Tribocatalytic Reaction of Polytetrafluoroethylene Sliding on an Aluminum Surface

Abstract

To improve the tribological performance of polytetrafluoroethylene (PTFE) resin sliding against a metallic surface, it is important to understand the chemical behavior of PTFE in this sliding system. The tribochemical reaction of PTFE on an aluminum surface has been strenuously studied by a series of computational chemistry methods [Onodera, T., et al. J. Phys. Chem. C 2014, 118, 5390−5396, and Onodera, T., et al. J. Phys. Chem. C 2014, 118, 11820−11826]. One of the most important insights was that PTFE reacted tribochemically with the oxidized surface of aluminum as a Lewis acid catalyst, forming a fluoride on the aluminum surface. The aluminum fluoride formed was a cause of decreasing tribological performance of PTFE because of less formation of a transfer film. In regard to this tribochemical reaction, it was suggested that preventing the fluoride formation is a key to improving the tribological performance of PTFE sliding against an aluminum surface. In this study, to investigate fluoride formation by a tribochemical reaction, the catalytic effect of an oxidized aluminum surface was investigated experimentally and theoretically. Two phases of an oxidized aluminum surface, namely, the α and γ phases of alumina, were chosen for investigating the catalytic tribochemistry of PTFE. A thermogravimetric analysis showed that the γ-alumina surface potentially exhibited a stronger catalytic effect in regard to PTFE since the reaction took place at lower temperature. The effect of the catalytic reaction on the tribological performance of PTFE was then investigated by a pin-on-disk tribometer. The results of this investigation show that the amount of wear of PTFE on the γ-alumina surface was higher than that on the α-alumina surface. By observing the wear scar on alumina surfaces by a scanning electron microscope (SEM) combined with energy-dispersive X-ray spectroscopy (EDX), it was clarified that the transfer film formed on the γ-alumina surface was less abundant, while it was regularly formed and more abundant on the α-alumina surface. In other words, the antiwear performance of PTFE was decreased because a lower amount of transfer film was formed by the catalytic effect during friction. In addition, a density-functional-theory (DFT) calculation also showed a stronger catalytic effect on the γ-alumina surface because the energy barrier for the chemical reaction producing fluoride was lower than that on the α-alumina surface. On the basis of these results, it was suggested that controlling the catalytic reaction of PTFE on the sliding surface is one of the ways to improve the antiwear performance of PTFE.

Tribological Behavior of Poly(tetrafluoroethylene) and Its Composites Reinforced by Carbon Nanotubes and Graphene Sheets: Molecular Dynamics Simulation

“…[33] In addition, the formation mechanism of the tribo-film in the metal/PTFE tribo-pairs was investigated through a combination of experiments, density functional theory, and MD simulations. Their results indicated that the adsorption energy of the metal oxide interface, [34] the ionic bonding of the friction interface, [30] the friction-induced tribo-film, [29,30] and tribochemical reactants [35] may change the interface adhesion energy and play an important role in the formation of PTFE transfer membrane. The previous studies have mainly focused on the friction and wear behavior and potential lubrication mechanism of pure PTFE materials at nanoscale, but the molecular-scale reinforcement mechanism of the fillers in PTFE composites during friction is still unclear.…”

Sustainable lubrication: low molecular weight PTFE micro-particles as extreme pressure additives for heavy duty grease applications

“…In case of extreme pressure conditions (high load/temperature) PTFE chemically attached over the surface of steel ball in the form of metal fluoride. In this case, rate of removal of metallic part is faster than metal fluoride film formation Komvopoulos et al (2004), Onodera et al (2015), Onodera et al (2014) as shown in Figure 7(b). Indicate the reaching the dynamic equilibrium, resulting in marginal decrease in load bearing characteristics.…”

Reductive Defluorination and Mechanochemical Decomposition of Per- and Polyfluoroalkyl Substances (PFASs): From Present Knowledge to Future Remediation Concepts

“…Thus, formation of Al−F species as well as the respective polyfluoroalkenes occur. Analogous behavior has been studied during the MC activation of PTFE with Al 2 O 3 [ 122 ]. Since the reaction barrier for a subsequent defluorination is too high, no further reductive dehalogenation reaction can be observed.…”