Friction and wear performance of functionally graded ductile iron for brake pads

Due to its wear resistance, functionally graded ductile iron (FGDI) is a potential material for automotive brake pads. In this investigation it was studied (i) under well controlled sliding conditions in a typical tribological model-test to show in detail the mechanism behind improved wear resistance, namely the “property on-demand” functionality in the sub-surface zone and (ii) in a validation experiment on a disc-brake machine that mimic realistic brake-contact conditions, at various elevated temperatures against a carbon-reinforced ceramic disc in order to understand its tribological behaviour in a real automotive brake application. The results revealed a positive effect of the functional gradient zone (FGZ) on the wear performance of the functionally graded ductile-iron pads. Most notably, improved wear resistance was observed under sliding model-test conditions as well as a more-stable wear and wear that is less sensitive to the contact temperature during a disc-brake test. In addition, the sliding model-tests revealed that FGZ leads to faster running in and a more stable coefficient of friction. The disc-brake tests also show that FGZ ensures a relatively constant coefficient of friction for all contact temperatures. Surface analysis of the pads further revealed that oxidized, smoothened plateaus containing graphite nodules were formed on the pads. On the other hand, a patchy and layered transfer film resulting from the pads was formed on the disc's surface.