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Article

Effects of Titanate on Brake Wear Particle Emission Using a Brake Material Friction Test Dynamometer

by
Emiko Daimon
* and
Yasuhito Ito
Otsuka Chemical Co., Ltd., 463 Kagasuno, Kawauchi-cho, Tokushima 771-0193, Japan
*
Author to whom correspondence should be addressed.
Lubricants 2024, 12(11), 387; https://doi.org/10.3390/lubricants12110387
Submission received: 30 September 2024 / Revised: 31 October 2024 / Accepted: 6 November 2024 / Published: 12 November 2024
(This article belongs to the Special Issue Emission and Transport of Wear Particles)
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Abstract

:
We investigated the effect of lepidocrocite-type layered titanate, which is compounded in brake pads, to reduce brake particle emissions. The dust reduction effect of titanate was evaluated using a small-scale inertial brake material friction test dynamometer. The results suggested that brake particle emissions are related to the microphysical structure of the pad surface, such as the uniformity of the friction film and secondary plateau formation, and that friction materials containing titanate contribute significantly to reducing both particle mass (PM) and particle number (PN) emissions of brake particles in both non-asbestos organic (NAO) and low-steel (LS) pads. In particular, LS pads generally have a problem of having more brake particles than NAO pads, but this study found that brake particles can be significantly reduced by compounding titanate instead of tin sulfide.

1. Introduction

Automobile disc brakes typically consist of a cast iron disc and a system of two or more brake pads integrated into calipers [1,2,3,4,5,6]. During braking, the pistons and calipers press the pads against the disc, generating high contact stresses and dissipating the kinetic energy of the car’s travel as heat [1,2,3,6]. Optimal brake-pad friction materials require not only mechanical durability but also excellent thermal conductivity and a relatively high and stable coefficient of friction [1,3,7,8]. Brake pads are manufactured from a blend of 10 to 20 different materials, depending on the specific application and expected life. The most widely applied materials for brake pads are organic resin binders [9]; fillers, such as barium sulfate [10,11]; friction modifiers, such as metal chips or abrasive metal powder [12,13]; organic polymer composites, such as aramid pulp [14,15] or steel fiber as reinforcement fiber; and solid lubricants [7,8,16]. Materials of various origins have been tested as friction modifiers and solid lubricants for brake pads [1,7,8], suggesting that the selection of brake-friction materials is still an open research topic and not fully understood [7].
Solid lubricant antimony sulfides [7,8] have been reduced or eliminated by most brake-pad manufacturers due to environmental concerns. As an alternative to antimony sulfide, tin sulfide is often used as a typical metal sulfide component of solid lubricants [16]. Tin-resource reserves, however, are gradually decreasing, posing a challenge to the stability of the tin-resource supply. The main causes are deteriorating resource–supply conditions, increasing demand, and heightened geopolitical risks [17]. Reports on the systematic study of metal sulfides are still scarce [7,8]. Small amounts of molybdenum disulfide in combination with graphite [18] or lepidocrocite-type-layered titanate [19,20,21] used in our study have also been reported as alternatives to the conventionally used whisker-like titanates [22].
Titanate plays an important role in improving friction stability and wear resistance by forming a strong friction film on the friction surface along with the synergistic effects of titanate shape [23] and aramid pulp [24]. Previous studies have pointed out that potassium ions in titanate promote the decomposition reaction of phenolic resin [19]. As a result, it has been pointed out that the friction coefficient is stable because no sticky resin residue remains during friction [20]. In addition, as the friction load increases and the temperature rises, phenolic resin not only decomposes but also carbonizes. The potassium ions in titanate also promote this carbonization reaction, so the friction coefficient is stable not only under low load but also under high load [21]. Considering the above, previous studies have mentioned the effect of the decomposition and carbonization of phenolic resin by titanate on the stability of the friction coefficient, but no research has been conducted on the reduction of wear and wear powder.
Pad-surface contact during braking also consists of two different contact regions: a primary plateau and a secondary plateau. A primary contact plateau consists of large hard components such as metal fibers and coarse abrasives, which are exposed to the friction material surface until they are worn away. A secondary contact plateau is formed by the compression of wear particles around the primary plateau, and their size and mechanical properties vary depending on the composition of the wear particles and braking conditions [25,26]. Thus, the formation of the contact plateau and brittleness of the friction film plays an important role in the wear resistance of titanates [27,28]. Restrictions on the release of brake particles into the atmosphere from brake wear have recently been introduced in Europe, raising concerns about brake dust [29]. However, little research has been conducted on the effects of titanates, which are used in brake pads on brake dust. Because the fundamental makeup of friction materials varies widely, it is nearly impossible to predict the effect that any single component will have on a friction material’s braking performance characteristics [6]. The friction test results and many observations obtained from dynamo tests help infer behavior during friction.
Brake pads for basic braking systems are available in a wide range of friction material compositions, including gray cast iron brake discs. The most important categories are low-steel (LS) pads (also called European performance “ECE”), which are developed and produced primarily for the European market, and non-asbestos organic (NAO) pads designed primarily for the US and Asian markets [6]. These two types of pads can be distinguished by, among other things, the percentage of steel. LS pads contain a significant percentage of steel fiber, while NAO pads are usually free of steel fiber [6,30,31]. Brake dynamometer experiments have verified that LS pads have higher brake-particle emissions and NAO pads have lower emissions when comparing commercial brake systems [30,32]. Although a reduction in brake-particle emissions with titanate has been suggested for commercial brake systems [30], there is a lack of knowledge on the role of the friction-modulating ability of titanate in brake-particle emissions for NAO and LS pads.
Brake dynamometer experiments typically involve different scales, depending on the purpose of the investigation. An inertial brake dynamometer with a full-scale braking system is used, for example, for measuring brake-particle emissions in the atmosphere, for which emission regulations have recently been established [29]. Following the Global Technical Regulation No. 24, the adjusting cooling-flow environment (temperature, humidity, wind speed, etc.) can be controlled, allowing for reliable emission measurements [33,34,35]. Although our test machine is different from the GTR No. 24 design, we set the conditions to reproduce a test environment similar to that of the GTR No. 24. Another possible method is to install a brake dust collector on the inertial brake dynamometer in accordance with JASO C470 [30,36]. By adjusting the environment for the cooling-flow rate, it is expected that the particle size and quantity of brake particles emitted from the test brakes will be equivalent to those in Global Technical Standard No. 24 and JASO C470 [30]. Inertial brake dynamometers are advantageous for reliable measurement of brake particle emissions from brake pads, but they have the disadvantage that the equipment is bulky and time-consuming to set and analyze. Pin-on-disc tribometers can be used to investigate material-level contact pairs in a controlled laboratory environment [37,38,39,40,41,42,43]. Material-level experiments using pin-on-disc tribometers are similar to those performed with inertial brake dynamometers in terms of wear and brake particle emissions [37,38,39,40,43]. Rapid evaluations using pin-on-disc tribometers are advantageous for analyzing friction and wear mechanisms, but a major disadvantage is that the braking mechanism is very different from inertial dynamometers, since it is not inertial. Therefore, the temperature, pressure, and friction energy applied to the friction surface are different. Therefore, it is not advantageous for evaluating friction-based brake particle emissions in urban driving.
We therefore aimed to investigate the role of lepidocrocite-type layered titanate used in our previous studies [19,20,21] in reducing brake-particle emissions, as an additive to NAO and LS friction materials and an alternative to metal sulfides. The effect of titanate on reducing brake-particle emissions in urban driving was also evaluated. Previous studies have focused on the role of titanate chemistry in μ-stability, but in this study, we focused on the role of titanates in reducing wear debris. A miniaturized inertial brake dynamometer was used to conduct experiments that enabled the evaluation of brake-particle emissions in urban driving for the specimen brakes. The corrosiveness of the specimen brakes due to metal sulfides after friction testing was also investigated.

2. Materials and Methods

2.1. Brake Material Friction Test Dynamometer; The 1/7-Scale Inertia Dynamometer

Brake-friction material tests were conducted by using the 1/7-scale inertia dynamometer shown in Figure 1a (custom-made 1/7-scale Inertia Dynamometer, Kobelco Machinery Engineering Co., Ltd., Gifu, Japan) [20,21,22,31,32,44,45,46]. The pad area was designed to be 1/7 of the specifications of the Toyota Corolla. Even with the reduced pad area, the inertia was adjusted so that the absorbed energy per unit area of the friction surface is the same for both when friction is performed at the same vehicle speed and deceleration as in an actual vehicle. A schematic of the testing apparatus used for our experiment is presented in Figure 1b. Two load cells were used to measure the applied load and brake torque. A thermocouple was inserted at a depth of 2.0 ± 0.2 mm for the disc surface to monitor the brake temperature. The load and initial velocity were obtained to give an inertia of 0.5 kg m2 from the rotating flywheel.

2.2. Friction-Test Conditions

The inertial mass was adjusted so that the energy per unit area was equal to approximately 2173 kg·m2/s2·cm2 for a typical passenger car when tested at 100 km/h. To achieve this value, the inertia of the scale dynamo was set to 0.5 kg m2. The experimental conditions were selected as urban driving conditions controllable by using the 1/7-scale dynamometer, first for bedding and then for emission measurement. For bedding, braking was repeated 500 times at an initial speed of 65 km/h, deceleration of 3.5 m/s2, and brake temperature (initial brake temperature) of 120 °C at the start of braking. The brake-emission measurements were conducted on the basis of the simplified driving mode (Otsuka Mode) shown in Figure 2a, which minimizes drag by not including cruising, and is referred to “Measurement Method for Particulate Matter Emissions from Passenger Car Braking Wear” JASO C470, as shown in Figure 2b [36]. During the brake-emission measurement, ten cycles of the Otsuka Mode (Figure 2a) were repeated two times (n = 2). The average vehicle speed was 22 km/h and the maximum speed was 120 km/h. The test was conducted at a constant deceleration of 1 m/s2 and the braking frequency was 15 brake stops/cycle. The total driving of sliding distance was 18.3 km.

2.3. Brake Disc and Friction Materials

The brake discs were made of FC-150 grade gray cast iron (carbon content 3.8%, disc outside diameter (O.D.) and the inside diameter (I.D) dimensions, 110 and 43 mm, respectively). The brake pads (friction materials) were prepared by Otsuka Chemical Co., Ltd. The brake pads were commonly composed of phenolic resin (powder novolak phenolic resin), aramid pulp (poly-paraphenylene terephthalamide), barium sulphate (BaSO4, ground barite), and Zircon (ZrSiO4, Zirconium (IV) silicate). The content of the lepidocrocite-type-layered titanate (Magnesium Potassium Titanium Oxide, Median Size 8 μm, Terracess® PM, Otsuka Chemical Co., Ltd., Osaka, Japan) [19,20,21,22,31,32], steel fiber (Cut wool BS1V, Bonster, Tokyo, Japan), and tin sulfide were adjusted in accordance with the brake-pad properties.
The brake pads were prepared as follows. The powders were mixed using an intensive mixer (Model R02, Nippon EIRICH Co., Ltd., Chiba, Japan) at a chopper speed of 3000 rounds/min.
We made both NAO and LS formulations. For the NAO formulation, the mixture was temporarily formed at a pressure of 10 MPa for 5 s and then hot pressed by heating at 150 °C for 300 s with degassing 6 times at a pressure of 20 MPa. The material was then fully cured in an oven at 160 to 200 °C for 2 to 6 h and cut into final fan-shaped brake pads (pad area: 5.53 cm2).
For the LS formulation, the mixture was hot pressed by heating at 150 °C for 240 s at a pressure of 15 MPa. The material was then fully cured in an oven at 160 to 210 °C for 6 h and cut into final fan-shaped brake pads (pad friction surface area: 5.53 cm2).
A series of brake pads were prepared, as shown in Table 1. The selection of the friction materials was based on the following evaluation parameters, assuming that the pads conformed to copper-free regulations [47,48]:
Brake 1: Cu-free NAO pads, free of potassium titanate.
Brake 2: Cu-free NAO pad, containing potassium titanate.
Brake 3: Cu-free LS pad, containing tin sulfide and free of potassium titanate.
Brake 4: Cu-free LS pad, containing tin sulfide and potassium titanate.
Brake 5: Cu-free LS pad, free of tin sulfide and containing potassium titanate.
Table 1. Brake-pad friction material selection, volume, and mass percentages.
Table 1. Brake-pad friction material selection, volume, and mass percentages.
MaterialsBrake 1Brake 2Brake 3Brake 4Brake 5
Selections
Pad typeNAONAOLSLSLS
Titanatenonewithnonewithwith
Tin sulfide------withwithnone
volume or mass percentages[vol%][wt%][vol%][wt%][vol%][wt%][vol%][wt%][vol%][wt%]
Titanate 15190014142325
Steel fiber 716716717
Tin sulfide 91491400
Di-antimony Tri-sulfide1212
Graphite2122969696
Aramid pulp5253313131
Phenolic resin2092092492492410
Cashew particles155155
Alumina 111111
Zirconium silicate5859565757
Magnetite5959
Mica5555211814121413
Barium sulfate35522031212814191420
Rock wool5555
Calcium hydroxide2222

2.4. Brake-Wear-Particle Measurement Instruments

Figure 3 shows the overall configuration and sampling section where brake particles were measured. In the sampling system shown in Figure 3a, cooling air (temperature 23 ± 3 °C, humidity 40 ± 20% relative humidity) through a high-efficiency particle filter (HEPA) flowed from upstream of the enclosure where the test brake was stored, and a constant flow of 2 m3/min was suctioned in the sampling tunnel downstream of the enclosure. Cooling air flowed from bottom to top when the brake disc was viewed from the front with the brake pad at 15 o’clock, and the rotation of the disc was clockwise in this experiment. The same policy as in GTR24 [33] was followed, except for the flow direction of the cooling air, which was from bottom to top.
Brake particles were measured by particle mass (PM) and the total particle number (PN) including volatile particles. PM and PN were aspirated from the isokinetic sampling nozzle shown in Figure 3b. The sampling-tunnel flow rate Q was 2 m3/min, and the sampling flow rate Qs was 15 L/min. The sampling nozzle diameter (B: 8.4 mm in Figure 3b) was chosen to have an isokinetic ratio of 1, considering the diameter of the sampling tunnel (duct inner diameter A: 97.4 mm in Figure 3b). For particle-mass measurement, brake particles were collected as >PM10, PM10-2.5 on a TX40 filter (Emfab TX40HI20-WW, Pall Corp., New York, NY, USA), and PM2.5 on a PTFE teflo filter (R2PJ47, Pall Corp., New York, NY, USA) using a multi-cascade impactor (MCI-15, Tokyo Dylec, Tokyo, Japan). The filters that collected brake particles were weighed on an electronic balance (Ultra-Microalance XP6V, METTLER TOLED, Greifensee, Switzerland) using a static electricity eliminator (ionizer PRX U Small set, HAUG, Echterdingen, Germany). PN was measured using a full-flow condensation particle counter (CPC) (model 3772, TSI Inc., Shoreview, MN, USA). Before the test, the test chamber and piping were cleaned, and the background PN value was measured without braking to confirm that the PN was 100 particles/cm3 or less before the test.
PM = PMfilter × Q/Qs,
here
PMfilter: Filter weighing mass for PM10–2.5 or PM2.5 (μg).
Qs: Sampling flow rate, 15 L/min.
Q: Sampling-tunnel flow rate, 2000 L/min.

3. Results

3.1. Friction Surface Conditions

Figure 4a,b compares the non-titanate (Brake 1) and with-titanate (Brake 2) friction surfaces of the NAO pads. The cross-sectional views of the brake pads are shown in backscattered electron (BSE) images obtained using a scanning electron microscope (SEM). To prepare these samples for observation, they were embedded in epoxy resin (Clear epox-2, Sankei, Tokyo, Japan), and the surfaces were smoothed with a rotary polisher (Daiya lap ML-150P, Maruto, Osaka, Japan) and sputtered using an ion milling system (Flat Milling IM-3000, Hitachi-hitec, Tokyo, Japan).
The friction films were formed on the friction surfaces of the non-titanate (Brake 1) and with-titanate (Brake 2) NAO pads. For the non-titanate pads shown in Figure 4a, the friction film was thick but rough and non-uniform, and some were cracked (damaged) or peeling off. For the with-titanate pads shown in Figure 4b, the friction film was found to be thin, widely uniform, and densely formed. We suggest that uniform and dense friction film leads to some of the film not peeling off.
Figure 4c,d compares the non-titanate (Brake 3) and with-titanate (Brake 4) friction surfaces of the LS pads. BSE images of the cross sections of the pads are shown. Both samples contained tin sulfide. The non-titanate (Brake 3) friction surface shown in Figure 4c had damaged steel fibers (the primary plateau) and no secondary plateau (no friction film). The with-titanate (Brake 4) friction surface shown in Figure 4d had no damaged steel fibers (primary plateau) compared with the non-titanate friction surface. Looking at the friction surface shown in Figure 4c in the sliding direction, wear particles gradually densified, forming the secondary plateau, and eventually the secondary plateau on the steel fibers disappeared, exposing the raw material. As shown in Figure 4c,d. we found significant differences in the observed results of steel fibers (the primary plateau) and secondary plateaus for the non-titanate and with-titanate friction surfaces.
The LS pad in Figure 4e is Brake 5 with the tin sulfide replaced with titanate from Brake 4 and the formation of a dense friction film was observed as the formation of a secondary plateau. The appearance of Brake 5 shown in the cross-section in Figure 4e was similar to the appearance of the widely uniform friction-film formation of the with-titanate NAO pad (Brake 2) shown in the cross-section in Figure 4b.
The above results indicate that the microphysical structure of the pad surface differs significantly depending on the friction material combination.
This study compares LS and NAO, and we have been able to confirm reasonable experimental differences. However, further research is needed to clarify the effect of the lying direction of steel fiber on plateau formation.

3.2. Brake-Wear-Particle Mass Amounts and Number Concentrations

Figure 5 shows the results of the PM and PN measurements for the brake pad series used for this study. In the JASO C470 testing, the PM and PN are typically represented as emission factors (mg/km or #/km). However, due to the use of a simplified test mode for C470 in this instance, we represent them in μg and #/cm3 instead of as emission factors. Most of the brake pads emitted more PM10-2.5 than PM2.5.
Comparing Brake 1 (the non-titanate) and Brake 2 (the titanate) in Figure 5, PM10-2.5 decreased by 70% with the addition of titanate. Similarly, a 40% reduction in PM10-2.5 was observed from the LS pads, Brake 3 (non-titanate) to Brake 4 (with titanate). The addition of titanate reduced abrasive wear for each of the NAO and LS pads in this study. The PM2.5 and PN were also found to have decreased for the NAO pads from Brakes 1 to 2 shown in Figure 5, while PM2.5 and PN did not significantly decrease for the LS pads from Brakes 3 to 4, as shown in Figure 5. The reduction in PM10-2.5 and the observation of the friction film or secondary plateau in Figure 4 suggest that the addition of titanate contributed significantly to the reduction in abrasive wear due to the homogeneous formation of the friction film or secondary plateau.
In Brake 5 shown in Figure 5, which is a blend in place of tin sulfide with titanate, even for the LS pad, both PM and PN measurements were observed to be comparable to the NAO Brake 2 measurement levels.
Comparing Brakes 4 and 5 in Figure 5, the LS pads with titanate, we found that both PM and PN measurements decreased in Brake 5. The friction material blends of these brakes shown in Table 1 are almost the same in terms of steel and organic resin content. The comparison of the LS pads with titanate, Brake 4 (with tin sulfide), and Brake 5 (non-tin sulfide) also indicates that the addition of titanate contributes to the reduction in both PM and PN.

4. Discussion

Titanate plays an important role in improving friction stability and wear resistance by forming a strong friction film on the friction surface along with the synergistic effects of titanate shape [23,28] and aramid pulp [24]. In our previous studies, we pointed out the importance of friction-film formation on the surface of NAO pads because the chemical reaction between titanate and phenolic resin produces carbides on the pad surface (see Appendix A, Figure A1), which improves friction stability (see Appendix B, Figure A2) [20,21]. When braking, the friction surface of the LS pads consists of two distinct contact regions, a primary plateau and a secondary plateau, in addition to a friction film. The primary plateau consists of large hard components such as metal fibers and coarse abrasives, which are exposed to the friction material surface and grind the disc and friction film until worn. A secondary plateau is formed by the compression of wear particles around the primary plateau, and the size and mechanical properties vary depending on the composition of the wear particles and braking conditions [26,49]. The frictional performance shown in previous studies with lepidocrocite-type layered titanate [20,21] and the frictional properties of the NAO pads shown in Figure A2 support the findings obtained with platelet-like titanates [28].
The results of our studies supported the finding that titanate plays an important role in the formation of contact plateaus and the fragility of the friction film in wear resistance, whereas wear resistance increases due to platelet-like titanates [27,28]. The improvement in wear resistance by platelet-like titanates [28] is supported by the reduction in PM and PN emissions of brake particles in the with-titanate NAO pad (Brake 2) and the with-titanate LS pads (Brakes 4 and 5) shown in Figure 5. In the LS pads, contact-plateau formation and friction-film fragility contribute significantly to wear resistance [27,28] and play an important role in terms of brake-particle emissions. Surface roughness, which is a concern with LS pads, is supported by the negatively sloped surface profile of the pad surface, which is a key factor in wear resistance affected by the formation of secondary plateaus near the primary contact plateaus (mainly steel fibers) of the brake pads [25,26].
It is also known that if the contact surface remains undamaged during braking, the pad surface becomes smoother, but if the contact surface is severely damaged, the surface becomes rougher [50]. The size of the secondary plateau is related to wear resistance, suggesting that platelet titanate may increase the friction level and decrease the wear rate when high molecular weight resins are used as binders in the low-temperature range [28]. The increased wear resistance of specimens with platelet titanates over whisker titanates at decomposition temperatures lower than 350 °C of organic resins is thought to be due to the accumulation of wear-resistant plate titanates on the surface during the wear process and actively participates in forming secondary plateaus [28].
Based on our experimental results and the findings obtained so far, it is possible that the friction film and secondary plateau caused by solid lubricants significantly affect the wear rate of the friction couple and brake-particle emission. So research on the effect of solid lubricants on brake-particle emission is needed.
While the solid lubricant tin sulfide is often used as an alternative to antimony sulfide, we propose lepidocrocite-type layered titanate as an alternative. Tin sulfide can provide various ribbon-layer structures depending on its oxidation state and is often used as an alternative to antimony sulfides [7,8]. However, the reserves of tin resources are gradually decreasing, and the stable supply of tin resources is a challenge [17]. There is also concern about the formation of corrosive substances from tin sulfide, such as sulfuric acid, a sulfur-oxidizing material, and that corrosive substances may lead to disc rust (see Appendix C) and stiction phenomena (see Appendix D). Of particular concern after friction testing of the LS pads used in this study was the stiction phenomenon caused by corrosive substances produced from tin sulfide [51]. Therefore, we evaluated not only environmentally friendly brake emissions (Figure 5) as a possible alternative to tin sulfide but also stiction testing (Appendix D) as a safety aspect of brake products. From the results of the stiction test shown in Figure A5, the LS pads with tin sulfide showed a stiction phenomenon between the pad and disc, while the LS pads without tin sulfide (replaced by the titanate) showed a decrease in the stiction phenomenon.
The experimental results suggest that pads with titanate not only have a positive impact on brake-system performance, safety, and reliability but also have a positive impact on the environment by reducing brake-particle emissions due to brake friction.

5. Conclusions

We evaluated the friction stability of NAO pads by using the l lepidocrocite-type-layered titanate we used in our previous studies. We investigated the role of titanate in reducing brake particle emissions as an additive to NAO friction materials, as an additive to LS friction materials, and as a substitute for metal sulfides, which are solid lubricants, with the aim of reducing brake particle emissions in an environmentally friendly manner.
The effect of titanate on reducing brake-particle emissions in urban driving was evaluated using a small inertial brake dynamometer. Images of pad cross-sections showed that the microphysical structures, such as the uniformity of friction film and secondary-plateau formation on the pad surface, differed significantly for NAO and LS pads, depending on the combination of friction materials including titanate. In terms of brake-particle emissions, it was shown that in relation to the microphysical structure of the pad surface, the friction material containing titanate can reduce emissions for both NAO and LS pads. Our experiments found that the addition of titanate contributed to the reduction in both PM and PN measurements for the LS pads in this study and were even comparable to brake-particle emissions from NAO pads with the addition of titanate.
In the future, we plan to conduct detailed analyses of the components and particle size of the wear debris to more precisely elucidate the wear reduction mechanism by titanates.

Author Contributions

Conceptualization, E.D. and Y.I.; methodology, E.D. and Y.I.; validation, E.D. and Y.I.; formal analysis, E.D. and Y.I.; investigation, E.D. and Y.I.; data curation, E.D. and Y.I.; writing—original draft preparation, E.D. and Y.I.; writing—review and editing, E.D. and Y.I.; visualization, E.D. and Y.I.; supervision, E.D.; project administration, E.D.; funding acquisition, E.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are not available to the public due to confidentiality agreements with material providers.

Acknowledgments

The authors would like to thank the co-workers who supported the set-up and operation of the dynamometer and the measurements. In addition, the authors would like to thank Hiroyuki Hagino with Japan Automobile Research Institute for proofreading support during the preparation of the original draft manuscript.

Conflicts of Interest

Emiko Daimon and Yasuhito Ito were employed by Otsuka Chemical Co., Ltd. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Appendix A. Hypothetical Overall View of the Role of Titanate to Phenolic Resin on the Substrate of Brake-Friction Products

Figure A1 shows the overall hypothesis of the role of titanate on the base brake-friction material containing phenolic resin obtained in our previous studies. We call the material between the pad and disc during breaking the “the friction products”.
During braking, thermal energy at the pad-disc interface breaks the chains of the phenolic resin contained in the pad-friction material. The chemical structure of phenolic resin is simply represented by circles and lines in Figure A1. The connections between the circles that represent the chemical structure of phenolic resin are so strong that it is difficult to break all of the chemical bonds. Therefore, char and tar, which are sticky substrates with high molecular weight, remain as a friction film on the pad surface [19,20,21,31]. It is assumed that this sticky substrate increases the amplitude of the friction coefficient μ [20,21]. After braking, the friction surface is exposed to large amounts of oxygen and residual heat, but our studies suggest that the high molecular weight substrate remains on the friction surface as a result of its resistance to decomposition by oxygen and heat alone [19,20,21]. When titanate is added to the brake-friction material, the crystalline structure of the titanate wears and breaks, isolating free potassium ions. The free potassium ions easily cut the carbon chains in the resin, resulting in a low molecular weight substrate. Since the low molecular weight substrate is less adhesive, the amplitude of μ decreases. After breaking, the low molecular weight substrate on the friction surface is further decomposed into carbon dioxide and water, so that little residue remains. Consequently, the titanate in the friction material leads to the control of friction stability [20,21] by decomposing the friction products to lower molecular weight [19].
Lubricants 12 00387 g0a1
Figure A1. Hypothetical overall view of role of titanate for phenolic resin on the substrate of brake-friction products.
Figure A1. Hypothetical overall view of role of titanate for phenolic resin on the substrate of brake-friction products.
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Appendix B. Friction Behavior

Figure A2 compares the frictional behavior of non-titanate (Brake 1) and with-titanate (Brake 2) NAO pads and the time variation in PN concentration as an example. Figure A2a,b shows the time trend of speed and the μ and disc temperature for the non-titanate and with-titanate NAO pads, respectively. Figure A2c,d shows the magnified μ for the non-titanate and with-titanate NAO pads, respectively. For the non-titanate NAO pad shown in Figure A2c, μ varied from 0.3 to 0.6 after the second time in the test mode, suggesting that adhesive behavior occurs during braking. With regard to the with-titanate NAO pad, μ was stable from 0.3 to 0.45 after the second time in the test mode shown in Figure A2d. Figure A2e,f shows the PN concentrations. Comparing the non-titanate and with-titanate pads, the PN concentration was higher for the non-titanate pad with adhesive behavior, while it remained low for the with-titanate pad with a stable μ. Previous studies have shown that above the critical temperature of 180 °C, the behavior of PM and µ changes simultaneously from an increasing trend to a decreasing trend with a hysteresis shape [52]. The correlation between PM and µ assumes that the decomposition of the organic binder leads to a decrease in PM emissions [52], which supports our findings. Wear and µ are discussed for commercial heavy truck drum linings tested in a Chase testing machine at constant load, constant temperature, and constant speed [50]. The results indicate that as wear on the friction material increases, the coefficient of friction increases linearly under constant load, speed, and temperature [50]. Hence, changes in µ lead to changes in brake wear [52], and the correlation between friction material wear and brake-particle emissions [30] suggests that this is a result that supports changes in brake-particle emissions. Therefore, our results support our previous studies [20,21] that the addition of titanate to NAO pads decreases adhesion behavior and highlight our new finding that brake-particle emissions, based on PN concentration, change with adhesion behavior.
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Figure A2. Friction behavior for NAO pads composed of the non-titanate (Brake 1) and the with-titanate (Brake 2) friction materials. (a,b) for time trends of speed, (c,d) for disc temperatures and µ, (e,f) for PN concentrations.
Figure A2. Friction behavior for NAO pads composed of the non-titanate (Brake 1) and the with-titanate (Brake 2) friction materials. (a,b) for time trends of speed, (c,d) for disc temperatures and µ, (e,f) for PN concentrations.
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Appendix C. Rust Resistance of Discs After Friction

The rust resistance of the disc after friction was evaluated by the following procedure. The disc was left in a constant temperature and humidity (35 °C, 95% relative humidity) environment. The left side of Figure A3 shows the appearance of the disc after the rust test. The upper row shows the results of the non-titanate pad and the lower row shows the results of the with-titanate pad. The graph in Figure A4 shows the time variation in the thickness of the disc surface after rusting. The X-axis is the number of days elapsed and the Y-axis is the increase in disc thickness. The black line is the non-titanate pad (Brake 3) and the blue line is the with-titanate pad (Brake 4). As the results of Figure A3 and Figure A4 show, titanate was effective in inhibiting post-braking disc rust in our experiments.
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Figure A3. Rust resistance of disc after friction. LS pad for non-titanate pad (Brake 3) (a) after 1 h and (b) after 14 h, and for the with-titanate pad (Brake 4) (c) after 1 h and (d) after 14 h.
Figure A3. Rust resistance of disc after friction. LS pad for non-titanate pad (Brake 3) (a) after 1 h and (b) after 14 h, and for the with-titanate pad (Brake 4) (c) after 1 h and (d) after 14 h.
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Figure A4. Disc-surface thickness after rusting for the LS pad of the non-titanate pad (Brake 3) and of the with-titanate pad (Brake 4).
Figure A4. Disc-surface thickness after rusting for the LS pad of the non-titanate pad (Brake 3) and of the with-titanate pad (Brake 4).
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Appendix D. Effect on Stiction of Tin-Sulfide Substitution into Titanate

Figure A5 shows the results of checking for stiction and rust. The stiction test was conducted as follows; the cut pad pieces were soaked in deionized water for 60 min. The pad and disc were then brought into contact at 2.0 MPa for 7 h at 25 °C with 95% relative humidity. Rust adhered to the (a) pad and (c) disc of the with-tin-sulfide pad (Brake 4) but did not adhere to the (b) pad and (d) disc of the non-tin-sulfide pad. Thus, our experiments confirm that the use of titanate instead of tin sulfide reduces stiction and rust.
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Figure A5. Stiction tests after friction. (a) With-tin-sulfide (Brake 4) and (b) non-tin-sulfide (Brake 5) LS pads. Discs for (c) with-tin-sulfide and (d) non-tin-sulfide LS pads.
Figure A5. Stiction tests after friction. (a) With-tin-sulfide (Brake 4) and (b) non-tin-sulfide (Brake 5) LS pads. Discs for (c) with-tin-sulfide and (d) non-tin-sulfide LS pads.
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Disc brake assemblies applied in automotive braking systems are complex multi-material systems. In actual use, they are exposed to several harsh environments, such as rain, snow, and chlorides in frozen road surface materials during winter driving [1]. Therefore, corrosion problems, especially those associated with cast iron discs, can adversely affect the performance, safety, and reliability of braking systems [53,54]. Under high oxidation conditions, electrochemical processes occurring on the surface of the gray iron disc can produce corrosion products (iron oxides) that can penetrate into the brake-pad material through the inherent porosity of the friction material used to manufacture the brake pads. For example, under static conditions in which the brake pads are pressed, stiction can occur [55], where the pad material adheres strongly to the disc [55,56,57]. This stiction phenomenon can damage the brake-pad material after the parking brake is released. In some cases, the stiction phenomenon can severely affect the performance of the brake system and, in the worst case, make the vehicle impossible to drive [55,56,57]. Another corrosion-related problem in automotive braking systems is the stick-slip phenomenon, which can generate noise and vibration in the braking system [57]. Tin sulfide is suggested to be a mechanism that generates corrosive substances such as sulfuric acid in brake-friction phenomena.

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Figure 1. Friction tester; the 1/7-scale Inertia Dynamometer. (a) Overall view of the dynamometer and brake-wear-particle measuring instrument and (b) schematic diagram of test apparatus in which brake pads and gray cast iron discs slide against each other. Flywheel is used to set the inertia, and two load cells are used to measure brake load and torque.
Figure 1. Friction tester; the 1/7-scale Inertia Dynamometer. (a) Overall view of the dynamometer and brake-wear-particle measuring instrument and (b) schematic diagram of test apparatus in which brake pads and gray cast iron discs slide against each other. Flywheel is used to set the inertia, and two load cells are used to measure brake load and torque.
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Figure 2. Driving mode on the brake-friction-test inertia dynamometer. (a) The simplified driving mode (Otsuka Mode); (b) the JASO C470 driving mode [36] based on the New European Driving Cycle (NEDC).
Figure 2. Driving mode on the brake-friction-test inertia dynamometer. (a) The simplified driving mode (Otsuka Mode); (b) the JASO C470 driving mode [36] based on the New European Driving Cycle (NEDC).
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Figure 3. Schematic diagram of the brake-wear-particle measurement. (a) Overall view of the measuring instrument configuration and airflow; (b) sampling tunnels and isokinetic nozzles.
Figure 3. Schematic diagram of the brake-wear-particle measurement. (a) Overall view of the measuring instrument configuration and airflow; (b) sampling tunnels and isokinetic nozzles.
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Figure 4. Friction surface conditions. Cross-sections of the NAO pad for (a) non-titanate (Brake 1) and (b) with-titanate (Brake 2) friction surface. Cross-sections of the LS pad for (c) with-tin-sulfide and non-titanate (Brake 3), (d) with-tin-sulfide and -titanate (Brake 4), and (e) non-tin-sulfide and with-titanate (Brake 5) friction surfaces.
Figure 4. Friction surface conditions. Cross-sections of the NAO pad for (a) non-titanate (Brake 1) and (b) with-titanate (Brake 2) friction surface. Cross-sections of the LS pad for (c) with-tin-sulfide and non-titanate (Brake 3), (d) with-tin-sulfide and -titanate (Brake 4), and (e) non-tin-sulfide and with-titanate (Brake 5) friction surfaces.
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Figure 5. Brake Wear Particle Mass (PM) Amounts and Particle Number (PN) Concentrations (n = 2).
Figure 5. Brake Wear Particle Mass (PM) Amounts and Particle Number (PN) Concentrations (n = 2).
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Daimon, E.; Ito, Y. Effects of Titanate on Brake Wear Particle Emission Using a Brake Material Friction Test Dynamometer. Lubricants 2024, 12, 387. https://doi.org/10.3390/lubricants12110387

AMA Style

Daimon E, Ito Y. Effects of Titanate on Brake Wear Particle Emission Using a Brake Material Friction Test Dynamometer. Lubricants. 2024; 12(11):387. https://doi.org/10.3390/lubricants12110387

Chicago/Turabian Style

Daimon, Emiko, and Yasuhito Ito. 2024. "Effects of Titanate on Brake Wear Particle Emission Using a Brake Material Friction Test Dynamometer" Lubricants 12, no. 11: 387. https://doi.org/10.3390/lubricants12110387

APA Style

Daimon, E., & Ito, Y. (2024). Effects of Titanate on Brake Wear Particle Emission Using a Brake Material Friction Test Dynamometer. Lubricants, 12(11), 387. https://doi.org/10.3390/lubricants12110387

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