Experimental Analysis of Friction and Wear of Carbon-Graphite at Low Contact Pressures Against Smooth and Rough Metallic Surfaces

Carbon-graphite is one of the most widely used materials for dynamic seals operating with very low or even almost zero clearance. Contact between the seal and the moving counterface inevitably leads to wear. Carbon-graphite is a soft and brittle material which, when worn, generates a fine powder that acts as a solid lubricant. The objective of the present work is to experimentally analyze the friction and wear of a planar contact between a carbon ring and a metallic disc. Only low contact pressures corresponding to a radial segmented seal used in aeronautical engines were analyzed. It was therefore possible to employ rheometers instead of traditional tribometers. The friction coefficient was measured for various contact pressures and rotational speeds. Results showed that it is almost constant and varies slightly with contact pressures. The surfaces of the carbon-graphite ring and the steel disc were measured before and after tests using a white-light interferometer and optical microscopy. Post-test surface analyses were performed on both uncleaned and cleaned samples. These measurements revealed the formation of a carbon layer covering the contact area. Tests were performed using discs with two different surface roughnesses in order to assess their influence on wear. Two distinct regimes were identified. With a smooth disc, graphite forms a continuous tribofilm on both the sample and the counterface, leading to negligible wear. On the other hand, with a rough disc, no continuous film develops on the disc surface; the tribofilm forms only on the graphite, resulting in more pronounced wear that is strongly dependent on contact pressure. These results highlight that the formation of the third-body film is highly sensitive to the nature of the mating surfaces, particularly their surface roughness.