Formability and corrosion properties of metal/ceramic multilayer coated strip steels

Recently, physical vapor deposition (PVD) coated strip steel has received growing attention. This work concerns the use of alternating ceramic and metallic coatings deposited by PVD to improve simultaneously the formability and corrosion properties of stainless and low carbon strip steels. The coatings were deposited by magnetron sputtering and alternatively electron-beam evaporation. In this study, the formability is characterized in terms of the area fraction cracks formed in the coating at strain levels of 5% and 20%, respectively. The results show that the crack density and crack patterns are affected by the substrate strength and topography as well as by the coating ductility and thickness. A single layer titanium coating showed the lowest crack density, whereas, among a number of multilayer nitrides, the 3 x (Al/AlN) multilayer coating performed the best. Unlubricated bah-on-hat tests revealed that the friction of a steel ball against the stainless material was significantly lowered with a thin Si coating, whereas none of the surface coatings on the low carbon steel led to decreased friction. The use of a ceramic ball resulted in lowered friction for many of the nitride coatings compared with the bare low carbon steel substrate. Potentiodynamic measurements show that the coated materials lack the active peak that is typical for the austenitic stainless substrate, which results in a 10- to 50-fold decrease in critical current density compared with the bare substrate. Most of the coatings on stainless steel result in increased resistance to general corrosion even after 20% plastic deformation. Similarly, the undeformed coatings on the low carbon steel showed 3 to 10 times lower corrosion current density compared with the substrate. After deformation, as the crack density increased over approximately 1%, the current density also increased. However, Ti and Ti/TiN still provide substantial protection of the mild steel substrate. In this respect, the metallic Ti coating clearly showed a superior behavior to the multilayer coatings, including the Ti/TiN coatings. (C) 1998 Elsevier Science S.A. All rights reserved.