Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 88-96).Bi-layer material systems are found in various engineering applications ranging from nano-scale components, such as thin films in circuit boards, to macro-scale structures such as adhesive bonding in aerospace and civil infrastructure applications. They are also found in many natural and biological materials such as nacre or bone. One of the most human-related applications of bi-layer material systems is the artificial tooth involving the bonding between the natural tooth and the metal cap glued with a polymer based material. The structural integrity of a bi-layer system depends on properties of both the interface and the constitutive materials. In particular, interfacial delamination has been observed as a major integrity issue. In this research, a comprehensive investigation on the interfacial debonding mechanism has been conducted both computationally and experimentally using an epoxy-silica system. In the computational approach, a multiscale model which can predict the intrinsic strength between organic and inorganic materials, based on a molecular dynamics simulation approach, is presented. The intrinsic strength between epoxy and silica derived from the molecular level can be used to predict the structural behavior of epoxy-silica interface at the macroscopic length-scale by invoking a finite element approach using a cohesive zone model developed in this research. In order to understand the moisture effect in a more comprehensive way, the free energy profile of the epoxy-silica bonded system describing the debonding process has been reconstructed for both dry and wet conditions and it is found that the adhesion between epoxy and silica, which is dominated by the van der Waals force and Coulombic interaction, can be weakened significantly (more than 68% reduction) in the presence of water. Experimental work involving two different approaches, namely "nanoindentation" and "superlayer" approaches, in characterizing the interfacial fracture toughness are presented and the advantages and disadvantages of these two approaches are discussed. The morphology of material in the vicinity of the interface has also been captured using the scanning electronic microscope (SEM). Experimental results show that the interface fracture energy decreases significantly after 4 weeks of moisture conditioning. Both the experimental and computational results show that water plays a main role in the interfacial deterioration. The mechanism of interfacial deterioration is explained using molecular dynamics simulation and a multiscale model of the epoxy-silica bonded system which is capable of predicting the macro-scale structural behavior based on the reconstructed free energy profile of the bonded system at the nano-scale. The multiscale modeling used in this research provides a powerful new approach to link nano-level to macro-level for complex material behavior.by Tak Bun Denvid Lau.Ph.D
