Tribochemical Reaction Dynamics Simulation of Hydrogen on a

ARTICLE pubs.acs.org/JPCC

Tribochemical Reaction Dynamics Simulation of Hydrogen on a Diamond-like Carbon Surface Based on Tight-Binding Quantum Chemical Molecular Dynamics Kentaro Hayashi,† Kotoe Tezuka,‡ Nobuki Ozawa,† Tomomi Shimazaki,† Koshi Adachi,‡ and Momoji Kubo*,† †

Fracture and Reliability Research Institute (FRRI), Graduate School of Engineering, Tohoku University, 6-6-11-703 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan ‡ Department of Nanomechanics, Graduate School of Engineering, Tohoku University, 6-6-01 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan ABSTRACT: Diamond-like carbon (DLC) has recently attracted much attention as a solid-state lubricant, because of its resistance to wear, low friction, and low abrasion. Several factors, such as the hydrogen atoms in DLC and transfer film formation are important for improving the tribological characteristics of DLC. In this paper, we discuss the low-friction mechanism of DLC by using our tightbinding quantum chemical molecular dynamics method. The method employs a DLC film sliding simulation in order to explore the effect of hydrogen atoms on the carbon-based transfer film. The formation of CC bonds between DLC films increases friction, while surface hydrogen atoms suppress CC bond formation, which results in the low-friction state. Moreover, the steric effect of hydrogen molecule generation was found to remove the load from the substrate, inhibiting CC bond formation. In addition, we determined that surface hydrogen atoms play a key role in the cleavage of CC bonds formed during sliding of DLC films.

1. INTRODUCTION Contact surfaces often suffer from mechanical problems, and therefore understanding the friction and wear of contact surfaces is important for both reliable operation and maximizing the lifespan of mechanical devices. In addition, the economic and environmental consequences of energy loss caused by friction are considerable. Liquid lubricants are frequently employed to reduce the friction on contact surfaces in many mechanical systems, such as vehicles and industrial robots. Conversely, solid lubricants have recently been used in precision mechanical equipment, such as microelectromechanical systems and aerospace instruments. Diamond-like carbon (DLC), which has an amorphous carbon structure, is one of the most promising solid lubricants because of its low abrasion, low friction, and chemical resistant properties.19 DLC is deposited on the surface of substrate materials by using various plasma and chemical vapor deposition processes. During the deposition of DLC films, elements such as hydrogen and silicon are added in order to control the tribological properties of the DLC film. For example, the tribological properties are improved by adding hydrogen atoms during the fabrication of the DLC film.1016 Moreover, films containing nitrogen have recently been reported.1721 However, electronic- and atomic-scale chemical knowledge of the additives is currently not adequate because tribological phenomena are usually investigated on a macroscopic scale. DLC shows superlow friction under specific conditions. Fontaine et al. performed rubbing tests using DLC against an r 2011 American Chemical Society

iron surface under high-vacuum conditions, and reported a superlow friction state with a friction coefficient of 0.002.22 They observed that a carbon-based transfer film is formed on the iron surface in the superlow friction state; the superlow friction state disappears when the transfer film is destroyed. However, the precise electronic- and atomic-scale mechanism of the superlow friction state is not yet sufficiently understood. Computer simulation studies are powerful tools for studying the microscopic behaviors of DLC, and many research groups have discussed the tribological phenomena observed in DLC and for other lubricants based on various computational simulations.2326 We have previously applied a classical molecular dynamics method to various tribological dynamics and obtained useful information on the mechanism of the friction process and the origin of high or low friction coefficients.2731 Recently, Schall et al.32 have investigated the atomic-scale effects of adhesion and transfer film formation on DLC contacts using a classical molecular dynamics simulation, where reactive empirical bond-order potential (REBO) was employed to describe the formation and cleavage of carboncarbon chemical bonds. They reported that the addition of hydrogen to the DLC film lowers the friction by Received: July 25, 2011 Revised: October 7, 2011 Published: October 11, 2011 22981

dx.doi.org/10.1021/jp207065n | J. Phys. Chem. C 2011, 115, 22981–22986

The Journal of Physical Chemistry C

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Table 1. Parameters for Tight-Binding (TB) Simulation atom Ia[eV]

s

C 21.4

p

13.4

s

1.98

1.50

p k

2.40 0.565

0.273

δ

0.065

0.065

a

1.145

0.742

b

0.080

0.078

ζb[1/Å]

H 13.6

a

Ionization energy from the valence orbital. b Exponent of Slater-type atomic orbital.

Figure 2. Sliding simulation results for non-hydrogen-terminated DLC films under a load pressure of 1 GPa. Figure 2a shows the friction coefficient. Figure 2b shows snapshots of the friction dynamics.

Figure 1. Sliding simulation model for DLC films.

reducing the unsaturated carbon bonds on the contact surface.32 However, this method is not sufficient for handling chemical reactions accurately because electrons are not considered in the classical molecular dynamics method, and most chemical reactions are induced by electron transfer. We therefore developed our original tight-binding quantum chemical molecular dynamics (TB-QCMD) simulator and successfully applied it to various tribochemical reaction dynamics phenomena.3336 Bond formation and bond breaking dynamics induced by electron transfer during tribological phenomena were examined using our TB-QCMD simulator. We demonstrated that our TB-QCMD simulator is effective for simulating the chemical reaction dynamics during tribological processes. We experimentally confirmed that chemical reactions strongly affect tribological phenomena and the properties of DLC films. Detailed mechanisms of the chemical reactions that occur in DLC films during tribological process remain unclear, although experimental works were recently done by Erdemir and co-workers using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS).3740 Therefore, in this paper, we employ our TB-QCMD simulator to explore the tribochemical reaction dynamics and mechanism in DLC contact surfaces on the electronic- and atomic-scale. We elucidate the relationship between the chemical reactions and the friction

coefficient, and focus on the role of surface hydrogen in DLC films during tribochemical reactions.

2. METHOD The TB-QCMD method was employed to analyze the superlow friction mechanism on the electronic- and atomic-scale.41 We applied our software package “Colors” to the tribochemical reaction dynamics of the DLC films. In the TB-QCMD method, the following Hamiltonian was used:42 1 Hrs ¼ Krs Srs ðHrr þ Hss Þ 2

ð1-1Þ

Krs ¼ f1 þ ðkr þ ks Þð1 Δ4 Þ þ Δ2 g exp½ ðδr þ δs Þfrrs ðdr þ ds Þg Δ¼

Hrr Hss Hrr þ Hss

ð1-2Þ ð1-3Þ

The diagonal matrix element, Hrr, is defined as the negative of ionization potential for valence electrons, Ir; that is, Hrr = Ir. The off-diagonal term Hrs is calculated from formula 1-1, where Srs is the overlap matrix. In formula (1-2), rrs is the distance between the two atoms to which the molecular orbitals belong, and kr and δr are the positive empirical parameters. These parameters for the DLC film are developed and listed in Table 1. The radius of each orbital dr is determined from the method reported by Calzafferi et al.43 The total energy Etotal of the system can be calculated from the eigenvalue εk and the repulsive 22982

dx.doi.org/10.1021/jp207065n |J. Phys. Chem. C 2011, 115, 22981–22986


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Figure 3. Sliding simulation results for hydrogen-terminated DLC films under a load pressure of 1 GPa. Panel a shows the friction coefficient. Panel b shows snapshots of the friction dynamics. Panel c shows the distance between substrates and the bond population for hydrogen atoms.

potential Erep as follows:42 Etotal ¼

occ

∑ 2 mi v2i þ ∑k εk þ i∑< j Erep ðrij Þ 1

ai þ aj rij Erep ðrij Þ ¼ ðbi þ bj Þ exp bi þ bj

ð2-1Þ

! ð2-2Þ

Here, k is the index for the molecular orbital, and i and j are indices for the atoms in the system. ai and bi are the parameters that are dependent on atom i (Table 1). These parameters are determined to reproduce the binding energy and the bond distance for diamond and hydrogen molecules. The TB-QCMD calculations using these parameters give the binding energy and the bond distance for diamond as 7.38 eV and 1.55 Å, respectively, which are similar to the experimental values of 7.37 eV and 1.54 Å. The calculated and experimental binding energies of a hydrogen molecule are 4.511 and 4.506 eV, respectively. The parameter set can also

reproduce the experimental bond length of 0.74 Å for a hydrogen molecule. In order to calculate the friction coefficient, the upper substrate was forcibly pushed at 100 m/s, whereas the bottom atoms of the lower substrate were fixed in position (Figure 1). The cumulative averaged friction coefficient μ was determined from the sum of forces on atoms of the upper substrate: μ = Fz‑averaged/Fx‑averaged, where Fx and Fz represent the sums of the horizontal and perpendicular forces with respect to the upper substrate, respectively. Fx‑averaged and Fz‑averaged are the averaged values of Fx and Fz for every step, respectively. In the sliding simulation, the simulation temperature was 300 K, achieved by velocity scaling. A molecular dynamics time step of 1.0 fs was used for the simulation model, which only included carbon atoms, and 0.2 fs was used for the model with hydrogen atoms. According to the previous simulation studies,28,32 we also employed 100 m/s for the sliding speed. Because friction surfaces have minute asperity, the load force is different at different points on the friction surface. In order to discuss the differences in load force, the simulations were performed at load pressures of 1 and 10 GPa. 22983



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Figure 4. Sliding simulation results for hydrogen-terminated DLC films under a load pressure of 10 GPa. Panel a shows the cumulative averaged and instantaneous friction coefficients as solid and dashed lines, respectively. Panels b, c, and d show the bond population for carbon atoms. Panel e shows snapshots of the friction dynamics.

3. RESULTS AND DISCUSSION First, we examine the sliding between non-hydrogen-terminated DLC films to clarify the effect of hydrogen on friction. The upper DLC film was slid at the velocity of 100 m/s under a load

force of 1 GPa. Figure 2a shows the friction coefficient between non-hydrogen-terminated DLC films, where the horizontal and vertical axes represent the simulation time and the cumulative averaged friction coefficient, respectively. Figure 2b shows 22984


The Journal of Physical Chemistry C snapshots of the friction dynamics for the non-hydrogen-terminated DLC model films. In the early stage of the sliding simulation from 0 to 10 ps, the friction coefficient is not a stable value because the number of values accumulated is insufficient. However, after 10 ps, the dispersion of the friction coefficient is small, and the value of the coefficient is ∼8.0. The high friction coefficient is caused by the formation of carboncarbon (CC) bonds between the DLC films. The shear deformation process rather than the friction behavior occurs between the non-hydrogen-terminated DLC films. The hydrogen-terminated DLC model was subjected to the sliding simulation under a load force of 1 GPa in order to investigate the effect of the surface hydrogen atoms on the friction coefficient of DLC films. Figure 3a shows the friction coefficient between hydrogen-terminated DLC films, where the horizontal and vertical axes represent the simulation time and the cumulative averaged friction coefficient, respectively. The hydrogen-terminated DLC film shows a friction coefficient of 0.05, which is considerably lower than that of non-hydrogen-terminated DLC films. In the non-hydrogen-terminated DLC film, the CC bonds increase the friction coefficient. The terminal hydrogen atoms suppress the formation of CC bonds; this effect on sliding DLC films has previously been reported by Schall et al.32 They employed a classical molecular dynamics simulation based on Brenner’s second-generation REBO potential, and demonstrated the relationship between CC bond generation and an increase in friction. The TB-QCMD method showed that terminal hydrogen atoms caused effects similar to those observed in the classical molecular dynamics study published by Schall et al.32 It was discovered that the hydrogen atoms played an additional role. Figure 3b shows a snapshot of the friction dynamics on the hydrogen-terminated DLC films. The snapshot shows the generation of hydrogen molecules at the friction interface, which contributes to the suppression of the generation of the CC bond. In order to confirm the effect, the atomic bond population for the hydrogen molecules and the distance between two hydrogenterminated DLC films is shown in Figure 3c. The bond population clearly shows the generation of chemical bonds between hydrogen atoms as well as the generation of molecular hydrogen; a value close to 1.0 represents a chemical bond between hydrogen atoms, and a value near zero indicates there is no hydrogenhydrogen bond. The change in the bond population in Figure 3b shows that one hydrogen molecule is generated at 3.5 ps, and the molecule enlarges the distance between the DLC films from 8.5 Å to 9.0 Å, because of the steric effect of the hydrogen molecule. The distance is measured between the centers of mass of the two substrates. The hydrogen molecule reduces the creation of CC bonds between the DLC films by moving them apart and thus reduces the friction. Experimentally, Eryilmaz and Erdemir reported that hydrogen molecules, induced between the DLC films, reduce the friction coefficient.40 The difference between the friction phenomena observed at high and low pressures was investigated via a sliding simulation under a load pressure of 10 GPa. In this simulation, we focus on the relationship between CC bond formation and the friction coefficient. In Figure 4a, the solid line indicates the cumulative averaged friction coefficient, which is ∼0.10. According to the cumulative averaged friction coefficient, there are several small peaks around 3 ps, 4 ps, and 8 ps. In order to confirm the changes, the instantaneous friction coefficient was determined, which was averaged every 1000 time-steps and corresponds to the dashed

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Figure 5. Participation of surface hydrogen atoms in the CC bond cleavage process.

line shown in Figure 4a. The instantaneous friction coefficient has a large dispersion over time, and can be useful for confirming changes in the friction coefficient. The instantaneous friction coefficient clearly shows three large peaks at 3 ps, 4 ps, and 8 ps. The bond population of the carboncarbon atoms was calculated in order to clarify the origin of these peaks (Figure 4b,c,d). The solid and dotted lines show the CC bond population and the instantaneous friction coefficients, respectively. There are three events before 8 ps that are related to the generation and cleavage of the CC bonds. Two CC bonds are generated at the beginning of the sliding simulation: one is broken around 3 ps (Figure 4b), and the other is cleaved around 8 ps (Figure 4c). The third CC bond is formed around 3 ps, and its cleavage occurs at around 4 ps (Figure 4d). The first CC bond cleavage causes a rapid decrease in the friction coefficient at 3 ps, and the decrease at 4 ps is related to the cleavage of the third CC bond. The third decrease at 8 ps is related to the cleavage of the second CC bond. Figure 4e shows snapshots of the friction dynamics on the hydrogen-terminated DLC films under a load pressure of 10 GPa. In the non-hydrogen-terminated DLC film sliding simulation, the generation of CC bonds increases the friction coefficient, and in hydrogen-terminated DLC films, cleavage of the CC bond results in the decrease in the friction coefficient. In addition, the hydrogen atoms that terminate the surface of the DLC films, contribute to the cleavage of the CC bonds. Figure 5 shows the participation of hydrogen atoms in the CC bond cleavage process. Initially, the terminal hydrogen atom approaches the CC bond (a), and the hydrogen atom is transferred to the neighboring carbon atom. The CH bond becomes unstable and elongated (b). In the final step, two hydrogen atoms are exchanged between the two carbon atoms (c), and the CC bond is broken (d). The TB-QCMD simulations in this paper suggest that the terminal hydrogen atoms not only suppress the formation of the CC bond but also participate in the CC bond cleavage. 22985



4. SUMMARY In this work, we have investigated the tribological behavior of two DLC films sliding against each other, based on TB-QCMD simulations. Recent experiments have reported the superlow friction behavior of DLC films, where carbon-based transfer films play a key role. Our simulation results suggest that terminal hydrogen atoms on the surface of DLC film are essential for superlow friction. Non-hydrogen-terminated DLC films display very high friction, which is similar to shearing deformation, caused by the formation of CC bonds. By contrast, hydrogen-terminated DLC films achieved low-friction sliding. The generation of hydrogen molecules during the sliding simulation increased the distance between the two DLC surfaces because of steric effects, which prevented the formation of CC bonds at the friction interface. Finally, we found that the surface hydrogen atoms play a key role on the CC bond cleavage process. These active characteristics of the surface hydrogen may be the most significant factor in reducing friction in DLC films. The effectiveness of our TB-QCMD method to clarify the effect of hydrogen termination and hydrogen molecule on the friction coefficient of the DLC films is validated in this paper. Further calculations on the effect of hydrogen within and on DLC films will be performed by our simulation code in the near future, and more detailed information will be expected to understand the effect of hydrogen during the friction process of DLC films. ’ REFERENCES (1) Erdemir, A.; Donnet, C. J. Phys. D: Appl. Phys. 2006, 39, R311. (2) Qi, Y.; Konca, E.; Alpas, T. Surf. Sci. 2006, 600, 2955. (3) Erdemir, A. Surf. Coat. Technol. 2001, 146147, 292. (4) Erdemir, A. Tribol. Int. 2004, 37, 1005. (5) Sanchez-Lopez, J. C.; Erdemir, A.; Donnet, C.; Rojas, T. C. Surf. Coat. Technol. 2003, 163, 444. (6) Erdemir, A.; Eryilmaz, O. L.; Nilufer, I. B.; Fenske, G. T. Diamond Relat. Mater. 2000, 9, 632. (7) Miyoshi, K.; Murakawa, M.; Watanabe, S.; Takeuchi, S.; Miyake, S.; Wu, R. L. C. Tribol. Lett. 1998, 5, 123. (8) Miyoshi, K.; Wu, R. L. C.; Garscadden, A. Surf. Coat. Technol. 1992, 54, 428. (9) Yoshida, K.; Horiuchi, T.; Kano, M.; Kumagai, M. Plasma Process. Polym. 2009, 6, S96. (10) Kim, S. G.; Kim, S. W.; Saito, N.; Takai, O. Diamond Relat. Mater. 2010, 19, 1017. (11) Chouquet, C.; Gerbaud, G.; Bardet, M.; Barrat, S.; Billard, A.; Sanchette, F.; Ducros, C. Surf. Coat. Technol. 2010, 204, 1339. (12) Jiang, J.; Hao, J.; Pang, X.; Wang, P.; Liu, W. Diamond Relat. Mater. 2010, 19, 1172. (13) Jiang, J.; Hao, J.; Wang, P.; Liu, W. J. Appl. Phys. 2010, 108, 033510. (14) Zhao, F.; Li, H.; Ji, L.; Wang, Y.; Zhou, H.; Chen, J. Diamond Relat. Mater. 2010, 19, 342. (15) Erdemir, A.; Eryilmaz, O. L.; Fenske, G. J. Vac. Sci. Technol. A 2000, 18, 1987. (16) Erdemir, A.; Eryilmaz, O. L.; Nilufer, I. B.; Fenske, G. R. Surf. Coat. Technol. 2000, 133134, 448. (17) Kato, K.; Bai, M.; Umehara, N.; Miyake, Y. Surf. Coat. Technol. 1999, 113, 233. (18) Kao, W. H.; Su, Y. L.; Yao, S. H.; Huang, H. C. Surf. Coat. Technol. 2010, 204, 1277. (19) Kato, K.; Umehara, N.; Adachi, K. Wear 2003, 254, 1062. (20) Zhou, F.; Adachi, K.; Kato, K. Thin Solid Films 2006, 514, 231. (21) Zhou, F.; Kato, K; Adachi, K. Tribol. Lett. 2005, 18, 153.

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Tribochemical Reaction Dynamics Simulation of Hydrogen on a

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