This study evaluated the friction, wear, and airborne brake wear particle (BWP) emissions of aluminum-based metal matrix composite brake discs fabricated from recycled aluminum alloy reinforced with silicon carbide particles (Al–SiCp MMC). The study further conducted a comparative analysis of the friction, wear, and BWP emissions of Al–SiCp MMCs against those of a commercial gray cast iron (GCI) brake disc, which served as a reference. The results show that the steady-state coefficient of friction for all Al–SiCp MMC brake discs remained consistently between 0.4 and 0.45, within the typical range for brake materials. A clear transfer layer was observed on the surfaces of Al–SiCp MMC discs after testing, resulting in apparently milder wear due to material transfer and reduced BWP emissions. Al–SiCp MMC brake discs resulted in higher wear rates for the mating pins than the GCI discs, with wear rates increasing as the fraction of secondary aluminum in the matrix increased. Within the measurement range of this study, both GCI and Al–SiCp MMC brake discs exhibited monomodal number-weighted particle size distributions in the steady state, with the mode size of approximately 0.5 µm. Future research should employ advanced particle samplers capable of detecting nanosized particles and explore more severe testing conditions, including higher contact pressures, speeds, and temperatures.

Automotive brake rotors are commonly made from gray cast iron (GCI). During usage, brake rotors are gradually worn off and periodically replaced. Currently, replaced brake rotors are mostly remelted to produce brand-new cast iron products, resulting in a relatively high energy consumption and carbon footprint into the environment. In addition, automotive brakes emit airborne particles. Some of the emitted particles are categorized as ultrafine, which are sized below 100 nm, leading to a series of health and environmental impacts. In this study, two surface treatment techniques are applied, i.e., high-velocity oxygen fuel (HVOF) and laser cladding (LC), to overlay wear-resistant coatings on conventional GCI brake rotors in order to refurbish the replaced GCI brake rotor and to avoid the remelting procedure. The two coating materials are evaluated in terms of their coefficient of friction (CoF), wear, and ultrafine particle emissions, by comparing them with a typical GCI brake rotor. The results show that the CoF of the HVOF disc is higher than those of the GCI and LC discs. Meanwhile, HVOF disc has the lowest wear rate but results in the highest wear rate on the mating brake pad material. The LC disc yields a similar wear rate as the GCI disc. The ultrafine particles from the GCI and LC discs appeared primarily in round, chunky, and flake shapes. The HVOF disc emits unique needle-shaped particles. In the ultrafine particle range, the GCI and HVOF discs generate particles that are primarily below 100 nm in the running-in period and 200 nm in the steady state. Meanwhile, the LC disc emitted particles that are primarily ~200 nm in the entire test run.