friction material (metal reinforcement) analysis of...
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FRICTION MATERIAL (METAL REINFORCEMENT) ANALYSIS OF BRAKE PAD
FOR
LIGHT RAIL TRAIN SYSTEM
ABDUL RASHID BIN ABDUL RAHMAN
A thesis submitted in
fulfillment of the requirement for the award of the
Degree of Master of Science in Railway Engineering
Faculty of Engineering Technology
Universiti Tun Hussein Onn Malaysia
OCTOBER, 2016
iii
To my beloved parents, siblings and friends for their endless love, support and tolerance
iv
ACKNOWLEDGEMENT
In the name of Allah, Most Gracious and Most Merciful, Praise is to Allah, the guidance
that brought this study to a completion.
I would like to take this opportunity to thank Assoc. Professor Dr Hasan Zuhudi
bin Abdullah as my supervisor for his support, encouragement, guidance and motivation
through the years. My special thanks to my parents, En. Abdul Rahman bin Ibrahim and
Pn Roziah Bte Abu Bakar as well as my siblings for the love and their continuous
support and willing to share with me all my joy and pain in completing this study.
My heartfelt thanks go to my co supervisor, Dr Zakiah bt Kamdi in providing
necessary guidance and help. So does the entire technician involved in this study for
their help and advice throughout the project.
I also highly appreciate the cooperation, guidance and unconditional support
from KKTSN committee members as well as my fellows, Sakinah, Aisyah, Nik and
Natasya.
Last but not least, I would like to thanks those who have directly or indirectly
contributed towards the accomplishment of this study.
v
ABSTRACT
Brake friction material is very important in braking system where they convert kinetic
energy of moving vehicles to thermal energy by friction during braking process. The
purpose of this research is to determine the optimal friction materials composition of
brake pad for light rail train system. Currently all the component of the train system
including brake pad is imported from overseas such as Germany. Hence, this research is
use to find the new formulation of the mixture ratio that may replace or compete with
the commercial available brake pad. Three different testing which are density and
porosity test, shore hardness test and wear test were done in order to select which metal
is the most suitable for railway application. Different composition were used,
(Cu30%BaSO430%), (Cu25%BaSO435%), (Cu20%BaSO440%), (Steel30%
BaSO430%), (Steel25% BaSO435%), (Steel20% BaSO440%), (Al30% BaSO430%),
(Al25% BaSO435%), and (Al20% BaSO440%) this study to determine the optimal
properties with lower wear rate. The selected material were mixed and compacted into
desired mould with 5 tons of pressure. The compacted samples were sintered using two
different temperatures which is 600oC and 800oC. Steel30% BaSO430% results in the
optimal composition since the result shows the lowest porosity, highest SD reading of
shore hardness and the lowest wear rate. The samples were analysed by using Scanning
Electron Microscopy (SEM) with an Energy Dispersive Spectrometry (EDS) system to
determine the morphology surface and overall composition of the samples. Comparing
different sintering temperature, the sintered sample of 800oC shows lower wear rate than
the sample sintered at 600oC. This is due to dense sample without crack showing by the
samples sintered at 800oC than at 600oC.
vi
ABSTRAK
Bahan geseran untuk brek adalah sangat penting dalam sistem brek di mana ianya
menukar tenaga kinetik kenderaan bergerak untuk tenaga haba melalui geseran semasa
proses brek. Tujuan kajian ini adalah untuk menentukan optimum komposisi bagi bahan
geseran pada pad brek untuk sistem kereta api aliran ringan. Pada masa ini semua
komponen sistem kereta api termasuk pad brek diimport dari luar negara seperti Jerman.
Oleh itu, kajian ini adalah untuk mencari formula baru dengan nisbah campuran yang
boleh menggantikan atau menjadi pesaing pad brek yang sedia ada. Tiga ujian yang
berbeza dilakukan iaitu ujian kepadatan dan ujian keliangan, ujian kekerasan dan ujian
kehausan untuk memilih logam yang paling sesuai untuk digunakan pada kereta api.
Sembilan komposisi, (Cu30%BaSO430%), (Cu25%BaSO435%), (Cu20%BaSO440%),
(Steel30% BaSO430%), (Steel25% BaSO435%), (Steel20% BaSO440%), (Al30%
BaSO430%), (Al25% BaSO435%), dan (Al20% BaSO440%) digunakan dalam kajian ini
untuk menentukan sifat-sifat optimum dengan kadar haus yang lebih rendah. Bahan
yang dipilih adalah bercampur dan dipadatkan ke dalam acuan yang dikehendaki dengan
5 tan tekanan. Sampel dipadatkan disinter menggunakan dua suhu yang berbeza yang
600oC dan 800oC. Steel30% BaSO430% menghasilkan komposisi yang optimum kerana
hasilnya menunjukkan keliangan yang paling rendah, bacaan SD tertinggi bagi ujian
kekerasan dan kadar haus yang paling rendah. Sampel kemudiannya dianalisis dengan
menggunakan SEM dengan sistem EDS untuk menentukan permukaan morfologi dan
komposisi keseluruhan sampel. Bagi perbandingan suhu pembakaran yang berbeza,
sampel yang disinter pada suhu 800oC menunjukkan kadar haus lebih rendah daripada
sampel disinter pada 600oC. Ini adalah kerana sampel padat tanpa menunjukkan retak
pada sampel yang disinter pada 800oC berbanding pada 600oC.
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLE x
LIST OF FIGURE xi
LIST OF SYMBOLS AND ABBREVIATIONS xiv
LIST OF APPENDICES xv
1 INTRODUCTION 1
1.1 Background of study 1
1.2 Problem statement 3
1.3 Objective 3
1.4 Scope of study 4
2 LITERATURE REVIEW 5
2.1 Introduction 5
viii
2.2 Brake pad 6
2.2.1 Application of brake pad for Light Rail Transit (LRT) 9
2.2.2 Type of brake pad for railway application 11
2.3 Composition of brake friction materials 11
2.3.1 Fiber 12
2.3.2 Filler 14
2.3.3 Binder 15
2.3.4 Friction modifier 15
2.3.5 Lubrication 16
2.4 Friction Materials 17
2.5 Fabrication of brake pad 23
2.51 Powder metallurgy 23
2.5 Characteristic for brake friction materials 25
2.5.1 Friction coefficient 25
2.5.2 Wear rate 26
2.5.3 Noise and judder 27
3 METHODOLOGY 28
3.1 Introduction 28
3.2 Preparation of materials 29
3.3 Methods 30
3.3.1 Composition 31
3.3.2 Mixing process 32
3.3.3 Compaction process 33
ix
3.3.4 Sintering process 35
3.4 Testing 37
3.4.1 Shore hardness 37
3.4.2 Porosity and Density Testing 38
3.4.3 Wear test 40
3.4.4 Scanning Electron Microscopy (SEM) 42
and Energy Dispersive Spectrometry (EDS)
4 RESULTS AND DISCUSSION 43
4.1 Introduction 43
4.2 Parameters of samples 44
4.3 Appearance of samples 45
4.4 Scanning Electron Microscopy (SEM) 48
and Energy Dispersive Spectrometry (EDS)
4.5 Porosity and density test 56
4.6 Shore hardness test 61
4.7 Wear test 64
5 CONCLUSION AND RECOMMENDATION 67
5.1 Conclusion 67
5.2 Recommendation 68
REFERENCES 69
APPENDIX
x
LIST OF TABLES
TABLE N0. TITLE PAGE
2.1
2.2
2.3
3.1
3.2
3.3
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
Different types of brake pad materials on various application
Candidate reinforcing fiber substitutes for asbestos
Literature review on friction materials used
The material selection for break pad fabrication
Composition of the raw material ingredients used for friction
materials
Common compaction pressure for various application
Composition of samples
Samples before and after sintered for copper metal fiber
Samples before and after sintered for steel metal fiber
Samples before and after sintered for aluminium metal fibre
Overall composition of Copper mixture by EDS system
Overall composition of Steel mixture by EDS system
Overall composition of Aluminium mixture by EDS system
Overall composition of Commercial Brake pad by EDS system
10
13
18
29
31
34
44
45
46
47
52
53
54
55
xi
LIST OF FIGURE
FIGURE N0. TITLE PAGE
2.1 The crescent-shaped piece that attach with lining tables is called
web
7
2.2 Bent tabs that hold the break pad is use to reduce break noise 8
2.3 (a) Front view of brake pad that use in ampang line LRT
(b) Back view of brake pad that use in ampang line LRT
10
2.4 Chopped steel wool (steel fibre) that used in medium sized-
vehicles brake pad
12
2.5 Coefficient of friction pattern for a typical lubricated contact. Z is
lubricant viscosity, N′ shaft speed, and P the unit load transferred
radially by the shaft to the bearing
16
3.1 Flow chart of the fabrication process and testing 30
3.2 Ball mill machine used for mixing process 33
3.3 Hydraulic press machine used for compaction process 34
3.4 Low temperature furnace 36
3.5 Sintering profile 36
3.6 Durometer type-D 37
3.7 Weighing machine used for density and porosity test 39
3.8 Grinding and polishing machine 41
3.9 All sample mounted using pipes before it is tested 41
3.10 Scanning Electron Microscopy (SEM) and Energy Dispersive
Spectrometry (EDS)
42
4.1 (a) SEM Micrograph for Cu 1
(b) SEM Micrograph for Cu 2
(c) SEM Micrograph for Cu 3
46
4.2 (a) SEM Micrograph for Steel 1 47
xii
(b) SEM Micrograph for Steel 2
(c) SEM Micrograph for Steel 3
4.3 (a) SEM Micrograph for Al 1
(b) SEM Micrograph for Al 2
(c) SEM Micrograph for Al 3
48
4.4 SEM micrograph for commercial brake pad 49
4.5 EDS for sample Cu 1 50
4.6 EDS for sample Steel 1 51
4.7 EDS for sample Al 1 52
4.8 EDS for commercial brake pad 53
4.9 Density result for copper metal fiber 55
4.10 Porosity result for copper metal fiber 55
4.11 Density result for steel metal fiber 56
4.12 Porosity result for steel metal fiber 57
4.13 Density result for aluminium metal fiber 58
4.14 Porosity result for aluminium metal fiber 58
4.15 Shore hardness test result for copper metal fiber 60
4.16 Shore hardness test result for steel metal fiber 61
4.17 Shore hardness test result for aluminium metal fiber 62
4.18 (a) The mounted sample before wear test 63
4.18(b) The mounted sample after wear test 63
4.19 Percentage of weight loss for copper metal fiber 64
4.20 Percentage of weight loss for steel metal fiber 65
4.21 Percentage of weight loss for aluminium metal fiber 65
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LIST OF SYMBOLS AND ABBREVIATIONS
Cu - Copper
Al - Aluminium
oC - Degree Celcius
cm3 - Cubic centimenters
g - Gram
kg - Kilogram
mm - millimeter
rpm - Rotation per minute
µ - micron
% - percent
LRT - Light Rail Transit
SEM - Scanning Electron Microscopy
EDS - Energy Dispersive System
xiv
LIST OF APPENDICES
APPENDIX
TITLE
PAGE
A 1 Porosity and density test for Copper Metal Fiber 75
A 2 Porosity and density test for Steel Metal Fiber 76
A 3 Porosity and density test for Aluminium Metal Fiber 77
A 4 Durometer reading for Copper Metal Fiber 78
A 5 Durometer reading for steel Metal Fiber 79
A 6 Durometer reading for Aluminum Metal Fiber 80
A 7 Wear test for Copper Metal Fiber 81
A 8 Wear test for Steel Metal Fiber 82
A 9 Wear test for Aluminum Metal Fiber 83
1
CHAPTER 1
INTRODUCTION
1.1 Background of study
Braking system is a crucial part in a rolling stock which where it convert mechanical
energy into heat energy through a friction to stop a vehicle (Yoong et al., 2010). Brake
friction materials play an important role in braking system. They convert the kinetic
energy of a moving car to thermal energy by friction during braking process. The ideal
brake friction material should have constant coefficient of friction under various
operating conditions such as applied loads, temperature, speeds, mode of braking and in
dry or wet conditions so as to maintain the braking characteristics of a vehicle (Jang et
al., 2004). Besides, it should also possess various desirable properties such as resistance
to heat, water and oil, has low wear rate and high thermal stability, exhibits low noise,
and does not damage the brake disc (Xin, Xu, & Qing, 2007). However, it is practically
impossible to have all these desired properties. Therefore, some requirements have to be
compromised in order to achieve some other requirements. In general, each formulation
of friction material has its own unique frictional behaviours and wear-resistance
characteristics.
2
Friction materials are generally consist of a number of different materials. Sometimes up
to 20 or 25 different components are used. These components including:
1. Binder - which holds the other components together and forms a thermally stable
matrix. Thermosetting phenolic resins are commonly used, often with the
addition of rubber for improved damping properties (Chan & Stachowiak, 2004).
2. Structural materials - providing mechanical strength. Usually fibres of metal,
carbon, glass, and/or Kevlar are used and more rarely different mineral and
ceramic fibres. Before its prohibition in the mid 80's, asbestos was the most
commonly used structural fibre (Maleque et al., 2012).
3. Fillers - mainly to reduce cost but also to improve manufacturability. Different
minerals such as mica and vermiculite are often employed. Barium sulphate is
another commonly used filler (Chan & Stachowiak, 2004).
4. Frictional additives - added to ensure stable frictional properties and to control
the wear rates of both pad and disc. Solid lubricants such as graphite and various
metal sulphides are used to stabilise the coefficient of friction, primarily at
elevated temperatures. Abrasive particles, typically alumina and silica, increase
both the coefficient of friction and the disc wear. The purpose of the latter is to
offer a better defined rubbing surface by removing iron oxides and other
undesired surface films from the disc (Selamat, 2006a).
3
1.2 Problem statement
Previously asbestos has been widely used as the main material in a brake pad; however it
has been avoided due to its cancer-causing nature (Idris et al., 2015). The brake pad is a
very important component in a railway where a regular replacement needed and it is
imported from overseas where it might cause higher maintenance cost (Bouvard et al.,
2011). Therefore a new asbestos free brake pads need to be developed and manufactured
locally to avoid health problem and reducing the production cost and eventually will
upgrade Malaysia railway industry level. Thus, the exact formula for the mixture ratio of
the brake pad need to be studied to formulate and create a stronger or at same level of
properties of the brake pad that had been use commercially by local rolling stock.
1.3 Objective
The purpose of this research is to study the wear behaviour of a brake pad for light rail
train system. Currently all the component of the train system including brake pad is
imported from overseas such as Germany. Hence, this research is conduct to find the
new formulation of the mixture ratio that can generate a better and stronger brake pad
for commercial used. The findings of this study will help in identifying which mixture
ratio will produce the finest friction material reinforcement that will generate a good
brake pad. In this study, a sample of specimen has been fabricated and gone through
several testing such as wear test and porosity test. The outcome of this study is expected
to be useful in Malaysia railway industry where it can be a starting point for Malaysia to
develop our own train from scratches including all component such brake pad.
4
The objectives of this study are to:
a) Determine the new ratio of the fibre reinforcement used in fabricating the brake
pad for light rail train (LRT) to enhance the friction material reinforcement.
b) Compare the porosity and density, hardness and wear behaviour of the fabricated
specimen with the commercial used brake pad.
1.4 Scope of study
This study is focus on light rail train system where it has different load and maximum
speed compare to heavy train system and it is focus on the range of study such:
i. Fabrication of brake pad with 9 different content of metal fibre used in the
mixture which is (Cu30%BaSO430%), (Cu25%BaSO435%),
(Cu20%BaSO440%), (Steel30% BaSO430%), (Steel25% BaSO435%), (Steel20%
BaSO440%), (Al30% BaSO430%), (Al25% BaSO435%), and (Al20%
BaSO440%) by using mould with diameter size 20mm and compacted with 5
tons pressure for every sample after mixing process.
ii. Sintering temperature for this study is at 800oC and 600oC.
iii. The characterisation of mechanical properties of the samples using shore
hardness test.
iv. The characterisation of the physical properties of the sample by using density
and porosity test.
v. Study the wear behaviour using weight loss test at 5 N pressures for 5 minutes
using 200 RPM speed.
5
CHAPTER 2
LITERATURE REVIEW
2.1 Introduction
The growth of railway transportation has been increase significantly this day which
require urgent attention in transportation industry. Since railway industry in such a
massive demand, it should be manufactured locally. In Malaysia, they only manage to
manufacture 80% of the train and some of the component was imported such as alarm
system component, seat, wheel, heating, ventilation and air conditioning (HVAC) and
brake pad. Brake pad is a component that been change regularly, hence, if it is
manufacture locally, it can reduce the cost. This study is particularly studied to produce
a local brake pad for railway application.
This study based on previous research in order to improve the outcome of
previous researcher. The theoretical analysis of the brake pad from the operation up until
the details of materials use in a brake pad particularly the one use for railway application
will be discussed in this chapter.
6
2.2 Brake pad
Braking system is the most important parts in a vehicles where it convert the kinetic
energy of a vehicles to thermal energy through friction to stop or slower the moving
vehicles (Talati & Jalalifar, 2009). There are basically made up by several parts of
equipment and brake pads are one of those that play a very important role where it
contact and apply pressure and friction to the brake disc. The pressure and the friction
that applied to the brake disc will eventually slow and stop the wheels. Thus, the
vehicles will stop running when the wheel stop turning.
Though the operation of brake pad looks simple, but the break pad itself play a
huge role where it undergo a great stress every time the vehicles is slow down or stop
because it is depends on the speed of the wheels rotate and the amount of weight that the
vehicles carry. In (Halderman & Mitchell, 2010) book’s explain that there basically two
types of brake system which is disc brake and drum brake and both of the system
applying the same rule where it slow down or stop the vehicles by engaging hydraulic
pistons to press brake pads or brake shoes against the brake disk or brake drums that
rotate along the wheels.
Brake pad and brake shoes is actually a different component which have the
same function. The main difference between those two is their position in the vehicle.
Brake shoes are specially designed to fit in drum brakes, while brake pad are placed on
top of brake disk, and act to apply pressure to the discs when applying the brakes
(Halderman & Mitchell, 2010). The brake shoes are attached to consumable surfaces in
brake systems called lining which use to support the brake shoes. Normally, the shoes
are made of two pieces of sheet steel welded together in a T-shaped cross section (Babel,
1933). Brake shoes are more expensive to produce and it is not good for high
temperature because it is not durable enough and obviously it is not suitable for railway
system because a train running with a huge system and it require equipment that can
sustain in high temperature. However, the interesting about brake shoes are it is a solid
part that can be relined and reused many time as long as the web (The crescent-shaped
7
piece which contains holes and slots as shown in Figure 2.1 ) and lining table are not
damage.
Figure 2.1 : The crescent-shaped piece that attach with lining tables is called web
(Jeffrey & Halderman, 2006)
Basically, the design of a brake pad is somewhat much simpler than brake shoes
where it consists of a block of friction material that attached to a stamped steel backing
plate. Some of the backing plate is installed with a bent tabs that use to prevent noise.
Figure 2.2 show the position of the bent tabs that will hold the pad tightly in place so
that it can prevent any brake noise. Usually, the edge of the lining material on a brake
pad is straight cut to reduce the vibration and noise. As for brake shoes, the lining
material can be fastened to the backing plate in several ways to reduce the vibration and
noise (Jeffrey & Halderman, 2006).
8
Figure 2.2 : Bent tabs that hold the break pad is use to reduce break noise (Jeffrey &
Halderman, 2006)
Brake pads are widely used in a large vehicles including train system. In the
early 1970’s, disk brake is getting on demand and been broadly produce by automobile
manufacturers because it had better braking performance than drum brakes (Idris et al.,
2013). Brake pads operate by converting the kinetic energy of the vehicles to thermal
energy by friction. It happens when the temperature of the brake pad is increase by the
contact with break disc to provide stopping power, thus the break pad starting to transfer
small amounts of friction materials to the disc. In general, to ensure the safety of the
vehicles, the friction materials are required to provide a firm friction coefficient and a
best wear rate at numerous operating speeds, pressures, temperatures and environmental
condition in order to reduce the extensive wear, vibration and noise during braking.
History shown from the past two decades, asbestos had been a common material used in
producing the brake pads due to its durability. However, since 1980s, research show that
asbestos is found out as harmful content and was banned from being used as the
ingredient in producing brake pads as it can cause severe disease (Liew & Nirmal,
2013).
9
The asbestos exposure was actually can cause scar tissue to form in the lungs and
it’s called asbestosis which it gradually will increase the shortness of breath and make a
permanent scarring to the lungs (Frank & Joshi, 2014). Besides, asbestos exposure also
can lead to lung cancer which normally takes about 15 to 30 years after the exposure.
The Occupational Safety and Health Administration (OSHA) have recognised three
levels of asbestos exposure in their standard as it is very important to ensure the safety
of human being. Any vehicles services company must ensure that the asbestos exposure
must less than 0.2 fibres per cubic centimetre (cc) as resolute by air sample and if the
level of exposure is exceed the limit, action will be taken by the OSHA organization
(Halderman, 2010).
Nowadays, the development of asbestos – free brake pads is increasing through
different researcher. There are different kinds of fibres used to replace asbestos such
metallic, glass, ceramic and carbon fibres (Solomon, Berhan, & Pad, 2007). Friction
materials for brake pad are normally contain metallic ingredient which to improve wear
resistance, thermal diffusivity and strength. Most of the vehicles usually using metallic
brake pad and made of iron, copper, steel and graphite. Different characteristic of the
different type of metallic ingredients can affect the friction and wear of the friction
materials (Jang et al., 2004). Metallic brake pad s are commonly used because it is cost
effective and durable despite that it is also provide great performance and best at shifting
heat that generate by friction with the rotor or brake disk.
2.2.1 Application of brake pad for Light Rail Transit (LRT)
Brake pad is the crucial part in vehicles where it faces the greatest wears when every
time the pads are pressed against the rotating disc. Thus, it needs a proper maintenance
to maintain the effectiveness of the vehicles. The friction material of a break pad plays a
very important role where much consideration must be considered such heat resistance
and durability. Table 2.1 shows the different types of brake pad materials on various
applications where every each of application uses different types of brake pad. Every
10
each of brake pads has different composition based own their application. Heavy
vehicles such as train especially the one with high speed need to choose a proper
material for break pad because such vehicles faces with great friction wear. Figure 2.3
(a) and 2.3 (b) show the brake pad that use in ampang line Light Rail Transit (LRT)
where it is imported from Becorit GmbH Company that base in Germany.
(a) (b)
Figure 2.3: (a) Front view of brake pad that use in ampang line LRT (b) Back view of
brake pad that use in ampang line LRT
11
Table 2.1: Different types of brake pad materials on various applications
Type of vehicle Brake pad material Advantages Ref.
Train/Freight Semi-metallic and
metallic (iron,
copper, steel and
graphite all mixed
together)
High stopping power but
expensive
High operating temperature
Sustain high temperature without
fade
(Gultekin et
al., 2010a)
Motorcycle Manufactured using
the same materials
used in automotive
brake pads (ceramic
and organic are the
one that commonly
used)
Organic brake pads does not wear
as fast in light weight vehicles
(Kumar &
Bijwe,
2011)
Commercial car Organic
Kevlar
Does not pollute the environment
as they are easy to dispose
Softer compared to other material
means quieter
(Kabir &
Ferdous,
2012)
Racing car Carbon fiber
Ceramic
Great braking performance
High operating temperature
Lightweight
Sustain high temperature without
fade
(Renz,
Seifert, &
Krenkel,
2012)
Trucks Ceramic and
metallic (iron,
copper, steel and
graphite all mixed
together)
High stopping power but
expensive
High operating temperature
(Renz et al.,
2012)
2.2.2 Type of brake pad for railway application
Brake pad is basically the same even for other application except they have different
sizes of brake pad based on the suitability of the application. Brake pad that is use in
railway industry is almost the same with the one that regularly use in car. Brake pad for
railway is slightly larger than car break pad because it needs a better braking system
especially with the high speed train that been use widely nowadays, the needs for a
better braking system is vital. The selection of brake disc and pad friction pairing has a
significant impact on brake safety and life cycle cost. In choosing a brake pad, especially
for railway application, the specific thermal simulation and dynamometer test to ensure
12
proper selection of friction pairing is very important so that it can deliver a reliable and
safe braking.
In producing a brake pad there are several important thing that need to consider
in the process such as (Schienenfahrzeuge, 2013):
i. Organic brake pads for even wear of friction disc
ii. Silent sinter brake pad for braking without squealing
iii. Flexible sinter brake pad with top braking performance
iv. Temperature resistance with constant friction behaviour
v. Long disc and pad life due to even temperature distribution on brake disc.
2.3 Composition of brake friction materials
Friction materials should keep high and constant friction coefficient, low wear at wide
ranges of pressure, speed, temperature and other aspects. In order to encounter those
needs, the friction materials typically consist of numerous ingredients such as
reinforcing fibres, binders, fillers and friction modifiers (Fei et al., 2015). The
composition of the friction material plays a major role in satisfying the target properties
of producing brake pads. Brake manufacturers have to meet a number of comprehensive
tribologyical and economic necessities demanded by their customers, predominantly,
acceptable friction level, friction stability, wear resistance irrespective of brake
conditions and cost reduction (Hentati et al., 2014). Friction and wear characteristic of
friction material play an important role in deciding which new formulation developed
are suitable for the brake system. The friction and wear is difficult to predict, depends on
the various parameters such as microstructure, metallic counter face, rotating speed,
pressure and contact surface temperature (Nagesh et al., 2014).
13
2.3.1 Fibre
Fibres play a critical role in a wet engagement process that absorb stresses and retain the
integrity of the composite at elevated temperatures (Fei et al., 2015). Reinforcing fibre
plays an essential role in improving the matrix, and avoiding mechanical destruction
such as crack and fracture, to the friction material subjected to impact and shearing
force. Additionally, it has also an important influence on the friction and wear properties
of the friction material. Presently, the commonly reinforcing fibres used for the friction
material contain mainly inorganic fibre and organic fibre as shown in Table 2.2, such as
ceramic fibre, metal fibre, glass fibre, mineral fibre and carbon fibre. Among these
fibres, the most frequent used metal fibre is steel fibre as shown in Figure 2.4. The steel
fibre is used widely to the semi-metallic and organic friction material due to its
outstanding properties in low cost, high strength and toughness. Alternatively, the steel
fibre can provide good friction stability and wear resistance over wide ranges of
temperatures. However, steel fibre is easy to be oxidation corrosion, and induce
excessive the counter disc wear and the brake squeal and vibration if they are formulated
in large proportions (Wang & Liu, 2014).
Figure 2.4: Chopped steel wool (steel fibre) that used in medium sized-vehicles brake
pad (Tradekorea, 2008)
14
Table 2.2: Candidate reinforcing fibre substitutes for asbestos
No Fibre Advantages Limitation Ref.
1 Aramid
(Kevlar)
High strength and toughness
High wear resistance
High temperature stability
- Non-melting and non-
softening
- Dimensionally stability
- Life exceeding common
binders
Good thermal and electrical
insulation
Low density
Processing characteristics
(Singh,
2012)
2 Steel High strength
High temperature performance
Processing characteristics
Abrasive
Low coefficient of friction
Poor wear at low temperatures
High density
(Kang, Lee,
Kim, &
Kim, 2011)
3 Glass High strength Processing characteristics
Abrasive
Softens at high temperature causing
fade
Brittle
High wear
(Holand &
Beall, 2012)
4 Cellulosics Non-melting Processing characteristics
Poor temperature resistance
(Johnston,
1980)
5 Carbon
High strength
Thermal stability
Processing characteristics
Brittle
(Dhakal et
al, 2013)
15
Fibre in friction materials used to be made primarily using asbestos because of its heat
absorbing properties and quiet operation. Asbestos is a mineral composed of a mixture
of silicates, mainly magnesium and iron silicates. The property of asbestos that makes it
attractive for use in friction material industry is its tough nature, which combines
strength and flexibility with resistance to heat and chemical. However, friction materials
(fibre) composition changed intensely when asbestos as the main composition is found
to be carcinogenic. Recently, the use of non-asbestos friction materials (fibre) is
increasing and many researchers come out with different formulation using organic and
inorganic non-asbestos friction material. Table 2.3 show the case study of previous
research on friction material (fibre) used in fabricating a brake pad. The brief
explanation on material used including the testing method and the results is illustrated in
the table. Based on previous study, iron and aluminium based material is the most used
material in brake friction material studies because of their strength and thermal
conductivity properties.
16
Table 2.3: Literature review on the friction material (fibre) used
Reference
(Researcher,
year)
Friction
material
(Fibre)
Purpose of finding
/application
Testing and
characterisation Results
(Maleque et
al., 2012)
Organic fibre
To develop new natural
fibre reinforced
aluminium composite for
automotive brake pad
application.
Density, porosity,
microstructural
analysis, hardness,
SEM
Coconut fibre have better
properties in terms of
higher density, lower
porosity and higher
compressive strength
(Liew &
Nirmal, 2013)
Asbestos
fibre, Steel
fibre
To study the tribological
properties difference of
potentially new designed
non-commercial brake
pad materials with and
without asbestos under
various speed and
nominal contact pressure.
Pin-on-disc tribo
test-rig under dry
contact condition
Non-asbestos maintained
stable frictional
performance and have
greater wear resistance
(Singh,
Patnaik, &
Satapathy,
2013)
Nanoclay,
Multiwalled
carbon
nanotubes,
lapinus,
kevlar
To fabricate graphite
lubricated phenolic-based
friction composites filled
with nanoclay and
Multiwalled carbon
nanotubes (MWCNT)
and reinforced with
lapinus and Kevlar fibres
to evaluate their physical,
chemical, mechanical
and tribological
properties, respectively.
Friction and wear
test, SEM
Higher nanoclay:MWCNT
ratio showed lowest fading
and excellent recovery
performance
(Maleque,
Dyuti, & n.d.)
Cast iron,
Al-alloy, Ti-
alloy
To develop the material
selection method and
select the optimum
material for the
application of brake disc
system emphasizing on
the substitution of this
cast iron by any other
lightweight material
Compressive
strength, friction and
wear test, thermal
conductivity and
specific gravity
The analysis led to
aluminium metal matrix
composite as the most
appropriate material for
brake disc system
(Asif, Chandra,
& Misra, 2011)
Iron
To prepare Iron based
friction material by using
powder metallurgy (PM)
processing utilizing hot
powder preform forging
Optical
metallography,
hardness test,
density and pin-on
disc test
The results indicate that the
coefficient of friction is
more stable and wear is
lower with respect to
temperature rise
17
2.3.2 Filler
Filler are normally a low cost mineral such as clay, calcium carbonate and barites that
are function to extend the brake lining other than to occupy space and minimize cost
(Selamat, 2006a). ASTM dictionary of engineering science and technology has define
filler as an inorganic material that used as an extender and it is added to modify its
strength, permanence, working properties and other qualities (ASTM, 2005). In friction
materials, fillers can be used either to increase or to decrease the density of a product.
This is because the density of filler can reach as high as 10 g/cm3 or as low as 0.03
g/cm3. Fillers may decrease the thermal conductivity of friction materials. The finest
insulation properties of composites are obtained with hollow spherical particles as filler.
Filler is normally classified by particle size since it is affect the performance and
in selecting the fillers it require certain levels of conductivity (thermal or electric) and
chemical interaction. In one publication, 33 materials were divided into particulates,
fibers, and colorants. These distinctions are not helpful for a material designer. For a
classification to be useful in filler applications, it must include the most important
properties of fillers which affect the resultant material such as (Wypych, 2000):
i. Particle size and distribution
ii. Aspect ratio
iii. Chemical composition of surface
iv. Mechanical properties of filler particles
v. Electric and thermal conductivity
vi. Quantitative description of interactions
vii. Composition of admixtures
viii. Optical properties
18
2.3.3 Binder
The binder basically a material that hold the brake lining together(Selamat, 2006b).
Basically, among all the ingredients used in friction materials, binder plays a major role
in determining the performance of friction materials. Although it is not a major
component by volume or cost, binder is the main part of friction materials. It is used to
bind the rest of ingredients in friction materials and maintain the structural integrity of
the composites under mechanical and thermal stresses (Cai et al., 2015). The binder
resin strongly affects the important aspects among the various ingredients used in the
friction material. Those aspect are includes the fade resistance, pedal feeling, wear
resistance, and noise propensity. This is because the physical and chemical properties of
the resin affect the wear process and friction characteristics of the friction material.
Straight phenolic resin has long been used for commercial brake friction materials and
various modified resins are available to improve compressibility, thermal stability,
damping capacity, and mechanical strength (Hong et al., 2009).
2.3.4 Friction modifier
Friction modifier is also known as abrasive where it is use to removes the pyrolyzed
friction film at the friction interface and controls friction level in the friction materials
formulation. Abrasive is a very sensitive ingredient where it may cause brake noise,
rotor wear and judder if the composition is not right. There are more than two different
abrasives need to be tried in a formulation to terminate negativities of one another in
order to balance the variation. Lubes are used to control friction which normally solid
lubes form a friction film over the rotor surface and controls noise propensity and rotor
wear. Normally Zircon has strong influence on the static friction and hence is optimized
with graphite, antimony trisulphide with low ratios say 1–2 % of zircon for a ratio of 10–
11 % graphite offset by the usage of Sb2S3 a third of lube used normally for optimization
(Sundarkrishnaa, 2013).
19
2.3.5 Lubricants
Lubricants are added to assist in part removal from the moulds. The example of material
used in lubricant is copper sulphide and graphite. In lubricated systems the starting
friction is frequently higher than the kinetic friction. When the surfaces slide, lubricant is
dragged into the contact region and separates the surfaces. Hence, this will initially
lower the coefficient of friction. However, at a higher sliding speed there is a viscous
drag which again causes to an increase of coefficient of friction as shown in Figure 2.5
(Sundarkrishnaa, 2013). This McKee-Petroff curve as shown in Figure 2.5 is typical for
a shaft rotated in a sleeve bearing. The abscissa is in units of ZN′/P where Z is the
viscosity of the lubricant, N′ is the shaft rotating speed, and P is the load transferred
radially from the shaft to the bearing.
Figure 2.5: Coefficient of friction pattern for a typical lubricated contact. Z is lubricant
viscosity, N′ shaft speed, and P the unit load transferred radially by the shaft to the
bearing (Sundarkrishnaa, 2013)
20
2.4 Fabrication of brake pad
2.4.1 Powder metallurgy
Powder metallurgy is basically a process in manufacturing science to producing solid
parts of preferred geometry and material from powders. The ultimate consideration need
in powder metallurgy is the powder used for the manufacturing process. The process of
powder metallurgy is include the process of mixing, compaction and sintering as shown
in the flowchart of the basic powder metallurgy process in Figure 2.6.
1) Powder production
- In this stage, the raw material will be process into powder form in order to
proceed to the blending or mixing process.
2) Mixing of powder
- In this very stage it often involve the additions alloy in elemental powder
form or the combination of a pressing lubricant.
3) Process of compaction the mixed powder
- The so called merging process is include the process of pressing in a rigid
toolset, comprising a die, punches and mandrels or core rods. Besides, there
are other process can be done based on the application.
4) Sintering the compact mixture
- The sintering process is done to enhance integrity and the strength of the
materials. This process is involves by heating the material in a temperature
that is below the melting point of the major constituent. At sintering
temperature, a minor constituent can form a liquid phase in certain cases.
21
Figure 2.6: A flowchart of the basic powder metallurgy process for structural press and
sintered components
Sintering
Optional
secondary
machining
Optional
secondary
finishing
Raw Materials
Additives
(Lubircants,
binders,etc)
Mixing
Compaction
Finished product
22
2.5 Characteristic for brake friction materials
All types of brake systems friction materials are having different complex mixtures of
filler, fibers and other components in a polymeric matrix in order to create the material
designs that meet specific applications. The materials design must meet the specified
friction coefficient and wear properties for the given application and also consider a
number of other requirements such as any possibilities of undue thermal damage or wear
in the opposing surface or induce brake squeal. In producing good friction materials,
some of the requirements are contradictory in nature where several compromises may be
necessary. Thus, after the design stage, it is supposed to undergo development work and
becomes tested and, modified until it is satisfactory in all aspects (Sundarkrishnaa,
2013).
2.5.1 Friction coefficient
Friction coefficient is the amount of friction between two objects or surfaces and it is
determined by dividing tensile force by weight force. The tensile force the pulling force
required to slide one of the surfaces across the other while the weight force is the force
that pushing down on the object being pulled. The friction coefficient can be calculated
by following Equation:
𝐹𝑡
𝐺 = µ (Eq 2.1)
Where:
Ft = tensile force in pounds
G = weight force in pounds
µ = friction coefficient
23
The friction coefficient Equation can also express by:
𝑇𝑒𝑛𝑠𝑖𝑙𝑒 𝑓𝑜𝑟𝑐𝑒
𝑊𝑒𝑖𝑔ℎ𝑡 𝑓𝑜𝑟𝑐𝑒 = Coefficient of friction (Eq 2.2)
The equation above can be used to show the effect different variables have on the
coefficient of friction. There are three factors that affect the friction coefficient of
vehicle brakes at any given weight force (Jeffrey & Halderman, 2006):
i. Surface finish
ii. Friction material
iii. Heat
2.5.2 Wear rate
In rail systems with such a little wear, failure is exhibited by the appearance of closely
spaced micro-cracks. Micro-cracks may continue to grow into larger cracks by one of
several crack propagation mechanisms. Examples include the shear mode driven by
accumulation of plastic flow due to ratcheting, and the crack opening mode due to crack
pressurization by fluids (Eadie et al., 2008). The wear rate of a friction material is
depends on several element such as temperature, speed and load. Besides, it is directly
proportional to applied normal load and speed.
The wear of the friction material will increase exponentially at high brake
temperature because of thermally induced degradation of organics resin binders and
other substances. When the operating temperature increase the wear rate will also
increase. There is a common relationship between friction and wear modification in the
formula that decreased wear tends to decrease friction. Wear might reduce by using solid
lubricants but it also reduces the friction. Wear is basically a slow process and is based
on service life and not on any measurements of varying operating conditions and
environment. Service life is not something that easy to determine, because in a brake
usage, different routes result in different operating temperatures (Sundarkrishnaa, 2013).
In particular, the wear process of brake friction materials at high temperatures has
24
attracted much attention since the exact understanding of the wear mechanism on a
molecular scale can provide significant improvement in wear resistance (Hong et al.,
2009).
2.5.3 Noise and judder
Normally, braking noise occur due to high friction coefficient and it is related to the
design of materials besides the contacting conditions. The braking noise can be eliminate
by increasing the material toughness. When the friction materials toughness is increase,
higher deformation strength will be achieve thus it will eventually eliminate the noise.
The toughness of friction materials is very important where it governs noise, braking
performance and wear.
Noise in a friction material brake pad is very complicated where it has different
types of noises like Groan, squeal, Gu, Go, as per numerous standards of classification.
All Noises in a brake either drum brake or disc brake has a common rule of varying
frequency due to temperature, pressure and sensitivity of certain raw materials in the
system. Noise in friction material can be easily diminish through matrix alteration by
eliminate the air gap between the contacting surfaces during braking. Either way it will
eventually achieve higher deformation strength and higher toughness to get better
resiliency in braking which eliminates noise (Sundarkrishnaa, 2013).
68
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