On the hydrogen lubrication mechanism(s) of DLC films: An imaging TOF-SIMS study

In this study, we explored the effect of hydrogen on sliding friction and wear behavior of a hydrogen-free DLC film in dry nitrogen. More specifically, we subjected this film to a very brief (~ 3 min) hydrogen plasma treatment in a sputter ion plating system to determine if the friction and wear behavior would change. All tests were performed in dry nitrogen using a pin-on-disk machine under a 0.5 N load and at 0.1 m/s sliding velocity. The friction coefficient of as-deposited DLC film was high and unsteady (varying between 0.1 and 0.8) and it could only last for a sliding distance of ~ 12 m. However, after the hydrogen plasma treatment, the same film was able to provide a friction coefficient of less than 0.01 and lasted for a sliding distance of more than 450 m. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to elucidate the nature and extent of the chemical changes that occurred during hydrogen plasma treatment. 2- and 3-D TOF-SIMS images of such surfaces revealed the formation of a hydrogen-rich top surface layer during the hydrogen-plasma treatment. Based on the combined results of tribological and surface analytical studies, we provided a mechanistic explanation for the very critical role of hydrogen on friction and wear of DLC films.