**2. Structural characterizations**

As one known, carbon-based curve structure is variety of things, such as carbon nanotubes, fullerenes, carbon nitride nanotubes and so on [35, 36]. FL-C:H films and its structures are very similar to fullerene-like carbon nitride (FL-CNx) films [37], within distorted multi-storey multilayer graphene intersecting and interlocked in an amorphous carbon matrix, show high hardness, high elastic recovery and high resistance to deformation. A typical high resolution transmission electron images (HRTEM) of FL-C:H films, grown via high impulse power (which is usually employed in magnetron sputtering, mentioned as high power impulse magnetron sputtering (HiPIMS) [26]) assistant PECVD, displays in **Figure 1a**. Atomic force acoustic microscope (AFM) imaging (**Figure 1b**) under tapping mode was used to inspect the cluster domains in the films and indicate that the film has a special nanocomposite structure with graphene domains dispersed in an amorphous carbon matrix [26].

materials. Nowadays, superlubricity is a most fascinating word not only in mind but also in practice [1–5]. Designing promising mechanical systems with ultra-low friction performance and establishing superlubricity regime is imperative not only to the most greatly save energy

Motivated by these issues, superlubricity has aroused extensive attention of many research groups and therefore an active topic in many fields [6–11]. Not surprisingly, superlubricity has been realized in some experiments associated with layered materials such as graphene, molyb-

nanotubes (MWCNT) [6, 12–15]. At the macroscale, hydrogenated diamond-like carbon (DLC) film with a supersmooth and fully hydrogen-terminated surface is the most promising material to realize superlubricity [4, 16]. However, the exact superlubricity of DLC film can only be observed under high vacuum or specific conditions and is not realized under ambient conditions for engineering applications [17]. The latest breakthrough in macroscale superlubricity is made by introducing fullerene-like nano-structure and designing graphene nanoscroll formation, which also demonstrates the structure-superlubricity (coefficient of friction ~0.002) relationship [5, 18]. In general, lubricating materials satisfying engineering application face some problems such as macro contact area (≥ mm × mm), withstanding high contact pressure (≥1 GPa) and expo-

realizing macroscale superlubricity [19–24]. But, luckily, Zhang et al. fabricated fullerene-like hydrogenated carbon (FL-C:H) films and achieved superlubricity under engineering conditions [5, 18, 24–28]. The curvature structure of fullerene-like structure extends the strength of graphite plane hexagon into three dimension space network, in turn, increasing the hardness and elasticity of carbon films, demonstrating super low friction in air, meaning solid superlubricity with engineering application value [18, 26]. Most interestingly, the fullerene-like structure of the carbon films could be adjusted via the hydrogen content, bias supply, in the deposition gas sources [29–33]. Thus, it is very interesting to design macroscale superlubricity

Usually, both plasma-enhanced chemical vapor deposition (PECVD) and reactive magnetron sputtering can be employed to growth FL-C:H films which one can find in early reports [5, 27]. When using PECVD, the pulse power and the reasonable atmosphere are necessary. FL-C:H films are growing in miexed atmosphere of CH₄ and H₂ with flow ratio at 1:2, but the deposition pressure can adjust from 10 to 20 Pa depending on different chambers [5, 34]. But for reactive magnetron sputtering, distinction from PECVD are that the additional magnetic field and

 anymore [21, 27]. The most important thing when using reactive magnetron sputtering is that a deep poisoning mode is employed [27, 28]. So, as we see, deep poisoning reactive magnetron sputtering can also be mentioned as magnetic field assistant PECVD. Such differ-

As one known, carbon-based curve structure is variety of things, such as carbon nanotubes, fullerenes, carbon nitride nanotubes and so on [35, 36]. FL-C:H films and its structures are

by prompting the in situ formation of these structures at the friction interfaces.

ence might influence the growth mechanisms which we will discuss later.

), highly oriented pyrolytic graphite (HOPG) and multi-walled carbon

, etc.), which are more prominent problems for designing and

but also to reduce hazardous waste emissions.

96 Fullerenes and Relative Materials - Properties and Applications

O, O<sup>2</sup>

denum disulphide (MoS<sup>2</sup>

sure to air environment (H<sup>2</sup>

**2. Structural characterizations**

no H<sup>2</sup>

These special nanostructures can also be inspected by Raman spectrum. Usually, FL-C:H films exhibit typical character of diamond-like carbon films in the region of 1000–2000 cm−1, but with some additional peaks at about 400 and 700 cm−1 and a distinct shoulder at around 1230 cm−1 **(Figure 2a)** [34]. The two low intermediate wave number bands near 400 cm−1 and 700 cm−1 are very similar to that of fullerene-like carbon nitride that can be assigned to relaxation of Raman selection rule due to the curvature in graphene planes, which appears to induce Raman scattering away from the G point [38]. The same bands are also present in the Raman spectra acquired from carbon onions and C60 that have been attributed to the transverse optic and transverse acoustic vibrations at the M point [38]. Thus, an acceptable fitting could only be reached via four Gaussian peaks at about 1230, 1350, 1470, and 1560 cm−1, respectively **(Figure 2b)** [9, 27, 28, 34]. Thus, we can consider that all peaks at around 400, 700, 1230 and 1480 cm−1 are all active from that of fullerene-like structure with curled graphene planes.

Thanks to the adjustable conditions and methods, the nanostructures of FL-C:H films can be tailored as one expecting. In fact, either a conventional PECVD or a magnetic field assistant

**Figure 1.** (a) High resolution transmission electron images of the FL-C:H films. Insets: carbon fingerprint and human fingerprint pattern. (b) An AFM phase image of the FL-C:H films deposited on silicon wafers. (Reproduced from Ref. [26] with permission from the Royal Society of Chemistry).

**Figure 2.** Raman spectra of hydrogenated carbon film deposited by dc-pulse plasma CVD. (a) Raman spectrum and magnified wave number region from 0 to 1000 cm−1 and (b) deconvoluted wave number region from 1000 to 2000 cm−1 in the Raman spectrum of (a).

PECVD (reactive magnetron sputtering), an important thing is that bias voltage has a main effects on nanostructures. Though the outfield auxiliary in reactive magnetron sputtering lowers the growth pressure directly, both high voltage and low duty cycle are crucial. During reactive magnetron sputtering process [21], the prominent FL-C:H films grown at −800 V bias exhibit the highest hardness of 20.9 GPa as well as elastic recovery of about 85%. The further increase of bias voltage cripples the mechanical due to further graphitization of the films. But very different nanostructure can be observed under low bias of −100 V, which is called amorphous carbon films dispersed with multilayer graphenes. It is very different from the films grown via PECVD that one can seen that with increasing the bias voltage, the hardness decreases while the elastic recovery keeps increasing [39]. The probable reason is that, at low pressure with the outfield auxiliary in reactive magnetron sputtering process, ions have higher free energy than that in PECVD which might induce easier graphitization.

On the other hand, the pulse duty cycles determine the local relaxation of the distorted chemical bonds. According to the typical subplantation model suggested by Robertson [40], three scales during ion interaction with the film are addressed that: the cascade, 10−14 s; the thermal spike, 10−12 s; and the longer time relaxation, ~10−9 s. The longer time relaxation time benefits the stress releasing and hydrogen removing which contributes to restructuring of carbon matrix. In our work, the annealing time is almost ~10−5 s, far than ~10−9 s, thus, the depositing pulse film has a completely surface, thereby restricting the formation of pentagonal rings which are associated with the ion bombardment at the pulse-on/off state [41]. As shown in **Figure 3**, one can see that with the pulse-on time increases while pulse-off time decrease, more curved graphene structures are arisen. These variations are also confirmed by Raman spectra, showing in **Figure 4**. With decrease in duty cycles, a shoulder at around 1230 cm−1 comes more obviously, which is believed from that of fullerene-like structure with curled graphene planes, in accordance well with HRTEM results. Here, Raman spectra of these films are simulated using four vibrational bands at 1260, 1380, 1470, and 1570 cm−1 [9, 27, 29, 34], respectively (**Figure 4**), these with A-type symmetry (from five-, six-, and seven-membered

rings) and one with E-type symmetry (from six-membered rings). The results show that even member rings and odd member rings have an opposite trend, and high odd member rings

**Figure 4.** (a) Raman spectra of the as-prepared films at different pulse bias duty cycle. (b) Contribution to the carbon Raman band from the vibrations of five-, six- and seven-member rings versus the pulse duty cycle. (Reproduced from

**Figure 3.** HRTEM images of the as-prepared films with pulse duty cycles of (a) 100%, (b) 80%, (c) 60%, (d) 40%, and (e)

Super-Lubricious, Fullerene-like, Hydrogenated Carbon Films

http://dx.doi.org/10.5772/intechopen.70412

99

20%. (Reproduced from Ref. [42] with permission from the Royal Society of Chemistry).

indicate the decrease and more fullerene-like structure.

Ref. [42] with permission from the Royal Society of Chemistry).

**Figure 3.** HRTEM images of the as-prepared films with pulse duty cycles of (a) 100%, (b) 80%, (c) 60%, (d) 40%, and (e) 20%. (Reproduced from Ref. [42] with permission from the Royal Society of Chemistry).

PECVD (reactive magnetron sputtering), an important thing is that bias voltage has a main effects on nanostructures. Though the outfield auxiliary in reactive magnetron sputtering lowers the growth pressure directly, both high voltage and low duty cycle are crucial. During reactive magnetron sputtering process [21], the prominent FL-C:H films grown at −800 V bias exhibit the highest hardness of 20.9 GPa as well as elastic recovery of about 85%. The further increase of bias voltage cripples the mechanical due to further graphitization of the films. But very different nanostructure can be observed under low bias of −100 V, which is called amorphous carbon films dispersed with multilayer graphenes. It is very different from the films grown via PECVD that one can seen that with increasing the bias voltage, the hardness decreases while the elastic recovery keeps increasing [39]. The probable reason is that, at low pressure with the outfield auxiliary in reactive magnetron sputtering process, ions have higher free energy than that in PECVD which might induce

**Figure 2.** Raman spectra of hydrogenated carbon film deposited by dc-pulse plasma CVD. (a) Raman spectrum and magnified wave number region from 0 to 1000 cm−1 and (b) deconvoluted wave number region from 1000 to 2000 cm−1

On the other hand, the pulse duty cycles determine the local relaxation of the distorted chemical bonds. According to the typical subplantation model suggested by Robertson [40], three scales during ion interaction with the film are addressed that: the cascade, 10−14 s; the thermal spike, 10−12 s; and the longer time relaxation, ~10−9 s. The longer time relaxation time benefits the stress releasing and hydrogen removing which contributes to restructuring of carbon matrix. In our work, the annealing time is almost ~10−5 s, far than ~10−9 s, thus, the depositing pulse film has a completely surface, thereby restricting the formation of pentagonal rings which are associated with the ion bombardment at the pulse-on/off state [41]. As shown in **Figure 3**, one can see that with the pulse-on time increases while pulse-off time decrease, more curved graphene structures are arisen. These variations are also confirmed by Raman spectra, showing in **Figure 4**. With decrease in duty cycles, a shoulder at around 1230 cm−1 comes more obviously, which is believed from that of fullerene-like structure with curled graphene planes, in accordance well with HRTEM results. Here, Raman spectra of these films are simulated using four vibrational bands at 1260, 1380, 1470, and 1570 cm−1 [9, 27, 29, 34], respectively (**Figure 4**), these with A-type symmetry (from five-, six-, and seven-membered

easier graphitization.

in the Raman spectrum of (a).

98 Fullerenes and Relative Materials - Properties and Applications

**Figure 4.** (a) Raman spectra of the as-prepared films at different pulse bias duty cycle. (b) Contribution to the carbon Raman band from the vibrations of five-, six- and seven-member rings versus the pulse duty cycle. (Reproduced from Ref. [42] with permission from the Royal Society of Chemistry).

rings) and one with E-type symmetry (from six-membered rings). The results show that even member rings and odd member rings have an opposite trend, and high odd member rings indicate the decrease and more fullerene-like structure.

Atmosphere influence on the nanostructures of FL-C:H films have been studied widely. Heterogeneous gases, such as H<sup>2</sup> and CF4 , introducing in growth process have different effects [29–33]. Interestingly, the growth of FL-C:H films show a conflicting to Hellgren's work [43] that intentional hydrogen addition to the discharge will terminate potential bonding sites for CNx precursors and hinder the growth of fullerene-like structures. But it seem that during growth FL-C:H films, hydrogen atoms during the deposition process may affect the production of odd rings by two competing ways: (1) stress induced by H+ leads to the introduction of odd ring into flat graphene plane; (2) H<sup>+</sup> preferentially etches the plane's sp2 phase and destroys the bond basis if forming odd rings. But more H<sup>2</sup> existing in growth condition has no much help on growing fullerene-like structures, so the effects of hydrogen need to be studied in detail. However, CF4 show different influence on the nanostructures of the FL-C:H films. At low fluorine content, many C sites bond to neighboring C and the films microstructure displays lots of well organized graphite-like and fullerene-like fragments. But as the amount of F incorporated in the network increase, F-terminated large rings, Branches and chains with sp<sup>2</sup> sites densify and start to interact with each other and features like interlocking pore and amorphousness strongly prevail in the nanostructure [30, 32].
