Improvement of the corrosion resistance of hard wear resistant coatings by intermediate plasma etching or multilayered structure

PVD hard coatings can be a serious alternative to electrochemical deposited coatings in the field of corrosion protection, if they are dense and without pores. This study is part of a series of experiments with the aim to deposit thin films onto high-speed steel by magnetron sputtering which possess a high corrosion resistance. Two different methods were chosen: (1) deposition of single layers (CrN, NbN) and (2) deposition of multilayer films (Cr/CrN). Additionally the effect of intermediate plasma etching was studied for these coating systems. That means for single layer systems, that the film growth was stopped several times and the surface of the as-grown film was exposed to an ion bombardment. For multilayer systems the intermediate plasma etching was applied between the different layers. It is assumed that the plasma etching of the film leads to a denser coating, due to defect-induced renucleation and improved adatom mobility. The corrosion resistance of the coatings was studied by potentiodynamic corrosion tests in sodium chloride solution. Further film properties were investigated by SEM, XRD, Scratch testing, Rockwell indentation tests and microhardness measurements. The results reveal that intermediate etching improves the corrosion resistance of monolithic CrN films, whereas for the multilayered coatings no further improvement could be observed.