamometer and car tests of pad materials and noted a maximum peak in particle size distribution at a particle diameter of approximately 1 μm for the LM brake pads. Iijima et al.  used a brake dynamometer to measure brake dust from NAO brake pads, and noted a peak in size distribution at a particle diameter of approximately 1 μm. Sanders et al.  used a particle density of 5,000 kg/m3 to calculate the aerodynamic diameter. If this density is used in our tests, the maximum peak in particle size distribution occurs at approximately 900 nm, comparable to their results. Iijima et al.  also discussed brake dust as a major atmospheric antimony source. Uexküll et al.  studied antimony in brake pads and presented SEM images of brake dust. None of the particles analyzed in this work, however, contained antimony.
Thorpe and Harrison  have reviewed the sources and properties of non-exhaust particulate matter from road traffic. They presented a summary of the metal concentrations present in brake linings and emitted brake dust and found that iron, copper, lead, and zinc have been repeatedly reported to be present in high concentrations in brake pads. No lead was found in the brake pads tested in this study, though titanium was registered. In addition, the presence of all metals detected by INCAEnergy mapping (Figs. 8–11) was confirmed by GDOES analysis (Fig. 5).
As can be seen in Figs. 8 and 10, most of the coarse particles appear as flakes and seem to be mechanically generated. These particles consist mainly of iron and iron oxide, which indicates that they originate from the disc. On the other hand, some of the coarse particles also contain titanium, copper, and aluminum, which indicates that they originate from the brake pads. Mosleh et al.  concluded that fine wear particles originate from the cast iron disc, since they were mainly composed of iron, oxygen, and carbon. This conclusion cannot be drawn here, since the fine particles are also composed of titanium, copper, and aluminum.
The results presented here indicate the presence of greater numbers of fine particles that are generally considered more toxic . In addition, the chemical composition may be an important factor affecting the toxicity. Although SEM and EDX yield information about the size, shape, and elemental composition of the airborne wear particles, they yield no information about the compounds constituting the wear particles. This information is needed if we are to understand the chemistry of the wear particles and estimate the temperature in the contact . Note that traditionally, mechanically generated particles have been associated with the generation of partcles several microns in diameter. The present study, however, found ultrafine wear particles. This result is in line with those of Wahlström et al.  and Riediker et al. , who noted a peak in particle concentration in the ultrafine fraction.
In the disc brake assembly test stand presented here, designed for the study of low cylinder pressure levels, it would be desirable to increase the applied load by using a stronger hydraulic motor. In addition, this test stand runs under constant load conditions. It would be of interest to extend the capacity of the test stand to handle more realistic (i.e., transient) brake events. This could be done in the disc brake assembly test stand by controlling the pressure exerted by the hydraulic system and the rotational speed of the motor. In future testing, it would also be of interest to study the influence of increased contact pressure on the airborne wear particles generated.
This article presents a first step toward separating, capturing, and analyzing airborne wear particles generated by a disc brake in a laboratory environment. In future testing, it would be interesting to perform tests at different load levels to determine whether the particle morphology changes with normal load and sliding speed.
The test method presented can be used to study the airborne part of wear particles generated by a disc brake.
The analyzed wear particles contained elements such as iron, titanium, zinc, barium, manganese, and copper.
Both the low-metallic and non-asbestos organic type of brake pads tested display a bimodal size distribution with peaks at 280 and 350 nm.
The authors acknowledge valuable help from Wubeshet Sahle (Royal Institute of Technology) with SEM/EDX, Mats Randelius (Swerea KIMAB) with GDOES, and Anders Jansson (Stockholm University) with filters and pumps.