# Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities
## 3 Results
### XRD and XPS Characterization of the Black Coatings  
XRD analysis was performed to evaluate the phase composition of coatings. Results are reported in Figure 5. The XRD was conducted on all treated samples, G2-30, G2-60, G5-30, and G5-60. All the pattern peaks are duly assigned using the 5.2.0 HighScore (Plus) software database. Similar results were obtained in all samples with a difference in the intensity of diffraction patterns. According to the database, the reflections of TiO\({}_{2}\) (anatase and rutile), Ti, Na\({}_{3}\)PO\({}_{4}\), Na\({}_{2}\)MoO\({}_{4}\), \(\gamma\)Fe\({}_{2}\)O\({}_{3}\) can be distinguished. The reflections of crystalline Ti are due to the substrate, while all the other compounds can be found in PEO coatings. Two different mineral forms of TiO\({}_{2}\), anatase, and rutile, are typical for PEO coatings on titanium alloy substrates [49]. Diffraction peaks assigned to them are situated  
Figure 4: Cross-section SEM micrographs of black PEO coatings of the sample: (**a**) G2-30, (**b**) G2-60, (c) G5-30, and (**d**) G5-60. at 20 = 24.9\({}^{\circ}\), 27.9\({}^{\circ}\), 36.4\({}^{\circ}\), 37.1\({}^{\circ}\), 41.1\({}^{\circ}\), 47.3\({}^{\circ}\), 61.7\({}^{\circ}\), respectively [ICSD 98-007-4532; ICSD 98-016-1908]. Na\({}_{3}\)PO\({}_{4}\) (20 = 20.9\({}^{\circ}\), 22.5\({}^{\circ}\), 33.1\({}^{\circ}\), 46.0\({}^{\circ}\), 47.5\({}^{\circ}\)) [ICSD 98-003-3719], Na\({}_{2}\)MoO\({}_{4}\) (20 = 21.0\({}^{\circ}\), 23.8\({}^{\circ}\), 29.6\({}^{\circ}\), 30.3\({}^{\circ}\), 47.1\({}^{\circ}\)) [ICSD 98-015-1972], \(\gamma\)Fe\({}_{2}\)O\({}_{3}\) (20 = 30.2\({}^{\circ}\), 35.6\({}^{\circ}\), 62.9\({}^{\circ}\), 74.8\({}^{\circ}\)) [ICSD 98-017-2906] are all present in the PEO coating due to the composition of the used electrolyte. These XRD analyses are in accordance with the EDS and XPS data. XPS analysis was carried out to investigate the kind and chemical state of elements on the samples' surface. The survey spectra of all PEO coatings on G2 and G5 titanium alloys reveal the presence on the surface of all the expected species, namely Ti, Mo, Fe, P, Na, O, and C. V contained in the G5 titanium alloy was not detected because of the small amount or not segregate on the surface. Traces of Al, N, and S derived from substrate and electrolytes were also observed. The peak fitting analysis of the more relevant elements is reported in Figure 6. The peak shape and binding energies at 458.4 and 464.2 eV, respectively, for Ti 2p\({}_{3/2}\) and 2p\({}_{1/2}\) suggest the presence of Ti(IV) in TiO\({}_{2}\) in all prepared PEO coatings regardless of the alloy or the time of the treatment [50]. The Mo 3d\({}_{5/2}\) and Mo 3d\({}_{3/2}\) XPS peaks at 231.8 and 235.0 eV, respectively, are ascribable to a 3d doublet of Mo(VI) in Na\({}_{2}\)MoO\({}_{4}\), in agreement with XRD  
Figure 5: XRD patterns of PEO-coated samples: G2-30, G2-60, G5-30, and G5-60. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{6}{c}{**Elements (Wt.\%)**} \\ \cline{2-9}  & **O** & **Na** & **P** & **Mo** & **Ti** & **Fe** & **V** & **Al** \\ \hline G2-30 surface & 52.8 & 2.8 & 17.3 & 3.1 & 22.8 & 1.2 & - & - \\ G2-30 cross-section & 40.2 & - & 17.6 & 4.3 & 36.0 & 1.9 & - & - \\ G2-60 surface & 51.0 & 3.5 & 17.0 & 2.9 & 24.3 & 1.3 & - & - \\ G2-60 cross-section & 47.0 & - & 17.5 & 3.1 & 31.2 & 1.2 & - & - \\ G5-30 surface & 50.4 & 4.0 & 18.1 & 3.2 & 20.3 & 1.1 & 0.9 & 2.0 \\ G5-30 cross-section & 55.5 & - & 19.7 & 4.4 & 19.0 & 1.4 & - & - \\ G5-60 surface & 49.1 & 3.4 & 17.9 & 3.0 & 22.5 & 1.2 & 0.9 & 2.0 \\ G5-60 cross-section & 47.6 & - & 14.1 & 4.2 & 32.8 & 1.3 & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of the Semi-quantitative EDS analysis of PEO-coated samples performed on the surface and the cross-section of samples. Results are present in Wt.%. data [51]. The doublet of Fe 2p (at 709.8 and 723.5 eV for Fe2p\({}_{3/2}\) and 2p\({}_{1/2}\), respectively) is ascribable to Fe(III) in Fe\({}_{2}\)O\({}_{3}\)[51] in agreement with XRD data.