Izabela Szlufarska

Friction laws at the nanoscale.

Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale, they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance. Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments. We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories of friction can be applied at the nanoscale.

Recent advances in single-asperity nanotribology

As the size of electronic and mechanical devices shrinks to the nanometre regime, performance begins to be dominated by surface forces. For example, friction, wear and adhesion are known to be central challenges in the design of reliable micro- and nano-electromechanical systems (MEMS/NEMS). Because of the complexity of the physical and chemical mechanisms underlying atomic-level tribology, it is still not possible to accurately and reliably predict the response when two surfaces come into contact at the nanoscale. Fundamental scientific studies are the means by which these insights may be gained. We review recent advances in the experimental, theoretical and computational studies of nanotribology. In particular, we focus on the latest developments in atomic force microscopy and molecular dynamics simulations and their application to the study of single-asperity contact.

Atomistic simulations of nanoindentation

Our understanding of mechanics is pushed to its limit when the functionality of devices is controlled at the nanometer scale. A fundamental understanding of nanomechanics is needed to design materials with optimum properties. Atomistic simulations can bring an important insight into nanostructure-property relations and, when combined with experiments, they become a powerful tool to move nanomechanics from basic science to the application area. Nanoindentation is a well-established technique for studying mechanical response. We review recent advances in modeling (atomistic and beyond) of nanoindentation and discuss how they have contributed to our current state of knowledge.

Multimillion-atom nanoindentation simulation of crystalline silicon carbide: Orientation dependence and anisotropic pileup

We have performed multimillion-atom molecular dynamics simulations of nanoindentation on cubic silicon carbide (3C-SiC) surfaces corresponding to three different crystallographic directions, (110), (001), and (111), using pyramidal-shaped Vickers indenter with 90° edge angle. Load-displacement (P-h) curves show major and minor pop-in events during loading. Detailed analysis of the (110) indentation shows that the first minor discontinuity in the P-h curve is related to the nucleation of dislocations, whereas the subsequent major load drops are related to the dissipation of accumulated energy by expansion of dislocation loops and changes of slip planes. Motion of dislocation lines in the indented films involves a kink mechanism as well as mutually repelling glide-set Shockley partial dislocations with associated extension of stacking faults during the expansion of dislocation loops. Our simulations provide a quantitative insight into the stress distribution on slip planes and stress concentration at kinks ...

Nanoindentation-induced amorphization in silicon carbide

The nanoindentation-induced amorphization in SiC is studied using molecular dynamics simulations. The load-displacement response shows an elastic shoulder followed by a plastic regime consisting of a series of load drops. Analyses of bond angles, local pressure, and shear stress, and shortest-path rings show that these drops are related to dislocation activities under the indenter. We show that amorphization is driven by coalescence of dislocation loops and that there is a strong correlation between load-displacement response and ring distribution.

Simultaneous enhancement of toughness, ductility, and strength of nanocrystalline ceramics at high strain-rates

Molecular dynamics simulations of tensile testing have been performed on nc-SiC. Reduction of grain size promotes simultaneous enhancement of ductility, toughness, and strength. nc-SiC fails by intergranular fracture preceded by atomic level necking. Conventionally, high strain-rate deformations of ceramics are limited by diffusion time scales, since diffusion prevents premature cavitation and failure. The authors report a nondiffusional mechanism for suppressing premature cavitation, which is based on unconstrained plastic flow at grain boundaries. Based on the composite’s rule of mixture, they estimate Young’s modulus of random high-angle grain boundaries in nc-SiC to be about 130GPa.

A molecular dynamics study of nanoindentation of amorphous silicon carbide

Through molecular dynamics simulation of nanoindentation of amorphous a‐SiC, we have found a correlation between its atomic structure and the load-displacement (P‐h) curve. We show that a density profile of a‐SiC exhibits oscillations normal to the surface, analogous to liquid metal surfaces. Short-range P‐h response of a‐SiC is similar to that of crystalline 3C‐SiC, e.g., it shows a series of load drops associated with local rearrangements of atoms. However, the load drops are less pronounced than in 3C‐SiC due to lower critical stress required for rearrangement of local clusters of atoms. The nanoindentation damage is less localized than in 3C‐SiC. The maximum pressure under the indenter is 60% lower than in 3C‐SiC with the same system geometry. The onset of plastic deformation occurs at the depth of 0.5A, which is ∼25% of the corresponding value in 3C‐SiC. a‐SiC exhibits lower damping as compared to 3C‐SiC, which is reflected in the longer relaxation time of transient forces after each discrete indenta...

Atomistic processes during nanoindentation of amorphous silicon carbide

Atomistic mechanisms of nanoindentation of a-SiC have been studied by molecular dynamics simulations. The load displacement curve exhibits a series of load drops, reflecting the short-range topological order similar to crystalline 3C–SiC. In contrast to 3C–SiC, the load drops are irregularly spaced and less pronounced. The damage is spatially more extended than in 3C–SiC, and it exhibits long-range oscillations consistent with the indenter size. Hardness is ∼60% lower than in 3C–SiC and is in agreement with experiment. The onset of plastic deformation occurs at depth ∼75% lower than in 3C–SiC.

Effect of Grain Boundary Stresses on Sink Strength

A fundamental understanding of the interactions between point defects and grain boundaries (GBs) is critical to designing radiation-tolerant nanocrystalline (nc) materials. An important consideration in this design is sink strength, which quantifies the efficiency of a sink to annihilate point defects. Contrary to the common belief that random high-angle GBs provide the upper limit for rate of defect annihilation, here we show that the sink strength of low-angle GBs can exceed that of high-angle GBs due to the effect of GB stress fields. This surprising finding provides a novel opportunity to enhance the radiation resistance of nc materials through GB engineering.

Dislocation controlled wear in single crystal silicon carbide

For better design and durability of nanoscale devices, it is important to understand deformation in small volumes and in particular how deformation mechanisms can be related to frictional response of an interface in the regime where plasticity is fully developed. Here, we show that when the size of the cutting tool is decreased to the nanometer dimensions, silicon carbide wears in a ductile manner by means of dislocation plasticity. We present different categories of dislocation activity observed for single asperity sliding on SiC as a function of depth of cut and for different sliding directions. For low dislocation density, plastic contribution to frictional energy dissipation is shown to be due to glide of individual dislocations. For high dislocation densities, we present an analytical model to relate shear strength of the sliding interface to subsurface dislocation density. Furthermore, it is shown that a transition from plowing to cutting occurs as function of depth of cut and this transition can be well described by a macroscopic geometry-based model for wear transition.