Self-healing Corrosion Protection Ability of Composition-Gradient Al ₂O₃·Nb Nanocomposite Thin Films

Al₂O₃·Nb nanocomposite thin films have been known as films having self-healing corrosion protection ability. To attain high self-healing ability and low pinhole defect density, the optimization of composition and structure of the films has been examined. The films were prepared on an Fe thin film substrate on a glass slide using ion beam sputter deposition technique. The self-healing ability of the films was evaluated by the repassivation behavior of potential and current of specimens coupled with an Au auxiliary electrode just after scratching by a knife edge in 0.1 M NaCl. Microscopic observation showed that the pinhole defect density increased with increasing Nb content. Therefore, to use the films as corrosion-protection coatings, decreasing Nb content would be needed for decreasing pinhole defect density but increasing Nb content for increasing self-healing ability. To solve this contradiction, composition-gradient Al₂O₃· Nb films were introduced. The composition-gradient film with increasing Nb content from the film surface to the substrate interface showed low pinhole defect density and high self-healing ability. A three-layer film of Al ₂O₃ (outer)/composition-gradient Al₂O ₃ · Nb (middle)/Nb (inner) gave the best result.