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Ý. SUGÖZÜ et al.: FRICTION AND WEAR BEHAVIOUR OF ULEXITE AND CASHEW IN AUTOMOTIVE BRAKE PADS
751–758
FRICTION AND WEAR BEHAVIOUR OF ULEXITE AND CASHEW
IN AUTOMOTIVE BRAKE PADS
ODPORNOST PROTI TRENJU IN OBRABI AVTOMOBILSKIH
ZAVORNIH OBLOG Z ULEKSITOM IN PRAHOM IZ INDIJSKEGA
OREHA
Ýlker Sugözü1,Ýbrahim Mutlu2, Ahmet Keskin3
1Mersin University, Tarsus Technology Faculty, Mersin, Turkey
2Afyon Kocatepe University, Technology Faculty, Afyonkarahisar, Turkey
3Abant Izzet Baysal University, Bolu Vocational School, Bolu, Turkey
ibrahimmutlu@aku.edu.tr
Prejem rokopisa – received: 2014-09-14; sprejem za objavo – accepted for publication: 2014-10-15
doi:10.17222/mit.2014.228
In the experimental studies, ulexite and cashew were investigated as new materials in brake pads. A newly formulated brake-pad
material with five different ingredients was produced using ulexite. Tribological properties of the friction materials were
obtained using the brake-test equipment. The friction and wear characteristics of the samples in contact with a disk made of cast
iron were studied. The change in the friction coefficient, the temperature of the friction surface and the amount of the wear were
examined to assess the performance of these samples. In addition, microstructural characterizations of the braking pads were
carried out using scanning electron microscopy (SEM). The results showed that the friction materials containing ulexite and
cashew have an important effect on the friction stability and fade resistance. The strategy proposed in this paper can be
considered for the alternative friction materials where ulexite and cashew can be used as friction materials in the brake pads.
Keywords: brake pad, composite materials, friction coefficient, ulexite, cashew
Preiskovan je bil vpliv uleksita in prahu iz indijskega oreha kot novega materiala v zavornih oblogah. Novo zasnovani material
za zavorne obloge iz petih sestavin je bil izdelan z uporabo uleksita. Tribolo{ke lastnosti tornega materiala so bile preizku{ene
na napravi za preizkus zavor. Preu~evano je bilo trenje in zna~ilnosti obrabe vzorcev v stiku z diskom iz sive litine. Za oceno
zmogljivosti vzorcev so bile preiskane spremembe koeficienta trenja, izmerjena temperatura na povr{ini stika in koli~ina obrabe.
Z vrsti~nim elektronskim mikroskopom (SEM) so bile dodatno preiskane mikrostrukturne zna~ilnosti zavornih oblog. Rezultati
so pokazali, da imajo torni materiali, ki vsebujejo uleksit in prah indijskega oreha, pomemben vpliv na stabilnost trenja in
odpornost proti odpovedi zavor pri vi{jih temperaturah. V tem ~lanku predlagana usmeritev se lahko upo{teva kot nadomestni
torni material za zavorne obloge na osnovi uleksita in prahu iz indijskega oreha.
Klju~ne besede: zavorne obloge, kompozitni material, koeficient trenja, uleksit, prah iz indijskega oreha
1 INTRODUCTION
The friction element of an automotive brake system
is one of the most important composite materials and
generally consists of more than ten ingredients. This is
because the friction materials have to provide a steady
friction force, a reliable strength, and a good wear resis-
tance in a broad range of braking circumstances.1
Recently, some studies have shown that the most fatal
accidents on the roads happen because of failed brake
systems.2,3 The performance of a brake system in a
vehicle is mainly determined by the tribological charac-
teristics of the friction couple, composed of a gray-iron
disk (or drum) and friction materials.1,4
The most important property of the friction materials
is a high friction coefficient so that they remain stable
under high forces and, especially, at high temperatures.2,3
The function of the brakes is to change the kinetic
energy into the heat energy by absorbing it and releasing
it into the atmosphere. If the generated heat exceeds the
capacity of a brake, there is a decrease in the friction
coefficient of the brake pads. When brake pads are ex-
posed to high temperatures for long time, they are
damaged. This damage results in a decrease in the brake
performance, a high brake-pad wear or noise.3,5 A great
deal of effort has been made to develop friction materials
with optimized tribological characteristics regardless of
the type of braking conditions.3,4
Currently, more than 800 raw materials cited in the
literature are used to produce friction materials for the
commercial brakes.6,7 Most automotive friction materials
contain a phenolic-resin binder with additions of mineral
fibers, fillers, friction-modifying compounds, abrasives
and metallic particles to modify the heat-flow characte-
ristics.8,9 In a simplistic sense, fibers are included for
their friction properties, heat resistance and thermal
conductivity. The physical and chemical properties of a
resin affect the wear process and friction characteristics
of the friction material. In addition, a straight phenolic
resin was used for brake friction materials and various
modified resins are available to improve the compressi-
bility, thermal stability, damping capacity and mecha-
nical strength.10 They play an important function of
Materiali in tehnologije / Materials and technology 49 (2015) 5, 751–758 751
UDK 539.92:531.43 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 49(5)751(2015)
toughening and strengthening the binder, which is quite
brittle in its pure form.11 Numerous compositions of
friction materials have been developed, largely by em-
pirical testing.8,9 Several projects were conducted to
investigate the friction materials that could improve the
braking effectiveness, while also contributing to the
energy efficiency.12 In the literature, there are a lot of
studies about using new materials for the brake pads in
order to increase the braking performance.13–18
Raw and refined borates are used in ceramic glazes;
thin coatings are applied onto ceramics. Borates are
consumed mostly in the glass and fiberglass industry.
Borates help the glass formation, reduce the thermal
expansion and give resistance and durability to the
ceramics. An addition of borates reduces the material
stresses caused by the temperature gradients, thus
making it more resistant to breaking. Borates are also
used in borosilicate glasses. Borosilicate glass is not only
highly resistant to a chemical attack, but it also has a
very low coefficient of thermal expansion and, as a
result, a high resistance to thermal shock. This thermal-
shock resistance exceeds that of the ordinary glass by a
factor of three.19
Ulexite is a mineral that combines calcium, sodium,
water molecules and boron in a complicated arrangement
with the following formula: NaCaB5O9·8H2O. It consists
of thin crystals that act like optical fibers. On the surface,
ulexite takes the shape of soft-looking masses and is
often called "cotton ball". This form also occurs beneath
the surface in veins similar to chrysotile, in which the
crystal fibers run across the width of the veins.20
Cashew friction dust is made of CNSL (cashew
nut-shell liquid). Due to its friction particles it is used as
a stability agent in brake products; it has a resilient
nature, acting as a cushioning agent of the engaging pro-
perty of the lining. Further, it is easily decomposed on
the surface of a brake lining at various elevated tempe-
ratures controlling the wear and acting as a protective
element by prohibiting the generation of excessive tem-
perature. It also easily absorbs the heat and disperses it
accross the whole area of the friction material. It is
mainly used as one of the prime raw materials for
heavy-automobile non-asbestos and asbestos brake
linings. It is also used for clutch facings and disc
pads.21,22
In order to test the performance of ulexite in auto-
motive friction materials, ulexite is sifted after grinding
to obtain dust as raw boron products. In the case of
ulexite and cashew being used together, when the
amount of ulexite is increased, the amount of cashew is
decreased. Five kinds of samples with different ingre-
dients were designed. The tribological properties of these
samples were determined using brake-test equipment.
The friction tests were performed up to 400 °C. The
change in the friction coefficient, the temperature of
friction surface and the amount of wear were examined
to identify the performance of these samples. In addition,
microstructural characterizations of the braking pads
were carried out using scanning electron microscopy
(SEM). The results showed that the friction materials
containing ulexite have an important effect on the
friction stability and fade resistance. Due to the heat and
abrasion resistance and the ability to reduce the thermal
expansion of ulexite, the use of this material in brake
pads can be attractive.
2 MATERIALS AND METHODS
In this study, a new automotive-brake friction mate-
rial was developed using an addition of ulexite. The
influence of ulexite on the brake’s friction characteristics
was especially examined. The friction materials investi-
gated in this study were variations of a NAO (non-asbes-
tos organic)-type material containing different ingre-
dients including ulexite. Ulexite was obtained from
Balikesir, the Bigadiç mine of Etibank in Turkey. The
composition of the ulexite studied in this work is shown
in Table 1.
Tab le 1 : Composition of ulexite studied in this work19
Tabela 1: Sestava uleksita, uporabljenega v tej {tudiji19
Element B2O3CaO Na2O SiO2
w/% 24–38 18–24 2–7 4–13
Five different samples were produced. These samples
contained ulexite, phenolic resin, steel fiber, copper,
aluminium oxide, graphite, brass particles, cashew and
barite. The friction-coefficient and temperature values
were stored in a databank. Friction coefficient/tempe-
rature/time graphs and the mean coefficients of friction
were obtained to identify the friction characteristics. An
analytical balance was used to weigh the ingredients.
The friction-material samples were produced in the
conventional procedure for a dry formulation following
dry mixing, pre-forming and hot pressing. These ingre-
dients were mixed for 10 min using a commercial
blender. The final mixture was loaded into a one-inch
square (small samples) mold for pre-forming under a
pressure of 9.8 MPa. The pre-formed samples were put
Ý. SUGÖZÜ et al.: FRICTION AND WEAR BEHAVIOUR OF ULEXITE AND CASHEW IN AUTOMOTIVE BRAKE PADS
752 Materiali in tehnologije / Materials and technology 49 (2015) 5, 751–758
Tab le 2 : Ingredients of the samples (w/%)
Tabela 2: Vsebnosti v vzorcih (w/%)
Sample code UC-4
UCH-4 UC-8
UCH-8 UC-12
UCH-12 UC-16
UCH-16 UC-20
UCH-20
Phenolic resin 22 22 22 22 22
Steel fibers 15 15 15 15 15
Al2O333333
Brass particles 55555
Graphite 33333
Barite 20 20 20 20 20
Cu particles 88888
Cashew 4 8 12 16 20
Ulexite 20 16 12 8 4
Total 100 100 100 100 100
into a hot-pressing mold and pressed at a pressure of
14.7 MPa and at 180 °C for 15 min. During the hot-
-pressing process, pressure was released several times to
release the gases that evolved from the cross-linking
reaction (polycondensation) of the phenolic resin.
Detailed conditions for each manufacturing step can be
found in the author’s other study.23 The compositions of
the friction materials studied in this work are shown in
Table 2.
In order to define the friction coefficients of the
automotive brake pad under different temperatures, a test
device was designed and manufactured. Figure 1 shows
a schematic view of the brake tester used in this study.
Using a real brake-disc-type tester, the friction-
coefficient characteristics of a pad next to the disc made
of cast iron were investigated by changing the pad. The
test sample was mounted on the hydraulic pressure and
pressed against the flat surface of the rotating disc.
Before performing the friction-coefficient tests, the
surfaces of the test samples and the cast iron discs were
ground with 320-grit sandpaper. The normal load was
varied to achieve a constant friction force. The braking
tests were carried out at a pressure of 1.05 MPa, a
velocity of 6 ms–1 and at temperatures from 50 °C to 400
°C for 500 s. An electrical heater was used in order to
achieve the friction-surface temperature of 400 °C. The
temperature and friction-coefficient values were stored in
the databank. The tests were repeated three times for
each sample.
Friction coefficient/temperature/time graphs were
obtained to identify the effects of these variables. The
friction coefficient of the surface-material couple needs
to be high and stable. The friction coefficient was cal-
culated by measuring the normal and tangential
pressures throughout the test 500 s. It was expressed as
the mean value of the entire braking dependence during
the friction-coefficient test. The specific wear rate was
determined with the mass method following the Turkish
Standard (TS 555) and British Standard (BS AU142) and
calculated with the following Equation (1):
V=(m1–m2)/(Lf
mr) (1)
where Vis the specific wear (mm3/MJ), m1is the mass
of the brake pad before the testing (kg), m2is the mass
of the brake pad after the testing (kg), Lis the friction
distance calculated using the number of revolutions and
the radius of the disc (m), fmis the average friction force
(N) and ris the density of the brake pad (kg/mm3).24,25
3 RESULTS AND DISCUSSION
3.1 Effect of the temperature on the friction perfor-
mance
In the present study, 10 samples were used. These
samples contained copper particles, phenolic resin,
Al2O3, steel fibers, brass particles, graphite, barite,
ulexite and cashew (Table 2). Only half of the samples
(with the UCH indices) were heat treated for4hata
temperature of 180 °C. The remaining 5 samples were
untreated (with the UC indices). These samples included
4–20 % ulexite and 20–4 % cashew, respectively, and the
mean friction coefficient ranged from 0.394 to 0.454.
When the coefficient of friction (μ) was studied it
varied significantly during the initial stage of the testing.
This can be attributed to the fact that the size of the
contact area increased and the friction layer was
developed on the surface. In order to determine the
variations in the friction coefficient and the temperature
of the friction surface with the testing time of the
samples, the tests were performed at applied tempera-
tures from 50 °C to 400 °C (Figures 2 to 6). As seen
from these figures, the friction coefficients show
different features depending on the content.
Generally, the friction-coefficient value gradually
increases for all the samples until the 100 s and then
gradually decreases after the 400 s (Figures 2 to 6). The
reason of the increasing friction coefficient is the contact
of the metallic material with the disc surface. The wear
of the ingredients of the metallic materials is tested.
Ý. SUGÖZÜ et al.: FRICTION AND WEAR BEHAVIOUR OF ULEXITE AND CASHEW IN AUTOMOTIVE BRAKE PADS
Materiali in tehnologije / Materials and technology 49 (2015) 5, 751–758 753
Figure 2: Change in the friction coefficient versus the temperature as
a function of time for samples UC-4 and UCH-4
Slika 2: Spreminjanje koeficienta trenja v odvisnosti od temperature
in ~asa pri vzorcih UC-4 in UCH-4
Figure 1: Schematic view of the brake tester
Slika 1: Shematski prikaz preizku{evalnika zavor
Therefore, these metallic materials detach from the
surface of the brake pad and the friction coefficient
decreases. This phenomenon continues until new friction
surfaces appear. A rapid increase in the coefficient of
friction may lead to a rapid increase in the temperature
of the friction surface.
It was found that the friction coefficient decreases
with the increasing testing temperature. Generally, the
friction coefficient decreased between 350 °C and 400
°C due to the softening of the phenolic resin (Figures 2
to 6). As a result, fading occurred during the braking
action. Furthermore, with the increasing temperatures,
the ingredients in the brake pads affected each other due
to a faster diffusion. This phenomenon is called the
thermal fade.4Therefore, one can say that the samples
remained unchanged only up to the temperature of 400
°C. There was an increase in μbefore the 200 s, followed
by a decrease just after the 350 s when μwas almost
constant for the UCH-4, UCH-16 and UCH-20 samples
(Figures 2, 5 and 6). This degradation is somewhat slow,
having slight fluctuations. By the 300 s, as a result of the
friction, a temperature of 300–350 °C was achieved on
the friction surface.
As seen from these figures, it is understood that all
the heat-treated samples (UCH) have higher friction-
coefficient values and more stable friction coefficients
than the others. The differences between the friction
coefficients of the UC samples and the UCH samples are
obvious especially at the beginning and after the 400 s.
The difference at the initial state shows a good fit of the
heat-treated samples to the surface. At the states after the
400 s the heat-treated samples have a lower temperature
then the others at this time (Figures 3 to 6). Thus, the
friction coefficients of the UC samples decreased due to
the increase in the temperature.26 It is observed that the
friction-coefficient and temperature values of the
non-heat-treated samples quickly change and fluctuate.
As a result, it can be said that heat treatment makes the
friction coefficients stable and increases the friction-
coefficient values.
Nonetheless, it should be noted that a good stability
of μis achieved using the samples under the working
condition considered. These results are consistent with
the behavior of the friction coefficients of all the sam-
ples. Therefore, if a μvalue of 0.39–0.45 is desired,
additions of both ulexite and cashew can be used in the
brake pads in the amounts of 4–20 %. Furthermore, if the
stability of μis desired and μhas higher values, the
UCH-20 sample is suggested as the best material for the
brake pads when compared to the others. Some middle
vibrations and noise were observed during the testing
with the friction assessment and screening test (FAST).
This vibration was typically observed at the beginning of
the test, before a stable friction layer was developed.
Ý. SUGÖZÜ et al.: FRICTION AND WEAR BEHAVIOUR OF ULEXITE AND CASHEW IN AUTOMOTIVE BRAKE PADS
754 Materiali in tehnologije / Materials and technology 49 (2015) 5, 751–758
Figure 6: Change in the friction coefficient versus the temperature as
a function of time for samples UC-20 and UCH-20
Slika 6: Spreminjanje koeficienta trenja v odvisnosti od temperature
in ~asa pri vzorcih UC-20 in UCH-20
Figure 5: Change in the friction coefficient versus the temperature as
a function of time for samples UC-16 and UCH-16
Slika 5: Spreminjanje koeficienta trenja v odvisnosti od temperature
in ~asa pri vzorcih UC-16 in UCH-16
Figure 4: Change in the friction coefficient versus the temperature as
a function of time for samples UC-12 and UCH-12
Slika 4: Spreminjanje koeficienta trenja v odvisnosti od temperature
in ~asa pri vzorcih UC-12 in UCH-12
Figure 3: Change in the friction coefficient versus the temperature as
a function of time for samples UC-8 and UCH-8
Slika 3: Spreminjanje koeficienta trenja v odvisnosti od temperature
in ~asa pri vzorcih UC-8 in UCH-8
3.2 Microstructural characterization of friction surfa-
ces
Apparently, friction layers are formed by the wear
particles generated during the friction. The chemistry
and structure of a friction layer depend on the bulk
materials (lining and disc), the testing conditions and the
environment. The role of the friction layer may vary
depending on its characteristics.27,28 The SEM micro-
graphs of the braking-pad surfaces after the braking test
are shown in Figures 7 and 8. The friction surfaces of
the samples were characterized using SEM (LEO 1430
VP). The sample surfaces for the SEM observations were
always coated with carbon. There are micro-voids on the
surfaces of almost all the samples. Micro-voids consist
of the metallic particles detached during the friction.
It is seen from Figures 7 and 8(UC-8 and UCH-4)
that larger micro-voids occurred in the samples due to
detached metallic particles. As seen in these figures,
some particles are detached from the body causing
micro-voids. The micro-voids on the surfaces of the
samples can be classified as smaller or bigger in size.
The bigger micro-voids were formed due to the pitting of
the metallic particles during the friction. The worn
metallic particles imply that they actively participated in
the friction during the braking test. It is known that if the
coherent surface of a metal component is bigger, the
friction and wear will be increased. In addition to
micro-voids, there are some micro-cracks on the surfa-
ces. It is also observed that Al2O3particles are distri-
buted homogeneously, therefore, contributing to the
effectiveness of the friction surfaces.
Several characteristic features can be observed on the
friction surfaces of the pads. White spots are seen on
Figure 7 (UC-8 and UC-20). Dark areas can also be seen
in Figures 7 and 8(UC-8 and UCH-4). A more loose
contact on the trailing edge facilitates the access of air
and an uneven wear related to a higher oxidation (a
burn-off) of the phenolic resin. This is due to the dis-
tribution of the friction force over a pad surface.28 When
the pads heat up during the braking, the resin tends to
expand at very high temperatures and turn into glassy
carbon. Carbonized resins weaken the matrix and acce-
lerate the pad wear (Figure 7, UC-8 and UC-16).28,29 The
glassy phase loses its support and is torn off from the
surface by the shear force.30 Figures 7 and 8relating to
UC-12, UC-16, UC-20, UCH-8 and UCH-12 (from the
SEM) show thick friction layers developed on the
surfaces of the pads.
In this particular case, the friction layer covering a
friction surface diminishes the abrasive effect of the
glassy phase by eliminating the sharp edge of the glass
and smoothing the friction surface. Hard glassy particles
typically act as an abrasive element and scratch off the
Ý. SUGÖZÜ et al.: FRICTION AND WEAR BEHAVIOUR OF ULEXITE AND CASHEW IN AUTOMOTIVE BRAKE PADS
Materiali in tehnologije / Materials and technology 49 (2015) 5, 751–758 755
Figure 8: SEM micrographs of brake-pad samples with the UCH code
Slika 8: SEM-posnetki vzorcev zavornih oblog z oznako UCH
Figure 7: SEM micrographs of brake-pad samples with the UC code
Slika 7: SEM-posnetki vzorcev zavornih oblog z oznako UC
cast-iron-disc counter face and the material adhering to
it.28 Apparently, the carbonaceous matrix was formed of
graphite, coke and degraded phenolic resin. The ingre-
dient with the plastic-deformation capability developed a
flake-like feature after the friction experiment (Figures 7
and 8; UC-16, UC-20, UCH-8 and UCH-12).
All matters were homogeneously distributed in the
matrix and, therefore, very few micro-voids were ob-
served in the structures (Figures 7 and 8; UC-4, UC-8,
UC-12, UCH-8 and UCH-20). The friction process is
characterized by the development of friction debris. Such
debris adheres to the friction surface and forms a friction
layer easily visible when examining a sample surface
after the testing (FAST) (Figures 7 and 8; UC-16,
UC-20, UCH-4 and UCH-8). A systematic analysis of
the surfaces of the composite materials indicated that the
friction process dominantly occurred on the friction
layer, which eventually covered the top of the bulk.
Well-developed friction layers on the friction surfaces as
well as their morphologies are easily visible. Detailed
views of the friction surfaces including the information
about the friction layers are shown in Figures 7 and 8.
Diffusion occurred in the Cu particles located in the
friction layers. Brighter areas marked as Cu in Figures 7
and 8(UC-4, UC-8, UC-16 and UCH-12) represent the
regions where Cu interacted with the friction layers.31
3.3 Wear behaviour
Table 3 gives the mean coefficients of friction, the
standard deviations of coefficients of friction, densities,
hardness values and the specific wear rates of the tested
samples. As can be seen from Table 3, the friction
coefficients are in the appropriate category according to
the Turkish Standard TS 555 and British Standard BS
AU142. Also, the standard deviations are very small,
which means that the materials have stable friction cha-
racteristics.
Table 3: Typical characteristics of the brake pads used in this study
Tabela 3: Zna~ilnosti zavornih oblog, uporabljenih v tej {tudiji
Sample
code
Mean
coeffi-
cient of
friction
Standard
deviation Density
(g cm–3)Hardness
(Brinell) Specific wear
(g mm–2)
UC–4 0.394 0.0182 1.935 18.2 0.26 × 10–6
UC–8 0.401 0.0172 1.842 16.2 0.27 × 10–6
UC–12 0.420 0.0192 1.765 15.9 0.34 × 10–6
UC–16 0.427 0.0216 1.704 14.7 0.37 × 10–6
UC–20 0.437 0.0223 1.680 13.9 0.38 × 10–6
UCH–4 0.410 0.0091 1.834 23.2 0.21 × 10–6
UCH–8 0.418 0.0127 1.782 20.3 0.22 × 10–6
UCH–12 0.430 0.0127 1.717 18.9 0.29 × 10–6
UCH–16 0.442 0.0140 1.652 18.2 0.33 × 10–6
UCH–20 0.454 0.0134 1.588 15.3 0.35 × 10–6
In the present study, no direct proportionality was
found among the density, the hardness and the wear
resistance due to the complexity of the composite
structures. However, the highest friction coefficient was
obtained for the UCH-20 sample. This sample includes
4%ofulexiteand20%ofcashewanditwasheat
treated. The lowest friction coefficient was obtained for
the UC-4 sample. This sample includes 20 % of ulexite
and4%ofcashew and it was heat untreated. In addition,
less wear was observed on the heat-treated samples,
which included 4–20 % of ulexite and 20–4 % of
cashew, and their mean values of the friction coefficients
ranged from 0.41–0.45 (Table 3). Besides, the minimum
wear was obtained for the UC-4 and UCH-4 samples,
including the heat-treated and untreated ones. It is seen
from Table 3 that the heat-treated samples generally
have a better wear resistance than the untreated samples.
It is also seen from Table 3 that heat treatment
facilitated a more homogeneous braking-pad structure.
In addition, a more stabilised friction coefficient was
obtained due to heat treatment. It is assumed that some
detached materials left the samples and then structural
variations were obtained. Furthermore, with the in-
creasing temperature the ingredients in the braking pad
affected each other due to a faster diffusion. Therefore, it
can be said that heat treatment is essential for braking
pads. It is also known that the hardness of a sample in-
creases and the density decreases due to heat treatment,
while the specific wear ratio changes (Table 3). In the
present work, heat treatment facilitated a better perfor-
mance of all the samples compared with the untreated
ones. These results are consistent with the earlier
works.32–35
As can be seen from Table 3, the friction coefficients
achieved for samples UCH-20 and UCH-16 are approxi-
mately 0.45, which is considered to be very good when
compared to the coefficients of friction achieved with the
current brake pads. The UC-4 sample has the highest
density and hardness among the untreated samples. It
includes 20 % of ulexite and4%ofcashew; the specific
gravity of ulexite is bigger than that of cashew. A higher
amount of ulexite causes a higher density. But the heat-
treated UCH-4 sample with the same ingredients has the
highest hardness among the all samples.
It is well known that the friction coefficient is usually
associated with an increase in the wear. In the present
work, all the samples confirmed this assumption (Table
3). But the UCH-16 and UCH-20 samples have better
values than the other samples. Therefore, one can say
that only samples UCH-16 and UCH-20 are preferred for
this kind of brake pads as they provide better mechanical
and microstructural properties than the other samples.
It can be seen from the results that there is no direct
correlation between the wear resistance and the hardness.
However, this is an unavoidable reality of these features
that affect each other. As can be seen from the table, a
high-hardness pad has a high wear resistance and lower
wear due to the amounts of the constituent components
of the samples. The components made of the pad mate-
Ý. SUGÖZÜ et al.: FRICTION AND WEAR BEHAVIOUR OF ULEXITE AND CASHEW IN AUTOMOTIVE BRAKE PADS
756 Materiali in tehnologije / Materials and technology 49 (2015) 5, 751–758
rials comprised of compounds of different contents can
have a hard structure and, consequently, a higher hard-
ness. But the amount of the resin component included in
the materials is not sufficient as the particle adhesion
surface is decreased and there is a quick separation of the
particles that make up the main structure at low strains,
while the friction can cause a wear that is higher than
expected.
The heating process generally affects the microstruc-
ture of a brake pad and, accordingly, the hardness
increases. It can be seen from Table 3 that the friction
coefficient of the UC-4 sample is 0.394 and its wear rate
is 0.26 · 10–6 and, after the heat treatment, these values
are 0.410 and 0.21 · 10–6, respectively. The situation is
similar for the other samples. Thus, after the heat treat-
ment, the coefficients of friction of the samples of the
same ingredients increase and their wear rates decrease.
Heat treatment has a positive impact on both the friction
coefficient and specific wear.
The mean coefficient of friction, hardness, and wear
rates of the heat-treated (UCH) and untreated (UC)
samples showing parallelism to each other exhibit the
characteristics of the material. The sample with a high
mean coefficient of friction also has a high wear rate. In
this case, the rubbing of the ruptures of large particles
from the brake pad due to the strain has a significant
impact. Also, since the pad’s hardness is lower than the
hardness of the disk, the wear naturally increases during
the process of creating a high coefficient of friction.
After the heat treatment, the densities of the samples
decrease. During the heat treatment at a low evaporation
temperature, volatile materials move away from the body
and form micro-scale porosity in the body. This situation
causes a decrease in the density, and the heat produced
during the friction (rubbing) is removed from the body
by the micro-pores, leading to a lower friction-surface
temperature and a stable friction performance.36
4 CONCLUSIONS
In this study, the effect of ulexite and cashew
amounts on the friction and wear behavior of a brake pad
used in the automotive industry is experimentally ana-
lyzed. As a result of the experiments, the structure and
chemical composition of the friction layer generated on
the friction surface differ significantly from those of the
bulk. It is apparent that no simple relationship exists
between the composition of the friction layer and the
bulk-material formulation. Heat treatment facilitated a
more homogeneous structure and, hence, microstructural
variations were minimized during the braking action. On
the other hand, heat treatment increased the hardness of
the samples and also decreased the density.
The highest friction coefficient was obtained for the
heat-treated samples. Smaller values were obtained for
the untreated samples. The UCH-4 sample had a more
stable friction coefficient during the FAST than the other
samples. As the UCH-4 and UCH-8 samples had lower
friction coefficients, their wear ratios and standard devi-
ations were also considerably lower. Out of 10 samples,
UC-20, UCH-12, UCH-16 and UCH-20 exhibited better
friction and wear properties. Therefore, these samples
can be suggested as brake-pad materials.
In the present work, the standard deviation was
within the acceptable range for all the samples. No direct
proportionality among the mean coefficients of friction,
the standard deviation and the wear resistance was found
due to the complexity of the composite structures. Some
micro-voids and micro-cracks were observed on the
worn surfaces.
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