Development of metakaolin-based geopolymer compositions for friction applications

In the past decade, extensive research has focused on substituting phenolic resin with alternative matrix as a binder for car brake pads. Phenolic resins and similar organic resins have several disadvantages, such as toxicity; decomposition and losing their friction coefficient at elevated temperatures (300°C – 400°C), which results in decreased braking performance. Among the various solutions, geopolymers appear to be a promising option. Indeed, metakaolin-based geopolymers, with an inorganic structure similar to natural zeolite, offer promising properties including excellent chemical stability and wear resistance enabling them to withstand harsh braking events where temperatures may reach 600°C – 700°C. Additionally, their green and low-cost production is a significant advantage for consumable parts. To this end, since the inception of the research project by previous researchers, various compositions have been designed and successfully tested. Building on the previous works, this master’s thesis aims to find the optimized molar ratio of the metakaolin-based geopolymer compositions in order to increase wear resistance while maintaining chemical stability. During the Design of Experiments (DoE) phase, a thorough investigation was conducted on the molar ratios of SiO2/Al2O3, H2O/Al2O3, K2O/Al2O3, keeping the K2O/Al2O3 ratio constant at 1. Samples were produced using cold sintering at a pressure of 30 MPa and a temperature of 150 °C. All samples underwent boiling tests, B3B flexural tests, Vickers microhardness tests, dynamic Young's modulus measurements via impulse excitation of vibration, and geometrical density tests. In the second phase of the research, the pure geopolymer best-performing composition identified in the previous step was reproduced at a higher pressure of 70 MPa and a lower temperature of 135 °C to simulate real process parameters during the production of the brake pads. For evaluation, a reference composition currently in production was synthesized under the same conditions. Both compositions were analyzed using the same tests as before. The final results demonstrated that the best-performing composition, which contained a higher amount of potassium silicate, exhibited greater Young’s modulus and hardness along with almost equal flexural strength, and lower density compared to the reference composition. Consequently, material loss would be minimized during loading at high temperatures and potentially leading to a reduction in the weight of the braking system.