Ferrite Nitrocarburizing of Grey Cast Iron Brake Discs: Thermodynamic Process Control, Layer Formation Kinetics and Self-Healing Mechanisms as Well as Service Properties

Abstract The Euro 7 regulation establishes binding limit values for particulate emissions from braking systems for the first time (PM₁₀ ≤ 7 mg/km by 2035; ≤ 3 mg/km from 2035). Recuperative braking in electric vehicles significantly reduces the frequency of use of the friction brake, which leads to increased corrosion of gray cast iron brake discs and increased particulate emissions when braking again. The present work describes the development of ferrite nitrocarburizing with optimized post-oxidation as a thermochemical surface treatment for the simultaneous improvement of corrosion and wear resistance. The thermodynamically based process control via the nitriding and carbonization potentials enables the targeted adjustment of compound layer thickness, phase composition and porosity. The modified post-oxidation produces a magnetite layer with an embedded inorganic polymer that extends into the pores of the compound layer, whose self-healing mechanism is based on the dissolution, diffusion and recombination of metal ions to form poorly soluble compounds.