Synthesis of carbon films with ultra-low friction in dry and humid air Christina A. Freyman, Yanfeng Chen, Yip-Wah Chung ⁎ Department of Materials Science and Engineering, Northwestern University, 2220 N Campus Dr., Evanston, IL 60208, USA Received 8 July 2005; accepted in revised form 7 November 2005 Available online 13 December 2005 Abstract Using pulsed dc magnetron sputtering, we synthesized hydrogenated amorphous carbon films with and without sulfur doping. These films were smooth and amorphous, with low compressive stress (b 1 GPa). Auger electron spectroscopy confirmed the incorporation of sulfur, and X-ray photoelectron spectroscopy showed that sulfur atoms are chemically bound. Most significant, ball-on-flat tribo-testing showed that hydrogenated carbon films doped with 5 at.% sulfur have ultra-low friction coefficients (b 0.01) in air with relative humidity from 0% to 50%. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogenated carbon films; Sulfur doping; Friction; Magnetron sputtering 1. Introduction Hydrogenated amorphous carbon films (CH x ) have been extensively investigated due to their wide range of properties. These films can be synthesized through a variety of physical and chemical techniques. Tribological properties of these films depend on testing conditions (environment, load, speed and geometry), film structure and composition, and surface rough- ness [1,2]. Heavily hydrogenated carbon films deposited by chemical vapor deposition have been shown to provide ultra- low friction coefficients (b 0.01) in vacuum and dry nitrogen environments. This ultra-low friction between two coated sur- faces has been attributed to hydrogen-terminated surfaces with weak van der Waals interaction, resulting in weak adhesion and hence low shear strength against sliding [3–7]. Their friction coefficients increase rapidly in the presence of oxygen and humidity. Oxygen can interact with both the carbon and hydro- gen atoms [8,9]. Tribochemical reactions resulting in oxidation of the film have also been observed to increase friction. Atomic oxygen increases the friction coefficient much more than mole- cular oxygen [10,11]. Tribochemical oxidation from oxygen or water has also been observed to affect the transfer layer, even at low partial pressures, increasing friction coefficients. This is attributed to the oxygen removal of sp 2 carbon atoms from the transfer layer [12,13]. The strong dependence on humidity has been attributed to viscous and capillary forces induced by adsorbed water [14]. There have been efforts to mitigate the humidity problem. Silicon, fluorine, boron, gold, and titanium doping have been attempted, but they have only marginal effects on the moisture sensitivity of CH x films. Friction coefficients of 0.04–0.1 were measured in ambient air, at least 10 times higher than those mea- sured in dry air by Erdemir et al. [1,15–18]. Silicon and boron dopants are thought to form a silicon oxide or boric acid lubricous layer on the surface in humid air, decreasing the ambient friction coefficient [1,19,20]. Gold acts as a solid lubricant because of its low shear strength [21]. Reduction in the surface energy using fluorine-doped films is believed to be the primary reason for the reduced sensitivity of friction towards moisture [22]. Tagawa et al. observed a direct link between water coverage on a hydrogenated carbon surface and friction coefficient. The friction coefficient increased markedly when the surface water coverage exceeded about one monolayer [23]. Shulka et al. measured the adsorption of water to be ∼ 1.5 monolayers on hydrogenated carbon films at 60% relative humidity [24]. There- fore, reduction in the surface water coverage under humid con- ditions may allow these films to retain their low friction properties. The ability of certain surface dopants to suppress gas adsorption is well-documented in the surface science litera- ture, due to physical site-blocking or modification of surface electronic properties [25]. In principle, one can minimize water Surface & Coatings Technology 201 (2006) 164 – 167 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +1 847 491 3112; fax: +1 847 491 7820. E-mail address: ywchung@northwestern.edu (Y.-W. Chung). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.11.075