Influence of Ferritic Nitrocarburizing with Smart-ONC® on the Modal and Damping Behavior of Gray Cast Iron Brake Rotors

Ferritic nitrocarburizing (FNC) with in-process post-oxidation has been developed as a production-capable surface engineering solution for gray cast iron (GCI) brake rotors to meet the Euro 7 non-exhaust particulate emission limits. While prior investigations have demonstrated significant PM₁₀ reduction, improved corrosion resistance, and stable braking performance, the influence of FNC on noise, vibration, and harshness (NVH) performance requires systematic evaluation. This study quantified the relative contributions of the alloy composition, rotor geometry, and FNC treatment to the modal frequency and damping behavior. Seven ventilated disc types from multiple foundries were characterized to assess the composition-driven variability. In addition, 120 production discs (ventilated and solid) were measured before and after FNC processing to isolate the treatment effects. Modal properties were obtained using impulse-hammer testing under free–free boundary conditions in accordance with VDA 301, and damping was evaluated using the half-power bandwidth method (Q-factor). The results show that the natural frequency is governed primarily by geometric parameters, scaling with the friction-ring thickness and disc diameter. In contrast, the damping behavior is dominated by the alloy composition and graphite morphology. Variations in silicon, chromium, and carbon equivalent produced a 3–4× difference in the Q-factor across foundries. FNC treatment had a negligible effect on the natural frequency (<1%) but produced a measurable increase in the Q-factor, typically 7–10% for solid discs and 22–32% for ventilated discs. The findings establish a clear hierarchy of influence: composition controls the damping, geometry controls the frequency, and the FNC introduces a secondary shift. Within production-relevant composition windows, FNC + Smart-ONC® does not represent a limiting factor for the NVH performance of Euro 7–compliant brake systems.