Determination of Diffusion Capability of Oxygen through Brake Pads from the Surface towards the Interior

The oxidation of raw materials, such as phenolic resin, in the pad during the braking depends on the temperature but also on the oxygen diffusion capability through the pad. Determination of oxygen diffusion is a key point in knowing how deep from the surface tribochemistry can take place. The diffusion of oxygen through the pad is a drawback because it induces the matrix decomposition that contributes to intra-stop CoF instability and consequently worsens NVH. This study is focused on determining the oxygen diffusion through brake pads using oxidized iron sulphide particles as indicator parameter. Iron sulphide has a rough microstructure when it becomes oxide that can be recognized easily, making it a good marker. The oxygen diffusion was determined through the pad characterization, after SAE J2522 and J2707 tests, with SEM and EPMA/WDS techniques. The results show that tribochemistry also takes place beneath the PAD surface during braking due to the oxygen presence and the high temperatures reached. Depending on the testing schedule the temperature profiles on the pad are different and so is the tribochemistry. In pads tested according to SAE J2707, the oxidized particles are found in deeper sections than in pads tested according to SAE J2522. The results of this research will contribute to understanding where the decomposition of the resin starts, and therefore, how reactive compounds such as iron sulphide that also react with oxygen helps to keep the integrity of the matrix by decreasing the decomposition and contributing to the stability of intra-stop CoF, and so NVH properties.