Time Strengthening of Crystal Nanocontacts

Interfacial processes underlying the temperature-dependence of friction and wear of calcite single crystals

HYPOTHESIS

This study posits that thermal effects play a substantial role in influencing interfacial processes on calcite, and consequently impacting its mechanochemical properties.

EXPERIMENTS

This work interrogates the temperature-dependence of friction and wear at nanoscale contacts with calcite single crystals at low air humidity (≤ 3-10 % RH) by AFM.

FINDINGS

Three logarithmic regimes for the velocity-dependence of friction are identified. BelowTc ∼ 70 °C, where friction increases with T, there is a transition from velocity-weakening (W1) to velocity-strengthening friction (S1). AboveTc ∼ 70 °C, where friction decreases with T, a second velocity-strengthening friction regime (S0) precedes velocity-weakening friction (W1). The low humidity is sufficient to induce atomic scale changes of the calcite cleavage plane due to dissolution-reprecipitation, and more so at higher temperature and 10 % RH. Meanwhile, the surface softens above Tc -likely owing to lattice dilation, hydration and amorphization. These interfacial changes influence the wear mechanism, which transitions from pit formation to plowing with increase in temperature. Furthermore, the softening of the surface justifies the appearance of the second velocity-strengthening friction regime (S0). These findings advance our understanding of the influence of temperature on the interfacial and mechanochemical processes involving calcite, with implications in natural processes and industrial manufacturing.

Velocity-weakening and -strengthening friction at single and multiasperity contacts with calcite single crystals

SignificanceThe empirical nature of rate-and-state friction (RSF) equations remains a drawback to their application to predict earthquakes. From nanoscale friction measurements on smooth and rough calcite crystals, a set of parameters is analyzed to elucidate microscopic processes dictating RSF. We infer the influence of roughness on the velocity dependence of friction in dry environment and that atomic attrition leads to stick-slip instabilities at slow velocities. In fault dynamics, stick-slip is associated with seismic slips. The aqueous environment eliminates atomic attrition and stick-slip and dissolves calcite under pressure. This yields remarkable lubrication, even more so in rough contacts, and suggests an alternative pathway for seismic slips. This work has implications for understanding mechanisms dictating fault strength and seismicity.

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.

Influence of Loading Conditions and Temperature on Static Friction and Contact Aging of Hydrogels with Modulated Microstructures

Biological tribosystems enable diverse functions of the human body by maintaining extremely low coefficients of friction via hydrogel-like surface layers and a water-based lubricant. Although stiction has been proposed as a precursor to damage, there is still a lack of knowledge about its origin and its relation to the hydrogel's microstructure, which impairs the design of soft matter as replacement biomaterials. In this work, the static friction of poly(acrylamide) hydrogels with modulated composition was investigated by colloidal probe lateral force microscopy as a function of load, temperature, and loading time. Temperature-dependent studies enable to build a phase diagram for hydrogel's static friction, which explains stiction via (polymer) viscoelastic and poroelastic relaxation, and a subtle transition from solid- to liquid-like interfacial behavior. At room temperature, the static friction increases with loading time, a phenomenon called contact aging, which stems from the adhesion of the polymer to the colloid and from the drainage-induced increase in contact area. Contact aging is shown to gradually vanish with increase in temperature, but this behavior strongly depends on the hydrogel's composition. This work scrutinizes the relation between the microstructure of hydrogel-like soft matter and interfacial behavior, with implications for diverse areas of inquiry, not only in biolubrication and biomedical applications but also in soft robotics and microelectromechanical devices, where the processes occurring at the migrating hydrogel interface are of relevance. The results support that modulating both the hydrogel's mesh size and the structure of the near-surface region is a means to control static friction and adhesion. This conceptual framework for static friction will foster further understanding of the wear of hydrogel-like materials.

High-resolution manipulation of gold nanorods with an atomic force microscope

The controlled manipulation and precise positioning of nanoparticles on surfaces is a critical requisite for studying interparticle interactions in various research fields including spintronics, plasmonics, and nanomagnetism. We present here a method where an atomic force microscope operating in vacuum is used to accurately rotate and displace CTAB-coated gold nanorods on silica surfaces. The method relies on operating an AFM in a bimodal way which includes both dynamic and contact modes. Moreover, the phase of the oscillating probe is used to monitor the nanoparticle trajectory, which amplitude variations are employed to evaluate the energy dissipation during manipulation. The nanoscale displacement modes involve nanorod in-plane rotation and sliding, but no rolling events. The transitions between these displacement modes depend on the angle between the scan axis direction and the nanorod long axis. The findings reveal the importance of mean tip-substrate distance and of oscillation amplitude of the tip. The role of substrate surface and of CTAB molecular bi-layer at nanorod surface is also discussed.

Plowing-Induced Structuring of Compliant Surfaces

The structures produced by a sharp tip scraping a compliant surface are modeled in the illustrative case of scan patterns formed by a series of parallel lines. This is made possible by a modified version of the Prandtl model for stick-slip friction, with an interaction energy landscape replicating the morphology of the evolving surface. As a result, a ripple motif emerges with a tilt angle increasing linearly with the distance between the scan lines, except for the region close to the left boundary of the scanned area, where the ripples are oriented at 90°. This region can penetrate considerably to the right, forming a complex branched pattern. These predictions are substantiated by atomic force microscopy nanolithography experiments on polystyrene surfaces at room temperature. A simple and robust theoretical protocol for reproducing early-stage wear processes (potentially going beyond single contacts) is thus made available.

Chemical aging of large-scale randomly rough frictional contacts

It has been shown that contact aging due to chemical reactions in single asperity contacts can have a significant effect on friction. However, it is currently unknown how chemically induced contact aging of friction depends on roughness that is typically encountered in macroscopic rough contacts. Here we develop an approach that brings together a kinetic Monte Carlo model of chemical aging with a contact mechanics model of rough surfaces based on the boundary element method to determine the magnitude of chemical aging in silica-silica contacts with random roughness. Our multiscale model predicts that chemical aging for randomly rough contacts has a logarithmic dependence on time. It also shows that friction aging switches from a linear to a nonlinear dependence on the applied load as the load increase. We discover that surface roughness affects the aging behavior primarily by modifying the real contact area and the local contact pressure, whereas the effect of contact morphology is relatively small. Our results demonstrate how understanding of chemical aging can be translated from studies of single asperity contacts to macroscopic rough contacts.

Recent highlights in nanoscale and mesoscale friction

Friction is the oldest branch of non-equilibrium condensed matter physics and, at the same time, the least established at the fundamental level. A full understanding and control of friction is increasingly recognized to involve all relevant size and time scales. We review here some recent advances on the research focusing of nano- and mesoscale tribology phenomena. These advances are currently pursued in a multifaceted approach starting from the fundamental atomic-scale friction and mechanical control of specific single-asperity combinations, e.g., nanoclusters on layered materials, then scaling up to the meso/microscale of extended, occasionally lubricated, interfaces and driven trapped optical systems, and eventually up to the macroscale. Currently, this "hot" research field is leading to new technological advances in the area of engineering and materials science.

Rate and State Friction Relation for Nanoscale Contacts: Thermally Activated Prandtl-Tomlinson Model with Chemical Aging

Rate and state friction (RSF) laws are widely used empirical relationships that describe macroscale to microscale frictional behavior. They entail a linear combination of the direct effect (the increase of friction with sliding velocity due to the reduced influence of thermal excitations) and the evolution effect (the change in friction with changes in contact "state," such as the real contact area or the degree of interfacial chemical bonds). Recent atomic force microscope (AFM) experiments and simulations found that nanoscale single-asperity amorphous silica-silica contacts exhibit logarithmic aging (increasing friction with time) over several decades of contact time, due to the formation of interfacial chemical bonds. Here we establish a physically based RSF relation for such contacts by combining the thermally activated Prandtl-Tomlinson (PTT) model with an evolution effect based on the physics of chemical aging. This thermally activated Prandtl-Tomlinson model with chemical aging (PTTCA), like the PTT model, uses the loading point velocity for describing the direct effect, not the tip velocity (as in conventional RSF laws). Also, in the PTTCA model, the combination of the evolution and direct effects may be nonlinear. We present AFM data consistent with the PTTCA model whereby in aging tests, for a given hold time, static friction increases with the logarithm of the loading point velocity. Kinetic friction also increases with the logarithm of the loading point velocity at sufficiently high velocities, but at a different increasing rate. The discrepancy between the rates of increase of static and kinetic friction with velocity arises from the fact that appreciable aging during static contact changes the energy landscape. Our approach extends the PTT model, originally used for crystalline substrates, to amorphous materials. It also establishes how conventional RSF laws can be modified for nanoscale single-asperity contacts to provide a physically based friction relation for nanoscale contacts that exhibit chemical bond-induced aging, as well as other aging mechanisms with similar physical characteristics.

Atomistic modeling of tribological properties of Pd and Al nanoparticles on a graphene surface

Background: The frictional properties of nanoparticles have been studied to gain insight into the fundamental origin of sliding friction. Results: Using molecular dynamics we investigate frictional properties of aluminum and palladium nanoparticles deposited on a graphene layer. We study the time evolution of the total momentum of the system, the total and potential energies, the temperature, the velocity and position of the center of mass, the dimensions of the nanoparticle, and the friction and substrate forces acting on the particle. We also study how the friction force depends on the nanoparticle-graphene contact area and the temperature. Conclusion: The tribological properties of nanoparticles strongly depend on the materials. The particles move in an irregular (saw-like) manner. The averaged friction force depends nearly linearly on the contact area and non-monotonously on temperature. We observe ordered crystalline domains of atoms at the bottom surface of the metal particles, but the peaks of radial distribution function are blurred indicating that the nanoparticles are amorphous or polycrystalline.

Interface bonding in silicon oxide nanocontacts: interaction potentials and force measurements

The interface bonding between two silicon-oxide nanoscale surfaces has been studied as a function of atomic nature and size of contacting asperities. The binding forces obtained using various interaction potentials are compared with experimental force curves measured in vacuum with an atomic force microscope. In the limit of small nanocontacts (typically <10³ nm²) measured with sensitive probes the bonding is found to be influenced by thermal-induced fluctuations. Using interface interactions described by Morse, embedded atom model, or Lennard-Jones potential within reaction rate theory, we investigate three bonding types of covalent and van der Waals nature. The comparison of numerical and experimental results reveals that a Lennard-Jones-like potential originating from van der Waals interactions captures the binding characteristics of dry silicon oxide nanocontacts, and likely of other nanoscale materials adsorbed on silicon oxide surfaces. The analyses reveal the importance of the dispersive surface energy and of the effective contact area which is altered by stretching speeds. The mean unbinding force is found to decrease as the contact spends time in the attractive regime. This contact weakening is featured by a negative aging coefficient which broadens and shifts the thermal-induced force distribution at low stretching speeds.