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Title:Interfacial spallation/delamination properties of self-assembled monolayers
Author(s):Zhang, Chen
Director of Research:Geubelle, Philippe H.
Doctoral Committee Chair(s):Geubelle, Philippe H.
Doctoral Committee Member(s):Sottos, Nancy R.; Chasiotis, Ioannis; Chew, Huck Beng
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Self-assembled monolayer, interfacial property, spallation, delamination, numerical simulation.
Abstract:Self-assembled monolayers (SAMs) are short (nanometer-size) organic chains terminated by functional groups that can be selected to tailor the electrical, thermal and/or mechanical properties of interfaces. In this thesis, we investigate how the presence of SAMs affects the failure properties of gold film/silicon/fused silica substrate interfaces. In particular, we study how the presence of SAMs affects (i) the spallation strength and (ii) the fracture toughness of the interface. The modeling work summarized in this thesis is motivated by previous results of molecular dynamic (MD) simulations and laser-induced spallation/delamination tests to quantify the strength and toughness of SAM-enhanced interfaces. Though the results obtained from MD simulations and experimental observations yielded similar trends in comparing the contribution of various SAMs, their actual values were off by significant amounts. The research presented in this dissertation involves the development of continuum-level numerical models to analyze the dynamic spallation and delamination events to fill the gap between MD simulations and experimental results. In the first part of the thesis, a continuum-level study is performed to investigate the influence of surface roughness on the cohesive strength of the interface between a fused silica/SAM substrate and a transfer-printed gold film. We approximate the film as a deformable continuum interacting with a rough substrate of SAM. Using the cohesive law predicted by MD, spallation is simulated to evaluate the effective traction-separation characteristics for the rough SAM-gold interface. The separation attributes based on roughness parameters and material properties of gold film are observed. The dependence of the interfacial cohesive strength of SAM-enhanced interface on incorporating roughness and the thin film properties is studied. In the laser-induced delamination test, the interface fracture energy is computed by assuming all of the kinetic energy imparted into the weak adhesion layer of the film is converted into fracture energy. However, part of this effective interface fracture toughness is associated with plastic deformations in the film. To quantify the plasticity contribution to the effective fracture toughness of the SAM-enhanced interface, we perform an implicit finite element numerical analysis of the dynamic delamination event that incorporated both large deformation and plasticity effects. Cohesive elements whose failure law is derived from MD simulations are introduced along the interface to simulate the failure initiation and debonding process. The amount of dissipated plastic energy is quantified and the film profile is depicted depending on the properties of the thin film and the interfacial attributes. The model is validated with experimental measurements of the crack propagation length, profile of the debonded thin film, and interfacial fracture energy.
Issue Date:2018-07-13
Rights Information:Copyright 2018 Chen Zhang
Date Available in IDEALS:2018-09-27
Date Deposited:2018-08

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