Bias voltage-induced structural change in PECVD-deposited diamond-like carbon coatings for optimizing the tribological performance

To improve the adhesion strength and enhance the wear resistance of diamond-like carbon (DLC) coatings, different negative bias voltages were utilized to deposit a-Si:C:H/H-DLC/DLC composite coatings on the 45 steel substrates using direct current plasma enhanced chemical vapor deposition technology. Influences of negative bias voltages on the surface morphology, microstructure, mechanical properties, and tribological performance were investigated. The results showed that as the bias voltages increased from −400 V to −600 V, the surface roughness and thickness of the coatings increased, reaching a maximum roughness of 0.426 μm and a maximum total thickness of 9.9 μm. Raman data illustrated the decrease of hydrogen contents and the increase in sp3 bonds content, matching well with the increasing hardness and elastic modulus. The residual stress tended to increase as the bias voltages, while the adhesion strength reached the maximum value of 62.45 N at −550 V. Under the synergistic effects of surface roughness, interfacial adhesion strength, hardness and elastic modulus, the average friction coefficient and wear rates of the coatings initially decreased and then increased, reaching the best tribological performance at −550 V.