Morphology and Composition of Brake Wear Particles Ameliorated by an Alumina Coating Approach

Abstract

A plasma-assisted electrochemical deposition (PAECD) technology was introduced to coat a cast iron brake disc for the possible reduction of brake wear and brake wear particle (BWP) emission. The majority of the coating consisted of alumina (Al₂O₃), determined by energy dispersive X-ray (EDX) analysis and X-ray diffraction (XRD) analysis. To validate the above strategy of the coating technology for automotive brake corners, one brake stock rotor was replaced by a PAECD-coated rotor for a vehicle road test. After the road test, weight loss of the brake components (rotors and pads) was measured, showing that the alumina coating can reduce the brake wear by more than 70%. BWPs were also collected from wheel barrels, spokes, and brake friction rings of the coated and uncoated rotors during the road test. A morphology and chemical composition analysis of the collected BWPs indicated that the coating could reduce BWP generation from the original sources and avoid a metal pick-up (MPU) issue, leading to less metallic content in BWPs. This alumina coating may provide the auto sector with a sustainable approach to overcome the brake dust emission problem, evidenced by less wear of the brake pads, minimal wear of the coated brake rotor, less MPUs, and a clean wheel rim on the coated brake corner.

1. Introduction

Particulate matter (PM) emissions from road traffic can be categorized into exhaust and non-exhaust, based on their sources. As exhaust PM emissions, such as vehicle tailpipe emissions, decrease due to stringent worldwide regulations and successful abatement technologies, non-exhaust PM emissions, resulting from the wear and tear of brakes, tires, clutches, road surfaces, and road dust resuspension, have witnessed a recent increase in percentage [1]. Many studies indicate that brakes constitute the most significant contributor to emissions among non-exhaust PM sources [2]. Approximately 40–50% of brake wear particles (BWPs) become airborne, while the remaining portion settles on the road and the brake components [3]. Non-airborne BWPs can be resuspended, partially becoming airborne again, or can be carried with precipitation through drains to reach the soil and water sources, posing a threat to human and aquatic life [2]. Exposure to BWPs may lead to health issues such as acute respiratory infections, lung cancer, and chronic respiratory and cardiovascular diseases [4].

Conventional grey cast iron brake rotors possess drawbacks such as excessive wear and poor corrosion resistance. The occurrence of corrosion on the cast iron rotors amplifies both the number and mass of BWPs [5]. Employing heavy braking to eliminate the corroded rusts on the rotor surface could lead to an increase in material loss induced by corrosion, thereby exacerbating brake emission issues.

Numerous original equipment manufacturers (OEM) and their suppliers are exploring surface treatments, including laser cladding [6] and thermal spray techniques [7], to tackle problems of wear and corrosion in cast iron brake rotor applications. However, the heavy metallic chemicals (such as Cr, Ni, and W) often employed in laser cladding and thermal spray coating can be released into the environment alongside BWP generation from the coated brake rotor surfaces, although those coated rotors have a much-improved wear resistance. Additionally, the metallic powders and the oxidation, melting, and vaporization of various metallic materials during these coating processes might also produce toxic fumes, carrying some health risks.

An environmentally friendly ceramic coating, devoid of toxic metallic elements, is desired to apply onto cast iron brake rotors. Such a kind of coating can be prepared using a plasma-assisted electrochemical deposition (PAECD) method [8,9]. Previous studies [10,11,12] have substantiated the capabilities and demonstrated the performances of the PAECD application on cast iron rotors in various laboratory testing conditions. The alumina coatings exhibit excellent wear resistance [13], as well as good corrosion resistance [8,13] when applied to ferrous metals. Specifically, the alumina coating distinguishes itself by its notable exclusion of toxic heavy metallic chemicals, setting it apart from alternative coating methods. However, the morphology and chemical composition of BWPs generated from an alumina-coated brake rotor sliding against a brake pad at a road driving condition remain unknown.

In this study, the PAECD method was used to prepare an alumina coating on one of the cast iron brake front rotors of a commercially available automobile. The rotors had shallow drilled and slotted friction rig surfaces (which may cause difficulty for other coating processes, like laser cladding and thermal spraying, to produce a high-quality coating). The alumina-coated rotor, alongside an uncoated stock cast iron brake rotor employed as a reference, was then installed on the sporty vehicle for a vehicle road test. During the road test, two pairs of commercial low-metallic brake pads were used as the counterparts of the tribological couplings. BWPs generated throughout the road test were gathered from brake systems with both coated and uncoated stock rotors. A scanning electron microscope (SEM) and an energy dispersive X-ray (EDX) spectroscopy were used to observe and analyze the collected particles. The research emphasized the comparison study, including morphology and chemical compositions of the BWPs originating from the alumina-coated and uncoated cast iron brake rotors, as well as their corresponding brake pads.

2. Materials and Methods

A PAECD method was used to prepare an alumina-based coating on a cast iron brake front rotor. To achieve the alumina-based coating, the rotor (anode) was polished with abrasive papers and then treated with an electrolyte containing 15–20 g/L sodium aluminate ($\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_2$) and 1–5 g/L sodium phosphate ($\mathrm{N}{\mathrm{a}}_3\mathrm{P}{\mathrm{O}}_4$). A stainless-steel plate served as the cathode.

The mechanism of the PAECD process in this study is as follows: when the high-voltage anodic current was applied, the iron from the cast iron substrate oxidized and dissolved into the electrolyte as Fe²⁺ cations, as follows:

$$\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)\to {\mathrm{F}\mathrm{e}}^{2+}\left(\mathrm{a}\mathrm{q}\right)+2{\mathrm{e}}^{-}$$

(1)

The $\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}$ anions (formed from $\mathrm{N}\mathrm{a}\mathrm{A}\mathrm{l}{\mathrm{O}}_2$ in the aqueous electrolyte) migrated toward the anode, reacting with ${\mathrm{F}\mathrm{e}}^{2+}$ cations to form a hercynite ($\mathrm{F}\mathrm{e}{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_4$) film on the iron surface, as follows:

$${\mathrm{F}\mathrm{e}}^{2+}\left(\mathrm{a}\mathrm{q}\right)+2\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}\left(\mathrm{a}\mathrm{q}\right)\to \mathrm{F}\mathrm{e}{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_4·4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)\downarrow$$

(2)

With the continuous hercynite ($\mathrm{F}\mathrm{e}{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_4$) film completing the coverage of the surface, plasma discharges commence. This thin film functioned as an interfacial bonding layer. Then, the $\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}$ anions migrated towards the anode, where the oxidation reaction occurs in the following manner:

$$\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}\left(\mathrm{a}\mathrm{q}\right)\to \mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_3\left(\mathrm{s}\right)\downarrow +{\mathrm{O}\mathrm{H}}^{-}$$

(3)

The insoluble $\mathrm{Al}{\left(\mathrm{OH}\right)}_3$ was simultaneously decomposed into aluminum oxide (${\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3$) through high temperature plasma sintering, forming a top layer, as follows:

$$2\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_3\left(\mathrm{s}\right)\to {\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)\downarrow +3{\mathrm{H}}_2\mathrm{O}$$

(4)

Previous research has reported on the details of the PAECD procedure on cast iron [8,10,11,12,13]. The morphology of the coating was examined using a SEM, while the elemental composition was assessed through an EDX. The phases and compositions of the coating were confirmed or determined using an XRD.

In this investigation, a commercial sporty passenger car (BMW 330i Xdrive) was chosen as the vehicle road test platform, and it had two drilled and slotted cast iron front rotors. The drilled and slotted surfaces would likely lead to increased BWPs compared to plain surfaces during brake events. One of the brake rotors was coated using the PAECD method. The coating thickness was ~30 µm. After the coating preparation, the coated rotor was slightly polished with abrasive papers, rinsed with water, and air-dried at room temperature before being installed in the driver’s side front brake corner. The as-coated rotor was polished to an arithmetic average roughness (Ra) of ~2.4 μm, while the roughness of the uncoated cast iron rotor is Ra ~1.3 μm. Simultaneously, an uncoated stock brake rotor was fitted in the front brake on the passenger’s side as a reference. Two pairs of commercial low-metallic front brake pads served as counterparts for the installed rotors.

The total test mileage was around 9000 km, with days that had sunny, rainy, and snowy weather. The roads included city, urban, country roads, highways, and mountain areas. The installed new rotors are presented in Figure 1a,b. Midway through the road test (after ~4500 km of driving), both sets of brake pads were replaced with new pairs. Before the road test, the chemical elemental distribution of the brake pad materials was observed and analyzed using a SEM and an EDX. When the driving mileage reached ~1000 km, BWPs generated during the road test from both the PAECD-coated and uncoated front cast iron brake rotors were collected using carbon tapes from the wheel barrels, spokes, and rotor surfaces. An example of BWP collecting locations is shown in Figure 2c,d. SEM and EDX were employed to investigate these BWPs. After the road test, the brake pads used for both PAECD-coated and uncoated rotors were reanalyzed using SEM and EDX. Thickness changes of brake rotors and pads were measured before and after the road test to assess their wear behaviors.

3. Results

3.1. Coating Characterization

The morphology of a ceramic coating deposited on a cast iron rotor using the PAECD method was examined using a SEM. Figure 2a,b displays SEM images of the coated cast iron rotor surface at different magnifications, revealing a characteristic dimple-like morphology. This surface texture is a result of the plasma discharge events occurring during the coating process, which contribute to the formation of porosity (~10–12%). An EDX surface analysis of Figure 2a was utilized to determine the elemental composition of the coating, confirming aluminum and oxygen as the primary elements, as shown in Figure 2c. This indicates that the coating primarily consists of aluminum oxide, with a minimal presence of other elements. To further investigate the phase composition, an XRD analysis was conducted. The XRD patterns, as shown in Figure 2d, confirmed that the coating is mainly composed of $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, with the presence of $\mathrm{F}\mathrm{e}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_4$. Additionally, peaks corresponding to Fe were detected, which are from the cast iron substrate. These observations align with the previous studies [8,10,11,12,13], which have reported similar phase compositions and morphology characteristics in the alumina coating prepared using the PAECD technique. Vickers hardness measurements were conducted in the previous studies [11], where the hardness of the alumina coating (with pores) is ~795 HV, higher than that of the grey cast iron (~185–250 HV). After the experiment, the roughness of the rotor surface with and without coating was Ra ~2.2 μm and Ra ~0.9 μm, respectively.

3.2. Brake Pad

Two pairs of commercial low-metallic front brake pads were used for the vehicle road test. Figure 3a shows a photo of the new brake pad before the test. Using SEM–EDX, the chemical elemental distribution of the brake pad materials was determined in EDX mappings. The SEM image with EDX mapping of four main chemical contents of the original brake pad surface are given in Figure 3d. Graphite in the brake pad material functions as a lubricant to reduce unexpectedly high coefficients of friction (COF), which can otherwise cause brake system overheating. Steel fibers working as reinforcing fibers are visible in the pad material shown in the SEM image. Mineral substances (containing iron (Fe), calcium (Ca), aluminium (Al), silicon (Si), magnesium (Mg), manganese (Mn), zinc (Zn), and barium (Ba)) are also used in the pad friction materials.

After the road test, the used brake pads for both PAECD-coated and uncoated brake rotors were removed and subjected to a second phase of the SEM–EDX analysis. Figure 3b,c shows photos of the brake pads working with coated and uncoated rotors, respectively. The surface of the utilized brake pad for the uncoated stock rotor (shown in Figure 3c) has some grooves that follow the rotational direction, while the surface of the pad for the PAECD-coated rotor (shown in Figure 3b) is rather smooth. Those grooves on the pad surface worked with the stock rotor were attributed to the abrasive braking events. The wear debris from the cast iron rotor can cause such surface grooves in the direction of rotation on the pad surface. After applying the PAECD coating to the rotor surface, the friction type was changed from abrasive wear between the pad and cast iron to adhesive friction between the pad and the coating [10,11,12], where the wear particles from the pad materials formed a smooth and thin transferred layer on the coated rotor surface due to the coating’s dimple-like morphology [10,11,12,13].

The EDX mappings illustrating the four main chemical contents of the used brake pads for both coated and uncoated rotors are presented at the bottom of Figure 3e,f. Comparing them to the EDX mapping of the original pad surface, the used pad surface for the PAECD-coated rotor exhibits an increased presence of Al. This elevation is attributed to the slight polishing wear of the alumina-based coating’s top layer. In contrast, the brake pad surface worked with the uncoated stock rotor shows a much higher concentration (indicated by the much brighter colour in EDX mapping) of Fe compared to other situations. This discrepancy arises from the cast iron rotor surface, where iron was worn off and subsequently picked up by the brake pad surfaces during the abrasive braking events [14].

In the middle of the entire road test (~9000 km), the brake pads tested with the as-installed rotors were replaced with two pairs of the same new pads. The thickness of the four pairs of brake pads, corresponding to both PAECD-coated and uncoated cast iron brake rotors, was measured both before the installation and after the road test. Figure 4 shows the average thickness loss of brake pads on both inboard and outboard sides for PAECD-coated and uncoated stock brake rotors during the first (~4500 km) and second (~4500 km) halves of the road test. In the first half of the test (shown by the grey bars), the thickness loss of the inboard pad for the PAECD-coated rotor was slightly lower than that for the uncoated stock rotor. At the same time, the thickness loss of brake pads installed on the outboard side for both coated and uncoated rotors was very similar. In the latter half of the test (shown by white bars), it became evident that the brake pads working with the uncoated rotor, both inboard and outboard, experienced greater material losses compared to those with the coated rotor. Considering the pad thickness loss and the density of the pad materials, the total weight loss of brake pads on the PAECD-coated side was 33.14 g throughout the entire road test, while it amounted to 38.94 g for the uncoated side. As a result, the coatings resulted in an approximately 15% reduction in brake pad weight loss in total compared to the uncoated side. The relatively excessive material loss of the brake pads could be attributed to the drilled and slotted surfaces of the brake rotors.

3.3. Brake Rotor

Before the road test, the sporty passenger car was installed with the new PAECD-coated and uncoated stock cast iron brake front rotors (Figure 2). During the road test, the coated and uncoated brake rotors had photos taken of them from time to time. In a rainy day after one night of outdoor parking, pictures of the rotors with and without the PAECD coating are shown in Figure 5a–d. Figure 5b illustrates that some corroded rust appeared at the fringes of the friction ring and drilled and slotted areas on the uncoated cast iron surface during the wet weather, whereas none was seen on the PAECD-coated surface, as shown in Figure 5a.

After parking outside, the road test with braking events proceeded again. Figure 5c,d shows photos of the PAECD-coated and uncoated cast iron brake rotors taken following the braking events. The coated rotor surface (shown in Figure 5c) looks the same as before, while the corroded rusty areas on the uncoated rotor (shown in Figure 5d) were cleaned during the heavy braking, in which iron oxides, as the form of BWPs, were generated from the cast iron rotor surface. It has been stated that the corrosion issue on the cast iron brake rotor can increase both the number and mass of particle emissions by over 50% [5]. The alumina-based ceramic coating can protect the cast iron brake rotors from their drawbacks of inferior corrosion resistance.

The PAECD coating on the cast iron rotor remains intact after the vehicle road test. Small worn steps are noticeable at the inner and outer edges of the friction ring surface of the uncoated rotor, whereas the coated rotor surface appears flat. Measurements of the coated and uncoated rotor thickness were taken before the installation and after the road test. Figure 5e depicts the average thickness loss for both rotors. The uncoated stock rotor experiences a loss of over 0.2 mm in overall thickness, whereas the coated rotor’s total thickness slightly increased.

The weight loss of the uncoated brake rotor throughout the road test is about 69.97 g based on thickness loss, nearly double the total pad weight loss on the same side (38.94 g). The uncoated rotor contributed to about 64% of the total weight loss in the brake rotor and pad system. The increased average thickness of the coated rotor is attributed to the thin transferred layer formed on the coating surface due to friction materials from brake pads. The difference in thickness variation indicates that the passenger’s side with the stock rotor produced significantly more wear particles from the brake rotor itself than the other side. Considering the total weight loss of the brake pads used for the coated and uncoated side, the brake pairs with PAECD coatings experienced a 70% reduction in total weight loss compared to those without the PAECD coatings. The PAECD coating demonstrates significant potential to increase the lifespan of the brake rotors and nearly eliminate the BWPs produced from the rotor itself.

3.4. BWP Analysis

BWPs were collected from the surfaces of wheel barrels, spokes, and the friction rings of both rotors. The collected particles were investigated using SEM–EDX. SEM images in Figure 6a,d depict BWPs collected from barrels of the brake systems with PAECD-coated and uncoated rotors under magnification of 1000. The particle size typically ranges from over 50 µm to less than 2.5 µm, categorized as fine particulate matter with a diameter of 2.5 µm or less (PM_2.5, particulate matter 2.5 µm or less in diameter). In Figure 6d, the bright large particle, as identified through EDX analysis, consists of iron debris torn off from the uncoated cast iron rotor. The presence of the cast iron debris indicates severe wear on the cast iron rotor surface during friction braking events. Compared to Figure 6a for the coated brake side, Figure 6d shows a much larger number of BWPs collected from the uncoated brake side, indicating that the uncoated brake system generated more airborne BWPs, and that some of the particles were stuck on the barrels of the wheel.

When comparing the BWPs collected from the friction ring surfaces of the PAECD-coated and uncoated rotors, a noteworthy disparity emerges. The coated rotor surface, as shown in Figure 6b,c, exhibits a significant number of BWPs. This elevated count is attributed to wear particles from the brake pad materials, which formed a thin transferred layer on the PAECD coating surface. Such a phenomenon was also previously found in small coupon-level lab tests [10,11,12,13]. This transferred layer not only provides additional protection for the substrate cast iron rotor, but also potentially retains wear particles produced during the braking events and prevents the immediate PM emission to brake corner component surfaces like spokes or the surrounding environment, owing to the dimple-like morphology of the PAECD coating surface. However, when a carbon tape was used to extract the BWPs from the coated rotor surface, the transfer layer was partially torn off and adhered to the carbon tape during the BWPs sampling collection, as shown by many particles in Figure 6b. Even if the clustered BWPs (Figure 6c), which appeared in the format of transfer layer [10], were chipped off, the size of the BWPs would be much larger than PM₁₀ (particulate matter 10 µm or less in diameter), which is considered to be less harmful than small particles.

On the other hand, the number of BWPs collected from the uncoated rotor friction ring surface (shown in Figure 6e,f) is relatively minimal. This is attributed to the scarcity of pad materials transferred onto the rotor surface. Considering the weight loss data from the preceding sections, the total weight loss from the uncoated side is about 3.28 times that of the PAECD-coated side. Consequently, the BWPs generated from the uncoated side may exceed three times that of the other side. This emphasizes the role of the PAECD coating in substantially mitigating the release of BWPs onto the surface of the wheels and into the surrounding environment.

Figure 7a,b presents the SEM images of BWPs collected from the spoke surfaces with PAECD-coated and uncoated cast iron brake rotors at a magnification of 5000. The particle diameter in both instances ranges from more than 10 µm, between 2.5 and 10 µm (PM₁₀), to less than 2.5 µm (PM_2.5). Multiple characteristic spots were selected and subjected to EDX analysis. The chemical composition weight percentages of the chosen spots in Figure 7a,b are shown in Figure 7c,d. Additionally, the total chemical contents of the entire collected BWPs shown in Figure 7a,b were also analyzed using EDX, and are presented in the late bars of Figure 7c,d.

As shown in Figure 7b, there are two different types of BWPs from the uncoated side, which are as follows: large particles with flat surfaces (Spots A–C) and smaller irregular particles (Spots D–F). Figure 7d shows that the flat particles possess higher Fe content (67.82 wt% in average) than the irregular ones (53.88 wt% in average), which indicates that Fe from the stock rotor was transferred to the contact plateaus and formed metal pick-up (MPU) on the pad surface during brake events before being released as BWPs [14,15]

However, the correlation between the BWP morphology and Fe content observed in BWPs collected from the spoke surface with an uncoated rotor does not apply to those collected from the spoke surface with a PAECD-coated rotor. Figure 7a obviously shows flat particles (Spots A and B) and irregular particles (Spots C–F). The flat particles are relatively small in both size and number, while the irregular particles tend to form small agglomerates. The small particles are agglomerated at the sliding interfaces, and the size of agglomerated particle is influenced by the strength of friction films on the contact plateaus [15], indicating that the larger irregular particle agglomerates on the PAECD-coated side can be attributed to the formed transferred layers on the PAECD coating. Compared to Figure 7a,b, the particles generated from the PAECD-coated brake corner are significantly larger in size. Furthermore, due to the absence of wear from the cast iron rotor matrix, there is almost no MPU from the brake rotor by the brake pad surface.

Shown in Figure 7a,c, Spot A contains 68.24 wt% carbon, suggesting that this flat particle could be graphite or an organic material that detached from the pad materials. An irregular particle (Spot D) possesses the highest Fe content (39.73 wt%), while another irregular one (Spot E) contains the lowest Fe (20.23 wt%), excluding Spot A. Therefore, with the application of the PAECD coating, flat BWPs may not necessarily have higher Fe content than irregular ones. The contents of Al and other chemical elements at Spots B–F range from 7–10 wt% to 15–21 wt%, respectively. The presence of the Al element may originate from both the friction materials and the polishing of the alumina coating’s top loosening surface during the bedding-in process. It is essential to highlight that the alumina coating could only release aluminium oxide during braking events, devoid of toxic heavy metallic elements.

Figure 7d shows chemical compositions of the representative BWPs generated from the brake pairs with the uncoated stock rotor, which exhibits 2–3 times the weight percentage of Fe compared to those collected from the PAECD-coated side. Increased metallic content heightens potential health risks upon exposure to released particles. In contrast, the Al contents (less than 0.8%) and the other elements in the BWPs collected from the uncoated side are substantially lower than those from the coated side. The notably higher Fe contents in the BWPs collected from the uncoated side align with the considerable wear observed on the stock cast iron rotor.

Based on the EDX results and the absence of thickness loss for the coated rotor shown in Figure 5e, nearly all of the Fe content in the BWPs on the coated side can be derived from the steel fibers and other friction materials containing Fe in the brake pads. Consequently, the BWPs predominantly originated from the brake pads when they were sliding against the PAECD-coated brake rotor. In contrast, a significant portion of the BWPs from the uncoated side comprises iron oxides formed from the cast iron rotor surface.

4. Discussion

Rotor-pad contacts involve the following two types of friction mechanisms: abrasive and adhesive friction [16]. Research has reported that the cast iron brake rotor undergoes abrasive-like friction sliding, while the PAECD-coated surface experiences adhesive-like friction sliding combined with compaction, chipping, and regeneration of the transferring layer [10,11]. In the case of the uncoated cast iron rotor, abrasive friction results from the breaking of bonds in both the pad material and the rotor upon contact, leading to wear on both the pad and rotor surfaces. However, with the application of a hard coating featuring dimple-like surface morphology, a shift to adhesive friction occurs, driven by the development of the transferred layer. This transition implies that both surfaces in contact become effectively the same or similar materials. The friction generated in this contact is achieved through the rupture or shear of bonds identical to those found in the pad materials. The transferred layer can also transfer back to the pad surface to compensate for pad material loss. Additionally, the transferred layer can isolate direct contact between the pad and the coating to reduce wear. As a result, the tribological couplings (both the pad and rotor) experience the reduced wear, eventually leading to decreased BWP generation.

The SEM–EDX analysis conducted in this study suggests that the BWP generation mechanism can be altered by applying an alumina coating to the cast iron brake rotor. In braking events involving the uncoated cast iron brake rotor and the low-metallic pad, a substantial quantity of BWPs, containing the Fe element (iron and iron oxide), were produced from both the cast iron rotor and steel fibers with other friction materials that contain Fe in the brake pad. Since only a few wear particles can adhere to the rotor surface, most BWPs are immediately emitted into the air or consequently adhere to other braking components. The iron particles from both the rotor and pad may undergo oxidation to form iron-containing oxide particles due to the high temperature during frictional braking. Additionally, some iron and iron oxide particles may contribute to the formation of MPU on the pad surface [14], which can subsequently corrode and detach as BWPs.

In the case of the PAECD-coated cast iron brake rotor, BWPs predominantly originate from the brake pad materials and, to a lesser extent, from the polishing the alumina-based ceramic coating during the brake bedding-in process. The dimple-like coating serves as firm mechanical interlocking anchor sites, facilitating the rapid formation of a transferred layer on the rotor surface. This thin layer developed during the braking events consists of friction materials transferred from the brake pad. The formation of the transferred layer protects the rotor surface from abrasive wear and, to some extent, capturing BWPs generated during the braking events. As a result, wear on the coated cast iron rotor is nearly nonexistent, and BWP emissions are primarily generated by the brake pads.

When the transferred layer accumulates to a certain thickness, its formation and disruption become dynamic, followed by some localized spalling of the transferred layer material. Subsequently, this material is released as a larger PM. Some wear debris generated during the polishing of the ceramic coating in the bedding-in process may also transfer to the pad surface. Furthermore, the PAECD coating acts as a protective barrier against corrosion on the cast iron substrate, significantly reducing the corrosion-induced wear emissions. Consequently, the PAECD coating not only minimizes BWP emissions related to corrosion issues, but also serves to mitigate the overall PM emissions from the brake system. This dual functionality can position the PAECD coating as a sustainable solution for environmental and health concerns associated with brake wear PM emission. 

Mass concentration has been a common approach used to correlate the atmospheric PM and its effects on human health [17]. One reason for that approach may be due to more research data and implemental instruments being available. However, the concentration of atmospheric PM is not the only relevant factor to be considered. For instance, the aerosol oxidative potential (OP) is considered to better represent the acute health hazards of aerosols as opposed to the mass concentration of fine particulate matter (PM_2.5) [18]. Oxidative potential (OP) measurements provide additional information that allows us to evaluate and integrate the toxic potential of PM samples in a way that connects emission sources, size distribution, and/or chemical composition. The OP of the PM is known to be largely related to the content of transition metals, such as Fe, V, Cr, Mn, Co, Ni, and others, and soluble transition metal species, which can initiate reactive oxygen species (ROS) formation both directly and indirectly through redox-mediated mechanisms [19].

Bio-reactive iron can catalyze the formation of damaging reactive oxygen species (ROS), leading to oxidative stress and cell damage or death. With iron overload, the iron-catalyzed Fenton reaction can transform less ROS, such as the superoxide anion (O₂(−)) and hydrogen peroxide (H₂O₂), into the highly reactive hydroxyl radical (HO·), which can attack proteins, lipids, and DNA, leading to cell and tissue damage [20,21].

The metal pick-up issue occurring from auto brake systems would result in more metal-enriched PMs, causing more cases of the toxicity mentioned above [20,21]. This study showed that a PAECD coating can alter the interface interaction between the brake rotor and brake pads, avoiding the metal pick-up phenomena from the cast iron brake rotor. Furthermore, the PAECD coating has demonstrated a benefit in reducing the wear of the brake rotor and pads, and, thus, less PM emission. In future works, OP estimations are needed to help understand whether the PM sources generated from the coated brake systems could provide less health hazards compared to an uncoated brake system. The results would lead to the proposal of a feasible coating strategy for PM emission control.

5. Conclusions

A plasma-assisted electrochemical deposition (PAECD) technology was introduced to coat a cast iron brake disc for the possible reduction of brake wear and brake wear particle (BWP) emission. The majority of the coating consisted of alumina (Al₂O₃). The alumina-based ceramic coatings were applied to drilled and slotted cast iron brake rotors using a PAECD technique. Due to the absence of toxic heavy metallic chemicals in the coating process, combined with the environmental friendliness of the coating materials, the PAECD coating method can be considered to be a sustainable and responsible technology. Through a road test of a sporty car installed with a PAECD-coated cast iron brake rotor (to replace one of the uncoated stock rotors), the implementation of the PAECD coating demonstrated a remarkable reduction of brake wear, primarily attributed to avoiding rotor wear. In detail, the alumina-coated rotor can result in a roughly 15% reduction in brake pad material loss compared to the uncoated rotor. The PAECD coating can reduce the total weight loss of the brake rotor and pads by 70%, compared to that of the uncoated brake corner.

Our analysis of the BWPs generated from the uncoated side suggests that the uncoated brake rotor produced the following two different types of BWPs: large flat particles with higher Fe content and small irregular particles with lower Fe content, on one hand. On the other hand, the application of PAECD coating altered the particle morphology as a result of the easy formation of agglomerated larger irregular particles due to the presence of the transferred layer, which, in turn, rendered the particle formation process and Fe content. Since the PAECD coating provided the cast iron rotor with an alumina ceramic surface, the ceramic surface eliminated MPU from the otherwise metallic rotor surface by the brake pads and subsequent BWPs. Notably, BWPs from the uncoated side contain Fe content almost three times higher than those from the coated side. The PAECD coating approach would thus reduce health risks due to it resulting in less BWP generation and lower metallic content. The PAECD-coated brake rotor could emerge as a promising sustainable solution for achieving significantly cleaner brakes and reducing the environmental and health impacts which arise from automotive brake systems.