Hydrogenated amorphous carbon (a-C:H) is capable of providing a near-frictionless lubrication state when rubbed in dry sliding contacts. Nevertheless, the mechanisms governing superlubricity in a-C:H are still not well comprehended, mainly due to the lack of spatially resolved structural information of the buried contact surface. Here, we present structural analysis of the carbonaceous sliding interfaces at the atomic scale in two superlubricious solid lubricants, a-C:H and Si-doped a-C:H (a-C:H:Si), by probing the contact area using state-of-the-art scanning electron transmission microscopy and electron energy-loss spectroscopy. The results emphasize the diversity of superlubricity mechanisms in a-C:Hs. They suggest that the occurrence of a superlubricious state is generally dependent on the formation of interfacial nanostructures, mainly a tribolayer, by different carbon rehybridization pathways. The evolution of such anti-friction nanostructures highly depends on the contact mechanics and the counterpart material. These findings enable a more effective manipulation of superlubricity and developments of new carbon lubricants with robust lubrication properties.

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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.

Friction tests using ceramic pins against fully hydrogenated diamond-like carbon (polymer-like carbon, PLC) film under H 2 /He mixed gas or pure H 2 gas environment were conducted. The test results of ZrO 2 (YSZ: yttria-stabilized zirconia) pin slid against PLC film with an applied load of 4.9 N showed that the friction coefficient dropped to the tribometer noise level (friction fade-out, FFO) as low as 0.0002. In another experiment with the same materials and with an applied load of 30.4 N, the friction coefficient dropped to 0.0001-0.0005, which continued more than 4 h. Optical microscope and scanning electron microscopic observations, nano-indentation, surface profiler, X-ray photoelectron spectroscopy, Raman, and time-of-flight secondary ion mass spectrometry measurements were conducted and the mechanism for inviting FFO is investigated. It is found by an optical microscopic observation that the transfer film has small blisters, indicating some gaseous substance is generated at the ZrO 2 surface. It is discussed based on the measurements that ZrO 2 catalysis plays very important roles for gaseous substances generation and for H-passivation on both PLC and transfer film surfaces, which are closely relating to FFO through gas-lubrication effect, and through reducing adhesive force, respectively.

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The ultra-high-speed performance of a hybrid ceramic 25-mm-bore

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