Influence of various nickel under-layers on the corrosion behaviour of AlN films deposited by reactive sputtering

Aluminium nitride films were deposited by reactive sputtering on steel substrates (H11) with the aim of realizing anti-corrosion coatings. Electrochemical impedance spectroscopy (EIS) tests performed in a 0.5-M NaCl solution showed a good corrosion resistance of the films, which, however, decreased drastically with immersion time. The relatively weak thickness (1 μm) of the AlN films combined with their columnar structure [observed by scanning electron microscopy (SEM)] permits the pathway of the electrolyte to the steel substrate. In order to create a corrosion barrier between AlN and steel, two types of nickel under-layers (obtained by both electroless and electrochemical techniques) were investigated. All these samples were analysed by X-ray diffractometry (XRD), atomic force microscopy (AFM) and SEM. The Würztite type hexagonal structure of AlN films remained unchanged despite the presence of a nickel under-layer. The electroless nickel samples have a micro crystalline structure and a flat surface, whereas the electrochemical coatings are well crystallized and present a relatively high roughness. During the deposition of AlN on electroless nickel a structural transition occurs, with the precipitation of a Ni3P phase, well known to enhance the hardness of the samples. EIS measurements pointed out the synergetic effect obtained by the combination of an AlN coating with a nickel under-layer, leading to impedance values superior to the sum of the two effects considered separately. The AlN sample with an electroless nickel under-layer presents the best corrosion resistance with time.