Friction is one of the decisive factors affecting the efficiency and service life of a mechanical system. Statistically, 23% of global energy consumption is due to the friction-related activities, of which 40% can be prevented through application of advanced surface, material and lubrication technologies (Holmberg and Erdemir, 2017). With the increasing demand on higher efficiency, power and life of mechanical systems, studies on reducing friction and wear are becoming increasingly important both from the economic and environmental perspectives. The discovery of superlubricity provides a possibility to reduce tribological consumption to an extremely low level, and has triggered increasingly extensive attention from researchers around the world over the past three decades.

The concept of superlubricity was firstly brought out by Hirano in a theoretical prediction based on calculations that the friction force could reach zero level when two crystal surfaces sliding in an incommensurable condition (Hirano and Shinjo, 1990). And it was soon verified by nanoscale friction test on cleaved mica surfaces with different contact angles (Hirano et al., 1991). Now this ideal friction vanishing state is generally redefined as structural superlubricity, and the concept of superlubricity is generally accepted as a sliding state with a kinetic fiction coefficient bellow 0.01 (Erdemir and Eryilmaz, 2007). Around 2000s, with the rapid increase of research investment, more superlubricity phenomena have been revealed, which can be generally divided into solid and liquid superlubricity, respectively. In nanoscale or microscale, superlubricity is achievable via a variety of solid materials such as layered materials like graphite, graphene (Dienwiebel et al., 2004), BN (Song et al., 2018), and MoS₂ (Martin et al., 1993), atomically smoothed crystals including covalent crystals like Si (001) (Hirano et al., 1997), ionic crystals (Socoliuc et al., 2004) like NaCl, metallic crystals like Ag (Goto and Honda, 2004). To date, various carbon-based materials have been found to be capable of achieving superlubricity (Chen and Li, 2020), for instance, C₆₀ intercalated graphite films (Miura et al., 2005), carbon nanotubes (Zhang et al., 2013), graphite or graphene, ultra-nanocrystalline diamond (Kumar et al., 2011), onion-like/fullerene-like carbon (Gong et al., 2017), and diamond-like carbon (Erdemir and Eryilmaz, 2014). Nevertheless, most of these superlubricity behaviors are based on the incommensurable contact of the ultra-smooth crystalline surfaces, which is still far from being applicable for engineering due to the diversified interferences in macroscopic tribo-systems.

Diamond-like carbon (DLC) film is a class of carbon-based amorphous coating, which exists in the form of a disordered covalent network of sp¹, sp², and sp³ hybridized carbon atoms, and has the ability to introduce other elements such as hydrogen into the film. According to the differences in structure and doped materials, DLC can be generally divided into amorphous carbon (a-C), hydrogenated amorphous carbon (a-C:H), tetrahedral amorphous carbon (ta-C), and hydrogenated tetrahedral amorphous carbon (ta-C:H) (Robertson, 2002). The synthesis methods of DLC films can be divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD). With the development of these technologies, DLC coatings can be deposited flexibly on a variety of materials such as silicon, ceramic, glass, metal, and rubber. Meanwhile, by changing the deposition conditions, their structure and composition are adjustable to obtain superior mechanical and tribological properties for different application conditions.

DLC was first synthesized around the 1950s. Systematic research on DLC started in the 1970s (Aisenberg and Chabot, 1971). Later in 1981, Enke reported the superlubricity behavior of DLC films with a friction coefficient of 0.005 in ultrahigh vacuum (Enke, 1981). In the 1990s, superlubricious capacity of a-C:H in vacuum was systematically studied by Donnet et al. (1994) and Donnet and Grill (1997). In 2000, an ultralow friction coefficient of 0.002 was achieved with self-mated a-C:H tested in nitrogen and argon atmosphere (Erdemir et al., 2000a), and an ultra-long superlubricity lifetime of 32 days was achieved (Erdemir et al., 2000b), as shown in Figure 1A. Recently, an extremely low friction coefficient of 0.0001 under heavy load was achieved with a-C:H in hydrogen atmosphere (Nosaka et al., 2017), as shown in Figure 1B. These results indicate that a-C:H films are capable of achieving stable superlubricity under dry inert atmosphere, vacuum and hydrogen, and the underling mechanism is closely related to friction-induced structural transformation of the contact area and tribo-chemical interaction between the sliding surfaces. Further researches have shown that introducing silicon (Chen et al., 2014, 2017) or sulfur (Freyman et al., 2006) into a-C:H films can significantly reduce its sensitivity to oxygen or water vapor, leading to superlubricity in humid air via novel pathways. Fullerene-like hydrogenated amorphous carbon (FL-C:H) films also exhibit superlubricious properties in both nitrogen and air (Wang et al., 2008). Kato et al. suggested that carbon nitride (CN_x,) can achieve superlubricity in nitrogen atmosphere without the presence of hydrogen (Kato et al., 2003; Adachi and Kato, 2008). Table 1 shows the timeline of the research progress on solid superlubricity achieved with DLC films. As for liquid lubrication, researches showed that hydrogen-free ta-C can achieve superlubricity with the assistance of ester additive (Kano, 2006), glycerol (De Barros Bouchet et al., 2007), unsaturated fatty acids (Kuwahara et al., 2019). DLC lubricated with nano boron nitride particle dispersed in PAO is also available to achieve superlubricity (Zeng et al., 2013).

To date, a growing interest in superlubricious DLC is aroused, and diversified superlubricity pathways have been discovered. Nevertheless, most of these superlubricity phenomena are achieved in strictly controlled lab conditions. The influencing mechanisms of environment and working conditions are complicated, and are still not fully comprehended, which poses uncertainties on the effective manipulation of superlubricious DLC films in mechanical engineering. In recent decades, numerous researches have been done, aiming at uncovering these mechanisms and optimizing the performance of DLC films in complicated environments. In the following sections, a review of superlubricity of DLC is provided in the perspective of influence factors as shown in Figure 2. Film composition is one of the key factors for the establishment of superlubricity. The influences of different elements, including hydrogen, fluorine, silicon, nitrogen, and metallic elements are discussed in section Influence of Film Elements. Stable superlubricity achieved with DLC films usually needs dry inert gaseous atmosphere or hydrogen environment. Though the friction of DLC films in vacuum is extremely low, their wear rate is very high. And it is still a challenge for DLC to achieve superlubricity in oxygen and water containing environments. The environmental sensitiveness and its mechanisms and controlling methods are discussed in section Influence of Environments. From the perspective of engineering applications, it is important to understand the influences of working conditions such as temperature, normal load and sliding velocity on the superlubricity of DLC films. The influence rules and their underlying mechanisms are introduced in section Influence of Testing Parameters. Finally, section Discussion and Conclusion presents the most prevailing theories for the mechanisms of superlubricity in the perspective of origin of friction. The fundamental restrictions of applying superlubricious DLC films in engineering and the approaches to combat these deficiencies are summarized.

The proper composition of DLC films is generally considered to be the precondition for achieving superlubricity state. Understanding the influencing mechanism of the film elements can enable a more effective manipulation of DLC with excellent tribological properties. In this section, the influence of film elements, for example, surface passivation induced by hydrogen and fluorine and low humidity sensitiveness enabled by silicon and sulfur, are presented. Superlubricity achieved via novel mechanisms in carbon nitride and metal-containing DLC films are also introduced.

Hydrogen is the most common element in DLC film, and its proportion can be controlled by changing deposition conditions such as the bias voltage (Chen and Kato, 2014) and the proportion of gas source (Erdemir et al., 2000c). Most of the hydrogen atoms exist in the form of bonded C-H, while some of hydrogen atoms are trapped in the interstice of carbon framework with the forms of unbonded atoms and not fully decomposed gas source molecules (Erdemir, 2004). During dry sliding contact, they are released in the forms of H₂, CH₄, and so on (Nevshupa et al., 2019). Studies have shown that the friction property of a-C:H is closely related with the proportion of hydrogen (Donnet et al., 1998, 1999; Erdemir, 2001; Fontaine et al., 2005). Generally, in vacuum or inert environments, hydrogen-rich a-C:H films, or polymer-like carbon (PLC) films (Casiraghi et al., 2005) with a hydrogen proportion higher than 40% are most favorable for achieving superlubricity (Donnet et al., 1994, 1999; Gao et al., 2003). Hydrogen-free DLC such as ta-C or a-C exhibits a very high friction coefficient up to 1 when tribo-tested in vacuum (Andersson et al., 2003). However, implanting hydrogen ion into the surface of hydrogen-free DLC can also lead to superlubricity (Eryilmaz and Erdemir, 2008). It is believed that introducing hydrogen into the film matrix will increase its void density and sp³ proportion, thereby reducing the residual stress in the film and softening the material (Chen and Li, 2020). Most importantly, hydrogen is speculated to passivate the carbon dangling bonds at the sliding interface, reducing tribochemical wear and friction caused by the formation of cross-interfacial carbon bonds and forming an none-adhesive sliding surface (Erdemir, 2001), as shown in Figure 3, which is proved by simulations (Pastewka et al., 2008, 2010; Schall et al., 2010). Meanwhile, simulations (Dag and Ciraci, 2004) also suggest that due to the low electronegativity of hydrogen, both hydrogenated surfaces are positively charged and generate repulsive force, thus reducing friction and wear (Eryilmaz and Erdemir, 2008). In addition to controlling the hydrogen proportion, introducing other elements is also an effective approach to improve the mechanical and tribological performance of DLC films. Fluorine is a case in point. Fluorine-doped a-C:H with lower hydrogen proportion (5 at.% H, 18 at.% F) could also achieve superlubricity with a friction coefficient of 0.005 in ultrahigh vacuum (Fontaine et al., 2004). They also exhibited higher thermal stability, even after annealing at a maximum temperature of 500°C (Nobili and Guglielmini, 2013). Fluorine, as another monovalent element like hydrogen, has the highest electronegativity and can effectively eliminate the dangling bond on the surfaces, providing negatively charged surfaces and repulsive force, leading to ultralow friction coefficients.

Silicon is another common dopant for DLC films, which could reduce their humidity sensitiveness and improve the thermal stability. Similar with a-C:H, a-C:H:Si films also exhibit superlubricious properties in vacuum (Sugimoto and Miyake, 1990), nitrogen, and hydrogen environments (Chen et al., 2014). As shown in Figures 4A,B, the lowest friction coefficient in nitrogen could reach 0.001 for self-mated a-C:H:Si (31.9 at.% H, 9.3 at.% Si), similar with a-C:H (~40 at.% H) (Chen et al., 2017). Moreover, under certain conditions, it could also achieve superlubricity in humid air (Chen et al., 2013), as shown in Figure 4C, and even under liquid water (Zhao et al., 2009). The superlubricity behavior of a-C:H:Si films is closely related to the formation of a softer polymerlike tribo-film in the contact area (Chen et al., 2017). Silicon is capable of reducing sp² proportion and stabilizing sp³ carbon network, improving thermal stability, and reducing residual stress. During dry sliding contact, silicon is expected to promote the phase transformation and the tribo-softening of the interface and involved in the evolution process of the tribofilm (Chen and Li, 2020). In humid conditions, the formation of a silica-like tribo-film and the hydroxyl groups attached to silicon atoms on the surface is supposed to be an important factor for their superlubricity behaviors, as they can directionally absorb water molecules and form layered-like structures under appropriate conditions.

To some extent, sulfur is similar to silicon in reducing humid sensitivity of DLC films. Experiments showed that a-C:H:S films (5.0 at. % S) are capable to realize superlubricity in humid environments with RH ranging from 0 to 50% (Freyman et al., 2006). Researchers suggested that sulfur can be involved in the formation of thiol-like (-C-S-H) groups on the sliding surfaces, leading to a weaker binding energy between water molecules and the sliding surface, and hence a low friction coefficient in humid environments.

Nitrogen doped DLC or CN_x, could achieve superlubricity via a different pathway without the assistance of hydrogen. The friction coefficient between CN_x (12 at.% N, 7 at.% O) and Si₃N₄ could reach a superlubricity state when tested in nitrogen environment, but lost this capacity in other inert ambient such as CO₂, He and vacuum (Kato et al., 2003). The mechanism is still not fully comprehended, and the formation of a transfer layer on the counterpart material and the establishment of a chemical inert surface terminated by nitrogen are thought to be the key factors.

The incorporation of metal elements such as titanium, chromium, and tungsten in DLC films could improve their hardness and toughness (Voevodin and Zabinski, 2000). However, superlubricity is difficult to achieve in these cases, mainly due to the abrasive wear effect caused by the metallic carbide nanocrystals. Generally, in order to achieve superlubricity, metallic elements should be controlled within a low proportion to limit the formation of nanocrystals in the film. Titanium-doped DLC was reported to achieve superlubricity with a friction coefficient of 0.005 in humid air with a relative humidity of 40%, and the authors suggested it was attributed to the anti-oxidation effect of the TiC-rich surface and the formation of fullerene-like structure with TiO₂ cores (Zhao et al., 2016). Titanium and silicon co-doped a-C:H films are also capable of achieving superlubricity in air with a friction coefficient of 0.007 (Jiang et al., 2010). Another study showed that aluminum and silicon co-doped a-C:H films could realize a superlow friction coefficient of 0.001 in high vacuum. Self-assembled nanostructure network and fullerene-like structures were observed to be the main factors for the superlubricity (Liu et al., 2012).

Previous researches have shown that a-C:H films can reach ultralow friction in clean, inert environments (such as nitrogen, carbon dioxide, noble gases, and vacuum) and hydrogen, but it is much challenging in oxygen or humid air (Erdemir et al., 2000c; Chen et al., 2014). Although DLC films are capable of achieving superlubricity in macroscale, their environmental sensitiveness is still a barrier to limit their engineering application. In this section, the experimental studies on superlubricious DLC films in dry inert gaseous atmosphere, vacuum, hydrogen, oxygen, or water containing environments are presented, respectively, and the influence mechanisms of these environments are discussed.

Understanding the performance of DLC films in different working conditions are important for the engineering application of these self-lubricating materials with superlubricity potential. In this section, the influence of temperature, normal load and sliding velocity are presented, respectively, and the most prevailing theories explaining their impacts on superlubricity are discussed.

The influencing factors and the mechanisms of superlubricity achieved with a-C:H films are multifaceted and interrelated. To date, diversified theories focusing on mechanical properties (Erdemir, 2004; Fontaine et al., 2004), establishment of transfer film (Chen et al., 2017; Liu et al., 2018; Liu Y. et al., 2019), surface passivation (Erdemir and Eryilmaz, 2014), and graphitization (Ma et al., 2015) have been brought up to explain the mechanism of solid superlubricity achieved with DLC films, and it seems that these factors are not working independently. Ultimately, the superlubricity state originates from the establishment of a sliding surface with low energy dissipation, meaning that the major interaction pathways between sliding surfaces are somehow sufficiently shielded. It is generally believed that the origin of friction force between solid surfaces can be qualitatively ascribed to three fundamental phenomena: abrasion, shearing, and adhesion (Fontaine et al., 2008), as shown in Figure 10. Abrasion is the plowing or scratching effect caused by micro-asperities or hard debris trapped between the sliding surfaces. Shearing is the energy consumption origins from the plastic or viscous flow of the material between the friction pairs. Adhesion is the resistance to break the micro-junctions bridging two sliding interfaces, mainly deriving from bonding force, electrostatic force, capillary force, and polarization forces. The realization of superlubricity needs all these friction contributors simultaneously to be limited at extremely low levels. DLC films exactly have the potential to accomplish this task owing to their numerous unique properties. Here, we can use superlubricious hydrogen-rich a-C:H film as an example for discussing.

From the mechanical perspective, DLC films exhibit special advantages in avoiding abrasion. Compared with other self-lubricating soft coatings, DLC films have higher hardness. For example, a hardness of 13.4 GPa for hydrogen-rich a-C:H film (Chen et al., 2017), is higher than that of common bearing steel or tool steel, which is generally around 10 GPa (Fontaine et al., 2008). Meanwhile, the surface roughness of DLC film is generally in accordance with their substrate, meaning that it can be controlled at an extremely low level, even atomically smooth. For example, the surface roughness of a-C:H film deposited on silicon wafer was 0.10 nm (Chen et al., 2017). The high hardness and ultrasmooth surface of DLC films can reduce the chance of abrasion in most cases. Moreover, due to the non-porous structure, hydrogen-rich DLC films also have unusual viscoelastic properties (Fontaine et al., 2004). As implied by the slow recovery of indents observed in nanoindentation experiments, a-C:H films may have some “healing” properties against scratches (Fontaine et al., 2008). These properties of DLC films can minimize the abrasive effect during sliding, paving the way for the establishment of stabilized, low-energy dissipation sliding interfaces.

When a-C:H film reaches a stable superlubricity state, the shearing behavior generally occurs inside an in-situ formed tribofilm or transferfilm with different mechanical properties, rather than in the film matrix (Fontaine et al., 2008). As shown in Figure 11, after friction against bare SUJ2 ball and Si₃N₄ ball, tribolayers of 27 and 5 nm thickness were generated on the balls, respectively. The establishment of these continuous, stably-attached carbonaceous tribofilms, together with the easy shearing nanostructure, will be a key factor for the superlubricity of DLC films (Liu et al., 2018; Liu Y. et al., 2019). High proportion of sp² rehybridized carbon phase can be observed within the tribofilms, and obvious reorientation will be taking place under heavy normal loads, forming layer-like structure (Chen and Li, 2020). Simulations suggest that the significant drop of friction during running-in process is also accompanied with shearing localization, limiting the occurrence of sliding in a thin region (Ma et al., 2015). Meanwhile, near the sliding surface, the as-formed carbon rings with higher sp² proportion are rotated parallel to the sliding direction, forming an interface with low proportion of out-of-plane sp³ bonds (Pastewka et al., 2010). The formation of easy-shearing sp²-C rich sliding interface (or called graphitization) and locally-ordered nanostructures, as well as the localization of shearing region, may be the key factors to keep the shearing resistance in a very low state, contributing to the stable superlubricious friction state.

Tribological tests with DLC films in microscale have shown that the friction vs. normal load curve does not go through the origin, indicating that adhesion cannot be ignored (Shi et al., 2020). For a well-established carbon-based sliding interface, the adhesion phenomenon caused by the establishment and breaking of micro junctions across sliding surfaces is supposed to be the main contributor for the friction force (Fontaine et al., 2008). Firstly, σ dangling bonds generated during mechanical and tribochemical interactions have potential to form strong covalent bonds bridging the sliding surfaces, causing high adhesive friction and wear (Schall et al., 2010). This is the reason for the high friction of hydrogen-free DLC films in dry inert environment or vacuum and its prevention of friction via hydrogen gaseous environment or hydrogen contained in the film. The hydrogen species could react and passivate the dangling bonds generated at sliding surface timely, and hence eliminate the high friction caused by cross-linking of the interfacial bridging (Robertson, 2002). A self-terminated structure with carbon rings could be established during running-in process, leading to a continuous passivated interface with fewer dangling bonds exposed (Pastewka et al., 2010). Secondly, the interaction between out-of-plane π-orbitals of sp² bonded carbon atoms, or π-π interaction, is another factor contributing to the friction force of DLC films. Likewise, this interaction can also be shielded by hydrogen, as they are reactive to carbon double bonds. Absorption of N₂ and CO₂ molecules can also restrain this phenomenon. Thirdly, for hydrogen-rich a-C:H films tested in humid environment, the hydrogen bond, and capillary force (mainly in micro-tribological experiments) induced by absorbed water layer are also contributors of friction force. Dry inert gaseous environments can prevent this phenomenon, thus superlubricity of a-C:H films can be achieved. Nevertheless, for hydrogen-free DLC, water can also act as a passivator preventing the strong π-π interactions and σ dangling bonds, leading to a relatively low friction, but has difficulty in achieving superlubricity. Fourthly, the van der Waals force is the weakest form for adhesion, and is the dominant factor for superlubricity state achieved with passivated DLC surfaces sliding in dry and inert environment. The repulsive coulombic force between two positively-charged C-H terminated interfaces can also account for the ultralow friction of DLC films.

To sum up, superlubricity of a-C:H films achieved in dry issnert environment mainly origins from three unique characteristics of the film: mechanical properties such as high hardness and viscoplastic properties and extremely low roughness, abilities in forming easy-shearing tribofilm or transferfilm through friction-induced phase transformation and bonding reorientation, abilities to passivate the sliding surface through self-contained hydrogen. All these factors work together to eliminate abrasion, shearing and adhesion effects to extremely low levels simultaneously, leading to superlubricity state. Basing on these understandings, involving other elements into the film is an effective approach to optimize their tribological properties since they could change the evolution process of tribofilms and state of sliding surface. As for gaseous ambient molecules, in addition to chemical involvement in the establishment of transfer film and passivation of sliding surface, they also influence friction force physically via thermal conduct and energy dissipation during desorption. Although the influence mechanisms of temperature, normal load, and sliding velocity are multifaceted, under the precondition that they are not crossing the threshold of the mechanical or thermal capacity of the film, their influencing trends mainly depend on whether they promote or deteriorate the evolution process of robust easy-shearing tribofilms and passivation of sliding surfaces.

Since the discovery of DLC films, their application as a self-lubricating and protective coating in mechanical engineering is increasingly broadened. For example, their applications in gearings (Michalczewski et al., 2019), bearings (Okamura et al., 2019), engine piston rings (Kumar et al., 2019), oil-well tubings (Liu L. et al., 2019), rubber seals (Liu J. Q. et al., 2019), and friction pairs in spacecraft (Donnet et al., 1999) have been testified to be effective and valuable. Superlubricity achieved with DLC coatings can provide a new concept to meet the increasing demand for lower energy consumption, longer life time, and higher power density of future mechanical systems, which has an enormous potential value from both economic and environmental perspectives. Nevertheless, it is still a challenge for engineering applications of superlubricious DLC films. More efforts should be concentrated on preventing their environmental sensitivity in humid condition and prolonging their service life in vacuum. Also, their performance still needs to be optimized in harsh working conditions such as extremely high or low temperature, extremely high pressure and velocity, and contaminated environment. Further researches on their superlubricity behaviors and mechanism can enable a more effective manipulation of DLC coatings with excellent self-lubrication properties.

JL, CZ, and XC gave the guidance on the structure and content of this review. QY completed this review. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.