Optical emission spectroscopy of plasma electrolytic ...

Surface & Coatings Technology 324 (2017) 18–25

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Optical emission spectroscopy of plasma electrolytic oxidation process on 7075 aluminum alloy Xuan Yang a,b, Lin Chen a,b, Yao Qu a,b, Run Liu c, Kejian Wei a,b, Wenbin Xue a,b,⁎ a b c

Key Laboratory for Beam Technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China Beijing Radiation Center, Beijing 100875, China Zhenjiang Watercraft College, Zhenjiang 212000, Jiangsu, China

a r t i c l e

i n f o

Article history: Received 15 February 2017 Revised 12 April 2017 Accepted in revised form 2 May 2017 Available online 3 May 2017 Keywords: Optical emission spectroscopy Plasma electrolytic oxidation 7075 aluminum alloy Electron temperature

a b s t r a c t Plasma electrolytic oxidation (PEO) on 7075 Al-Zn-Mg-Cu alloy was performed to produce the ceramic coatings in silicate electrolyte at constant voltage. The plasma electron temperature, electron density and atomic ionization degree in plasma zone were evaluated by analyzing the spectral lines of optical emission spectroscopy (OES), and the high spike peaks on plasma temperature profiles were emphatically discussed. The average electron temperature in plasma zone was about 3000 K–15,000 K, and the electron density was about 4.95 × 1021 m−3–1.65 × 1022 m−3, meanwhile the atomic ionization degree of Al was less than 10−3, while the temperature inside the alloy is below 120 °C. It was believed that the high spike peaks on plasma temperature profiles appeared in the later stage of PEO process resulted from the calculation deviation of plasma temperature from weak OES spectral line intensities. The generation of these spike peaks depended on the spark density and illumination intensity rather than the appearance of large discharge sparks, which was different from the previous viewpoint. © 2017 Published by Elsevier B.V.

1. Introduction Plasma electrolytic oxidation (PEO) is widely used to produce ceramic coatings on light metals and their alloys such as Aluminum [1– 3], Titanium [4] etc. to improve their hardness, wear resistance and corrosion resistance [5,6]. During PEO process, a great quantity of sparks moves rapidly on the surface of sample [7–10]. However, the evolution of sparks with treating time depends on applied power mode. Under a constant voltage mode, the sparks in the later stage of PEO gradually become weak and little without large sparks [11,12]. But under a constant current mode, the voltage will gradually increase with the increase of coating thickness, thus a few relatively large and long-lasting sparks can be still observed [13]. When the atom drops from one excited state to a lower level, the energy will be emitted in the form of light to generate atomic emission spectrometry. Hussein et al. [13] analyzed the optical emission spectroscopy (OES) during PEO process on 1100 pure aluminum to calculate the electron temperature in plasma zone and suggested a phenomenological model of plasma discharge. Further papers showed that the OES analysis was an effective method to understand the mechanism of ⁎ Corresponding author at: Key Laboratory for Beam Technology and Materials Modification of Ministry of Education, College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China. E-mail address: [email protected] (W. Xue).

http://dx.doi.org/10.1016/j.surfcoat.2017.05.005 0257-8972/© 2017 Published by Elsevier B.V.

discharge and coating growth during PEO process on Al, Mg, Ti and metal matrix composites [11–19]. Jovovic et al. [16] determined electron densities from the shape of the Hβ line of OES and the Stark broadening parameters of Al II lines on AA5754 aluminum alloy, and the electron temperature was determined from relative intensities of Mg I and O II lines using Boltzmann plot technique. Klapkiv et al. [17] calculated the ionization degree in plasma zone as 1.4 × 10−5. Liu et al. [11] evaluated the evolution of plasma parameters with oxidation time for PEO treatment of 2024 aluminum alloy on the basis of OES, and suggested a discharge breakdown model of PEO coating different from the model in Ref. 13. Hussein et al. [13] found that the electron temperature under a constant current mode on 1100 pure aluminum had some high spike peaks in the later stage of PEO process. It was considered that these spike peaks resulted from the large sparks rather than the discharge population density. On basis of OES measurement and plasma temperature calculation, they proposed three discharge types to analyze the breakdown mechanism of PEO coating. However, under a constant voltage mode, the sparks will become weak and less with the increase of oxidation time in the later stage of PEO process, and only some fine sparks without large sparks may be observed. In this case, the high spike peaks on plasma temperature profile should not appear in terms of Hussein's explanation in Ref. 13. Actually, we find that some high spike peaks of plasma temperature under a constant voltage mode for PEO on 7075 Al-Zn-Mg-Cu alloy can also appear even if these sparks are very weak.

X. Yang et al. / Surface & Coatings Technology 324 (2017) 18–25

So it is necessary to analyze plasma discharge behaviors on basis of OES and discuss the formation mechanism of high spike peaks again. In this work, the optical emission spectra during the PEO process of 7075 Al-Zn-Mg-Cu alloy under a constant voltage mode were recorded, and the electron temperature, electron density and atomic ionization degree in plasma discharge zone with oxidation time were calculated on basis of OES. The microstructure and compositions of PEO coatings were analyzed, and the dissolution of alloying elements in 7075 alloy into aqueous solution during PEO process was determined. The discharge behaviors, high spike peaks of electron temperature profiles and formation process of PEO coatings were discussed. 2. Experimental procedure 2.1. Materials and PEO process Disc samples of 7075 aluminum alloy (wt%, Zn 5.1–6.1, Mg 2.1–2.9, Cu 1.2–2.0, Fe 0.5, Si 0.4, Mn 0.3, Cr 0.18–0.28, Ti 0.20 and Al balance) with dimensions of Φ30 × 10 mm were used as anode material, and the stainless steel bath as cathode. Each disc was ground with SiC abrasive papers, and dried with hot air after cleaning in water. The PEO process was carried out using a bipolar pulsed power supply with a frequency of 75 Hz under constant voltages of + 500 V/− 120 V. The aqueous solution contained 1 g l−1 KOH and 6 g l−1 Na2SiO3. The temperature of the electrolyte solution was controlled using a water cooling system. To measure the temperature inside the sample, a thermocouple (No.1) was inserted into the aluminum alloy anode at a distance of 0.2 mm away from the surface. Meanwhile, the other two thermocouples (No.2 and 3) were inserted into the solution to measure its temperature at 5 mm and 150 mm away from sample, respectively as shown in Fig. 1. 2.2. Optical emission spectroscopy (OES) Fig. 1 displays a schematic diagram of PEO process for measurements of OES and temperature. An optical emission spectrometer (AvaSpec3648) with a resolution of about 0.08 nm was used to collect the spectra over a period of 60 min with the scanning time of 200 ms–2000 ms. This spectrometer has two channel slots with the spectral wavelength region of 200 nm–750 nm. This spectrometer detector consists of a 3648 pixel CCD-array. The plasma emission light is transmitted and focused through a quartz window, and collected by an optical fiber with less than 1 mm in diameter placed at 2 cm away from the disc surface. In addition, the illumination during the PEO process was recorded by a digital 1336A light meter, which replaced the optical emission spectrometer. The collected spectra were analyzed by Plasus Specline 2.1 and NIST database. When the plasma is in the state of partial local thermodynamic

Fig. 1. Schematic diagram of PEO process for measuring optical emission spectra and temperature.

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equilibrium (PLTE), its electron temperature can be calculated using relative spectral line intensities of the same atomic or ionic species [11,13]. In the local discharge zone, the electron temperature is approximately equal to the ion temperature, then the electron density Ne and plasma atomic ionization degree can be calculated [11,13,20]. In order to determine the electron density in this work, the Stark broadening of the Hβ line was used, for it was wide enough to record without perturbing the plasma. The background line of the Hβ line was eliminated to calculate its FWHM. 2.3. Analyzing and testing The cross-sectional microstructure and compositions of the PEO coatings were analyzed by scanning electron microscope (SEM, Hitachi S-4800) with energy dispersive spectroscopy (EDS), and their phase constituents were analyzed by X-ray diffraction (XRD, X’ PERT PRO MPD). The concentrations of the Al, Zn, Mg, Cu elements dissolved from 7075 alloy substrate into the electrolyte solution during PEO treatment were measured by inductively coupled plasma atomic emission spectrometer (ICP-AES, SPECTRO ARCOS EOP). The silicate electrolyte before the PEO process was taken as a blank solution to calibrate their concentrations. The solution was taken out at a fixed position of bath during the process. 3. Results and discussion 3.1. Optical emission characterization The illumination intensity and video images with oxidation time during PEO process are shown in Fig. 2. There are numerous white sparks on the surface of 7075 alloy in the initial stage of process (see photo a). That leads to a remarkable rise of illumination intensity, reaching 31 lx within 2 min. While the thickness of oxidation film increases, the white sparks gradually transform into large yellow sparks (see photo b and c). However, the reduction of the total amount of sparks results in the rapid decline of illumination intensity below 3 lx. In this stage, the current density will quickly decrease. After 20 min, these large yellow sparks disappear and many fine white sparks cover on the surface (see photo e), then these fine sparks gradually become weak and less and eventually disappear (see photo f). Meanwhile, it is found that the illumination intensity still remains stable at about 3 lx. Fig. 3 displays a typical optical emission spectrum obtained at 110 s oxidation in the wavelength scope of 200 nm–750 nm with the scanning time of 1000 ms during PEO process. The notation I and II refer to neutral and singly ionized atoms, respectively. It is found that the OES

Fig. 2. Dependence of illumination intensity and video images on oxidation time during PEO process.

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Fig. 3. Typical optical emission spectrum obtained at 110 s oxidation during PEO process at constant voltages of +500 V/−120 V.

from PEO plasma discharge contains aluminum, zinc, magnesium, and copper from the 7075 alloy substrate, and sodium, potassium, silicon, OH, hydrogen α and β Balmer lines, and oxygen from the electrolyte. The upward trend from 450 to 700 nm in Fig. 3 results from collision radiative recombination [21,22] and bremsstrahlung radiation of electrons [17,20]. The intensity variation of Al emission lines at 309.27 nm and 394.40 nm with oxidation time during the PEO process is presented in Fig. 4. The intensity of the two Al lines dramatically increases to a maximum value within 75 s, then the Al emission line at 309.27 nm gradually drops to a lower intensity at about 40 min while the Al line at 394.40 nm decreases quickly within about 1200 s. This low intensity implies that the large sparks disappear. 3.1.1. Plasma electron temperature The intensity ratio of Al I lines at 309.27 and 394.40 nm is chosen to calculate the plasma electron temperature (Te). The corresponding parameters are displayed in Table 1. Fig. 5 displays the variation of plasma electron temperature (Te) with oxidation time at different scanning time (200, 1000, and 2000 ms), and their fitting lines are also given. These Te curves were calculated using the same method. The Te in spark discharge zone is about 3000 K– 15,000 K. The three Te curves display a similar variation trend. In the initial stage of PEO process, they keep stable without obvious fluctuation. After this stage, the high spike peaks on the Te profiles appear. The Te shows an upward trend with a strong fluctuation before it gradually drops. It is found that the earlier stable stage of Te profiles prolongs from the 900 s at 200 ms scanning time to the 1500 s at 2000 ms scanning time. Furthermore, the large spike peaks become fewer and the amount of small spike peaks also decrease with increasing the scanning time, and the spike peaks at 2000 ms scanning time are apparently less than that at 200 ms and 1000 ms scanning time. The plasma discharge behaviors during the PEO process can be well described by the electron temperature. In the earlier stable stage of Te, the numerous fine white sparks appear on the surface of 7075 Al disc within 2 min as shown in Fig. 2, and Te is about 3000 K. After the oxide film increases to a certain thickness, the applied voltage reaches its breakdown value, which leads to the decrease of fine white sparks and the increase of large yellow sparks. The Te rises and fluctuates considerably after these large sparks gradually disappear. Then some fine sparks cover the surface of disc, and the average Te is up to about 10,000 K at 40 min for 1000 ms scanning time. After that, the average Te gradually decreases until the end of PEO process. 3.1.2. Plasma electron density and atomic ionization degree As one of the important parameters, the electron density (Ne) is used to characterize plasma discharge. Fig. 6 shows that the Ne is in a range of 4.95 × 1021 m−3 to 1.65 × 1022 m−3, which slightly decreases with

Fig. 4. The intensity variation of Al emission lines with oxidation time. (a) Al I 309.27 nm, (b) Al I 394.40 nm.

oxidation time. This result is in good agreement with the Ne obtained by Ref. 13 and 18. Klapkiv [19] suggests that the partial LTE condition in plasma zone can reach while the Ne is over 4 × 1021 m−3. The Ne in Fig. 6 is higher than 4 × 1021 m−3, thus the plasma parameters in this work can be calculated under the assumption of a partial local thermal equilibrium (LTE) condition. Fig. 7 displays the primary ionization degree of aluminum during the PEO process. It is found that the Al atomic ionization degree keeps steady at about 3.5 × 10−9 at the initial 10 min, and increases gradually to about 7.5 × 10−4 at 15 min. The atomic ionization degree is not given after 15 min in Fig. 7 due to the limit of our optical emission spectrometer. With the appearance of large sparks at 10 min, it is easier to excite the electrons up to high electron temperature, which leads to the increase of atomic ionization degree. 3.2. Characterization of morphology, microstructure and phase constituents of PEO coatings The surface morphology of the PEO coatings on 7075 aluminum alloy with 20 min, 40 min and 60 min oxidation time is showed in

Table 1 Observed spectral lines with the wavelength, transition, statistical weights of the upper state gk, respectively, energy difference and the transition probabilities Aki [21]. Line Al I Al I Hβ

λ (nm) 309.27 394.4 486.13

Transition 2

2

2

2

3s 3d D → 3s 3p p 3s24s 2S → 3s23p 2p 4d 2D → 2p 2P

gk

Energy (eV)

Aki (108S−1)

6 2 4

4.02 3.14 2.55

0.74 0.493 0.172


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Fig. 6. Dependence of the plasma electron density on oxidation time.

substrate participate in the coating formation although some Al, Mg, Cu, Zn elements are dissolved into solution during PEO process. As shown in Fig. 9(a) and (b), the coating at 20 min is 20 μm thick, and its loose outer layer with 13 μm is thicker than the compact inner layer with 7 μm. Then the compact layer of 40 min sample quickly grows to 18 μm, while the thickness of the loose layer only slightly increases about 4 μm (see Fig. 9(c) and (d)). As shown in Fig. 9(e) and (f), the coating at 60 min has 58 μm thick, but its compact layer can reach 35 μm. Generally, in the initial stage of PEO process, the loose layer of coating grows quickly. Then the compact layer grows faster than the loose layer. Finally, the compact layer has a high ratio of total coating thickness. It is believed that the location of the initial substrate surface is approximately close to the interface between compact layer and loose layer [23,24]. The PEO coating grows outward to form the loose outer layer and grows inward to form the compact inner layer. Fig. 10 shows that the PEO coatings on 7075 alloy at different oxidation time consist of γ-Al2O3, α-Al2O3 and mullite (3Al2O3·2SiO2) phases, but the content of γ-Al2O3 phase is much higher than that of other two phases. By increasing the oxidation time, the relative intensity of diffraction peaks of γ-Al2O3 phase significantly increases while the diffraction peak of 3Al2O3·2SiO2 phase becomes weak. It is also observed that the diffraction peaks of 3Al2O3·2SiO2 for the coating at 60 min treatment are very weak, however, as shown in Fig. 10, they can be obviously detected again after the outer layer of coating is polished off. As shown in Fig.8, a lot of sediments are deposited on the surface of coating at 60 min. They might contain amorphous SiO2 Fig. 5. Plasma electron temperature as a function of oxidation time at different scanning time. (a) 200 ms, (b) 1000 ms, (c) 2000 ms.

Fig. 8. There are some large discharge channels on the surface of coating at 20 min. Then these large channels reduce, meanwhile many large grains are observed on the coating at 40 min. For the coating at 60 min, the large discharge channels are hardly left, but a lot of sediments are deposited on its surface. Fig. 9 displays the cross-sectional micrographs of PEO coatings with composition analyses on 7075 aluminum alloy at different oxidation time. The thicknesses of three coatings with 20 min, 40 min and 60 min treating time reach 20 ± 2 μm, 35 ± 2 μm, and 58 ± 3 μm, respectively. Furthermore, the oxide coatings contain a loose outer layer and a compact inner layer. The Si element from electrolyte obviously enriches in the loose outer layer, meanwhile the Al content in the compact layer is higher than that in the loose layer. The Mg, Cu, and Zn contents in the coatings are lower than that in the 7075 aluminum alloy substrate. It implies that the alloying elements from 7075 aluminum alloy

Fig. 7. Dependence of Al atomic ionization degree on oxidation time.

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concentrations of the dissolved Al, Zn, Cu, and Mg in the aqueous solution determined by ICP-AES. In the initial stage of PEO process, the dissolution speed of alloying elements from 7075 alloy into solution is noticeably fast. After 20 min, the concentrations of Al, Zn, Cu, and Mg begin to rise slowly. As described above, the sparks in the later stage of PEO process gradually become weak and the large sparks disappear, then the increase of coating thickness is ascribed to the growth of the compact layer. Hence, the dissolution rate of alloying elements from 7075 alloy into solution declines while the oxide coating grows thicker. On the other hand, as shown in Fig. 11, the Cu concentration in solution is higher than the Mg concentration, but the Cu content in 7075 alloy is lower than the Mg content. It means that the Cu dissolution rate into solution from 7075 substrate is faster than the Mg element during PEO treatment. 3.4. Temperature of alloy substrate and solution

Fig. 8. Surface morphology of PEO coatings on 7075 Al alloy at different oxidation time. (a) 20 min, (b) 40 min, (c) 60 min.

phase, which suppresses the appearance of diffraction peaks of 3Al2O3·2SiO2 phase in the inner layer of coating. However, it is identified that all coatings at 20 min, 40 min, 60 min contain the 3Al2O3·2SiO2 phase. In addition, the appearance of 3Al2O3·2SiO2 phase also implies that the silicon element from electrolyte involves in the formation of oxide coatings, which is in accordance with the composition profiles in Fig. 9. 3.3. Concentrations of the dissolved alloying elements During PEO process, the alloying elements of 7075 Al-Zn-Mg-Cu alloy are oxidized to fabricate the oxide coatings, meanwhile some alloying elements will also dissolve into solution. Fig. 11 reveals the

Fig. 12 displays the dependence of temperature inside the 7075 aluminum alloy and solution temperature at different distance away from the Al sample on oxidation time. The solution temperature at 150 mm away from sample keeps stable below 27 °C. However, the solution temperature at 5 mm away from sample gradually increases, reaching a maximum value of 38 °C at 2100 s. Then the solution temperature begins to decrease slowly until the end of PEO process. In fact, the solution temperature is related to the evolution of sparks. When the quantity of large sparks decreases, the heat produced by plasma discharge reduces, which leads to the decrease of solution temperature around the sample. On the other hand, the temperature inside alloy sample close to the surface about 0.2 mm increases dramatically to 63 °C in the initial 55 s of PEO process, and then quickly drops to 51 °C within 125 s. The temperature grows slightly from 51 °C at 180 s to 60 °C at 1200 s, then it shows a great increase after 1200 s and reaches 103 °C at 2100 s. After that, the rise of temperature slows down again, and the final temperature is only 113 °C at 60 min. While the PEO process stops, the temperature inside the sample rapidly reduces to solution temperature within 100 s. The variation of temperature inside alloy sample with oxidation time corresponds to the video images as shown in Fig. 2. In the initial 55 s, the passive film on the alloy is very thin, so the current density under high applied voltage is high enough to make much heat, which results in a suddenly rise of sample's temperature up to 63 °C. The breakdown of thin oxide film results in a large amount of sparks on the surface of 7075 alloy. The oxide film grows quickly to cause the decrease of current density and temperature inside sample. After 180 s, some large sparks appear and the loose layer grows. In this stage, the sample's temperature increases slowly. After 1200 s, the large sparks vanish and many fine sparks cover on the sample, meanwhile the compact layer grows fast. Because the alumina has a low thermal conductivity, a thick alumina coating will hinder the heat loss of alloy sample into solution, therefore the temperature of sample quickly increases to 103 °C at 2100 s. In the final stage, only a few fine sparks are left, which generate a little heat. Furthermore, a thick coating over 40 μm significantly suppresses the heat loss inside alloy sample. So the temperature inside sample gradually increases to 113 °C at the end of PEO process. 4. Discussion As shown in Fig. 2, the PEO process accompanies the evolution of sparks. In fact, the plasma parameters and coating growth during the PEO process depend on the size and density of sparks. In the first stage of 20 min, the spark density and the illuminance are rather high, furthermore, these white sparks gradually transform to some large sparks. In this stage, it has the strong optic emission lines and quick dissolution rate of alloying elements into solution. The coating mainly grows outwards, and the loose layer of oxide coating grows faster than the compact layer, meanwhile, the temperature inside sample maintains at 50 °C–60 °C. After 20 min, the illuminance is very low


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Fig. 9. Cross-sectional micrographs and composition profiles of PEO coatings on 7075 Al alloy at different oxidation time. I: loose layer; II: compact layer; III: aluminum substrate. (a) (b) 20 min, (c) (d) 40 min, (e) (f) 60 min.

Fig. 10. XRD patterns of the PEO coatings with different oxidation time.

Fig. 11. Dependence of concentration of the dissolved alloying elements in solution on oxidation time.

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appear in the later stage of PEO process under constant current mode even if a few large sparks are left at end of oxidation. On the other hand, when the discharge on 2024 Al-Cu-Mg alloy under constant voltage is strong, the large spike peaks do not appear obviously [11]. In general, the spike peaks in Te profiles might appear under both constant voltage and constant current modes as long as the spark density and illumination intensity are rather low. 5. Conclusions

Fig. 12. Dependence of temperature on oxidation time. (a) inside 7075 Al alloy sample, (b) temperature of solution at 5 mm away from sample, and (c) temperature of solution at 150 mm away from sample.

and the fine sparks gradually become less and weak, thus the dissolution rate of alloying elements slows down. The increase of coating thickness is ascribed to the growth of compact layer. The growth process of loose and compact coatings is in good agreement with results in Ref. 23 and 24. On the other hand, the weak sparks lead to the decrease of large discharge channels and heat, but the thick alumina coating can suppress the heat loss from the alloy into solution. So the temperature inside sample rises quickly from 20 min to 40 min and then increases slowly to 113 °C at end of PEO process as shown in Fig. 12. Hussein et al. [13] has reported there are many spike peaks in Te profiles in the later stage under constant current. The voltage will gradually increase with the increase of coating thickness under constant current, so some large sparks appear in the later stage. They suggested that these spike peaks resulted from the large sparks rather than the quantity of sparks. However, it is difficult to understand our results using their explanation. As shown in Fig. 2, the large sparks vanish and only some weak sparks cover on the sample under constant voltage after 20 min, but many spike peaks also appear in Te profiles (see Fig. 5). Therefore, the Hussein's viewpoint in Ref. 13 cannot explain our results. The intensity ratio of Al emission lines at 309.27 nm and 394.40 nm is used to calculate the Te, but their intensities are low in the later stage of PEO process as shown in Fig. 4. So the deviation of their intensity ratio will increase obviously, which will result in the spike peaks of Te profiles. The average lifetime of a single spark is about 0.14 ms [25]. When the scanning time of OES prolongs from 200 ms to 2000 ms, more sparks can be recorded, so the intensities of two Al emission lines will enhance, which decreases the deviation of their intensity ratio. Then the quantity of spike peaks can be reduced, in the meantime the time of spike generation will be also prolonged. This explanation is in accordance with the results in Fig. 5. Hence, the spike peaks in Te profiles is not related to the generation of large sparks, but related to the spark density and illumination intensity. It is believed that the spike peaks in Te profiles might appear when the spark density and illumination intensity decrease to a certain extent. The diameter of optical fiber for OES measurement is usually less than 1 mm, thus the OES spectra will collect light from a very small area. The scanning time in Ref. 13 is less than 4 ms, thus the weak emission line intensity is easy to fluctuate, which might result in the spike peaks in Te profiles. Under constant current mode, the voltage gradually increases with the coating growth and some large sparks will appear in the later stage of PEO process. However, in this stage, the thick oxide coating is difficult to break down, thus the density of sparks reduces significantly, meanwhile, the illumination intensity and OES emission line intensity decrease greatly as well. Hence, some high spike peaks may

(1) The plasma parameters in spark discharge zone during PEO process on 7075 alloy at constant voltage are evaluated on basis of OES. The electron temperature in plasma discharge zone is about 3000 K–15,000 K, and electron density is about 4.95 × 1021 m−3 to 1.65 × 1022 m− 3. The atomic ionization degree of Al is about 3.5 × 10−9–7.5 × 10−4, while the temperature inside the alloy is less than 120 °C. (2) At the initial stage of PEO process with large sparks and spark density, the dissolution rate of Al, Zn, Cu, and Mg elements from 7075 alloy into solution is fast. Then the sparks become weak and less, and their concentrations in solution increase slowly. In the later stage, the compact layer of coatings grows quickly while the thickness of the loose outer layer keeps stable. (3) The high spike peaks on plasma temperature profiles appeared in the later stage of PEO process result from the calculation deviation of plasma temperature from weak OES spectral line intensities. The appearance of these spike peaks depends on the spark density and illumination intensity rather than the large discharge sparks.

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