Stainless steel brake discs significantly reduce particulate emissions and are exceptionally durable
A stainless steel brake disc exhibits extremely low wear and has a projected service life of up to 300,000 km. In a wheel brake fitted with an inorganic brake lining, wear is reduced by over 85 per cent compared with a grey cast iron brake disc and an organic friction lining.
Until now, particulate matter resulting from tyre and brake abrasion has not been taken into account in European emissions legislation. It is only the Euro 7 standard – which comes into force from the end of 2026 for newly developed (type-approved) vehicles and from the end of 2027 for all newly registered cars and light commercial vehicles – that introduces binding limit values. The aim is to limit emissions of fine particulate matter with a diameter of less than 10 micrometres, which can penetrate deep into the respiratory tract. Such particles are considered particularly harmful to health. A project consortium involving the Fraunhofer IWU is now presenting a stainless steel brake disc that easily meets the strict EU requirements.
The stainless steel brake disc, manufactured using a forming process, exhibits extremely low wear and has a projected service life of up to 300,000 km. In a wheel brake with an inorganic brake lining material, wear is reduced by over 85 per cent compared with the current standard solution comprising a grey cast iron brake disc and an organic friction lining.
Why stainless, hardened (nitrided) stainless steel is particularly suitable
The project team opted early on for nitrided stainless steel, which is particularly suitable due to its tribological and thermal properties. Positive experiences with stainless steel brake discs in motorcycles also support this choice. Expensive specialist solutions such as carbon/ceramic are only considered for particularly high-end vehicles; coating grey cast iron is extremely challenging – laser cladding methods are not yet ready for series production. Structural steel discs, on the other hand, do not meet the requirements for dimensional stability in the temperature range above 650 °C; structural transformations occurring in this temperature range can also alter the properties of the steel.
Manufacture using forming techniques, weight advantages
The project team produced brake discs with a slightly larger diameter than conventional grey cast iron discs to ensure sufficient surface area for the required deceleration performance (braking performance). Conversely, the thickness of a stainless steel brake disc can be reduced. As the raw material for the workpiece is initially in a square format, cutting offcuts are produced; however, these can be remelted. Depending on the vehicle, four stainless steel brake discs are up to 5 kg lighter than grey cast iron alternatives. The lower weight not only reduces the vehicle’s energy consumption but also reduces unsprung masses. This allows springs and dampers to work more efficiently, which improves vertical dynamics and thus overall handling.
Option of a lifetime brake, positive cost-benefit analysis
The manufacturing costs for grey cast iron brake discs only appear low as long as there are no legal requirements regarding particulate emissions caused by brake wear. However, with the introduction of the Euro 7 standard, only 3 mg/km is permitted for battery electric vehicles and 7 mg/km for all other drive types in passenger cars and light commercial vehicles with a gross vehicle weight of up to 3.5 tonnes. In most cases, a conventional wheel brake ‘falls short’ here, even when combined with high-quality brake pads – it cannot achieve such low emission levels. But even without the regulatory requirements of the Euro 7 standard, their overall cost-benefit balance, based on a vehicle’s service life of up to 300,000 km, is unfavourable. Once the wear limit is reached, they must be replaced (often together with the brake pads), with labour costs frequently accounting for the largest share of the total cost. Replacement may be required after less than 40,000 km if they have been subjected to long periods of inactivity and road salt (corrosion), short-distance journeys or a sporty driving style (increased abrasion, grooving).
Initial tests successfully passed
The stainless steel brake disc, developed by Fraunhofer IWU in collaboration with Chemnitz University of Technology, automotive supplier ElringKlinger and engineering firm Andritz Aweba, has already been successfully tested on Chemnitz University of Technology’s flywheel test rig in accordance with SAE J2522. The system, comprising a stainless steel brake disc and inorganic friction material, showed approximately 85 per cent less wear compared with solutions currently available on the market.
Source: idw