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
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 50’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.
ADVANTAGES OF CERAMIC BRAKE DISCS
- 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.
- 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 Daewoo’s Nexia, it takes about 4 seconds to stop the vehicle.
- 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.
- Ceramics retain their resistance up to 2000 degree celcius.Only if the temperature is excess of this, they loose their dimensional stability.
- 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.
- They are not subject to wear, are maintenance free and are heat and rust resistant.
- Heavily commercial vehicles can be braked safely over long distances without having to undergo brake maintenance. This dispenses with the need for expensive maintenance.
- Ceramic brake discs do not rust under high oxygen concentration.
- Ceramic brake discs do not warp or wear under constant use.
- Dry and wet performances are excellent. Ceramics are water proof materials and the brake pads always remain dry.
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.