Date of Award
Master of Science
In recent decades, significant requirements of changes in composition of brake friction materials in order for faster and more reliable transportation as well as their environmental friendly characterizations attracted attentions. However, the relation between performance and formulation/composition is not clear since friction processes are accompanied by many complex problems such as instability in the coefficient of friction, noise, vibration and wear. Creep-groan is a low frequency vibration which could originate in different part of the system (vehicle). The resulting resonant vibration in the passenger compartment causes discomfort and often leads to complaints of customers and related increase of warranty costs covered by manufacturers. In spite of relatively large amount of publications addressing the creep-groan phenomena, there is not an universal solution addressing the engineering aspects of brake/vehicle design. In addition, Relevance of wear occurring in brake materials increased particularly with relation to the released chemicals and corresponding health and environmental hazards. It is well known that humidity can considerably modify the adhesion of rubbing counterfaces by creating menisci and increasing the contact area. The chemistry, morphology and phase composition of the friction layers (third body) generated on the friction surfaces could play a determining role when amounts of adsorbed water on brake surfaces is concerned. The friction layer is typically a complex mix of numerous materials and, as a rule, contains the agglomerated or sintered nanoparticles. Hence, quantum effects could further modify the adsorption of water. This work addresses the impact of humidity on wear and creep groan of two commercial brake material types: the so called i) "non-asbestos organic" (NAO) and the ii) "semi-metallic" (SM) brake materials rubbed against pearlitic gray cast iron rotors typically used in the passenger vehicles. The friction and wear tests were performed with the Universal Mechanical Tester (UMT) manufactured by Bruker and the wear surfaces/mechanisms were studied by using of scanning electron microscopy, energy dispersive X-ray microanalysis, and optical topography methods. The applied wear testing conditions were designed as a series of particularly designed drag tests and were performed at several different relative humidity levels ranging between 50% and 80%. The major findings confirmed the considerable effect of humidity on wear of brake materials. Both pad types wore noticeably less at increased humidity. This was ascribed to a better capacity to form a protective friction layer. The complex wear mechanisms including abrasive, adhesive, fatigue, and corrosion wear were observed on both material types, irrespectively of humidity levels, and they dependent on the chemistry and phase composition of the friction layer. Humidity also influenced the level of friction.
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