The corrosion protection ability of TiAlN coatings produced with CA-PVD under superimposed pulse bias

In this study we have investigated the contribution of superimposing high voltage pulse on DC bias on the corrosion protection ability of TiAlN coatings deposited with cathodic arc physical vapor deposition (CA-PVD). Superimposing high voltage on DC bias resulted in the decrease of the number of attached macroparticles (MP) and also densified the columnar structure. These differences significantly improved the corrosion protection ability of the coatings that were determined by electrochemical impedance spectroscopy (EIS) in 0.1 N HCl solutions. An approximately 6 fold increase in the polarization resistance for the samples deposited by using −1000 V superimposed pulse voltage on 40 V DC bias when compared to the ones deposited only by DC bias. For further elaborating these results, the distribution, size and morphology of the corrosion sites were determined with ferroxyl test, scanning electron microscopy (SEM) and 3D optical profilometry. Results of the ferroxyl tests indicated a dramatic decrease of corrosion sites by superimposing −1000 V on DC bias. Closer investigation of these sites showed that corrosion of the substrate is first initiated at the bottom of the defects extending through the coating and then expanded below the coating. The depths and diameters of the corroding sites, revealed by removing their caps with ultrasonication were substantially lower for the samples produced with superimposed bias. Results of the study indicated the necessity of combining electrochemical corrosion test results with the morphology and distribution of corrosion sites for a proper understanding of the corrosion protection ability of hard coatings.