The role of the growth defects on the corrosion resistance of CrN/NbN superlattice coatings deposited at low temperatures

Abstract

CrN/NbN nano-scale multilayered coatings have been successfully deposited at a temperature of 250 °C on 304 stainless steel substrates by the combined cathodic arc/unbalanced magnetron technique (Arc Bond Sputter) with a thickness typically 3.0 μm at bias voltages UB of −75 and −95 V. The microstructure and surface morphology of the coatings were examined in the as-deposited and corroded condition using an environmental scanning electron microscope (ESEM) in the gaseous secondary electron mode. The corrosion resistance of the coatings was determined by a potentiodynamic polarisation measurement in a 3% NaCl solution open to air at room temperature. In the ESEM images defects could be observed on the surface of the coatings and the number of these defects decreased with increasing bias voltage. ESEM images of the as-deposited coatings showed that microstructure adjacent to growth defects is under dense. These defects have been shown to be sites for preferential corrosive attack However, even with these drawbacks the corrosion resistance as-measured by potentiodynamic polarisation was shown to be superior to that of the uncoated 304 stainless steel substrate, whilst the corrosion resistance of the CrN/NbN nano-scale multilayered coatings increased with increasing bias voltage. In all cases the corrosion resistance was inferior to similar CrN/NbN nano-scale multilayered coatings deposited at higher temperatures of 450 °C.