Structural and chemical study of C/C composites before and after braking tests

The oxygen, nitrogen, and hydrogen contents of three kinds of C/C composites used for brake discs, before and after braking tests, have been measured by a combustion-infrared method. The morphologies of wear debris have been observed by SEM, and the relationships between their oxygen, nitrogen, and hydrogen contents and crystal structures and friction properties have been established. The results have shown that the oxygen, nitrogen, and hydrogen contents of the C/C composites are mainly determined by structural defects, and are independent of the degree of anisotropy; the greater the number of lattice defects, the higher the contents of oxygen, nitrogen, and hydrogen. Oxygen, nitrogen, and hydrogen were desorbed during the braking tests, but they were re-adsorbed on the friction surface and wear debris in greater amounts after braking. The oxygen content directly affects the friction behavior; the greater the amount of oxygen desorbed, the greater the increase in friction coefficient, that is, the worse the stability of friction.