Effect of the Incorporation of Bactericidal Agents Ag, Cu, and Zn Through Micro-arc Oxidation on the Properties of the Ti-25Ta Alloy Surface

Abstract

The main objective of this paper is to produce an anodic layer of TiO₂ rich in calcium, phosphorus, and magnesium using the anodizing technique known as micro-arc oxidation (MAO) on the Ti-25Ta alloy together with the incorporation of silver (Ag), copper (Cu) and zinc (Zn) metallic ions using different electrolytes. The structural changes and microstructures on the MAO surfaces were analyzed using the X-ray diffraction (XRD) technique and scanning electron microscopy (SEM). The chemical composition to verify whether silver, copper, and zinc were incorporated via energy dispersive spectroscopy (EDS) and surface characteristics such as wettability and hardness of the coatings were also investigated. From the EDS mapping results, it is observed that the MAO process was effective in incorporating the ions present in the electrolyte. Regarding X-ray diffraction patterns, the TiO₂ phases formed in the Ti-25Ta alloy has a mixture of anatase and rutile, making the MAO surface hydrophilic. The Ag element surface is thicker than the films containing Cu and Zn, as the Ag ions have high electrical conductivity, accelerating the oxidation process during surface treatment. Regarding hardness, coating containing Cu have a higher hardness value.

Keywords:
Bactericidal agents; surface modification; Ti alloy; ion incorporation; biomaterial

1. Introduction

Commercially pure titanium (CP-Ti) and its alloys have excellent properties, including biocompatibility, good corrosion resistance, high specific strength, and low density, making them interesting for use in materials for implants and medical devices¹,². Tantalum (Ta), as niobium (Nb) and molybdenum (Mo), are β-stabilizing element that reduces the transition temperature from the α phase (hexagonal closest-packed (hcp) crystalline structure) to the β phase (body-centered cubic (bcc) crystalline structure) of the Ti³. For applications in orthopedic implants, it is interesting to use these alloy elements since the β phase of Ti has better mechanical properties than the α phase, which can reduce stress shielding effects⁴,⁵.

Thus, the Ti-Ta alloy system was extensively studied for potential use in the biomedical field⁶-¹¹. Zhou et al.⁸,⁹ studied the Ti-Ta system and observed that its alloys exhibited better cytotoxicity results than the Ti-6Al-4V ELI alloy. The addition of Ta improved the corrosion resistance of Ti due to the formation of a more resistant Ta₂O₅ passivation film than TiO₂. Moreover, alloys between 20 and 30 wt% of Ta had better elastic modulus results. In a more detailed study of the quenched Ti-25Ta alloy, Zhou et al.¹⁰ found that the presence of the α-orthorhombic phase, with a larger unit cell volume, gave the lowest elastic modulus for this alloy among the studied alloys of the system.

Although the good properties of Ti alloys are attractive for their use as biomaterials for orthopedics implants, their surface still exhibit relatively low biological activity and resistance to corrosion and wear¹². Thus, surface modification techniques such as plasma spray¹³,¹⁴, sol-gel¹⁵,¹⁶, and micro-arc oxidation (MAO)¹⁷,¹⁸ have been studied to improve the surface properties of Ti and its alloys.

In addition, problems associated with infection of the implants are also very frequent, making it interesting to obtain a mechanism of bacterial action on the surface of the alloys, which can prevent the formation of biofilms after the surgical procedure¹⁹. Therefore, antibacterial agents can be incorporated into the surface through the MAO method to reduce bacterial adhesion and proliferation on the material’s surface. For this purpose, zinc (Zn)²⁰,²¹, silver (Ag)²²,²³, and copper (Cu)²⁴,²⁵ are examples of elements that can be used during MAO treatment to attribute antimicrobial properties to Ti and its alloys²⁶,²⁷.

The bactericidal action mechanisms of these elements have some differences between them. Zn or ZnO nanoparticles generate reactive oxygen species (ROS), which kill bacteria. Ag particles infiltrate the bacteria's nucleus, damaging their DNA. Finally, bacteria contact with Cu ions generates cavities in the cell wall¹⁹.

The surface modification of the Ti-25Ta alloy through MAO treatment was studied by Kuroda et al.²⁸. They found that the oxide layer formed on the surface of the alloy by this method improved the material’s wear resistance. Furthermore, it was observed that in the MAO coating produced on Ti-25Ta, titanium dioxide (TiO₂) forms in two crystalline structures, anatase and rutile. According to other works in the literature, TiO₂ rich in rutile has more excellent resistance to wear due to its high hardness value compared to TiO₂ in the form of anatase²⁹,³⁰. Thus, this study aimed to continue the investigations regarding the surface modification of the Ti-25Ta alloy, verifying the effect of adding different bactericidal agents (Ag, Cu, and Zn) on the characteristics and properties of the coating produced by the MAO technique. This study also sought to compare the effect of each bactericidal element produced on the MAO surface characteristics, which can also influence the material's response against infectious agents.

2. Material and Methods

A 60 g ingot of Ti-25Ta alloy was prepared using an arc melting furnace with a water-cooled copper crucible, a non-consumable tungsten electrode, and a controlled argon atmosphere. MAO treatments were performed in a DC power source (Keysight, N5751A) at 300 V, 2.5 A, at room temperature, for 60 s. The electrolyte used consisted of 0.35 M calcium acetate monohydrate ((CH₃COO)2Ca · H₂O), 0.02 M β-glycerol phosphate pentahydrate (C₃H₇Na₂O₆P · 5H₂O), and 0.1 M magnesium acetate tetrahydrate ((CH₃COO)2Mg · 4H₂O) to incorporate Ca, P, and Mg³¹. To incorporate Ag, Cu, and Zn, were used 2.5 mMol silver nitrate (AgNO₃), 2.5 mMol copper chloride (CuCl₂), and 1.0 mMol zinc chloride (ZnCl₂). The concentration of each bactericidal element was determined after a literature analysis²¹,²²,³².

The MAO coatings were characterized by x-ray diffraction using a MiniFlex600 diffractometer (Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 1.54056 Å). Optical microscope (Olympus BX51M), scanning electron microscopy, SEM (Carl Zeiss Inc), and confocal laser scanning microscopy, CLSM (Leica, DC3M) were used to analyze microstructure, topographic images, roughness by CLSM, and cross sections of MAO coating. Wettability was obtained using contact angle measurements by the droplet technique on a Krüss DSHAT HTM Reetz GmbH. Deionized water, 2.5 μl volume, was used as the test fluid.

To obtain the Vickers microhardness values, a Shimadzu HMV G20ST was used by applying 0.5 kgf for 15 s following ASTM-E384³³ through an average of 15 measurements. In order to analyze the adhesion of the MAO films to the Ti-25Ta substrates, the Rockwell-C adhesion test was performed³⁴. This test was carried out using a Sussen Wolpert brand Rockwell C indenter with a load of 150 kg.

3. Results and Discussions

Figure 1 shows the variation in current during the MAO layer formation on the Ti-25Ta alloy with incorporation of Ca, P and Mg as well as Ag, Cu and Zn, considering bactericidal elements. The MAO process carried out in this work initially operates in the galvanostatic regime (constant current value). As the MAO coating grows during the surface modification process, the voltage value increases until reaching its limit value (300 V). After reaching 300V, the current value begins to decrease, and the process start to operate in the potentiostatic regime. More details of the surface modification process used in this work can be seen in previous works²⁹.

Figure 1
Current x time curves of the Ti-25Ta alloy during the MAO layer formation with incorporation of Ca, P and Mg as well as Ag, Cu and Zn elements.

From Figure 1 also it can be seen the incorporation of Ag and Cu present a similar curve of current (A) x time (s), with dwell time in the galvanostatic regime of approximately two seconds. For the Zn incorporation process, the galvanostatic regime lasted just one second. This difference is due to the change in the electrical conductivity of the electrolyte, as the elements Ag and Cu have similar electrical conductivity values (62.5 and 61.7 Sm/mm²) the material can remain in the galvanostatic regime for longer. However, for the incorporation of Zn which has an electrical conductivity of 17.8 Sm/mm², the system reaches the limit voltage value (300V) to produce the MAO film.

The x-ray diffraction results performed on the surfaces of the Ti-25Ta alloy after the MAO process for incorporating the elements Ca, P, and Mg, with elements considered bactericidal, such as Ag, Cu, and Zn, are shown in Figure 2. Information about the XRD results of the substrate can found a previous study³⁰. The diffractograms showed characteristic peaks of the hexagonal closest-packed structure (HCP), known as the α phase in titanium alloys³⁵. In addition, characteristic peaks of titanium dioxide (TiO₂) in anatase and rutile forms can be visualized³⁶, indicating that the MAO process oxidized the Ti present in the Ti-25Ta alloy substrate. The diffraction peaks of the brookite structure on TiO₂ were not observed.

Figure 2
XRD of the MAO coatings containing Ag, Cu, or Zn on the Ti-25Ta substrate.

The anatase and rutile ratio (A/R) remains constant at approximately 2.3 in all diffractograms, indicating that the addition of CuCl₂, ZnCl₂, and AgNO₃ compounds to the electrolyte does not affect the formation of TiO₂ phases in the MAO coating.

No crystalline structures of compounds that can form easily during the MAO process, such as calcium and magnesium carbonates (CaCO₃ and MgCO₃), were observed due to the low concentrations of elements present in the electrolytes, specifically Ag, Cu, and Zn, which can have cytotoxic character in high amounts³⁷,³⁸.

Figure 3 shows the SEM images at 1000x and 10,000x magnifications on the MAO surface of the Ti-25Ta alloy. Kuroda et al.³⁰ presented the SEM image of the Ti-25Ta substrate, without the MAO coating. It was observed that the MAO process produced a porous surface typical of the MAO coatings. The pores come from oxygen bubbles forming in the electrolyte during the formation of electric arcs, which can reach temperatures above 2000 °C³⁹,⁴⁰. Generally, the more intense the electric arcs produced during the MAO process, the larger the oxygen bubbles from boiling water, forming anodic films with high porosity.

Figure 3
SEM images at 1000x (left) and 10000x (right) of the MAO coatings containing Ag, Cu, or Zn on the Ti-25Ta substrate.

From the SEM topographic images, histograms were produced (Figure 4) to demonstrate the size and quantity of pores visualized on the surfaces of the Ti-25Ta alloy containing Ag (Figure 4a), Cu (Figure 4b) and Zn (Figure 4c). The histograms showed that most pores present in the MAO layer have an average area of 0.25-0.50 μm². Figure 4d shows the relationship between the number of pores per μm², considering the error bar it can be highlighted that the surfaces of the alloys have the same number of pores per unit area.

Figure 4
Histograms produced from SEM images of the ceramic layer formed on the Ti-25Ta alloy. a), b) and c) number of pores per area. d) relation of pores/area from each surface modified with Ag, Cu and Zn ions.

Kuroda et al.³⁰ produced a MAO layer rich in Ca, P and Mg in the Ti-25Ta-Zr system alloys (Zr = 0, 25 and 50% by weight), and was observed that the majority of ceramic pores produced on the alloy surfaces have a size of approximately 0.50 μm², and α-type alloys tend to have larger pores compared to β-type alloys. It is observed that the results obtained by Kuroda corroborate the results obtained in this work. Figure 5 shows the cross-section images of the MAO coating with the incorporation of Ag, Cu, or Zn ions and the chemical mapping images of the elements present in the substrate (Ti and Ta) on the MAO coating.

Figure 5
Chemical mapping by EDS of elements in the MAO coatings and cross-sectional images of the produced coatings on Ti-25Ta substrate.

It was observed that the MAO process effectively incorporated the chemical elements in the electrolytes, noting that Ca, P, and Mg, along with Ag, Cu, or Zn, are present in all coatings. Furthermore, the coatings exhibit a significant level of carbon content. This carbon originates from the Ca, Mg, and P acetates, which may hinder the formation of electric arcs due to its property of reducing the electrolyte's electrical conductivity. However, based on the EDS results, it is observed that the MAO process successfully incorporated the metallic ions. To complement the semi-quantitative chemical composition results, Figure 6 shows the chemical composition of Ca, P, Mg, Ag, Cu, and Zn in the MAO coatings produced on the Ti-25Ta alloy.

Figure 6
Elemental quantification obtained using EDS for Ti-25Ta alloys.

Previous studies in the literature have shown that in MAO coatings, which were produced in a similar way as described in this, Zn, Cu, and Ag are commonly found in the form of ZnO²⁰,⁴¹, Cu₂O and CuO²⁴,⁴¹, and Ag₂O and metallic Ag⁴², respectively.

Regarding the cross-section images, it was observed that the coating containing Ag has a thickness of (31.8 ± 2.5) μm. The surface containing Cu has a thickness of (9.1 ± 1.2) μm, while the MAO surface containing Zn has a thickness of (9.9 ± 1.0) μm. The MAO coatings with Cu and Zn are thinner because chlorides (ZnCl₂ and CuCl₂) were used instead of the nitrate (AgNO₃).

Figure 7 is indicating the wettability results of the MAO surface and the 2D images of the contact of distilled water with the MAO coating on the Ti-25Ta alloy. Kuroda et al.³⁰ presented that the contact angle value of the Ti-25Ta alloy, without the MAO coating, is approximately 45°. The contact angle in all samples of this present study was less than 90°, indicating that the MAO surfaces produced in this study are hydrophilic. Previous studies reported that TiO₂ in the anatase form is generally hydrophilic, while TiO₂ in the rutile form creates hydrophobic surfaces. However, a mixture of these two phases generally results in hydrophilic surfaces⁴³. Based on XRD results, the MAO coatings on the Ti-25Ta substrate have an anatase/rutile ratio of approximately 2.3 and hydrophilic behavior as predicted by the literature, which contributes to increasing its bioactivity, improving its osseointegration⁴⁴.

Figure 7
Contact angle of the MAO coatings containing Ag, Cu, or Zn on the Ti-25Ta substrate.

Figure 8 presents the results of the surfaces' hardness by the Vickers microhardness technique. The coatings contain Ag, Cu, and Zn exhibit hardness levels of 337, 404, and 286 HV, respectively. According to Kuroda et al.⁴⁵, the Ti-25Ta alloy, after a solution heat treatment, has a hardness of approximately 250 HV. Therefore, it is observed that the coatings have higher hardness than the substrate, even with the high amounts of pores, which tend to decrease the hardness value of the MAO surfaces.

Figure 8
Vickers microhardness of the MAO coatings containing Ag, Cu, or Zn on the Ti-25Ta substrate.

The surface with Cu has a higher hardness value than the MAO surfaces containing Ag and Zn. This behavior is due to the lower radius value and Cu's atomic weight than Ag and Zn. The element with a small atomic radius and low atomic weight is more easily incorporated into the MAO coating process. Therefore, the Cu-containing coating probably has a higher entropy, which results in a higher hardness. It is important to note that a hard anodic surface in Ti alloys is preferred to increase resistance to micro-abrasive wear. This is because surfaces with high hardness tend to have good wear resistance, as demonstrated by Kuroda et al.⁴⁶.

4. Conclusions

From the results obtained in this study, it can be concluded that:

• It was possible to incorporate bactericidal elements (Ag, Cu, and Zn) into the surface of the Ti-25Ta alloy using the MAO method.

• The Ag-containing coating has the highest thickness due to its superior electrical conductivity.

• The anatase/rutile ratio remained close to 2.3 in all coatings produced, indicating that adding Ag, Cu, or Zn to the electrolyte did not affect the formation of the TiO₂ phases.

• The proportion of anatase and rutile phases on the coatings ensured the surfaces were hydrophilic, with contact angles between 64-79°.

• The MAO surface containing Cu has higher hardness compared to the MAO surface containing Ag and Zn

5. Acknowledgments

The authors would like to thank the Faculdade de Ciências de Bauru, UNESP, for the XRD and SEM measurements. This study was supported by the following funding agencies FAPESP (São Paulo Research Foundation) grants #2019/26517-6 and through “Projeto Temático” # 2018/18293-8. This study was financed partly by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior of Brazil (CAPES), Finance Code 001", and CNPq (Brazilian National Research Council), grant # 314.810/2021-8.

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Publication Dates

• Publication in this collection
  26 July 2024
• Date of issue
  2024

History

• Received
  19 Sept 2023
• Reviewed
  11 Jan 2024
• Accepted
  03 Apr 2024