Article pubs.acs.org/JPCC
Friction Reduction Mechanism of Hydrogen- and FluorineTerminated Diamond-Like Carbon Films Investigated by Molecular Dynamics and Quantum Chemical Calculation Shandan Bai,† Tasuku Onodera,† Ryo Nagumo,‡ Ryuji Miura,† Ai Suzuki,‡ Hideyuki Tsuboi,‡ Nozomu Hatakeyama,† Hiromitsu Takaba,† Momoji Kubo,§ and Akira Miyamoto*,‡,† †
Department of Chemical Engineering, Graduate School of Engineering, Tohoku University, 6-6-10, Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan ‡ New Industry Creation Hatchery Center, Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan § Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, 6-6-11-701 Aoba, Sendai, 980-8579, Japan ABSTRACT: The friction reduction mechanisms of diamond-like carbon (DLC) and H- or F-terminated DLC films were investigated using molecular dynamics (MD) and tight-binding quantum chemistry (TBQC) calculations. Atomistic-scale friction dynamics of both DLC and the surface-terminated DLC model in which the unsaturated bonds on their surface were terminated with H or F atoms were investigated by MD. The F-terminated DLC model showed lower friction than that of the H-terminated DLC model because of the stronger repulsive Coulombic force between F atoms at the surfaces. On the other hand, strong van der Waals interaction acting on the interface was observed for the H-terminated DLC model compared to that for the F-terminated DLC models. TBQC calculation indicates a bonding interaction between the surfaces of DLC, while the antibonding interaction was observed for the surface-terminated DLC model. Those interactions would make the difference in the friction properties among the studied models. decreased as the increase in the proportion of fluorine atoms on the DLC surface that was indicated by atomic force microscopy (AFM).10 The superlow friction coefficient about 0.005 for fluorine contents at 18% of DLC film was reported by Fontaine et al.9 Recently, the atomic-scale friction behavior investigated by molecular dynamics (MD) and density functional theory (DFT) has been paid much attention.11−14 We have successfully studied the chemical reaction of the lubricant additive on the atomic scale by tight-binding quantum chemistry (TBQC) MD.15,16 Sen et al. reported a first principles calculation that showed the strong ionic bond formation between C and F atoms at the interface of the Fterminated carbon film. The lower surface energy for the Fterminated diamond film was observed compared to that for the H-terminated diamond film. The negatively charged F atoms are expected to develop a large repulsive electrostatic interaction, indicating the lower adhesion and friction between F-terminated surfaces.17 Although these state observations suggest that the F-terminated diamond-like film would show the lowest friction properties, the more dynamic behavior of friction during a sliding of the tribo-film is to be investigated.
1. INTRODUCTION Diamond-like carbon (DLC) film is generally described as a family of the amorphous carbon, including types of nonhydrogenated DLC (a-C), tetrahedral amorphous carbon (taC), hydrogenated DLC (a-C:H), and tetrahedral amorphous carbon (ta-C:H).1 DLC coated materials shows excellent tribological properties, such as hardness, wear resistance, and low friction. Therefore, it is widely applied to molding and automotive industry, which requires the reduction of friction and the elimination of adhesive transfer of plastics to the tool.2−5 It has been successfully applied in the automotive industries on the engines and standard production vehicles.6 In the past years, many researchers have studied the lubrication performance of DLC film using conventional and some special experimental techniques. Konca et al. reported that nonhydrogenated DLC film during a friction test obtained a high friction coefficient in the ultrahigh vacuum (UHV).7 Andersson reported that the DLC film with high hydrogen content obtained a low friction coefficient in a different gas environment.8 Hydrogenated DLC film with 40% hydrogen showed a coefficient lower than 0.003 in the UHV.9 These reports suggest that hydrogen plays an important role in reduction of the friction of DLC. In order to improve tribological properties, surface modified DLC films were studied in detail. F-terminated DLC film showed lower friction and excellent chemical resistance.10 The frictional coefficient and adhesive force © 2012 American Chemical Society
Received: January 29, 2012 Revised: April 10, 2012 Published: May 9, 2012 12559
dx.doi.org/10.1021/jp300937n | J. Phys. Chem. C 2012, 116, 12559−12565
The Journal of Physical Chemistry C
Article
Table 1. The Parameter for Hrr of eq 6 Used in the TBQC Calculations element
atomic orbital
b0
b1
b2
b3
b4
b5
H C
s s p s p
−12.6190 −16.6353 −11.8040 −20.3228 −8.2749
−10.2160 −15.6704 −11.6714 −17.3687 −17.2986
8.5404 −2.0665 −1.3864 −1.5194 −1.7116
17.6430 −0.1607 0.1867 0.0000 0.0000
14.2410 0.0000 0.0000 0.0000 0.0000
4.4969 0.0000 0.0000 0.0000 0.0000
F
Shall et al. investigated no hydrogenated DLC and 20% hydrogenated DLC films using a MD method with Brenner’s second-generation reactive empirical bond-order potential (REBO). In the study, the chemical reactions occurred at the interface, and the number of C−C bonds decreased with the increase in the H content in DLC.18 However, the mechanism of the reduction friction for H-terminated DLC film is still illusive. Therefore, the tribo-phenomenon at the interface of H- or Fterminated DLC films and the details of the friction reduction mechanism have not yet been unveiled. In this paper, to address such tribological phenomena, we investigated the atomistic scale friction mechanism of DLC film and H- or F-terminated DLC film using computational chemistry methods.
U=
i
+ +
−
Bij ⎞ ⎟ rij6 ⎟⎠
∑ ∑ Dij{e−2β (r − r ) − 2e−β (r − r )} ij ij
0
ij ij
0
j
