The “leaves on the line” problem—a study of leaf residue film formation and lubricity under laboratory test conditions

The problem of leaf residue and loss of adhesion in the wheel/rail track contact has been studied in ball-on-disc test device. The friction properties of sycamore leaf samples were measured for a range of speeds and imposed slip at a maximum contact pressure of 1.0 GPa. At the end of the test the leaf lubricant films were examined under a low-power microscope and the organic content analysed by Infra-Red (IR) Reflection-Absorption Microspectroscopy. The test samples included water-saturated leaves and the supernatant soaking water to measure the effect of water-soluble leaf components. The results were compared to pure water. During the initial film formation the leaf samples rapidly formed a slurry composed of small black particles, this dried to a thick adherent black film as the test proceeded. This suggested that the black film was due to a chemical reaction between the water-soluble leaf component and steel rather than as the result of charring of the organic material. Friction coefficients of the leaf slurry were in the range μ = 0.03–0.06 (50% slip) compared to values of μ = 0.15–0.2 for pure water. Friction was also reduced in tests with the leaf-soaking water, suggesting that water-soluble leaf components (identified as pectin) play a role in the low adhesion mechanism. IR analysis showed that the black residue films contained pectin (pectate) and cellulose derived from the leaf samples. Pectin appears to play an important role in the lubrication process. In the presence of metal ions pectin will gel and thus could form a thin but highly viscous layer on the track surface. This gel will also flocculate cellulose to create the black biomass, which provides the lubricating film.