The effect of each ingredient on the tribological behaviour of a brake friction material is not fully understood because of the difficulty involved in the study of interactions between the ingredients over time, under a wide range of pressures, temperatures and stress. Graphite is one of the most used ingredients in friction materials, due to its lubricant properties and thermal stability. Different types of graphite are available with a wide range of characteristics, leading to variations on final product performance depending on the graphite used. This work shows the influence of the graphite characteristics on tribological behaviour of brake friction materials. The following characteristics were elected as the most important regarding graphite: particle size, interlayer distance, amount of hexagonal phase and amount of impurities. A scale dynamometer was used to evaluate disc and pad wear, coefficient of friction levels and its stability. Correlations were noticed between crystallinity parameters and fade performance, amount of impurities and wear measurements, among others. The stronger correlations were detected with regards to particle size, showing higher coefficient of friction along with lower disc and pad wear. This paper offers new criteria to help choosing graphite among the wide variety available on the market, aiming cost reduction, wear control and performance stability. The strongest correlations observed were: coefficient of friction proportional to particle size, wear rates inversely proportional to particle size, and fade resistance proportional to crystallinity degree.
Felipe Pandolfo - Fras-le Friction Materials, Diego Jara - Universidad Mayor de San Andres