From Temperature-Controlled Wear Testing to Real-Cycle Emissions: A Multi-Analytical Study on Automotive Friction Materials

This PhD research, carried out in collaboration with Raicam Industrie S.r.l., aimed to investigate the tribological and environmental behaviour of automotive friction materials, focusing on the influence of temperature on wear mechanisms, tribolayer evolution, and particulate emissions. The study was motivated by the recent introduction of the UNECE Global Technical Regulation No. 24 (GTR No. 24), which defines the WLTP-Brake test procedure for measuring brake particle emissions, soon to become mandatory for vehicle approval in Europe. In this regulatory and industrial context, the work sought to develop an experimental and analytical framework capable of linking laboratory-scale investigations to full-scale emission testing. A temperature-dependent wear protocol, the Block Wear Test in Temperature (BWTT), was designed to isolate the effect of thermal load on friction stability, pad and disc wear, and surface morphology. The test was applied to two commercial reference formulations — a Non-Asbestos Organic (NAO) and a Low- Steel (LS_1) material — across controlled temperature steps (100 °C, 300 °C, and 500 °C). Complementary multi-technique analyses, including Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM/EDS and FESEM), and Optical 3D Profilometry, were employed to characterise the degradation and transformation of the tribolayer. These methods enabled the correlation between chemical decomposition, microstructural evolution and surface roughness parameters (Ra, Rsk, Rku). The same methodologies were subsequently extended to the WLTP-Brake dynamometer test performed by the company's Dyno Testing Department, allowing direct comparison between temperature-controlled and real-cycle braking conditions. The results demonstrated strong qualitative consistency between the two approaches, confirming that the 100 °C BWTT block represents the mild-wear regime typical of WLTP braking. The combination of pad reformulation and tribolayer analysis also proved effective for interpreting emission factors (PM10, PM2.5 and PN) and for linking frictional behaviour with particle generation mechanisms. Finally, the methodology was applied to a set of prototype low-steel materials (LS_3, LS_4, LS_5) with progressively reduced steel-fibre content and to TiC-coated discs, in order to evaluate their emission performance and industrial scalability. The prototypes showed that decreasing metallic content reduces total particulate emissions but increases the fine organic fraction, whereas coated discs reduce disc wear by up to 50 %, stabilising the tribolayer and mitigating coarse particle release. Overall, this work establishes a comprehensive BWTT–WLTP analytical framework capable of bridging laboratory and regulatory testing. It highlights temperature as the key parameter controlling the degradation of friction materials and the formation of airborne particles, providing a scientific and methodological basis for the development of next-generation, low-emission braking systems aligned with future Euro 7 requirements