Date of Award

12-1-2018

Degree Name

Master of Science

Department

Mechanical Engineering

First Advisor

Filip, Peter

Abstract

Carbon-carbon (C/C) composite exhibits high mechanical and thermal properties due to their fiber reinforce matrix. C/C composites are lightweight, has low coefficient of thermal expansion and high impact resistance. Hence, C/C composites are suitable for friction applications such as brake discs and pad, mainly due to their low wear rates and low coefficient of friction. However, environmental conditions such as temperature and humidity affect the friction performance. When gases diffuse across the surface and the cracks of the carbon composite at high temperature, carbon reacts with oxygen vigorously causing oxidation process to occur. In addition, the moistures in air could generate menisci that could cause adhesion between the contact surfaces or provide lubrication effects, which increase or decrease the coefficient of friction and friction wear. The moisture adsorption or desorption process on the brake’s surface attributed to the braking capacity as well. A relatively large amount of research have been carried out addressing these issues with a dynamometer, however it is very costly, time consuming and complicated. The friction performance of real systems can be simulated in a smaller and simpler test using Universal Mechanical Tester (UMT). In this study, friction test under different humidity and temperature were performed at various normal landing energies with a Universal Mechanical Tester (UMT) manufactured by Bruker. The application of “Scaling law” was implemented to scale down testing parameters from previous research and applied it into the UMT. Besides, the microstructure of the C/C composite was characterized and analyzed through a polarized light microscopy and wear mechanisms were analyzed and studied under a scanning electron microscopy. The surface topography was measured using a coordinate measuring machine (CMM). The coefficient of friction of C/C composite brake discs were found to be less dependent on humidity but very dependent to temperature. The coefficient of friction increased when the temperature increased to 550°C. Less wear was observed on high humidity and low temperature condition. The wear mechanisms included abrasive, adhesive, erosion, corrosion, fatigue and oxidation wear were observed in all friction surface of the composites. Friction is a system property. The characteristic, stability and vibration of the system are dissimilar. Thus, the coefficient of friction at small scaled did not attain the similar friction performance from a dyno test.

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