Structural, mechanical and tribological behavior of fullerene-like and amorphous carbon nitride coatings J. Neidhardt a, * , L. Hultman a , E. Broitman b , T.W. Scharf c,1 , I.L. Singer c a Thin Film Physics Division, Department of Physics, IFM, Linko ¨ping University, S-58183 Linko ¨ping, Sweden b Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA c U.S. Naval Research Laboratory, Code 6176, Washington, DC 20375-5342, USA Received 8 March 2004; received in revised form 14 May 2004; accepted 20 May 2004 Available online 27 July 2004 Abstract The mechanical and tribological properties of fullerene-like (FL) and amorphous carbon nitride (CN x ) coatings synthesized by reactive unbalanced magnetron sputtering were analyzed and compared to carbon coatings deposited without N 2 . Elastic recoil detection analysis (ERDA) was used to determine the amount of incorporated nitrogen while X-ray photoelectron spectroscopy (XPS) was used to study the local bonding environment. Nano-indentation revealed a large spread in hardness, elastic modulus and elastic recovery. The tribological performance of the coatings was tested with a reciprocating sliding tribometer that allowed in situ visualization of the sliding contact. Tests were performed with a sapphire hemisphere sliding at 4 mm/s in dry and ambient humidity air. Friction coefficients of the FL– CN x coatings in both humidities were slightly higher than those of diamond-like carbon coatings (DLC); the values were between 0.1 and 0.25, dropping as the bias voltage increased. FL – CN x coatings had higher wear resistance than amorphous CN x , DLC and graphite coatings in sliding contact although the hardnesses of the coatings were comparable. Wear rates of the FL– CN x coatings in ambient air were lower than in dry air and orders of magnitude lower than the other carbon coatings. Furthermore, the most wear-resistant high-temperature FL – CN x coating had the highest hardness, but the shortest wear life of the three high-temperature FL – CN x coatings. In situ visualization indicated films transferred from the coating to the sapphire hemisphere; the transfer film isolated the hemisphere from the coating, thereby controlling both the friction coefficient and the wear behavior. Effects of deposition conditions, structure and hardness on wear and friction behavior are discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nitride; Nanostructures; Mechanical properties; Tribology 1. Introduction Over the last decade, carbon nitride (CN x ) coatings have attracted substantial research interest [1]. While crystalline h-C 3 N 4 coatings remain elusive [2–4], amorphous coatings have already found large-scale applications as protective overcoats on hard disks and read/write heads [5,6] due to their superior properties. Compared to hydrogenated dia- mond-like carbon coatings (DLC), CN x has been shown to exhibit a higher scratching and wear resistance while retaining the low-friction properties [7,8]. Another benefit of incorporating nitrogen in DLC coatings is an increase in surface energy, which in turn provides for the desired high wettability with common hard disk lubricants [9]. It also has been shown that CN x under certain conditions forms a structured, nonamorphous allotrope consisting of bent, cross-linked and frequently intersecting nitrogen-con- taining graphite sheets, denoted ‘‘fullerene-like’’ (FL) [1,10]. Coatings with this unique microstructure, designated FL–CN x , have properties such as extreme elasticity that can be beneficial for tribological applications [11,12]. The curved sheet-like structure extends the extraordinary in- plane strength of a sp 2 -coordinated carbon network in three dimensions. The alleged nitrogen-induced cross-linkage between the sheets on sp 3 sites [13] or sp 2 sites [1] contributes considerably by preventing interplanar slip, 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.05.012 * Corresponding author: Tel.: +46-13-28-1232; fax: +46-13-28-8918. E-mail address: jorne@ifm.liu.se (J. Neidhardt). 1 Current address: Microsystems Materials, Tribology, and Technology, Sandia National Laboratories, MS0889 Albuquerque, NM 87185-0889, USA. www.elsevier.com/locate/diamond Diamond & Related Materials 13 (2004) 1882 – 1888