Understanding the hydrogen and oxygen gas pressure dependence of the tribological properties of silicon oxide-doped hydrogenated amorphous carbon coatings

Silicon oxide-doped hydrogenated amorphous carbons (a-C:H:Si:O) are amorphous thin films used as solid lubricants in a range of commercial applications, thanks to its increased stability in extreme environments, relative to amorphous hydrogenated carbons (a-C:H). This work aims to develop a fundamental understanding of the environmental impact on the tribology of a-C:H:Si:O. Upon sliding an a-C:H:Si:O film against a steel counterbody, two friction regimes develop: high friction in high vacuum and low gas pressure (oxygen pressure < 10 mbar; hydrogen pressure < 50 mbar), and a low friction regime at higher gas pressures (10 mbar < oxygen pressure < 500 mbar; 50 mbar < hydrogen pressure < 1000 mbar). Scanning electron microscopy (SEM) revealed that the tribological behavior of a-C:H:Si:O is governed by adhesive junctions at the sliding interface. At low gas pressures, material transfer from the steel pin to the a-C:H:Si:O flat occurs. At higher gas pressures, a tribofilm forms on the steel countersurface. Raman and near edge X-ray absorption spectroscopy (NEXAFS) spectroscopies demonstrate that upon sliding under the higher gas pressure, low friction regime, a surface layer with an elevated fraction of sp2-bonded carbon atoms forms. These changes indicate that these gases favor the release of the adhesive junctions by dissociatively reacting with the mechanically-stressed sp2 carbon-rich surface layer.