Quantifying the Pressure-dependence of Work of Adhesion in Silicon-Diamond Contacts

Continuum mechanics models for contacting surfaces assume a constant interfacial energy, or work of adhesion, between materials. Recent studies have challenged this assumption, instead demonstrating that stress-dependent chemical reactions across the interface modify the work of adhesion. Here, we perform 77 adhesion tests on diamond-silicon contacts using in situ TEM and atomistic simulations to quantify how the adhesion changes as a function of applied pressure. The results show a 7-fold increase in work of adhesion (from approximately 1 to 7 J/m2) with an increase in mean applied pressure from 0 to 11 GPa, where the most significant increase occurs above 5 GPa. We rule out alternative explanations for the changing work of adhesion, such as electron-beam artifacts, bulk shape change by inelastic deformation, and time-dependent processes such as creep. Therefore, these results confirm the presence of stress-driven chemical reactions in the contact and quantify the resulting change in adhesion of these materials with applied pressure

In situ Mechanical Testing of Contacts Between Nanoscale Bodies: Measuring the Load-dependence of Contact Area.

Mechanical tests were performed on a silicon/diamond nanocontact. Using in situ transmission electron microscopy (TEM) and matched atomistic simulations, the contact area was measured during loading and unloading. The results agreed within uncertainty, and both experiment and simulation data showed significant hysteresis. While the unloading curves could be fit to a continuum model, yielding a realistic value for elastic modulus, this model overpredicted the contact area upon loading by an average of 40%. The implications of these results for real-world nanoscale contacts are that the contact area upon loading can deviate significantly from continuum predictions, even when the behavior upon unloading is well described by these models

Matching atomistic simulations and in situ experiments to investigate the mechanics of nanoscale contact

Many emerging devices and technologies rely on contacts between nanoscale bodies. Recent analytical theories, experiments, and simulations of nanocontacts have made conflicting predictions about the mechanical response as these contacts are loaded and separated. The present investigation combined in situ transmission electron microscopy (TEM) and molecular dynamics (MD) simulation to study the contact between a flat diamond indenter and a nanoscale silicon tip. The TEM was used to pre-characterize the materials, such that an atomistic model tip could be created with identically matched materials, geometry, crystallographic orientation, loading conditions, and degree of amorphization. A large work of adhesion was measured in the experiment and attributed to unpassivated surfaces and a large compressive stress applied before separation, resulting in covalent bonding across the interface. The simulations modeled atomic interactions across the interface using a Buckingham potential in order to reproduce the experimental work of adhesion without explicitly modeling covalent bonds, thereby enabling larger time- and length-scale simulations than would be achievable with a reactive potential. Then, the experimental and simulation tips were loaded under similar conditions with real-time measurement of contact area and deformation, yielding three primary findings. First, the results demonstrated that significant variation in the value of contact area can be obtained from simulations, depending on the technique used to determine it. Therefore, care is required in comparing measured values of contact area between simulations and experiments. Second, the contact area and deformation demonstrated significant hysteresis, with larger values measured upon unloading as compared to loading. Therefore, continuum predictions, in the form of a Maugis-Dugdale contact model, could not be fit to full loading/unloading curves. Third, the load-dependent contact area could be accurately fit by allowing the work of adhesion in the continuum model to increase with applied force from 1.3 to 4.3 J/m^2. The most common mechanisms for hysteretic behavior—which are viscoelasticity, capillary interactions, and plasticity—can be ruled out using the TEM and atomistic characterization. Stress-dependent formation of covalent bonds is suggested as a physical mechanism to describe these findings, which is qualitatively consistent with trends in the areal density of in-contact atoms as measured in the simulation. The implications of these results for real-world nanoscale contacts are that significant hysteresis may cause significant and unexpected deviations in contact size, even for nominally elastic contact

Characterization of small-scale surface topography using transmission electron microscopy

Multi-scale surface topography is critical to surface function, yet the very smallest scales are not accessible with conventional measurement techniques. Here we demonstrate two separate approaches for measuring small-scale topography in a transmission electron microscope (TEM). The first technique harnesses “conventional” methods for preparation of a TEM cross-section, and presents how these methods may be modified to ensure the preservation of the original surface. The second technique involves the deposition of the material of interest on a pre-fabricated substrate. Both techniques enable the observation and quantification of surface topography with Ångström-scale resolution. Then, using electron energy loss spectroscopy (EELS) to quantify the sample thickness, we demonstrate that there is no systematic effect of thickness on the statistical measurements of roughness. This result was verified using mathematical simulations of artificial surfaces with varying thickness. The proposed explanation is that increasing the side-view thickness of a randomly rough surface may change which specific features are sampled, but does not significantly alter the character (e.g., root-mean-square (RMS) values and power spectral density (PSD)) of the measured topography. Taken together, this work establishes a new approach to topography characterization, which fills in a critical gap in conventional approaches: i.e., the measurement of smallest-scale topography

Understanding Atomic-Scale Mechanisms of Adhesion and Deformation at Contacting Surfaces: Quantitative Investigations Using In situ TEM

Nanoscale contacts are relevant in advanced technologies like nanomanufacturing, scanning probe microscopy, micro- and nanoelectromechanical systems, nanodevices, and nanostructured catalysts. In all cases, functional properties such as adhesion, friction, electrical, and thermal transport depend on the size and nature of the contact. Continuum-based contact mechanics models are routinely applied to describe the behavior of these contacts in real-world applications, despite evidence of breakdown of their underlying assumptions at the nanoscale. In order to understand the applicability of contact mechanics at the nanoscale, and also the nature of any observed deviations, the present dissertation research uses in situ transmission electron microscopy (TEM) experiments and matched molecular dynamics (MD) simulations to perform loading and adhesion tests on nanoscale contacts. Specifically, the true contact area at varying loads is measured in experiment and atomistic simulation and compared against the predictions of continuum models for three different classes of materials: noble metals, covalently bonded materials, and metal oxides. First, for noble-metal contacts, it is observed that direct measurements of contact radius exceed the predictions of contact mechanics due to dislocation activity in the near-surface material, which is fully reversed upon unloading. Second, for same contacts, electron transport models under-predict the contact size by more than an order of magnitude. It is due to a robust monolayer of surface species on the contact interface, and the contact size is predicted better with tunneling theory. Third, for silicon-diamond contacts, the work of adhesion increases with applied stress which is contrary to the underlying continuum assumption that adhesion energy is a constant for a given material system. Such behavior is also observed for self-mated contacts of titania. This suggests that, for covalently bonded systems, the loading modifies the atomic-scale interactions at the interface and increases the adhesion strength. The primary implications of the present dissertation are two-fold: first, these findings demonstrated that commonly-used contact mechanics models are insufficient in predicting the contact properties in real-world nanostructures, and suggest modifications to account for atomic-scale phenomena; and second, these findings reveal the different physical mechanisms that govern the contact behavior of metals and covalent solids

Origin of Pressure-Dependent Adhesion in Nanoscale Contacts.

The adhesion between nanoscale components has been shown to increase with applied load, contradicting well-established mechanics models. Here, we use in situ transmission electron microscopy and atomistic simulations to reveal the underlying mechanism for this increase as a change in the mode of separation. Analyzing 135 nanoscale adhesion tests on technologically relevant materials of anatase TiO2, silicon, and diamond, we demonstrate a transition from fracture-controlled to strength-controlled separation. When fracture models are incorrectly applied, they yield a 7-fold increase in apparent work of adhesion; however, we show that the true work of adhesion is unchanged with loading. Instead, the nanoscale adhesion is governed by the product of adhesive strength and contact area; the pressure dependence of adhesion arises because contact area increases with applied load. By revealing the mechanism of separation for loaded nanoscale contacts, these findings provide guidance for tailoring adhesion in applications from nanoprobe-based manufacturing to nanoparticle catalysts