Date

2013

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Vinci, Richard P.

Other advisers/committee members

Harmer, Martin P.; Chan, Helen M.; Nied, Herman F.

Abstract

Grain boundaries play a very important role in determining the mechanical properties of polycrystalline ceramics. In particular, the addition of certain dopants or impurities can cause significant changes in fracture path and fracture toughness. While substantial progress has been made in the past few decades, there remains an active debate about the fundamental mechanism of how the grain boundary segregation influences the mechanical properties. Prior studies have revealed limitations of the conventional macro-scale mechanical testing technique since dopants not only alter grain boundary structure they also influence the grain size, grain anisotropy and porosity that all contribute to the mechanical properties of polycrystalline bulk samples. Small-scale mechanical testing offers an opportunity to minimize the number of variables to obtain information about the mechanical properties of a single grain boundary, thus enabling direct correlations between grain boundary structures and mechanical properties. Three different micro-scale mechanical testing techniques--including Compression Splitting, Double Torsion and Micro-cantilever Deflection test methods--were examined and the Micro-cantilever Deflection test was selected for this project. At the beginning, tests were performed in single crystal magnesium aluminate spinel (MgAl2O4). Five distinct orientation cantilever specimens were prepared by Focused Ion Beam (FIB) technique and loaded by a dedicated in-situ nano-indentation system. Anisotropic finite element analysis was performed and the influence of the material's anisotropy on the stress intensity factor was determined. Orientation dependent fracture behaviors of different specimens were observed and associated with a mixed-mode loading condition and anisotropic mechanical properties of spinel single crystal. A maximum tensile stress fracture criterion for anisotropic materials was used to determine the fracture toughness values of fracture planes as the crack deflected at certain angles. Good agreement of fracture toughness values for {100} planes was obtained between the micron-scale measurements and macroscopic results available in the open literature. Lower bounds for the fracture toughness of {110} and {111} orientations were also obtained.After the technique had been validated, the Micro-cantilever Deflection test technique was then applied to test bi-crystal grain boundaries. The effect of ytterbium (Yb) on magnesium aluminate spinel (MgAl2O4) grain boundary structure and strength has been evaluated. Doped and updoped interfaces were characterized using atomic resolution High Angle Annular Dark Field (HAADF) imaging. Interface fracture toughness was determined using a micro-scale cantilever beam test. A 30% increase in fracture toughness was found to correlate with grain boundary Yb segregation.Finally, the Micro-cantilever Deflection beam deflection test was introduced to measure the fracture toughness and investigate fracture mechanisms of electrodeposited Ni-W coatings exposed to different heat treatment conditions. As deposited Ni-W films were found to be amorphous and to exhibit a ductile fracture behavior while after heat treatment nanocrystalline Ni-W exhibited a brittle fracture behavior. By introducing nano-scale sharp pre-cracks, the nanocrystalline Ni-W fracture surface exhibited an intergranular morphology that has never been reported before to our knowledge. In addition, the fracture path of nanocrystalline Ni-W was along boundaries of grain boundary agglomerates that can be directly correlated with an agglomerate cooperative motion model proposed in previous reports.

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