R. ERTAN: SYNERGISTIC EFFECT OF ORGANIC- AND CERAMIC-BASED INGREDIENTS ...
223–228
SYNERGISTIC EFFECT OF ORGANIC- AND CERAMIC-BASED
INGREDIENTS ON THE TRIBOLOGICAL CHARACTERISTICS OF
BRAKE FRICTION MATERIALS
SINERGISTI^EN VPLIV SESTAVIN Z ORGANSKO IN
KERAMI^NO OSNOVO NA TRIBOLO[KE ZNA^ILNOSTI
MATERIALOV ZA TORNE ZAVORE
Rukiye Ertan
Uludag University, Faculty of Engineering, Department of Automotive Engineering, Görükle, 16059, Bursa, Turkey
rukiye@uludag.edu.tr
Prejem rokopisa – received: 2014-09-11; sprejem za objavo – accepted for publication: 2015-04-08
doi:10.17222/mit.2014.225
In this study, the composition of a brake friction material was experimentally investigated with respect to the effects of the
proportions of organic (cashew dust) and ceramic (ZrSiO4 and Fe2O3) based ingredients on the tribological properties. The
tribological properties of the friction materials were evaluated using a Chase-type friction tester. The effect of the ingredient
proportions on the wear resistance and friction stability were obtained in relation to the test temperature and the number of
brakings. A scanning electron microscope was used to study the effect of braking on the sliding surface of the friction material.
Results showed that the complementary nature of the organic- and ceramic-based ingredients provided the optimum friction
behaviour, such as the coefficient of friction stability and the wear resistance.
Keywords: sliding friction, brakes/clutches, wear testing, electron microscopy
V {tudiji je bila eksperimentalno preiskovana sestava materiala za torne zavorne obloge, glede na razmerje organskih (prah
indijskih ore{~kov) in kerami~nih sestavin na osnovi ZrSiO4 in Fe2O3 ter na tribolo{ke lastnosti. Tribolo{ke lastnosti tornih
materialov so bile ocenjene z uporabo preizku{evalca trenja vrste Chase. Vpliv razmerja sestavin na odpornost proti obrabi in
stabilnost trenja je bil ugotovljen glede na temperaturo preizkusa in {tevilo zaviranj. Vrsti~ni elektronski mikroskop je bil
uporabljen za {tudij u~inka zaviranja na torni povr{ini tornega materiala. Rezultati so pokazali, da komplementarna narava
sestavin na organski in kerami~ni osnovi, zagotavlja optimalno obna{anje pri trenju kot sta koeficient stabilnega trenja in
odpornost na obrabo.
Klju~ne besede: drsno trenje, zavore/sklopke, preizku{anje obrabe, elektronska mikroskopija
1 INTRODUCTION
In a brake system, the energy input begins as a driver
presses the brake pedal, is then mechanically, hydro-
pneumatically, electrically or with a hybrid method
transmitted to the other components and ends at the
disc/drum brakes. This kinetic energy is mostly distri-
buted as the heat resulting from the friction between the
brake friction material and the disc/drum interface.1
Thus, the design and material characteristics of the
friction material, the disc/drum material where friction is
generated and energy transformation occurs are
important for the brake system. Especially brake friction
materials are crucial for the stopping distance and noise
propensity of a vehicle.2,3
A commercial friction material generally contains
more than ten ingredients including metallic-, organic-,
ceramic-, polymeric-based powders or fibre materials in
a thermoset polymeric matrix. The understanding of the
synergistic interaction between the ingredients has lar-
gely relied on hands-on experiences and systematic
studies of friction materials for the optimum brake per-
formance.4 In particular, hard ingredients used as abra-
sives in brake friction materials, with a relatively high
hardness control the level of the friction force and
remove pyrolyzed friction films at the sliding interface.5
The amount of abrasive is limited in vehicle brake pads
because it does a lot of damage to the disc.6 The abra-
sives used in commercial brake friction materials are
generally ceramic-based, in various sizes and forms of
oxides and silicates, such as zircon, alumina, quartz,
magnesia, etc. The organic ingredients of the friction
composites such as resin, cashew dust, aramid pulp, etc.,
are softer than the ceramic ones and responsible for the
fade (a decrease in the braking efficiency or coefficient
of friction with an increase in the average temperature of
the braking surface) which is an extremely undesirable
feature.7
From references list, it is seen that only a few studies
report about the synergistic effect of the organic- and
ceramic-based ingredients in friction materials and their
roles in the brake performance.8 On the other hand,
individual effects of organic- and ceramic-based ingre-
dients were extensively investigated.8–11 The purpose of
the present investigation is to investigate the organic- and
ceramic-based ingredients together, with regard to the
tribological characteristics of a brake friction material.
Materiali in tehnologije / Materials and technology 50 (2016) 2, 223–228 223
UDK 539.92:620.178.1:539.538 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(2)223(2016)
2 EXPERIMENTAL WORK
2.1 Material preparation
The brake friction materials used in this study are
non-asbestos organic (NAO) materials and the propor-
tions of the ingredients are given in Table 1, including
the average densities of the finished products. Friction-
material specimens were manufactured by mixing, hot
pressing, and sintering. The ingredients were weighed
and mixed in the given proportions in a plough shear
mixer for 10 min. Aramid fibbers were added initially,
followed by other pulpy materials and finally by
powdery materials. The manufacturing parameters were
chosen according to the study of Ertan and Yavuz.12 The
mix was moulded at 150 °C under a pressure of 7.5 MPa
for 5 min in a steel die. Heat treatment was carried out in
a mechanical convection oven at 165 °C for 12 h. The
total amount of ZrSiO4, Fe2O3 and cashew dust was not
changed and was set as 15 % of mass fractions for all the
specimens.
2.2 Friction testing and microstructure analysis
Friction and wear performances were conducted
using a Chase-type friction tester (Figure 1), according
to the national standard of the Society of Automotive
Engineers (SAE) J661, determining the friction coeffi-
cient, the friction force, the wear loss and the types of
wear. Gray cast iron with a 280-mm diameter and a hard-
ness of 210 HB was used as the counterpart. The applied
load was exerted on the specimen in the holder with a
closed-loop servo system and the maximum hydraulic
pressure was 540 N. The speed was held constant at
411 min–1 and controlled by a variable speed drive.
The test procedure consisted of a burnishing stop, the
fade and recovery tests. The test procedure began with
the baseline-I operation of 20 applications. This was
followed by the fade-I test at constant speed and load,
where the frictional force was recorded continuously at
28 °C intervals while the drum temperature rose to
289 °C. Then, the drum was cooled to 93 °C and the fric-
tional force was recorded continuously at 56 °C intervals
during the recovery-I test. This was followed by the
baseline-II, fade-II and recovery-II test, similar to the
first one, but with the temperatures going up to 345 °C.
The wear test, which consisted of 100 applications, was
conducted at the end of the testing.
The weight of the pads for each sample was
measured before and after the friction test, and the spe-
cific wear was determined with the mass method follow-
ing the standard of TSE 555 (1992)13 and calculated with
the following Equation (1):

 
= × ×
−1
2
1 1 2
R f n
m m
m
(1)
where  is the specific wear rate (cm3/N m), R is the
distance between the centre of the specimen and the
centre of the drum (m), n is the number of revolutions of
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224 Materiali in tehnologije / Materials and technology 50 (2016) 2, 223–228
Figure 1: Chase-type friction tester
Slika 1: Preizku{evalna naprava trenja, vrste Chase
Table 1: Ingredients of the friction materials investigated in this work (in mass fractions, w/%)
Tabela 1: Vsebnosti tornih materialov, preiskovanih v tem delu (v masnih dele`ih, w/%)
Ingredients A1 A2 A3 A12 A13 A23 A123
Ceramic-based abrasives ZrSiO4
Fe2O3
10
2
3
9
3
2
6.5
5.5
6.5
2
3
5.5
6
4
Organic friction modifiers Cashew dust 3 3 10 3 6.5 6.5 5
Reinforcements 25 25 25 25 25 25 25
Binders 10 10 10 10 10 10 10
Lubricants 20 20 20 20 20 20 20
Fillers 30 30 30 30 30 30 30
Density (g/cm3) 2.13 2.11 1.80 2.03 2.01 1.95 2.05
the rotating disk, m1 and m2 are the average weights of a
specimen before and after the test (g),  is the density of
the brake lining (g/cm3) and fm is the average friction
force (N). The densities of the specimens were deter-
mined with the Archimedean principle in water, and the
density calculations were repeated five times for each
specimen after the sintering.
The friction surfaces after the testing were analysed
using a scanning electron microscope (Fa. LEO 1455
VP). For all the observations, the samples were carefully
cut from an actual-size brake pad in order to avoid any
modification of the friction surface, with a sample size of
2 cm × 2 cm × 1 cm.
3 RESULTS AND DISCUSSION
Experimental observations were made to determine
the effects of the ceramic and organic constituents on the
changes in the COF related to the temperature, the ave-
rage COF, the braking number and the specific wear rate.
The friction test results for the A1, A2 and A3 specimens
are given in Figure 2, showing that the COF was gener-
ally decreased at elevated temperatures. For all the
specimens, it is seen that the COF continued to increase
until 150 °C. The explanation of this behaviour is that
the growth of hard particles in the brake material gene-
rated a large shear strength and the maximum COF at
150 °C.
After this temperature, the COF started to decrease.
This behaviour can be explained with the destruction of
the resin structure and the loss of the local binding
properties as well as the formation of the friction film on
the surface, called the fade. A compaction of the wear
debris generated at the friction interface accounts for the
formation of the friction film.14,15 After 250 °C the COF
exhibited a stable change, with the increasing tempera-
ture, in the A1 and A3 specimens, but the COF stability
of the A2 specimen containing a high proportion of
Fe2O3 was the lowest, especially at elevated tempera-
tures.
The abrasive effect of the Fe2O3 powders increased
the wear rate (Figure 2b) and the wear debris were built
at the friction interface and formed a friction film. This
friction film reduces the contact between the pad and the
disc. This film (with loosened debris, a stable friction
level, and low wear rates) can be maintained at various
temperatures, as long as it is not destroyed.16 These
results were confirmed with the microstructure analysis
given in Figure 3. The areas covered with a disconti-
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Materiali in tehnologije / Materials and technology 50 (2016) 2, 223–228 225
Figure 2: Friction test results for A1, A2 and A3 brake-pad materials: a) COF depending on the test temperature (°C), b) the average COF at
elevated temperatures and specific-wear rates
Slika 2: Rezultati preizkusov trenja A1, A2 in A3 materialov zavorne plo{~ice: a) COF v odvisnosti od temperature preizkusa (°C), b) povpre~je
COF pri povi{anih temperaturah in specifi~ne stopnje obrabe
Figure 3: SEM micrographs of the worn surfaces of the brake-pad
specimens after the friction test: a) A1-general, b) A1-detailed, c)
A2-general, d) A2-detailed, e) A3-general and f) A3-detailed
Slika 3: SEM-posnetki obrabljene povr{ine vzorcev zavornih plo{~ic
po preizkusu trenja: a) A1-splo{no, b) A1-podrobno, c) A2-splo{no, d)
A2-podrobno, e) A3-splo{no in f) A3-podrobno
nuous friction film on the friction surface are shown in
Figures 3a and 3b.
The brake pads containing more Fe2O3 powders than
ZrSiO4 and cashew dust show an unstable friction beha-
viour and a low fade resistance, and the areas covered
with the friction film on the friction surface are increased
(Figures 3c and 3d). The surface of brake pad A2 is very
rough and exhibits large, locally delaminated friction-
film regions. Several reasons may explain these differen-
ces, some of which are merely physical, such as the
particle-size distribution and the hardness.17
The average COF of A3 at elevated temperatures
(between the 165–345 °C) was very high (Figure 2b).
The abrasive proportion in this brake pad was minimum.
The SEM images indicate very thin and locally delami-
nated friction-film regions on the surface of the specimen
(Figures 3e and 3f) that do not reduce the contact bet-
ween the pad and the disc. As shown in Figure 2b,
sample A3 exhibited a high wear rate. The increase in
the wear rate of sample A3 is associated with the tem-
perature sensitivity of the organic-based cashew dust
(Figure 3e).
When both the ZrSiO4 and Fe2O3 amounts were
increased in a mixture (A12), the average COF at ele-
vated temperatures (between the 165–345 °C) exhibited
the lowest value (Figure 4b). The decrease in the COF
and a high wear rate (Figure 4), therefore, appeared due
to the low fade resistance and the large friction-film
regions at the sliding interface (Figures 5a and 5b). The
degradation of the organic particles and the increased
abrasive effect caused a decrease in the COF at elevated
temperatures. When the worn surfaces of A12 were exa-
mined, the areas covered with the friction film were
larger and thicker than in the cases of the other speci-
mens. The friction-film areas were locally delaminated
from each other. These areas decreased the average COF
and the stability of the friction material because a
friction film reduces the contact between the pad and the
disc.18 The abrasive effect of the ceramic particles caused
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226 Materiali in tehnologije / Materials and technology 50 (2016) 2, 223–228
Figure 4: Friction-test results for A12, A13, A23 and A123 brake-pad materials: a) COF depending on the test temperature (°C), b) the average
COF at elevated temperatures and specific wear rates
Slika 4: Rezultati preizkusov trenja A12, A13, A23 in A123 materialov zavornih plo{~ic: a) COF v odvisnosti od temperature preizkusa (°C), b)
povpre~ni COF pri povi{anih temperaturah in specifi~ne stopnje obrabe
Figure 5: SEM micrographs of the worn surfaces of the composites in
the Chase test machine: a) A12-general, b) A12-detailed, c) A13-ge-
neral, d) A13-detailed, e) A23-general, f) A23-detailed, g) A123-ge-
neral, h) A123-detailed
Slika 5: SEM-posnetki obrabljene povr{ine kompozitov v Chase pre-
izku{evalni napravi: a) A12-splo{no, b) A12-podrobno, c) A13-
splo{no, d) A13-podrobno, e) A23-splo{no, f) A23-podrobno, g)
A123-splo{no, h) A123-podrobno
an increase in the wear rate. Therefore, the maximum
wear rate was obtained for sample A12.
The brake pad with a high relative amount of ZrSiO4
and cashew dust (A13) has a stable COF at elevated
temperatures, but the values are not so high. This result
can be explained with the lubricating effect of the
cashew dust and the stable friction behaviour of ZrSiO4.
The friction-film formation is more homogeneous and
locally delaminated than in the case of A12 (Figures 5c
and 5d). However, the wear rate is lower than that of
A12.
When Figure 5e is examined, it is seen that the areas
covered with the friction film increased and the stability
decreased with the increasing amounts of Fe2O3 and
cashew dust. The abrasive effect of Fe2O3 decreased the
stability and eliminated the cashew-dust lubrication
effect. The COF exhibited an unsteady change, espe-
cially at elevated temperatures, as shown in Figure 4a. It
can be observed that large, locally delaminated friction-
film regions formed on the surface of sample A23. A low
wear rate with the increased amount of Fe2O3 and
cashew dust is associated with the strong and durable
friction film (Figures 5e and 5f).
Sample A123 exhibited the highest fade resistance
among the brake-pad samples due to its stable COF
changing with high values. When the worn surface of
this sample is examined, it can be seen that the friction
film was barely formed on the friction interface (Figure
5g). The synergistic effect of the ceramic- and organic-
based constituents is clearly seen from the wear rate
(Figure 4b). The wear rate for this sample is the lowest
among the tested samples. This optimum tribological
behavior of the A123 brake-pad material can be ex-
plained with a combination of the stable friction
behavior of ZrSiO4, the aggressive behavior of the high
COF of Fe2O3 and the lubricating effect of the cashew
dust.
The COF change related to the number of brakings
can be seen in Figure 6. From the literature review, it is
observed that the number of braking applications has the
strongest effect on the friction-interface temperature.19
Consequently, the COF change decreases with the
number of brakings. The stability of the COF is slightly
reduced after the 50th braking. It can be seen that A1,
A13 and A123 exhibit high and stable COFs, while A3,
A12 and A23 exhibit low and unstable COFs.
4 CONCLUSION
The friction characteristics of the brake pads
including organic (cashew dust) and ceramic (ZrSiO4 and
Fe2O3) based ingredients in different combinations were
examined. The results showed that the organic and
ceramic ingredients used as abrasive and friction modi-
fiers have strong synergistic effects providing the ave-
rage COF, the friction stability and the wear resistance.
The friction film on the brake-pad material was mainly
affected by the proportions of the ceramic-based ingre-
dients. Increasing the cashew-dust proportion increased
the average COF in the specimens. The wear resistance
of a brake pad was decreased with an increase in the
ceramic ingredients. On the other hand, an addition of a
certain amount of cashew dust greatly improved the wear
resistance. This complementary nature of the organic and
ceramic ingredients provided the optimum friction beha-
vior. In this study, the best friction behavior was obtained
in the brake-pad material with the proportions of the
ingredients close to the composition of specimen A123
(6 % ZrSiO4, 4 % Fe2O3 and 5 % cashew dust).
Acknowledgements
The author deeply appreciates the Frentek Automo-
tive and Brake Lining Industry Corporation, where the
brake-pad production and the experimental work were
carried out.
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228 Materiali in tehnologije / Materials and technology 50 (2016) 2, 223–228