Effects of Alloying Elements on Hard-ceramic Layer Formation on Surfaces of Biomedical Ti-29Nb-13Ta-4.6Zr and Ti-6Al-4V ELI during Gas Nitriding

Masaaki Nakai¹, Mitsuo Niinomi¹, Toshikazu Akahori¹, Naofumi Ohtsu¹, Hideki Nishimura²*,
Hiroyuki Toda², Hisao Fukui³ and Michiharu Ogawa⁴
¹Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
²Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi
441-8580
³School of Dentistry, Aichi-Gakuin University, Nagoya 464-8650
⁴Daido Steel Co., Ltd., Nagoya 457-8545
*Graduate Student, Toyohashi University of Technology (Present address: Bridgestone Corp.)

Formation of the reaction product layer on the surface of biomedical titanium alloys, Ti-29Nb-13Ta-4.6Zr (TNTZ) and Ti-6Al-4V ELI (Ti64), during gas nitriding was investigated. These alloys were exposed to nitrogen atmosphere at 1023, 1073, 1123 and 1223K as shown in Fig. 1. After the gas nitriding, the cross-sections of TNTZ and Ti64 were observed by optical microscopy as shown in Figs 2 and 3. In every specimen, three kinds of phases—regions A, B, and C as described in the figureare formed. A layer with a thickness of a few micrometers is formed as a reaction product on the outermost surface (region A). On the inner side of this layer, acicular phases are precipitated (region B), followed by a matrix of each alloy (region C). Then, the reaction product layers on the outermost surface of both alloys were analyzed using an X-ray diffraction (XRD), Auger electron spectroscopy (AES) and X-ray Photoelectron spectroscopy (XPS). The layer was comprised of two phases, which were outer oxide layer (mainly TiO₂) and inner nitride layer (mainly TiN and Ti₂N) as shown in Figs 4 and 5. In these layers, the thickness of the oxide layer particularly depended on the kinds of alloys. According to the thermodynamics and point defect theory, the growth rate of oxide layer is expected to be increased by the presence of Al in TiO₂. Thus, the oxide layer formed on Ti64 was thicker than that of TNTZ. On the other hand, the elements that accelerate the growth of the nitride layer are not contained in both TNTZ and Ti64. Thus, the nitride layers with similar thicknesses may be formed on TNTZ and Ti64 during gas nitriding.

[Published in J. Japan Inst. Metals, Vol. 71, No. 4 (2007), pp. 415-422.]

Fig. 1 Schematic drawing of gas nitriding process.

Fig. 2 Optical micrographs of cross-sections of TNTZ subjected to gas nitriding at (a) 1023 K, (b) 1073 K, (c) 1123 K, and (d) 1223 K.

Fig. 3 Optical micrographs of cross-sections of Ti64 subjected to gas nitriding at (a) 1023 K, (b) 1073 K, (c) 1123 K, and (d) 1223 K.

Fig. 4 Compositional depth profiles obtained from each surface of TNTZ after (a) solution treatment, (b) nitriding at 1073 K, and (c) nitriding at 1223 K using AES.

Fig. 5 Compositional depth profiles obtained from each surface of Ti64 after (a) solution treatment, (b) nitriding at 1073 K, and (c) nitriding at 1223 K using AES.