Distinct effects of endogenous hydrogen content and exogenous hydrogen supply on superlubricity of diamond-like carbon

Abstract

Hydrogenated diamond-like carbon (HDLC) has drawn significant interest as a solid lubricant coating due to its superlubricity. However, HDLC exhibits drastically different frictional behavior depending on hydrogen (H) content or sp2/sp3 carbon ratio. Various structural aspects of HDLC can be analyzed with Raman spectroscopy; but analyzing the wear track on the HDLC surface could not provide insightful information about the shear plane structure because the probe depth of Raman is not shallow enough to discriminate the contribution from the bulk film. When a dissimilar material is rubbed on HDLC, the transfer film is always formed on the counter-surface. Since the transfer film is the direct outcome of frictional shear, one can assume that analyzing the transfer film can depict characteristic features of the shear plane during friction without convolution from the bulk contribution. This study employed microscopic Raman analysis to capture hyperspectral images of the entire transfer films on a stainless-steel counter surface formed from HDLCs with different endogenous H-contents (30 and 40 at.% in the film) in gas environments of N2 and H2 (exogenous supply). This analysis provided statistically meaningful data showing how the degrees of graphitization and hydrogenation of the shear plane during the run-in and steady-state friction periods vary with the HDLC structure as well as the environment condition, which is critically needed information to understand how the superlubricity is induced and altered when HDLC is used as a solid lubricant film.