University of Leeds Team Puts the Brake on Inefficient Cars

Professor David Barton from the University of Leeds on the search for lightweight materials for automotive parts

Virtually all vehicles on the road– including electric cars, supposed to be very energy efficient – use cast iron as the brake rotor material. It’s a high-density material, so you are carrying around excess mass. With electric vehicles, which have regenerative braking, you may hardly ever be using that extra mass, but you have to have it anyway for emergency braking if the electrical system fails.

One of the areas of our work is to cut the weight of the rotor by using a light alloy alternative, primarily an aluminium alloy. We have been looking at a metal matrix composite, where you reinforce the aluminium alloy with particles such as silicon carbide, to give it strength, rigidity, and a bit of exothermal resistance.

We have also coated the surface of the alloy with a ceramic material called aluminate. It creates a temperature-resistant layer on the brake rotor, enabling it to resist the braking duty without safety concerns. We are testing it on our brake dynamometer, which replicates what happens on a vehicle when braking. There could be sudden stops under emergency braking, or it could be a long, continuous, drag-braking event such as going down a long mountain descent. The risks could be very serious if the aluminium alloy softens or, in the worst case, melts. Then you potentially lose all your braking performance. It is important to make sure you are nowhere near the limit of the aluminium alloy.

The good thing about cast iron is that, even though it is heavy, it is quite heat resistant. You have to have a really high temperature before the cast iron starts to degrade, whereas the aluminium alloy will start to soften around 500°C.

So that is the challenge. By reducing the weight of the rotor, we could easily be saving 2-3kg per wheel station, meaning 10kg or more per vehicle. There are constant efforts to cut the weight of engine and drivetrain components, and the chassis itself. Jaguar Land Rover, for example, produces virtually an all-aluminium chassis now for its cars. Still, there are certain components – such as the brake rotor – where there has been no progress with cutting the weight. It is not going to reduce fuel consumption by a fantastic percentage, but it all contributes to producing fuel-efficient vehicles that release less CO2.

The other focus is the debris that comes out of friction braking, such as particulate debris from diesel engines. But the other main sources of debris are the tyres, which wear and produce tiny rubber particles, and the brakes.

There have been some suggestions of a link – that has not been proven – between tiny iron oxide particles, magnetite in particular, and people suffering from Alzheimer’s. If there is iron in a brake, it is going to be oxidised and released as small iron oxide particles. They can get into people’s lungs, so it’s best to switch to a non-iron rotor such as an aluminium alloy. Also, rather than letting the debris be released into the atmosphere, you could capture it as you do with particulates from a diesel engine. That’s another aspect we are starting to work on.

The car industry is very conservative. But potentially, within five years, you could start seeing cars with these brake discs on the streets of our cities.