Particulate wear debris of coated lightweight disc brakes

Stringent regulations on exhaust emissions (e.g. Regulation (EC) No 443/2009) have put automotive companies under tremendous pressure to improve vehicle fuel economy and reduce the carbon dioxide emissions. The use of lightweight disc brake rotors is one of the solutions to meet this legislative requirement. Due to significant reductions in exhaust emissions, the proportion of non-exhaust emissions has increased in recent years. Airborne brake wear emissions are the major contributor to the non-exhaust emissions and they can adversely affect human health and the environment (Grigoratos and Martini, 2015). In this study, the performance of conventional grey cast iron (GCI) and novel Plasma Electrolytic Oxidation Aluminium (PEO-Al) rotors with regard to particle size and mass distribution, morphology and chemical composition was investigated and characterised for comparison purposes.
Experiments were conducted using a newly developed test rig which was based upon the existing Leeds full-scale disc brake dynamometer. The brake head of the dynamometer was enclosed and a ducting system was used to supply clean air from the laboratory and exhaust it to the outside environment. A sampling probe in the outlet duct was connected to a Dekati ELPI®+ Electrical Low Pressure Impactor to collect and measure airborne particles in different size ranges from 10 μm down to the nano-particle range. Computational Fluid Dynamics (CFD) simulations were performed to predict airflow behaviour within the brake enclosure and in the vicinity of the sampling point. The CFD results suggested good evacuation of brake wear particles from the enclosure and a near uniform air flow regime in the outlet duct near the sampling probe.
Both GCI and PEO-Al brake rotors were tested under steady-state drag brake test conditions. Braking test parameters of sliding speed, hydraulic pressure, rotor surface temperature and coefficient of friction were monitored during the tests for each rotor material. Hydraulic pressures of 5, 10 and 15 bar at 135 rpm rotational speed were selected as three typical braking test conditions to be studied. Measurement of brake wear emissions was carried out using the Dekati ELPI®+ along with an isokinetically designed sampling probe.
Generally, the coefficient of friction measured during steady-state conditions was greater for the GCI rotor (0.43-0.49) as compared to the PEO-Al rotor (0.25-0.28). However, the ELPI®+ results showed that the PEO-Al rotor tended to produce lower wear debris particle emissions than the conventional GCI rotor in the critical size range of 1 μm – 10 μm especially under low applied pressures. Based on SEM-EDX analysis, the percentage of Fe in the PEO-Al debris was about threefold lower than that from the GCI rotor. This is an important result given the reported link between sub-micron airborne particles containing Fe and its oxides with neurological diseases such as Alzheimer's.