Effect of tribofilm formation on the tribological performance of hydrogenated carbon coatings

Amorphous hydrogenated carbon (a-C:H) films were deposited by the r.f. plasma technique on stainless steel substrates. Pin-on-disc experiments were carried out over a wide range of normal loads (5-40 N) and sliding velocities (0.1-3.0 m s-1) in order to study the friction and wear performance of the coating against steel and alumina. The friction coefficient of a-C:H films against both steel and alumina pins decreased with increasing load and sliding velocity. On the pin wear surface, tribolayer formation was detected. The wear of the alumina pins increased with increasing load and sliding velocity when they contacted the coating. However, the thick tribolayer formed on the wear surface of the steel pin protected it from excessive wear when high loads and sliding velocities were applied. The wear surfaces were analysed by secondary ion mass spectroscopy and Auger electron spectroscopy. The analyses revealed that the thick tribolayer formed on the pin wear surface mainly consisted of the oxides of the pin material. However, evidence of carbon was found in the sliding deposit formed in front of the contact area of the pin and also in some cases on the pin wear surface. Carbon played an important role in the low friction behaviour although the amount of carbon was low. It is assumed that a thin tribolayer with low shear strength, consisting of carbon species, is formed on the disc wear surface. The coating wear increased when the normal load was increased. Some transfer of pin material was observed on the coating wear surface.