Crystal orientation mapping of NiO grown on cube textured Ni tapes

Samples of cube textured Ni tapes were oxidized in flowing oxygen at different temperatures. Crystal orientation maps (COMs) of the resulting oxide layers were produced by electron backscatter diffraction. The oxide layers were also analysed by X-ray diffraction (XRD), scanning electron microscopy and atomic force microscopy (AFM). The oxide grain size of a sample oxidized at 600 degrees C was similar to that of the substrate and the oxide was highly textured, both indicating epitaxial growth. The orientation relationship between the substrate and the oxide was directly observed from XRD to be (111)NiO//(001)Ni, [101]NiO//[110]Ni with four, equivalent, in-plane variants. In each variant, the oxide has both <110>- and <211>-type directions parallel to the Ni <110> directions. Differences in oxide thickness and surface roughness on neighbouring grains were revealed by AFM and these were attributed to the existence of a range of oxide growth conditions resulting from small differences in the orientation of each substrate grain. Similar macrotexture and microstructure were observed on a sample oxidized at 1300 degrees C, but additional, facetted oxide crystals had formed at the oxide grain boundaries. COMs showed that these crystals were either cube or 45 degrees rotated cube orientated, a texture different to that of the large oxide grains. The grain boundary crystals were thought to form by inward diffusion of oxygen at defects in the growing oxide scale.