Surface analytical investigation of nearly-frictionless carbon films after tests in dry and humid nitrogen O.L. Eryilmaz, A. Erdemir ⁎ Argonne National Laboratory, Energy Systems Division, Argonne, IL 60439, USA Received 12 October 2006; accepted in revised form 2 February 2007 Available online 13 February 2007 Abstract In this paper, we attempt to elucidate the near-surface chemistry of the sliding contact interfaces of nearly-frictionless carbon (NFC) films subjected to sliding in dry and humid nitrogen environments. Using time-of-flight secondary ion mass spectrometry (ToF-SIMS), along with X-ray photoelectron spectroscopy (XPS), we determined the chemical state of the rubbing surfaces of NFC films and correlated these findings with the friction and wear test results. Tribological tests were performed in a ball-on-disk machine under a 5 N load and at sliding velocities of 0.2 to 0.3 m/ s. An air-tight flexi-glass enclosure was used to create and maintain the dry and wet nitrogen environments. When tested in dry nitrogen, the NFC film provided a friction coefficient of ≈ 0.006 at steady state. However, when tests were performed in humid nitrogen, the friction coefficients increased by factors of 5 to 10 (depending on the humidity level). In both cases, the amount of wear was very low and hard to quantify, especially on the disk side. Imaging 3-D ToF-SIMS of the sliding contact surfaces of the NFC films revealed mostly C 2 H and C 2 H 2 within the wear tracks after testing in dry nitrogen. XPS confirmed the presence of mostly C–C and C–H bonded species within the same wear tracks. However, the surfaces that were rubbed in humid nitrogen contained large amounts of C–O and C_O bonded species, in addition to C–C and C–H. We concluded that super-low friction of NFC in dry nitrogen was mainly associated with the hydrogen termination of its surface carbon atoms, while the much higher friction of NFC in humid nitrogen was perhaps due to oxidation and/or hydroxyl ion termination of its surface carbon atoms. © 2007 Published by Elsevier B.V. Keywords: DLC; Friction; ToF-SIMS; XPS 1. Introduction Diamond-like carbon (DLC) films have attracted over- whelming interest in recent years mainly because of their unusual friction and wear properties [1–5]. As a class, these films encompass a broad range of structure, property, and hence performance. At present, there exist certain types that can provide friction coefficients as low as 0.001; others possess friction coefficients as high as 0.7 or more [1–3]. Recent systematic studies have confirmed that such a disparity in friction and wear properties of these films mainly stems from some differences in their chemical, structural, and mechanical properties, which are primarily determined by the relative amounts of sp 2 vs. sp 3 bonded carbon atoms and by the presence or absence of hydrogen and/or other alloying elements within their structures [6–9]. Furthermore, the same studies have demonstrated that test conditions and environments can also play major roles in their friction and wear performance. In particular, it was shown that the chemical nature of the test environment has a profound effect on the friction and wear of these films [10–20]. Previous researchers have shown that the presence of water molecules in the test environment is very beneficial to the frictional behavior of hydrogen-free DLC films. For hydroge- nated DLC, however, water molecules have an adverse effect [6,7,12]. Conversely, in inert or vacuum test environments, certain forms of hydrogenated DLC films appear to provide very low friction coefficients, while hydrogen-free DLC exhibits some of the highest friction coefficients [16–24]. Based on such contrasting observations, researchers have proposed hypothetical friction models for both hydrogen-free and hydrogenated DLC films. For example, they attributed low friction behavior of highly hydrogenated DLC films in inert or Surface & Coatings Technology 201 (2007) 7401 – 7407 www.elsevier.com/locate/surfcoat ⁎ Corresponding author. Tel.: +1 630 252 6571; fax: +1 630 252 4798. E-mail address: erdemir@anl.gov (A. Erdemir). 0257-8972/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2007.02.005