Machining induced transformation of graphite flakes to graphite/graphene nanoplatelets Wazeem Nishad, Sathyan Subbiah ⇑ Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu, India article info Article history: Received 18 December 2018 Received in revised form 23 June 2019 Accepted 11 August 2019 Available online 12 August 2019 Keywords: Few-layer graphene Tool-chip interface Solid lubricant additives, Aqueous graphite lubricants Graphene exfoliation abstract A mechanical exfoliation technique to produce exfoliated graphite-graphene nanoplatelet dispersion as a byproduct of aqueous graphite-based lubricant-assisted metal cutting is presented here. Low cutting speed oscillatory orthogonal metal cutting process has been carried out completely immersed in the lubricant containing graphite flakes. Reduction in the thickness of the graphite flakes from around 30 mm to graphite-graphene nanoplatelets with a size range of 1–15 nm is observed after machining. Characterization of the particles depicts low structural order and induced defects. Optimizing the param- eters can tune this machining-induced exfoliation technique to produce high-quality defect and disorder free few-layer graphene sheets. Ó 2019 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved. 1. Introduction Graphene, the first two-dimensional material to be isolated, has been found in various promising applications due to its interesting electrical and mechanical properties [1–3]. The applicability of these properties largely depends on the method of its synthesis [4]. These methods can be classified broadly into epitaxial growth, colloidal suspension, unconventional methods, and exfoliation (thermal, chemical and mechanical) [1,5–8]. Exfoliation, a top- down approach of synthesis, involves breaking the weak forces of attraction between the layers using external energy like electrical, chemical, thermal or mechanical [9]. Such exfoliations can create few-layer graphene – FLG (1–5 lay- ers), multilayer graphene – MLG (5–10 layers), graphene nanosheets (less than 100 nm lateral dimension), graphene micro sheets (100 nm to 100 mm lateral dimension) and/or graphite nanoplatelets (up to 100 nm thick) [10]. Single layer graphene does not produce a bandgap, while few-layer graphene can be tuned to open bandgap and enhance its applicability in nano-electronics [11]. Few-layer and multilayer graphene nanosheets and graphene-like amorphous carbon nanosheets have also been used as reinforcements in composites due to its improvements in the mechanical properties [12]. Mechanical exfoliation involves mechanical separation of the layers in bulk graphite by peeling off, cutting and/or shearing using a various method [13–16]. The natural lubricating action of the graphite flakes between two solid mechanically rubbing surfaces may also cause exfoliation. Graphite flakes/nano-platelets have been explored as a lubri- cant in machining [17,18]. The shearing process of machining [19] involves intense high-stress contact rubbing of the chip with the rake face of the cutting tool. If the graphite flakes can be intro- duced at this tool-chip interface, then the consequent rubbing can shear the flakes into few-layer graphene (FLG) (Fig. 1). This can be expected since the shear stress generated at the tool-chip interface during ductile-metal cutting, estimated between 150 and 300 MPa [19], is in the range of interlayer shear stress required to shear the layers of graphite that were reported to be about 140 MPa [20]. Hence, layered materials (e.g. graphite, hBN, MoS 2 ) while working effectively as a lubricant in metal cutting result in few-layer gra- phene dispersion as a byproduct. This phenomenon has been explored in the current paper. Machining conditions are optimized with the goal of enhancing the exfoliations and hence are not related to practical machining conditions. 2. Experimental design and methodology 2.1. Materials and methods One gram of sodium cholate (Alfa Aesar (A17074)) and 100 g of graphite flakes (mesh +70, 20) were mixed with one liter of dis- tilled water to prepare the lubricant solution [16]. Sodium cholate prevents aggregation of finer particles. A special double tubular https://doi.org/10.1016/j.mfglet.2019.08.002 2213-8463/Ó 2019 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: sathyans@iitm.ac.in (S. Subbiah). Manufacturing Letters 21 (2019) 66–69 Contents lists available at ScienceDirect Manufacturing Letters journal homepage: www.elsevier.com/locate/mfglet