Surface analytical investigation of nearly-frictionless carbon films after tests in dry and humid nitrogen

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 C2H and C2H2 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 CO 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.