Research Papers

High Rake Angle Orthogonal Machining of Highly Ordered Pyrolytic Graphite Parallel to the Basal Plane

[+] Author and Article Information
B. Jayasena

School of Mechanical and
Aerospace Engineering,
Nanyang Technological University,
Singapore 639798
e-mail: buddhikaphd@gmail.com

S. Subbiah

Indian Institute of Technology, Madras,
Chennai 600036, India
e-mail: sathyans@iitm.ac.in

C. D. Reddy

Institute of High Performance Computing,
Singapore 138632
e-mail: reddy@ihpc.a-star.edu.sg

1Corresponding author.

*work was carried out when author was at NTU Singapore.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received October 8, 2014; final manuscript received May 29, 2015; published online September 9, 2015. Assoc. Editor: Radu Pavel.

J. Manuf. Sci. Eng 138(1), 011010 (Sep 09, 2015) (8 pages) Paper No: MANU-14-1515; doi: 10.1115/1.4030756 History: Received October 08, 2014

High rake angle orthogonal machining of highly ordered pyrolytic graphite (HOPG) parallel to the basal plane was carried out to synthesize few layers of graphene. The quality of the graphite sheets was found to be an alliance of any pre-existing defects in the HOPG and the nature of the machining process itself. Presence of pre-existing defects such as kinks and discontinuous layers were observed during the lateral examination of HOPG structure prior to machining. Evidence of flat, folded, and rolled structures were found in exfoliated graphite sheets in addition to defects such as two types of kink bands. Multiple spikes in measured cutting forces were seen during machining due to disturbances in tool movement. Molecular dynamic simulations were carried out to support the argument that specific pre-existing defects such as discontinuous layers cause the marked disturbances during machining.

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Fig. 1

Microtome mechanical exfoliation technique: (a) schematic diagram of specimen and tool, (a-1) unoriented machining of graphite, and (a-2) oriented sectioning of HOPG

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Fig. 2

The experimental setup: the HOPG sample is mounted on the moving stage of the ultramicrotome while the wedge is mounted on a force sensor held on the fixed portion of the microtome. The force sensor detects machining forces.

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Fig. 3

Pre-existing defects in HOPG prior to machining: (a) sidewalls of the as-received HOPG material show parallel striation marks, (b) enlarged view of HOPG sidewalls showing presence of powdered material, (c) kinks (circle 1) and discontinuous layers (circle 2) seen on the sidewalls after several adhesive tape peelings to remove the powdered layer, (d) distorted layers can also be seen on some side wall locations, (e) cleavage step, and (f) wrinkles on the top surface of the HOPG after several peelings

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Fig. 4

Optical microscope images of thin section layer morphologies: (a) flat and several folded layers showing the thickness variations in colors (online version), (b) carbon particles attached to the edge of the layers, and (c) enlarged image of a single continuous layer. The marked regions of 1 show pink color (online version) that indicates thickness less than 5 nm, and 2 shows yellow color (online version) regions representing the thicker (tens of nanometers) sections.

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Fig. 5

Optical microscope images of thick sections: (a) suspected deformed graphite-stand regions, (b) wedge marks, (c) tearing marks, and (d) large area-flat region without any folds or defects

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Fig. 6

SEM images of sectioned layers: (a) sheared layers, (b) folded layers, (c) step shaped kink, (d) peak-cap shaped kink, and (e) crumpled layers with sharp folds

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Fig. 7

TEM Images of sectioned layer morphologies: (a) flat and sheared layers, (b) torn layers, (c) and (d) kinks, (e) and (f) multiple bent layers, and (g) and (h) partially rolled and axially slid out structures

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Fig. 8

The surface of HOPG after machining: (a) uneven surface after machining, (b) enlarged image indicating patches of cleaved regions, (c) enlarge image of a sectioned surface indicating wedge marks, and (d) cracked regions on the HOPG surface where cleaved sections are torn at this point

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Fig. 9

Machining and normal force trend with time. Several intermittence force-peaks were observed during the machining process. The first peak is due to the wedge and material engagement, while subsequent peaks could be caused by pre-existing defects.

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Fig. 10

Schematic diagram of simulation models: (a) defect-free layers before tool penetration [35], (b) chip is coming on the tool after penetration into HOPG [35], (c) machining force profile of defect free model and observed only initial peak, which is due to the initial tool engagement, (d) a discontinuous layer representing a pre-existing defect in the HOPG, (e) shape of the layers generated ahead of the wedge during machining, and (f) machining force profile now displays two peaks, first one due to the initial tool engagement and later one is due to the defective layer




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