Identification of the Shear Plane During Sliding of Solid Boundary Films: Potassium Chloride Films on Iron

HY Gao and WT Tysoe and A Martini, TRIBOLOGY LETTERS, 62, 4 (2016).

DOI: 10.1007/s11249-016-0654-2

The commonly observed linear variation in shear strength with contact pressure is explored using a model system consisting of a potassium chloride (KCl) film between two iron counterfaces, for which a linear dependence has been found experimentally. The effect of artificially changing the KCl lattice spacing is explored using molecular dynamics (MD) simulations, where the interaction potential between the iron and KCl was previously optimized using surface science experiments for KCl on iron. It is found that the shear plane can change from occurring within the KCl film to taking place between the film and iron, depending on lattice spacing. In addition, shear within the KCl film leads to an increase in friction force with lattice spacing, while shear at the KCl- Fe interface leads to a decrease in friction with lattice spacing. The transition between the two regimes depends on the applied load. MD simulations of an equilibrium KCl film reveal that shear occurs at the KCl-iron interface, consistent with experiment, where no KCl transfer is found. Accordingly, the friction force increases with load, where the MD simulations yield a proportionality constant between shear strength and contact pressure of 0.115 +/- 0.004, in excellent agreement with the experimental value of 0.14 +/- 0.02. This result can be interpreted using the Prandtl-Tomlinson model where the pressure dependence arises from the external work carried out on the system due to the change in inter-layer spacing at the sliding interface.

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