The Role of Waste Rubber and Ceramic Fibers in Brake Friction Materials: Tribofilm Stability, Wear, and Creep-Groan Propensity

The current research aims to investigate the effects of waste rubber particles on the tribological characteristics and noise tendency of brake friction materials. Five formulations were prepared by partially replacing ceramic fibers with waste rubber particles up to 10 wt.%. Density, hardness, porosity, and compressibility were examined for each material, and their tribological characteristics were assessed using a braking tribometer with an SAE J2522 test and a creep groan stick–slip test. Additionally, surface analysis was conducted to observe contact plateaus on the friction material and tribofilm on the disk. The results revealed that increasing rubber content led to reduced hardness and density, while porosity and compressibility increased. Although the hardness was reduced with increasing rubber content, wear resistance was improved (up to 35%) due to the formation of larger plateaus and homogenous tribofilm that have a protective action. Friction coefficients were found to decrease up to 10% with increasing rubber content. The trend in creep groan propensity was different. The highest rubber content formulation (R10) showed the lowest wear and the most stable tribofilm but also showed the highest level of stick–slip events (SSP = 37 Nm/s), indicating higher noise propensity because of stronger adhesive junctions. On the other hand, the hybrid formula containing 5 wt.% rubber showed the lowest level of stick–slip activity (SSP = 0.91 Nm/s) while still showing stable friction behavior. This demonstrates that the balance between damping capacity (rubber) and structural stiffness (ceramic fibers) governs the tradeoff between tribofilm stability and adhesive stick–slip behavior.