A Design of Experiments-Based Investigation of Solid Lubricants in Heavy Vehicle Brake Pads

Brake pad formulations for heavy-duty vehicles must balance wear resistance, fade resistance, and friction stability under extreme braking conditions. This study systematically investigates the impact of solid lubricant compositions, including graphite, antimony trisulfide (Sb₂S₃), molybdenum disulfide (MoS₂), zinc sulfide (ZnS), and calcium fluoride (CaF₂), on the tribological performance of copper-free brake pads. A Design of Experiments (DOE) approach was employed to develop 29 unique formulations and evaluate them under the ECE R90 test procedure using a Krauss friction tester. Statistical analyses were used to determine the effects of lubricant combinations on performance metrics. The findings revealed that increasing CaF₂ content led to a significant rise in the coefficient of friction (up to 0.46), whereas Sb₂S₃ enhanced fade resistance by reducing loss of friction coefficient to as low as 0.03 but increased wear. MoS₂ was associated with superior thermal stability, while graphite primarily contributed to reduced wear loss (minimum 0.009 g). The formulation containing 10 wt% graphite, 2.5 wt% MoS₂, and 2.5 wt% CaF₂ demonstrated the most optimal balance of performance. These results underscore the importance of synergistic solid lubricant combinations in improving the durability and effectiveness of environmentally friendly brake pad formulations for heavy-duty applications.