Todayâ„¢s technology is in need for speed, but at the same time, we need safety as well. For safety, we need deceleration to the maximum extent. These two things are moreover contradictory factors. For speed, we need engines of maximum efficiency and for keeping this speed in bounds, we need brakes of latest technology. For coping up with todayâ„¢s speed, new materials are introduced in the manufacture of brakes.
In my seminars, I am introducing to you the ceramic disc brakes. Cast iron is extensively used as the material for manufacturing disc brakes. This is much heavier and thus reduces initial acceleration and causes more fuel consumption. For reducing these effects, we use ceramic brakes.
The ceramic disc brakes possess the following advantages:
Its weight is half the weight of conventional disc brakes.
It increases the fuel efficiency of the vehicle.
It functions well in wet conditions as well.
The major disadvantage is its cost.
One of the most important control system of an automobile is BRAKE SYSTEM .They are required to stop the vehicle within the smallest possible distance and is done by converting kinetic energy of the vehicle into heat energy which is dissipated into atmosphere.
The main requirements of brakes are given below:-
1. The brakes must be strong enough to stop the vehicle within the minimum possible distance in an emergency. But this should also be consistent with safety. The driver must have a proper control over the vehicle during emergency braking and the vehicle must not skid.
2. The brakes must have good antifade characteristics and their effectiveness should not decrease with constant prolonged application.
The actual stopping distance of vehicle while braking depends on the following factors:-
1. Vehicle speed
2. Condition of the road surface
3. Condition of tyre tread
4. Coefficient of friction between the tyre tread and the road surface
5. Coefficient of friction between the brake drum/disc and brake lining/friction pad
6. Braking force applied by the driver
TYPES OF BRAKES
The brakes for automotive use may be classified according to the following consideration
a). Service or primary brakes
b). Parking or secondary brakes
a). Drum brakes
b). Disc brakes
3. Method of Actuation
a). Mechanical Brakes
b). Hydraulic Brakes
c). Electric Brakes
d). Vacuum Brakes
e). Air Brakes
4. Extra Braking Effort
a). Servo Brakes or Power assisted brakes
b). Power Operated Brake
As shown in fig a disc brake consists of a cast iron disc bolted to the wheel hub and a stationary housing called caliper. The caliper is connected to some stationary part of the vehicle, like the axle casing or the stub axle and is cast in two parts, each part containing a piston. In between each piston and disc there is a friction pad held in position by retaining pins, spring plates etc., passages are drilled in the caliper for the fluid to enter or leave each housing. These passages are also connected to another one for bleeding.
When the brakes are applied hydraulically actuated pistons move the friction pads into contact with the disc, applying equal and opposite forces the later. On releasing the brakes the rubber sealing rings act as return springs and retract the pistons and the friction pads away from the disc.
Two types brake discs are generally used the solid type and the ventilated type. The ventilated type more efficient since it provides better cooling. But they are thicker and heavier than solid type, they are liable to wrap at severe braking conditions, the dirt accumulates in the vents which affects cooling and apart produces wheel imbalance.
The discs of the brakes are made of pearlite gray cast iron. The material is cheap and has good antiwear properties. Cast steel discs have also been employed in certain cases, which wear still less and provide higher coefficient of friction. Their main drawback is the non uniform frictional behavior.
The other materials used for the manufacture of disc are
Obviously, cast-iron disc is the heaviest part of a brake - about 8 kg each, or 32 kg per car. Aluminium alloy discs are used in the Lotus Elise. Though light, they were less resistant to heat and fade, thus more powerful Elises still employ conventional cast-iron disc.
In contrast, carbon-fiber disc is most heat-resisting yet is by far the lightest, however, it requires very high working temperature, and otherwise braking power and response will be unacceptable.
Ceramics are inorganic, non-metallic materials that are processed and used at high temperatures. They are generally hard brittle materials that withstand compression very well but do not hold up well under tension compared to the metals. They are abrasive-resistant, heat resistant (refractory) and can sustain large compressive loads even at high temperatures. The nature of the chemical bond in the ceramics is generally ionic in character, and the anions play an important role in determination of the properties of the material. Typical anions present are carbides, borides, nitrides and oxides. The different types of ceramics are clays, refractories, glasses etc.
Cast iron has been the material of choice for brakes rotors since the introduction of disc brakes during the 50â„¢s.Elise made a new era, being the first road production road car to use aluminium metal matrix composite discs inserted on four wheels. Aluminium is better rotor material than cast iron due to two main reasons: its density is as one third as cast iron but its thermal conductivity is three times greater. These factors made it possible to construct a much lighter brake disc.
COMMPARISON OF CERAMIC BRAKES
AND CONVENTIONAL BRAKE DISC
Until now brake discs have been made up of grey cast iron, but these are heavy which reduces acceleration, uses more fuel and has a high gyroscopic effect.
Ceramic disc brake weigh less than carbon/carbon discs but have the same frictional values with more initial bite and cost a fraction of price. Carbon /carbon discs are used only in Formula 1 racing cars etc, because it is so expensive. More over ceramic brake discs are good even in wet conditions which carbon / carbon disc notoriously fails to do.
But comparing their weight, you will see right away that we are looking at two different worlds, with ceramic brake discs more than 61 per cent lighter than conventional cast iron discs. In practice this reduces the weight of the car, depending on the size of the brake discs, by up to 20 kg. And apart from saving fuel, resulting in better and lower emission for the same mileage, this also means a reduction in unsprung masses with a further improvement of shock absorber response and behavior. Another is the manufacturer can add more safety features without adding to current weight.
Disc brake fitted to a vehicle
MANUFACTURING OF CERAMIC BRAKE DISCS
In the earlier days, the brake discs were made from the conventional brittle ceramic materials would have disintegrated into a thousand pieces under slightest pressure.
The research division of Daimler Chrysler has developed techniques to make carbon fiber reinforced brake discs which avoid the brittleness problem. In the earlier days, long carbon fibers were used. Later the use of short carbon fibers increased the efficiency.
The composites for producing fiber reinforced ceramic brake discs are short carbon fibers, carbon powder, and resin mix. The process involves first compressing the carbon fibers, carbon powder and the resin mix together and then sintering at1000 degree Celsius. In the furnace a stable carbon frame work created. This consists of carbon fibers in a carbon matrix. Once cooled this material can be ground like wood and the break disk obtains its final shape.
Together with silicon the ground break disk blank is then inserted into the furnace a second time. The pores in the carbon framework absorb the silicon melt like a sponge; the fibers themselves remain unaffected by this process.
The ceramic material is created when the matrix carbon combines with liquid silicon. This fiber reinforced ceramic material cools over night and the gleaming dark grey break disk is ready.
Resin is a binder, which holds the different constituents together. Resins are of two types : 1.Thermosetting resins
2. Thermoplastic resins.
Thermoplastic resins are those, which can be softened on heating harden on cooling. Repeated heating and cooling does not affect their chemical nature of materials. These are formed by addition polymerization and have long chain molecular structure.
Thermosetting resins are those resins which, during molding process (by heating) get hardened and once they have solidified, they cannot be softened i.e. they are permanent setting resins. Such resins during moldings, acquire three dimensional cross linked structure with predominantly strong covalent bonds. They are formed by condensation polymerization and are stronger and harder than thermoplastic resins. They are hard, rigid, water resistant and scratch resistant.
COATING OF CERAMICS ON CONVENTIONAL BRAKE DISC
Earlier brake disc have been made of grey cast iron, but these are heavy which reduces acceleration, uses more fuel, etc. The new technology developed by Freno Ltd uses metal matrix composite for the disk, basically an alloy of aluminum for lightness and silicon carbide for strength. However it was found that, the ceramic additive made the disk highly abrasive and gave a low and unstable coefficient of friction. So it was realized that the surface had to be engineered in some way to overcome this problem. After experiments, Sulzer Metco Ltd found an answer in the form of a special ceramic coating. They developed thermal spray technology as well as manufacturing plasma surface engineering machinery used for the task and coating materials.
In use, the ceramic face requires a special carbon metallic friction pad, which deposits a layer of material on the brake disc. This coupling provides the required conditions of exceptional wear resistance, high and stable coefficient of friction.
The coated matrix composite discs were first used on high performance motor cycles, where the reduced gyroscopic effect had the additional advantage of making the cycles easier to turn.
Another company named Lanxide used aluminium as the disc material. To provide necessary abrasion resistance, aluminium discs have to be reinforced with a ceramic material, hence metal composite. They used silicon carbide also to increase the strength.
PORSCHE CERAMIC DISC BRAKES (P C C B)
After a long period of research and tests Porsche has developed new high performance disc brakes, P C C B (Porsche Ceramic Composite Brakes). Porsche has succeeded as the first car manufacturer in the world to develop ceramic brake discs with involute cooling ducts for an efficient cooling. The new brake system offers a substantial improvement in the car braking technology and sets entirely new standards in terms of decisive criteria such as braking response, fading stability, and weight and service life.
PORSCHE CERAMIC COMPOSITE BRAKE
Porsche's new brake system also offers obvious advantages in emergencies at low speeds: In such a case emergency application of the brakes with PCCB technology does not require substantial pedal forces or any technical assistance serving to build up maximum brake forces within fractions of a second. Instead, the Porsche Ceramic Composite Brake ensures maximum deceleration from the start without requiring any particular pressure on the brake pedal. And the new brake system is just as superior in its response under wet conditions, since the new brake linings cannot absorb water in the same way as conventional linings. The final point, of course, is that the cross-drilled brake discs help to optimize the response of the brakes also in wet weather.
The process involves heating carbon powder, resin and carbon fibers in a furnace to about 1700 degree Celsius and is a high vaccum process.
P C C B FITTED IN PORSCHE 911 TURBO
ADVANTAGES OF CERAMIC BRAKE DISCS
1. Ceramic brake discs are 50% lighter than metal brake discs. As a result, they can reduce the weight of car by up to 20kg. In case of a high speed ICE like train with 36 brake discs, these savings amount to 6 tons. And apart from saving fuel, this also means a reduction in unsprung masses with a further improvement of shock absorber response and behavior.
2. The ceramic brake disc ensures very high and, in particular, consistent frictional values
throughout the entire deceleration process. With Porsche ceramic brake discs, a car was able to decelerate from 100Km to 0Km in less than 3 seconds. In the case of Daewooâ„¢s Nexia, it takes about 4 seconds to stop the vehicle.
3. Brake temperature - a factor crucial to stopping distances with metal brake discs - is now only a minor factor, both the brake lining and ceramic brake disc retaining their high level of friction regardless of whether they are hot or cold. This not only shortens stopping distances by a couple of - often decisive - meters, but also spares the driver unpleasant surprises whenever having to apply the brakes, say, from a high speed.
4. Ceramics retain their resistance up to 2000 degree celcius.Only if the temperature is excess of this, they loose their dimensional stability.
5. Initial field studies have shown that ceramic brake discs can still reliably bring an automobile to standstill even after 300,000 kilometers. Brake disc changes will in future be unnecessary.
6. They are not subject to wear, are maintenance free and are heat and rust resistant.
7. Heavily commercial vehicles can be braked safely over long distances without having to undergo brake maintenance. This dispenses with the need for expensive maintenance.
8. Ceramic brake discs do not rust under high oxygen concentration.
9. Ceramic brake discs do not warp or wear under constant use.
10. Dry and wet performances are excellent. Ceramics are water proof materials and the brake pads always remain dry.
It was first introduced in Formula One, but applying to road cars seems impractical (F1 cars have warm up lap to bring the discs into appropriate working temperature), although the short-lived French sports car specialists Venturi made history by applying it to its road cars in the mid-90s
Porsche's fastest production model, the 911 Turbo, with a top speed of 305 km/h (189 mph) and acceleration from rest to 100 km/h (62 mph) in 4.2 s. Its engine is a 3.6-L producing 309 kW (420 hp), with maximum torque of 560 NÂ¢m (413 lbÂ¢ft) available from 2700 rpm is available with new Porsche Ceramic Composite Brake System.
PORSCHE 911 TURBO
PORSCHE 911 GT2
The new 911 GT2 comes with the most effective braking system ever featured on a production Porsche: the Porsche Ceramic Composite Brake (PCCB) - a powerful new technology designed to cope with even the most extreme conditions on racetrack and road
Mercedes-Benzâ„¢s the futuristic Vision GST concept car features 22 inch wheels, 'butterfly' doors, three-dimensional instruments, a 360 horsepower 5.5 litre V8 engine, and carbon-fiber reinforced ceramic disc brakes.Mercedes Benz SLR IS ALSO available with ceramic disc brakes.
The main disadvantage of ceramic brake discs is their high initial cost. Initially the ceramic matrix composite brake discs will be more expensive than the current technology metal ones due to the low manufacturing volumes and high cost of production. But , because of the advantages listed above , the ceramic brakes will workout to be cheaper in the long run.
Ceramic brake discs due to its advantages over the conventional brake discs are going to be the brake discs for cars in future. The special combination in the ceramic brake discs had turned the conventional brake disc into a material most suited for making brake discs. With the success of this in Porsche turbo car , many other racing cars and commercial vehicles are going to implement the ceramic disc in their cars.
1. AUTOMOTIVE MECHANICS â€œ CROUSE / ANGLIN
3. Automotive Engineering International Online Global Viewpoints, Nov_ 1999
1. INTRODUCTION 1
2. TYPES OF BRAKES 1
3. DISC BRAKES 2
4. CONSTRUCTIONAL FEATURES 2
5. COMPARISON OF CERAMIC BRAKES
AND CONVENTIONAL BRAKES DISC 5
6. MANUFACTURING OF CERAIC BRAKE DISCS 6
7. COATING OF CERAMICS ON CONVENTIONAL BRAKE DISC 7
8. PORSCHE CERAMIC DISC BRAKES (PCCB) 8
9. ADVANTAGES OF CERAMIC BRAKE DISCS 9
10. APPLICATION 11
11. DISADVANTAGES 12
12 CONCLUSION 13
13. REFERENCES 14