The breakdown mechanism of diamond-like carbon coated nickel in chloride solution

The breakdown mechanism of chemical vapor deposited nickel (CVD Ni) coated with 5 micrometers of diamond-like carbon (DLC) produced by plasma source ion implantation (PSII) in chloride solution was investigated. PSII differs from traditional implantation techniques in that the targets are placed directly in a plasma source and then pulse biased to produce a non-line-of-sight process to complex-shaped targets without complex fixturing. Electrochemical Impedance Spectroscopy (EIS) measurements indicated the presence of small pores in the DLC presumably formed in the deposition process. These pores were also observed in the scanning electron microscope. These pores exposed a layer of CVD Ni implanted with carbon (C). EIS results were analyzed within the context of an equivalent circuit model which incorporated three time constants representing the Ni-C implant layer, the DLC/solution interface, and the pore/solution interface. Good correlation between CNLS fitting of the impedance data and this circuit model was obtained. Upon immersion in chloride solution, the onset of breakdown of the DLC coating was rapid. Breakdown failed to initiate in chloride solution buffered with sodium berate. Mixed potential theory was used to describe these results and mechanisms for the initiation and propagation of coating failure in chloride solutions. Published by Elsevier Science Ltd.