| Carbon/carbon composites are well suited to high energy-friction applications due to their excellent thermal conductivity and capacity, low density, and their ability to withstand the temperatures up to 3000°C. Since friction performance is critical, understanding the fundamental principles of controlling frictional performance is a crucial step in tailored design of Carbon/carbon (C/C) composite friction materials. The frictional performance of C/C composites is known to be dependent on many factors, of which material microstructure and properties (mechanical, thermal, chemical), and serving environment are primary.;This dissertation presents experimental research to understand the tribological principles of PAN-fiber reinforced carbon matrix composites. Samples were subjected to three different heat treatment temperatures (1800, 2100 and 2400°C), which altered the microstructure, properties and friction performance. Microstructure was characterized utilizing a combination of light and high-resolution transmission/scanning electron microscopy, focused ion beam, and x-ray diffraction methods. A nanoindentation technique was used to characterize the nano-mechanical properties of individual components. Thermal conductivity was calculated from thermal diffusivity, specific heat capacity and density of the samples. The subscale aircraft dynamometer, equipped with a mass spectroscopy to analyze evolving gases, was used for simulations of aircraft landing at various energy and humidity levels.;Increased heat treatment temperature (HTT) led to formation of a better-organized microstructure of fiber and matrix, and also to formation of thermal cracks. Heat treatment produced an increase in average carbon crystallite size from 103 to 193A. The elastic modulus of rough laminar CVI pyrocarbon decreased from 18 GPa to 12 GPa with increased heat treatment temperatures. In contrast, the isotropic CVI pyrocarbon and charred resin matrix displayed only a slight change of elastic modulus. As expected, the elastic modulus of PAN fiber also changed significantly with the development of a better-organized microstructure in the fiber axial direction from 18 GPa to 34 GPa. Thermal conductivity increases as a function of increased heat treatment temperature.;A friction layer, different from the bulk material, formed on the contact surface after friction tests. While a continuous friction layer formed after the friction tests at 100% normal landing energy (NLE) simulations, after 25% NLE level friction tests, the friction layer either did not form or only partially formed on the contact surface. HRTEM studies revealed an amorphous carbon friction layer after 100% NLE simulations. This new type friction layer was found to be responsible for low wear of C/C composites at high energy landing simulations. Friction test results showed that adsorbed moisture on the friction surface is one factor, which reduces the CoF as well as the wear of investigated C/C composites in low energy tests. However, in high energy level tests, adsorbed vapor is not effective due to high, friction induced, contact temperatures. The oxidation itself (increased CO2 content) does not necessarily produce lubricating effect. Coefficient of friction (CoF) were always high in the simulated landing stops when the amount of CO 2 released was high. This research showed that by understanding the relationships between the factors affecting wear, CoF and friction layer formation, it is possible to optimize the bulk microstructure of the bulk material in order to tailor the properties of C/C composites friction materials. |
