Traction reduction effect of iron–tannic acid complex film formation at friction interface between rail and wheel

The black leaf film on the top surface of the rail reduces the tangential force between rails and wheels of the railway vehicles. Previous research has indicated that this film is an iron-tannin complex film formed by a reaction between iron in the rail and tannin, a constituent of fallen leaves. This study aims to understand the properties of the artificially prepared iron-tannin complex film and to clarify its effect on tribological properties when formed at the friction interface between rail and wheel. Rolling-sliding friction tests were conducted using rail specimens cut from the head of a 50 kg N standard rail and wheel specimens cut from the block material of a Japanese railway train wheel. When the complex film was formed in advance on the friction surface of the rail, the traction with distilled water supplied was lower than when it was not formed. The complex film formed on the friction surface from tannin supplied as an aqueous solution also significantly reduced the traction between the rail and wheel. The evaluation of complex film properties, using immersion tests of rail specimens, showed that the complex film was soft and reduced the contact angle on the formed surface.