Abstract
To reduce the overgrowth of marine organisms on titanium alloy, this paper presents an antifouling scheme to process ceramic coatings on Ti6Al4V (TC4) using micro-arc oxidation (MAO) combined with nanotechnology. In the study, different concentrations of Cu2O nanoparticles are added into the electrolytes prepared for MAO process to produce the ceramic coatings. The antifouling performances of the samples are observed by scanning election microscope (SEM), energy dispersive spectrometer and X-ray diffraction. The results demonstrate that nanoparticles of Cu2O are found in the coating films and the antifouling performance of titanium alloy after MAO treatment is obviously enhanced, in which antibacterial rate is up to 95.25%. The Ti-MAO-10.0g/LCu2O group has the highest antibacterial rate of 99.74%.
Introduction
Titanium and its alloys, due to the excellent performances in rigidity, hardness, melting temperature, strength-to-weight ratio, friction, wear and corrosion resistance, have an increasing utilisation in various industrial fields including marine field [1-3]. However, its poor antifouling performance will increase drag on ship equipment when they attach marine microorganism, and the attachment will accelerate material corrosion, block pipe and cause failure in mechanism [4]. Therefore, great efforts should be taken to study antifouling methods of titanium and its alloys.
In recent years, environment-friendly methods to improve antifouling properties have been advocated to take the place of traditional measures such as toxic coatings. Although toxic paint can effectively protect the metal surface, and prolong the service life of the metal parts, the certain toxic substances it releases may be limited in industries and manufacturing companies to prevent the pollution to the sea environment and marine lives [5]. Hence, micro-arc oxidation (MAO), an advanced surface treatment process of titanium and its alloys [6], has been considered as a new idea for the antifouling of titanium alloys. Researchers believe that antibacterial and antifouling have the same nature in function since the biofilm is formed by bacteria and its secretion is the adnexed foundation of all subsequent periphyton, so it is positive that the antifouling can be achieved by antibacterial on the condition that the adnexed foundation is eliminated. Since TiO2 with a photocatalytic effect produced during MAO process on titanium alloys can catalyse and photolyse organic matter and micro-organisms attached to the surface, antibacterial and antifouling on titanium alloys can be accomplished on its dependence [7-10]. In view of this, MAO has been studied as an effective treatment to prevent biological adhesion. Wu et al. [11] studied antibacterial effects and corrosion resistant of porous Cu-TiO2 coatings, and the results showed that the MAO coating had an obvious antibacterial effect. Necula et al. [12] studied the growth of TiO2-Ag antibacterial coatings on Ti6Al7Nb biomedical alloy, verifying the antibacterial properties of TiO2 film containing silver ion. Zhao et al. [13] studied the antifouling properties of micro-arc oxidation coatings containing Cu2O/ZnO nanoparticles on Ti6Al4V (CT4), proving that the additive has a promoting effect on the antibacterial and antifouling property of MAO film. All previous researches have concluded that taking titanium dioxide and metal ions as a means of antifouling for titanium alloy is feasible and worthy of attention.
As environment-friendly antifouling substances against marine organisms, such as Cu and Cu2O, have been widely known in marine antifouling fields [14,15], various attempts are made to produce devices like surface coatings that contain copper or cupric composites. Mungkalasiri et al. [16] obtained TiO2-Cu composite films by means of DLI-CVD and found that the antifouling capability can be positively influenced by the content proportion of Cu. Stranak et al. [17] produced Ti-Cu films with an improved antifouling capability by magnetron sputtering. These methods showed effective antifouling performance in some circumstances, but could not ensure an effective and constant ion release rate, which may limit the utilisation of certain antifouling substance.
Therefore, an integration scheme synthesising micro-arc oxidation (MAO) and nanotechnology (NT) is presented to improve the antifouling property of TC4. In this study, the oxide coatings on TC4 alloy are produced by the MAO method in phosphate electrolyte samples containing different concentrations of Cu2O nanoparticles. The characteristics of the coatings are observed and analysed by the scanning election microscope (SEM), the energy dispersive spectrometer (EDS) and the X-ray diffraction (XRD). To verify the antifouling effect of micro-arc oxidation film containing Cu2O, an antibacterial assay is taken on the samples by Escherichia coli, and the test result is evaluated.
Experimental details
Materials preparation
The chemical composition of TC4 alloy (wt-%).
Before the MAO process, TC4 specimens in the size of 20 × 15 × 3 mm are prepared by a metal wire cutting. All the specimens are polished by emery papers of 200#, 400#, 600#, 800# and 1200# grit in turn, washed and dried at room temperature.
The MAO process
Groups and Cu2O additive concentration in electrolyte.
The MAO processes are carried on the same home-made MAO equipment, of which a stainless steel container works as the cathode electrode and TC4 plate serves as the anode electrode in phosphate electrolytes. All the devices and parameters of different experimental groups should be maintained the same except for the electrolyte composition, which is the main parameter to be evaluated (refer to Table 2 for detailed information). A constant voltage DC pulse power is supplied for the equipment. During the experimental process, the working voltage is 450 V, working current is 10 A, pulse frequency is 200 Hz, duty ratio is 15% and the oxidation time is 15 min. Subsequently, the coated specimens are washed with distilled water and dried at room temperature.
Analyses of the coating surface
Surface morphologies of coatings obtained though different MAO process are observed by means of the scanning election microscope (SEM, Quanta 200FEG, FEI, America) with the factors of 1000 and 200 000. Chemical composition of the surface layers is examined by the energy dispersive spectrometer (EDS, Quanta 200, FEI, U.S.A). Crystalline structures of the composite are evaluated by X-ray diffraction (XRD, D/max-rB, Rigaku, Japan) with the angle range of 20°–80° to obtain the phase composition of MAO coatings.
Microorganism attachment test
Microorganism attachment is prepared and studied to evaluate the antifouling performance of MAO coatings. Since the mucosa produced by bacteria can serve as the adhering medium for other marine organisms, the Escherichia coli, as a kind of typical attaching bacteria, is chosen as the experimental microorganism sample.
This assay is conducted based on the American Society Practice for Testing and Materials (ASTM) G21–13. In the antibacterial test, a certain amount of Escherichia coli [CMCC (B) 44102] is inoculated into the primary culture solution which was sterilised by high temperature previously. The specimens are sterilised by ultraviolet light and suspended in the culture solution for 24 h at 37°C. Then, the specimens are extracted by 20 mL sterile liquid and banister brush repeatedly to obtain extracting solutions. Subsequently, 1/5 mL attenuated (1:100) solutions are evenly spread on lysogeny broth (LB) and cultivated in the incubator for another 24 h at 37°C. As the amount of bacterial colonies will reflect the amount of bacterial attached to the samples, it can be used to evaluate the antibacterial performance.
The attached bacterial colonies are counted. The antifouling rate can be calculated by the formula
Results and discussion
Surface morphology
The SEM micro-graphs on MAO coatings amplified by the factor of 1000 are shown in Figure 1. The surfaces present uneven and contain many micro-pores whose diameters range from several micron to about 30 µm. In particular, the coatings in Figure 1(a)–(c) are more compact than that in Figure 1(d) which indicates that MAO coatings are improved by Cu2O nanoparticles. Results show that coatings structure can be improved at a certain amount of additive agents.
SEM micro-graphs on MAO coatings amplifying by the factor of 1000: (a) Ti-MAO-2.0 g/LCu2O, (b) Ti-MAO-6.0 g/LCu2O, (c) Ti-MAO-10.0 g/LCu2O and (d) Ti-MAO.
Figure 2 is the diagram of the SEM micro-graphs on MAO coatings amplified by the factor of 200 000. The coatings in Figure 2(a)–(c) display some nanoparticles whose diameters are about dozens of nanometre on its surface and the number grows with the content of particles, while there is no particle in Figure 2(d). This demonstrates that nanoparticles appearing in coatings come from additive agents and the amount is influenced positively by the concentration of Cu2O nanoparticles.
Surface morphology under SEM amplifying by the factor of 200 000: (a) Ti-MAO-2.0 g/LCu2O (b) Ti-MAO-6.0 g/LCu2O (c) Ti-MAO-10.0 g/LCu2O and (d) Ti-MAO.
Chemical composition
Composition of coatings (wt-%).
Phase compositions
Figure 3 shows the XRD spectra of coatings with the angle range of 20–80. It is observed that all the MAO coatings contain TiO2 in the phase of anatase and rutile, and a few Ti. Cu2O and a small amount of CuO can be found in the experimental groups that include additive agents, wherein the cupreous phases become clearer with the increase of the additive concentration.
XRD spectra of MAO coatings: (a) Ti-MAO-2.0 g/LCu2O (b) Ti-MAO-6.0 g/LCu2O (c) Ti-MAO-10.0 g/LCu2O and (d) Ti-MAO.
The phase of MAO coatings varies with the change of additive concentration. Specifically, the nanoparticles observed in SEM are Cu2O, Cu and a small amount of CuO, which indicates that the nanoparticles of Cu2O enter the MAO coatings and a few Cu2O converts into Cu and CuO on redox reaction during the MAO process.
Antifouling performance
Figure 4 shows the cultured result of each vaccinated medium and Figure 5 schematically describes the amount of bacterial colonies in Figure 4.
The culture medium after microorganism attachment experiment: (a) Ti-MAO-2.0 g/LCu2O (b) Ti-MAO-6.0 g/LCu2O (c) Ti-MAO-10.0 g/LCu2O (d) Ti-MAO and (e) Ti-without MAO. Statistical results of the colonies.
As shown in the graphs, the amount of attached microorganism in Ti-MAO group is lower than that of Ti-without MAO group, which indicates a relatively good antibacterial capacity of MAO treatment. The improvement in antifouling performance can be attributed to the anatase and rutile TiO2 components. As the positive and negative charges counteract each other, bacteria with a negative charge can be attracted by the MAO coatings with positive charge, causing the breakdown and death of bacterial cell, wherein the organics can be oxidised to CO2 and H2O in oxygenous water environment during the photocatalysis process of TiO2. In addition, the combination of rutile and anataseTiO2 has a better antifouling capability [18].
Moreover, it can be observed that the groups within Cu2O give a better antifouling performance when compared with the other groups. Figure 6 demonstrates the antibacterial rate of each group. Results show that micro-arc oxidation on titanium alloy has good antimicrobial properties whose efficiency can reach 95.25%. The addition of Cu2O improves the antibacterial rate (positively related to the concentration), and the highest antibacterial rate, 99.74%, appears in the Ti-MAO-10 g/LCu2O group. That is to say, the antifouling rate also has a positive relationship with the concentration of Cu2O additive. Therefore, it can be proved that the antifouling properties of the experimental specimens are improved by MAO coating combining Cu2O nanoparticles, and there is a positive correlation between the antifouling performance and the content of nanoparticles within limits.
Statistical results of the antibacterial rates.
Conclusion
The special MAO coatings incorporating nanoparticles show a better morphology and antifouling performance than plain MAO coatings, which raises the antibacterial rate from 95.25 to 99.74%. Besides, as it is observed, the antifouling properties of the MAO coatings with nanoparticles have a positive relationship with the increasing concentration of additives. It is further proved that the cuprous ions released from the additives are the basic elements to improve the antifouling performance. On the basis of that, this article presents a potential to optimise the antifouling performance by varying the amount of cuprous ions.
Footnotes
Acknowledgements
This work was supported by projects Study on Intelligent Control and Structure Parameter Optimization of Valve used in Warship (GH2012107) and Optimization of a Kingston Valve (GH2012024).
Disclosure statement
No potential conflict of interest was reported by the authors.