Development of novel eco-friendly friction materials for disc brake systems

Due to new environmental regulations, the demand for brake systems producing low polluting emissions and keeping high performance, is increasing. Therefore, a reduction in the toxic substances contained in brake friction materials is required that still meets all applicable safety standards and retaining properties such as low wear, high temperatures resistance, friction coefficient stability and consistency. Among the various ingredients used in formulations, copper has recently become the subject of different regulations on brake pad materials and will be reduced, or even prohibited, in the coming years. Recent studies have found that brake pads are major contributors to the deposit of copper in rivers and lakes, and this has a toxic impact on the environment. This metal is currently added in brake material formulations because of its good physical properties and its contribution to the formation of a uniform and stable friction layer. Its characteristics make it a constituent that is not easy to replace and an adequate substitute covering all the roles of copper has not been found as yet. The present research aims at developing, producing and testing new formulations for brake pads without copper. A relatively wide range of friction materials was investigated, in order to understand the role that selected constituents have in friction and wear behaviour. In the first place, an investigation on the role of copper in friction materials was performed. The study was followed by the selection of a suitable replacement for this constituent in the formulation. To do so, different friction components were studied. Additionally, other aspects relating to friction materials, such as the deterioration of the binder, were subject of investigation. The novel formulations, produced starting from commercially available compositions, were ranked in terms of wear and friction behaviour by means of a pin-on-disc tribometer. Cylindrical specimens were produced directly from powders, so that constituents could be easily modified based on the test outcome. This is a very effective method to study the role of individual constituents in the mixture, considering the relatively small amount of each specific composition to be prepared and the ease of processing it. In order to identify the principal wear mechanisms and their dependence on material properties and test conditions, the worn materials were analysed via scanning electron microscopy (SEM) techniques and Energy Dispersive X-Ray Spectroscopy (EDXS). Part of the acquired knowledge from the first part of the work was used in the initial stages of the ECOPADS project (the project started during the doctoral period) to develop and manufacture real brake pads that were tested on brake dynamometers and evaluated in terms of both performance and emissions.