Origin of Superlubricity in a-C:H:Si Films: A Relation to Film Bonding Structure and Environmental Molecular Characteristic

Superlubricity of Si-containing hydrogenated amorphous carbon (a-C:H:Si) films has been systematically investigated in relation to the film bonding structure and the environmental atmosphere. Structural diversity induced by hydrogen incorporation (i.e., 17.3-36.7 at. % H), namely sp(2)-bonded a-C, diamond-like or polymer-like, and tribointeractions activated by the participation of environmental gaseous molecules mainly determine the frictional behaviors of a-C:H:Si films. A suitable control of hydrogen content in the film (i.e., the inherent hydrogen coverage) is obligate to obtain durable superlubricity in a distinct gaseous atmosphere such as dry N2, reactive H2 or humid air. Rapid buildup of running-in-induced antifriction tribolayers at the contact interface, which is more feasible in self-mated sliding, is crucial for achieving a superlubric state. Superior tribological performances have been observed for the polymer-like a-C:H:Si (31.9 at. % H) film, as this hydrogen-rich sample can exhibit superlow friction in various atmospheres including dry inert N2 (μ ∼ 0.001), Ar (μ ∼ 0.012), reactive H2 (μ ∼ 0.003) and humid air (μ ∼ 0.004), and can maintain ultralow friction in corrosive O2 (μ ∼ 0.084). Hydrogen is highlighted for its decisive role in obtaining superlow friction. The occurrence of superlubricity in a-C:H:Si films is generally attributed to a synergistic effect of phase transformation, surface passivation and shear localization, for instance, the near-frictionless state occurred in dry N2. The contribution of each mechanism to the friction reduction depends on the specific intrafilm and interfilm interactions along with the atmospheric effects. These antifriction a-C:H:Si films are promising for industrial applications as lubricants.