Enhanced oxygen-induced properties of bulk oxygenated amorphous carbon films deposited with an anode layer ion source

Bulk oxygenated amorphous carbon films (a-C:H:O) with different oxygen concentrations are deposited on high-speed steel with a chromium carbide (CrC_x) interlayer using an anode layer ion source (ALIS) and their structural, mechanical, and tribological properties are determined. Films with 7 at%, 9 at%, 13 at%, and 27 at % oxygen are prepared by changing the O₂ to C₂H₂ ratio in the gas mixture during deposition. The film deposited without intentionally introducing oxygen has an oxygen concentration of 5% and serves as the control sample. The surface of the control film contains nanodots which can be reduced with 10% O₂ in the gas mixture. The best properties are obtained from the film containing 7 at% oxygen. The hardness increases from 19.5 GPa for the control film to 29.0 GPa of the a-C:H:O film with 7 at% oxygen. At the same time, the wear rate decreases by 2.5 times to 2.0 × 10⁻⁷ mm³/m-N and the friction coefficient by 10 times to 0.09. If the oxygen concentration in the film exceeds 7%, the hardness begins to decrease and wear rate increases while the friction coefficient is reduced slightly. Raman scattering reveals a smaller I_D/I_G ratio and more disorder in the 7% sample and XPS shows that the improvement stems from increased sp³ hybridization and C–C bonding. However, if the oxygen concentration in the gas mixture is increased further, oxygen ions begin to replace carbon atoms in the sp³ structure and more O–C–O, C=O, and O–C=O bonds are created. The carbonyl and carboxylic groups soften the matrix and degrade the mechanical properties. Our results clarify the effects and mechanism of oxygen on the mechanical and tribological properties of bulk amorphous carbon films.