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UNIERSITY OF NAIROBI

DEPARTMENT OF MECHANICAL AND MANUFACTURING ENGINEERING

FINAL YEAR PROJECT

PROJECT NO. JMO/01/2011

TITLE: DESIGN OF BRAKEPAD FRICTION MATERIAL

COMPILED BY:

JOEL JEREMIAH GACHOKI : F18/1853/2006

MARANGU DICKSON KATHENYA : F18/1838/2006

SUPERVISED BY: DR. JULIUS M.OGOLA

A final year project submitted in partial fulfillment of the requirements for the award of the degree of Bachelor of science in Mechanical Engineering

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DECLARATION

Students

This report is our original work and has not been published or presented for

award of degree in any university there before.

Signed……………………………………… Date……………………………………………………

JOEL JEREMIAH GACHOKI

Signed...………………………………… Date…………………………………………………

MARANGU DICKSON KATHENYA

Supervisor

This report has been submitted by the above students for examination with

my approval as a university lecturer and supervisor of the project.

Signed………………………………….. Date ………………………………….

Dr. JULIUS M. OGOLA

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DEDICATION

To our families and friends for their abundant and relentless love.

We respect you and above all we love you.

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ACKNOWLEDGEMENT

The undertaking and completion of this research work was made possible by a number of

people to whom we are profoundly grateful to.

We are particularly indebted to our supervisor Dr. J.M. Ogola for his guidance,

encouragement and constructive criticism which has immensely contributed to the

successful completion of this project.

We would wish to give thanks to mechanical and manufacturing engineering department

staff for their in-time support and also the department of physics Chiromo campus for their

support.

Our gratitude also goes to Mr. Mwitari from KIRDI and Mr. Tioni from KEBS for their

continued support throughout the project.

We are eternally grateful to our parents for their support and encouragement, but most

important for their prayers.

We appreciate and wish to extend our best wishes to our fellow colleagues and friends for

their invaluable support and encouragement throughout the entire period.

Lastly but most important, we thank God for abilities and opportunity he has bestowed

upon us. Glory is to Him.

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ABSTRACT

The project was aimed at designing a frictional material that can replace asbestos in the

manufacture of brake pads.

Frictional material used in brake pads is made up of four subcomponents which play

different roles.

These are; abrasives materials to modify friction, lubricants to stabilize developed friction,

binders to hold different constituents together and prevent disintegration and fillers to

improve manufacturability as well as lower the cost.

Brake pads are required to maintain; a sufficiently high friction coefficient with the brake

disc, not decompose or break down at high temperatures and exhibit a stable and

consistent friction coefficient. In this project the materials used included; alumina, kaolin,

vermiculite, graphite, barium sulphate, fibre glass, steel fibre, silica and sodium silicate

based resin which were locally obtained. Hence three formulations were made, which had

different percentage material composition. The mixtures were moulded by use of a locally

made mould and cured before subjecting them to a temperature of 1500c in an oven for

three hours. The samples were then tested for internal shear strength and hardness as well

as a study for macrostructure.

The internal shear strength was conducted on test specimens of dimensions 20mm by

20mm by 10mm and the results obtained were 1.7235, 1.88515 and 1.9066 N/mm2 for

samples 1, 2 and 3 respectively. Hardness test was conducted on the surfaces of the

samples and results obtained were 86, 81 and78 Rockwell hardness for samples 1, 2 and 3

respectively. The macrographs from the macrostructure study are shown in the results

section.

The results obtained showed that increasing the amount of binder results to increase in

hardness and a slight increase in internal shear strength. Further to this, an increase in the

quantity of reinforcing fibres led to increase in internal shear strength with a decrease in

hardness as shown in the plotted graphs. The study has also shown that locally available

materials can be used instead of importing materials to manufacture brake pads.

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TABLE OF CONTENTS DECLARATION .............................................................................................................................................. i

DEDICATION ............................................................................................................................................... iii

ACKNOWLEDGEMENT ............................................................................................................................... iv

ABSTRACT .................................................................................................................................................... v

CHAPTER ONE ............................................................................................................................................. 1

INTRODUCTION ........................................................................................................................................... 1

1.1 Background ........................................................................................................................................ 1

1.2 Problem statement ............................................................................................................................ 2

1.3 Objectives ........................................................................................................................................... 2

CHAPTER TWO ............................................................................................................................................ 3

LITERATURE REVIEW ................................................................................................................................ 3

2.1 Brakes ................................................................................................................................................. 3

2.2 Braking system .................................................................................................................................. 3

2.3 Drum brake ........................................................................................................................................ 5

2.4 Disc brake .......................................................................................................................................... 6

2.5 Disc rotor ........................................................................................................................................... 6

2.6 Calliper ............................................................................................................................................... 7

2.7 Brake pads ......................................................................................................................................... 7

2.7.1 Steel backing plate ..................................................................................................................... 8

2.7.2 Wear indicator ............................................................................................................................ 9

2.7.3 Frictional lining .......................................................................................................................... 9

2.8 Categories of frictional lining ........................................................................................................... 9

2.8.1 Metallic pads ................................................................................................................................ 9

2.8.2 Asbestos pads ............................................................................................................................ 10

2.8.3 Ceramics pads ............................................................................................................................ 10

2.8.4 Semi-metallic pads ..................................................................................................................... 10

2.9 Constituents of brake pad frictional lining .................................................................................... 11

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2.9.1 Reinforcing materials ............................................................................................................... 12

2.9.2 Fillers ......................................................................................................................................... 14

2.9.3 Binders ...................................................................................................................................... 15

2.9.4 Frictional additives .................................................................................................................. 17

CHAPTER THREE....................................................................................................................................... 19

FRICTIONAL LINING MATERIALS ............................................................................................................ 19

3.1 Materials requirement .................................................................................................................... 19

3.2 Materials used in the frictional lining formulation ....................................................................... 20

3.2.1 Reinforcing materials ............................................................................................................... 20

3.2.2 Fillers ......................................................................................................................................... 21

3.2.3 Frictional additives .................................................................................................................. 23

3.2.4 Binders ...................................................................................................................................... 24

3.3 Choice of sample composition ........................................................................................................ 26

CHAPTER FOUR ......................................................................................................................................... 31

MANUFACTURING PROCESS .................................................................................................................... 31

4.1 Materials preparation ..................................................................................................................... 31

4.3 Moulding process ............................................................................................................................ 33

CHAPTER FIVE ........................................................................................................................................... 36

LAB TESTING ............................................................................................................................................. 36

5.1 Hardness testing .............................................................................................................................. 36

5.2 Shear strength ................................................................................................................................. 38

5.3 Macrostructure study ..................................................................................................................... 41

CHAPTER SIX ............................................................................................................................................. 43

DISCUSSION, CONCLUSION AND RECOMMENDATION.......................................................................... 43

6.1 Discussion ........................................................................................................................................ 43

6.2 Conclusion........................................................................................................................................ 44

6.3 Recommendations ........................................................................................................................... 45

REFERENCES ............................................................................................................................................. 46

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CHAPTER ONE

INTRODUCTION

1.1 Background

In the past asbestos was used to manufacture brake pads. Asbestos became increasingly

popular among brake pad manufacturers because of its sound absorption, average tensile

strength, and its resistance to heat, electrical and chemical damage However, since 1970s,

asbestos had gained widespread acknowledgement as a carcinogen .This is because

inhalation of asbestos fibers can cause serious illnesses, including malignant lung cancer,

mesothelioma (a formerly rare cancer strongly associated with exposure to amphibole

asbestos) and asbestosis (a type of pneumoconiosis).Other asbestos-related diseases

include;

Asbestos warts: caused when the sharp fibers lodge in the skin and are overgrown causing

benign callus-like growths.

Pleural plaques: discrete fibrous or partially calcified thickened area which can be seen on

X-rays of individuals exposed to asbestos. Although pleural plaques are themselves

asymptomatic, in some patients this develops into pleural thickening.

Diffuse pleural thickening: similar to above and can sometimes be associated with

asbestosis. Usually no symptoms shown but if exposure is extensive, it can cause lung

impairment.

This led to the use and development of other materials which could take the place of

asbestos in the manufacture of brake pads in automotive industry. These other materials

which are currently in use include; metals, ceramics, carbon and organic materials.

However these materials have different advantages and disadvantages when solely used to

construct a brake pad. Hence more research is being carried on a combination of different

materials so as to optimize the performance requirements.

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1.2 Problem statement

From the previous project research undertaken by Kevin Loye Wadeya and Peter Ochieng

Ombewa, three brake pads were constructed and tested in the laboratories. The tests

carried out were internal shear strength test and the Rockwell hardness test.

Macrostructure study was also carried out. The results obtained from the Rockwell

hardness test were satisfactory since they met the standard required by the Kenya Bureau

of Standards. However the values of internal shear strength for the three samples

constructed were below the value required by Kenya Bureau of Standards. As a result,

internal shear strength was the major drawback, hence more research needed to be carried

out to improve the pads. In this research the aim was to use the locally available asbestos

free materials to construct a brake pad with improved internal shear strength and

optimum friction coefficient while maintaining the hardness.

1.3 Objectives

The objectives of the project are as follows

(i) To design a heat resistant brake pad that has optimum friction coefficient and

internal shear strength

(ii) To use locally available materials that will not cause environmental pollution as

well as health problems.

(iii) To construct and test the designed brake pad so as to determine its feasibility

for use in motor vehicles.

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CHAPTER TWO

LITERATURE REVIEW

2.1 Brakes

A brake is a device which inhibits motion. Most brakes use friction to convert kinetic

energy into heat, though other methods of energy conversion may be employed. For

example regenerative braking converts much of the energy to electrical energy, which may

be stored for later use. Other methods convert kinetic energy into potential energy in such

stored forms as pressurized air or pressurized oil. Still other braking methods even

transform kinetic energy into different forms, for example by transferring the energy to a

rotating flywheel. In automobiles friction brakes are commonly used.

A friction brake is a type of an automotive brake that slows or stops a vehicle by

converting kinetic energy into heat energy, via friction. The braking heat is stored in the

brake drum or disc while braking, then conducted to the air gradually.

2.2 Braking system

The modern automotive brake system has been refined for over 100 years and has become

extremely dependable and efficient. The typical brake system consists of disk brakes in

front and either disk or drum brakes in the rear connected by a system of tubes and hoses

that link the brake at each wheel to the master cylinder. When one steps on the brake

pedal, he or she pushes against a plunger in the master cylinder, which forces hydraulic oil

(brake fluid) through a series of tubes and hoses to the braking unit at each wheel. On a

disk brake, the fluid from the master cylinder is forced into a caliper where it presses

against a piston. The piston in-turn squeezes the two brake pads against the rotor that is

attached to the wheel forcing it to slow down.

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A picture illustrating how brake works

With drum brakes, fluid is forced into the wheel cylinder, which pushes the brake shoes out

so that the friction linings are pressed against the drum, which is attached to the wheel,

causing the wheel to stop.

A friction brake in automobile may either be a drum brake or a disc brake.

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2.3 Drum brake

A drum brake is a brake in which friction is caused by a set of brake shoes that press

against the inner surface of a rotating drum. The drum is connected to the rotating road

wheel hub.

A picture of a brake drum

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2.4 Disc brake

This is a brake in which the pads pinch a rotating disc. The disc brake is a device for

slowing or stopping the rotation of a road wheel. A brake disc, usually made of cast iron or

ceramic, is connected to the wheel or the axle. To stop the wheel, friction material in the

form of brake pads (mounted in a device called a brake caliper) is forced mechanically,

hydraulically or pneumatically against both sides of the disc. Friction causes the disc and

attached wheel to slow or stop. A disc brake assembly consists of a

• Cast iron disc or the disc rotor that rotates with the wheel

• Caliper assembly attached to the steering knuckle

• Friction materials (disc pad) that are mounted to the caliper assembly

When hydraulic pressure is applied through the caliper piston, it forces the inside pad to

contact the disc. As pressure increases the caliper moves and causes the pad to contact the

disc. Braking force is generated by friction between the disc pads as they are squeezed

against the disc rotor.

The friction surface is constantly exposed to air ensuring good heat dissipation minimizing

brake fade. It also allows for self cleaning as dust and water are thrown off reducing friction

differences

2.5 Disc rotor

The disc rotor is made of grey cast iron and is either solid or ventilated. The ventilated disc

rotor consists of a wider disc with cooling fins cast through the middle to ensure good

cooling. Proper cooling prevents fading and ensures longer pad life. Some ventilated rotors

have spiral fins which creates more air flow and better cooling .Spiral finned rotors are

directional and are mounted on a specific side of the vehicle.

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2.6 Calliper

The caliper houses one to four pistons and is mounted to the Torque plate and steering

knuckle or wheel carrier. There are two caliper designs; floating caliper and fixed caliper

2.7 Brake pads

Brake pads are a component of disk brakes used in automotive and other applications.

Brake pads are steel backing plates with friction material bound to the surface that faces

the disk brake rotor.

Brake pads convert the kinetic energy of the car to thermal energy by friction. Two brake

pads are contained in the brake caliper with their friction surfaces facing the rotor. When

the brakes are hydraulically applied, the caliper clamps or squeezes the two pads together

into the spinning rotor to slow/stop the vehicle. When a brake pad is heated by contact

with a rotor, it transfers small amounts of friction material to the disc, turning it dull gray.

The brake pad and disc (both now with friction material), then "stick" to each other,

providing the friction that stops the vehicle.

Although it is commonly thought that the pad material contacts the metal of the disc to

stop the car, the pads work with a very thin layer of their own material and generate a

semi-liquid friction boundary that creates the actual braking force. Friction can be divided

into two parts: Adhesive and abrasive.

Depending on the properties of the material of the pad and the disc, the configuration and

the usage, pad and disc wear rates will vary considerably. The properties that determine

material wear involve trade-offs between performance and longevity. The friction

coefficient for most standard pads will be in the region of 0 .40 when used with cast iron

discs. Racing pads with high iron content designed for use with cast iron brake discs reach

0.55 to 0 .60 which gives a very significant increase in braking power and high temperature

performance. High iron content racing pads wear down discs very quickly and usually

when the pads are worn out so are the discs.

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Different brake design applications require different kinds of friction materials, several

considerations are weighed in development of brake pads; the coefficient of friction must

remain constant over a wide range of temperatures .The brake pads must not wear out

rapidly nor should they wear the disc rotor, should withstand the highest temperatures

without fading and it should be able to do all these without any noise. Materials that make

up the brake pad include; friction modifiers, powdered metal, binder, fillers and curing

agents. Friction modifiers such as graphite and cashew nuts shells alter the friction

coefficient. Powdered metals such as lead, zinc, brass and Al increase a materials resistant

to heat fade .Binders are the glues that hold the friction materials together. Phenolic resin is

the most common binder in current use. Fillers are added to friction materials in small

quantities to accomplish specific purposes such as rubber chips to reduce brake noise. The

brake pad material is bonded to a stamped steel backing plate with a high temperature

adhesive to which heat and pressure are applied to cure the assembly .A slit is provided on

the face of the pad to indicate the allowable limit of pad wear and provide a path for brake

dust and gas to escape .A metal plate or in some applications multiple plates called anti

squeal shims are provided on the piston side of the pad to minimize brake squeal. Various

springs and clips are used to reduce rattle as well as reduce brake noise. Shims and plates

should be inspected for wear and rust and can be reused when replacing pads. Fresh

approved grease should be applied to the shims prior to installation.

The brake pad consists of the following parts

2.7.1 Steel backing plate

This part is usually fabricated from low-carbon steels. The function of the backing plates

is to transmit the force from the caliper pistons to the friction material. It may also

contain retention features such as holes or serrated edges to provide a better anchor

point for the friction material.

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2.7.2 Wear indicator

Since all friction materials gradually wear out, a wear indicator is fitted to each pair of

brake pads to inform the driver that replacement is required. In most cases a simple

steel spring is riveted to the brake pad backing plate to serve this purpose. When the

friction material wears to the point that replacement is necessary, the spring will contact

the rotor, emitting a high-pitched squeal.

2.7.3 Frictional lining

The brake pad friction lining is the primary wear element for the brake system. This is a

part of the brake pad that is designed to contact with the rotor, converting kinetic

energy into heat and as a result, it gradually wears out until it needs replacing. Of all the

brake pad parts, the friction material is arguably the most critical from a high-

performance perspective.

2.8 Categories of frictional lining Frictional linings can be categorized according to the material composition used in the

manufacture. The categories include

2.8.1 Metallic pads

These pads are typically made of iron, copper, steel and graphite all mixed together and

bonded to form the pad material.

The pads are cost-effective and durable. They are also good at transferring the heat

generated by friction with the brake rotors .However, being made of metal the pads are

very hard. This makes them to cause more wear on the brake rotors.

They do not work well when cold, they are noisy and dust

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2.8.2 Asbestos pads

Asbestos was the best choice material in the manufacture of brake pads. This is because of

its good properties such as low thermal conductivity and ability to withstand high

temperatures which are essential in brake pads. Asbestos was also readily available.

However, in the early 1980s it was discovered to be carcinogenic and was capable of casing

Asbestosis and Mesothelioma.

2.8.3 Ceramics pads

These pads are made of ceramics compounds which are a composition of about 15 percent

metal fibres and other ingredients such as fillers binders and lubricants. The various fillers

and lubricants help dampen vibration and noise and hence these pads are quiet in

operation,

These pads wear less, transfer heat better because of metal fibres and are lighter in weight

2.8.4 Semi-metallic pads

These pads consist of about 40 percent metallic fibres and other ingredients which include

fillers binders and lubricants. They also include pads that are carbon based

Semi-metallic pads are strong, conduct heat away from rotors, generate noise and are

abrasive enough to increase rotor wear.

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2.9 Constituents of brake pad frictional lining

Brake pad frictional lining formulations comprises of a composition of four different

categories of materials. The formulation must meet the following demands on the brake

pad

(a) Maintain a sufficiently high friction coefficient with the brake disc;

(b) Not decompose or break down in such a way that the friction coefficient with the

brake disc is com- promised, at high temperatures

(c) Exhibit a stable and consistent friction coefficient with the brake disc.

Brake pads frictional lining comprises of the following categories of materials

(a) Frictional additives, which determine the frictional properties of the brake pads and

comprise a mixture of abrasives and lubricants

(b) Fillers, which reduce the cost and improve the manufacturability of brake pads

(c) A binder, which holds components of a brake pad together

(d) Reinforcing fibers, which provide mechanical strength

Enlarged isometric view of a used brake pad

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2.9.1 Reinforcing materials

These are materials used is to provide mechanical strength to the frictional lining.

Research has shown that the braking load is actually carried by tiny plateaus that rise

above the surrounding lowlands on the friction materials. These plateaus are formed by

the reinforcing fibres surrounded by the softer compacted components. Therefore the

importance of the reinforcing fibres in friction material cannot be underestimated. Friction

materials typically use a mixture of different types of reinforcing fibres with

complementing properties.

Reinforcing material Description /comment

Glass fibre (i) Glass fibres bonded together with resinous binder are physically strong and hence are suitable for use as reinforcing fibres.

(ii) They also exhibit thermal resilience having a high melting point of 1430 0C.

(iii) However, typical glass fibres have a low thermal conductivity of 0.04 W/m K. The brittleness of glass means that it cannot be the sole reinforcement in brake friction materials.

Aramid (i) These fibres are made from the condensation product of isophthalicr terephthalic acids and m- or p-phenylenediamine.

(ii) Aramid fibres such as Kevlar fibres are widely used as reinforcing fibre.

(iii) They are much light and exhibit excellent thermal stability, with a very good stiffness weight ratio.

(iv) They have superior wear resistance (v) Due to their relative softness, it is unlikely that they

will be the only fibres supporting the braking load and hence they have to be used together with other fibres.

Sepiolite (i) Sepiolite is a hydrated magnesium silicate mineral that occurs as a fibrous chain-structure mineral

(ii) Thermally resilient (high melting point of approximately 1550 °C); able to absorb traces of fluid

(iii) Sepiolite is a potential health hazard since it is

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associated with the development of inflammation in lung and pulmonary interstitial fibrosis and therefore not a suitable replacements for asbestos.

Potassium titanate (i) They are fibres prepared from highly refined, single

crystals. (ii) They are thermally resilient with a melting point of

approximately 1371 °C (iii) They are very hard and exhibit good wear

resistance (iv) However they are a health hazard since they are

associated with mesothelioma.

Ceramic (i) These are made of metal oxides such a alumina (aluminum oxide) as well as carbides such as silicon carbide

(ii) They have high thermal resistance with melting points ranging from 1850 to 30000 C. They have high strength-weight ratio.

(iii) However, since they are brittle they cannot be used alone.

Metallic (i) Metallic chips or granules are commonly used as reinforcing fibres and hence they are referred to as metallic ‘fibres’. Examples of metallic fibres include steel, brass and copper.

(ii) The advantage of using metal fibres is that they have very high conductivities able to remove heat from the frictional surfaces very quickly.

(iii) However some metallic fibres such as steel will rust, especially if the vehicle has been operating in a corrosive environment, thereby compromising their functionality as reinforcing fibres.

(iv) Addition metallic fibres such as steel might cause excessive wear of the brake disc if they are present in large proportions as well as increase friction coefficient fluctuations.

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2.9.2 Fillers

It is a loose term which could also mean anything used in a large proportion in a brake

friction material for the purpose of improving its manufacturability as well as to reduce the

overall cost of the brake pad. Fillers are also used to maintain the overall composition of

the friction material.

Fillers are of two types; functional fillers used to improve particular characteristic feature

of composites such as resistance to fade and space/inert fillers mainly used to cut the cost.

Generally, they can be classified as organic and inorganic fillers.

(a)Organic fillers

They include rubber and cashew nuts in form of dust. They are mainly used for the

purpose of noise reduction due to their superior viscoelastic characteristics. However,

these particles, especially cashew, fall off the friction surface easily, leaving behind large

pores that act as stress concentrators leading to crack growth. At elevated temperatures

cashews can be used to reduce fluctuations in coefficient of friction.

(b)Inorganic fillers

Some common inorganic filler include

Filler Description

Barium sulphate (i) Imparts heat stability to friction material and improves frictional characteristic.

Calcium carbonate (i) Imparts heat stability to friction material Mica (i) Suppresses low-frequency brake noise, but causes

interlayer splitting in friction material Molybdenum trioxide (i) Prevents thermal fade and cracking of friction

lining under high-temperature conditions Vermiculite (i) Suppresses low-frequency brake noise, and has

high heat resistance (ii) Capable of resisting abrasion. (iii) Has some amount of porosity enabling it to

absorb water.

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2.9.3 Binders

The function of a binder is to maintain the brake pad's structural integrity in various

braking condition. Binders hold all constituents of a brake pad together and prevent them

from disintegrating. Selection of binders for brake pads is hence an important issue. The

binder should have a high heat resistance.

Some commonly used binders are;

Binder Description/comment

Phenolic resin (i) This is a type of polymer formed by a condensation reaction between phenol and formaldehyde

(ii) It is the most common binder in use. When used in little amount it leads to lining weakening and when used in much large quantities it leads to frictional drop at high temperatures.

(iii) It is cheap and easy to produce, has a good combination of mechanical properties such as high hardness, moderate thermal resistance, compressive strength, creep resistance and very good wetting capability with most of the material.

(iv) However, these resins are sensitive to heat and humidity and in situ polymerization starts slowly even at ambient temperature leading to its poor shelf life. They are also brittle and have a very low impact resistance.

COPNA resin

(i) COPNA resin is an abbreviation of ‘condensed poly-nuclear aromatic’ resin

(ii) It has high bonding strength with graphite (a common lubricant); therefore has better wear resistance than pure phenolic resin

(iii) However, it decomposes at relatively low temperatures( between 450 and 500 °C)

Silicone-modified resin( phenolic siloxane resins)

(i) Made by reacting silicone oil or silicone rubber with phenolic resins.

(ii) It is modified with tougheners to reduce

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brittleness (iii) However, the original characteristics of

thermal and chemical resistance of phenolic resins would be compromised and the modified resin becomes highly toxic.

(iv) Kane and Mowers succeed in combining phenolic resins with silicone to form phenolic siloxane resins having enhanced impact resistance

Cyanate ester resin (i) They are formed from polyfunctional

cyanate monomers. (ii) They are stable at elevated temperatures,

chemically inert and have damping properties.

(iii) However they are brittle like the phenolic- based resins

Epoxy-modified resin

(i) At temperatures above 2600C, typical epoxy resin binders degrade, to increase the operating temperatures of epoxy resins, special curing agents have to be used.

(ii) Epoxy is used to modify phenolic resins, resulting in the synergistic effect of having a higher heat resistance than phenolic resin or epoxy resin alone and has a high frictional stability.

Thermoplastic polyimide resin (i) This is a product of fluoro resin and calcium carbonate.

(ii) It is abrasion resistant and does not exhibit thermal fade commonly experienced with phenolic - based resins or induce excessive brake disc wear.

(iii) However, its thermal conductivity is approximately three times lower than that of phenolic resins, so it is less able to dissipate heat away from the friction surface.

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2.9.4 Frictional additives

These are components added to frictional lining to aid in the modification of the friction co-

efficient and wear rates. They can be categorized into two classes: lubricants and abrasives.

According to experiment done by Kim and Jang on brake pads with varying quantity of

lubricants (antimony sulphide) and abrasive (zirconium silicate) it was discovered that

friction coefficient stability largely depended on the quantity of either lubricant or abrasive

and hence an optimum quantity of each must be used. This is because increase in the

quantity of lubricants materials in the lining leads to increase in friction co-efficient

stability while increase in quantity of abrasive materials leads to instability in friction co-

efficient

(a) Lubricants

They are mainly used to stabilize the developed friction coefficients during braking,

especially at high temperatures. Commonly used lubricants include

Lubricant Description /comment

Graphite (i) Graphite is one of the most widely used lubricants. It is available in natural or synthetic forms and as flakes or powder. Graphite in the flake form has better lubrication properties, while graphite in powder form is superior in dissipating heat generated during braking

(ii) It is able to form a lubricant layer on the opposing counter friction material rapidly

(iii) However, graphite cannot be used too liberally in phenolic resins because the bonding strength between graphite and phenolic resin is weak, leading to low shear strengths.

Metal sulphides (i) These have good lubricating properties, and low thermal conductivities when compared to graphite.

(ii) They include antimony, tin, copper and lead sulphides

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(b)Abrasives

These are additives which increase the friction coefficient and the rate of wear of the

counter face material. They remove iron oxides from the counter friction material as well

as other undesirable surface films formed during braking. However, friction materials with

higher abrasive content exhibit a greater variation of friction coefficient. Examples of

abrasives are hard particles of metal oxides and silicates.

Abrasive Description/comment

Aluminum oxide (i) Hydrated form added as a polishing agent and for wear resistance, but can produce fade.

(ii) Anhydrous form is still more abrasive

(iii) Fused is very hard and is the most abrasive form

Iron oxides (i) Hematite (Fe2O3) can act as a mild abrasive; magnetite(Fe3O4) also mildly abrasive

Silica (i) May be natural or synthetically-produced (SiO2)

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CHAPTER THREE

FRICTIONAL LINING MATERIALS

3.1 Materials requirement

Desirable Properties for friction linings brakes

(i) The frictional lining as well as the rotor materials should have a high

coefficient of friction.

(ii) The frictional lining material in contact with the rotor should resist

wear effects.

(iii) The value of coefficient of friction should be constant over a range of

temperatures and pressures.

(iv) The materials should be resistant to the environmental effects such as

moisture, dust, pressure.

(v) The materials should possess good thermal properties, high heat

capacity, and good thermal conductivity, withstand high temperature.

(vi) The materials should be able to withstand high contact pressures

which are experienced during braking.

(vii) The materials should have good shear strength which is transferred to

friction forces.

(viii) Should be safe to use and not cause environmental pollution

(ix) The materials should have low specific weight so as to improve on the

fuel economy.

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3.2 Materials used in the frictional lining formulation

Evidently, it is difficult to find a material which has all the above characteristics and hence a

mixture of different materials is used to obtain characteristics which are close to the

requirement. These different materials used are grouped into different categories. These

categories are: reinforcing materials, binders, frictional additives and fillers.

3.2.1 Reinforcing materials

a) Steel

When steel is used in form of powder, granules or chips it serves as an abrasive. However

when used in form of fibre it serves as a reinforcing material. Steel is used in brake pad

manufacture because of its good thermal conductivity and high strength. Steel can also

withstand high temperatures of above 10000c.

b) Fibre glass

It is also known as glass-reinforced plastic (GRP). It is a fiber reinforced polymer made of a

plastic matrix reinforced by fine fibers made of glass. It is a lightweight, strong .has

relatively high thermal stability and can retain most of its properties at temperatures as

high as 3720c. It has ability to resist corrosion and chemical distortion although it has low

conductivity of heat hence it was used together with steel

21

3.2.2 Fillers

a) Vermiculite

It exists naturally as a mineral and expands with the application of heat. It may expand

even up to 16 times when heated to about 20000F (exfoliation) to form a very light weight

aggregate with good insulating properties. It can withstand very high temperatures of up to

20000F and is capable of resisting abrasion. Like mica it has stratified structure with some

amount of porosity enabling it to absorb water. This is advantageous to brake pad as it

absorbs any stray moisture keeping the surface of the pad dry and ensuring the pads

friction coefficient is stable enough. The porosity also allows vermiculate to absorb sound

and thus it’s able to suppress brake noise. In Kenya the mineral is commonly found in

Eastern province a place known as wote.

22

b) Barium sulphate

Barium sulphate is a white crystalline solid with the chemical formula BaSO4. It occurs as

barite mineral and is insoluble in water. Barite is processed by carbothermal reduction

(heating with coke) to give barium sulphide in a blast furnace. To produce highly pure

barium sulphate, the sulphide is treated with sulphuric acid or sulphate salts. It has a high

melting point of 1580 °C and hence it is able to withstand high temperature of the brake

pad during braking. It is also used to lower the hardness of Alumina and steel granules to

ensure the brake pad formed is not very hard to abrade the rotor

23

3.2.3 Frictional additives

a) Abrasives

(i) Silica

The chemical compound silicon dioxide, also known as silica, is an oxide of silicon with the

chemical formula SiO2. Silica is most commonly found in nature as sand or quartz crystals

In solid form, silicon dioxide is white and powdery. It is a mineral that is highly resistant to

heat, with a melting point of 1650 degrees Celsius and has high hardness hence it is used in

brakes as an abrasive.

(ii) Alumina

Aluminum oxide commonly referred to as alumina or corundum in its crystalline form is an

amphoteric oxide with the chemical formula Al2O3. Its most significant use is in the

production of aluminum metal, although it is also used as an abrasive due to its hardness

and as a refractory material due to its high melting point of about 2072 °C. It has a

relatively high thermal conductivity (30 Wm−1K−1)

It density of about 3.95-4.1 g/cm is low and hence makes it suitable for brake pads because

of fuel economy

24

b) Lubricants

a) Graphite

Graphite is one of the allotropes of carbon. It is commonly used as a lubricant especially for

dry lubrication since it is able to form a lubricant layer on the opposing counter friction

material rapidly. It is used for stabilizing friction coefficient. It was chosen because of its

good lubrication properties and thermal conductivity. In Kenya graphite is found in

Kariandusi

3.2.4 Binders

a) Kaolin

Kaolin is a name given to fireclay. This is natural clay processed by firing at 7500c for 24hrs

and then milled. Commonly used in kilns and ovens. It was used because of its ability to

withstand high temperatures and its ability to combine with resin to form a strong binder.

25

b) Resin

A sodium silicate resin was used during the construction of the brake pads. This resin can

withstand temperatures of above 8000c. The resin was obtained from Athiriver mining

company.

26

3.3 Choice of sample composition

Sample 1

Material Weight composition Percentage composition

Binder Resin

24g 13.9

Kaolin

10g 5.8

Filler Vermiculite

45.4g 26.4

Barium sulphate

27.3g 15.9

Reinforcing Steel

8g 4.7

Fibre glass 10g 5.8 Abrasive Silica

15.1g 8.8

Alumina 22.7g 13.2 Lubricant Graphite

9.5g 5.5

13.90%5.80%

26.40%

13.20%

5.80%

15.90%

8.80%

5.50% 4.70%

Percentage composition of sample 1

Resin

Kaolin

Vermiculite

Alumina

Fibre glass

Barium sulphate

Silica

Graphite

Steel

27

Sample 2

Material Weight composition Percentage

composition

Binder Resin 31g 15.7

Kaolin 20g 10.1

Filler Vermiculite 45.4g 23.0

Barium sulphate 27.3g 13.8

Reinforcing Steel 11g 5.6

Fibre glass 15g 7.6

Abrasive Silica 15.4g 7.8

Alumina 22.7g 11.5

Lubricant Graphite 9.5g 4.8

19.70%

42.30%

10.50%

22.00%

5.50%

Percentage composition of material categories. sample 1

Binder

Filler

Reinforcing

Abrasive

Lubricant

28

15.70%

10.10%

4.80%11.50%

23.00%

13.80%

7.80%

5.60%7.60%

Percentage composition of sample 2

Resin

Kaolin

Graphite

Alumina

Vermiculite

Barium sulphate

Silica

Steel

Fibre glass

25.80%

36.80%

13.20%

19.30%

4.80%

Percentage composition of material categories. sample 2

Binder

Filler

Reinforcing

Abrasive

Lubricant

29

Sample 3

Material Weight composition Percentage

composition

Binder Resin 37g 16.1

Kaolin 30g 13.0

Filler Vermiculite 45.4g 19.7

Barium sulphate 27.3g 11.9

Reinforcing Steel 14g 6.1

Fibre glass 29g 12.6

Abrasive Silica 15.4g 6.7

Alumina 22.7g 9.9

Lubricant Graphite 9.5g 4.1

16.10%

13.00%

6.70%

9.90%19.70%

11.90%

4.10%12.50%

6.10%

Percentage composition of sample 3

Resin

Kaolin

Silica

Alumina

Vermiculite

Barium sulphate

Graphite

Fibre glass

Steel

30

29.10%

31.60%

18.70%

16.60%

4.10%

Percentage composition of material categories. sample 3

Binder

Filler

Reinforcing

Abrasive

Lubricant

31

CHAPTER FOUR

MANUFACTURING PROCESS

4.1 Materials preparation

The brake pads needed different materials for its construction as listed above. They were

obtained from different locations and also existed in different forms. Hence they needed

preparations to transform them into a state that was more appropriate for the brake pads

construction.

The materials were first collected into a central point before preparation. The preparation

process involved

(i) Crushing in a crushing machine to obtain fine powder. These materials included;

Alumina, barium sulphate, kaolin and silica.

(ii) Vermiculite obtained in natural form was expanded by a process of exfoliation

which involved heating it to temperatures of about 2000F.

(iii) Steel was obtained from the steel wool manufacturers in form of fibre and was

chopped into small lengths of approximately 5mm.

(iv) The above process for steel was also done for fiberglass

Using predetermined weights, the materials were weighed on a weighing balance for every

sample formulation that is samples 1, 2, and 3.

32

4.2 Mould

A mould made of sheet metal to be used in the moulding process was fabricated. This was

made in a shape identical to that of a brake pad already existing in the market.

It was made by folding a sheet metal of height 3mm to the required shape and the joint was

arc welded. For testing purpose a 30mm by40mm by 40mm mould was also fabricated. The

moulds prepared are shown below.

A photo of the two moulds fabricated

33

4.3 Moulding process

The weighed contents were put in a bowl and manually mixed by use of stirring rod until a

homogeneous mixture was obtained.

The steel backing plate was smeared with binder to ensure proper sticking of lining to the

backing plate and positioned into the mould.

The homogeneous mixture was transferred into the mould and pressed severally before

being ejected.

The ejected brake pad was put into a polythene bag for 18 hours to allow slow curing

process.

The brake pads were then hardened by putting them under controlled a temperature of

1500c for 3 hours in an oven.

A photo of brake pads hardening in the oven.

34

The testing samples were also prepared in a similar way and are shown below

Each of the samples was cut to produce three testing specimens of dimensions 20mm

by20mm by10mm for testing internal shear strength.

4.4 Finishing process

After the moulding process finishing operations were done on the brake pads. These

processes included;

The heat groove was cut by use of a grinding machine. The cut was done to a depth of about

3mm on the lining.

The surfaces of the constructed brake pads were also leveled to acquire a uniform

thickness as well as a uniform surface finish. This was done by the use of a grinding

machine to eat away excess materials.

35

The brake pad was finally painted to look presentable. The resulting pads are shown below.

A photo of the resulting brake pads

36

CHAPTER FIVE

LAB TESTING

5.1 Hardness testing

After the brake pads were constructed a hardness test was to be conducted. The hardness

test used was Rockwell hardness test. This was preferred because; it can be used for non

metals, indenters are small hence it could not destroy the specimen, it is a high speed test

and the surface does not have to be reflective

This test consists of an indenter which is a diamond cone with an angle of 1200 and a tip

radius of 0.2mm or steel balls of various diameters. The Kenya standard for this test is KS

06-71:1980. Main scale available are Rockwell C which uses a diamond cone indenter and a

load of 150 kg, Rockwell B where the indenter is 1/16 inch steel ball of 100kg and

Rockwell A which has same indenter as Rockwell C among others. In this case Rockwell C

was used.

The test was done on different positions of the brake pad and an average reading recorded.

SAMPLE TEST RESULTS (Rockwell hardness values)

Test 1 Test 2 Test 3 Average

1 84 88 86 86

2 79 83 81 81

3 77 81 76 78

37

74

76

78

80

82

84

86

88

19.7 25.5 29.1

Har

dn

ess

val

ue

Percentage binder

Graph of Hardness value against binder

74

76

78

80

82

84

86

88

10.5 13.2 18.7

Har

dn

ess

val

ue

Percentage reinforcing fibre

Graph of hardness against reinforcing fibre

38

5.2 Shear strength

This is one of the most important tests in a brake pad. It tests the strength of a brake pad

under shear force hence helps to determine whether the formulation is strong enough to

withstand the forces experienced during braking operation.

The test consists of a special test rig which is used to apply shear load as a normal tension

load. A specimen of dimensions 20mmby 20mm by10mm is loaded on the test rig leaving

it with no room for play. The rig is then mounted on a tensile testing machine and tensile

load is applied until the specimen fails by developing a crack.

The maximum load to failure is determined from the difference between initial and final

loading. The value of ultimate tensile strength is determined as follows:

A photo the testing rig used

39

Results obtained

Sample Test Maximum load

(Newton)

1 1 349.2

2 332.1

3 352.8

Average 344.7

2 1 386.3

2 364.7

3 377.9

Average 376.3

3 1 371.5

2 392.4

3 379.7

Average 381.2

Analysis

Ultimate tensile strength (UTS) = Average maximum load to crack formation

shear area

Shear area = length х width

= 20mm х 10mm= 200mm2

Sample calculations

For sample 1

UTS = 344.7N

200mm = 1.7235N/mm2

40

A table of shear strength for the three samples

SAMPLE SHEAR STRENGTH(N/mm2)

1 1.7235

2 1.8815

3 1.9060

1.6

1.65

1.7

1.75

1.8

1.85

1.9

1.95

19.7 25.8 29.1

She

ar s

tre

ngt

h(N

/mm

2 )

Percentage binder

Graph of shear strength against binder

41

5.3 Macrostructure study

In this test an Optika vision microscope was used. The microscope has an inbuilt digital

camera connected to a computer via a USB cable which enables images of specimen to be

seen on a computer screen. The images seen on the computer were studied.

The specimens were first ground to obtain a flat surface. After placing the specimen in the

microscopes field of view, the lenses were adjusted until a sharp image was obtained.

The image was then transferred to a computer for study. The process was repeated for the

samples 2 and 3.

The results obtained are shown in the next page.

1.6

1.65

1.7

1.75

1.8

1.85

1.9

1.95

10.5 13.2 18.7

She

ar s

ten

gth

(N/m

m2 )

Percentage reinforcing fibre

Graph of shear stregth against reinfocing fibre

42

Macrograph for sample 1

Macrograph for sample2

Macrograph for sample3

43

CHAPTER SIX

DISCUSSION, CONCLUSION AND RECOMMENDATION

6.1 Discussion

Three tests carried out on the constructed samples were; internal shear strength, Rockwell

hardness test and macrostructure study. The results from the internal shear strength test

showed an increase in internal shear strength in samples 1, 2 and 3 respectively.

Sample Internal shear strength (N/mm2)

1 1.7235

2 1.8815

3 1.9060

The increase was attributed to the increase in the percentage of reinforcing fibres (10.5%,

13.2% and 18.7% in samples 1, 2 and 3 respectively) and binder (19.7%, 25.8 and29.1% in

samples 1,2and3 respectively). The values of internal shear strength obtained were lower

than the standard value of 2.5N/mm2 as required by the Kenya Bureau of Standards. The

low shear strength could be attributed to lack of sufficient pressure during pressing

process.

The results obtained from Rockwell hardness test indicated that the hardness of samples

decreased in order of samples 1, 2 and3 respectively.

Sample Hardness(Rockwell hardness values)

1 86

2 81

3 78

The decrease in hardness values was attributed to a decrease in the percentage volume of

abrasives and fillers which are generally hard materials which have an effect of improving

overall hardness.

44

The values obtained from the hardness test for sample 1 and 2 were within to the

minimum range of between 80-85 as required by the Kenya Bureau of Standards while

that of sample 3 was below.

The results from the macrostructure showed that the four subcomponents were not

uniformly distributed as required. Pores were also observed on the samples which gave an

indication that pressure was not sufficient.

Several challenges were faced during the period this project was conducted. These

included;

(i) Lack of facilities such as mixing and moulding machine for use in construction

(ii) Lack of facilities for testing other parameters such as friction coefficient and wear

rate which are also important.

(iii) Lack of co-operation from manufacturers who were not ready to give important

information as wells as allow the use of their facilities.

6.2 Conclusion

We were able to design heat resistant brake pads of materials free of asbestos with

improved internal shear strength. The materials used in the design were locally available.

The locally available materials were used to construct formulations for three samples 1, 2

and 3 which were then tested for shear strength, hardness and macrostructure study.

Since the materials were free of asbestos they can be used as brake pad manufacturing

materials. This indicates that materials used for manufacture of brake linings can be

obtained locally instead of importing.

45

6.3 Recommendations

More research could still be done to achieve optimum brake pads from locally available

materials. Some of the recommendations are;

(i) Materials should be processed to a fine finish before use as opposed to their use in

natural form which would help in improving the final quality of the brake pads

(ii) Machines should be designed for use in the manufacture of brake pads using the

formulations.

46

REFERENCES

(i)Carlson, R. A. and Headley, J. L. Fiber mixtures for brake pads, US Pat. 5871159,

1999 United States Patent and Trademark Office.

(ii) Institution of Mechanical engineers journal vol. 218 part D: J Automobiles

Engineering, Chan and G.W. Stochowiak, Review of automobile brake fiction

materials.

(iii) H. Jang, et al. “The effect of metal fibres on the friction performance of automotive

brake friction materials” Wear, Vol.88

(iv) J. Bijwe, composites as friction material: recent development in non-asbestos fiber

reinforced friction material- a review, polym.comp.18 (3) (1997)378-396

(v)P. J. Blau. Compositions, Function and testing of Brake Materials and their Additives,

RNL/TM-2001/64.September 2001.

(vi)G.O Rading. Concise notes on materials science and engineering, 2006

(vii)Kenya standards for brake pads and brake linings from Kenya Bureau of

standards

(viii) Patent applications by Dickinson Wright Pllc.

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