2016 Volume 57 Issue 12 Pages 2015-2019
Ti-15Zr-7.5Mo alloy was melted and its structure and mechanical properties were evaluated, followed by micro-arc oxidation (MAO) treatment to add bioactivity. Melted Ti-15Zr-7.5Mo alloy was consisted of mainly β containing of metastable α'' and ω phases. The Vickers hardness of the alloy was 420 HV and larger than those of Ti-6Al-4V alloy (320 HV) and Ti-29Nb-13Ta-4.6Zr ally (TNTZ) (180 HV). The Young's modulus of the alloy was about 104–112 GPa and almost the same as that of Ti-6Al-4V alloy (113 GPa) and larger than that of TNTZ (80 GPa). The MAO treatment was performed in a mixed electrolyte of 0.1-mol L−1 calcium glycerophosphate and 0.15-mol L−1 calcium acetate with a positive maximum voltage of 400 V and a 31.2 mA cm−2 for 600 s. Porous composite oxide of Ti, Zr, and Mo containing large amounts of Ca and P was formed on Ti-15Zr-7.5Mo alloy by micro arc oxidation (MAO) treatment. Zr was preferentially enriched and Ti and Mo were depleted in the oxide layer. Pore size was larger than that of CP Ti. The ability of calcium phosphate formation of the alloy in Hanks' solution after MAO treatment was less than those of CP Ti and TNTZ. It is necessary for the alloy to conduct a chemical treatment to accelerate bone formation.
Ti alloys are successfully used in medicine and dentistry due to their excellent corrosion resistance, tissue compatibility, and low elastic modulus. In particular, the requirement for a low Young's modulus to prevent stress shielding in bone fixation is fulfilled by a β-type alloy in orthopedics. Various β-type alloys, Ti-12Mo-6Zr-2Fe alloys,1) T-15Mo2) and Ti-15Mo-2.8Nb-0.2Si-0.28O3) have been developed in the United States and Ti-15Mo-5Zr-2Al alloy has been specified. Also, Ti-29Nb-13Ta-4.6Zr (TNTZ) has been developed as a β-type alloy.4) This alloy is transformed to the β phase by heat treatment and forging, and shows the smallest Young's modulus among β-type alloys. Elsewhere, the Ti-Nb-Sn alloy system is in the process of development.5) Ti alloys consisting of elements with low toxicity have been developed. The basic design of the alloys is the substitution of V and Al with Nb, Ta, Zr, and Hf, which are in groups 4 and 5 in the periodic table.
Ti-15Mo-Zr and Ti-15Zr-Mo system alloys has been investigated. The structure and microstructure of alloys were sensitive to Mo and Zr concentration, presenting α', α'', and β phases. Mo proved to have greater β-stabilizer action than Zr. Micro hardness was changed with addition of Mo and Zr, showing 436 HV in Ti-15Zr-10Mo and 378 HV in Ti-15Mo-10Zr.6) The addition of over 5% Mo forms β phase. In addition, Ti-15Zr-7.5Mo has larger hardness than Ti-6Al-4V alloy and TNTZ. Therefore, this composition is attractive as a biomedical β-type Ti alloy.6)
To add the ability of bone bonding to Ti surface, anodic oxidation of Ti is conducted to form a TiO2 layer on the surface by applying a positive voltage to a Ti specimen immersed in an electrolyte. When the applied voltage is increased to a certain point, a micro arc occurs as a result of the dielectric breakdown of the TiO2 layer. At the moment the dielectric breakdown occurs, Ti ions in the implant and OH− ions in the electrolyte move very quickly in opposite directions to form TiO2 again. This process is generally referred to as micro arc oxidation (MAO) or plasma electrolysis.7) The newly formed TiO2 layer is both porous and firmly adhered to the substrate, which is beneficial for the biological performance of the implants. Another advantage of the MAO process is the possibility of incorporating Ca and P ions into the surface layer by controlling the composition and concentration of the electrolytes.8,9) The biological response of Ti implants demonstrates that the MAO process constitutes one of the best methods for modifying the implant surface.10–15) The phase and morphology of the oxide layer are dependent on the voltage applied during the oxidation treatment.16) MAO-Ti alloys show good biocompatibility and Ca-P inducement capability in vivo and could accelerate bone tissue growth and shorten the osseointegration time.17) A novel technique for preparing the Ca- and P-containing ceramic coating on the Ti-6Al-4V alloy by MAO has been developed successfully. In the new technique, the Ti alloy is first micro-arc oxidized in a P-containing electrolyte and then in a Ca-containing electrolyte. The effect of MAO treatment on the bioactivity of the Ti-29Nb-13Ta-4.6Zr alloy (TNTZ) has been investigated. Ca, P, and Mg ions from the electrolyte are incorporated into the resultant oxide layer by MAO treatment.18) The oxides grown by MAO on pure Ti and its alloys, Ti-6Al-4V and Ti-6Al-7Nb, in a pH 5 phosphate buffered saline exhibited breakdown potentials of about 200 V, 130 V, and 140 V, respectively, indicating that the oxide formed on the pure metal is the most stable.19) MAO treatment helps form a porous surface with a biologically active bone-like apatite layer on the Ti-24Nb-4Zr-7.9Sn alloy specimens, which may improve the biological response.20)
The purpose of this research was to add bone bonding property to Ti-15Zr-7.5Mo alloy. The alloy was first melted, followed by characterization of the structure and evaluation of the hardness and Young's modulus. Thereafter, MAO treatment was conducted to the alloy and bioactivity was evaluated by immersion in Hanks' solution.
Ti-15Zr-7.5Mo alloy buttons were prepared by arc melting from commercially pure Ti (CP Ti; Grade 2), pure Zr (99.5%), and pure Mo (99.9%) under an Ar atmosphere on a water-cooled copper hearth in an arc-melting furnace. The button was flipped and re-melted at least 10 times to homogenize the composition. Surface of button ingot were analyzed at 5 points using X-ray fluorescent spectroscopy (XRF, XGT-1000WR, HORIBA) with a standard material of Ti-20Zr-20Mo-1Sn-1Hf-1Fe-1Cr alloy for quantification. The button alloys were cast by a vacuum centrifugal cast machine to rods and plate. Rods with 8-mm diameter, rods with 5-mm diameter, and plate with a dimension of 600 mm × 10 mm × 2 mm were used for structure characterization, mechanical testing, and MAO treatment, respectively. On the other hand, the density of rods with 5-mm diameter and three kinds of length (64.1 mm, 63.4 mm, and 63.7 mm in length) were measured with Archimedes' method that were compared with theoretical values calculated with mass and volume of the rod specimens to judge the accuracy of the compositions of the resultant alloy.
2.2 Structural characterization 2.2.1 X-ray diffractionDisks with 1.5-mm thickness were obtained from cutting the rod alloy. The disks were metallographically polished and finally polished with suspensions containing 9-μm diamond particles and 0.04-μm SiO2 particles. The polished disks were ultrasonically rinsed in acetone twice and ethanol once for 600 s, respectively. X-ray diffractometory (XRD) was performed using X-ray diffractometer (D8 ADVANCE, Bruker). X-ray source was Cu Kα with a voltage of 40 kV and a current of 40 mA.
2.2.2 Optical and transmission electron microscopiesThe above disks were metallographically polished and finally polished with #1000 grid abrasive paper and finally sheets with 0.1-mm thickness were obtained. The sheet was polished with suspensions containing 9-μm diamond particles and 0.04-μm SiO2 particles. The polished sheets were electrolytically polished in 5 vol% of H2SO4-MeOH for 9 s. The surface was observed using an optical microscope (LEXT OLS 4000, OLIMPUS).
The above sheet was again electrolytically polished in 8 vol% of H2SO4 in methanol at −33.9℃ for 116 s. The electrolytic voltage was 20 V.
2.3 Mechanical property evaluation 2.3.1 Vickers hardnessThe rod with 8-mm diameter was cut into disks with 1.5-mm thickness at top, medium, and bottom parts of the rod. The disks were metallographically polished and finally polished with suspensions containing 9-μm diamond particles and 0.04-μm SiO2 particles. Vickers hardness was measured using with a load of 4.903 N for 15 s at 12 points per each specimen. Maximum and minimum values were omitted.
2.3.2 Young's modulusThree kinds of rod specimens with 5 mmϕ × 64.1 mm, 5 mmϕ × 63.4 mm, and 5 mmϕ × 63.7 mm were prepared. The Young's modulus was evaluated for the rod specimens using the free resonance vibration method (JE-RT3, Nippon Techono-Plus co. Ltd.).
2.4 MAO treatmentMAO treatment was performed for the disk with 8-mm diameter and 1.5-mm thickness and plate with a dimension of 10 mm × 2 mm × 600 mm. These specimens were finally polished with a #800 grid paper. The plates were additionally sandblasted to form uniform oxide layer. Specimens were ultrasonically rinsed in acetone twice and ethanol once for 600 s, respectively. The disk specimen was fixed in a polytetrafluoroethylene holder with an o-ring. The exposed area contacting an electrolyte was 39 mm2 (7.0 mm in diameter). Details of the working electrode are described elsewhere.18) A 304 type stainless steel plate was used as a counter electrode. The electrolyte for MAO treatment was 0.1-mol L−1 calcium glycerophosphate and 0.15-mol L−1 calcium acetate. After pouring the electrolyte into the electrochemical cell, both electrodes were connected to a DC power supply (PL-650–0.1, Matsusada Precision), and then a positive maximum voltage of 400 V with a 31.2 mA cm−2 was applied for 600 s. In the case of the plate specimen, the plate was sealed with an insulate resin except an area of 25.04 cm2. After MAO treatment, specimens were washed in ultrapure water.
2.5 MAO surface layer characterizationSurface of MAO treated surface was characterized using scanning electron microscopy with energy dispersive X-ray spectrometry (SEM/EDS, S-3400NX HITACHI) and the XRD. To observe the cross-section of MAO layer, the plate specimen was cut to a dimension of 10 mm × 2 mm × 1.5 mm by a diamond cutter and the cut surface was polished with suspensions containing 9-μm diamond particles and 0.04-μm SiO2 particles, followed by ultrasonically rinsed in ultrapure water for 10 min.
2.6 Evaluation of ability of calcium phosphate formationThe ability of calcium phosphate formation was evaluated by immersion in Hanks' solution without glucose with pH 7.4. The electrolyte concentration was similar to that of extracellular fluid. The composition of the solution is shown in Table 1. Polytetrafluoroethylene bottles were washed with nitric acid and rinsed with ultra-pure water in advance. Specimens with and without the above treatments were immersed in bottles with 20 ml of Hanks' solution at 37℃ for 7 d. Hanks' solution was changed at 3.5 d. After immersion, specimens were rinsed in ultrapure water. Calcium phosphate formation on the specimens during immersion in Hanks' solution was evaluated using SEM.
| Ion | Concentration (mol L−1) |
|---|---|
| Na+ | 1.42 × 10−1 |
| K+ | 5.81 × 10−3 |
| Mg2+ | 8.11 × 10−4 |
| Ca2+ | 1.26 × 10−3 |
| Cl− | 1.45 × 10−1 |
| PO43− | 7.78 × 10−4 |
| SO42− | 8.11 × 10−4 |
| CO32− | 4.17 × 10−3 |
The composition of the alloy determined by XRF as Ti: 77.26–77.66 mass%, Zr: 14.72–15.10 mass%, Mo: 7.42–7.56 mass%, and Fe: 0.03–0.08 mass% at five points on button ingot. Appropriate composition was obtained and no segregation was observed on melted button ingots. Densities of specimens were summarized in Table 2. Little difference in density between theoretical values and empirical values was observed. Therefore, the accuracy of the composition is sufficiently high.
| Length of specimen, d/mm |
Theoretical density, ρ/g cm−3 |
Empirical density, ρ/g cm−3 |
|---|---|---|
| 641 | 4.98 | 4.93 |
| 634 | 4.97 | 4.94 |
| 637 | 4.75 | 4.78 |
XRD pattern of Ti-15Zr-7.5Mo alloy is shown in Fig. 1. Multi peaks origination from β phase appeared, indicating that the Ti-15Zr-7.5Mo alloy consisted of β phase. Image of the alloy obtained from optical microscope is shown in Fig. 2. Needle-shape structure that is expected as α'' phase near β matrix phase is observed.
XRD pattern of Ti-15Zr-7.5Mo alloy.
Image of the alloy obtained from optical microscope.
Figures 3 and 4 shows TEM image (a) and the corresponding SAED (selected area electron diffraction) pattern (b and c). Diffraction dot of α'' phase separating from that of β phase was observed from Fig. 3.21) In addition, ω phase was observed between diffraction dots of β phase from Fig. 4.22) Therefore, Ti-15Zr-7.5Mo alloy consisted of mainly β phase containing metastable α'' phase and ω phase.
TEM image (a), the corresponding SAED (selected are electron diffraction) pattern (b and c).
TEM image (a), the corresponding SAED (selected are electron diffraction) pattern (b and c).
Vickers hardness values of the alloy were 421, 422, and 420 HV at top, medium, and bottom parts, respectively and they are almost the same, indicating the uniformity of the cast specimen. These values are much larger than those of α + β type Ti-6Al-4V alloy (320 HV) and β type TNTZ (180 HV), indicating that the Ti-15Zr-7.5Mo alloy is much harder than conventional Ti alloys. This possibly caused by metastable ω phase.
Young' modulus values of 3 kinds of specimens were 110, 112, and 104 GPa, respectively, that is almost identical as that of Ti-6Al-4V alloy (113 GPa) and larger than that of TNTZ (80 GPa). Therefore, Young's modulus of the alloy is similar to those of α + β type Ti alloys and large as β-type Ti alloy. This possibly caused by metastable α'' phase and ω phase.
3.3 MAO oxide layerMicrographs of CP Ti and Ti-15Zr-7.5Mo alloy are shown in Fig. 5. The metallic color was eliminated and the formation of oxide layer is expected on both materials, while the color of the surface on Ti-15Zr-7.5Mo alloy was darker than that of CP Ti.
Micrographs of CP Ti (a) and Ti-15Zr-7.5Mo alloy (b).
SEM images of CP Ti and Ti-15Zr-7.5Mo alloy after MAO treatment are shown in Fig. 6. Porous layer was formed on both materials, while size of pore on Ti-15Zr-7.5Mo alloy was 1.7 ± 0.7 μm that was much larger than that on CP Ti (1.1 ± 0.4 μm): Pore size on Ti-15Zr-7.5Mo alloy was larger than that on CP Ti. In the case of pure Zr, ZrO2 layer with a pore size of 2 μm in maximum were formed.23) In Ti-15Zr-7.5Mo alloy, Zr in the alloy substrate possibly influenced the size of pore that is medium between CP Ti and pure Zr.
SEM images of CP Ti (a) and Ti-15Zr-7.5Mo alloy (b) after MAO treatment.
XRD pattern of Ti-15Zr-7.5Mo alloy after MAO treatment is shown in Fig. 7. Only peaks origination form β phase appeared, indicating that the peaks were originated from Ti-15Zr-7.5Mo alloy substrate. Therefore, surface oxide layer formed on Ti-15Zr-7.5Mo alloy by MAO treatment was amorphous. SEM image and EDS mapping of surface of Ti-15Zr-7.5Mo alloy are shown in Fig. 8. According to the image of cross-section of the surface oxide layer, the thickness of the layer was about 8 μm. The oxide layer consisted of composite oxides of Ti, Zr, and Mo and Zr was preferentially enriched and Ti and Mo were depleted. In addition, Ca and P were incorporated in the oxide layer, as the same as CP Ti. Relative concentrations of elements in the oxide layer are summarized in Table 3, indicating that large amounts of Ca and P were incorporated in the oxide layer. Cation fractions of Ti, Zr and Mo in the oxide layer calculated from Table 3 and nominal composition of the substrate alloy are summarized in Table 4. From this table, the enrichment of Zr and depletion of Ti and Mo are again revealed.
XRD pattern of Ti-15Zr-7.5Mo alloy after MAO treatment.
SEM image and EDS mapping of crosssection of Ti-15Zr-7.5Mo alloy surface after MAO treatment.
| Relative concentration of elements (mol%) | ||||||
|---|---|---|---|---|---|---|
| Ti | Zr | Mo | Ca | P | O | C |
| 7.3 (1.0)* | 1.6 (0.3) | 0.2 (0.2) | 4.2 (1.2) | 5.6 (0.0) | 53.4 (3.4) | 27.7 (4.1) |
* Standard deviation
| Ti | Zr | Mo | |
|---|---|---|---|
| Cation fraction in the oxide layer (%) | 87.0 | 8.8 | 4.2 |
| Nominal composition (mol%) | 80.2 | 17.6 | 2.2 |
SEM images of Ti-15Zr-7.5Mo alloy before and after immersion in Hanks' solution are shown in Fig. 9. Even after immersion in Hanks' solution, no precipitate was observed, so calcium phosphate was not formed on the layer. Therefore, the ability of calcium phosphate formation of Ti-15Zr-7.5Mo alloy after MAO treatment was lower than those of CP Ti and TNTZ.16) When over 50% of Zr is added to Ti, Ti-Zr alloy does not form calcium phosphate.24) A large amount of Zr in the oxide layer possibly decreased the ability of calcium phosphate formation. Therefore, the effect of Ca and P on the acceleration of calcium phosphate formation was canceled due to the existence of Zr in the oxide layer in this study. If the concentration of Zr is small in the oxide layer, calcium phosphate may form on itself. Ability of calcium phosphate formation is given to even pure Zr by chemical treatment as immersion in NaOH and H2SO4.25) Therefore, chemical treatment could be effective also for Ti-15Zr-7.5Mo alloy to accelerate bone formation.
SEM images of Ti-15Zr-7.5Mo alloy before (a) and after (b) immersion in Hanks' solution.
On the other hand, it is well known that the ingrowth of bone tissue into pores of the oxide occurs even in the small pore size and finally anchoring effect of bone tissue due to the porous surface is completed after healing in commercial MAO-treated dental implant.26) Therefore, bone bonding with the surface could be occurred also in this MAO-treated alloy.
The Vickers hardness of the alloy was 420 HV and larger than those of Ti-6Al-4V alloy (320 HV) and Ti-29Nb-13Ta-4.6Zr ally (TNTZ) (180 HV). The Young's modulus of the alloy was about 109 GPa and almost the same as that of Ti-6Al-4V alloy (113 GPa) and larger than that of TNTZ (80 GPa). Porous composite oxide of Ti, Zr, and Mo containing large amounts of Ca and P was formed on Ti-15Zr-7.5Mo alloy by MAO treatment. Zr was preferentially enriched and Ti and Mo were depleted. Pore size was larger than that of CP Ti. The ability of calcium phosphate formation of the alloy in Hanks' solution after MAO treatment was less than that of CP Ti and TNTZ. It is necessary for the alloy to conduct a chemical treatment to accelerate bone formation.
This research was supported by Japan Agency for Medical Research and Development, International Collaborative Research Program: Strategic International Research Cooperative Program (SICP), No. 16jm0310021h0004 and The Light Metal Educational Foundation, Inc.
