Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond Investigations on tribological properties of non-catalytic CVD synthesized carbon spheres in lubricant Radhika Panickar a,b, ⁎ , C.B. Sobhan a,b , Sivaji Chakravorti a a National Institute of Technology, Calicut 673 601, India b School of Materials Science and Engineering, National Institute of Technology, Calicut 673 601, India ARTICLE INFO Keywords: Carbon spheres Tribology Lubricants Additives Four-ball test ABSTRACT The effect of Carbon Spheres (CS) on the tribological properties of lubricating oil was studied. Relatively uniform Carbon Spheres of size ranging from 200 to 350 nm were obtained using the non-catalytic Chemical Vapor Deposition (CVD) method. The synthesis of CS was carried out using the C 2 H 2 /H 2 /Ar gas mixture at 950 °C. Morphological studies and material structure characterizations were carried out using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Raman Spectroscopy and X-ray Diffraction (XRD) techniques. The tribological properties of oil (Castrol 20W-40) samples containing different wt% of CS were explored using a four-ball tester conforming to the ASTM 4172-94 standard for wear prevention properties of lubricating oil. The concentration of CS in neat oil was varied from 0.01 wt% to 0.5 wt%, and the maximum reduction in the coefficient of friction obtained was 10–11% corresponding to 0.3 wt% of CS without the ad- dition of surfactants. The optical image of the contact surface of the balls confirms a reduction in the wear corresponding to the diameter of the worn scar. 1. Introduction Nano tribology investigates the effects of nano-additives on the tribological performance of lubricating oils and greases. Numerous studies have been carried out to learn the impact of micro and nano- additives on the tribological properties of lubricating oils and greases. The reported studies on additives in lubricants show remarkable im- provement in the tribological properties [1–5]. Apart from micro and nano-additives, its composites are also explored in the friction and wear studies [6–8]. The lubricant additives show excellent enhancement in tribological properties like friction, anti-wear properties and higher load capacities at different loads [9,10]. Different mechanisms like mending effect [11], rolling effect [12], the formation of tribo-films [13,14], the formation of protective films [15] and third body layer formation [16] are responsible for the reduction in friction. The studies on additives in lubricants reveal that these additives form a thin lubricant film with an average thickness of 0.01 and 1 μm that separates the lubricating surfaces [17]. The lubricating film formed from the additives separates the contact surface, and improve the friction and wear performances. Among different material additives, carbon materials like graphene [18], carbon nanotubes [19], carbon black [20], carbon onions [21], nanodiamonds [22] and carbon dots [23–26] also have been investigated for their tribological performance. These carbon materials have applications in almost all fields of en- gineering owing to their exciting morphology, corrosion resistance, high thermal conductivity, and outstanding mechanical properties [27–29]. Carbon Spheres (CS) are also fascinating amorphous carbon material that significantly improves the tribological properties of lu- bricating oils. Numerous studies have been reported on the tribological performance of CS. Table 1 illustrates the work published so far on CS, including the method of synthesis, optimized wt% in neat oil, the base oil used and the tribometer used for the friction and wear studies. From the reported tribological studies of CS, the optimized weight percentage was different for CS synthesized from ultrasonic irradiation and thermal decomposition methods. However, for CS synthesized using the hy- drothermal carbonization method of D-glucose, the optimized disper- sion of 0.5 wt% CS was found similar in two different base oils [30,31]. This result infers that the method of synthesis has significant im- portance on the tribological property of the CS in different lubricants. The present work focuses on the study of the tribological property of the lubricating oil dispersed with CS synthesized from the non-catalytic CVD method. The CS was synthesized under non-catalytic APCVD (Atmospheric Pressure Chemical Vapor Deposition) at 950 °C in C 2 H 2 / H 2 /Ar gas mixture and was characterized using various means. The tribological investigation of CS in lubricant was performed using a four- ball tester conforming to the ASTM standard. The neat oil, Castrol 20W- https://doi.org/10.1016/j.diamond.2020.107834 Received 29 January 2020; Received in revised form 29 March 2020; Accepted 31 March 2020 ⁎ Corresponding author at: National Institute of Technology, Calicut 673 601, India. E-mail address: radhika_p160009ns@nitc.ac.in (R. Panickar). Diamond & Related Materials 106 (2020) 107834 Available online 12 April 2020 0925-9635/ © 2020 Published by Elsevier B.V. T