Corrosion performance and mechanical stability of 316L/DLC coating system: Role of interlayers

In the present work, the corrosion performance and mechanical stability of diamond-like carbon (DLC) coatings were investigated in the context of their biomedical applications. DLC was prepared by radio-frequency (RF) plasma-enhanced chemical vapor deposition (PECVD) onto medical grade 316L stainless steel. Interlayers of amorphous hydrogenated silicon-based materials such as a-Si, a-SiNx, a-SiCx, and a-SiCxNy, and a nitrided interlayer, were studied in order to optimize its adhesion strength. Potentiodynamic polarization tests were performed to evaluate the corrosion performance of the 316L/DLC coating system. Electrochemical impedance spectroscopy (EIS) was used to determine the stability of the coating system during long-term tests of exposure to a simulated body fluid solution. The evolution of EIS spectra was monitored during two years of immersion in Ringer's solution. In addition to providing the best adhesion, the a-SiNx interlayer was found to significantly improve the corrosion resistance of the DLC system since it is highly impervious to the liquid. This is demonstrated by a two-order of magnitude improvement in the corrosion current density compared to the DLC with the nitrided interlayer. The a-SiNx interlayer substantially enhances the mechanical stability of the DLC coating system in the simulated body fluid environment, indicated by a slight reduction (less than 20%) in the adhesion strength and fivefold increase in the charge transfer resistance after two years of immersion. Moreover, Si-doped DLC coatings show improved corrosion barrier properties, due to the formation of a passive silicon oxide film at the electrode/electrolyte interface.