Friction characteristics of Cd-rich carbonate films on calcite surfaces: implications for compositional differentiation at the nanometer scale

摘要

Lateral Force Microscopy (LFM) studies were carried out on cleaved calcite sections in contactwith solutions supersaturated with respect to otavite (CdCO3) or calcite-otavite solid solutions(SS) as a means to examine the potential for future application of LFM as a nanometer-scale mineralsurface composition mapping technique. Layer-by-layer growth of surface films took place eitherby step advancement or by a surface nucleation and step advancement mechanisms. Friction vs.applied load data acquired on the films and the calcite substrate were successfully fitted to theJohnson Kendall Roberts (JKR) model for single asperity contacts. Following this model, frictiondifferences between film and substrate at low loads were dictated by differences in adhesion,whereas at higher load they reflect differences in contact shear strength. In most experiments atfixed load, the film showed higher friction than the calcite surface, but the friction-load dependencefor the different surfaces revealed that at low loads (0–40 nN), a calcian otavite film has lowerfriction than calcite; a result that is contrary to earlier LFM reports of the same system. Multilayerfilms of calcian-otavite displayed increasing friction with film thickness, consistent with theexpectation that the film surface composition will become increasingly Cd-rich with increasingthickness. Both load- and thickness-dependence trends support the hypothesis that the contactshear strength correlates with the hydration enthalpy of the surface ions, thereby imparting frictionsensitivity in the LFM to mineral-water interface composition.