Decoupling the role of pad materials in brake wear particulate emissions using the UN GTR-24 test method toward non-exhaust PM management.

Brake wear particles (BWPs) are emerging as a serious and significant source of non-exhaust emissions. This study investigates the effects of friction material type on BWP emission characteristics, employing a testing platform compliant with the UN GTR-24 standard. The results show that the pads of non-asbestos organic (NAO) material operated at higher average temperatures than low-metallic (LM) pads. Although both materials generally showed similar total particles number of 10 nm (TPN10) magnitudes (∼104) under most conditions, a dominant peak (∼106) occurred for LM material under high-speed sudden deceleration, which was absent for NAO material. Overall, the primary factors influencing TPN10 levels are, in order: friction material type, high-speed sudden deceleration, and temperature. An anomalous observation was that the solid particles number of 10 nm (SPN10) emission factor for NAO exceeded its TPN10, which be attributed to thermal fragmentation of weakly bonded aggregates in the volatile particle remover. Particulate mass (PM) emission factors from LM material were substantially higher, with PM2.5 and PM₁₀ averaging ∼3.0 mg/brake/km and ∼6.0 mg/brake/km respectively-about an order of magnitude greater than NAO-consistent with its higher metallic content and friction coefficient. Elemental analysis showed that BWPs from both LM and NAO materials are dominated by iron oxides (Fe2O3). FTIR spectra confirmed Fe2O3 presence and detected surface-adsorbed water, organic compounds, and carbon dioxide. Microstructurally, LM-derived BWPs form compact aggregates, while NAO-derived particles exhibit chain-like structures like soot emitted from internal combustion engines.