Anne-Lise Cristol

Braking performance and influence of microstructure of advanced cast irons for heavy goods vehicle brake discs

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.

Influence of Hot Molding Parameters on Tribological and Wear Properties of a Friction Material

The manufacturing process of organic friction material used for braking applications is of importance with regard to their properties and performance. This article deals with a friction material manufactured according to two different hot molding conditions. An original analysis of polymerization of the phenolic resin matrix led to the choice of temperature and duration of the hot molding process. Two materials were developed, changing the hot molding process according to these two dependent parameters, and their friction and wear behaviors were investigated for various thermal severities of sliding conditions. The thermal conductivity was also analyzed and found to be higher when the hot molding temperature was low and the hot molding duration was long. Wear test results showed that friction was not significantly affected by the process change. On the contrary, wear appeared to be sensitive to hot molding temperature and duration. Hot molding at elevated temperature for a short duration led to higher wear resistance because the rubbing conditions were not severe. On the contrary, hot molding at low temperature for a long duration was in favor of the wear resistance for higher severities of the rubbing conditions.

Relationships between the heterogeneous microstructure, the mechanical properties and the braking behaviour of an organic brake lining material

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.

A New Method of Mixing Quality Assessment for Friction Material Constituents Toward Better Mechanical Properties

Friction materials used for brake lining are highly heterogeneous composites for which the link between formulation, the resulting material properties, and performances is not well understood. Their heterogeneity is induced by the variety of ingredients (morphology, size, properties etc.) and the manufacturing process which includes a succession of steps: mixing, preforming, hot molding, and post-curing. Among these steps, mixing have a great impact on the material microstructure in terms of ingredient distribution and, therefore, on its mechanical properties. However, mastering the mixing process is very difficult, since it is still based on the empirical experience of manufacturers. In this study, a new method and methodology of mixing state evaluation of friction material constituents was developed. First, it consists on studying constituents’ physical properties permitting to facilitate the investigation of mixing state evolution. This investigation includes two steps: binary and multi-constituent mixture study. This work suggests a non-time-consuming image analysis method using two statistical coefficients to evaluate the mixing state, which are Kurtosis and Coefficient of Variation (C.V). The latter enable to describe the mixing quality and state evolution at the surface of the mixing volume as well as to evaluate the mixing time.