Combined cathodic arc/unbalanced magnetron grown CrN/NbN superlattice coatings for corrosion resistant applications

CrN/NbN superlattice coatings have been developed as a potential alternative to electroplated hard chrome for specialised industrial applications. The coatings have been grown at 400 degrees C by combined cathodic arc/unbalanced magnetron technique. Two types of coating, stoichiometric (N/Me = 1) and substoichiometric (N/Me = 0.5) with superlattice wavelength 3.7 nm and 6.5 nm, respectively have been deposited on single-phase CrN and two-phase Cr2N + CrN; 0.35 mu m thick base layer. It has been found that the residual stress levels in the CrN/NbN superlattice coating depend on the chemical composition as well as the phase composition of a monolithically grown base layer. The corrosion resistance of CrN/NbN superlattice coatings has been investigated by potentiodynamic polarisation measurements in aerated acetic acid/sodium acetate buffer solution and compared to 20 mu m thick electroplated chromium as well as 304L stainless steel. CrN/NbN coatings showed clear passivation behaviour with pitting potentials in the range 230 mV to 400 mV depending on their chemical composition, residual stress levels and CrN base layer phase composition. The lower stressed stoichiometric CrN/NbN superlattice coating deposited on a single-phase CrN base layer showed the best performance with passive current density as low as 0.35 mu A cm(-2). In contrast the electroplated chromium did not show any evidence of passivity with a factor of 10 higher corrosion current density when compared to stoichiometric CrN/NbN superlattice coating. (C) 1999 Elsevier Science S.A. All rights reserved