Wear-emission correlation in brake materials

This study is based on a test campaign of 10 brake couples on a reduced-scale dynamometer, with the aim of exploring the relationship between wear and fine airborne particle emissions from brake materials. Different Non-Asbestos Organic (NAO) and low-metallic friction materials sliding against grey cast iron discs, also against High-Velocity Oxygen Fluid (HVOF) coating, were investigated. The tribological tests reproduce mild sliding conditions through brake cycles designed using a factorial approach, with contact pressure levels of 1–3.5 MPa and sliding velocities ranging from 4 to 16 m/s.The wear rate is quantified using the specific wear coefficient, Ka, and the emissions were quantified with the concept of Emission Factors (EF). NAO materials exhibit an EF of approximately 1.5–2.2 mg/km/corner when coupled with cast iron discs. Conversely, low-metallic materials show EF in this range only when sliding against the hard-coated disc. When coupled with cast iron discs, the EF of low-metallic materials ranges from 5.7 to 11.1 mg/km/corner. A strong correlation between the system Ka and EF values is found and this is leveraged to derive considerations on the permissible Ka to adhere to emission limitations in the context of the EU7 regulation.