Friction force microscopy (FFM) in aqueous environments has recently proven to be a very effective method for lattice-resolution imaging of crystal surfaces. Here we demonstrate the use of ethanol for similar measurements on water-soluble materials. Lattice resolved frictional stick-slip traces of a cleaved NaCl(100) surface submerged in ethanol are compared with previous obtained FFM results in ultrahigh vacuum (UHV). We use the Prandtl-Tomlinson framework to estimate the amplitude of the corrugation potential and the contact stiffness. The surface potential amplitude scales with the applied normal loads are in good agreement with data obtained for NaCl measured under UHV conditions, but demonstrates deviations from the ideal periodic potential given by the Prandtl-Tomlinson model. An additional finding is that the use of ethanol allows us to explore higher load ranges without detectable evidence of surface wear. The contact stiffness does not vary significantly with the normal load up to 38 nN, while above it a sudden increase by almost one order of magnitude was observed. Comparing this to previous results suggests that considerable atom rearrangements may occur in the contact region, although the (100) surface structure is preserved by ethanol-assisted diffusion of Na and Cl ions.

Friction force microscopy (FFM) of materials with well-defined crystalline surfaces is interpreted within the framework of the Prandtl–Tomlinson (PT) model. This model portrays the interaction with a surface through a deterministic periodic potential. While considering materials with polycrystalline or amorphous surfaces, the interpretation becomes more complex, since such surfaces may lack distinct lattice constant and/or corrugation energy amplitude. Here, we utilize an approach to describe the nanofriction measured on a catalyst with an irregular surface by describing the slip forces in terms of static disorder (SD) in the corrugation potential. We performed FFM measurements of the Fe–Al–O spinel catalyst powder, which is involved in reverse water–gas-shift reaction. The FFM measurements resulted in intermittent stick–slip pattern with large variance in the slip forces and their spatial distribution. We compare our results with a mean version of the PT model. The two models showed close proximity of the surface energy values and their trend with the applied normal load, where the SD model estimations were less scattered. The approach presented in this work may provide a useful tool to interpret the FFM measurements of materials with irregular surfaces.

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Friction Force Microscopy (FFM) measurements on NaCl immersed in ethanol display an increase of the effective contact stiffness with the applied load. This stiffness is estimated from the measured local contact interaction of the tip with the NaCl surface and the Prandtl-Tomlinson (PT) parameter, which reflects the relation between the corrugation stiffness and the effective contact stiffness. Different from FFM measurements in ultrahigh vacuum, for measurements in ethanol surroundings the PT parameters showed a maximum with the applied load. We incorporated this measured load-dependent effective stiffness together with the load-dependent amplitude of the corrugation energy into simulations based on the PT model, and studied its effect on the lateral friction for symmetric 1D and 2D potentials, as well as for an asymmetric 1D potentials. The simulations reproduced the experimentally observed non-monotonous behavior of the PT parameter, and enabled a glimpse on the relation of the characteristic observables (mean maximal slip forces and stiffness) with respect to their governing parameters (corrugation energy, effective stiffness). In all, apart from large deviations from symmetry in the interaction potential, the PT parameter provides a reliable estimate for nanoscale friction over periodic surfaces.

Friction force microscopy (FFM) explores the interaction in a sliding contact on the nanoscale, providing information on the frictional dynamics and lateral contact stiffness with lattice resolution. Recent FFM measurements on a NaCl crystal immersed in liquid (ethanol) surroundings displayed an increase of the effective contact stiffness, Keff, with the applied load, differently from similar measurements performed under ultra-high vacuum (UHV) conditions, where Keff showed negligible load dependency. Additionally, under UHV conditions multiple slip length friction with increasing load was reported, while in ethanol surroundings only single (lattice unit length) slips were observed. Our current understanding of this behavior relates the transition from single jumps to multiple jumps dynamics to the normal load (manifested through the amplitude of the interaction potential at the contact, U0) and to the damping of the system. Here we have incorporated the effect of the load dependency on both U0 and Keff within Prandtl—Tomlinson based simulations, accompanied by variations in the damping coefficient of the system. Introducing the experimentally observed load dependency to Keff resulted indeed in single slip jumps at critical damping, while multiple slip jumps were obtained at constant Keff. The average slip length increased with the normal load, particularly when the system became underdamped. Our work provides a glimpse on the relation between the characteristic observables in atomic-scale sliding friction (maximal slip forces, stiffness, and slip dynamics) with respect to their governing parameters (corrugation energy, effective stiffness, and damping). While common understanding in nanotribology relates the effect of surrounding media mainly to the interaction potential at the contact, here we show that the media can also greatly affect the elastic interaction, and consequently play an important role on the transition from single to multiple stick-slip.