Proceedings of the 12 th World Conference of Titanium 2116 Oxidation behavior and effect of layer thickness on whiteness and exfoliation behavior of oxide film formed on Ti-Nb-Ta-Zr alloy E. Miura-Fujiwara 1 , S. Yamada 2 , A. Obata 2 , H. Sato 2 , Y. Watanabe 2 , T. Kasuga 2 and M. Niinomi 3 1 Graduate School of Engineering, University of Hyogo, 2160 Shosha, Himeji, Hyogo, 671-2280, Japan 2 Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan 3 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Metallic materials for dentistry such as Ti alloys, the Pt group or Au group alloys have excellent mechanical properties as a load bearing material, however, they are inferior in point of esthetic. Ti-29Nb-13Ta-4.6Zr alloy is β-type Ti alloy with excellent mechanical property as a biomedical load-bearing material, and on which it is found that a firm yellowish-white oxide layer is formed by heat treatment at a particular temperature. Therefore, the detailed study regarding the oxide film formation in Ti-29Nb-13Ta-4.6Zr and its properties will be interesting in terms of biomedical applications. In the present study, the oxide formation behavior investigation was carried out with focusing on oxide exfoliation behavior and esthetic property. Our recent results revealed that multilayer oxide film covers Ti-29Nb-13Ta-4.6Zr substrate, and which each layer has different color. From cross sectional observation, a dense white oxide layer formed on top, and whose thickness increased with increasing heat treatment temperature and duration time. CIELab measurement results suggest that L* increases with increasing layer thickness. Keyword : titanium-niobium-tantalum-zirconium (Ti-Nb-Ta-Zr) alloy, oxide coating, biomaterials, color difference, microstructure 1. Introduction Metals, ceramics and polymers are used in many kinds of dental devices. Especially, metallic materials have been used for principal parts of prosthesis and orthodontic devices such as dental implant, crown, denture, bracket, wire, and so forth. Although ceramic materials are becoming popular and starting to substitute for metallic parts of these devices, metals are superior to in terms of ductility. On the other hand, they are inferior in point of esthetic. For example, a ceramic fused metallic core crown are commonly used, however, exfoliation between metallic core and ceramic surface is one of the most serious clinical problem (1, 2) . Thus, in dental prosthesis field, white-colored materials that own high ductility, high strength and excellent esthetic properties is the most long-awaited ones. Not only Pt group or Au group alloys also Ti alloys have been used for dental materials because of their high corrosion resistance and excellent mechanical properties (3) . Surface coating on Ti alloys with oxide, calcium phosphate such like hydroxyapatite (4, 5) or other ceramic (6-8) has been investigated to improve corrosion resistance (9, 10) and to enhance osseointegration and regeneration of bone tissue (8, 11) on a surface of the device. It is known that a TiO 2 layer is formed by high temperature oxidation of Ti, and it is recently found that a yellowish-white firm oxide film is formed by heat treatment at above 1273 K on Ti-29Nb-13Ta-4.6Zr alloy. Ti-29Nb-13Ta-4.6Zr alloy is β-type Ti alloy with excellent mechanical property as a biomedical load-bearing material developed by Niinomi et al. (12) . The ideal metallic white materials could be realized when esthetically and mechanically excellent oxide coating having durability against exfoliation of Ti-29Nb-13Ta-4.6Zr can be made. Therefore, study of the oxide film formation in Ti-29Nb-13Ta-4.6Zr and its properties will be interesting in terms of biomedical applications. In this study, we investigated relationships between oxide film growth and heat treatment condition or brightness of oxidized surface, and cross-sectional microstructure of the oxide layer were studied in order to optimize the condition. 2. Experimental Procedure Hot rolled Ti-29Nb-13Ta-4.6Zr bars with a diameter of 20 mm and 10 mm were used as a substrate material. The bar was sliced with a thickness of about 1 mm. The slices were polished with emery paper up to #1500, and some of them were polished with colloidal silica to obtain mirror surface. Polished samples were oxidized in air furnace at 1073 K, 1273 K, and 1473 K for from 1.8 ks to 5.4 ks. The samples were air-cooled after heat treatment. To investigate oxidation behavior of the substrate, thermal analysis was conducted using a thermogravimetric- differential thermal analyzer (TG-DTA). Heating rate was 0.33 K/s. Color, especially brightness, of the oxidized surface was measured using a spectrophotometric colorimeter. Color was expressed in terms of a L*a*b* color system (CIE1976). In this color system, color space was expressed by the parameter of brightness L* and chromaticity coordinates a* and b*. L* indicates brightness, and a* and b* express redness (+) ~ greenness (-) and blueness (+) ~ yellowness (-), respectively. Saturation C* is obtained from the equation; C* = (a* 2 + b* 2 ) 1/2 . Color difference ΔEab* was adopted to evaluate the effect of heat treatment condition on surface color. ΔEab* is calculated as the following equation; ΔEab* = (Δa* 2 + Δb* 2 + ΔL* 2 ) 1/2 . In this study, ΔEab* was defined the color difference between polished substrate without heat treatment and the heat-treated surface.