Tribochemistry of Diamond-like Carbon: Interplay between Hydrogen Content in the Film and Oxidative Gas in the Environment

Abstract

The lubricity of hydrogenated diamond-like carbon (HDLC) films is highly sensitive to the hydrogen (H) content in the film and the oxidizing gas in the environment. The tribochemical knowledge of HDLC films with two different H-contents (mildly hydrogenated vs highly hydrogenated) was deduced from the analysis of the transfer layers formed on the counter-surface during friction tests in O2 and H2O using Raman spectroscopic imaging and X-ray photoelectron spectroscopy (XPS). The results showed that, regardless of H-content in the film, shear-induced graphitization and oxidation take place readily. By analyzing the O2 and H2O partial pressure dependence of friction of HDLC with a Langmuir-type reaction kinetics model, the oxidation probability of the HDLC surface exposed by friction as well as the removal probability of the oxidized species by friction were determined. The HDLC film with more H-content exhibited a lower oxidation probability than the film with less H-content. The atomistic origin of this H-content dependence was investigated using reactive molecular dynamics simulations, which showed that the fraction of undercoordinated carbon species decreased as the H-content in the film increased, corroborating the lower oxidation probability of the highly-hydrogenated film. The H-content in the HDLC film influenced the probabilities of oxidation and material removal, both of which vary with the environmental condition.

Competition-Induced Macroscopic Superlubricity of Ionic Liquid Analogues by Hydroxyl Ligands Revealed by in Situ Raman

“…Superlubricants can be broadly classified into two categories: solid and liquid . For the former, typical ones include molybdenum disulfide, graphene, diamond-like carbon films, and multiwalled carbon nanotubes. − However, solid superlubricity materials usually require specific conditions (e.g., vacuum/inert gas atmosphere, incommensurate contact, micrometer size, and absolute surface rigidity) to achieve superlubricity, whereas the liquid ones easily achieve macroscopic superlubricity on routinely flat surfaces under atmospheric conditions such as aqueous phosphoric acid solutions . As a result, liquid superlubricants have a much greater potential than solid superlubricants to meet the real-world requirements of a wide range of applications and are compatible with most existing lubrication systems.…”