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 equipped with an inorganic brake lining, wear is reduced by over 85 percent compared to a gray cast iron brake disc and an organic friction lining.
Until now, fine particulate matter generated by tire and brake wear has not been addressed in European emissions legislation. Only with the introduction of the Euro 7 standard – taking effect at the end of 2026 for newly developed (type-approved) vehicles and at the end of 2027 for all newly registered passenger cars and light commercial vehicles – will binding limit values be introduced. The goal is to restrict the emission of fine particulate matter with diameters less than 10 micrometers, particles that can penetrate deep into the respiratory tract and are considered particularly harmful to human health. A project consortium involving Fraunhofer IWU has now introduced a stainless-steel brake disc that easily meets the strict EU requirements.
The “Ufo-Brems” project was funded by the German Federal Ministry for Economic Affairs and Energy and coordinated by Projektträger Jülich.
The brake disc, manufactured using forming processes, exhibits extremely low wear and has a projected service life of up to 300,000 km. When used in a wheel brake system with an inorganic friction material, wear is reduced by more than 85 percent compared to today’s standard solution, consisting of a gray cast iron brake disc and an organic friction lining.
Why Corrosion-Resistant, Hardened (Nitrided) Stainless Steel Is Particularly Suitable
At an early stage, the project team opted for nitrided stainless steel, which is particularly well-suited due to its tribological and thermal properties. A positive experience with stainless-steel brake discs on motorcycles also supports this choice. Specialized solutions such as carbon–ceramic brakes are only feasible for very high-end vehicles, while coating gray 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 at temperatures above 650 °C; phase transformations occurring in this temperature range can also adversely affect the material properties.
Forming-based manufacturing and weight advantages
The project team produced brake discs with a slightly larger diameter than conventional gray cast iron discs in order to provide sufficient surface area for the required deceleration performance (braking force). In return, the thickness of a steel brake disc can be reduced. Since the starting material is initially available in a square format, cutting scrap is generated, but this material can be remelted. Depending on the vehicle, a set of four stainless-steel brake discs can be up to 5 kg lighter than comparable brake discs made of gray cast iron. The reduced weight not only lowers the vehicle’s energy consumption but also decreases unsprung mass. This allows the suspension and dampers to operate more efficiently, improving vertical dynamics and overall driving behavior.
Option for a lifetime brake system and a positive cost balance
The manufacturing costs of gray cast iron brake discs appear low only as long as there are no statutory requirements for fine particulate emissions caused by brake wear. With the introduction of the Euro 7 standard, however, only 3 mg/km for battery-electric vehicles and 7 mg/km for all other powertrain types will be permitted for passenger cars and light commercial vehicles with up to 3.5 tons gross vehicle weight. Conventional wheel brake systems generally fail to meet these limits – even when combined with high-quality brake pads. But even without the regulatory requirements of Euro 7, their overall cost balance over a vehicle lifetime of up to 300,000 km becomes unfavorable. Once the wear limit is reached, they must be replaced (often together with the brake pads), with labor costs frequently accounting for the largest share of total expenses. Replacement may be necessary after less than 40,000 km, if long idle periods combined with road salt (corrosion), short-distance driving, or a sporty driving style (increased wear and grooving) accelerate degradation.
Initial tests successfully completed
The stainless-steel brake disc developed by Fraunhofer IWU in cooperation with the Chair of Vehicle System Design at TU Chemnitz, ElringKlinger and Andritz Aweba has already been successfully tested on the inertia dynamometer at TU Chemnitz. The brake disc passed testing in accordance with SAE J2522 (AK Master). The tests demonstrated excellent tribological performance. The system – consisting of a stainless-steel brake disc and inorganic friction material – showed approximately 85 percent less wear compared to solutions currently available on the market.
Source: iwd
