Doctor of Philosophy (PhD)
Civil and Environmental Engineering
Clay minerals, ubiquitous in the geosphere, are hydrous layered silicates or phyllosilicates that are made of nanometer-thick 2:1 or 1:1 layers. Understanding their mechanical properties of clay minerals is of vital importance to a variety of disciplines, such as geomechanics, geophysics, mineral physics, and nanocomposites. Owing to their complex crystal structure and size, however, accurate determination of their mechanical properties is a significant challenge. This dissertation presents the first research effort to study systematically the fundamental mechanical properties along the c-axis (e.g., elastic modulus, hardness) of a wide range of phyllosilicates with varying crystal structure and chemical compositions. To explore the applications of clay minerals in nanocomposites and natural geological deposits, research was also extended to study the mechanical behavior of a class of synthesized nanocomposites – clay-oxide nanostructured multilayers possessing a similar layered structure and of the clay aggregates with highly preferred orientation – a layered packing of clay crystals. The experimental program employed an array of nanocharacterization and nanomechanical testing techniques, including nanoindentation under both static and dynamic loading modes, scanning probe microscopy, and atomic force microscopy (AFM), in order to elucidate the mechanical behavior of the tested materials at the nanoscale and to establish a fundamental understanding of the elastic and plastic deformation mechanisms for these complex layered nanostructured materials. For the nanocomposites and clay aggregates, the characterization was also accompanied by various sample preparation and pre-treatment methods to examine dynamic nature and variability of a material’s properties. Moreover, a simple empirical method was developed of extracting the elastic moduli of both thin films and underlying substrates. The success of this study proves that nanoindentation is a viable tool to probe the mechanical properties of hydrous phyllosilicates and to study their nanoscale deformation mechanisms. Results reveal that the mechanical properties of clay minerals are significantly dependent upon the characteristics of the crystal structure: layer charge, interlayer complexes, interlayer spacing, and even chemical compositions. In general, higher layer charges result in stronger interlayer cohesion forces and hence higher stiffness and resistance to permanent penetration. A simple empirical model was also proposed to predict the c-axis elasticity of clay minerals.
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Wei, Zhongxin, "Nanoidentation behavior of clay minerals and clay-based nonstructured multilayers" (2009). LSU Doctoral Dissertations. 3033.