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8/16/2019 Characteristics and Morphology of Disk Brake Material
1/7
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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M. Mosleh et al. / Wear 256 (2004) 1128–1134 1131
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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1132 M. Mosleh et al. / Wear 256 (2004) 1128–1134
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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M. Mosleh et al. / Wear 256 (2004) 1128–1134 1133
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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1134 M. Mosleh et al. / Wear 256 (2004) 1128–1134
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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