*3.2. Surface Characterization*

Coating morphology and cross-sections images of the produced surfaces by MAO processing are shown in Figures 2 and 3.

**Figure 2.** SEM images of surface morphology of the alloy Ti-6Al-4V treated by MAO with various current pulses: (**a**) Sam–200 Hz; (**b**) Sam–400 Hz; (**c**) Sam–600 Hz; (**d**) Sam–800 Hz; (**e**) Sam–1000 Hz.

**Figure 3.** SEM images of cross-sections of the alloy Ti-6Al-4V treated by MAO with various current pulses: (**a**) Sam–200 Hz; (**b**) Sam–400 Hz; (**c**) Sam–600 Hz; (**d**) Sam–800 Hz; (**e**) Sam–1000 Hz.

Investigation of the surface morphology presented by SEM in Figure 2 revealed numerous micro-sized and sub-micron pores. These pores are formed in the process as the result of dielectric breakdowns at the extreme high temperature. Surface porosity change was also detected on the morphology images where sub-micron pores tend to disappear with the frequency increase. This was also clearly detected on the cross-section images presented in Figure 3.

MAO treatment is a high-energy process associated with a high current and a high voltage that affect surface morphology formation. The distribution of this current and voltage over the surface is not uniform and strongly depends on the localized dielectric properties of the developed coating. Thus, appearance of pores on the surface is spontaneous during the dielectric breakdowns, which are also influenced by their size and form. Therefore, control of the coating porosity may be done by the variety of the current frequency.

Additionally, to reduce porosity, the cross-sectional images also evaluated compaction of the coating with the growth of current pulses in MAO process. Thickness of the coatings was determined on the SEM cross-sectional images. It is clearly seen that the thickness of the produced coating reduced with the current pulse increase. Sam–200 Hz has a coating thickness of 2 μm, which was reduced up to 1 μm for the Sam–1000 Hz. This can be attributed to the reduction of current peak time duration with the increase of the frequency as the result of τon and τoff reduction.

Observation of Figure 3a–c revealed almost the same coating thickness with different pores size and their number. The large pores detected in Sam–200 Hz became smaller—mostly sub-micron—in Sam–600 Hz due to the dielectric breakdowns caused by localized high temperature. This phenomenon may be attributed to the decrease of the localized temperature as the result of a specific power density reduction. With the increase of the current frequency (Figure 3d,e), the local re-melt of the initial porous coating occurs, resulting in compaction and reduction of its porosity.

Examination of the cross-sectional images showed in Figure 3 also revealed thickness of all coatings. The thickness was found to be almost the same over the surface depth of each sample.
