Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Materials Science & Engineering

First Advisor

David J. Srolovitz

Abstract

The properties of contacting interfaces are strongly affected by the bulk and surface properties of contacting materials, and the ubiquitous presence of adsorbed contaminants. We focus on the properties of single asperity contacts in the presence of adsorbates within a molecular dynamics description. A platinum-platinum asperity contact is modeled with adsorbed oligomers with variable properties. This system is particularly tailored to the context of nano-electro-mechanical system (NEMS) contact switches, but the results are generally relevant to metal-metal asperity contacts in non-pristine conditions.

Even though mechanical forces can displace adsorbate out of the contact region, increasing adsorbate layer thickness and/or adsorbate/metal adhesion makes it more difficult for metal asperity/surface contact to occur, thereby lowering the electrical contact conductance. Contact separation is a competition between plastic necking and decohesion. The mechanism which operates at a lower tensile stress dominates. Necking dominates when the adsorbate/metal adhesion is strong and/or the adsorbate layer thickness is small. In broad terms, necking implies larger asperity deformation and mechanical work, as compared with decohesion. Optimal NEMS switch performance requires substantial contact conductance and minimal asperity deformation; these results indicate that these goals can be achieved by balancing the quantity of adsorbates and their adhesion to the metal surface.

As the number of contact cycle increases, the system settles into a steady-state where the morphologies, Pt/Pt contact area and deformation stabilize. The stress generated during asperity contact increases the rate of reactions amongst adsorbates in contact region. This leads to an increase in the size of adsorbate molecules, and thus more exposed metal. This implies higher electrical conductance in closed contact, but more plastic deformation and mechanical work expended in each cycle. This implies that mechano-chemistry is important in the formation/structure of tribopolymers formed in multi-cycle contacts in NEMS switches. The evolution of asperity contacts in environment containing gaseous species that form tribopolymers is controlled by device operation conditions, gas composition, and fundamental reaction rates that describe adsorption of species from gas onto metal and adsorbate surfaces and reactions between adsorbates. This provides guidance for thinking about the complex interactions that control the long-term performance of NEMS switches.

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