Synergistic Effect of Silica Nano-Particles and Carbon Fiber on the Antiskid and Wear Resistance Properties of Brake Composites

This study aims to investigate the antiskid and wear resistance properties of friction brake materials in slippery environments. For this purpose, the hydrophobic silica nano-particles (SiO₂NPs), with superior friction-enhancing properties, and high-strength carbon fiber (CF) were chosen as additives to modify thermoplastic polyurethane (TPU) and novel TPU matrix composites composed of SiO₂NPs, CF and TPU were fabricated. The tribological measurements were conducted by using a ball-on-disk tribometer with a rotary sliding contact mode in aqueous environments to reveal the antiskid performance and mechanism of modified TPU composites. The impacts of varying mass fraction of SiO₂NPs and CF on the antiskid performance and wear resistance of modified TPU composites were investigated under different load (10, 30, and 50 N) and sliding speed (60, 120, and 180 r/min) conditions. The typical mechanical properties, water contact angle, friction coefficient, wear rate, wear morphology of TPU composites were analyzed. The results showed that the meshing effect between the SiO₂NPs and the micro-asperities of counterpart surfaces changed the wear behavior of TPU composites from adhesive wear to abrasive wear, resulting in an increased frictions coefficient and improved antiskid properties. However, the wear resistance of modified TPU composites was impaired. The TPU composite modified with 12% SiO₂NPs exhibited the largest frictions coefficient, fluctuating around 0.6, although the mechanical properties were weakened. The introduction of CF significantly enhanced the mechanical properties and thermal stability of modified TPU composites. The tensile strength of the TPU composite modified with 3% SiO₂NPs and 9% CF reached 37.09 MPa, which was 246.90% larger than that of the TPU composite modified with 12% SiO₂NPs. This enhancement facilitated the inhibition of failure behavior during friction and reduced the wear rate. Additionally, the CF peeled off from the matrix was prone to fill the defects on the wear surface and formed a tribo-film, protecting the matrix material from severe wear and eventually enhancing the wear resistance. Furthermore, the water contact angle of modified TPU composites increased from 67.55° to 112.10° due to the hydrophobicity of SiO₂NPs and CF, changing the TPU from hydrophilic to hydrophobic. As a result, the absorption of water molecules at the friction interface weakened, reducing its lubricating effect and further enhancing the antiskid property of modified TPU composites. The synergistic effect of the SiO₂NPs and CF made the TPU composite modified with 3% SiO₂NPs and 9% CF exhibit both high frictions coefficient and low wear rate. Its frictions coefficient curve stabilized at about 0.5, and the average frictions coefficient increased by 204.26% compared to that of pure TPU. The wear rate was reduced by 96.28% compared to that of composite modified with 12% SiO₂NPs, demonstrating excellent antiskid and anti-wear properties. The findings obtained in this study provided a reference for designing and fabricating polymer-based friction brake composites with prominent anti-slip and high wear resistance in slippery environments.