Effect of Tribochemical Interactions on Friction and Wear of Highly Hydrogenated DLC Films

In this paper, we attempted to elucidate the nature and extent of tribochemical interactions between sliding surfaces of highly-hydrogenated diamondlike carbon (DLC) films (containing about 40 at.% hydrogen) and correlated these findings with their friction and wear behaviors in dry and humid nitrogen environments. For the tribochemical studies, we used a time-of-flight secondary ion mass spectrometer (ToF-SIMS) in 2- and 3-D imaging modes, while for tribological tests, we used a ball-on-disk machine that was operated under a 5 N load and at sliding velocities of 0.2 to 0.3 m/s. In dry nitrogen, the DLC films provided a friction coefficient of ≈ 0.007, but in humid nitrogen, the friction coefficient 3 to 4 times higher. 3-D surface profilometry and optical microscopy did not reveal much wear on sliding ball and disk surfaces after the tests regardless of the test environment, but the 2- and 3-D ToF-SIMS images of the same surfaces revealed very rich tribochemistry for films tested in humid nitrogen but not much chemistry for those films tested in dry nitrogen. Complementary x-ray photoelectron spectroscopy of these sliding surfaces showed similar results further confirming extensive tribo-oxidation (in the forms of both C-O and C=O) of DLC surfaces tested in humid nitrogen but no major chemical changes in DLC films tested in dry nitrogen. Based on the results of surface analytical and tribological studies, we concluded that superlow friction behavior of highly hydrogenated DLC films was perhaps due to the hydrogen termination of their surface carbon atoms, while the much higher friction coefficients observed in humid nitrogen was primarily due to tribo-oxidation and/or oxygen termination of these surfaces.