Effect of silicon and oxygen co-doping on structure and properties of non-hydrogenated amorphous carbon

Introducing silicon and oxygen dopants into amorphous carbon (a-C) is known to improve its structural and functional properties, making it suitable for specialized applications. Yet, given the limited compositional control offered by the complex deposition methods, the relationship between dopant concentration and coating properties is not well understood. This study systematically investigates the effect of increasing silicon (0 - 30 at.%) and oxygen (0 - 20 at.%) content on structure, hardness, reduced modulus, and surface wettability of non-hydrogenated a-C coatings, deposited by magnetron sputtering. The coating composition was controlled by adjusting the power applied to the silicon target and the oxygen gas flow rate during the sputtering process. Our results indicate that with increasing silicon content, the sp(3)/sp(2) fraction, hardness, reduced modulus, and surface energy progressively increased. Moreover, while high oxygen content, in general, promoted graphitization, leading to loss of mechanical properties, it also helped in reducing surface energy by terminating silicon bonds. Nonetheless, it was observed that the otherwise detrimental effect of oxygen doping on mechanical properties could be avoided by maintaining a low concentration of oxygen (similar to 10 at.%), combined with high silicon content (> 18 at.%). This work establishes optimized doping conditions for tailoring the structure and properties of silicon and oxygen co-doped non-hydrogenated a-C coatings, potentially extending their usage in protective and functional applications.