Morphological Investigation of Cracks in TiN and Ti Films Coated on Stainless Steel Coils Caused by Bulge-Press Forming

Abstract

A new horizontal-type high current (1500A) HCD gun was installed in continuous ion plating apparatus of the Air-to-Air type, making it possible to do high-speed plasma coating on wide and large materials with a high evaporation rate of 5 μm/min and a high ionization of 75%. This method was used to coat TiN and Ti on stainless coils. Samples were then taken from the stainless coils and bulge-press-formed. After bulge-press-forming, the samples showed no apparent peeling of the TiN- and Ti-coated films and no obvious difference in surface integrity between the central and corner areas. In the scanning electron microscope (SEM) observation of TiN and Ti films taken from the central area, the TiN films showed comparatively sharp, straight characteristic of ceramics, while the Ti films cracked in a slightly elongated manner characteristic of metal, with scribing on the surface due to the soft Ti film, accompanied by a complicated curvature of the cracks. In cross-sectional observation of TiN and Ti films, the TiN films showed a series of regular cracks in the TiN layer perpendicular to the steel surface, while the Ti films opened out slightly at the surface and the steel sheet was masked in places by the Ti films. In the corrosion test of these films, the Ti films were more susceptible to corrosion than the Ti films, presumably because the cavities created by internal peeling of the TiN films exposed a greater area of steel and tended to trap corrosive substances.