Effect of Tribochemical Reaction on Transfer-Film Formation by Poly

May 12, 2014 - Understanding tribochemical reaction mechanisms is necessary to develop a novel resin material that can easily slide on metallic parts...
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Effect of Tribochemical Reaction on Transfer-Film Formation by Poly(tetrafluoroethylene) 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: Understanding tribochemical reaction mechanisms is necessary to develop a novel resin material that can easily slide on metallic parts. For this purpose, the chemical reaction dynamics between poly(tetrafluoroethylene) (PTFE) resin and an aluminum surface were studied by using a quantum chemical molecular dynamics simulation [Onodera, T., et al. J. Phys. Chem. C 2014, 118, 5390−5396]. The study showed that the PTFE tribochemically reacted with the oxidized surface of aluminum, forming two chemical products, namely, aluminum fluoride and depolymerized PTFE with a carbon double bond at the terminus of the PTFE polymer chain. The carbon backbone was exposed by changing to a double bond configuration, although that in genuine PTFE is fully covered by fluorine atoms. The subsequent chemical reaction of the polymer that reacts with gaseous molecules in the atmosphere (i.e., nitrogen, oxygen, and water vapor) was first studied by density functional theory (DFT). The DFT calculation results show that the chemical reaction of PTFE with water vapor was the most likely to occur and that a carboxyl group was finally formed on the terminus of the PTFE chain. The effect of the chemical reaction with water vapor on formation of a PTFE transfer film on an aluminum surface, which directly affects tribological performance of the focusing system, was then investigated by a classical molecular dynamics method. By forming a carboxyl group as a reaction product with water vapor, the amount of PTFE transfer film on an aluminum fluoride surface (one of the tribochemical reaction products) was increased. On the other hand, less genuine PTFE (without a carboxyl group) was transferred to the aluminum fluoride. This study clarified that the transfer film is formed easily by the reaction of PTFE with atmospheric water vapor, which thereby improves the tribological performance of the PTFE/aluminum lubrication system. there is less carbonyl formation.3 This change in PTFE’s chemical structure during the friction process is probably a key factor in improving the tribological performance of PTFE-based materials. To develop a novel PTFE-based resin composite for low-humidity environments, it is therefore necessary to determine the mechanism by which the chemical reaction of PTFE depends on the working environment and its related transfer-film formation. However, obtaining knowledge about a chemical reaction and transfer film of PTFE at the nanoscale level, which corresponds to the size of a transfer film, is generally difficult by conventional experimental and analytical techniques only because of difficulties with the in situ observation during a friction experiment. For solving the above-described problem concerning the “nanotribology” of PTFE, it has been recognized that a computational chemistry method, such as molecular dynamics

1. INTRODUCTION Poly(tetrafluoroethylene) (PTFE) is a useful bearing material because it always has a low friction coefficient when rubbed against metallic surfaces. In nature, the wear resistance of genuine PTFE is rather low, thereby impairing its application as an engineering material. To overcome this problem, PTFE has been used as composites in combination with carbon,1,2 metal,2 and ceramics3,4 fillers to increase its mechanical properties such as hardness, Young’s modulus, and so on. The origin of lubrication by PTFE is considered to be formation of a “transfer film” on a metallic counter surface.5−9 The film inhibits direct contact of the PTFE resin matrix and metallic surface despite its nanometricscale thickness,8 i.e., 10 nm. The coverage and morphology of the transfer film on a metallic surface is also important in regard to the self-lubrication mechanism of PTFE.4 On the other hand, the tribological performance of PTFE is strongly influenced by its working environment whether the humidity is high or low.3,10 As an example, rubbing under a low-humidity condition significantly increases the amount of wear of PTFE because its chemical structure changes less; viz., © 2014 American Chemical Society

Received: April 4, 2014 Revised: May 8, 2014 Published: May 12, 2014 11820

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3. RESULTS AND DISCUSSION 3.1. Chemical Reaction of PTFE with Gaseous Molecules. The chemical reactivity of PTFE with gaseous molecules, especially those found under a humid air condition, was first studied by DFT. During our previous elucidated tribochemical reaction on an aluminum oxide surface,20 PTFE lost its fluorine atoms and depolymerized by formation of the carbon double bond. To reflect this situation through the reaction, the model to be used should be composed of the reacted PTFE and the aluminum oxide, which acts as catalytic Lewis acid. One of the DFT-optimized model structures for a reactant is shown in Figure 1. This model is composed of a C5F10 molecule, an

(MD), is a powerful tool for revealing the mechanism of friction on the atomistic scale. Such methods have thus been extensively applied,11−16 and the PTFE system focused on in this study has been no exception.17−20 Recently, the tribochemical reaction dynamics between PTFE and an aluminum oxide surface was investigated by an original quantum chemical MD (QCMD) method developed by the authors.20 This method was used to clarify the tribochemical reaction including the fluorination of the aluminum oxide surface and depolymerization by forming a carbon double bond in the PTFE molecule. Note that the double bond formation in PTFE has been experimentally observed in a process of high-energy irradiation.21 Attaching the transfer film to a metallic surface was prevented by forming aluminum fluoride by electrostatic repulsion between fluorine atoms in the fluoride and PTFE. This aluminum fluoride formation may be one of the reasons that wear resistance of PTFE is reduced under a low-humidity condition. However, the effect of the double bond formation in a PTFE molecule on the tribological performance still remains an unclear point. The carbon chain in genuine PTFE is fully covered by fluorine atoms, resulting in high chemical stability. On the other hand, the carbon atoms are exposed to PTFE’s working environment through the double bond formed by the tribochemical reaction. Gaseous components such as nitrogen, oxygen, and water molecules in humid air attack the double bond during the friction process. Accordingly, in this study, the effect of the chemical reaction of PTFE with molecules in the environment on the tribological property of PTFE was further investigated, and the dependence of the tribological property of a PTFE/aluminum system on its working environment was determined. For this purpose, methods of computational chemistry, including density functional theory (DFT) and classical MD, were used.

Figure 1. Model for analyzing chemical reactivity of PTFE together with a gaseous molecule.

aluminum oxide cluster (Al4O6), and a gaseous molecule. Because the main objective of this calculation was to check the possibility of the chemical reaction to take place, DFT calculations were performed using nonperiodic cluster models so as to reduce the computation cost of the calculations. A C5F10 molecule was a model compound of the depolymerized PTFE and includes the carbon double bond at its terminus. A water molecule was arranged on the upper side of the double bond in the C5F10 molecule. Two other models (including gaseous oxygen or nitrogen molecules) were also built and optimized by DFT calculation. Accordingly, three geometries were obtained as reactants. The following chemical reactions for three different models of reactant were considered.

2. COMPUTATIONAL METHOD DFT was used for elucidating the chemical reactivity and reaction mechanism of a PTFE/aluminum system in which the carbon double bond formed in the PTFE molecule interacts with environmental gaseous molecules. DFT calculation was performed by the DMol3 program implemented in Accelrys Materials Studio package Version 4.3. In the DMol3 calculation, all core electrons were represented by effective core pseudopotentials to reduce the computation cost. Double numerical basis sets with polarization were employed, and a generalized gradient approximation in terms of Perdew−Burke−Ernzerhof (PBE) exchange-correlation functionals22,23 was adopted to optimize geometries and to evaluate energies. The transitionstate (TS) geometries were optimized by the complete linear synchronous transit (LST) and quadratic synchronous transit (QST) methods. To observe the dynamic behavior of the rubbing interface between PTFE and aluminum, classical MD simulation was also performed by using the LAMMPS24 code developed at the Sandia National Laboratory. In the present study, the consistent valence force field (CVFF)25 was used to express the atomic and molecular interaction. The system temperature was controlled by scaling the velocity of atoms. The velocity Verlet algorithm26 was used to solve the equation of motion. The molecular structure was visualized by the Materials Studio software.

Here, R denotes C3F7. In reaction 1, oxygen molecules are inserted between double-bonded carbon atoms which is one of the probable chemical reactions suggested by experiment.6 The formed molecule is then separated, and the resultant product is two types of ketone. In reaction 2, a nitrogen molecule is inserted into the carbon chain in the same manner as the oxygen is inserted, and an azo group is formed. Reaction 3 is for the case of a water molecule. Because the carbonyl group has been experimentally found in actual wear tracks on PTFE materials,3 a molecule composed of a fluorocarbon with a carbonyl group is considered as reaction product. A hydrogen fluoride molecule is liberated by this reaction and, in fact, has been detected in a previous friction experiment on fluorocarbon lubricant.27 The geometries of the products were constructed and optimized by DFT, and the TS geometry was then optimized on the basis of the superimposed structure between 11821

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Figure 2. DFT-optimized structure and its relative energies for the chemical reaction of a C5F10 molecule with environmental oxygen or nitrogen or gaseous water molecules. The color of atoms is the same as that in Figure 1, and R denotes C3F7. The energy of each reactant is set to zero.

the reactant and product geometries. Single-point-energy calculations were subsequently performed to obtain electronic energy for reactant, TS, and product. The DFT-optimized structure (and its relative electronic energy) for each of the three above-described chemical reactions is shown in Figure 2. According to the figure, TS energy, which corresponds to the reaction barrier, in the case of the water reaction is almost one-fourth of that in the case of oxygen, while it is the highest in the case of nitrogen. The chemical reaction between PTFE and the water molecule is thus the most likely to occur as far as the humid-air condition is considered. As for the oxygen reaction, the reaction product is the most stable of those generated in three considered reactions. This stability results from the formation of two molecules including a carbonyl group. The strong covalency of two carbonyl bonds contributes to formation of an energetically stable structure. However, the reaction of PTFE with oxygen does not typically occur easily because the activation energy is higher. Obviously, the chemical reaction with nitrogen would hardly take place because of significantly high activation energy. On the basis of these DFT results, it is concluded that the carbon double bond in a PTFE molecule probably reacts with a water molecule to form the carbonyl group. As mentioned above, the carbonyl group has been experimentally observed on a friction surface of PTFE.3 Hence, it is inferred that the change in the chemical structure of PTFE is due to the reaction with environmental water vapor, that is, not oxygen. The tribological property of PTFE thus might be involved by this change in chemical structure and is discussed in section 3.3. 3.2. Chemical Reaction Pathway for PTFE/Water-Vapor System. The reaction mechanism of PTFE with a water molecule was studied in more detail because there might be other chemical reactions that produce the carbonyl group. Moreover, the formed carbonyl group probably interacts with another water molecule. Based on this expectation, the following chemical reaction pathways are thus considered.

Reactions 4-1 and 5-1 include formation of a hydroxyl group on the terminus of the fluorocarbon molecule. Reactions 4-2 and 5-2 correspond to the formation of the carbonyl group by attacking the second water molecule. The main difference between these two reactions is formation of an enol species in reaction 5-1. For these reactions, the role of the secondaryacting water molecule is to exchange the hydrogen atom, like a catalyst; it provides one hydrogen atom to a fluorocarbon and removes one hydrogen atom from a fluorocarbon. It is important to note that the final product in reactions 4-1 and 5-2, i.e., fluorocarbon reacting with the carbonyl group, is the same as reaction 3. The DFT-optimized structures of the reactant, TS, and product and their electronic energy are shown in Figure 3. The activation energy in reaction 4-1 is almost two times higher than that in reaction 3. Although reaction 5-1 also produces the hydroxyl group as well as reaction 4-1, it shows not such a different energy value from that for reaction 3. This result was probably caused by the formation of the enol and hydrogen fluoride in reaction 5-1; in other words, a strong covalency between hydrogen and fluorine atoms in hydrogen fluoride, and the carbon double bond in the enol, contributed to keeping a stable structure and reducing the energy barrier. The product in reactions 4-1 and 5-1 again reacted with the water molecule via reactions 4-2 and 5-2, respectively. Reaction 4-2 happened 11822

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The subsequent reaction between the formed carbonyl group and a water molecule (see reaction 6 below) was then examined. The carbonyl group reacts with the water molecule to form a carboxyl group and hydrogen fluoride.The DFT-optimized geometries for the reaction system and these energies are shown in Figure 4. It can be found in the figure that the activation energy is lower than that in reaction 3. The formation of hydrogen fluoride and the hydroxyl group contributes to reducing the reaction barrier in addition to the existing carbonyl group. The carbonyl group and finally a carboxyl group are therefore formed by sequentially reacting with environmental water molecules in humid air. 3.3. Effect of Chemical Reaction to Transfer Film Formation. On the basis of the above static DFT calculations, it is suggested that the carbon double bond, acting as a tribochemical product of PTFE, reacts with water vapor and produces a carboxyl group in its molecular structure. The influence of this chemical reaction on lubrication by PTFE is

easily due to lower activation energy. Similar to reaction 4-1, the formation of strong covalent bonds, such as in hydrogen fluoride and carbonyl group, may reduce the energy barrier. On the other hand, reaction 5-2 shows high activation energy.

From these results, it is inferred that the carbonyl group is formed on the terminus of the PTFE chain through reaction 3, which shows the lowest activation energy. In this reaction, the formation of the carbonyl group coincides with the liberation of hydrogen fluoride; in other words, as a result, the system hardly loses covalency, contributing to a stable structure. In addition, the formation of a hydroxyl group is also likely to occur through reaction 5-1. The carbonyl group generally interacts strongly with a water molecule because of the polar nature of an oxygen atom.

Figure 3. DFT-optimized structure and its relative energies for the chemical reaction of a C5F10 molecule with environmental water vapor. The color of atoms is the same as that in Figure 1, and R denotes C3F7. The energy of each reactant is set to zero.

Figure 4. DFT-optimized structure and its relative energies for the chemical reaction generating a carboxyl group by water vapor. The color of atoms is the same as that in Figure 1, and R denotes C3F7. The energy of each reactant is set to zero. 11823

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horizontal velocity. On a real contact area of PTFE/aluminum system, the pressure was much higher than a macroscopic contact pressure (typically in the megapascal order3). Hence, the contact pressure of a few hundred megapascals has been adopted for studying PTFE tribology by molecular-level simulation.18 Accordingly, a contact pressure of 500 MPa was used in our simulation. The sliding was given parallel to PTFE chains at a velocity set at 10 m/s to observe the sliding effects with reasonable computation time. The bottom layer of the PTFE was completely fixed. The MD simulation was performed over 5 × 106 steps with an integration time of 0.1 fs. The final structures of the models (5.0 nm sliding) for the three above-described models are shown in Figure 6. In the figure, the periodic boundary was ignored for clear understanding. In model a, almost 17 layers of PTFE were tracked with the sliding α-Al2O3, meaning that PTFE transfer film was generated on the OH-terminated surface. In this manner, the main sliding interface appeared between the PTFE layers, not at the PTFE/α-Al2O3 interface. On the other hand, in model b, including a F-terminated α-Al2O3 surface, just two layers of PTFE were transferred to the α-Al2O3 surface. On the basis of our original QCMD method, the difference in the amounts of PTFE transfer given by models a and b has been explained by the chemical interaction between PTFE and the α-Al2O3 surface.20 In model a, many hydroxyl groups stay on the α-Al2O3 surface, and hydrogen atoms interact with fluorine atoms in PTFE by electrostatic attraction. This process leads to high adhesion and ease of transfer of PTFE. The situation upon transfer of PTFE was drastically changed when the α-Al2O3 surface was terminated by fluorine atoms. That is, electrostatic repulsive interaction acted between the fluorine atoms in PTFE and the F-terminated α-Al2O3 surface. Transferring a PTFE layer was thus inhibited by this repulsive interaction. On the other hand, the amount of PTFE transferred was increased by the formation of the carboxyl group, which is a chemical reaction product with water vapor. Three or four layers of PTFE were transferred to the α-Al2O3 surface even though the surface was terminated by fluorine atoms (see Figure 6c). In nature, a PTFE/F-terminated α-Al2O3 interface is supposed to show electrostatic repulsive interaction; however, part of the carboxyl group showed electrostatic attraction with the F-terminated α-Al2O3 surface (interaction between hydrogen in the carboxyl group and fluorine on the F-terminated α-Al2O3 surface). The surface nature of PTFE resin was therefore changed by introducing carboxyl groups so as to increase the amount of PTFE transfer. The difference between the structure

discussed hereafter. For this purpose, the classical MD method was used to track the change in the structure of the sliding interface. The simulation model used for MD (shown in Figure 5)

Figure 5. Simulation model for analyzing transfer-film formation on three types of interface: (a) PTFE/OH-terminated α-Al2O3, (b) PTFE/F-terminated α-Al2O3, and (c) PTFE with carboxyl group/ F-terminated α-Al2O3.

is composed of 20 layers of crystalline PTFE and α-Al2O3 and includes over 104 atoms in total. All the PTFE chains were aligned in parallel to the α-Al2O3 surface. The MD simulation was conducted for three types of simulation model, reflecting different situations concerning surface chemistry of PTFE/aluminum interface. Model a represents the interface between OH-terminated α-Al2O3 and genuine PTFE, reflecting the situation prior to their tribochemical reaction. Model b includes F-terminated α-Al2O3 and genuine PTFE and represents the tribochemical reaction on the PTFE/α-Al2O3 interface. Model c represents surface termination of α-Al2O3 in the same manner as model b, except the top layer of PTFE includes several carboxyl groups as a product of the chemical reaction of PTFE with water vapor. The atomic charge was referred from a previous QCMD simulation (see Table 1 in ref 20); that is, a different value of atomic charge was adopted on the basis of the surface chemistry, viz., OH-termination or F-termination of the α-Al2O3 surface. In the MD simulation, 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

Figure 6. Final structure of classical MD simulation for three types of interface: (a) PTFE/OH-terminated α-Al2O3, (b) PTFE/F-terminated α-Al2O3, and (c) PTFE with carboxyl group/F-terminated α-Al2O3. Sliding distance is 5 nm for all models. 11824

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Figure 7. Schematic illustrations for the effect of tribochemical reaction on transfer-film formation.

that in the case of a humid condition. Also the hydroxyl group is not regenerated (exposing many Lewis-acid centers). The tribochemical reaction, namely, fluorination of the aluminum surface and PTFE depolymerization, is further progressed because the aluminum surface is not protected by the PTFE transfer film.

with and without a carboxyl group will become clearer when a longer sliding distance is applied or a greater amount of carboxyl groups are introduced into the model. 3.4. Summary of Environmental-Dependent PTFE Tribology. On the basis of the results of our previous20 and present simulations, the effect of the chemical reaction with water vapor on the tribological performance of PTFE on an aluminum surface is schematically illustrated in Figure 7. The top surface of aluminum is typically oxidized, and the environmental water vapor is chemisorbed, forming hydroxyl groups (passivation). Although no clear chemical reaction between PTFE and the passivated aluminum surface can occur during the sliding process, they start to react when the thermochemical28 or mechanochemical29 dehydration occurs on the passivated aluminum surface. By this surface dehydration, the Lewis-acid center (a bare aluminum atom) is exposed and tribochemically removes fluorine atoms from the PTFE polymer chain. The resultant products are aluminum fluoride and depolymerized PTFE including a carbon double bond. The carbon double bond is chemically reactive and likely to react with an environmental water molecule. As the PTFE/aluminum system is working under a humid-gas condition, the carboxyl group, which increases the adhesiveness of the resin transfer film to the metallic counter surface, is thereby formed on the terminus of the depolymerized PTFE chain. Direct contact between the resin and metal is inhibited by generating this “adhesive” transfer film, leading to good antiwear performance. An environmental water molecule may react with the dehydrated aluminum surface, and then the hydroxyl group is partially regenerated. This reaction reduces the extent of exposure of the Lewis-acid center, which initiates the double bond formation in the PTFE polymer chain. On the actual contact surface, both the depolymerization of PTFE and the regeneration of hydroxyl group would simultaneously take place and balance on the active Lewis-acid center. On the other hand, under a dry-gas condition with less or no water content, the carboxyl group is, of course, hardly formed; accordingly, the transfer film is not well generated because of electrostatic repulsion on the aluminum fluoride surface. The antiwear performance in a dry-gas condition is thus lower than

4. CONCLUSIONS Chemical interaction of poly(tetrafluoroethylene) (PTFE) with environmental gaseous molecules and its effect on the tribological performance was investigated by a computational chemistry approach. The results of this investigation revealed one of the possibilities to explain the mechanism of the experimentally observed tribological phenomena that the antiwear performance of PTFE deteriorates under a dry-gas condition.3 The chemical reaction between tribochemically depolymerized PTFE and vapor water was most likely to occur while the reaction with oxygen or nitrogen gas did not take place. As a result of the chemical reaction of PTFE with water vapor, a carboxyl group was formed at the terminus of the PTFE molecule. The ability to form a transfer film was improved by forming a carboxyl group, even though fluoride stayed on the aluminum counter surface and potentially reduced the amount of PTFE transferred. Under a dry-gas condition, the transfer film was less generated because of the electrostatic repulsive interaction between the PTFE and the fluoride surface, causing severe wear of the PTFE resin matrix. The most important insight drawn from this study is that a tribochemical reaction produces aluminum fluoride and depolymerized PTFE. When the PTFE is sheared under a humid-gas condition, the tribochemical reaction triggers a subsequent chemical reaction with water vapor to form an adhesive transfer film. In this manner, good antiwear performance is attained. Under a dry condition, however, the antiwear performance is reduced by the tribochemical reaction because less transfer film is formed. Moreover, progression of metal corrosion and depolymerization are also thought to cause of the reduction in antiwear performance. It is concluded that the role of the tribochemical reaction is dramatically changed by different working environments of PTFE. 11825

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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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