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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

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)

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(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

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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

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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.

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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

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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)

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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

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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

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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

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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%

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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.

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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.

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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.

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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.