Transfer-film formation mechanism of polytetrafluoroethylene: A computational chemistry approach

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.