Chemical Reaction Mechanism of Polytetrafluoroethylene on

Article pubs.acs.org/JPCC

Chemical Reaction Mechanism of Polytetrafluoroethylene on Aluminum Surface under Friction Condition Tasuku Onodera,*,† Kenji Kawasaki,† Takayuki Nakakawaji,† Yuji Higuchi,‡ Nobuki Ozawa,‡ Kazue Kurihara,§,∥ and Momoji Kubo‡ †

Hitachi Research Laboratory, Hitachi, Ltd., 7-1-1 Omika-cho, Hitachi 319-1292, Japan Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, 6-6-11 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan § WPI Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ∥ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡

ABSTRACT: To develop a novel shearing resin material, it is necessary to understand the mechanism of friction-induced chemistry during the friction process. For this purpose, the chemical reaction of the polytetrafluoroethylene (PTFE) resin on an aluminum surface during friction was first focused on and investigated by a quantum chemical molecular dynamics method. From our simulation, an aluminum atom on a native oxide of aluminum surface led to a tribochemical reaction, which included defluorination of PTFE and aluminum fluoride formation. It was inferred that the aluminum surface acted as a catalytic Lewis acid in which the fluorine atom was removed from the PTFE polymer chain. The tribological performance of the investigated system was reduced by the forming of aluminum fluoride since a self-lubrication by PTFE was inhibited by an interfacial electrostatic repulsion. On the basis of our study, it was suggested that the key to increase tribological performance was a chemical reaction between reactive defluorinated PTFE and environmental water vapor to form a novel functional group on the PTFE chain.

1. INTRODUCTION Polytetrafluoroethylene (PTFE) resin is usually used as a sliding material with and without liquid lubricants because of its excellent chemical stability, low friction, antiwear, and sealing performances. For using engineering materials, PTFE has been applied as composites with carbon,1,2 metal,2 and ceramics3,4 fillers to increase mechanical and, sometimes, thermal properties. Whereas the resin or its composite is always naturally much softer than any conventional metallic materials, it can actually be applied as sliding parts that contact metallic parts, especially steel and aluminum alloys. It is generally recognized that lubrication by PTFE against metallic surfaces is based on formation of a transfer film onto a metallic counter surface.5−8 The thickness of the formed transfer film is typically in the nanometric scale,8 i.e., 1−10 nm, and this prevents the PTFE resin and metallic surface directly contacting. This selflubrication by PTFE reduces friction and wear to a sliding system. On the other hand, several aspects of working environments of PTFE such as humidity2,3,9 and inert gas10 strongly affect the tribological properties of PTFE. For example, the wear amount of PTFE is increased under low humidity due to less change in chemical structure and the nature of transfer film during the friction process.3 Actually, this tribochemical effect has limited © 2014 American Chemical Society

the use of PTFE resin to only humid conditions. Thus, a novel PTFE-based resin composite needs to be developed for dry conditions (without water vapor). To develop such materials, it is necessary to work out the mechanism of friction-induced transfer film formation and its detailed chemistry depending on working environments. However, obtaining knowledge about a transfer film at 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, a computational chemistry method is a powerful tool for analyzing the mechanism of friction at the atomistic scale, and they have been extensively applied so far.11−18 The PTFE sliding system has also been investigated by this method.12,19 Recently, the initial stage of the transfer film formation onto metallic surface, which was supported by interfacial chemical bonds, was successfully simulated by our original quantum chemical molecular dynamics (QCMD) method.19 Accordingly, in this study, the tribochemical reaction between PTFE and metallic surface after Received: December 16, 2013 Revised: February 17, 2014 Published: February 20, 2014 5390

dx.doi.org/10.1021/jp412461q | J. Phys. Chem. C 2014, 118, 5390−5396

The Journal of Physical Chemistry C

Article

forming transfer film was focused on since the change in the chemically stable polymer structure is probably caused by interaction with environmental molecules, viz. water vapor. For this purpose, the QCMD method was applied to the friction interface between PTFE and metallic surface and their chemical reaction was investigated.

2. COMPUTATIONAL METHOD QCMD simulation was performed by the “Colors” program,14,19−21 which is based on the LCAO (linear combination of atomic orbital) molecular orbital method. The program uses our original tight-binding approximation, in which a long-range Coulombic interaction is explicitly considered.22 The Colors program thus allows the study of a system including both covalent and ionic interactions. The total potential energy, ET, used in the Colors program is expressed as occ

ET =

∑ nkεk + ∑ ∑ k

i

j>i

ZiZje 2 rij

⎡

+

⎛ aij − rij ⎞⎤ ⎟⎟⎥ ⎝ bij ⎠⎥⎦

∑ ∑ ⎢⎢bij exp⎜⎜ i

j>i

⎣

(1)

Figure 1. Model for simulating tribochemical reaction on the PTFE/ α-Al2O3 interface.

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; 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 self-consistently by solving the deformed Schrödinger equation as follows: HC = εSC

(2)

CTSC = I

(3)

α-Al2O3 surface. The orientation of PTFE was the same for all layers. The model includes 448 atoms in total. The Colors program requires several parameters for accelerating computation as described in section 2. In this study, the parameters from our previous study were referred to.19 It is worth mentioning that the QCMD simulation by Colors with these parameters could well-represent electronic states of the focusing materials obtained by accurate density functional theory (DFT) calculations. Using the constructed model and parameters, the QCMD simulation by Colors was subsequently performed for 10 000 steps with an integration time of 0.2 fs. A periodic boundary condition was used, and the system temperature was set to 300 K. To simulate the friction condition, vertical pressure was applied to the topmost surface of the α-Al2O3 layer while it was slid at a certain horizontal velocity. Experimentally, a wear test for PTFE/metal couple is usually done under contact pressure of MPa order (6.3 MPa3), which is a few ten percent of the yield strength of PTFE resin (20−30 MPa at room temperature25). However, on a real contact area of our focusing interface, the pressure is probably much higher than a macroscopic pressure due to point contact. To reflect this fact to molecular level simulation, a contact pressure of a few hundred MPa was always employed for studying PTFE tribology by MD.26 Therefore, the contact pressure in our simulation was also set as 500 MPa, which corresponds with a typical pressure of real contact area under a boundary friction condition. The sliding was given parallel to PTFE chains because this direction would be energetically stable in sliding process.13 Also, the velocity was set as 100 m/s to observe the sliding effects with reasonable computation time. The bottom layer of the carbon backbone in PTFE was completely fixed during the QCMD simulation. The atomic structure of the model system was visualized by Materials Studio software developed by Accelrys. Figure 2a shows the snapshots of overall structure of our sliding system. The figure ignores the periodic boundary condition for ease of understanding. The top layer of PTFE adsorbs onto the α-Al2O3 surface and tracks with the sliding αAl2O3 layer, while the middle layer of PTFE hardly deforms. By this, the main sliding plane appears inside the PTFE layer, that

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.23 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 ref 21.

3. RESULTS AND DISCUSSION 3.1. Formation of Transfer Film on Passivated Aluminum Surface. Formation of a PTFE transfer film onto metallic surface was first investigated by using the Colors program. Aluminum oxide (α-Al2O3), which was recognized as a typical surface of aluminum alloy, was chosen as the metallic part because the material was used for frictional parts in many machines. Note that actual surface of aluminum alloy is more complex since it contains other metal elements (magnesium, silicon, copper, and so on) for increasing its hardness. The model used consisted of α-Al2O3 and three layers of crystalline PTFE (see Figure 1). The topmost surface of α-Al2O3 was fully terminated by the hydroxyl (OH) groups to mimic the dissociative chemisorption of water vapor in atmosphere. To reflect an experimental insight for polymer reptation under rubbing process,24 all PTFE chains were aligned parallel to the 5391



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Figure 2. (a) Overall structure of the used model during friction simulation and (b) enlarged illustration for PTFE/α-Al2O3 interface.

is, not in the PTFE/α-Al2O3 heterointerface. Moreover, the chemical interaction between the hydrogen atom in terminated OH group and the fluorine atom in PTFE can be seen on the interface as shown in Figure 2b. The interatomic distance of this H−F pair is 0.22 nm on average, which is less than that in the hydrogen fluoride molecule (about 0.10 nm). Therefore, the observed chemical bond may have less covalency. It is thus inferred that weak hydrogen bonds between the OH group and fluorine atoms act as an adhesive joint for PTFE transfer film formation on aluminum surface. The interlayer slippage as a resultant of this resin transfer film formation may cause selflubricity and thereby reduce friction and inhibit wear. 3.2. Tribochemical Reaction between PTFE and Aluminum Surface. Although the process of transfer film formation was observed in our QCMD simulation under friction condition, no clear chemical reactions in PTFE polymer backbone or its surface could be observed on the fully terminated α-Al2O3 surface. During the tribological process, it is generally known that the surface OH group, which is a resultant product of chemisorptions of water vapor in atmosphere, thermochemically27 or mechanochemically28 react with others to re-form the water molecules under a friction condition. This means that the bare aluminum atoms coordinating with less oxygen atoms would be exposed on the α-Al2O3 surface during the friction process. According to this hypothesis, the dehydration situation should be reflected in our simulation model. Thus, the QCMD simulation was subsequently performed for the model considering partial dehydration on the OH-terminated α-Al2O3 surface. The model was similar to that described in section 3.1, but two water molecules (two OH groups and two hydrogen atoms) were removed from the αAl2O3 surface so as to expose aluminum atoms as shown in Figure 3a. The QCMD simulation considering friction was performed with the same conditions as those in section 3.1. Figure 3b shows the final structures of this system, and a bond dissociation reaction between carbon and fluorine atoms was observed in the vicinity of the exposing aluminum atom. Moreover, the bond formation reaction was found in which the dissociated fluorine atom bonded to a bare aluminum atom. The results indicate that PTFE chemically reacts with a partially dehydrated aluminum surface to form aluminum fluoride as a product. To understand the mechanism of this chemical reaction deeply, the electron transfer on PTFE/α-Al2O3 interface was

Figure 3. (a) Aluminum atom exposed on α-Al2O3 surface and (b) the same atom during friction simulation.

Figure 4. Change in atomic charge of aluminum atom and bond overlap population between reacted aluminum and fluorine atoms.

then analyzed because it always triggers a chemical reaction. Figure 4 shows the change in atomic charge of the focusing aluminum atom and the bond overlap population, BP, of the newly formed chemical bond between fluorine and bare aluminum atoms. Here, the BP indicates the degree of covalency between atoms and is defined by the following equation29,30 for atom pair A and B. occ

A

B

BPA − B = 2 ∑ nk ∑ ∑ CkrSrsCks k

r

s

(4)

From the figure, the atomic charge of the aluminum atom was negatively increased during the simulation, meaning that the 5392



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formed. Therefore, the α-Al2O3 surface acts as a catalyst. By this defluorination by aluminum surface, the unsaturated carbon atom is formed inside PTFE backbone. Since this molecular structure is chemically unstable, the chemical bond between carbon atoms is dissociated and its conformation changes into the double bond to stabilize. Note that a frictional contact would be necessary for driving the unveiled chemical reaction as far as the partially dehydrated surface is considered in the system because no clear chemical reaction can be found under the condition of atmospheric pressure, i.e., 0.1 MPa (see Figure 7). Under a low contact pressure, PTFE cannot be contacted

electron was transferred from PTFE polymer to aluminum. At the simulation time of 0.5 ps, which corresponds to the time at which this electron transfer was almost accomplished, the BP between aluminum and fluorine atoms was increased, indicating bond formation. Hence, it is inferred that the bare aluminum atom on the α-Al2O3 surface acts as an electron acceptor, viz. Lewis acid, and removes fluorine atom from PTFE backbone. A chemical change in PTFE polymer chain induced by this chemical reaction was then focused on. Figure 5 shows the BP

Figure 5. Bond overlap population between carbon atoms in PTFE polymer chain. Figure 7. Final structure of QCMD simulation for PTFE/α-Al2O3 interface under atmospheric pressure.

between two carbon atoms in PTFE backbone. Focusing on the red line, one of the chemical bonds between carbon atoms is completely dissociated after the simulation time of 0.5 ps, that the neighboring fluorine atom was removed by the aluminum surface. In the figure, BP for another carbon pair that neighbors this dissociated bond is also represented by a blue line. It can be understood that the focusing bond is not dissociated, and its bond strength starts to increase after 0.5 ps. This means that the single bond in PTFE backbone was changed into a double bond, which is not contained in the original PTFE chain, by neighboring carbon bond scission. Figure 6 schematically shows the mechanism of the chemical reaction in the PTFE/α-Al2O3 interface. The exposed

with the acid center of α-Al2O3 surface due to its structural constraint of polymer chain. On the other hand, under a high contact pressure, viz. real contact area, PTFE is forced into contact with the acid center, and then it overcomes the reaction barrier for forming the aluminum fluoride. It is thus concluded that the reaction between PTFE and the α-Al2O3 surface is likely to occur tribochemically. The driving force of this system is not only the catalytic effect of the α-Al2O3 surface but also the forced contact between PTFE and α-Al2O3 surfaces during friction process. The similar defluorination reaction of PTFE polymer chain in which the double or triple bond is formed is known to occur in an electrochemical process with metallic magnesium, lithium, and sodium31 that originally showed a low ionic potential. In the tribological process, although the aluminum fluoride formation and defluorination of PTFE have been found by analyzing bonding states by X-ray photoelectron spectroscopy (XPS),3,32 the detailed mechanism of chemical reaction of PTFE/aluminum interface has not well understood. On the basis of our QCMD simulations, it is suggested that the extreme frictional contact could also introduce defluorination for the chemically stable PTFE polymer chain as well as the highly reactive electrochemical process. 3.3. Effect of Tribochemical Reaction on Transfer Film Formation. To summarize the above, the tribochemical reaction on the PTFE/α-Al2O3 interface provides two main reaction products. One is the aluminum fluoride on the α-Al2O3 surface, and other is the double bond between carbon atoms inside PTFE backbone. The influence of these tribochemical

Figure 6. Schematic illustrations for the mechanism of chemical reaction on PTFE/α-Al2O3 interface under friction condition.

aluminum atom on the α-Al2O3 surface is positively charged strongly due to losing electrons because of surface dehydration. Highly reactive vacant orbital is thus exposed on the surface. The vacant orbital of aluminum obtains electrons via removing the fluorine atoms from PTFE polymer chain so as to stabilize the α-Al2O3 surface, and as a result, aluminum fluoride is 5393



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aluminum surface. That is, as shown in Table 1, the fluorine atom on the surface shows a negative charge, whereas the hydrogen atom in OH-terminated surface has a positive charge. Furthermore, the negativity of the fluorine atom in PTFE contacted with fluorine-terminated surface is much larger than that in the case of OH-terminated surface. The fluorineterminated surface thus interacts with PTFE resin with an electrostatic repulsion between negatively charged fluorine atoms, while the PTFE adheres to an OH-terminated surface by hydrogen bonding as described in section 3.1. Hence, the chemical reaction of PTFE on the fluorine-terminated surface cannot be taken place due to interfacial separation, although the surface potentially behaves as Lewis acid. The PTFE resin will barely be transferred to the aluminum fluoride surface as the tribochemical product as long as the contact pressure of 500 MPa is applied. The tribological performance of the system is thus worsened by fluoride formation. The second discussion is about the double bond formation of carbon in PTFE backbone. The genuine PTFE polymer chain shows a high chemical stability because the carbon chain is fully covered by fluorine atoms. On the other hand, the backbone carbon is exposed to its working environments by the double bond formation via the unveiled tribochemical defluorination reaction on the α-Al2O3 surface. The gaseous components, such as nitrogen, oxygen, and water molecules in humid atmosphere, attack the double bond during the friction process. If water molecules react with the double bond, the hydroxyl or carboxylic group, which has been experimentally found in the PTFE transfer film formed,3 will probably form on the terminal of PTFE polymer chain. One of the merits for forming these functional groups is to increase the adhesion strength of PTFE transfer film onto metallic surface due to the polar nature of oxygen atoms in these functional groups. By forming such oxygen-including functional groups, the PTFE can be transferred also for the fluoride surface to which the PTFE without any functional group cannot be attached by the electrostatic repulsion. This is because stable hydrogen bond between fluorine atom in fluoride surface and hydrogen in the functional group. In our observed tribochemical reaction, the fluoride and double bond of carbon formation are expected to take place in both humid and dry environments. However, the transfer film barely forms under a dry condition. On the other hand, in a humid environment, the hydroxyl or carboxylic group is introduced into PTFE polymer chain, and as a result, its transfer film is able to form easily. From a tribochemical point of view, this is one of the reasons the tribological performance of PTFE for use in atmosphere is much better than in a dry condition such as inert gas.3 It can be suggested that the subsequent chemical interaction with environments after the tribochemical reaction unveiled in this study is important to reduce friction and wear caused by the transfer film formation. The possibility and mechanism of chemical reaction between double bond in PTFE and environmental gaseous components are now investigated by our methodology of a QCMD simulation.

reaction products on the tribological performance of PTFE is next discussed in this section. The first point is the effect of aluminum fluoride formation on the α-Al2O3 surface. Since the fluorine atom is more electronically negative than the oxygen atom in oxide, significant positive charge may lie on the aluminum atom in its fluoride structure. Thus, the catalytic effect of Lewis acid is probably enhanced by the fluoride formation. To check the effect of the fluoride formation, the QCMD simulation for the model as shown in Figure 8a was conducted with the same simulation condition with section 3.1. The model consists of three layers of PTFE and α-Al2O3, but the surfaces of Al2O3 are terminated by fluorine atoms instead of OH groups used in the previous sections. Table 1 shows the atomic charge and BP

Figure 8. (a) Model for confirming the effect of aluminum fluoride formation on tribological property and (b) enlarged illustration for interface.

Table 1. Atomic Charge and Bond Overlap Population for the α-Al2O3 Surface Terminated by a Fluorine or Hydroxyl Group functional group on Al2O3 surface

atomic charge (e)

BP

Al surface F surface H F in PTFE Al−F Al−O

F

OH

+0.45 −0.33

+0.39

−0.11 +0.26

+0.17 −0.04 +0.41

from the simulation. The table shows the results obtained in the previous section for comparison. The atomic charge for the aluminum atom on fluorine-terminated surface is larger than that on the OH-terminated surface, meaning a significant catalytic effect of fluoride. From the analyzed BP, it can be inferred that the chemical bond between aluminum and fluorine is more ionic in nature (less covalency) than that between aluminum and oxygen. This result also suggests that the fluoride is a strong Lewis acid. The final structure of this QCMD simulation is shown in Figure 8b, and any clear chemical reaction between PTFE and fluorine-terminated α-Al2O3 cannot be observed even though a significant catalytic effect is expected on the surface. The reason for this mismatch between catalytic effect and the observed chemical reaction is explained by the electronic state of an

4. CONCLUDING REMARKS The formation mechanism of polytetrafluoroethylene (PTFE) transfer film and its tribochemical reaction with aluminum surface was investigated by a quantum chemical molecular dynamics (QCMD) method. The PTFE was transferred onto an OH-terminated α-Al2O3 surface, which reflects the product 5394



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(9) McNicol, A.; Dowson, D.; Davies, M. The Effect of Humidity and Electrical Fields upon the Wear of High Density Polyethylene and Polytetrafluoroethylene. Wear 1995, 181−183, 603−612. (10) Podgornik, B.; Borovšak, U.; Megušar, F.; Košir, K. Performance of Low-Friction Coatings in Helium Environments. Surf. Coat. Technol. 2012, 206, 4651−4658. (11) Mosey, N. J.; Müser, M. H.; Woo, T. K. Molecular Mechanism for the Functionality of Lubricant Additives. Science 2005, 307, 1612− 1615. (12) Jang, I.; Burris, D. L.; Dickrell, P. L.; Barry, P. R.; Santos, C.; Perry, S. S.; Phillpot, S. R.; Sinnott, S. B.; Sawyer, W. G. Sliding Orientation Effects on the Tribological Properties of Polytetrafluoroethylene. J. Appl. Phys. 2007, 102, 123509. (13) Barry, P. R.; Jang, I.; Perry, S. S.; Sawyer, W. G.; Sinnott, S. B.; Phillpot, S. R. Effect of Simulation Conditions on Friction in Polytetrafluoroethylene (PTFE). J. Comput.-Aided Mater. Des. 2007, 14, 239−246. (14) Onodera, T.; Morita, Y.; Suzuki, A.; Koyama, M.; Tsuboi, H.; Hatakeyama, N.; Endou, A.; Takaba, H.; Kubo, M.; Dassenoy, F.; et al. A Computational Chemistry Study on Friction of h-MoS2. Part I. Mechanism of Single Sheet Lubrication. J. Phys. Chem. B 2009, 113, 16526−16536. (15) Onodera, T.; Morita, Y.; Nagumo, R.; Miura, R.; Suzuki, A.; Tsuboi, H.; Hatakeyama, N.; Endou, A.; Takaba, H.; Dassenoy, F.; et al. A Computational Chemistry Study on Friction of h-MoS2. Part II. Friction Anisotropy. J. Phys. Chem. B 2010, 114, 15832−15838. (16) Mikulski, P. T.; Workum, K. V.; Chateaueuf, G. M.; Gao, G.; Schall, J. D.; Harrison, J. A. The Effects of Interface Structure and Polymerization on the Friction of Model Self-Assembled Monolayers. Tribol. Lett. 2011, 42, 37−49. (17) Pastewka, L.; Moser, S.; Gumbsch, P.; Moseler, M. Anisotropic Mechanical Amorphization Drives Wear in Diamond. Nat. Mater. 2011, 10, 34−48. (18) Martin, J. M.; Onodera, T.; Minfray, C.; Dassenoy, F.; Miyamoto, A. The Origin of Anti-Wear Chemistry of ZDDP. Faraday Discuss. 2012, 156, 311−323. (19) Onodera, T.; Park, M.; Souma, K.; Ozawa, N.; Kubo, M. Transfer-Film Formation Mechanism of Polytetrafluoroethylene: A Computational Chemistry Approach. J. Phys. Chem. C 2013, 117, 10464−10472. (20) Bai, S.; Onodera, T.; Nagumo, R.; Miura, R.; Suzuki, A.; Tsuboi, H.; Hatakeyama, N.; Takaba, H.; Kubo, M.; Miyamoto, A. Friction Reduction Mechanism of Hydrogen- and Fluorine-Terminated Diamond-Like Carbon Films Investigated by Molecular Dynamics and Quantum Chemical Calculation. J. Phys. Chem. C 2012, 116, 12559−12565. (21) Hayashi, K.; Sato, S.; Bai, S.; Higuchi, Y.; Ozawa, N.; Shimazaki, T.; Adachi, K.; Martin, J. M.; Kubo, M. Fate of Methanol Molecule Sandwiched between Hydrogen-Terminated Diamond-Like Carbon Films by Tribochemical Reactions: Tight-Binding Quantum Chemical Molecular Dynamics Study. Faraday Discuss. 2012, 156, 137−146. (22) Kubo, M.; Ando, M.; Sakahara, S.; Jung, C.; Seki, K.; Kusagaya, T.; Endou, A.; Takami, S.; Imamura, A.; Miyamoto, A. Development of Tight-Binding, Chemical-Reaction-Dynamics Simulator for Combinatorial Computational Chemistry. Appl. Surf. Sci. 2004, 223, 188−195. (23) Ewald, P. P. The Calculation of Optical and Electrostatic Grid Potential. Ann. Phys. 1921, 64, 253−287. (24) Breiby, D. W.; Sølling, T. I.; Bunk, O.; Nyberg, R. B.; Norrman, K.; Nielsen, M. M. Structural Surprises in Friction-Deposited Films of Poly(tetrafluoroethylene). Macromolecules 2005, 38, 2383−2390. (25) Rae, P. J.; Dattelbaum, D. M. The Properties of Poly(tetrafluoroethylene) (PTFE) in Compression. Polymer 2004, 45, 7615−7625. (26) Barry, P. R.; Chiu, P. Y.; Perry, S. S.; Sawyer, W. G.; Phillpot, S. R.; Sinnott, S. B. The Effect of Normal Load on Polytetrafluoroethylene Tribology. J. Phys.: Condens. Matter 2009, 21, 144201. (27) Bhattacharya, I. N.; Das, S. C.; Mukherjee, P. S.; Paul, S.; Mitra, P. K. Thermal Decomposition of Precipitated Fine Aluminum Trihydroxide. Scand. J. Metall. 2004, 33, 211−219.

in atmospheric working conditions, by forming an interfacial hydrogen bond between fluorine and hydrogen atoms. The aluminum atom was exposed on the α-Al2O3 surface during friction process, and this could lead to tribochemical reaction including defluorination of PTFE and aluminum fluoride formation. In this reaction, the dehydrated α-Al2O3 surface acted as a catalytic Lewis acid in which the fluorine atom was removed from PTFE polymer chain. The tribological performance was reduced by the forming of an aluminum fluoride due to its electrostatic repulsive interaction with fluorine atoms in PTFE. It is expected that the key to modify the tribological performance is a chemical reaction between environmental water vapor and reactive carbon double bond in PTFE backbone as a product of the above tribochemical reaction. The chemical reaction mechanism with water vapor will be considered in our next paper. Although the fluorination of aluminum oxide surface was only observed during our QCMD simulation, there still remained a possibility to form aluminum carbide by reacting with the double bond in PTFE backbone. To see this subsequent chemical reaction with double bond, it is necessary to extend the simulation step. Also, the tribochemical reaction dynamics with a nascent aluminum surface exposed by wear of native oxide layer was not unveiled. Several QCMD simulations will be next conducted with a new model for revealing these chemical reactions.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +81-294-525111; Fax +81-294-52-7622 (T.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by “Tohoku Innovative Materials Technology Initiatives for Reconstruction (TIMT)“ funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Reconstruction Agency, Japan.

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Chemical Reaction Mechanism of Polytetrafluoroethylene on

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