A methodology for characterizing the electrochemical stability of DLC coated interlayers and interfaces

Abstract

DLC coatings are often deposited on Si and Cr based adhesion-promoting interlayers to mitigate stress at the DLC/substrate interface. Interlayers reduce the likelihood of mechanical coating delamination. However, their chemical stability at miniature DLC coating defects, such as pores and micro-pinholes, may be adversely affected in a corrosive environment, such as the human body where DLC coated implants are envisaged. An experimental methodology is presented for accessing and characterizing the electrochemical reactivity of buried interlayers and interfaces. The interlayer, with its interfaces, is revealed by ion beam polishing at an angle, forming a wedge-like profile of the substrate/interlayer/DLC system. The chemical binding and composition of the interlayer/interface is determined by Auger electron spectroscopy (AES), and the susceptibility of the different interfaces to carbide formation and oxidation is identified. It is shown that pure interlayer/interface materials can be obtained for the model Co interlayer. On the other hand, for more reactive materials such as Si, Ti and Cr, the formation of new interface phases was observed. These different new materials are characterized for their corrosion susceptibility with a local-electrochemical microcapillary technique both at open circuit potential (OCP) and under potentiodynamic polarization. Cr and Si-DLC based interlayers presented good passive behavior, while a Si interlayer corroded in bovine-based wear test fluid (HyClone® WTF). An analogous degradation is found after a long-term immersion experiment and failed implant cases. The influence of coating defects distribution on the current response of DLC surfaces during electrochemical measurements was also investigated by varying the exposed area (from the cm2 to the μm2 range). Microscale characterization allowed for a better representation of the intrinsic reactivity of DLC, while larger areas were highly dependent on the underlying interlayer.