WRITE A REPORT ON

ANTIFRICTION MATERIALS USED

IN CAR BRAKES
Manoj Saikia, RB4911B36, 10906756
B.Tech (Mechanical), Lovely Professional University,
NH1, Phagwara, Punjab, India

Abstract:- The term paper presents about in 1989. All forms of asbestos are
the analysis of antifriction materials carcinogenic. Figure 1 shows a typical disc
brake commonly found in passenger
used in car brakes. It focuses on
vehicles. When a driver steps on the brake
various antifriction materials that pedal, brake fluid is effectively pushed
exist in nature and use of them in against the pistons of the brake caliper,
automobiles. which in turn forces the brake pads against
the brake rotor. This clamping action of the
brake pads retards the rotational movement
I. INTRODUCTION of the brake rotor and the axle that it is
mounted on. Hence the kinetic energy of the
vehicle is converted into thermal energy
The gradual phasing-out of asbestos in which is primarily borne by the rotor and
automotive brake friction materials in many brake pads. Drum brakes operate based on
parts of the world has sparked the onset of similar principles.
extensive research and development into
safer alternatives. As a result, the brake
friction industry has seen the birth of
different brake pads and shoes in the past
decade, each with their own unique
composition, yet performing the very same
task and claiming to be better than others.
This suggests that the selection of brake
friction materials is based more on tradition
and experimental trial and error rather than
fundamental understanding. In this paper, a
review of the materials and constituents
currently used in automotive brake friction
material after the phasing-out of asbestos is
presented. Since the 1970s, asbestos had
gained widespread acknowledgement as a
carcinogen although the introduction of the Fig:- 1
asbestos ban in the United States only came
 II. BASIC Figures 2 and 3 show two different brake
pads and their corresponding images taken
INTRODUCTION
using a scanning electron microscope. An
TO BRAKE energy-dispersive X-ray probe was used to
FRICTION identify the constituents present in the brake
pads. Brake pad sample X was formulated
MATERIAL for use on light motorcycles and is intended
to have a moderate friction coefficient;
brake pad sample Y was formulated for use
on trains and is intended to develop a higher
An automotive brake functions by
friction coefficient. Therefore sample X
converting the vehicle’s kinetic energy into
contains softer fibres made of brass whereas
heat energy. During braking, the heat energy
sample Y contains harsher ingredients such
is first borne by the two contact surfaces of
as zirconium silicate particles and steel
the brake, namely the brake disc and the
fibres.
brake pad (or drum and shoe in the case of
drum brakes), and is then transferred to the
Complete compositional disclosure of brake
contacting components of the brake such as
friction material is rare because this
the calipers of the brake, as well as the
information is treated as proprietary and
surroundings. Table 1 shows the general
manufacturers are not obliged in any way to
classification of brake pads used in the brake
disclose it to customers. The avenues of
industry. The demands on the brake pads are
compositional disclosure can be narrowed
such that they must:-
down to academic research papers and
patent offices, where information disclosure
(a) maintain a sufficiently high friction
is a prerequisite for patent application.
coefficient with the brake disc;
(b) not decompose or break down in such a
way that the friction coefficient with the
brake disc is compromised, at high
temperatures;
(c) exhibit a stable and consistent friction
coefficient with the brake disc.

Brake pads typically comprise the following

sub- components:-

(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.
 III. DRY FRICTION
MATERIALS When you step on the brake pedal, you are
actually pushing against a plunger in
This section deals with the materials used in the master cylinder, which forces hydraulic
formulating friction linings where the oil (brake fluid) through a series of tubes
frictional contact surfaces are intended to be and hoses to the braking unit at each wheel.
dry most of the time, even though they may Since hydraulic fluid (or any fluid for that
be unintentionally lubricated (such as matter) cannot be compressed, pushing fluid
braking a car in the rain). The four main through a pipe is just like pushing a steel bar
components of a brake pad, namely the through a pipe.  Unlike a steel bar, however,
reinforcing fibres, binders, fillers and fluid can be directed through many twists
frictional additives, are discussed below. It and turns on its way to its destination,
is important to note that certain substances arriving with the exact same motion and
perform multiple functions and may be pressure that it started with.  It is very
placed in more than one classification. important that the fluid is pure liquid and
that there are no air bubbles in it.  Air can
compress, which causes a sponginess to the
pedal and severely reduced braking
IV. BRAKES efficiency.  If air is suspected, then the
system must be bled to remove the air.
The modern automotive brake system has There are "bleeder screws" at each wheel
been refined for over 100 years and has cylinder and caliper for this purpose.
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.  Other systems
that are connected with the brake system
include the parking brakes, power
brake booster and the anti-lock system.

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 two brake pads against the disk
(rotor), which is attached to the wheel,
forcing it to slow down or stop.
This process is similar to a bicycle brake
where two rubber pads rub against
the wheel rim creating friction.
 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.

Brake Fluid
Brake fluid is a special oil that has specific
properties. It is designed to withstand cold
temperatures without thickening as well as
very high temperatures without boiling. (If
In either case, the friction the brake fluid should boil, it will cause you
surfaces of the pads on a disk brake system, to have a spongy pedal and the car will be
or the shoes on a drum brake convert the hard to stop.) Brake fluid must meet
forward motion of the vehicle into heat. standards that are set by the Department of
Heat is what causes the friction surfaces Transportation (DOT). The current standard
(linings) of the pads and shoes to eventually is DOT-3, which has a boiling point of 460º
wear out and require replacement. F.  But check your owners manual to see
what your vehicle manufacturer
Master Cylinder recommends.
The master cylinder is located in the engine The brake fluid reservoir is on top of the
compartment on the firewall, directly in master cylinder. Most cars today have a
front of thedriver's seat.  A typical master transparent reservoir so that you can see the
cylinder is actually two completely separate level without opening the cover. The brake
master cylinders in one housing, each fluid level will drop slightly as the brake
handling two wheels. This way if one side pads wear. This is a normal condition and no
fails, you will still be able to stop the car. cause for concern.  If the level drops
The brake warning light on the dash will noticeably over a short period of time or
light if either side fails, alerting you to the goes down to about two thirds full, have
problem. Master cylinders have become your brakes checked as soon as possible.
very reliable and rarely malfunction; Keep the reservoir covered except for the
however, the most common problem that amount of time you need to fill it and never
they experience is an internal leak. This will leave a can of brake fluid uncovered. Brake
cause the brake pedal to slowly sink to the fluid must maintain a high boiling point. 
floor when your foot applies steady Exposure to air will cause the fluid to absorb
pressure. Letting go of the pedal and moisture, which will lower that boiling
immediately stepping on it again brings the point.
pedal back to normal height.
BrakePads have your brakes checked as soon as
There are two brake pads on each caliper. possible.
They are constructed of a metal "shoe" with
the lining riveted or bonded to it.   The pads Power Brake Booster
are mounted in the caliper, one on each side The power brake booster is mounted on the
of the rotor.  Brake linings used to be made firewall directly behind the master cylinder
primarily of asbestos because of its heat and, along with the master cylinder, is
absorbing properties and quiet operation; directly connected with the brake pedal.  Its
however, due to health risks, asbestos has purpose is to amplify the available foot
been outlawed, so new materials are now pressure applied to the brake pedal so that
being used.  Brake pads wear out with use the amount of foot pressure required to stop
and must be replaced periodically. There are even the largest vehicle is minimal.  Power
many types and qualities of pads available. for the booster comes from engine vacuum.
The differences have to do with brake life The automobile engine produces vacuum as
(how long the new pads will last) and noise a by-product of normal operation and is
(how quiet they are when you step on the freely available for use in powering
brake). Harder linings tend to last longer and accessories such as the power brake
stop better under heavy use but they may booster.  Vacuum enters the booster through
produce an irritating squeal when they are a check valve on the booster. The check
applied. Technicians that work on brakes valve is connected to the engine with a
usually have a favorite pad that gives a good rubber hose and acts as a one-way valve that
compromise that their customers can live allows vacuum to enter the booster but does
with. not let it escape. The booster is an empty
shell that is divided into two chambers by a
rubber diaphragm. There is a valve in the
diaphragm that remains open while your
foot is off the brake pedal so that vacuum is
allowed to fill both chambers.  When you
step on the brake pedal, the valve in the
diaphragm closes, separating the two
chambers and another valve opens to allow
air in the chamber on the brake pedal side. 
This is what provides the power assist.  
Brake pads should be checked for wear
periodically. If the lining wears down to the
metal brake shoe, then you will have a
"Metal-to-Metal" condition where the shoe
rubs directly against the rotor causing severe
damage and loss of braking efficiency.
Some brake pads come with a "brake
warning sensor" that will emit a squealing
noise when the pads are worn to a point
where they should be changed.  This noise
will usually be heard when your foot
is off the brake and disappear when you
step on the brake.  If you hear this noise,
Power boosters are very reliable and cause system are connected to a lever that is pulled
few problems of their own, however, other by the parking brake cable to activate the
things can contribute to a loss of power brakes. The brake "drum" is actually the
assist. In order to have power assist, the inside part of the rear brake rotor.
engine must be running. If the engine stalls On cars with automatic transmissions, the
or shuts off while you are driving, you will parking brake is rarely used.   This can cause
have a small reserve of power assist for two a couple of problems.  The biggest problem
or three pedal applications but, after that, the is that the brake cables tend to get corroded
brakes will be extremely hard to apply and and eventually seize up causing the parking
you must put as much pressure as you can to brake to become inoperative.  By using the
bring the vehicle to a stop. parking brake from time to time, the cables
stay clean and functional. Another problem
comes from the fact that the self adjusting
Parking Brakes mechanism on certain brake systems uses
the parking brake actuation to adjust the
The parking brake (a.k.a. emergency brake)  brakes.  If the parking brake is never used,
system controls the rear brakes through a then the brakes never get adjusted.
series of steel cables that are connected to
either a hand lever or a foot pedal.  The idea Anti-Lock Brakes (ABS)
is that the system is fully mechanical and The most efficient braking pressure takes
completely bypasses the hydraulic system so place just before each wheel locks up. When
that the vehicle can be brought to a stop you slam on the brakes in a panic stop and
even if there is a total brake failure.  the wheels lock up, causing a screeching
On drum brakes, the cable pulls on a lever sound and leaving strips of rubber on the
mounted in the rear brake and is directly pavement, you do not stop the vehicle nearly
connected to the brake shoes. this has the as short as it is capable of stopping.  Also,
effect of bypassing the wheel cylinder and while the wheels are locked up, you loose all
controlling the brakes directly. steering control so that, if you have an
Disk brakes on the rear wheels add opportunity to steer around the obstacle, you
additional complication for parking brake will not be able to do so.  Another problem
systems.  There are two main designs for occurs during an extended skid is that you
adding a mechanical parking brake to rear will burn a patch of rubber off the tire,
disk brakes. The first type uses the existing which causes a "flat spot" on the tread that
rear wheel caliper and adds a lever attached will  produce an annoying thumping sound
to a mechanical corkscrew device inside the as you drive.
caliper piston.  When the parking brake Anti-lock brake systems solve this lockup
cable pulls on the lever, this corkscrew problem by rapidly pumping the brakes
device pushes the piston against the pads, whenever the system detects a wheel that is
thereby bypassing the hydraulic system, to locked up. In most cases, only the wheel that
stop the vehicle.  This type of system is is locked will be pumped, while full braking 
primarily used with single piston floating pressure stays available to the other wheels.
calipers, if the caliper is of the four piston This effect allows you to stop in the shortest
fixed type, then that type of system can't be amount of time while maintaining full
used.  The other system uses a complete steering control even if one or more wheels
mechanical drum brake unit mounted inside are on ice.  The system uses a computer to
the rear rotor. The brake shoes on this monitor the speed of each wheel. When it
detects that one or more wheels have
stopped or are turning much slower than the (ii) Metallic
remaining wheels, the computer sends a
signal to momentarily remove and reapply Metallic chips or granules are commonly
or pulse the pressure to the affected wheels used as reinforcing fibres and hence they are
to allow them to continue turning. This referred to as metallic ‘fibres’ although they
"pumping" of the brakes occurs at ten or may not strictly be thread like. Examples of
more times a second, far faster then a human metallic fibres include steel, brass and
can pump the brakes manually.  If you step copper. The drawback of using steel fibres is
on the brakes hard enough to engage the that they will rust, especially if the vehicle
anti-lock system, you may feel a strong has an extended rest period or if the vehicle
vibration in the brake pedal. This is a normal has been operating near a coastal
condition and indicates that the system is environment. Steel fibres attacked by rust
working, however, it can be disconcerting to will be less resilient, thereby compromising
some people who don't expect it.  If your their functionality as reinforcing fibres.
vehicle has anti-lock brakes, read your
owner's manual to find out more about it.
The system consists of an electronic control (iii) Ceramic
unit, a hydraulic actuator,  and wheel speed
sensors at each wheel.  If the control unit Ceramic fibres are a relatively new addition
detects a malfunction in the system, it will in brake pads compared to metallic fibres
illuminate an ABS warning light on the dash such as steel. They are typically made of
to let you know that there is a problem.   If various metal oxides such as alumina
there is a problem, the anti-lock system will (aluminium oxide) as well as carbides such
not function but the brakes will otherwise as silicon carbide. With a high thermal
function normally.  resistance (melting points ranging from
1850 to 3000 °C). Their high strength
weight ratio means that they are preferred
over metallic fibres, which are much
V. REINFORCING heavier.
FIBRES

(i) Glass (iv) Aramid

Glass fibres have been used as reinforcing Aramid fibres (a generic expression
fibres since the mid 1970s. Being physically denoting fibres made from the condensation
strong when bonded together with resinous product of isophthalic or such as Kelvar
binders, glass fibres are suitable for use as fibres are also widely used as reinforcing
reinforcing fibres as they also exhibit fibres, but they are a different class of fibres
thermal resilience. Typical glass has a in that they are relatively soft fibres. They
melting point of 1430 °C which is much are very light and exhibit excellent thermal
higher than the melting point of 800–850 °C stability, with a very good stiffness– weight
for asbestos. The brittleness of glass means ratio.
that it cannot be the sole reinforcement in
brake friction materials.
 of antifriction materials as a result of their
VI. ANTI-FRICTION antifriction capability under all friction
MATERIALS conditions is their inability or poor ability to
seize (adhere) with the material of the
associated part. The greatest tendency to
seizure in friction is with ductile metals of
the same composition in a pair having face-
Materials employed for machine parts centered and body-centered cubical lattices.
(bearings, bushings, and others) which For steel the greatest tendency to seize in
operate with sliding friction and which friction is with silver, tin, lead, copper,
under certain conditions have a low cadmium, antimony, bismuth, and alloys
coefficient of friction. They are notable for based on them.
low adhesion, good wear-in, thermal
conductivity, and stable characteristics. The most common antifriction materials are
Under hydrodynamic lubrication conditions the bearing materials used for sliding
when parts (which are not distorted by the bearings. Besides antifriction properties they
pressure in the lubricating layer) are must possess the requisite strength,
completely separated by a relatively thick resistance to corrosion in the lubricating
layer of lubricant, the properties of the medium, suitability for production, and
material from which the parts are made has economy. Because of the difference in
no effect on the friction. The antifriction requirements between a bearing material
capability of materials becomes apparent forming a friction surface (antifriction
under imperfect lubrication conditions (or capability) and a bearing part providing
for friction without lubrication) and is a support (adequate strength), the bearing
function of the physical and chemical material is distributed on a bearing having a
properties of the material, including high strong structural material as a base (steel, for
thermal conductivity and heat capacity, the example) and a friction surface consisting of
ability to form strong boundary layers that a layer of antifriction material, such as
reduce friction, and the capability of being babbitt metal. The antifriction materials are
easily (elastically or plastically) deformed or applied to a semifinished bearing or to a
worn away so that the load can be uniformly continuously moving steel strip by casting;
distributed over the contacting surface (the from the bimetallic thicknessgauged strip,
wear-in feature). Also related to the bearings (inserts and bushings) are
antifriction capability are the microstructure fabricated by stamping.
of the surface (the specific degree of
roughness or porosity in whose recesses the Bearing materials are divided
lubricating oil is retained) and the ability of into metallic and nonmetallic types. The
the material to “absorb” solid abrasion metallic bearing materials include alloys
particles that fall on the friction surface, based on tin, lead, copper, zinc, aluminum,
thereby protecting the associated part from and also some cast irons; the nonmetallic
wear. Antifriction capability is displayed bearing materials include certain kinds of
under dry friction conditions by a material plastics, materials based on wood, graphitic
containing components that have a carbon materials, and rubber. Some bearing
lubricating action and make for low friction materials are a combination of metals and
by being present on the friction surface (for plastics—for example, a porous layer
example, graphite, molybdenum disulphide, formed by sintered bronze beads
and others). One of the important properties
impregnated with tetrafluoroethylene working surface. The best antifriction
(Teflon) or tetrafluoroethylene with fillers. properties are found in an aluminum alloy
with 20 percent of tin having a
Bearing materials based on tin or lead microstructure obtained by means of plastic
(babbitt metal) are employed in bearings in deformation and annealing. Alloys having a
the form of a layer cast on steel (or Brinell hardness (HB) of HB < 350 mega-
sometimes on bronze). A strong adhesion is newtons per sq m (35 kilograms-force per sq
achieved by special cleaning of the steel; it mm) are used in producing by rolling jointly
is also possible to melt the babbitt metal (for with steel a bimetallic strip or stock from
large bearings) and line the bearing surface which bearing inserts are subsequently
with cavities or grooves for better adhesion. stamped. Alloys having a higher hardness
Automobile bearings are fabricated from (HB=450 meganewtons per sq m, or 45
bimetallic steel-babbitt strip. kilograms-force per sq mm) are used to
make diesel bearings.
The bearing materials having a copper base
are the tin bronzes, the tin-lead bronzes, the Gray pearlitic cast iron having a specific
leaded bronzes, some tin-less bronzes, and microstructure (average to large laminar
also some brasses. For the very high stress pearlite, graphite of average coarseness,
bearings of internal combustion engines, phosphide eutectic in the form of isolated
leaded laminated bronzes (25 percent or inclusions) has antifriction properties and is
more lead) are used in the form of a thin employed for bearings operating under high
layer poured on the steel. loads at low speeds.

Bearing materials having a zinc base serve Bearing materials based on a plastic with
as substitutes for bronzes—for instance the fillers of fabric (textolite) and wood chips
type TsAm9–1.5 alloy used in steam are used successfully in bearings—liberally
locomotive bearings both for the fabrication lubricated with water—for low shaft rotation
of entire inserts and for lining the steel. rates. Even more widely used as bearing
There is also the well-known method of materials are plastics (polyamides,
cladding steel with this alloy during the tetrafluoroethylene, and others) operating
production of bimetallic strips by rolling. with lubricating oil or water. Polyamides are
also used in the form of a thin coating (for
Bearing materials having an aluminum base, instance, 0.3 mm) on the metallic base of a
used extensively for internal combustion bearing, thus increasing the permissible
engine bearings, can be subdivided into two load. The operating condition of bearings
groups according to the degree of ductility made of plastic is limited by the temperature
(hardness rating). Compared with the at the friction surface (for example, no more
babbitts, the ductile aluminum alloys have than 80° to 100°C for polyamides). A
greater heat conductivity and better feature of certain bearings made of
mechanical properties at elevated polyamides is the nearly complete absence
temperatures; they are far cheaper but worse of wear on an associated steel shaft. The
on wear-in and less capable of “absorbing” best antifriction capability at low sliding
hard particles, and they wear the associated velocities without lubrication is that of
steel shaft somewhat more. Their properties tetrafluoroethylene, which retains its low
are improved by depositing a thin (25 friction over a temperature operating range
micron) layer of a tin-lead alloy on the from -200°C up to 260°C.
Bearing materials based on wood generally VII. FUTURE TENDS
include natural wood, compressed woods,
and laminated woods. An example of a It is envisioned that future developments in
natural bearing material is guaiacum or the field of brake friction materials will
lignum vitae, which is used with a water closely mimic the current trends of the
lubricant. Bearing materials based on wood automotive industry. The future emphasis on
are used with liberal water lubrication in cars will be on lower emissions and fuel
bearings for rolling mills, water turbines, efficiency as environmental regulations
and ships’ propellers. become more stringent. This shift towards
environmentally friendly cars has already
The graphitic carbon bearing materials are seen the release of hybrid cars such as
produced by pressing and heat-treating Toyota Prius, Honda Insight and Ford
mixtures of petroleum coke and coal tar with Escape SUV. The focus on vehicle fuel
a small amount of natural graphite. They are efficiency and lower emissions will mean
used as bearing materials to operate without that brakes will have to be lighter and not
lubrication under low specific loadings, at release any toxic and carcinogenic
temperatures up to 480°C, and in air. substances into the atmosphere during use.
Graphitic carbon bearing materials This means that the choice of brake friction
impregnated with metals or resin are also materials will need to be more
prepared. environmentally friendly and not include
toxic substances such as asbestos.
Rubber is used as a bearing material when
well lubricated with water at low specific
loadings and low sliding velocities. The VIII. CONCLUSION
operating condition is limited to a
temperature of 50° to 70°C at the friction The present invention relates to antifriction
surface. materials, such as plastic and methods of
manufacturing articles there from which,
Metal-ceramic self-lubricating bearing articles are intended for use in dry friction
materials are employed in the form of assemblies. A disadvantage of the known
porous bushings (mainly of small size, antifriction materials prepared on the basis
operating at low speeds, and with no of said cyclic polymers is that at high
external supply of lubricant). They are temperatures the value of their coefficient of
fabricated by sintering previously friction is great and not stable. 
compressed blanks made of tin bronze
powders (10 percent Sn) with a graphite
admixture or of iron with graphite. The
degree of porosity is about 25 percent. The
bushings are impregnated with oil.
 IX. REFERENCE

 http://www.familycar.com/brakes.ht
m
 http://ask.alibaba.com/Q/808720296-
Most-of-the-car-brake-pads-made-
with-what-material.html
 http://en.wikipedia.org/wiki/Parking
_brake
 http://en.wikipedia.org/wiki/Disc_br
ake
 http://www.sciencedirect.com
 http://www.answers.com/topic/antifri
ction-material
 http://manufacturing.kellysearch.com
/suppliers/Friction+and+Antifriction
+Products+and+Materials/US/993
 http://www.freepatentsonline.com/36
52408.html