Tribological Properties of a-C:H Sliding on a-C:H:F Films

Reducing friction not only promotes the life and safety of the mechanical part, but also saves energy by lowing the friction force. Many films, like carbide, nitride and boride films, have been employed to reduce friction for moving parts. Among them, carbon based films have been thought to be the best choice due to their low friction, high anti-wear and high anti-corrosion properties, etc. However, these films are combined with sp2 and sp3 bonding structures and some times they are doped with hydrogen (H) or fluorine (F) or other elements, such as Si, P, Ti, Cr, etc. However, since H and F have single electrons that can terminate on the dangling σ-bond of amorphous carbon films to give repulsion force, contributing low friction. However, no one knows what will happen if the H and F doped amorphous carbon (a-C:H and a-C:H:F) films are taken to slide on each other. The work aims to study the tribological properties of a-C:H sliding on a-C:H:F film and explore the mechanism of tribology between the a-C:H and a-C:H:F films with the variation of loads. a-C:H:F film was prepared on Si substrate by plasma enhanced chemical vapor deposition and a-C:H film was deposited with magnetron sputtering of a carbon target in methane and argon atmosphere, respectively. Then, the tribology behaviors were evaluated on a commercial reciprocating ball-on-disk tribometer (CSM TRB 3 TRIBOMETER, Switzerland) with the test frequency and the total sliding times were 9 000 at 5 Hz, respectively. Normal load was set to 2, 4, 6, 8 and 10 N, respectively. Surface morphology of the a-C:H:F films were analyzed by SEM, and the mechanical properties were investigated by nanoindentation tests, with the depth no more than 1/10 of the a-C:H:F film thickness. The structure and tribology properties of the a-C:H:F films were tested by Raman spectroscopy, XPS spectroscopy and FTIR spectroscopy, respectively. Through this characterization results, it was found that the effect of fluorine doping played a key role in reducing friction coefficient as well as wear rate. The friction coefficient of a-C:H:F film decreased gradually and the curve was becoming stable with the increase of normal load, and the friction coefficients were 0.037, 0.027, 0.025, 0.021, and 0.018, respectively. While the wear rates of a-C:H:F film reduced with increase of the load from 2 to 10 N, which were 1.991×10−8, 1.212×10−8, 0.881×10−8, 0.721×10−8, 0.557 ×10−8 mm3/(N·m), respectively. The low friction coefficient can be ascribed to the transformation of sp3 to sp2 structure and promote the increase of sp2-C clusters in the film which can be proved by typical Raman spectrum. The low wear rate can be attributed to the wear debris, which prevent the direct contact of the a-C:H and a-C:H:F films. Fluorine also has effect of passivating film surface and electrostatic repulsion as hydrogen do. Generally, a-C:H:F films prepared by PECVD have a superior anti-friction and anti-wear ability to couple with a-C:H balls. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.