Shear-Induced Structural Changes and Origin of Ultralow Friction of Hydrogenated Diamond-like Carbon (DLC) in Dry Environment.

The origins of run-in and ultralow friction states of a sliding contact of hydrogenated diamond-like carbon (H-DLC) and sapphire were studied with an in situ Raman tribometer as well as ex situ analyses of transmission electron microscopy (TEM), Raman spectroscopy, and nanoindentation. Prior to ultralow friction behavior, H-DLC exhibits a run-in period. During the run-in period in dry nitrogen atmosphere, the transfer film was formed and its uniformity and thickness as well as structure were varied. The duration and friction behaviors during the run-in depended on the initial surface state of the H-DLC coatings. A comparative study of pristine and thermally oxidized H-DLC revealed the role of surface oxide layer on run-in friction and transfer film formation. Attainment of the ultralow friction state appeared to correlate with the uniformity and structure of the transfer film evolved during the run-in, rather than its final thickness. TEM cross-section imaging of the wear track and the counter surfaces showed a trace of nanocrystalline graphite and a thin modified surface layer on both rubbing bodies. The comparison of hardness and reduced modulus of the wear tracks and the unworn surfaces as well as the ex situ Raman spectra suggested the densification of the wear track surfaces. Combining the in situ and ex situ analysis results, a comprehensive model was proposed for the formation and structure of the ultralow friction sliding contact of H-DLC.