Influences of structure and composition on growth of anodic oxide films on Ti-Zr alloys

Sputter-deposited Ti-Zr alloys of various compositions have been anodized at a constant current density in 0.1 mol dm-3 ammonium pentaborate electrolyte at 298 K. The resultant anodic films have been characterized using transmission electron microscopy (TEM), glow discharge optical emission spectroscopy (GD-OES), Rutherford backscattering spectroscopy and capacitance measurements to elucidate the influence of alloy composition on their structure, growth behavior and dielectric properties. The Ti-10.5 at.% Zr alloy reveals oxygen formation within the film above ∼40 V, associated with crystallization of the anodic oxide to anatase. In contrast, amorphous anodic films, with a thin outer layer consisting of zirconium-free TiO2, grow up to about 250 V on Ti-23.0 at.% Zr and Ti-42.0 at.% Zr alloys at high current efficiency. Further increase in zirconium content to 62.5 at.% results in the formation of a film containing nanocrystals of monoclinic ZrO2 phase in an amorphous matrix. The crystalline oxide is formed predominantly in the inner region of the film that is developed by anion migration inward. An anodic film composed of monoclinic ZrO2 throughout the film thickness is formed on the zirconium. The influences of structure and composition of the anodic films on their growth behavior and dielectric properties are discussed and compared with findings for Al-Ta alloys, which form amorphous oxides over the entire composition range