Interfacial structure and enhanced adhesion between anodized ZrO2 nanotube films and Zr substrates by sedimentation of fluoride ions

We reported a modified anodization to form ZrO2 nanotube films with significantly enhanced adhesion to zirconium substrates, which contains a traditional procedure to form nanotubular structure in a fluoride-containing electrolyte and an additional procedure to sedimentate fluoride (F−) ions in the electrolyte by addition of MgCl2 during the later period of anodization. With the aid of the F− sedimentated procedure, the adhesion of ZrO2 nanotube films, as characterized by critical load (Lc) using scratch test, can be significantly improved from about 4.5 ± 1 N to a maximum value of 23 ± 2 N. The surface morphologies and phase components show no obvious changes between ZrO2 nanotube films formed by anodization with and without F− sedimentated procedure. For the nanotube film anodized without F− sedimentated procedure, an ultra-thin fluoride-rich layer in the form of almost amorphous structure is present between the nanotube bottoms and Zr substrate, and it weakens the adhesion strength. However, the nanotube films anodized with F− sedimentated procedure consist of two layers, in which one is a thick nanotube layer and the other is a thin compact layer as an intermediate layer between the nanotubes and Zr substrate, while the portion of the compact layer adjacent to the Zr substrate consists of randomly nanograined cubic ZrO2 without fluoride-rich compound. Such interfacial structure at ZrO2/Zr interfaces leads to significant improvement in the adhesion strengths of nanotube films anodized with F− sedimentated procedure.