Tribological performance
characterization of brake friction
materials: What test? What
coefficient of friction?
Mohamed Kchaou
1, Amira Sellami
1, Jamal Fajoui
2, Recai Kus
3,
Riadh Elleuch
1and Fre´de´ric Jacquemin
2Abstract
This article describes and explains the tribological tests and methods for the evaluation of the performance of the brake friction materials. It starts by discussing the particularities of these materials and the variation of characterization tests, which can experimentally simulate many aspects of brake situation but with a large field of tribo-test, from standard to specific protocol. Examples of preparation, procedures, instrumentation, and analysis results for the tribological aspect testing ranging from the scale of vehicle braking performance (by methods including inertia dynamometers, Krauss testing, friction assessment screening test, and Chase testing) to simplified test using reduced-scale prototypes for small-sample friction, are explained. A particular attention is attributed to the discussion of the viability of the friction coefficient report in relation to the material properties and brake compound performance. At the end of this article, the guarantee of the performance output or ranking evaluated by such experimental methods is discussed.
Keywords
Friction composite material, performance tests, coefficient of friction, experimental rigs
Date received: 2 December 2017; accepted: 13 February 2018
Introduction
The brake performance of a vehicle is primarily deter-mined by the tribological properties of the friction couple of the brake system (disc (or drum) and pad friction material).1,2Basically, required performances are summarized on a relatively higher and stable fric-tion level at wide range of braking condifric-tions, i.e. temperature, humidity, third body established in the interface pad-disc, external road particle, etc.3–5 Safety requirements, long life of the brake parts, and high comfort (especially dynamic and acoustic aspects) are not lacking interest.6–8 To develop brake pads with a maximum of performances, full-vehicle road and laboratory-scale testing under differ-ent loading, sliding speed, temperature, and pressure are requested by manufacturers and researchers.9 In fact, the innovation in the brake pad design required numerous test methods based on laboratory developments and in service control, ranging from ‘‘small coupon rub tests’’ to ‘‘full-sized vehicle, on-road tests’’. Characterization tests are the most used tests to study new formulation of brake pads based on experimental and empirical approach while
performance tests concentrate on the pad service effi-ciency, i.e. fading and recovery behaviors at different braking situations (urgency, accumulative, and decel-eration braking).9In fact, for the friction material, the fade, i.e. effectiveness decrease at elevated tempera-ture, and the recovery, i.e. return to acceptable levels of friction at lower temperature after braking application, are influenced by many parameters and braking conditions involving friction materials prop-erties, brake system technology, and braking solicita-tion (particularly mechanical, thermal, and tribological solicitations).10,11These parameters act on synergy and increases the complexity of the study.12,13 As a
1
Universite´ de Sfax, Ecole Nationale d’Inge´nieurs de Sfax, LASEM, Sfax, Tunisia
2
GeM – Institut de Recherche en Ge´nie Civil et Me´canique, Universite´ de Nantes – Centrale Nantes – CNRS UMR, Saint-Nazaire, France 3
Faculty of Technology, Department of Mechanical Engineering, Selcuk University, Konya, Turkey
Corresponding author:
Mohamed Kchaou, Universite´ de Sfax, Ecole Nationale d’Inge´nieurs de Sfax, LASEM, BP No. 153, 3013 Sfax, Tunisia.
Email: kchaou.mohamed@yahoo.fr
Proc IMechE Part J: J Engineering Tribology 2019, Vol. 233(1) 214–226 !IMechE 2018 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/1350650118764167 journals.sagepub.com/home/pij
consequence, the choice of adequate test parameters and conditions, experimental rig, and controlled data is not evident and interrogates several questions about the meaning of the representative results, particularly the coefficient of friction.14 In fact, the investigation on the friction characteristics of the brake materials is well underway in literature. However, no reflections about the viability of such characterization in relation to the adequacy of the experimental protocol have been reported. In this paper, we reported the performance and characterization tests used to qualify friction mater-ials and discussed the challenge, since no equipment seems to be able to perform all the required tests. A particular attention is given to the different friction coefficients defined and used by researchers to study the friction material performance. We conclude this paper by our tribological team’s characterization of a nonasbestos organic friction material in relation to the molding process parameters.
Particularities of the friction
composite materials
Friction materials are commonly composites with complex formulations comprising multiple constitu-ents (about 10 and up to 30 ingrediconstitu-ents), which are different in nature, shape, size and morphology, and interact on synergies to obtain high temperature bility, high strength, improvement of frictional sta-bility, high thermal conductivity, and oxidation and wear resistances.4These brake performances are not only affected by the selection of the formulation, but also by the manufacturing process and parameters, which are considered as a second source of hetero-geneity.15 Besides, it was clarified by many authors that the friction material behavior i.e. mechanical, tribological, vibro-acoustical behaviors, depends not only on the global characteristics of the brake system but also especially on the local evolution of the contact in terms of dynamical behavior of the pad, heating concentration and evolution on the disc, sliding accommodation, friction wear mechan-isms, etc.2,3,5,12 Therefore, the study of the friction material performance requires investigation at vari-ous scales, considering the effect of its heterogenevari-ous microstructure and its anisotropy with regard to syn-ergy activated between solicitations and material properties.16
Standard test of performance
Many standard tests are used by researchers and manufacturers to study the friction material perform-ance, particularly:9
– J Standards or Recommended Practices from the Society of Automotive Engineers Procedures - SAE (examples: J 1802 test procedure for drum brake linings, J 886 laboratory-scale for determining
lining friction, J 2430 multi-stage dynamometer test for disc brakes),
– Economic Commission for Europe regulation (ECE R-90),
– Federal Motor Vehicle Safety Standards (FMVSS) from the National Highway Traffic Safety Administration (NHTSA),
– International Standards from the International Organization for Standardization (ISO).
Tribological performance analysis
Definition of the required modes of performance
Performance testing of the friction materials is gener-ally related to the study of fading and recovery behav-ior.17 As the fade characterizes the loss in braking effectiveness at elevated temperatures, Anderson18 has classified fade performance modes as thermal, delayed, blister, flash, and contamination fades, as defined below:
– Delayed fade: Specific phenomenon that occurs well after a period of hand brake usage and usually with no warning signs. During the fade recovery, brake effectiveness may drop unexpectedly, causing a temporary, but pronounced increase of brake pedal force requirement. Therefore, delayed fade is insidious in that it is unexpected.
– Blister fade: It is shown as a near-surface blistering resulting in a rapid and brief loss of brake effect-iveness. It is associated with the reactivity of vola-tile materials induced by manufacturing and attached to the friction material, when they are not released by the end of the burnish process, and able to cause high internal gas pressures upon rapid heating.
– Flash fade: It occurs only at very high brake power levels, usually at very high speeds.
– Contamination fade: It is induced by the gener-ation of elasto-hydrodynamic fluid film from water, oil, or a combination of these, which react on the surface of the brake lining and can make a bearing from a brake.
Therefore, as performance test, we can quote the following tests:19–21
Fade test. The test consists of the application of high energy conditions of friction at very short intervals for the speed and high presser level to thermally solicit the friction material. As a result, the graph of temperature vs. coefficient of friction evolution is drawn and the fade behavior is deduced.
Residual stop test. The residual stop test took place immediately after fade test. The coefficient of friction after the hot performance is deduced and percentage fades are given.
Recovery test. Recovery, which is carried out immedi-ately after residual stop test, describes the return of the same pad to acceptable friction level at lower temperatures. It is generally tested at the speed of 120 to 60 km/h.
Post recovery test. Post recovery test is generally per-formed immediately and at the same conditions of the recovery test to give the performance after the recovery of the coefficient of friction. The percentage recovery is given.
Friction performance parameters
The coefficient of friction (COF) is considered to be the most important characteristic of friction mater-ials, which qualify the braking safety of any vehi-cle.22,23 This coefficient is sensitive to the operating conditions such as sliding speed, braking pressure, humidity, etc. For composite friction materials, it was reported that temperature is the most influential parameter on the COF when the interfacial thermo-mechanical behavior is affected by friction heating during different kinds of braking, associated with the increase in the temperature.24 Researchers used many friction coefficient parameters to qualify the friction level of the friction materials. The most com-monly used parameters are:23–26
– -Minimum (min): this is the lowest COF in all
the studied modes (cold, fade, and recovery). – -Maximum (max): this is the highest COF in all
the three modes.
– Performance : this is an average COF considered between N and N/2 braking operation (N is the total number of brake applications).
– Fade : minimum coefficient of friction for fade mode (measured after 270C).
%Fade : Performance fadeð Þ=Performance 100
ð1Þ – Recovery : average coefficient of friction in the
recovery mode, measured after 100C.
%Recovery : Recovery =Performance 100 ð2Þ – Friction variation
ðÞ ¼ maxmin ð3Þ
Full-scale performance test rigs and
experimentation
As described in the introduction part of this article, full-scale and reduced-scale tests are used to study the performance of the friction materials. However, as the
usage of a brake lining in the vehicle scale under these tests required sophistical equipment and a long duration of test in-service, the test sample geometry is simplified with an adequate representative elementary volume (REV). One of the primary aims of the brake materials reduced-scale testing is to quantify the fric-tion coefficient as a funcfric-tion of pad pressure, sliding velocity, and temperature under conditions equivalent to the real brake systems. Therefore, it is critical to maintain these parameters in ‘‘one-to-one’’ relationship between the full-scale and reduced-scale testing. This means that the nominal values of these parameters remain the same in both types of testing systems. In this section, we distinguish full-scale per-formance tests (carried out using different rigs and machines) to reduced-scale tests (mostly achieved on pin or pad-on-disc tribometer).
based friction material rig. Dynamometer-based friction material rig is used typically to apply a series of complex test sequence to characterize the braking performance (Figure 1) (burnishing, fade, fade recovery, etc.). As concluded by Wu et al.27 and Swarbric and Wu28in their studies, the clarifica-tion acquired from braking tests on a reduced-scale dynamometer is significant enough in supporting the understanding of the testing vehicle results. To match the requirements of modern fast moving vehicles and the continuous evolution of the vehicle comfort, full-scale inertia dynamometer testing is very essential.29 Since such facility is generally available with indus-tries, the number of research papers on the evaluation of materials in this respect is limited. Many reduced-scale dynamometer are developed for different topics of study of brake friction materials.30–32 Parc et al.3 studied the influence of steel fibers on the tribological behavior of a brake lining material using an inertial brake dynamometer (the experimental steps, summar-ized in Table 1, consist of a burnish stops, stops test, short drag, and extended drag). Attention was particularly focused on the stick-slip phenomenon in relation to the steel fiber percentage. They have reported that the friction coefficient of the nonsteel friction material was not strongly affected by the
Table 1. Test procedure from El-Tayeb and Liew.3
1. Burnish stops: initial brake temperature ¼ 100C, deceler-ation ¼ 0.35 g, initial speed ¼ 60 km/h, number of
applications ¼ 200
2. Stop tests: initial brake temperature ¼ 50C, 100C, 150C, 200C, 250C, applied pressure ¼ 3.75 MPa, initial speed ¼ 100 km/h, number of applications ¼ 5 3. Short drag: initial brake temperature ¼ 250C,
speed ¼ 10 km/h 100 km/h (10 km/h interval), number of applications ¼ 9
4. Extended drag: initial brake temperature ¼ 100C, torque ¼ 110 kgfcm, speed ¼ 70 km/h, duration ¼ 4 min
sliding speed and the stick-slip phenomenon of the low-steel friction material was pronounced at slow sliding speeds. Gweon et al.33 studied friction and vibration behaviors of brake friction materials containing short
glass fibers. A one-fifth-scale brake dynamometer was used according to the test procedure summarized in Table 2. Jan et al.34 experimented four tribological test procedures to study the friction effectiveness of C/C-SiC brake discs using a scale brake dynamometer (Table 3). Particular attention was given to the friction level and wear evolutions under different conditions. Chase testing. Many researchers used Chase-type friction machine for friction and wear study of auto-motive brake lining material (Figure 2).10,11 Arjmand’s team has developed a chase-type friction machine;11 the test stages comprised three sequences composed of preliminary, thermal, and complemen-tary steps. Preliminary and complemencomplemen-tary steps were conducted at constant temperature of 150C
for 10 braking cycles with constant interval of 10 s application (braking) and 10 s release (no braking). During the thermal step, which was initiated upon completion of the preliminary step, continuous brak-ing was applied and the temperature of the drum was allowed to increase. Using this protocol, average value of COF in both preliminary and complementary as well as variation of COF with temperature during the thermal stage were characterized (Figure 3). Ozturk et al.35 studied the effect of resin type and fiber length on the mechanical properties and friction characteristics of automotive brake materials using a
Figure 2. Chase-type friction machine.10
Figure 1. (a) Reduced-scale brake dynamometer with (b) disc and (c) 2 pads.30 Table 3. Dynamometer test procedure.34
1. Burnish: 200 stops, initial brake temperature ¼ 100C, speed ¼ 80 km/h, deceleration ¼ 0.35 g
2. Pressure sensitive: initial brake temperature ¼ 100C, speed ¼ 80 km/h, P ¼ 10,20,30,40,50 kgf/cm2
3. Friction instability: initial brake temperature ¼ 100C, speed ¼ 100 km/h, deceleration ¼ 0.4 g
4. Wear: 1000 stops, initial brake temperature ¼ 100C, speed ¼ 100 km/h, deceleration ¼ 0.3 g
Table 2. The test procedures used for burnishing and the dynamical behavior test with a one-fifth-scale brake dynamometer.33
1. Burnishing: speed ¼ 70 km/h, pressure ¼ 60 kgf/cm2, Initial brake temperature ¼ 110C Repeated 50 8 times in a constant pressure, Drag velocity ¼ 0.1–30.0 mm/s, Line pressure ¼ 10, 20, 40, 60 kg f/cm2, Initial brake tempera-ture ¼ 25–27C
Chase-type friction tester. Performance test condi-tions are summarized in Table 4. The COF showed a good correlation with the wear resistance of the fric-tion composites. However, no clear correlafric-tion with the mechanical and tribological properties of the fric-tion composites was identified.
Krauss testing. A Krauss-type friction machine is also widely used to measure the wear rate and the COF of brake composite material. Typical protocol consists of the application of burnish followed by a constant interval test mode (fade test) or a high temperature wear test mode.24 Table 5 shows an example of the experimental protocol. Assif36 have used a Krauss-type RWDC 100C (450 V/50 Hz) machine for tribo-evaluation of aluminum-based metal matrix composites (ABMMC) used for heavy duty vehicles brake pad applications carried at a temperature range of 40–200C. The same was used by Cho et al.37
to simulate intermittent brake applications at downhill
to slowdown a vehicle in order to study the effect of potassium titanate on high-temperature friction sta-bility. Friction and wear of the brake pad were exam-ined during the extended drag situation at high temperatures and not intended for normal stops in the city traffic situation (Figure 4). Lee et al.38 used a constant interval test (CIT) to evaluate the effect of
Figure 3. COF and temperature evolution curves under preliminary, thermal, and complementary stages.11
Table 4. Chase-type test procedure from Ozturk and Ozturk.35
Block Speed (r/min)
Temperature (C)
Load (N)
On time Off time
No. of applications
Min Max Increment min s min s
Burnish 308 – 93 – 450 20 – – – 1 Reset 205 82 93 – 230 5 – – – 1 Baseline-I 411 82 104 – 670 – 10 – 20 20 Fade-I 411 82 289 28 670 10 – – – 1 Recovery-I 411 261 93 56 670 – 10 – – 1 Wear 411 193 204 – 670 – 20 – 10 100 Fade-II 411 82 345 28 670 10 – – – 1 Recovery-II 411 317 93 56 670 – 10 – – 1 Baseline-II 411 82 104 – 670 – 10 – 20 20
Table 5. Experimental test procedures from Shin et al.24 Burnishing: initial brake temperature ¼ 25C, sliding speed ¼
3 m/s, pressure ¼ 0.7 MPa, application duration ¼ 300 s, 2 times
2. Fading: initial brake temperature ¼ 100C, sliding speed ¼ 4.0 m/s, pressure ¼ 1.0 MPa, application duration ¼ 600 s (50 s drag—10 s interval, 10 times)
3. High temperature wear test: constant tested temperature ¼ 350C, sliding speed ¼ 1.2–4.2 m/s, pressure ¼ 0.7 MPa, sliding distance ¼ 25 km
antimony trisulfide (Sb2S3) on stick-slip propensity of
the friction materials using a Krauss-type tribometer. Kim and Jang39 studied the fade and wear behavior of the friction composite material reinforced with chopped glass fibers over different temperature ranges maintained at constant pressure and velocity. Friction assessment screening test. Fast assessment screening test (FAST) is considered as a quality con-trol test for screening brake and clutch friction mater-ial (effectiveness, fade, recovery, speed sensitivity, and wear characteristics). Teo et al.40 studied carbon– carbon composites friction material. By conducting friction tests using FAST machine (Figure 5), they noticed a decrease in the friction level and wear rate when the temperature increased to 950C. They
attributed this behavior to the oxidation effect and the chemisorption of impurities to form lubricating film. Philip et al.41 applied an experimental protocol of severe conditions on brake lining materials, consist-ing of a succession of (i) 10 instrument check stops (ii) effectiveness testing, (iii) burnishing, (iv) fade test, (v) recovery test, and (vi) 600C and 800C wear test.
The purpose was to qualify the friction stability under these different stages of service (Figure 6). In
their study on the effect of using FAST machine, Muzathik et al.42 tried to control brake performance of aluminum–boron–carbide friction material. No evi-dence of fade with temperature increases was reported since this ceramic friction material has high toughness and thermal conductivity relative to other ceramics.
Reducing scale tribo-testing
Laboratory-scale tribo-testing protocols commonly use constant speed and load (force or torque)
Figure 4. Examination of friction coefficient during drag situation at high temperatures, in relation to sliding time: (a) whisker; (b) platelet; (c) splinter.37
Figure 5. The friction assessment screening test (FAST) machine.40
though pin-on-disc or pad-on-disc configurations were used for comprehensive studies of the friction material behaviors and properties.43 In fact, these techniques allow the measurement of friction and wear levels when a disc rubs facing to a pin, providing valuable information on the tribological characteris-tics of the brake system. According to the scale theory, the environment conditions and friction solici-tations are defined based on a similitude approach between full- and reduced-scale machines (sliding velocity, density of energy dissipated in the pads, deceleration, and braking stop time).44 In fact, as it is difficult to reproduce by sliding friction a realistic full-scale thermal loading using a reduced-scale rig, compromises between thermal, mechanical, and tribo-logical aspects are needed.45Experimental parameters are defined with respect to scale shifting and simili-tude rules with regard to rise of temperatures and heat transfer between pad and disc, allowing sameness at reduced-scale braking situations observed at full vehi-cle scale. Otherwise, the choice of shape and size of pad and disc used for these tests should lead to stres-ses equivalent to those in full-scale machine, and be sufficient toward the heterogeneity of the microstruc-ture and the phenomena of loadbearing localization.46 By exploitation of the wide range of parameters vary-ing with the tribometer, the sensitivity of the friction materials to chosen brake situations are discussed, particularly, the temperature, pressure, and sliding velocity influences on the friction coefficient and its sta-bility, third body establishment, effect of ingredient size or nature on the friction level, etc., Complementary tests are useful to study braking safety and comfort of automobiles.
Effect of solicitation conditions. The study of the solicita-tion condisolicita-tions effect consists of the characterizasolicita-tion of the sensitivity of friction material to the loading, sliding speed, duration, effective pressure, etc. Yin et al.13 attributed the friction coefficient sensitivity
to the increase of the braking pressure and sliding velocity when there is rise in the temperature by fric-tion sliding. They concluded that when automobile is moving with a high velocity, it is not reliable to exe-cute an emergency braking only by raising the braking pressure. A small-scale tribo-testing brake pad disc machine was used by EL-Tayeb and Liew3to conduct tribo-tests under dry and wet conditions. The aim was to study the effect of continuous braking at constant sliding speeds and contact pressures on friction and wear characteristics of composites brake materials tested against gray cast iron (GCI) disc. The coeffi-cient friction () and sliding velocity (v) relationship was established and discussed in relation to the steel fibers–GCI counter disc interaction. Zhu et al.47and Shi et al.48had respectively elucidated the tribological performance of the asbestos and non-asbestos brake shoe of mine hoister by varying load, sliding speed, duration, effective pressure parameters to test friction materials sensitivities with regard to the operating parameters.
Effect of environmental conditions. Humidity and tem-perature are the environmental parameters controlled by the study of the behavior and properties of the friction materials. Blau and McLaughlin49 investi-gated the effect of the presence of water films at the interface friction material pad–GCI disc on the fric-tion behavior. It was reported that, under water spray conditions, the friction coefficient was inversely proportional to the square of the sliding speed. Hydrodynamic effects on the specificity of the transfer film on the cast iron surface were discussed when the sliding speed was increased. Bian and Wu50examined the friction performance of an organic pad rubbing face to carbon and silicon carbide ceramic discs in air and water spray conditions. They particularly revealed a lower friction coefficient down 0.1 under humid environment for both brake friction materials. Lee et al.51,52investigated the automotive creep groan
noise. They reported that the water molecules trapped in the interface changed the level of static and kinetic COFs during sliding with regard to the hydrophobic properties of the studied friction material. Verma et al.53conducted wear tests at large range of tempera-tures (25C, 170C, 200C, 250C, 300C, and
350C), which correspond to different brake heating.
The critical temperature corresponding to the transi-tion from mild to severe wear was identified. Kasem et al.54 investigated the temperature changes in dry sliding contact for braking applications. By using pad-on-disc tribometer, the viability of thermal results reported from an original in situ thermal metrology method especially developed for sliding contact in braking applications was discussed. Ferrer et al.55 dis-cussed the friction material capacity to dissipate energy in the form of surface heat under controlled environmental conditions using pin-on-disc test, par-ticularly hot points and strips spreading.
Effect of the operational regime (loading mode). Cyclic, continuous and intermittent loadings and reciprocating sliding are used in the study of the effect of operational regimes on the friction material behaviors and the gen-eration of airborne wear particles.56 Eriksson et al.57 simulated the internal loading effect on the friction layer establishment of a pad-disc system due to the behavior between the disc and the pad, induced by normal load variation (Figure 7). The investigation elucidates rapid changes of the contact situation on a microscale. Kristol et al.16proposed temperature-con-trolled tribological tests defined by increasing rubbing conditions severity under periodic sliding contact of 30 braking cycles. These tests correspond to low, medium, and high energy wear tests, which permit reaching three temperatures ranges representative of road braking severities (Figure 8). Average, maximum, and minimum friction coefficients fewer than 30 braking cycles were identified and correlated to the thermo-mechanical properties of the brake system materials.
Effect of the antagonist material category (disc material). Usually, GCI, steel, and ceramic are used
as disc brake material.58 The tribological behavior of the friction material is in strong relation with the antagonist, which particularly influences the thermal transfer and friction layers establishment and com-position. Ferrer et al.55 compared friction coefficient properties of brake lining material in contact with a sintered copper alloy disc. They reported a completely different evolution of the friction coefficient. Maleque et al.58discussed a method to optimize braking mater-ial efficiently by the substitution of the cast iron disc by other lightweight materials. The use of pin-on-disc test has allowed a board performance comparison between cast iron and 5 other Ti-alloy category fric-tion materials. Gunjal et al.59 developed a compara-tive protocol to study the tribological behavior of different disc materials i.e. GCI, structural steel, alu-minum, and high speed steel (HSS) based on the cri-teria of low frictional force, low coefficient of friction, low wear rate, low cost, and better mechanical properties.
Effect of ingredients properties and distribution on the pad surface. Friction materials are typically made of a composition of binder, reinforcement additives, fillers, and friction modifiers. The nature, the quantity as well as the shape and the geometry of these ingredi-ents influenced the friction material behavior, and consequently, the friction coefficient stabilities. The studies of the compositional impact are widely investigated in the literature and a complete analysis of the synergic effect of all the ingredients in the fric-tion material performance is seldom reported. Menapace et al.60 studied the possible substitution of copper ingredient in the brake friction materials by changing the microstructure of the metallic ingre-dients using high energy ball-milling. Based on pin-on-disc tests, they controlled the friction coeffi-cient behavior of the new material with regard to the formation of Cu–ZrO2clusters on the pad-disc rubbed surfaces. Kolluri et al.61 studied the friction potential of different synthetic graphite with different sizes (from 21 to 486 mm). They identified a critical size of synthetic graphite particle, which can give the best tribological performance properties in the friction composite materials. Cho et al.37studied the effect of
Figure 7. Nominal normal load vs. time loading mode to simulate operational regime effect.57
Figure 8. Temperature-controlled loading of tribological tests of friction material characterization.16
phenolic resin, Ca(OH)2, rockwool, MgO, and
zirco-nium silicate (zircon) weight percentage on friction and noise propensity. Aigbodian team’s tried to develop friendly friction material by the valorization of natural wastes. Different formulations were tested using pin-on-disc tribometer to evaluate the braking potential of the new materials particularly to keep stable and high level friction coefficient with an optimized formulation.62,63
Case study: Representative friction test
for studying the influence of hot molding
duration on the friction–wear behavior
of a friction composite material
The friction material is manufactured by a succession of steps, essentially mixing, molding, and hot curing. It was suggested that process parameters can influence the friction material properties, especially by the dis-tribution of ingredients on the surface and the forma-tion of pores.12,16,64 Consequently, the frictional behavior was affected by the heterogeneity induced by the manufacturing process. Beside, researchers try to understand the effect of such parameters to optimize the manufacturing process for a more effi-cient material.
Experimental approach
As reported in our previous study,12the phenolic resin composite material is molded generally at 160C for
18 min. The alternative is to reduce this period to half according to a best combination between resin reticula-tion and tribological properties of the composite. Based on the previous work of our team,65two formulations namely C1 and C2 molding under 18 and 9 min
respectively were characterized using a pin-on-disc trib-ometer tests (Figure 9). The material composition is given in Table 6. GCI material is chosen for the antag-onist. The investigated friction materials were manufac-tured by a conventional procedure consisting of dry mixing, pre-forming, hot molding, and post curing. The friction test consists of 600 cyclic rubbing, intermit-tent by the control of the sliding temperature, from 200C to 250C, induced by surface friction. Table 7
gives the imposed parameters, which correspond to temperature reached under service and moderate pres-sure conditions during severe braking, leading to mod-erate thermal atmosphere on the pad–disc contact. The test loading is shown in Figure 10.
Figure 9. (a) Pin-on-disc tribometer, (b) details of the materials contact, (c) experimental protocol.
1. Motor, 2. Table, 3. Guidance, 4. Pneumatic loading, 5. Normal force sensor, 6. Transducer, 7. Spindle, 8. Four thermocouples type K, 9. Slip ring, 10. Acquisition.
Table 6. Brake friction material composition.
Classification Constituent Weight%
Binder Phenolic resin 14
Reinforcement additives Mineral fiber 22 Filler Friction modifiers Barite 47 Aluminum oxide Graphite 17 Rubber
Table 7. Experimental parameters.
Load (N) 185
Pressure (MPa) 0.6
Sliding speed (m/s) 6
Temperature range 200–250C
Friction-wear results
Figure 11 shows the friction coefficient (COF) evolu-tion of the two fricevolu-tion materials versus time for 600 braking cycles. For the C1 material, COF does not
exceed 0.46 for all cycles of braking with stable fric-tion behavior. On the other hand, C2 materials show higher COF (globally 0.5) but with some fluctuation of friction along the braking test. This difference in behavior is particularly noticed at the end of the cycling. If we related this behavior to unique modified parameters between the two formulations, i.e. hot molding duration, manufacturing process, it is note-worthy that the decrease of hot molding duration from 18 min to 9 min involves an increase of the fric-tion coefficient. Controlled pin and disc temperatures evolution is shown in Figure 12. It is noticed that the temperature increases more rapidly for C1 material, for both pin and disc (Figure 12(a)). However, the pin temperature level of C2 material is less important with few relative fluctuations (Figure 12(b)). Hentati et al.65 reported that hot molding conditions act on thermal properties, particularly thermal conductivity but not on mechanical properties. Cristo et al.66 sug-gested that hot molding impacts thermal localization during braking (intensity and amplitude of thermal
Figure 11. Evolution of friction coefficient in relation to the test duration and braking number of: (a) C1 (18 min), (b) C2 (9 min). Figure 10. Friction experimental protocol.
radiation) that is more intense for the materials man-ufactured under a longer molding time, while the fluc-tuations of the thermal radiation are more compacted when the molding time is reduced. Consequently, we can confirm that the temperature parameter affects the friction responses of the material, which probably con-firm the friction coefficient instability of C2. Figure 13 shows the wear rate of both studied materials. It exhi-bits that the wear rates of C2 is 1.5 times superior to the material C1. The decrease in the molding duration seems to effect wear rates inversely. If we correlate this wear behavior to the friction and temperature evolu-tion, we suggest that this process parameter has a little influence on friction evolution over the time. However, wear is sensitive to hot molding conditions.
Conclusion
In this article, tribological tests and methods for the evaluation of the performance of the brake friction materials are discussed in order to emphasize the via-bility of these trials and their representativeness of studied the problem. The following conclusions are established:
– Friction testing of brake friction materials is a complex job attributed to the heterogeneity of the materials induced by process, constitutions, micro-structure, etc. and the number of activated
phenomenon induced by the synergy between solicitations and materials properties under varying operating conditions.
– Standard tribological tests are required for the fric-tion material performance testing. However, the need for the reduced scale testing is justified for the development of new formulations with the optimiza-tion of manufacturing process and materials com-position in a sustainable development approach. – Different COFs (m, mmind, mmax, mmin) are defined
and used by researchers to characterize the friction behavior not only in the overall braking system but also in relation to each specific braking condition. The variability of brake situation induces a large range of parameters that condition the braking per-formance necessities of some COF to describe local problem (heating interface exchange, localization phenomenon evolution from the first to the last cycle of braking) and other the global system (taking in to consideration the dynamical aspect of the braking system and the synergy between dif-ferent active phenomenon like tribology-brake squeal-dynamic triplet).
– Not all full-scale tribo-testing rigs i.e. Dynamometer, Chase, Krauss, and FAST tribo-testing, have the same potential of performance qualification (for evaluating COF–temperature sensitivity). However, significant evidence about the friction loss at high temperature (fading effect) could be established, which cannot be observed in more global tests, such as those per-formed on vehicle tests. Therefore, they should be carefully designed to assure more realistic brake friction performance evaluation.
– Additional measurements carried out using experi-mental apparatuses are required to extend and val-idate pin-on-disc indicators of the tribological behavior of friction materials.
Acknowledgements
The authors thank the Tunisian Ministry of Higher Education and Scientific Research for their continuous sup-port of research at the Laboratory of Electro-Mechanical Figure 12. Evolution of pin (black) and disc (grey) friction temperatures: (a) C1 (18 min), (b) C2 (9 min).
Systems of Sfax. The authors thank STUGA Frem Society (Tunisia) for providing them with the brake lining samples and GeM Laboratory (France) and Selcuk University (Turkey) for their scientific cooperation.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.
References
1. Findik F. Latest progress on tribological properties of industrial materials. Mater Des 2014; 57: 218–244. 2. Liew KW and Nirmal U. Frictional performance
evalu-ation of newly designed brake pad materials. Mater Des 2013; 48: 25–33.
3. EL-Tayeb NSM and Liew KW. On the dry and wet sliding performance of potentially new frictional brake pad materials for automotive industry. Wear 2009; 266: 275–287.
4. Jang H and Jin Kim S. Brake friction materials. In: Polymer Tribology2009; 1: 506–532.
5. Kchaou M, Mat Lazim AR, Abdul Hamid MK, et al. Experimental studies of friction-induced brake squeal: Influence of environmental sand particles in the inter-face brake pad-disc. Tribol Int 2017; 110: 307–317. 6. Bijwe J. Multifunctionality of nonasbestos organic
brake materials. In: Multifunctionality of polymer composites, challenges and new solutions 2015; 17: 551–572.
7. Hong C, Xun G, Yun-Feng H, et al. Automotive con-trol: The state of the art and perspective. Acta Autom Sin2013; 39: 322–346.
8. Song J. Performance evaluation of a hybrid electric brake system with a sliding mode controller. Mechatronics2005; 15: 339–358.
9. Agudelo CE and Ferro E. Technical overview of brake performance testing for Original Equipment and Aftermarket industries in the US and European mar-kets. Technical Report FEV205-01, 2005; pp.1–27. 10. Ertan R and Yavuz N. An experimental study on the
effects of manufacturing parameters on the tribological properties of brake lining materials. Wear 2010; 268: 1524–1532.
11. Saffar A, Shojaei A and Arjmand M. Theoretical and experimental analysis of the thermal, fade and wear characteristics of rubber-based composite friction materials. Wear 2010; 269: 145–151.
12. Sellami A, Kchaou M, Elleuch R, et al. Study of the interaction between microstructure, mechanical and tribo-performance of a commercial brake lining mater-ial. Mater Des 2014; 59: 84–93.
13. Yin Y, Bao J and Yang L. Frictional performance of semimetal brake lining for automobiles. Ind Lubr Tribol 2012; 64: 33–38.
14. Dante RC. Tribology of friction materials. In: Handbook of friction materials and their applications. Woodhead Publishing, 2016, p. 174.
15. Makni F, Cristol A, Kchaou M, et al. Heterogeneity of organic brake lining materials: impact of the mixing of ingredients. In: Europe’s braking technology conference & exhibition EURO BRAKE 2015, Dresden, Germany, 4–6 May 2015.
16. Cristol A, Desplanques Y, Baklouti M, et al. Formulation and process impact on friction material properties and performances. Transport Research Arena TRA2014, Paris, France, 14–17 April 2014. 17. Satapathy BK and Bijwe J. Performance of friction
materials based on variation in nature of organic fibres: Part I. Fade and recovery behavior. Wear 2004; 257: 573–584.
18. Anderson AE. Friction, lubrication and wear technol-ogy. ASM hand book. Materials Park, OH: ASM Materials Information Society, 1990, pp.569–577. 19. Yin Y, Bao J, Xiao X, et al. Objective characterization
on the dynamic friction coefficient of disc brake. Tribol Trans2016; 59: 1122–1133.
20. Arjmand M and Shojaei A. Tribological characteristics of rubber-based friction materials. Tribol Lett 2011; 41: 325–336.
21. Kermc M, Kalin M, Vizˇintin J, et al. A reduced-scale testing machine for tribological evaluation of brake materials. Tribol Interf Eng Ser 2015; 48: 799–806. 22. O¨sterle W, Do¨rfel I, Prietzel C, et al. A comprehensive
microscopic study of third body formation at the inter-face between a brake pad and brake disc during the final stage of a pin-on-disc test. Wear 2009; 267: 781–788.
23. Kumar M and Bijwe J. Studies on reduced scale trib-ometer to investigate the effects of metal additives on friction coefficient – Temperature sensitivity in brake materials. Wear 2010; 269: 838–846.
24. Shin MW, Cho KH, Lee WK, et al. Tribological characteristics of binder resins for brake friction mater-ials at elevated temperatures. Tribol Lett 2010; 38: 161–168.
25. Kchaou M, Sellami A, Elleuch R, et al. Friction char-acteristics of a brake friction material under different braking conditions. Mater Des 2013; 52: 533–540. 26. Ostermeyer GP. On the dynamics of friction coefficient.
Wear2003; 254: 852–858.
27. Wu H, Wang Y, Pindar D, et al. Interaction between ceramic matrix composite and organic pad materials and its impact on the friction performance. SAE Technical Paper 2011-01-2350, 2011.
28. Swarbric A and Wu H. Surface conditioning of carbon-fibre ceramic rotors against organic pads. SAE Technical Paper 2012-01-1833, 2012.
29. Ikeuchi K, Handa K, Lunde´n R, et al. Wheel tread profile evolution for combined block braking and wheel–rail contact: Results from dynamometer experi-ments. Wear 2016; 366–367: 310–315.
30. Sanders PG, Dalka TM and Basch RH. A reduced-scale brake dynamometer for friction characterization. Tribol Int2001; 34: 609–615.
31. Shin MW, Kim YH and Jang H. Effect of the abrasive size on the friction effectiveness and instability of brake friction materials: A case study with zircon. Tribol Lett 2014; 55: 371–379.
32. Bin Park S, Hyung Cho K, Jung S, et al. Tribological properties of brake friction materials with steel fibers. Metals Mater Int2009; 15: 27–32.
33. Gweon JH, Joo BS and Jang H. The effect of short glass fiber dispersion on the friction and vibration of brake friction materials. Wear 2016; 362–363: 61–67. 34. Jang GH, Cho KH, Park SB, et al. Tribological
proper-ties of C/C-SiC composites for brake discs. Metals Mater Int2010; 16: 61–66.
35. Ozturk B and Ozturk S. Effects of resin type and fiber length on the mechanical and tribological properties of brake friction materials. Tribol Lett 2011; 42: 339–350. 36. Asif M. Tribo-evaluation of aluminium based metal matrix composites used for automobile brake pad appli-cations. Plast Polym Technol 2012; 1.
37. Hyung Cho K, Hyung Cho M, Jin Kim S, et al. Tribological properties of potassium titanate in the brake friction material: Morphological effects. Tribol Lett2008; 32: 59–66.
38. Lee K, Hee Rhee T, Seong Kim H, et al. Effects of antimony trisulfide (Sb2S3) on sliding friction of auto-motive brake friction materials. Metals Mater Int 2013; 19: 1101–1107.
39. Hwan Kim S and Jang H. Friction and vibration of brake friction materials reinforced with chopped glass fibers. Tribol Lett 2013; 52: 341–349.
40. Teo KM and Lafdi K. Friction and wear of carbon-carbon composite, part 1: Friction transition and struc-tural changes of friction film. Carbon reinforcements and carbon/carbon composites1998; 2: 263–280.
41. Hee KW and Filip P. Performance of ceramic enhanced phenolic matrix brake lining materials for automotive brake linings. Wear 2005; 259: 1088–1096.
42. Muzathik AM, Mohd Nizam YB and Wan Nik WB. Effect of boron on friction and temperature character-istics of brake pad materials. Mater Sci 2013; 9: 481–487.
43. Cristol-Bulthe AL, Desplanques Y, Degallaix G, et al. Mechanical and chemical investigation of the tempera-ture influence on the tribological mechanisms occurring in OMC/cast iron friction contact. Wear 2008; 264: 815–825.
44. Desplanque Y, Roussette O, Degallaix G, et al. Analysis of tribological behaviour of pad–disc contact in railway braking: Part 1. Laboratory test develop-ment, compromises between actual and simulated tribo-logical triplets. Wear 2007; 262: 582–591.
45. Sanders PG, Dalka TM and Basch RH. A reduced-scale brake dynamometer for friction characterization. Tribol Int2001; 34: 609–615.
46. Neis PD, Ferreira NF and Lorini FJ. Contribution to perform high temperature tests (fading) on a labora-tory-scale tribometer. Wear 2011; 271: 2660–2664. 47. Zhu ZC, Shi ZY and Chen GA. Tribological behaviors
of asbestos-free brake shoes for hoist winder disc brakes. J Harbin Inst Technol 2008; 40: 462–5. 48. Shi ZY, Zhu ZC and Chen GA. Experimental study on
friction behaviors of brake shoes materials for hoist winder disc brakes. Lubr Eng 2006; 12: 99–102. 49. Blau PJ and McLaughlin JC. Effects of water films and
sliding speed on the frictional behavior of truck disc brake materials. Tribol Int 2003; 36: 709–715.
50. Bian G and Wu H. Friction performance of carbon/ silicon carbide ceramic composite brakes in ambient air and water spray environment. Tribol Int 2015; 92: 1–11.
51. Kyu Lee W and Jang H. Moisture effect on velocity dependence of sliding friction in brake friction mater-ials. Wear 2013; 306: 17–21.
52. Kyu Lee W, Wook Shin M, Hwan Kim S, et al. The influence of humidity on the sliding friction of brake friction material. Wear 2013; 302: 1397–1403.
53. Chandra Verma P, Ciudin R, Bonfanti A, et al. Role of the friction layer in the high-temperature pin-on-disc study of a brake material. Wear 2016; 346–347: 56–65. 54. Kasem H, Dufrenoy P, Desplanques Y, et al. On the use of calcium fluoride as an infrared-transparent first body for in situ temperature measurements in sliding contact. Tribol Lett2011; 42: 27–36.
55. Ferrer C, Pascual M, Busquets D, et al. Tribological study of Fe–Cu–Cr–graphite alloy and cast iron railway brake shoes by pin-on-disc technique. Wear 2010; 268: 784–789. 56. Wahlstrom J, Olander L and Olofsson U. A pin-on-disc study focusing on how different load levels affect the concentration and size distribution of airborne wear particles from the disc brake materials. Tribol Lett 2012; 46: 195–204.
57. Eriksson M, Lord J and Jacobson S. Wear and contact conditions of brake pads: Dynamical in situ studies of pad on glass. Wear 2001; 249: 272–278.
58. Maleque MA, Dyuti S and Rahman MM. Material selection method in design of automotive brake disc. In: Proceedings of the world congress on engineering 2010 Vol III WCE, London, UK, 2010, June 30 to July 2 2010.
59. Gunjal PN, Galhe DS and Mishra H. Selection of the disc brake material using pin on disc apparatus. Int J Innov Eng Res Technol2015; 2: 1–6.
60. Menapace C, Leonardi M, Perricone G, et al. Pin-on-disc study of brake friction materials with ball-milled nanostructured components. Mater Des 2017; 115: 287–298.
61. Kolluri D, Ghosh AK and Bijwe J. Analysis of load-speed sensitivity of friction composites based on various synthetic graphites. Wear 2009; 266: 266–274.
62. Idris UD, Aigbodion VS, Abubakar IJ, et al. Eco-friendly asbestos free brake-pad: Using banana peels. J King Saud Univ – Eng Sci2015; 27: 185–192. 63. Aigbodion VS, Akadike U, Hassan SB, et al.
Development of asbestos – free brake pad using bagasse. Tribol Ind 2010; 32: 45–50.
64. Aleksendric´ D and Senatore A. Optimization of manu-facturing process effects on brake friction material wear. J Compos Mater 2012; 46.
65. Hentati N, Cristol A-L, Najjar D, et al. Influence of hot molding parameters on tribological and wear properties of a friction material. Tribol Trans 2014; 57: 387–395. 66. Cristol A, Hentati N, Limodin N, et al. Impact of hot
molding on friction material microstructure. In: Europe’s braking technology conference & exhibition EURO BRAKE 2014, Lille, France, 13–15 May 2014.
