1 Introduction

Particulate matter (PM) in the atmosphere is a source of air pollution and, at sizes below 10 μm and 2.5 μm, is commonly known as PM10 and PM2.5, respectively. The PM is known to be, in part, originated from exhaust emissions from incomplete fuel combustion and from lubricant volatilization during the combustion process; however, non-exhaust emissions are also created through brake, tyre, and general vehicle wear processes, and through the resuspension of road wear particles [1]. The aerodynamic diameters of non-exhaust particulate emissions range from nanometers to micrometers according to temperature and pressure conditions during the wear process. Bukowiecki et al. reported that 21% of traffic-related PM10 originates from brake wear [2]. Several studies have estimated that 35−55% of brake wear particles become airborne [3,4,5,6] and may enter the respiratory system of humans where they may lead to adverse health effects [7].

Many studies have shown that the emissions and compositions of brake wear particles vary depending on type of the friction materials (non-steel, low-steel, semi-metallic, carbon composites, and gray cast iron of disk/drum) [4, 5], assembly structure (disks, drums, caliper) [4]. Vehicle operating conditions, including initial speed, deceleration, pressure, torque, and brake temperature, are also important for brake wear particle emission [4, 5, 8, 9]. To develop a technology to reduce non-exhaust particle emissions, it is very crucial to establish a reliable and realistic particle measurement platform considering the impacts of (1) materials, (2) assembly, and (3) driving conditions.

Even though many methods such as on-road [5, 10], pin-on-disk [11, 12], and dynamometer [8, 13] tests have been employed to measure brake wear particles, the most reproducible approach to measure brake wear particulate is by brake dynamometers [14]. Currently, a standard procedure for particle emission measurement using a dynamometer is being developed in the UNECE-GRPE-PM program (PMP-Group [14]). Several on-road tests have been applied in different environments (e.g., tunnels, street canyons, or roadside environments) to estimate brake wear emission factors for light duty vehicles (LDVs), and these studies have yielded quite a range of values, from 1.0 to 8.8 mg/km/vehicle (PM10), depending on the observation site and conditions [1]. In contrast, direct measurements using brake dynamometers with isokinetic sampling systems estimated brake wear emission factors in the range of 3.0–8.0 mg/km/vehicle (PM10) and 2.1–5.5 mg/km/vehicle (PM2.5) [1]. Compared to pin-on-disk and on-road tests, the dynamometer test produces much more reliable and reproducible results when the methods of sampling and drive cycle are properly controlled [15]. Isokinetic sampling is crucial for the accurate quantification of PM emission [16], and drive cycles should be simulated based of realistic driving and braking. Several drive cycles, such as LACT (Los Angeles City Traffic), WLTC (Worldwide harmonized Light duty driving Test Cycle), JE, JC, and so on, are being studied for the measurement of airborne PM [17, 18].

Brake pad (friction materials) typically comprises several components, including reinforcing fiber, binder, filler, and friction modifier. The reinforcing fiber provides the mechanical strength of the pads, which are made of metallic fiber or powder (Fe, Cu), ceramic fiber (potassium titanate), and aramid fiber. A binder holds the components of the pad together. Fillers reduce cost and improve the manufacturability of brake pads. The friction modifier determines the frictional properties of the brake pads and is comprised of a mixture of abrasives and lubricants.

According to the components, brake pads are generally classified into three categories, non-steel, semi-metallic, and low-steel. However, most commercial linings for mid- or full-sized cars fall into the low-steel or non-steel category. Non-steel friction material consists of organic binders and non-steel substances such as ceramic fiber, copper fiber, graphite, and various frictional modifiers [19]. The metal content of non-steel materials is typically lower than 10% and the coefficient of friction is quite low. The non-steel pad produces relatively low brake noise, low wear rate, and poor fade, which is the loss of braking efficiency during sustained high braking temperatures. The non-steel materials are commonly used in the US and Japan markets.

Low-steel pads predominantly consist of metallic components such as steel fiber. Total Fe content is typically in the range of 10–50%. The low steel is developed from non-steel materials in order to improve braking efficiency. Low-steel materials contain not only higher concentrations of reinforcing fiber (Fe), but also higher concentrations of abrasives. For this reason, low-steel pads have high friction and good fade, but with the drawbacks of high wear rates and noise. Low-steel friction materials are produced mainly for the European market.

Several studies can be found in the scientific literature concerning the relationship between wear mechanisms in disk brakes and relevant emissions of airborne PM [20,21,22,23,24,25], and furthermore, research interests have focused on not only the quantification of brake wear particle emission but also systematic study on the influences of materials on the issue via REBRAKE and Lowbrasys project.

In this work, the main aim was to elucidate the effect of composition of brake pad materials on the amount of airborne particulate matter from brake wear in a drive cycle of WLTC using a dynamometer. Three low-steel and four non-steel brake pad materials, representing the main classes of commercial products available now on the market, were investigated. The evolution of PM emission during the driving cycle was presented by time-resolved and temperature-dependent particle emission measurement. Furthermore, the relationship between the structure of the worn surface of the pads/disk and airborne PM emissions was investigated to clarify the impact of frictional mechanism, due to the pad component, on PM emission factors.

2 Materials and Methods

Direct measurement using a 1/5-scaled brake dynamometer, which was fully enclosed in a completely sealed chamber (800 × 370 × 555 mm, W × D × H) , was employed for a quantitative determination of airborne brake wear particle emissions. (Fig. 1) The chamber was air conditioned, and a constant flow of air (prefiltered air, 50% humidity, 20 °C) was drawn by a blower through the brake into a constant volume sampling (CVS) system [4, 8]. Air flow was cleaned using a high-efficiency particulate air (HEPA) filtering system.

Fig. 1
figure 1

A schematic of brake dynamometer and PM measurement devices. Brake assembly is situated in a constant environmental condition (temperature, humidity) controlled by environment control unit (ECU). Incoming air filtered by HEPA prevents experimental errors from PM outside. Isokinetic sampling of particles is made in the center of long straight transport line

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The brake assembly, consisting of a gray cast iron disk and a caliper mounted on a pair of pads, was installed in a closed chamber of stainless steel. The caliper for the disk brake was connected to the drive shaft via a gearbox (reduction ratio 1/5) and a 110-kW DC dynamometer was placed outside the chamber. The drive motor coupled to a speed-reducing gearbox delivers the torque and shaft speed needed for the dynamometer and the brake system, which was manipulated by movement of the piston of the master cylinder controlled by an actuator valve. A data acquisition and control system using personal computer controlled the temperature and friction of the rotor, speed, rotation, torque, and intake airflow rate.

The air flow rate, created by the negative pressure of a blower, was set at 3.2 m3/min, and was measured directly in front of the blower, using an air flow meter (TSI 9565-P, USA). Constant air flow was maintained using the feedback control of the blower. Vertical and isoaxial sampling took place at the center of the straight transport line. The inlets were situated in the sampling tunnel > 1.0 m from the driving shaft, at the center of the automotive brake system.

The real-time particle number concentration was measured by optical particle counter (OPC, GRIMM 11-A, Germany) with a time series resolution of 1 s. The particle mass concentration was calculated from the amount of scattered light according to Eq. 1 [26]. First, particle numbers in separate 32 channels of OPC are collected as a time series (1–1800 s). Then, particle mass for each channel is calculated from the number values assuming uniform density and spherical shape, volume: 4π/3*(di/2)3, at each measurement time. The parameter di represents a diameter of particle collected in each channel. Finally, total mass emission factor was calculated from the summation of particle mass for total measurement period (1–1800 s). Density was set to be calibrated value for brake wear particulate using gravimetric method. Previous studies showed that calibration factor of ~ 6 was used when the detector calibrated according to Arizona Road dust (SAE J726) that has a different composition and light scattering characteristics [13, 18].

$${\text{BEF}}\left( {{\text{mg}}/{\text{km}}} \right) = \mathop \sum \limits_{{i = 1}}^{{31}} PN*C*K*\rho *\pi *{{d_{i}^{3} } \mathord{\left/ {\vphantom {{d_{i}^{3} } 6}} \right. \kern-\nulldelimiterspace} 6}*f/l$$
(1)

PN Number concentration of particle emitted (#/L), l WLTC driving distance (23.27 km), C detector flow rate (L/min), 1.2, ρ density (g/cm3), 5, K = (R/r)2 duct diameter correction factor, R duct diameter, r detector line diameter. f scale dynamometer correction factor, front brake area correction factor *2 + rear brake area correction factor *2. di average diameter of ith channel (μm).

To quantify the mass emission of airborne brake wear particles, an air sampler equipped with Teflon membrane filters for PM10 (Microvol 1100, Ecotech Ltd., Australia) (PE47S05, 47 mm, Tisch scientific) was used. The Teflon filters used to determine the PM mass were kept in a constant humidity (50%) and temperature (25 °C) chamber for 24 h before and after sampling. The Teflon filters were weighed using a microbalance (MCA3.6P-2S01-M, Satorius, Germany) capable of weighing 0.1 µg. Filter blanks were also weighed before, during, and after each weighing session to verify the accuracy and consistency of the microbalance.

Before the abrasion tests, the friction surface of the brake assemblies was burnished more than 200 times at initial driving speeds of 80 km/h, deceleration of 0.3G, and not exceeding 120 °C of friction temperature, to remove all roughness [8]. The disk temperatures were monitored with a thermocouple contacted to the friction surface. In the test cycles, three successive runs of the WLTC were investigated without maintenance of the material (disk and pair of pads) between the runs (Table 1). After each run, the disk and pads were cooled for > 30 min before the next run.

Table 1 Composition of friction materials
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For the PM emission investigations, a sequence of the worldwide harmonized light vehicles test cycles (WLTC) was conducted. The cycle was developed by the UN ECE GRPE (working party on pollution and energy) group [14]. Among WLTC, a class 3B cycle with Vmax > 120 km/h and the highest power-to-mass ratio was employed. The WLTP was divided into 4 different sub-parts, each one with a different maximum speed (low, medium, high, extra-high). Compared to the 3 h-LACT-based brake cycle, the WLTC is categorized as a realistic, representative cycle since the driving phases simulate urban, suburban, rural, and highway scenarios, respectively, with an equal division between urban and non-urban paths (52% and 48%) [17].

3 Result

3.1 Friction materials

The materials from the front brake assembly of a typical medium-size passenger car were applied in the evaluation, which included a gray cast iron disk and two brake pads. The disk for the non-steel pads was made of gray cast iron with a 4.1% carbon equivalent content and a hardness of 200 HB (FC200). On the other hand, the disk for low-steel pads was cast iron with carbon of 4.3 wt% and a hardness of 170 HB.(FC170).

Three low-steel (LS-1/-2/-3) and four non-steel friction materials (NS-1/-2/-4/-5), representing the main classes of commercial products nowadays available on the market, were investigated. The phase composition of the studied friction materials was determined using XRD, FE-SEM, EDX, and XRF (chemical analysis) techniques, and the identified phases are listed in Table 1. The characteristics of the worn surface and the PM emission results of the friction materials studied in this work are shown in Table 2.

Table 2 Dynamometric results of friction materials
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In Table 2, the PM emission parameters obtained by dynamometric testing of the studied friction materials are listed together with the relevant tribological parameters (friction coefficient, disk wear loss, pad wear loss) and surface analysis results (roughness, contact plateau area). The PM2.5 and PM10 emission factors, expressed as the amount of airborne particulate matter emitted per distance (km) per vehicle, were reported.

LS (Low-steel) friction materials consisted of reinforcements of steel fiber with abrasives (Al2O3 and Cr3C2) accompanied by mild abrasives such as MgO. The LS formulation is considered to have more abrasive character than non-steel, while the abrasive effect of this friction material is balanced mainly by the presence of graphite and metal lubricants such as SnS2. The LS-3 friction material contains a slightly lower amount of copper (~ 4.5%) than LS-2 (~ 7%).

The formulation of the NS (non-steel) friction material was very different from LS, with high amounts of ZrO2 accompanied by low amounts of Al2O3 as the hard abrasive, which was balanced by the graphite, Sb2S3, and MoS2. The NS-1 formulation is a typical one for Cu-free non-steel friction materials using K2Ti6O13 as reinforcements. The NS-2 formulation uses K2Ti6O13 and zinc fiber as reinforcements, and the friction behavior is balanced by Sb2S3. The NS-4/5 friction material obviously contains considerable amounts of Cu fiber (1.9%/2.6%) and Sn (2.4%/0.8%), respectively. The NS-5 friction material also contains a considerable amount of MoS2.

3.2 Time Series Profile

First, to study the evolution of airborne brake wear particle emissions as a function of vehicle operating conditions and cycles, time-resolved measurements of the wear particles were carried out with a time resolution on the order of 1 s. Time series variations in PM2.5 and PM10 mass emissions during WLTC cycles were measured using light scattering laser photometers (OPC). Figure 2 shows an example of the time series for number concentration (#/s/vehicle) of PM10 during typical WLTC-3B cycle experiments (runs #1–3). Run #1 for each vehicle, which, in general, resulted in lower PM emissions compared with run #3. As shown Fig. 3, fine particles (< 2.5 μm) and coarse particles (2.5–10 μm) are significant components of brake wear particles in the PM number and mass patterns, respectively. Even though the number concentration of PM2.5 was much higher than that of PM10, the calculated mass of fine particles was smaller than that of coarse particles.

Fig. 2
figure 2

a Time-resolved profiles of particle number concentration for LS-3 materials. b Time-resolved profile of particle number concentration for NS-5 materials

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Fig. 3
figure 3

Time evolution of a particle number (PN) and b particle mass (PN) concentration profiles for LS-1, LS-3, NS-1, and NS-5 materials

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In general, large peaks of brake wear particles appeared during the extra-high-speed region between 1500 and 1800s for WLTC-3B. (Fig. S1). In our measurements, no consistent trend of PM emission versus run number was found; however, the details of the emission patterns varied with different brake pad materials (especially for the LS-3 and NS-5 materials), as shown in Fig. 2a, b. A discrepancy in PM emission occurred from run #1 to #3 with the LS-3 and NS-5 friction material, as shown in Table 2. However, there were no significant and consistent trends between runs in the PM emission patterns except LS-3 and NS-5. (Fig. S2). The LS-3 material shows an emergence of large peaks at 800 ~ 1500 s, showing increased PM emission with increasing run. The LS-3 material exhibited a constant increase in PM emission factor and the number concentration of PM10 with increasing dynamometric runs as shown in Fig. 2(a). The NS-5 exhibited much higher emission peaks in the region of 1700–1800s; however, with increasing runs, significant PM emission peaks were found in the low-speed region (0~1200 s). The NS-5 material showed an increase of PM emission factor and number concentration of PM2.5/PM10 with increasing runs.

In Fig. 3, the emission curves for the friction materials in Table 1 according to time are displayed. These results show that there are notable differences in PM (size range 0.25–32 μm) emissions for the different friction materials. In general, it turns out that compared with the LS friction material, the NS materials exhibited lower PM emission curves over the entire range of airborne particle sizes. A significant reduction was achieved by NS-4/-5, whose emission curves lay below the NS-1/-2 curve, indicating a reduction in the total amount of emitted airborne particulate of a non-steel material containing Cu and Sn. The LS materials containing Cu, LS-2, and LS-3 also exhibited smaller PM emission values than LS-1, as shown in the NS materials. Table 2 shows that the best performing low-steel and non-steel friction materials were LS-3 and NS-5, which is consistent with the PM emission factors determined by gravimetric analysis using filtration. For the gravimetric method using filtration, the mass emission factors of PM10 for the LS and NS materials were estimated to be 2.92–1.39 and 2.47–0.25 mg/km/vehicle, respectively, which is consistent with the optical data obtained by OPC.

The PM emission factors in our work are rather smaller than those reported by Garg et al. [4], Sanders et al. [5], and Perricone et al. [27]. It should be note that airborne PM emission is dependent on driving cycle, analysis method, and friction materials. In the previous works, driving cycles of BSL-035 [4], UDP/AMS [5], and SAE J2707 B [27] were applied, in which wear condition is more severe than the present WLTC. As airborne PM emission is significantly related to temperature, the emission factor is also varied according to driving cycle. Perricone et al. showed the composition of friction materials and property of disk also affect PM emission factor significantly because tribology mechanism would be changed according to the composition and surface profile. These effects will be discussed in the following Sects. (4.1–4.3).

Figure 3 shows the particle mass size distributions of emissions from the LS and NS friction materials, from the low-speed region (1~550 s) to the extra-high-speed region (1500~1800s) under the transient driving cycle tests. The mass distributions from the LS materials were a bimodal shape with a small peak in the 0.25–2.5 μm, and a large peak in the 2.5–10 μm. NS materials showed similar distributions with two peaks in the 0.25–2.5 μm and 2.5–10.0 μm range. A significant contribution was also found from ultrafine particles (< 0.25 μm) for LS-3 materials. This means that larger amounts of fine particles (< 0.25 μm) were generated from NS materials even though the number of fine particles was not collected because of a detection limitation.

The particle number of PM2.5 and PM10 for the LS-1 materials was roughly 1.0–2.0 times that of LS-3 both for the low-speed and extra-high-speed regions. This is consistent with the total PM mass emission factor for the LS materials (3.14, 1.78 mg/km/vehicle for LS-1 and LS-3, respectively). The particle number of PM2.5 for the NS-1 materials was ~ 2.0 times that of NS-5 both for the low-speed and extra-high-speed regions; however, on the other hand, the PM10 for NS-1 was much higher than that of NS-5. This discrepancy was more conspicuous in the extra-high-speed region than in the low-speed region. The PM10 value for NS-1 was ~ 10 times that of NS-5, and this value corresponds to the total PM mass emission factor (2.34, 0.3 mg/km/vehicle for NS-1 and NS-5, respectively). This result implies that the PM10 emission in the high-speed region was much higher than in the low-speed region, which determines the total PM mass emission factor. This result was also shown in the time series profiles in Fig. 3, showing the PM emission in the extra-high-speed region (1500–1800s) represents the brake wear emission of the materials.

3.3 Surface Profile Analysis

Figure 4b illustrates a schematic of a surface profile of brake pads worn by the dynamometric tests. The surface profile of a tested specimen (45 × 18 mm) separately analyzed 20 sections of a pad as shown in Fig. 4a. In Fig. 4c, a color map of 20 separate regions of a pad was constructed according to relative height measured by laser confocal microscope. The red area represents a contact plateau formed by friction between the pad and disk, which is known to be constructed by the precipitation of wear debris due to high temperature and pressure during braking. On the other hand, the dark area (black, purple, blue) represents an area having relatively low height, which is abraded or pulled out during friction. The green area indicates an area suffering relatively minor abrasion.

Fig. 4
figure 4

a Sampling points of surface profile for friction materials by laser confocal microscope. b An example of 3D surface profile in color map of (c). c Color map of worn surface of NS-4 materials. d histograms of relative height in worn surface (Color figure online)

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In general, NS materials exhibited a relatively low proportion of black area (low height, low land) and higher green area (average height). On the other hand, there was no conspicuous difference in the red areas (high height) of the NS and LS materials. This is probably due to the less aggressive character of the NS materials toward the gray iron disk, resulting in low surface roughness in the surface profiles. This also implies that the low emission factor for the NS materials is also related to the low surface roughness after driving cycles.

4 Discussion

4.1 PM Emission vs Composition

Figure 5 shows the relationship between the area/roughness and PM emission factor of the NS and LS materials. The red or black areas with high or low heights, respectively, do not show a distinct relationship to emission factor; however, the green area (average height) shows a negatively proportional relation to PM emission values, indicating that LS materials with low green area have relatively larger PM emission values. Figure 5b clearly shows that surface roughness is positively proportional to PM emission factors for both the NS- and LS materials.

Fig. 5
figure 5

Relationship between particle mass emission factor with respect to a green area and b surface roughness of worn pad surface after dynamometric test (Color figure online)

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It is well known that copper plays a dual role, as a reinforcing hard metal providing primary contact sites between a pad and disk, and as solid lubricant to enable smooth sliding conditions and reduced noise generation [22]. The first function is fulfilled when copper is present as a macro particle, whereas the second function is achieved when copper nanocrystals are detached and incorporated in the interface between the pad and disk, also called a tribolayer or third body layer. It has been revealed that a soft graphite inclusion improves sliding performance and minimizes friction force fluctuation; however, the carbonaceous material may lose this lubrication property at elevated temperature [28, 29]. On the other hand, recrystallized nanocrystalline copper contributes to the formation of lubricating tribofilms above the dynamic recrystallization temperature (> 530 °C) [30, 31]. This property has also been reported for soft constituents such as Sn, Sb2S3, and SnS2, which incorporate easily into the tribolayer.

In our work, friction materials containing metallic ingredients of Cu and/or Sn show much reduced PM emission compared to the other materials having graphite, barite, and MoS2. Figure 5 and Table 2 show the resulting PM emission factor as follows: LS-1(Cu-free) > LS-2(Cu-containing) > LS-3(Cu-containing); NS-1(Cu/Sn-free) ~ NS-2 (Cu/Sn-free) ≫ NS-4(Cu/Sn-containing) > NS-5(Cu/Sn-containing). In particular, the NS-5 material, having both Cu and Sn, was found to be superior in PM emission although it showed almost the same performance in frictional characteristics as CoF. However, the friction -modifying ingredients of chalcogenides with low melting point such as Sb2S3, and SnS2 do not exhibit a beneficial effect on the reduction of PM emission. Therefore, further study on the role of Cu and Sn on frictional behavior affecting PM emissions in comparison to chalcogenide ingredients such as MoS2, Sb2S3, and SnS2 is needed.

4.2 PM Emission vs Wear vs Surface Topography

As shown in Table 2, the wear amounts of the LS pads (0.102–0.190 mm) were much larger than those of the NS (0.013–0.052 mm). The disk wear was also found to be considerably higher when the wear test was carried out using the LS (0.311–0.413 g) than the NS (0.017–0.035 g). The substantial disk wear induced by the LS pads was attributed to the steel fibers that aggravated the gray iron surface through metallic adhesion and abrasion [32]. Such aggressiveness of steel fibers toward gray iron disks have been reported by previous studies [33, 34]. On the other hand, the disk and pad wear for the NS pads were considerably smaller than those for the LS pads, thereby indicating lower aggression of the NS pads against gray iron disks considering the smoother disk surface after the wear tests, as shown in Tables 3 and 4.

Table 3 Surface analysis results of low-steel pads
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Table 4 Surface analysis results of non-steel pads
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In the case of the LS, the pad surface exhibits smaller contact plateaus (red area, 47(3)%) and larger lowlands (black area, 11(3)%) than NS (50(7)%, 9(2)%), as shown in Tables 2 and 3. The contact plateaus are generated during sliding due to the hard ingredients exposed on the sliding surface (Primary contact plateau), and the wear debris compacts accumulated in front of the ingredients (Secondary contact plateau). On the other hand, the surface of the NS pads was relatively uniform with larger plateau area and smaller lowlands. Average roughness (Ra) of the LS pads ranged from 3.6 to 4.87 μm, whereas that of the NS was less than 2.3 μm.

It has been reported that rough surfaces were more susceptible to wear because of increased pressure on the contact area with increasing surface roughness, which is because the wear mode changed from adhesion to flake formation and fracture [35]. Such local pressure on the contact plateaus increases wear rate and wear debris generation [36]. In this work, smaller area of contact plateau and larger area of lowlands on the surface of LS pads should induce higher local pressure on the contact plateaus, compared to the NS having relatively uniform surfaces [37]. As the wear rate is proportional to PM emission factor, the LS pads exhibit higher emission factor than the NS, as a result. In addition, the lowlands could provide open channels for the wear debris, preventing them from accumulation to formation of secondary plateaus. The wear debris stored in the lowlands could be a possible source of airborne PM, which is released during driving.

4.3 Temperature Effect on PM Emission

The number and mass of particles emitted for each brake stop were analyzed in relation to the temperature of the disk because the PM emission is closely correlated to the temperature. Each of the 28 brake stops was classified into intervals between the nearest two stops. The total number and mass of emitted particles for each stop were calculated for each time interval using OPC data providing time-resolved data. In Fig. 6a, the total number (Ntot) is plotted versus mean disk temperature (Tdisk) for 1–3 cycles. Ntot increases steeply beyond the critical temperature, ~ 150 °C. The increase in PN at distinct brake temperatures has also been reported recently by Alemani et al. [12] in a pin-on-disk study. In contrast to the previous study, the shift in Tcritical toward a higher temperature with cycle repetition was not observed, probably because of the rather mild braking conditions of the WLTC compared to LACT [18].

Fig. 6
figure 6

Relationship between a particle number (PN) concentration b particle mass(PN) concentration with respect to disk temperature during dynamometric test

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Figure 6b also shows a clear correlation between PM emission and temperature. Furthermore, the LS-1 (copper-free) and NS-5(copper/tin-containing) exhibited distinct differences in PN and PM data. The copper/tin-containing material (NS-5) in particular showed remarkably reduced particle emission beyond the critical temperature. This result supports the hypothesis that PM emission is suppressed in the presence of copper or tin, particularly at high temperature, as expected from the surface profile analysis (Sect. 3.3), which is also consistent with the comparisons among friction materials.

For low steel, the copper containing LS-2/-3 exhibited a lower PM emission factor than LS-1, and, for non-steel, NS-4/-5 with copper and Sn additives showed much lower emission factor than NS-1/-2. Soft metallic components such as copper and tin seem to enable solid lubrication of the frictional surface, as shown in the profile analysis, which is beneficial to the reduction of PM emissions. This effect is particularly effective at severe conditions (high temperature, high speed, high deceleration) as shown in the correlation between PM and temperature.

5 Conclusion

In this work, we investigated airborne PM emissions from various brake pads under brake dynamometer operation. Three low-steel and four non-steel friction materials, which are commercial products currently available on the worldwide market, were examined in relation to surface topography analysis.

  1. 1.

    The time-resolved data show that PM10 emissions in the extra-high-speed region in WLTC were much higher than in the low-speed region, which determines the total PM mass emission factor.

  2. 2.

    The friction materials containing Cu and Sn exhibited much reduced PM emissions, suggesting that solid lubrication of the frictional surface at severe condition (disk temperature > 150 °C, high speed) is crucial to reducing PM emissions.

  3. 3.

    According to a surface profile analysis of the brake pads worn by dynamometric tests, the surface roughness was positively proportional to PM emissions. Contact plateaus and lowlands produced on the pad surface are closely related to the amount of brake emission.

  4. 4.

    In general, it turns out that compared with the LS friction material, the NS materials exhibit lower PM emissions based on the optical and gravimetric measurement methods.