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A study of airborne wear particles from automotive disc brakes
Jens Wahlström
Doctoral thesis Department of Machine Design Royal Institute of Technology SE-100 44 Stockholm
TRITA – MMK 2011:04 ISSN 1400-1179
ISRN/KTH/MMK/R-11/04-SE ISBN 978-91-7415-871-7
TRITA – MMK 2011:04 ISSN 1400-1179 ISRN/KTH/MMK/R-11/04-SE ISBN 978-91-7415-871-7
A study of airborne wear particles from disc brakes
Jens Wahlström
Doctoral thesis
Academic thesis that, with the approval of the Royal Institute of Technology, will be presented for public review in fulfillment of the requirements for a Doctorate of Engineering in Machine Design. Public review: The Royal Institute of Technology, Flodissalen (F3), Lindstedtsvägen 26, Stockholm, on May 27, 2011 at 10.00.
Abstract During braking, both the disc and pads in disc brakes are worn. Since disc brakes are not sealed, some of the wear particles generated can become airborne. Several studies have found an association between adverse health effects and the concentration of particles in the atmosphere, so it is of interest to improve our knowledge of the airborne wear particles generated by disc brakes.
This thesis deals with experimental and computational methods focusing on airborne wear particles from disc brakes. The eight appended papers discuss the possibility to both measure and numerically determine the concentration and size distribution of airborne wear particles that originate from the pad-to-disc contact. The objective is to increase the scientific knowledge of airborne wear particles generated from disc brakes.
Papers A, B and C describe tests of disc brake materials conducted in a modified pin-on-disc machine. The results show that the test set-up can be used to measure and rank disc brake materials with respect to the concentration of airborne particles generated. Ultrafine (nanosized), fine and coarse airborne wear particles that contain metals such as iron, copper and tin were found.
Papers D and E describe a novel disc brake assembly test stand and tests of disc brake materials conducted in it. The results show that the test set-up can be used to measure the concentration and size distribution of airborne wear particles generated from disc brake materials. The results also indicate an ability to rank different pad/disc combinations with respect to the concentration of airborne wear particles. Furthermore, the results suggest that this test stand can be used to study rust layer removal from the disc and that airborne particles are generated even at low brake pressures, such as used to remove dirt from the disc.
Paper F compares measurements made in passenger car field tests with measurements made in a disc brake assembly test stand and in a pin-on-disc machine. A promising correlation between the three different test methods is found.
Paper G presents and discusses a simulation methodology that numerically determines the concentration and size distribution of airborne wear particles generated from the pad-to-disc contact in disc brakes by using general-purpose finite element software.
Paper H discusses a cellular automaton model that describes the microscopic contact situation between the pad and disc in disc brakes. This model is used to numerically determine the amount of wear that leaves the contact. The results correlate qualitatively with experimental observations found in the literature.
Keywords: Disc brakes, Airborne wear particles, Nanoparticles, Cellular automaton
To Whom It May Concern
List of appended papers This thesis consists of a summary and the following eight appended papers:
Paper A
Wahlström J, Söderberg A, Olander L, Jansson A, and Olofsson U. 2010. A pin-on-disc simulation of airborne wear particles from disc brakes. Wear, 268, 763-769.
Paper B
Wahlström J, Olander L, and Olofsson U. 2010. A pin-on-disc study of automotive disc brake materials focusing on airborne wear particles. Submitted to Journal of Tribology 2010-07-11. Paper Id: TRIB-10-1122.
Paper C
Wahlström J, Gventsadze D, Olander L, Kutelia E, Gventsadze L, Tsurtsumia O, and Olofsson U. 2011. A pin-on-disc investigation of nanoporous composite-based and conventional brake pad materials focusing on airborne wear particles. Submitted to Tribology International 2011-02-11. Paper Id: TRIBINT-D-11-00064.
Paper D
Wahlström J, Söderberg A, Olander L, and Olofsson U. 2009. A disc brake test stand for measurements of airborne wear particles. Lubrication Science, 21, 241–252.
Paper E
Wahlström J, Olander L, and Olofsson U. 2010. Size, shape, and elemental composition of airborne wear particles from disc brake materials. Tribology Letters, 38, 15-24.
Paper F
Wahlström J, Söderberg A, Olander L, Jansson A, and Olofsson U. 2010. Airborne wear particles from passenger car disc brakes: a comparison of measurements from field tests, a disc brake assembly test stand, and a pin-on-disc machine. Proc. IMechE Part J: Journal of Engineering Tribology, 224. 179-188.
Paper G
Wahlström J, Söderberg A, and Olofsson U. 2009. Simulation of airborne particles from disc brakes. SAE Technical Paper 2009-01-3040.
Paper H
Wahlström J, Söderberg A, and Olofsson U. 2011. A cellular automaton approach to numerically simulate the contact situation in disc brakes. Accepted for publication in Tribology Letters.
The author's contributions to the appended papers The work presented in this thesis was initiated and supervised by Professor Ulf Olofsson.
Paper A
The author contributed to the writing, experimental work and evaluation.
Paper B
The author performed most of the planning, writing, experimental work, and evaluation.
Paper C
The author performed most of the planning, writing, experimental work, and evaluation.
Paper D
The author performed most of the planning, writing, experimental work, and evaluation.
Paper E
The author performed most of the planning, writing, experimental work, and evaluation.
Paper F
The author performed most of the writing and contributed to the planning, experimental work and evaluation.
Paper G
The author performed most of the writing and experimental work, and contributed to the planning, simulation and evaluation.
Paper H
The author performed most of the planning, writing and evaluation.
List of published papers not included in this thesis
Jansson A, Olander L, Olofsson U, Sundh J, Söderberg A, and Wahlström J. 2010. Ultrafine Particle Formation from Wear. The International Journal of Ventilation, 9, 83-88.
Abbasi S, Wahlström J, Olander L, Sellgren U and Olofsson U. 2010. A study of airborne wear particles generated from organic railway brake pads and brake discs. Submitted to Wear 2010.
Jansson A, Olander L, Olofsson U, Sundh J, Söderberg A and Wahlström J. 2009. Airborne particle generation at metal wear. NOSA Aerosol Symposium 2009, Lund, Sweden, November 12-13, 2009.
Kutelia E, Gventsadze D, Eristavi B, Maisuradze N, Tsurtsumia O, Gventsadze L, Olofsson U, Wahlström J, and Olander L. 2011. The tribological efficiency and the mechanism of action of nano-porous composition base brake lining materials. Georgian Engineering News (GEN), 1.
Abbreviations LM - Low Metallic type of brake pad
NAO - Non Asbestos Organic type of brake pad
PM - Particulate Matter
PM0.1 - Particulate matter with an aerodynamic diameter below 0.1 µm
PM1 - Particulate matter with an aerodynamic diameter below 1 µm
PM10 - Particulate matter with an aerodynamic diameter below 10 µm
OPC - Optical Particle Counter
CNC - Condensation Nucleus Counter
DMA - Differential Mobility Analyzer
SMPS - Scanning Mobility Particles Sizer
SEM - Scanning Electron Microscopy
EDX - Energy-Dispersive X-ray spectroscopy
FEA - Finite Element Analysis
FEM - Finite Element Method
CA - Cellular Automaton
MCA - Movable Cellular Automaton
Contents
1 Introduction .........................................................................................................................................1
1.1 Automotive disc brakes.............................................................................................................1
1.2 Airborne particles and health effects.......................................................................................2
1.3 Particle instruments ...................................................................................................................4
1.4 Contact situation in disc brakes ...............................................................................................5
1.5 Cellular automaton.....................................................................................................................6
1.6 Friction and wear testing of disc brake materials ..................................................................7
1.7 Objectives....................................................................................................................................8
2 Summary of appended papers ...........................................................................................................9
3 Discussion ..........................................................................................................................................11
4 Conclusions........................................................................................................................................15
5 References ..........................................................................................................................................16
Appended papers
A. A pin-on-disc simulation of airborne wear particles from disc brakes
B. A pin-on-disc study of automotive disc brake materials focusing on airborne wear particles
C. A pin-on-disc investigation of nonporous composite-based and conventional brake pad materials focusing on airborne wear particles
D. A disc brake test stand for measurements of airborne wear particles
E. Size, shape and elemental composition of airborne wear particles from disc brake materials
F. Airborne wear particles from passenger car disc brakes: a comparison of measurements from field tests, a disc brake assembly test stand, and a pin-on-disc machine
G. Simulation of airborne particles from disc brakes
H. A cellular automaton approach to numerically simulate the contact situation in disc brakes
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1 Introduction This thesis deals with experimental methods at three test levels for testing disc brake materials with respect to the generation of airborne wear particles. Furthermore, two computational methods to numerically simulate wear and airborne wear of disc brake materials are discussed. The eight appended papers provide methods for both measuring and numerically determining the concentration and size distribution of airborne wear particles originating from the pad-to-disc contact.
An introduction to automotive disc brakes is given in Section 1.1. Airborne particles are an essential part of the thesis. Section 1.2 introduces the necessary terminology and provides an overview of the origins of atmospheric particles and their health effects. An overview of particle measurement techniques and instruments are given in Section 1.3. The airborne wear particles originate from the disc brake contact, which is discussed in Section 1.4. Cellular automaton is introduced in Section 1.5. Friction and wear testing of disc brake materials is discussed in Section 1.6. Finally, the objectives of this thesis are presented in Section 1.7.
1.1 Automotive disc brakes
Modern cars have disc brakes on the front wheels, and there is a growing trend to have them on the rear wheels as well. The main purpose of a disc brake is to slow down a vehicle by transforming kinetic energy into frictional heat. A brake disc (rotor) is firmly fitted to and rotates with the wheel. Two brake pads (linings) are positioned inside a caliper mounted on the knuckle, which is mounted on the chassis. When the driver hits the brakes, the brake cylinder pressure increases and the piston pushes the pads into contact with the disc. The friction force between the brake pads and disc exerts braking torque on the disc, which is connected to the wheel, and the subsequent friction between the tire and the road makes the car slow down. An example of a disc brake assembly that consists of a ventilated disc, a cross-section of a sliding caliper with a single piston, and two brake pads is presented in Figure 1.
Figure 1. Disc brake assembly with a single-piston floating caliper and a ventilated disc.
Disc
Caliper
Pads
Piston
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Most discs used in 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 [1]. 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 made of metal, glass, carbon, and ceramic fibers, is to give mechanical strength to the brake pad. Fillers are used partly to reduce cost and partly to alter the brake pad properties, for example to reduce noise and improve 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. [2], NAO-type 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.
1.2 Airborne particles and health effects
Particulate matter (PM) is made up of solid or liquid particles suspended in a gas or liquid, and atmospheric aerosols refer to the particles and gas together. The aerodynamic diameter is the diameter of a sphere of unit density that has the same gravitational settling velocity as the particle in question. In this thesis, particles with an aerodynamic diameter less than 10 µm (PM10) are divided into a coarse fraction (>1 µm), a fine fraction (<1 µm, PM1), and an ultrafine fraction (<0.1 µm, PM0.1). Figure 2 (modified from the Environmental Protection Agency [3]) presents typical size distributions of atmospheric particulate matter with the size classification marked.
Figure 2. Typical size distribution of atmospheric particulate matter with the size classifications marked.
The coarse particles mainly originate from natural sources such as dust and pollen, but also from anthropogenic sources (e.g., mechanical processes). In an urban environment, these
Ultrafine Fine Coarse
Particle diameter [µm]
Fre
quen
cy [%
]
Typical size distribution of atmospheric particulate matter
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anthropogenic particles can come from various sources, including demolition and construction [4], resuspended road dust [5], wheel-to-rail contact [6], [7], and tire-to-road contact [8], [9].
Fine and ultrafine particles are usually formed from gases, mainly due to fossil fuel combustion. Primary fine particles are introduced directly to the atmosphere, and secondary fine particles are formed by chemical reactions in the atmosphere. Ultrafine particles, such as metallic vapor, coagulate (or condense) without chemical reactions from primary fine particles. The combustion of gasoline and diesel forms fine particles, whereas the combustion of coal and heavy fuel oil yields both fine and coarse particles.
Due to the well-documented relationship between adverse health effects and the concentration of particles in the atmosphere (e.g., [10]-[12]), there are limit levels for PM2.5 and PM10. These limits have been established by the European Commission in the European Union [13] and by the Environmental Protection Agency in the United States [14]. Research show that particle size is an important factor influencing how particles are deposited in the respiratory tract and affect human health [11], [12]. Most coarse particles are deposited in the nose and throat, while fine and ultrafine (nanosized) particles can generally penetrate deep into the lungs. Inflammation is believed to be one way particles affect health, 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 [15]. Furthermore, the metal content of inhalable particles has also been suggested to have a great influence on their toxicity level [16], [17]. Gasser et al. [18] investigated the toxic effect of brake wear particles on lung cells, and concluded that metals (iron, copper and manganese) on brake wear particles damage the lung cells’ tight junctions with a mechanism involving oxidative stress and an increase of pro-inflammatory responses.
Unlike drum brakes, disc brakes are not sealed off from the ambient air. During braking, both the disc and pads are worn and this wear process generates wear particles, some of which become airborne. In urban environments, airborne particles come from various sources and occur in all size intervals. Querol et al. [4] 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. [5] 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. [8] 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. [19] demonstrated that brake wear was one of the major sources of metal particulate emissions in Stockholm. Iijima et al. [20] also concluded that airborne wear particles originating from brake wear contribute considerably to levels of PM10. Furthermore, Furusjö et al. [21] identified brake wear as one of the major sources of PM10 during urban driving.
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1.3 Particle instruments
A number of different particle instruments exist that measure concentration and size distribution of airborne particles. The main physical principles used in particle instruments are inertial, optical and electrical properties of the measured particles.
The impactor [22] and the cyclone [23] are instruments based on the inertia properties of the measured particles. In an impactor, the aerosol is pumped through a nozzle after which it is forced into a sharp bend by a plate placed near the end of the nozzle. Particles with a large enough aerodynamic diameter deviate so much from the air flow lines that they hit the plate and get stuck. In the cyclone, the aerosol is moving in a spiral path in a cylindrical chamber. Particles with high inertia are hurled against the walls and get stuck, while smaller particles can pass. A cascade impactor [24] consists of a number of impactors that can be used to study particle size distributions. By gradually reducing the subsequent impactor stage nozzle diameters, the particulate fractions that deposit will have a decreasing aerodynamic particle size.
The optical particle counter (OPC) is an instrument based on the principle of light scattering from particles. Instruments based on this technique have an advantage because they present the results online and have a high sensitivity (they can count down to a single particle). Since the OPC is based on light scattering, it can only measure particles with a diameter larger than the wavelength of light. Another light scattering instrument is the condensation nucleus counter (CNC) [25], see Figure 3. The aerosol passes through an alcohol, which is heated so that a supersaturated alcohol vapor is obtained above the bath. After passing the bath, the aerosol is saturated with alcohol vapor. The aerosol is then transported through a cooled column. Alcohol vapor then condenses on the particles, and they grow to a size that can be detected by an OPC. A CNC can count particles down to nanometers. In a photometer, many particles are simultaneously in the measuring volume and the measured light scattering is detected. An OPC is sensitive to the form and refractive index of the particles, which means that the measured particle sizes and number distributions should be regarded as approximate [26].
Figure 3. Schematic illustration of a condensation nucleus counter (CNC) [27].
5
The differential mobility analyzer (DMA) is an instrument based on the electrical properties of the particles. The DMA consists of a central electrode inside a vertical cylinder. A radial electric field is obtained between the grounded cylinder and a negative voltage source connected electrode. The aerosol enters the cylinder by a narrow annular slit at the end of the cylinder, while clean air is supplied to the rest of the cylinder. Particles with a positive charge will move towards the central electrode, and different particles with the same electrical mobility will be deflected in the same way. At the bottom of the central electrode are gaps through which particles are drawn out with a distinct electrical mobility within a narrow range. With increased voltage on the electrode, the particles with decreasing mobility pass through the slits. The resulting aerosol contains simple charged particles, but also a few larger particle sizes with many charges [28]. Each mobility fraction can be counted with a CNC to obtain a size distribution. An example of a DMA combined with a CNC is the scanning mobility particles sizer (SMPS).
1.4 Contact situation in disc brakes
The wear (and airborne wear particles) generated from disc brakes originate from contact between the pad and disc. Eriksson et al. [29] presented an explanatory model of the complex contact situation between an organic brake pad and a cast iron disc (Österle et al. [30] and Ostermeyer [31] presented similar results). In this model (Figure 4), metal fibers (or other hard materials) in the pad material form stable primary plateaus (or patches), which carry the main part of the load. Wear particles (originating from both the pad and disc) can flow in the boundary layer between the pad and disc since there are gaps large enough between the primary plateaus. The particle flow wears down the matrix material of the pad by three-body abrasion. Some of these wear particles will stack up against the primary plateaus and create secondary plateaus, and some will escape the contact into the surroundings. A small change of the contact situation can disturb the stability, and the secondary plateau will break loose only to be milled down into wear particles again. The wear particles will be reused in another secondary plateau or escape the contact into the surroundings. The destruction of plateaus depends on the wear of the surrounding plateaus, the plateau temperature, and the mechanical stress in the plateau, which can lead to crack growth.
Figure 4. An illustration of the contact situation between the pad and disc. A transparent disc is moving from left to right. Some of the wear particles pile up against the contact plateaus and create secondary plateaus. A flow of wear particles in the gap between the pad and disc wear the lowlands of the pad through three-body abrasion. [29].
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Eriksson [32] concluded that the hardness of primary and secondary plateaus is about the same, and that the matrix material hardness is about 20 times lower. That is, the hard plateaus protect the matrix material below from being worn. The size of the plateaus is roughly between 50 μm and 500 μm. Furthermore, it has been shown ([32]-[34]) that the plateau surface is covered by a nanocrystalline third body formed by the wear particles. This third body is mainly made of iron oxides. The main part of the frictional energy is dissipated into heat in the plateaus, and increases the temperature of the pad and disc, which leads to the rather high temperature of a plateau. To summarize, the macroscopic friction and wear behavior of a disc brake can be explained by the microscopic contact situation (growth and the destruction of contact plateaus) in the boundary layer between the pad and disc.
Numerical simulations of the friction and wear behavior of disc brakes have been studied with different approaches, including movable cellular automaton (MCA) [35] and finite element analysis (FEA) ([36]-[38]). In [35], a model to numerically calculate the third body behavior is used. This technique can be used to understand the particle flow on a nanoscopic length scale. Due to the necessary length scale and very short time scale, scaling up to a macroscopic behavior is hard. Furthermore, it would be difficult to simulate the contact behavior for the aforementioned complex contact situation using FEA, since both the length and time scales are microscopic. In order to numerically simulate the macroscopic behavior of disc brakes, a numerical method that can handle the complex contact situation with the microscopic length and time scale is needed. As such, Müller and Ostermayer ([39], [40]) used a cellular automaton approach to describe the three-dimensional friction and wear behavior of disc brakes.
1.5 Cellular automaton
The idea of a cellular automaton is to simulate complex systems with a few simple rules. A cellular automaton is composed of a grid whose elements are cells, a finite set of inner variables and neighborhood relations given to each cell, and a set of rules (transition functions) that determine the new state at time t + Δt of the inner variables from the current state at time t. Cellular automaton models are used in many applications, for example to model surface topography [41], fire spread [42], and traffic flow [43]. The most well-known cellular automaton is John Conway’s Game of Life [44], and it is presented below as an introduction to cellular automaton. The universe of the game of life is a two-dimensional orthogonal grid of square cells, each of which is in one of two possible states—live or dead. Every cell interacts with its eight closest neighbors. At each step in time, the following rules are applied:
Any live cell with fewer than two live neighbors dies (underpopulation)
Any live cell with more than three live neighbors dies (overcrowding)
Any live cell with two or three live neighbors lives on to the next generation
Any dead cell with exactly three live neighbors becomes a live cell (reproduction)
The first generation is created by simultaneously applying the above rules to every cell in the initial population. The rules continue to be applied repeatedly to create further generations. A simple system like this, with a few rules, results in complex patterns.
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1.6 Friction and wear testing of disc brake materials
Friction and wear parameters for sliding contacts are determined on different test scales, from simplified model tests, where the contact-pair are replaced with simplified models, to full field tests, where the system is tested under real conditions.
The most common tribometer for conducting model tests with disc brake materials is the pin-on-disc machine (for example [45]-[49]) with a horizontal rotating disc and a dead weight or piston-loaded pin. Disc brake materials can be studied in the pin-on-disc machine using disc-shaped brake disc samples and pin-shaped brake pad samples. The usual output parameters of a pin-on-disc test are the coefficient of friction and the wear coefficient. By weighing the test samples before and after testing, the latter can be determined. A displacement gauge could be used to measure the total wear of both the pin and disc sample online, making it possible to observe changes in the wear rate due to run-in effects or changes in wear mechanisms. The bulk temperature in the pad and disc could be measured with thermocouples fitted inside the test specimens.
Brake friction and wear tests at the component level are usually conducted in an inertia dynamometer (for example [50], [51]). A motor rotates a number of inertia flywheels (depending on the car weight simulated), and the disc is fitted at the end of the shaft. The caliper is attached to the bench and a controlled load is applied by a hydraulic system. During testing, the rotational speed of the disc and the brake torque is measured. The coefficient of friction can be calculated from the measured brake torque and the effective radius of the applied braking load. By weighing the disc and pads before and after testing, the wear coefficient can be determined. The bulk temperature in the pad and disc could be measured with thermocouples fitted inside the test specimens.
The representativity of the laboratory test stands should be verified with field tests; however, few field tests [2] are reported in literature.
Although several studies have focused on wear and friction of disc brakes, only some have focused on the measurement of airborne wear particles in model tests [52], in component/bench tests ([20], [53]-[55]), and field tests [2]. Few studies have examined the actual size, shape and elemental composition ([56]-[58]) of the generated wear particles using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). None of those studies have attempted to control the cleanliness of the air surrounding the test specimens and collect only the airborne portion of the wear particles on filters.
Olofsson and Olander [59] used a sealed box to control the cleanliness of the air surrounding the test samples in a pin-on-disc machine. In this set-up, a fan took in air from the room and passed it into the box via a filter. The air in the chamber transported the generated particles to the air outlet, where the particle measurements and filter sampling were made. This technique ensures that the particles measured or collected on the filter are airborne and generated from the studied contact. Vorbau et al. [60] developed a test method for quantification of the nanoparticle release into the air from surface coatings, but they did not use a clean air supply to control the background concentration.
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1.7 Objectives
With this background in mind, the aim of this thesis is to increase the scientific knowledge about airborne wear particles originating from disc brakes. The objective is divided into five sub-objectives:
Develop experimental methods for measuring the concentration and size distribution of airborne wear particles generated in the pad-to-disc contact in model and component tests, and to verify the validity of these by comparison with field tests.
Investigate the possibility to rank disc brake materials with respect to the generated concentration of airborne wear particles in laboratory test stands.
Investigate the size, shape and elemental composition of airborne wear particles from disc brake materials.
Investigate how different braking loads effect the concentration and size distribution of airborne wear particles from disc brake materials.
Investigate computational methods to numerically simulate wear and airborne wear from disc brakes.
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2 Summary of appended papers This thesis comprises eight appended papers (Appendices A–H) that discuss different aspects of airborne wear particles generated by disc brakes. A summary of the eight appended paper are presented below.
Papers A and D describe two test stands for testing brake friction materials at model and component levels. The test stands were screened off from their surroundings by a closed box in which the cleanliness of the air surrounding the test specimens could be controlled. Both test stands were equipped with particle counting instruments. In papers A-C, a pin-on-disc machine was used as test equipment, and in papers D and E, a novel disc brake assembly test stand was used as test equipment. Paper F discusses the validity of the results obtained from these test stands by comparing them with field test results.
Paper A presents a study on the concentration and size distribution of airborne wear particles from disc brake materials. Material from four different NAO pads and four different LM pads were tested against material from gray cast iron discs at one load level. The results indicate that the LM pads caused more wear to the disc material than did the NAO pads, resulting in higher concentrations of airborne wear particles. Although there were differences in the measured particle concentrations, similar size distributions were obtained in the size interval 250 nm to 32 µm.
Paper B presents an investigation of the concentration, size distribution, morphology, and elemental content of the airborne wear particles generated from the contact between a LM pad material and a gray cast iron disc at four different load levels. Trimodal size distributions were registered during run-in in the size interval 250 nm to 32 µm for all load levels. After run-in, bimodal size distributions were registered for all load levels save for the highest load level, where multimodal size distributions were registered. At the two highest load levels, the concentration of ultrafine/fine particles showed an increase of a factor of 100, indicating a change in wear mechanism. SEM images show ultrafine (nanosized), fine and coarse airborne wear particles that contain metals such as iron, copper and tin.
Paper C investigates novel nanoporous composite-based brake pads and conventional brake linings with respect to the concentration of airborne particles generated. LM, NAO and nanoporous composite-based pins were tested against cast iron discs. The results show that two of the nanoporous composite-based materials generated considerably fewer airborne wear particles than the conventional materials.
Paper D presents a disc brake assembly test stand used to study the number concentration and size distribution of airborne wear particles from disc brake materials. The test stand consisted of a front right brake assembly mounted in a sealed chamber. A braking load was applied by a pneumatic system, and the disc was driven by an electric motor. The number and size of airborne wear particles was then measured. NAO and LM brake pads were tested at low braking loads against material from gray cast iron discs, which had been pre-conditioned with a rust layer to simulate a car standing parked overnight in a wet environment. The results suggest that this test
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stand can be used to study rust layer removal from the disc and that airborne particles are generated even at low brake pressures, including at dragging conditions.
Paper E presents a study of one pair each of LM and NAO brake pads tested against gray cast iron discs. The analyzed wear particles contained elements such as iron, titanium, zinc, barium, manganese, and copper. SEM images of ultrafine, fine and coarse particles are presented. Both the LM and NAO type of brake pads tested display a bimodal size distribution in the size interval 250 nm to 32 µm. Furthermore, the LM pads seem to generate more coarse particles than the NAO pads.
Paper F presents a comparison of the number and volume distributions of airborne wear particles measured online in a passenger car field test, in a disc brake assembly test stand at the component level, and in a pin-on-disc machine at the model level. In all cases, gray cast iron discs and LM pads were tested. Promising correlation between the three test methods was shown.
Paper G presents and discusses a simulation methodology that numerically determines the concentration and size distribution of airborne wear particles generated from the pad-to-disc contact in disc brakes by using general-purpose finite element software. The idea of the methodology is to first determine the wear and particle coefficient (airborne wear coefficient) by experiments in a pin-on-disc tribometer at the material level. These coefficients are then used in a wear simulation in which a FE model of a subsystem consisting of a piston, brake pad and disc is used to compute the pressure distribution at the interface. On the basis of the resulting contact pressure distribution, the concentration and size distribution of airborne wear particles from both contact surfaces is computed using a modified form of Holm-Archard’s wear law and an explicit Euler integration scheme. Finally, to validate the simulation methodology, the simulated concentration and size distributions should be compared to experimental measurements in a disc brake assembly test stand at the component level. A simple simulation case with known expected outputs shows that the methodology has been implemented correctly in the finite element software.
Paper H presents a cellular automaton approach that describes the microscopic contact situation between the pad and disc in disc brakes. First, the contact pressure was determined, followed by pad temperature and wear. Finally, based on these results, the creation, growth and destruction of contact plateaus are simulated using a cellular automaton model. Based on these results, the amount of wear that leaves the contact is determined. The results correlate qualitatively with experimental observations found in the literature.
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3 Discussion Traditionally, mechanically generated particles have been associated with the generation of particles several microns in diameter. This thesis shows that a lot of ultrafine and fine wear particles, which are generally considered more toxic [61], are generated from the pad-to-disc contact. SEM images (papers B and E) show that ultrafine and some of the fine particles seem to be smoother (some of them nearly spherical) and have fewer sharp edges than the coarse particles, which suggests that they are thermal and/or chemical generated rather than mechanical generated ([62], [63]). Note that these ultrafine (nanosized) particles are in the same size scale as exhaust particles [64].
After repeated braking, the temperature induced by frictional heating can be high enough to decompose the phenolic resin [65]. Phenolic resins carbonize at approximately 450 °C, and at higher temperatures it decomposes by charring and evaporation [1]. This could be one explanation of the step in concentration of ultrafine and fine airborne particles presented in paper B. Another scenario is that the frictional induced temperature is high enough to melt metals in the contact. After following solidification, this would create particles that are nearly spherical. More information is needed to decide when the contact temperature is high enough to decompose the phenolic resins and/or melting of metals. Note that ordinary driving doesn’t result in an average pad bulk temperature higher than 120 °C (paper F). Repeated high energy braking is needed to reach these kinds of mean temperatures in the field.
Oberdörster et al. [66] recommend a number of key factors (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. Although SEM and EDX yield information about the morphology and elemental composition of the airborne wear particles, they yield no information about the compounds constituting the wear particles. This information can be used to understand the chemistry of the wear particles and ensures it is possible to estimate the temperature in the contact [67].
Overall, the airborne particles generated during the steady state in the fine and coarse size interval form a bimodal size distribution with peaks around 350 nm and 550 nm for all tested load levels (papers A-F), except the highest in paper B. Similar results have been reported in model tests [52], component tests ([2], [20]) and bench tests [52]. Note that none of these studies uses a technique that ensures that the particles measured are airborne and generated from the pad-to-disc contact. The bimodal size distribution indicates that the airborne wear particles are generated by the same wear mechanism. At the highest load level in paper B, it seems that there has been a change in wear mechanism since a lot of particles that are smaller than 300 nm are generated.
Mosleh et al. [52] conducted pin-on-disc tests on disc brake materials and analyzed wear particles collected in a cup that surrounded the disc sample. They found a peak of fine particles about 350 nm for all tests regardless of the test conditions (pad material, sliding speed and contact pressure), and a peak of coarse particles that varied between 2 and 15 μm. Furthermore, they concluded that the fine wear particles originate from the cast iron disc, since they mainly consist of iron, oxygen and carbon. Overall, this is also the case for the fine particles analyzed in this
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thesis, although some of the fine particles consist of titanium, copper, tin, and barium, which are elements of the pads. The coarse peak in number distribution was not found in this work, which can be explained because only airborne particles were collected. However, in the volume distribution peaks of coarse particles can be found. In all tests conducted in this thesis, the same type of disc material was used.
Most of the coarse particles analyzed contained elements such as titanium, copper, tin, and barium, which indicate that they originate from the pads. It seems that most of the fine particles originate from the disc and most of the coarse particles originate from the pad. This could be one possible explanation for the fact that around 350 nm the peak is registered regardless of pad material. Österle et al. [53] sampled brake dust during fading cycles in a dynamometer. They concluded that the micro- and macro-constitutes of pads and discs are transformed to a nanocrystalline third body, which is partly released to the environment as dust. They also concluded that primary nanoparticles formed agglomerates in the size range 100-300 nm. These agglomerates can also be seen in the SEM images presented in this thesis.
Gasser et al. [18] exposed lung cells to brake wear particles and concluded that the metal (iron, copper and manganese) contents of the particles have an adverse effect on lung cells (i.e., an increase in oxidative stress and pro-inflammatory responses). A lot of the brake wear particles analyzed with EDX in this thesis consist of iron, copper and manganese.
When standing parked overnight in a wet environment or driving in high air humidity [32], a rust layer that could decrease the coefficient of friction builds on the pads and disc. Also, dirt (such as road salt [46] or mud) from the surroundings can soil the disc and decrease the coefficient of friction. Therefore, to ensure robust brake performance, some brake systems require the pads to frequently be in low pressure contact with the disc. This dragging may remove any rust and/or dirt layer from the disc and keep the contact surfaces clean. However, the resulting drag torque increases the fuel consumption and generates wear particles (paper D) because the pads are still in contact with the disc after the rust and/or dirt layer has been removed. It may, therefore, be desirable to reduce the dragging without affecting the performance of the brakes.
Some of the pad materials tested in the pin-on-disc machine generated more airborne wear particles than other pad materials, but the coefficient of friction was higher for the pad materials with a higher concentration of airborne wear particles (papers A and C). That is, the total amount of frictional power generated by the sliding contact during a pin-on-disc test is smaller for some of the tested pad materials. It is important to keep this in mind when comparing two types of pad material with respect to wear and airborne wear particles. If the objective is to decrease the amount of airborne particles generated by changing pad material, one must remember that the normal brake force and/or pad area must be increased to obtain the same brake properties, which in turn probably results in an increase of wear and airborne wear particles. Also, when changing pad material, it is important to keep in mind that the properties of the airborne wear particles generated may change and result in an increase or decrease of their toxicity [66].
Sometimes when conducting pin-on-disc tests, the test time is chosen to be much longer than the run-in time. By weighting the samples before and after the test, this gives a measurement of the steady state mass loss per sliding distance, which can be used to calculate the wear coefficient
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according to Holm-Archard’s wear law. In the case of disc brakes, the run-in period could be more interesting than the steady state level, since the brakes are only applied for a short time with different brake pressure each time, in other words the brake pressure may differ between each brake event, which results in a new run-in taking place for each brake event. This means that the run-in may better simulate a real brake event. For example, in paper B it has been shown that the measured size distribution changes from a trimodal to a bimodal distribution after run-in. With the above discussion in mind, the trimodal size distribution may better correlate with the real situation.
Test conditions varied due to the limitations of the test stands (papers A-F). In the disc brake assembly test stand presented in paper D, designed to allow the study of low cylinder pressure levels, it would be desirable to increase the applied load by using a stronger hydraulic motor. Both of the test stands occurred under stationary load conditions. It would be of interest to extend the capacity of the test stands to handle more realistic (transient) brake events. This could be done in the disc brake assembly test stand by controlling the pressure from the hydraulic system and the rotational speed of the motor. It is harder to control the pin-on-disc test stand (paper A), where dead weights are used to apply a load, though the rotational speed can be controlled. Also, due to the limitations of the particle instruments, it is not possible to sample faster than one hertz, which makes it hard to capture transient brake events faster than this.
To ensure that the wear models used in the simulation methodology (paper G) have been implemented correctly in the finite element software, a simple simulation case with known expected outputs was conducted. This simple simulation case was conducted under steady state load conditions and the scaling up to the component level can easily be done by hand calculations. That is, the results show that the implementation is correct, but it remains to validate the methodology by comparing simulation results with experimental measurements conducted at the component level. Also, in order to simulate more realistic brake events with transient load conditions, where the contact pressure distribution and sliding velocity spread more, there is a need to map the wear and particle coefficient under different running conditions.
It should be noted that no thermoelastic effects were included in the FE model presented in paper G. Thermoelastic effects are important because the frictional heat at the contact surface may lead to thermal expansion of the disc and brake pad materials, which could significantly affect the behavior of the interface. Frictional heating also influences the oxidation rate on the contact surfaces, and thus indirectly the wear process. The use of finite element software allows for coupled thermo-mechanical analysis to include the effects of frictional heating.
Eriksson et al. [68] used a pin-on-disc machine to test pad material against a glass disc and recorded the development of the contact situation on video during the test. Their study visually showed the creation, growth and destruction of primary and secondary plateaus. Although the fiber size is smaller, the behavior of the contact simulation presented in paper H seems similar to their observations.
Ostermeyer and Müller ([39], [40]) used a cellular automaton model to simulate the contact situation between the pad and disc. They presented graphs of the creation, growth and destruction of contact patches (plateaus) that are similar to the results presented in paper H. In
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this work, the specific wear rate is not regarded as constant, which differs from their work. Also, in their structural analysis, Ostermeyer and Müller take into account the influence of the surrounding cells.
The models in papers G and H require a number of input parameters such as wear coefficients, particle coefficients, specific heats, thermal conductivities, pad composition, and elastic modulus. These parameters are crucial for the resulting output if the purpose is to predict the wear and airborne wear particles. To be able to determine these parameters, a great number of complex and time-consuming experimental tests must be conducted.
It should be mentioned that if the objective is to decrease the amount of airborne wear particles generated from disc brakes, it could be interesting to study other brake designs, where the airborne particles are taken care of at the source. For example, the use of some kind of enclosed brake system such as a wet brake or a drum brake, or the use of some type of filtering in the disc brake.
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4 Conclusions This thesis presents experimental methods to separate, capture and analyze airborne wear particles generated by disc brake materials in laboratory environments. Also, computational methods that numerically simulate the wear and airborne wear are presented. Five main conclusions can be drawn from this thesis:
The pin-on-disc machine (paper A) and the disc brake assembly test stand (paper D), both proven to produce results correlated to measurements made in car field tests (paper F), can be used to measure and rank the concentration of airborne wear particles generated from disc brake materials.
Ultrafine (nanosized), fine and coarse airborne wear particles generated from disc brake materials that contain metals such as iron, copper and tin were found (papers A-F).
The results presented in paper D suggest that the disc brake assembly test stand can be used to study rust layer removal from the disc, and that airborne particles are generated even at low brake pressures, used to keep the disc clean from dirt and/or rust.
The simulation methodology presented in paper G may be used to numerically calculate the number and size distribution of airborne particles generated by disc brakes.
The results in paper H show that the proposed cellular automaton approach can be used to computationally simulate the behavior of contact plateaus.
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