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.
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