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Characterisation of tip wear during AFM probe-based nanomachining

Mukhtar, Nur Farah Hafizah 2017. Characterisation of tip wear during AFM probe-based nanomachining. PhD Thesis, Cardiff University.
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Atomic force microscope (AFM) probe-based mechanical nanomachining has been considered as a potential low-cost alternative method for the generation of nanoscale features on the surface of components and devices. Therefore, it is important to understand the factors that influence the tip wear of AFM probes in order to achieve reliable and accurate machining operations when implementing this process. Despite the fact that the basic applicability of AFM probe-based machining has been demonstrated for many years, studies focussing on the wear of the tips as a function of processing conditions are relatively scarce. In addition, the accuracy and practical suitability of in-situ techniques to monitor the condition of AFM probes is not adequately acknowledged. To address these issues, a series of experimental studies were conducted in this PhD research when implementing the AFM probe-based machining process on a single crystal copper workpiece at selected values of applied normal loads, machining distances and for different machining directions. First, the assessment of the wear of AFM silicon probes was carried out based on two dimensional (2D) tip profile data. This particular study also presented a simple method for improving the accuracy of the tip wear assessment procedure when conducted on 2D profiles. Next, AFM silicon probes with diamond-coated tips were used as cutting tools for a different range of applied normal loads and along various processing directions. For this particular study, the AFM probes wear assessment relied on two different three dimensional (3D) in-situ measurement techniques, namely the ultra-sharp tip scan approach and the reverse imaging method. Reliability and practical suitability aspects between these two in-situ techniques were also assessed and discussed. For each set of experiments, different qualitative and quantitative ii wear metrics were observed and analysed. Particularly, from the qualitative perspective, the evolution of the AFM tip apex profiles along selected machining distances and directions was considered. As for the quantitative measurement, tip radius and tip volume loss measurements were estimated. The most important findings reached in this study are given as follows. First, it was shown that the error associated with the traditional method of assessing the tip volume from 2D profiles could be 26% in comparison with the simple method proposed here. In addition, among the 3D in-situ AFM probe characterisation methods considered, the reverse imaging approach was judged to be the most reliable technique. This study also showed that tips in silicon were very prone to initial tip fracture during the AFM probe-based nanomachining process. This phenomenon could also take place, albeit to a lesser extent, when silicon tips coated with diamond were utilised. When the nanomachining process is not in control due to such tip fracture, it is difficult to extract firm conclusions about the influence of the processing parameters on the tip wear. Besides, this rules out the application of a design of experiments approach where the minimisation of the tip wear volume may be the objective. The study also showed that the AFM probe-based nanomachining process was more likely to be in control when using silicon tips coated with polycrystalline diamond with no nitrogen doping. In this case, a much reduced likelihood of tip fracture could be achieved accompanied with negligible tip wear. In addition, the associated results suggested that the evolution of the tip wear was not equal in all machining directions investigated, with the largest wear occurring in a direction parallel and away to the cantilever long axis. The reason for this should be due to the fact that this was also the direction where the process was most likely to be conducted in the ploughing-dominated regime. Finally, when the nanomachining process was realised in iii control, the wear volume was seen to increase with the increase in the normal load for all directions considered.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TJ Mechanical engineering and machinery
Uncontrolled Keywords: Nanotechnology; AFM Probe Based Nanomachining; Nanomachining.
Date of First Compliant Deposit: 6 April 2017
Last Modified: 20 Apr 2021 09:44

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