Role of humidity in reducing sliding friction of multilayered graphene Sukanta Bhowmick, Anindya Banerji, Ahmet T. Alpas * Engineering Materials Program, Department of Mechanical, Automotive and Materials Engineering, University of Windsor, 401 Sunset Avenue, Windsor, ON N9B 3P4, Canada ARTICLE INFO Article history: Received 14 November 2014 Accepted 27 January 2015 Available online 3 February 2015 ABSTRACT The sliding friction of multilayered graphene was examined by conducting pin-on-disk tests in ambient air with different relative humidity (RH) levels (10–45% RH) and in dry N 2 (0% RH) where a high coefficient of friction (COF) of 0.52 was measured. Tests carried out at 10% RH showed a low steady state COF of 0.17. Progressively lower friction values were observed when the RH of the atmosphere was increased, with the lowest COF of 0.11 reached at 45% RH. Micro-Raman analyses of sliding surfaces of graphene transfer layers formed on the Ti–6Al–4V counterfaces showed an increase in the intensity of D-band with an increase in the RH suggesting defect formations in the C-network as a result of sp 2 to sp 3 transformation and amorphization during sliding. The H and OH passivation of gra- phene, as indicated by X-ray photoelectron spectra (XPS) of the contact surfaces, was a possible reason for low friction of graphene in humid atmospheres. Transfer layers incor- porated stringers of graphene stacks with d-spacings of 0.34–0.38 nm, which were larger than that of pristine graphite suggesting that repulsion between the adsorbed molecules caused an increase in the lattice spacing. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Minimizing friction and wear losses remains one of the greatest challenges for components operating under sliding contact conditions and requires development new surface coatings, as well as liquid and solid lubricants [1–3]. Graphite and other carbon based materials such as diamond like car- bon coatings are also being used to reduce friction [4–6]. The friction behaviour of graphene, a new carbon based material, has attracted interest and is currently being studied [7–11]. Kim et al. [8] investigated the frictional characteristics of CVD grown graphene under a normal load of 70 mN and in an atmosphere with 33% RH and observed that the coefficient of friction (COF) was affected by the substrate as the graphene grown on Ni substrate demonstrated a low COF of 0.10 compared to the coatings grown on Cu with a COF of 0.20. Berman et al. [9] conducted tribological tests in air (30% RH) on graphene flakes suspended in ethanol (spread on stainless steel) sliding against stainless steel balls at 2.0 N and recorded a low COF of 0.15. In another study [10], a COF of 0.18 was observed during sliding of stainless steel lubricated by ethanol processed graphene in a dry N 2 atmosphere and the low friction of graphene was attributed to the occurrence of ‘easy shear’ at the contact interface. Some studies investigated the frictional characteristics of graphene using atomic force microscope (AFM) tips using very low loads [12–17]. Lee et al. [12] studied friction http://dx.doi.org/10.1016/j.carbon.2015.01.053 0008-6223/Ó 2015 Elsevier Ltd. All rights reserved. * Corresponding author: Fax: +1 (519) 973 7085. E-mail address: aalpas@uwindsor.ca (A.T. Alpas). CARBON 87 (2015) 374 – 384 Available at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/carbon