Structural control and self-organization of the functional oxide films formed on titanium

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1.Formation of self-organized porous anodic oxideWe have successfully developed self-organized porous anodic oxide films on Ti-Si alloys in hot glycerol solution containing dibasic phosphate. The key issue for the formation of the porous films is suppression of the crystallization of anodic oxide, which induces gas evolution during anodic film growth. The similar porous films are also developed on niobium and aluminum substrates. The cell size of the porous films on aluminum is dependent upon the formation voltage, similar to those formed in acid aqueous solutions.2.Formation of barrier-type anodic oxide with excellent dielectric propertiesIncorporation of foreign species from substrate suppresses effectively the development of crystalline anodic oxide on titanium. By increasing alloying element content, the structure of the air-formed film present before anodizing is modified, such that growth of amorphous oxide continues to higher formation voltages. The compositional dependence of the dielectric properties of anodic oxides formed on a range of titanium alloys has been examined systematically.3.Hard coating of titanium alloys by spark anodizingSpark anodizing of titanium, Ti-6Al-4V and Ti-15V-3Al-3Cr-3Sn has been performed in alkarine electrolyte containing aluminate and phosphate. Highly crystalline Al2TiO5-based oxide coatings,〜20μm thick, have been developed by dc-biased ac anodizing to high voltages. Regardless of the materials used, the coatings with hardnes of 〜6GPa have been obtained. The coatings have porosity, associated with discharge channel. From optical measurements during spark anodizing, the temperature at the discharge region has been estimated to be 4500〜5000℃. Such high temperature allows us to develop the highly crystalline coatings with high concentration of electrolyte species even in an aqueous solution at ambient temperature.