Investigation of Initial and Steady-State Sliding Behavior of a Nearly Frictionless Carbon Film by Imaging 2- and 3-D TOF-SIMS

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 surfaces. 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 undisturbed 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.