Tribochemistry of Hydrogenated Diamond-like Carbon: Superlubricity Assisted by CO₂

This study investigates the tribological behavior of mildly hydrogenated diamond-like carbon (HDLC) under various CO₂ partial pressures, focusing on the tribochemical effects on surface chemistry, friction, and wear characteristics. A nonmonotonic run-in period was observed at intermediate CO₂ partial pressures, characterized by a momentary increase in friction following an initially low coefficient of friction (COF), which subsequently decreases to a superlow COF. This peculiar behavior is somewhat similar to the frictional behavior of mildly HDLC observed in H₂ gas environments, where shear-induced reactive sites at the HDLC surface are passivated by H₂. Raman and X-ray photoelectron spectroscopy analyses revealed that the evolution of frictional states is closely associated with changes in surface chemistry, especially surface hydrogenation, graphitization, and oxidation. These findings collectively suggest that the tribochemical reaction of HDLC with CO₂ leads to the selective removal of reactive carbon species via the Langmuir-Hinshelwood process, which involves desorption as CO(g). This study advances the fundamental understanding of HDLC tribochemistry in CO₂-rich environments, offering valuable insights for applications of HDLC coatings to instruments used in carbon capture and storage, combustion systems, and industrial processing.