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Journal of Minera ls & Materials Ch ar ac teri zatio n & Engineeri ng, Vol. 10, No.3, pp.231-244, 2011
jmmce.org Printed in the USA. All rights reserved
Development of Iron Based Brake Friction Material by Hot Powder
Preform Forging Technique used for Medium to Heavy Duty Applications
Mohammad Asif *, K. Chandra, P.S. Misra
Metallurgical and Materials Engineering Department, Indian Institute of Technology,
Roorkee. Roorkee- 247667, India
* Corresponding author: firstname.lastname@example.org
A promising friction material, Iron -based friction material, was prepared by powder
metallurgy (PM) processing utilizing hot powder preform forging (near net-shape).The
preparation of the product and its characterization are presented in this paper. These
products are useful in heavy duty Military Aircraft applications such as AN-32. In order to
eliminate costly environmental control systems to protect products during their high
temperature processing (as is conventionally practiced employing hydrogen gas), the present
investigation relies on carbon (mixed in the brake pad formulation) as reducing agent and
high temperature oxidation resistant glassy coating (separately developed) applied over the
product’s surface after cold compacting. After conducting an initial characterization such as
hardness, density and Pin-on Disc tests, the samples were tested in sub-scale dynamometer
under Rejected Take Off conditions. It was observed that the obtained density in the present
investigation is higher than the reported density obtained by sintering route, and wear is on
the lower side of the range as per the Aeronautical Standards. Optical metallography was
used to investigate the microstructure of friction, interface and backing layer. It was
observed that the distribution of ingredients in matrix was homogeneous. The results also
indicate that the coefficient of friction is more stable, and wear is lower with respect to
temperature rise. .
Keywords: Forging, Powder Metallurgy, Friction material, brake pad
Friction materials which are made by powder metallurgical technique, have been widely used
in commercial/fighter aircrafts; high speed trains; brakes; clutches and gear assemblies of
different kinds of automobiles .
232 Mohammad Asif , K. Chandra, P.S. Misra Vol.10, No.3
The traditional powder metallurgical friction materials mainly include iron and copper based,
which are prepared by pressing and sintering of metal powders with the addition of solid
lubricants (BaSO4, Sb2S3) and abrasives. . In recent years, the speed and load of vehicles
and engineering machines have increased greatly, which brings forward higher requirements
for friction materials . So it is important to develop alternative approach with good
combination of properties. Iron based friction materials, retain some metallic characteristics
(stable thermal stability, and hardness), as well as some particular properties such as high
density level, favorable oxidation and corrosion resistance .
In this paper, Iron, as the friction material matrix is chosen due of its stability under higher
temperatures (1100oC) and can be applied under heavy duty dry operating conditions because
of its high melting point and other improved properties which include strength, hardness,
ductility, heat resistance and stability etc. . Friction elements based on metallo-ceramic
constituents contain number of materials such as metals, lubricants, abrasives etc. These are
known for high strength, high temperature stability, oxidation resistance, high thermal
conductivity, wear resistance, and fade resistance. The chemistry of the friction elements
obviously becomes extremely complex because such expectations cannot be met with
restricted number of constituents. In view of this, it becomes necessary to process these
starting from powder mixture having different constituents in suitable proportions.
Compacting and sintering technology  is one such technology through which these
products are currently being made. However, it suffers from certain major limitations such as
inadequate joining of friction element with backing plate, poor density levels achieved in
friction element owing to limited application of pressure during compacting, poor thermal
conductivity due to high levels of porosity in the product, poor strength due to segregation of
the impurities along prior particle boundaries (PPB’s), anisotropy in the strength of the
product owing to preferred direction due of pressing, and, wide variations in final
characteristics due to large number of variables involved.
Besides above, cost of raw material and consumables along with heavy capital equipments
makes this technique costly and only large scale production is possible where these costs are
distributed in the large volume of production. In contrast to these limitations, the present
technique can made brake pads of much simpler chemistry but with improved performance
on account of simultaneous application of pressure and temperature and with better control of
variables. The technique does not involve any custom built equipments and is also likely to
become economical even with small volume production.
Chemistry in the present investigation relies on use of ferro-phosphorus which forms
phosphide eutectic network along the grain boundaries of alloyed ferrite . It is expected
that this will provide improved frictional characteristics in the brake pad. Use of copper in the
brake pad helps in increasing thermal conductivity and stabilization of coefficient of friction
. Concerns have been shown that increasing content of copper may cause growth during
sintering. However, this problem is not likely to occur in the present investigation since
Vol.10, No.3 Development of Iron Based Brake Friction Material 233
sintering has been replaced by hot powder preform forging with built in powder based
backing plate, which is purely solid state processing. Solid lubricants such as graphite,
sulphates and sulphides give rise to improved frictional stability  and damping of vibration
during engagement, apart from anti-seizure characteristics at higher temperatures . Due to
these reasons, iron as base metal matrix have been chosen in the present investigation.
2. EXPERIMENTAL DETAILS
Brake pads made in the present investigation consist of two major parts namely friction layer
and backing plate which are formed simultaneously, such that these two layers do not
separate out during operation. Joining of these two layers by sintering is usually difficult
because of widely varying chemistry and processing involved. So the present investigation
relies on manufacturing of these two parts simultaneously employing different chemistry of
powders but having similar constituents. This results in better joint quality between them.
Table 1 provides details of chemistry of friction layers along with backing plate. Five
compositions along with backing plate namely FA01, FA02, FA03, FA04 and FA05 were
formulated. Sequence of manufacture is as described below:
(a) Powder processing:
1. For backing plate:
Powder mixture for backing plate as per Table1, is subjected to mechanical alloying in
the Attritor Mill with a ball to charge ratio in the range of 10:1 and duration of
Mechanical alloying is 2hours and rotational speed of mill is 200 rpm.
Powder mixture for friction layer as per the chemistry given in Table 1 is prepared as per
the following sequence.
2. For Friction Materials
- SIC powder is mechanically alloyed with sulphide / sulphate powders and of graphite
powder (having size range 0-120micron as given in Table 2.). The parameters of
mechanically alloying are:
- Attritor speed (200rpm), Ball to charge ratio (10:1), Duration (60mins.). This will
ensure coating of soft powders on hard SiC powder.
- Entire amount of iron and other powders as per the chosen chemistry is mechanically
alloyed with graphite powder in Attritor Mill as mentioned above.
- The two powder mixtures so prepared are then mechanically mixed with each other
(b) Powder compacting:
Powder compaction takes place in a suitably designed die in accordance with the given shape
of disc brake pad for example, suitable for AN-32 Transport aircraft. Requisite quantity of
backing plate powder mixture is first filled upto a uniform height. Thereafter, friction layer
powder mixture in requisite quantity is filled in the die. The filling of these layers of powder
234 Mohammad Asif , K. Chandra, P.S. Misra Vol.10, No.3
is done on fixed weight basis as per the design (say in Fig.1) and densities of respective
layers. Both the layers of powder mixture are simultaneously pressed in a Screw forging
press with the help of upper and lower punches at a pressure in the range of 450 to 600MPa.
The compact so pressed is ejected out of the die. The die is suitably lubricated employing
graphite /Zinc stearate in the suspension of methyl alcohol/ethyl alcohol/acetone prior to
filling for easy ejection without cracking of green compact.
Table 1. Chemistry of friction elements along with backing plate:
Lubricants Backing Plate
FA01 Fe-70.3, Cu-10,
C- 0.3,Cu – 1.5, P -
0.3, Fe –97.9
Table 2. Sizes of powders employed:
S.No. Powder Size range Source
1 Iron powder -120µm Hoganas Industries Ltd.
2 Copper powder -120µm Electrolytic
3 Graphite -200 to +150µm Natural Crystalline
4 Ferro-phosphorus -45µm Commercial grade
5 Tin powder -75µm Atomized
6 Silicon Carbide -180 to +150µm Chemical grade
7 Calcium Sulphate/
-45µm Chemical grade
Vol.10, No.3 Development of Iron Based Brake Friction Material 235
Fig.1. Die and Punch set up for Military air craft brake pad.
(c) Preform coating:
The green compact so produced is coated with an indigenously developed high temperature
oxidation resistant ceramic coating. This is used to protect our samples from oxidation when
subjected to heating. The coating is baked to remove moisture from it at a temp of 120 oC for
2 hrs in an oven.
The coated green compacts are heated in a furnace. The operating temperature of the furnace
ranges from 1050oC for iron based brake pads, holding time 1 hr.
236 Mohammad Asif , K. Chandra, P.S. Misra Vol.10, No.3
(e) Hot pwder preform forging:
The hot powder performs are taken out from the furnace and quickly transferred into the hot
die duly lubricated with graphite and fitted in the forging press. The forging is done at a
speed of 500mm/s. The preform fully consolidates to its near theoretical density on forging.
The forged component is later on ejected out of the die.
The forged pads are subjected to high temperature resistant ceramic glassy coating again and,
then annealed at a temperature of 710oC for 2 hours to adjust hardness of friction layer to <
On cooling the coating peels off by itself and residual coating if any is removed by minor
surface finishing operations. .
2.1. Density and Hardness Measurement:
Densities of the friction material were estimated by measuring the volume of the given piece
of the material by water displacement method. Prior to density measurement the sample is
wax coated to seal surface pores.
Hardness measurements were carried using Brinell’s hardness testing machine employing a
31.25 kg Load with ball diameter of 2.5mm and kept constant throughout the experiment for
all friction materials. The load was applied for 30 seconds on a sample. At least 5
indentations were made on each sample as shown in Table 3.
Table3. Density and Hardness measurement:
2.2. Pin on Wear Disc Test:
To analyze the wear studies of friction materials, an initial laboratory level based pin-on-disc
wear test was performed under different set of parameters (applied load, sliding speed, and
sliding time/distance) according to ASTM G99 standard test procedure. The pin circular or
square or rectangular cross section can be used to clamp between the fixed jaw and the
A pin made from friction material of square size of 30mmx7mmx7mm was used in the
present study. The counter face disc is made of EN-32 steel hardened to 45 HRC. To ensure
Samples FA01 FA02 FA03 FA04 FA05
ρ(gm/ cm2) 5.2 5.4 5.7 5.8 5.7
(BHN) 94 69 78 79 84
Vol.10, No.3 Development of Iron Based Brake Friction Material 237
proper contact between the test pin and the steel disc, test pin adjusting screw is provided.
Normal loading on the test piece is provided by the slotted weight, which are hanged on pan.
This load is transferred through the frictionless wheel to the specimen holder. Electric motor
is directly connected to the steel disc and the speed of the disc can be controlled by variable
regulator. It may be noted that the wear in case of GCI (Grey cast iron) is excessively high in
comparison to friction materials developed in present investigation. It is justifiable since GCI
does not contain wear resistant constituents in it, so we will test samples on steel disc. The
pin on contact side of the disc was rubbed on polish paper of 1/0, 2/0 before clamping and
counter disc is also made smooth surface finish by the polish paper of 2/0 at each cycle. The
load applied is 8kg respectively. Sliding speed of 9m/s (1140 ±10 rpm at a track radius of
75mm) was kept constant for the whole experiment. Sliding time is 50 minutes. The wear
loss after each cycle, the frictional force generated at every 1 minute was recorded by
indigenously designed pin on disc wear testing machine. The noise generated during the wear
process is also recorded by means of special microphone based noise level meter in decibels
(dB). This gives an idea about the noise behavior of friction materials during the test. The
density and hardness of the hot powder preform forged samples are measured; the values are
given in Table 3.
Measurement of rise in temperature of the wear surface as a function of time and during wear
test was carried out using a thermocouple made of Chormal-Alumel wire is placed at 5mm
away from the wearing surface by making hole of 2mm diameter with 3mm depth
approximately. The rise in temperature is recorded till the temperature at the wear surface
becomes stable. Care must be taken while brazing thermocouple bead inside the hole at
friction material. Simultaneous measurement of coefficient of friction and temperature rise
during the test would provide brake fading .
3. RESULTS AND DISCUSSIONS
Density and Hardness of Friction Materials are summarized in Table 3. Both these parameters
depend on forging and annealing treatments. This also depend upon, chemistry of friction
materials. The technology developed in the present investigation is capable of developing
these parameters in wide ranges which is not possible in sintering technology. It is amply
clear that the processing technique employed in the present investigation is superior with
regard to density and hardness in comparison with the compacting and pressure sintering
technology. Although hardness obtained in these brake pads are similar to the conventional
brake materials, higher hardness of present materials are primarily due to higher density
levels and therefore are likely to show improved performance. An initial laboratory level
based pin-on-disc wear test has been performed under the parameter of applied load 8kg and
the sliding speed 9m/s, as tabulated in Table 4. The noise generated during the wear process
is also recorded by placing a microphone based noise level meter in decibels (dB) at a
distance of 25 mm from the point of contact between friction pin and rotor disc. Rise in
temperature during the wear test is also measured by placing thermocouple 5mm away from
the initial wearing surface. It has been inferred that the performance of our brake pads is
238 Mohammad Asif , K. Chandra, P.S. Misra Vol.10, No.3
better than that of brake pads made by compacting and sintering technology . The results
of wear test are summarized in Table 4.
Table 4. Pin-On-Disc Wear Test results at 8 kg Load, 9m/s Sliding speed:
Area of the pin = 49mm2
TemperatureNoise Level, (dB)
Max. Min. Avg. oCMax.Min Avg.
FA01 0.63 0.38 0.35 0.365 116 38 34 36.0
FA02 0.49 0.58 0.53 0.555 168 35 32 33.5
FA03 0.61 0.39 0.38 0.385 145 30 29 29.5
FA04 0.42 0.48 0.47 0.475 169 36 30 33.0
FA05 0.64 0.46 0.45 0.455 151 32 28 30.0
Figure2 shows that the sample FA02and FA04 has the higher wear resistance than Sample
FA01. FA02, and has higher coefficient of friction (COF) shown in Fig. 4, which is suitable
for heavy duty applications. The range of coefficient of friction for our composites varies
from about 0.16 to 0.55, which in the Aeronautical standard range except for the samples
FA02 and FA04 respectively. It is noticed that the composites have better combination of
solid lubricants (graphite, BaSO4, Sb2S3) with addition of SiC powder, showed better overall
performance. It may be concluded that frictional characteristics can be improved with the
addition of solid lubricants in bulk amount but on the other hand, its excessive amount
weakens their stiffness resulting in cracks during testing. It may be inferred from the graphs
that wear is within the Aeronautic standard range. In our compositions, Phosphorous (0.4-0.7)
is present which develops wear resistance, but some wear resistant constituents were
incorporated such as Silicon carbide. This is apparent because wear resistant materials, like
silicon carbide, increase COF, then the net wear loss. The noise level during the test is not
significantly altered. This is because of the fact that the noise level is related to metal content
of the brake elements which in our case varies from 82% to 90%. The noise level for the
samples FA03, FA05 is marginally constant; as shown in Fig 5.The temperature is also
marginally increases in samples FA02 and FA04. For the friction materials developed in the
present investigation, temperature rise is low due to high thermal conductivity of matrix
material. The temperature rise ranges from 1010C to 168 0C. It is noticed that higher wt. % of
metallic matrix improves heat transfer rate (due to higher thermal conductivity), resulting
lower temperature rise.
Vol.10, No.3 Development of Iron Based Brake Friction Material 239
Fig.2. Wear (gm) for friction materials
Fig.3. Temperature rise for friction materials
Fig 4. Coefficient of Friction for friction material
240 Mohammad Asif , K. Chandra, P.S. Misra Vol.10, No.3
Fig.5. Noise level for friction materials
3.1. Sub Scale Dynamometer Test
The test input parameters for Military transport such as AN-32 rotor aircraft applications is
shown in Table 5. The required test output parameter values are shown in Table 6. The test
results are tabulated in Table 7. The run down time are nearly in the specified ranges as that
of output parameters (Table 6). These pads seem to be suitable for Military transport rotor
aircraft applications. The test results for AN-32 aircraft for rotor application are best suitable
in terms of run down time; wear in thickness, coefficient of friction.
Table 5. Test input parameters:
Brake pad contact area: 25.8 cm2
Brake Force, kgf
Heavy duty air craft
17300 1000 160
Table 6.Test output parameters:
RD Time, sec COF Max. Wear for 50 cycles
Min Max Min Max gm Pad thickness, mm
6 12 0.18 0.4 14 1.25
Vol.10, No.3 Development of Iron Based Brake Friction Material 241
Table 7.Test results for Heavy duty Military air craft rotor application:
Pad1 Pad2 Pad1 Pad2
FA01 Max 120 14.4 0.28 27 65 148 359
Min 96 11.6 0.22 21 43 119 240
Avg 107 12.8 0.25 24 52 133 290
FA02 Max 106 12.7 0.28 28 62 154 343
Min 94 11.2 0 0 41 0 229
Avg 101 12.1 0.28 26 51 143 282
3.2. Metallographic Analysis
On the basis of detailed metallographic examination carried out and shown in Fig.6., we have
observed that the microstructures of friction layer as well as backing plate are homogeneous
in this hot powder preform forged sample, backing plate in this sample which is shown in
Fig.6(c)., is essentially a low carbon steel but with very fine ferrite grains, and it is
Widmanstattan ferrite. This is because of high speed forging employed during processing
providing fast cooled fine grained structure which in turn yields high strength backing plate
without resorting to higher carbon steel. It is also expected to have better toughness of
backing plate due to fine ferrite grain structure which may help better engagement of brakes
in actual applications. Further, friction material which is shown in Fig.6 (a) is well distributed
structure. Friction layer consists of largely continuous coarse iron matrix separated by coarse
Yellow colored particles are clearly appearing in the friction as well as backing layer, which
is probably a Copper phase. Their revelation has been highlighted by adjusting the contrast
and intensity of the image and recorded pictures separately. SiC particles in friction material
are embedded in the regions of graphite flake and are clearly visible like a junky particle and
are recorded in microstructures [as shown in Fig.6 (a)]. The distribution of other constituents
in this sample is also uniform; at interface carbon can easily diffuse towards the backing plate
side leading to development of fully pearlitic structure in the adjacent backing plate.
Microstructures of hot powder preform forged sample at the interface indicate strong bonding
between backing plate and friction element because of enhanced diffusion of carbon from
friction element side to backing plate side, and interface is clearly defined with diffusion of
SiC across the interface, signifying adequate adhesion.
242 Mohammad Asif , K. Chandra, P.S. Misra Vol.10, No.3
Fig.6.(a)Friction Layer(FA05 at 100μm) (b)Interface (FA05 at 100μm) (c)Backing
Layer (FA05 at 100μm)
3.3 SEM with EDAX Analysis
The surface morphology of the friction material, backing plate, and interface layer are
analyzed through SEM. EDAX analysis of fracture surface of different constituents present in
the samples, which have been done to check homogeneity of the constituents in friction
elements and backing plate as shown in Fig. 7(a) to Fig.7 (b).To identify the element
wherever required was carried with the help of point/surface area analysis of EDAX analysis.
QUANTA FEG 200 FEI (Netherland make) was used in the present study.
1. Elements added in the iron matrix during powder mixing are retained after powder
2. Development of reaction/secondary phases has been arrested.
3. The fracture surfaces exhibit a ductile appearance (dimples); indicative of micro-void
1. Hot powder preform forging (with built-in powder based backing plate) near net-shape,
a process, for fabricating iron based brake pads has been successfully developed.
2. Complete elimination of H2 gas by protective high temperature oxidation resistant
ceramic/glassy coating on iron based preform like paints.
3. The technology completely eliminates the problem for joining of backing plate to
friction element on account of simultaneous processing.
4. Higher density levels can be achieved by hot powder pre-form forging technique.
CuPart ic le s
Vol.10, No.3 Development of Iron Based Brake Friction Material 243
(a)Friction layer (FA05) 100μm
(b)Interface (FA05) 100μm
(c)Backing plate (FA05) 100μm
Figure 7(a-c).SEM and EDAX of Fracture surface of hot powder preform forged brake pad.
Authors are highly thankful to Foundry and Forge Division, HAL Bangalore, India for
providing testing facilities to test our samples on their sub-scale dynamometer.
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