High-Temperature Tribological Performance of Al2O3/a-C:H:Si Coating in Ambient Air

The study investigates thermal stability and high temperature tribological performance of a-C:H:Si diamond-like carbon (DLC) coating. A thin alumina layer was deposited on top of the a-C:H:Si coating to improve the tribological performance at high temperatures. The a-C:H:Si coating and alumina layer were prepared using plasma-activated chemical vapour deposition and atomic layer deposition, respectively. Raman and X-ray photoelectron spectroscopy were used to investigate the structures and chemical compositions of the specimens. The D and G Raman peaks due to sp(2) bonding and the peaks corresponding to the trans-polyacetylene (t-Pa) and sp bonded chains were identified in the Raman spectra of the a-C:H:Si coating. Ball-on-disc sliding tests were carried out at room temperature and 400 degrees C using Si3N4 balls as counter bodies. The a-C:H:Si coating failed catastrophically in sliding tests at 400 degrees C; however, a repeatable and reproducible regime of sliding with a low coefficient of friction was observed for the Al2O3/a-C:H:Si coating at the same temperature. The presence of the alumina layer and high stress and temperature caused structural changes in the bulk a-C:H:Si and top layers located near the contact area, leading to the modification of the contact conditions, delivering of extra oxygen into the contact area, reduction of hydrogen effusion, and suppression of the atmospheric oxidation.