Hardness and surface roughness of hydrogenated amorphous carbon-based films synthesized by atmospheric pressure-plasma enhanced chemical vapor deposition at low temperature

► Hydrogenated amorphous carbon (a-C:H) films were deposited at atmospheric pressure. ► Effect of dilution gas on film properties and chemical composition is investigated. ► The films deposited in nitrogen dilution contained nitrogen (a-C:H:N). ► As C2H2 gas ratio decreased, nitrogen incorporation into films was induced. ► a-C:H was harder and smoother than a-C:H:N and maximum hardness of a-C:H was 1.1 GPa.