published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Civil Engineering / Master / Master of Philosophy

Anodic alumina nanohoneycombs (AAO) have been widely used because of its convenient fabrication and controllable pores’ geometry. A lot of investigations have been conducted to study its physical and chemical properties. However, the mechanical properties, especially tribological properties, of such a popular nanomaterial still remain almost unknown. In this project, a series of scratch experiments were conducted on AAO films fabricated by a two-step anodisation method. The testing system is a G200 Nanoindenter provided by Agilent Corporation. A standard diamond Berkovich tip is used as the scratch tip. A scanning electron microscope was used to image the microstructure of the material deformation after scratching. Strengths of AAO domains with different pore regularities were compared by performing the scratch tests at constant normal loads crossing the boundaries separating these domains. Ramping load tests were carried out to show the effects of the normal load on the deformation and friction of the AAO. Scratch cycles and velocities were also varied to see their influence on friction and wear. The results show that the more ordered AAO structure has higher strength than disordered counterparts under scratch testing. The friction coefficient reduces rapidly on increasing normal load, and an explanation of this unusual behaviour is offered by considering the row-by-row deformation of the AAO structure. In multicycle scratch tests, the friction reaches a maximum at the fourth cycle. A step-like deformation behaviour was observed when the scratch velocity became extremely large, and this is probably due to the dynamic response of the instrument. / published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Self-ordered anodic porous alumina with a nano-honeycomb structure has recently been extensively used as templates for the synthesis of various nanomaterials for diverse applications. However, due to the insufficient knowledge on the combined electro-chemo-mechanical processes, the formation mechanism of self-ordering has been under debate for decades without clear conclusions. Also, fast fabrication of highly self-ordered and mechanically stable anodic porous alumina is still a challenge. Furthermore, the actuation behavior of anodic porous alumina upon external mechanical and electrical triggering in an electrochemical cell has not been exploited. In this work, firstly, we investigated the self-ordering mechanism by establishing a kinetics model involving the Laplacian electric potential distribution and a continuity equation for current density within the oxide body. Current densities governed by the Cabrera-Mott equation are formed by ion migration within the oxide as well as across the interfaces. The pore channel growth, due to electric-field-assisted reactions, is governed by Faraday’s law. Real-time evolution of pre-patterned pore channel growth was simulated in two-dimensional cases by finite element method. The simulations revealed a parameter domain within which pre-patterned pore channels will continue to grow in a stable manner during the subsequent anodization if the pre-patterns are commensurate with the self-ordered configurations, or these are driven into stable if the pre-patterns do not initially match the self-ordered configurations. This was verified in experimentally observed pore channel growth under the guidance of pre-patterns made by focused-ion-beam milling. Furthermore, the simulations revealed that ionization reaction on (001) oriented Al grain is relatively easier than that on (101) grain, which results in stable and unstable pore channel growth on (001) and (101) Al grains, respectively, both of which were observed from the simulations and experiments. Secondly, a scheme on quantitative evaluation of self-ordering qualities in anodic porous alumina has been developed, based on which we systematically searched the optimum self-ordering conditions, by varying the key anodization factors, including substrate grain orientation, electrolyte concentration, temperature, voltage, and time. A high acid concentration and high temperature anodization method was found. Compared with conventional methods, the present method can realize fast formation of highly self-ordered, and mechanically stable anodic porous alumina under a continuous range of anodization voltage with tunable interpore distances. Thirdly, reversible bending was found in anodic porous alumina-Al composites upon cyclic electric actuation, as directly observed by an optical microscope and detected by in situ nanoindentation. The bending is thought to be the result of charge-induced surface stresses in the nanoporous alumina. The results suggest a new type of composite materials for applications as micro-scale actuators to transform electrical energy into mechanical energy. Furthermore, the composite exhibits significant softening during in situ nanoindentation when the estimated maximum stress underneath the indenter is exerted on the metal/oxide interface. Softening was further verified by in situ microindentation. Electron microscopy examination indicated that the softening is due to a combination of high compression stress and electric field acting near the interface, which enhance ionization reaction and cause the interface to move faster into the substrate. / published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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published_or_final_version / abstract / toc / Mechanical Engineering / Master / Master of Philosophy

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