Ing. Franco Arosio, Oerlikon, GERMANY
Dr.-Ing. Ingo Lange, Oerlikon, SWITZERLAND
The rise of Electrical Vehicles (EVs) is unstoppable and EVs will become a key part of the mainstream automotive market. According to recent post-COVID-19 scenarios based on IHS data, EVs will surge up to 14% of global passenger car sales in 2027 and go up to 57% in 2040. The electrification of future mobility concepts is going along with new requirements also for the brake system. EVs with regenerative braking applications utilize the traditional friction brakes in fewer circumstances due to recuperation: therefore the risk of superficial corrosion increases. In case of an emergency brake situation the basic requirement is that the braking surface will be free of corrosion to have maximum brake power. Thus, the corrosion-free condition on the braking surface is a safety requirement at any time.
The state of the art solution consists of paintings or “coatings”, such as ultraviolet (UV)-hardening paint, Zn or Zn/Al paints, which can perform well in new conditions (e.g. up to 120 hours in standard UNI ISO 9227 salt fog chamber). But these solutions will be easily abraded within approximately 20 standard-condition braking applications. The corrosion-free condition during the lifetime of the disc is not achieved yet in the current state of the art; rust or corrosion can seriously downgrade the braking performances.
This paper is describing an innovative 2-step process to improve the corrosion and wear resistance of standard cast iron brake discs. In the first step, the amount of undesired graphite lamellae will be reduced from the surface with customized parameters, according to the individual types of grey cast iron material of the substrate. This pre-process is followed by a thermochemical diffusion process including controlled oxidation of the substrate resulting in high corrosion protection of the rotors.
The authors will produce proof of corrosion resistance up to 300 hours in salt conditions according to UNI ISO 9277. In addition, bench tests and vehicle endurance tests have been performed in cooperation with Tier 1 and OEMs and have shown increased wear resistance even with non-electric cars and with standard ECE brake pads.
The novel surface solution could be also applied to non-functional areas of the brake disc like cooling channels, bell and swan neck in order to substitute the current paintings.
In summary, the new 2-step heat treatment process is a price competitive solution for corrosion protection of functional and non-functional areas of iron casted brake discs over the entire lifetime, especially on EVs with strong recuperation. But this solution also works for hybrid and conventional cars in preferably on the rear axis with low-abrasive brake pads (e.g. NAO pads). Finally, even when the vehicle fleet goes all-electric, dust emission from brakes and tyres will still pollute the environment. Addressing this topic, the authors will provide an outlook of the ongoing activities to reduce brake dust emissions with innovative surface solutions.