122 CHAPTER 6 EFFECT OF VARYING FRICTION DUST AND RESIN ON NON ASBESTOS DISC BRAKE PAD: STABILITY AND SENSITIVITY OF μ TO PRESSURE, SPEED AND TEMPERATURE 6.1 GENERAL A lot of work was reported to effect of inorganic abrasives namely Al 2 O 3 and ZrSiO4 on friction performance of Automotive brake friction materials (Mustafaboz 2007). The cashew friction dust is an organic friction modifier and is used as one of the prime raw material because it serves as a stability agent in brake products (Yuji handa 2008). Cashew friction dust is an organic based spongy material and MOHS hardness is virtually in the scale of 0 to 1 from the theoretical point of view. This can be further validated by its rotor kindliness effect. Cashew containing friction particles has the ability to absorb the heat created by friction while retaining braking efficiency. It is a major export product of India and the Asian subcontinent. Cashew friction particles are cross linked Phenolic Polymers derived from Cashew Nut Shell Liquid (A Natural Phenol) by using an exclusive process to give the desired friction properties. Like organic friction modifiers, inorganic modifiers also boost up the friction level, but the MOHS hardness of the material like alumina and silica are between 7 and 8. This shows excessive aggressiveness against the mating surface, generating more disc rotor wear dust, which tends to create the sticking problem and increases the amount of disc rotor wear.
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122
CHAPTER 6
EFFECT OF VARYING FRICTION DUST AND RESIN
ON NON ASBESTOS DISC BRAKE PAD: STABILITY
AND SENSITIVITY OF µ TO PRESSURE, SPEED
AND TEMPERATURE
6.1 GENERAL
A lot of work was reported to effect of inorganic abrasives namely
Al2O3 and ZrSiO4 on friction performance of Automotive brake friction
materials (Mustafaboz 2007). The cashew friction dust is an organic friction
modifier and is used as one of the prime raw material because it serves as a
stability agent in brake products (Yuji handa 2008). Cashew friction dust is an
organic based spongy material and MOHS hardness is virtually in the scale of
0 to 1 from the theoretical point of view. This can be further validated by its
rotor kindliness effect. Cashew containing friction particles has the ability to
absorb the heat created by friction while retaining braking efficiency. It is a
major export product of India and the Asian subcontinent. Cashew friction
particles are cross linked Phenolic Polymers derived from Cashew Nut Shell
Liquid (A Natural Phenol) by using an exclusive process to give the desired
friction properties. Like organic friction modifiers, inorganic modifiers also
boost up the friction level, but the MOHS hardness of the material like
alumina and silica are between 7 and 8. This shows excessive aggressiveness
against the mating surface, generating more disc rotor wear dust, which tends
to create the sticking problem and increases the amount of disc rotor wear.
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Another important reason to use the friction dust is to improve a performance
parameter called compressibility of the brake pads. Compressibility
determines the elastic behavior of the pure friction material of a pad. This is
the amount of squishiness in the pad. This characteristic is important for the
noise reduction aspect of the pad. The chemistry, particle size and loading
level of the friction dust are used to control the compressibility characteristics.
Also, the various ingredients in the brake pads are held together by the
powder resin. If the binder amount is too less it results in material weakness
and if too much is used then there is a friction drop in high temperatures.
Hence it becomes necessary to find out the optimum loading levels of these
organic components and their effect on fade and recovery and wear
performance. Three pads with varying resin (10.11, 11.11, 12.11 percentage
by weight) and the cashew dust (9.33, 10.33, 11.33 percentage by weight) are
fabricated and their effect in relation to frictional stability (fade and recovery
performances) is studied by carrying out the test on the Inertia dynamometer
following JASO C 406 schedule.
A lot of reports are available on fade and recovery behaviour
(µ-temperature sensitivity) of brake pads (Bijwee 2005). However, very less
is reported on µ-pressure and µ-speed sensitivity of brake pads (Satapathy
2005). Gopal 1995 studied the load-speed sensitivity of developed FMs based
on various fibers like: aramid, glass, steel wool and carbon.
Rhee 1974 in his study reported that the influence of speed on the
tribo-performance is via abrupt changes in interfacial temperatures. Beyond a
threshold speed value stabilized wear was reported. However, these
observations were based on reduced scale composites and not on realistic
FMs. Satapathy 2006 in his work reported that the braking pressure was the
most influential operating parameter on the wear performance of FMs rather
than speed. Moreover, the pressure and speed sensitivity, especially on the
amount of organic contents in the formulation is very limited. Hence, in this
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chapter an attempt is made to carry out the regression analysis by considering
the experimental results in the second effect of the test design which is
imperative to further validate the test results.
6.2 ORGANIC FRICTION MODIFIER
The properties of the resin have been characterised in the previous
chapter. Hence, the organic friction modifier namely, the cashew friction dust
characterization is carried out here.
6.2.1 Decomposition Temperature
Figure 6.1 TGA of the cashew friction dust
The degradation temperature of the cashew friction dust is found to
be well above 400°C. From the TGA (fig 6.1) it is clear that decomposition
of resin starts after 325°C while that of friction dust starts after 400°C
6.3 FABRICATION OF THE BRAKEPADS
The friction materials are fabricated in three steps which are mixing
of the ingredients, preforming and curing in a compression molding machine
and post baking. Three pads are developed by varying the resin content and
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friction dust content which is shown in Table 6.1. The compensation is carried
out by the inert filler that is barytes. For reference they are designated as BPL,
BPM and BPH.
Table 6.1 Formulation of the brake pad
S.No Raw Material BPL BPM BPH
% by wt % by wt % by wt
1 Kevlar 2.00 2.00 2.00
2 Cellulose fiber (Arbocel ZZ8-1R)
1.00 1.00 1.00
3 Barytes powder fine 8.86 6.86 4.86
4 MCA Rockwool fiber 15.00 15.00 15.00
5 Lapinus fiber RB 250 17.16 17.16 17.16
6 Vermiculate 7.25 7.25 7.25
7 Green Chrome Oxide 1.04 1.04 1.04
8 Steel wool 10.15 10.15 10.15
9 Synthetic Graphite 5.15 5.15 5.15
10 Alkyl Benzene Modified Phenolic resin
10.11 11.11 12.11
11 Crumb rubber 2.07 2.07 2.07
12 Chemigum Rubber NBR 2.07 2.07 2.07
13 Cashew Friction dust 9.33 10.33 11.33
14 China clay 4.66 4.66 4.66
15 Yellow Iron Oxide(Natural)
3.11 3.11 3.11
16 Zinc oxide 1.04 1.04 1.04
Total 100 100 100
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The brake friction composites in the form of Pads were moulded in
hydraulic Press (Table 6.2). The surfaces of the pads were then polished with
the grinding wheel to attain the desired thickness.
Table 6.2 Detail of the processing condition for brake pad
Procedure Conditions
Sequential mixing Total duration 12 mins feeder RPM 300 Chopper RPM 3000 Sequence
(a) Power ingredients
(b) Pulps and Fibers
Curing Temp. 145°C;
Compression 17 MPa; Curing time: 9 mins
Post- curing 120°-160°C, 8 hr.
6.4 TEST SET-UP AND PROCEDURE FOR BRAKE
EFFECTIVENESS TEST AS PER JASO C-406 SCHEDULE
Table 6.3 Conditions for effectiveness studies
Description Speed
( Kmph)
Brake deceleration
( g ) m/s2
Initial Brake
Temp oC
No. of
Applications
Air Blower
Bedding Test 50 3.0 & 6.0 120°C 200 ON
Effectiveness I 50,100 3.0 & 6.0 80oC 20 ON
Effectiveness II 50,100,130 3.0 & 6.0 < 80°C 20 ON
Effectiveness III 50,100,130 3.0 & 6.0 <100°C 20 ON
The condition for effectiveness studies of the dynamometer used for
testing the brake pads for effectiveness I, II and III is given in the table 6.3.
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Figure 6.2 Disc brake assembly and caliper
Figure 6.3 Inertia brake dynamometer Setup for testing brake performance
6.4.1 Effectiveness studies
Effectiveness study measures the efficiency of a brake pad to
function more reliably under different pressures and speeds. The tests were
carried out mainly to study the influence of pressure and speed. The test is
conducted by first establishing at least 90% conformal contact between the
mating surfaces (pad and the disc) by running for nearly four hours during
bedding. It is done at three different braking speeds viz., 50, 100 and 130
Km/h and at the starting temperature of 80°C. The tests are conducted at two
different decelerations (3.0 and 6.0 m/s2). As the deceleration increased, the
severity of the braking conditions also increased. The amount of deceleration
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is controlled by pressure, which was programmed to achieve a selected rate of
deceleration.
6.5 EFFECT OF RESIN AND FRICTION DUST ON PHYSICAL,
CHEMICAL AND MECHANICAL PROPERTIES
The physical, mechanical and chemical properties of the selected
composites are listed in table 6.4. The specific gravity and the hardness values
are found to increase with the decrease in the wt% of resin and organic
friction modifier. The possible reason may be due to the addition of hard
barytes by replacing the organic resin and the friction dust. Loss of ignition
indicates the mass loss subjected to elevated temperature. Higher amount of
the Loss of ignition of BPH indicates its quicker thermal degradation due to
more amounts of organic substances involved in it. Generally, specimens with
high hardness tend to exhibit low compressibility. But in the present study
compressibility didn’t show any fixed pattern.
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Table 6.4 Physical, Chemical and mechanical properties of the composites
Properties Unit BPL BPM BPH
Test Method as per IS 2742 of the 1994 standard
Specific gravity - 2.63, 2.75 2.56,2.60 2.13, 2.15