Wear 269 (2010) 826–837
Contents lists available at ScienceDirect
Wear
journal homepage: www.elsevier.com/locate/wear
NAO friction materials with various metal powders: Tribological evaluation on
full-scale inertia dynamometer
Mukesh Kumar, Jayashree Bijwe ∗
Industrial Tribology Machine Dynamics and Maintenance Engineering Centre, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110 016, India
a r t i c l e
i n f o
Article history:
Received 31 March 2010
Received in revised form 31 July 2010
Accepted 12 August 2010
Available online 19 August 2010
Keywords:
Polymer–matrix composite
Brake/clutch materials
Brake dynamometer
Metallic fillers in friction materials
Thermal conductivity
a b s t r a c t
A right selection of filler, its type, shape, size, amount and its compatibility with other co-fillers contribute to the performance of a friction material. Among various types of fillers, metallic fillers are very
important and multifunctional. In spite of well-accepted fact that these affect all performance properties
of friction materials, in depth studies and systematic evaluation as per standard procedure are not documented. Hence, in this work three friction composites were developed in the laboratory with identical
parent composition (90 wt.%) except metallic filler (10 wt.%) viz. brass, copper and iron powders while
one more composite was developed without any metal contents for comparison’s sake. These were characterized for physical, thermo-physical, chemical and mechanical properties. For tribo-evaluation inertia
brake-dynamometer testing which reflects the most realistic performance was selected. The various
performance properties like friction, sensitivity to load, speed and temperature were studied as per
industrial schedule.
It was concluded that inclusion of metal contents, improved almost all the performance properties
and copper containing composite showed best tribo-behavior followed by brass and then iron powder
containing composite. Composite without metal powder proved poorest. Worn surface analysis and wear
mechanism are also discussed in detail.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
The heart of the braking device is friction material (FM), which
is expected to continue its functioning reliably and efficiently
for a long time in adverse operating conditions. Non-asbestosorganic (NAO) fiber reinforced-low metallic friction composites are
increasingly being used in automotive brake disc pads, shoes, linings, blocks, clutch facings, etc. primarily because of awareness of
health hazardness of asbestos. The performance requirements of
FMs are very complex and conflicting [1]. FMs are essentially multiingredient systems in order to achieve the desired amalgam of
performance properties [2,3]. The influence of these ingredients on
performance properties is so complex that formulation of friction
materials is still referred as an art rather than science [4]. It is a practice to add metallic fillers such as copper, brass, iron, aluminum,
etc. in FMs in various shapes, sizes, amounts and combinations. In
spite of the fact that these are expected to affect all performance
properties such as physical, thermal, thermo-physical, (specific
heat, thermal conductivity (TC), diffusivity, thermal effusivity, etc.),
mechanical apart from tribological, hardly any systematic stud-
∗ Corresponding author. Tel.: +91 11 26591280; fax: +91 11 26591280.
E-mail address: [email protected] (J. Bijwe).
0043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.wear.2010.08.011
ies are reported in this aspect [5–9]. Handa and Kato [5] studied
the influence of variation of three fillers (powders of Cu, CNSL
and barite) in quaternary composition based on phenolic resin for
friction and wear properties on a reduced scale tribometer. They
concluded that Cu powder inclusion resulted in increase in fade
resistance but decrease in wear resistance. BaSO4 inclusion led to
the exactly opposite behavior. Jang et al [6] studied the effect of different metallic fibers viz. steel, Al and Cu (15 vol.%) on performance
of NAO friction composites using a small-scale friction tester. The
studies were focused on investigating the influence of addition
of these fibers individually in composites under various operating conditions. Overall benefits or limitations of each fiber were
discussed. Jia and Ling [7] observed the effect of four fillers viz.
resin, cast-iron powder, brass fiber (short), and graphite powder
(variable) in increasing amount on tribo-properties of friction composites on a block-on-ring tribometer. They observed that, when
the mass fraction of brass fibers was approximately below 19%, the
showed very small variation. However, when it exceeded 19% the
increased. Ju and co-workers [8] studied the effects of addition of
different fibers (steel, brass, cellulose, carbon, ceramic and copper)
on the mechanical and tribological properties of friction materials. A small-scale pin on disc rig was used for tribological study. It
was reported that composites with fibers of steel, brass and copper
showed highest, moderate and lowest wear, respectively.
M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
Tribo-testing of FMs is equally complex job because FMs are
used in various forms (pads, shoes, linings, blocks against different
rotors such as discs, drums, wheels, etc.) in varying operating conditions. Various performances evaluation methods for these FMs
such as brake-dynamometer testing [9], Krauss testing [10], the
friction assessment screening test (FAST) [11], Chase testing [12],
etc. are very popular globally. Among these Chase and FAST are
most commonly used and are considered as a quality control test.
Such rigs are more useful for comparative performance evaluation
of materials in the laboratory to have some preliminary idea about
the performance. The screened materials on prototypes are supposed to be evaluated as per globally accepted standards so that
real potential under various operating parameters (relatively) can
be evaluated before field tests followed by commercialization [1].
In light of the advent of modern fast moving vehicles, the friction
material requirements became more complex in accordance with
the applied braking pressure, speed, road conditions, and environmental conditions. Hence, to match these requirements full-scale
inertia dynamometer testing is very essential. Since such facility
is generally available with Industries, number of research papers
on the evaluation of materials in this respect is limited [13–17].
The schedules for evaluating material performance vary from country to country, in general. Federal Motor Vehicle Safety Standards
from US prescribes schedules (FMVSS 121) for passenger brake pads
braking. This is for air braked trucks and FMVSS 105 is for hydraulic
brakes and Japanese equivalent of this is JASO 406 and 407. Apart
from this SAE J 2522 (AK-master) is also equally important and very
popular in Europe.
Thus three FMs with identical parent composition (90%) but
with difference in metallic powders viz. copper, brass and iron (10%)
were developed and characterized. These were tribo-evaluated on
full-scale inertia dynamometer for various performance properties.
One more composite without any metallic contents was also developed and compared to quantify the benefits of metallic fillers. The
paper discusses the studies in this background.
2. Materials and methodology
2.1. Fabrication of the composites
The fabrication of three composites containing 12 ingredients was based on keeping parent composition of 11 ingredients
(90 wt.%) constant and varying metal ingredients (brass, copper and
iron powder) by 10 wt.%, respectively in each composition. Morphology of selected metallic fillers is shown in SEM micrographs
(Fig. 1). One composite was developed without any metal content and 10% amount was compensated with space filler (barite)
as shown in Table 1. These composites were designated as BP1 ,
CP1 , IP1 and Ref. The ingredients were mixed in a plough type
shear mixer to ensure the macroscopic homogeneity using a chopper speed of 2800 rpm and feeder speed of 1450 rpm. The mixing
Table 1
Design and designations of composites.
Ingredients/designation
Parent compositiona
Barite (BaSO4 )
Brass powder
Copper powder
Iron powder
Composition by weight %
827
schedule was of 10 min duration. The mixture was then placed into
a four-cavity mould supported by the adhesive-coated back plates.
Each cavity was filled with approximately 80 g of the mixture and
heat cured in a compression-molding machine under a pressure of
8 MPa for 7–8 min at a curing temperature of 150 ◦ C. Five intermittent ‘breathings’ were also allowed during the initiation of curing
to expel volatiles. The pads were then post-cured in an oven at
150 ◦ C for 4 h [18]. The surfaces of the pads were then polished
with a grinding wheel to attain the desired thickness and surface
finish.
2.2. Characterization of the composites
Composites were characterized for physical and chemical (density, porosity, and acetone extraction), thermo-physical (thermal
conductivity, diffusivity, specific heat, etc.) and mechanical (tensile
strength, flexural strength, compressibility and hardness) properties as per standard practice. Thermo-physical properties were
measured as per ASTM-E1461-01 standard on FL-3000 Flash line
instrument supplied by Anter Corporation, USA. Samples of square
size (10 mm × 10 mm) and thickness 2–2.5 mm were used for these
measurements at room temperature.
2.3. Test set-up and procedure of testing on brake inertia
dynamometer
This dynamometer (Dyno) in ITMMEC, IIT Delhi was supplied by
Pyramid Precision Engineering Pvt. Ltd., Chennai, India. It applies
variable inertial load on the engine (175 kW motor) and measures
the braking force, wear of the brakes, torque applied and the coefficient of friction through a computer connected to it. Both braking
mode viz. hydraulic and pneumatic can be applied on this machine.
Main Chasis is of ladder type construction and fabricated out of
I-section girders of about 300 mm height with adequate diagonal
members for rigidity. Prime mover is a 175 kW, 1500 rpm DC motor
with external cooling facility. A variable speed drive with tachometer feedback is provided to vary the speed of the motor. The drive
motor is fixed on the separate box type bed which is mounted on the
main chassis. Main shaft is directly coupled to the motor through
Gear coupling. The motor rpm is to set such that the main shaft
will run at the speed set by the computer. Emergency push buttons
are provided on the machine and the control panel can be used to
stop the motor and engage with emergency disc brake with shaft
to stop the rotation. Disc/drum temperature is monitored by a noncontact IR sensor, while a provision of nine thermocouples is made
to measure the pad/lining temperature. The analog output from the
sensor is suitably signal conditioned and fed to the interface card
of the computer to monitor and control the temperature at which
brake is to be applied. All the wheels are properly enclosed with
metallic sheet for security purpose. Fig. 2a, shows the schematic of
the dynamometer (Dyno) while Fig. 2b shows the photograph of
the braking end. Some important specifications of Dyno are listed
in Table 2. Lab view(TM) -based software (version 4.0) is provided to
calculate the desired output parameters.
2.4. Testing schedule
BP1
CP1
IP1
Ref
90
0
10
0
0
90
0
0
10
0
90
0
0
0
10
90
10
0
0
0
Abbreviations used: B, brass; C, copper; I, iron; P, powder; Ref, reference and subscript
1 for 10 wt.%.
a
Binder (St. Phenolic): 10 wt.%; fibers (Aramid, rock wool, ceramic, acrylic, potassium titanate): 23 wt.%; additives (graphite, alumina): 12 wt.% and fillers (NBR
powder, cashew dust, vermiculite, barite): 45 wt.%.
In the present work, a Japanese Automobile Standards (JASO C
406) was used for testing. The purpose of this schedule is to evaluate performance of service brake devices for passenger car brake
pads through simulation of braking conditions on road in laboratory (complete schedule is attached in Appendix A). This is one
of the important schedules generally practiced in Asian countries.
Instead of following complete schedule, two parts were selected
for testing because of complexity, long time and cost involved in it
[19]. Two major parts of the JASO schedule, effectiveness-II and fade
828
M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
Table 3
Dynamometer test schedule.
Table 2
Specifications of the dynamometer.
Motor
Max. motor torque
Base speed
Shaft speed
Min. inertia
Max. inertia
No of wheels
Max. braking torque
Max. pressure (air)
Max. pressure (hydraulic)
Gross vehicle weight (GVW) range
Max. dissipated energy
175 kW, 1500 rpm DC motor
1000 kg m
1400 rpm
1500 rpm
1.5 kg m2
1570 kg m2
11
1000 kg m
10 bar
120 bar
1000–16,000 kg s
20 MJ
and recovery-I were followed in the present study (these parts are
marked in blue colored in Appendix A). Effectiveness studies comprise the influence of operating parameters (pressure and speed) on
performance of a material while; fade and recovery part is useful to
examine the influence of temperature on performance properties.
Schedule
Inertia
Rolling radius (Tyre radius)
Caliper piston diameter
Effective radius (pad on disc sliding radius)
JASO C 406
4.00 kg m s2
279 mm
51 mm
97 mm
The vehicle and other specifications of the test schedule are given
in Table 3.
2.4.1. Effectiveness studies (pressure–speed sensitivity)
Effectiveness studies comprise the influence of operating
parameters (pressure and speed) on the performance of material.
It measures the efficiency of a friction material to function more
reliably under different pressures and speeds. Table 4 shows the
experimental design for effectiveness studies. Each pair of pads
was subjected to 100 brake applications to establish at least 80%
conformal contact with the disc during bedding test prior to final
Fig. 1. Scanning electron micrographs of selected metallic fillers; brass powder (particle size: 650–850 ␮m), Cu powder (particle size: 280–430 ␮m), and iron powder (particle
size: 87–225 ␮m).
Fig. 2. (a) Schematic of brake inertia dynamometer: (1) motor, (2) flywheel of 0.5 kg m2 , (3) flywheel of 2 kg m2 , (4) flywheel of 4 kg m2 , (5) flywheel of 8 kg m2 , (6) flywheel of
16 kg m2 , (7) and (8) flywheel of 32 kg m2 , (9) flywheel of 64 kg m2 , (10) couplings, (11) housings, (12) shaft bearings, (13) braking assembly, (14) load cell, (15) air chamber,
(16) torque arm, (17) emergency brakes, (18) dyno bed. (b) Close view of braking end.
M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
829
Table 4
Experimental parameters for effectiveness studies.
Description
Bedding test
Effectiveness-II
Speed (kmph)
65
50, 80, 100
Deceleration (g)
0.35
0.1 to 0.8
Initial Temp. (◦ C)
◦
120 C
< 80 ◦ C
Nos. of applications
Air Blower
100
24
ON
ON
Table 5
Experimental parameters for fade and recovery studies.
Description
Speed (kmph)
Deceleration (g)
Initial temperature (◦ C)
Baseline check
Fade & Recovery-I
Fade test
Recovery test
50
0.30
80
80
50
0.45
0.30
80 (for 1st brake)
<80
testing. Once it was established, the disc was replaced by a fresh
disc. Test was done at three different braking speeds viz., 50, 80 and
100 kmph. The tests were conducted under eight different decelerations (0.1–0.8 g). The deceleration is varied by increasing the
pressure and the pressure was programmed to achieve a given rate
of deceleration depending on the friction level of the tested material. As the deceleration increased, the severity of the braking also
increased and stopping distance got reduced to a set value. At each
deceleration (g value) one brake was applied. Thus total eight brake
applications were applied at each speed. Brake duration was variable as each brake starts from desired speed and ends with zero
speed (till halt). Total 24 brake applications were applied in this
part of effectiveness studies. Initial braking temperature of each
brake was 80 ◦ C.
2.4.2. Fade and recovery behavior (temperature sensitivity)
Fade and recovery characteristics highlight the effect of temperature on . The selected part (Fade and Recovery-I) from the JASO
standard (Appendix A) is for evaluating sensitivity of towards
temperature of the surfaces during braking in more realistic conditions. As per requirement of the schedule, same pads and disc
should be continued for further test schedule. Hence, brake pads
and disc used in earlier effectiveness studies were retained for these
studies.
The test starts with baseline check mode having initial speed of
50 kmph and deceleration (g) of 0.3. Total three brakes were applied
in this segment and temperature was kept below 80 ◦ C by using air
blower. After this fade test started with initial speed of 80 kmph,
deceleration of 0.45 and initial temperature 80 ◦ C. Total 10 brakes
were applied in this mode and temperature of the disc was allowed
to rise uninterruptedly during this mode to observe the influence of
temperature (by keeping air blower switched off). Finally recovery
test of 12 brakes started with initial speed of 50 kmph and deceleration of 0.30. Air blower was switched on during this test and initial
temperature of disc before each brake was kept under 80 ◦ C. Table 5
No of applications
Air blower
3
On
10
12
Off
On
shows the experimental parameters for fade and recovery test. All
the desired output parameters were collected synchronously on a
computer.
Definitions of required performance parameters from fade and
recover studies:
-Fade = lowest recorded during fade test: higher, the better.
max (fade) = highest recorded during fade test.
min (fade) = lowest recorded during fade test.
% fade ratio = (min /max ) × 100: higher, the better.
Max. disc temperature = highest temperature rise in the disc
during fade test: lower, the better
-Recovery = highest recorded during recovery test: higher, the
better.
max (recovery) = highest recorded during recovery test.
min (recovery) = lowest recorded during recovery test.
% Recovery ratio = (min /max ) × 100: higher, the better.
3. Results and discussion
3.1. Physical, thermo-physical and mechanical properties
Results are collected in Table 6. As seen from the table, most
of the properties such as density, porosity, tensile strength, compressibility, thermal conductivity (TC), thermal diffusivity (TD), etc.
increased due to metallic contents. However, hardness and flexural
properties deteriorated due to inclusion of metallic contents. It was
also observed that with metallic fillers, the mechanical properties
did not conform to any defined trend in general. In an attempt to
study the relation between properties and formulation of brake linings, Filip et al. [20] have revealed that the rule of mixture cannot
be applied to predict the mechanical properties based on contents
of structural constituents. Regression analysis has also confirmed
Table 6
Physical, thermo-physical and mechanical properties of the composites.
Properties
BP1
CP1
IP1
Ref
Density (g/cc)
Porosity (%) (JIS D 4418:1996)
Acetone extraction (%)
Tensile strength (MPa) ASTM D 638
Young’s modulus (GPa) ASTM D 638
Flexural strength (MPa) ASTM D 790
Flexural modulus (GPa) ASTM D 790
Rockwell Hardness (S-scale) (ASTM D 785)
Compressibility (%) ISO 6310
Thermal Conductivity (W m−1 K−1 )
Thermal diffusivity ×10−4 (cm2 s−1 )
Specific heat (J kg−1 K−1 )
Effusivity (J m−2 K−1 s−1/2 )
2.30
4.33
1.38
12.37
2.28
24.15
4.33
78–88
1.07
2.22
80
1207
2482
2.31
4.30
1.18
12.65
2.30
23.89
4.80
88–93
1.02
2.41
97
1076
2447
2.29
4.10
1.28
14.56
2.16
30.43
5.70
85–95
1.26
2.11
71
1298
2504
2.14
4.05
1.34
12.14
2.19
27.38
5.05
90–95
0.68
1.55
61
1187
1985
830
M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
Fig. 4. Variation in speed spread with increase in deceleration at (a) mild conditions
(transition from 50 to 80 kmph) and (b) severe conditions (transition from 50 to
100 kmph).
Fig. 3. Variation in with increase in deceleration at (a) 50 kmph, (b) 80 kmph, and
(c) 100 kmph.
that apparently no correlation was observed between hardness and
friction or wear properties. Kim et al. [21] also reported hardness
values which were not in consistency with their level of inclusions.
In case of composites as seen from Table 6, TC, TD and thermal effusivity of the composites increased with metallic contents,
but no specific trends were observed for specific heat. TC of the
composites was in the range of 1.55–2.41 W m−1 K−1 which is in
tune to the literature range [22]. Composite CP1 measured highest
value of TC because of the highest TC of copper powder followed
by composites with brass and iron.
3.2. Tribological properties of composites
3.2.1. Effectiveness studies (pressure–speed sensitivity)
Change in as a function of sliding speed and applied pressure
is a very important issue during braking and it should show minimal changes because drivers expect the same level of friction force
under various braking conditions.
3.2.1.1. Pressure sensitivity. Variation in with increase in deceleration (g) at each constant speed reflects the sensitivity towards
pressure and is shown in Fig. 3. For an ideal one, slope of the curve
and undulations in curve should be minimum. General observations from Fig. 3 are collected in Table 7.
3.2.1.2. Speed sensitivity. Variation in speed spread with deceleration (g) reflects the speed sensitivity and shown in Fig. 4.
Speed spread (SS) (in terms of %), is the stability in when speed
changes from first to second level, i.e. from 50 to 80 kmph in these
studies and this corresponds to mild conditions, while from first
to third level 50–100 kmph corresponds to severe condition. For
an ideal one SS should be in the range of 85–100% and slope of
the curve should be as minimum as possible. General observations
from Fig. 4 are collected in Table 8.
In present results, of the composites was in the range of
0.30–0.41 which is in the acceptable range for industrial applications. With increase in pressure and speed, decreased for all
the composites which are as per trends in the literature [23–25].
It is a well-known fact that Amonton’s law does not hold good
for polymers and composites and is not constant as pressure
and speed varies. With increase in pressure, it decreases (generally referred as pressure-fade), though the slope is not necessarily
linear. This is mainly because with increase in pressure, real area
of contact increases excessively and disproportionately since polymeric materials are visco-elastic rather than elasto-plastic (as in
case of metals). This leads to eventually disproportionately low
reduction in pressure on the asperities in spite of increase in load.
Hence falls down [26,27]. However, with speed this relationship
is more complex than pressure. With increase in speed generally
does not show fixed trends. Increased slightly, showed maxima
followed by a sharp decrease in some cases [23–25]. In the present
case, decreased with increase in speed which is expected since
increase in speed leads to generation of more frictional heat and is highly dependent on temperature. It lowers down with increase
in temperature of the surface. The variation in speed leads to thermal and mechanical overloading of the asperities. The shear film
disruption results in exposing the surface underneath to frictional
contact and the friction changes accordingly [28,29]. One more reason stated is that with increase in speed, kinetic energy of the
system increase and lot of fictional heat generated at the interface causing decomposition of ingredients and finally fluctuations
in .
3.2.1.3. Effect of metallic fillers on magnitude of . Inclusion of each
type of metallic filler led to increase in . This was basically due to
the reason that metal contents were added at the cost of barite and
metal powders are known to be slightly harder and more abrasive
than very fine and smooth barite powder.
3.2.1.4. Effect of metallic fillers on sensitivity. Among all metallic
fillers, copper proved best followed by brass while iron was poor
from p–s (pressure and speed) sensitivity point of view. One of the
M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
831
Table 7
General observations and behavior observed from Fig. 3.
Observations
50 kmph
80 kmph
100 kmph
BP1
Range: 0.38–0.40
No slope, less undulations
Good behavior
Range: 0.37–0.38
Low slope, less undulations
Good behavior
Range: 0.35–0.37
Low slope, more undulations
Good behavior
CP1
Range: 0.38–0.40
No slope, less undulations
Good behavior
range: 0.37–0.38
Low slope, less undulations
Good behavior
range: 0.36–0.37
Low slope, less undulations
Good behavior
IP1
Range: 0.39–0.41
Low slope, more undulations
Moderate behavior
Range: 0.36–0.38
Low slope, less undulations
Good behavior
Range: 0.35–0.36
Low slope, more undulations
Moderate behavior
Ref
Range: 0.36–0.39
High slope, more undulations
Moderate behavior
Range: 0.33–0.37
High slope, more undulations
Poor behavior
Range: 0.30–0.35
Highest slope, most undulations
Poorest behavior
Overall, performance order from sensitivity to pressure point of view was: CP1 ≥ BP1 ≥ IP1 Ref.
Table 8
General observations and behavior observed from Fig. 4.
Observations
Mild conditions
Severe conditions
BP1 – Moderate range of SS, moderate slope, less undulations, moderate behavior
BP1 – High range of SS, low slope, modearte
undulations – good behavior
CP1 – Highest range of SS, lowest slope, less
undulations, best behavior
IP1 – Moderate range of SS, high slope, more
undulations, poor behavior
Ref – Lowest range of SS, highest slope, more
undulations, poorest behavior
CP1 – Highest range of SS, lowest slope, less undulations, best behavior
IP1 – Low range of speed spread (SS), moderate slope, more undulations, poorest behavior
Ref – Low range of speed spread (SS), moderate slope, more undulations, poor behavior
Overall, performance order from -speed sensitivity point of view was:CP1 > BP1 Ref ≈ IP1.
onwards, started to decrease and hence performance also. Almost
similar behavior was observed in recovery test.
reasons for this could be the appreciable difference in TC of the
fillers. TC of Copper, brass and iron are 400, 110 and 60 (W m−1 K−1 ),
respectively, while those of their composites were 2.41, 2.22 and
2.11 (W m−1 K−1 ), respectively. High TC is responsible to carry away
the frictional heat generated at the interface very efficiently and
hence extent of thermal degradation of organic material is reduced.
This leads to maintain the integrity of the composite as well as surface properties and hence also very effectively. These features
were observed in micrographs of worn surfaces as discussed in SEM
section.
3.2.2.1. Essence of the fade and recovery studies. The fade-recovery
characteristics (performance parameters) of composites are shown
in Table 9 as per the pattern of the report of the JASO C406.
Following are the inferences drawn from Table 9.
3.2.2.2. Resistance to fade (% fade ratio): performance order – higher
the better. This is the most important parameter which reflects
deterioration in when operating conditions are demanding. This
fade behavior was due to increase in surface temperature as the
test was conducted at constant pressure and speed. Generally fade
% ratio in the range of 80–100 are acceptable as per industry norms.
The % fade ratio (higher the better) and hence performance was in
the following order for composites;
3.2.2. Fade and recovery behavior (temperature sensitivity)
Fig. 5 shows the fade and recovery behavior of composites. For
an ideal performance should be in good range (0.3–0.4) and fade
curve ( vs. number of brake applications) should be straight with
minimum slope. In case of recovery mode, curve should be flat with
low slope and should be in the range of pre-fade value.
It was observed that with inclusion of metallic fillers fade and
recovery (F & R) performance improved. Ref composites (without metal content) showed poorest fade behavior as started to
decrease from 2nd braking onwards. Amoung metallic fillers CP1
showed best fade behavior follwed by BP1 as more consistancy in
was observed. IP1 showed high initially but from 5th braking
CP1 (97) = BP1 (96) IP1 (91) > Ref(88)
Fade is caused by thermal decomposition of ingredients due to
accumulation of frictional heat on the surface. Metallic fillers in FMs
on the other hand, help to conduct away the heat from the surface
and effectively leading to lower amount glaze on the surface. These
particles being somewhat hard and abrasive to some extent, try
Table 9
Fade and recovery responses of the composites as per JASO schedule.
Segment
Parameters
Fade
-Fade
% Fade ratio
Max. disc temperature (◦ C)
Recovery
-Recovery
% Recovery ratio
Ref
BP1
CP1
IP1
0.331
88
317
0.410
96
270
0.390
97
251
0.395
91
245
0.410
86
0.436
94
0.425
88
0.434
90
832
M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
Fig. 5. Fade and recovery behavior of the composites
to abrade the glaze and help in rejuvenating the surface and tend
to modify the quality of transferred tribo-layer. This layer is also
responsible to modify the topography of the pad surface and hence
wear. Thus metallic fillers improve not only the thermo-physical
properties of the composites but also the quality of a transfer film
on the friction interface to improve fade resistance [6–8].
Once again TC of metallic fillers based composites proved very
influencing factor because it conducted away the frictional heat
generated at the interface very efficiently and finally led to lower
temperature rise in the disc.
3.2.2.3. The recovery behavior (% recovery ratio): performance order
– higher the better. Composites which recover their friction level
considerably after fade cycles are rated as good FMs. In general for
acceptable composite, recovery should be in the range of 75–100%.
The recovery % of composites is shown below.
Wear of the composites was measured by weight loss method
after completing the effectiveness and fade and recovery studies.
Wear volume was finally calculated by using density data of the
composites.
BP1 (94) IP1 (90) > CP1 (88) > Ref(86)
3.3.1. Wear behavior of the composites
Fig. 6 shows the wear performance of composites. Lower the
wear, better is the performance. Inclusion of metallic powder
improved the wear performance significantly. CP1 showed highest wear resistance followed by BP1 . Composite IP1 was moderate
performer while composite without metal contents (Ref) exhibited
highest wear.
Wear mechanisms during wearing of friction materials
are extremely complex phenomena since several interactions
and mechanisms are simultaneously operative during material
removal/gain process from both the surfaces. These are dynamic
processes which in turn depend on composition of contacting surfaces including that of third phase (tribo-layer), which in turn
depends on operating parameters such as load, speed and especially
interface temperature.
In the present study following was the order of the wear performance and thermal conductivity of composites;
BP1 showed highest recovery performance followed by IP1 and CP1
while Ref composite recorded lowest values.
In the recovery performance the temperature rise of the pad and
the disc was restricted by using the air blower. As the braking surface is cooled down to room temperature and allowed for re-testing
under same conditions with existing friction film on both the surfaces, the rheology of surface layer changes during next run. The
layer is viscous at high temperature while during recovery run, it is
in solid form. The original film during fade run consisting of loosely
attached wear debris, disintegrates and the surface underneath the
friction film gets hardened. This wear debris during the recovery
run when entrapped in the mating zone of the composite and disc
act as hard abrasives leading to third body abrasion contributing
to the enhanced abrasive action via rolling abrasion mechanism
[30]. Hence the characteristic frictional response of the composites as observed usually in the fade test, gets restored, leading to
a better recovery. There is slight difference in mechanisms during
recovery and fade. During cooling of the composite surface prior to
recovery run friction film deformability gets reduced. This extent
of reduction in the elasticity of the film on both the friction surfaces
is dependent on the composition of the friction film. The elastically
and plastically deforming nature of the friction films at the braking
interfaces has been reported to be dependent on composition by
Trefilov [31]. Thus the in situ changes in the formation and deformation of friction films during recovery cycle are responsible for
the differences in the recovery and fade behavior.
3.3. Studies on wear and worn surfaces
Wear performance order: CP1 > BP1 > IP1 Ref.
Thermal conductivity (TC) order: CP1 > BP1 > IP1 Ref.
3.2.2.4. Effect of metallic fillers on counterface friendliness. The rise
in the disc temperature during fade cycles decides the counterface
friendliness of the pad materials. Lower the rise, better is the friendliness. The performance of composites was in following order:
IP1 (245) > CP1 (251) > BP1 (270) > Ref(317)
Composite IP1 proved best in this aspect followed by CP1 . Ref composite proved poorest showing highest disc temperature.
Fig. 6. Wear behavior of the composites
M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
833
Fig. 7. SEM micrographs of composite CP1 (10% copper powder) highest WR : general features—quite smooth topography, appreciable damage on the surface, some amount
of secondary or primary plateaus, (a) X1000—quite good adhesion of ingredients with matrix and transferred layer, hardly any thick primary plateaus, a thin uniform layer
possibly enriched with metal contents, covering all ingredients, long pieces of broken fibers also lying on the surface indicating some wear. (b) X1000—still longer fibrous
debris, little thicker back transferred layer. Overall—best WR was perfectly correlated with moderately damaged surface.
Fig. 8. SEM micrographs of composite BP1 (10% brass powder)-II best in WR : general features—substantial damage to the surface and ingredients, slightly larger sized wear
debris in large amount lying on the surface, Appreciable amount of secondary plateaus but not very thick and patchy (a) X1000—a cavity where from powdery and fibrous
ingredients were extracted. (b) X1000—considerable amount of wear debris, thin layer of possibly of metallic material spread over the primary surface. Overall—low WR
perfectly matched with considerably damaged surface.
Thus it seems that a fairly good correlation emerged between
TC and wear performance of composites. Liu and Rhee [32] in
their study on semi-metallic composites stated that TC of the composites plays a major role in wear performance and concluded
that semimetallic proved better than organic friction materials in
this aspect. Another reason why copper and brass powder-based
composites showed better wear performance than the iron-based
composite lies in the fact that copper and brass have better malleability and plasticity than that of iron. During braking, Cu also gets
transferred in the tribo-layer on the counterface due to combined
action of frictional heating and interfacial stresses. This Cu in transfer film has more adhesion to the counterface rather than Fe in the
tribo-layer transferred from the composite containing Fe powder.
(It is a well-accepted fact that similar metals are less desirable for
good friction and wear properties). Moreover, the tribo-layer could
be thinner and uniform because of malleability and ductility of Cu
and brass powders apart from higher conductivity rather than Fe
containing tribo-layer which could be patchy, thick and less conducting. The SEM analysis of these composites was also done and is
presented in successive section, which presents the evidences for
thin transferred layers.
3.3.2. Worn surface analysis (SEM observations)
The SEM micrographs of worn surfaces of the composites are
arranged as per their decreasing wear resistance WR in Figs. 7–10.
Fig. 9. SEM micrographs of composite IP1 (10% iron powder)-III best in WR : general features—substantial damage to the surface and ingredients, square shaped, sharp
particles (possibly of iron) exposed on the surface, appreciable amount primary plateaus and still large wear possibly because of weak adhesion of iron powder with resin:
(a) X500—Fe particles in the middle of photograph showing excessive de-bonding. (b) X1000—large damage, large amount of wear debris, heavy transfer of material from
the tribo-layer. Overall—low WR was fairly correlated with heavily damaged surface.
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M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
Fig. 10. SEM micrographs of Ref composite (0% metal filler)-poorest WR : General features—very heterogeneous surface, substantial damage to the surface and ingredients,
indications of excessive removal of ingredients, pulled out fibers and particles, thick transferred layer on some portions and most of the places, heavy transfer of fine wear
debris looking like porous surface (a) X1000—large damage, large amount of wear debris, heavy transfer of material from the tribo-layer sitting on the damaged surface
loosely leading to more wear in successive braking. (b) X2000—extremely heterogeneous surface with very rough topography, loosely placed debris confirming extremely
poor quality of the surface. Overall—lowest WR was fairly correlated with very heavily damaged surface.
The main features are described in the captions. The surface of CP1
(highest WR ) showed smoothest surface topography and evidence
of very thin fine layer of metal (Cu contents) which was responsible for its stable friction behavior and highest WR . In the literature
Jia and Ling [7] also reported about frictional film transfer on the
counterface with rich in Cu due to the combined action of frictional
heating and interface stress. This was claimed to be responsible for
high WR of the composite containing copper. Micrographs of BP1
(Fig. 8) showed appearance of small cracks on the surface indicating
slightly higher wear of this composite.
Secondary plateaus with shiny patches and some degradation of ingredients were observed on micrographs. The friction
layer/friction film/tribo-layer/secondary plateaus terms are used
by the researchers [33–37] to describe the compacted wear debris.
This is generally responsible for the deterioration in friction and
wear behavior. These secondary plateaus are because of back
transfer of organic material such as resin (charred, degraded, and
softened), Aramid fibers, cashew nut shell liquid (CNSL) powder,
graphite, etc. These plateaus do not have adhesion to the pad material as good as that of the original ingredients. Hence these do
not have good load bearing properties resulting in deterioration
in friction and increase in wear.
4. Conclusions
Based on the experimental studies conducted on NAO friction composites developed in this work with variation in metallic
fillers (powdery) for evaluating effectiveness and fade and recovery behavior on brake inertia dynamometer, following conclusions
were drawn:
• Use of metallic ingredients affected most of the performance
properties in a beneficial way. Most of the properties such as
density, porosity, tensile strength, compressibility, thermal conductivity and thermal diffusivity, etc. increased due to metallic
contents. (Cu was most effective in boosting TC significantly
followed by brass and iron). However, hardness and flexural properties deteriorated due to inclusion of metallic contents.
• All composites showed adequate (0.35–0.45) which is in the
desired range as per industrial practice. Decreased with pressure and speed in general for all composites.
• Inclusion of metallic fillers led to appreciable increase in magnitude of , decrease in sensitivity towards pressure–speed,
temperature and wear resistance also.
• Thermal conductivity of the composites played an important role
in enhancement of the performance properties of composites
(fade resistance, recovery performance and counterface friendliness).
• Copper proved best performing metallic filler in all the three
major performance properties viz. performance (friction), sensitivity studies (pressure–speed and F & R) and wear behavior,
followed by brass. Iron powder proved moderate in these aspects.
Acknowledgement
Authors acknowledge the funding by Department of Science and
Technology (Govt. of India) to carry out this work.
Appendix A. Japanese automobile standard organisations
Braking device Dynamometer Test Procedure-Passenger car
(JASO 406 C).
A.1. Scope
This standard specifies the dynamometer test procedure to
check on performance of normally operated service brake devices
for passenger cars, however, except two wheeled vehicles.
Remark: in this standard and unit and numerical values given in
{} are based on the international system of units (SI) and are given
for references.
A.2. Purpose
This standard aims to establish a unified dynamometer test procedure in order to make comparisons of performances of service
brake devices for passenger cars, through the simulation of road
test and use conditions.
A.3. Definitions
Definitions of major terms (initial speed, braking interval and
brake initial temperature before braking) used in this standard are
as defined in A.3 of JASO C 446 (general rules of brake of automobile and motorcycles). The brake initial temperature before braking,
where plural brake device are tested at the same time, small be
represented by the highest value.
A.4. Vehicle classification
Test vehicle will be classified in accordance with the provision
given in A.4 of JASO C 446 and shall be expressed respectively as
P1, P2, P3 and P4 in terms of their nominal maximum speeds.
M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
Category P1: vehicles with their nominal maximum speed
exceeding 140 km/h.
Category P2: vehicles with their nominal maximum speed
exceeding 110 km/h and up to 140 km/h.
Category P3: vehicles with their nominal maximum speed
exceeding 90 km/h and up to 110 km/h.
Category P4: vehicles with their nominal maximum speed up to
90 km/h.
A.5. Condition of each part of brake device
Condition of each part of brake device upon testing shell be, as
a rule, as specified in A.5 of JASO C 446.
A.6. Test conditions
(1) Moment of inertia: The calculation equation and setting of the
moment of inertia shall be as specified in A.7.1 of JASO C 446.
Test load, however, shall be as follows, according to the type
of test equipment such as dual or single dynamotors, and each
test procedure.
(a) In this case where a front and rear combination test is carried out using a dual dynamometer: 1/2 of the total vehicle
load.
(b) In case where a front and rear combination test is carried
out using single dynamometer or a dual dynamometer.
Value obtained by dividing 1/2 of the total vehicle load (the
full value of the total vehicle load if the test is done using a dual
dynamometer) by the braking force ratio between the front and
rear wheels upon braking with the deceleration of 0.45 g.
(2) Braking torque: The calculation equation for the braking torque
shall be as specified in A.7.2 of JASO C 446. The equation thus
used shall be recorded on the recording sheet.
(3) Temperature measurement: a thermocouple shall be installed at
the fixed side, as a rule, according to A.4 of JASO C 446.
Remark: If the test was not done according to the above, it
shall be recorded on the recording sheet to that effect.
(4) Cooling wind: The wind velocity shall be, as a rule, 11 m/s and the
wind shall be so adjusted that it may blow against the projected
surface of the brake device uniformly and continuously. The
temperature shall be normal.
A.7. Test procedure
A.7.1. Preparation for test
Make sure that the brake device has no abnormalities before
installing it onto the test equipment. Make also sure that no grease,
paint of any other foreign matte is adhered onto the lining.
In order to measure lining wears accurately, determine measuring locations on brake shoes or pads prior to the test. The number
of measuring points for a shoe shall be 5 on each side, which shall
total to 10 points for both sides. For a pad, on the other hand, measuring points shall be 4–6, as a rule. Clean up the friction surface of
the drum of disc with acetone, etc.
Install a thermocouple at a proper location on the shoe or pad,
install the brake device on the test equipment and center it properly, and record the deviation of the drum or disc accordingly.
A.8. Test items and sequence of tests
Test Items and sequence shall be as follows. Each braking shall
be so applied that the vehicle comes to a complete stop. For a vehicle
on which the brake device temperature dies not rise easily, the test
may be carried out by changing the initial brake temperature from
80 to 60 ◦ C or 120 to 80 ◦ C, respectively.
835
(1) Initial measurements: The thickness of the lining or pad, of any
other dimensions of the brake device may be measured and
recorded as called for.
(2) Preburnish check
Initial speed:
Braking deceleration:
Braking interval:
Number of applications:
50 km/h
0.3 g
120 s
10 times
(3) First (Preburnish) effectiveness test
Initial speed: 50 and 100 km/h for P1 and P2vehicle s 50 and
80 km/h for P3 vehicles.
Braking deceleration: between 0.1 and 0.8 g
Initial temperature: 80 ◦ C
Numbers of applications: The brake applications shall be
repeated until the record can be obtained for 5 or more
measuring points as equally as possible, within the range of
specified braking deceleration.
Remark 1: The lower initial speed conditions shall be
employed first in the test, and the lower deceleration shall
be used, then the higher deceleration, respectively.
Remark 2: If it is necessary to increase the brake temperature to a certain degree prior to the braking the burnish
conditions shall be used.
Remark 3: Braking shall be done with the constant output
or input. It shall be recorded whether a constant output or a
constant input was used in the test.
(4) Burnish
Initial speed: 65 km/h
Braking deceleration: 0.35 g
Initial temperature: 120 ◦ C
Number of applications: 200 times
(5) Second effectiveness test
The first effectiveness test specified in (3) above shall be
repeated. The initial speeds however shall be as follows.
P1 vehicles: 50, 100 and 130 km/h
P2 vehicles: 50, 80 and 100 km/h
P3 vehicles: 50 and 80 km/h
P4 vehicles: 50 and 65 km/h
(6) First Reburnish repeat the (4): The number of applications
shall be however, 35 times.
(7) Emergency brake test (optional item): Assuming the failure
of a single brake system, the following test may be carried out.
Initial speed:
P1 vehicles, 80 km/h (100 km/h)
P2 vehicles, 80 km/h
P3 vehicles, 65 km/h
P4 vehicles, 50 km/h
Braking declarations: between 0.1 and 0.25 g
Initial temperature: 80 ◦ C
Number of applications: The brake applications shall be
repeated until the record can be obtained for 4 or more
measuring points as equally as possible, within the range of
specified braking deceleration.
Remark 1: for four wheel and dual dynamometers the test
shall be carried out for each system according to the failure. For
single dynamometers, the moment of inertia corresponding to
test.
Remark 2: The initial speed for P1 vehicles shall be as a rule,
80 km/h but 100 km/h may be used for exceptional cases.
Remark 3: After the completion of the emergency brake test,
a burnish test with the same conditions as in (6) first Reburnish
shall be repeated, then move over to the following tests.
(8) First fade-recovery test
(a) Baseline check
Initial speed:
50 km/h
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M. Kumar, J. Bijwe / Wear 269 (2010) 826–837
Braking deceleration: 0.3 g constant or a constant pressure to obtain 0.3 g
Initial temperature: 80 ◦ C
Number of applications: 3 times
Remark 1: When the pressure is set constant in the test,
select an appropriate pressure before the test that may
allow the declaration of 0.3 g.
Remark 2: If it is necessary to increase the brake initial
temperature to a certain degree in the fade-recovery test,
refer to (4) Burnish conditions.
(b) Fade test
Initial speed:
(9)
(10)
(11)
(12)
(13)
(14)
100 km/h for P1 vehicles
80 km/h for P2 vehicles
65 km/h for P3 and P4 vehicles.
Braking declarations: 0.45 g constant, or a constant pressure that allows 0.45 g
Initial temperature: 60 ◦ C at the first time
Braking interval: 35 s for P1 vehicles
40 s for P2 vehicles
45 s for P3 and P4 vehicles
Numbers of applications: 10 times
Remark 1: When the test is to be done with a constant
pressure, select an appropriate pressure in advance so that
a deceleration equivalent to 0.45 g may be obtained, after
the completion of the baseline test. The number of brake
applications, however, shall be up to 3 times.
Remark 2: The interval between the vehicle stop and
the restarting shall be as short as possible. Carry out the
recovery test immediately after the completion of test. The
braking interval up to the first recovery test shall be 120 s.
Remark 3: If the test cannot be done with the abovementioned braking interval, the time cycle upon the fade
test may be extended to 40 or 60 s. In such case the braking
interval thus used shall be recorded.
(c) Recovery test
Initial speed: 50 km/h
Braking declarations: 0.3 g constant or a constant pressure obtained in baseline check
Braking interval: 120 s
Numbers of applications: At least 12 times.
(d) Effective spot check (optional item)
Initial speed: the same as in (b) fade Test.
Braking declarations: the same as in (b) fade Test.
Initial temperature: 60 ◦ C
Numbers of applications: 2 times.
Second Reburnish
The first Reburnish specified in (6) above shall be repeated.
Second fade-recovery test
The fade-recovery test specified in (8) above shall be
repeated. The number of application shall be, however, 15
times.
Third Reburnish
The first Reburnish specified in (6) above shall be repeated.
Third effectiveness test
The second Effectiveness test specified in (5) above shall be
repeated. For initial speed however, attest according to following initial speeds may be dine as an optional test.
Fourth Reburnish
The first Reburnish specified in (6) above shall be repeated.
Water recovery test
(a) Baseline check
Initial speed: 50 km/h
Braking deceleration: 0.3 g constant or a constant pressure to obtain 0.3 g
Initial temperature: 80 ◦ C
Number of applications: 3 times
Remarks: When the pressure is set constant in the test,
select an appropriate pressure before the test that may
allow the declaration of 0.3 g.
(b) Water immersion
With the brake released, allow the frictional material
surface to be thoroughly immersed in water for 120 s while
rotating the brake slowly at 10–30 rpm. In case of a drum
brake, the brake may be taken out (of the system) and
immersed in water.
Remarks: Test shall be started immediately after the
water immersion
(c) Recovery test
Initial speed: 50 km/h
Braking declarations: 0.3 g constant or a constant pressure obtained in baseline check
Braking interval: 60 s
Numbers of applications: at least 15 times.
(15) Final measurement and inspection
The brake shall be inspected and the observation results
shall be recorded. The initial measurement specified in (1) in
the foregoing shall then be repeated.
(8) Recording
(1) All noise, vibrations and other abnormalities shall be
recorded.
(2) Braking torque, pressure, temperature and braking initial
speed (rotational speed) during tests shall be recorded.
(3) Pressure and braking torque shall preferably be measured continuously.
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