A significant amount of vehicle weight resides in the conventional cast iron brake discs. This “unsprung mass” further compounds the effects of rotational inertia and impacts fuel consumption and subsequent CO2 emissions. In addition, non-exhaust air pollution from brake dust (disc and pad wear comprising nano-particles, PM2.5 and PM10) is of high concern due to public health issues, premature deaths and reduction in life-expectancy. Thus, there is a strong impetus to develop alternative approaches that reduce both CO2 and PM emissions in future vehicles.
Light materials such as aluminium alloys present attractive options as potential solutions to help reduce vehicle weight, provide for better fuel economy and improve overall sustainability credentials. The main challenge for using an aluminium alloy is the maximum operating temperature (MoT) limitation as well as surface properties deemed insufficient leading to excessive corrosion and wear. Keronite plasma electrolytic oxidation (PEO) is an environmentally safe coating process which offers aluminium alloys multifunctional characteristics such as high hardness, strong adhesion, low stiffness and a continuous barrier capable of offering protection against corrosion, wear and extreme heat likely to be experienced by brake discs.
Brake discs for the rear axle of a high performance sports car have been manufactured in three different materials; AA6xxx, AA2xxx and 7xxx series aluminium alloys, with and without vents. These were subsequently coated with a Keronite PEO ceramic layer. Full-scale dynamometer tests have been conducted against a standard Low Met brake pad using a modified AK Master test at increasing levels of IBT until a physical limit is reached. Microstructural characterisation of coating and brake pad wear and tribolayer have been studied extensively using SEM and EDX. In addition, post-dyno test discs have been exposed to salt fog testing, and their corrosion behaviour and relative interactions with the tribolayer have been further studied using electrochemical corrosion test methods. The overall relative performance and the degradation mechanisms have been discussed and the potential to use PEO coated light alloy discs have been proposed.