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

Vishnubhotla, Sai Bharadwaj (2020) Understanding Atomic-Scale Mechanisms of Adhesion and Deformation at Contacting Surfaces: Quantitative Investigations Using In situ TEM. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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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.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Vishnubhotla, Sai Bharadwajsav40@pitt.edusav400000-0002-4611-3313
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairJacobs,
Committee MemberWiezorek, Jö
Committee MemberMao,
Committee MemberVeser, Gö
Date: 31 July 2020
Date Type: Publication
Defense Date: 13 March 2020
Approval Date: 31 July 2020
Submission Date: 26 March 2020
Access Restriction: 2 year -- Restrict access to University of Pittsburgh for a period of 2 years.
Number of Pages: 112
Institution: University of Pittsburgh
Schools and Programs: Swanson School of Engineering > Materials Science and Engineering
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: In situ TEM Electron Microscopy Adhesion Contact Mechanics Dislocations Nanomechanics Electrical Transport Nanocrystalline Metals Noble Metals Metallic Contacts Metal Oxides
Date Deposited: 31 Jul 2020 18:47
Last Modified: 31 Jul 2022 05:15


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