Frictions materials that have been used in automotive brake pads were
formulated almost a century ago. Although, in the early 1920s, asbestos
fiber was chosen as a friction material for use in all kinds of vehicles,
due to its harmful effects on human health, Non Asbestos Organic (NAO)
materials have become main stream nowadays.
A characteristic friction material is a multicomponent polymer matrix composite
with a formulation, which is often developed empirically (Filip
et al., 2002). The high-energy conditions and complex mechano-chemical
interactions on the friction surface during braking make it difficult to predict
the chemistry and particle size of newly formed species. Even if the raw materials
selected for manufacturing a brake lining are in conformation with the environmental
requirements, it is possible that newly formed wear particles will have different
chemistry and structure. In contrast to other tribological applications, a relatively
high friction coefficient in the range of 0.3-0.7 is normally desirable when
using brake lining materials (Roubicek et al., 2008).
The friction coefficient should be moderately high, but most significantly must
be unchanging during of braking. It should have a stable level, independence
of temperature, humidity, age, degree of wear and corrosion, existence of dirt
and water spray from the road, etc. (Filip et al.,
Industrial pads usually contain a large number of different constituents like
ceramic particles and fibers, minerals, metallic chips, solid lubricants and
elastomers in a matrix material such as phenolic resin. Up to now, the development
of new friction materials has been done empirically, starting from well-known
base compositions which have been successively optimized by adding friction
modifiers (Osterle et al., 2001).
Agricultural products are emerging as new and inexpensive materials in the
friction material development with commercially viability and environmental
acceptability (Bledzki and Gassan, 1999). Among these
kinds of materials, lignocellulosic fillers are considered as attractive candidates
to be used as fillers of thermoplastic polymers (Cyras et
al., 2001). In this way, it is possible to obtain composite materials
with properties quite similar to the already known synthetic-filler reinforced
plastics with their superior properties such as low cost, low density, enhanced
energy recovery, biodegradability and recyclability. However, the high sensitivity
to moisture makes their use limited (Bledzki and Gassan,
1999; Garcia et al., 2007).
One of the agricultural residues which can be potentially used as fillers of
polymeric materials is the rice husk. Rice is the most important food crop grown
in the world and planted on an estimated 540 million tons every year. The rice
husks are grinded and burned at low temperature. After this process, white ash
is gained. The white ash consists of 80% silica. The rice straw on the other
hand consists of 30% cellulose, 20% hemicelluloses and lignin, about 10% water
and 15% mineral ash. The mineral ash consists mainly of silica (95%), insoluble
silicates of aluminum, iron, magnesium and calcium (Seki,
2006). Rice straw and rice husk are known to have low lignin and high silica
as shown in Table 1 (Van hoest, 2006).
A variety of epoxy-hardener systems with lignin (up to 20%, in some cases) have
been shown to improve the adhesive joint shear strength (Wang
et al., 1992).
||Lignin and silica amounts in rice straw and husk (Van
In this study, Rice Straw Dust (RSD) and Rice Husk Dust (RHD) are used
to obtain friction material for the brake pads, since, it includes high
proportion of silica. Effect of using friction materials obtained from
RSD and RHD on friction coefficient and abrasion resistant has been investigated.
Different amounts of RSD and RHD were mixed with other regular ingredients
in the brake pad. In the experimental studies, the change of friction
coefficient and the amount of wear were measured. In addition, micro-structural
characterizations of braking pads were looked at by using a Scanning Electron
Microscopy (SEM). The results revealed that RSD and RHD can in fact be
used for friction materials in the brake lining pad.
MATERIALS AND METHODS
In this study, a new automotive brake friction material was developed by using
RSD and RHD and their performance on brake friction characteristics was specifically
examined. Friction materials investigated in this study were Non Asbestos Organic
(NAO) type materials containing four different ingredients including RSD and
RHD. This study was carried out for 4 different mixtures of brake pads. The
ingredients in the friction material comprise binder resin, friction modifiers
and space filler. Friction material specimens were produced by a conventional
procedure for a dry formulation following dry-mixing, pre-forming and hot pressing.
Detailed conditions for each manufacturing step can be found in the researchers
other study (Mutlu et al., 2007). The composition
of the friction materials studied in this research is shown in Table
An analytical balance was used to weigh each of the ingredients which were
mixed for 10 min using a commercial blender. The final mixture was loaded into
a cylindrical (small samples) or a brake lining (brake lining samples) mold.
The mixtures in both mold types were hot pressed at 180°C for 15 min and subsequently
post cured. 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. Post-curing was carried out at a constant temperature
of 180°C by placing the samples in a preheated furnace (Fisher scientific Isotherm
Furnace) for 4 h (Kristkova et al., 2004).
Using the Friction Assessment and Screening Test (FAST) machine, friction
tests were performed for each material. For each sample, three friction
test procedures were applied and the average of these three tests was
recorded. For comparison purposes, FAST testing was also repeated with
samples obtained. The FAST machine uses a pearlitic gray cast iron disc
(diameter of 180 mm, thickness 38 mm) and a brake lining test sample with
dimensions of 12.7×12.7×5.00 mm. The test sample was mounted on the load
arm and pressed against the flat surface of the rotating disc. The rotating
cast iron disc moved with a constant sliding speed of v = 7 m sec-1
for 90 min and the temperature was increased from room temperature to
around 300°C. Before performing the FAST testing, the surfaces of
the test samples and the cast iron discs were ground with 320-grid sandpaper.
The normal load was varied to achieve a constant friction force. The friction
coefficient was calculated by measuring normal and tangential pressures
every 5 sec throughout the 90 min test. The weight and thickness of two
pads and a disc for each sample were taken before and after the friction
test. In order to obtain average thickness, six measurements (three at
the beginning and three at the end) were taken at different locations
on the pads and disc before and after the friction test. Wear rate was
calculated as weight loss for per mm2 of the sample during
||The ingredients of samples (wt.%)
RESULTS AND DISCUSSION
The coefficient of friction (μ) varied significantly in the initial stage
of testing, since the size of the contact area increased and the friction layer
was developed on the surface (Filip et al., 2002).
The variations of friction coefficient with test time are given in Fig.
1. The characteristic time dependences of μ as detected in FAST for all
samples are shown in Fig. 1.
RH4 and RS4 coded samples showed a continuous initial increase in the
friction coefficient (μ) between 10th and 20th min of testing in
FAST. Such increase can often be attributed to the adhesion of metal chips
in the brake lining to the friction surface of the cast iron disc. The
μof RS4 in Fig. 1
slowly decreased after a little increase (i.e., between 10th and 20th
min) and then stabilized which is known as the typical friction manner
in the literature. Note that this sample has 4% RSD which has 130 g kg-1
of SiO2 and 35-70 g kg-1 of Lignin as can be seen
from Table 1.
||The change of friction coefficient as a function of
time for all samples
While μ of RH4 slowly decreased between 10th and 20th min as in
case of RS4, later it increased more than that of RS4 and finally stabilized
at the value of 0.35. This sample has 4% RHD which has 230 g kg-1
of SiO2 and 160 g kg-1 of Lignin as can be seen
from Table 1. It is important to observe that the difference
between the two used samples (RS4 and RH4) became apparent after 40th
min. This can be explained as follows: Since the temperatures become higher
(i.e., 350-400°C) after 40th min, it is conjectured that the micro-structural
changes on the brake pad were completed and thus μ does not change
The same experiments were conducted by changing the RSD and RHD rates to 20%,
respectively. RS20 and RH20 shows similar behavior until 40th min as observed
in the case of RS4 and RH4. However, since the RS and RH rates were higher (i.e.,
20%), the increase in μ is higher until 55th min as shown in Fig.
1. After the 55th min, with the effect of high temperature and unforeseen
changes in the brake pad material, the binder material lost its bounding capability
and thus a slow decrease in μ is observed. This decrease is more significant
in RS20 than RH20 since the rate of lignin and silica were lower in RS20. As
a result of such fading in μ, it finally stayed around 0.33-0.36. As can be
shown from the Table 3, the μ achieved in all samples are
0.315 and 0.381, respectively which are considered to be very good when compared
to friction coefficients achieved in current brake pads (Blau,
As can be seen from Fig. 2-5, the silica particles
are homogenously distributed in the body (white points). As seen, some
particles are detached from the body causing micro-voids. The micro-voids
on the surface of the samples can be classified as smaller and bigger
size. The bigger sized micro-voids are due to falling of the metallic
particles during the friction. The worn metallic particles imply that
they actively participated in friction during braking test. It is known
that if the metal-component coherent surface is bigger, friction and wear
will be increased. In addition to micro-voids, there are some micro cracks
on the surface. Therefore, they stayed as effective in friction surface.
All matters were homogeneously distributed in the matrix and therefore,
very few micro voids were observed in the structure.
||SEM micrographs of brake pad sample for RS4
||SEM micrographs of brake pad sample for RS20
The photos of RS4 and RH4 samples are blurred when compared to the photos
of RS20 and RH20. This can be attributed to the fact that the barite rate
is higher in RS20 and RH20 (Fig. 2, 4).
Note that in only Fig. 5, the ingredient having the
plastic deformation capability has taken a flake like feature after the
friction experiment. Overall, it is observed that the used ingredients
for the brake pad created a fine structure.
||SEM micrographs of brake pad sample for RH4
||SEM micrographs of brake pad sample for RH20
||Typical characteristics of the brake pad used in this
Table 3 presented the mean of friction coefficient,
standard deviation of friction coefficient and wear per mm2
of the sample during the tests. As can be seen from the Table
3, RHD provided better results in terms of friction coefficient
when compared to RSD. This observation is also valid for the standard
deviation. However, as far as the wear rate is considered, RS4 has a little
better wear rate than RH4 which can be ignored. In general 20% rate causes
more wear rate than that of 4%. These because; there are more organic
material in the mix with 20% of RHD and RSD which causes increased wear
rates. As a conclusion, RH20 with 20% of RHD use gives the best result
in terms of friction coefficient and standard deviation.
In this study the following conclusions were drawn:
||RSD and RHD can be effectively used in brake pad formulations when
properly combined with other additives
||The experimental results have shown that the friction layer, with
the use of RHD significantly improved the overall performance. Simultaneous
faded reduction and stabilized friction coefficient
||Wear rates were slightly increased with 20% of RSD and RHD. Some
micro voids and micro cracks are observed on the worn surface
||The best mean friction coefficient was achieved with RH20 samples.
Furthermore, the standard deviation is in the acceptable range for
||Friction surface and chemistry strongly affect the frictional performance
on the friction layer. The structure of such friction layer differs
from the bulk material formulation and also depends on the environment
and applied testing condition
This research is funded by the Scientific and Technological Research
Council of TURKEY (TŰBYTAK) (Grant Reference No. BIDEP 2219)
and conducted in Center for Advanced Friction Studies (CAFS) at Southern
Illinois University Carbondale (SIUC), IL, USA. The author wishes to acknowledge
the TŰBYTAK and SIUC CAFS for their support.