Wear associated with growth defects in combined cathodic arc/unbalanced magnetron sputtered CrN/NbN superlattice coatings during erosion in alkaline slurry

The erosive wear associated with growth defects was studied in deaerated 0.5 M (Na2CO3-NaHCO3) buffer solutions containing 150-200 mum alumina particles on a rotating cylinder erosion-corrosion system for PVD CrN/NbN superlattice coatings grown by the are bond sputtering (ABS) process on mild steel BS6323. Corrosion was minimised by selective control of the sample's potential, according to potentiodynamic polarisation, during the slurry erosive wear test. The morphology of the coatings, particularly of the defects, was examined in planar and cross-section views by means of scanning electron microscopy before and after the test. It is found that wear of the coating is typically preferential to the defects, progressing from the particle exterior top to the interior with erosion time, while the coating matrix (areas free of such defects) is largely intact, after 24 h prolonged exposure to erosion. (C) 2000 Elsevier Science B.V. All rights reserved