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A comprehensive microscopic study of third body formation at the interface between a brake pad and brake disc during the final stage of a pin-on-disc test
Wear 267 (2009) 781–788
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
A comprehensive microscopic study of third body formation at the interface between a brake pad and brake disc during the final stage of a pin-on-disc test W. Österle a,∗ , I. Dörfel a , C. Prietzel a , H. Rooch a , A.-L. Cristol-Bulthé b , G. Degallaix b , Y. Desplanques b a b
Bundesanstalt für Materialforschung und – prüfung, 12200 Berlin, Germany Ecole Centrale de Lille, Laboratoire de Mécanique de Lille, BP 48, F-59651 Villeneuve d’Ascq Cedex, France
a r t i c l e
i n f o
Article history: Received 29 August 2008 Received in revised form 25 November 2008 Accepted 25 November 2008 Keywords: Brake Third body Wear Friction Nano-scale TEM
a b s t r a c t In order to preserve contact situations occurring during a tribological test, an experiment was performed to investigate the pin–disc interface while keeping the pin pressed against the disc. The pin was made from a commercial brake pad material and the disc from gray cast iron. Thus the experiment simulated a moderate brake application. After fixation of the friction-couple, macro- and micro-cross-sections were prepared by conventional metallography and focused ion beam technique, respectively. The amount of third body between the two first bodies and its structure and chemical composition was determined by microscopic and micro-analytical techniques including SEM, FIB, TEM and EDX. Furthermore, the size and structure of wear particles ejected from the contact area during the test was examined. Although testing conditions were moderate in respect to energy input and corresponding temperature rise, at least the main features of the third body structure are comparable to features observed after severe braking conditions. The observations confirm that during braking the first bodies are separated by a third body which is mainly responsible for the friction response. Lacking evidence of third body films after braking with certain pad materials can be attributed to film loss after braking due to weak adherence to the disc surface. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Brake manufacturers have to meet a number of requirements demanded by their customers. For regional inter-city railway application, as regarded here, the following expectations have been communicated: A constant coefficient of friction (CoF) between 0.3 and 0.4, irrespective of pressure, temperature and humidity, smooth braking without vibrations and noise generation, regeneration of original properties even after severe brake applications, e.g., after emergency braking, low wear rates of pad and disc, cost reduction by using cheap but environment-friendly raw materials. Besides geometric and system considerations the design of the pad material plays a major role in reaching the target properties. Brake pad materials are multi-constituent composites. The formulations have to be optimized by trial and error until the desired performance properties are reached. The objective of this work was to contribute to the knowledge of the basic friction mechanisms which finally promote the desired behaviour described above. Referring to Godet’s fundamental work [1] we suggest the following hypothetical scenario for a disc brake.
∗ Corresponding author. Tel.: +49 3081041511; fax: +49 3081041517. E-mail address: [email protected] (W. Österle). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.11.023
In the course of a bedding procedure, a fine-grained wear debris comprising a mix of pad and disc constituents is produced and trapped in troughs at the surface of the matching first bodies. Since the trapped material differs in respect to composition and microstructure from both first bodies, Godet defined it as third body (TB). During braking the TB is spread over contact areas providing separation of the first bodies, thus preventing adhesion and micro-welding of metallic constituents and providing velocity accommodation between the rotating disc and fixed pad. As long as enough TB-material is available at the interface and the TB does not change its structure, stable friction behaviour can be expected. Even after severe braking conditions, when the unique structure of the TB might have been destroyed, or all wear debris removed from the contact, the system is able to regenerate its original friction properties during subsequent recovery cycles during which the desired TB is produced again. Godet also proposed the continuous formation and destruction of surface films, so-called screens which cover the contact areas providing the main source of wear particle production. Experimental evidence of films produced by debris compaction of melted and pyrolyzed products from brake pads was first provided by Jacko et al. [2]. Several authors reported on films containing BaSO4 which is a frequently used filler for brake pads [3–5]. Later, iron oxide was identified as a major constituent of surface films of brake pads and discs [6–9]. Filip et al. [10] and Blau and Meyer [11]
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Nomenclature CoF EDX EF FIB LM NAO rpm SAED SEM Td Tp TB TEM XFA
coefficient of friction Energy Dispersive X-ray spectroscopy Energy Filter Focused Ion Beam Light-optical Microscope None Asbestos Organic disc rotation in revolutions per minute Selected Area Electron Diffraction Scanning Electron Microscope disc mass temperature pin mass temperature third body Transmission Electron Microscopy X-ray Fluorescence Analysis
identified mixtures of oxides and carbonaceous products in films and wear particles, respectively. Obviously, the chemical composition of wear debris and friction films depends on the formulation of pad materials. In addition to flat plates of compacted wear particles, Cristol-Bulthé et al. observed loose particles with the same composition as the plates, trapped at the pad surface [12]. They also recognized an increased amount of TB-material after braking conditions leading to high temperatures and related this finding to resin degradation which was measured in situ by mass spectroscopy. All experimental findings reported up to now refer to observations at either the pad or the disc surface after braking, but not to the situation during braking, when the pads are pressed against the rotating disc. This might explain, why only fragments of third body layers were observed experimentally while in situ studies of pad on glass suggested the formation of continuous films spreading over the contacting areas [13]. The motivation for the present work was, to investigate the situation at the pad–disc interface before non-adhering TB-material is removed from the contacting surfaces. This approach was inspired by the work of Kasem et al. Those authors were able to show a 20 m wide gap filled with TB-material between the pin and the disc of a carbon/carbon composite friction-couple [14]. Further motivation was to verify assumptions which were made to simulate material flow and friction response at micro-contacts at the pad–disc interface [15]. The latter study implied that only if the first bodies are screened by oxide layers with a certain amount of solid lubricant, a smooth friction response and medium CoF value can be expected. 2. Experimental procedure 2.1. Materials The same polymer matrix composite pad material as in previous studies [12,16,17] was investigated. Table 1 depicts the main constituents of this formulation. A commercial gray cast iron disc was used as counter body. The cast iron had a pearlitic matrix with a small quantity of free ferrite. The graphite flakes were homogeneously distributed and oriented in the matrix.
mercial cast iron disc. Thus the friction-couple comprises of the same materials as the real brake, but energy input and resulting temperature correspond to very moderate braking conditions only. The tribological tests were performed on a pin-on-disc tribometer with a maximal disc rotation speed of 1000 rpm and a maximal normal load applicable on the pin of 500 N. A deformable steel parallelogram linked to the pin and equipped with deformation gauges allowed the measurement of normal and tangential loads on the pin-on-disc contact. Using this data, the CoF was calculated. The pin and the disc were both equipped with a thermocouple located at 2 mm from the contact surface on the mean friction radius. They permitted mass temperature measurements Td and Tp for disc and pin, respectively. A cylindrical pin with a diameter of 20 mm and a height of 14 mm was machined from a real brake pad. The disc had a diameter of 266 mm and was 13 mm thick. The contact occurred on a 105 mm mean friction radius (Fig. 1). The test was performed with a constant rotation speed of 900 rpm which corresponds to a sliding velocity of 10 m/s, and a normal load of 230 N. The time interval was 225 s. After the rotation speed was reached, the load was manually applied. Before the test, the disc surface was ground with sand paper (grade 80). A preliminary run-in was also performed before the test to obtain geometric adjustment of the rubbing surfaces. 2.3. Specimen preparation and interface characterization After tribological testing the pin, still under pressure, was clamped to the disc and fixed with the aid of a glue, thus preserving the interface situation during the final stage of the test. Then the disc-segment with fixed pin was cut from the disc and embedded completely in epoxy resin. After curing of the resin the clamp was removed. Next the major part of the remaining disc-segment and also part of the pin was cut off, leaving only a thin specimen containing the whole interface embedded in resin at its circumference. This specimen was thoroughly cut in the middle, perpendicular to the rubbing direction, with a low-speed diamond saw. It was not possible to cut cast iron with a diamond wire saw, as suggested by Kasem et al. for the C/C-material [14]. Inspection of the cut crosssection in a Light-optical Microscope (LM) revealed closed contact between pin and disc. After having removed material by grinding and polishing a horizontal crack appeared indicating separation of pin and disc due to pressure release or thermal stresses during cutting. Obviously, this crack had been filled with wear debris from the cutting and grinding process and therefore did not appear before final polishing. Further investigations were performed at higher magnification in a Scanning Electron Microscope (SEM) equipped with an Energy Dispersive X-ray spectrometer (EDX). Finally a thin lamella, approximately 20 m wide, 10 m deep and 100 nm thick was cut from the interface region, where the third body had been identified by SEM/EDX, with a Focused Ion Beam (FIB) instrument. This lamella was lifted with the aid of a micro-manipulator under LM control at a magnification of 500:1 and put on a carbon-coated nickel grid. After covering with a second approximately 10 nm thick carbon film supported by nickel grid (200 mesh per inch), the micro-specimen was investigated in a conventional TEM (JEOL JEM4000FX) and in an analytical TEM (JEOL JEM2200FS) equipped with EDX, Energy Filter (EF) and Field Emission Gun (FEG). 2.4. Wear particle characterization
2.2. Tribological testing Since it was not possible to do the planned interface investigation with a brake dynamometer or a full-scale brake test, we decided to perform a simple pin-on-disc test with a cylindrical pin cut from an original brake pad which was tested against a com-
Loose wear particles were collected with a brush around the pin before applying the epoxy resin for fixation. These particles had been released from the contact and were recirculated on the disc but then were not able to enter the contact again, but were blocked on the pin periphery. After dispersion of the particles in
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Table 1 Composition of brake pad in vol%. Mineral fibers
Metallic fibers
Organic fibers
Solid lubricants
Abrasive particles
Metallic particles
Phenolic resin
20
8.5
4
18
12
5.5
Balance
Fig. 1. Test device: photo of pin actuating against disc (a) and schematic of test (b).
pure ethanol under sonication single droplets were transferred to carbon-coated support grids and allowed to dry in air. Then the grids were investigated in the analytical TEM. A second site of particle sampling was outside the disc, approximately 10 cm away from the pin, in the direction of disc rotation. On this site, ejected particles, definitely lost for the contact, were sampled on aluminium foil covered by a glue for further SEMinvestigation.
disc. Here the matrix appears bright and the graphite lamellae gray. There always exists a gap between pin and disc which is filled mostly with material displaying dark contrast in the LM. Since there is considerable roughness on both surfaces, the width of the gap changes from site to site. The gap becomes very narrow if a metal constituent of the pad material protrudes somewhat from the worn pin surface, but it never disappears completely. 3.3. Scanning Electron Microscopy
3. Results 3.1. Thermal and tribological behaviour The initial temperatures of both, pin and disc, were 23 ◦ C and after the friction test, the mass temperatures reached in a depth of 2 mm from the friction surfaces were Td = 67 ◦ C and Tp = 55 ◦ C. Fig. 2 shows the evolution of the CoF which was measured during the test. From the beginning of the test (at t = 20 s, when the load was applied) until t = 80 s, the CoF increased from 0.26 to 0.4. Then, the CoF stabilized at a mean value of 0.4. From t = 50 s until the end of the test, the CoF was prone to minor fluctuations with a maximum amplitude of 0.08. 3.2. Light-optical Microscopy Approximately 10% of the cross-section is shown in Fig. 3. In the pad material, at the top, the metallic constituents copper and steel appear in bright contrast, whereas mineral fibers, ceramic constituents and phenolic resin are shown with different gray levels. The lower half of the cross-section corresponds to the cast iron
Fig. 2. CoF as function of time at 900 rpm and 230 N.
The area marked by the frame in Fig. 3 is shown at higher magnification by SEM in Fig. 4a. The material filling the gap between pin and disc is now shown in light gray contrast. As will be shown later, this material is the TB, i.e., compacted and mixed wear debris from pin and disc. A horizontal crack is mostly visible within the TB. It can run along its interfaces with pin or disc or sometimes also through the TB. This crack most likely is due to pressure release during specimen preparation. The EDX-spectra from several sites within the gray layer (TB) mostly reveal a similar composition. One typical example is shown in Fig. 4b. The main constituent is always iron oxide followed by silicon and Ca, corresponding to the two major pad constituents steel fibers and mineral fibers, respectively. Furthermore, small amounts of elements from other pad constituents like Cu, Zn, C and S are visible. Although the TB is always a mixture of pad constituents with iron oxide, the volume fractions of the different constituents may change from site to site. 3.4. FIB–TEM investigation of third body at the pad–disc interface The frame termed FIB-cut 1 in Fig. 4a depicts the site of TEM-specimen preparation by Focused Ion Beam technique. The
Fig. 3. LM-micrograph of the pin–disc interface.
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Fig. 4. SEM-micrograph (a) and EDX-spectrum corresponding to marked area 5 of the TB between pin and disc (b).
overview-micrograph produced by FIB imaging mode after micromachining of the lamella is shown in Fig. 5a. The almost uniform gray contrast on the imaged surface indicates that the TEM-lamella consists completely of TB-material. Nevertheless there are some coarse particles (diameters approximately 1 m) showing darker contrast than the embedding matrix. Two of them were identified to be silicon oxide later by EDX in the TEM. There are also some cracks and voids visible in the middle of the lamella, indicating that adhesion between compacted wear debris filling the gap between pad and disc has partly broken down during specimen preparation. Fig. 5b is a TEM overview-micrograph showing part of the lamella after having cut it off the specimen, and having lifted it out and deposited on a TEM-grid. The contrasts are different, often complementary to the FIB image, but corresponding sites are obvious, especially in the cracked region. The frames indicate areas were further TEM investigations were performed at higher magnifications. We will focus on site 1 to show the nanostructure of the TB by conventional and analytical TEM. Fig. 6a shows a conventional bright field image and corresponding Selected Area Electron Diffraction (SAED) pattern. A coarse particle is visible in bright contrast surrounded by an agglomerate of fine particles (diameter approximately 10 nm). Some of the fine particles appear in dark diffraction contrast, thus contributing to the ring pattern shown in Fig. 6b. The rings can be attributed to the magnetite phase (Fe3 O4 ), though not unambiguously, because their intensity is very weak and many theoretically expected rings are missing. A similar area, although rotated and at a different specimen tilt (due to investigation in another instrument, the analytical TEM) is shown in Fig. 7. These energy-filtered images using signals cor-
responding to Fe and O provide useful chemical information. The coarse particle is aluminum oxide, as revealed by the high oxygen signal and EDX-mapping with Al-signal (not shown here). An iron particle (bright in Fig. 7a and dark in Fig. 7b) is attached to the bottom of the aluminum oxide. The surrounding matrix consists mainly of granular iron oxide, as revealed by the iron image (Fig. 7a). The granular pattern of the iron distribution suggests that there is at least one further phase in the space between the iron oxide nanoparticles. In order to proof this suggestion, further energyfiltered images were taken at even higher magnification in a region close to the aluminum oxide particle. As revealed in Fig. 8, Fe, C and Cu depict inhomogeneous distribution at the regarded scale: 100 nm × 100 nm, suggesting a mixture of different particles. This was not revealed by EDXmapping because of the lower spatial resolution of this technique. The EDX-spectrum corresponding to the region depicted in Fig. 8 is shown in Fig. 8d. Although the sampled area is much smaller, it is similar to the one obtained by SEM (Fig. 4b). Again the Si-signal is quite high. Therefore it was assumed that particles or seams enriched in silicon could be revealed, but this was not the case. Ni- and Ga-signals in Fig. 8d are artefacts from support grid and FIB-machining with Ga-ions, respectively. Further TEM investigations at other sites, as well as other TEMlamellae, revealed a large variety of “coarse particles” (diameters of several 100 nm), e.g., steel or copper micro-chips as well as corundum or calcite particles. Nevertheless, a common feature of all investigated sites was that the coarse particles were embedded in a matrix of the same structure, as the one shown in Fig. 8.
Fig. 5. FIB-micrograph (a) and corresponding TEM overview (b).
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Fig. 6. TEM-micrograph, higher magnification of site 1 (a) and corresponding SAED pattern (b).
Fig. 7. EF–TEM-micrograph showing Fe-distribution (a) and O-distribution (b) in and around a “coarse” particle in the TB.
Fig. 8. EF–TEM-micrographs showing Fe- (a), C- (b) and Cu-distribution (c), as well as EDX-spectrum of this area (d).
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Fig. 9. Wear particle from collection site 1: TEM-micrograph (a), SAED pattern (b) and EDX-spectrum (c).
3.5. Investigation of wear particles Fig. 9 shows results of the TEM-characterization of a wear particle collected at site 1 (Fig. 1). Obviously this is a fragment of the TB, because it shows a similar microstructure, diffraction pattern and EDX-spectrum as discussed in the previous section. Particles collected at site 2 usually were much bigger than the ones from site 1. Those particles were often coarse wear fragments from the disc. Actually, cast iron chips up to 1 mm in length were observed frequently. Furthermore, agglomerates of TB-material and coarse particles of pad constituents were observed as well. 4. Discussion The results show that a TB with a different structure and composition as the first bodies has been trapped in an approximately 10 m wide gap between pin and disc. As already mentioned in Section 2.3 the gap seen after cutting the cross-section may not only contain TB-material produced during the tribological test, but also wear particles from cutting or grinding. Since cutting and grinding always was performed in one direction, namely from disc to pin, the latter material can only consist of iron, silicon, carbon and probably some oxygen, whereas TB-material always contains a certain amount of other pad constituents. Indeed, a SEM-study of the cross-section prior to polishing revealed a slight difference of element-related contrast during imaging with back-scattered electrons within the gap, whereas after polishing a uniform contrast was observed. The origin of TB-formation during the tribological test is wear, which occurs at different length and time scales by different mechanisms. One of these mechanisms is severe plastic deformation at the disc surface, cracking of the interface between graphite lamellae and pearlitic matrix of the cast iron and corresponding production of coarse cast iron particles. A previous investigation of a brake disc and modeling of the graphite–pearlite composite [9], corroborate this statement. Such coarse wear particles normally are ejected from the friction contact. If not, they may be transferred to the pad and cause problems [18]. A second very important wear mechanism is tribo-oxidation leading to the formation of nano-crystalline iron oxide [6–9]. At temperatures above 250 ◦ C (not relevant for our experiment) pyrolysis of the phenolic resin leads to a considerable increase of pad wear [12]. But also at the moderate temperature of our experiment, particles from the pad were released. Hard particles like aluminum oxide (corundum) or silicon oxide (quartz) will retain their original size, whereas soft particles like copper or metal sulfides will be milled together with the iron oxide and finally end up with a very small grain size. While one part of the milled wear particles will be ejected from the contact, another part may be stored in wear troughs or incorporated into films adhering to the surfaces of the first bodies. During tribological testing the two first
bodies are separated by a layer of TB which consists of a matrix of the nano-crystalline wear debris in which some coarser particles are embedded, as shown in the schematic sketch (Fig. 10). All our published [8,9,15,16] and not yet published experiences suggest that the scenario described above does not only apply for pin-on-disc testing, but also for praxis-relevant braking conditions, although the quantitative composition of TB-material of course will depend on testing conditions, pad formulation and location at the pad surface. The EF–TEM results (Fig. 8) showed for the first time that the matrix of the TB actually consists of at least 3 nano-constituents, namely iron oxide, carbon and copper, mixed on a very fine scale. Recently, modeling on the nano-scale revealed that if films with such a peculiar structure are covering the surfaces of the first bodies, a constant friction force with a minimum of fluctuations can be expected [15]. With our experiment we could show that such films actually exist in the contact zone, although after release of the pin from the disc the major part of these films seems to be destroyed and ejected as wear particles, and only some residual film fragments remain at the actuating surfaces. Brake manufacturers and customers know that the formulation of the pad material exerts an impact on the adhesion of transfer films on the disc surface. Whereas so-called Lowmet pads, like the one regarded here, do not promote visible film formation at the disc surface, so-called NAO pads do. Depending on braking conditions, 10–100 m thick films were observed on the disc surface in the latter case [19]. Furthermore, we expect that the pad formulation will also determine the chemical composition of the TB. In fact, a higher silicon content compared to previous studies [8,9,15] was observed in the EDX-spectra (Figs. 4a, 8d and 9c). This can be attributed to the mineral fibers which were missing in the other pad formulations. It is also interesting to compare the chemical composition
Fig. 10. Schematic presentation of results obtained by cross-sectional investigations of the pad–disc interface.
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Fig. 11. XFA sum spectrum of a representative pad area prior to braking.
of the TB with the integral composition of the pad material. The latter was obtained by XFA of a large area at the original pad surface. The result is shown in Fig. 11. Although light elements are not revealed well in the XFA-spectrum, similarities with the EDX-spectra of the TB are obvious, suggesting that almost all constituents of the pad contribute to TB-formation. It should be mentioned that Rh is an artefact from the X-ray source. The minor signals from Pb, Cr, Mn, Ti, Sn and K all correspond to pad constituents, but their mass fraction is too small for being detected by EDX within the small volume sampled by EDX in the SEM or TEM. Our hypothesis from Section 1 has been that braking conditions should not affect the structure and properties of the TB. We can check this by comparing the present results with previous findings for the same materials but different braking parameters [20]. In the latter study emergency braking was simulated leading to surface temperatures of approximately 250 ◦ C. In spite of that remarkable difference compared to our test parameters, the nano-crystalline structure and qualitative composition of the TB was the same. The main difference compared to the present study was the volume of the TB which should increase with increasing temperature due to higher wear rates. Consequently, more TB was stored at the surface at so-called powder bands and thicker TB-layers were formed [20]. Thus the system is able to adjust its surface state to different stressing conditions without changing its friction properties. Though it is likely that during real braking under certain conditions hot spots will be formed which may lead to a local change of the surface structure, usually the system will have the ability to regenerate and restore its original properties. This is because the surface films are not static but permanently created and destroyed during the process of braking. 5. Conclusions - Wear debris production is an essential prerequisite for obtaining desired friction properties during braking. - Although not always visible after braking, a Third Body (TB) forms which consists of milled and compacted wear debris and determines friction properties. - In the regarded case the TB consisted of a matrix of nanocrystalline material which contained some particles in the size range 100–1000 nm. - Whereas coarser wear particles are preferentially ejected from the contact region, fine particles adhere to the surface or get trapped in wear troughs. - The observed fluctuations during tribological testing suggest that not enough fine wear particles were present at the inter-
face to provide a stable and constant coefficient of friction (CoF). Acknowledgements Financial support from German Research Foundation (OS77/) and German Academic Exchange Service (DAAD) is gratefully acknowledged. References [1] M. Godet, The third body approach: a mechanical view of wear, Wear 100 (1984) 437–452. [2] M.G. Jacko, P.H.S. Tsang, S.K. Rhee, Wear debris compaction and friction film formation of polymer composites, Wear 133 (1989) 23–38. [3] A. Wirth, D. Eggleston, R. Withaker, A fundamental tribochemical study of the third body layer formed during automotive friction braking, Wear 179 (1994) 75–81. [4] M. Börjesson, P. Eriksson, C. Kylenstrirna, The role of friction films in automotive brakes subjected to low contact forces, ImechE (1993) 259–267. [5] W. Österle, M. Griepentrog, Th. Gross, I. Urban, Chemical and microstructural changes induced by friction and wear of brakes, Wear 251 (2001) 1469–1476. [6] M. Eriksson, Friction and contact phenomena of disc brakes related to squeal, PhD thesis, Uppsala University 2000, Acta Universitatis Upsaliensis ISBN 91554-4716-3. [7] M. Eriksson, S. Jacobson, Tribological surfaces of organic brake pads, Tribol. Int. 33 (2000) 817–827. [8] W. Österle, I. Urban, Friction layers and friction films on PMC brake pads, Wear 257 (2004) 215–226. [9] W. Österle, H. Kloß, I. Urban, A.I. Dmitriev, Towards a better understanding of brake friction materials, Wear 263 (2007) 1189–1201. [10] P. Filip, Z. Weiss, J. Rafaja, On friction layer formation in polymer matrix composite materials for brake applications, Wear 252 (2002) 189–198. [11] P.J. Blau, H.M. Meyer, Characteristics of wear particles produced during friction tests of conventional and unconventional disk brake materials, Wear 255 (2003) 1261–1269. [12] A.-L. Cristol-Bulthé, Y. Desplanques, G. Degallaix, Y. Berthier, Mechanical and chemical investigation of the temperature influence on the tribological mechanisms occurring in OMC/cast iron friction contact, Wear 264 (2008) 815–825. [13] M. Eriksson, J. Lord, S. Jacobson, Wear and contact conditions of brake pads: dynamic in situ studies of pad on glass, Wear 249 (2001) 272–278. [14] H. Kasem, S. Bonnamy, B. Rousseau, H. Estrade-Szwarckopf, Y. Berthier, P. Jacquemard, Interdependence between wear process, size of detached particles and CO2 production during carbon/carbon composite friction, Wear 263 (2007) 1220–1229. [15] W. Österle, H. Kloß, A.I. Dmitriev, Modeling of friction evolution and assessment of impacts on vibration excitation at the pad–disc interface, Proc. SAE Brake Colloquium 2008, paper No. 2008-01-2582, 303-311. [16] W. Österle, U. Müller, Y. Desplanques, A.-L. Bulthé, G. Degallaix, A.I. Dmitriev, Third body formation of two different brake pad materials designed for railway application, Proceedings of the 5th European Conference on Braking (JEF 2006), Lille, France, GRRT (ed.) 20, rue Elisee Reclus-B.P.317 59666 Villeneuve d’ AscQ CEDEX-France, pp. 107–115, in English. [17] Y. Desplanques, A.-L.-Bulthé, G. Degallaix, L. Sabatier, J. Bertheau, Transient aspects of local physical mechanisms induced by friction in braking, Proceedings of the 5th European Conference on Braking (JEF 2006), Lille, France, GRRT
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(ed.) 20, rue Elisee Reclus-B.P.317 59666 Villeneuve d’ AscQ CEDEX-France, pp. 89–97, in English. [18] D. Severin, S. Dörsch, Friction mechanism in industrial brakes, Wear 249 (2001) 771–779. [19] M.H. Cho, K.H. Cho, S.J. Kim, D.H. Kim, H. Jang, The role of transfer layers on friction characteristics in the sliding interface between
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Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
A comprehensive microscopic study of third body formation at the interface between a brake pad and brake disc during the final stage of a pin-on-disc test W. Österle a,∗ , I. Dörfel a , C. Prietzel a , H. Rooch a , A.-L. Cristol-Bulthé b , G. Degallaix b , Y. Desplanques b a b
Bundesanstalt für Materialforschung und – prüfung, 12200 Berlin, Germany Ecole Centrale de Lille, Laboratoire de Mécanique de Lille, BP 48, F-59651 Villeneuve d’Ascq Cedex, France
a r t i c l e
i n f o
Article history: Received 29 August 2008 Received in revised form 25 November 2008 Accepted 25 November 2008 Keywords: Brake Third body Wear Friction Nano-scale TEM
a b s t r a c t In order to preserve contact situations occurring during a tribological test, an experiment was performed to investigate the pin–disc interface while keeping the pin pressed against the disc. The pin was made from a commercial brake pad material and the disc from gray cast iron. Thus the experiment simulated a moderate brake application. After fixation of the friction-couple, macro- and micro-cross-sections were prepared by conventional metallography and focused ion beam technique, respectively. The amount of third body between the two first bodies and its structure and chemical composition was determined by microscopic and micro-analytical techniques including SEM, FIB, TEM and EDX. Furthermore, the size and structure of wear particles ejected from the contact area during the test was examined. Although testing conditions were moderate in respect to energy input and corresponding temperature rise, at least the main features of the third body structure are comparable to features observed after severe braking conditions. The observations confirm that during braking the first bodies are separated by a third body which is mainly responsible for the friction response. Lacking evidence of third body films after braking with certain pad materials can be attributed to film loss after braking due to weak adherence to the disc surface. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Brake manufacturers have to meet a number of requirements demanded by their customers. For regional inter-city railway application, as regarded here, the following expectations have been communicated: A constant coefficient of friction (CoF) between 0.3 and 0.4, irrespective of pressure, temperature and humidity, smooth braking without vibrations and noise generation, regeneration of original properties even after severe brake applications, e.g., after emergency braking, low wear rates of pad and disc, cost reduction by using cheap but environment-friendly raw materials. Besides geometric and system considerations the design of the pad material plays a major role in reaching the target properties. Brake pad materials are multi-constituent composites. The formulations have to be optimized by trial and error until the desired performance properties are reached. The objective of this work was to contribute to the knowledge of the basic friction mechanisms which finally promote the desired behaviour described above. Referring to Godet’s fundamental work [1] we suggest the following hypothetical scenario for a disc brake.
∗ Corresponding author. Tel.: +49 3081041511; fax: +49 3081041517. E-mail address: [email protected] (W. Österle). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.11.023
In the course of a bedding procedure, a fine-grained wear debris comprising a mix of pad and disc constituents is produced and trapped in troughs at the surface of the matching first bodies. Since the trapped material differs in respect to composition and microstructure from both first bodies, Godet defined it as third body (TB). During braking the TB is spread over contact areas providing separation of the first bodies, thus preventing adhesion and micro-welding of metallic constituents and providing velocity accommodation between the rotating disc and fixed pad. As long as enough TB-material is available at the interface and the TB does not change its structure, stable friction behaviour can be expected. Even after severe braking conditions, when the unique structure of the TB might have been destroyed, or all wear debris removed from the contact, the system is able to regenerate its original friction properties during subsequent recovery cycles during which the desired TB is produced again. Godet also proposed the continuous formation and destruction of surface films, so-called screens which cover the contact areas providing the main source of wear particle production. Experimental evidence of films produced by debris compaction of melted and pyrolyzed products from brake pads was first provided by Jacko et al. [2]. Several authors reported on films containing BaSO4 which is a frequently used filler for brake pads [3–5]. Later, iron oxide was identified as a major constituent of surface films of brake pads and discs [6–9]. Filip et al. [10] and Blau and Meyer [11]
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Nomenclature CoF EDX EF FIB LM NAO rpm SAED SEM Td Tp TB TEM XFA
coefficient of friction Energy Dispersive X-ray spectroscopy Energy Filter Focused Ion Beam Light-optical Microscope None Asbestos Organic disc rotation in revolutions per minute Selected Area Electron Diffraction Scanning Electron Microscope disc mass temperature pin mass temperature third body Transmission Electron Microscopy X-ray Fluorescence Analysis
identified mixtures of oxides and carbonaceous products in films and wear particles, respectively. Obviously, the chemical composition of wear debris and friction films depends on the formulation of pad materials. In addition to flat plates of compacted wear particles, Cristol-Bulthé et al. observed loose particles with the same composition as the plates, trapped at the pad surface [12]. They also recognized an increased amount of TB-material after braking conditions leading to high temperatures and related this finding to resin degradation which was measured in situ by mass spectroscopy. All experimental findings reported up to now refer to observations at either the pad or the disc surface after braking, but not to the situation during braking, when the pads are pressed against the rotating disc. This might explain, why only fragments of third body layers were observed experimentally while in situ studies of pad on glass suggested the formation of continuous films spreading over the contacting areas [13]. The motivation for the present work was, to investigate the situation at the pad–disc interface before non-adhering TB-material is removed from the contacting surfaces. This approach was inspired by the work of Kasem et al. Those authors were able to show a 20 m wide gap filled with TB-material between the pin and the disc of a carbon/carbon composite friction-couple [14]. Further motivation was to verify assumptions which were made to simulate material flow and friction response at micro-contacts at the pad–disc interface [15]. The latter study implied that only if the first bodies are screened by oxide layers with a certain amount of solid lubricant, a smooth friction response and medium CoF value can be expected. 2. Experimental procedure 2.1. Materials The same polymer matrix composite pad material as in previous studies [12,16,17] was investigated. Table 1 depicts the main constituents of this formulation. A commercial gray cast iron disc was used as counter body. The cast iron had a pearlitic matrix with a small quantity of free ferrite. The graphite flakes were homogeneously distributed and oriented in the matrix.
mercial cast iron disc. Thus the friction-couple comprises of the same materials as the real brake, but energy input and resulting temperature correspond to very moderate braking conditions only. The tribological tests were performed on a pin-on-disc tribometer with a maximal disc rotation speed of 1000 rpm and a maximal normal load applicable on the pin of 500 N. A deformable steel parallelogram linked to the pin and equipped with deformation gauges allowed the measurement of normal and tangential loads on the pin-on-disc contact. Using this data, the CoF was calculated. The pin and the disc were both equipped with a thermocouple located at 2 mm from the contact surface on the mean friction radius. They permitted mass temperature measurements Td and Tp for disc and pin, respectively. A cylindrical pin with a diameter of 20 mm and a height of 14 mm was machined from a real brake pad. The disc had a diameter of 266 mm and was 13 mm thick. The contact occurred on a 105 mm mean friction radius (Fig. 1). The test was performed with a constant rotation speed of 900 rpm which corresponds to a sliding velocity of 10 m/s, and a normal load of 230 N. The time interval was 225 s. After the rotation speed was reached, the load was manually applied. Before the test, the disc surface was ground with sand paper (grade 80). A preliminary run-in was also performed before the test to obtain geometric adjustment of the rubbing surfaces. 2.3. Specimen preparation and interface characterization After tribological testing the pin, still under pressure, was clamped to the disc and fixed with the aid of a glue, thus preserving the interface situation during the final stage of the test. Then the disc-segment with fixed pin was cut from the disc and embedded completely in epoxy resin. After curing of the resin the clamp was removed. Next the major part of the remaining disc-segment and also part of the pin was cut off, leaving only a thin specimen containing the whole interface embedded in resin at its circumference. This specimen was thoroughly cut in the middle, perpendicular to the rubbing direction, with a low-speed diamond saw. It was not possible to cut cast iron with a diamond wire saw, as suggested by Kasem et al. for the C/C-material [14]. Inspection of the cut crosssection in a Light-optical Microscope (LM) revealed closed contact between pin and disc. After having removed material by grinding and polishing a horizontal crack appeared indicating separation of pin and disc due to pressure release or thermal stresses during cutting. Obviously, this crack had been filled with wear debris from the cutting and grinding process and therefore did not appear before final polishing. Further investigations were performed at higher magnification in a Scanning Electron Microscope (SEM) equipped with an Energy Dispersive X-ray spectrometer (EDX). Finally a thin lamella, approximately 20 m wide, 10 m deep and 100 nm thick was cut from the interface region, where the third body had been identified by SEM/EDX, with a Focused Ion Beam (FIB) instrument. This lamella was lifted with the aid of a micro-manipulator under LM control at a magnification of 500:1 and put on a carbon-coated nickel grid. After covering with a second approximately 10 nm thick carbon film supported by nickel grid (200 mesh per inch), the micro-specimen was investigated in a conventional TEM (JEOL JEM4000FX) and in an analytical TEM (JEOL JEM2200FS) equipped with EDX, Energy Filter (EF) and Field Emission Gun (FEG). 2.4. Wear particle characterization
2.2. Tribological testing Since it was not possible to do the planned interface investigation with a brake dynamometer or a full-scale brake test, we decided to perform a simple pin-on-disc test with a cylindrical pin cut from an original brake pad which was tested against a com-
Loose wear particles were collected with a brush around the pin before applying the epoxy resin for fixation. These particles had been released from the contact and were recirculated on the disc but then were not able to enter the contact again, but were blocked on the pin periphery. After dispersion of the particles in
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Table 1 Composition of brake pad in vol%. Mineral fibers
Metallic fibers
Organic fibers
Solid lubricants
Abrasive particles
Metallic particles
Phenolic resin
20
8.5
4
18
12
5.5
Balance
Fig. 1. Test device: photo of pin actuating against disc (a) and schematic of test (b).
pure ethanol under sonication single droplets were transferred to carbon-coated support grids and allowed to dry in air. Then the grids were investigated in the analytical TEM. A second site of particle sampling was outside the disc, approximately 10 cm away from the pin, in the direction of disc rotation. On this site, ejected particles, definitely lost for the contact, were sampled on aluminium foil covered by a glue for further SEMinvestigation.
disc. Here the matrix appears bright and the graphite lamellae gray. There always exists a gap between pin and disc which is filled mostly with material displaying dark contrast in the LM. Since there is considerable roughness on both surfaces, the width of the gap changes from site to site. The gap becomes very narrow if a metal constituent of the pad material protrudes somewhat from the worn pin surface, but it never disappears completely. 3.3. Scanning Electron Microscopy
3. Results 3.1. Thermal and tribological behaviour The initial temperatures of both, pin and disc, were 23 ◦ C and after the friction test, the mass temperatures reached in a depth of 2 mm from the friction surfaces were Td = 67 ◦ C and Tp = 55 ◦ C. Fig. 2 shows the evolution of the CoF which was measured during the test. From the beginning of the test (at t = 20 s, when the load was applied) until t = 80 s, the CoF increased from 0.26 to 0.4. Then, the CoF stabilized at a mean value of 0.4. From t = 50 s until the end of the test, the CoF was prone to minor fluctuations with a maximum amplitude of 0.08. 3.2. Light-optical Microscopy Approximately 10% of the cross-section is shown in Fig. 3. In the pad material, at the top, the metallic constituents copper and steel appear in bright contrast, whereas mineral fibers, ceramic constituents and phenolic resin are shown with different gray levels. The lower half of the cross-section corresponds to the cast iron
Fig. 2. CoF as function of time at 900 rpm and 230 N.
The area marked by the frame in Fig. 3 is shown at higher magnification by SEM in Fig. 4a. The material filling the gap between pin and disc is now shown in light gray contrast. As will be shown later, this material is the TB, i.e., compacted and mixed wear debris from pin and disc. A horizontal crack is mostly visible within the TB. It can run along its interfaces with pin or disc or sometimes also through the TB. This crack most likely is due to pressure release during specimen preparation. The EDX-spectra from several sites within the gray layer (TB) mostly reveal a similar composition. One typical example is shown in Fig. 4b. The main constituent is always iron oxide followed by silicon and Ca, corresponding to the two major pad constituents steel fibers and mineral fibers, respectively. Furthermore, small amounts of elements from other pad constituents like Cu, Zn, C and S are visible. Although the TB is always a mixture of pad constituents with iron oxide, the volume fractions of the different constituents may change from site to site. 3.4. FIB–TEM investigation of third body at the pad–disc interface The frame termed FIB-cut 1 in Fig. 4a depicts the site of TEM-specimen preparation by Focused Ion Beam technique. The
Fig. 3. LM-micrograph of the pin–disc interface.
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Fig. 4. SEM-micrograph (a) and EDX-spectrum corresponding to marked area 5 of the TB between pin and disc (b).
overview-micrograph produced by FIB imaging mode after micromachining of the lamella is shown in Fig. 5a. The almost uniform gray contrast on the imaged surface indicates that the TEM-lamella consists completely of TB-material. Nevertheless there are some coarse particles (diameters approximately 1 m) showing darker contrast than the embedding matrix. Two of them were identified to be silicon oxide later by EDX in the TEM. There are also some cracks and voids visible in the middle of the lamella, indicating that adhesion between compacted wear debris filling the gap between pad and disc has partly broken down during specimen preparation. Fig. 5b is a TEM overview-micrograph showing part of the lamella after having cut it off the specimen, and having lifted it out and deposited on a TEM-grid. The contrasts are different, often complementary to the FIB image, but corresponding sites are obvious, especially in the cracked region. The frames indicate areas were further TEM investigations were performed at higher magnifications. We will focus on site 1 to show the nanostructure of the TB by conventional and analytical TEM. Fig. 6a shows a conventional bright field image and corresponding Selected Area Electron Diffraction (SAED) pattern. A coarse particle is visible in bright contrast surrounded by an agglomerate of fine particles (diameter approximately 10 nm). Some of the fine particles appear in dark diffraction contrast, thus contributing to the ring pattern shown in Fig. 6b. The rings can be attributed to the magnetite phase (Fe3 O4 ), though not unambiguously, because their intensity is very weak and many theoretically expected rings are missing. A similar area, although rotated and at a different specimen tilt (due to investigation in another instrument, the analytical TEM) is shown in Fig. 7. These energy-filtered images using signals cor-
responding to Fe and O provide useful chemical information. The coarse particle is aluminum oxide, as revealed by the high oxygen signal and EDX-mapping with Al-signal (not shown here). An iron particle (bright in Fig. 7a and dark in Fig. 7b) is attached to the bottom of the aluminum oxide. The surrounding matrix consists mainly of granular iron oxide, as revealed by the iron image (Fig. 7a). The granular pattern of the iron distribution suggests that there is at least one further phase in the space between the iron oxide nanoparticles. In order to proof this suggestion, further energyfiltered images were taken at even higher magnification in a region close to the aluminum oxide particle. As revealed in Fig. 8, Fe, C and Cu depict inhomogeneous distribution at the regarded scale: 100 nm × 100 nm, suggesting a mixture of different particles. This was not revealed by EDXmapping because of the lower spatial resolution of this technique. The EDX-spectrum corresponding to the region depicted in Fig. 8 is shown in Fig. 8d. Although the sampled area is much smaller, it is similar to the one obtained by SEM (Fig. 4b). Again the Si-signal is quite high. Therefore it was assumed that particles or seams enriched in silicon could be revealed, but this was not the case. Ni- and Ga-signals in Fig. 8d are artefacts from support grid and FIB-machining with Ga-ions, respectively. Further TEM investigations at other sites, as well as other TEMlamellae, revealed a large variety of “coarse particles” (diameters of several 100 nm), e.g., steel or copper micro-chips as well as corundum or calcite particles. Nevertheless, a common feature of all investigated sites was that the coarse particles were embedded in a matrix of the same structure, as the one shown in Fig. 8.
Fig. 5. FIB-micrograph (a) and corresponding TEM overview (b).
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Fig. 6. TEM-micrograph, higher magnification of site 1 (a) and corresponding SAED pattern (b).
Fig. 7. EF–TEM-micrograph showing Fe-distribution (a) and O-distribution (b) in and around a “coarse” particle in the TB.
Fig. 8. EF–TEM-micrographs showing Fe- (a), C- (b) and Cu-distribution (c), as well as EDX-spectrum of this area (d).
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Fig. 9. Wear particle from collection site 1: TEM-micrograph (a), SAED pattern (b) and EDX-spectrum (c).
3.5. Investigation of wear particles Fig. 9 shows results of the TEM-characterization of a wear particle collected at site 1 (Fig. 1). Obviously this is a fragment of the TB, because it shows a similar microstructure, diffraction pattern and EDX-spectrum as discussed in the previous section. Particles collected at site 2 usually were much bigger than the ones from site 1. Those particles were often coarse wear fragments from the disc. Actually, cast iron chips up to 1 mm in length were observed frequently. Furthermore, agglomerates of TB-material and coarse particles of pad constituents were observed as well. 4. Discussion The results show that a TB with a different structure and composition as the first bodies has been trapped in an approximately 10 m wide gap between pin and disc. As already mentioned in Section 2.3 the gap seen after cutting the cross-section may not only contain TB-material produced during the tribological test, but also wear particles from cutting or grinding. Since cutting and grinding always was performed in one direction, namely from disc to pin, the latter material can only consist of iron, silicon, carbon and probably some oxygen, whereas TB-material always contains a certain amount of other pad constituents. Indeed, a SEM-study of the cross-section prior to polishing revealed a slight difference of element-related contrast during imaging with back-scattered electrons within the gap, whereas after polishing a uniform contrast was observed. The origin of TB-formation during the tribological test is wear, which occurs at different length and time scales by different mechanisms. One of these mechanisms is severe plastic deformation at the disc surface, cracking of the interface between graphite lamellae and pearlitic matrix of the cast iron and corresponding production of coarse cast iron particles. A previous investigation of a brake disc and modeling of the graphite–pearlite composite [9], corroborate this statement. Such coarse wear particles normally are ejected from the friction contact. If not, they may be transferred to the pad and cause problems [18]. A second very important wear mechanism is tribo-oxidation leading to the formation of nano-crystalline iron oxide [6–9]. At temperatures above 250 ◦ C (not relevant for our experiment) pyrolysis of the phenolic resin leads to a considerable increase of pad wear [12]. But also at the moderate temperature of our experiment, particles from the pad were released. Hard particles like aluminum oxide (corundum) or silicon oxide (quartz) will retain their original size, whereas soft particles like copper or metal sulfides will be milled together with the iron oxide and finally end up with a very small grain size. While one part of the milled wear particles will be ejected from the contact, another part may be stored in wear troughs or incorporated into films adhering to the surfaces of the first bodies. During tribological testing the two first
bodies are separated by a layer of TB which consists of a matrix of the nano-crystalline wear debris in which some coarser particles are embedded, as shown in the schematic sketch (Fig. 10). All our published [8,9,15,16] and not yet published experiences suggest that the scenario described above does not only apply for pin-on-disc testing, but also for praxis-relevant braking conditions, although the quantitative composition of TB-material of course will depend on testing conditions, pad formulation and location at the pad surface. The EF–TEM results (Fig. 8) showed for the first time that the matrix of the TB actually consists of at least 3 nano-constituents, namely iron oxide, carbon and copper, mixed on a very fine scale. Recently, modeling on the nano-scale revealed that if films with such a peculiar structure are covering the surfaces of the first bodies, a constant friction force with a minimum of fluctuations can be expected [15]. With our experiment we could show that such films actually exist in the contact zone, although after release of the pin from the disc the major part of these films seems to be destroyed and ejected as wear particles, and only some residual film fragments remain at the actuating surfaces. Brake manufacturers and customers know that the formulation of the pad material exerts an impact on the adhesion of transfer films on the disc surface. Whereas so-called Lowmet pads, like the one regarded here, do not promote visible film formation at the disc surface, so-called NAO pads do. Depending on braking conditions, 10–100 m thick films were observed on the disc surface in the latter case [19]. Furthermore, we expect that the pad formulation will also determine the chemical composition of the TB. In fact, a higher silicon content compared to previous studies [8,9,15] was observed in the EDX-spectra (Figs. 4a, 8d and 9c). This can be attributed to the mineral fibers which were missing in the other pad formulations. It is also interesting to compare the chemical composition
Fig. 10. Schematic presentation of results obtained by cross-sectional investigations of the pad–disc interface.
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Fig. 11. XFA sum spectrum of a representative pad area prior to braking.
of the TB with the integral composition of the pad material. The latter was obtained by XFA of a large area at the original pad surface. The result is shown in Fig. 11. Although light elements are not revealed well in the XFA-spectrum, similarities with the EDX-spectra of the TB are obvious, suggesting that almost all constituents of the pad contribute to TB-formation. It should be mentioned that Rh is an artefact from the X-ray source. The minor signals from Pb, Cr, Mn, Ti, Sn and K all correspond to pad constituents, but their mass fraction is too small for being detected by EDX within the small volume sampled by EDX in the SEM or TEM. Our hypothesis from Section 1 has been that braking conditions should not affect the structure and properties of the TB. We can check this by comparing the present results with previous findings for the same materials but different braking parameters [20]. In the latter study emergency braking was simulated leading to surface temperatures of approximately 250 ◦ C. In spite of that remarkable difference compared to our test parameters, the nano-crystalline structure and qualitative composition of the TB was the same. The main difference compared to the present study was the volume of the TB which should increase with increasing temperature due to higher wear rates. Consequently, more TB was stored at the surface at so-called powder bands and thicker TB-layers were formed [20]. Thus the system is able to adjust its surface state to different stressing conditions without changing its friction properties. Though it is likely that during real braking under certain conditions hot spots will be formed which may lead to a local change of the surface structure, usually the system will have the ability to regenerate and restore its original properties. This is because the surface films are not static but permanently created and destroyed during the process of braking. 5. Conclusions - Wear debris production is an essential prerequisite for obtaining desired friction properties during braking. - Although not always visible after braking, a Third Body (TB) forms which consists of milled and compacted wear debris and determines friction properties. - In the regarded case the TB consisted of a matrix of nanocrystalline material which contained some particles in the size range 100–1000 nm. - Whereas coarser wear particles are preferentially ejected from the contact region, fine particles adhere to the surface or get trapped in wear troughs. - The observed fluctuations during tribological testing suggest that not enough fine wear particles were present at the inter-
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