Characteristics and Morphology of Disk Brake Material

8/16/2019 Characteristics and Morphology of Disk Brake Material

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Wear 256 (2004) 1128–1134

Characteristics and morphology of wear particles fromlaboratory testing of disk brake materials

Mohsen Mosleh a,∗, Peter J. Blau b, Delia Dumitrescu c

a Department of Mechanical Engineering, Architecture and Computer Sciences, College of Eng ineering, Howard University,

Rm 2036-C, 2300 6th Street NW, Washington, DC 20059, USAb Metals and Ceramic Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

c Department of Mechanical Engineering, University of M ichigan, Ann Arbor, MI 48109, USA

Received 19 December 2001; received in revised form 8 July 2003; accepted 8 July 2003

Abstract

The geometrical characteristics and morphology of wear particles generated from brake materials are important for environmental and

tribological reasons. Low- and high-speed, pin-on-disk friction and wear testing of a commercial truck brake pad material against cast iron

was conducted in which wear debris was collected. The sliding speed was held constant either at 0.275 or at 5 m/s, and the nominal contact

pressure was varied between 0.125 and 1.25 MPa in room temperature air. In low-speed experiments, some tests were conducted with the

pin in continuous sliding contact and others in which the pin specimen was raised and lowered periodically. Laser scattering examination of

wear debris revealed two distinct peaks in the plot of frequency versus the mean particle size. The first peak occurs around 350 nm and does

not vary with respect to the pressure and the sliding speed. The location of the second peak varies between 2 and 15 m, depending on the

pressure and the sliding speed. Energy dispersive X-ray (EDX) analysis of wear particles revealed particles having a high concentration of

carbon, silicon, aluminum, iron, oxygen, molybdenum, and sulfur. It was also found that the continuity or discontinuity of sliding contact

affects the size distribution of wear particles. In general, when the motion was discontinuous, as is the case in a repeated braking action,

smaller wear particles are generated.

© 2003 Elsevier B.V. All rights reserved.

Keywords: Wear particles; Disk brake materials; Debris size distribution; Wear particle morphology; Brake dust

1. Introduction

The friction and wear of vehicle brake materials have been

widely studied by numerous researchers for many decades

[1–5]. While gray cast iron has been the dominant material

for brake drums and disks, brake shoe and pad materials

are constantly evolving into more complex composites [6].

Today’s friction brake materials consist of abrasives, fric-

tion modifiers, fillers and reinforcements, and binder mate-

rials. The concentration of additives as well as their form,

distribution, and particle size greatly affects the tribological

performance of brake materials. As the braking materials

become more complex and contain more species, so are the

wear debris generated during sliding. These particles cause

third body action at the interface and may be a concern when

they are expelled into the environment.

It has been shown in laboratory experiments that wear

particles adversely affect friction and wear [7]. With respect

∗ Corresponding author. Tel.: +1-202-806-6622.

E-mail address: [email protected] (M. Mosleh).

to friction, wear debris entrapped at the sliding interface

agglomerates to produce larger particles. The larger ag-

glomerate intensifies the third body abrasion and plowing,

and causes the friction coefficient to increase. The increase

in the friction coefficient of most metals from a low ini-

tial value to a higher steady-state value was attributed to

increased plowing by wear particles and agglomeration.

If their hardness is similar to or greater than that of the

original wear surfaces, large wear particle agglomerates

also adversely affect the wear by causing higher abrasion

[8,9].

The examination of wear particles of tribological materi-

als can yield useful information with respect to wear mech-

anisms and the surface interaction phenomena. Often, larger

sheet-like wear debris is attributed to delamination and fa-

tigue wear, whereas smaller particles with smaller aspect

ratio are known as abrasive and/or adhesive ones [10]. The

morphology of wear debris and any transferred films cre-

ated during contact is also important, and reveals valuable

information on the nature of wear, effects of environment,

and test conditions.

0043-1648/$ – see front matter © 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.wear.2003.07.007


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M. Mosleh et al. / Wear 256 (2004) 1128–1134 1129

The objective of the work described here was to inves-

tigate the geometrical and morphological characteristics of

wear particles that were generated from a typical commer-

cial truck disk brake pad material sliding against gray cast

iron under controlled, laboratory conditions. The effects of

contact pressure, sliding speed, and continuous versus inter-

mittent sliding contact on debris were specifically addressed.

2. Experimental

2.1. Materials

Gray cast iron and commercial truck brake pads1 were

used as mating surfaces in wear testing. The microstructure

of the Type 40 gray cast iron is shown in Fig. 1(a). The aver-

age Vickers microindentation hardness of the matrix phase,

using a 200 gf load, was HV = 2.92GPa(S.D. = 0.29 GPa).

This compares favorably with the average Vickers microin-dentation hardness number of a polished section of a com-

mercial automotive brake disk tested at the same indenter

load (HV = 3.06 GPa, S.D. = 0.34 GPa).

The heterogeneous microstructure of the pad material is

shown in Fig. 1(b). Microindentation hardness tests were

also performed on selected constituents within the brake pad

material, but at a load of 10 gf to enable the indentation

to sample individual grains. HV values of these particles

ranged from 0.23 to 3.07 GPa, and as a result, no specific

hardness number can be given for the pad material.

2.2. Procedure

The wear tests were conducted using a pin-on-disk config-

uration in which the disk was rotating in a horizontal plane.

The pin samples (made of pad materials) had a square cross

section whose side was varied to obtain different nominal

contact pressures. The cast iron disks were 3.81 cm in diame-

ter and the diameter of the wear track was 2.54 cm. The slid-

ing speed was 0.275 and 5 m/s for low- and high-speed tests,

respectively. The initial arithmetic average surface rough-

ness ( Ra) of the disks was 0.125m, as obtained with a

Talysurf 10TM profiling instrument with a 2.5 m tip ra-

dius. The nominal contact pressures were 0.125, 0.375, and

0.625MPa for low-speed tests and 0.75, 1.0, and 1.25 MPafor high-speed ones. The total sliding distance, i.e. 8 km,

was kept the same for all wear tests. Relative humidity in

the air was measured by the wet bulb/dry bulb method and

varied between 70 and 77% RH. The ambient air temper-

ature was 20–24 ◦C. Aluminum foil was used to make a

cup around the disk to collect wear particles. Wear of the

pins was calculated by weight measurement prior and af-

ter the tests. A comparison of the total wear mass from

1 Jurid 539 friction material (semi-metallic), manufactured by Knorr-

Bremse.

Fig. 1. Microstructure of the (a) gray cast iron disk specimen and (b) pad

material used in wear tests.

a disk and a pin with the weight of collected wear de-

bris in the aluminum cup revealed that about 90% of the

wear particles generated during the tests were trapped in

the cup.

The pin-on-disk tester was also equipped with a rotating

cam mechanism that could lift the pin from contact with the

disk with a desired frequency while the disk was rotating.

Therefore, in low-speed sliding, besides the continuous con-

tact wear testing, discontinuous contact wear tests were also

conducted. The contact and separation times were 5 and 3 s,

respectively. However, all high-speed tests were continuous.

The size distribution of wear particles was obtained using

an LA-700 laser scattering analyzer. The analyzer is capa-

ble of detecting particle sizes in the range of 0.04–262m.

The main dispersant used during the size analysis was dis-

tilled water. However, two wetting agents namely Ammo-

nium Polyacrylate (Darvan) and Monawet, proved to be most

effective for detecting the smallest wear particles. In this

study, 0.1% of the former was used as the dispersant.


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1130 M. Mosleh et al. / Wear 256 (2004) 1128–1134

3. Results

A wear coefficient defined as the measured weight of the

pin wear divided by the pressure and the total sliding dis-

tance is determined and shown in Table 1. The table shows

only a small difference between the results for continuous

and discontinuous loading. Also, the sliding speed does not

0

1

2

3

4

5

6

7

8

0.01 0.1 1 10 100

Diameter ( m)

F r e q u e n c y %

0.125 MPa

0.375 MPa

0.625 MPa

0

1

2

3

4

5

6

7

8

0.01 0.1 1 10 100

Diameter ( m)

F r e q u e n c y %

0.6250.125 MPa

0.375 MPa

0

1

2

3

4

5

6

7

0.01 0.1 1 10 100

DIAMETER ( m)

F r e q u e n c y

%

1.0 MPa

1.25 MPa

0.75 MPa

(a)

(b)

(c)

Fig. 2. Size distribution of wear particles generated during the (a) low-speed, continuous tests (sliding speed = 0.275 m/s); (b) low-speed, discontinuous

tests (sliding speed = 0.275m/s) and (c) high-speed tests (sliding speed = 5 m/s).

Table 1

Wear coefficient (g/MPam) of the slider, defined as the measured wear

weight divided by the nominal contact pressure and the sliding distance

Low-speed

sliding

continuous

Low-speed

sliding dis-

continuous

High-speed

sliding

Wear coefficient 1.4 × 10−6 1.1 × 10−6 0.89 × 10−6


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Table 2

Summary of friction coefficients measured for a variety of pressures and

speeds

Pressure (MPa) Low-speed

sliding

continuous

Low-speed

sliding

discontinuous

High-speed

sliding

0.125 0.55 0.5

0.375 0.5 0.425

0.625 0.475 0.55

0.75 0.475

1.0 0.375

1.25 0.35

significantly affect the wear rate. Table 2 summarizes the

friction coefficients obtained during the wear tests. The fric-

tion coefficient is between 0.4–0.6 and 0.35–0.5 for the low-

and high-speed tests, respectively.

The size distribution of wear particles in the continuous,

low-speed tests is shown in Fig. 2(a). There are two peaks

in the frequency plots. The first peak occurs around 350 nmfor all nominal contact pressures. The particle size at which

the second peak in frequency occurs becomes bigger as the

pressure increases. The particle size for the second peak is

2, 7, and 15m for 0.125, 0.375, and 0.625MPa pressure,

respectively. The use of discontinuous contact conditions did

not affect the size of sub-micron wear particles, as shown

in Fig. 2(b). However, it tended to reduce the particle size

at which the second frequency peak occurs. Fig. 2(c) shows

the size distribution of wear debris for high-speed tests. As

in previous tests, the first peak occurs around 350 nm. The

location of the second peak is 3, 5, 6 m for 0.75, 1.0, and

1.25 MPa pressure, respectively.Scanning electron microscopy (SEM) of wear particles

collected from continuous tests showed large particle ag-

glomerates. These agglomerates consist of sub-micron and

micron-sized particles in a variety of shapes. Fig. 3(a) and

(b) are SEM micrographs of wear particles agglomerates

generated during low- and high-speed tests. Sub-micron to

few-micron sized particles with high concentration of iron,

silicon, carbon, aluminum, oxygen, and molybdenum were

found in the energy dispersive X-ray (EDX) analysis of par-

ticles. Fig. 4(a) shows the EDX spectrum of a 5 m× 5m

area in the center of Fig. 3(a). These wear particles are

formed from the brake material as shown by their high con-

centration of silicon, aluminum, and molybdenum. The spec-

trum of a 5m × 5m area in the lower right corner of

Fig. 3(b) is shown in Fig. 4(b). The spectrum suggests the

existence of antimony in the wear particles. It cannot be de-

termined whether the detected carbon came from the pad

material or from the graphite flakes in the cast iron disk.

Larger particles of both cast iron and brake pad material

were also identified in the EDX analysis. When individual

sub-micron wear particles were examined, a high concen-

tration of iron, oxygen, and carbon was observed. The wear

track on the cast iron disks was smoother than the original

machined surfaces. This is due to the smoothing of the harder

Fig. 3. Wear particles of (a) low-speed continuous tests (contact

pressure = 0.125MPa, sliding speed = 0.275m/s) and (b) high-speed

tests that (contact pressure = 0.75 MPa, sliding speed = 5 m/s) were ex-

amined by EDX analysis.

surface by abrasive particles in the softer brake pad material

and the possible filling of machining grooves with debris.However, numerous craters were formed on the smoothened

wear track as shown in Fig. 5(a) and (b).

4. Discussion

The authors recognize that the friction and wear behav-

ior of brake materials, as they relate to vehicle performance,

are more quantitatively measured using a full-scale inertial

brake dynamometer; however, the intent of the present work

was not to produce an exact simulation. Rather it was our


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Fig. 4. EDX analysis of a 5m × 5m area in the (a) center of Fig. 3(a) and (b) lower right corner of Fig. 3(b).


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Fig. 5. (a) Wear track on a cast iron disk tested under 0.625 MPa contact

pressure. The smoothening of the track is apparent while some craters

are also created by delamination wear. A rounded agglomerate of debris

particles rests on the surface. (b) Close-up of a crater created on the wear

track due to the delamination wear. The resultant delaminated sheet may

have been crushed into smaller particles when it became entrapped at the

interface.

intent to investigate, under controlled laboratory-scale con-

ditions, certain effects of speed and intermittent loading on

debris particles generated during the sliding of typical brake

materials. Despite the differences from the full-scale hard-

ware tests, the experimental results presented in this paper

can still provide insights into wear phenomena of friction

brakes for the following reasons: first, the flat-on-flat con-

tact geometry of our pin-on-disk tests was similar to the

flat-on-flat geometry of a disk brake. Second, while the slid-

ing speeds encountered in commercial truck disk brakes can

exceed 11 m/s (assuming a maximum operating truck speed

of 112 km/h (70 mph), a wheel diameter of 1.25 m, and a

disk brake diameter of 0.43 m), the speeds used in this studycovered the low to mid-range of typical operation. Third, the

nominal contact pressure in this investigation is within the

low-end of the range encountered in real applications, i.e.

0–10 MPa. Finally, the ability to isolate and capture most of

the ‘fresh debris’ produced during sliding offered us a better

opportunity for studying debris agglomeration and size dis-

tributions without a major portion of the debris being lost to

the surroundings, as it would in a dynamometer or road-test.

Two modes of contact, i.e. continuous and discontinuous,

were used during the low-speed wear tests. The latter mim-

ics intermittent braking action and experiences a different

process of particle agglomeration. In previous studies [8], it

has been shown that the entrapped wear particles agglomer-ate at the interface, which separates the sliding surfaces. The

deposit grows larger until it reaches a critical size beyond

which it collapses and new agglomerate is formed. The size

distribution of wear particles indicates that the discontinuous

contact resulted in smaller particles. This is because the dis-

continuity of contact interrupts the agglomeration process,

resulting in smaller wear debris.

Another factor that affected the size distribution of wear

particles is the normal pressure. The experimental results

suggest that as the normal load increases, the wear particles

become bigger. This may also be explained in terms of the

compacting pressure on the agglomerate. A higher contactpressure augments particulate bonding within the wear par-

ticle agglomerates.

While SEM and EDX provided valuable morphological

and compositional information about elements in the wear

debris, they did not give any information about specific com-

pounds in the debris. The sliding environment of brakes in-

volves significant heating and that can alter the chemistry

of the contact surfaces, the transfer films, and the ejected

debris. Therefore, the chemistry of brake wear particles is

likely to be considerably different than that of the start-

ing materials. That issue is a fruitful subject for further

studies.

Clearly, more information is needed on the characteris-

tics of friction brake wear particles as a function of operat-

ing conditions and starting composition, and that remains a

subject for further research.

5. Conclusions

Pin-on-disk tests were conducted on a pair of typical disk

brake materials to study the effects of speed, pressure, and

constancy of contact on wear particle formation and ag-

glomeration. Wear debris tends to follow a bi-modal size


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distribution. The first population of wear debris averages ap-

proximately 350 nm in diameter. EDX analysis revealed that

these sub-micron particles are generated from the cast iron

disk because they have a high concentration of iron, carbon,

and oxygen. The sliding speed, the normal load, and the con-

tinuity or discontinuity of the sliding motion does not affect

the size of these sub-micron particles. On the other hand,the second population of wear debris is larger than a 1 m

and its mean size does depend on the applied contact pres-

sure at both low and high sliding speeds. The existence of

aluminum, magnesium, antimony, silicon, sulfur, and cop-

per in these particles suggests that the larger particles are

generated from the brake pad material.

Acknowledgements

The authors wish to thank Ronald D. Ott for his help

in performing wear tests, Harry M. Meyer for the Auger

analysis, Dixie L. Barker for help with the laser scattering

system to obtain size distributions of particles, and Shirley

B. Waters for her help in EDX analysis. This research

sponsored by the US Department of Energy, Assistant Sec-

retary for Energy Efficiency and Renewable Energy, Office

of FreedomCAR and Vehicle Technologies, under contract

DE-AC05-00OR22725 with UT-Battelle LLC. Participation

by the first author was supported by an appointment to the

US Department of Energy (DOE) Higher Education Re-

search Experiences (HERE) for Faculty at the Oak RidgeNational Laboratory (ORNL) administered by the Oak Ridge

Institute for Science and Education.

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[4] Y. Handa, T. Kato, Tribol. Trans. 39 (2) (1996) 346–353.

[5] T. Kato, H. Soutome, Tribol. Trans. 44 (1) (2001) 137–141.

[6] P.J. Blau, Oak Ridge National Laboratory Technical Report

ORNL/TM-2001/64, 2001, 24 pp.

[7] N.P. Suh, N. Saka, Ann. CIRP 36 (1987) 403–408.[8] S.T. Oktay, N.P. Suh, J. Tribol. 114 (2) (1992) 379–393.

[9] M. Mosleh, N. Saka, N.P. Suh, Wear 252 (1–2) (2002) 1–8.

[10] M. Mosleh, N.P. Suh, Tribol. Trans. 39 (4) (1996) 843–848.

Characteristics and Morphology of Disk Brake Material

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