*3.1. Process Characterization*

The recorded electric characteristics in the process are illustrated in Figure 1.

**Figure 1.** Typical plots of: (**a**) voltage- and current-time behavior; (**b**) the waveform of the MAO process applied on alloy Ti-6Al-4V. V—voltage-time curve; C—current-time curve.

Figure 1 presents a typical voltage- and current-time behavior for examined samples; the obtained plots are the same for all five examined samples. The voltage-time plot contains three main stages, which are clearly detected on the plot (Figure 1a). In the initial stage, the voltage increases rapidly up to 80 V. At this stage the amorphous layer is formed (Figure 1a, area I) and the process is accompanied by the gas bubbles appearance on the materials' surface, which was also shown previously in the work of Snizhko et al. [22]. In the second stage of the process, the voltage has reached 105 V and

dielectric breakdowns came out with the uniformly appeared electric sparks on the alloys' surface (Figure 1a, area II). At this stage, an amorphous to crystalline structure transition of the oxide coating occurred, as described in work of Hussein et al. [23]. In the last stage of the MAO process, the voltage was near the plateau (Figure 1a, area III). Here, sparks became larger and more intense, which corresponds with the increase in the charge transfer resistance. This behavior is also confirmed by work of Mortazavi et al. [24].

On the current-time plot, the rapid drop of the current at the initial stage is detected (Figure 1a, area I). However, the process current begins at 0 A, and it extremely quickly reaches the maximum value as a result of the quasi short circuit. Simultaneously, the amorphous layer is rapidly growing. Its growth terminates as sufficient layer thickness is reached. In the second stage of the processing, the transformation of the oxide layer occurs resulting in the formation of low conductive coating (Figure 1a, area II), and finally stabilizes (Figure 1a, area III). The same behavior was also observed by Ahounbar et al. [25].

The obtained waveform that was determined by the nature of the process is shown in Figure 1b. τoff refers to the period when the current is not supplied. At this period of time, titanium ions may release to electrolyte or may form a thin amorphous layer on the alloys' surface. When the voltage value is τon positive (anodic polarization), the current is supplied, and micro discharges appear over the dielectric breakdown potential. At this period of time, the MAO process is accompanied by a strong electric field and a high temperature and pressure [6]. Thus, several components of the electrolyte, the coating, or the substrate may ionize and decompose resulting in titanium cations release. This strong electric field also affects the diffusion of the oxygen anions towards the Ti substrate. Oxygen ions interact with Ti ions, producing titanium oxide amorphous structure that later transforms into the crystalline phase. Newly formed TiO2 coating is subjected to the compaction during subsequent dielectric breakdowns. When the value is τon negative (cathodic polarization), the dielectric breakdowns do not occur due to the high electrical conductivity of the created oxide coating [9].
