Growth of oriented anodic films on single grains of Zr: structure and epitaxy from anisotropy-micro-ellipsometry

Abstract

Anodic passive films formed potentiodynamically (20 mV s −1) on polycrystalline zirconium in 0.5 M H 2SO 4 have been investigated on single grains by Anisotropy-Micro-Ellipsometry (AME). Substrate grain orientations were determined from AME-curves ( Δ Ψ vs rotation angle α around the surface normal) recorded on freshly electropolished samples. For Zr ZrO 2 , an increase in amplitude along with a shift in the phase relation of the AME curves is observed with increasing anodic layer thickness on all grain orientations except (0001). These effects are caused by the formation of crystalline, optically anisotropic oxide films exhibiting grain-dependent epitaxy relations with the Zr substrate. From the resulting curves, the crystal orientation of the passive film in addition to layer thickness was determined for several substrate grains. For basal (0001)-oriented grains ( ϑ MET = 0 °), amorphous or nanocrystalline ZrO 2 films are formed as indicated by the absence of anisotropy effects. On less densely packed surfaces ( ϑ s = 10 °–60 °), ordered crystalline ZrO 2 films of the monoclinic phase are grown exhibiting orientations of 45 and 22.5 deg. to (001), respectively, in systematic dependence on the substrate grain orientation. Similar differences found in the oxide formation factors correlate with packing density and structure of the anodic oxides. Comparison with photocurrent measurements at 4.8 eV sub-bandgap photon energy yields additional differences in the electronic properties of the films. The results prove a strong texture-dependence of passive film growth on polycrystalline Zirconium.