Initial Corrosion and Wear Behavior of Brake Rotors Treated with Plasma Electrolytic Aluminating or Ferritic Nitrocarburizing Process

To combat corrosion and wear issues of automotive brake discs, many manufacturers have introduced various surface treatment technologies, such as thermal spraying, laser cladding, and ferritic nitrocarburizing (FNC). Besides those surface treatment technologies, a plasma electrolytic aluminating (PEA) process has also shown to be effective in producing alumina-based ceramic coatings on cast iron substrates, providing an enhanced corrosion resistance. In this study, the PEA-coated brake rotor and FNC-treated brake rotor were comparatively tested in various corrosion conditions, including an electrochemical corrosion test and simulative corrosion experiment, before and after a road driving test. A scanning electron microscope (SEM) and an energy-dispersive X-ray (EDX) were used to observe and analyze morphology and chemical compositions of the surfaces and cross-sections of the tested rotors. The results showed that the new PEA-coated brake rotor demonstrated the best corrosion resistance in the electrochemical corrosion test among all given tested cases. After the vehicle test, the PEA-coated rotor surface had an obvious materials transfer layer which can protect the rotor from abrasive wear. The transfer layer materials sourcing from the low-met brake pads however contained metallic elements, leading to appearance of a lower corrosion resistance during the electrochemical corrosion test. In duration of the vehicle road test (1000 braking events at 0.3-0.4 g), the FNC brake rotor showed some loss of its white layer but maintained its nitrogen diffusion layer, which still showed protection to the brake rotor base material from corrosion. It was also found that some surface areas where the graphite flakes were located on FNC-treated cast iron brake disc surface were exposed to the ambient environment, which may be a reason why the localized corrosion appeared on the FNC-treated rotor in the late stage of the vehicle road test.