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Wear 269 (2010) 826–837

Contents lists available at ScienceDirect

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AO friction materials with various metal powders: Tribological evaluation onull-scale inertia dynamometer

ukesh Kumar, Jayashree Bijwe ∗

ndustrial Tribology Machine Dynamics and Maintenance Engineering Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India

r t i c l e i n f o

rticle history:eceived 31 March 2010eceived in revised form 31 July 2010ccepted 12 August 2010vailable online 19 August 2010

eywords:olymer–matrix compositerake/clutch materials

a b s t r a c t

A right selection of filler, its type, shape, size, amount and its compatibility with other co-fillers con-tribute to the performance of a friction material. Among various types of fillers, metallic fillers are veryimportant and multifunctional. In spite of well-accepted fact that these affect all performance propertiesof friction materials, in depth studies and systematic evaluation as per standard procedure are not doc-umented. Hence, in this work three friction composites were developed in the laboratory with identicalparent composition (90 wt.%) except metallic filler (10 wt.%) viz. brass, copper and iron powders whileone more composite was developed without any metal contents for comparison’s sake. These were char-acterized for physical, thermo-physical, chemical and mechanical properties. For tribo-evaluation inertiabrake-dynamometer testing which reflects the most realistic performance was selected. The various

rake dynamometer

etallic fillers in friction materialshermal conductivity

performance properties like friction, � sensitivity to load, speed and temperature were studied as perindustrial schedule.

It was concluded that inclusion of metal contents, improved almost all the performance propertiesand copper containing composite showed best tribo-behavior followed by brass and then iron powdercontaining composite. Composite without metal powder proved poorest. Worn surface analysis and wearmechanism are also discussed in detail.

. Introduction

The heart of the braking device is friction material (FM), whichs expected to continue its functioning reliably and efficientlyor a long time in adverse operating conditions. Non-asbestos-rganic (NAO) fiber reinforced-low metallic friction composites arencreasingly being used in automotive brake disc pads, shoes, lin-ngs, blocks, clutch facings, etc. primarily because of awareness ofealth hazardness of asbestos. The performance requirements ofMs are very complex and conflicting [1]. FMs are essentially multi-ngredient systems in order to achieve the desired amalgam oferformance properties [2,3]. The influence of these ingredients onerformance properties is so complex that formulation of frictionaterials is still referred as an art rather than science [4]. It is a prac-

ice to add metallic fillers such as copper, brass, iron, aluminum,tc. in FMs in various shapes, sizes, amounts and combinations. In

pite of the fact that these are expected to affect all performanceroperties such as physical, thermal, thermo-physical, (specificeat, thermal conductivity (TC), diffusivity, thermal effusivity, etc.),echanical apart from tribological, hardly any systematic stud-

∗ Corresponding author. Tel.: +91 11 26591280; fax: +91 11 26591280.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.2010.08.011

© 2010 Elsevier B.V. All rights reserved.

ies are reported in this aspect [5–9]. Handa and Kato [5] studiedthe influence of variation of three fillers (powders of Cu, CNSLand barite) in quaternary composition based on phenolic resin forfriction and wear properties on a reduced scale tribometer. Theyconcluded that Cu powder inclusion resulted in increase in faderesistance but decrease in wear resistance. BaSO4 inclusion led tothe exactly opposite behavior. Jang et al [6] studied the effect of dif-ferent metallic fibers viz. steel, Al and Cu (15 vol.%) on performanceof NAO friction composites using a small-scale friction tester. Thestudies were focused on investigating the influence of additionof these fibers individually in composites under various operat-ing conditions. Overall benefits or limitations of each fiber werediscussed. Jia and Ling [7] observed the effect of four fillers viz.resin, cast-iron powder, brass fiber (short), and graphite powder(variable) in increasing amount on tribo-properties of friction com-posites on a block-on-ring tribometer. They observed that, whenthe mass fraction of brass fibers was approximately below 19%, the� showed very small variation. However, when it exceeded 19% the� increased. Ju and co-workers [8] studied the effects of addition of

different fibers (steel, brass, cellulose, carbon, ceramic and copper)on the mechanical and tribological properties of friction materi-als. A small-scale pin on disc rig was used for tribological study. Itwas reported that composites with fibers of steel, brass and coppershowed highest, moderate and lowest wear, respectively.

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M. Kumar, J. Bijwe / W

Tribo-testing of FMs is equally complex job because FMs aresed in various forms (pads, shoes, linings, blocks against differentotors such as discs, drums, wheels, etc.) in varying operating con-itions. Various performances evaluation methods for these FMsuch as brake-dynamometer testing [9], Krauss testing [10], theriction assessment screening test (FAST) [11], Chase testing [12],tc. are very popular globally. Among these Chase and FAST areost commonly used and are considered as a quality control test.

uch rigs are more useful for comparative performance evaluationf materials in the laboratory to have some preliminary idea abouthe performance. The screened materials on prototypes are sup-osed to be evaluated as per globally accepted standards so thateal potential under various operating parameters (relatively) cane evaluated before field tests followed by commercialization [1].

n light of the advent of modern fast moving vehicles, the frictionaterial requirements became more complex in accordance with

he applied braking pressure, speed, road conditions, and environ-ental conditions. Hence, to match these requirements full-scale

nertia dynamometer testing is very essential. Since such facilitys generally available with Industries, number of research papersn the evaluation of materials in this respect is limited [13–17].he schedules for evaluating material performance vary from coun-ry to country, in general. Federal Motor Vehicle Safety Standardsrom US prescribes schedules (FMVSS 121) for passenger brake padsraking. This is for air braked trucks and FMVSS 105 is for hydraulicrakes and Japanese equivalent of this is JASO 406 and 407. Apartrom this SAE J 2522 (AK-master) is also equally important and veryopular in Europe.

Thus three FMs with identical parent composition (90%) butith difference in metallic powders viz. copper, brass and iron (10%)ere developed and characterized. These were tribo-evaluated on

ull-scale inertia dynamometer for various performance properties.ne more composite without any metallic contents was also devel-ped and compared to quantify the benefits of metallic fillers. Theaper discusses the studies in this background.

. Materials and methodology

.1. Fabrication of the composites

The fabrication of three composites containing 12 ingredi-nts was based on keeping parent composition of 11 ingredients90 wt.%) constant and varying metal ingredients (brass, copper andron powder) by 10 wt.%, respectively in each composition. Mor-hology of selected metallic fillers is shown in SEM micrographsFig. 1). One composite was developed without any metal con-

ent and 10% amount was compensated with space filler (barite)s shown in Table 1. These composites were designated as BP1,P1, IP1 and Ref. The ingredients were mixed in a plough typehear mixer to ensure the macroscopic homogeneity using a chop-er speed of 2800 rpm and feeder speed of 1450 rpm. The mixing

able 1esign and designations of composites.

Ingredients/designation Composition by weight %

BP1 CP1 IP1 Ref

Parent compositiona 90 90 90 90Barite (BaSO4) 0 0 0 10Brass powder 10 0 0 0Copper powder 0 10 0 0Iron powder 0 0 10 0

bbreviations used: B, brass; C, copper; I, iron; P, powder; Ref, reference and subscriptfor 10 wt.%.a Binder (St. Phenolic): 10 wt.%; fibers (Aramid, rock wool, ceramic, acrylic, potas-

ium titanate): 23 wt.%; additives (graphite, alumina): 12 wt.% and fillers (NBRowder, cashew dust, vermiculite, barite): 45 wt.%.

69 (2010) 826–837 827

schedule was of 10 min duration. The mixture was then placed intoa four-cavity mould supported by the adhesive-coated back plates.Each cavity was filled with approximately 80 g of the mixture andheat cured in a compression-molding machine under a pressure of8 MPa for 7–8 min at a curing temperature of 150 ◦C. Five intermit-tent ‘breathings’ were also allowed during the initiation of curingto expel volatiles. The pads were then post-cured in an oven at150 ◦C for 4 h [18]. The surfaces of the pads were then polishedwith a grinding wheel to attain the desired thickness and surfacefinish.

2.2. Characterization of the composites

Composites were characterized for physical and chemical (den-sity, porosity, and acetone extraction), thermo-physical (thermalconductivity, diffusivity, specific heat, etc.) and mechanical (tensilestrength, flexural strength, compressibility and hardness) prop-erties as per standard practice. Thermo-physical properties weremeasured as per ASTM-E1461-01 standard on FL-3000 Flash lineinstrument supplied by Anter Corporation, USA. Samples of squaresize (10 mm × 10 mm) and thickness 2–2.5 mm were used for thesemeasurements at room temperature.

2.3. Test set-up and procedure of testing on brake inertiadynamometer

This dynamometer (Dyno) in ITMMEC, IIT Delhi was supplied byPyramid Precision Engineering Pvt. Ltd., Chennai, India. It appliesvariable inertial load on the engine (175 kW motor) and measuresthe braking force, wear of the brakes, torque applied and the coef-ficient of friction through a computer connected to it. Both brakingmode viz. hydraulic and pneumatic can be applied on this machine.Main Chasis is of ladder type construction and fabricated out ofI-section girders of about 300 mm height with adequate diagonalmembers for rigidity. Prime mover is a 175 kW, 1500 rpm DC motorwith external cooling facility. A variable speed drive with tachome-ter feedback is provided to vary the speed of the motor. The drivemotor is fixed on the separate box type bed which is mounted on themain chassis. Main shaft is directly coupled to the motor throughGear coupling. The motor rpm is to set such that the main shaftwill run at the speed set by the computer. Emergency push buttonsare provided on the machine and the control panel can be used tostop the motor and engage with emergency disc brake with shaftto stop the rotation. Disc/drum temperature is monitored by a non-contact IR sensor, while a provision of nine thermocouples is madeto measure the pad/lining temperature. The analog output from thesensor is suitably signal conditioned and fed to the interface cardof the computer to monitor and control the temperature at whichbrake is to be applied. All the wheels are properly enclosed withmetallic sheet for security purpose. Fig. 2a, shows the schematic ofthe dynamometer (Dyno) while Fig. 2b shows the photograph ofthe braking end. Some important specifications of Dyno are listedin Table 2. Lab view(TM)-based software (version 4.0) is provided tocalculate the desired output parameters.

2.4. Testing schedule

In the present work, a Japanese Automobile Standards (JASO C406) was used for testing. The purpose of this schedule is to eval-uate performance of service brake devices for passenger car brakepads through simulation of braking conditions on road in labora-

tory (complete schedule is attached in Appendix A). This is oneof the important schedules generally practiced in Asian countries.Instead of following complete schedule, two parts were selectedfor testing because of complexity, long time and cost involved in it[19]. Two major parts of the JASO schedule, effectiveness-II and fade

828 M. Kumar, J. Bijwe / Wear 269 (2010) 826–837

Table 2Specifications of the dynamometer.

Motor 175 kW, 1500 rpm DC motorMax. motor torque 1000 kg mBase speed 1400 rpmShaft speed 1500 rpmMin. inertia 1.5 kg m2

Max. inertia 1570 kg m2

No of wheels 11Max. braking torque 1000 kg mMax. pressure (air) 10 bar

amppe

Table 3Dynamometer test schedule.

Schedule JASO C 406Inertia 4.00 kg m s2

It measures the efficiency of a friction material to function more

Fs

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Max. pressure (hydraulic) 120 barGross vehicle weight (GVW) range 1000–16,000 kg sMax. dissipated energy 20 MJ

nd recovery-I were followed in the present study (these parts are

arked in blue colored in Appendix A). Effectiveness studies com-

rise the influence of operating parameters (pressure and speed) onerformance of a material while; fade and recovery part is useful toxamine the influence of temperature on performance properties.

ig. 1. Scanning electron micrographs of selected metallic fillers; brass powder (particle siize: 87–225 �m).

ig. 2. (a) Schematic of brake inertia dynamometer: (1) motor, (2) flywheel of 0.5 kg m2, (36 kg m2, (7) and (8) flywheel of 32 kg m2, (9) flywheel of 64 kg m2, (10) couplings, (11) ho16) torque arm, (17) emergency brakes, (18) dyno bed. (b) Close view of braking end.

Rolling radius (Tyre radius) 279 mmCaliper piston diameter 51 mmEffective radius (pad on disc sliding radius) 97 mm

The vehicle and other specifications of the test schedule are givenin Table 3.

2.4.1. Effectiveness studies (pressure–speed sensitivity)Effectiveness studies comprise the influence of operating

parameters (pressure and speed) on the performance of material.

reliably under different pressures and speeds. Table 4 shows theexperimental design for effectiveness studies. Each pair of padswas subjected to 100 brake applications to establish at least 80%conformal contact with the disc during bedding test prior to final

ze: 650–850 �m), Cu powder (particle size: 280–430 �m), and iron powder (particle

) flywheel of 2 kg m2, (4) flywheel of 4 kg m2, (5) flywheel of 8 kg m2, (6) flywheel ofusings, (12) shaft bearings, (13) braking assembly, (14) load cell, (15) air chamber,

M. Kumar, J. Bijwe / Wear 269 (2010) 826–837 829

Table 4Experimental parameters for effectiveness studies.

Description Speed (kmph) Deceleration (g) Initial Temp. (◦C) Nos. of applications Air Blower

Bedding test 65 0.35 120 ◦C 100 ONEffectiveness-II 50, 80, 100 0.1 to 0.8 < 80 ◦C 24 ON

Table 5Experimental parameters for fade and recovery studies.

Description Speed (kmph) Deceleration (g) Initial temperature (◦C) No of applications Air blower

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Baseline check 50 0.30Fade & Recovery-IFade test 80 0.45Recovery test 50 0.30

esting. Once it was established, the disc was replaced by a freshisc. Test was done at three different braking speeds viz., 50, 80 and00 kmph. The tests were conducted under eight different decel-rations (0.1–0.8 g). The deceleration is varied by increasing theressure and the pressure was programmed to achieve a given ratef deceleration depending on the friction level of the tested mate-ial. As the deceleration increased, the severity of the braking alsoncreased and stopping distance got reduced to a set value. At eacheceleration (g value) one brake was applied. Thus total eight brakepplications were applied at each speed. Brake duration was vari-ble as each brake starts from desired speed and ends with zeropeed (till halt). Total 24 brake applications were applied in thisart of effectiveness studies. Initial braking temperature of eachrake was 80 ◦C.

.4.2. Fade and recovery behavior (temperature sensitivity)Fade and recovery characteristics highlight the effect of temper-

ture on �. The selected part (Fade and Recovery-I) from the JASOtandard (Appendix A) is for evaluating sensitivity of � towardsemperature of the surfaces during braking in more realistic con-itions. As per requirement of the schedule, same pads and dischould be continued for further test schedule. Hence, brake padsnd disc used in earlier effectiveness studies were retained for thesetudies.

The test starts with baseline check mode having initial speed of0 kmph and deceleration (g) of 0.3. Total three brakes were applied

n this segment and temperature was kept below 80 ◦C by using airlower. After this fade test started with initial speed of 80 kmph,eceleration of 0.45 and initial temperature 80 ◦C. Total 10 brakesere applied in this mode and temperature of the disc was allowed

o rise uninterruptedly during this mode to observe the influence ofemperature (by keeping air blower switched off). Finally recoveryest of 12 brakes started with initial speed of 50 kmph and deceler-tion of 0.30. Air blower was switched on during this test and initialemperature of disc before each brake was kept under 80 ◦C. Table 5

able 6hysical, thermo-physical and mechanical properties of the composites.

Properties BP1

Density (g/cc) 2.30Porosity (%) (JIS D 4418:1996) 4.33Acetone extraction (%) 1.38Tensile strength (MPa) ASTM D 638 12.37Young’s modulus (GPa) ASTM D 638 2.28Flexural strength (MPa) ASTM D 790 24.15Flexural modulus (GPa) ASTM D 790 4.33Rockwell Hardness (S-scale) (ASTM D 785) 78–88Compressibility (%) ISO 6310 1.07Thermal Conductivity (W m−1 K−1) 2.22Thermal diffusivity ×10−4 (cm2 s−1) 80Specific heat (J kg−1 K−1) 1207Effusivity (J m−2 K−1 s−1/2) 2482

80 3 On

80 (for 1st brake) 10 Off<80 12 On

shows the experimental parameters for fade and recovery test. Allthe desired output parameters were collected synchronously on acomputer.

Definitions of required performance parameters from fade andrecover studies:

�-Fade = lowest � recorded during fade test: higher, the better.�max (fade) = highest � recorded during fade test.�min (fade) = lowest � recorded during fade test.% fade ratio = (�min/�max) × 100: higher, the better.

Max. disc temperature = highest temperature rise in the discduring fade test: lower, the better

�-Recovery = highest � recorded during recovery test: higher, thebetter.�max (recovery) = highest � recorded during recovery test.�min (recovery) = lowest � recorded during recovery test.% Recovery ratio = (�min/�max) × 100: higher, the better.

3. Results and discussion

3.1. Physical, thermo-physical and mechanical properties

Results are collected in Table 6. As seen from the table, mostof the properties such as density, porosity, tensile strength, com-pressibility, thermal conductivity (TC), thermal diffusivity (TD), etc.increased due to metallic contents. However, hardness and flexuralproperties deteriorated due to inclusion of metallic contents. It wasalso observed that with metallic fillers, the mechanical properties

did not conform to any defined trend in general. In an attempt tostudy the relation between properties and formulation of brake lin-ings, Filip et al. [20] have revealed that the rule of mixture cannotbe applied to predict the mechanical properties based on contentsof structural constituents. Regression analysis has also confirmed

CP1 IP1 Ref

2.31 2.29 2.144.30 4.10 4.051.18 1.28 1.34

12.65 14.56 12.142.30 2.16 2.19

23.89 30.43 27.384.80 5.70 5.05

88–93 85–95 90–951.02 1.26 0.682.41 2.11 1.55

97 71 611076 1298 11872447 2504 1985

830 M. Kumar, J. Bijwe / Wear 269 (2010) 826–837

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ig. 3. Variation in � with increase in deceleration at (a) 50 kmph, (b) 80 kmph, andc) 100 kmph.

hat apparently no correlation was observed between hardness andriction or wear properties. Kim et al. [21] also reported hardnessalues which were not in consistency with their level of inclusions.

In case of composites as seen from Table 6, TC, TD and ther-al effusivity of the composites increased with metallic contents,

ut no specific trends were observed for specific heat. TC of theomposites was in the range of 1.55–2.41 W m−1 K−1 which is inune to the literature range [22]. Composite CP1 measured highestalue of TC because of the highest TC of copper powder followedy composites with brass and iron.

.2. Tribological properties of composites

.2.1. Effectiveness studies (pressure–speed sensitivity)Change in � as a function of sliding speed and applied pressure

s a very important issue during braking and it should show mini-al changes because drivers expect the same level of friction force

nder various braking conditions.

.2.1.1. Pressure sensitivity. Variation in � with increase in decel-ration (g) at each constant speed reflects the � sensitivity towardsressure and is shown in Fig. 3. For an ideal one, slope of the curvend undulations in curve should be minimum. General observa-ions from Fig. 3 are collected in Table 7.

.2.1.2. Speed sensitivity. Variation in speed spread with decelera-ion (g) reflects the speed sensitivity and shown in Fig. 4.

Speed spread (SS) (in terms of %), is the stability in � when speedhanges from first to second level, i.e. from 50 to 80 kmph in these

tudies and this corresponds to mild conditions, while from firsto third level 50–100 kmph corresponds to severe condition. Forn ideal one SS should be in the range of 85–100% and slope ofhe curve should be as minimum as possible. General observationsrom Fig. 4 are collected in Table 8.

Fig. 4. Variation in speed spread with increase in deceleration at (a) mild conditions(transition from 50 to 80 kmph) and (b) severe conditions (transition from 50 to100 kmph).

In present results, � of the composites was in the range of0.30–0.41 which is in the acceptable range for industrial appli-cations. With increase in pressure and speed, � decreased for allthe composites which are as per trends in the literature [23–25].It is a well-known fact that Amonton’s law does not hold goodfor polymers and composites and � is not constant as pressureand speed varies. With increase in pressure, it decreases (gener-ally referred as pressure-fade), though the slope is not necessarilylinear. This is mainly because with increase in pressure, real areaof contact increases excessively and disproportionately since poly-meric materials are visco-elastic rather than elasto-plastic (as incase of metals). This leads to eventually disproportionately lowreduction in pressure on the asperities in spite of increase in load.Hence � falls down [26,27]. However, with speed this relationshipis more complex than pressure. With increase in speed � generallydoes not show fixed trends. � Increased slightly, showed maximafollowed by a sharp decrease in some cases [23–25]. In the presentcase, � decreased with increase in speed which is expected sinceincrease in speed leads to generation of more frictional heat and �is highly dependent on temperature. It lowers down with increasein temperature of the surface. The variation in speed leads to ther-mal and mechanical overloading of the asperities. The shear filmdisruption results in exposing the surface underneath to frictionalcontact and the friction changes accordingly [28,29]. One more rea-son stated is that with increase in speed, kinetic energy of thesystem increase and lot of fictional heat generated at the inter-face causing decomposition of ingredients and finally fluctuationsin �.

3.2.1.3. Effect of metallic fillers on magnitude of �. Inclusion of eachtype of metallic filler led to increase in �. This was basically due tothe reason that metal contents were added at the cost of barite andmetal powders are known to be slightly harder and more abrasivethan very fine and smooth barite powder.

3.2.1.4. Effect of metallic fillers on sensitivity. Among all metallicfillers, copper proved best followed by brass while iron was poorfrom p–s (pressure and speed) sensitivity point of view. One of the

M. Kumar, J. Bijwe / Wear 269 (2010) 826–837 831

Table 7General observations and behavior observed from Fig. 3.

Observations

50 kmph 80 kmph 100 kmph

BP1 � Range: 0.38–0.40 � Range: 0.37–0.38 � Range: 0.35–0.37No slope, less undulations Low slope, less undulations Low slope, more undulationsGood behavior Good behavior Good behavior

CP1 � Range: 0.38–0.40 � range: 0.37–0.38 � range: 0.36–0.37No slope, less undulations Low slope, less undulations Low slope, less undulationsGood behavior Good behavior Good behavior

IP1 � Range: 0.39–0.41 � Range: 0.36–0.38 � Range: 0.35–0.36Low slope, more undulations Low slope, less undulations Low slope, more undulationsModerate behavior Good behavior Moderate behavior

Ref � Range: 0.36–0.39 � Range: 0.33–0.37 � Range: 0.30–0.35High slope, more undulations High slope, more undulations Highest slope, most undulationsModerate behavior Poor behavior Poorest behavior

Overall, performance order from � sensitivity to pressure point of view was: CP1 ≥ BP1 ≥ IP1 � Ref.

Table 8General observations and behavior observed from Fig. 4.

Observations

Mild conditions Severe conditions

BP1 – Moderate range of SS, moderate slope, less undulations, moderate behavior BP1 – High range of SS, low slope, modearteundulations – good behavior

CP1 – Highest range of SS, lowest slope, less undulations, best behavior CP1 – Highest range of SS, lowest slope, lessundulations, best behavior

IP1 – Low range of speed spread (SS), moderate slope, more undulations, poorest behavior IP1 – Moderate range of SS, high slope, moreundulations, poor behavior

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Ref – Low range of speed spread (SS), moderate slope, more undulations, poor be

verall, performance order from �-speed sensitivity point of view was:CP1 > BP1 �

easons for this could be the appreciable difference in TC of thellers. TC of Copper, brass and iron are 400, 110 and 60 (W m−1 K−1),espectively, while those of their composites were 2.41, 2.22 and.11 (W m−1 K−1), respectively. High TC is responsible to carry awayhe frictional heat generated at the interface very efficiently andence extent of thermal degradation of organic material is reduced.his leads to maintain the integrity of the composite as well as sur-ace properties and hence � also very effectively. These featuresere observed in micrographs of worn surfaces as discussed in SEM

ection.

.2.2. Fade and recovery behavior (temperature sensitivity)Fig. 5 shows the fade and recovery behavior of composites. For

n ideal performance � should be in good range (0.3–0.4) and fadeurve (� vs. number of brake applications) should be straight withinimum slope. In case of recovery mode, curve should be flat with

ow slope and � should be in the range of pre-fade value.It was observed that with inclusion of metallic fillers fade and

ecovery (F & R) performance improved. Ref composites (with-ut metal content) showed poorest fade behavior as � started toecrease from 2nd braking onwards. Amoung metallic fillers CP1howed best fade behavior follwed by BP1 as more consistancy in

was observed. IP1 showed high � initially but from 5th braking

able 9ade and recovery responses of the composites as per JASO schedule.

Segment Parameters Ref

Fade�-Fade 0.33% Fade ratio 88Max. disc temperature (◦C) 317

Recovery�-Recovery 0.41% Recovery ratio 86

Ref – Lowest range of SS, highest slope, moreundulations, poorest behavior

IP1.

onwards, � started to decrease and hence performance also. Almostsimilar behavior was observed in recovery test.

3.2.2.1. Essence of the fade and recovery studies. The fade-recoverycharacteristics (performance parameters) of composites are shownin Table 9 as per the pattern of the report of the JASO C406.

Following are the inferences drawn from Table 9.

3.2.2.2. Resistance to fade (% fade ratio): performance order – higherthe better. This is the most important parameter which reflectsdeterioration in � when operating conditions are demanding. Thisfade behavior was due to increase in surface temperature as thetest was conducted at constant pressure and speed. Generally fade% ratio in the range of 80–100 are acceptable as per industry norms.The % fade ratio (higher the better) and hence performance was inthe following order for composites;

CP1(97) = BP1(96) � IP1(91) > Ref(88)

Fade is caused by thermal decomposition of ingredients due toaccumulation of frictional heat on the surface. Metallic fillers in FMson the other hand, help to conduct away the heat from the surfaceand effectively leading to lower amount glaze on the surface. Theseparticles being somewhat hard and abrasive to some extent, try

BP1 CP1 IP1

1 0.410 0.390 0.39596 97 91

270 251 245

0 0.436 0.425 0.43494 88 90

832 M. Kumar, J. Bijwe / Wear 269 (2010) 826–837

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Wear performance order: CP1 > BP1 > IP1 � Ref.Thermal conductivity (TC) order: CP1 > BP1 > IP1 � Ref.

Fig. 5. Fade and recover

o abrade the glaze and help in rejuvenating the surface and tendo modify the quality of transferred tribo-layer. This layer is alsoesponsible to modify the topography of the pad surface and henceear. Thus metallic fillers improve not only the thermo-physicalroperties of the composites but also the quality of a transfer filmn the friction interface to improve fade resistance [6–8].

.2.2.3. The recovery behavior (% recovery ratio): performance orderhigher the better. Composites which recover their friction level

onsiderably after fade cycles are rated as good FMs. In general forcceptable composite, recovery should be in the range of 75–100%.he recovery % of composites is shown below.

P1(94) � IP1(90) > CP1(88) > Ref(86)

P1 showed highest recovery performance followed by IP1 and CP1hile Ref composite recorded lowest values.

In the recovery performance the temperature rise of the pad andhe disc was restricted by using the air blower. As the braking sur-ace is cooled down to room temperature and allowed for re-testingnder same conditions with existing friction film on both the sur-aces, the rheology of surface layer changes during next run. Theayer is viscous at high temperature while during recovery run, it isn solid form. The original film during fade run consisting of looselyttached wear debris, disintegrates and the surface underneath theriction film gets hardened. This wear debris during the recoveryun when entrapped in the mating zone of the composite and discct as hard abrasives leading to third body abrasion contributingo the enhanced abrasive action via rolling abrasion mechanism30]. Hence the characteristic frictional response of the compos-tes as observed usually in the fade test, gets restored, leading tobetter recovery. There is slight difference in mechanisms during

ecovery and fade. During cooling of the composite surface prior toecovery run friction film deformability gets reduced. This extentf reduction in the elasticity of the film on both the friction surfacess dependent on the composition of the friction film. The elasticallynd plastically deforming nature of the friction films at the brakingnterfaces has been reported to be dependent on composition byrefilov [31]. Thus the in situ changes in the formation and defor-ation of friction films during recovery cycle are responsible for

he differences in the recovery and fade behavior.

.2.2.4. Effect of metallic fillers on counterface friendliness. The risen the disc temperature during fade cycles decides the counterfaceriendliness of the pad materials. Lower the rise, better is the friend-iness. The performance of composites was in following order:

P1(245) > CP1(251) > BP1(270) > Ref(317)

omposite IP1 proved best in this aspect followed by CP1. Ref com-osite proved poorest showing highest disc temperature.

avior of the composites

Once again TC of metallic fillers based composites proved veryinfluencing factor because it conducted away the frictional heatgenerated at the interface very efficiently and finally led to lowertemperature rise in the disc.

3.3. Studies on wear and worn surfaces

Wear of the composites was measured by weight loss methodafter completing the effectiveness and fade and recovery studies.Wear volume was finally calculated by using density data of thecomposites.

3.3.1. Wear behavior of the compositesFig. 6 shows the wear performance of composites. Lower the

wear, better is the performance. Inclusion of metallic powderimproved the wear performance significantly. CP1 showed high-est wear resistance followed by BP1. Composite IP1 was moderateperformer while composite without metal contents (Ref) exhibitedhighest wear.

Wear mechanisms during wearing of friction materialsare extremely complex phenomena since several interactionsand mechanisms are simultaneously operative during materialremoval/gain process from both the surfaces. These are dynamicprocesses which in turn depend on composition of contacting sur-faces including that of third phase (tribo-layer), which in turndepends on operating parameters such as load, speed and especiallyinterface temperature.

In the present study following was the order of the wear perfor-

Fig. 6. Wear behavior of the composites

M. Kumar, J. Bijwe / Wear 269 (2010) 826–837 833

Fig. 7. SEM micrographs of composite CP1 (10% copper powder) highest WR: general features—quite smooth topography, appreciable damage on the surface, some amountof secondary or primary plateaus, (a) X1000—quite good adhesion of ingredients with matrix and transferred layer, hardly any thick primary plateaus, a thin uniform layerpossibly enriched with metal contents, covering all ingredients, long pieces of broken fibers also lying on the surface indicating some wear. (b) X1000—still longer fibrousdebris, little thicker back transferred layer. Overall—best WR was perfectly correlated with moderately damaged surface.

F al featd aus bi layerp

Ttpttccltaft

Fp(t

ig. 8. SEM micrographs of composite BP1 (10% brass powder)-II best in WR: generebris in large amount lying on the surface, Appreciable amount of secondary plate

ngredients were extracted. (b) X1000—considerable amount of wear debris, thinerfectly matched with considerably damaged surface.

Thus it seems that a fairly good correlation emerged betweenC and wear performance of composites. Liu and Rhee [32] inheir study on semi-metallic composites stated that TC of the com-osites plays a major role in wear performance and concludedhat semimetallic proved better than organic friction materials inhis aspect. Another reason why copper and brass powder-basedomposites showed better wear performance than the iron-basedomposite lies in the fact that copper and brass have better mal-

eability and plasticity than that of iron. During braking, Cu also getsransferred in the tribo-layer on the counterface due to combinedction of frictional heating and interfacial stresses. This Cu in trans-er film has more adhesion to the counterface rather than Fe in theribo-layer transferred from the composite containing Fe powder.

ig. 9. SEM micrographs of composite IP1 (10% iron powder)-III best in WR: general fearticles (possibly of iron) exposed on the surface, appreciable amount primary plateausa) X500—Fe particles in the middle of photograph showing excessive de-bonding. (b) Xhe tribo-layer. Overall—low WR was fairly correlated with heavily damaged surface.

ures—substantial damage to the surface and ingredients, slightly larger sized wearut not very thick and patchy (a) X1000—a cavity where from powdery and fibrousof possibly of metallic material spread over the primary surface. Overall—low WR

(It is a well-accepted fact that similar metals are less desirable forgood friction and wear properties). Moreover, the tribo-layer couldbe thinner and uniform because of malleability and ductility of Cuand brass powders apart from higher conductivity rather than Fecontaining tribo-layer which could be patchy, thick and less con-ducting. The SEM analysis of these composites was also done and ispresented in successive section, which presents the evidences forthin transferred layers.

3.3.2. Worn surface analysis (SEM observations)The SEM micrographs of worn surfaces of the composites are

arranged as per their decreasing wear resistance WR in Figs. 7–10.

atures—substantial damage to the surface and ingredients, square shaped, sharpand still large wear possibly because of weak adhesion of iron powder with resin:

1000—large damage, large amount of wear debris, heavy transfer of material from

834 M. Kumar, J. Bijwe / Wear 269 (2010) 826–837

Fig. 10. SEM micrographs of Ref composite (0% metal filler)-poorest WR: General features—very heterogeneous surface, substantial damage to the surface and ingredients,i k trand r debrl neoup ily da

T(obJchh(s

tlbTwtsgrni

4

tfiew

•

•

•

•

ndications of excessive removal of ingredients, pulled out fibers and particles, thicebris looking like porous surface (a) X1000—large damage, large amount of wea

oosely leading to more wear in successive braking. (b) X2000—extremely heterogeoor quality of the surface. Overall—lowest WR was fairly correlated with very heav

he main features are described in the captions. The surface of CP1highest WR) showed smoothest surface topography and evidencef very thin fine layer of metal (Cu contents) which was responsi-le for its stable friction behavior and highest WR. In the literatureia and Ling [7] also reported about frictional film transfer on theounterface with rich in Cu due to the combined action of frictionaleating and interface stress. This was claimed to be responsible forigh WR of the composite containing copper. Micrographs of BP1Fig. 8) showed appearance of small cracks on the surface indicatinglightly higher wear of this composite.

Secondary plateaus with shiny patches and some degrada-ion of ingredients were observed on micrographs. The frictionayer/friction film/tribo-layer/secondary plateaus terms are usedy the researchers [33–37] to describe the compacted wear debris.his is generally responsible for the deterioration in friction andear behavior. These secondary plateaus are because of back

ransfer of organic material such as resin (charred, degraded, andoftened), Aramid fibers, cashew nut shell liquid (CNSL) powder,raphite, etc. These plateaus do not have adhesion to the pad mate-ial as good as that of the original ingredients. Hence these doot have good load bearing properties resulting in deterioration

n friction and increase in wear.

. Conclusions

Based on the experimental studies conducted on NAO fric-ion composites developed in this work with variation in metallicllers (powdery) for evaluating effectiveness and fade and recov-ry behavior on brake inertia dynamometer, following conclusionsere drawn:

Use of metallic ingredients affected most of the performanceproperties in a beneficial way. Most of the properties such asdensity, porosity, tensile strength, compressibility, thermal con-ductivity and thermal diffusivity, etc. increased due to metalliccontents. (Cu was most effective in boosting TC significantlyfollowed by brass and iron). However, hardness and flexural prop-erties deteriorated due to inclusion of metallic contents.All composites showed adequate � (0.35–0.45) which is in thedesired range as per industrial practice. � Decreased with pres-sure and speed in general for all composites.

Inclusion of metallic fillers led to appreciable increase in mag-nitude of �, decrease in � sensitivity towards pressure–speed,temperature and wear resistance also.Thermal conductivity of the composites played an important rolein enhancement of the performance properties of composites

sferred layer on some portions and most of the places, heavy transfer of fine wearis, heavy transfer of material from the tribo-layer sitting on the damaged surfaces surface with very rough topography, loosely placed debris confirming extremelymaged surface.

(fade resistance, recovery performance and counterface friend-liness).

• Copper proved best performing metallic filler in all the threemajor performance properties viz. performance � (friction), sen-sitivity studies (pressure–speed and F & R) and wear behavior,followed by brass. Iron powder proved moderate in these aspects.

Acknowledgement

Authors acknowledge the funding by Department of Science andTechnology (Govt. of India) to carry out this work.

Appendix A. Japanese automobile standard organisations

Braking device Dynamometer Test Procedure-Passenger car(JASO 406 C).

A.1. Scope

This standard specifies the dynamometer test procedure tocheck on performance of normally operated service brake devicesfor passenger cars, however, except two wheeled vehicles.

Remark: in this standard and unit and numerical values given in{} are based on the international system of units (SI) and are givenfor references.

A.2. Purpose

This standard aims to establish a unified dynamometer test pro-cedure in order to make comparisons of performances of servicebrake devices for passenger cars, through the simulation of roadtest and use conditions.

A.3. Definitions

Definitions of major terms (initial speed, braking interval andbrake initial temperature before braking) used in this standard areas defined in A.3 of JASO C 446 (general rules of brake of automo-bile and motorcycles). The brake initial temperature before braking,where plural brake device are tested at the same time, small berepresented by the highest value.

A.4. Vehicle classification

Test vehicle will be classified in accordance with the provisiongiven in A.4 of JASO C 446 and shall be expressed respectively asP1, P2, P3 and P4 in terms of their nominal maximum speeds.

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Category P1: vehicles with their nominal maximum speedxceeding 140 km/h.

Category P2: vehicles with their nominal maximum speedxceeding 110 km/h and up to 140 km/h.

Category P3: vehicles with their nominal maximum speedxceeding 90 km/h and up to 110 km/h.

Category P4: vehicles with their nominal maximum speed up to0 km/h.

.5. Condition of each part of brake device

Condition of each part of brake device upon testing shell be, asrule, as specified in A.5 of JASO C 446.

.6. Test conditions

1) Moment of inertia: The calculation equation and setting of themoment of inertia shall be as specified in A.7.1 of JASO C 446.Test load, however, shall be as follows, according to the typeof test equipment such as dual or single dynamotors, and eachtest procedure.(a) In this case where a front and rear combination test is car-

ried out using a dual dynamometer: 1/2 of the total vehicleload.

(b) In case where a front and rear combination test is carriedout using single dynamometer or a dual dynamometer.

Value obtained by dividing 1/2 of the total vehicle load (thefull value of the total vehicle load if the test is done using a dualdynamometer) by the braking force ratio between the front andrear wheels upon braking with the deceleration of 0.45 g.

2) Braking torque: The calculation equation for the braking torqueshall be as specified in A.7.2 of JASO C 446. The equation thusused shall be recorded on the recording sheet.

3) Temperature measurement: a thermocouple shall be installed atthe fixed side, as a rule, according to A.4 of JASO C 446.

Remark: If the test was not done according to the above, itshall be recorded on the recording sheet to that effect.

4) Cooling wind: The wind velocity shall be, as a rule, 11 m/s and thewind shall be so adjusted that it may blow against the projectedsurface of the brake device uniformly and continuously. Thetemperature shall be normal.

.7. Test procedure

.7.1. Preparation for testMake sure that the brake device has no abnormalities before

nstalling it onto the test equipment. Make also sure that no grease,aint of any other foreign matte is adhered onto the lining.

In order to measure lining wears accurately, determine measur-ng locations on brake shoes or pads prior to the test. The numberf measuring points for a shoe shall be 5 on each side, which shallotal to 10 points for both sides. For a pad, on the other hand, mea-uring points shall be 4–6, as a rule. Clean up the friction surface ofhe drum of disc with acetone, etc.

Install a thermocouple at a proper location on the shoe or pad,nstall the brake device on the test equipment and center it prop-rly, and record the deviation of the drum or disc accordingly.

.8. Test items and sequence of tests

Test Items and sequence shall be as follows. Each braking shalle so applied that the vehicle comes to a complete stop. For a vehiclen which the brake device temperature dies not rise easily, the testay be carried out by changing the initial brake temperature from

0 to 60 ◦C or 120 to 80 ◦C, respectively.

69 (2010) 826–837 835

(1) Initial measurements: The thickness of the lining or pad, of anyother dimensions of the brake device may be measured andrecorded as called for.

(2) Preburnish check

Initial speed: 50 km/hBraking deceleration: 0.3 gBraking interval: 120 sNumber of applications: 10 times

(3) First (Preburnish) effectiveness testInitial speed: 50 and 100 km/h for P1 and P2vehicle s 50 and

80 km/h for P3 vehicles.Braking deceleration: between 0.1 and 0.8 gInitial temperature: 80 ◦CNumbers of applications: The brake applications shall be

repeated until the record can be obtained for 5 or moremeasuring points as equally as possible, within the range ofspecified braking deceleration.

Remark 1: The lower initial speed conditions shall beemployed first in the test, and the lower deceleration shallbe used, then the higher deceleration, respectively.

Remark 2: If it is necessary to increase the brake temper-ature to a certain degree prior to the braking the burnishconditions shall be used.

Remark 3: Braking shall be done with the constant outputor input. It shall be recorded whether a constant output or aconstant input was used in the test.

(4) BurnishInitial speed: 65 km/hBraking deceleration: 0.35 gInitial temperature: 120 ◦CNumber of applications: 200 times

(5) Second effectiveness testThe first effectiveness test specified in (3) above shall be

repeated. The initial speeds however shall be as follows.P1 vehicles: 50, 100 and 130 km/hP2 vehicles: 50, 80 and 100 km/hP3 vehicles: 50 and 80 km/hP4 vehicles: 50 and 65 km/h

(6) First Reburnish repeat the (4): The number of applicationsshall be however, 35 times.

(7) Emergency brake test (optional item): Assuming the failureof a single brake system, the following test may be carried out.

Initial speed: P1 vehicles, 80 km/h (100 km/h)P2 vehicles, 80 km/hP3 vehicles, 65 km/hP4 vehicles, 50 km/h

Braking declarations: between 0.1 and 0.25 gInitial temperature: 80 ◦CNumber of applications: The brake applications shall be

repeated until the record can be obtained for 4 or moremeasuring points as equally as possible, within the range ofspecified braking deceleration.

Remark 1: for four wheel and dual dynamometers the testshall be carried out for each system according to the failure. Forsingle dynamometers, the moment of inertia corresponding totest.

Remark 2: The initial speed for P1 vehicles shall be as a rule,80 km/h but 100 km/h may be used for exceptional cases.

Remark 3: After the completion of the emergency brake test,a burnish test with the same conditions as in (6) first Reburnish

shall be repeated, then move over to the following tests.

(8) First fade-recovery test(a) Baseline check

Initial speed: 50 km/h

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36 M. Kumar, J. Bijwe /

Braking deceleration: 0.3 g constant or a constant pres-sure to obtain 0.3 g

Initial temperature: 80 ◦CNumber of applications: 3 timesRemark 1: When the pressure is set constant in the test,

select an appropriate pressure before the test that mayallow the declaration of 0.3 g.

Remark 2: If it is necessary to increase the brake initialtemperature to a certain degree in the fade-recovery test,refer to (4) Burnish conditions.

(b) Fade test

Initial speed: 100 km/h for P1 vehicles80 km/h for P2 vehicles65 km/h for P3 and P4 vehicles.

Braking declarations: 0.45 g constant, or a constant pres-sure that allows 0.45 g

Initial temperature: 60 ◦C at the first timeBraking interval: 35 s for P1 vehicles40 s for P2 vehicles45 s for P3 and P4 vehiclesNumbers of applications: 10 timesRemark 1: When the test is to be done with a constant

pressure, select an appropriate pressure in advance so thata deceleration equivalent to 0.45 g may be obtained, afterthe completion of the baseline test. The number of brakeapplications, however, shall be up to 3 times.

Remark 2: The interval between the vehicle stop andthe restarting shall be as short as possible. Carry out therecovery test immediately after the completion of test. Thebraking interval up to the first recovery test shall be 120 s.

Remark 3: If the test cannot be done with the above-mentioned braking interval, the time cycle upon the fadetest may be extended to 40 or 60 s. In such case the brakinginterval thus used shall be recorded.

(c) Recovery testInitial speed: 50 km/hBraking declarations: 0.3 g constant or a constant pres-

sure obtained in baseline checkBraking interval: 120 sNumbers of applications: At least 12 times.

(d) Effective spot check (optional item)Initial speed: the same as in (b) fade Test.Braking declarations: the same as in (b) fade Test.Initial temperature: 60 ◦CNumbers of applications: 2 times.

(9) Second ReburnishThe first Reburnish specified in (6) above shall be repeated.

10) Second fade-recovery testThe fade-recovery test specified in (8) above shall be

repeated. The number of application shall be, however, 15times.

11) Third ReburnishThe first Reburnish specified in (6) above shall be repeated.

12) Third effectiveness testThe second Effectiveness test specified in (5) above shall be

repeated. For initial speed however, attest according to fol-lowing initial speeds may be dine as an optional test.

13) Fourth ReburnishThe first Reburnish specified in (6) above shall be repeated.

14) Water recovery test

(a) Baseline check

Initial speed: 50 km/hBraking deceleration: 0.3 g constant or a constant pres-

sure to obtain 0.3 gInitial temperature: 80 ◦C

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Number of applications: 3 timesRemarks: When the pressure is set constant in the test,

select an appropriate pressure before the test that mayallow the declaration of 0.3 g.

(b) Water immersionWith the brake released, allow the frictional material

surface to be thoroughly immersed in water for 120 s whilerotating the brake slowly at 10–30 rpm. In case of a drumbrake, the brake may be taken out (of the system) andimmersed in water.

Remarks: Test shall be started immediately after thewater immersion

(c) Recovery testInitial speed: 50 km/hBraking declarations: 0.3 g constant or a constant pres-

sure obtained in baseline checkBraking interval: 60 sNumbers of applications: at least 15 times.

(15) Final measurement and inspectionThe brake shall be inspected and the observation results

shall be recorded. The initial measurement specified in (1) inthe foregoing shall then be repeated.

(8) Recording(1) All noise, vibrations and other abnormalities shall be

recorded.(2) Braking torque, pressure, temperature and braking initial

speed (rotational speed) during tests shall be recorded.(3) Pressure and braking torque shall preferably be mea-

sured continuously.

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