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Theoretical and experimental investigations about the AFM tip-based nanomachining process

Al-Musawi, Raheem 2016. Theoretical and experimental investigations about the AFM tip-based nanomachining process. PhD Thesis, Cardiff University.
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In the last two decades, technological progress towards the miniaturisation of products and components has increased significantly. This trend has also been driven by demands for the manufacture of devices with functional features on the nanoscale. One of the nanofabrication processes, which has been proposed by researchers to meet such needs, relies on the mechanical machining of the surface of a workpiece with the tip of an atomic force microscope (AFM) probe. In this case, the AFM probe is utilised as a cutting tool as it enables the direct contact between its sharp tip, which is fixed on a flexible micro cantilever, and the workpiece surface. A relatively large numbers of studies have been reported in the field of AFM tip-based nanomachining since the invention of the AFM instrument itself just over thirty years ago. However, such studies have typically neglected the fact that AFM probes should be considered as flexible tools when investigating this process. Thus, this shortcoming constitutes the main motivation behind this PhD research. Following a review of the literature, the work reported in this Thesis starts by a study of the bending orientation of cantilevers during AFM tip-based nanomachining operations along different processing directions. To achieve this, an advanced experimental set-up is developed first in order to monitor a number of output signals, which characterise the motions of both the fixed and the free ends of the cantilever together with the displacements of the AFM stage. A refined theoretical analysis is also presented to express the bending orientation of an AFM probe cantilever at its free end as a function of the forces acting on the tip when machining in a direction pointing away from the probe. This refined model shows that the bending orientation depends on both geometric parameters of the cantilever and on the cutting forces. Complementary experiments, which are designed to determine the quasi-static bending behaviour of cantilevers in practice, show that, contrary to assumed knowledge, both concave and convex bending orientations could take place when machining along this direction. The occurrence of a change of the cantilever deflected shape from convex to concave bending during machining can principally change the depth and width of grooves produced. For instance, the depth of grooves machined on a single crystal copper specimen may increase up to 70% following this phenomenon. iv Following this, another refined model is also developed to measure the normal force acting on the tip when the AFM stage is static by taking in account the cantilever geometry and its inclination angle with respect to the sample surface. This work leads to the introduction of a correction factor that should be applied when using the conventional equation for determining the normal load in this configuration. Results obtained when implementing this model based on the dimensions of typical commercial AFM probes show that the conventional approach always leads to an underestimation of the normal applied force. In addition, it is demonstrated, both theoretically and experimentally, that the conventional method for determining the applied normal load during AFM tip-based nanomachining, i.e. when the stage is not static, is wrong. Based on this shortcoming, a novel procedure is proposed to estimate all three force components (i.e. thrust, axial, and lateral forces) acting on the tip during AFM tip-based nanomachining. To achieve this, two novel methods are also developed to assess the actual value of normal force during machining, which in this case is referred to as the thrust force. Based on experimental data, a good agreement is found between both methods for different physical quantities evaluated. Another refined theoretical model, based on the classical beam theory, is also employed in this procedure to determine the axial force acting on the tip and subsequently, the lateral force. Using this novel procedure to estimate the cutting forces, it is also shown that even if the deflection angle at free end of probe is constant, this does not mean that the associated cantilever vertical deflection is constant between the configurations when the AFM stage is static (i.e. for nanoindentation) and when it is moving (i.e. during an actual cutting operation). Finally, in order to gain further insights into the material removal mechanisms that influence the process, a series of post-machining investigations on the topography of produced grooves is reported for different applied loads and processing directions. This particular experimental study takes advantage of the prior knowledge established in this Thesis. Indeed, the understanding of the cantilever deflected shape and the accurate assessment of cutting forces provide key inputs when the groove formation process is analysed.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TJ Mechanical engineering and machinery
Uncontrolled Keywords: Atomic force microscope; Nanomachining; Cantilever deflection; Cutting forces measurement; Normal force; Normal sensitivity; nanoscale groove.
Date of First Compliant Deposit: 13 April 2017
Last Modified: 04 Jun 2017 09:47

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