Influence of Disc Topography on Generation of Brake Squeal

Ž .Wear 225–229 1999 621–628

Influence of disc topography on generation of brake squeal 1

Filip Bergman, Mikael Eriksson ), Staffan Jacobson˚Department of Materials Science, The Angstrom Laboratory, Uppsala UniÕersity, Box 534, S-751 21 Uppsala, Sweden¨

Abstract

The influence of the disc topography on the generation of automotive disc brake squeal has been studied. Two brake discs wereshot-blasted to produce small pits in the disc surface. The discs were then tested in a special brake squeal rig. During the tests, thecoefficient of friction increased from about 0.3 to 0.45 as pits were gradually reduced in size as the discs were worn. A removable sectionin one of the discs made it possible to record the size and location of the surface defects by SEM-investigations before, during and afterthe test. For the tested padrdisc combination, there were no brake squeals generated as long as the friction coefficient was below acritical level of 0.4. The use of shot-blasted discs thus provides a unique possibility to investigate the correlation between brake disctopography, friction coefficient and brake squeal generation without changing neither the composition nor the macroscopic geometry ofbrake pad or disc. q 1999 Published by Elsevier Science S.A. All rights reserved.

Keywords: Brake squeal; Coefficient of friction; Surface topography; Disc brake; Brake disc

1. Introduction

Traditionally, brake squeal in automotive brakes hasbeen studied as a phenomenon caused by the design of thebrake system and the macroscopic friction properties. Manydifferent mechanisms have been suggested as being re-sponsible for brake squeal generation. Most of the workhas concerned macroscopic frictional behaviour and com-monly a stick–slip motion or a friction coefficient decreas-

w xing with sliding velocity has been blamed 1–7 .However, lately it has been shown that a brake system

theoretically will generate squeal even with a stable fric-w xtion coefficient, if it exceeds a certain level 4,8 . The

question then turns over to why many commercial brake

) Corresponding author. E-mail: [email protected] Prime NoÕelty of the Paper: It is shown that small pits in the brake

disc surface lower the coefficient of friction. The friction coefficientgradually increases as the spits are worn away. Brake squeals areprevented as long as the coefficient of friction is below a critical value.This is a unique test for investigation of the correlation between disctopography, friction coefficient and brake squeal generation withoutchanging any other test parameters.

systems do not generate squeal although the friction coeffi-cient is above this critical level? The answer can probablybe found within the microscopic friction characteristics ofthe brake, an aspect not included in the theoretical modelsand also almost neglected in the literature on brake squeal.

In some technical areas, it is well known that roughsurfaces excite less noise than do smooth. Despite thisknowledge among practicioners, there is very limited in-formation in the literature on its physical background.

An earlier investigation has shown that the surface ofthe tested pads exhibit well-defined contact spots slidingagainst the disc. For an ordinary pad, it has been shownthat the number of contact spots is as high as in the order

5 w xof 10 9 . These spots are considerably harder than themean hardness of the pad and are composed mainly ofstructural fibres, abrasive particles and compacted weardebris. Obviously, the macroscopic friction force is thesum of the forces on the individual contact spots.

The objective of this paper is to investigate the effect ofa change in the microscopic friction conditions on thegeneration of brake squeal and the macroscopic coefficientof friction. The microscopic friction conditions were modi-fied by the introduction of shallow pits evenly distributedover the disc surface. The gradual wear of the pits wasrepeatedly investigated in the SEM and correlated to the

0043-1648r99r$ - see front matter q 1999 Published by Elsevier Science S.A. All rights reserved.Ž .PII: S0043-1648 99 00064-2

( )F. Bergman et al.rWear 225–229 1999 621–628622

gradual change of squeal generation and macroscopic fric-tion.

2. Test material and experimental

One set of brake pads and two standard cast iron brakediscs, both made for a front wheel of a Volvo 850, werefirst subjected to a run-in procedure of 42 brakings identi-cal to one sequence of the following test program. Bothdiscs were then shot-blasted with angular SiC-particlesŽ .f 1–2 mm causing about 100mm wide pits in the discsurface.

The pads selected are a slightly modified, noncommer-cial version of the production pads used in the Volvo 850.These pads are suitable for fundamental investigations ofbrake squeal as they generate squeal over a wide range ofbrake pressures and temperatures. The nominal composi-tion of the discs is given in Table 1 and the approximatecomposition of the pads are shown in Table 2. The exactcomposition of the pad material was not available from themanufacturer.

All tests were conducted in a special brake squeal rigw x10 . It is based on a Volvo 850 front-wheel suspensionwith an electric motor rotating the wheel via the originaldrive-shaft, see Fig. 1. The rotational speed is controlledseparately and is not influenced by friction forces as in anormal brake dynamometer. The brake-line pressure iscontrolled with a servo-valve and the brake torque ismeasured using a torque gauge mounted on the drive-shaft.

Two squeal tests were performed, one for each disc.Basically, both tests were based on a test sequence consist-ing of 42 brakings, see Fig. 2a. This sequence was re-peated 30 times for disc 1 in Test 1 and 14 times for disc 2in Test 2, making it a total of 1260 and 588 brakings,respectively. Each single braking lasted for 20 s duringwhich the rotational speed was reduced from 3 rps down to1 rps, see Fig. 2b. The brake-line pressure was heldconstant during each braking but was shifted from onebraking to another, following the sequence shown in Fig.2a. Between consecutive brakings, there was a 100 s idleperiod to avoid overheating of the brake components. Thistest sequence was designed to cover a wide range ofpressure–temperature combinations, including high pres-sures at low disc temperatures and vice versa.

During each single braking the brake pressure, braketorque, rotational speed and emitted sound were measuredonce every third second, see Fig. 2b. Sound was registered

Table 1Nominal compositions of the cast iron discs

Element C Si Mn Cr S P Ni

Wt.% 3.5 1.9 0.60 0.15 0.10 0.03 -0.05

Table 2Approximate composition of the pad

w xStructural component Ingredient Amount wt.%

Fibres Steel, aramid and glass fibres 30Ž .Matrix organic Binder 8

Other 11Ž .Friction modifiers Metallic brass, bronze 15

Graphite 15Metal sulphides 8

Ž .Quartz abrasive 5Filler Clay minerals, iron oxide 8

only when the sound level exceeded 84 dB at frequenciesbetween 500 and 20,000 Hz.

ŽTest 1 was interrupted five times after 4, 9, 16, 23 and.30 sequences to make silicon rubber replicas of the disc

surface. These replicas were later investigated using awhite light optical profilometer. Test 2 was interrupted

Ž .four times after 1, 5, 10 and 14 sequences to study theŽ .disc surface using a scanning electron microscope SEM .

Micrographs were made using both enhanced topographi-cal mode and enhanced atomic number contrast mode inorder to study the shape of the pits and how they werefilled up with wear debris. The replicas and the SEM-in-vestigations were made to monitor the running-in of theshot-blasted surface and to correlate this to squeal genera-tion.

In order to facilitate the repeated SEM-investigations ofthe disc surface in Test 2 the disc was equipped with aremovable section, see Fig. 3. This section could easily beremounted without noticeably affecting the sliding surfaceof the disc.

3. Results

The coefficient of friction and the accumulated numberof squeals for the full test programs are shown in Fig. 4.As a comparison, the squeal generation prior to the shot-blasting is shown. Fig. 4d shows the gradual increase inthe fraction of smooth area on the brake disc surface, thatis part of the area not occupied by the pits. The surfacearea increases as the pits are worn away, more rapidly atthe earlier stages of the test and considerably slower at theend. After 1260 brakings, the pits still cover approximately13% of the surface. Further, the surface profiles obtainedfrom the replicas revealed a maximum pit depth of 30–40mm after 378 brakings and roughly 20 mm after 966brakings. The average diameter to depth ratio was gener-ally estimated to 5, i.e., the pits were five times as wide asthey were deep.

The gradual reduction of the size of the blast pits duringTest 2, due to the wear of the disc surface is shown in Fig.5. During the initial period, the pits exhibited diameters

( )F. Bergman et al.rWear 225–229 1999 621–628 623

Ž . Ž .Fig. 1. a Schematic of the test-rig. An electric motor drives the wheel via the original drive-shaft. b Photograph of the rig. The torque gauge is mountedon the drive-shaft. The wheel that was mounted during the tests is removed in the photo to improve visibility.

Ž .Fig. 2. a Squeal test sequence of 42 brakings at predetermined brake-line pressures. This sequence was repeated 30 and 14 times, respectively, to formŽ .the two complete test programs. b Pressure and rotational speed during one braking. Squeal measurements are made every third second.

Fig. 3. The brake disc and its removable wedge used to facilitate SEM-investigation of the disc surface in Test 2.


Ž . Ž .Fig. 4. a The coefficient of friction and accumulated number of registered squeals in Test 1. b The coefficient of friction and accumulated number ofregistered squeals in Test 2. The small step in friction after 420 brakings is a measurement artefact due to drift of the torque gauge between test sequences

Ž . Ž .number ten and eleven and should be disregarded. Dashed vertical lines in a and b indicate where the tests were interrupted for replication or discŽ . Ž .surface analysis. c The run-in sequence prior to shot-blasting of the disc used in Test 1. d Area fraction of disc surface not covered by pits and

corresponding friction coefficient in Test 1.

from 10 up to 100 mm. Wear debris was collected andcompacted in some of the smaller pits, as shown in thelower row of micrographs in Fig. 5. EDX-analysis showedthat this compacted wear debris contained elements suchas Al and Cu and a high concentration of carbon, alltypical of the pad. As smaller pits were filled to a higherdegree than larger ones, the share of filled pits increased asthe disc was worn.

In both tests, the friction coefficient was initially lowŽ .0.3 but quickly increased to 0.4 during the first 300brakings. Even though the friction then started to stabilise,

it was still 25% below that of a nonprepared disc. As thetests continued, the discs became smoother with the fric-tion gradually increasing. When the average friction levelexceeded 0.4, squeal started to be generated more fre-quently. Fig. 6 shows that no squeals were generated at acoefficient of friction lower than 0.4. The same criticalcoefficient of friction was obtained in Test 2, see Fig. 4b.

Squeals were first initiated at one specific brake-linepressure but with further friction increase the system startedto generate squeals at other, neighbouring pressures as

Ž .well. In Test 1, the first 50 squeals up to 350 brakings


Fig. 5. Gradual change of the disc surface and corresponding friction and squeal generation curves during Test 2. The top row of SEM-micrographs showsthe topographical contrast and the bottom row the compositional contrast. Dark compositional contrast corresponds to wear debris from the pad that hasbecome compacted into the pits. The test was interrupted for SEM investigation after 1, 5, 10 and 14 sequences, corresponding to 42, 210, 420 and 588individual brakings, respectively.

were generated almost exclusively at a brake-line pressureof 8 bar, see Fig. 7. Between 350 and 450 brakings, squealwas mainly generated at 7 and 8 bar but also at 10 bar.After 450 brakings, squeal was generated also at 6, 9 and11 bar. This should be compared to the run-in sequenceprior to shot-blasting, where squeal was generated at alltested brake-line pressures except 3 and 4 bar.

4. Discussion

The shot-blasted brake discs have proven an excellenttool for studying the influence of macroscopic friction onbrake squeal generation. This technique allows the uniquepossibility to study a system where the friction coefficientgradually increases while the material combination and

other test parameters are kept constant. The results showthree important features.

Ž .1 No squeals are generated when the coefficient offriction is below a critical value.

The friction coefficient had to exceed 0.4 in both Test 1and Test 2 before squeals were generated. This corre-sponds well to the rule of thumb, practiced within thebrake industry, stating that brake pads with a high coeffi-cient of friction also are more prone to generate squeals.

In the literature, several reports have concerned theinfluence of macroscopic friction on squeal generation.Commonly a stick–slip motion or a friction coefficient

w xdecreasing with sliding velocity has been blamed 1–7 .Recently, a numerical model by Hulten suggests that disc´brake squeals can be generated also when the coefficient

w xof friction is velocity independent 8 . This model suggeststhat the vibration in the pad is a nonsynchronous wave


motion rather than a simple harmonic oscillation and thatŽ .no instabilities i.e. brake squeals occur when the coeffi-

cient of friction is below a certain value. The model islimited to two dimensions while optical measurementshave shown brake pad vibrations to be rather complicated

w xwith both bending and torsional movements 11,12 . A fulldescription would thus require 3D-modelling. However,within the limitations of Hulten’s model, our findings´basically support the assumption of a critical friction coef-ficient for brake squeal generation.

However, we have previously shown that brake padswith similar friction coefficient values can have very dif-

w xferent squeal propensities 13 . Thus, the value of thefriction coefficient threshold must depend on other param-eters such as pad and disc geometry, state of wear, stiff-ness, modulus of elasticity, damping and the frictionalproperties on both macroscopical and microscopical level.Accordingly, the present experimental threshold value of0.4 should be regarded as unique for this special combina-tion of pad and disc.

Ž .2 A very small friction increase may lead to a dra-matic increase in squeal generation.

The present investigation has clearly demonstrated thesharpness of the friction threshold for squeal generation.The increase in the average friction coefficient betweenbraking no. 200 and 500 was less than 0.03 while thegeneration of squeals increased dramatically.

At the threshold value, the system squealed only at onespecific brake-line pressure. As the friction increased,squeals were generated at a growing number of pressures,as shown in Fig. 7.

Ž .3 The shot-blasted brake disc has a lower coefficientof friction than a regular disc.

It is noteworthy that the uneven, shot-blasted discsexhibited lower friction coefficients than do normal smooth

Fig. 6. Friction level for the registered squeals during Test 1. Note that allsqueals were registered at a friction coefficient above 0.4.

Ž .Fig. 7. Brake-line pressures that generated squeals. a During the run-inŽ .sequence prior to shot-blasting of the disc used in Test 1. b During Test

Ž .1 shot-blasted disc .

discs. As the discs were gradually worn, the pits dimin-ished and the friction coefficient slowly approached thelevel of smooth discs.

Typically, the coefficient of friction also increases dur-ing each individual braking, see Fig. 8. This behaviourreflects that it takes some time for the pad and discsurfaces to adapt to the new contact situation, and that theadapted contact will exhibit a higher friction force.

The explanation to these two friction variation phenom-ena is to be found in the nature of the contact between discand pad.

An earlier investigation has shown that the surface ofthe tested pads exhibit well-defined contact spots sliding

w xagainst the disc 9 . Each spot exhibit a very inhomoge-neous structure with widely varying mechanical properties.

ŽThe spots are composed mainly of structural fibres typi-


Fig. 8. Example on the increase in the coefficient of friction duringŽindividual brakings. The examples are from brakings no. 279 and 280 in

.Test 1 .

. Ž .cally steel , abrasive particles typically alumina and com-Ž .pacted wear debris mainly polymeric . They typically

have a diameter of 100 mm, a height of 1 to 10 mm andare separated by a few hundred microns, see Fig. 9. Thetotal number of spots on a pad is around 105 and hence

Žcontact pressures are typically very low e.g., 4 MPa at a.brake-line pressure of 10 bar .

The spot size is not constant, but can grow by theaddition of more compacted debris, etc. The ‘steady state’

Žsize depends on the contact situation brake pressure,.temperature, surface topography, etc. and the pad compo-

sition. The main mechanism of friction is believed to beshearing of the top layer of these contact spots in theirmotion relative to the disc.

Given this general description of the contact situation,the mechanism explaining the lower friction against the

shot-blasted disc surface with shallow pits must be either adecreased contact area or a reduced shear stress within thecontact area.

Is there a mechanism by which the presence of pitscould reduce the shear stress within the areas of realcontact? Probably, the pits could increase the amount ofwear debris or alter the debris morphology. This wouldchange the conditions in the sliding interface and possiblyreduce the friction.

However, simple tests of cleaning the disc surface fromwear debris and surface contaminants by rubbing with acloth soaked in acetone or alcohol did not result in anynoticeable friction increase. This indicates that any differ-ence in amount of wear debris in the contact area haslimited effect, or only a short-lived effect.

w xAs reported by the present authors 14 , decreasing thenominal brake pad area down to 50% only has marginaleffect on the friction coefficient. Thus, the brakes behaveas expected for materials following the classical frictionlaws. This indicates that the presently achieved frictionreduction is not due to the pits causing a reduced nominalcontact area.

Based on this discussion, both the low initial friction ineach individual braking and the low friction against theshot-blasted surface, in general, are proposed to be due toa reduced real contact area.

It is believed that for each new braking the pad surfacehas to adapt geometrically to the new position on the disc.This geometrical adaptation will take some time and re-sults in a gradual increase of the real contact area andsubsequently an increased friction force.

The low friction against the shot-blasted surface, ingeneral, is proposed to be due to the frequent encountersbetween the contact spots and the pits in the disc surface.These encounters damage the contact spots thus preventingthem from growing to their normal ‘steady state’ size. Bythis mechanism, the real contact area is kept smaller than

Ž . Ž .Fig. 9. a SEM-micrograph of the disc surface taken after 420 brakings in Test 2. b SEM-micrograph of a pad surface. The contact spots, the flat islandsin the picture, are mainly constituted by metal fibres, abrasive particles and compacted wear debris.


against a smooth disc and the friction force thereby stayslower.

5. Conclusions

The run-in of a shot-blasted disc has been shown to be aunique test where the effects of a slowly increasing frictioncoefficient on squeal generation can be studied for onespecific disc–pad combination while other test parametersare kept constant.

Brake squeal will not be generated if the coefficient offriction is kept below some critical level. For the testedpadrdisc combination, this level was found to be 0.4.

Also a small increase in the coefficient of friction in theclose vicinity of the critical level causes a dramatic in-crease in squeal generation, indicating some thresholdbehaviour of the excitation mechanism.

The small impressions caused by shot-blasting of thedisc lowers the macroscopic friction coefficient and canthus help to prevent brake squeals. The friction reductionis proposed to be due to a reduction of the real contactarea. The frequent encounters with the pits in the discsurface damage the contact spots, preventing them fromgrowing to their normal ‘steady state’ size. By this mecha-nism, the real contact area is kept smaller than against asmooth disc and the friction force thereby stays lower.

When the pits are worn down to become small andshallow, material originating from the brake pads is com-pacted in the pits making the surface virtually flat. Thissurface still reduces the friction by 25% as compared to aconventional, smooth disc.

The squeal reducing effect from shot-blasting is limitedin time due to the wear of the disc surface.

Acknowledgements

The authors gratefully acknowledge the financial sup-port from the Swedish board for Technical DevelopmentŽ .NUTEK , Volvo Technological Development for provid-

ing test materials and Claes Kuylenstierna for valuablediscussions.

References

w x1 S.K. Rhee, P.H.S. Tsang, Y.S. Wang, Friction-induced noise andŽ .vibration of disc brakes, Wear 133 1989 39–45.

w x2 M.R. North, Disc brake squeal, in: Braking of Road Vehicles,Loughborough, United Kingdom, 1976.

w x3 S.W.E. Earles, M.N.M. Badi, Oscillatory instabilities generated in adouble-pin and disc undamped system: a mechanism of disc-brake

Ž .squeal, Proc. of the Institution of Mechanical Engineers 198 4Ž .1984 43–50.

w x4 J.O. Hulten, Friction phenomena related to drum brake squeal´instabilities, in: ASME Design Engineering Technical Conferences,ASME, Sacramento, CA, 1997.

w x5 T. Borchert, Dynamical behaviours of the disc brake pad, SAETechnical Papers Series 912656, 1991.

w x6 R.A. Ibrahim, Friction-induced vibration, chatter, squeal, and chaos:Part II. Dynamics and modeling, friction induced vibration, chatter,squeal and chaos, American Society of Mechanical Engineers, De-

Ž .sign Engineering Division 49 1992 123–138.w x Ž .7 N. Millner, An analysis of disc brake squeal, SAE Trans. 87 1978 .w x8 J. Hulten, Drum brake squeal—a self-exciting mechanism with´

constant friction, in: SAE Truck and Bus Meeting, SAE, Detroit,MI, USA, 1993.

w x9 M. Eriksson, F. Bergman, S. Jacobson, Surface characterisation ofbrake pads after running under silent and squealing conditions, in:

Ž .S.S. Eskildsen Ed. , Proc. Nordtrib’98, Ebeltoft, Denmark, DTITribology Centre, Aarhus, Denmark, 1998, pp. 657–663.

w x10 F. Bergman, M. Eriksson, S. Jacobson, A software based measure-ment system for test and analysis of automotive brake squeal,Submitted to TriboTest, 1997.

w x11 A. Felske, G. Hoppe, H. Matthai, Oscillations in squealing disk¨brakes—analysis of vibration modes by holographic interferometry,SAE Technical Paper Series 780333, 1978.

w x12 J.D. Fieldhouse, T.P. Newcombe, Double pulsed holography used toŽ .investigate noisy brakes, Optics and Lasers in Engineering 25 1996

455–494.w x13 F. Bergman, L. Gudmand-Høyer, M. Eriksson, S. Jacobson, The

effect of Cu S, PbS, Sb S solid lubricants on the occurence of2 2 3

brake squeals for three automotive brake pad matrix types, in: S.S.Ž .Eskildsen Ed. , Proc. Nordtrib’98, Ebeltoft, Denmark, DTI Tribol-

ogy Centre, Aarhus, Denmark, 1998, pp. 665–672.w x14 F. Bergman, M. Eriksson, S. Jacobson, The effect of reduced contact

area on the occurence of brake squeals for an automotive disc brakepad, To be submitted to Journal of Automobile Engineering.

Documents

Influence of Disc Topography on Generation of Brake Squeal