Hardness and surface roughness of hydrogenated amorphous carbon films synthesized by atmospheric pressure plasma enhanced CVD method with various pulse frequencies

► A-C:H films were synthesized by AP-PECVD method from C2H2 diluted with N2. ► Hardness increased up to 0.92 GPa as pulse frequency decreased to 2 kHz. ► Surface roughness and N/C molar ratio decreased by decreasing pulse frequency. ► Smooth-surface film densely packed with particles led to high hardness.