The corrosion behaviour of macroparticle defects in arc bond-sputtered CrN/NbN superlattice coatings

The investigation concerned the corrosion behaviour of macroparticle and gr owth defects in PVD CrN/NbN superlattice coatings formed by are bond-sputte ring (ABS) process on a mild steel BS6323. The electrochemical behaviour of the coatings was firstly studied by potentiodynamic polarising in de-aerat ed 0.5 M (Na2CO3-NaHCO3) buffer and 5% NaCl solutions, respectively. The co ating and defects were then examined in planar view and cross-section by sc anning electron microscopic analyses and the results were compared with tho se prior to the electrochemical measurement. It is found that the overall c oating/substrate corrosion process is closely related to the deleterious ef fect of the macroparticles and growth defects in the PVD coatings. It is fu rther demonstrated that for through thickness macroparticle inclusions, cor rosion initiates by galvanic or crevice corrosion between the defect and th e coating matrix, subsequently permitting solution access to those defects with eventual substrate pitting and corrosion at the coating/substrate inte rface. On the basis of the experimental findings and the macroparticle form ation theory, the mechanisms of the growth defect-related coating/substrate corrosion are finally proposed. (C) 2000 Published by Elsevier Science S.A . All rights reserved.