ORIGINAL PAPER Investigation of Initial and Steady-State Sliding Behavior of a Nearly Frictionless Carbon Film by Imaging 2- and 3-D TOF-SIMS Osman L. Eryilmaz Æ Ali Erdemir Received: 4 February 2007 / Accepted: 4 September 2007 / Published online: 11 October 2007 Ó Springer Science+Business Media, LLC 2007 Abstract Using imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS), we investigated the initial and steady-state sliding behavior of a nearly frictionless carbon (NFC) film. Specifically, TOF-SIMS images (both 2- D and 3-D) of these surfaces were constructed to highlight the spatial distributions of ionized and molecular species that were present on as-received and friction-tested NFC surfaces and as a function of depth. As a complementary technique, we used X-ray photoelectron spectroscopy (XPS) to gain further insight into the chemical nature of the sliding sur- faces. The NFC films were produced on Si wafers and steel substrates in a gas discharge plasma that consisted of 25 vol.% methane and 75 vol.% hydrogen using a plasma- enhanced chemical vapor deposition (PECVD) system. They were then subjected to sliding friction and wear experiments in a pin-on-disk machine under 5- and 10-N loads and at sliding velocities of 0.2–0.5 m/s in dry nitrogen. The initial friction coefficients of the NFC films were in the range of 0.05–0.1, but decreased rapidly to values less than 0.01 at steady state. Positive and negative TOF-SIMS spectra and 2- and 3-D images reconstructed from selected masses revealed that the elemental distribution of certain chemical species differs substantially between undisturbed and tribo-tested areas of the NFC films. Specifically, the tribo-tested areas are essentially made up of carbon and hydrogen, while undis- turbed or as-received areas are covered by a layer that is rich in oxygen and other species. These findings correlate well with the initial and steady-state friction coefficients of these films and help further explain their superlubricity in inert test environments. Keywords DLC Á Initial and steady-state friction Á Wear Á Superlubricity Á Lubrication mechanism Á TOF-SIMS 1 Introduction Diamondlike carbon (DLC) films have attracted huge sci- entific and commercial interest during the past decade, mainly because of their impressive tribological properties [1–3]. Systematic studies of such films in our laboratory for more than a decade have led to the development of a new class of DLC film that can consistently provide friction coefficients below 0.01 at steady state and wear rates of 10 –11 to 10 –9 mm 3 /N.m when tested in inert gases or high-vacuum environments [4–9]. Their initial or break-in friction coef- ficients are in the range of 0.05–0.2, but after tens of sliding cycles, these high friction values decrease rapidly and sta- bilize between 0.001 and 0.01. Due to such extremely low steady-state friction coefficients, we call these films ‘‘nearly frictionless carbon’’ or NFC for short. As opposed to conventional DLC films, which may have been derived from solid carbon targets or pure hydrocarbon gases such as methane (CH 4 ) and acetylene (C 2 H 2 ), the NFC films are derived from hydrogen-rich hydrocarbon plasmas. A typical plasma composition may consist of 75– 90 vol.% H 2 and 10–25 vol. % CH 4 or 90 vol.% H 2 and 10 vol.% C 2 H 2 . The most typical deposition method is plasma-enhanced chemical vapor deposition (PECVD), but other deposition methods (such as cathodic arc-physical vapor deposition, magnetron sputtering, ion plating, and pulse laser deposition) can also be used [4–6]. Tribologically, the films that are derived from pure car- bon targets and/or hydrogen-poor hydrocarbon gases (such as C 2 H 2 ) exhibit very high and erratic friction coefficients O. L. Eryilmaz Á A. Erdemir (&) Energy Systems Division, Tribology Section, Argonne National Laboratory, 9700 South Cass Avenue, Building 212, Room D218, Argonne, IL 60439, USA e-mail: erdemir@anl.gov 123 Tribol Lett (2007) 28:241–249 DOI 10.1007/s11249-007-9268-z