An automotive friction brake pad is a complex system consisting of several components with unique and balanced properties related to operation conditions. There are efforts to develop brake pads with longer lifetime and better friction performance and wear properties. Those properties are related to composition of the pads, and therefore, new materials are being evolved. Tuning the friction and wear properties can be achieved with the selection of a functional filler and optimizing its amount in a formulation of friction brake pad. Laboratory-developed and laboratory-prepared nanocomposite material kaolin/TiO2 (KATI) has been introduced to formulation of the commercially available automotive low-steel brake pad. Kaolin was utilized as a matrix for anchoring TiO2 nanoparticles. New unused pads and pads after AK master, a standard dynamometer testing procedure of friction performance, were investigated using light and scanning electron microscopy providing information on the structure and its changes after the friction processes. Moreover, MTK wear test was used to compare wear rate of the newly developed pad with the reference low-steel pad. Improved durability of the brake pad formulation has been observed together with sufficient friction performance. Microscopic analysis shown homogenous distribution of the KATI nanocomposite in the friction layer. From the obtained results, it can be assumed that the new formulation is promising regarding to the life cycle of the pads and reduction of wear rate and thus potential production of wear particulate emissions.
Typically, a brake pad is a multicomponent composite material characteristically consisting of more than 10 constituents. Phenolic resin usually serves as a matrix, and several forms of metals, ceramics, minerals, polymers, and carbonaceous ingredients are present in a typical brake pad. Due to significantly different material properties of single components, the resulting friction layer established on the surface of friction composites is strongly heterogeneous and also its surface is rough with many asperities across it [
Significant phase transformations of brake pad material occur during friction processes. These transformations lead to origination of newly formed friction layer and have a great effect on the friction efficiency of brake pads as well as an effect on wear debris formation and its properties. While braking, both parts of braking couple in the disc brake system (i.e., pads and rotating disc) are worn and thus wear debris is generated. It is well known that the wear debris may have a negative impact on the environment and human health [
Nowadays, environmental issues are being discussed due to the potential influence of wear products on the air quality in urban areas. It has been proved that the nonexhaust emission comes also from the wear of the pads and rotors [
Therefore, nanostructured composite material kaolin/TiO2 could be one of the suitable candidates and thus the aim of the study was the introduction of this nanomaterial into the brake pad formulation together with subsequent detailed characterization of friction surfaces of tested brake pads and their chemical and phase composition as well as evaluation of their changes during the friction process.
Federal-Mogul Motorparts (Germany) provided reference commercial brake pads and manufactured the brake pads with modified composition used in this study. The composition of the reference commercially available low-steel brake pad was adjusted, and nanocomposite kaolin/TiO2 (KATI) (Figure
SEM image of KATI [
Hence, the phase composition of the pads is confidential; therefore, Table
Description of brake pad composition in volume%.
Reference pad | Pad with KATI | |
---|---|---|
Abrasives | 12.5 | 4.9 |
Organic binder | 23.0 | 22.0 |
Metals | 17.0 | 17.4 |
Organic fiber | 3.5 | 3.7 |
Carbon | 25.0 | 22.0 |
Lubricants | 7.0 | 7.0 |
Filler | 12.0 | 9.0 |
Kaolin/TiO2 | 0 | 14 |
AK Master and MTK wear test with both samples were performed using full-scale dynamometer at Federal-Mogul Motorparts, Germany. These procedures simulate various braking scenarios with various brake pressures and speed, and thus, these studied materials were exposed to different conditions, which may occur during usage of automotive friction brakes. The dynamometer program AK Master describes the friction value behavior of a friction material with regard to the influences of pressure, temperature, and speed. Its main purpose is to compare friction materials under the most equal conditions possible. To take account of the different cooling behavior of the different test stands, the fading series are temperature-controlled. Project-related brakes and brake discs must be used. Test conditions are defined by inertia, press rate increase, sampling frequencies, temperature measurement, and cooling conditions. The friction performance is tested under various braking conditions, such as speed, pressure, and disc temperature representing different situations during vehicle driving. Firstly, pads and rotor go through a friction check in new conditions (30 brake applications), followed by a bedding section to get pad and rotor surface adapted (62 Ba). After that, a characterization of the friction value at standard pressure, speed, and initial temperature is measured (6 Ba). Next point is speed/pressure sensitivity of the friction coefficient (5 × 8 Ba). Again, a characterization as before is added (6 Ba). A cold temperature check followed by a highway simulation with elevated speed is next (3 Ba). This section is closed by a newly conducted characterization (18 Ba). The fade characteristic is tested by continuously increasing disc temperature up to 550°C initial temperature by executing 15 brake applications. The next characterization (18 Ba) is followed by pressure lines. This means 8 Ba with increasing pressure at 100°C initial temperature, an increase of disc temperature up to 500°C by 9 Ba, and again a pressure line at 500°C starting temperature (8 Ba). Next sections are again a characterization (18 Ba), followed by a second fade (15 Ba), and finally a characterization (18 Ba ≥ total 274 Ba).
Friction values are determined by the following formula. Efficiency factor
Evaluation of wear behavior of pad material in Main-Taunus endurance course based on vehicle data was done. The course is a driving cycle containing 240 brake applications each, depending on the driving conditions tracked from the vehicle driving. 20 test cycles are run during the MTK test, which means 4800 brake applications in total. The wear of friction pads is determined by thickness measurement before and after the test at four defined positions.
The inner brake pad (placed in the inner side of the disc) was selected for further experiments due to higher pressures and temperatures achieved during braking. For better manipulation with the studied samples, back plates were removed from the pads. Since friction composites could be considered as homogenous material on the macro level, randomly selected area of 25 mm2 of the initial sample surface was labelled and the rest of the sample was covered with aluminum foil to ensure that the same one will be analyzed by all of the techniques used. Since the friction layer formation depends on many factors and varies at the sample surface, two spots (called site 1 and site 2) of 25 mm2 from different areas of the tested sample surface were labelled. Rest of the sample was covered with aluminum foil. The samples prepared this way were analyzed using a combination of selected analytical methods.
Light digital microscope VHX-500 (Keyence Corporation, Japan) was used for macroscale characterization of the friction surface of the studied samples together with surface roughness.
The EDS mapping of the selected (400 × 400
The samples of the tested brake pads were also measured using TESCAN TIMA (TESCAN Integrated Mineral Analyzer) equipped with tungsten emitter at the following working conditions: acc. voltage 25 kV, working distance 15 mm, and probe current 7 nA. The EDAX Element 30 detectors were used for collecting the characteristic X-rays. The detectors are Peltier-cooled SDD (silicon drift detectors) with 30 mm2 active area. Two areas of CCA 5 × 5 mm were measured from each sample. Liberation analysis with high-resolution mapping mode was used. Pixel spacing was 1
Table
AK Master braking procedure.
Unit | Reference pad | Pad with KATI | |
---|---|---|---|
(1) | 0.44 | 0.44 | |
(1) | 0.34 | 0.37 | |
(1) | 0.48 | 0.50 | |
(1) | 0.42 | 0.44 | |
(1) | 0.41 | 0.42 | |
(1) | 0.40 | 0.34 | |
(1) | 0.45 | 0.43 | |
(1) | 0.43 | 0.41 | |
(1) | 0.34 | 0.37 | |
(1) | 0.46 | 0.43 | |
(1) | 0.35 | 0.37 | |
(1) | 0.45 | 0.46 | |
(1) | 0.46 | 0.38 | |
(1) | 0.40 | 0.41 | |
Average pad wear | (mm) | 0.92 | 0.41 |
Disc wear | (g) | 11.6 | 11.6 |
Table
MTK wear braking procedure.
Unit | Reference pad | Pad with KATI | |
---|---|---|---|
Circuit 3–20 average pad life | (km) | 27,367 | 57,665 |
Circuit 3–20 average disc life | (km) | 181,154 | 279,380 |
MTK—inner pad wear (mean) | (mm) | 1.22 | 0.56 |
MTK—outer pad wear (mean) | (mm) | 1.11 | 0.56 |
MTK—disc wear | (g) | 9.2 | 8.0 |
Pad condition MPU (min) | (1) | 9 | 10 |
Disc scoring/grooving (min) | (1) | 10 | 8 |
Light digital microscopy is a useful technique for studying sample surface in the macroscale. It enables to image the surface in real colors, which may help to identify some of the present structures. For instance, in the case of the reference pad, it can be presumed that shiny gold-like features could be brass fibers (Figure
Selected area of the reference low-steel brake pad surface with its corresponding 3D reconstruction (a), site 1 (b), and site 2 (c) of the tested reference pad.
The surface of the reference pad is quite rough with remarkable fibrous structures. The surface of two selected areas after the dynamometer testing is more flat, but still exhibits notable roughness and contains cavities. The first area (Figure
The surface of the initial pad containing KATI nanocomposite is less rough. Some fibers can be seen, but they are not made of brass as in the reference pad. Some cavities can be seen at the surface of the tested pad (Figures
Selected area of the surface of the brake pad containing KATI nanocomposite with its corresponding 3D reconstruction (a), site 1 (b), and site 2 (c) of the tested pad containing KATI.
Combination of electron image with EDS mapping enables visualization of elemental composition of each component of pad surface. Due to the lack of colors in the software, several elements, which were presented at the same areas of the sample (most probably create together a compound), have the same color (e.g., Cu and Zn—brass or K, Si, Al, and Mg—clay minerals). Oxygen is not included in these maps because it was present almost in every point, and thus, such map including oxygen would be useless. Figure
Selected magnified area of the reference pad before testing (a), EDS spectral map overlapped with electron image (b), and its corresponding spectrum (c).
Contact plateaus based on iron visible in Figure
Selected magnified area of site 1 of the reference pad after dynamometer testing (a), EDS spectral map overlapped with electron image (b), and its corresponding spectrum (c).
The larger part of the surface of site 2 of the reference pad (Figure
Selected magnified area of site 2 of the reference pad after dynamometer testing (a), EDS spectral map overlapped with electron image (b), and its corresponding spectrum (c).
The surface of the selected area of the pad with KATI nanocomposite (Figure
Selected magnified area of the pad with KATI nanocomposite before testing (a), EDS spectral map overlapped with electron image (b), and its corresponding spectrum (c).
The surface of site 1 pad with KATI nanocomposite (Figure
Selected magnified area of site 1 of the pad with KATI nanocomposite after dynamometer testing (a), EDS spectral map overlapped with electron image (b), and its corresponding spectrum (c).
The surface of site 2 pad with KATI nanocomposite (Figure
Selected magnified area of site 2 of the pad with KATI nanocomposite after dynamometer testing (a), EDS spectral map overlapped with electron image (b), and its corresponding spectrum (c).
EDS mapping is a useful technique, which provides information about the distribution of single elements; however, multicomponent samples such as brake pads are hitting limits of this technique and even knowledge and experience with this kind of samples cannot guarantee a good result. Thus, there are tendencies to find other complementary techniques to obtain results with limited misinterpretations. Therefore, TESCAN TIMA microscope was used in this pilot study, and based on the published research paper available, it was the first time when it was used for this kind of sample.
Since initial pads have some defined composition (even industrially protected) and significant chemical and morphological changes could not be expected, only pads after dynamometer testing were further examined. Additionally, phase mapping provided by TIMA was also very time-consuming (approx. 12–15 hours). Signal from backscattered electrons and X-ray was collected to create a map of phase composition as can be seen in the following images.
The phase composition of site 1 (25 mm2) of the reference pad after dynamometer testing is shown in Figure
Panorama map of phase composition of site 1 of the reference pad after dynamometer testing.
The phase composition of site 2 of the reference pad is given in Figure
Panorama map of phase composition of site 2 of the reference pad after dynamometer testing.
The surface of site 1 of the pad with KATI nanocomposite (Figure
Panorama map of phase composition of site 1 of the pad with KATI nanocomposite after dynamometer testing.
The last examined surface is site 2 of the pad with addition of KATI nanocomposite (Figure
Panorama map of phase composition of site 2 of the pad with KATI nanocomposite after dynamometer testing.
Exported elemental maps and cumulative spectra of individual grains (the cumulative spectra are summed from all analyzed pixels comprising the grain Figure
Representative spectrum of a grain of Fe-Cu oxide from site 1 of the pad with KATI nanocomposite after dynamometer testing.
Image of a representative grain of Fe-Cu oxide: (a) Cu-K elemental map, (b) BSE image, (c) Fe-K, and (d) Sn-L.
The average composition of 100 largest grains of Fe-Cu oxides from all samples.
Spectra of 100 grains | O | Mg | Al | Si | S | K | Ti | Cr | Fe | Cu | Zn | Sn |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Average content wt% | 26.55 | 2.04 | 3.74 | 2.50 | 1.38 | 0.49 | 0.50 | 1.23 | 34.87 | 16.99 | 4.67 | 4.71 |
It can be said that addition of TiO2 to the brake pad formulations is still not frequently studied, and this study with KATI nanocomposite is one of the first in this field; however, there are some studies with titanates. Mahale et al. [
Nanostructured composite material kaolin/TiO2 (KATI) has been successfully introduced to the existing automotive brake pad formulation. This pad with modified formulation has been tested together with commercially available low-steel pad on full-scale brake dynamometer to determine its friction performance and wear properties. It has been found that friction performance of the pad with KATI nanocomposite is comparable with the commercial low-steel pad; however, the average pad durability was significantly higher expressed by pad thickness loss which was about 50% lower compared to the reference pad formulation. Microscopic techniques revealed structures of the friction surface where some intermetallic phases have been found, what indicates occurrence of local high temperatures. The nanocomposite KATI was homogenously distributed on the surface of the pad before and also after the dynamometer testing which indicates good thermal stability of the modified composite. This modified brake pad formulation may be promising regarding the durability of pads and the needs to control wear and reduce particulate emissions. Most expected phases of the initial compounds were determined; however, some of the detected phases do not correspond to possible initial components. It can be interpreted as a result of phase transformation. When selecting a suitable nanomaterial for application in automotive friction composites, nanocomposite materials appear to be suitable candidates due to decrease of potential environmental risk, because of anchoring of nanoparticles in a matrix. New useful properties of the friction composite can also be achieved by using a lower amount of suitable nanomaterial than conventional bulk material.
No data were used to support this study.
The authors of the paper certify that they have no affiliations with or involvement in any organization or entity with any financial interest.
This study was supported by the project no. LO1203 “Regional Materials Science and Technology Centre - Feasibility Program” funded by the Ministry of Education, Youth and Sports of the Czech Republic and by the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 636592.