Tribological properties of chemically modified diamond like carbon films in hydrogen plasma S.R. Polaki n , N. Kumar n , Nanda Gopala Krishna, T.R. Ravindran, M. Kamruddin, S. Dash, A.K. Tyagi Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India article info Article history: Received 16 March 2014 Received in revised form 14 August 2014 Accepted 8 September 2014 Available online 16 September 2014 Keywords: Diamond like carbon films Tribological properties Chemical composition Surface topography abstract Diamond like carbon (DLC) films were deposited on silicon substrates by plasma enhanced chemical vapor deposition (PECVD) technique and chemically modified by H 2 plasma treatment. The Raman spectra and XPS analysis, revealed a decrease in sp 2 C ¼C/sp 3 C–C ratio with increase in H 2 plasma exposure duration up to 30 min. This is due to the etching of sp 2 C ¼C sites by hydrogen ions/radicals during plasma treatment. However, this ratio increased fairly for longer H 2 plasma exposure time. This might be due to the stabilization of sp 2 C ¼C bonding by passivation of film surface by hydrogenation. Friction coefficient of these films is found to decrease with an increase in sp 2 C ¼C fraction. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Diamonds like carbon (DLC) films are extensively studied owing to their excellent characteristics like high hardness, optical transparency, biocompatibility and enhanced tribological proper- ties. Low friction coefficient and high wear resistance of DLC films are considered to be ideal for several sliding applications [1–4]. DLC films are amorphous in nature and their physical and chemical characteristics mainly depend on the hydrogen content and sp 2 C ¼ C to sp 3 C–C ratio [2]. Various deposition techniques have been used to grow the DLC films. Physical sputtering techniques are mainly employed to deposit hydrogen free films where as chemical vapor deposition (CVD) based techniques result in hydrogenated films [5]. Moreover, hydrogen during film growth process creates dangling bonds and allows the adsorption of CH x species to the unsaturated carbon atoms which enhances sp 3 C–C bonding network in the film [6]. Presence of hydrogen in the films is also an advantageous factor because it determines the cross linking across sp 3 C–C bonding fraction. This, in turn, influences the physical and chemical properties of the films. It was shown that the hydrogen ions and radicals stabilize sp 3 C–C bonding [7]. Specifically, hydrogen ion etches the amorphous carbon more efficiently compared to hydrogen radicals. Surface mod- ification influences the properties of DLC films considerably due to the change in surface chemical characteristics, roughness and wettability [8]. Such modified DLC films are useful in biomedi- cal, electronic and tribological applications due to their improved surface functionalities [9]. Ion implantation, plasma treatment and doping with desired elements are the most commonly used processes to modify the surfaces functionalities of the DLC films. The plasma exposure of thin films is a suitable method for modification of film surface chemistry which nor- mally forms catalytically active sites required for functional interaction [8,9]. In this respect, chemically hydrogenated DLC films exhibit low friction coefficient due to hydrogen termina- tion [8]. This altered surface chemistry enhances the friction and wear properties of the DLC films [2,8]. In addition, another way to improve the tribological properties is altering environment during the tribo-test [3,10–12]. Furthermore, DLC films exhibit super lubricity when the tribo-test is carried out in hydrogen atmosphere. Such a characteristic is related to the saturation of covalent dangling carbon bonds by forming C–H bonds at the sliding interface [11]. Moreover the surface hydrogen in amor- phous hydrogenated (a-C:H) films play a significant role in super lubricity [3]. In this respect, mainly the hydrogen content and nature of carbon hybridization determine the friction and wear behavior of DLC films [10]. In the present paper, the chemical characteristic of DLC film after H 2 plasma exposure was studied. Change in surface topo- graphy and contact angle of surface were examined to understand the effect of hydrogen plasma treatment on DLC films. Tribological properties of these films were studied in respect of physical and chemical changes after the H 2 plasma treatment. Subsequently, chemical and physical characteristics of wear tracks were investi- gated to understand the tribological mechanism. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/triboint Tribology International http://dx.doi.org/10.1016/j.triboint.2014.09.009 0301-679X/& 2014 Elsevier Ltd. All rights reserved. n Corresponding authors. Tel./fax: þ91 4427480081. E-mail addresses: polaki@igcar.gov.in (S.R. Polaki), niranjan@igcar.gov.in (N. Kumar). Tribology International 81 (2015) 283–290