||Truck brakes have to dissipate several megajoules of energy in few seconds. The high thermal stresses obtained in this way in brake discs lead to their premature failure by thermal cracking. Improvement of disc brake lifespan can be achieved through design modification or material evolving. The present study focuses on the latter aspect. Literature shows introduction of plentiful additional elements, for example molybdenum, vanadium, niobium, titanium or nickel, to improve thermal fatigue strength or wear resistance . However, such alloying elements are expensive and do not always bring about marked improvements. Thus, the chosen way to evolve the commonly used grey cast iron is to modify the graphite morphology. On the one hand, a nitrogen-enriched lamellar cast iron is produced. It has been shown that tensile strength is increased because of modified form of graphite flakes . On the other hand, a cast iron with a microstructural gradient in the depth is manufactured: on the surface, graphite lamellae are very short to minimize the notch effect and in the depth their length increases to provide good thermal shock resistance. The behaviour of these advanced cast irons has to be validated under braking conditions.
The first step of the study has been dedicated to the identification of critical thermo-mechanical loadings . The commercial grey cast iron disc and semi-metallic pads have been submitted to various braking situations on a full-scale brake bench. FEM thermomechanical analysis considering experimental data for loadings permits to determinate the most critical thermal stresses as the highest energy dissipated stop braking. The second step, presented here, consists in checking the friction behaviour of the 3 studied cast irons; the manufactured cast iron, the nitrogen-enriched cast iron and the cast iron with a microstructural gradient. Tests are performed on a braking tribometer with various conditions: stop-brakings and series of slowdowns with heat accumulation. During tests, thermal behaviour is monitored at the disc surface using an infrared camera and in the mass by thermocouple measurements. The three cast irons have equivalent braking performance. Reducing graphite size does not penalize the thermal behaviour. Surface analyses performed after tests show iron patches coming from steel fibres of the pad and also alumina particles resulting from the thermal degradation of the ceramic binder of the friction material. This can be observed on all the studied cast irons but differences appear in terms of quantity due to various trapping effect due to graphite lamella size.