LILLE - France Nov. 2010!"#$%&'&$ PROCEEDINGS RECUEIL DES CONFÉRENCES

Transcription

1 LILLE - France Nov. 2010!"#$%&'&$ 6 th European Conference on Braking 6 ème Journées Européennes du Freinage PROCEEDINGS RECUEIL DES CONFÉRENCES 1

2 Editeur : GRRT 20, rue Elisée Reclus B.P VILLENEUVE D ASCQ CEDEX FRANCE Imprimé en novembre 2010 par : Reprographie Polytech-Lille (Université de lille 1) Conception Couverture : Daniel BOURBOTTE : GRRT VILLENEUVE D ASCQ 2

3 6 th European Conference on Braking JEF 2010, Lille, France Investigation on the role of metallic fillers in brake friction materials for counterface friendliness M. Kumar 1, X. Boidin 2-3-4, Y. Desplanques 2-3-4, J. Bijwe 1 1 Industrial Tribology Machine Dynamics and Maintenance Engineering Centre, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi , India 2 Univ Lille Nord de France, F Lille, France 3 ECLille, LML, F Villeneuve d Ascq, France 4 CNRS, UMR 8107, F Villeneuve d Ascq, France xavier.boidin@ec-lille.fr Abstract: The frictional heat generated between brake pad and brake disc during high speed braking induces thermal distortion on the mating surfaces. This leads to the generation of localized high temperature contact regions and hot spots on the disc surface. The nature and extent of phenomenon are highly influenced by the mechanical and thermal properties of a tribo-couple and are also sensitive to their friction behaviour. In the present work, three non-asbestos organic friction composites with identical base composition but varying in three various metallic fillers (iron, copper and brass powder) in equal amount were developed. One composite was developed without any metallic content as a reference composite. They were evaluated on specially designed tribometer for enabling the studies of hot spots and thermal localization phenomena. Tendency of metal ingredients to produce hot bands and hot spots and related phenomenon on the counter face was studied. Composites with iron powder as a filler showed better results from hot-spot sensitivity point of view. Key words: Thermal localization; Metallic ingredients; Hot band; Hot spots. 1. INTRODUCTION In order to meet friction material quality requirements of high comfort level, high life expectancy of disc, efforts are required to avoid the occurrence of hot spots. During past decade, most of the work on hot spots has been reported particularly for railways [1]. It is well understood that hot spots phenomenon is highly influenced by the thermo-physical properties (specific heat, thermal conductivity, diffusivity, thermal effusivity, ) of tribo-pair. These play vital role in performance of brake pads, especially when braking is severe. In spite of this fact, no literature on this aspect is available. Metallic fillers are important in friction materials since they control the conductivity of composites apart from additional functions such as wear resistance and strength. A little is reported on the role of metallic contents in non-asbestos friction materials from friction and wear point of view [2-4]. However, influence of increasing amount of different types of metallic fillers on thermal conductivity of friction composites and their sensitivity on hot spots occurrence is not reported so far. In the present work, three NAO composites were developed by varying three different metals ingredients (iron, copper, brass). One composite 87

4 6 th European Conference on Braking JEF 2010, Lille, France was developed without any metal ingredients for comparative purpose. These were characterized for physical, chemical, thermal and mechanical properties. These were further studied on specially designed tribometer to investigate the sensitivity of friction composites on thermal localization (TL) phenomenon and hot-spot (HS) occurrence. 2. NAO FRICTION COMPOSITES The fabrication of composites containing 12 ingredients was based on keeping parent composition of 11 ingredients (90 % by wt.) constant and varying the rest (10% by wt.) with selected metallic fillers (iron, copper and brass powder). Parent composition contains binder 10%, fibers 25%, friction modifiers 12% and rest fillers 43%. One composite was developed without any metal ingredient and the amount of 10% was compensated with the space filler barite. Composites were characterized for physical, chemical and mechanical properties. Principal results are shown in Table 1. Parameters NM BP CP IP Tensile strength (MPa) Young s modulus (GPa) Compressibility (%) Thermal conductivity (W m -1 K -1 ) Thermal diffusivity x10 4 (cm 2 s -1 ) Effusivity (J m -2 K -1 s -1/2 ) Table 1. Physical and mechanical properties of the selected composites 3. EXPERIMENTS 3.1. Experimental set-up The thermal localization tests were conducted using a pin-on-disc braking tribometer developed to reproduce at reduced-scale railway-type stopbraking and is discussed elsewhere [5]. An infrared camera was used during the tests to monitor the evolution of HS and TL on the discrubbing surface. One eighth of the disc-track was observed through the test-chamber window. Acquisition frequency was of 176 Hz, corresponding to one shot every 80 at 2400 rpm, which was the maximum speed used in this study. A very short integration time of 35 µs reduced the impact on thermogram of the disc rotation during the shot (0.5 at 2400 rpm) Preparation of test specimens Friction composite was cut as shown in Fig. 1 and ground to have the flat surface with final thickness of 14 mm and a width of 16 mm. Figure 1. Shap and dimensions of the specimen (pad) A fresh grey cast iron disc with 100 mm radius was used for each composites material. The disc has friction track of 17 mm wide and 22 mm thick. All the discs were progressively polished with abrasive papers of grade 80, 180 and 320 for fine polishing. Metrology of the disc was done to locate hollows and bumps on the disc surface. The circumferential flatness of the track was reduced below 15!m Test procedure In order to have insight in the influence of dissipated energy and power on HS and TL phenomenon, series of stop braking tests were implemented taking account of a progressive increase of braking severity. It consists of 15 stopbrakings with gradual increase of initial rotation speed from 1000 to 2400 rpm, with a rotor inertia of 4 kg m 2. The same normal load was kept constant during the whole test procedure and a cooling time was observed before each stop braking to ensure low initial temperature. Table 2 gives the details of test parameters for present study. A succession of 80 bedding stop-brakings was performed to ensure an initial apparent area of pad-disc contact of more than 90% of the pad surface. Initial rotational speed Mean contact pressure rpm, by step of 100 rpm 0.87 MPa Spindle inertia 4 kg m 2 Stop-braking dissipated energy Initial disc temperature kj! 70 C (1 st braking ambient) Table 2. Stop-braking test series parameters 4. RESULTS 4.1. Friction coefficient Figure 2 shows evolutions of friction coefficient (!) versus braking time for all the four composites for brakings at 1000 and 1500 rpm. The instantaneous! was determined using load 88

5 Investigation on the role of metallic fillers in brake friction materials for counterface friendliness components measured with a 3D piezoelectric sensor. At 1000 rpm, dissipated energy is low, leading to a short braking duration. For each braking of the test series, friction behaviour is quite similar. During the few first seconds, fluctuations are observed, more marked and during a longer period for composite IP. Then the! stabilizes around the same range of mean value (0.35 to 0.4) for all materials. The final stage is marked by a final rise at low speed, except for composite CP, which presents the most stable friction behaviour of the four pad materials. Braking duration is most the same for all the materials, close to 12 s for braking at 1000 rpm and 20 s for braking at 1500 rpm, confirming, for each braking, the very close values of the! for all materials, then the very close dissipated powers. appearance of thermal gradients inside the hot band, leading to the formation of HS for the last brakings of the test series. Since the data produced were large and the patterns for all the composites were similar, presentation of all the data would be repetitive. Hence results for composite IP, well representative of the whole materials, are presented in the following. Results presented below refer to the stop braking at 1900 rpm, which is especially interesting from TL point of view because it corresponds to the first appearance of HS in the braking series for material IP. Figure 3 shows six successive thermograms of the disc track, taken during the stop braking. They show the formation and the migration of the hot band during the first 10 s of the braking, and the progressive appearance of circumferential thermal localization in the hot band, from t = 7.5 s, leading to the HS formation on the inner surface of the track, well visible from t = 12,5 s. The second half of the braking is characterized by the HS expansion, their intensity been decreasing while the sliding velocity and the dissipated energy decrease, leading to a quite homogeneous discsurface temperature at the end of the braking. (a) t=1.5 s (b) t=7.5 s (c) t=12.5 s (d) t=17.5 s (e) t=20 s (f) t=22.5 s Figure 2. Friction coefficient versus time (all composites - stop brakings at 1000 and 1500 rpm) 4.2. Thermal behaviour Same kind of TL phenomenon was observed with each pad formulation. At low dissipated power and energy, TL consists of the formation of a hot band at the beginning of brakings, on the outer perimeter of the disc track, followed by its radial migration towards the inner side of the disc. At higher energy, the braking severity leads to the Figure 3. Thermal localization phenomena observed by infrared thermography during stop braking: (a-b) hotband migration, (c-d) hot-spot formation and expansion, (e-f) surface temperature homogenization (composite IP - stop braking at 1900 rpm) Figure 4 shows the entire disc-track thermogram at half time of the braking duration, when HS are well marked. Since the infrared camera can observe one eighth of the disc, this thermogram is reconstituted using eight successive infrared 89

6 6 th European Conference on Braking JEF 2010, Lille, France images. For composite IP five hot spots dominated during the braking, located on the inner periphery of the disc track. Since the disc flatness is not perfect, HS are not regularly distributed and they vary in shape and intensity. Hot spots Figure 4. Entire disc-track thermogram at half braking time and location of the five dominant hot spots (composite IP - stop braking at 1900 rpm) 5. DISCUSSION Since disc flatness imperfections lead to circumferential thermal localizations and HS appear very gradually along the braking test series, it is not easy to detect on thermograms their first occurrence and then to study the HS sensitivity of composite Correlations between HS appearance and induced vibration The origin of HS lies in the progressive waviness distortion of the disc, leading to the alternating formation of bumps and hollows on the disc-track circumference. The disc undulation indeed excites the normal force applied on the pad by the servohydraulic actuator. These excitations are induced by the disc rotation, and then their frequencies are of course in the relation with the frequency of the disc rotation. The appearance of HS can be detected by analyzing induced vibration. Figure 5 compares time-frequency analysis of the normal load with infrared thermograms of the disc track at half time of the braking duration. The same scale of grey levels is used for all thermograms, from light to dark, corresponding to infrared luminance ranging from low to high values. Each diagram presents normal-load excited frequencies versus braking time. Colours indicate the intensity of excitation, from blue, corresponding to no excitation, to red for the highest intensity (results are given in db). a) b) Figure 5. Detection of hot-spot appearance by timefrequency analysis: (a) infrared thermograms of the disc-track at half braking time, (b) time-frequency analysis of the normal load (composite IP - stop brakings at 1500, 1700, 1900 and 2100 rpm) For stop braking at 1500 rpm, 3 straight lines characterize each diagram. They correspond to 3 excited frequencies by the initial imperfections of the disc flatness: a fundamental frequency H1, the lowest one equal to the disc-rotation frequency, and the two first harmonic frequencies H2 and H3. As the rotational speed is decreasing along braking test, these frequencies also decrease from a maximum value, related to the initial rotational frequency (H1=31,7 Hz, leading to H2=63,2 Hz and H3 = 95,1 Hz), down to zero at standstill. As the friction coefficient is more or less constant along the braking, the disc deceleration is quasi constant. Each excited frequency decreases regularly until braking ended with the rotational speed decreasing. At 1900 rpm, two new harmonic frequencies H4 and H5 appear, been excited during a short time interval at middle braking time. They correspond to the first occurrence of the disc undulation in the braking test series, then to the early stage of the HS formation. Harmonic frequency H5 has to be correlated with the five dominant HS observed on the disc track for composite IP (Fig. 6). For the last braking at 2100 rpm, the same phenomenon is repeated, been more and more marked in 90

7 Investigation on the role of metallic fillers in brake friction materials for counterface friendliness correlation with higher intensity of HS observed by infrared thermography. Figure 6 shows the frequency of the normal-load excitation versus braking time for all 4 composites NM, BP, CP and IP. Each diagram presents timefrequency results for the stop-braking test where the first HS induced excitation of the normal force is observed. The number of harmonic frequencies excited by hot spots is indicated. Figure 6. Time-frequency analysis of the normal load during braking of the test series with first hot-spot occurrence for each formulation 5.2. Correlations between TL phenomena and composite properties It was interesting to examine if any structureproperty correlation emerge between properties of composites (Table 1) and TL behaviour. Compressibility of the composites showed a good correlation with their TL performance (Fig. 7). Higher the compressibility better is the TL performance. This is attributed that high compressibility of composites helps to more homogeneous pressure distribution avoiding contact localization and therefore, preventing drastic rise of thermal gradients at the friction interface. Abbasi et al. [6] also concluded in their study that a higher value of compression modulus was one of the effective parameters to avoid hotspot phenomenon. TL performance appears to be more influenced by thermal effusivity. Under high power dissipation, a higher thermal effusivity leads to a faster heat absorption and conducts to a less rise of temperature and thermal gradients. That is reason composite IP with highest effusivity showed the best TL performance and composite Ref with the lowest effusivity proved the poorest. Table 3 summarizes the main results. Composite IP has the best performance with a first occurrence of HS late in the braking series, during stop braking at 1900 rpm, while composite NM presents the worst performance, HS appearing early at 1500 rpm. Results for composites CP and BP are intermediate, the best for brass particles. For all composite, results show a correlation between the number of dominant HS and the highest excited harmonic frequency. Pad formulation : NM BP CP IP Initial rotational speed at the first hotspot occurrence in the stop-braking test series (rpm) Figure 7. Correlation of first hot-spot occurrence in the braking series with composite properties Fundamental frequency H1 (Hz) Highest harmonic frequency excited by hot spots (Hz) Number of dominant hot spots observed by thermography H4 H6 H4 H Table 3. Results at hot-spot appearance for all pad formulations Hence, time-frequency analysis of the normal-load excitation appears to be very relevant to analyze HS occurrences, given useful information on their number and the time interval during braking concerned by the phenomenon. Thus higher the compressibility and effusivity of the composite, less is the tendency to produce the hot spots and better the TL performance and hence counterface friendliness. 6. CONCLUSIONS For each pad, thermal localization phenomena are similar, characterized by the migration of a hot band, which progressively undergoes by circumferential thermal localization in hot spots for the most severe brakings. Since thermal localizations slowly rise along the series, the first 91

8 6 th European Conference on Braking JEF 2010, Lille, France occurrence of hot spots is faintly discernable. Hot spots inducing vibration, time-frequency analysis of the contact normal load allowed precise detection of their occurrences. The number of excited harmonic frequencies of the disc rotation exactly matches the number of dominant hot spots observed by infrared thermography while duration of induced vibration makes clear the exact time interval of hot-spot phenomenon. Inclusions of metallic contents improve the overall thermal-localization performance of friction material. The best results are obtained with iron powder, avoiding hot-spot occurrence up to high power and energy brakings, while the use of brass and copper leads to quite comparable behaviour, a little better for the formulation with brass particles. Interesting correlations are made with thermal effusivity and compressibility of composites: higher the effusivity and the compressibility, better the avoidance of hot-spot occurrence. REFERENCES [1] Panier S., Dufrenoy P., Brunel J.F., Weichert D., Progressive waviness distortion: a new approach of hot spotting in disc brakes, J. of Thermal Stresses [2] Jang H., Koa K., Kim S.J., Basch R.H., Fash J.W., The effect of metal fibers on the friction performance of automotive brake friction materials, Wear [3] Shojaei A., Hahimian M., Derakhshandeh B., Thermally conductive rubber-based composite friction materials for railroad brakes - Thermal conduction characteristics, Composite Science and Technology [4] Bijwe J., Kumar M., Gurunath P.V., Desplanques Y., Degallaix G., Optimization of brass contents for best combination of tribo-performance and thermal conductivity of non-asbestos organic friction composites, Wear [5] Desplanques Y., Rousette O., Degallaix G., Copin R., Berthier Y., Analysis of tribological behaviour of paddisc contact in railway braking. Part 1: laboratory test development, compromises between actual and simulated tribological triplets, Wear [6] Abbasi F., Shojaei A., Katbab A.A., Thermal interaction between polymer-based composite friction materials and counterfaces, J. Appli. Poly. Sci