Effect of relative humidity and kinetic energy on frictional performance of carbon-carbon composite materials (disc-on-disc configuration)

Carbon/carbon composites are used as frictional components in braking systems because of their unique properties, high melting temperature, excellent thermal conductivity, high thermal capacity, low density and their ability to be used as both frictional and structural materials. However, the sensitivity of carbon/carbon brake materials to the environment, which has been attributed to moisture adsorbtion/desorption on the brake's surface, is known to be a significant factor influencing frictional performance [1]. In particular, the decrease in braking capacity ("morning sickness") and transition phenomenon (sudden increase in friction) has been attributed to moisture adsorption/desorption process [1]. In this work, the impact of kinetic energy dissipated during friction process and the impact of different environmental conditions on the frictional performance of selected carbon/carbon composite materials was studied.; The average coefficient of friction at landing energies of 2/D, carbon-carbon composite, composed of randomly chopped pitch fibers with smooth laminar CVI and charred resin matrix material (HWCCA) was found to be very sensitive to the energy level applied during braking, but it is less sensitive to humidity. At all humidity levels, the average CoF drastically decreased when the amount of kinetic energy dissipated in friction process increased. The average coefficient of friction at landing energies of 3/D, carbon-carbon composite composed of non-woven needled felt PAN fiber with a rough laminar CVI matrix material (HWCCD) was found to have modest sensitivity to the landing energy and humidity level. Slightly lower values of average CoF were detected at lower energies and higher humidity levels. Values of the average CoF were significantly influenced by the development of the transition phenomenon.; For HWCCA material, transients developed at all energy levels. At the 100% normal energy level, two transients were detected. Increases in energy level shorten the time for onset of the transient. At 50% NLE, the CoF is high from the beginning of the stop. Increase of the humidity level in the environment decreased oscillations of the friction coefficient and noise at all energy levels. For HWCCD material, the coefficient of friction was relatively stable at lower energy conditions (12.5 and 25% NLE). Drastic changes in the coefficient of friction occurred the when energy dissipated during the stop reached the 50% NLE. Two transients were observed when the energy dissipated during the stop increased to the level of 100% NLE. The kinetic energy dissipated during landing stops had no significant influence on the oscillation and noise level. Relative humidity level did not have a significant influence on the landing stops as far as oscillation and noise are concerned.; The time dependence of the CoF was shown to be closely correlated with the increase of temperature on the friction surfaces, in particular the occurrence of the transients. It was shown that the beginning and the end of the first transients occurred at the similar friction surface temperatures for both investigated materials. It was shown that oxygen (in dry air) slightly postponed the development of the transient, while the water vapor had significant influence. Water vapor also caused a decrease in the CoF at the very end of the stop, while no drop in the CoF was detected when tests were performed in dry nitrogen and dry air. It is therefore, reasonable to conclude that re-adsorption of water vapor on the friction surface is responsible for such behavior.; Comparison of the friction surface microstructures at two different transients of the CoF, of very similar values showed the microstructure of the friction surface was turbostratic at the first and highly ordered at the second transient. Therefore, no direct relationship between microstructure of the friction surface itself and frictional performance for investigated materials and simulated conditions c...