CrN/NbN superlattice structured coatings with enhanced corrosion resistance achieved by high power impulse magnetron sputtering interface pre-treatment

In physical vapour deposition the ion etching pre-treatment prior to thin film deposition strongly influences the substrate/coating interface structure and chemistry and consequently properties such as adhesion and corrosion performance. This study compares CrN/NbN superlattice coatings grown by unbalanced magnetron sputtering on 304L stainless steel (SS) and M2 high speed steel (HSS) pre-treated with Nb ions at bias voltages of -600 V with three different methods: (1) high power impulse magnetron sputtering (HIPIMS), (2) HIPIMS with subsequent Nb HIPIMS interlayer deposition and (3) steered cathodic arc (CA) metal ion etching with CA interlayer deposition. The discharge during the pre-treatment step was analysed by optical emission spectroscopy of the Nb plasma, revealing a HIPIMS plasma dominated by singly charged Nb1+ ions. Two-fold charged Nb2+ ions were also present. HIPIMS pre-treatment promoted the formation of a clean and droplet-free interface and growth-defect free and dense coatings. Localized epitaxial growth of the films, as examined by cross-sectional transmission electron microscopy, resulted in significantly improved coating adhesion (L-C=56 N) compared to CA pre-treatment (L-C=45 N). The introduction of a 30 rim, thick Nb metal interlayer resulted in clearly defined and sharp interfaces with coherent base layer growth further improving the adhesion (L-C=65 N). Furthermore, the corrosion performance was studied by potentiodynamic polarisation in 3% NaCl solution. Results show superior corrosion performance for the HIPIMS pre-treated coatings. The HIPIMS pre-treatment prior to coating deposition provided passivation up to +1000 mV on 304 stainless steel (SS) substrates. An additional Nb interlayer significantly enhanced the corrosion performance with lower current densities of 4 x 10(-5) A cm(-2). CrN/NbN superlattice structured coatings were also sufficient to effectively protect non-noble M2 high speed steel (HSS) substrates. However, passivation is only retained up to potentials of +660 mV. The substrate/coating system corroded unimpeded when exceeding these values. The CrN/NbN coating with CA etch did not show passivation on both SS and HSS. CrN/NbN nanoscale multilayer coatings both with HIPIMS and CA pre-treatment outperform 20 gin thick commercial electroplated hard chrome with the HIPIMS pre-treated coatings being superior to the CA pre-treated ones. (c) 2006 Elsevier B.V. All rights reserved