Analysis of the acid, base and air oxidized carbon microspheres synthesized in a single step from waste engine oil Abheek Datta, Anustup Sadhu, Bhaskar Sen, Manpreet Kaur, Rajhans Sharma, Santosh Ch Das, Sayan Bhattacharyya ⇑ Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur 741 252, Nadia, WB, India article info Article history: Received 24 December 2012 Accepted 10 April 2013 Available online 17 April 2013 Keywords: B. Weight loss B. SEM B. IR spectroscopy B. Raman spectroscopy C. Oxidation abstract Carbon microsphere (C-lS) ball bearings are prone to corrode after long exposure to oxidizing environ- ments. 4.2 lm diameter C-lSs synthesized from waste engine oil (at 600 and 900 °C) by dry autoclav- ing, were exposed to nitric acid (30 and 60 vol.%), piranha (H 2 SO 4 :H 2 O 2 60:40 and 90:10), base (NH 4 OH:H 2 O 2 15:30 and 30:15) and air oxidation at 300 and 450 °C for 12–48 h. The oxidation processes were monitored by weight loss analyses, scanning electron microscopy, X-ray diffraction, Raman spec- troscopy, chemical functionalization and electrical conductivity measurements. Crystallinity and higher graphitic content in the C-lSs results in better resistance to oxidation. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Carbon microspheres (C-lSs) have tremendous potential as ball bearing lubricants to reduce friction and material wear in technol- ogies such as internal combustion engines, turbines, hydraulics, vehicle gears and compressors, microelectromechanical systems and computer disk drives [1]. The microspheres of carbon [1] and silica [2] demonstrate better frictional performance and improved rolling behavior than the nanometric spherical particles such as bucky ball fullerene C 60 [3,4] WS 2 /MoS 2 nanoparticles [5–7], car- bon nanotubes [8], nanopearls [9] and nano-onions [10,11]. Since the diameter of the microspheres is larger than the roughness of the shearing surfaces, they can easily roll over the surface. How- ever, the small size of the nanoparticles traps them in surface asperities. In fact the ball bearing efficiency of C-lSs is evident from its reported tensile strength, greater than 8 GPa before frac- ture [12]. The ball bearings undergo failure, corrosion and reduced life- time with increased time of usage which ultimately affect the effi- ciency and lifetime of the machinery [12]. The factors responsible for such corrosion include high oil temperatures, exposure to oxy- gen, fuel spillage, contamination from engine coolants, extended oil change intervals and unwanted emission of by-products such as formic acid, nitric acid, and carbon monoxide. from combustion of engine fuels. All these events essentially thicken the lubricant and cause deposits on the engine. The ball bearings made of C- lSs are prone to such corrosion in the presence of chemicals and at elevated temperatures. Carbon starts oxidizing to carbon mon- oxide (CO) and carbon dioxide (CO 2 ) from 300 °C in air. Smaller the carbon particles and higher the amorphous content, greater is its tendency to get oxidized. If the corrosion extended to the C-lSs is monitored it will provide a database for the extreme envi- ronments in which the C-lS ball bearings can be used. In fact, the oxidation behavior has been utilized to chemically functionalize carbon nanotubes [13,14] and graphene [15] for targeted applications. The solid C-lSs researched so far were synthesized by chemical vapor deposition [16], hydrothermal method [17] and chemical activation using acid-pretreated resin beads [18]. However, to meet the technological demands, there is a need to produce C- lSs in kilogram quantities from cheap and/or waste resources, by environmentally friendly processes. One such method is a high temperature high pressure reaction inside a closed cell. Waste plastics were successfully converted to C-lSs by this method [12,19], which is one-pot, solvent-free and does not allow the pro- duced greenhouse gases to escape from the reactor. In the present work, the C-lSs were synthesized in large quantities by the same approach from waste engine oil at two different temperatures. The degree of corrosion/oxidation of the C-lSs was systematically monitored by treating with two different concentrations each of nitric acid, piranha, ammonium hydroxide–hydrogen peroxide mixture and air heating. The electrical measurements demonstrate the changes in electrical conductivity due to oxidation. 0010-938X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2013.04.024 ⇑ Corresponding author. Tel.: +91 9051167666; fax: +91 33 25873020. E-mail address: sayanb@iiserkol.ac.in (S. Bhattacharyya). Corrosion Science 73 (2013) 356–364 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci