Friction Layer of Automotive Brake Pads
Electron Microscopy - Raman Spectroscopy Correlation
- Fig. 1: Image of friction layer of the tested brake pad.
- Fig. 2: Magnified selected area of the friction layer (left) and corresponding EDS maps (right) of pad after breaking procedure
- Fig. 3: (a) Light microscopy image of the brake pad with marked selected area, (b) corresponding spectral map: red color represents graphite compounds, green color represents silicon carbide, and blue color represents zirconium oxide. The black color is mainly amorphous carbon, (c) Raman spectra of brake pad after braking proces: barite (B), graphite (G), molybdenite [MoS2] (M), hematite [Fe2O3] (H), aluminum oxide [Al2O3] (A), and magnetite [Fe3O4] (Ma), amorphous carbon (AC), copper oxide [Cu2O] (C), and hematite [Fe2O3] (H)
Wear of friction materials symbolizes a challenge since several chemical compounds are being released. There is still a lack of information about chemical composition of friction layer and the wear debris. It is necessary to develop analytical methods, which make it possible to correlate microstructure and chemistry of the friction layer. SEM, EDS with Raman microspectroscopy were introduced as a possible combination of analytical techniques.
The brake pad is a multicomponent system, which is typically created by more than 10 constituents – reinforcing agents, abrasives, lubricants, binders and fillers . Phenolic resin is used as a matrix and several metals, ceramics, minerals, carbons or polymers are present in the typical brake pad. Exact formulation is know-how of producer and it variates according to applications. Several studies presented chemical composition and study of the morphology of wear particles released from brake pads [2,3]. It is assumed that chemical composition of wear debris is almost similar to the chemical composition of friction layer. Distribution of constituents in friction layer is crucial for the braking process. High pressure and temperature occur during braking, therefore various changes of morphology and chemistry can occur. Formation of friction layer in respect to wear performance has been studied  and it revealed that particles, which escaped the friction interface create the friction layer. Electron microscopy made it possible to uncover [2,5,6] that the presence of smaller particles has a high influence on wear debris emission. Properties of the friction layer determine the friction performance of the brake pad. However, there is still lack information about the relationship between the chemical composition of initial pad formula and composition of friction layer, and also between the composition of friction layer and wear debris composition. Additional studies of friction layer formation and its contribution to brake process are therefore necessary. The aim of this study is to propose new possible combination of analytical methods focused on mapping of friction layer, which would enable the correlation of microstructure and the chemistry of the friction layer.
A model brake pad of the following composition (CuS, CuZn, CuSnP, Sn, iron fibers, graphite, coke, aramid fibers, BaSO4, ZrO2, FeCr2O4, SiO2, SiC, MgO, ZnO, MoS2, Al2O3, SnS, rubber, phenolic resin) was prepared and tested according to the procedure ISO 26867:2009 Road vehicles -- Brake lining friction materials -- Friction behavior assessment for automotive brake systems was used for the automotive brake dynamometer test .
Surface/friction layer was analyzed by use of scanning electron microscope (SEM) Quanta FEG 450 (FEI) equipped with EDS analyzer Apollo X (EDAX).
Raman spectra of surface/friction layer were obtained using Smart Raman Microscopy System XploRA (Horiba Jobin Yvon, France) with integrated light microscope Olympus BX41/51.
Raman spectra were acquired with 532 nm excitation laser source, and 1 200 g×mm-1 grating.
Scanning electron microscopy is a useful tool for study and characterization of friction layer of brake pads. A typical friction layer is displayed in figure 1. Back scattered electron imaging enables us to see material contrast in the studied sample. Metals, such as iron, are shown as bright white, where carbon-based compounds are black. Shades of black-gray-white depend on the relative atomic mass of components. Without further EDS analysis, it is almost impossible to distinguish each components of the pad. Therefore, EDS mapping was performed (fig. 2).
From a comparison of EDS maps of elements, it is possible to deduce the chemical origin of particles or aggregates. For instance, the map of barium and the map of sulfur correlates together and therefore it can be said that the particle is created by barite. However, it is not always possible to determine all particles due to the overlapping of maps and signal in general. Therefore, additional analysis, which enables determination of phases, is needed.
Raman spectral imaging is an analytical method based on creation of spectral maps. figure 3a displays the friction layer of the brake pad under a light microscope integrated in Raman microspectrometer, where a selected area is marked. Spectral map of the selected area is given in figure 3b, where each detected compound in real size and distribution in the friction layer can be seen. Red color represents graphite, green color represents silicon carbide and blue color represents zirconium oxide. The rest of the friction layer in the selected area was created by amorphous carbon, which is represented by black color. According to the comparison with the scale bar, it can be assumed that graphite particles are approximately 1 µm wide. Some particles consisted of several compounds. For example silicon carbide (green) is surrounded by graphite (red), as well as zirconium oxide (blue) contained a small amount of graphite. The selected analyzed area was created mainly by amorphous carbon (black). Apart from mapping, Raman microspectroscopy allows also phase point analysis and the obtained spectra are given in figure 3c. The presence of iron oxides (mainly Fe2O3, and Fe3O4), barite (BaSO4), graphite, amorphous carbon, molybdenite (MoS2), aluminum oxide (Al2O3), copper oxide (Cu2O) in the friction layer of the tested brake pad were confirmed.
This pilot study showed that the combination of scanning electron microscopy, EDS, and Raman microspectroscopy is useful for friction layer characterization. The combination of these methods enables studying of the sample morphology, elemental and phase composition. A better understanding of the process of friction layer formation and distribution of components in friction layer is important in terms of wear debris emission and its reduction.
This paper was created at the Faculty of Metallurgy and Materials Engineering within the Project No. LO1203 „Regional Materials Science and Technology Centre - Feasibility Program“ funded by Ministry of Education, Youth and Sports of the Czech Republic.
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Kateřina Dědková1, Kristina Čabanová1, Miroslav Vaculík1
1Regional Materials Science and Technology Centre, VŠB, Technical University of Ostrava, Ostrava-Poruba, Czech Republic