Diamond-like carbon coatings for vacuum and space tribology

Abstract

Solid lubricant coatings for vacuum and space mechanisms are widely used when the operating conditions (extreme temperatures, ultrahigh vacuum) and the cleanliness requirements become so severe that conventional liquid lubrication is prohibited. While the well-known MoS2 lamellar solid lubricant is the one most extensively today, diamond-like carbon (DLC) coatings are studied as potential candidates for a wear resistant material with low friction in vacuum conditions. The paper discribes the pin on flat tests which were performed on several DLC coatings under air and ultrahigh vacuum (<10-8 Torr). However, depending on the properties of the DLC which are in turn dependent on the deposition system, tested DLC coatings present a wide range of tribological behavior, with friction coefficients in UHV ranging from less than 0.01 to more than 0.5 and very low wear under air or UHV. Typical DLC structures and compositions allowing the achievement of extremely low friction in vacuum and good behavior under air are identified and discussed. Such properties can be obtained for specific combinations of hydrogen content, fraction of hydrogen bonded to carbon, and film stress, factors which may be accurately controlled by the deposition process.