Masataka Nosaka

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.

Structural and environmental dependence of superlow friction in ion vapour-deposited a-C?:?H?:?Si films for solid lubrication application

Understanding the tribochemical interaction of water molecules in humid environment with carbonaceous film surfaces, especially hydrophilic surface, is fundamental for applications in tribology and solid lubrication. This paper highlights some experimental evidence to elucidate the structural and environmental dependence of ultralow or even superlow friction in ion vapour-deposited a-C?:?H?:?Si films. The results indicate that both surface density of silicon hydroxyl group (Si?OH) and humidity level (RH) determine the frictional performance of a-C?:?H?:?Si films. Ultralow friction coefficient ? (?0.01?0.055) is feasible in a wide range of RH. The dissociative formation of hydrophilic Si?OH surface and the following nanostructure of interfacial water molecules under contact pressure are the origin of ultralow friction for a-C?:?H?:?Si films in humid environment. The correlation between contact pressure and friction coefficient derived from Hertzian contact model is not valid in the present case. Under this nanoscale boundary lubrication, the friction coefficient tends to increase as the contact pressure increases. There even exists a contact pressure threshold for the transition from ultralow to superlow friction (????0.007). In comparison, when tribotested in dry N2, the observed superlow friction (????0.004) in the absence of water is correlated with the formation of a low shear strength tribolayer by wear-induced phase transformation.

Evolution of tribo-induced interfacial nanostructures governing superlubricity in a-C:H and a-C:H:Si films

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.Hydrogenated amorphous carbon is a promising solid lubricant, but the underlying mechanisms surrounding its superlubricity remain unclear. Here the authors reveal that the attainment of a superlubricious state is dependent on the in-situin-situ formation of a nanostructured tribolayer through different carbon rehybridization pathways.

Friction fade-out at polymer-like carbon films slid by ZrO2 pins under hydrogen environment:

Friction tests using ceramic pins against fully hydrogenated diamond-like carbon (polymer-like carbon, PLC) film under H2/He mixed gas or pure H2 gas environment were conducted. The test results of ZrO2 (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 ZrO2 surface. It is discussed based on the measurements that ZrO2 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.

Cryogenic Tribology in High-Speed Bearings and Shaft Seals of Rocket Turbopumps

In recent years, as a rule, improvement of the reliability of liquid propellant rockets becomes an international technical problem for built-up of safe space transport systems. The high performance, liquid propellant rocket engines require high-pressured turbopumps to deliver extremely low temperature propellants of liquid oxygen (LO2, boiling point 90 K) and liquid hydrogen (LH2, boiling point 20 K) to a combustion chamber in engine [1]. In LO2/LH2 turbopumps, cryogenic high-speed bearings and rotating-shaft seals are very important parts to sustain high reliability of the high-rotating-shaft systems. The turbopump bearings are directly equipped in cryogenic propellants in pump side [2]. The shaft seal systems are also set up between the cryogenic pumps and the hot turbines to restrain the leakage of cryogenic propellants and hot turbine gas [3].

Stability of friction fade-out at polymer-like carbon films slid by ZrO2 pins under alcohol-vapored hydrogen gas environment

Friction tests using ZrO2 (Y-PSZ: yttria partially stabilized zirconia) pin slid against polymer-like carbon film of bilayer and multilayer structures under H2 gas environment are conducted. It will be shown that friction coefficients of the level of 0.0001 (friction fade-out) is stably realized by adding alcohol vapors to H2 gas during run-in stage, then by stepping up the load from 19.8 N to 63.7 N after run-in stage. Four kinds of vapors of alcohol aqueous solutions are tested using bilayer samples, and ethanol-vapored H2 gas shows the longest friction fade-out duration. Polymer-like carbon/diamond-like carbon multilayer sample shows long-life friction fade-out of 4 h, and it will be shown that the friction trace of 4 h reflects wear process of the first layer of polymer-like carbon and the second layer of diamond-like carbon. ZrO2 surface is observed by an optical microscope and scanning electron microscopy and measured by surface profiler after friction fade-out test, and it is shown that flat contact area at the central region has many blisters and crimps, and is surrounded by peripheral bumps. It is also shown that the sliding marks are seen only at the top of crimps at the central region. Raman measurements indicate that short-chain carbons are predominant at blisters and ring carbons of small clusters are predominant at bumps. With these observations friction fade-out mechanism is discussed.

Possibility of elasto-hydrostatic evolved-gas bearing as one of the mechanisms of superlubricity:

In this article, firstly the experiments previously reported have been reviewed. That is, in the experiments conducted earlier it was seen that the friction coefficient reduced to the friction-tester noise-level of 0.0001, and only a small amount of sliding marks was seen on the tribofilm by scanning electron microscopic observation even though the maximum Hertzian contact pressure was 2.6 GPa. Based on these measurements, the possibility of elasto-hydrostatic gas bearing as the mechanism of super-low friction coefficient, where gas is not supplied from outside of the contact area but evolved from the contact surface, is investigated in the present report. Assuming the evolution of hydrocarbon gas mixture by ZrO2 catalytic reaction, which is represented by C2H4 in this report, the hydrostatic pressure distribution at the contact area and the evolved gas amount required for supporting a heavy applied load of 60 N are calculated.