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Wear 270 (2011) 371–381

Contents lists available at ScienceDirect

Wear

journa l homepage: www.e lsev ier .com/ locate /wear

nfluence of various metallic fillers in friction materials on hot-spot appearanceuring stop braking

ukesh Kumara, Xavier Boidinb,c,d, Yannick Desplanquesb,c,d, Jayashree Bijwea,∗

Industrial Tribology Machine Dynamics and Maintenance Engineering Centre, Indian Institute of Technology Delhi, HauzKhas, New Delhi 110 016, IndiaUniv Lille Nord de France, F-59000 Lille, FranceECLille, LML, F-59650 Villeneuve d’Ascq, FranceCNRS, UMR 8107, F-59650 Villeneuve d’Ascq, France

r t i c l e i n f o

rticle history:eceived 9 May 2010eceived in revised form4 November 2010ccepted 29 November 2010

a b s t r a c t

The heat generated between friction couple during high speed braking induces thermal distortion inthe disc and leads to appearance of hot spots. The nature and extent of intensity of this phenomenonare highly influenced by the mechanical and thermal properties of materials. In the present work, threenon-asbestos organic (NAO) friction composites with three metallic particulate fillers (iron, copper andbrass) were developed. They were evaluated on a specially designed tribometer for enabling the studies

vailable online 4 December 2010

eywords:rakes and clutchhermal effectsot-bands

of hot spots and thermal localization phenomenon under braking conditions. Overall, composite withiron powder proved best by showing least tendency for hot-spot generation.

© 2010 Elsevier B.V. All rights reserved.

ot-spotsetallic particulates

. Introduction

NAO fiber-reinforced low-metallic composites are increasinglyeing used in automotive brake pads, shoes, linings, blocks, clutchacings, etc. and are essentially multi ingredient systems in ordero achieve the desired amalgam of performance properties [1–3].uring the braking action kinetic energy of the moving vehicle con-erts into heat and dissipates primarily by thermal conduction tohe contacting components of the brake followed by convectionnd radiation to the atmosphere and surrounding. At high speedshe appearance of hot bands and hot spots can be observed onhe disc surface. Due to an unavoidable irregular distribution ofressure and speed along the disc radius, dissipated power gra-ients appear widthwise, leading to the formation of hot bands.heir radial migrations are driven by a coupling between thermalxpansion effect and local increase of wear debris flow. At higherissipated energy, hot spots form in hot bands. Anderson and Knapp

4] first proposed hot spots phenomenon in automotive brake sys-ems. Four types of hot spots were defined according to their width,emperature and duration. These were characterized as: asperity,ocal, distortional, and regional. Later on more in-depth studies on

∗ Corresponding author. Tel.: +91 11 2659 1280; fax: +91 11 2659 1280.E-mail address: jbijwe@gmail.com (J. Bijwe).

043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2010.11.009

hot spots were done by Panier et al. [5–7]. The origin of hot spotslies in the progressive waviness distortion of the disc, leading to thealternate formation of bumps and hollows on the disc-track circum-ference. The pad-disc contact being localized on bumps, hot spotsare formed in the hot band and localize on the bumps of the track,leading to the formation of circumferential thermal gradients inthe hot band. Such hot spots are mostly periodically distributed inthe disc’s circumference. Occurrence of hot spots leads to followingeffects:

• Thermal crazing, cracks and fatigue, especially of the disc braking[4,8].

• Cycling of tensile and compressive stresses with plastic strainvariation [9].

• Undesirable low frequency vibrations called hot judder [10,11].• Brake torque variation (BTV) and brake pressure variation (BPV)

[12].

During past decade, most of the work on hot spots has beenreported particularly for railways [4–7,13,14]. It is well accepted

that hot spots phenomenon is highly influenced by the thermo-physical properties (specific heat, thermal conductivity (TC),diffusivity, thermal effusivity, etc.) of a tribo-pair (disc and pad).These properties are expected to play vital role in performance ofbrake pads, especially when braking is severe. However, hardly any

372 M. Kumar et al. / Wear 270 (2011) 371–381

Table 1Physical properties of selected metallic ingredients [27].

Properties Iron powder Brasspowder

Copperpowder

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Table 2Physical and mechanical properties of the selected composites [27].

Parameters NM BP CP IP

Density (g/cm3) 2.14 2.30 2.31 2.29Porosity (%) (JIS D 4418:1996) 4.05 4.33 4.30 4.10Acetone extraction (%) 1.34 1.38 1.18 1.28Tensile strength (MPa) 12.14 12.37 12.65 14.56Young’s modulus (GPa) 2.19 2.28 2.30 2.46Flexural strength (MPa) 27.38 24.15 23.89 30.43Flexural modulus (GPa) 5.05 4.33 4.80 5.70Rockwell hardness (R-scale) 90–95 78–88 90–95 85–95Compressibility (%) 0.68 1.07 1.02 1.26Thermal Conductivity (W m−1 K−1) 1.55 2.22 2.41 2.11

Diameter (�m) 87–225 650–850 280–430Density (kg/m3)a 7864 8520 8940Thermal conductivity (W/(m K))a 62 111 396

a Data from literature.

iterature on this aspect is available. Metallic filler and their amountre important in friction materials since they control the TC of com-osites apart from additional functions such as friction, wear andtrength. A little is reported on the role of metallic contents in NAOriction materials from friction and wear point of view [15–19].owever, influence of different types of metallic fillers on TC of

riction composites and their sensitivity to hot spots occurrences not reported so far. In the present work three NAO composites

ere developed by varying three different metallic powdery ingre-ients viz. iron, copper and brass. One composite was developedithout any metal ingredient for comparative purpose. These were

haracterized for physical, chemical, thermal and mechanical prop-rties. These were further studied on specially designed tribometero investigate the sensitivity of friction composites towards ther-

al localization (TL) phenomenon and hot spots (HS) occurrence.ll the results are described in present article.

. Material and methodology

.1. Procurement and characterization of raw materials

Powder of iron, brass and copper were procured from local ven-or. Fig. 1 (SEM micrographs) shows the morphology of selectedetallic ingredients. Table 1 comprises the measured particles size

ata and other physical properties for selected metal ingredients.

.2. Fabrication of the composites

The fabrication of composites containing 12 ingredients wasased on keeping parent composition of 11 ingredients (90% by wt.)onstant and varying the rest (10% by wt.) with selected metallicller (brass, copper and iron powder). Parent composition con-ains binder 10%, fibers 25%, friction modifiers 12% and rest fillers3%. One composite was developed without any metal ingredi-

nt and the amount of 10% was compensated with the space fillerarite. The four composites are designated by BP (Brass Powder),P (Copper Powder), IP (Iron Powder) and NM (No Metal). The

ngredients were mixed in a plough type shear mixer to ensurehe macroscopic homogeneity using a chopper speed of 2800 rpm

Fig. 1. SEM micrographs of sele

Thermal diffusivity (×10−4 cm2 s−1) 61 80 97 71Specific heat (J kg−1 K−1) 1187 1207 1076 1298Effusivity (J m−2 K−1 s−1/2) 1985 2482 2447 2504

and feeder speed 1450 rpm. The addition of ingredients during mix-ing was in a particular sequence [17]. The mixing schedule was of18 min duration. The mixture was then placed into a four-cavitymould supported by adhesive-coated back plates. Each cavity wasfilled with approximately 8 × 10−2 kg of the mixture and heat curedin a compression-moulding machine under a pressure of 8 MPafor 7–8 min at a curing temperature of 150 ◦C. Three intermittent‘breathings’ were also allowed during the initiation of curing toexpel volatiles. The pads were then removed and were post-curedin an oven at 70 ◦C, 120 ◦C and 150 ◦C for 1 h, 1 h and 6 h respec-tively [17,19]. The surfaces of the pads were then polished with agrinding wheel to attain the desired thickness and surface finish.

2.3. Characterization of the composites

For mechanical strength testing (flexural and tensile), testbars were fabricated as per ASTM standards. Composites werecharacterized for physical, chemical and mechanical properties.Thermo-physical properties were measured on thermal diffusiv-ity analyzer (FL-3000), supplied by Anter Corporation USA. Detailsof measurement procedure of these properties are discussed else-where [19]. Hardness was measured by a Rockwell hardness testeron R scale. Compressibility was measured as per standard (ISO6310). All the results are shown in Table 2.

3. Experiments

3.1. Experimental set-up

The thermal localization tests were conducted using a spe-cially designed tribometer developed to reproduce railway-typestop-braking and is discussed elsewhere [13,14]. A photograph of

cted metallic fillers [27].

M. Kumar et al. / Wear 270 (2011) 371–381 373

t benc

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Fig. 2. Braking tribometer: (a) tes

raking tribometer is shown in Fig. 2. An infrared camera was useduring the tests to monitor the evolution of HS and TL on the disc-ubbing surface. One eighth of the disc-track was observed throughhe test-chamber window. Acquisition frequency was of 176 Hz,orresponding to one shot every 80◦ at 2400 rpm, which was theaximum speed used in this study. A very short integration time

f 35 �s reduced the impact on thermogram of the disc rotationuring the shot (0.5◦ at 2400 rpm). In addition, an optical pyrom-ter focused on the mean radius of the disc-track (spot diameterf 2.2 mm). Its long response time (2 ms) implies a large rotationf the disc during each measurement. Then, circumferential gra-ients of TL are not detectable and the pyrometer only gives anvaluation of a “mean” value of thermal radiation of the disc-track.easuring surface temperature with infrared device requires the

omplete data on disc-track emissivity, which exactly could note determined. It varies with both time and space during braking,specially due to disc oxidation and third-body flowing on the trackuring friction [20]. The results hence are presented in terms of

uminance temperature. The disc surface is considered as a black-ody (emissivity equal to 1) and only a rough value of the surfaceemperature is obtained. Nevertheless, luminance temperature isseful to compare results and to analyze surface thermal gradients.

.2. Preparation of test specimens

Brake pad was cut as shown in Fig. 3 and ground to have the flaturface with final thickness of 14 mm and a width of 16 mm. Theomposite was inserted with a thermocouple at 2 mm away from

he contact surface.

A fresh grey cast iron disc with 100 mm diameter was used forach composites material. The disc has friction track of 17 mm widend 22 mm thick. All the discs were polished with abrasive papersith decreasing roughness (grades 80, 180 and 320) to get fine pol-

Fig. 3. Shape and dimensions of the specimen (pad).

h; (b) test chamber, pad and disc.

ishing. Metrology of the disc was done to locate hollows and bumpson the disc surface. The circumferential flatness of the track wasreduced below 15 �m. Each disc was inserted with four thermo-couples, two of them were at 2 mm and 5 mm distances from thecontact surface at bump and rest two at 2 mm and 5 mm distancesfrom the contact surface at hollow.

3.3. Test procedure

In order to study the influence of dissipated energy and poweron HS and TL phenomenon, series of stop braking tests were imple-mented taking account of a progressive increase of braking severity.It consists of 15 stop-brakings with gradual increase of initial rota-tion speed from 1000 to 2400 rpm, with rotor inertia of 4 kg m2. Thesame normal load was kept constant during the whole test proce-dure and a cooling time was observed before each stop brakingto ensure low initial temperature. In terms of densities of energyand power absorbed by the disc, these testing conditions are to berelated to a real disc brake system of a 3.5-ton commercial vehi-cle, from medium (braking at 60 kmph) to very severe (brakingat 150 kmph) stop-braking conditions. Environmental conditions,such as room temperature and humidity, were fairly the same dur-ing the tests. Data acquisition was performed at a frequency of1 kHz. Table 3 gives the details of test parameters for present study.A succession of 80 bedding stop-brakings was performed to ensurean initial apparent area of pad-disc contact of more than 90% of thepad surface.

4. Results

4.1. Friction coefficient

Friction coefficient (�) versus braking time is shown in Fig. 4for all the four composites for brakings at 1000 and 1500 rpm,

which represent the braking series. The friction coefficient wasdetermined using load components measured with a 3D piezo-electric sensor. At 1000 rpm, dissipated energy is low, leading toa short braking duration. For each braking of the test series, frictionbehavior is quite similar. During the few first seconds, more fluc-

Table 3Stop-braking test series parameters.

Initial rotational speed 1000–2400 rpm, by step of 100 rpmMean contact pressure 0.87 MPaSpindle inertia 4 kg m2

Stop-braking dissipated energy 21.9–126 kJInitial disc temperature ≤70 ◦C (1st braking ambient)

374 M. Kumar et al. / Wear 270 (2011) 371–381

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ing duration corresponded to the hot-band radial migration. Whenthe braking initiated, hot band formed on the outer perimeter wasnot observed by the optical pyrometer and the luminance tempera-ture was low. While the band migrated, the luminance temperature

Fig. 4. Friction coefficient versus time (all c

uations in � were observed, however, during a longer period thestabilizes around the same range of mean value (0.35–0.4) for

ll materials. The final stage is marked by a final rise at low speed,xcept for composite CP, which presents the most stable frictionehavior among four composites. Braking duration is almost sameor all the materials, close to 12 s for braking at 1000 rpm and 20 s forraking at 1500 rpm. For each braking, the value of � and dissipatedowers were very close for all the composites.

.2. Thermal behavior

For each composite, TL phenomena were rather similar. At lowissipated power and energy, TL consists of the formation of a hotand at the beginning of brakings, on the outer perimeter of the discrack, followed by its radial migration towards the inner side of theisc. At higher energy, the braking severity leads to the appearancef thermal gradients inside the hot band, leading to the forma-ion of hot spots for the last brakings of the test series. Since theata produced were large and the patterns for all the compositesere similar, presentation of all the data would be repetitive. Hence

esults for only composite IP, which would represent all materials,re presented here. Results for other three composites are shownn Appendix A. Results presented below refer to the stop brak-ng at 1900 rpm, which were especially important from TL point

f view because these correspond to the first appearance of hotpots in the braking series for composite IP. Fig. 5 shows pad andisc bulk temperatures versus time measured on the mean radiusf the rubbing surfaces. A thermocouple was located at the centerf the pad friction surface at 2 mm in deep, while disc-bulk tem-

sites; stop braking at 1000 and 1500 rpm).

peratures were measured by thermocouples, embedded in a bump(red and green curves) and in a hollow (black and blue curves) ofthe track, respectively at 2 and 5 mm below the rubbing surface.In addition, the luminance temperature of the disc-track was mea-sured by optical pyrometry on the mean radius. The peak of theluminance temperature observed during the first half of the brak-

Fig. 5. Pad and disc bulk temperatures versus time compared to disc-track lumi-nance temperature measured by optical pyrometer (composite IP; stop braking at1900 rpm).

M. Kumar et al. / Wear 270 (2011) 371–381 375

F g stop( rpm).

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using eight successive infrared images. For composite IP five hotspots were observed during braking and were located on the innerperiphery of the disc track. Since the disc flatness was not per-fect, hot spots were not regularly distributed and were varying in

ig. 6. Thermal localization phenomena observed by infrared thermography durinh and i) surface-temperature homogenization (composite IP; stop braking at 1900

ncreased, reached a maximum value a little before 5 s when theand passes through the view of pyrometer on the mean radius.fter 5 s of braking, the luminance temperature started to decrease

ill to the end of the braking. In accordance, bulk temperaturesose from the beginning of the braking, reached their maximumalue later than the luminance temperature, considering the delaynduced by thermal conduction. Logically, the low pad conductivityeads to low bulk temperature, and disc temperatures at 5 mm areower than disc temperatures at 2 mm. It has to be noted that theow value of the disc-surface luminance temperature compared toisc-bulk temperatures measured by thermocouples, especially athe beginning of braking when the disc temperature is homoge-eous. That confirms the “grey body” nature of the disc-rubbingurface. It is clear that surface temperatures of the disc remainigher than its bulk temperatures during the whole braking. Fig. 6hows nine successive thermograms of the disc track, taken duringhe stop braking. These show the formation and the migration ofhe hot band during the first 10 s of the braking, and the progressiveppearance of circumferential thermal localization in the hot band,rom t = 5 s, leading to the formation of hot spots on the inner sur-ace of the track, well visible from t = 12.5 s. The second half of theraking was characterized by the hot-spot expansion. The intensityf HS was decreased during the decrease in the sliding velocity and

he dissipated energy, leading to a quite homogeneous disc-surfaceemperature at the end of the braking. Fig. 7 shows the entire disc-rack thermogram at half time of the braking duration, when hotpots were well marked. Since the infrared camera could observenly one eighth of the disc, this thermogram was reconstructed

braking: (a–d) hot-band migration, (d–h) hot-spot formation and expansion, and

Fig. 7. Entire disc-track thermogram at half braking time and location of the fivedominant hot spots (composite IP; stop braking at 1900 rpm).

3 ear 270 (2011) 371–381

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76 M. Kumar et al. / W

hape and intensity. Similarly entire disc-track thermograms wereeconstructed for the other composites also and are presented inppendix A.

. Discussion

Since disc flatness imperfections lead to circumferential thermalocalizations and hot spots appear very gradually along the brakingest series, it is not easy to detect on thermograms their first occur-ence and then to study the hot-spot sensitivity of composite. Withhis objective, results were analyzed in details, firstly infrared datand surface thermal gradients.

.1. Analysis of luminance-temperature variations of the discurface

Luminance-temperature variations constitute a good indica-or of TL sensitivity, especially in the circumferential directionor hot spots. These variations were analyzed from data obtainedy infrared thermography. On each thermogram, four arcs wereefined to describe the luminance temperature change, numberedrom 1 to 4 respectively for the outer, mean, intermediate and

nner radius of the disc track (Fig. 8), these radial locations beingignificant considering the hot band migration and hot-spot devel-pment.

Fig. 9 presents the change of the maximum luminance tem-erature computed on each arc for composite IP and braking at

ig. 9. Change of maximal luminance temperature per eighth of disc-track during brakinraking at 1900 rpm).

Fig. 8. Location of the 4 arcs used for luminance-temperature variation analysis: (1)inner, (2) intermediate, (3) mean and (4) outer radius.

1900 rpm. Correlated to thermograms shown on Fig. 6, these curvesare useful to describe TL evolution. The luminance temperatureincreased from the initiation of the braking until a peak valuereached when the hot band passed through each radius. Then, the

hot-band migration clearly appeared like a peak on each curve: att = 1.5 s on the outer radius (Fig. 9a), t = 5 s on the mean one (Fig. 9b),t = 10 s on the intermediate one (Fig. 9c) and t = 15 s on the inner one(Fig. 9d). For each radius, fluctuations in the luminance temperatureappeared and was more strongly marked for the intermediate and

g on (a) outer, (b) mean, (c) intermediate and (d) inner radius (composite IP; stop

M. Kumar et al. / Wear 270 (2011) 371–381 377

Table 4Results at hot-spot appearance for all pad formulations.

Pad formulation NM BP CP IP

Initial rotational speed at the first hot-spot occurrence in the stop-braking test series (rpm) 1500 1700 1600 1900Fundamental frequency H1 (Hz) 25 28.3 26.7 31.7

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Highest harmonic frequency excited by hot spots (Hz)Number of dominant hot spots observed by thermography�Tmax at first hot-spot occurrence (◦C)

nner radius. The fluctuation appearances correspond to the pro-ressive formation of thermal gradients leading to the formationf the hot spots. They arise when the hot band reaches the inte-ior of the track. That explains the high fluctuations in luminanceemperature at the intermediate and the inner radius. For eachhermogram, the circumferential variation of luminance temper-ture �T was defined on the intermediate radius as the differenceetween the maximum luminance temperature (Tmax) and the min-

mum luminance temperature (Tmin) for an eighth of the disc-track:

T = (Tmax − Tmin)thermogram

hen hot spots are formed, �T corresponds to the differenceetween the maximum luminance temperature reached in hotpots and the minimal luminance temperature measured betweenot spots. Then its maximum �Tmax is defined as an indicator ofot-spot sensitivity. Fig. 10 gives the evolution of �Tmax along thetop-braking series for all four composites. Composite NM showedoorest thermal localization performance with higher �Tmax cor-esponding to the highest circumferential gradients. Composite IPxhibited the best thermal localization performance. Results for thewo other composites BP and CP are quite comparable, a little betteror the composite BP with brass particles.

.2. Correlations between hot-spot appearance and inducedibration

Since the origin of hot spots lies in a disc distortion, theirppearance can be detected by analyzing induced vibration. Dur-

ng braking, when hot spots occur, bump-hollow alternation of theisc undulation indeed excites the normal force applied on the pad.hese excitations been induced by the disc rotation, their frequen-ies are in coordination with the frequency of the disc rotation:perfect sinus-type undulation (i.e. one bump and one hollow

ig. 10. Maximum of luminance-temperature circumferential variation along theraking series from 1500 to 2000 rpm on the intermediate radius of the disc-trackor pad NM, BP, CP, IP.

H4 H6 H4 H54 6 4 543 45 44 55

along one disc circumference) will lead to one excitation of thenormal force for each revolution. Then, it is interesting to comparetime–frequency analysis of the normal load with infrared ther-mograms of the disc track. In Fig. 11a, infrared thermograms areshown at half time of the braking duration for composite IP atsuccessive stop brakings, from the initial rotation speed increas-ing from 1500 to 2200 rpm. The same scale of grey levels is usedfor all thermograms, from light to dark, corresponding to infraredluminance ranging from low to high values. Similarly, Fig. 11bshows time–frequency analysis of the normal load for the samebrakings. Each diagram presents normal-load excited frequenciesversus braking time. On these diagrams, colors indicate the inten-sity of excitation, from blue, corresponding to no excitation, to redfor the highest intensity (results are given in dB). For stop brak-ings at 1500–1800 rpm, 3 straight lines characterize each diagram.They corresponded to 3 frequencies excited by the initial imperfec-tions of the disc flatness; a fundamental frequency H1, the lowestone equal to the disc-rotation frequency, and the two first har-monic frequencies H2 and H3. As the rotational speed decreasedalong braking, these frequencies also decreased from a maximumvalue, related to the initial rotational frequency (H1 = 31.7 Hz, lead-ing to H2 = 63.2 Hz and H3 = 95.1 Hz), down to zero at standstill. Thelinear decrease can be explained by the almost constant frictioncoefficient during the braking. At 1900 rpm, the two harmonic fre-quencies H4 and H5 appear, as excited during a short time intervalat mid-braking. They correspond to the first occurrence of the discundulation in the braking test series, then to the early stage of thehot-spot formation. Harmonic frequency H5 has to be correlatedwith the five dominant hot spots observed on the disc track forcomposite IP (Fig. 12). For the last brakings, from 2000 to 2200 rpm(Fig. 11), the same phenomenon is repeated, been more and moremarked as the braking series progresses: harmonic frequencies H4and H5 are excited earlier and for a longer time interval during thebraking, in correlation with higher intensity of hot spots observedby thermography. These results confirm the progressiveness of thedisc waviness distortion at the origin of hot spots proposed byPanier et al. [7].

Fig. 13 shows time–frequency diagrams of the normal loadversus braking time for all composites viz. NM, BP, CP and IP. Eachdiagram corresponds to the first occurrence of hot spots in thetest series and shows the number of harmonic frequencies excitedby hot spots. Table 4 summarizes the main results. They confirmTL performance of composites obtained by the analysis of disc-surface luminance-temperature variations. Composite IP has thebest performance with a first occurrence of hot spots late in thebraking series, during stop braking at 1900 rpm, while compos-ite NM presents the worst performance, hot spots appearing atthe earliest (at 1500 rpm). Results for composites CP and BP areintermediate, little better for BP.

Interestingly, first occurrence of hot spots has been reportedwith a plain mark in Fig. 10, showing that hot spots occur the first

time for a maximum value of the luminance-temperature circum-ferential variation �T in the range of 40–50 ◦C. Similar results wereobtained by Kolluri et al. [21], suggesting that �Tmax could be usedas an indicator of hot-spot appearance. For all composites, resultsshow a correlation between the number of dominant hot spots and

378 M. Kumar et al. / Wear 270 (2011) 371–381

Fig. 11. Detection of hot-spot appearance by time–frequency analysis: (a) infrared thermograms of the disc-track at half braking time and (b) time–frequency diagrams ofthe normal load measured with a piezoelectric sensor (composite IP; stop brakings from 1500 to 2200 rpm).

Fig. 12. Correlation between the number of dominant hot spots and excited harmonic frequencies of the disc rotation (composite IP; stop braking at 1900 rpm).

M. Kumar et al. / Wear 270 (2011) 371–381 379

g of the test series with first hot-spot occurrence for each formulation.

tatot

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Fig. 13. Time–frequency diagram of the normal load during brakin

he highest excited harmonic frequency. Hence, time–frequencynalysis of the normal-load excitation appears to be very relevanto analyze occurrences of hot spots and to obtain useful informationn their number and the time interval during braking concerned byhe phenomenon.

.3. Correlation between TL phenomena and composite properties

It was interesting to examine if any structure–property corre-ation emerges between properties of composites (Table 2) and TLehavior. As seen from Fig. 14(a), compressibility of the compositeshowed a good correlation with their TL performance. Higher theompressibility better was the TL performance. This is attributedhat high compressibility of composites helps to a more homo-eneous pressure distribution avoiding contact localization andherefore, preventing drastic rise of thermal gradients at the fric-ion interface. Abbasi et al. [22] also concluded in their study thathigher value of compression modulus was one of the effective

arameters to avoid hot-spot phenomenon. However, no strict cor-elations were observed with other properties. Fig. 14(b) showshe changes between thermo-physical properties with TL perfor-

ance, which appears to be more influenced by thermal effusivity.nder high power dissipation, a higher thermal effusivity leads tofaster heat absorption and conducts to a less rise of temperaturend thermal gradients. Hence composite IP with highest effusiv-ty showed the best TL performance while composite NM with theowest effusivity proved the poorest. Since the four compositesave very close values of thermal conductivity, no clear depen-ency could emerge with thermal localization. It is always desired

o consider the thermal conductivity at high temperatures (suchs 300–350 ◦C) rather than at room temperature since this shoulde more realistic. However, because of constraints in measurementf high temperature thermal conductivity, this could not be done.hus higher the compressibility and effusivity of the composite,

Fig. 14. Correlation of first hot-spot occurrence in the braking series with (a)mechanical and (b) thermo-physical properties.

3 ear 27

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tion and Research, and the National Center for Scientific Researchfor their continuous support of research on braking developed atthe Laboratory of Mechanics of Lille.

Appendix A.

80 M. Kumar et al. / W

ess is the tendency to produce the hot spots and better the TLerformance and hence counterface friendliness. The appearancef hot spots being strongly dependent of thermal loading [4–7],hese results show the great importance of physical pad proper-ies, which favor homogeneous surface distribution of pressure andemperature. Nevertheless, local friction conditions are driven byhe formation of a third body at the interface, which constituteoad-bearing areas between the pad and the disc [23–25]. Then, theissipation of power and energy localized on these small areas leadso very high thermal gradients and localization phenomena resultrom a coupling between friction and wear mechanisms activatedt the interface, the location of heat dissipation and pad and dischermo-mechanical distortions [26]. From this point of view, thead microstructure and microscopic material properties (homo-eneity of the pad, nature of pad ingredients, size of particles, etc.)hould also have a significant impact on the appearance of thermalocalization phenomena.

. Conclusions

Based on the thermal localization (TL) studies conducted onelected NAO friction composites containing various types ofetallic fillers viz. iron, brass and copper, following conclusionsere drawn:

TL phenomenon was similar in nature for each composite. It wascharacterized by the migration of a hot band, which progres-sively underwent into hot spots by circumferential TL for the mostsevere brakings.Since TL slowly rose as severity of braking increased (rpmincreased), the first occurrence of hot spots was faintly discern-able. Hot spots inducing vibration, time–frequency analysis ofthe contact normal load allowed precise detection of their occur-rences.The number of excited harmonic frequencies of the disc rotationexactly matched with the number of dominant hot spots observedby infrared thermography, while duration of induced vibrationmakes clear the exact time interval of hot-spot phenomenon.The progressiveness of hot-spot appearance during braking con-firmed that the origin of hot spots lies in a progressive wavinessdistortion of the disc.Inclusions of metallic contents improved the overall thermal-localization performance of friction material. The best resultswere obtained with iron powder (IP), avoiding hot-spot occur-rence up to high power and energy brakings (1900 rpm), whilethe performance of composites with brass and copper was almostsimilar and moderate, brass being little better than copper. Com-posite without metal powder (NM) proved worst as hot spotsappeared at the earliest (1500 rpm).Interesting correlations were observed between thermal effusiv-ity and compressibility of composites. Higher the effusivity andthe compressibility, lower was the tendency to produce hot spotsand higher was the counterface friendliness.

cknowledgements

First author would like to acknowledge the fellowship (EGIDE)y Embassy of France, New Delhi, to carry out this researchork at Ecole Centrale de Lille, France. Indian authors gratefully

cknowledge the financial support by Department of Science and

echnology, New Delhi, India to carry out this research work.rench authors gratefully acknowledge the International Campusn Safety and Intermodality in Transportation, the Nord-Pas-de-alais Region, the European Community, the Regional Delegation

or Research and Technology, the French Ministry of Higher Educa-

0 (2011) 371–381

Reconstitution of the entire disc-track thermogram for pad for-mulations BP, CP and NM respectively for the stop brakings at 1700,1600 and 1500 rpm at tf/2 and location of the hot spots.

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M. Kumar et al. / W

eferences

[1] G. Nicholson, Facts About Friction, Gedoran Pub., Winchester, VA, 1995.[2] J. Bijwe, Composites as friction materials: recent developments in non-asbestos

fiber reinforced friction materials—a review, Polym. Compos. 18 (3) (1997)378–396.

[3] D.M. Elzey, R. Vancheeswaran, S. Myers, R. McLellan, Multi-criteria optimiza-tion in the design of composites for friction applications, in: Intl. Conf. on Brakes2000, Automotive Braking-Technologies for the 21st Century, Leeds, UK, 11–12July, 2000, pp. 197–205.

[4] A.E. Anderson, R.A. Knapp, Hot spotting in automotive friction systems, Wear135 (1990) 319–337.

[5] S. Panier, P. Dufrenoy, D. Weichert, Macroscopic hot spots occurrence in fric-tional organs, in: Proc. of the Thermal Stresses 2001, Osaka, 2001.

[6] S. Panier, P. Dufrenoy, D. Weichert, An experimental investigation of hot spotsin railway disc brakes, Wear 256 (2004) 764–773.

[7] S. Panier, P. Dufrenoy, J.F. Brunel, D. Weichert, Progressive waviness distortion:a new approach of hot spotting in disc brakes, J. Therm. Stresses 28 (2005)47–62.

[8] T. Kao, J.W. Richmond, A. Douarre, Brake disc hot spotting and thermal judder:an experimental and finite element study, Int. J. Vehicle Des. 23 (2000) 276–296.

[9] P. Dufrénoy, D. Weichert, A thermomechanical model for the analysis of discbrakes fracture, in: Proc. of the Thermal Stresses 2001, Osaka, 2001.

10] W. Kreitlow, F. Schrodter, H. Matthai, Vibration and hum of disc brakes underload, SAE 850079, 1985.

11] E. Thomas, Disc brakes for heavy vehicles, Inst. Mech. Eng., Intl. Conf. on DiscBrakes for Commercial Vehicles, C464/88, pp. 133–137.

12] J. Fieldhouse, C. Beveridge, An experimental investigation of hot judder, SAEpaper 2001-01-3135, 2001, pp. 1–10.

13] Y. Desplanques, O. Roussette, G. Degallaix, R. Copin, Y. Berthier, Analysis oftribological behaviour of pad-disc contact in railway braking. Part 1: labora-tory test development, compromises between actual and simulated tribologicaltriplets, Wear 262 (2007) 582–591.

14] A.-L. Cristol-Bulthe, Y. Desplanques, G. Degallaix, Coupling between frictionphysical mechanisms and transient thermal phenomena involved in pad–disccontact during railway braking, Wear 263 (2007) 1230–1242.

15] Y. Handa, T. Kato, Effects of Cu powder BaSO4 and cashew dust on the wear andfriction characteristics of automotive brake pads, Tribol. Trans. 39 (2) (1996)346–353.

[

0 (2011) 371–381 381

16] H. Jang, K. Koa, S.J. Kim, R.H. Basch, J.W. Fash, The effect of metal fibers on thefriction performance of automotive brake friction materials, Wear 256 (2004)406–414.

17] J. Bijwe, M. Kumar, Optimization of steel wool contents in non-asbestos organic(NAO) friction composites for best combination of thermal conductivity andtribo-performance, Wear 263 (2007) 1243–1248.

18] A. Shojaei, M. Fahimian, B. Derakhshandeh, Thermally conductive rubber-basedcomposite friction materials for railroad brakes—thermal conduction charac-teristics, Compos. Sci. Technol. 67 (2007) 2665–2674.

19] J. Bijwe, M. Kumar, P.V. Gurunath, Y. Desplanques, G. Degallaix, Optimizationof brass contents for best combination of tribo-performance and thermal con-ductivity of non-asbestos organic (NAO) friction composites, Wear 265 (2008)699–712.

20] H. Kasem, J. Thevent, X. Boidin, M. Siroux, P. Dufernoy, B. Desmet, Y.Desplanques, An emissivity-corrected method for the accurate radiometricmeasurement of transient surface temperatures during brakings, Tribol. Int.43 (2010) 1823–1830.

21] D. Kolluri, X. Boidin, Y. Desplanques, G. Degallaix, A. Ghosh, M. Kumar, J.Bijwe, Effect of natural graphite particle size in friction materials on ther-mal localization phenomenon during stop brakings, Wear 268 (2010) 1472–1482.

22] F. Abbasi, A. Shojaei, A.A. Katbab, Thermal interaction between polymer-basedcomposite friction materials and counterfaces, J. Appl. Polym. Sci. 81 (2001)361–369.

23] M. Eriksson, J. Lord, S. Jacobson, Wear and contact conditions of brake pads:dynamical in situ studies of pad on glass, Wear 249 (2001) 272–278.

24] M. Eriksson, S. Jacobson, Tribological surfaces of organic brake pads, Tribol. Int.33 (2000) 817–827.

25] W. Osterle, I. Dorfel, C. Prietzel, H. Rooch, a-L. Cristol-Bulthe, G. Degallaix, Y.Desplanques, A comprehensive microscopic study of third body formation atthe interface between a brake pad and brake disc during the final stage of apin-on-disc test, Wear 267 (2009) 781–788.

26] Y. Desplanques, G. Degallaix, Interactions between third-body flows and

localisation phenomena during railway high-energy stop braking, SAE Inter-national Journal of Passenger Cars – Mechanical Systems 1 (2009) 1267–1275.

27] M. Kumar, J. Bijwe, Studies on reduced scale tribometer to investigate the effectsof metal additives on friction coefficient – temperature sensitivity in brakematerials, Wear 269 (2010) 838–846.