MICROSTRUCTURAL CHANGES DUE TO FRICTION PROCESSES IN BRAKES

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1 MICROSTRUCTURAL CHANGES DUE TO FRICTION PROCESSES IN BRAKES T. POESTE, A. PYZALLA Hahn-Meitner-Institut, Abt. Werkstoffe, D-1419 Berlin, GERMANY S. TREPTE, U. FRANKE, D. SEVERIN TU Berlin, Fachgebiet Fördertechnik und Getriebetechnik, D-1623 Berlin, GERMANY SUMMARY During the contact between the disk and the friction lining both parts of the brake system, the disk as well as the friction lining, are subjected to thermal as well as mechanical loading. The mechanical and the thermal loading induce structural changes in the material of the disk and the lining. These structural changes are accompanied by the generation of residual stresses in the brake disk. In order to study the influence of temperature and mechanical loading, thermo-shock trials of the brake disks as well as different braking tests on a testing device are performed. The influence of the loading on the thermo-shock samples and the brake disk are investigated using microscopy and X-rays. Keywords: Brake, friction, microstructure, microscopy, stress 1 INTRODUCTION Automobile brake disks are safety parts. Although new developments employing ceramics are underway [1], still most conventional automobile brake disks are made of steel or gray cast iron. In the contact zone between the disk and the lining, friction leads to mechanical loading which can be accompanied by high temperatures. The mechanical loading as well as the temperature rise lead to structural changes of the material within the contact zone. These structural changes include material removal from the disk and the lining surface, plastic deformation of the brake disk surface as well as phase transformations at the surface of the brake disk and in the lining. The plastic deformation and the phase transformations at the disk surface induce the formation of residual stresses. The residual stresses may lead to crack formation [2] as well as distortions, which might bring about the unwelcome phenomenon of brake judder [3]. Up to now the residual stress state of brake disks [3] and the residual stress state of brakes used in heavy army motor vehicles [2] have been studied. The aim of this investigation is a systematic study of the effect of the thermal and the mechanical loading on the microstructural changes of the disk and the lining during a braking cycle. In order to differentiate, as far as possible, between the thermal and the mechanical loading, thermo-shock trials, braking trials from low speeds, which lead to predominantly mechanical loading, and braking trials from high speeds, which induce thermal and mechanical loading, are performed. 2 EXPERIMENTAL DETAILS 2.1 Samples The material chosen for the brake trials is steel German Grade St Samples sized 3 mm 4 mm 15 mm were machined from normalized material and annealed in an argon atmosphere at 5 C for 2 h, cooling with a cooling rate of 4 K / h. The microstructure of the samples is ferrite - pearlite. The material chosen for the thermal shock trials is also St 52-3 with sample dimensions of 19 mm 3 mm 1 mm. 2.2 Loading of the samples inductor sample thermocouples Fig. 1: Thermo-shock testing device at the TU Berlin Thermo-shock trials Since the thermal loading during a braking cycles occurs at a very high speed, thermo-shock trials were performed on the samples by inductive heating. The samples were heated to a maximum temperature of 6 C. An inductor was used which was placed 3 mm above the surface. The maximum temperature was reached within 5 seconds. Then the sample was cooled off in air down to 2 C, before it was heated again. The temperature was controlled by thermocouples, which were placed on the back side of the samples in drill holes.3 mm beneath the surface. After thermal load cycles of 1, 1, 1 and 1 the samples were demounted and analyzed.

friction lining brake disk sample temperature reached during the braking was well below A c1

2 friction lining brake disk sample temperature reached during the braking was well below A c Samples subjected to heavy loading in braking trials The stronger loading causes significant annealing colours on the surface and in the in-depth direction (heat affected zone). This indicates that the samples were subject to an inhomogeneous loading and an inhomogeneous temperature distribution (Fig. 3). 3 heat affected zone Fig. 2: Reciprocal testing device at Fachgebiet Förderund Getriebetechnik of the TU Berlin 4 transversal Brake Trials The brake trials were performed on a reciprocal testing device which has been developed and manufactured at the Fachgebiet Förder- und Getriebetechnik of the Berlin Technical University [4]. The brake trials were carried out using a commercially available JURID lining with a diameter of 125 up to 185 mm. The load applied was 24 N. The time interval used for cooling down the brake disk inbetween the decelerations was chosen to 3 s. The intensity of the loading was varied by slowing down the brake disk from a speed of U = 9 revolutions / min to U = 85 revolutions / min (referred to as heavy loading ) respectively, by slowing down from 5 revolutions / min to revolutions / min ( weak loading ). The number of braking cycles were chosen as 1, 2, 1 and X-ray Diffraction X-ray diffraction analysis for determining the phase composition and the residual stresses of the samples was performed on a diffractometer in Ψ - configuration using Co-K α radiation. The penetration depth of the radiation is 7 µm, approximately, the irradiated area on the sample surface has a diameter of 1 mm. For residual stress analysis the sin 2 ψ - method [5] was employed. 2.4 Synchrotron Radiation Microstructure analyses were done at HASYLAB beamline G3 with a 4-cycle diffractometer using a CCD mounted on the 2Θ -arm. The experiments were done with an X-ray energy of 6.9 kev [6]. 3 RESULTS 3.1 Microstructure Samples subjected to weak loading in braking trials X-ray phase analyses of the surface revealed that the microstructure of the samples subjected to a weak loading is still ferrite pearlite. Thus the maximum contact zone Fig.3: Brake disk, after loading with F = 24 N, slowing down from U = 9/min to U = 85/min; number of brakings n = 2 This optical visible inhomogenity of the surface is due to an incomplete contact between brake disk and lining. With an increasing number of brakings the region where microstructural changes are apparent increases, and after 1 brake trials with strong loading the contact area covers the whole sample surface. In the contact zone optical and scanning electron micrographs reveal, that the morphology of the transformed region of the sample shows three different types of microstructures (Fig. 4). 1 µm Fig. 4: Cross section picture of the near surface region with martensite phase and retained austenite, sample with heavy loading, F = 24 N, slowing down from U = 9/min to 85/min; number of brakings n = 2. At the surface martensite is present, in a distance of 15 µm from the surface the microstructure is bainite and in a distance of 3 µm to the surface the original ferrite - pearlite matrix is visible. X-ray analysis reveals that in case of the samples subjected to strong loading the originally ferrite pearlite microstructure near the edges of the samples has changed into martensite and retained austenite. In case of the martensite the 2 reflection shows broadening and a splitting-up of the profile into an 2/2 and an 2 part. Thus, while in case of the weakly loaded sample the temperature in the contact area stayed below Ac1, in case of the strong loading the temperature Ac1 was exceeded. An analysis of the surface of the contact

zone with synchrotron radiation (Fig. 5c, Fig. 5d) further on reveals the presence of retained austenite and magnetite (Fe 2 O 3 ).

5d shows the sum reflection profiles of the austenite and the magnetite corresponding to the respectively shaded surface areas of the contact zone in Fig. 5c. 1 mm 3.2

The temperature distribution and the coordinate system used for residual stress analysis are indicated. temperature 5b transversa X [mm] 5a Fig.

5b: Photograph of the sample with marked region 5c Fig.

Due to the geometry of the sample and due to the different heat transfer in the and transversal directions of the specimen the resulting residual stress values depend on the specimen direction.

1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 2θ [ ] Fig. 5c: X-ray image of the 111- austenite reflection of a strong loaded sample in the contact zone Fig.

3 zone with synchrotron radiation (Fig. 5c, Fig. 5d) further on reveals the presence of retained austenite and magnetite (Fe 2 O 3 ). Because of the high resolution obtainable using synchrotron radiation, it was possible to separate the 111 austenite reflection from the Fe 2 O 3 4 reflection. Fig. 5d shows the sum reflection profiles of the austenite and the magnetite corresponding to the respectively shaded surface areas of the contact zone in Fig. 5c. 1 mm 3.2 Residual Stresses Residual stresses after thermo-shock tests Fig. 6 shows a typical specimen used for the thermoshock tests. The temperature distribution and the coordinate system used for residual stress analysis are indicated. temperature 5b transversa X [mm] 5a Fig. 5: Microstructure analysis of a sample after heavy loading and n = 2 Fig. 5a: Surface color etching micrograph of martensite (black) and austenite (white) Fig. 5b: Photograph of the sample with marked region 5c Fig. 6: Specimen St 52-3 after thermal load of Tmax = 6 C, thermal cycles n = 1 In the initial state in the thermo shock sample compressive residual stresses of 2 MPa are the are present. Due to the geometry of the sample and due to the different heat transfer in the and transversal directions of the specimen the resulting residual stress values depend on the specimen direction. The residual stresses in transversal direction are higher than in direction. The residual stress distribution is shown in fig. 7 for different numbers of load cycles. intensity [a.u.] 5d austenite iron oxide θ [ ] Fig. 5c: X-ray image of the 111- austenite reflection of a strong loaded sample in the contact zone Fig. 5d: Spectra obtained by averaging over pixels with dark (full line) and light gray (dash) values in fig. 5c σ ± 4 MPa cracks heat spot 5µm number of cycles n=1 n=1 n=3 n=1 initial state: -2 MPa distance to the middle of the heat spot [mm] Fig. 7: Dependence of residual stresses at the surface of thermal shock samples on the number of cycles T max = 6 C Due to the symmetry of the sample, the heat distribution and the load transfer, only the right half of the residual stress distribution is displayed. A comparison of the residual stress curves given in fig. 8 reveals that already after one load cycle the residual stress state at the surface has changed distinctly from the initial state. A local residual stress maximum appears in the area of the sample which was subjected to the highest temperature, directly under the inductor loop, in a distance of 6 mm from the centre of the sample. With increasing number of load cycles the residual stress maximum increases up

4 to a maximum of 15 MPa and the area containing tensile residual stresses widens. After 1 load cycles the residual stress distribution changes distinctly. In the location of the maximum temperature instead of the stress maximum a local residual stress minimum arises. This change in the residual stress distribution is due to crack initiation (Fig. 7) Residual stresses in samples subjected to weak loading in braking trials In the as-manufactured and annealed state at the surface of the brake disk samples, due to machining, compressive residual stresses of 5 MPa are present in both directions of the samples. During the braking tests, inhomogeneous plastic deformation evolves and compressive residual stresses in as well as in transversal direction develop in the contact region (Fig. 8). transversal contact zone σ = 4 MPa number of brakings n=2 n=1 n= distance to the left boundary of the sample [mm] Fig. 8: Dependence of transversal residual stresses at the surface of samples on the number of brakings. Weak loading, F = 24 N, slowing down from U = 5/min to U = /min The maximum of these compressive residual stresses both in and in transversal direction is located near the edge of the samples. In transversal direction the compressive residual stresses are higher than in direction. The elementary processes of the residual stress formation, similar to those in deeprolling [7, 8], are pressing and squeezing of the material on the disk surface, which leads to a stretching of the surface near zone [7] especially in the direction, which is the direction of rotation of the lining. This stretching is obstructed by the bulk material, thus compressive residual stresses evolve. In transversal direction friction also induces surface stretching and successively the formation of compressive residual stresses. The material below the surface in both and transversal direction has to be under tensile residual stresses for reasons of stress equilibrium. With increasing number of load cycles the zone containing the compressive residual stresses widens due to a levelling of the surface and thus an increase of the contact area between disk and lining. Within the contact zone the residual stress level is almost constant and nearly independent of the number of load cycles. At the boundary of the contact area the compressive residual stresses decrease steeply towards the region containing the original stress state before loading Residual stresses in samples subjected to heavy loading in braking trials In case of the heavily loaded samples (Fig. 9) within the contact zone in the martensite and retained austenite are present. 4 transversal α- Fe 3 transversal γ- Fe 2 α- Fe 1 γ- Fe contact -4 zone distance to the left boundary [mm] Fig. 9: Dependence of residual stresses at the surface of samples on the direction; heavy loading, F = 24 N, slowing down from U = 9/min to 85/min; number of brakings n = 2 The compressive residual stresses in the martensite reach up to 6 MPa. The retained austenite contains up to 4 MPa tensile residual stresses. The maximum of these tensile residual stresses is near the boundary between contact zone and undisturbed material. The high compressive residual stresses in the martensite are due to both the mechanical loading, similar to the formation of the compressive residual stresses in the weakly loaded samples, but they are also caused by volume increase brought about by the martensite formation, which might also cause a distortion of the disk. In the retained austenite high tensile residual stresses are present. These high tensile residual stresses can be detrimental with respect to the lifetime of the brake disk since they allow for crack propagation. 4 SUMMARY Thermo-shock tests and braking tests have been performed in order to determine the influence of mechanical and thermal loading on brake disks. The samples were characterized using microscopy and diffraction methods. The results of the investigations reveal that due to the thermal and the mechanical loading in the brake disk microstructural changes and residual stresses arise. The thermo-shock samples show, that these tensile residual stresses evolve. In weakly loaded samples the microstructure in the contact zone remains ferrite pearlite. The predominant mechanical loading gives rise to compressive residual stresses in the contact zone. The level of these compressive residual stresses is almost uninfluenced by the number of brake trials performed.

5 In the heavily loaded samples, in the contact zones near the edges, the temperature generated by the braking is high enough and the cooling is fast enough for the martensitic phase transformation to occur. The mechanical loading and the martensite transformation induce compressive residual stresses in the martensite, which are significantly higher than in the weakly loaded samples. Tensile residual stresses evolve in the retained austenite. Also in the ferritic regions near the heat affected zone at the surface. These tensile residual stresses can be detrimental with respect to the lifetime of the brake disk. 5 ACKNOWLEDGEMENTS This work has been funded by the Deutsche Forschungsgemeinschaft (DFG), in the Sonderforschungsbereich Sfb 65, projects A1 and B3. 6 REFERENCES [1] T. Henk, W. Krenkel: Ultralight and Wear Resistant Ceramic Brakes EUROMAT 99 Volume 1, Wiley VCH Verlag, Weinheim (2), 89. [2] H. Richter: Residual Stress Measurements on Cracked Brake Disks of Motor Vehicles, 3rd European Conference on Residual Stresses (1992), 451. [3] S. Spooner, E.A. Payzant, C.R. Hubbard, W.T. Donion, G.M. Vyletel: Neutron Scattering Residual Stress Measurements on Gray Cast Iron Brake Disks. Proc. 2nd Int. Conf. on Quenching and the Control of Distortion, Cleveland, Ohio, (4 7 November 1996), 491. [4] A. Marquardt: Dissertation, Technische Universität Berlin (1992). [5] E. Macherauch and P. Müller: Z. angew. Physik 13 (1961), 35. [6] T. Wroblewski, E. Wild, T. Poeste, A. Pyzalla: Journal of Mat. Sc. Letters 19 (2), 975. [7] A. Pyzalla, W. Reimers: Materialprüfung 7/8 (1998), 33. [8] B. Scholtes: Eigenspannungen in randschichtverformten Werkstoffzuständen, DGM Informationsverlag, Oberursel (1991). [9] H. M. Mayer, A. Pyzalla, W. Reimers, C. Achmus: Mat. Sci. Forum Vols (2), 34.

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