Evaluation of corrosion performance of ultra-thin Si-DLC overcoats with electrochemical impedance spectroscopy

Diamond-like carbon (DLC) incorporating 3.6 at.% Si and with thickness ranging from 2 to 100 run were deposited on conducting Al2O3-TiC substrate by means of the PECVD technique. Electrochemical behaviour has been studied in 2 M HCl solution using AC impedance and polarisation measurements. The electrochemical impedance (EI) spectra were analysed in the context of an equivalent circuit model, which incorporated two times constants representing the Si-DLC coating and the corrosion reaction occurring at electrolyte Al2O3-TiC interface. However, 5 and 10 nm thick films displayed three phase constants suggesting the existence of interlayers, density gradients and inhomogeneities in the films. Results indicate that ultrathin films in the range 2-10 nm provide barrier properties, evidenced by increases of one order of magnitude relative to the substrate in the charge transfer resistance, which is a measure of the rate of electrochemical reactions occurring at the electrolyte/substrate interface. After the potentiodynamic scan, films thicker than 20 nm remained intact while their thinner counterparts exhibited a severely pitted surface. Pitting occurred preferentially in the TiC sites. The corrosion resistance of the ultrathin films increased substantially with immersion time in the electrolyte solution due to the filling of pores with a passivating material, thus stopping access of the electrolyte to the substrate. (C) 2002 Elsevier Science B.V. All rights reserved.