Transfer-Film Formation Mechanism of Polytetrafluoroethylene: A

Article pubs.acs.org/JPCC

Transfer-Film Formation Mechanism of Polytetrafluoroethylene: A Computational Chemistry Approach Tasuku Onodera,*,† Minseok Park,† Kenichi Souma,‡ Nobuki Ozawa,§ and Momoji Kubo§ †

Hitachi Research Laboratory, Hitachi, Ltd., 7-1-1 Omika-cho, Hitachi 319-1292, Japan Hitachi Industrial Equipment Systems Co., Ltd., AKS Bldg., 3 Kanda, Neribei-cho, Chiyoda-ku, Tokyo 101-0022, Japan § Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, 6-6-11 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan ‡

ABSTRACT: A method based on computational chemistry is used to theoretically investigate the mechanism by which a transfer film, which strongly contributes to reducing friction and wear of a resin material, of polytetrafluoroethylene (PTFE) forms. The formation mechanism of a PTFE transfer film on an aluminum surface was investigated by a quantum chemistry simulation that showed that interfacial ionic interaction plays an important role in forming the transfer film on the counter aluminum surface. A tribochemical reaction, namely, the PTFE reacts with the aluminum surface as a Lewis acid to form a carbon radical, was found. It is inferred that this reaction is one of the key processes that produces the good tribological performance of PTFE. The influence of ambient water and nitrogen molecules on the transfer film formation was then studied by using a molecular dynamics method. The degree of PTFE transfer to a metallic surface in nitrogen gas was less than that in water vapor because the nitrogen molecules shielded the interfacial ionic interaction, which is a key factor in forming the transfer film. It is concluded that to form the transfer film a polar gas molecule or several polar groups in PTFE polymer chain is necessary.

1. INTRODUCTION Polytetrafluoroethylene (PTFE) is widely used as a lubricious sliding material, which especially contacts with metallic surfaces, because of its excellent chemical stability, low friction, and antiwear performances. Because PTFE enables dry friction with metallic materials without any oils or liquids, it can be applied as an alternative to various oil-lubricated mechanical parts. It is generally recognized that lubrication by PTFE is based on the formation of a transfer film on a metallic counter surface.1−4 This film achieves a self-lubrication; namely, the formed solid film (with nanometric thickness, i.e., 1−10 nm4) reduces friction and wear of a sliding system by preventing direct contact of metallic surface. Moreover, a previous report by Wittmann et al. suggested that PTFE can easily transfer to a counter surface;5 this excellent tribological performance is another advantage of PTFE. Some researchers have suggested that friction and wear of PTFE are strongly influenced by its working environment, such as humidity,6−8 inert gas,9 and vacuum.10,11 For example, Krick et al. reported that environmental humidity affects the tribochemistry of PTFE composite (i.e., not its mechanical properties) and is responsible for its wear behavior;8 that is, the amount of PTFE wear is increased under low humidity. The kind of metal composing in counter face also affects the tribochemistry of PTFE.12,13 Gong et al. observed the formation of a metal fluoride when aluminum or iron was rubbed with PTFE, but they did not observe it when copper was rubbed with PTFE.12 While the above-described breakthrough results give an important insight into tribochemical effects on friction © 2013 American Chemical Society

and wear behavior, the mechanism by which a friction-induced transfer film is formed and its detailed chemistry are still unclear. However, obtaining knowledge about a transfer film on the nanoscale level, which corresponds to the size of a transfer film, is not easy by only conventional experimental and analytical techniques owing to some difficulties of in situ observation during the sliding process. In the field of nanotribology, methods applying computational chemistry have emerged as a powerful tool for analyzing the mechanism of friction on the atomistic scale, and they have been extensively applied so far.14−21 In fact, they have been used to study the friction of PTFE.15,16 Using a moleculardynamics (MD) method, Jang et al. analyzed the friction between a PTFE polymer chain and itself, and they found that the structural orientation of PTFE contributes to its tribological properties.15 However, to the authors’ knowledge, there has been no theoretical study that focuses on the behavior and formation of a PTFE transfer film on a metallic surface. Accordingly, in this study, to investigate a tribochemical reaction mechanism on a sliding interface between PTFE and metallic surface, we applied a method based on density functional theory (DFT) and a method based on quantum chemical MD (QCMD). To study the influence of several ambient gases on the formation of the PTFE transfer film, we also applied a classical MD method. Received: January 16, 2013 Revised: April 15, 2013 Published: April 22, 2013 10464

dx.doi.org/10.1021/jp400515j | J. Phys. Chem. C 2013, 117, 10464−10472

The Journal of Physical Chemistry C

Article

⎡ Z Z e2 ⎛ ai + aj − rij ⎞⎤ i j ⎟⎟⎥ + f0 (bi + bj) exp⎜⎜ U = ∑∑⎢ ⎢ + b b r ⎝ ⎠⎥⎦ ij i j ⎣ i j>i

2. COMPUTATIONAL METHODS 2.1. Quantum Chemistry Methods. Two quantum chemistry methods, viz., DFT and QCMD, were used to investigate the interfacial tribochemical reaction mechanism of PTFE adsorbed on a metallic surface. For the DFT simulation, DMol3 implemented in Accelrys Materials Studio package version 4.0 was used. To reduce the computation cost of the DMol3 calculation, all core electrons were represented by the effective core pseudopotentials. Double numerical basis sets with polarization were employed, and a generalized gradient approximation in terms of Perdew−Burke−Ernzerhof exchangecorrelation functions22 was used to optimize geometries and evaluate electronic energy. The charge population was analyzed by the Hirshfeld method.23 QCMD simulation was performed by the “Colors” program,17,24,25 which is based on the extended Hückel method,26 namely, the LCAO (linear combination of atomic orbital) molecular orbital method. The program uses our original tightbinding approximation, in which a long-range Coulombic interaction is explicitly considered.27 The Colors program thus allows the study of a system including both covalent and ionic interactions, although the original Hückel method does not treat any ionic materials. Total potential energy, ET, used in the Colors program is expressed as occ

ET =

∑ nkεk + ∑ ∑ k

i

j>i

ZiZje2 rij

⎡

+

+

i

+

j>i

⎣

+

(3)

⎛A

∑ ∑ ⎜⎜

ij 12 r j > i ⎝ ij

−

Bij ⎞ ⎟ rij6 ⎟⎠

The first term corresponds to the Coulomb potential, and the second term corresponds to the short-range exchange repulsion potential ( f 0 is a constant for unit adjustment, a represents size, and b represents stiffness), which gives a good account of the repulsive interactions arising from the overlap of electronic clouds. These terms are well known as the Born−Mayer− Huggins-type potential function, which has been successfully applied to various ionic solid materials.29−31 The third, fourth, and fifth terms in eq 4 represent covalent interaction; Morsetype potential (Dij is bond energy, βij is a form factor, and r0 is equilibrium bond length), angle potential (Hθ is a force constant, θ is bending angle, and θ0 is equilibrium bending angle), and torsion potential (Hφ is a force constant, n is a repeating number, φ is dihedral angle, and φ0 is equilibrium dihedral angle), respectively. The sixth term corresponds to the intermolecular Lennard-Jones (LJ) potential (A and B are constants for each atom), which represents the vdW interaction. The system temperature was controlled by scaling atomic velocity. The velocity Verlet algorithm32 was adopted to solve the equation of motion. The Ewald method28 was used to compute the long-range Coulombic interaction.

On the right-hand side of eq 1, the first term is the summation of the eigenvalues for all of the occupied molecular orbitals (“occ” means the occupied molecular orbital and nk and εk are the number of the electrons occupying the kth molecular orbital and its energy, respectively), the second term represents the long-range Coulombic interaction (rij is interatomic distance, Z atomic charge, and e elementary electric charge), and the third term corresponds to short-range exchange repulsion energy. (aij and bij are constants for each atomic pair). The electronic state of the atomic system was calculated selfconsistently by solving the deformed Schrödinger equation as follows:

CTSC = I

φ

(4)

(1)

(2)

∑ Hθ(θ − θ0)2 + ∑ Hφ[1 + cos(n·φ − φ0)]

i

⎛ aij − rij ⎞⎤ ⎥ ⎟⎟ ⎝ bij ⎠⎥⎦

HC = εSC

j>i

θ

∑ ∑ ⎢⎢bij exp⎜⎜ i

∑ ∑ Dij{exp[−2βij(rij − r0)] − 2 exp[−βij(rij − r0)]}

3. RESULTS AND DISCUSSION 3.1. Mechanism for Formation of Transfer Film on PTFE/Al2O3 Interface. The formation of a PTFE transfer film contacting a metallic surface was first investigated by using the Colors program. For the metallic surface, aluminum was chosen because it is used as a conventional material for frictional parts in many machines. The model used for the interface between crystalline PTFE (144 atoms) and amorphous Al2O3 (120 atoms), which is the native oxide layer on the aluminum surface, is shown in Figure 1a. In the initial stage of the tribological process, the topmost surface of Al2O3 was terminated by the hydroxyl (OH) groups, namely, an aluminum hydroxide, due to the dissociative chemisorption of water molecules in atmosphere. Thus, another model including Al2O3 surface with OH groups (156 atoms) shown in Figure 1b was also used for considering the surface passivation by water. During the tribological process, it is well known that the surface OH group reacts thermochemically33 and mechanochemically34 with others to reform the water molecules under a shear condition, and a bare Al2O3 surface is exposed. The model shown in Figure 1a mimics this situation. The Colors program requires several parameters for accelerating computation. Thus, before the simulation by Colors was run, the validity of QCMD parameters, such as the Hamiltonian term and Slater exponent, was first checked. The determined parameters used are listed in Table 1. To show the validity of the determined parameters, the atomic charge calculated by

Here H, C, ε, S, and I refer to the Hamiltonian matrix, eigenvectors, eigenvalues, overlap integral matrix, and unit matrix, respectively. In the Colors program, the double-ζ Slatertype basis set was employed, and long-range Coulombic interactions were computed by the Ewald method.28 Moreover, our tight-binding approximation introduces several parameters, such as a Hamiltonian term and Slater exponent, ζ, to accelerate computation; it thus permits faster computation than a conventional first-principles MD method. Details of the Hamiltonian expression and its parameters are given in the references.25 2.2. Molecular Dynamics Method. To study the influence of ambient gaseous molecules on the formation of a PTFE transfer film, we conducted the classical MD simulation by inhouse program. To consider the ionic, covalent, and weak van der Waals (vdW) interactions among atoms, we employed the following potential functions. 10465



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Figure 1. Model for simulating tribochemical reaction dynamics between PTFE and (a) bare or (b) OH-terminated Al2O3surface.

reaction dynamics could be observed with reasonable computation time. Moreover, the bottom layer of PTFE was completely fixed during the QCMD simulation. Snapshots of the interface between the PTFE and bare Al2O3 (without OH-termination) for several simulation times are shown in Figure 2. In snapshot (b), a first formation of an Al−F chemical bond during friction process can be seen. As shown from snapshots (c) to (f), the Al−F bond was not dissociated completely and always existed at the interface. The overall structure at the simulation time of 25 ps is shown in Figure 3a. Note that the Figure ignores the periodic boundary condition for the ease of understanding. According to the Figure, the top layer of PTFE adsorbs onto the Al2O3 surface via Al−F chemical bonds and tracks with the sliding Al2O3 layer, while the middle layer of PTFE does not form any clear chemical bonds with the top or bottom layers. It is thus inferred that the PTFE molecules transfer to the Al2O3 counter face by forming Al−F chemical bonds and that the main sliding plane appears inside the PTFE layer, that is, not in the PTFE/Al2O3 heterointerface. This interlayer slippage may cause self-lubricity and thereby reduce friction and inhibit wear. Similar to the bare Al2O3 surface, it can be seen that PTFE transfers also to the OH-terminated Al2O3 surface via mainly H−F interactions. (See Figure 3b.) The nature of the Al−F bond, which is a key factor in the transfer of PTFE to Al2O3, was then analyzed. The bond overlap population of Al−F pairs, indicating the degree of covalency between atoms, is shown in Figure 4. Here Mulliken’s bond overlap population between atoms A and B, BPA−B, is computed by35,36

Colors was compared with that calculated by accurate DFT calculation by DMol3. Charges on atoms in PTFE and Al2O3 calculated by Colors and DMol3 are listed in Table 2. Note that the calculations for both PTFE and Al2O3 bulk models were done separately. The results of the calculations by Colors and DMol3 agree fairly well. Also, the bond energy for C−F and Al−F bonds was compared between the results by Colors and DMol3 programs because the PTFE layer chemically interacts with Al2O3 via its fluorine atoms (not a carbon backbone). This comparison was done for C3F8 and AlF3 molecules. The bond energy by Colors was calculated by eq 13 in ref 17, and that by DMol3 was calculated by the following equation. EC − F = EC3F8 − (EC3F7 + E F)

(5)

Here EC−F is the C−F bond energy and EC3F8, EC3F7, and EF are the electronic energy of C3F8 molecule, C3F7, and F radicals, respectively. The Al−F bond energy was calculated by the same scheme. The C−F and Al−F bond energies calculated by Colors and DMol3 are also listed in Table 2, and it can be seen that the result by Colors agrees with that by DMol3 with almost 5% accuracy. From these comparisons, it is hence confirmed that the Colors program with our determined parameters well reproduces the results obtained by the accurate DFT method. It is worth mentioning that the energy barrier of bond formation or dissociation should be actually considered for observing tribochemical reaction dynamics between PTFE and Al2O3. The present study does not consider well the energy barrier. Therefore, the QCMD simulation with more accurate definition of electronic state is our next target. Using the constructed model and determined parameters, the QCMD simulation by Colors was subsequently performed for 50 000 steps with integration time of 0.5 fs. A periodic boundary condition was used, and the system temperature was set to 300 K. To simulate the friction condition, a vertical pressure was applied to the topmost surface of the Al2O3 layer, while it was slid at a certain horizontal velocity. The pressure was set as 0.5 GPa in correspondence with typical Hertzian contact pressure under a boundary friction condition. The sliding velocity was set as 100 m/s so that the tribochemical

occ

A

B

BPA − B = 2 ∑ nk ∑ ∑ CkrSrsCks k

r

s

(6)

In the Figure, the interatomic distance between targeted Al and F atoms is also shown. When the interatomic distance between Al and F closes to 0.21 nm, the bond overlap population pairs is slightly increased, but it remains almost zero on average throughout the simulation. This result means that the nature of the chemical bond between Al and F atoms is not covalent but 10466



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Table 1. Parameters for the QCMD Simulation by Colors Program atom H C 0 F Al atom H H H H H C C C C O O O F F Al atom H H H H H H H H H a

orbital

orbital

Ir (eV)a

ζr (Å−1)b

atom

orbital

atom

orbital

κrs

δrs

1s 2s 2p 2s 2p 2s 2p 3s 3p atom

−13.6 −21.4 −11.4 −32.3 −14.8 −40.0 −18.1 −12.3 −6.5 aij (Å)

1.3000 1.8087 1.6851 3.1080 2.5240 3.7701 2.4947 1.5166 1.3063 bij (Å)

C C C C C C C C C C C C C C C O O O O O O O O O O O F F F F F F F Al Al Al

2s 2s 2p 2s 2s 2p 2p 2s 2s 2p 2p 2s 2s 2p 2p 2s 2s 2p 2s 2s 2p 2p 2s 2s 2p 2p 2s 2s 2p 2s 2s 2p 2p 3s 3s 3p

C C C O O O O F F F F Al Al Al Al O O O F F F F Al Al Al Al F F F Al Al Al Al Al Al Al

2s 2p 2p 2s 2p 2s 2p 2s 2p 2s 2p 3s 3p 3s 3p 2s 2p 2p 2s 2p 2s 2p 3s 3p 3s 3p 2s 2p 2p 3s 3p 3s 3p 3s 3p 3p

1.020 0.375 0.705 0.001 0.001 0.001 0.001 0.500 0.600 1.500 0.400 0.001 0.001 0.001 0.001 0.360 0.300 0.300 0.001 0.001 0.001 0.001 0.200 0.700 1.200 0.700 0.300 0.300 0.300 0.200 1.000 0.450 0.478 0.300 0.300 0.300

0.344 0.161 0.070 0.300 0.070 0.070 0.050 0.300 0.150 0.150 0.070 0.300 0.070 0.070 0.050 0.300 0.150 0.050 0.300 0.070 0.070 0.050 0.300 0.070 0.070 0.050 0.100 0.100 0.100 0.200 0.100 0.080 0.080 0.100 0.100 0.100

H C O F Al C O F Al 0 F Al F Al Al atom

1s 1s 1s 1s 1s 1s 1s 1s 1s

H C C O O F F Al Al

1.800 2.200 1.900 1.600 2.200 1.880 2.400 1.600 1.650 2.800 2.400 2.170 1.950 1.700 2.800 orbital 1s 2s 2p 2s 2p 2s 2p 3s 3p

κrs 0.200 0.001 0.001 0.100 0.300 0.200 0.600 0.001 0.001

0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 0.100 δrs 0.938 0.300 0.070 0.200 0.050 0.300 0.070 0.300 0.070

Valence state ionization potential. bExponent of Slate-type orbital.

Table 2. Atomic Charge and Bond Energy Calculated by Colors and DMol3 Programs

3.2. Tribochemical Reaction Dynamics in PTFE/Al2O3 System. The PTFE/bare Al2O3 interfacial structure during the dry friction process was then carefully checked in the QCMDobtained snapshots, and an interesting chemical reaction between other C−F pair in the PTFE molecule could be seen. That is, a bond dissociation of a C−F pair in a PTFE molecule was observed (see Figure 5a), while the molecule transfers to the Al2O3 surface via Al−F ionic bonds. The dissociated F atom from the PTFE backbone makes a new chemical bond with the Al atom in the Al2O3 surface. As a result of this chemical reaction, an unsaturated carbon radical was formed in the PTFE molecule, and the Al2O3 surface was partially terminated by a fluorine atom. This chemical reaction is explained as follows. The change of interatomic distance and bond overlap population for the targeted C−F and Al−F pairs are shown in Figures 6 and 7, respectively. Note that the focused atoms here are not the same as those discussed in Section 3.1. According to Figure 6, the C−F interatomic distance was elongated to 0.3 nm at a simulation time of 8.0 ps, while the Al−F interatomic distance was reduced. As shown in Figure 7, the Al−F pair shows clear covalency after 8.0 ps because its bond overlap population is positive. The bond overlap population of C−F

atomic charge atom PTFE A12O3

C3F8 A1F3

C F Al O

colors

DMol3

0.178 0.173 −0.089 −0.086 0.432 0.434 −0.288 −0.289 bond energy (kJ/mol)

bond

colors

DMol3

C−F Al−F

−559.1 −697.9

−546.9 −663.5

ionic. For the model with OH-terminated Al2O3 surface, the bond overlap population between H and F at the interface was almost zero while they closed to 0.14 to 0.16 nm, also indicating ionic nature. Hence, it is worth suggesting that the formation of interfacial ionic bonds plays an important role in forming a PTFE transfer film on both bare and passivated aluminum counter faces. 10467



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Figure 2. Snapshots of PTFE/Al2O3 interface at simulation times of (a) 0, (b) 5, (c) 10, (d) 15, (e) 20, and (f) 25 ps.

Figure 4. Bond overlap population and interatomic distance between aluminum and fluorine atoms during friction simulation.

pair was zero after 8.0 ps, indicating complete bond dissociation between them. To further study the radical formation mechanism in the PTFE/Al2O3 system, accurate DFT simulation was performed, and the geometry of the PTFE adsorbed on Al2O3 surface was optimized by using the DMol3 program. The model includes an Al4O6 cluster and a C3F8 molecule, which mimic a surface adsorption site on Al2O3 and a small fragment of a fluorocarbon

polymer chain, respectively. According to the DFT-optimized structure of a C3F8 molecule adsorbed on Al4O6, the C−F bond distance in the vicinity of the aluminum site is increased from 0.135 to 0.144 nm, while fluorine and aluminum atoms are 0.208 nm apart. The C−F bond weakening was caused by the charge transfer from the C3F8 molecule to the Al4O6 cluster; the total charges of C3F8 and Al4O6 were 0.267 and −0.267,

Figure 3. Atomic structure of the model including (a) bare or (b) OH-terminated Al2O3 surface at the simulation time of 25 ps. In both Figures, a periodic boundary condition is ignored for clear understand. 10468



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Figure 7. Bond overlap population of C−F and Al−F pairs.

charge was 0.246 and −0.246 for C3F8 and Al4O6, respectively.) It is thus considered that the observed phenomenon, namely, the C−F bond is dissociated on the rubbing Al2O3 surface, is initiated by charge transfer at the interface. According to the DFT calculation, the C−F bond did not completely dissociate. Introducing a friction process including compression, shear force, and frictional heat is probably needed for forming a radical species by breaking the C−F bonds in PTFE because the radical formation always requires high activation energy (such as provided by ultraviolet irradiation). To clarify the effect of sliding on radical formation in PTFE, QCMD simulation was conducted for the model shown in Figure 1 with sliding velocity of 0 m/s. The Al−F bond formation and C−F bond dissociation were not observed during the simulation. (See Figure 5b.) This result suggests that the Al2O3 surface acts as a Lewis acid and tribochemically removes fluorine atoms from the PTFE chain, thereby forming radical sites. It also suggests the reason that the C−F bond dissociation was not indicated by the static DFT calculation by DMol3 because the program did not consider finite temperature or any friction effects. In fact, the Al−F bond formation was found experimentally by analyzing bonding states on a rubbing surface by X-ray photoelectron spectroscopy (XPS).12 The XPS results also suggest the C−F bond dissociation and radical formation paradoxically. Also, the C−F bond dissociation and Al−F bond formation were not observed in the model including OH-terminated Al2O3 surface during the QCMD simulation with sliding. (See Figure 5c.) This result tells us that the passivated Al2O3 surface by hydroxyl groups hardly activates the tribochemical reaction of PTFE and the exposure of bare Al2O3 as Lewis acid is thus needed to trigger the reaction. The radicalized PTFE formed on the bare Al2O3 surface potentially has a high reactively and probably reacts easily with ambient gas molecules, such as water and oxygen, providing several tribochemical products that may show different transfer film formability from that of genuine PTFE. Actually, on the rubbing surface of PTFE under atmospheric conditions, a carboxyl group, which is likely to be the product of a chemical reaction between PTFE and water and oxygen, was detected by XPS.8 We believe that the formation of tribochemical radicals in PTFE by the Al2O3 surface is one of the key processes that achieve the low friction and antiwear performance of PTFE. The chemical reaction dynamics between radicalized PTFE and gas molecules is the target of a future study.

Figure 5. Atomic structure of the PTFE/Al2O3 interface at the simulation time of 25 ps under sliding conditions (a) 100 and (b) 0 m/s. (c) Atomic structure of the interface between PTFE and Al2O3 terminated by hydroxyl groups at the simulation time of 25 ps under 100 m/s sliding condition.

Figure 6. Interatomic distance of C−F and Al−F pairs.

respectively, meaning that the Al4O6 cluster plays a role as a Lewis acid. Note that the Colors program also showed a similar trend in the charge transfer for the same model. (The total 10469



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3.3. Influence of Ambient Gas Molecules on Formation of PTFE Transfer Film. As described in Section 1, friction and wear of PTFE are influenced strongly by its working environment, such as atmospheric humidity.6−8 It is thus important to determine how the ambient gas affects the formation of a PTFE transfer film. Accordingly, the formation of a transfer film under ambient gas conditions was investigated. For this purpose, the classical MD method, which can treat a largescale system including gas molecules, was used. Although the simulation described in the following cannot reflect the clarified chemical reaction of PTFE in Section 3.2 due to classical mechanics, it can explicitly consider interaction between the PTFE and the ambient gas molecules. We believe that the simulation provides some chemical insights into the effect of ambient gas on formation of a PTFE transfer film. Use of a reactive potential such as ReaxFF37 and REBO potential38 for the classical MD simulation is consequently our future target to consider the chemical reaction between the solid surface (aluminum surface and PTFE) and ambient gas molecules. To determine the influences of humidity on transfer film formation, two types of the simulation model were built: one included water molecules at the interface between PTFE and bare Al2O3 surface, and the other included nitrogen molecules instead of water molecules. (See Figure 8.) Because nitrogen

For constructed models, MD simulation was performed for 500 000 steps with integration time of 1.0 fs. Temperature was set as 300 K. In the same manner as the QCMD simulation described in Section 3.1, to simulate the friction condition, a vertical pressure was applied to the topmost surface of the Al2O3 layer, while it was forcedly slid with a horizontal velocity. Pressure and sliding velocity were set as 0.5 GPa and 10 m/s, respectively. The bottom layer of PTFE was fixed. The parameters used in eq 4 were referred from a previous study39 for Al2O3 and from a consistent valence force field40 for PTFE. For PTFE/Al2O3 interface, only Coulombic and weak LJ interactions (not a covalent interaction) were introduced because the QCMD study described in Section 3.1 showed that ionic interaction is dominant at the interface including both bare and OH-terminated Al2O3 surfaces. The MD simulations for both water- and nitrogen-including models showed that PTFE molecules in the vicinity of the interface tracked with the sliding Al2O3 (both bare and OHterminated surfaces) and that the slip in these models occurred inside the PTFE layer, indicating transfer film formation (not shown here). To compare the difference of transfer-film formation between the water vapor and nitrogen conditions, we analyzed the movement of atoms in transferred PTFE onto a counter Al2O3 surface at the final MD step (500 ps). Figure 9a,b

Figure 8. Model of the influence of ambient gaseous molecules on formation of PTFE transfer film.

gas is chemically inert under room-temperature conditions, it is suitable to reflect a dry condition (0% humidity) for the classical MD simulation. The number of inserted gas molecules is 40. It should be mentioned that to determine a suitable inserting number, we preliminarily performed the MD simulation with several numbers of molecules, that is, 20, 40, 80, and 160. As a result, no clear difference between transfer of PTFE molecules under the water and nitrogen conditions was seen when 20 molecules were inserted into PTFE/Al2O3 interface, and PTFE could not contact the Al2O3 surface (gas lubrication state) when 80 or more molecules were inserted. According to this result, the formation of a PTFE transfer film by insertion of 40 molecules was chosen to simulate the boundary lubrication. In addition, the model including OH-terminated Al2O3 surface was also used for the MD simulation to consider the system in the initial stage of the tribological process (not shown here). Therefore, the MD simulation was conducted for totally four types of model (PTFE/bare Al2O3 with water or nitrogen molecule, PTFE/OH-terminated Al2O3 with water or nitrogen molecule).

Figure 9. Movement of atoms in PTFE layer at the simulation time of 500 ps for the models including (a) bare or (b) OH-terminated Al2O3 surface. 10470



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Figure 10. Schematic illustration of PTFE/Al2O3 interface with (a) water and (b) nitrogen molecules.

charges in classical MD are 0.41 for hydrogen atom and −0.82 for oxygen atom). Thus, a water molecule plays a role as an adhesive agent in the transfer film formation as far as tribochemical reaction between water and radicalized PTFE is not considered. Nitrogen is a nonpolar and electrically neutral molecule. In the case of nitrogen, the PTFE molecule hardly adheres to not only bare Al2O3 surface but also OH-terminated Al2O3 surface because of shielding of the ionic interaction between the two surfaces. (See Figure 10b.) This is the reason that the amount of PTFE transfer is reduced under the dry condition. It is important to mention that a polar molecule in the gas phase or several polar substituents in the PTFE polymer chain (hydroxyl or carboxyl groups) are necessary to form the transfer film and finally reduce wear when the aluminum is used as counter surface of PTFE.

shows the movement of atoms in PTFE in the sliding direction for the models including bare and OH-terminated Al2O3 surfaces, respectively. In the Figures, the distribution of atomic movement in the perpendicular direction to the sliding is shown. In panel a, the atomic movement in the model including water is much larger than that in the nitrogen-including model. As shown in panel b, a larger movement of the atoms in PTFE can be seen in the water-including model also on the OH-terminated surface. By comparing the panels a and b, for both water- and nitrogen-including models, it can be clearly understood that the movement of atoms in PTFE on the OH-terminated surface is less than that on bare surface. This is due to the OH-passivation; that is, the hydrogen atoms in the surface hydroxyl group have a less positive charge than the aluminum atoms in the bare aluminum oxide surface, meaning that the interfacial ionic interaction (necessary for transferring) on passivated surface is weaker than bare surface. From these results, therefore, the degree of PTFE transfer in nitrogen is less than that in water without relation to the surface chemical state of aluminum (OH termination or not) during tribological process. This result indicates that the low-friction and antiwear performances of PTFE under humid conditions are better than that under the dry condition because the role of the transfer film is to protect the PTFE surface from direct contact with the metallic surface. Experimental study also showed an increment of the amount of wear under low humidity;8 thus, the MD simulations well-reproduced the tribological trend concerning humidity. The difference of PTFE transfer caused by humidity is explained by the chemical nature of ambient gas molecules. The PTFE/Al2O3 interface with water and nitrogen molecules is illustrated schematically in Figure 10a,b, respectively. As mentioned in Section 3.1, the ionic interaction is dominant on the PTFE/Al2O3 interface. As shown in Figure 10a, when water molecules exist between the two surfaces, the PTFE molecule can adhere to the counter Al2O3 surface with and without OH-termination because the ionic interaction is not shielded. This is due to a high polarizability of a water molecule (atomic

4. CONCLUDING REMARKS The mechanism by which a transfer film composed of PTFE/ Al2O3 is formed was investigated by an approach based on computational chemistry. The mechanism of PTFE transfer film formation on the Al2O3 surface was traced by using QCMD simulation. The topmost surface of the PTFE layer tracked with the sliding Al2O3 layer and interlayer slippage inside the PTFE layer was observed, causing self-lubricity. It was also found that interfacial ionic bonds played an important role in the formation of the transfer film on the counter aluminum surface. The PTFE molecule tribochemically reacted with Al2O3 as a Lewis acid to form radicalized PTFE by removing fluorine atoms. Because the radical potentially has a high reactively, it probably reacts easily with the ambient gaseous molecules, such as water vapor (humidity) and oxygen. This tribochemical reaction in PTFE is one of the key processes that produces the good tribological performance of PTFE. The transfer-film formation of PTFE interacting with water or nitrogen molecules was also studied by using the classical MD method. The degree of PTFE transfer in nitrogen was less than that in water vapor because the nitrogen molecules shields the interfacial ionic 10471



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interaction, which is important to form the transfer film, while the water molecules play a role as an adhesive agent.

■

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-294-52-5111. Fax: +81-294-52-7622. Notes

The authors declare no competing financial interest.

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