Diamond-like carbon-based functionally gradient coatings for space tribology

Solid lubricant coatings for vacuum and space mechanisms are widely used wh en conventional liquid lubrication is prohibited, either when the operating conditions become too severe (extreme temperatures, ultrahigh vacuum) or w hen a clean environment is required. While the well-known MoS2 lamellar sol id lubricant is the most extensively used material today, diamond-like carb on (DLC) coatings are studied as potential candidates for a wear resistant material with low friction in vacuum conditions. Diamondlike carbon-based f unctionally gradient Ti/a-CH(Ti) films have been deposited by the hybrid te chnique of magnetron sputtering and d.c. plasma-enhanced chemical vapor dep osition, in various conditions. Analytical characterization coupled with tr ibological tests in ultrahigh vacuum and ambient humid air have been perfor med to identify relationships between the deposition conditions, the compos ition and the properties (stress, friction) of the films. Depending on the properties of the DLC which are in turn dependent on the deposition procedu re, the investigated films present a wide range of tribological behavior, i ncluding friction coefficients in UHV below 0.02. Typical DLC structures an d compositions allowing the achievement of extremely low friction in vacuum and good behavior tinder air are identified and discussed. (C) 1999 Elsevi er Science S.A. All rights reserved.