**1. Introduction**

People have a love and hate relationship with friction because we need high friction some times (like braking) but we expect the friction to be as small as possible in the machines which might help to save energy. The energy issue pushes us to develop more robust lubricious

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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 but also to reduce hazardous waste emissions.

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

Super-Lubricious, Fullerene-like, Hydrogenated Carbon Films

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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

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).

with graphene domains dispersed in an amorphous carbon matrix [26].

planes.

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, molybdenum disulphide (MoS<sup>2</sup> ), highly oriented pyrolytic graphite (HOPG) and multi-walled carbon 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 exposure to air environment (H<sup>2</sup> O, O<sup>2</sup> , etc.), which are more prominent problems for designing and 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 by prompting the in situ formation of these structures at the friction interfaces.

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 no H<sup>2</sup> 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 difference might influence the growth mechanisms which we will discuss later.
