Nanoscale wear as a stress-assisted chemical reaction

Wear of sliding contacts leads to energy dissipation and device failure, resulting in massive economic and environmental costs. Typically, wear phenomena are described empirically, because physical and chemical interactions at sliding interfaces are not fully understood at any length scale. Fundamental insights from individual nanoscale contacts are crucial for understanding wear at larger length scales, and to enable reliable nanoscale devices, manufacturing and microscopy. Observable nanoscale wear mechanisms include fracture and plastic deformation, but recent experiments and models propose another mechanism: wear via atom-by-atom removal ('atomic attrition'), which can be modelled using stress-assisted chemical reaction kinetics. Experimental evidence for this has so far been inferential. Here, we quantitatively measure the wear of silicon-a material relevant to small-scale devices-using in situ transmission electron microscopy. We resolve worn volumes as small as 25 ± 5 nm 3, a factor of 10 3 lower than is achievable using alternative techniques. Wear of silicon against diamond is consistent with atomic attrition, and inconsistent with fracture or plastic deformation, as shown using direct imaging. The rate of atom removal depends exponentially on stress in the contact, as predicted by chemical rate kinetics. Measured activation parameters are consistent with an atom-by-atom process. These results, by direct observation, establish atomic attrition as the primary wear mechanism of silicon in vacuum at low loads. © 2013 Macmillan Publishers Limited. All rights reserved

On the application of transition state theory to atomic-scale wear

The atomic force microscope (AFM) tip is often used as a model of a single sliding asperity in order to study nanotribological phenomena including friction, adhesion, and wear. In particular, recent work has demonstrated a wear regime in which surface modification appears to occur in an atom-by-atom fashion. Several authors have modeled this atomic-scale wear behavior as a thermally activated bond breaking process. The present article reviews this body of work in light of concepts from formal transition state theory (also called reaction rate theory). It is found that this framework is viable as one possible description of atomic-scale wear, with impressive agreements to experimental trends found. However, further experimental work is required to fully validate this approach. It is also found that, while the Arrhenius-type equations have been widely used, there is insufficient discussion of or agreement on the specific atomic-scale reaction that is thermally activated, or its dependence on stresses and sliding velocity. Further, lacking a clear picture of the underlying mechanism, a consensus on how to measure or interpret the activation volume and activation energy is yet to emerge. This article makes suggestions for measuring and interpreting such parameters, and provides a picture of one possible thermally activated transition (in its initial, activated, and final states). Finally, directions for further experimental and simulation work are proposed for validating and extending this model and rationally interrogating the behavior of this type of wear. © 2010 Springer Science+Business Media, LLC

Understanding the tip-sample contact: An overview of contact mechanics from the macro- to the nanoscale

The field of contact mechanics was pioneered in 1880 by Heinrich Hertz who examined the problem of elastic deformation for two spheres being pressed into contact. As it forms the basis of many other contact situations including nanoscale contacts, the Hertz model is treated in this chapter in some detail. The Hertz model neglects any adhesion and friction forces between the two bodies in contact-a significant assumption given that these forces are always present to some degree and become more significant at smaller length scales. The chapter also discusses contact models to be used when adhesion cannot be neglected. It discusses how to address problems where one (or more) assumption in this model is explicitly violated, as well as special considerations that become relevant for nanoscale contacts. Many samples of interest for atomic force microscopy (AFM) studies are thin films

Simulated adhesion between realistic hydrocarbon materials: Effects of composition, roughness, and contact point

The work of adhesion is an interfacial materials property that is often extracted from atomic force microscope (AFM) measurements of the pull-off force for tips in contact with flat substrates. Such measurements rely on the use of continuum contact mechanics models, which ignore the atomic structure and contain other assumptions that can be challenging to justify from experiments alone. In this work, molecular dynamics is used to examine work of adhesion values obtained from simulations that mimic such AFM experiments and to examine variables that influence the calculated work of adhesion. Ultrastrong carbon-based materials, which are relevant to high-performance AFM and nano- and micromanufacturing applications, are considered. The three tips used in the simulations were composed of amorphous carbon terminated with hydrogen (a-C-H), and ultrananocrystalline diamond with and without hydrogen (UNCD-H and UNCD, respectively). The model substrate materials used were amorphous carbon with hydrogen termination (a-C-H) and without hydrogen (a-C); ultrananocrystalline diamond with (UNCD-H) and without hydrogen (UNCD); and the (111) face of single crystal diamond with (C(111)-H) and without a monolayer of hydrogen (C(111)). The a-C-H tip was found to have the lowest work of adhesion on all substrates examined, followed by the UNCD-H and then the UNCD tips. This trend is attributable to a combination of roughness on both the tip and sample, the degree of alignment of tip and substrate atoms, and the surface termination. Continuum estimates of the pull-off forces were approximately 2-5 times larger than the MD value for all but one tip-sample pair. © 2014 American Chemical Society

The effect of atomic-scale roughness on the adhesion of nanoscale asperities: A combined simulation and experimental investigation

The effect of atomic-scale roughness on adhesion between carbon-based materials is examined by both simulations and experimental techniques. Nanoscale asperities composed of either diamond-like carbon or ultrananocrystalline diamond are brought into contact and then separated from diamond surfaces using both molecular dynamics simulations and in situ transmission electron microscope (TEM)-based nanoindentation. Both techniques allow for characterization of the roughness of the sharp nanoasperities immediately before and after contact down to the subnanometer scale. The root mean square roughness for the simulated tips spanned 0.03 nm (atomic corrugation) to 0.12 nm; for the experimental tips, the range was 0.18-1.58 nm. Over the tested range of roughness, the measured work of adhesion was found to decrease by more than an order of magnitude as the roughness increased. The dependence of adhesion upon roughness was accurately described using a simple analytical model. This combination of simulation and experimental methodologies allows for an exploration of an unprecedented range of tip sizes and length scales for roughness, while also verifying consistency of the results between the techniques. Collectively, these results demonstrate the high sensitivity of adhesion to interfacial roughness down to the atomic limit. Furthermore, they indicate that care must be taken when attempting to extract work of adhesion values from experimental measurements of adhesion forces. © 2013 Springer Science+Business Media New York

Ultrananocrystalline diamond tip integrated onto a heated atomic force microscope cantilever

We report a wear-resistant ultrananocrystalline (UNCD) diamond tip integrated onto a heated atomic force microscope (AFM) cantilever and UNCD tips integrated into arrays of heated AFM cantilevers. The UNCD tips are batch-fabricated and have apex radii of approximately 10 nm and heights up to 7 μm. The solid-state heater can reach temperatures above 600 °C and is also a resistive temperature sensor. The tips were shown to be wear resistant throughout 1.2 m of scanning on a single-crystal silicon grating at a force of 200 nN and a speed of 10 μm s-1. Under the same conditions, a silicon tip was completely blunted. We demonstrate the use of these heated cantilevers for thermal imaging in both contact mode and intermittent contact mode, with a vertical imaging resolution of 1.9 nm. The potential application to nanolithography was also demonstrated, as the tip wrote hundreds of polyethylene nanostructures. © 2012 IOP Publishing Ltd

Advances in manufacturing of molded tips for scanning probe microscopy

A common method for producing sharp tips used in scanning probe microscopy (SPM) and other applications involving nanoscale tips is to deposit thin-film materials, such as metals, silicon nitride, or diamond-based films, into four-faceted pyramidal molds that are formed by anisotropic etching into a (100) silicon substrate. This well-established method is capable of producing tips with radii as small as a few nanometers. However, the shape of the tip apex is difficult to control with this method, and wedge-shaped tips that are elongated in one dimension are often obtained. This limitation arises due to the practical difficulty of having four planes intersecting at a single point. Here, a new method for producing three-sided molds for SPM tips is demonstrated through the use of etching in (311) silicon wafers. It is shown that silicon nitride and ultrananocrystalline diamond tips fabricated with this new method are wedge free and sharp (< 10 nm radius), thereby restoring tip molding as a well-controlled manufacturing process for producing ultrasharp SPM tips. © 2012 IEEE