New insights into the microstructure of the friction surface layer of C/C composites

An attempt has been undertaken to obtain a better understanding of the friction and wear mechanisms of C/C composites by microstructural analysis of the friction surface layer and wear debris using scanning electron microscopy and transmission electron microscopy. The friction and wear properties of C/C composites of rough laminar pyrocarbon with added resin-derived carbon have been investigated with a home-made laboratory-scale dynamometer to simulate airplane normal landing. The results have shown that a friction layer with a thickness of several micrometers was formed on the bulk material. It was characterized as consisting mainly of amorphous carbon, but with a few microsized particles as well. This friction layer protected the bulk materials from serious degradation. In addition to the friction layer, a friction film with a thickness of less than 200 nm was observed for the first time. It partly covered the top surface of the friction layer. Transmission electron microscopy analysis has demonstrated that the friction film was characterized by a laminar structure, the belts of which were revealed to consist of highly oriented graphene sheets. The constant friction coefficient was attributed to this lubricant film.