Synergistic effects of fibre arrangements on the microstructure and properties of organic composite materials

Abstract

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.

Organic Brake Friction Composite Materials: Impact of Mixing Duration on Microstructure, Properties, Tribological Behavior and Wear Resistance

“…SEM micrography of M2 (Figure 2 (b) provides evidence for the poor interface adhesion between rockwool fibers and the matrix. Morphology of fiber bundles for the two materials seem to be similar since they result from the re-entanglement of fibers during the mixing step [20].…”

“…Resin, calcium carbonate and alumina present very fine particles (mean diameter < 6 µm) and constitute the matrix of these composite materials. This material simplification was detailed in previous work [20]. The elaboration process of brake friction materials includes several steps (mixing, cold preforming, hot molding, post curing).…”

“…The size distribution, arrangement and spatial repartition of constituents of the two materials M1 and M2 were analyzed by 3D X-ray tomography using a laboratory equipment with a procedure already described in detail in Makni et al [20]. Cylindrical specimen with a length of 16 mm and a radius of 6 mm were sampled from each material in the normal direction (Z direction) and scanned using an Xray microtomographic apparatus: Ultratom, RxSolution©.…”

“…Where ρm presents the bulk specific density (g/cm 3 ), mi corresponds to the weight (%) of each ingredient of the formulation and ρi presents the absolute density of each ingredient of the formulation (g/cm 3 ). Thermal Expansion (α) , thermal conductivity (λ) and specific heat capacity (𝐶𝑝) measurements were performed according to defined parameters and using specific equipments detailed in previous study [20]. The thermal effusivity (ε) was calculated using values of the measured conductivity, density and specific heat capacity as follows :…”

“…Each stress level was applied five times. The speed of plateaus displacement is set at 0.01 mm/s [20]. Digital Image Correlation (DIC) method was employed to identify strain/ stress localizations [23].…”