Crecimiento epitaxial de calcita sobre las caras (104) de dolomita y Kutnahorita

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Numerical study of pattern formation in compliant surfaces scraped by a rigid tip

The emergence of surface patterns on the surfaces of compliant materials subject to plowing wear is a complex problem which can be quantitatively characterized, e.g., on polymer surfaces scraped by an atomic force microscope (AFM) tip. Here we explore the applicability of a phenomenological model recently introduced to describe this phenomenon. Based on the competition between the viscoplastic indentation and the elastic shear stress caused by the tip, the model is able to reproduce the wavy features (ripples) observed when the tip is scanned along a series of parallel lines. For low values of the driving velocity v and the spacing b between scan lines, the existence of dotted areas formed by variously oriented pit alignments is observed. Moreover, coexistence of rippled with dotted domains is also observed at suitable parameter values. The formation process of the ripples is also described in detail. The amplitude, period, and orientation of these features are estimated numerically for different values of v and b parameters. We have also revisited the formation of the wavy patterns formed when a single line is scanned, and derived an equation which correctly describes their period and depth, and the static friction as well. This equation is not applicable when several lines are scanned one after the other and the ripples emerge as result of a cooperative process which involves the scanning of several lines

Estudio nanotribológico de superficies minerales en condiciones acuosas

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Surroundings affect slip length dynamics in nanoscale friction through contact stiffness and damping

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, K$_{eff}$ with the applied load, differently from similar measurements performed under ultra-high vacuum (UHV) conditions, where K$_{eff}$ 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, U${_o}$) and to the damping of the system. Here we have incorporated the effect of the load dependency on both U${_o}$ and K$_{eff}$ within Prandtl—Tomlinson based simulations, accompanied by variations in the damping coefficient of the system. Introducing the experimentally observed load dependency to K$_{eff}$ resulted indeed in single slip jumps at critical damping, while multiple slip jumps were obtained at constant K$_{eff}$. 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

Multiple Slips in Atomic-Scale Friction: An Indicator for the Lateral Contact Damping

The occurrence of multiple jumps in 2D atomic-scale friction measurements is used to quantify the viscous damping accompanying the stick-slip motion of a sharp tip in contact with a NaCl(001) surface. Multiple slips are observed without apparent wear for normal forces between 13 and 91nN. For scans parallel to [100] directions, the tip jumps between minima of the substrate corrugation potential in a zigzag fashion. An algorithm is applied to determine histograms of lateral force jumps which characterize multiple slips. The same algorithm is used to classify multiple slips occurring in calculated lateral force maps. Comparisons between simulations and experiments indicate that the nanometer-sized contact is underdamped at intermediate loads (13-26nN) and becomes slightly overdamped at higher loads. The proposed procedure is a novel way to estimate the lateral contact damping which plays an important role in the interpretation of measurements of the velocity and temperature dependence of friction, of slip duration, and of the reduction of friction by applied perpendicular or parallel oscillation

Atomic Friction Investigations on Ordered Superstructures

We review recent friction measurements on ordered superstructures performed by atomic force microscopy. In particular, we consider ultrathin KBr films on NaCl(001) and Cu(001) surfaces, single and bilayer graphene on SiC(0001), and the herringbone reconstruction of Au(111). Atomically resolved friction images of these systems show periodic features spanning across several unit cells. Although the physical mechanisms responsible for the formation of these superstructures are quite different, the experimental results can be interpreted within the same phenomenological framework. A comparison between experiments and modeling shows that, in the cases of KBr films on NaCl(001) and of graphene films, the tip-surface interaction is well described by a potential with the periodicity of the substrate which is modulated or, respectively, superimposed with a potential with the symmetry of the superstructur

Epitaxial Growth of Calcite Crystals on Dolomite and Kutnahorite (104) Surfaces

Epitaxial growth of calcite on dolomite and kutnahorite (104) surfaces has been promoted at room temperature by immersing cleavage rhombohedra of these minerals in highly supersaturated solutions with respect to calcite (βcalcite = [a(Ca2+)·a(CO3 2−)/Ksp,calcite] > 20). Scanning electron microscopy (SEM) images revealed an inhomogeneous coverage of dolomite and kutnahorite surfaces by large calcite crystals. In situ atomic force microscopy (AFM) observations showed that, while calcite islands rapidly grow perpendicularly to the substrates, their lateral spreading is slower. Furthermore, the accumulated strain associated with the relatively high calcite−substrate lattice misfits (δ > 2.2%) is accommodated by the generation of screw dislocations, which are evidenced by growth spirals on calcite three-dimensional islands. These observations are consistent with the Volmer−Weber epitaxial growth mode, characteristic of high overgrowth−substrate lattice misfits. Additional nanomanipulation experiments conducted with the AFM tip allowed us to remove calcite islands on both dolomite and kutnahorite (104) surfaces and to provide first estimates of shear strength

Friction and Wear of Mineral Surfaces in Liquid Environments

Lateral Force Microscopy (LFM) is a very suitable technique to investigate the structure and reactivity of mineral surfaces in liquids. Studies performed in the last two decades have shown that the dissolution and growth of mineral surfaces immersed in water and aqueous solutions can be monitored by recording friction signals with LFM. Moreover, the sensitivity of lateral forces to both structure and chemistry makes possible to use LFM to obtain information about monolayers formed on mineral faces. Finally, numerous mineral surfaces are excellent substrates on which nanoparticles and complex organic molecules can be deposited and subsequently imaged and manipulated. This opens the way to future applications in molecular electronics. This chapter presents an overview of the recent use of LFM in liquid to investigate mineral surfaces and processes occurring on them