Exfoliation Resistance, Microstructure, and Oxide Formation Mechanisms of the White Oxide Layer on CP Ti and Ti-Nb-Ta-Zr Alloys.

Eri Miura-Fujiwara,Masahiko Nishijima,Keisuke Mizushima,Toshihiro Kasuga,Mitsuo Niinomi,Soichiro Yamada,Yoshimi Watanabe

doi:10.3390/ma14216599

Abstract

We found that specific biomedical Ti and its alloys, such as CP Ti, Ti–29Nb–13Ta–4.6Zr, and Ti–36Nb–2Ta–3Zr–0.3O, form a bright white oxide layer after a particular oxidation heat treatment. In this paper, the interfacial microstructure of the oxide layer on Ti–29Nb–13Ta–4.6Zr and the exfoliation resistance of commercially pure (CP) Ti, Ti–29Nb–13Ta–4.6Zr, and Ti–36Nb–2Ta–3Zr–0.3O were investigated. The alloys investigated were oxidized at 1273 or 1323 K for 0.3–3.6 ks in an air furnace. The exfoliation stress of the oxide layer was high in Ti–29Nb–13Ta–4.6Zr and Ti–36Nb–2Ta–3Zr–0.3O, and the maximum exfoliation stress was as high as 70 MPa, which is almost the same as the stress exhibited by epoxy adhesives, whereas the exfoliation stress of the oxide layer on CP Ti was less than 7 MPa, regardless of duration time. The nanoindentation hardness and frictional coefficients of the oxide layer on Ti–29Nb–13Ta–4.6Zr suggested that the oxide layer was hard and robust enough for artificial tooth coating. The cross-sectional transmission electron microscopic observations of the microstructure of oxidized Ti–29Nb–13Ta–4.6Zr revealed that a continuous oxide layer formed on the surface of the alloys. The Au marker method revealed that both in- and out-diffusion occur during oxidation in Ti–29Nb–13Ta–4.6Zr and Ti–36Nb–2Ta–3Zr–0.3O, whereas only out-diffusion governs oxidation in CP Ti. The obtained results indicate that the high exfoliation resistance of the oxide layer on Ti–29Nb–13Ta–4.6Zr and Ti-36Nb-2Ta-3Zr-0.3O are attributed to their dense microstructures composing of fine particles, and a composition-graded interfacial microstructure. On the basis of the results of our microstructural observations, the oxide formation mechanism of the Ti–Nb–Ta–Zr alloy is discussed.

Highlights

The development of regenerative medicine has gained increasing attention and high expectations
scanning electron microscopy (SEM) observations revealed that a firm and robust oxide continuously formed on Ti–29Nb–13Ta–4.6Zr [4] and Ti–36Nb–2Ta–3Zr–0.3O [6] substrates. These results suggest that oxide layers on Ti–29Nb–13Ta–4.6Zr and Ti–36Nb–2Ta–3Zr–0.3O alloys can be used for dental devices when their exfoliation resistance is sufficiently high
In the case of commercially pure (CP) Ti, the exfoliation stress was relatively low; our results indicated that this was attributable to its layered structure and not to interfacial compression stress due to the Pilling–Bedworth ratio (PBR) being > 1

Summary

Introduction

The development of regenerative medicine has gained increasing attention and high expectations. Inorganic and organic biomedical materials still play important roles in the medical field; in particular, many such materials are used in dental prostheses. New alloys and surface treatments that aim to improve biocompatibility and/or osseointegration are frequently proposed. In this field, white-colored materials with mechanical properties similar to those of metal, i.e., “white metals” have long been sought after. The shade of color on an artificial tooth or orthodontic device is an important property, as are the tooth or device’s corrosion resistance and mechanical properties [1,2]. The simplest method of obtaining white-colored metals is to coat the metal with a whitecolored material. The issue of exfoliation resistance has always been a problem with such hybrid materials

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