*3.2. Morphology and Elemental Analysis*

The surface morphology and chemical compositions of the specimen treated by MAO process were investigated by SEM and EDS, respectively. The corresponding SEM and EDS images are shown in Figure 3a–c.

**Figure 3.** SEM image of surface morphology of the alloy Ti-6Al-4V treated by MAO with magnification: (**a**) 1000×; (**b**) 10,000× and (**c**) the elemental composition obtained by EDS.

The surface of the oxide coating has a typical morphology usually obtained by the MAO process upon valve metals [23]. The homogeneously distributed round-shaped pores formed on the surface in the locations where the electrical high-temperature breakdowns took place.

During the electrical breakdown, the temperature of the discharge reached several thousand degrees, as noted by estimations made by Hussein et al. [24], resulting in the creation of a newly formed oxide layer that was first melted and then recrystallized. Surface morphology investigation showed that the formed coating has no cracks on the surface, indicating a low cooling rate of the newly formed oxides. Additionally, the obtained surface consists of round pores of a diameter ranging from 0.15 μm up to 0.5 μm. These pores are significantly smaller than the pores formed in the MAO process conducted in aqueous electrolyte (usually 2.5–15 μm, depending on the applied potential and processing time [25]).

The atomic composition of titanium, aluminum, and oxygen obtained by EDS analysis were 32.3%, 3.0%, and 64.7%, respectively. These values clearly indicate that the formed coating is free of impurities. That is contrary to the coating obtained by MAO treatment in aqueous electrolyte, which usually includes additional components originating from the electrolyte [26,27].
