Abstract
Different types of bi-layered scales of NiO can be formed in air on both sides of a thin nickel specimen at temperatures from 900 to 1200°C. When the black/green bi-layer is formed on one side and the black/black bi-layer on the other, a flexure of the specimen (99.9% pure) is achieved during oxidation; the former is on the convex side. When removed electrochemically from the substrate of nickel (99 or 99.9% pure), the black/green bi-layer was elongated causing bending to the opposite side. On the contrary, the black/black bi-layer formed on the 99.9% pure nickel remained straight with a negligibly small elongation as compared with the black/green bi-layer. It is found that high compressive stresses occur in the green layer, and negligibly small stresses in the black/black bi-layer. Macroscopic elastic stresses in the green layer at room temperature are estimated to be the order of magnitude 109 dynes/cm2. Stress gradients across the scale are roughly estimated. Discussions are given of the formation of bi-layered structures.
