Influence of Nanothick Layers of DLC on the Rates of Electrochemical Reduction Reactions on Magnetic Hard Disks

Abstract

Potentiodynamic cathodic polarization tests and electrochemical impedance spectra were obtained for magnetic hard disks coated with a layer of diamond-like carbon (DLC) and immersed in aqueous borate buffer (pH8.4) containing 0.1 M NaCl. The log of the cathodic current was linearly related to (Deltaphi- Deltaphi(corr))(1/2) for disks coated with a layer of DLC and polarized between 0.2 and 0.4 V below the corrosion potential. In addition, the low frequency impedance of the disks coated with a layer of DLC decreased by a factor of similar to50 as the potential was decreased from 0.2 to 0.4 V below the corrosion potential. The results were independent of the DLC's thickness (2-5 nm), dopant (H and H + N) and method of deposition (sputtering and ion-beam deposition). The results suggest the presence of a thin (approximate to12-19 Angstrom) charge-denuded layer that forms by a reaction between the DLC and either air or the aqueous solution. At potentials within - 0.4 V of the corrosion potential, the kinetics of the electrochemical reduction reactions on DLC-coated hard disks are explained equally well by Schottky emission or quantum mechanical tunneling of electrons through the charge-denuded, reaction product layer. Both charge-transport mechanisms suggest similar remedial actions for decreasing corrosion of the magnetic layer. (C) 2004 The Electrochemical Society.