An Experimental Study on the Fine Particle Emissions of Brake Pads According to Different Conditions Assuming Vehicle Deceleration with Pin-on-Disc Friction Test

Abstract

Fine particles from vehicles occur in a range of particulate matter (PM) sizes and influence the roadside atmosphere. The contribution of fine dust from automobiles to road pollution has reportedly been extremely high. Researchers have estimated that non-exhaust fine dust originating from brakes, tires, clutches, and road surface wear rate is increasing. Several studies have shown that brake pads account for a significant proportion of non-exhaust emissions. In this study, a friction test using vehicle brake pads was carried out with a friction tester to reveal the harmfulness of brake pad particles by the driver’s driving habits. Conditions were made considering the pressure, vehicle speed, and temperature and assuming the amount of deceleration of the vehicle. Particle collection devices were used to analyze the concentration of number and the mass distribution of particles produced in the experiment, with a range from 6 nm to 7.3 μm to gauge the toxicity of particles. The results showed that the number concentration of fine particles tended to increase linearly with changes in vehicle deceleration (braking energy) in the particle diameter region around 0.75–7.3 μm. The number concentration of fine particles tended to increase exponentially in the particle diameter region around 71–120 nm. The rapid occurrence of ultrafine particles in nanometers varied depending on the test conditions.

1. Introduction

Emission regulations have significantly reduced the number of floating particles on roads since the 1990s. Although the amounts of sulfur oxides and nitrogen oxides floating in the air have decreased, they have remained at certain concentration levels. According to the World Meteorological Agency, the concentration of suspended solids in Seoul has been relatively high compared to those in major cities worldwide for the last 10 years. In addition to exhaust emissions, particles such as road dust resuspension, road surface abrasion, brakes, and tire wear also contribute to air particle concentrations [1]. As the amount of fine dust emitted by engine combustion has greatly reduced, there has been increasing interest in fine dust generated by non-exhaust sources. In the road pollutant sector, as announced by the German Environment Agency, the relative contribution of non-exhaust emission fine dust will account for 93% of PM10 and 74% of PM2.5 by 2030, and similar results were predicted by the NAEI (National Atmospheric Emissions Inventory) [2]. The most important factors that result in direct particulate matter emissions are tires, brakes, clutches, and road wear [3].

Braking rotors that require the ability to absorb the high temperature and friction energy generated by vehicle braking and that can quickly release heat energy have been inspected for decades. Gray cast iron has been widely selected because of its poor wear properties in contact with brake pads and the low generation of harmful substances owing to wear processes [4]. However, there have been continuous developments and changes in the materials of brake pads. Brake pads are typically made of asbestos, semi-metallic (SM) materials, non-asbestos organics (NAO), and low-strength and low-carbon materials [5,6]. Asbestos fibers are largely avoided because of their carcinogenic properties, and new asbestos-free friction materials and brake pads have been developed [7]. Automotive brake pads typically include reinforcing fibers, binders, abrasives, lubricants, and fillers. Reinforced fibers such as brass, copper, steel, aramid, carbon, and glass are used to determine the main components of brake pads. A binder holding the ingredients of a brake pad provides bonding strength, and phenolic resin is generally used [8]. Studies have found that reinforced fibers and abrasive components are used for significant changes in the friction coefficients and strength. The friction coefficient depends on the features of the contact surfaces and the operating conditions, such as contact pressure, sliding velocity, and temperature. The formulation of brake pad components mainly affects the wear rate of the brake pad system [5,8,9,10,11]. Therefore, the ingredients of the pads used in the friction experiment vary from study to study [12]. Although the components of brake materials may vary according to the manufacturer, the main composition of particulate matter emitted by brake applications shows common components: iron, copper, antimony, and barium [1,13].

Several challenges arise when studying non-exhaust traffic-related sources and particulates resulting from brake wear. One reason is that there is a lack of an experimental method for releasing and collecting fine particles from brake pads. Euro published the official method of sampling and measuring brake particle emissions from full-friction brakes in a dynamometer system in November 2022, but it still needs a standard form in other methods. This leads to different approaches of our study for comparing results. To analyze the fine dust generated from brake pads, it is necessary to collect fine particles generated from an actual vehicle. However, when measuring brake particles in the air during field tests, it may be difficult to distinguish them from particles produced from other sources, such as construction, tires, and road resuspended dust [14,15]. Brake dynamometer experiments have implemented brake discs, pads, hydraulic systems, and vehicle power sources. These can be made similar to the actual vehicle environment, but the collected resultants of fine particles are quite different from the real field, and the main reason is the difference in the sampling system. The generated fine particles changed rapidly with some distance between the vehicle and the sampling spot by coagulation and molecularization [16,17]. Pin-on-disc distribution meters have been frequently used in many studies to identify the friction and wear behaviors of disc brake materials in controlled environments, but each experiment has different results by researchers due to different test methods [18,19,20]. In this study, the pin-on-disk method was adopted to find out the friction characteristics in low-temperature areas that are not feasible in actual vehicles and dynamometers. Previous studies controlled the cleanliness of the air surrounding the pin-on-disc, which makes it possible to measure the air abrasion particles generated by the contact pair [14,21]. The change in the amount of wear of metal materials according to atmospheric components is a well-known fact. As a result, fine particles in the normal state and the experimental state were observed in real time and were tested analyzing the difference of each one.

When analyzing brake pad particulate matter, the particle number concentration is an important factor because several studies have found significant values among the distributions. Sanders found peaks in a relatively large one micrometer-sized fine particle section during a sampling of light-duty vehicle brake wear [22]. Iijima found number concentration peaks around 1 to 6 μm with different types of pads at different temperatures [23]. There are also results with peaks in sections of tens to hundreds of nanometers in size. Studies on brake dynamometer tests have found several nanoparticles of 100–200 nm analyzed by SMPS around 300 °C [24]. Olofsson found the characteristic particle number concentrations of airborne brake wear particles with different brake pads, resulting in peaks at 100, 280, 350, and 550 nm [14,25]. Studies of brake pad specimens have shown a section where the incidence rate of ultrafine particles increased at 160 °C [26]. In the low temperature region, an inspection of the first reaction of a brake pad revealed that it corresponded to the evaporation of substances inside the pads, which led to the occurrence of fine particles [27].

In this study, considering the increasing contribution to air pollution from the friction of brake pads, a tribo-meter was used to conduct a friction experiment. In order to simulate the friction shape between the brake pad and the rotor, the surface contact method was selected. In the case of actual brake pads, the temperature rises sharply even if friction proceeds for a few seconds due to heat. Therefore, a pin-on-disc specimen test was designed to minimize the variables caused by temperature. By using a friction tester to analyze the concentration and distribution of particles produced under different driving conditions, the study provides valuable insights into the impact of vehicle deceleration on roadside air pollution. Number concentration data were obtained by analyzing fine particles generated from the friction test. We investigated and analyzed the characteristics of fine particles that depend on the conditions of the experiments.

2. Materials and Methods

A friction experiment is required to study the fine particles generated from brake pads in vehicles. In this study, a specimen of the brake pad and rotor was manufactured, and a pin-on-disk friction experimental device was designed. H-company brake pad products that are generally well-known and commercially available, with non-asbestos materials, were selected as disc specimens. XRF composition analysis is listed in

Table 1. Composition of the brake pad specimen.

Formula	Concentration (wt%)
Ba	37.5
Fe	26.35
Ca	14.8
S	6.36
Al	5.43
Si	4.77
K	1.72
Zn	1.63
Mg	0.6
Sr	0.596
Mn	0.23

. A pin of 10 × 5 × 10 mm (width × depth × height) gray cast iron was held with the jig connected to the load bar as the relative specimen. The contact surface was round shaped to prevent severe abrasion of the endpoint of the specimen and to fix the contact area. The pump was connected to the top of the environmental stage for analyzing airborne particles. A K-type thermocouple was attached to the zig of the pin specimen for temperature measurement. For PM analysis, Electrical Low-Pressure Impactor (ELPI+, DEKATI, manufactured in Finland) equipment related to the pump was used to draw air into the ELPI equipment and perform a real-time analysis of the fine particles. The average values of the number concentration of particles during the friction test period were used to obtain the results. A schematic of the entire system is shown in Figure 1.

It is necessary to select a vehicle type to establish detailed test conditions. We set the passenger car, which accounts for the largest proportion of roadside pollution, as the reference model. The assumed vehicle state values are listed in

Table 2. Values and units of the conditions.

	Values	Units
Vehicle mass (m)	1600	kg
Correction factor for rotating masses (k)	1	-
Heat distribution on rotor	0.9	-
Tire slip	0.08	-
Swept area of one brake side	67	cm2
Initial brake temperature	300	K
Speed of vehicle (V)	60	km/h
Deceleration (a)	4.91–17.66	m/s2

.

A theoretical equation for calculating the energy of brake power is required to match the condition of a real vehicle brake system. To solve this problem, the method of calculating the heat energy applied per unit area of the actual pad was selected [28]. The thermal energy generated when a passenger car with arbitrary conditions was applied in the brake was calculated according to the following process.

When the vehicle decelerates a higher velocity $V_1$ to $V_2$, the braking energy $E_b$ is as follows:

$$E_b=\left(\frac{m}{2}\right)\left({V_1}^2-{V_2}^2\right)+\left(\frac{I}{2}\right)\left({{\omega }_1}^2-{{\omega }_2}^2\right), \mathrm{N}\mathrm{m}\left(\mathrm{l}\mathrm{b}\mathrm{f}\mathrm{t}\right)$$

(1)

where $I$ = mass moment of inertia of rotating parts, ${kgm}^2\left({lbfts}^2\right)$

m = vehicle mass, $kg({lbs}^2/ft)$

$V_1$ = velocity at begin of braking, $m/s(ft/s)$

$V_2$ = velocity at end of braking, $m/s(ft/s)$

${\omega }_1$ = angular velocity of rotating parts at beginning of braking, $(1/s)$

${\omega }_2$ = angular velocity of rotating parts at end of braking, $(1/s)$

If the vehicle comes to a complete stop, $V_2={\omega }_2=0$, then Equation (1) becomes the following:

$$E_b=\frac{{m{V}_1}^2}{2}+\frac{{I{\omega }_1}^2}{2}, Nm\left(lbft\right)$$

(2)

When all rotating parts are expressed relative to the revolutions of the wheel, then with $V=R\omega$, Equation (2) becomes the following:

$$E_b=\frac{m}{2}\left(1+\frac{I}{R^{2}m}\right){V_1}^2\approx \frac{km{V_1}^2}{2}, Nm\left(lbft\right)$$

(3)

where $k$ = correction factor for rotating masses $(k\approx 1+I/R^{2}m)$

R = tire radius, m(ft)

Typical values of k for passenger cars range from 1.05 to 1.15 in high gear to 1.3 to 1.5 in low gear. The corresponding values for trucks are from 1.03 to 1.06 and from 1.25 to 1.6, respectively. Braking power $P_b$ is equal to braking energy divided by the time t during which braking occurs.

$$P_b=d\left(E_b\right)/dt, Nm/s(lbft/s)$$

(4)

If the deceleration a is constant, then the velocity V(t) is given by the following:

$$V\left(t\right)=V_1-at, m/s(ft/s)$$

(5)

where a = deceleration $m/s^2(ft/s^2)$

T = time, s

Equation (3) through (5) yield the brake power as follows:

$$P_b=kma\left(V_1-at\right), Nm/s(lbft/s)$$

(6)

Equation (6) reveals that braking power does not stay constant during the braking process. At the beginning of braking (t = 0), the brake power is at a maximum, and it decreases to zero when the vehicle stops. The time $t_s$ for the vehicle to come to a stop is calculated as follows:

$$t_s=V_1/a, \mathrm{s}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}$$

(7)

The average braking power $P_b$ over the braking time $t_s$ for a vehicle coming to a stop is calculated as follows:

$$P_b=\frac{kmaV}{2}, Nm/s(lbft/s)$$

(8)

Equation (8) shows the basic general calculations of the energy absorption. The tire slip, heat distribution, brake area, and percentage braking of brake pads should all be considered. Tire slip is defined by the ratio of the difference between vehicle forward speed and circumferential speed to vehicle forward speed. The friction energy varies by deceleration, which can be controlled by the load and rpm of the friction experiment. The conditions of the friction test are listed in

Table 3. Friction test conditions.

Test pressure	0.25 MPa, 0.66 MPa
Test rpm	120, 180, 240, 300
Temperature	292–295 K
Cycle	7200
Humidity	Under 30%

. The test was conducted for a total of 7200 cycles for 60 min based on 120 rpm. This is 40 min based on 180 rpm, 30 min based on 240 rpm, and 24 min based on 300 rpm. For the ELPI+ equipment, airborne particles were captured and separated into stages according to their sizes. The concrete size of each stage is listed in

Table 4. Average and max diameter of particle sizes.

Stage	Average Diameter (μm)	Max Diameter (μm)
1	0.006	0.010
2	0.016	0.022
3	0.030	0.040
4	0.054	0.071
5	0.094	0.12
6	0.15	0.19
7	0.25	0.31
8	0.38	0.48
9	0.60	0.75
10	0.94	1.2
11	1.6	2.0
12	2.5	3.0
13	3.6	4.4
14	5.3	7.3

. The experimental data were set by obtaining the difference between the average of the data values stabilized after the run-in phase and the average of the data values before the initial experiment. An example used to extract data values is shown in Figure 2.

3. Results

The experiments were conducted based on the conditions detailed in Section 2. The surface profiles and micro-images of brake pads are shown in Figure 3. The surface profiles prove that there was a clear difference between the base material and worn surface. The run-in effect might have lowered the height deviation of the worn area. The materials mixed in the brake pad caused adhesive-like wear due to friction (Figure 3 D, circle). The results of the number concentrations of the particles are shown in Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10. The horizontal axis represents the average diameter of the collected particles, and the vertical axis represents the number concentration. A total of 14 chambers were used to collect the particles, but if there was no difference in the amount collected before and after the test, then it was not marked in the graph. Specific numbers were labeled on the data marked with dots.

4. Discussion

Experimental results for each condition are shown in Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10. In Graphs 3 and 4, which had relatively low pressure and low speeds, the occurrence of ultrafine particles was not confirmed. The emission of relatively large particles between 1 micrometer and 10 μm can be found. In Graphs 5 and 6, the emission of ultrafine particles of less than 10 nanometers increased with respect to rpm increase. In Figure 8, the pressure increased to 0.66 MPa, and the overall pattern of the graph was similar to 0.25 MPa. Figure 8, Figure 9 and Figure 10 show that the amounts of ultrafine particles produced increased several times to several tens of times as rpm increased. Comprehensive data are shown in Figure 11. Wear particles larger than 0.75 μm in diameter appeared to increase as the friction condition became harsh; this was found by comparing visibly seen red dots and black dots. However, some conditions showed similar results, and in order to quantitatively evaluate them, the brake power equation was substituted [28]. Accordingly, the conditions changed from the pressure and speed values to the deceleration of the vehicle. If listed sequentially through the calculation formula, values such as those in

Table 5. Deceleration values corresponding to friction conditions.

Pressure (MPa)	0.25				0.66
RPM	120	180	240	300	120	180	240
Deceleration (m/s2)	4.91	7.36	9.81	12.3	8.83	13.24	17.66

could be obtained.

Assuming the number concentration as the amount of wear, the wear factor could be obtained for each particle diameter. Comparing the data between the deceleration and the amount of wear in Table 5, the form can be shown in a graph like that of Figure 12. The graph was linear overall, and the slope varied slightly from diameter to diameter. Between 1.2 and 3 μm, the average diameter showed the highest number, and the number of moments when it falls below 0.75 and above 7.3 μm decreased dramatically (black, brown dots). As the above graph shows, it is reasonable to insist that the fine particles from the micrometer region were caused by mechanical abrasive wear. The abrasion of coarse particles dominated the friction process, and the worn particles rolled against the bulk materials, which caused multiple body abrasions and led to increasingly complicated friction. We assume that the main reason for this result was the difference in the contact area of the brake pad specimen and the complexity of the pad material. The surface of the pad was not flat, and friction occurred in the actual contact area but not in the apparent area. Consequently, the temperature of the actual pad surface was distributed irregularly, and the pressure distribution changed continuously for the same reason [29]. This imbalance affected the sheer force of the junction, resulting in a change in the amount of wear debris.

By sorting the fine particles in region B in the same way, a graph, such as that shown in Figure 13, could be derived. In the case of fine particles, in the region under 1 micrometer, the focus was on chemical action rather than mechanical wear in the junction. As the deceleration increased, the number of ultrafine particles generated tended to increase linearly at the log scale of number concentration. Overall, a large number of abrasion particles with a diameter of 0.1 micrometer occurred. It can be seen that, when the generated particles were replaced with the amount of wear, it increased exponentially as the deceleration increased. The number of particles generated varied greatly depending on the particle size, which is known as the difference in the mechanism of fine particle generation.

A related study found that the mechanism of ultrafine particles results from the process of nucleation and coagulation when the friction surface exceeds a certain level and evaporates [30]. It can be assumed that the average 0.071 micrometer particles in Figure 13 were caused by the aggregation of 0.12 micrometer particles. Additionally, the detection of fine particles before 100 °C resulted from the evaporation of condensed oils or ethanol, which are mostly contained inside the brake pads [27]. In this experiment, a thermometer was stuck to the zig to observe temperature changes. Due to small contact areas and pressure, the temperature never exceeded 100 °C during the tests (Figure 14). According to Equation (8) used in this experiment, an increase in velocity increases the contact heat energy, and the heat energy absorption may also increase. The increased heat energy absorption might promote subsequent processes and lead to an increase in ultrafine particle emissions.

We suspect the occurrence of specific substances due to the increase in fine particles of certain regions. For the consideration of this phenomenon, the results of the research related to the generation of ultrafine particles should be inspected.

Critical temperatures exist in some studies, and many fine particles are generated at specific temperatures during brake pad friction tests. However, studies have shown that the intensification of particles only occurs at the size of ultrafine particles with a change in heat generation. A study that changed the temperature from 100 to 300 °C reported that the emission of ultrafine particles intensified, whereas that of coarse particles decreased. Moreover, the critical temperatures between 165 and 190 °C facilitated the emission of ultrafine particles at a rate of four to six orders of magnitude [31]. A significant release of nanoparticles was observed when the average temperature of the rotor reached 300 °C because of the chemical decomposition of the organics in a brake pad [24]. Reportedly, 1.3–4.4 nm particles predominate in number at temperatures above 160 °C with brake pads of low metallic and non-asbestos organics [26]. The detection of fine particles before 100 °C is caused by the evaporation of condensed oils or ethanol, which are mostly contained inside the brake pads. Between 250 and 475 °C, decomposition of the binder system begins, and the minimum temperature at which oxidation occurs is 300 °C [27]. Ultrafine particles seem to have a large effect on temperature. Reference data showed that only 10 repetitive braking actions at a speed of 91 km/h at a speed of 159 km/h resulted in the temperature of the brake pad exceeding 300 °C [32]. In order to analyze the driver’s breaking habits originally planned, the temperature of the initial brake pad had to be considered. In future studies, we plan to supplement this part and include it in the results and use it for analysis.

5. Conclusions

The conclusions of this study are as follows: The number concentration of fine particles tended to increase linearly with changes in vehicle deceleration (braking energy) in the particle diameter region from 0.75 μm to 7.3 μm. The number concentration of fine particles tended to increase exponentially in the particle diameter region from 71 nm to 120 nm. It was confirmed that, when replaced by the deceleration of the vehicle according to each condition, small fine particles were rapidly released at a level of about 8 m/s² (assuming that the vehicle is the general passenger car, running at 16.7 m/s; detailed conditions are shown in Table 2). This research is particularly significant in the context of current air pollution challenges and the increasing contribution of non-exhaust sources, such as brake pads, to the problem. The study’s unique approach and findings have the potential to inform the development of new brake pad materials and the implementation of measures to reduce non-exhaust emissions from vehicles, ultimately contributing to improved public health and environmental quality, especially in urban areas where air pollution is a major concern.