The electrolyte temperature versus process time is shown in Figure 2. The electrolyte temperature without cooling system increased gradually from 12.3˚C till boiling at 90.5˚C, when the processing time was about 1800 s, and then kept almost constant. The electrolyte temperature was around 30˚C in thermostatic control mode. At the beginning of the MAO process without cooling, there was no micro-arc on the surface of the sample due to the anodizing of the Al-alloy substrate at low pulse voltage (see the photo at 10 s in Figure 2). The discharges began to appear a light-white color (the image at 30 s in Figure 2) and then became to an orange-red color at the later stages (the images at 720 s and 1800 s in

Figure 2). The amount of the bubbles in the MAO process increased with the electrolyte temperature growing up and the average-size of the bubbles at the beginning stage and the later stage in boiling electrolyte seemed to be smaller than that in intermediate stage.

An interesting phenomenon is that the noise variation of the micro-arc discharge without electrolyte cooling was different compared with that in conventional micro-arc discharge in the electrolyte with constant temperature. Generally, the noise is getting stronger and stronger due the growing discharge voltage applied to the workpiece under the constant-current control mode of power supply. In this case, the noise gradually grew up before the electrolyte boiling, which was similar with traditional MAO process. However, the noise intensity became small when the electrolyte was boiling as shown in the image at 1800 s in Figure 3. The acoustic signals at different time were detected by the sound transducer and oscilloscope and depicted in time domain as shown in Figure 3(a). The amplitude of the acoustic signal had the maximum value at 18 min. In order to analyze the frequency characteristics of the MAO noise, Fast Fourier transform was used to transform the acoustic signals from time domain to frequency domain in Figure 3(b). The characteristic frequencies of the MAO noise were related to the frequency of the pulse voltage applied to the substrate (600 Hz in this case). At the beginning stage of 1-min treatment time, a very low peak at the characteristic frequency of 600 Hz appeared and its amplitude was lower than that of background noise mainly come from the fans in power supply. In intermediate stage at 12 min, there were several characteristic frequencies of 600 Hz, 1225 Hz, 1820 Hz, 2440 Hz and 3664 Hz at 12 min. The peak with the maximum amplitude occurred at 2440 Hz. When the electrode is boiling (30 min), the characteristic frequencies of discharge noise were almost the same as the intermediate state. But the peak with the maximum amplitude of characteristic frequency was located at 600 Hz, which was similar with the beginning state.

OES spectra recorded at different stages of MAO process without the electrolyte cooling are shown in Figure 4. The spectra were obtained with an integration time of 0.1 s. The presence of the continuous spectrum indicates that large amount of free electrons are generated during MAO processes. Albella et al. reported the electron avalanche to associate with the dielectric breakdown during anodization of valve metals and developed a model, in which the primary electronic current of the avalanche was attributed to the electrons released by the electrolyte species in the oxide [10]. Figure 4(a) shows that the most salient lines in spectrum belongs to sodium (Na I 588.99 & 589.59 nm) and potassium (K I 766.57 & 769.94 nm), which come from the electrolyte. In constant-current control mode of power supply, the applied voltage grew gradually to sustain breaking

through the oxide layer to the substrate as powerful arcs, which leads the OES signals of Na and K increased to a higher intensity level from 1 min to 20 min. After the first 20 min, the OES signals declined to a lower intensity level. Especially when the electrolyte was severe boiling after 30 min, the intensity of Na and K lines dropped to a very low level. There were some other species coming from electrolyte include hydrogen (H_α line at 656.27 & 656.28 nm and H_β 486.13 nm, respectively), OH radical (at 306.54 and 306.78 nm), oxygen (O I line at 777.19 nm), aluminum (Al I duplets 394.4 and 396.1 nm) and magnesium (Mg I line at 788.17 & 793.08 nm), as is shown in Figure 4(b). It was proposed that the high energy electron collision with H₂O, O₂ molecules will generate the H atoms, OH radicals, O atoms [11]. The signals of the Al I and Mg I lines comes from the A6N01 substrate and partly from the anions in the electrolyte, which is existed in the form of Al ( OH ) 4 − [12]. The intensity-decreasing of Al I and Mg I lines in the spectra indicates that less and less elements of Al and Mg were involved in the oxidation reaction, which was normally unfavorable to the growth of MAO barriers layer. The presence of the OES continuous spectra in the present was similar with traditional MAO process [13]. It seems that the electrolyte without cooling during MAO has little influence on the composition of the active plasma species. But the discharge at the beginning (e.g. at 1 min) is similar spectra with the discharge in boiling electrolyte (e.g. at 50 min) except for the differences in intensity, which means that the most salient lines in spectrum belongs to sodium.

The characteristics of the MAO process at a constant current density, which generally consisted of both ionic current and electronic current, were clearly controlled by the pulse voltage response in terms of the process time of 1800 s. Three successive discharge stages I, II and III, which differentiate by the variable trend of the pulse voltage versus process time, were recognized in both electrolyte-temperature control modes, as shown in Figure 5. Although the overall trends of the voltage vs. time curve in three stages are similar, the pulse voltage in the electrolyte without cooling was always higher than that in the thermostatic

electrolyte, and the discharge characteristics in the second and third stages were diverse depending on the control mode of the electrolyte temperature. In the first stage, the curves exhibit breakdown voltages of 305 V and 307 V with the almost fixed slopes of 19 V/s and 15 V/s in 16 s and 20 s of anodizing, respectively, this suggests that the control mode of the electrolyte temperature slightly modified the barrier film characteristics. The breakdown voltages are similar possibly because the breakdown voltage was mainly determined by the substrate alloy with similar electrolyte temperatures at a relatively low level, as shown in Figure 5.

In stage I of MAO without control of electrolyte temperature, compared to the conventional anodizing process, the substrate and metal oxide barriers more slowly dissolved, so the barrier layer has grown faster when the cations and anions transported through the initial alumina film, which was mostly controlled by the ionic current density, which was driven by the electric field [14]. Immediately after the dielectric breakdown occurred on the barrier layer, fine and short-lived sparks were observed, which moved and were homogeneously distributed on the anode surface. Then, the MAO process was considered in the micro-arc oxidation regime, which was simultaneously controlled by the ionic and electric currents, instead of the traditional anodizing regime like the first stage [15].

In the intermediate stage, the pulse voltage continued to successively increase but at a substantially slower rate than before with increasing gas evolution and micro-arc discharges, and the spark color changed from light-white to orange-red. With identical current densities, the consumed MAO electric power in the solution decreased because of the enhanced conductivity of the electrolyte with the increase in solution temperature. Therefore, the desired ionic current to maintain the growth of the coating is decreased because there are more active anions in the solution with increasing temperature. Although the applied voltage to the film increased to a certain extent, the electric field strength per unit volume film evidently decreased because the film rapidly thickened. As a result, the electric current to sustain breakdown and discharge relatively increased, which caused the more intensive motion and higher collision probability of the particles in the plasma. Then, the duration of the instable discharges increased, and the length of this stage obviously increased with the increasing electrolyte temperature.

In the final stage, the pulse voltage reached an almost stable plateau [15]. In MAO process with constant electrolyte temperature, the electric current density became the dominant constituent of the total current density, which implies that the films attained a constant resistance configuration, and no significant increase in anodic voltage was necessary to maintain the current value. With the increase in processing time, even stronger micro discharges and more intensive gas liberation were observed, but the number of sparks decreased, and the sparks became more widely spaced. This result is consistent with a previous study in the literature [16].

However, the discharge characteristics that we observed without electrolyte cooling were notably different from the above discussion. In the entire period of this stage, the electrolyte with a high temperature was boiling, and the solution was rolling around the sample because of the gaseous emission. The micro discharges reverted to being fine, and evenly distributed on the surface of the anodic sample, where the sound of the micro-arc discharges gradually fainted. Hence, this process may be closer to the first stage of the MAO process, which is an enhanced anodizing process because the composition of the electric current rapidly decreased so that the plasma activity was suppressed (see the characteristic frequency of acoustic signals in Figure 3(b) and OES spectra in Figure 4(a)). Accordingly, over the processing time in the boiling water, the ionic current was again a dominant component of the current density into the oxide conduction band, as in the first period [17]. The film maintained a rapid electrochemical formation with the cations and anions, which actively migrated through the initial alumina film because of the electronic field. That distinction is essential between two control modes of the electrolyte temperature in the final stage.

However, the micro-arc discharge process cannot be steady for a long term in the boiling electrolyte. The sample as the anode was isolated from the electrolyte when large amounts of gas sometimes formed a barrier to inhibit the discharge breakdown and interrupted the circuit, which resulted in the sudden increase in pulse voltage.

Altering the control mode of the electrolyte temperature changes the activity of the anions in the electrolyte and the duration of different stages in the MAO process. As a result, the micro-discharge characteristic is affected, which determines the thermal and chemical conditions during MAO and affects the morphology [18]. To achieve a deeper understanding of the effect of a high-temperature process on the coating growth, the MAO film morphology of samples treated in MAO bath with cooling system at 30˚C and without cooling system was compared.

The surface morphology of the MAO coatings fabricated for different treatments are shown in Figure 6(a) and Figure 6(b). Each surface SEM image inserted by the CLSM picture shows the topographies of the surface. The surface topographies comprised of hills and valleys (Figure 6). The shapes and dimensions of the micro pores, which were created by discharges, were notably affected by the MAO treatment temperature. Comparing the surface morphologies of the samples that were coated for different temperature in Figure 6(a) and Figure 6(b), note that when the MAO treatment was performed to 30 min, many pores turned into elongated oval pores because of the interconnection of the circle pores and the average size of the pores of 9.06 μm. The pores were discharge channels through which cations, anions and the molten materials transporting because of the high gas pressure and strong electric field. Furthermore, the surface

morphology of the coatings is characterized by a noticeable change with a 30 min treatment at 30˚C, as shown in Figure 6(b). Some pores were sealed, and the micro pore diameter decreased to 2.9 μm, which formed a flatter structure. Nevertheless, the observed surface of the alumina film had many crack defaults. The treatment in constant temperature mode had a notably long period with reasonably intensive discharges, which caused the increase in residual stress in the coating and ultimately the cracking of the coating. The surface topographies of the coating in Figure 6(b) show that the surface was much less rough than that in Figure 6(a).

The backscattered micrograph of the cross-sectional features in Figure 6(c) and Figure 6(d) show the structure of the MAO coatings. The resulting coatings contained porosities because oxygen was trapped in the molten oxides formed by localized discharges [19]. The coating in Figure 6(c) was produced for 30 min in the electrolyte without cooling. The coating typically has two layers: a considerable compact barrier layer adjacent to the substrate and a relatively porous layer with discharge channels. The higher-temperature electrolyte possibly penetrated deeper into the growing film during the treatment through the porosities, so that a thicker inner barrier film was formed than that formed in the solution with a constant temperature of 30˚C as shown in Figure 6(d).