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

CrN/NbN superlattice coatings have been developed as a potential alternativ e to electroplated hard chrome for specialised industrial applications. The coatings have been grown at 400 degrees C by combined cathodic arc/unbalan ced magnetron technique. Two types of coating, stoichiometric (N/Me = 1) an d 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 Cr 2N + CrN; 0.35 mu m thick base layer. It has been found that the residual s tress levels in the CrN/NbN superlattice coating depend on the chemical com position as well as the phase composition of a monolithically grown base la yer. The corrosion resistance of CrN/NbN superlattice coatings has been investig ated by potentiodynamic polarisation measurements in aerated acetic acid/so dium acetate buffer solution and compared to 20 mu m thick electroplated ch romium as well as 304L stainless steel. CrN/NbN coatings showed clear passi vation behaviour with pitting potentials in the range 230 mV to 400 mV depe nding on their chemical composition, residual stress levels and CrN base la yer phase composition. The lower stressed stoichiometric CrN/NbN superlatti ce coating deposited on a single-phase CrN base layer showed the best perfo rmance 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 f actor of 10 higher corrosion current density when compared to stoichiometri c CrN/NbN superlattice coating. (C) 1999 Elsevier Science S.A. All rights r eserved.