Ultralow friction regime from the in situ production of a richer fullerene-like nanostructured carbon in sliding contact

Abstract

Hitherto, among carbon-based thin films, fullerene-like hydrogenated carbon (FL-C:H) films exhibit unique ultralow friction and wear in open or humid air, but the mechanisms responsible for the friction regime are still not clear. Here, we provide definitive experimental evidences obtained from the wear tracks and debris of FL-C:H films, and show that FL-C:H films’ surface undergoes gradual transformation into a richer and more stable fullerene-like nanostructure, due to the increasing content of pentagonal and heptagonal carbon rings in addition to the six-membered graphene rings, as a result of thermal and strain effects from the repeated friction. The sliding-induced in situ promotion of the FL structures at frictional contact leads to ultralow friction and wear in open air. The results establish an excellent low friction and wear regime for the structural film, and develop the lubrication mechanisms of carbon-based films.

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1. Introduction

A sustainable society is a long-term target being chased by human beings, which is to some extent dependent on energy efficiency. In machines, a large amount of energy is generally consumed to overcome the friction of moving parts. Novel lubricant materials are desired to reduce the friction of machines, and among them, carbon-based thin films such as diamond-like carbon (DLC) thin films are considered as potential next generation solid lubricants, and it is found that the construction of nanostructures bestow carbon thin films with excellent low friction behaviors. For instance, although hydrogenated diamond-like amorphous carbon (a-C:H) films exhibit excellent friction and wear resistance and extended lifetime in vacuum or inert gas, their frictional properties remain poor in the presence of humidity or oxygen, thereby limiting their further applications since machines are generally used in open air.^1–4 But designing special structures such as fullerene-like (FL) ones^5–9 and composite structures^10,11 can improve their performances. Recently, an important advancement in these areas is the discovery of FL nanostructures in hydrogenated carbon thin films. The shape of the structures is represented by the curved fragments with five- and seven-membered carbon rings in addition to six-membered rings, and it is similar to those found in carbon nanotubes and bucky onions. The FL-C:H film,^8,9,12 with a high hardness (19–26 GPa) and extremely high elastic recovery (75–95%), exhibits ultralow friction (10⁻³) and wear in humid environments, but the mechanism responsible for the ultralow friction behaviors of these films is not well understood.

Hitherto, even if DLC films contain special nanostructures, such as fullerene-like nanostructures,^8,13 their tribological studies have fixed patterns, and their low friction regimes have been attributed to the friction-induced rehybridization of an interfaced amorphous structure as well as to the passivation of surface dangling bonds. Passivation is proposed because friction and wear for DLC films are lower in environments containing H₂, H₂O and glycerol, than in a vacuum and inert gas.^14–16 A similar mechanism has also been reported for the lubrication of ultrananocrystalline diamond by water molecules.¹⁷ However, these molecules are not able to totally passivate dangling bonds, especially in situations where dissociation and removal of surface termination occurs during sliding.^18,19 In other words, sliding inevitably induces the sp³ → sp² rehybridization of carbon. An sp²-rich tribofilm forming on the sliding surface of DLC films has been observed in experiments^18,20–26 and simulations.^27,28 It is believed that the richer sp²-bonded carbon atoms compared to the starting carbon-based materials are grouped into amorphous (rehybridization) or layered graphite-like (graphitization) structures. But the fixed patterns lead to a lack of detail on how FL nanostructures may evolve under friction and how they may affect the ultralow friction and wear behavior of FL-C:H films. For example, are they graphitized or involved in the formation of other lubricious structures?

Previously, FL nanostructures within FL-C:H films have been shown to contribute to ultralow friction and wear in open air.²⁹ However, an in situ formed tribofilm is always present at the sliding interface, which is known to be a key factor to control the frictional properties of the films. This underlines the need to characterize not only the film bulk composition, but also the interfacial structure of the tribofilm, for use in practical tribological applications. Here, we provide definitive experimental evidence, and show that the richer sp²-bonded carbon atoms compared to the starting FL-C:H film evolve towards richer FL structures. We think that the ultralow friction comes from both the inherent FL structure within the films and the newly formed FL structures within the tribofilm, apart from environmental conditions. We expect that the result can enrich the understanding of FL-C:H films’ lubrication mechanism and develop the lubrication mechanism of carbon-based films.

2. Experimental

2.1 Film preparation

FL-C:H films were produced on Si (001) substrates using a direct current plasma chemical vapor deposition system. Prior to the deposition, the deposition chamber was pumped down to 10⁻⁴ Pa followed by introducing a mixed gas of methane and hydrogen in the feedstock. The mixed gas was CH₄ = 10 SCCM, and H₂ = 0, 2.5, 5, 7.5 and 10 SCCM. The distance between the two electrodes was fixed at about 5 cm, with a DC negative voltage of 800 V. The thickness is about 500 nm.

2.2 Characterization of wear tracks and debris

Friction tests were performed on a reciprocating ball on-disc tribotester in open air and at room temperature (25 °C). The Al₂O₃ balls were loaded against the films by the normal force of 20 N generating a contact pressure of about 2000 MPa, the sliding speed was fixed at 60 cm s⁻¹, and the amplitude was 5 mm. The diameter of all balls was 5 mm, and their surface roughness was 0.052 μm. The sliding time was 30 min. Such load and speed conditions in humid air with a relative humidity (RH) of about 20% drive the system into a realistic working state. The HRTEM images and Raman spectra of the wear tracks and debris were revealed by HRTEM (FEI Tecani F30) and a Raman spectrometer (Jobin-Yvon HR-800) with an excitation wavelength of 532 and 632 nm respectively. Due to the debris floating on the surface of deionized water, the TEM samples were prepared by dipping the holey Cu grids into the water and scooping out the debris from the surface. The chemical states of the films were determined using PHI-5702 multifunctional X-ray photoelectron spectroscopy (XPS, operating with Al Kα radiation and a detecting chamber pressure of below 10⁻⁶ Pa). The XRD patterns were collected by powder X-ray diffraction (XRD, D/max2550HB+/PC, 40 kV, 450 mA, with Cu KR (λ = 1.54 Å)).

3. Results and discussion

In this work, FL-C:H films on Si (001) substrates were deposited by a plasma-enhanced chemical vapor deposition technique using CH₄ and H₂ as feedstock.²⁹ An analysis method of their Raman spectra is introduced that permits the amorphous and FL nanostructures of these films to be distinguished.^12,30,31 This method consists of four vibrational bands centred at about 1200, 1380, 1470 and 1560 cm⁻¹, three with A-type symmetry (from five- (5A₁), six- (6A_1g), and seven-membered rings (7A₁)) and the last one with E-type symmetry (from six-membered rings (6E_2g)),^32–34 respectively. These rings are present in fullerenes and FL carbon, and the 1200 cm⁻¹ mode usually has a companion mode at around 1480 cm⁻¹.³⁰ The total relative intensity fraction of 7A₁ and 5A₁ is a measure of FL nanostructures,^29,31,35 as curvature and cross-linking are caused by the introduction of odd membered rings in an otherwise hexagonal carbon structure.^6,36 The films at 5 SCCM have ultralow friction and wear (0.011 and 1.48 × 10⁻⁸ mm³ Nm⁻¹).²⁹ The effect of H₂ flow rate on the structure and frictional behavior is discussed in a manner described elsewhere.²⁹

3.1 Characteristics of the wear tracks of the as-deposited FL-C:H films

As shown in Fig. 1b, a new and low peak near 710 cm⁻¹ is observed in the wear tracks, but seems to be non-existent in the originally deposited surface (OS₅). This peak is previously observed in the Raman spectra acquired from the curvature in graphitic planes and carbon nano-onions.^12,37–39 Due to the presence of the near 1200 cm⁻¹ peak for OS₅ and the central wear tracks of the films, their 532 nm Raman spectra in the region from 1000 to 2000 cm⁻¹ are decomposed into four Gaussian peaks at about 1230, 1365, 1470 and 1560 cm⁻¹, respectively corresponding to 7A₁, 6A_1g, 5A₁ and 6E_2g, as introduced in the method above. The total relative intensity fraction of 7A₁ and 5A₁ is a measure of FL nanostructures, and thus the odd carbon ring fraction in the samples is summarized in Fig. 2. After sliding friction, the odd ring fraction (62%) in the wear tracks is higher than that (52%) of OS₅, indicating that the structural changes may be the promotion of their own FL nanostructures during sliding. Moreover, each of the FL-C:H films prepared at 0, 2.5, 5, 7.5 and 10 SCCM exhibit lower friction, and the related wear track has a higher odd ring fraction than its originally deposited surface. Therefore, the Raman results from the wear tracks may infer that an ultralow friction coefficient measured for the films may simply be a consequence of the creation of FL nanostructures during sliding.

Fig. 1 532 nm Raman spectra from the original surface (OS₅) and central wear tracks of FL-C:H films grown at a H₂ flow rate of 5 SCCM (a). The Raman spectrum in the region from 1000 to 2000 cm⁻¹ is deconvoluted into four peaks at about 1230, 1365, 1470, and 1560 cm⁻¹, respectively. (b) Magnified wave number region from 200 to 1000 cm⁻¹ in the Raman spectrum of (a).

Fig. 2 Friction coefficient (FC) and odd ring fraction of the FL-C:H films as a function of the gas flow rate of H₂. All the central wear tracks (WK) have a higher odd (pentagonal and heptagonal) carbon ring fraction than that of the originally deposited surfaces (OS).

3.2 Characteristics of the wear debris of the as-deposited FL-C:H films

At the beginning of sliding tests, wear materials containing FL nanostructures mainly deposited in front of the sliding ball and later covered the main part of the contact area,^40,41 resulting in the formation of a tribolayer. After ultimate sliding, the tribolayer still adhered to the ball surface. When the ball surface with the tribolayer slid on a new slip way of the films, the tribolayer was detached from the counterface as a result of plough of protruding regions of the new asperity contacts, or extruded out of the contact interface during sliding, and thus wear debris was formed (Fig. 3a). Among sliding, the tribolayer was subject to friction heat and shear strain effects. If possible, the films’ tribo-chemical and structural changes may be triggered, and the extent of reaction or rehybridization can vary with the subjected time. In other words, wear debris around wear track is an important representation of the films’ interfacial structure changes. All friction experimental results were repeatedly performed more than three times, and the mean friction coefficient was 0.011.

Fig. 3 XPS spectra of the FL-C:H film grown at 5 SCCM (a and b).

In order to gain more insight into the interfacial structure changes of the FL-C:H films, XPS analysis has been performed. The C 1s spectra of the films deposited at 5 SCCM, shown in Fig. 3a and b, can be deconvoluted into three bands at around 284.3, 285.3, and 286.1 eV. The bands are respectively assigned to sp² bonds in graphite, sp³ bonds in diamond and C–O bonds formed on the film surface due to the free σ-bonds passivated by adsorbates such as water and oxygen molecules in open air.^42,43 The relative content of each bond is listed in Table 1. As a result of cyclic sliding, the sp² fraction obviously rises from 65.0% in the original surface to 80.4% in the wear debris, and the sp³ fraction decreases from 27.2% to 10.7%, but the C–O bonds’ fraction slightly increases from 7.8% to 8.9%. The low oxidation during the friction process in humid air is consistent with the studies in which FL-C:H films have less sensitivity to H₂O and O₂ molecules in air.^8,29,41,44 FL-C:H films are known to have high hardness and elastic recovery originating from their unique FL nanostructures. Thus, under friction, the structures with rich sp² bonds can stably exist rather than be broken at a longer time after the beginning of sliding, due to elastic deformation by bond angle deflection.⁴⁴ This indicates that during sliding, the sp³-hybridized bonds are preferentially broken and primarily converted to sp²-hybridized bonds but not free σ-bonds within the film tribo-surface, shown as follows:

C(sp³ bond) → primary C(sp² bond) (1)

Table 1 The content of the three chemical bonds determined as the ratio of the corresponding peak area over the total C 1s peak area

	FL-C:H film
	Original surface (OS5)	OS5 + wear debris
sp^2 content (%)	65.0	80.4
sp^3 content (%)	27.2	10.7
C–O content (%)	7.8	8.9
C 1s position (eV)	284.47	284.36
Friction	0.011

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What is a primary C(sp² bond)? Where is it formed? Is it formed at the tribolayers’ exposed surface, or within the tribolayers? Is it grouped into amorphous (rehybridization) or layered graphite-like (graphitization) structures mentioned in the introduction? X-ray diffraction (XRD) analysis has been performed at the original surface (OS₅) and wear debris and tracks of the FL-C:H films, to obtain further information about their chemical composition and structure (Fig. 4). The film pattern shows three peaks at about 2θ = 69.1°, 33° and 22.4°. The two peaks at 2θ = 69.1° and 33° are from the silicon substrate, i.e., Si (004) and Si (002), respectively. The weak peak at 2θ = 22.4° is consistent with these studies^12,29 which revealed the presence of fullerene-like or onion-like nanoparticles using HRTEM and Raman peaks near 400, 710 and 1200 cm⁻¹. Provided that this peak corresponds to the (002) reflection, it shows a significantly downward shift compared to the normal graphite at 2θ = 26.5°. It decreases with the increase of the curvature of the graphene layers,⁴⁵ as is expected for highly curved sheets with many non-six-membered rings. After sliding, the peak about 2θ = 22.4° becomes obvious, while a new broad peak around 2θ = 15° shows up. The peaks stem from the creation of pentagonal and heptagonal carbon rings with partial enrichment of the FL structures in the wear debris and tracks, because the characteristic peaks of 3D sp²-coordinated carbon materials, such as C₆₀ or C₇₀ including odd carbon rings, are mainly distributed in the range below 30°.^46,47 In fact, diffraction methods do not provide unequivocal evidence that pentagons and heptagons are present. Nevertheless, in the range below 25°, fullerenes, nanotubes and carbon onions containing these odd rings have characteristic diffraction peaks but graphite has no such features. Therefore, it may be inferred that under friction, a C(sp² bond) evolves toward crystalline order, analogous to a fullerene structure, though such order is poorly crystallized. The tribo-chemical reaction is shown as follows:

C(sp³ bond) → primary C(sp² bond) → pentagons and heptagons (2)

Fig. 4 Experimental XRD patterns from the films (OS₅) and wear debris showing that the debris has a structure analogous to that of C₆₀, far from that of nc-graphite.

Raman spectroscopy is an effective way to distinguish fullerene-like nanostructures within hydrogenated carbon films, due to its sensitivity to pentagonal and heptagonal rings.^29,31,35 In this paper, we have tried to superimpose the Raman signal of the debris on the films’ original surface signal. Fig. 5a shows the wear debris accumulated at the surrounding area of the wear tracks. The debris volume increases from D₅1 to D₅4 in sequence. When the OS₅ and wear debris at D₅1 are observed (in Fig. 5b), both the spectra exhibit similar profiles, due to the fact that there is too little D₅1 debris to influence the Raman signal of OS₅. However, as the debris at D₅2 and above is detected, the Raman shapes are quite different from that of OS₀, suggesting a structural transition. From Fig. 5d, the G peak dramatically shifts from 1527 at D₅1 to 1556 cm⁻¹ at D₅4, and the I_D/I_G shifts towards a higher value in order from 1.61 at D₅1 to 2.65 at D₅4. These are indicative of the conversion of sp³- to sp²-bonded carbon during sliding, consistent with the XPS result above.

Fig. 5 532 nm Raman analysis of the original surface (OS₅) and wear debris of the FL-C:H films grown at a H₂ flow rate of 5 SCCM. (a) Olympus microscope image of OS₅ and wear debris. (b) Raman spectra from OS₅ and wear debris at D₅1, D₅2, D₅3, and D₅4 of (a). (c) Raman spectrum of D₅4 debris. The Raman spectrum in the region from 1000 to 2000 cm⁻¹ is deconvoluted into four peaks at about 1230, 1365, 1470, and 1560 cm⁻¹, respectively. (d) Odd ring fraction, G peak position and I_D/I_G as a function of OS₅ and wear debris at D₅1, D₅2, D₅3, and D₅4.

These are usually not final results, however, some interesting potential data have not been exposed. For example, apart from those observed peaks (710, 850 and 1200 cm⁻¹) and a new peak at around 520 cm⁻¹ from the silicon substrate,³⁰ two additional peaks near 400 and 1050 cm⁻¹ appear in all the Raman spectra from the wear debris of the FL-C:H films deposited at 5 SCCM (Fig. 5b), and are more obviously observed in Fig. 5c. A similar band also appears in the Raman spectra of nanoparticle carbon nitride films³⁷ and carbon nano-onions.³⁸ Recently, we investigated in detail the Raman spectra of fullerene-like carbon nitride (FL-CN_x)³⁰ and FL-C:H films¹² and confirmed that the peak at about 400 cm⁻¹ is attributed to FL or onion-like nanostructures. Interestingly, with extended analysis of the Raman spectra from the wear debris, the odd ring (7A₁ and 5A₁) fraction increases from 52% at OS₅ to 70% at D₅4. Combined with the higher odd ring fraction in the central wear tracks than in the corresponding original surfaces of the FL-C:H films grown at 0, 2.5, 5, 7.5 and 10 SCCM (Fig. 2), it is inferred that the sp³-bonded carbon atoms in the films undergo tribo-chemical reactions whereby they may not simply form sp²-bonded carbons but probably form pentagonal and heptagonal carbon rings across the interface, leading to the promotion of the FL nanostructures and the decrease of the friction.

Fig. 6 shows 632 nm Raman spectra taken from the wear debris particles present on the 5 SCCM FL-C:H films. The spectra show changes in shape and intensity similar to the 532 nm spectra. Some additional peaks at about 490, 710, 860, 1050, 1200 and 1360 cm⁻¹ can be distinguished as broad shoulders or as local maxima, especially for D4 and D5. The spectra in the range of 1000–2000 cm⁻¹ were evaluated by a Gaussian fit with four Gaussian peaks at about 1230, 1365, 1470, and 1560 cm⁻¹. These peaks from pentagons (5A₁) and heptagons (7A₁) are more obvious and intense from Fig. 6a–d, indicating the formation of richer FL structures with a higher content of pentagons and heptagons compared to the starting film.

Fig. 6 Experimental 632 nm Raman spectra of the film and wear debris from 5 SCCM. Raman spectra taken at different places of the debris show changes in shape and intensity. Some additional peaks can be distinguished as broad shoulders or as local maxima, especially for D4 and D5. The changes were evaluated by a Gaussian fit (the spectra can be fitted very well by four Gaussian curves centered at about 1230, 1365, 1470, and 1560 cm⁻¹ from 5A₁, 6A_1g, 7A₁ and 6E_2g (G)) (a–d). The peaks from pentagons (5A₁) and heptagons (7A₁) are more obvious and intense, and their fractions show a significant increase of the relative intensity of the 7A₁ and 5A₁ bands from (c) and (d).

In addition, the tribo-chemical reaction of C(sp³-bond) → pentagons and heptagons (eqn (2)) can be further confirmed by the 532 nm Raman analysis of the films and wear debris at 0 SCCM (Fig. 7a–d). The films have the lowest FL content (Fig. 2). From Fig. 7d, the G peak dramatically shifts from 1525 at D₀1 to 1547 cm⁻¹ at D₀4, and I_D/I_G shifts towards a higher value in the order from 1.57 at D₀1 to 2.38 at D₀4. These are indicative for the conversion of sp³- to sp²-bonded carbon during sliding. From extended analysis of the Raman spectra from the wear debris, the odd ring (7A₁ and 5A₁) fraction increases from 40% at OS₀ to 67% at D₀4 (Fig. 7d), and additional peaks near 400 and 1050 cm⁻¹ compared to the starting film are observed.

Fig. 7 532 nm Raman analysis of the original surface (OS₀) and wear debris of the FL-C:H films grown without H₂. (a) Olympus microscope image of OS₀ and wear debris. (b) Raman spectra from OS₀ and wear debris at D₀1, D₀2, D₀3, and D₀4 of (a). (c) Raman spectrum of D₀4 debris. The Raman spectrum in the region from 1000 to 2000 cm⁻¹ is deconvoluted into four peaks at about 1230, 1365, 1470, and 1560 cm⁻¹, respectively. (d) Odd ring fraction, G peak position and I_D/I_G as a function of OS₀ and wear debris at D₀1, D₀2, D₀3, and D₀4.

HRTEM has been performed to confirm the interfacial structure changes of the FL-C:H films at 5 SCCM. Fig. 8a shows the film wear debris around the track, and the superposition and agglomeration of the wear debris due to the rubbing process can be observed by comparing the sets in Fig. 8b and c. Fig. 8b shows that the FL nanostructures are different from both graphite and turbostratic carbon. The basal planes in the structure were covalently curved and cross-linked by tetrahedral sp³ bonds which have a much shorter bond length than van der Waals bonds.^5,48 The planes are therefore unlike the normally flat sp²-hybridized planes of graphite. From Fig. 8c, the restacking of disordered FL-C clusters in thicker regions confuses the image and prevents clear observation of the curved sheets. Nevertheless, an obviously increasing number of curved sheets are observed in the direction of the arrow (Fig. 8c), and in thinner areas (Fig. 8d). From Fig. 8d, we can also see open spherical particles much like carbon onion nanoparticles. The broken curved lines of the particles may mean that it has remaining amorphous carbon and lacks the lamellar structure of well-crystallized onions.

Fig. 8 Experimental HRTEM images showing changes of the a-C film before or after friction. (a) The red circles show wear debris and tracks on the film. (b) The red circle indicates a few sp²-hybridized curved planes (local FL-C structures) embedded in an amorphous matrix of the film. (c) The red arrow indicates that the wear debris has richer FL-C than the film, and has partial and poorly crystallized fullerene-like or onion-like nanoparticles (d).

Given the above, we reveal that friction inevitably induces interfacial structure changes of FL-C:H films, and with the continuous sliding of the film against Al₂O₃ under a contact pressure of about 2000 MPa and a sliding speed at 60 cm s⁻¹, there are an increasing number of sp²-bonded carbon atoms compared to at the starting film grouped into FL nanostructures with richer pentagons and heptagons, as shown by eqn (3) and in Fig. 9. The newly formed FL structures within the tribofilm, with the film’s inherent FL structure, contribute to the ultralow friction and wear (0.011 and 1.48 × 10⁻⁸ mm³ Nm⁻¹).²⁹ We expect that the results can enrich the understanding of FL-C:H films’ lubrication mechanism and develop the lubrication mechanism of carbon-based films.

C(sp³ bond) → primary C(sp² bond) → pentagons and heptagons → FL structures (3)

Fig. 9 Schematic diagrams showing the products of a richer fullerene-like hybrid carbon, and the mechanism responsible for the ultralow friction of FL-C:H films. Pentagons and heptagons (red) are embedded in amorphous (silver-gray) and fullerene-like (yellow) matrices.

3.3 The friction-induced structural evolution analysis of the as-deposited FL-C:H films

We report on the structural changes within the FL-C:H films as a result of cyclic annealing and strain relaxation during sliding. Simulations⁴⁹ have proven highly successful in understanding the formation mechanism of FL structured carbon, whereby a variety of precursors including amorphous carbon films and even nanodiamonds form large, well-aligned and closed structures if a sufficient annealing temperature and annealing time are provided. Experimentally, modest thermal annealing (from 300 °C) of amorphous carbon films containing considerable fractions of sp² carbon has resulted in the formation of carbon nanocomposites with the evolution of five- and seven-membered rings.³⁴ Moreover, FL-C:H films are shown to be annealed at 200 and 300 °C in a vacuum, the structures of the films will be reorganized with the creation of odd rings, and the films will be endowed with excellent tribological properties.³⁵ In this paper, the flash temperature at the sliding contact area is tentatively estimated as 200–1250 °C during the first cycle, according to the literature.^50–52 Repeated sliding friction is expected to further raise the temperature, probably up to the point where nanostructured sp³ clusters transform to curved sp² clusters, i.e. FL structures.

Besides, frictional shear inevitably causes dissociation and removal of surface termination (for example, hydrogen removal),^16,18,19 and high local stress during initial sliding.²⁷ This drives the formation of dangling σ-bonds or radicals from the cleavage of C–C or C–H bonds on the top layer of the films,⁵³ and the atomic migration or rearrangements at the friction interface region.²⁷ In the presence of water, these radicals chemisorb water molecules to form hydroperoxide groups (–COOH) by a series of tribo-chemical interactions.⁵³ Meanwhile, the C=C bonds at the edge of the FL clusters react with the O and OH radicals (atmospheric), and thus transform into –C–OH groups. The friction load brings the –COOH and –C–OH groups closer, leading to the trigger of reactions as ambient solid-state mechano-chemical reactions between functionalized carbon nanotubes⁵⁴ and the formation of six-membered graphene rings at the edge of FL clusters. So, the graphite-like sheets at the edge of the clusters are extended. The highly strained environment within the tribolayer will force curvature in large extended graphite-like sheets, and thus pentagons and heptagons are grouped into the sheets, leading to structure relaxation of the interfacial C–C network, analogous to their stress model in the films;³¹ at this stage, these rings may favor the passivation of edge dangling bonds in the sheets,^55,56 reducing adhesive interaction between two sliding surfaces. Additionally, along with repeated sliding, an increasing sp² content is directly related to a lower density, which further gives the tribolayer lower compressive stress surroundings. As a result, cross-linking among the planes and distortion of the graphite-like sheets by odd rings is produced gradually, and finally a richer FL structural carbon network is formed. When this network resides in the midst of the interfacial region, it forms a lubricating layer, which leads to low friction and small wear in open air.

4. Conclusions

In summary, we provide definitive experimental evidence obtained from the wear tracks and debris of FL-C:H films, and show that a richer FL nanostructure tribolayer compared to the starting film residing in the midst of an interfacial region leads to ultralow friction and wear in open air. The cyclic annealing and strain relaxation induced by sliding is attributed to an increase in sp² bonding and the pentagonal and heptagonal carbon ring fraction, leading to the promotion of a 3D fullerene-like nanostructure within the film surface. The results can enrich the understanding of the lubrication mechanism of FL-C:H films.

Acknowledgements

Y. W. and J. Z. designed the study and wrote the paper. Y. W. prepared the FL-C:H films and analyzed the resulting data. Y. W. and J. G. collected the data. All authors commented on the manuscript and discussed the results. This work was supported by the Major State Basic Research Development Program of China (973 Program) (No 2013CB632304).

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