(Ti,Cr)N and Ti/TiN PVD coatings on 304 stainless steel substrates: wear-corrosion behaviour

When hard nitrides coatings work in aggressive environments, wear phenomena occur in conjunction with corrosion processes; hence the characterisation of wear and corrosion resistance, considered as two distinct aspects, cannot be regarded as representative of the real degradation process, where the interactions between the two phenomena become extremely important. In this work wear-corrosion tests were performed by means of a laboratory-made apparatus, where the electrochemical characterization can be easily carried out. TiN coatings (with and without the presence of a Ti underlayer) and (Ti,Cr)N coatings were tested under different wear conditions in a sodium chloride solution. The results were compared and the different behaviour was interpreted in terms of film texture and residual stresses due to the PVD process. The (Ti,Cr)N deposits, which have the best intrinsic corrosion resistance, turned out to be the less resistant coatings to the wear-corrosion process, probably because residual stress was much higher than in TiN, making the coating more brittle and poorly adherent to the substrate. The TiN/Ti samples showed the best behaviour under wear-corrosion, which can be explained by a better adhesion of the coating, enhanced by the titanium underlayer. Moreover, the presence of the Ti underlayer proved to be effective in reducing the residual stresses in the TiN layer.