*3.3. Chemical and Phase Analysis*

The elemental composition of the developed coatings detected by EDS is presented in Table 2.


**Table 2.** Elemental composition of the alloy Ti-6Al-4V treated by MAO with various current pulses detected by EDS investigation (at. %).

EDS study revealed that the appearance of Ti, Al and V originated from the Ti-6Al-4V alloy and Si originated from the electrolyte decomposition. The chemical composition of all samples is almost the same and is not affected by the applied current pulse.

Figure 4 shows PIXE spectra of the formed coatings.

**Figure 4.** PIXE spectra of the alloy Ti-6Al-4V coatings subjected to MAO treatment with various current pulses.

PIXE analysis clearly detected the following chemical elements: C, O, Al, Na, Si and Ti. Titanium and oxygen are main elements of the developed titanium oxide coating, aluminum is the major alloying element in Ti-6Al-4V alloy, and carbon peak was detected because the surface was coated in order to eliminate charge effect. Sodium and silicon were detected because of their appearance in the coating as the result of electrolyte decomposition during MAO processing. As it is clearly seen in Figure 4, there are no changes in the detected peaks were found in all examined samples.

The phase composition of the obtained coatings was revealed by the XRD measurements and are shown in Figure 5.

**Figure 5.** X-ray diffraction patterns of the alloy Ti-6Al-4V surfaces after MAO treatment with various current pulses.

XRD measurement evaluated the presence of the titanium phase (ICDD 44-1294) along with two titanium oxide phases, rutile (ICDD 21-1276) and anatase (ICDD 21-1272). It is detected that the content of the rutile phase was reduced with the growth of the processing frequency. It is clearly seen on the rutile analytical peak intensity change at 27.6◦, that the peak is relatively low for Sam–1000 Hz as compared to the same peak for Sam–200 Hz alloy. Additional peaks of rutile at 36.2 and 41.2◦ were also clearly detected in the pattern obtained in the Sam–200 Hz alloy, and their intensity was significantly reduced with the current frequency increase. Simultaneously, the analytical peak of anatase at 25.5◦ and its additional peak at 55.5◦ intensity were detected to be increased with the current frequency increase. This behavior may be attributed to the anatase-to-rutile transformation that occurred when the process reached a dielectric breakdown temperature, as described in by Hanaor et al. [26]. A metastable anatase phase is irreversible and transforms into rutile phase at a low frequency where the contribution of the current is more significant. No more phases were detected by XRD analysis. However, additional phases originated from the electrolyte decomposition may appear in the coating in amorphous structure or even in crystalline with a content that is below the detection limit of XRD.

Apart from XRD analysis, the TiO2 coatings obtained by MAO process were also investigated by Raman spectroscopy (Figure 6).

Raman spectra illustrated in Figure 6 are almost the same for all examined samples and show that the TiO2–anatase phase, which was detected with the active vibrations at: 147 (Eg1), 198 (Eg2), 398 (B1g), 515 (A1g + B1g doublet band), 640 (Eg3), and 796 cm−<sup>1</sup> (B1g[F], first overtone of B1g at 398 cm<sup>−</sup>1) [27]. The intensity of the Raman spectra peaks increased with the current frequency increase as the result of the produced surface porosity variation.

Due to the low penetration, the rutile phase was not detected by Raman spectroscopy. However, the anatase-to-rutile phase transformation was found by XRD and described above. Consequently, rutile was formed as the result of the transformation in the inner layer of the coating.

**Figure 6.** Raman spectra of the alloy Ti-6Al-4V surfaces after MAO treatment with various current pulses.
