Mechanical Properties and Deformation Behaviors of Nanoporous Anodic Aluminum Oxide Membrane

Open Access
Author:
Dai, Jingyao
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 10, 2017
Committee Members:
  • Namiko Yamamoto, Thesis Advisor
  • Jogender Singh, Committee Member
  • Philip John Morris, Committee Member
Keywords:
  • Ceramics
  • Fracture
  • Nanoporous
  • Anodic Aluminum Oxide
  • Nanoindentation
Abstract:
In this thesis, mechanical deformation behaviors of nanoporous anodic aluminum oxide (AAO) membranes and associated mechanical properties were studied using nanoindentation testing; the effects of porosity, inter-pore distance, and material phase were evaluated. With unique combination of properties such as low density, high strength, thermal stability and corrosion resistance, ceramics are essential for aerospace and other engineering applications involving extreme environments, including gas turbines, thermal protection tiles and heat exchangers for corrosive agents. However, current application of ceramics is limited due to their low fracture toughness. In recent studies, nanoporous ceramics demonstrated unusual deformation mechanisms such as shear banding, pore collapse, and pore compression. While these behaviors introduced by nanoporosity can possibly enhance macro-scale toughness of ceramics, implementation of such mechanisms in macro-scale still requires more thorough multi-scale understanding of the mechanisms' drivers and limitations. In this study, the effects of nanoporosity on mechanical properties and deformation behaviors, particularly pseudo-plasticity, of ceramics were studied by conducting nanoindentation tests on anodic aluminum oxide (AAO) membranes. More specifically, key deformation mechanisms of nanoporous ceramics and their transition, along with the correlation between the above mechanical deformation mechanisms with measured micro-scale mechanical properties such as elastic modulus and hardness were studied. The AAO samples with porosity ranging from ~10-30%, pore size from ~38-210 nm and different material phases (amorphous and polycrystalline) were tested using Berkovich and cube-corner tips with indentation depth up to 2 μm and 4 μm and resulting indentation load up to ~400 mN. Mechanical properties including elastic modulus and hardness were measured. Deformation behaviors were observed by post-indentation scanning electron microscopy (SEM) and transmission electron microscopy (TEM) inspections and were correlated with the mechanical properties. Inspection of the indented AAO sample surface revealed the existence and trends of multiple deformation mechanisms including radial/subsurface fracture, nanocracks, shear banding in the form of arrays of collapsed nanopores and localized pore compression for AAO samples with varying pore size, porosity and material phase. For example, shear banding and localized pore collapse, compression behaviors are more easily triggered at medium to high porosity (~20-30 %) and small inter-pore distance (~107 nm). For AAO samples with large inter-pore distance and low porosity, only nanocracks or radial/subsurface fracture can be observed. Among these deformation mechanisms, shear banding forming around indentation impression created by cube-corner tip are of the most interest, as they can potentially result in quasi-ductility with limited compromise of materials mechanical properties. In future, these studies can be extended to correlating nano-pore deformation with macro-scale fracture toughness, potentially leading to development of toughened ceramics without weakening.