Experimental analysis of friction and wear of carbon-graphite at low contact pressures against smooth and rough counterpart surfaces

Carbon–graphite is widely used in dynamic sealing applications operating with very small, or nearly zero, clearances. Contact between the seal and the moving counterface inevitably leads to wear. As a soft and brittle material, carbon–graphite generates fine debris during wear, which acts as a solid lubricant. The objective of this work is to experimentally investigate the friction and wear behavior between a carbon ring and a metallic disk under low contact pressures representative of radial segmented seals used in aeronautical engines. It was therefore possible to employ rheometers instead of traditional tribometers. The friction coefficient was measured for several contact pressures and rotational speeds. The results showed that the friction coefficient remains nearly constant and varies slightly as contact pressure increases. Surface topographies of the sample and the steel disk were characterized before and after testing using white-light interferometry and optical microscopy. Post-test analyses, performed on both uncleaned and cleaned samples, revealed the presence of a carbon layer covering the contact surface. To evaluate the influence of counterface roughness, tests were carried out using disks with two different surface roughness. Two different wear regimes were identified. With a smooth disk, a continuous tribofilm formed on both contacting surfaces, resulting in negligible wear. In contrast, with a rough disk, the tribofilm developed only on the graphite surface, leading to higher wear levels strongly dependent on contact pressure. These results highlight that the formation of the tribofilm is highly sensitive to the surface roughness of the mating surfaces.