In the present research, DLC films are produced by asymmetric bipolar pulsed plasma enhanced CVD after preparing oxynitride-treated PM30 steel. The experimental parameters include various CH 4 gas flows (3, 7, 11, 15 and 19 sccm) with asymmetric bipolar pulsed voltages (− 15 + 10%) and a deposition time of 90 min, respectively. In order to evaluate the properties of the DLC films for oxynitride-treated PM30 steel, Raman spectroscopy analysis, adhesion tests, nano-indentation, wear tests, and corrosion resistance inspections were performed. The experimental results reveal that the DLC/oxynitride duplex coating layers have better properties when the DLC films are treated with an appropriate CH 4 gas flow (15 sccm). Meanwhile, the coatings possess the highest hardness and elastic modulus (5.91 and 89.87 GPa, respectively), the lowest wear loss volume (for axial loads of 2 and 5 N of 3.82 × 10 − 3 and 5.49 × 10 − 3 mm 3 , respectively), the lowest corrosion current (1.43 × 10 − 4 A•cm − 2), and the highest polarization resistance (5.32 × 10 2 Ω•cm 2) after a 3.5 mass% NaCl corrosion test. This study also confirms that the wear and corrosion resistance of DLC/oxynitride duplex-treated PM30 steel can be effectively improved via asymmetric bipolar pulsed plasma enhanced CVD.