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The 'friction-induced graphitization' mechanism for hydrogenated amorphous carbon (a-C:H) films has been postulated, but it requires concrete analyses at specific conditions. Here, we have discussed the reason for higher sp2 bond content on the interface of self-mated fullerene-like a-C:H films when the relative humidity (RH) increases from dry N 2 to 70%. The reason is the failure of the role of FL structures and the change of interface structure evolution. Before RH 50%, FL structures work properly, and the films remain low graphitization to form nanoparticle-like debris having onion-like structures. After RH 50%, the failure of the role of FL structures leads to the abruptly increasing wear, and the formation of larger flake-like debris. Higher sp2 content, is not only caused by formation of longer and order graphene sheets than the original surfaces, but also linked with the increase of C O bonds. This is unlike the general friction-induced graphitization that higher sp2 content indicates higher graphitization. The result reminds us to further understand friction-induced graphitization mechanism at special condition. Unlabelled Image • The 'friction-induced graphitization' mechanism for hydrogenated amorphous carbon (a-C:H) films is discussed. • Higher graphitization induced higher friction is observed under higher humidity condition. • The reason for higher graphitization is the failure of FL structure and the change of interface structure evolution. • Higher sp2 bonds' content is caused by formation of graphitic sheets and increase of C O bonds at higher humidity. [ABSTRACT FROM AUTHOR]

Self-mated fullerene-like hydrogenated carbon films (FL-C:H films) realized the conversion from low friction (~0.012) to super-low friction (~0.006) state due to the increasing normal load. The structure changes in the conversion and the effect of environments (N 2 and humid air) on the structure changes were discussed by the combination of SEM, TEM, Raman and XPS spectra. With the increasing normal load, smaller wear scar area (or contact area) and thicker tribofilm could be observed. At smaller scale (such as nanoscale), not only more perfect graphitic structure can be formed, but newly formed longer graphitic sheets also tended to curve. [ABSTRACT FROM AUTHOR]

Fullerene-like hydrogenated carbon films deposited by plasma enhanced chemical vapor deposition (PECVD) as compared with other methods can attain super-low friction and wear. But they often have bad adhesion to steel substrate and their structures are tailored by non-metal elements as F, N etc. In this work, we have prepared the films on steel balls by the combination of PECVD and plasma nitriding, among which iron nitride particles are sputtered and introduced in the films. The films have longer wear life and better adhesion to the steel substrate due to the N diffusion from iron nitride particles to the atom matrix of the ball. [ABSTRACT FROM AUTHOR]

In this study, we carried out the transition experiments of graphite-like (GL) to fullerene-like (FL) structures by placing high temperature steel substrates in the depositing environment which can form FL hydrogenated carbon films. We investigated the changes of bond mixtures, H content, aromatic clusters and internal stress at the transition process, and proposed the transformation mechanism inferred from Raman, TEM cross-section, FTIR and XPS results. It was found that the size of aromatic clusters and accordingly graphene planes and the formation of edge dangling bonds were the key steps. H + bombardment leaded to the splitting of large graphene planes (at GL stage) into more and smaller planes (at FL stage) and the formation of edge dangling bonds; Some of these dangling bonds were reduced by the formation of pentagons and subsequent curving of the smaller planes, which were an indicator of FL structures. [ABSTRACT FROM AUTHOR]

Fullerene-like hydrogenated carbon films have outstanding mechanical and frictional properties, but their structures have never enjoyed elaboration. In this study, we investigate the relation between nano-hardness and elasticity and fullerene-like clusters by changing energy supply form (direct current and pulse) and H 2 concentration in the feedstock. It is found that the films have a network of H-rich amorphous carbon and H-poor or -deficient fullerene-like carbon, and the network change can affect hardness and elastic recovery. This is due to the energy minimization process of the film growing system in a very short pulse period at low temperature. [ABSTRACT FROM AUTHOR]

High performance solid lubricant materials with long wear life remain as one of the challenges in space technology. It is of great significance to lower the solid film's friction and wear and to prolong its service life in vacuum. Hydrogenated fullerene-like carbon film produced by dc-pulsed power source behaves ultralong wear life (≫1.8 × 105 cycles) and ultralow wear rate (2.2 × 10−8 mm3 Nm−1) as well as low friction (≈0.14) in high vacuum (2.3 × 10−4 Pa). To understand its long wear life reason, the interfacial structural evolution is investigated after steady state friction is achieved. By observing the interfacial nanostructure evolution, it is found that the friction and wear properties are related to the friction-induced phase transformation from fullerene like structure to nanocrystalline graphite and the deposition of tribofilm on Al2O3 counterface. And this process is analyzed by X-ray photoelectron spectroscopy, micro-Raman spectra and is observed through transmission electron microscopy. The results enrich hydrogenated fullerene-like carbon film friction mechanisms and expand its potential application in space. [ABSTRACT FROM AUTHOR]

Fullerene-like nanostructural hydrogenated amorphous carbon (FL-C:H) films were prepared by dc- and pulse- plasma enhanced chemical vapor deposition technique (PECVD). Both the films exhibit relatively stresses (0.63 GPa) in spite of their FL features and nanostructural bonding configurations, especially the pentagonal carbon rings. The creation of pentagonal rings is not fully driven by thermodynamics, but is closely related to compressive stress determined by the ion bombardment at the discharged state of the pulse- and dc- discharged plasmas methods. The dc method leads to FL's basal planes which contain less cross-linkages, and causes amorphous strongly hydrogenated structures. [ABSTRACT FROM AUTHOR]

Fullerene-like hydrogenated carbon (FL-C:H) films as carbon materials were prepared by direct current plasma enhanced chemical vapor deposition (dc-PECVD) technique. The content of FL nanostructure was confirmed by high-resolution transmission electron microscopy (HRTEM), visible Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The effect of fullerene-like nanostructure on the friction behavior of the films was studied using a reciprocating ball-on-flat tribometer in humid environment. It is concluded that the curved FL nanostructure provide the film excellent mechanical properties and friction performance. Interestingly, combining with the results of Raman analyses of the wear debris, we find that new FL nanostructure form during the friction process. These new FL nanostructure may originate from the rapid annealing and stress relaxation of unstable carbon clusters. [ABSTRACT FROM AUTHOR]

The development of highly reliable, low-friction, durable lubrication materials and technologies applicable to the space environment will be of great importance to improve the lubrication status and life of space equipment. Fullerene-like films exhibit excellent ultra-low friction and wear resistance under atmosphere and vacuum, and are considered to be one of the most promising materials for space lubrication. In this work, the effect of vacuum atomic oxygen on the mechanism of action and tribological properties of fullerene-like carbon and MoS 2 films were investigated. After atomic oxygen irradiation, the content of odd-membered rings in the fullerene-like carbon films decreased, the sp2 carbon converted to sp3. Besides, the total proportion of carbon atoms in the form of oxides did not increase significantly (from 11.3% to 13.0% only), and the hydrogen content in the films did not change significantly due to the existence of the cage-like structure of the fullerene-like carbon films. After atomic oxygen irradiation, it was more difficult to form fullerene-like nanostructures at the interface of the friction surface during the friction process, resulting in the increase of friction coefficient. For the MoS 2 film, the hardness was drastically decreased after atomic oxygen irradiation, causing it to wear out quickly under high contact stress. [ABSTRACT FROM AUTHOR]

In this article, we prepared the fullerene-like hydrogenated carbon film by using the chemical vapor deposition (CVD) technique with pure CH 4 and low ion energy supply (500 V). The film exhibits high hardness (~16.8 GPa), good elasticity recovery (~82%) and ultra-low friction (~0.023) and wear in humid air. Meanwhile, compared with previous deposition methods, this synthesis method offers advantages such as simpler operation, easier controlling, and lower cost. [ABSTRACT FROM AUTHOR]

Friction force microscopy experiments at the nanometer scale are applied to study low friction of hydrogenated fullerene-like carbon films. The measured friction coefficients indicate that lower hydrogen concentration during preparation is beneficial to enter the low friction regime, especially in combination with only methane as precursor. Furthermore, two regions are found with distinct friction coefficients and surface roughnesses related to different surface structures. One is rich in amorphous carbon and the other is rich in fullerene-like carbon, dispersed on the same surface. Transmission electron microscopy and Raman spectroscopy images verify this observation of the two separated structures, especially with the extracted fullerene-like structures in the wear debris from macro friction experiments. It is speculated that hydrogen may tend to impair the growth of fullerene-like carbon and is therefore detrimental for lubricity. [ABSTRACT FROM AUTHOR]

• FL-C:H films can exhibit macroscale ultra-low friction in humid air. • The ultra-low friction mechanisms of FL-C:H films in humid air is discussed. • The tribochemistry is interpreted by structure, counterparts and environmental molecules. Hydrogenated fullerene-like carbon (FL-C:H) films can exhibit macroscale superlubricity, which is expected to make energy and material losses in the mechanical motion systems very small and can be used in various applications. However, the mechanism under ambient conditions and internal structure characterization for FL-C:H films need to be understood in depth. Here, we have discussed the tribochemistry of ultra-low friction FL-C:H films in humid air with the help of our previous studies. The ultra-low friction mechanism in open air can be interpreted in terms of their internal structure features, interfacial electron density from sliding counterparts, oxidation from environmental molecules, and friction induced in-situ formation of graphitic nanoparticles. [ABSTRACT FROM AUTHOR]

Driven by the advent of fullerenes, carbon nanotubes and graphene, designing or tailoring sp 2 -rich carbon clusters in carbon based films attains very interesting properties which can be ideal in various applications. Depositing sp 2 -rich clusters evolve toward a three-dimensional arrangement (composed of extended, bent, and cross-linked graphite basal planes, i.e. fullerene-like carbon (FLC)) or nanocrystalline planar graphitic configurations with the promotion in size and ordering of six-membered ring clusters (i.e. graphite-like carbon (GLC)). However, the films are studied separately and their structure effects on sliding-induced structure changes has never enjoyed elaboration. Here we built the self-mated friction groups of single structural films coated at two sliding surfaces, and designed a smart device to gather the transformed products under different load and sliding cycles. Super-low friction was realized under higher load by low-shear strength from graphene formed at the GLC interface (0.005), or reduced contact area from spherical nanoparticles with outer graphite shells produced at the FLC interface (0.009), resulting by different rehybridization pathways from different film structures. The role of film and rehybridized structures under super-low friction contact was discussed. The results could enrich the understanding of friction-induced rehybridization mechanism and help to deposit suitable films to significantly reduce friction. [ABSTRACT FROM AUTHOR]

Richer fullerene-like (FL) structure interface layers with sp 2 ‑carbon rich are observed in the sliding contacts of ultra-low friction fullerene-like hydrogenated carbon (FL-C:H) films, which can offer a new design possibility to further optimize these films' tribological performances by element doping such as hydrogen, fluorine and nitrogen. In this paper, we provide an understanding about the structure-performance relationship such as the effects of FL structures and incorporated elements such as fluorine and nitrogen. Meanwhile, the detailed structure, mechanical and frictional behaviors and surface properties of the doped films have been investigated. The results indicate that N atoms promote the formation of more cross-linked FL structure, while F atoms terminate sp 2 bond linking state of curved graphitic similar to hydrogen, and thus destroy the FL continuous network structure. Moreover, the N-6 FL-C:H film exhibits higher hardness (~ 18.6 GPa), better elasticity recovery (~ 83%) and a very smooth morphology (rms ~ 0.242 nm), lower friction (0.017) and wear (1.25 × 10 − 9 mm 3 /Nm). [ABSTRACT FROM AUTHOR]

The work presents data on mechanical properties, tribological performances and microstructure changes of hydrogenated amorphous, fullerene-like and graphite-like carbon films (a-C:H, FL-C:H and GL-C:H) after post cryogenic treatment. It is shown that post cryogenic treatment more effectively improves the mechanical and tribological performances of carbon nanostructure films as compared to the a-C:H films. By the combination of Raman and TEM studies, the performance improvement can be attributed to the introduction and enhancement of defects such as pentatomic and heptatomic carbon rings in their carbon structure matrices, which facilitate cross-linking between the graphitic basal planes via sp3-hybridized carbons. The results provide an important reference for achieving solid lubrication under the cryogenic environment that is both reliable and presents longevity. • Cryogenic treatment can affect structure performance of DLC films. • Cryogenic treatment optimizes the films' structure by increasing sp3 fraction. • Cryogenic treatment improves wear-resistant and structure stability of the films. [ABSTRACT FROM AUTHOR]

Abstract Fullerene-like hydrogenated carbon (FL-C:H) films hold superlubricity properties at room condition, which important for saving energy, for engine use, however, high temperature serving (below 500 °C) are needed considering. In this paper, FL-C:H films were annealed with protecting of nitrogen. The tribological test show that all films have superlubricity properties, of which friction coefficient are 0.008 (200 °C), 0.005 (250 °C), 0.004 (300 °C), 0.005 (400 °C), 0.004 (500 °C), respectively. In addition, the hardness is incremental when the annealed temperature increased from 200 to 300 °C, and decremental from 300 °C to 500 °C. Interestingly, the changes of hydrogen content can be neglect in present work. Combined HRTEM, Raman spectra and XPS results, one can speculate that the degree of order for FL-C:H films increase along with the argument of annealed temperature, but competition of growth and ordering of short-chain hydrocarbon and graphene stacks that determine the hardness and H3/E2 ratio, which further influences the wear volumes. However, the friction coefficient only depends on curved grahene which can formation scroll graphene between contact surface to low friction force. Highlights • Though the highest hardness turn out under annealing at 300 °C, the elastic recovery of films is the highest at 500 °C. • The C-C sp2 content raises and the C-C sp3 content reduces with increase of annealing temperature. • All samples, show superlubricity (0.004∼0.005) properties, which is correlated with fullerene-like structure. [ABSTRACT FROM AUTHOR]

High load-bearing and ambient robust superlubricity of graphene/MoS 2 heterojunctions enabled by contact stress dispersion. [Display omitted] • A novel principle for achieving high load-bearing structural superlubricity based on contact stress dispersion is demonstrated. • The contact stress dispersion maintains 2D material integrity and promotes heterointerfacial contact acquirement. • Macroscale structural superlubricity is investigated for the first time at 15N engineering load under ambient air. • A ball-on-disk sliding shearing technique is developed to produce almost less edge- and plane-defect graphene paper. Superlubricity from two dimensional (2D) materials has aroused increasing interest in recent years; however, it can only be achieved below millinewton loads under ambient condition. How to obtain high load-bearing and ambient robust superlubricity remains a challenge but is highly desirable. Here, we use the deformation of highly elastic fullerene-like hydrogenated carbon (FL-C:H) substrate on a large area, to disperse high contact pressure applied on graphene/MoS 2 heterojunctions and avoid structure destruction caused by stress concentration. The contact pressure diffusion ensures the structural integrity of 2D materials and promotes their heterointerfacial contact, resulting in high load-bearing superlubricity. Moreover, the maintenance of structural integrity, combined with the preparation of graphene with few defects by using a vacuum ball-shearing exfoliated graphene (BSEG) technique, remarkably weakens environmental chemical interaction to reach ambient robust superlubricity. The BSEG/MoS 2 /FL-C:H system achieves macroscale superlubricity under vacuum (0.009) and ambient conditions (0.007 at 20% RH, 0.009 at 40% RH) and at engineering load of 15 N. The applied load is far higher than 35 mN that is the known largest values. Atomistic simulations reveal the overall link between contact stress dispersion and experimental observations. This work reveals a novel principle of achieving high load-bearing and ambient robust superlubricity based on contact stress dispersion, and helps to bridge structural superlubricity to practical applications, potentially dramatically reducing energy dissipation. [ABSTRACT FROM AUTHOR]

• The graphite-like and fullerene-like carbon films' pair exhibits superlubricity. • The superlubricity originates from graphene nanoscroll formation at high load. • Their formation leads to an incommensurate sliding contact and a superlow COF of ~0.005. Recently, macroscopic superlubricity has been realized by forming spherical geometries at a sliding interface to cause nanoscopic point contact. Here, we showed macroscopic superlubricity (~0.005) enhanced by graphene nanoscrolls, which were formed at the sliding interface of a tribological pair composed of graphite-like and fullerene-like hydrogenated carbon films. [ABSTRACT FROM AUTHOR]