Stripping of CrN from CrN-coated high-speed steels

Stripping of hard ceramic coatings from metallic substrates is needed before re-coating of expensive tools, such as gear cutters, and of defective coated parts. In this study, a process similar to the stripping of metallic chromium coatings on steel substrates — the stripping of CrN coatings from high-speed steel (HSS) substrates by electrolytic anodic dissolution in alkaline solutions — is investigated. Arc-PVD CrN-coated HSS substrates were used. Anodic stripping experiments were conducted in a 100 g l −1 NaOH solution with 6 V cell voltage. This procedure worked effectively and the coating was stripped away rapidly. In order to follow the stripping procedure, further experiments were conducted by the application of a lower, constant current density of 40 mA cm −2. The results of this investigation revealed that it is possible to strip CrN coatings on HSS steels by anodic polarization in concentrated NaOH solutions similar to the dissolution of chromium coatings on steel substrates. Three effective simultaneous reactions taking place during stripping of CrN coatings were the dissolution of chromium droplets, delamination of the CrN layer by the dissolution of the chromium interlayer between the substrate and the coating, and finally dissolution of CrN. The stripping process proceeded mainly by delamination of the coating that occurred via dissolution of the chromium interlayer. Among the three reactions, the slowest was the dissolution of the CrN layer. The anodic dissolution process also affected the substrate material to some extent.