Coating a heat shield on Composite disks

Metal matrix composite material combined with plasma spraying has solved a problem on vehicle disk brakes. Until now, the discs on disc brakes have been mostly made of grey cast iron, but these are heavy, which reduces acceleration and contributes to increased fuel consumption. A significant decrease in weight has been achieved using carbon discs, but these are so expensive that they can only be used on the highest profile racing machines such as Formula 1 cars and Grand Prix motorcycles. Moreover, they are not so effective in wet conditions.

Aluminium offers advantages because it is light, cheap and easily formed, but it is soft compared to high grade steel. Temperatures on vehicle disc brakes can reach 400iC and more, which is less than the 700iC melting point of aluminium, but a disc of pure aluminium would distort at this temperature. To solve the problem, Metal Composite Technology added 20% of silicon carbide particles to the aluminium to form a metal composite. This increased its strength and stiffness, particularly at high temperatures.

Not all that simple

Casting and machining the composite is not easy. Various new techniques had to be developed to achieve a satisfactory result. Yet even the discs made of the composite material were not the whole answer: the ceramic additive made the discs highly abrasive, so the discs and pads wore out too quickly. It was realised that a coating was needed, and this was provided in consultation with Sulzer Metco.

The coating itself consists of a ceramic in powder form that is applied by plasma spraying. In this, a jet is created by passing an inert gas, such as argon, between a water cooled nozzle and the electrode in a spray gun. The electric arc struck between them ionises the inert gas stream and forms a plasma. The powder is then introduced radially and reaches a temperature of 16000iC in the spray gun. The gas jet reaches a velocity of over 600m/s and carries the powder to impact on the disc where the powder particles are flattened into plate-like shapes, which cool and solidify immediately. The result is a smooth coating with very high density and cohesion. The coating acts like a heat shield so there is no risk of thermal distortion, cracking or warping.

In use, the ceramic face requires a special carbon disc pad, which is supplied together with the new disc rotors. These deposit a layer of material on the disc that helps to reduce wear. Some adjustment is also necessary to suspensions to take account of the altered load. Tests show that the new brake discs perform as effectively as carbon discs with carbon pads, weigh less, have more bite and function as well in the wet.

In comparison with conventional discs of cast iron and stainless steel, they are half the weight and more effective, but they cost more initially, although this is offset by reduced fuel costs and longer wear. The new discs will become commercially available for motor bikes this year on the Honda Fireblade and the Suzuki GSX-R750.