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Investigations into AFM-tip based vibration-assisted nanomachining

Baqain, Sameeh 2022. Investigations into AFM-tip based vibration-assisted nanomachining. PhD Thesis, Cardiff University.
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The recent global shortage of microchips highlighted the exponential increase in demand in the past three decades. It also showcased the fragile supply chain and its vulnerability to bottlenecking. Hence, it became evident that there is a need to explore additional manufacturing methods for miniature device manufacturing. The use of the atomic force microscope (AFM) has gained traction due to it being more environmentally friendly and its lower cost of operation when compared to other nanofabrication methods. The use of the AFM tip as a cutting tool is well established, especially when silicon workpieces are machined. In addition, the introduction of vibrations to the nanomachining process was found to provide improvements. However, the majority of research looks into silicon, with copper being investigated to a much lesser degree. Hence, this thesis investigates AFM tip-based vibration-assisted nanomachining of single crystal copper theoretically and experimentally. Following the introduction, a literature review of nanomachining is done, followed by a review regarding Atomic Force Microscopy. After that, conventional and ultraprecision machining are reviewed. Stagnation zones that play an important role at micro and nanoscales are examined. Additionally, vibration-assisted nanomachining and its various methods are explained. Also, the advantages of vibration-assisted nanomachining over conventional nanomachining are discussed. Finally, the analysis necessary for the determination of the one-direction vibration-assisted nanomachining parameter is done. Equations that govern the vibrations are explained in detail, including values extraction necessary for selecting the various frequencies and amplitudes used during experimentation. Chapter 3, in turn, studies the material deformation mechanisms at different scales and how they differ. Since the motion of dislocations is common among all size scales, it is analysed in addition to slip and the resolved shear stress. Four categories are agreed upon in the research community and are studied: (i) atomic scale, (ii) nanoscale, (iii) microscale (iv) macroscale. A brief discussion of the atomic scale is done with the inverse Hall-Petch effect on the grain size being mentioned. The nanoscale is studied in most detail due to its close relation to the experimental work of this thesis, with emphasis on nucleation of dislocations and the variations between metals and macroscopically brittle materials like silicon. After that, the microscale is discussed with its most prominent plastic deformation theory, referred to as the strain gradient plasticity (SGP). Also examined is the applicability of SGP at the nanoscale. In addition, the macroscale plastic deformation is studied, including classical theories, which in turn is followed by hardness analysis. Fatigue and specifically low-cycle fatigue are analysed then, with the impact of the size scale on the latter also reviewed. Finally, an analytical study of an experiment similar to one done in this thesis wherein proof is provided regarding the prominence of dislocation nucleation in the nanomachining of copper. As for Chapter 4, characterisation of the AFM tip shape and condition is performed. Following an introduction to tip characterisation, a review on how the shape of the tip impacts AFM operations, including nanomachining, is carried out. Methods including in-situ and ex-situ used to characterise the tip are then showcased. Later, images using the SEM are taken of the tip and later processed for tip shape extraction. Consequently, a power-law function is used to characterise the tip shape using a non-integer bluntness value. Bluntness values were extracted for both vertical and 12° tilt angle orientation, where the latter is observed in practice. Afterward, a force-displacement experiment of the same tip is done to validate the results obtained from the SEM images, using the Borodich rescaling formula. The bluntness impact of the work of adhesion and pull-off force is analysed using values extracted during experimentation, in addition to other arbitrary but practical non-integer values. In the end, an analytical study is done to investigate the factors affecting the effective rake angle. Such angle plays a vital role in the nanomachining process, including forces exerted and surface quality. However, the tip bluntness, depth of penetration, and tilt angle all impact this angle to varying degrees. Chapters 5 and 6 cover the vibration-assisted nanomachining experimental work and consequent results. Chapter 5 discusses the test setup and all the necessary equipment to complete the experiment. The cutting conditions and parameters of the tests are discussed, such as the three different amplitudes and frequencies of the vibrations resulting in nine combinations plus a conventional test used as reference. Parameters also include the cutting velocity and direction in each test and the data acquisition software settings. The chapter then briefly explains the imaging and scanning techniques used to investigate the resultant grooves. Chapter 6 discusses the outcomes and results of the experiment. The chapter starts by presenting the first test's results with no vibrations induced. Following that, the nine tests with vibrations induced are presented with specific parameters compared to the first test. These include groove depths, pile-up heights, surface roughness, chip formation, and live signal acquired from AFM nanomachining experiment. The results are then discussed, highlighting the cases where the induced vibrations did provide improvements, where they did not, and where no significant changes are noticed. Later, trends observed during testing are presented. Finally, evaluation and comparisons between the results and what is available in the research community, including theoretical and analytical studies with consequent conclusions, are presented. Finally, Chapter 7 concludes with the knowledge gaps addressed in this thesis, in both the characterisation of AFM tips as well as the optimisation of AFM tip-based vibration-assisted nanomachining and followed by a summary of the work done in this thesis and conclusions encompassing work done across the thesis. Then, suggestions regarding the additional future work that can be done to expand and improve upon what was done throughout this thesis.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Atomic Force Microscope AFM, Scanning Electron Microscope SEM, Nanomachining, Bluntness of AFM tip probes, Load-displacement curve, Three-sided pyramid
Date of First Compliant Deposit: 6 October 2022
Last Modified: 07 Oct 2022 10:10

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