Performance of Low-metallic Cu-free Brake Pads with Two Different Graphite Types 2015-01-2677
Automotive brake lining materials are complex composites consisting of numerous ingredients allowing for their optimal performance. Since regulations are increasingly limiting Cu content in brake pads and Cu exhibits extremely high thermal conductivity, graphites being excellent heat conducting materials themselves, are often considered for use as potential Cu replacement. This paper surveys the role of two types of carbons (Superior Graphite) with high thermal conductivity but different mechanical properties and morphology: the so-called i) purified flake graphite (PFG) and the ii) resilient graphitic carbon (RGC). A successful “high-end” commercial low-metallic brake pad was re-formulated (SIU Carbondale) by removing of over 20 wt. % of Cu and replacing it with a cocktail of ingredients including 15 wt. % of these two graphite types (RGC and PFG). Original equipment manufacturer (OEM) Crown Victoria 1999 mold was used to prepare the pads and they were subjected to the SAE J2430 test and BEEP evaluation using the full-scale automotive brake dynamometer (Link Engineering M 2800) and original hardware (rotor and caliper). After friction tests, the surfaces of pads were explored using scanning electron microscopy equipped with the energy dispersive X-ray microanalysis (FEG450 and Inca System) and X-ray diffraction (Rigaku Max-Flash-B). The performance of two different low-metallic pads was different. Both formulations exhibited extremely good stability of friction during fade section. The different friction levels and different wear of samples were related to the specific surfaces developed on two different pads containing RGC and PFG graphites and encountering rotors were covered by a discontinuous (patchy) friction layer. The capacity of the PFG to reduce surface oxides is considerably higher when compared to the RGC. Proper understanding of role of individual graphitic forms in particular formulations can be very beneficial when optimizing the performance of brake pads.