Surface Analysis of Anodized Ti-45Nb Alloy

Abstract

Titanium is extensively applied for dental implants manufacture due to its remarkable mechanical properties, corrosion behavior, and biocompatibility. To further improve these attributes, some TiNb alloys have been developed. This study investigates the Ti-45Nb alloy, aiming to optimize its biomechanical performance and osseointegration properties through micro-arc anodization. The research encompasses the analysis of the alloy's microstructure, surface modification via micro-arc anodization using H₂SO₄ solutions at concentrations of 1M, 1.5M, and 2M, characterization of surface properties, surface topography and roughness profiling, and surface energy analysis. The findings indicate that the micro-arc anodization treatment with 1.5M H₂SO₄ solution is the most effective to enhance biomechanical performance and osseointegration of the Ti-45Nb alloy.

Keywords:
Ti-45Nb alloy; Wettability; Electrochemical anodization; Surface energy

1. Introduction

Titanium is a metal renowned for its exceptional mechanical strength and corrosion behavior, properties that have led to its extensive application across several industries throughout the 20th century. These characteristics, combined with its biocompatibility and chemical stability, become titanium a material of choice for medical applications, particularly in orthopedic and dental implants¹.

One of the most commonly used titanium alloys for medical implants is Ti-6Al-4V (ASTM F136), known for its favorable mechanical properties. However, recent studies have highlighted the possibility of concerns regarding to the release of cytotoxic elements from this alloy, which can be detrimental to the human body. Evidences suggest that these elements can be released in ionic form from the implant surface when it is exposed to bodily fluids, potentially leading to cytotoxic and systemic effects².

Vanadium, in particular, has been associated with toxicity of biological tissues such as the liver, kidneys, bones, and spleen. Its toxicity is related to the release of ionic species, such as vanadate (V⁵⁺) and oxidovanadium (V⁴⁺), which can interact with intracellular biogenic ligands, participate in redox reactions, and form complexes with diverse biological activities. These species may interfere with essential cellular processes, such as the regulation of phosphatases — due to their structural similarity to phosphate — as well as induce oxidative stress, apoptosis, and DNA alterations. Aluminum, in its turn, is also recognized as a neurotoxic element, potentially contributing to neurological disorders and acute toxicity. Moreover, vanadium has been associated with adverse cardiovascular effects²,³

Another limiting aspect of the Ti-6Al-4V alloy is its high elastic modulus, exceeding 110 GPa, which represents a significant mismatch with the modulus of human cortical bone (10 – 30 GPa). This disparity can result in mechanical incompatibility between the implant and bone tissue, promoting the stress shielding effect and compromising osseointegration²,³.

Given these limitations, new β-type titanium alloys containing elements such as molybdenum, niobium, tantalum, zirconium, and tin have been developed to reduce the elastic modulus while enhancing mechanical strength. Among these alloys, the TiNb binary alloy noteworthy due to its non-toxic nature, excellent biocompatibility, and outstanding corrosion performance, making it a promising candidate for orthopedic and dental implants⁴.

The Ti-45Nb alloy (45 wt% niobium), has emerged as a promising alternative for biomedical implants, particularly for orthopedic and dental use. Among its main features figure excellent biocompatibility, a reduced elastic modulus to the range between 55 – 65 GPa — closer to that of human bone — as well as good mechanical strength, high corrosion behavior in physiological environments, and the absence of potentially toxic elements. Furthermore, the alloy exhibits good processability and formability, enabling its use in several implant shapes and geometries. These properties give to the Ti-45Nb enhanced structural and functional stability in biological environments, thereby supporting the longevity and performance of the implantable devices⁵,⁶.

Furthermore, surface modification of biomaterials becomes an area of growing interest, particularly to enhance the osseointegration. Anodic oxidation, an electrolytic passivation technique, is commonly applied to increase the thickness of the natural oxide layer on metal surfaces. This process not only improves corrosion and wear resistance but also produces a nanoporous surface layer that promotes cell adhesion and bone growth, thereby enhancing mechanical stability and corrosion performance⁷,⁸.

Anodizing parameters, including applied voltage, electrolyte type, and anodizing duration, significantly influence the characteristics of the oxide layer formed, such as pore size and thickness⁷. In this context, the present study investigates the surface properties of the Ti-45Nb alloy subjected to anodic oxidation in sulfuric acid solution, with the objective of optimizing its performance for biomedical use. Accordingly, micro-arc oxidation (MAO) was carried out at a constant voltage of 250 V for 15 minutes. The samples were immersed in H₂SO₄ solutions at three different concentrations (1.0 M, 1.5 M, and 2.0 M), using platinum as the counter electrode.

2. Materials and Methods

This study deal with the preparation and analysis of Ti-45Nb alloy samples, which were initially cut and polished for anodic oxidation treatment. The alloy was given in the form of cold-rolled bars with deformation of 0.9. Following cutting and polishing, the samples were cleaned by ultrasonic bath with isopropyl alcohol and subsequently subjected to the micro-arc anodization, in which platinum served as the auxiliary electrode, while the titanium alloy acted as the anode. The samples were immersed in three concentrations of sulfuric acid (H₂SO₄) - 1M, 1.5M, and 2M - for 15 minutes at the applied potential of 250 V.

For the analysis of surface roughness, the ImageJ software (Win64 version) was used to perform three-dimensional profiling based on scanning electron microscopy (SEM) images. The original images were converted from 32-bit to 8-bit format to optimize processing. The Ripple and Roughness Analysis module was applied to generate two-dimensional (2D) images representing surface roughness. These 2D images were then used to perform the three-dimensional (3D) topographical representations using the Surface Plot tool, allowing for a more detailed evaluation of the surface features of the treated samples.

The surface profiling performed using ImageJ was based on the variation in grayscale intensity of the SEM images, where the brightest regions (white) represent peaks, and the darkest regions (black) correspond to valleys. The intermediate gray levels were interpreted as height variations between these two extremes, enabling three-dimensional surface reconstruction based on brightness gradients. This approach provides a visual and qualitative estimation of the surface roughness and topography of the analyzed samples.

The wettability of the samples was assessed by measuring the contact angles of distilled water and ethylene glycol droplets on the surfaces. Measurements were conducted at different time intervals post-anodization (1, 5, 15, and 20 days) to evaluate the temporal changes in surface properties and to calculate the average contact angles, which were measured using a HAIZ 1600x digital microscope. A static sessile drop of 5 µL of each fluid (distilled water and ethylene glycol) was dripped on the surface of each sample, and images of the droplet were obtained using the microscope. The angles were subsequently analyzed and processed using ImageJ software.

Finally, the surface energy of the samples was given by the Owens-Wendt method, which calculates the total surface energy based on its dispersive and polar components, as described in Equation 1. This method applies two liquids with known properties, enabling the evaluation of the surface characteristics of the Ti-45Nb alloy following the micro-arc anodization⁹.

The model provides long-range dispersion (γd) and short-range polar dispersion (γp), where γs refers to solid surface energy and γL to liquid surface energy. In the Owens-Wendt model, there are two unknowns, which requires the use of two liquids with known surface tensions but differing polarities for the tests.

$$\gamma L\left(1+cos\theta \right)=2{\left({\gamma }_S{}^d{\gamma }_L{}^d\right)}^{1/2}+2{({\gamma }_S{}^p{\gamma }_L{}^p)}^{1/2}{}^{}$$ (1)

Where:

● ${\gamma }_L:\text{Liquid surface tension}$

● ${\theta }_L:Contactangleofliquidonsolidsurface$

● $$\begin{array}{l}{\gamma }_S^{d}and{\gamma }_L^d:Dipersivecomponentsofthe\\ surfacetensionofthesolidandliquid,respectively.\end{array}$$

● $\begin{array}{l}{\gamma }_S^{p}and{\gamma }_L^p:Polarcomponentsofthe\\ surfacetensionofthesolidandliquid,respectively.\end{array}$

The polar and dispersive components of water are 51.0 mN/m and 21.8 mN/m, respectively, while for ethylene glycol, they are 30.0 mN/m and 34.0 mN/m.

3. Results and Discussion

The surface of the anodized Ti-45Nb alloy was analyzed to evaluate the surface topography, roughness and wettability.

Table 1 presents the angles after micro-arc anodization by H₂SO₄ at concentrations of 1M, 1.5M, and 2M using distilled water, while Table 2 exhibits the results for ethylene glycol. The data denoted the variations observed at the different time intervals.

Table 1
Measured degrees for distilled water.

Concentration (H2SO4)	Angle (Day 1)	Angle (Day 5)	Angle (Day 15)	Angle (Day 20)
1M	40º	98º	62º	58º
1,5M	49º	63º	71º	63º
2M	21º	46º	50º	70º

Table 2
Measured degrees for ethylene glycol.

Concentration H2SO4)	Angle (Day 1)	Angle (Day 5)	Angle (Day 15)	Angle (Day 20)
1M	37º	44º	38º	36º
1,5M	44º	42º	47º	40º
2M	5º	43º	34º	25º

The angles measured after drops with distilled water on the anodized Ti-45Nb alloy varied depending on the H₂SO₄ concentration. At the concentration of 1M, the surface exhibited increasing hydrophobicity over time until it stabilized. At 1.5M, the surface exhibited a more consistent and stable hydrophobic behavior. At 2M, the initially hydrophilic surface gradually changed to a more hydrophobic state.

The observed changes in surface energy can be attributed to phase transformations of titanium oxides. During the micro-arc oxidation, performed under high voltages, the initially amorphous oxide undergoes crystallization. This process may initially form anatase, an unstable phase at room temperature, which over time converts into rutile, a thermodynamically more stable phase under standard temperature and pressure conditions. This phase transformation significantly influences the surface energy of the samples².

However, the behavior of droplet angles with ethylene glycol for the 1M of H₂SO₄ remained relatively stable over time. Although minor changes were observed, the surface consistently exhibited hydrophilic characteristics. For the 1.5M concentration, the angles denoted moderate fluctuations over time, indicating that the surface's interaction with ethylene glycol was not significantly impacted.

At the 2M concentration, an initial variation was observed, with the angles on the first day being noticeably low, indicating an hydrophilic surface. However, a significant increase of the angle was noted on the 5th day, followed by stabilization in subsequent 15th and 20th days. This initial behavior can be attributed to the formation of a porous oxide layer, which promotes the rapid absorption of ethylene glycol, eventually stabilizing the interaction over time.

Surface topography plays a critical role in the interaction between materials and biological tissues, particularly in biomedical implants, as it affects protein adsorption and cellular behavior. Surfaces with micro- and nanostructures can enhance osseointegration, a key factor for the success of bone implants¹⁰.

Figure 1 illustrates the surface of the Ti-45Nb alloy, both untreated and after 20 days of micro-arc anodization by H₂SO₄ at varying concentrations. At 1M, the surface appears uniform with moderate microporosity. At 1.5M, a porous oxide layer is present, exhibiting changes in pore size and distribution. At 2M, the surface is more porous and irregular, with larger cavities resulting from more aggressive oxidation.

Figure 1
Scanning electron microscopy (SEM) images of the Ti-45Nb alloy surface after micro-arc anodization treatment. a) Untreated surface (standard, #600 sandpaper); b) Anodized with H₂SO₄ at 1 M concentration; c) Anodized with H₂SO₄ at 1.5 M concentration; d) Anodized with H₂SO₄ at 2 M concentration, all after 20 days of micro-arc anodization.

To better understand and enhance the performance of the Ti-45Nb alloy for biomedical devices, the topography and roughness profile obtained from SEM images given at 20 days after micro-arc anodization were analyzed. The qualitative analysis of the anodized surfaces is correlated with the acid concentration used and the density of the oxide layer formed on the surface of the Ti-45Nb alloy.

Figure 2 exhibits the roughness profile of the sanded sample. It can be observed that its surface exhibits homogeneous surface density with sanding marks, indicating that the surface has not undergone significant structural changes.

Figure 2
Surface roughness profile of the Ti-45Nb alloy anodized at different H₂SO₄ concentrations. a) Untreated surface (standard, #600 sandpaper); b) Anodized with 1 M H₂SO₄; c) Anodized with 1.5 M H₂SO₄; d) Anodized with 2 M H₂SO₄.

The anodized surfaces shown in Figures 2b, 2c, and 2d denote noticeable morphological changes if compared to the sanded sample (Figure 2a). It is evident that, with the variation in color intensity on the surface during the oxidation, there is an increase of the density of peaks and valleys if compared to the sanded sample. This change is further evidenced by the lighter pink tones observed in the areas near the peaks of the anodized samples. In contrast, the sanded sample exhibits a darker purple hue, indicating lower topographical elevations, if compared to the anodized samples. While 3D image characterization reveals significant morphological changes in the anodized samples relative to the sanded samples, specific differences in roughness among the anodized samples could not be discerned. To achieve a more accurate assessment, is required a quantitative roughness analysis of both the anodized and sanded samples. This approach will provide an objective quantification of the changes in surface topography, which were not clearly identified by visual inspection of the three-dimensional images alone.

Surface energy consists of two components: polar and dispersive, the sum of which determines the total free surface energy. The polar component influences protein adsorption¹¹. Figure 3 presents the surface free energy calculated from the angle’s measurements. On the 1st day after anodization, the alloy treated with 1.5M of H₂SO₄ exhibited the highest total surface free energy, both polar and dispersive, indicating a more reactive surface. The sample treated with 2M of H₂SO₄ showed significantly lower surface free energy, particularly the dispersive component, suggesting a less interactive and more hydrophobic surface.

Figure 3
Surface free energy of the Ti-45Nb alloy sample anodized with H₂SO₄ at different concentrations, calculated from contact angle measurements one day after treatment.

Figure 4 shows the surface free energy after the 5th day of anodization, where it can be observed that the alloy anodized with 1M exhibits a significant increase in the polar component of free energy, indicating a structural change in the oxide layer that enhances its affinity for polar liquids¹². The 1.5M and 2M samples display a more balanced relationship between the polar and dispersive components.

Figure 4
Surface free energy of the Ti-45Nb alloy sample anodized with H₂SO₄ at different concentrations, calculated from contact angle measurements five days after treatment.

Figure 5 exhibits the free energy after the 15th day of anodization, indicating a noticeable stabilization of the free energy for all concentrations. The 1.5M continues to exhibit the highest polar energy, suggesting that its surface maintains a stronger interaction with water molecules. In contrast, the 2M concentration, while still undergoing lower energies, demonstrates an increase in polar energy if compared to the energy measured at day 5. This increase can be attributed to the changes in the surface composition or modifications in the microstructure of the oxide layer.

Figure 5
Surface free energy of the Ti-45Nb alloy sample anodized with H₂SO₄ at different concentrations, calculated from contact angle measurements fifteen days after treatment.

Figure 6 exhibits the free energy after 20 days of anodization. For the 1M and 1.5M, a slight decrease in free energy is observed, suggesting a possible stabilization of the oxide layer. In contrast, the 2M concentration exhibits a significant increase in polar energy, indicating a likely change in the surface that enhances its affinity for polar liquids.

Figure 6
Surface free energy of the Ti-45Nb alloy sample anodized with H₂SO₄ at different concentrations, calculated from contact angle measurements twenty days after treatment.

The surface energy of the micro-arc oxidation samples changes significantly over the 20 days after anodization, with more intense transformations occurring during the first 15 days. During this period, a significant change in polar energy is observed, which determines the surface reactivity and its ability to interact with biological tissue.

The medical device implant between the 1st and 10th day after micro-arc anodization is most recommended, as in this time range, the surfaces treated with 1 M and 1.5 M of H₂SO₄ exhibit high levels of polar energy. This condition promotes protein adsorption and cell adhesion, which are essential for the onset of osseointegration. This phase concentrates the peak of surface reactivity, optimizing the initial integration of the implant with the tissue.

After the 15th day, the oxide layer tends to stabilize, with a reduction in surface energy for the 1 M and 1.5 M samples. At this stage, the surface becomes chemically more stable, improving the uses that require greater long-term predictability, although with less initial stimulation of cellular interaction.

Thus, the range between the 1st and 10th day after anodization treatment is the most suitable for implantation, as it offers optimal conditions of reactivity and the onset of stabilization, allowing for the biological integration between the bone tissue and implant.

4. Conclusion

This study was dedicated to investigate the micro-arc anodization of the Ti-45Nb alloy subjected to 0.9 true cold strain. The process was carried out using sulfuric acid (H₂SO₄)-based electrolyte at three different concentrations: 1 M, 1.5 M, and 2 M. Surface energy evaluation was conducted through contact angle measurements with distilled water and ethylene glycol, performed at 1, 5, 15, and 20 days after anodization, as well as surface morphology analysis by scanning electron microscopy (SEM) and roughness profile characterization. The following conclusions were drawn:

Analysis of the contact angles with distilled water and ethylene glycol over time revealed significant changes in surface wettability. Initially hydrophilic, the surfaces became progressively more hydrophobic over time, particularly at the 2M concentration.

Characterization of surface energy indicated that, after 15 days of the anodization, the values stabilized. The 1.5M concentration maintained the highest polar energy, suggesting a more reactive surface, while the 2M concentration showed an increase in polar energy following an initial period of low energy.

The SEM images showed that the surface morphology of the anodized samples was significantly altered if compared to the sanded sample. The formation of denser and more porous oxide layers was related to the electrolyte concentration, revealing a more irregular surface with an increase in peaks and valleys. Qualitative roughness profile analysis reinforced this observation: while the sanded sample exhibited a homogeneous surface with sanding marks, the anodized samples displayed complex topographies, evidenced by the color changes in the 3D images.

These results have direct implications for biomedical applications, as surface morphology, roughness, and surface energy influence protein adsorption, cell adhesion, and interaction with biological tissues. The stabilization observed on the surfaces treated with 1 M and 1.5 M indicates prolonged reactivity, which is beneficial for the initial integration of implants with tissue.

5. Acknowledgments

Authors would like to thank the financial support of agencies Capes, CNPq (grant number 304187/2023-2), and FINEP (grant number 01.22.019600).

6. References

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Publication Dates

• Publication in this collection
  09 June 2025
• Date of issue
  2025

History

• Received
  30 Jan 2025
• Reviewed
  05 May 2025
• Accepted
  17 May 2025