Date of Award
Master of Science
Automotive brake lining materials are complex composites consisting of numerous ingredients allowing for their optimal performance. Different types of carbons have been used in all types of brake pad formulations for more than century. The role of carbon, however, does not seem to be completely understood, since carbon materials possess a wide variety of characteristics and properties and they interact with other ingredients present in the friction process. Carbons/graphites are commonly considered to be solid lubricants but this role is only available at relatively low temperatures and at a sufficient humidity. 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 when potential Cu replacement options in the low-metallic, non-asbestos organic, hybrid, and ceramic pads are explored. This paper surveys the role of two types of graphites 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. 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) 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). Although the brake pad formulations were otherwise identical and were prepared at the same conditions, the performance of two different low-metallic pads was different. The effectiveness of PFG sample reached value 0.5 and wear of pad was 5.3 g, while the effectiveness was 0.4 and wear detected was 5.9 g in the case of RGC containing sample. Both formulations exhibited extremely good stability of friction during fade section and only low sensitivity to speed variations between 50 and 160 km/h. Surfaces of both pads were covered by a discontinuous (patchy) friction layer which was formed as a consequence of a gentle abrasive mechanisms involved. The different friction levels and different wear of samples were related to the specific character/differences in detected different friction layers. Importantly, the capacity of the PFG to reduce surface oxides is considerably higher when compared to the RGC. It was concluded that the proper understanding of role of individual graphitic forms in particular formulations can be very beneficial when optimizing the performance of brake pads.
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