Carbon-Ceramic Brake Disks
At the IAA in Frankfurt in 1999, the carbon-ceramic brake disk had its world premiere. The use of the high-tech material had revolutionized the brake technology: In comparison to the conventional grey cast iron brake disk the carbon-ceramic brake disk weighed round 50 per cent less reducing the unsprung mass by almost 20 kilograms. Further significant advantages are: improved brake response and fading data, high thermal stableness, no hot judder, excellent pedal feel, improved steering behavior, high abrasion resistance and thus longer life time and the advantage of avoiding almost completely brake dust. At first Porsche AG built the carbon-ceramic brake disk in 2001 into the 911 GT2 as series equipment. Since that time also other premium brands use the advantages of this innovative brake technology for more security and comfort. These are for example sports cars and luxury class limousines from Audi, Bentley, Bugatti and Lamborghini.
Dimensioning and Design
The overall car braking system is designed to match a car´s layout and take advantage of the ceramic brake disk material´s properties. We cover the designing of the brake – the construction of the brake disk as well as the selection of the friction layers and the caliper – and adjust the brake into the concept of the vehicle. The main parameters determining the braking system design are a car´s maximum speed, the time sequence of full brake applications possible to bring a car to a stop from top speed and the mass to be braked, in addition to the axle load distribution and the car´s aerodynamics. The purpose of brake disk dimensioning and design is to ensure that a car can be stopped safely under any conceivable driving conditions. Braking system design also needs to ensure that neither the disk itself nor any other component in its direct vicinity is exposed to excessive thermal loads. The optimal cooling vane geometry is determined by numerical methods (Computational Fluid Dynamics) for each car model. The design calculation also takes account of the air pressure building up underneath the car and inside the wheel arch as a function of the car´s aerodynamic design and traveling speed.
A special feature of carbon-ceramic brake disks is the ceramic composite material they are made from. Both the carbon-ceramic brake disk body and the friction layers applied to each side consist of carbon fiber-reinforced silicon carbide. The main matrix components are silicon carbide (SiC) and elemental silicon (Si). The reinforcement of the material is provided by carbon fibers (C). Silicon carbide, the main matrix component governs great hardness for the composite material. The carbon fibers make for high mechanical strength and provide the fracture toughness needed in technical applications. The resulting quasiductile properties of the ceramic composite material ensure its resistance to high thermal and mechanical load. Carbon fiber-reinforced silicon carbide materials thus combine the useful properties of carbon fiber-reinforced carbon (C/C) and polycrystalline silicon carbide ceramics. The elongation at break of C/SiC materials ranges from 0.1 to 0.3%. This is exceptionally high for ceramics. The entire characteristic profile makes fiber-reinforced silicon carbide to a fist-choice material for high-performance brake systems: Particulary the low weight, the hardness, the stable characteristics also in case of high pressure and temperature, the resistance to thermal shock and the quasiductility provide long live time of the brake disk and avoid all problems resulting of loading, which are typical for the classic grey cast iron brake disks.
|C/SiC material, general||C/SiC for carbon- ceramic brake disk||Gray cast iron (GG-20)|
|Density||g cm-3||1,8 ... 2,9||2,45||7,25|
|10 ... 240||20 ... 40||200 ... 250|
|Modulus of elasticity||GPa||20 ... 240||30||90 ... 110|
|20 ... 210||50 ... 80||150 ... 250|
|Elongation at break||%||0.05 ... 0.8||0.3||0.3 ... 0.8|
|Thermal shock resistance (second thermal coefficient K')||W m-1||26.500 ... 46.000||> 27.000||< 5.400|
|Thermal stability||°C||1350||1350||approx. 700|
|Maximum operating temperature (brake disk)||°C||1400 non-oxidizing||900||700|
|Linear coefficient of thermal expansion||K-1||1.0 ... 3.5||2.6 ... 3.0||9 ... 12|
|Thermal conductivity||W m-1K-1||20 ... 150||40||54|
|Specific heat capacity (cp)||kJkg-1K-1||0.6 ... 1.7||0.8||0.5|
The secret of the advantages of the carbon-ceramic brake disk is the unique production process over approximately 20 days. To produce carbon-ceramic brake disks, we use carbon fibers which are given a special protective coating and then cut into short fiber sections of defined thickness and length. The production process includes preparation of the fiber mixture, the production process for the disk body and the bell mounting as well as the final machining of the assembled brake disk. The entire production process is monitored with various tests and ends with one final testing. The production process of the ceramic brake body continues with a preform pressed with binding resin to a so called green body which will be converted in the ceramic component by carbonizing at 900 °C and siliconizing at 1700 °C in high vacuum. The complex feature of the manufacturing process is the use of the “lost core” technology – a plastics matrix which defines the design of the cooling vane geometry and which burns out without residues at carbonizing – as well as the different fiber components of the brake disk body, the friction layers on the ring exterior side and the point-shaped abrasion indicators which are integrated into the friction layer.
A carbon-ceramic brake is developed in three main stages to match a car´s particular layout: numerical modeling, the construction and testing of prototypes, and testing on an actual car. The brake disk is first simulated numerically on the computer, using the car´s particular model data. The carbon-ceramic brake disk´s diameter, its thickness and the height of the friction path are only some of the parameters calculated on the computer. Calculations for assembled carbon-ceramic brake disks include the design of the bell connection. This is a highly demanding design task because of differences in coefficients of thermal expansion need to be compensated for at any operating temperature possible. The numerical model also provides the design of the cooling vanes configured to optimise fluid dynamics. In the second development stage, prototypes (test specimens) of the carbon-ceramic brake disks are constructed on the basis of numerical model results and bench-tested, together with the matching brake pads and calipers. In the third and final stage, the disk prototypes are tested on the car. They complete not only high-speed runs on a test circuit but also mountain pass descents and road tests. On these test runs, the driver evaluate brake behavior, in particular braking performance and braking comfort, and the computer provides a detailed analysis of measured results. Together with the bench test results, the car test runs determine whether a disk prototype can be approved or not.
Quality Assurance and Testing
The braking system is the most important safety system in any car. Carbon-ceramic brake disks therefore need to be manufactured to consistently high quality standards. This is why we have introduced a comprehensive Quality Management System conforming to VDA 6.1 and ISO 9001:2000. The system describes all production and operating processes and also ensures the consistent monitoring and documentation of product quality – from the raw material used to the finished product. The Manufacturing Execution System and a Computer-Aided Quality System are used to implement it. We record all production data and test results for each individual carbon-ceramic brake disk. This allows each disk to be identified both during production and also later when the disk has been mounted on the car. All operations concerned are documented in an Enterprise Resource Planning System and a CAQ System, together with all tests and inspections, the staff and equipment involved, and the results of the tests. Each carbon-ceramic brake disk thus generates around 600 data items during its complete production run.