OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 value for an ideal capacitor is 90°.At this phase angle value, there is an ideally dense oxide film on the surface that can effectively inhibit charge transfer processes [31]. For all samples with copper-titanium coatings, the largest phase angle values were lower than 60°. This indicates that the oxide film formed on the surface of copper-titanium coatings was looser compared to the titanium alloy. Moreover, the coatings have two convexity maxima, which indicate a more complex oxide film compared to the titanium alloy. Comparing the convexity width on the phase angle graphs, it can be concluded that it decreases monotonically with increasing copper concentration in the coatings. Thus, the stability of the oxide film formed on the samples decreased with decreasing titanium concentration. In general, the results of impedance spectroscopy are in good agreement with the polarization data of the samples (Table 4). Thus, in the SBF solution, coppertitanium coatings Cu10–Cu70 have better corrosion resistance compared to the titanium alloy, but the barrier films formed on it are more permeable than on the Ti-6Al-4V alloy. Table 5 shows the concentration of dissolved copper ions after immersion of Cu-Ti coated samples in SBF solution for 24 h. It can be found that the concentration of copper ions was in the range of 98.6 to 484.9 μg/L; the minimum concentration was characteristic of Cu50 sample, and the maximum concentration was characteristic of Cu70 sample. According to the works on copper-titanium alloy (Ti – 5 wt. % Cu), the release of copper ions into 0.9 wt. % NaCl solution after anodizing was in the range of 52 to 239 μg/L [32], and after acid etching it was in the range of 26 to 386.9 μg/L [33], despite the fact that the copper concentration in the alloy was several times less than in the case of our coatings. The safe concentration of copper in drinking water, according to the World Health Organization, is < 2 mg/L [34]. Thus, the copper concentration released into the SBF solution from the developed Cu-Ti coatings is many times lower than the permissible values and therefore it can be used as biocompatible coatings. Another significant element is aluminum, which is released from the Ti-6Al-4V alloy and accumulates in the body of patients with implants. Despite the low concentration of aluminum in the Ti-6Al-4V alloy (~ 6 wt. %), it was released from the surface of the samples in quantities comparable to copper. Ta b l e 5 Content of metals released from the samples into the SBF solution Samples Concentration of metals, µg/dm3 Al Ti V Cu Cu10 188.45 1.67 6.15 193.98 Cu30 57.98 3.71 4.39 243.50 Cu50 54.14 1.17 6.82 98.55 Cu70 198.02 0.90 3.88 484.92 Cu90 98.37 5.17 4.69 145.15 The antibacterial activity (AA) of the samples with copper-titanium coatings was calculated using the method [35] in accordance with expression (1): ( ) 100 A B AA A − × = , (1) where: A is the number of bacterial colonies in the Petri dish for the reference metal; B is the number of bacterial colonies in the dish for the bactericidal metal. According to the calculations, the antibacterial activity of Cu-Ti coatings to the culture of non-pathogenic Escherichia coli monotonously increased from 25.5 ± 4.2 to 62.8 ± 5.4 % (Fig. 4). In real conditions of use, a longer contact (> 24 h) of the environment with the surface of copper-titanium coatings will lead to its complete disinfection. The result of the experiment showed that all deposited Cu-Ti coatings exhibited bactericidal activity. The highest concentration of copper in the Cu90 sample resulted in the
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