Effect of Transfer Film Forming on Super-low Friction Properties of Hydrogenated Amorphous Carbon Films

Hydrogenated amorphous carbon (a-C:H) films exhibit super-low friction properties in inert atmospheres with a friction coefficient of down to an order of 10^–3. In this study, a experiment was designed to demonstrate that the formation of transfer film was a necessary condition for the super-low friction properties of a-C:H films, among which the tribological properties of a-C:H film sliding against steel in dry N₂ were evaluated by a ball-on-disk rotation test in two contact modes. One was a steel ball sliding against a-C:H film coated steel disk, and the other is the same a-C:H film coated steel ball sliding against steel disk. For the first mode, the a-C:H film on steel disk was prone to transfer to the steel ball surface, exhibiting super low friction. For the second mode, the a-C:H film on steel ball was hard to transfer to the steel disk surface forming continuous transfer film, and showed high friction. While the transfer film was characterized containing high sp² bonded carbon content and H, which was beneficial to obtain low friction because H passivated carbon dangling bonds to ensure the low chemical activity of contact surface, and sp² planar molecular structure had low shear strength. Therefore, the formation of transfer film also was as important as H passivation being found in previous studies in contribution to the super-low friction properties of a-C: H films.