Surface oxidation of cube-textured Ni–Cr for the formation of a NiO buffer layer for superconducting coated conductors

Abstract

The surface oxidation epitaxy behaviour of the rolling assisted biaxially textured substrates of cube-textured Ni–10%Cr and Ni–13%Cr foils were investigated, in air, at 1050 °C. For both alloys, the optimum out-of-plane texture of cube oriented NiO was obtained for the shortest oxidation times, t, of a few minutes, although a strong texture still remained for longer times. For t<40 min, the NiO layer showed both 0° and 45° in-plane orientations with respect to the underlying Ni–Cr, whereas for t⩾40 min, a single, 45° in-plane orientation was observed. The surface roughness of the NiO layer was the lowest after ∼10–40 min oxidation. For oxidation times longer than 40 min, the macroscopic NiO roughness (mm 2 scale) increased dramatically due to a festooning effect. A fully connected Cr 2O 3 layer formed at the interface between the Ni and NiO for even the shortest oxidation times (few minutes). The Cr 2O 3 layer acted as a diffusion barrier to Ni, and limited the thickness of the surface NiO to a few microns, almost independent of oxidation time. The surface NiO layer showed better adherence to the Ni–10%Cr than to the Ni–13%Cr. For superconducting coated conductor applications, the optimum NiO buffer layer is formed on Ni–10%Cr foil after oxidation at 1050 °C for ∼40 min. The important columnar surface layer is ∼ 7 μ m thick, is highly textured with an r.m.s. surface roughness of ∼200 nm on a mm 2 scale, and ∼100 nm on the scale of the grain size. The layer shows good adherence to the underlying Cr 2O 3/Ni–Cr.