Vacuum tribological properties of a-C:H film in relation to internal stress and applied load

Amorphous hydrogenated carbon (a-C:H) films were deposited by medium frequency unbalanced magnetron sputtering with Ar/CH4 gases. The microstructure and vacuum tribological properties of the films in relation to internal stress and applied load (1, 2, 4 and 5 N) were investigated. The results show that the bonded hydrogen in the film decreases and the graphitization of the film increases with increasing Ar ratio in the source gases. Meanwhile, all the films have ultra-low friction coefficients (about 0.005) but relatively short sliding lifetimes (shorter than 900 s) in vacuum. Furthermore, the film can maintain a sliding lifetime longer than 3600 s either by lower the internal stress (annealed at 200 °C) or sliding at a small applied load (1 N). It has been found that relaxation of the internal stress is the main case of the short sliding lifetime in vacuum, whereas it does not occur in air under the same tested conditions.