Grey cast iron
Grey cast iron is the material most commonly used in passenger cars and commercial vehicles: EN-GJL-150, EN-GJL-200, EN-GJL-250 and in part also EN-GJL-300 (flake graphite cast iron). The casting compound is adapted to the specific application in the vehicle by adding various alloying components such as silicon and manganese.
Explanation of designation:
GJL = flake graphite cast iron, 250 = minimum tensile strength 250 N/mm2
High-carbon brake discs (HC / high carbon)
The brake discs or the friction ring on composite brake discs are made of grey cast iron with a higher carbon content for improved heat transfer. The brake disc heats up more uniformly in the braking process and, conversely, also cools down more uniformly. This results in lower thermal deformation of the brake discs with a positive effect on the judder characteristics of the brake. Due to the casting compound, the wear properties and tensile strength of high-carbon brake discs are slightly higher than those of conventional cast iron brake discs.
Example: EN-GJL-200 HC
Brake discs made from stainless steel are mainly used on motorcycles as well as bicycles. Brake pads made of sintered metal or semimetal are used in connection with such discs. Drilled and slotted versions of stainless steel brake discs are also available. Two brake discs are sometimes used on the front wheel of high-powered motorcycles. The brake discs can be mounted in a floating arrangement to isolate vibration.
Carbon ceramic brake discs are around 50% lighter than conventional brake discs made of cast iron. This reduces the unsprung mass of the chassis, thus having a positive effect on the handling characteristics of the vehicle. Carbon ceramic brake discs are made up of a basic body and two friction layers on both sides. Both the basic body and the friction layers are made of carbon fibre-reinforced silicon carbide. This material is extremely hard – the carbon fibres increase its strength and fracture toughness – the impact strength however is low.
In connection with the specifically developed brake pads, carbon ceramic brake discs provide greatly improved response properties, increased fading stability and high thermal stability. This combination produces less brake dust and minimises judder. Due to the less favourable thermal conductivity of carbon ceramic brake discs, all components are subject to greater thermal stress compared to cast iron brake discs. The carbon ceramic discs are however considerably more resistant to high temperatures and have a very long service life. They have a wear indicator hole in the friction ring. It shows when the brake discs are worn and need to be replaced. Under normal usage conditions, carbon ceramic brake discs should be able to last for the complete service life of the car.
The intricate process to produce carbon ceramic brake discs takes about 20 days and is extremely cost-intensive. Consequently they are considerably more expensive than cast iron brake discs and are predominantly used on luxury and sports cars.
These extremely expensive brake discs are used in motorsport (e.g. formula 1). The optimum operating temperature of such brakes is between 350 °C and 550 °C, enabling deceleration of up to 5 g. The brake pads and brake discs are made of the same material.