Role of Surface Mechanical Attrition Treatment (SMAT) on the Electrochemical and Mechanical Properties of Functional Metallic Materials


Student thesis: Doctoral Thesis

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Award date24 Oct 2018


Anodization, an effective fabrication technique consists of electrochemical film passivation and oxidization of metal passive layer, is widely used in the synthesis of nanoporous metals. Modern fabrication techniques involved in anodic nanoporous metal production focus on the variation of extrinsic properties during the electrochemical process. As a result, experimental enhancements on fabricating new anodic nanoporous metals are constrained by electrochemical parameters ranging from voltage, electrolyte, temperature, and time. In this thesis, an alternative approach under the perspective of how intrinsic property can facilitate anodization is examined. This intrinsic property is material nanocrystallization. The following chapters in this thesis will explain how nanocrystallization can enhance anodization, fabricate anodic metal oxides with new morphology, and consequently equipped with enhanced mechanical and electrochemical performances.

To achieve nanocrystallization, a small plastic deformation technique, surface mechanical attrition treatment (SMAT), is used prior anodization to convert the metal of interest from polycrystalline into nanocrystalline. The conversion results in grain size refinement and establishment of high density of grain boundaries. A comparison between the resulting anodic metals subject with, and without nanocrystallization will be contrasted to demonstrate the significance of how nanocrystallization can enhance anodic kinetics, and the anodic metal oxides’ improvements in both mechanical and electrochemical performances. 

The structure of this thesis is arranged into six chapters. The first chapter is an introduction providing an understanding on SMAT, and anodization. It will explain their respective theories and mechanisms. A background on nanoporous materials and their modern fabrication methods are discussed. The chapter concludes with the principles of the applications performed by the resulting anodic metal oxides. The second chapter presents the methodology of the design and engineering of SMAT. Various SMAT prototypes were developed and repeatedly tested with revision to generate desirable amount of nanocrystallization. This chapter highlights the evolutionary modifications of SMAT, and how it is designed and improved in order to create sufficient grain refinements in the nanometer regime for better acceleration of chemical reaction kinetics during the fabrication of anodic metal oxides. 

The sequential chapters selected three different metals as individual experimental studies to demonstrate how nanocrystallization contributes in the enhancements of anodization. In the third chapter, tin subjected to SMAT resulted an additional 72 % in the layer thickness of anodic tin oxide. More interestingly, this tin oxide consists of paralleled nanostructures in confined domains, and when multiple domains assembled and viewed altogether, displays a zigzagged patterned morphology. Through its morphological characteristic, this type of nanostructure demonstrates enhanced light absorption ability with a performance increase of 76 % in kinetics of photocatalytic degradation. The fourth chapter examines the effects of SMAT on aluminum when integrated with anodization. By applying different SMAT conditions, the diameters of the anodic pores can be controlled and tuned. The pore diameter in anodic aluminum subjected to SMAT is 5.6 times smaller than normal aluminum under the same anodic conditions. Moreover, the anodic pores tuned by SMAT demonstrated better mechanical properties in hardness and elastic modulus when compared to their counterpart. The fifth chapter discusses the temperature effects on SMAT when it is applied to nickel. The integration of cryogenic condition has the ability in facilitating the formation of nanocrystallization compared to ordinary room temperature SMAT. It results an anodic layer thickness of 4.34 µm when contrasted with room temperature SMAT’s 1.35 µm under identical anodization conditions. Its superior thickness growth in anodic nickel oxide resulted in excellent photocatalytic performances. Finally, the last chapter concludes and summarizes the findings of this thesis.

The discoveries in this thesis open a new pathway in anodic fabrication of nanoporous materials. Through the integration of SMAT and anodization, the metal firstly experiences nanocrystallization, followed by the facilitation of anodic kinetics, and last but not least, the formation of nanomaterial with new morphology equipped with enhanced mechanical and electrochemical abilities. The discoveries in this thesis setup a new perspective in the future developments of nanomaterials.