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The arrangement of the constituents of organic-composite friction materials is a key factor of their microstructure and thermal and mechanical properties which can influence braking performance. Among these constituents, fibres can present complex morphologies and different arrangements depending on their type and the process of manufacturing. Besides, synergistic effects acting between these constituents and the resulting properties are still not well investigated. This work relates to rock- wool used for brake friction materials, and for which the process can lead to various arrangements. The focus is on synergies between these fibre arrangements and the other material constituents, in ways that reveals the link between the resulting microstructural characteristics and properties of organic composite materials. To achieve these objectives, two simplified formulations are elaborated with two distinct arrangements of rock wool fibres. The friction materials are investigated in terms of microstructure, thermo-physical and mechanical properties. It is found that fibre arrangements affect carbonaceous particle distribution, porosity, and fibre-matrix adhesion. On one side, homogeneous distribution and regular size of fibre bundles results in a better connectedness of conductive particles and thus enhances thermal conductivity. On the other side, a regular fibre bundles repartition lead to a more homogeneous distribution of strain localizations and a softer mechanical response.

Industrial brake lining materials are composite with complex formulations consisting of multiple constituents. Resulting from the fabrication process, the morphology and distribution of the constituents have significant influences on the future properties and braking performance. In this study, an in-depth analysis ranging from the microscale to the macroscale was performed to assess the relationships between the microstructure, the mechanical properties and the braking performance of an industrial brake lining material formulated for heavy vehicles. It was observed that the manufacturing process had different effects on the morphology and size of constituents and on their distribution in the phenolic binder. The morphologies of large organic particles such as rubber and graphite were affected by the mixing procedure, contrary to those of fibres and mineral particles. A transverse anisotropy consistent with fibre orientation due to cold preforming and hot moulding was observed. The microstructure displayed a strong local heterogeneity right up to the mesoscopic scale at which friction and wear mechanisms typically occur. The mechanical properties were analysed with regard to the heterogeneity of the microstructure to determine the scale at which these properties could be considered to be associated with a homogenised behaviour. The rubbing surface after braking showed that load-bearing localisation depends on the nature, morphology and orientation of constituents but that this heterogeneity can be of interest with regard to the braking ability.

It is well known that truck brakes dissipate several megajoules of energy every few seconds, which leads to high thermal stresses in the rubbing parts. Therefore, premature failure by cracking of truck brake discs is a matter of major concern. Improving the design and material of brake discs may enhance braking performance. This study focuses on the latter aspect and was carried out with the aim of developing new material solutions for increasing disc lifespan. To do so, braking experiments were conducted on a specially designed braking tribometer. The brake pads that were used were made from a commercial brake lining material. Two advanced cast irons with different graphite morphology were studied in comparison with the lamellar grey cast iron commonly used for brake disc. To verify the friction and thermal behaviour of the two cast irons, braking tests were carried out as a series of stop-brakings with increasing dissipated power and energy and as a series of slowdowns to achieve heat accumulation effects. Thermal phenomena were studied through bulk temperature measurements and infrared monitoring of the disc surface. Friction behaviour, braking performance and variations in thermal loading were analysed in relation to the level of energy dissipation. The two advanced cast irons and lamellar cast iron had equivalent braking performance and stored similar amounts of heat, according to their thermophysical properties. Observations of the rubbing surfaces indicated damage mechanisms affected by the graphite morphology. Less plastic deformation on the surface was observed with an interdendritic graphite.