Effects of Bias Voltages on the Tribological Behaviors of DLC Coatings

Ti/TiN/(Ti,N)-DLC/Ti-DLC/DLC coatings were deposited on 431 stainless steel using direct current magnetron sputtering technology under different bias voltages(0 V, -100 V, -200 V and -300 V). The microstructure and tribocorrosion performance of these DLC coatings in seawater was investigated. The results indicated that under the bias voltages, a denser and smoother surface of DLC coatings with a higher bonding strength between coatings and substrates was observed related to the increased incident kinetic energy of deposited ionized atoms. When the bias voltage was -200 V, the surface roughness reduced from 9.81 nm to 7.03 nm, and the bonding strength enhanced from 8.23 N to 8.86 N. What is more, the sp3 bond proportion and the disorder degree in DLC coatings both increased, which resulted in improved hardness and deformation resistance. However, when the bias voltage was -300 V, the increase of the amorphization was associated with a simultaneous rise in internal stress, which reduced the hardness and bond strength a little (8.72 N). DLC coatings can effectively improve the tribocorrosion properties of 431 stainless steel in seawater. When the voltage was -200 V, the average friction coefficient decreased from 0.35 to 0.07, with shallower wear traces and the wear loss of the DLC coating also being the smallest. The abrasive wear caused by metal oxides falling off the grinding ball, and the plastic deformation of the DLC coatings are the main wear forms. The high-density structure of DLC coatings under bias voltages can not only prevent the rapid expansion of cracks during deformation, but also provides a physical barrier to the erosion, which improves the corrosion and friction resistance in seawater. The optimization of bias voltage can improve the tribological performance of DLC coatings by regulating the carbon chain bond and microstructure. These results provide reference for DLC preparation and their potential engineering applications in stainless steel.