Combined Effects of Structural Transformation and Hydrogen

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Combined Effects of Structural Transformation and Hydrogen Passivation on the Frictional Behaviors of Hydrogenated Amorphous Carbon Films Yi-Nan Chen, Tian-Bao Ma, Zhe Chen, Yuanzhong Hu, and Hui Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04533 • Publication Date (Web): 18 Jun 2015 Downloaded from http://pubs.acs.org on June 23, 2015

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The Journal of Physical Chemistry

Combined Effects of Structural Transformation and Hydrogen Passivation on the Frictional Behaviors of Hydrogenated Amorphous Carbon Films Yi-Nan Chen, Tian-Bao Ma*, Zhe Chen, Yuan-Zhong Hu, Hui Wang State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China

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Abstract: Tribological behaviors of hydrogenated amorphous carbon (a-C:H) films under single asperity contact are investigated by molecular dynamics (MD) simulations. Hydrogen concentration and normal load are found to play essential roles in the frictional behavior of aC:H films. With low hydrogen concentration, the a-C:H film shows high adhesion and friction even at very low normal loads (1.75 nN). The sp3-to-sp2 rehybridization is observed in the aC:H films with all studied hydrogen concentrations, which is greatly enhanced with increasing normal load. At high normal loads, formation of nanocrystalline graphene-like lamellar structures is observed locally, usually beneath the instantaneous contact area, acting as a lubricating agent, demonstrating local graphitization due to combined effects of the compression and shear process. With high hydrogen concentration, the friction shows a linear increase along with the normal load and higher load bearing capability. Hydrogen atoms accumulate on the film surface during the sliding process, reducing friction significantly. Hydrogen passivation becomes more obvious with higher hydrogen content especially at low normal loads, where a hydrogen-rich monolayer is found to attach onto the asperity, functioning as a lubricating transfer layer. This work shows the combined effects of structural transformation and surface passivation on the frictional behaviors of a-C:H films, and may shed light on the well-known but not well-understood superlubricity mechanism of a-C:H films.

Keywords: hydrogenated amorphous carbon film; molecular dynamics simulation; superlow friction; hydrogen passivation; structural transformation

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1. INTRODUCTION In recent years, among various kinds of solid materials, amorphous carbon films have become an excellent candidate of solid lubricant, demonstrating good friction-reduction and wearresistance properties.1-6 According to hydrogen content, amorphous carbon films could be classified into hydrogen-free amorphous carbon (a-C) and hydrogenated amorphous carbon (aC:H) films, and the latter is experimentally found to exhibit superlow friction behavior or superlubricity. Various mechanisms have been proposed to explain the superlow friction and wear properties of a-C:H films, among which the most popular postulations are surface passivation and structural transformation (or sp3-to-sp2 rehybridization). Erdemir et al.7 has put special emphasis on the effect of hydrogen-to-carbon (H/C) ratio of source gas in a-C:H film preparation on its frictional and wear properties, showing that high H/C ratio in source gas is fundamental to achieve superlubricity. They8 further proposed that it is the hydrogen atoms saturating the dangling σ-bonds of surface carbon atoms and the resultant electrostatic repulsion between the two sliding surfaces that accounts for the superlubricity of a-C:H film. However, it should be mentioned that an intact surface passivation is difficult to achieve in real applications, as the interfacial σ covalent bonds are inevitably formed and broken during the sliding process, resulting in lubrication failure and high friction,9,10 especially under high normal pressure11 and when hydrogen desorption remarkably happens,12 as indicated by previous atomistic simulations. On the other hand, the relationship between friction evolution and structural transformation during sliding, especially during the running-in stage has also been studied both experimentally and theoretically.13-16 Buildup of carbonaceous transfer film or tribofilm has been found to play an essential role in friction reduction, which is generally characterized experimentally by Raman spectroscopy and transmission electron microscopy (TEM). Wang et al.17 found that the curved


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graphitic structure in fullerene-like hydrogenated carbon film is beneficial for achievement of low friction. Quite recently, Song et al.18 found that instead of existing in as-prepared a-C:H films, lubricious onion-like structure can be spontaneously formed at the sliding interface, which could account for the extremely low friction coefficient. These layered structures may lead to superlubricity due to possible interlayer sliding. Several in-situ TEM studies showed evidences for graphitization during shearing of a-C film, which provides promising tools to an in-depth understanding of the superlubricity mechanism.21, 22 From the theoretical aspects, the transfer film was characterized by the atomic density overlap between the two sliding surfaces.9 The materials transfer was found to be much less for a-C:H than hydrogen-free a-C film. It revealed that the addition of hydrogen causes a large decrease in the unsaturated interfacial C-C bonds which reduces the friction. Pastewka et al.10 further analyzed the formation of transfer film and sliding interface from the velocity profile, and found that a hydrogen-rich film can achieve steady-state low friction at a pressure of about 5 GPa, however a hydrogen-depleted film could only bear a much lower pressure. In the work of Romero et al.19 sliding-induced rehybridization (transformation from sp3 to sp2) was observed and the shear resistance was reduced by hydrogen passivation. We previously20 studied the shear localization behavior of a-C films during the friction process. The shear deformation of a-C films is gradually localized to a very thin shear band, the shear band is characterized quantitatively by the distribution of ‘covalent bond orientation angle’ along the depth direction, which represents localized deformation in the transfer layer or tribolayer and causes the lubricious properties of the a-C films. All the mechanisms mentioned above provide significant advances in understanding the frictional behavior of a-C:H films. However, the combined effect of the structural transformation and hydrogen passivation on the frictional behavior of a-C:H films, is yet to be elucidated. In this


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paper, by using a single asperity model, the frictional behavior during the sliding process between the diamond tip and a-C:H film is firstly analyzed. Then the roles of structural transformation and hydrogen passivation in the friction behaviors of a-C:H films are systematically discussed, which depend closely on the normal load and hydrogen concentration (7.5%, 20.7%, and 42.8%) of the film.

2. SIMULATION MODEL Molecular dynamics (MD) simulations employing a second-generation reactive empirical bond order (REBO) potential23 with increased C-C cutoff radius24 were adopted to reveal the friction process of a diamond tip with the a-C:H films using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). REBO potential could well describe the covalent bond breaking/forming in carbon-based materials and accurate calculation of bond energies, length, and force constants for solid carbon and hydrocarbon molecules from the atomistic perspective, hence complex chemistry in large many-atom systems could be modeled. The diamond tip was made up of 9 atomic layers with a radius of curvature of 3 nm. And it was brought into sliding contact with an a-C:H film with a lateral dimension of 6 nm × 4 nm in x and y directions, respectively. This simulation setup was not only useful in understanding the role of surface roughness in macroscopic tribotest, but also shed light on the nanoscopic tribological behaviors by using AFM tip scanning. The supercell of a-C:H film was obtained by a 3 × 2 replication of the a-C:H film with a lateral cell dimension of 2 nm × 2 nm to save the computational cost for film deposition. The a-C:H film was deposited by MD simulations. Details about the molecule-by-molecule deposition simulation of the a-C:H films were described elsewhere.25 In order to investigate the influence of the hydrogen concentration of the a-C:H film


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on the friction process, three groups of a-C:H films grown on the diamond substrate, were selected with the hydrogen concentration of 7.5%, 20.7%, and 42.8%, marked as films I, II and III, respectively. Periodic boundary conditions were applied along both x and y directions. The key deposition parameter and structural properties of the films were quantitatively shown in Table 1.

Film

Table 1 Deposition parameter and structural properties of films Incident energy Hydrogen sp3 (%) Source gas (eV/ carbon atom) concentration (%)

sp2 (%)

I

C2 H

80

7.5

58.7

39.8

II

C2H2

60

20.7

63.9

34.1

III

C2H2

20

42.8

59.8

37.3

Here a no-wear assumption of the diamond tip was made by setting the atoms rigid in order to focus on the frictional response of the a-C:H films, otherwise the tip could wear out rapidly as simulated by Sha et al.26 The real contact pressure was kept constant during sliding. The bottom two atomic layers of the diamond substrate were fixed to prevent spatial monolithic motion. And the next 10 atomic layers of the substrate were coupled to a Langevin thermostats27 with a temperature of 300 K to avoid overheating. All remaining atoms were free to move. Firstly the entire system was relaxed for 20 ps to reach equilibration. And then the diamond tip was slid against the a-C:H film along x direction at a constant speed of 20 m/s, under a fixed normal load along -z direction, which was applied uniformly on every rigid atom in the diamond tip, ranging from 1.75 nN to 210.25 nN. The time step here was set to 0.2 fs and eventually the sliding distance was 1000 Å (100 nm).


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3. RESULTS AND DISCUSSION The variation of friction force during the sliding process is shown in Figure 1a,b for films I and III respectively, obtained under the normal load of 105.12 nN. Here, the friction force is calculated by the sum of lateral forces (along x direction) on the atoms in the fixed layers of the substrate. In both cases, the friction force rapidly increases and reaches the peak value due to the shear deformation of the film, then decreases corresponding to the running-in period. The friction decrease is closely related to the structural transformation which will be explained in detail in following text. Afterwards, the friction force remains basically stable till the end of the sliding process. The stable value of the friction force for film III is much lower than that of film I.

Figure 1. Evolution of the friction force along with the sliding distance, under a normal load of 105.12 nN, for (a) film I or (b) film III.


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Furthermore, in order to get a fundamental understanding of the relationship between the friction force and the normal load, the steady-state friction forces of these three films are compared. Here, the friction force as plotted in Figure 2 is calculated by averaging over the steady-state stage after the running-in period. Friction force increases with normal load at all three cases. A general linear friction-load relationship is observed for both films II and III. In addition, under a certain normal load, higher hydrogen concentration leads to lower friction force, demonstrating the crucial role of hydrogen in reducing friction. Furthermore, films with higher hydrogen concentration exhibit better load-bearing capability, for example, the friction for film III at the normal load of 210.25 nN has the comparable value with that for film II at 31.54 nN. However, for film I, the linear relationship is kept only at the low load range (< 78.84 nN), while for higher load, the friction deviates from the previous linear trend as shown by the dashed line and increases with a smaller slope. Hence it can be inferred that the sliding friction behaves differently in two loading ranges for film I, which could be attributed to the load dependence of shear induced structural transformation as will be discussed later. Here, the friction coefficient, which is defined as the slope of the friction-load curve, is calculated further to better investigate the tribological performance. Obviously, the lowest friction coefficient of about 0.36, is achieved for film III with the highest hydrogen concentration, again indicating the role of hydrogen in the tribological behavior of the film. The friction coefficient of film I is about 1.2 or 0.43 at the two loading ranges, respectively, and the latter is close to that of film III. This implies that structural transformation reduces the friction coefficient of a-C:H film, which is obvious at high normal load range.


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Figure 2. Variations of the steady-state friction forces of three a-C:H films, along with the normal loads.

So far, there have been two primary mechanisms proposed to explain the friction reduction. One is shear-induced structural transformation, or specifically sp3-to-sp2 rehybridization. The other is surface passivation by hydrogen, saturating the dangling σ-bonds of surface carbon atoms, generating repulsive interaction between the two sliding surfaces. In light of this, we study the combined effects of structural transformation and passivation on the frictional behavior of the film. In order to clearly view the structural changes and chemical compositions of the film, the atomistic snapshot of a 1-nm-thick slice in the x-z plane is shown in Figure 3 for films I and III, before and after steady-state sliding, respectively. The sp3-to-sp2 rehybridization and structural transformation can be viewed more clearly by comparing Figure 3a and 3b. The hybridization of each carbon atom is depicted by the coordination number, which is defined as the number of nearest neighbor atoms within the cutoff length to this atom, where the cutoff length corresponds to the first minimum of the pair correlation function curve (2.05 Å for C/C pairs, 1.50 Å for C/H pairs). For example, 4-fold atoms represent sp3 hybridization and 3-fold atoms represent sp2 hybridization. With the lowest hydrogen content in film I, the rehybridization


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happens at all applied normal load in the present study, however friction is quite high as shown in Figure 2 (black line). This rehybridization is caused by the shear induced film deformation exerted by the sliding asperity, especially at the interfacial region. Some carbon and hydrogen atoms which originally belong to the a-C:H film, adhere to and slide along with the diamond tip, and simultaneously get removed from the tip and transfer back to the a-C:H film, showing dynamic atomic transfer process at the sliding interface. Some linear chains of carbon atoms are also formed and broken dynamically. Furthermore, we observe local layered structures (marked with a circle in Figure 3b), comprised of sp2- hybridized carbon atoms, which tend to appear on the surface of a-C:H film underneath the contact region at rather high normal load. This graphitic structure is progressively formed upon shear and vanished after the diamond tip passes. This local graphitization happens as a consequence of the shear force as predicted theoretically and recently observed experimentally by in-situ TEM.20,

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The formation of these local layered

structures only commences at rather high load (>100 nN), exhibiting more and more ordered and perfect structure with increasing load, as shown in the inset of Figure 4. Interestingly, it shows planar and smooth graphene-like honeycomb structure, which consists of five, six and sevenatom rings. When the load is below the critical point, however, this layered graphene-like structure can hardly be formed. The graphene-like layer can exhibit a certain lubrication effect, which explains the slowdown of the friction increase at higher load range for film I. We note that the graphitization only occurs locally rather than globally, hence, we do not observe significant reduction of friction or superlubricity in the present study. It should also be mentioned that rehybridization is a dynamic process. Not only sp3-to-sp2 rehybridization occurs, the opposite trend, i.e., sp2-to-sp3 recrystallization is also likely to happen under high load, that is, some original sp2- hybridized carbon atoms on the surface of the a-C:H film change into sp3-


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hybridized ones when involved in the contact region.

Figure 3. Typical atomistic configurations of the sliding system during the friction process, obtained from a 1-nm-thick slice in the x-z plane. (a) initial configuration of film I before loading. In order to save the computational cost for film deposition, the supercell of a-C:H film was obtained by a 3 × 2 replication of the a-C:H film with a lateral cell dimension of 2 nm × 2 nm, which was obtained from the molecule-by-molecule deposition simulation; (b) steady-state sliding configuration of film I under the highest load of 210.25 nN, when the sliding time is 2066 ps. (c) initial configuration of film III before loading; (d) steady-state sliding configuration of


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film III under the lowest load of 1.75 nN, when the sliding time is 1566 ps. Yellow balls represent one-fold carbon atoms, red balls are two-fold (sp1) atoms, white balls are three-fold (sp2) atoms, cyan ones are four-fold (sp3) atoms, and blue ones are hydrogen atoms.

With the highest hydrogen content and lowest normal load, rehybridization is hardly observed, as shown in Figure 3c,d. The chain-like structure, made up of sp2- hybridized carbon atoms and hydrogen atoms, is found on the surface of the a-C:H film. Once formed, the atomic chain does not easily fracture or react with other atoms to be shortened or elongated under low normal load, which means that it is chemically stable and passivated, which may act as a lubricating agent during the friction process. Some carbon and hydrogen atoms originally from the a-C:H film, adhere to the bottom of the diamond tip and generate a stable hydrogen-rich transfer layer, as marked with a circle in Figure 3d. The hydrogen-rich transfer layer, along with the high hydrogen concentration on the surface of the a-C:H film, forming apparent hydrogen passivated sliding interface, which results in the low friction of film III.


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Figure 4. Variations of the average values of sp2 fraction in the top surface layer for film I, along with the normal loads. The insets show the film configurations, as well as the enlarged top-view snapshots of the graphitic structures, under the normal loads of 105.12 nN and 210.25 nN, respectively. The color coding is the same as in Figure 3.

To quantify the structural transformation, the depth profiles of sp3 and sp2 fractions of the films at the beginning (Figure 5a) and the end (Figure 5b) of the friction process are calculated for the case of film III, under the highest load of 210.25 nN. Along the film depth (z) direction, the a-C:H film, as well as the diamond tip, is divided into a set of thin slabs with a thickness of 0.89 Å. Both the sp3 fraction and sp2 fraction are calculated in each slab. By comparing Figure 5a with Figure 5b, although there exist slight variations of both sp3 and sp2 fractions in the inner aC:H film at depths ranging from 5 to 16 Å, we mainly focus on the interfacial region at depths


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ranging from about 16 to 32 Å, extending inwards from the original film surface to the inner aC:H film. This interfacial region is defined as the tribolayer, where sliding induced sp3-to-sp2 rehybridization happens significantly. At the beginning, the sp3 fraction in the tribolayer is around 50.2%, almost identical to the sp2 fraction, which is approximately 47.7%. However, at the end of the friction process, the sp2 fraction in the tribolayer achieves a peak value of 85.3%, in contrast, the sp3 fraction declines to a lowest value of 11.5%, indicating dramatic structural transformation during the friction process.

Figure 5. Depth profiles obtained from film III, under the highest load of 210.25 nN, (a) at the beginning, (b) at the end of the friction process. Dashed lines denote the tribolayer where significant structural transformation happens.


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In order to reveal the structural transformation throughout the whole friction process, the evolution of sp2 and sp3 fractions during the sliding process under the normal load of 1.75 nN is recorded as shown in Figure 6. The sp2 (sp3) fraction here is averaged in the 2-nm-thick top surface layer where structural transformation happens most obviously. It can be seen that, at the initial sliding stage, the sp2 fraction in all cases increases more or less. Specifically, the sp2 fraction rises the slowest for film III and reaches the stable value soon, while it increases significantly to a relatively stable value for film I. The variation of sp3 fraction shows the opposite trend. The significance of structural transformation defined by the variation of sp2 (or sp3) fraction between the original and final structure of a-C:H film is compared clearly by Figure 6 among the three films: film I > film II > film III. Based on the above analysis, it can be deduced that structural transformation happens the most dramatically for film I, which has the lowest hydrogen concentration. This is consistent with Figure 3a,b that film I shows significant sp3-to-sp2 rehybridization. It has been assumed that sp2 fraction in the film could affect the friction coefficient. In order to elucidate the mechanism, we study the relationship between sp2 content, normal load and friction coefficient. In Figure 4, under each normal load for film I, the sp2 content in the surface layer of film I averaged over the steady-state sliding stage increases from 63.4% to 74.1%, with the normal load increasing from 1.75 to 210.25 nN. Together with the change of slope of the friction-load curve for film I in Figure 2, we could deduce that a critical sp2 value may exist (68.5% in the present study) to determine whether high or low friction coefficient can be achieved. The reduction of friction coefficient above a critical normal load is essentially determined by the formation of the local graphitic structure at the sliding interface, which is closely related to the sp2 content of the film. With higher normal load, the shear induced sp3-to-sp2 rehybridization and structural transformation become more pronounced,


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a more ordered and perfect graphene-like structure is consequently formed when the sp2 content exceeds a certain critical value, which accounts for the lower friction coefficient.

Figure 6. Evolutions of (a) sp2 fraction and (b) sp3 fraction of the top surface layer throughout the friction process, under the lowest normal load (1.75 nN) for each film.

From Figure 6, it is found that for film III, hydrogen could hinder the film deformation and structural transformation under low normal load, however, the structural transformation is still quite obvious under high normal load. Figure 7 shows the variations of sp3 and sp2 fractions of film III under different loading conditions. It is clear that at the beginning of sliding, varying degrees of increase in sp2 fraction and decrease in sp3 fraction are observed in all cases, until all quantities remain largely unchanged during the steady-state friction stage. The trend of sp3 fraction at the normal load of 105.12 nN as depicted by the red line is in good agreement with the friction curve shown in Figure 1b, which again proves that structural transformation does influence the friction behavior of a-C:H films, even with the highest hydrogen content. Normal load has an important impact on the structural transformation of film III. At the lowest normal load (1.75 nN), the value of sp2 fraction increases by approximately 1.3% during the sliding


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process; while at the highest normal load (210.25 nN), sp2 fraction increases by 17.5%, rising from 47.5% to 65%, throughout the sliding process. This demonstrates that structural transformation from sp3 structure to sp2 structure becomes more and more drastic along with the increasing normal load, which also applies to other films. It is necessary to point out that, although structural transformation significantly happens under high normal load for film III, the friction force of film III still has a good linear relation with the normal load, different from film I as shown in Figure 2, suggesting that structural transformation does not contribute significantly to the reduction of friction for film III, which has the highest hydrogen concentration. Instead, hydrogen passivation plays an essential role in the present loading range for film III, resulting in friction reduction.

Figure 7. Variations of sp2 fraction and sp3 fraction of film III throughout the friction process, under different normal loads of 1.75 nN, 105.12 nN and 210.25 nN, respectively.


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Then the distribution of hydrogen atoms, especially those near the sliding interface as shown in Figure 8, are investigated to reveal the role of hydrogen passivation during the sliding process. The instantaneous system is divided by a series of concentric circles, using the bottom profile of the diamond tip as the reference circle (denoted by the solid circle in the inset of Figure 8), extending inward and outward for several layers, marked with sequence numbers of -1, 1, 2, 3, 4, etc. Each layer is set to be 0.89 Å thick. Then the hydrogen content in each circular ring layer is calculated, which is referred to as the ratio of the number of hydrogen atoms to the total number of atoms. Figure 8 presents the variation of the hydrogen content along with layer number for film II, under the lowest normal load (1.75 nN), before, during and after sliding. Apparently, at all these three stages, the hydrogen distribution profiles show a sharp peak in layer 2. This demonstrates that hydrogen atoms tend to distribute on surface or in the transfer layer as shown in Figure 3d and inset in Figure 8. More importantly, significant accumulation of hydrogen atoms during the sliding process, as indicated by the increasing hydrogen content peak value, suggests increasingly sufficient hydrogen passivation on the surface or transfer layer, which gives rise to the long-standing low friction.


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Figure 8. Variations of hydrogen content along with the circular ring layer for film II, under the load of 1.75 nN, when the sliding time t = 0 ps, 2500 ps, and 5000 ps, which corresponds to the beginning, intermediate and end of the sliding process, respectively. The inset shows the atomistic structure obtained near the end of the sliding process. Each circular ring layer is marked with a number, such as -1, 1, 2, 3, 4, etc, where the radius of curvature is 3 nm, and the thickness (labeled as d) of each layer is 0.89 Å. Cyan balls are carbon atoms, and blue ones are hydrogen atoms. The hydrogen content refers to the ratio of the number of hydrogen atoms to the total number of atoms in each circular ring layer.

The combined effects of structural transformation and hydrogen passivation can be explained as follows: for all the films with different hydrogen content, these two mechanisms both play


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roles in the friction behavior of a-C:H films, but the tribological properties such as the friction force or friction coefficient, eventually depend more on a leading factor. For hydrogen-depleted films, structural transformation becomes more drastic along with increasing normal load, generating more and more ordered and perfect graphene-like structures, hence the effect of structural transformation dominates as compared with that of hydrogen passivation, resulting in the reduction of the friction coefficient at higher normal loads. For hydrogen-rich films, lower friction force is obtained due to hydrogen passivation. Although structural transformation is apparent at high normal load, the effect of hydrogen passivation still dominates as compared with that of structural transformation, hence the friction force keeps a good linear relation with normal load, without any change in friction coefficient in the present loading range. It should be mentioned that superlow friction has already been achieved in macroscopic ballon-disk tribotests. So there still remains some discrepancy of friction coefficient between MD simulations and experiments. Firstly, in the present simulation, a single-asperity contact model is used to avoid the complexity of interactions between multiple asperities, and may reflect a tipsample sliding process to some extent, which should be different from the macroscopic contact and friction. Secondly, the time and length scales are quite different between MD simulation and experiments. Also the frictional behavior is affected by complex experimental conditions and factors. The scaling laws to bridge the macro- and microscopic phenomenon still remain a big challenge till now.

4. CONCLUSIONS The low friction property of a-C:H films is postulated to be due to two primary mechanisms: surface passivation, and sp3-to-sp2 rehybridization. Here, MD simulation is conducted to learn


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the relationship between the microstructures and tribological properties of three types of a-C:H films during friction process. The combined effects of the structural transformation and hydrogen passivation on the friction behavior of a-C:H films are elucidated. For films with low hydrogen concentration, a linear relationship between steady-state friction force and normal load is observed only at the low load range; while at higher loads, structural transformation remarkably happens, accompanied by formation of tribolayer and local graphitic structures in the contact region, resulting in the deviation of the friction force from the original linear trend, demonstrating the reduction of friction coefficient. For films with high hydrogen concentration, although structural transformation also takes place significantly at high normal load, hydrogen passivation still plays a more important role in the friction process, leading to lower friction force and a consistent linear relationship between steady-state friction force and normal load. Furthermore, under low normal load, the high hydrogen concentration of the a-C:H film suppresses the structural transformation, and generates a hydrogen-rich passivation layer on the surface or in the transfer film, resulting in low friction. The results shed light on the long-lasting debate of whether surface passivation or structural transformation determines the low friction property of hydrogenated amorphous carbon film, and provide guidance in design of tribological systems in real applications.

AUTHOR INFORMATION Corresponding Author *Phone: +861062788310. E-mail: [email protected]

ACKNOWLEDGEMENT


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The authors would like to acknowledge the support of National Natural Science Foundation of China (Grant Nos. 51375010, 51422504, 91323302) and the National Key Basic Research Program of China (2013CB934200). Simulations are executed on the “Explorer 100” cluster system of Tsinghua National Laboratory for Information Science and Technology.

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TABLE OF CONTENTS


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Combined Effects of Structural Transformation and Hydrogen

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