Fluorine and sulfur co-doped amorphous carbon films to achieve ultra-low friction under high vacuum

Fluorine and sulfur co-doped amorphous carbon (a-C:S:F) films were synthesized from C2H2 and SF6 plasma mixture, and the bonding states of S and F in films were scrutinized. The a-C:S:F films exhibited a more ordered carbon structure than hydrogenated films. The friction behavior of films was tested against GCr15 bearing steel balls under high vacuum (HV) conditions. Ultra-low steady-state friction coefficient (0.01-0.02) was achieved for a-C:S:F films containing about 2.0 at.% H, depending on the concentrations of S and F in films. Graphite-like transfer layer on the ball surface provided a low-adhesive sliding surface terminated by -CF2- and -CF3 groups. The electrostatic repulsive interaction between the F-terminated transfer layer and the S- and F-terminated carbon structure was probably responsible for the ultra-low friction behavior. Intriguingly, the sp2-C clusters containing 'thiophene-S' (-C-S-C-) structure in films might contribute to the low friction when sliding against an highly fluorinated surface. These results demonstrate that the lubricating properties of a-C films in vacuum can be achieved through the S and F co-doping, which is of great significance for developing a-C films as vacuum lubricants.