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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.
123
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
124
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
125
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
126
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.
127
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
128
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.
129
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
Hardness HRS 105,110,
118,120, 125 89,91,97,
98, 99 85,86,88, 90,91,95
Acetone Extraction % 0.58, 0.87 0.78, 0.95 0.89, 0.90
Heat Swell In mm 0.14 0.15 0.12
Loss of Ignition % 32.2,32.6 37.2, 37.1 36.2,36.8
Test method as per ISO 6312
Cold Shear Strength MPa 3.952, 4.089 4.118 4.216 3.971, 4.138
Test Method as per ISO 6310
Compressibility % 0.072 0.126 0.057
6.6 FRICTION BEHAVIOUR OF THE BRAKD PADS –(Pressure
and Speed sensitivity)
6.6.1 Effectiveness studies (pressure–speed sensitivity)
Change in µ 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 a variety of
braking conditions.
6.6.2 Pressure sensitivity
Variation in µ with an increase in deceleration (g) at each constant
speed reflects the µsensitivity towards pressure and is shown in figures 6.4(a)
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–(d). For an ideal one, slope of the curve and undulation in curve should be
minimum.
(a)
(b)
(c)
131
(d)
Figure 6.4 (a) I Effectiveness (b) II Effectiveness (c) III Effectiveness (d)
Overall Effectiveness
In present results, µ of the composites was in the range of 0.32–
0.41 which is in the acceptable range of automotive applications. With an
increase in pressure and speed, µ decreased for all the composites which are
as per trends in the literature.
µavg: average coefficient of friction of 20 brake applications during one test
µmax: highest µ observed during 20 brake applications for one particular test
Friction stability : (µavg/ µmax x 100) as a function of braking pressure and
sliding speed.
Increase in the amount of resin and friction dust led to decrease in
µ. This was basically due to the reason that the barytes is replaced by the
binder and the friction dust, which are organic in nature and tends to burn off
at elevated temperatures. It was observed that µavg was high for all the
composites at low speeds and at low loads. A similar observation was
reported earlier by (Erikson 2000) in the case of non asbestos friction
composites, while studying the contact phenomena and squealing behaviour
132
of brakes. As the operating conditions of load and speed became more severe,
the frictional fluctuations and magnitudes tended to reduce. It was also
observed that for all the speeds, the composites registered highest values of
µavg at low pressure (3 m/s2). This was attributed to the fact that the load
carrying elements which are strongly held by the matrix on the friction
surface operate below their load limits (Blau 2001). Hence the shear stress
(input) is converted to frictional output to the maximum extent leading to high
µavg. When the pressure and speed increase, the friction couple gets
overloaded mechanically and thermally which redistributes the input shear
stress. A greater share of these shear stresses is released through thermal
overloading of the contacting asperities, leading to the fracture of the same
and hence less is left to be reflected as a frictional output. This caused
decrease of µavg at higher pressures. Similarly, an increase in speed leads to
the thermal and mechanical overloading of the asperities. This causes shear
film disruption which results in exposing the surface underneath to frictional
contact and the friction changes accordingly(Person 2000 and Severin 2001).
Another reason may be due to the higher heat energy developed to increase in
speed causing decomposition of the organic ingredients which causes the
fluctuations in coefficient of friction.
6.7 FRICTION STABILITY OF THE BRAKE PADS
With the increase in amount of resin and friction dust, the friction
stability decreased due to fine and smoothness of friction dust with very low
Mohs hardness as shown in the figures (6.5 (a) & (b)). Among the three
composites, BPH showed relatively broader variation. Such broader variations
may be attributed to the relatively unstable characteristics of the operating
friction films (friction layers) at the braking interface (Wirth 1994). The
performance order is as follows: BPL>BPM>BPH
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(a)
(b)
Figure 6.5 (a) Friction stability for 3.0 m/s2, (b) Friction stability for 6.0
m/s2
134
However to have a quantitative appreciation of the performance
sensitivity to the selected operating variables, further analysis of the friction
data is carried out.
6.8 REGRESSION ANALYSIS OF FRICTION DATA
In the previous section, the performance of all the composites was
evaluated under different speeds and pressures (deceleration). The focus was
mainly on the tribological performance namely the frictional stability. That is
the consistence of friction at various operating conditions. But these efforts
did not divulge the degree of involvement of three concurrently operating
parameters (viz pressure, speed and their mutual interaction) and the material
itself for frictional stability. Such an analysis is imperative to have more
understanding about the behaviour of the brake pads. Also, it is interesting to
note down the most dominating parameter on the frictional properties, which
the JASO test could not reveal properly (operating parameter or composition).
Hence, in this section, the experimental data of second effect are further
analyzed with regression model based on test design methods. The regression
coefficients of the variety of individual and interactive parameters were
calculated. The selected coefficients were then analyzed for their favored
roles in the friction performance of the brake pad.
6.9 TEST ARRANGEMENT AND RESULTS:
The contributing effects of braking pressure (Z1:MPa) and sliding
speed (Z2:ms-1) and their interactions on the coefficient of friction were
investigated. The test arrangement and results are listed in Table 6.5. The
basis for the statistical analysis was drawn from the experimental design
matrix showing the level of each factor used for each run.
135
Table 6.5 Experimental arrangement of coefficient of friction during second effect in JASO test and results
Experiment Number
Speed (m/s) Z1
Pressure (m/s2) Z2
Z12=Z1*Z2 BPL Y1
BPM Y2
BPH Y3
1 180(low temp) 3.0 540 0.38 0.36 0.37
2 180(Low temp) 6.0 1080 0.35 0.33 0.42
3 180 3.0 540 0.36 0.36 0.35 4 180 6.0 1080 0.35 0.35 0.45 5 360 3.0 1080 0.47 0.46 0.42 6 360 6.0 2160 0.44 0.44 0.45 7 468 3.0 1404 0.43 0.42 0.41 8 468 6.0 2808 0.36 0.35 0.37
Regression equations between friction coefficient and its influencing factors
To make the data processing and the calculation of the regression
coefficients convenient according to the experimental level codes, the
investigated parameters were encoded to give the following normalized
variables oscillating within the range of (-1,1) (28,30)
X11 = (Z Z ) / D1 1 1 (6.1)
Z1 = Mean value of the speed
D1 = Difference between the consecutive speeds
X12 = (Z Z ) / D2 2 1 (6.2)
Z2 = Mean value of the pressure
136
Where X11 and X12 stand for the first order code of sliding velocity
and braking pressure respectively.
Coding of the parameters summarized in Table 6.6 with the
statistical calculation and regression coefficients bj, where Bj,Dj and bj are
expressed as follows:
Bi = ji i
8(X . )i 1
(6.3)
Di = 2
8(X ji)i 1
(6.4)
137
Table 6.6 Analysis of regression coefficients of selected composites
Codes Speed (m/s) X1 Pressure (m/s2) X2 X12=X1*X2
1 -0.65 -0.5 0.325
2 -0.65 0.5 -0.325
3 -0.65 -0.5 0.325
4 -0.65 0.5 -0.325
5 0.58 -0.5 -0.29
6 0.58 0.5 0.29
7 0.95 -0.5 -0.475
8 0.95 0.5 0.475
Dj 4.1678 2 1.04195
Bj(1) 0.3423 -0.07 -0.0289
bj(1) 0.0821 -0.035 -0.0277
Bj(2) 0.35 -0.06 -0.293
bj(2) 0.0839 -0.03 -0.2815
Bj(3) 0.2121 0.07 -0.591
bj(3) 0.0508 0.035 -0.5672
bj = Bj / Dj (6.5)
In the above equations (eqns (6.3)-(6.5)), Bj is the statistical
parameter signifying the interactive roles of the operating variables on the
coefficient of friction. Dj on the other hand, is the statistical parameter
signifying the net normalized magnitude of the operating variables over all
the experiments, while bj is the regression coefficient. The regression
coefficient bj can be taken as a quantitative measure of an extent of
influencing parameter on the µ. Higher the coefficient of bj, stronger is the
138
role of that parameter. Thus, it projects a quantitative estimation of the
sensitivity of friction coefficient to the braking pressure, sliding speeds and
their mutual contributions apart from the material itself. The stronger the
influence of a parameter, the higher is the sensitivity of µ to that parameter
and poorer is the performance. Based on the calculated coefficients as the
statistical parameters, neglecting those with minor contributions and the
second order interactions on the friction performance of the materials, the
regression equations between the coefficient of friction (µ) and each of the
individual contributing parameters along with their interactions for the three
varying proportions of the friction dust and the resin based friction material
systems have been obtained. The respective equations relating to the
coefficient of friction µ ( µ1, µ2, µ3, corresponding to BPL,BPM and BPH
respectively), corresponding to the three selected composites and the various
codes of operating variables are expressed in the following order:
µ1= 0.3903+ 0.0391X11 -0.0336X12 -0.0241X11*12
µ2=0.3801+ 0.0420 X11 -0.0286 X12 -0.0250 X11*12
µ3= 0.4046+ 0.0062 X11+ 0.0385 X12 -0.0609 X11*12
The above equations have been converted into the following
equations (Eqs. (6)-(8)), using Eqs.(1) and (2):
µ1= 0.3480 + 0.0003Z1 -0.0008Z2 (6.6)
µ2 = 0.3191 +0.0004 Z1 + 0.0019 Z2 (6.7)
µ3 = 0.2250+ 0.0005 Z1 + 0.0386 Z2 -0.0001Z12 (6.8)
From Equations (6.6)-(6.8), it was evident from the magnitude of
coefficients that the most influencing parameter on the friction sensitivity was
139
due to the material formulation itself followed by the first order contribution
from sliding speed and then the braking pressure. This is in agreement with
the work carried out by Lei Xu 2012 , who conducted the similar statistical
analysis for brake applications for hoist in coal mines.
Influence of the mutual interaction of the parameters, however, was
negligible. Selected amount of resin and friction dust had the most dominant
influence on the µ rather than the applied pressure and speed. The order was
as follows:
BPL > BPM > BPH
BPH composite showed the maximum influence on µ-sensitivity
followed by an BPM. Composite BPL is found to be less sensitive and hence
the best performer.
In case of earlier section also, the friction composite with a higher
percentage of resin and friction dust proved to be the worst performer with
more sensitivity towards pressure and speed, with inconsistent friction, while
the medium and lower percentage level of friction dust and resin namely BPM
and BPL proved least sensitive. Thus, the regression analysis proved to be a
beneficial tool to finalize the performance order of the composites from
sensitivity to both, speed and pressure point of view.
6.10 RESPONSE SURFACE METHODOLOGY
Response surface methodology (RSM) is a collection of
mathematical and statistical techniques for empirical model building. By
careful design of experiments, the objective is to optimize a response (output
variable) which is influenced by several independent variables (input
variables). An experiment is a series of tests, called runs, in which changes
140
are made in the input variables in order to identify the reasons for changes in
the output response. Here, the objective is to optimize a response (friction
coefficient and wear resistance) which is influenced by several independent
variables (ingredients in the formulation & operating variables namely the
speed and pressure). Also, it is difficult to analyze the varying percentage of
the ingredients in the formulation. In our experimental analysis, it was found
that the lower amount of resin contributes for higher friction with slightly
higher amount of wear. And for most of the tribological properties lower the
amount of resin and friction dust, satisfactory is the performance. Still, it is
interesting to see what happens if the amount of resin and the friction dust go
for lower levels. Hence, it becomes important to use a mathematical technique
like RSM to optimize the loading levels of the ingredients in the formulation.
Figure 6.6 loading level of friction dust by (a)6.33%,(b)9.33%,(c)11.33% &(d)15.33%
141
From the plots 6.6 (a – d) it is clear that, the loading level of 10.11
by weight percentage of resin and 9.33 by weight percentage of friction dust
gives the desired level of friction coupled with the lower amount of wear.
6.11 FADE AND RECOVERY BEHAVIOR (TEMPERATURE
SENSITIVITY)
Fig. 6.7 shows the fade and recovery behavior of composites. For
an ideal performance µ should be in good range (0.3–0.4) and fade curve (µ
vs. number of brake applications) should be straight with less undulation. In
case of recovery mode, the curve should be flat with low slope and µ should
be in the range of pre-fade value.
Figure 6.7 Fade and recovery behaviour and Temperature rise in the rotor disc.
142
Table 6.7 Fade and recovery behavior
Fade & recovery –I Parameters BPL BPM BPH
Fade
µ-fade 0.35 0.27 0.29
% fade ratio 20 24 32
Max. disc temp (oC) 380 395 432
Recovery µ-recovery 0.37 0.46 0.35
% recovery ratio 93 70 73
The % fade ratio (lower the better) and hence performance was in
the following order for composites;
BPL(20) > BPM(24) > BPH (32)
The recovery % of composites is shown below.
BPL(93) > BPH(73) > BPM (70)
The change of friction coefficient during sliding depends on the
changes of the real area of contacts at the friction interface, the strength of the
binder resin and the frictional properties of the ingredients at elevated
temperatures. Hence µ seems to be related to the change of real contact at the
friction interface and the change of the mechanical properties of resin and
other organic components above the glass transition temperature. The addition
of cashew friction dust particles provides adequate elasticity to the friction
material (Yuji Shishido 2009) and, as a result, the frictional characteristics are
stabilized, since the contact area of friction surfaces increases through the
elastic deformation on engaging with partner component of friction at lower
temperatures.
143
Due to rise in temperature as the braking is applied continuously,
the pads are forced to work outside their temperature continuum (above the
degradation temperature of the resin) the resins are burned off rapidly which
causes them to melt and the melted resin acts as a lubricant between the pad
and the disc which causes the brake to slip, that is, it reduces the coefficient of
friction µ. At 400°C, the degradation of the friction dust also happens, based
on the thermal data shown earlier. The temperature rise in the disc is recorded
as follows:
BPH (412°C) >BPM (390°C) >BPL (376°C)
From TGA it is reported that the friction dust starts to burn off only
after 400° C. The data points of the temperature rise in the disc of the three
composites show only one which is above 400°C. This BPH material is the
one showing significantly lower µ at 130 kmph where the temperature goes
above 400°C. Hence it is observed that the burning off of the friction dust
does not happen in all stops except the high speed stops in BPH material.
Thus, it is possible to correlate the thermal stability from TGA of organic
contents in the brake pad to the friction performance results in fade and
recovery sections. Therefore, to retain the integrity of the friction dust, the
formulation can be designed in such a way that the peak temperature does not
go above 400°C.
6.12 SUMMARY
All composites showed sufficient µ (0.30-0.45), which is in
the desired range as per the industrial practice. The friction
coefficient decreased with speed and pressure in general for
all the composites.
144
Decreasing the amount of resin and friction dust led to an
appreciable increase in magnitude of µ and stability of
friction.
Fade testing of all composites tested resulted in temperature
rise higher than the decomposition of the resin. Only in the
case of BPH, the temperature rise in fading exceeded the
decomposition temperature of friction dust.
Response surface methodology indicates the optimum
percentage of resin and friction dust, which is in agreement
with the test results.
Based on the regression analysis, it was concluded that the
major influences on µ were due to the material itself followed
by first order contributions from the speed. Speed was
observed to be a more dominant parameter, which influenced
the µ rather than the pressure. However, the contribution of
the mutual interaction of braking pressure and speed was
negligible.
Hence, this optimum level of organic contents (10.11 wt% of
resin and 9.33 wt% of friction dust) tend to decrease the µ
sensitivity towards pressure and speed and hence the overall
performance of the composites.
Having studied the effect of resin and cashew dust, the other organic
component namely the different organic fibers aramid, cellulose and acrylic
was studied which is detailed in the chapter 7.