- Email: [email protected]
Influence of electrolyte components on the microstructure and growth mechanism of plasma electrolytic oxidation coatings on 1060 aluminum alloy
Journal Pre-proof Influence of electrolyte components on the microstructure and growth mechanism of plasma electrolytic oxidation coatings on 1060 aluminum alloy
Shuaixing Wang, Xiaohui Liu, Xiaole Yin, Nan Du PII:
S0257-8972(19)31204-6
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125214
Reference:
SCT 125214
To appear in:
Surface & Coatings Technology
Received date:
11 September 2019
Revised date:
7 November 2019
Accepted date:
28 November 2019
Please cite this article as: S. Wang, X. Liu, X. Yin, et al., Influence of electrolyte components on the microstructure and growth mechanism of plasma electrolytic oxidation coatings on 1060 aluminum alloy, Surface & Coatings Technology (2019), https://doi.org/ 10.1016/j.surfcoat.2019.125214
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier.
Journal Pre-proof
Influence of electrolyte components on the microstructure and growth mechanism of plasma electrolytic oxidation coatings on 1060 aluminum alloy Shuaixing Wang*, Xiaohui Liu, Xiaole Yin, Nan Du National Defense Key Discipline Laboratory of Light Alloy Processing Science and Technology, Nanchang Hangkong University, Nanchang 330063, P. R. China
of
*Corresponding author. E-mail address: [email protected] (S. X. Wang)
Abstract: Electrolyte systems have a significant impact on plasma electrolytic oxidation (PEO). In this
ro
work, the surface, aluminum/coating (A/C) interface and fracture cross-section microstructure and element
-p
distribution of PEO coatings prepared in silicate, phosphate and mixed electrolyte were characterized in detail by FESEM, EDS, XRD and XPS. The growth mechanisms of PEO coatings in different electrolytes
re
were also discussed. Results demonstrated that the deposition of silicate dominated the growth of coating in
lP
silicate system, many loose nodules were present on the coating surface. Si element occupied almost the entire coating and was enriched at the nodules and cavity edges. In phosphate system, the growth of coating
na
was primarily dependent on the oxidation of aluminum. There are a large number of “hill”-like protrusions at A/C interface. The main component of this coating was a-Al2O3, and a small amount of P was distributed
Jo ur
at A/C interface. In the mixed electrolyte, the film-formation process was mainly the matrix oxidation, and supplemented by deposition of electrolytes. Meanwhile, the growth models of PEO coating in different electrolyte were established.
Keywords: aluminum alloy; plasma electrolytic oxidation; electrolyte components; microstructure; growth mechanism
1. Introduction Plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), is an environment-friendly surface-modification technique to prepare ceramic coatings on the valve metals (Al, Mg, Ti, etc) in-situ by using the effects of electrochemistry, thermo-chemistry and plasma chemistry [1-3].
Journal Pre-proof This ceramic coating is usually composed of matrix oxides and the electrolyte species, which can meet the needs of corrosion resistance, wear resistance and electrical insulation. The researches show that the surface hardness of 2024 aluminum alloy treated by PEO can reach to 1500 HV, the wear resistance can be increased by 3~5 times compared with the matrix [4, 5], and the corrosion current density of the coated specimen is about 4000 order lower than uncoated one [5]. However, some studies confirmed that the structure, composition and properties of PEO coatings are closely related to the electrolyte components [6, 7]. For aluminum alloys, PEO can be carried out in an acidic or alkaline electrolyte, but it is less used due to
of
the greater contamination of the acidic electrolyte. Many literatures have found that the alkaline species,
ro
such as SiO32-, PO43-, VO43- or MoO42-, are the main substance for the formation of PEO coatings [6-8]. During PEO process, these species are easily adsorbed on the surface of the initial oxide layer to form
-p
discharge points of foreign particles, and hence generated a plasma discharge [8, 9]. In addition, some
re
literature believes that in the alkaline system, the metal ions generated by anodic reaction and some other cations in the solution can be easily transformed into negatively charged particles and re-enter the coating
lP
[10], which can adjust and change the microstructure of coating. Therefore, the PEO electrolyte of
9-11].
na
aluminum alloy mostly uses the weak alkaline system composed of NaOH, Na2SiO3, Na3PO4 or NaAlO2 [7,
Jo ur
However, in the sodium hydroxide system, the electrolyte concentration is the key determining factor whether the micro-arc discharge can proceed. The high concentration of sodium hydroxide is not conducive to the formation of a high-resistance film on the sample surface, and the plasma discharge is difficult to occur [11]. During the same oxidation time, the growth rate of PEO coating in the silicate system is faster, but the coating surface is very rough [11, 12]; the coating prepared in the phosphate electrolyte is dense, but the wear resistance is poor due to the insufficient thickness [11, 13]. Aluminate system is beneficial to the formation of -Al2O3 in the coating, but the stability of electrolyte is poor [14, 15]. Therefore, the composite electrolyte is often used for PEO treatment to prepare a coating with good comprehensive performance. Currently, many researches about PEO electrolyte are also focused on the effects of soluble additive [14, 15] or the second phase particles on the structure and properties of PEO coating [17-19]; while the little research involves the three-dimensional structure, composition and growth mechanism of the coating under different electrolytes. Some researches have found that the size of outward growth for PEO coating in the silicate electrolyte is
Journal Pre-proof larger than that in the phosphate electrolyte [20]; and the distribution of Si and P elements in the coating is also different [21]. Silicon-rich material is deposited on the surface of alumina-based coating, but phosphorus species are primarily found in a region next to the metal [21, 22]. There are different interpretations between scholars for this phenomenon [21, 23]. Some scholars believe that discharge channels are the main ways to form the new coating. The discharge channel in silicate electrolyte is dumbbell-shaped, while that in phosphate electrolyte is trumpet-shaped, which lead to the above differences [21]. Another viewpoint is that the growth process within the discharge region result in the separation of the silicate- and phosphate-derived species [23]. In order to further understand the growth
of
mechanism of coating in different electrolytes, especially the local physical process, it is necessary to
ro
characterize the fine microstructure of coating formed in different electrolytes.
In this paper, the free-standing coatings were obtained using an electrochemical dissolution method [13,
-p
24]. A large number of nice pictures for coating structure (including the surface, aluminum/coating
re
interface and fracture cross-section) and detailed composition analysis data are given based on FESEM, EDS, XRD and XPS. Meanwhile, the growth mechanisms of PEO coatings in different electrolytes were
lP
fully discussed by combining existing theories. It is expected to provide a basis for the regulation of
Jo ur
2. Experimental
na
microstructure and performance of PEO coating.
2.1 Materials and PEO Treatment
The 1060 aluminum alloy (99.6% purity) was used as the substrate for preparing the PEO coating, and the aluminum plate was cut into samples with a the dimension of 20 × 20 × 2 mm. In preparation for PEO process, all samples were ground with SiC abrasive (from 320# to 1000#) and cleaned ultrasonically with alcohol for 5min. Then, all samples were cleaned with deionized water and air dried. An asymmetrical pulsed power supply (WHD–20, Harbin Institute of Technology, Harbin, China) was employed for the PEO process. The electrolytes are selected from silicate system, phosphate system and a mixed electrolyte system, respectively. The specific composition of electrolytes and the sample number are shown in Table 1. The PEO of all aluminum samples was carried out under a constant current density of 15 A/dm2, a frequency of 500 Hz, a duty ratio of 60% and the oxidation time of 60 min. During the PEO treatment, stainless steel tank was used as the cathode, a circulating water-cooling system was used to
Journal Pre-proof maintain the electrolyte temperature below 40 °C. After the PEO treatment, the coated samples were cleaned with deionized water and dried at ambient temperature. Table 1 Sample codes and electrolyte composition of different PEO coatings on aluminum. Sample codes Si-coating P-coating Si-P-coating
Electrolyte composition Na2SiO3·9H2O (g/L)
(NaPO3)6 (g/L)
NaOH (g/L)
20 0 10
0 20 10
0 2 1
of
2.2 Growth regularity measurement of PEO coatings During PEO process, the oxidation voltage was collected by the software that comes with the power
ro
supply, and the sampling interval is 1 s. The discharge sparks in the PEO process was also observed in real
-p
time through a long working distance microscope (QUESTAR QM-100) equipped with a CMOS image collector (MDX-4T). The acquisition rate set at 15 frames per second. Besides, the growth regularity of
re
PEO coating was obtained by measuring the change in sample dimension in the process of PEO [25], as shown in Fig.1. Here, in order to avoid dimensional errors caused by double-sided oxidation; the sample
lP
was oxidized on one side, and the non-oxidized side was covered by a sealant. h0 and h1 represented the sample thickness berfore and after PEO respectively, which can be measured by a digimatic micrometer
na
(INSIZE 6353-25C, unit. ± 2μm). h was the thickness of PEO coating, which was obtained from average of three measurements on each sample using a thickness gauge (Elcometer 345). The difference between h1 and
Jo ur
h0, that was a, can be defined as the outward growth thickness of the coating. Besides, the part growing towards the Al substrate can be calculated by h-a. The linear surface profiles and the roughness parameters of PEO coatings were measured using a SJ-310 surface profile meter. Fig. 1
2.3 Microstructure characterization of PEO coatings In order to accurately characterize the fine structure of PEO coating, a free-standing coating was obtained by removing the aluminum substrate using an electrochemical dissolution method. The specific operation method is described in the literature [13, 24]. Then, the surface morphology, aluminum/coating (A/C) interface morphology and the fracture cross-sectional morphology of PEO coatings formed in different electrolytes were analyzed by a field emission gun SEM (FE-SEM, Nova Nano-SEM 450). The element distribution on the surface and cross-section of coating was examined by energy dispersive spectroscopy (EDS, INCA 250) equipped on the FE-SEM system. Besides, X-ray photoelectron
Journal Pre-proof spectroscopy (XPS, Axis Ultra DLD) equipped with a standard Al Ka X-ray source (1486.6 eV) was used to determine the chemical composition at the coating surface. Phase composition of the coatings was investigated by X-ray diffraction (XRD, Bruker D8-Advance) scanning from 10°to 80°. Before XRD, the free-standing coatings are ground into powder samples to perform test.
3. Results and disscusion 3.1 Influence of electrolyte on the growth process of PEO coating Fig. 2a shows the voltage-time curve of 1060 aluminum alloy during PEO in different electrolytes. It can
of
be seen that the oxidation voltage exhibits a similar change trend under different electrolytes, except that
ro
the breakdown voltage and the termination voltage are different. In this work, the PEO process is observed in real time through a long working distance microscope, and the point at which the first spark discharge
-p
occurs is defined as the breakdown voltage. It was found that the plasma discharge appeared earlier in the
re
silicate electrolyte, the breakdown voltage was about 240 V, and termination voltage was 476 V after 60 minutes of PEO, as shown in Fig. 2(a). In the phosphate electrolyte, breakdown discharges occurred at
lP
about 300 V. The termination voltage after PEO of 60 min was about 10% higher than that in silicate system. In the mixed electrolyte of silicate and phosphate, the breakdown voltage and termination voltage
na
are 280 V and 503 V, respectively.
Jo ur
Fig. 2b gives a variation curve of coating thickness in different electrolytes. It can be seen that the coating thickness approximately increased linearly during PEO of 60 min, regardless of the electrolyte system. However, the growth rate of coating in silicate electrolyte is the fastest in the same time. The average growth rate of coating in this system is about 0.51 μm/min, but the growth rate of coating in the phosphate and mixed electrolyte is only 0.45μm/min and 0.34 μm/min, respectively. Most studies believed that PEO coatings are formed by polarizing the metal to the dielectric breakdown voltage [2, 23, 26]. The oxidation was similar to the conventional anodizing process before the breakdown discharge occurred. At this stage, a very thin alumina layer was produced at the interface, the working voltage was also rapidly increased. When the primary oxide layer was punctured, a turning point appeared on the voltage curve, and so a large number of small and short-lived plasma sparks were formed and moved rapidly on the electrode surface. Herein, the coating grew almost linearly under the cycle action of “discharge – melt – quench” due to the high temperature and high pressure (~ 7000K and 102 MPa) [26-28]. The micro-pores were also left on the sample surface because of the breakdown discharge. The oxidation
Journal Pre-proof time further prolonged, the voltage increased slowly, the electrical breakdown became more difficult, and the local large and long-lasting plasma arcs caused the coating to grow slowly around the discharge channel. According to this analysis, it was known that a lower breakdown voltage in a silicate system means that more discharge energy was used for the coating formation, and so the coating grew faster. A higher termination voltage in the phosphate system indicated that the density of coating might be higher, which could be also demonstrated by the coating morphology (Fig. 4b, 8b). In the mixed electrolyte, the high breakdown voltage and dense structure may result in the lower growth rate of the coating.
of
Fig. 2
ro
3.2 Influence of electrolyte on the surface structure of PEO coating
-p
Fig. 3 shows the linear surface profiles of various PEO coatings prepared in different electrolytes. It can
re
be seen that the surface of all the coatings were in the uneven state. Among them, the surface profile of the coating prepared in silicate electrolyte fluctuated the most, followed by the phosphate electrolyte, and the
lP
smallest in the mixed electrolyte. The surface roughness of PEO coating was evaluated using the parameters Ra, Rt and Rz, and the results were shown in Table 2. Where, Ra was the average roughness, Rt
na
and Rz was the the maximum peak and average peak to valley height, respectively. It can be seen that the roughness of PEO coating prepared in silicate electrolyte was the largest, and Ra value was up to 2.512 μm;
Jo ur
while the surface of the coating prepared in the phosphate electrolyte and the mixed electrolyte were relatively smooth and the roughness were low. Fig. 3
Fig. 4 shows the surface morphology and element distribution of PEO coating prepared in different electrolytes. It can be seen that all the coating surface was mainly occupied by the pancake structure, and each of pancakes had a tiny pore in the center, which was closely related to the plasma discharge channel and gas release [13, 27-28]. However, the electrolyte system had a great influence on the surface morphology. As shown in Fig. 4a, in addition to the pancakes, the surface of PEO coating prepared in the silicate electrolyte also had a large number of loose nodular protrusions, and the nodules are always discontinuously distributed around the pancakes. As shown in Fig. 4b, pancakes with the central closed pore were the main feature of PEO coating prepared in phosphate electrolyte, yet some pores were also
Journal Pre-proof distributed around the pancakes. For the coating formed in the mixed electrolyte, see Fig. 4c, pancakes were still the dominant feature, but the pores on the coating surface were significantly less than those in the phosphate system. Besides, a few granular nodules were present around the pancakes, but whose shape and size were much smaller than that formed in the silicate electrolyte. Fig. 4 The coating surface image (1000 ) was processed by “Image J” processing software, and the surface porosity of PEO coating was calculated by the ratio of the area occupied by the pores to the total image
of
area [29]. The results are shown in Table 2. It can be seen that the surface of PEO coating prepared in the
ro
silicate electrolyte was porous and had a porosity of up to 18.2%; while the surface of the coating prepared in phosphate and mixed electrolyte was relatively dense, and the porosity was 7.3% and 6.0%, respectively.
-p
The surface roughness of PEO coating was largely determined by surface protrusion and porosity [11, 29].
re
The difference in surface profile can also well reflect the change in the surface structure of PEO coating formed in the different electrolytes. It can be determined that the silicate electrolyte promoted the growth of
lP
PEO coating, but the coating surface had a large number of pores left by the plasma discharge and the
na
irregular nodules formed due to product accumulation [12, 27]. The phosphate electrolyte was favorable for preparing a smooth and flat coating [6, 11]. For PEO coating prepared in the mixed electrolyte, surface
Jo ur
porosity is greatly reduced, pancakes are increased, and surface quality was superior to other coatings. Table 2 Porosity and the roughness of PEO coatings formed in different electrolytes. Sample code
Ra (μm)
Rt (μm)
Rz (μm)
Surface porosity (%)
Si-coating P-coating Si-P-coating
2.512 0.995 0.771
20.960 9.337 9.289
9.321 4.582 3.730
18.2 7.3 6.0
3.3 Influence of electrolyte on the surface composition of PEO coating Fig. 5 shows the EDS analysis results for the surface of PEO coating prepared in different electrolytes. As shown in Fig. 5a, the surface of PEO coating prepared in silicate electrolyte contained O, Al and Si, and Si element content was as high as 24.18 at.%. However, O and Al were the main components of PEO coating prepared in a phosphate electrolyte, and only a very small amount of P (1.16 at.%) was present. For the coating prepared in the mixed electrolyte, O, Al and Si were still the main components, but the Si content was only 1/6 of that of the pure silicate electrolyte, the P content was still as small as that of the
Journal Pre-proof phosphate system. The distribution of O, Al, Si and P on the surface of different coatings is also shown in Fig. 4. It can be seen that the distribution of O element on each coating was relatively uniform. For the coating formed in silicate electrolyte, the pancakes and nodules on the surface were the Al-rich and Si-rich regions, respectively. In the phosphate electrolyte, the Al-rich phenomenon was also appeared in the pancakes, and a small amount of P was also accumulated around the pancakes. The weak aggregation of Si and P was also appeared in the edge of pancakes in the coating formed in mixed electrolyte. Fig. 5
of
Besides, the chemical composition at the coating surface and phase compositions for each coating were
ro
analyzed by XPS and XRD, and the results are shown in Fig. 6. As shown in Fig. 6a, and the coating prepared in silicate electrolyte contained O, Al, Si and Na, if the contamination of C element was removed.
-p
After fitting, Al 2p spectrum of this coating could be decomposed into two peaks with binding energies at
re
74.80 eV and 74.10 eV (see Fig. 6b), corresponding to Al6Si2O13 [30] and Al2O3 [31] respectively. The binding energy of Si 2p peaks (Fig. 6c) was 102.70 eV, which was a typical characteristic peak of Al6Si2O13
lP
[30]. XRD spectrum (Fig. 6e) also showed that PEO coating prepared in the silicate electrolyte is mainly composed of mullite, -Al2O3 and a small amount of -Al2O3. Wherein, mullite is a solid solution
na
composed of Al, Si and O, which has different Al/Si atomic ratios, Al6Si2O13 belongs to one type of mullite [32].
Jo ur
For the phosphate system, PEO coating was mainly composed of O, Al and P, which was basically consistent with EDS results. The binding energy of Al 2p peak (Fig. 6c) was 74.70 eV, which was assigned into Al2O3. Moreover, the binding energy of P 2p peak (134.50 eV) indicated the presence of a few metaphosphate in the coating [33]. Therefore, it could be seen from XRD that this coating was composed of -Al2O3, -Al2O3 and -Al2O3, meanwhile XPS results displayed that amorphous metaphosphate was also present on the coating surface. In addition to O, Al and Si, a small amount of P was detected in the coating prepared in the mixed electrolyte, as shown in Fig. 6a. Among them, one peak of Al 2p spectrum (74.10 eV) correspond to Al2O3, the other peak of Al 2p spectrum (74.30 eV) and Si 2p peak (102.34 eV) together indicated the presence of SiO2(Al2O3)0.22 [34], and P 2p spectrum demonstrated the presence of metaphosphates. It could be determined synthetically form Fig.6 that this coating contained a little mullite and amorphous metaphosphate in addition to -Al2O3, -Al2O3 and -Al2O3, but the Al/Si atomic ratio of the mullite was
Journal Pre-proof quite different from that of the mullite obtained in silicate system. The different film-formation processes in different electrolyte may be the main reason for the distinction in surface morphology and chemical composition of PEO coatings. The literature [20, 24] believed that the formation of PEO coating was the synergism results of the oxidation of aluminum and the deposition of electrolyte compounds. The pancake-like product usually resulted from the matrix oxidation, whereas the nodules were the product of electrolyte hydrolysis and deposition [20, 24]. Combined with SEM, EDS, XRD and XPS analysis, it could be inferred that the oxidation of aluminum and the deposition of silicate simultaneously occurred on the coating surface for the silicate system, which formed the pancake-like
of
alumina and the nodular mullite (Al6Si2O13), respectively. However, the film-formation process in the
ro
phosphate system was mainly the oxidation of matrix, and the main component of coating was -Al2O3. In the mixed electrolyte, the film-formation process was dominated by the matrix oxidation, and
re
amount of Si-containing mullite SiO2(Al2O3)0.22.
-p
supplemented by the deposition of electrolytes. In addition to Al2O3, the coating also contained a small
lP
Fig. 6
na
3.4 Influence of electrolyte on the aluminum/ coating interface structure of PEO coating Fig. 7 gives the aluminum/coating (A/C) interface morphology of PEO coatings prepared in different
Jo ur
electrolytes. It can be seen that the A/C interface morphology was completely different from the surface morphology of PEO coating, and irregular undulations often appeared at the interface. However, the volume and height difference of “hill”-like protrusion at the interface were different in different electrolytes. In silicate system, the A/C interface was relatively flat, as shown in Fig. 7a. In the phosphate and mixed electrolyte, the obvious “hill”-like protrusions were present at A/C interface, and the boundaries between these protrusions were distinct. However, regardless of the electrolyte system, the “hill”-like protrusions at A/C interface were composed of regular spherical cells. Moreover, the diameter of cells had no significant difference and always maintained at a constant size of 0.5 ~1 μm, as shown in Fig. 7d ~ f. It was well known that the molten zones could be formed along with the plasma discharges during PEO process [24]. The molten aluminum was continuously oxidized to form a coating, in this way, the coating was thickened toward the substrate. Thus the “hill”-like protrusion at A/C interface represented the oxidation consumption of Al [13, 24, 35]. In the silicate system, the micro-melting zone at the interface was
Journal Pre-proof flat, indicating that the oxidation of Al was not severe, the advancement of coating toward the substrate was relatively gentle, and the thickening of coating was mainly performed toward the electrolyte. However, the consumption of Al matrix was more violently in the phosphate system, and a large number of deep and dense micro-melting zones were formed. The growth of coating in this electrolyte mainly depended on the continuous consumption of aluminum. In the mixed electrolyte, the film-formation process was still dominated by the matrix oxidation, and the oxidation degree of aluminum was close to that of the phosphate system, so the A/C interface morphology of the two was similar.
of
Fig. 7 In addition, in order to more accurately explain the growth rule of the coating, the part growing towards
ro
the Al substrate and the outward growth thickness of coating in different systems were calculated
-p
respectively by the method shown in Fig. 1. The corresponding results were shown in Table 3. It can be found that PEO coating in the silicate system did grow mainly toward the electrolyte, and the size of
re
outward growth was as high as 95%. In the phosphate and mixed electrolytes, the coatings both grow
lP
inward and outward. However, in the phosphate electrolyte, the part growing toward the substrate has always dominated, with a ratio of about 70%. In the mixed electrolyte, the growth mode of coating changed
na
with oxidation time; the coating initially predominantly grew outward, and the internal growth gradually plays a leading role after a certain time (~60min). This change law basically confirmed the above analysis.
Jo ur
Table 3 The growth rule of PEO coatings formed in different electrolytes. electrolyte system Si-coating
oxidation time
h (μm)
a (μm)
b (μm)
a/h
b/h
30 min
17.55
16.8
0.75
95.7%
4.3%
90 min
44.90
42.5
2.40
97.2%
2.8%
30 min
12.60
3.9
8.70
30.9%
69.1%
90 min
38.15
10.2
27.95
26.7%
73.3%
30 min
10.75
8.7
2.05
80.9%
19.1%
90 min
28.34
16.8
11.54
59.3%
40.7%
P-coating
Si-P-coating
3.5 Influence of electrolyte on the cross-sectional morphology of PEO coating Fig. 8 gives the fracture cross-sectional morphology and chemical composition of PEO coating prepared in different electrolytes. As shown in Fig. 8a, the PEO coating formed in silicate electrolyte showed a
Journal Pre-proof distinct layering characteristic. The outer surface of this coating exhibited loose nodules, the inside of coating had a large number of cavities. Besides, there was a barrier layer with thickness of ~1 µm at the A/C interface inside the cavity, a relative dense thick layer with numerous closed holes was present on the outside of cavity. For the coating formed in phosphate electrolyte (Fig. 8b), although there was a dense barrier layer at A/C interface and cavities near the barrier layer, the size of cavities were very small and discontinuous, and the outer surface of coating was relatively flat. As shown in Fig. 8c, a large cavity was also found inside, but the surface had no obvious nodular protrusion for the coating formed in mixed electrolyte. This coating could be divided into three parts: an outer layer, an inner layer containing
of
discontinuous holes, and a barrier layer near A/C interface.
ro
The corresponding element distribution showed that the denser part of PEO coating prepared in silicate system was the Al-enriching zone, while the loose nodules outside the coating were almost completely
-p
occupied by Si and O elements. In addition, Si element was almost absent in the barrier layer near the
re
matrix, but some Si also enriched at the edge of cavity. The coating prepared in phosphate electrolyte had a uniform distribution of O and Al along the entire coating; interestingly, P element was mainly located at the
lP
A/C interface, and only a little P element existed on the coating surface. For the coating prepared in mixed
na
electrolyte, Si element was mainly located on the outer side of coating and the edge of cavities, but the distribution of P element was opposite to Si element, and was mainly present in the inner layer close to the
Jo ur
cavity.
Fig. 8
3.6 Elemental distribution along the cross-section of PEO coating formed by stepwise oxidation In order to further understand the action mechanism of anions in the film-formation processes, the samples were subjected to stepwise oxidation in two electrolytes, and then the structure and element distribution of coating was studied. Fig. 9 shows the SEM image and EDS mapping along the cross-section of PEO coating formed in silicate electrolyte for 45 min and transferred into phosphate electrolyte for another 10 min. As shown in Fig. 9a, PEO coating prepared by stepwise oxidation in two electrolytes had no obvious chasm phenomenon, and the coating thickness after two-step oxidation was about 30 μm. EDS mapping showed that Si element was mainly located at the middle and outside of PEO coating, and P element was still mainly located at A/C interface, and the surface only contained a little P. Since P element could only be
Journal Pre-proof introduced from the phosphate electrolyte during the second oxidation, this result indicated that P in the electrolyte passed through the Si-rich outer surface and the relative dense inner layer to arrive in the A/C interface and cavities during the second oxidation. This transmission of P element might be related to holes, cracks, and discharge channels [6, 11, 21, 23]. Fig. 9 Fig. 10 gives the SEM image and EDS mapping along the cross-section of PEO coating formed in phosphate electrolyte for 45 min and after in silicate electrolyte for 10 min. The figure showed that P
of
element was still mainly distributed in the inner side of coating, and a little P was present at the coating
ro
surface, even the sample was treated by in phosphate electrolyte in first and followed in silicate electrolyte. However, Si element was still mainly distributed in the outside of PEO coating. Besides, a discontinuous
-p
distributed Si-rich region also appeared in the inside of coating, and it exhibited a shape similar to the
re
discharge channel, connecting the inner and outside of the coating.
lP
Fig. 10
na
3.7 Discussion
As shown in Figs. 4, 6, 7 and 8, it was known that there were great difference on the three-dimensional
Jo ur
structure (whether the coating surface, A/C interface or cross-sectional morphology) of PEO coating formed in different electrolytes. Besides, the distribution characteristics of Si and P elements in the coating also had a difference. A large amount of Si was present in the coating and showed Si-enriching phenomenon in the nodules; however, only a little P was present at the A/C interface. It was believed that these differences were likely related to the growth mechanism of PEO coating and the deposition pattern of anions in the electrolyte. Researches had confirmed that the entry of anions into the coating depended on the chemical reaction/deposition under the action of plasma [15]. The anions in the silicate electrolyte were mainly SiO32-, meanwhile the hydrolysis of silicate produced partial OH- (Reac. 1); and the following reactions may occur during the PEO process [11, 13, 20]:
SiO32 2H 2O
H 2SiO3 +2OH -
(1)
Al Al3 3e
(2)
2Al3 +6OH Al2O3 3H 2O
(3)
Journal Pre-proof 2SiO32 4e 2SiO2 +O2
(4)
SiO2 xAl2O3 SiO2 （ x Al2O3）
(5)
The reactions (2) and (3) caused the oxidation of aluminum matrix, and the reaction (4) produced the deposition of the electrolyte compound. Since SiO32- had a strong adsorption capacity [20], the reaction (4) was easily generated under the high temperature action of plasma discharge. The rapid deposition of SiO2 resulted in a high growth rate and a rough surface for PEO coating, while the deposition of SiO2 did not consume the aluminum matrix, and the flat structure at A/C interface (Fig. 7a) also strongly proved that the
of
consumption of substrate was very weak. Therefore, the growth of PEO coating in silicate electrolyte was
ro
mainly dominated by the deposition of SiO2, accompanied by weak oxidation of aluminum matrix. Fig. 11a shows the growth model of PEO coating in silicate system. As shown in the figures, the growth of PEO
-p
coating was mainly pushed toward the electrolyte side, and a large amount of Si-rich compound was
re
deposited on the coating surface, and then repeatedly underwent the "remelting – mixing – solidification" process [24-27, 36, 37], so that the distribution of Si elements along the cross-section gradually increased
lP
from inner to outside, as shown in Fig. 8a. In addition, since SiO2 was stable in the coating, it could be sintered with Al2O3 to form mullite (Reac. 4) [38] in the subsequent formed micro-melting zone.
na
The reactions (2) and (3) also occurred in the phosphate electrolyte. However, sodium hexametaphosphate was easily hydrolyzed to orthophosphate by the following steps (Reac. 6-10) in an
Jo ur
alkaline environment [39]. Meta-phosphate might also be deposited (Reac. 11), but P2O5 could not be stable in alkaline electrolyte (Reac. 12) [40]. As can be seen from Figs. 5, 6 and 8, P content in the coating prepared in phosphate electrolyte was extremely low, and XPS results (Fig. 6d) showed that a small amount of P was mainly present in the form of metaphosphate, which was most likely the residual of electrolyte. Therefore, the growth of PEO coating in phosphate electrolyte was mainly occurred by the oxidation consumption of aluminum matrix, which inevitably led to a higher buldges at A/C interface (See Fig. 7b). That was to say, the growth of PEO coating in this system was mainly pushed toward the aluminum substrate, as shown in Figs. 11b.
( N a P 3O 6 )
6 N a3P O
2NaPO3 H2O Na 2 H2 P2O7 H2O
Na 2 H2 P2O7 2NaH 2PO4
(6) (7) (8)
Journal Pre-proof H 2 PO4 HPO4 2
H HPO4 2
(9)
H PO43
(10)
4PO3 4e 2P2O5 +O 2
(11)
P2O5 2OH 2PO3 +H 2O
(12)
For the coating prepared in a mixed electrolyte, the A/C interface morphology (see Fig. 7c) was close to that of the phosphate system, and no obvious nodules were present on the surface (see Fig. 4c); the
of
distribution characteristics of Si and P were also similar to those of the coatings prepared in the single electrolyte (Fig. 8c); and there was no change in the distribution of Si and P for the coating (Figs. 9 and 10)
ro
prepared through two-step oxidation, regardless of the order of two electrolytes. That was to say, the
-p
film-formation process was dominated by the matrix oxidation, and supplemented by the deposition of electrolytes. Therefore, the growth model of PEO coating in the mixed electrolyte was established, as
re
shown in Fig. 10(c).
lP
Fig. 11
na
In addition, the discharge mechanism is critical in the formation of PEO coating. Hussein et al. [41] believed that three types of discharges (type A, B and C) might occur during PEO process, which in turn
Jo ur
produced different structures. In general, type A and type C discharge reflected the gas discharge near the surface and in the micropores of coating respectively, which easily produced electrolyte-riching deposition structures. Type B discharge was the dielectric breakdown discharge originated in the substrate/ coating interface, which caused the oxidation of matrix and formed the pancake structure. Cheng et al. [42] also proposed type D discharge, that is, the breakdown discharge occurring in the large cavities inside the coating, resulting in the Si-rich phenomenon around the cavities. Combined with the above results, it could be dermined that the PEO system of silicate was more likely to breed type A, type C and type D discharges, meanwhile type B discharge was also present. However, type B discharge was dominant in phosphate system. Under the action of type B discharge, it was more likely to generate obvious "hill" -like protrusions at the aluminum/coating interface. Meanwhile, all the four types of discharge were also present in the mixed electrolyte. Different discharge types were also the essential cause of the difference in the microstructre and chemical composition of prepared coatings in these electrolytes..
Journal Pre-proof 4. Conclusions (1) The deposition of silicate dominated the growth of PEO coating in the silicate electrolyte. The coating was mainly pushed toward the electrolyte side, a large amount of loose nodules (mullite) were present on the surface, and A/C interface was relatively gentle. The Si element gradually increased from inside to outside and was enriched at the nodules and the edge of cavities. (2) In the phosphate system, the growth of PEO coating mainly depended on the oxidation of aluminum matrix, and the coating mainly advanced to the substrate. The coating surface was smooth and dense, but a large number of “hill”-like protrusions were present at A/C interface. The main component of this coating
of
was a-Al2O3, and a liitle P element was mainly distributed at A/C interface.
ro
(3) In the mixed electrolyte, the film-formation process was dominated by the matrix oxidation, and
-p
supplemented by the deposition of electrolytes. The morphology of A/C interface was close to that of the phosphate system, the distribution characteristics of Si and P were also similar to those of the coatings
lP na
Conflict of interest
re
prepared in a single electrolyte.
Jo ur
None.
Acknowledgements
The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (Nos. 51801094 and 51361025) and the Natural Science Foundation of Jiangxi Province (No. 20171BAB216006).
References [1] A. L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S. J. Dowey, Plasma Electrolysis for Surface Engineering, Surf. Coat. Technol. 122 (1999) 73-93. [2] V. Antia, N. Saito, O. Takai, Microarc plasma treatment of titanium and alunminum surfaces in electrolytes, Thin Solid Films, 507 (2006) 364-368.
Journal Pre-proof [3] R. O.Hussein, D. O. Northwood, X. Nie, The effect of processing parameters and substrate composition on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloys, Surf. Coat. Technol. 237 (2013) 357-368. [4] T. Wei, F. Yan, J. Tian, Characterization and wear- and corrosion-resistance of microarc oxidation ceramic coatings on aluminum alloy, J. Alloy. Compd. 389 (2005) 0-176. [5] H. Fadaee, M. Javidi, Investigation on the corrosion behaviour and microstructure of 2024-T3 Al alloy treated via plasma electrolytic oxidation, J. Alloy. Compd. 604 (2014) 36-42. [6] H. Ma, D. Li, C. Liu, Z. Huang, D. He, Q. Yan, P. Liu, P. Nash, D. Shen, An investigation of (NaPO3)6
of
effects and mechanisms during micro-arc oxidation of AZ31 magnesium alloy. Surf. Coat. Technol.
ro
266 (2015) 151-159.
[7] Y. Mori, A. Koshi, J. Liao, H. Asoh, S. Ono, Characteristics and corrosion resistance of plasma
-p
electrolytic oxidation coatings on AZ31B Mg alloy formed in phosphate – silicate mixture electrolytes,
re
Corros. Sci. 88 (2014) 254-262.
[8] S.V. Stojadinovic, R. Belca, I. Petkovic, M. Kasalica, B. Nedic, Z. Zekovic, Lj, Characterization of the
lP
plasma electrolytic oxidation of aluminium in sodium tungstate, Corros. Sci. 52 (2010) 3258-3265.
na
[9] L. O. Snizhko, A. L. Yerokhin, A. Pilkington , N. L. Gurevina,D. O. Misnyankin, A. Leyland, A. Matthews, Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions,
Jo ur
Electrochim. Acta 49 (2004) 2085-2095.
[10] W. Mu, Y. Han, Study on micro-arc oxidized coatings on magnesium in three different electrolytes, Rare Metal Mater. Eng. 39 (2010) 1129-1134. [11] W. C. Gu, G. H. Lv, H. Chen, G. L. Chen, W. R. Feng, G. L. Zhang, S. Z. Yang, Investigation of morphology and composition of plasma electrolytic oxidation coatings in systems of Na2SiO3-NaOH and (NaPO3)6-NaOH, J. Mater. Process. Technol. 182 (2007) 28-33 [12] J. H. Lee, S. J. Kim, Effects of silicate ion concentration on the formation of ceramic oxide layers produced by plasma electrolytic oxidation on Al alloy, Jpn. J. Appl. Phys. 56 (2017) 01AB01. [13] Y. K. Wu, Z. Yang, R. Q. Wang, G. R. Wu, D. Chen, D. D. Wang, X. T. Liu, D. L. Li, C .H. Guo, S. X. Yu, D. J. Shen, P. Nash, An investigation of microstructure evolution for plasma electrolytic oxidation (PEO) coated Al in an alkaline silicate electrolyte, Surf. Coat. Technol. 351 (2018) 136-152.
Journal Pre-proof [14] Y. L. Cheng, M. K. Mao, J. H. Cao, Z. M. Peng, Plasma electrolytic oxidation of an Al-Cu-Li alloy in alkaline aluminate electrolytes: a competition between growth and dissolution for the initial ultra-thin films, Electrochim. Acta 138 (2014) 417-429 [15] S. Ono, S. Moronuki, Y. Mori, A. Koshi, J. Liao, H. Asoh, Effect of electrolyte concentration on the structure and corrosion resistance of anodic films formed on magnesium through plasma electrolytic oxidation, Electrochim. Acta 240(2017) 415-423. [16] M. Kaseem, M.P. Kamil, J.H. Kwon, Y.G. Ko, Effect of sodium benzoate on corrosion behavior of 6061 Al alloy processed by plasma electrolytic oxidation, Surf. Coat. Technol. 283 (2015) 268-273.
of
[17] X. Lu, M. Mohedano, C. Blawert, E. Matykina, R. Arrabal, K. U. Kainer, Plasma electrolytic
ro
oxidation coatings with particle additions – a review, Surf. Coat. Technol. 307 (2016) 1165-1182. [18] C. J. Hu, M. H. Hsieh, Preparation of ceramic coatings on an Al–Si alloy by the incorporation of ZrO2
-p
particles in microarc oxidation, Surf. Coat. Technol. 258 (2014) 275-283.
re
[19] E. Matykina, R. Arrabal, F. Monfort F, P. Skeldon, G.E. Thompson, Incorporation of zirconia into
Surf. Sci. 255 (2008) 2830-2839.
lP
coatings formed by DC plasma electrolytic oxidation of aluminium in nanoparticle suspensions, Appl.
na
[20] J. M. Li, H. Cai, X. N. Xue, B. L. Jiang, The outward–inward growth behavior of microarc oxidation coatings in phosphate and silicate solution, Mater. Lett. 64 (2010) 2102-2104.
Jo ur
[21] F. Monfort, E. Matykina, A. Berkani, P. Skeldon, G. E. Thompson, H. Habazaki, K. Shimizu, Species separation during coating growth on aluminium by spark anodizing. Surf. Coat. Technol. 201 (2007) 8671-8676.
[22] Z. Yang, Y. K. Wu, X. Z. Zhang, D. D. Wang, X. T. Liu, G. R. Wu, D. L. Li, S. X. Yu, D. J. Shen, An interesting anodic oxidation behavior of plasma electrolytic oxidation coatings fabricated on aluminum in alkaline phosphate electrolyte, Surf. Interfaces 16(2019) 199-205. [23] X. H. Liu, L. Q. Zhu, H. C. Liu, W. P. Li, Investigation of MAO coating growth mechanism on aluminum alloy by two-step oxidation method. Appl. Surf. Sci. 293 (2014) 12-17. [24] X. H. Liu, S. X. Wang, N. Du, X. Y. Li, Q. Zhao, Evolution of the three-dimensional structure and growth model of plasma electrolyticoxidation coatings on 1060 aluminum alloy, Coatings 8 (2018) 105 [25] W.B. Xue, Z.W. Deng, R.Y. Chen, T. H. Zhang, Growth regularity of ceramic coatings formed by microarc oxidation on Al–Cu–Mg alloy, Thin Solid Films 2000, 372 (1-2), 114-117.
Journal Pre-proof [26] A. L. Yerokhin, I. O. Snizhko, A. Leyland, Discharge characterization in plasma electrolytic oxidation of aluminium, J. Phys. D: Appli. Phys. 36 (2003) 2110-2120. [27] F. Monfort, A. Berkani, E. Matykina, P. Skeldon, G.E. Thompson, H. Habazaki and K.Shimizu, Development of anodic coatings on aluminium under sparking conditions in silicate electrolyte, Corros. Sci. 49 (2007) 672-693. [28] C. S. Duleav, I. O. Golosnoy, J. A. Curran, T. W. Clyne, Characterization of Discharge Events during plasma electrolytic oxidation, Surf. Coat. Technol. 203 (2009) 3410-3419. [29] E. Erfanifar, M. Aliofkhazraei, H. F. Nabavi, A. S. Rouhaghdam, Growth kinetics and morphology of
of
microarc oxidation coating on titanium. Surf. Coat. Technol. 315 (2017) 567-576.
ro
[30] A. E. Hughes, M. M. Hedges, B. A. Sexton, Reactions at the Al/SiO2 /SiC layered interface, J. Mater. Sci. 25 (1990) 4856-4865.
-p
[31] E. A. Koblova, A. Y. Ustinov, V. S. Rudnev, I. V. Lukiyanchuk, I. V. Chernykh, An x-ray photoelectron
re
spectroscopy study of Ni, Cu-containing coatings formed by plasma electrolytic oxidation on aluminum and titanium, J. Struct. Chem. 58 (2017) 1129-1136.
lP
[32] H. Schneider, J. Schreuer, B. Hildmann, Structure and properties of mullite—A review, J. Eur. Ceram.
na
Soc. 28 (2008) 329-344.
[33] C. D. Wagner, X-ray photoelectron spectroscopy with x-ray photons of higher energy. J. Vac.Sci.
Jo ur
Technol. 15 (1978) 518-523.
[34] A. Ayame, T. Kitagawa, X-Ray photoelectron spectroscopic measurement and chemical characteristics of silica, alumina and silica-alumina, Bunseki Kagaku 40 (1991) 673-678. [35] R. Q. Wang, Y. K. Wu, G. R. Wu, D. Chen, D. L. He, D. L. Li, C. H. Guo, Y. F. Zhou, D. J. Shen, P. Nash, An investigation about the evolution of microstructure and composition difference between two interfaces of plasma electrolytic oxidation coatings on Al, J. Alloy. Compd. 753 (2018) 272-281. [36] X. Y. Li, X. G. Li, Y. Li, C. F. Dong, H. P. Tian, S. X. Wang, Q. Zhao, Growth mechanism of micro-arc oxidation film on 6061 aluminum alloy, Mater. Res. Express 6 (2019) 066404. [37] T. W. Clyne, S. C. Troughton, A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals, Int. Mater. Rev. 64 (2019) 127-162. [38] P. D. D. Rodrigo, P. Bocha, High purity mullite ceramics by reaction sintering, Int. J.High Technol. Ceram. 1 (1985) 3-30.
Journal Pre-proof [39] D. D. Wang, X. T. Liu, Y. K. Wang, H. P. Han, Z. Yang, Y. Su, X. Z. Zhang, G. R. Wu, D. J. Shen, Evolution process of the plasma electrolytic oxidation (PEO) coating formed on aluminum in an alkaline sodium hexametaphosphate ((NaPO3)6) electrolyte, J. Alloy. Compd. 2019, 798,129-143. [40] Q. B. Li, W. B. Yang, C. C. Liu, D. A. Wang, J. Liang, Correlations between the growth mechanism and properties of micro-arc oxidation coatings on titanium alloy: Effects of electrolytes, Surf. Coat. Technol. 316 (2017) 162-170. [41] R.O. Hussein, X. Nie, D.O. Northwood, A. Yerokhin, A. Mattews, Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process. J. Phys. D:
of
Appl. Phys. 2010, 43 (10), 105203-105215.
ro
[42] Y. L. Cheng, Z.G. Xue, Q. Wang, X. Q. Wu, E. Matykina, P. Skeldon, G. E. Thompson, New findings on properties of plasma electrolytic oxidation coatings from study of an Al–Cu–Li alloy. Electrochim.
Jo ur
na
lP
re
-p
Acta 2013, 107, 358-378.
Journal Pre-proof
Figure captions Fig. 1 The schematic drawing of the change in sample dimension in the process of PEO Fig. 2 Effects of electrolyte system on the working voltage (a) and coating thickness (b) during PEO process of 1060 aluminum alloy. Fig. 3 Surface linear profiles of the PEO coatings formed in different electrolytes. Fig. 4 Surface morphology and element distribution for PEO coatings formed in different electrolytes:
of
Si-coating (a), P- coating (b) and Si-P- coating (c).
ro
Fig. 5 EDS spectra of the corresponding square area for Si- coating (a), P- coating (b) and Si-P- coating (c)
-p
in Fig. 4.
re
Fig. 6 XPS wide-survey spectrum (a), Al 2p spectrum (b), Si 2p spectrum (c) and P 2p spectrum (d) and XRD spectrum (e) of PEO coatings formed in different electrolytes.
lP
Fig. 7 SEM morphology of aluminum/coating interface of PEO coatings formed in different electrolytes:
na
Si-coating (a, d), P-coating (b, e) and Si-P-coating (c, f). Fig. 8 Fracture cross-section morphology and EDS mapping images of PEO coatings formed in different
Jo ur
electrolytes: Si-coating (a), P- coating (b) and Si-P- coating (c). Fig. 9 Fracture cross-section SEM image (a), elemental maps of O, Al, Si and P (b) and mixed elemental maps of Si and P (c) for PEO coating formed in silicate electrolyte for 45 min and transferred into phosphate electrolyte for another 10 min. Fig. 10 Fracture cross-section SEM image (a), elemental maps of O, Al, Si and P (b) and mixed elemental maps of Si and P (c) for PEO coating formed in phosphate electrolyte for 45 min and transferred into silicate electrolyte for another 10 min. Fig. 11 The growth model diagrams for PEO coatings in silicate electrolyte (a), phosphate electrolyte (b) and the mixed electrolyte (c).
Journal Pre-proof
Jo ur
na
lP
re
-p
ro
of
Fig.1 The schematic drawing of the change in sample dimension in the process of PEO
ro
of
Journal Pre-proof
Fig. 2 Effects of electrolyte system on the working voltage (a) and coating thickness (b) during PEO
Jo ur
na
lP
re
-p
process of 1060 aluminum alloy.
re
-p
ro
of
Journal Pre-proof
Jo ur
na
lP
Fig. 3 Surface linear profiles of the PEO coatings formed in different electrolytes.
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 4 Surface morphology and element distribution for PEO coatings formed in different electrolytes: Si-coating (a), P- coating (b) and Si-P- coating (c).
of
Journal Pre-proof
ro
Fig. 5 EDS spectra of the corresponding square area for Si- coating (a), P- coating (b) and Si-P- coating (c)
Jo ur
na
lP
re
-p
in Fig. 4.
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 6 XPS wide-survey spectrum (a), Al 2p spectrum (b), Si 2p spectrum (c) and P 2p spectrum (d) and XRD spectrum (e) of PEO coatings formed in different electrolytes.
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 7 SEM morphology of aluminum/coating interface of PEO coatings formed in different electrolytes:
Jo ur
Si-coating (a, d), P-coating (b, e) and Si-P-coating (c, f).
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 8 Fracture cross-section morphology and EDS mapping images of PEO coatings formed in different electrolytes: Si-coating (a), P- coating (b) and Si-P- coating (c).
Journal Pre-proof
of
Fig. 9 Fracture cross-section SEM image (a), elemental maps of O, Al, Si and P (b) and mixed elemental
ro
maps of Si and P (c) for PEO coating formed in silicate electrolyte for 45 min and transferred into
Jo ur
na
lP
re
-p
phosphate electrolyte for another 10 min.
Journal Pre-proof
of
Fig. 10 Fracture cross-section SEM image (a), elemental maps of O, Al, Si and P (b) and mixed elemental
ro
maps of Si and P (c) for PEO coating formed in phosphate electrolyte for 45 min and transferred into
Jo ur
na
lP
re
-p
silicate electrolyte for another 10 min.
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 11 The growth model diagrams for PEO coatings in silicate electrolyte (a), phosphate electrolyte (b) and the mixed electrolyte (c).
Journal Pre-proof
Author contrbutions section
The experimental scheme was framed by S. X. Wang and N. Du. The tests and characterization were carried out by S. X. Wang, X. H. Liu and X. L. Yin. The manuscript was composed by S. X. Wang and X. H. Liu, and revised by S. X.
Jo ur
na
lP
re
-p
ro
of
Wang.
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Graphical abstract
Journal Pre-proof
Research highlights 1. The deposition of silicate dominates the growth of PEO coating in silicate electrolyte.
2. Many loose Si-riching nodules are present on the surface of coating formed in silicate system.
of
3. The growth of PEO coating in phosphate system mainly depends on the oxidation of Al.
ro
4. Obvious “hill”-like protrusions are present at A/C interface of coating formed in phosphate
-p
system.
re
5. The oxidation of matrix accompanying electrolyte deposition forms PEO coating in the
lP
mixed electrolyte.
Jo ur
na
6. The growth model diagrams of PEO coatings in different electrolytes are established.
Shuaixing Wang, Xiaohui Liu, Xiaole Yin, Nan Du PII:
S0257-8972(19)31204-6
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125214
Reference:
SCT 125214
To appear in:
Surface & Coatings Technology
Received date:
11 September 2019
Revised date:
7 November 2019
Accepted date:
28 November 2019
Please cite this article as: S. Wang, X. Liu, X. Yin, et al., Influence of electrolyte components on the microstructure and growth mechanism of plasma electrolytic oxidation coatings on 1060 aluminum alloy, Surface & Coatings Technology (2019), https://doi.org/ 10.1016/j.surfcoat.2019.125214
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier.
Journal Pre-proof
Influence of electrolyte components on the microstructure and growth mechanism of plasma electrolytic oxidation coatings on 1060 aluminum alloy Shuaixing Wang*, Xiaohui Liu, Xiaole Yin, Nan Du National Defense Key Discipline Laboratory of Light Alloy Processing Science and Technology, Nanchang Hangkong University, Nanchang 330063, P. R. China
of
*Corresponding author. E-mail address: [email protected] (S. X. Wang)
Abstract: Electrolyte systems have a significant impact on plasma electrolytic oxidation (PEO). In this
ro
work, the surface, aluminum/coating (A/C) interface and fracture cross-section microstructure and element
-p
distribution of PEO coatings prepared in silicate, phosphate and mixed electrolyte were characterized in detail by FESEM, EDS, XRD and XPS. The growth mechanisms of PEO coatings in different electrolytes
re
were also discussed. Results demonstrated that the deposition of silicate dominated the growth of coating in
lP
silicate system, many loose nodules were present on the coating surface. Si element occupied almost the entire coating and was enriched at the nodules and cavity edges. In phosphate system, the growth of coating
na
was primarily dependent on the oxidation of aluminum. There are a large number of “hill”-like protrusions at A/C interface. The main component of this coating was a-Al2O3, and a small amount of P was distributed
Jo ur
at A/C interface. In the mixed electrolyte, the film-formation process was mainly the matrix oxidation, and supplemented by deposition of electrolytes. Meanwhile, the growth models of PEO coating in different electrolyte were established.
Keywords: aluminum alloy; plasma electrolytic oxidation; electrolyte components; microstructure; growth mechanism
1. Introduction Plasma electrolytic oxidation (PEO), also known as micro-arc oxidation (MAO), is an environment-friendly surface-modification technique to prepare ceramic coatings on the valve metals (Al, Mg, Ti, etc) in-situ by using the effects of electrochemistry, thermo-chemistry and plasma chemistry [1-3].
Journal Pre-proof This ceramic coating is usually composed of matrix oxides and the electrolyte species, which can meet the needs of corrosion resistance, wear resistance and electrical insulation. The researches show that the surface hardness of 2024 aluminum alloy treated by PEO can reach to 1500 HV, the wear resistance can be increased by 3~5 times compared with the matrix [4, 5], and the corrosion current density of the coated specimen is about 4000 order lower than uncoated one [5]. However, some studies confirmed that the structure, composition and properties of PEO coatings are closely related to the electrolyte components [6, 7]. For aluminum alloys, PEO can be carried out in an acidic or alkaline electrolyte, but it is less used due to
of
the greater contamination of the acidic electrolyte. Many literatures have found that the alkaline species,
ro
such as SiO32-, PO43-, VO43- or MoO42-, are the main substance for the formation of PEO coatings [6-8]. During PEO process, these species are easily adsorbed on the surface of the initial oxide layer to form
-p
discharge points of foreign particles, and hence generated a plasma discharge [8, 9]. In addition, some
re
literature believes that in the alkaline system, the metal ions generated by anodic reaction and some other cations in the solution can be easily transformed into negatively charged particles and re-enter the coating
lP
[10], which can adjust and change the microstructure of coating. Therefore, the PEO electrolyte of
9-11].
na
aluminum alloy mostly uses the weak alkaline system composed of NaOH, Na2SiO3, Na3PO4 or NaAlO2 [7,
Jo ur
However, in the sodium hydroxide system, the electrolyte concentration is the key determining factor whether the micro-arc discharge can proceed. The high concentration of sodium hydroxide is not conducive to the formation of a high-resistance film on the sample surface, and the plasma discharge is difficult to occur [11]. During the same oxidation time, the growth rate of PEO coating in the silicate system is faster, but the coating surface is very rough [11, 12]; the coating prepared in the phosphate electrolyte is dense, but the wear resistance is poor due to the insufficient thickness [11, 13]. Aluminate system is beneficial to the formation of -Al2O3 in the coating, but the stability of electrolyte is poor [14, 15]. Therefore, the composite electrolyte is often used for PEO treatment to prepare a coating with good comprehensive performance. Currently, many researches about PEO electrolyte are also focused on the effects of soluble additive [14, 15] or the second phase particles on the structure and properties of PEO coating [17-19]; while the little research involves the three-dimensional structure, composition and growth mechanism of the coating under different electrolytes. Some researches have found that the size of outward growth for PEO coating in the silicate electrolyte is
Journal Pre-proof larger than that in the phosphate electrolyte [20]; and the distribution of Si and P elements in the coating is also different [21]. Silicon-rich material is deposited on the surface of alumina-based coating, but phosphorus species are primarily found in a region next to the metal [21, 22]. There are different interpretations between scholars for this phenomenon [21, 23]. Some scholars believe that discharge channels are the main ways to form the new coating. The discharge channel in silicate electrolyte is dumbbell-shaped, while that in phosphate electrolyte is trumpet-shaped, which lead to the above differences [21]. Another viewpoint is that the growth process within the discharge region result in the separation of the silicate- and phosphate-derived species [23]. In order to further understand the growth
of
mechanism of coating in different electrolytes, especially the local physical process, it is necessary to
ro
characterize the fine microstructure of coating formed in different electrolytes.
In this paper, the free-standing coatings were obtained using an electrochemical dissolution method [13,
-p
24]. A large number of nice pictures for coating structure (including the surface, aluminum/coating
re
interface and fracture cross-section) and detailed composition analysis data are given based on FESEM, EDS, XRD and XPS. Meanwhile, the growth mechanisms of PEO coatings in different electrolytes were
lP
fully discussed by combining existing theories. It is expected to provide a basis for the regulation of
Jo ur
2. Experimental
na
microstructure and performance of PEO coating.
2.1 Materials and PEO Treatment
The 1060 aluminum alloy (99.6% purity) was used as the substrate for preparing the PEO coating, and the aluminum plate was cut into samples with a the dimension of 20 × 20 × 2 mm. In preparation for PEO process, all samples were ground with SiC abrasive (from 320# to 1000#) and cleaned ultrasonically with alcohol for 5min. Then, all samples were cleaned with deionized water and air dried. An asymmetrical pulsed power supply (WHD–20, Harbin Institute of Technology, Harbin, China) was employed for the PEO process. The electrolytes are selected from silicate system, phosphate system and a mixed electrolyte system, respectively. The specific composition of electrolytes and the sample number are shown in Table 1. The PEO of all aluminum samples was carried out under a constant current density of 15 A/dm2, a frequency of 500 Hz, a duty ratio of 60% and the oxidation time of 60 min. During the PEO treatment, stainless steel tank was used as the cathode, a circulating water-cooling system was used to
Journal Pre-proof maintain the electrolyte temperature below 40 °C. After the PEO treatment, the coated samples were cleaned with deionized water and dried at ambient temperature. Table 1 Sample codes and electrolyte composition of different PEO coatings on aluminum. Sample codes Si-coating P-coating Si-P-coating
Electrolyte composition Na2SiO3·9H2O (g/L)
(NaPO3)6 (g/L)
NaOH (g/L)
20 0 10
0 20 10
0 2 1
of
2.2 Growth regularity measurement of PEO coatings During PEO process, the oxidation voltage was collected by the software that comes with the power
ro
supply, and the sampling interval is 1 s. The discharge sparks in the PEO process was also observed in real
-p
time through a long working distance microscope (QUESTAR QM-100) equipped with a CMOS image collector (MDX-4T). The acquisition rate set at 15 frames per second. Besides, the growth regularity of
re
PEO coating was obtained by measuring the change in sample dimension in the process of PEO [25], as shown in Fig.1. Here, in order to avoid dimensional errors caused by double-sided oxidation; the sample
lP
was oxidized on one side, and the non-oxidized side was covered by a sealant. h0 and h1 represented the sample thickness berfore and after PEO respectively, which can be measured by a digimatic micrometer
na
(INSIZE 6353-25C, unit. ± 2μm). h was the thickness of PEO coating, which was obtained from average of three measurements on each sample using a thickness gauge (Elcometer 345). The difference between h1 and
Jo ur
h0, that was a, can be defined as the outward growth thickness of the coating. Besides, the part growing towards the Al substrate can be calculated by h-a. The linear surface profiles and the roughness parameters of PEO coatings were measured using a SJ-310 surface profile meter. Fig. 1
2.3 Microstructure characterization of PEO coatings In order to accurately characterize the fine structure of PEO coating, a free-standing coating was obtained by removing the aluminum substrate using an electrochemical dissolution method. The specific operation method is described in the literature [13, 24]. Then, the surface morphology, aluminum/coating (A/C) interface morphology and the fracture cross-sectional morphology of PEO coatings formed in different electrolytes were analyzed by a field emission gun SEM (FE-SEM, Nova Nano-SEM 450). The element distribution on the surface and cross-section of coating was examined by energy dispersive spectroscopy (EDS, INCA 250) equipped on the FE-SEM system. Besides, X-ray photoelectron
Journal Pre-proof spectroscopy (XPS, Axis Ultra DLD) equipped with a standard Al Ka X-ray source (1486.6 eV) was used to determine the chemical composition at the coating surface. Phase composition of the coatings was investigated by X-ray diffraction (XRD, Bruker D8-Advance) scanning from 10°to 80°. Before XRD, the free-standing coatings are ground into powder samples to perform test.
3. Results and disscusion 3.1 Influence of electrolyte on the growth process of PEO coating Fig. 2a shows the voltage-time curve of 1060 aluminum alloy during PEO in different electrolytes. It can
of
be seen that the oxidation voltage exhibits a similar change trend under different electrolytes, except that
ro
the breakdown voltage and the termination voltage are different. In this work, the PEO process is observed in real time through a long working distance microscope, and the point at which the first spark discharge
-p
occurs is defined as the breakdown voltage. It was found that the plasma discharge appeared earlier in the
re
silicate electrolyte, the breakdown voltage was about 240 V, and termination voltage was 476 V after 60 minutes of PEO, as shown in Fig. 2(a). In the phosphate electrolyte, breakdown discharges occurred at
lP
about 300 V. The termination voltage after PEO of 60 min was about 10% higher than that in silicate system. In the mixed electrolyte of silicate and phosphate, the breakdown voltage and termination voltage
na
are 280 V and 503 V, respectively.
Jo ur
Fig. 2b gives a variation curve of coating thickness in different electrolytes. It can be seen that the coating thickness approximately increased linearly during PEO of 60 min, regardless of the electrolyte system. However, the growth rate of coating in silicate electrolyte is the fastest in the same time. The average growth rate of coating in this system is about 0.51 μm/min, but the growth rate of coating in the phosphate and mixed electrolyte is only 0.45μm/min and 0.34 μm/min, respectively. Most studies believed that PEO coatings are formed by polarizing the metal to the dielectric breakdown voltage [2, 23, 26]. The oxidation was similar to the conventional anodizing process before the breakdown discharge occurred. At this stage, a very thin alumina layer was produced at the interface, the working voltage was also rapidly increased. When the primary oxide layer was punctured, a turning point appeared on the voltage curve, and so a large number of small and short-lived plasma sparks were formed and moved rapidly on the electrode surface. Herein, the coating grew almost linearly under the cycle action of “discharge – melt – quench” due to the high temperature and high pressure (~ 7000K and 102 MPa) [26-28]. The micro-pores were also left on the sample surface because of the breakdown discharge. The oxidation
Journal Pre-proof time further prolonged, the voltage increased slowly, the electrical breakdown became more difficult, and the local large and long-lasting plasma arcs caused the coating to grow slowly around the discharge channel. According to this analysis, it was known that a lower breakdown voltage in a silicate system means that more discharge energy was used for the coating formation, and so the coating grew faster. A higher termination voltage in the phosphate system indicated that the density of coating might be higher, which could be also demonstrated by the coating morphology (Fig. 4b, 8b). In the mixed electrolyte, the high breakdown voltage and dense structure may result in the lower growth rate of the coating.
of
Fig. 2
ro
3.2 Influence of electrolyte on the surface structure of PEO coating
-p
Fig. 3 shows the linear surface profiles of various PEO coatings prepared in different electrolytes. It can
re
be seen that the surface of all the coatings were in the uneven state. Among them, the surface profile of the coating prepared in silicate electrolyte fluctuated the most, followed by the phosphate electrolyte, and the
lP
smallest in the mixed electrolyte. The surface roughness of PEO coating was evaluated using the parameters Ra, Rt and Rz, and the results were shown in Table 2. Where, Ra was the average roughness, Rt
na
and Rz was the the maximum peak and average peak to valley height, respectively. It can be seen that the roughness of PEO coating prepared in silicate electrolyte was the largest, and Ra value was up to 2.512 μm;
Jo ur
while the surface of the coating prepared in the phosphate electrolyte and the mixed electrolyte were relatively smooth and the roughness were low. Fig. 3
Fig. 4 shows the surface morphology and element distribution of PEO coating prepared in different electrolytes. It can be seen that all the coating surface was mainly occupied by the pancake structure, and each of pancakes had a tiny pore in the center, which was closely related to the plasma discharge channel and gas release [13, 27-28]. However, the electrolyte system had a great influence on the surface morphology. As shown in Fig. 4a, in addition to the pancakes, the surface of PEO coating prepared in the silicate electrolyte also had a large number of loose nodular protrusions, and the nodules are always discontinuously distributed around the pancakes. As shown in Fig. 4b, pancakes with the central closed pore were the main feature of PEO coating prepared in phosphate electrolyte, yet some pores were also
Journal Pre-proof distributed around the pancakes. For the coating formed in the mixed electrolyte, see Fig. 4c, pancakes were still the dominant feature, but the pores on the coating surface were significantly less than those in the phosphate system. Besides, a few granular nodules were present around the pancakes, but whose shape and size were much smaller than that formed in the silicate electrolyte. Fig. 4 The coating surface image (1000 ) was processed by “Image J” processing software, and the surface porosity of PEO coating was calculated by the ratio of the area occupied by the pores to the total image
of
area [29]. The results are shown in Table 2. It can be seen that the surface of PEO coating prepared in the
ro
silicate electrolyte was porous and had a porosity of up to 18.2%; while the surface of the coating prepared in phosphate and mixed electrolyte was relatively dense, and the porosity was 7.3% and 6.0%, respectively.
-p
The surface roughness of PEO coating was largely determined by surface protrusion and porosity [11, 29].
re
The difference in surface profile can also well reflect the change in the surface structure of PEO coating formed in the different electrolytes. It can be determined that the silicate electrolyte promoted the growth of
lP
PEO coating, but the coating surface had a large number of pores left by the plasma discharge and the
na
irregular nodules formed due to product accumulation [12, 27]. The phosphate electrolyte was favorable for preparing a smooth and flat coating [6, 11]. For PEO coating prepared in the mixed electrolyte, surface
Jo ur
porosity is greatly reduced, pancakes are increased, and surface quality was superior to other coatings. Table 2 Porosity and the roughness of PEO coatings formed in different electrolytes. Sample code
Ra (μm)
Rt (μm)
Rz (μm)
Surface porosity (%)
Si-coating P-coating Si-P-coating
2.512 0.995 0.771
20.960 9.337 9.289
9.321 4.582 3.730
18.2 7.3 6.0
3.3 Influence of electrolyte on the surface composition of PEO coating Fig. 5 shows the EDS analysis results for the surface of PEO coating prepared in different electrolytes. As shown in Fig. 5a, the surface of PEO coating prepared in silicate electrolyte contained O, Al and Si, and Si element content was as high as 24.18 at.%. However, O and Al were the main components of PEO coating prepared in a phosphate electrolyte, and only a very small amount of P (1.16 at.%) was present. For the coating prepared in the mixed electrolyte, O, Al and Si were still the main components, but the Si content was only 1/6 of that of the pure silicate electrolyte, the P content was still as small as that of the
Journal Pre-proof phosphate system. The distribution of O, Al, Si and P on the surface of different coatings is also shown in Fig. 4. It can be seen that the distribution of O element on each coating was relatively uniform. For the coating formed in silicate electrolyte, the pancakes and nodules on the surface were the Al-rich and Si-rich regions, respectively. In the phosphate electrolyte, the Al-rich phenomenon was also appeared in the pancakes, and a small amount of P was also accumulated around the pancakes. The weak aggregation of Si and P was also appeared in the edge of pancakes in the coating formed in mixed electrolyte. Fig. 5
of
Besides, the chemical composition at the coating surface and phase compositions for each coating were
ro
analyzed by XPS and XRD, and the results are shown in Fig. 6. As shown in Fig. 6a, and the coating prepared in silicate electrolyte contained O, Al, Si and Na, if the contamination of C element was removed.
-p
After fitting, Al 2p spectrum of this coating could be decomposed into two peaks with binding energies at
re
74.80 eV and 74.10 eV (see Fig. 6b), corresponding to Al6Si2O13 [30] and Al2O3 [31] respectively. The binding energy of Si 2p peaks (Fig. 6c) was 102.70 eV, which was a typical characteristic peak of Al6Si2O13
lP
[30]. XRD spectrum (Fig. 6e) also showed that PEO coating prepared in the silicate electrolyte is mainly composed of mullite, -Al2O3 and a small amount of -Al2O3. Wherein, mullite is a solid solution
na
composed of Al, Si and O, which has different Al/Si atomic ratios, Al6Si2O13 belongs to one type of mullite [32].
Jo ur
For the phosphate system, PEO coating was mainly composed of O, Al and P, which was basically consistent with EDS results. The binding energy of Al 2p peak (Fig. 6c) was 74.70 eV, which was assigned into Al2O3. Moreover, the binding energy of P 2p peak (134.50 eV) indicated the presence of a few metaphosphate in the coating [33]. Therefore, it could be seen from XRD that this coating was composed of -Al2O3, -Al2O3 and -Al2O3, meanwhile XPS results displayed that amorphous metaphosphate was also present on the coating surface. In addition to O, Al and Si, a small amount of P was detected in the coating prepared in the mixed electrolyte, as shown in Fig. 6a. Among them, one peak of Al 2p spectrum (74.10 eV) correspond to Al2O3, the other peak of Al 2p spectrum (74.30 eV) and Si 2p peak (102.34 eV) together indicated the presence of SiO2(Al2O3)0.22 [34], and P 2p spectrum demonstrated the presence of metaphosphates. It could be determined synthetically form Fig.6 that this coating contained a little mullite and amorphous metaphosphate in addition to -Al2O3, -Al2O3 and -Al2O3, but the Al/Si atomic ratio of the mullite was
Journal Pre-proof quite different from that of the mullite obtained in silicate system. The different film-formation processes in different electrolyte may be the main reason for the distinction in surface morphology and chemical composition of PEO coatings. The literature [20, 24] believed that the formation of PEO coating was the synergism results of the oxidation of aluminum and the deposition of electrolyte compounds. The pancake-like product usually resulted from the matrix oxidation, whereas the nodules were the product of electrolyte hydrolysis and deposition [20, 24]. Combined with SEM, EDS, XRD and XPS analysis, it could be inferred that the oxidation of aluminum and the deposition of silicate simultaneously occurred on the coating surface for the silicate system, which formed the pancake-like
of
alumina and the nodular mullite (Al6Si2O13), respectively. However, the film-formation process in the
ro
phosphate system was mainly the oxidation of matrix, and the main component of coating was -Al2O3. In the mixed electrolyte, the film-formation process was dominated by the matrix oxidation, and
re
amount of Si-containing mullite SiO2(Al2O3)0.22.
-p
supplemented by the deposition of electrolytes. In addition to Al2O3, the coating also contained a small
lP
Fig. 6
na
3.4 Influence of electrolyte on the aluminum/ coating interface structure of PEO coating Fig. 7 gives the aluminum/coating (A/C) interface morphology of PEO coatings prepared in different
Jo ur
electrolytes. It can be seen that the A/C interface morphology was completely different from the surface morphology of PEO coating, and irregular undulations often appeared at the interface. However, the volume and height difference of “hill”-like protrusion at the interface were different in different electrolytes. In silicate system, the A/C interface was relatively flat, as shown in Fig. 7a. In the phosphate and mixed electrolyte, the obvious “hill”-like protrusions were present at A/C interface, and the boundaries between these protrusions were distinct. However, regardless of the electrolyte system, the “hill”-like protrusions at A/C interface were composed of regular spherical cells. Moreover, the diameter of cells had no significant difference and always maintained at a constant size of 0.5 ~1 μm, as shown in Fig. 7d ~ f. It was well known that the molten zones could be formed along with the plasma discharges during PEO process [24]. The molten aluminum was continuously oxidized to form a coating, in this way, the coating was thickened toward the substrate. Thus the “hill”-like protrusion at A/C interface represented the oxidation consumption of Al [13, 24, 35]. In the silicate system, the micro-melting zone at the interface was
Journal Pre-proof flat, indicating that the oxidation of Al was not severe, the advancement of coating toward the substrate was relatively gentle, and the thickening of coating was mainly performed toward the electrolyte. However, the consumption of Al matrix was more violently in the phosphate system, and a large number of deep and dense micro-melting zones were formed. The growth of coating in this electrolyte mainly depended on the continuous consumption of aluminum. In the mixed electrolyte, the film-formation process was still dominated by the matrix oxidation, and the oxidation degree of aluminum was close to that of the phosphate system, so the A/C interface morphology of the two was similar.
of
Fig. 7 In addition, in order to more accurately explain the growth rule of the coating, the part growing towards
ro
the Al substrate and the outward growth thickness of coating in different systems were calculated
-p
respectively by the method shown in Fig. 1. The corresponding results were shown in Table 3. It can be found that PEO coating in the silicate system did grow mainly toward the electrolyte, and the size of
re
outward growth was as high as 95%. In the phosphate and mixed electrolytes, the coatings both grow
lP
inward and outward. However, in the phosphate electrolyte, the part growing toward the substrate has always dominated, with a ratio of about 70%. In the mixed electrolyte, the growth mode of coating changed
na
with oxidation time; the coating initially predominantly grew outward, and the internal growth gradually plays a leading role after a certain time (~60min). This change law basically confirmed the above analysis.
Jo ur
Table 3 The growth rule of PEO coatings formed in different electrolytes. electrolyte system Si-coating
oxidation time
h (μm)
a (μm)
b (μm)
a/h
b/h
30 min
17.55
16.8
0.75
95.7%
4.3%
90 min
44.90
42.5
2.40
97.2%
2.8%
30 min
12.60
3.9
8.70
30.9%
69.1%
90 min
38.15
10.2
27.95
26.7%
73.3%
30 min
10.75
8.7
2.05
80.9%
19.1%
90 min
28.34
16.8
11.54
59.3%
40.7%
P-coating
Si-P-coating
3.5 Influence of electrolyte on the cross-sectional morphology of PEO coating Fig. 8 gives the fracture cross-sectional morphology and chemical composition of PEO coating prepared in different electrolytes. As shown in Fig. 8a, the PEO coating formed in silicate electrolyte showed a
Journal Pre-proof distinct layering characteristic. The outer surface of this coating exhibited loose nodules, the inside of coating had a large number of cavities. Besides, there was a barrier layer with thickness of ~1 µm at the A/C interface inside the cavity, a relative dense thick layer with numerous closed holes was present on the outside of cavity. For the coating formed in phosphate electrolyte (Fig. 8b), although there was a dense barrier layer at A/C interface and cavities near the barrier layer, the size of cavities were very small and discontinuous, and the outer surface of coating was relatively flat. As shown in Fig. 8c, a large cavity was also found inside, but the surface had no obvious nodular protrusion for the coating formed in mixed electrolyte. This coating could be divided into three parts: an outer layer, an inner layer containing
of
discontinuous holes, and a barrier layer near A/C interface.
ro
The corresponding element distribution showed that the denser part of PEO coating prepared in silicate system was the Al-enriching zone, while the loose nodules outside the coating were almost completely
-p
occupied by Si and O elements. In addition, Si element was almost absent in the barrier layer near the
re
matrix, but some Si also enriched at the edge of cavity. The coating prepared in phosphate electrolyte had a uniform distribution of O and Al along the entire coating; interestingly, P element was mainly located at the
lP
A/C interface, and only a little P element existed on the coating surface. For the coating prepared in mixed
na
electrolyte, Si element was mainly located on the outer side of coating and the edge of cavities, but the distribution of P element was opposite to Si element, and was mainly present in the inner layer close to the
Jo ur
cavity.
Fig. 8
3.6 Elemental distribution along the cross-section of PEO coating formed by stepwise oxidation In order to further understand the action mechanism of anions in the film-formation processes, the samples were subjected to stepwise oxidation in two electrolytes, and then the structure and element distribution of coating was studied. Fig. 9 shows the SEM image and EDS mapping along the cross-section of PEO coating formed in silicate electrolyte for 45 min and transferred into phosphate electrolyte for another 10 min. As shown in Fig. 9a, PEO coating prepared by stepwise oxidation in two electrolytes had no obvious chasm phenomenon, and the coating thickness after two-step oxidation was about 30 μm. EDS mapping showed that Si element was mainly located at the middle and outside of PEO coating, and P element was still mainly located at A/C interface, and the surface only contained a little P. Since P element could only be
Journal Pre-proof introduced from the phosphate electrolyte during the second oxidation, this result indicated that P in the electrolyte passed through the Si-rich outer surface and the relative dense inner layer to arrive in the A/C interface and cavities during the second oxidation. This transmission of P element might be related to holes, cracks, and discharge channels [6, 11, 21, 23]. Fig. 9 Fig. 10 gives the SEM image and EDS mapping along the cross-section of PEO coating formed in phosphate electrolyte for 45 min and after in silicate electrolyte for 10 min. The figure showed that P
of
element was still mainly distributed in the inner side of coating, and a little P was present at the coating
ro
surface, even the sample was treated by in phosphate electrolyte in first and followed in silicate electrolyte. However, Si element was still mainly distributed in the outside of PEO coating. Besides, a discontinuous
-p
distributed Si-rich region also appeared in the inside of coating, and it exhibited a shape similar to the
re
discharge channel, connecting the inner and outside of the coating.
lP
Fig. 10
na
3.7 Discussion
As shown in Figs. 4, 6, 7 and 8, it was known that there were great difference on the three-dimensional
Jo ur
structure (whether the coating surface, A/C interface or cross-sectional morphology) of PEO coating formed in different electrolytes. Besides, the distribution characteristics of Si and P elements in the coating also had a difference. A large amount of Si was present in the coating and showed Si-enriching phenomenon in the nodules; however, only a little P was present at the A/C interface. It was believed that these differences were likely related to the growth mechanism of PEO coating and the deposition pattern of anions in the electrolyte. Researches had confirmed that the entry of anions into the coating depended on the chemical reaction/deposition under the action of plasma [15]. The anions in the silicate electrolyte were mainly SiO32-, meanwhile the hydrolysis of silicate produced partial OH- (Reac. 1); and the following reactions may occur during the PEO process [11, 13, 20]:
SiO32 2H 2O
H 2SiO3 +2OH -
(1)
Al Al3 3e
(2)
2Al3 +6OH Al2O3 3H 2O
(3)
Journal Pre-proof 2SiO32 4e 2SiO2 +O2
(4)
SiO2 xAl2O3 SiO2 （ x Al2O3）
(5)
The reactions (2) and (3) caused the oxidation of aluminum matrix, and the reaction (4) produced the deposition of the electrolyte compound. Since SiO32- had a strong adsorption capacity [20], the reaction (4) was easily generated under the high temperature action of plasma discharge. The rapid deposition of SiO2 resulted in a high growth rate and a rough surface for PEO coating, while the deposition of SiO2 did not consume the aluminum matrix, and the flat structure at A/C interface (Fig. 7a) also strongly proved that the
of
consumption of substrate was very weak. Therefore, the growth of PEO coating in silicate electrolyte was
ro
mainly dominated by the deposition of SiO2, accompanied by weak oxidation of aluminum matrix. Fig. 11a shows the growth model of PEO coating in silicate system. As shown in the figures, the growth of PEO
-p
coating was mainly pushed toward the electrolyte side, and a large amount of Si-rich compound was
re
deposited on the coating surface, and then repeatedly underwent the "remelting – mixing – solidification" process [24-27, 36, 37], so that the distribution of Si elements along the cross-section gradually increased
lP
from inner to outside, as shown in Fig. 8a. In addition, since SiO2 was stable in the coating, it could be sintered with Al2O3 to form mullite (Reac. 4) [38] in the subsequent formed micro-melting zone.
na
The reactions (2) and (3) also occurred in the phosphate electrolyte. However, sodium hexametaphosphate was easily hydrolyzed to orthophosphate by the following steps (Reac. 6-10) in an
Jo ur
alkaline environment [39]. Meta-phosphate might also be deposited (Reac. 11), but P2O5 could not be stable in alkaline electrolyte (Reac. 12) [40]. As can be seen from Figs. 5, 6 and 8, P content in the coating prepared in phosphate electrolyte was extremely low, and XPS results (Fig. 6d) showed that a small amount of P was mainly present in the form of metaphosphate, which was most likely the residual of electrolyte. Therefore, the growth of PEO coating in phosphate electrolyte was mainly occurred by the oxidation consumption of aluminum matrix, which inevitably led to a higher buldges at A/C interface (See Fig. 7b). That was to say, the growth of PEO coating in this system was mainly pushed toward the aluminum substrate, as shown in Figs. 11b.
( N a P 3O 6 )
6 N a3P O
2NaPO3 H2O Na 2 H2 P2O7 H2O
Na 2 H2 P2O7 2NaH 2PO4
(6) (7) (8)
Journal Pre-proof H 2 PO4 HPO4 2
H HPO4 2
(9)
H PO43
(10)
4PO3 4e 2P2O5 +O 2
(11)
P2O5 2OH 2PO3 +H 2O
(12)
For the coating prepared in a mixed electrolyte, the A/C interface morphology (see Fig. 7c) was close to that of the phosphate system, and no obvious nodules were present on the surface (see Fig. 4c); the
of
distribution characteristics of Si and P were also similar to those of the coatings prepared in the single electrolyte (Fig. 8c); and there was no change in the distribution of Si and P for the coating (Figs. 9 and 10)
ro
prepared through two-step oxidation, regardless of the order of two electrolytes. That was to say, the
-p
film-formation process was dominated by the matrix oxidation, and supplemented by the deposition of electrolytes. Therefore, the growth model of PEO coating in the mixed electrolyte was established, as
re
shown in Fig. 10(c).
lP
Fig. 11
na
In addition, the discharge mechanism is critical in the formation of PEO coating. Hussein et al. [41] believed that three types of discharges (type A, B and C) might occur during PEO process, which in turn
Jo ur
produced different structures. In general, type A and type C discharge reflected the gas discharge near the surface and in the micropores of coating respectively, which easily produced electrolyte-riching deposition structures. Type B discharge was the dielectric breakdown discharge originated in the substrate/ coating interface, which caused the oxidation of matrix and formed the pancake structure. Cheng et al. [42] also proposed type D discharge, that is, the breakdown discharge occurring in the large cavities inside the coating, resulting in the Si-rich phenomenon around the cavities. Combined with the above results, it could be dermined that the PEO system of silicate was more likely to breed type A, type C and type D discharges, meanwhile type B discharge was also present. However, type B discharge was dominant in phosphate system. Under the action of type B discharge, it was more likely to generate obvious "hill" -like protrusions at the aluminum/coating interface. Meanwhile, all the four types of discharge were also present in the mixed electrolyte. Different discharge types were also the essential cause of the difference in the microstructre and chemical composition of prepared coatings in these electrolytes..
Journal Pre-proof 4. Conclusions (1) The deposition of silicate dominated the growth of PEO coating in the silicate electrolyte. The coating was mainly pushed toward the electrolyte side, a large amount of loose nodules (mullite) were present on the surface, and A/C interface was relatively gentle. The Si element gradually increased from inside to outside and was enriched at the nodules and the edge of cavities. (2) In the phosphate system, the growth of PEO coating mainly depended on the oxidation of aluminum matrix, and the coating mainly advanced to the substrate. The coating surface was smooth and dense, but a large number of “hill”-like protrusions were present at A/C interface. The main component of this coating
of
was a-Al2O3, and a liitle P element was mainly distributed at A/C interface.
ro
(3) In the mixed electrolyte, the film-formation process was dominated by the matrix oxidation, and
-p
supplemented by the deposition of electrolytes. The morphology of A/C interface was close to that of the phosphate system, the distribution characteristics of Si and P were also similar to those of the coatings
lP na
Conflict of interest
re
prepared in a single electrolyte.
Jo ur
None.
Acknowledgements
The authors gratefully acknowledge the financial support of National Natural Science Foundation of China (Nos. 51801094 and 51361025) and the Natural Science Foundation of Jiangxi Province (No. 20171BAB216006).
References [1] A. L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S. J. Dowey, Plasma Electrolysis for Surface Engineering, Surf. Coat. Technol. 122 (1999) 73-93. [2] V. Antia, N. Saito, O. Takai, Microarc plasma treatment of titanium and alunminum surfaces in electrolytes, Thin Solid Films, 507 (2006) 364-368.
Journal Pre-proof [3] R. O.Hussein, D. O. Northwood, X. Nie, The effect of processing parameters and substrate composition on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloys, Surf. Coat. Technol. 237 (2013) 357-368. [4] T. Wei, F. Yan, J. Tian, Characterization and wear- and corrosion-resistance of microarc oxidation ceramic coatings on aluminum alloy, J. Alloy. Compd. 389 (2005) 0-176. [5] H. Fadaee, M. Javidi, Investigation on the corrosion behaviour and microstructure of 2024-T3 Al alloy treated via plasma electrolytic oxidation, J. Alloy. Compd. 604 (2014) 36-42. [6] H. Ma, D. Li, C. Liu, Z. Huang, D. He, Q. Yan, P. Liu, P. Nash, D. Shen, An investigation of (NaPO3)6
of
effects and mechanisms during micro-arc oxidation of AZ31 magnesium alloy. Surf. Coat. Technol.
ro
266 (2015) 151-159.
[7] Y. Mori, A. Koshi, J. Liao, H. Asoh, S. Ono, Characteristics and corrosion resistance of plasma
-p
electrolytic oxidation coatings on AZ31B Mg alloy formed in phosphate – silicate mixture electrolytes,
re
Corros. Sci. 88 (2014) 254-262.
[8] S.V. Stojadinovic, R. Belca, I. Petkovic, M. Kasalica, B. Nedic, Z. Zekovic, Lj, Characterization of the
lP
plasma electrolytic oxidation of aluminium in sodium tungstate, Corros. Sci. 52 (2010) 3258-3265.
na
[9] L. O. Snizhko, A. L. Yerokhin, A. Pilkington , N. L. Gurevina,D. O. Misnyankin, A. Leyland, A. Matthews, Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions,
Jo ur
Electrochim. Acta 49 (2004) 2085-2095.
[10] W. Mu, Y. Han, Study on micro-arc oxidized coatings on magnesium in three different electrolytes, Rare Metal Mater. Eng. 39 (2010) 1129-1134. [11] W. C. Gu, G. H. Lv, H. Chen, G. L. Chen, W. R. Feng, G. L. Zhang, S. Z. Yang, Investigation of morphology and composition of plasma electrolytic oxidation coatings in systems of Na2SiO3-NaOH and (NaPO3)6-NaOH, J. Mater. Process. Technol. 182 (2007) 28-33 [12] J. H. Lee, S. J. Kim, Effects of silicate ion concentration on the formation of ceramic oxide layers produced by plasma electrolytic oxidation on Al alloy, Jpn. J. Appl. Phys. 56 (2017) 01AB01. [13] Y. K. Wu, Z. Yang, R. Q. Wang, G. R. Wu, D. Chen, D. D. Wang, X. T. Liu, D. L. Li, C .H. Guo, S. X. Yu, D. J. Shen, P. Nash, An investigation of microstructure evolution for plasma electrolytic oxidation (PEO) coated Al in an alkaline silicate electrolyte, Surf. Coat. Technol. 351 (2018) 136-152.
Journal Pre-proof [14] Y. L. Cheng, M. K. Mao, J. H. Cao, Z. M. Peng, Plasma electrolytic oxidation of an Al-Cu-Li alloy in alkaline aluminate electrolytes: a competition between growth and dissolution for the initial ultra-thin films, Electrochim. Acta 138 (2014) 417-429 [15] S. Ono, S. Moronuki, Y. Mori, A. Koshi, J. Liao, H. Asoh, Effect of electrolyte concentration on the structure and corrosion resistance of anodic films formed on magnesium through plasma electrolytic oxidation, Electrochim. Acta 240(2017) 415-423. [16] M. Kaseem, M.P. Kamil, J.H. Kwon, Y.G. Ko, Effect of sodium benzoate on corrosion behavior of 6061 Al alloy processed by plasma electrolytic oxidation, Surf. Coat. Technol. 283 (2015) 268-273.
of
[17] X. Lu, M. Mohedano, C. Blawert, E. Matykina, R. Arrabal, K. U. Kainer, Plasma electrolytic
ro
oxidation coatings with particle additions – a review, Surf. Coat. Technol. 307 (2016) 1165-1182. [18] C. J. Hu, M. H. Hsieh, Preparation of ceramic coatings on an Al–Si alloy by the incorporation of ZrO2
-p
particles in microarc oxidation, Surf. Coat. Technol. 258 (2014) 275-283.
re
[19] E. Matykina, R. Arrabal, F. Monfort F, P. Skeldon, G.E. Thompson, Incorporation of zirconia into
Surf. Sci. 255 (2008) 2830-2839.
lP
coatings formed by DC plasma electrolytic oxidation of aluminium in nanoparticle suspensions, Appl.
na
[20] J. M. Li, H. Cai, X. N. Xue, B. L. Jiang, The outward–inward growth behavior of microarc oxidation coatings in phosphate and silicate solution, Mater. Lett. 64 (2010) 2102-2104.
Jo ur
[21] F. Monfort, E. Matykina, A. Berkani, P. Skeldon, G. E. Thompson, H. Habazaki, K. Shimizu, Species separation during coating growth on aluminium by spark anodizing. Surf. Coat. Technol. 201 (2007) 8671-8676.
[22] Z. Yang, Y. K. Wu, X. Z. Zhang, D. D. Wang, X. T. Liu, G. R. Wu, D. L. Li, S. X. Yu, D. J. Shen, An interesting anodic oxidation behavior of plasma electrolytic oxidation coatings fabricated on aluminum in alkaline phosphate electrolyte, Surf. Interfaces 16(2019) 199-205. [23] X. H. Liu, L. Q. Zhu, H. C. Liu, W. P. Li, Investigation of MAO coating growth mechanism on aluminum alloy by two-step oxidation method. Appl. Surf. Sci. 293 (2014) 12-17. [24] X. H. Liu, S. X. Wang, N. Du, X. Y. Li, Q. Zhao, Evolution of the three-dimensional structure and growth model of plasma electrolyticoxidation coatings on 1060 aluminum alloy, Coatings 8 (2018) 105 [25] W.B. Xue, Z.W. Deng, R.Y. Chen, T. H. Zhang, Growth regularity of ceramic coatings formed by microarc oxidation on Al–Cu–Mg alloy, Thin Solid Films 2000, 372 (1-2), 114-117.
Journal Pre-proof [26] A. L. Yerokhin, I. O. Snizhko, A. Leyland, Discharge characterization in plasma electrolytic oxidation of aluminium, J. Phys. D: Appli. Phys. 36 (2003) 2110-2120. [27] F. Monfort, A. Berkani, E. Matykina, P. Skeldon, G.E. Thompson, H. Habazaki and K.Shimizu, Development of anodic coatings on aluminium under sparking conditions in silicate electrolyte, Corros. Sci. 49 (2007) 672-693. [28] C. S. Duleav, I. O. Golosnoy, J. A. Curran, T. W. Clyne, Characterization of Discharge Events during plasma electrolytic oxidation, Surf. Coat. Technol. 203 (2009) 3410-3419. [29] E. Erfanifar, M. Aliofkhazraei, H. F. Nabavi, A. S. Rouhaghdam, Growth kinetics and morphology of
of
microarc oxidation coating on titanium. Surf. Coat. Technol. 315 (2017) 567-576.
ro
[30] A. E. Hughes, M. M. Hedges, B. A. Sexton, Reactions at the Al/SiO2 /SiC layered interface, J. Mater. Sci. 25 (1990) 4856-4865.
-p
[31] E. A. Koblova, A. Y. Ustinov, V. S. Rudnev, I. V. Lukiyanchuk, I. V. Chernykh, An x-ray photoelectron
re
spectroscopy study of Ni, Cu-containing coatings formed by plasma electrolytic oxidation on aluminum and titanium, J. Struct. Chem. 58 (2017) 1129-1136.
lP
[32] H. Schneider, J. Schreuer, B. Hildmann, Structure and properties of mullite—A review, J. Eur. Ceram.
na
Soc. 28 (2008) 329-344.
[33] C. D. Wagner, X-ray photoelectron spectroscopy with x-ray photons of higher energy. J. Vac.Sci.
Jo ur
Technol. 15 (1978) 518-523.
[34] A. Ayame, T. Kitagawa, X-Ray photoelectron spectroscopic measurement and chemical characteristics of silica, alumina and silica-alumina, Bunseki Kagaku 40 (1991) 673-678. [35] R. Q. Wang, Y. K. Wu, G. R. Wu, D. Chen, D. L. He, D. L. Li, C. H. Guo, Y. F. Zhou, D. J. Shen, P. Nash, An investigation about the evolution of microstructure and composition difference between two interfaces of plasma electrolytic oxidation coatings on Al, J. Alloy. Compd. 753 (2018) 272-281. [36] X. Y. Li, X. G. Li, Y. Li, C. F. Dong, H. P. Tian, S. X. Wang, Q. Zhao, Growth mechanism of micro-arc oxidation film on 6061 aluminum alloy, Mater. Res. Express 6 (2019) 066404. [37] T. W. Clyne, S. C. Troughton, A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals, Int. Mater. Rev. 64 (2019) 127-162. [38] P. D. D. Rodrigo, P. Bocha, High purity mullite ceramics by reaction sintering, Int. J.High Technol. Ceram. 1 (1985) 3-30.
Journal Pre-proof [39] D. D. Wang, X. T. Liu, Y. K. Wang, H. P. Han, Z. Yang, Y. Su, X. Z. Zhang, G. R. Wu, D. J. Shen, Evolution process of the plasma electrolytic oxidation (PEO) coating formed on aluminum in an alkaline sodium hexametaphosphate ((NaPO3)6) electrolyte, J. Alloy. Compd. 2019, 798,129-143. [40] Q. B. Li, W. B. Yang, C. C. Liu, D. A. Wang, J. Liang, Correlations between the growth mechanism and properties of micro-arc oxidation coatings on titanium alloy: Effects of electrolytes, Surf. Coat. Technol. 316 (2017) 162-170. [41] R.O. Hussein, X. Nie, D.O. Northwood, A. Yerokhin, A. Mattews, Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process. J. Phys. D:
of
Appl. Phys. 2010, 43 (10), 105203-105215.
ro
[42] Y. L. Cheng, Z.G. Xue, Q. Wang, X. Q. Wu, E. Matykina, P. Skeldon, G. E. Thompson, New findings on properties of plasma electrolytic oxidation coatings from study of an Al–Cu–Li alloy. Electrochim.
Jo ur
na
lP
re
-p
Acta 2013, 107, 358-378.
Journal Pre-proof
Figure captions Fig. 1 The schematic drawing of the change in sample dimension in the process of PEO Fig. 2 Effects of electrolyte system on the working voltage (a) and coating thickness (b) during PEO process of 1060 aluminum alloy. Fig. 3 Surface linear profiles of the PEO coatings formed in different electrolytes. Fig. 4 Surface morphology and element distribution for PEO coatings formed in different electrolytes:
of
Si-coating (a), P- coating (b) and Si-P- coating (c).
ro
Fig. 5 EDS spectra of the corresponding square area for Si- coating (a), P- coating (b) and Si-P- coating (c)
-p
in Fig. 4.
re
Fig. 6 XPS wide-survey spectrum (a), Al 2p spectrum (b), Si 2p spectrum (c) and P 2p spectrum (d) and XRD spectrum (e) of PEO coatings formed in different electrolytes.
lP
Fig. 7 SEM morphology of aluminum/coating interface of PEO coatings formed in different electrolytes:
na
Si-coating (a, d), P-coating (b, e) and Si-P-coating (c, f). Fig. 8 Fracture cross-section morphology and EDS mapping images of PEO coatings formed in different
Jo ur
electrolytes: Si-coating (a), P- coating (b) and Si-P- coating (c). Fig. 9 Fracture cross-section SEM image (a), elemental maps of O, Al, Si and P (b) and mixed elemental maps of Si and P (c) for PEO coating formed in silicate electrolyte for 45 min and transferred into phosphate electrolyte for another 10 min. Fig. 10 Fracture cross-section SEM image (a), elemental maps of O, Al, Si and P (b) and mixed elemental maps of Si and P (c) for PEO coating formed in phosphate electrolyte for 45 min and transferred into silicate electrolyte for another 10 min. Fig. 11 The growth model diagrams for PEO coatings in silicate electrolyte (a), phosphate electrolyte (b) and the mixed electrolyte (c).
Journal Pre-proof
Jo ur
na
lP
re
-p
ro
of
Fig.1 The schematic drawing of the change in sample dimension in the process of PEO
ro
of
Journal Pre-proof
Fig. 2 Effects of electrolyte system on the working voltage (a) and coating thickness (b) during PEO
Jo ur
na
lP
re
-p
process of 1060 aluminum alloy.
re
-p
ro
of
Journal Pre-proof
Jo ur
na
lP
Fig. 3 Surface linear profiles of the PEO coatings formed in different electrolytes.
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 4 Surface morphology and element distribution for PEO coatings formed in different electrolytes: Si-coating (a), P- coating (b) and Si-P- coating (c).
of
Journal Pre-proof
ro
Fig. 5 EDS spectra of the corresponding square area for Si- coating (a), P- coating (b) and Si-P- coating (c)
Jo ur
na
lP
re
-p
in Fig. 4.
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 6 XPS wide-survey spectrum (a), Al 2p spectrum (b), Si 2p spectrum (c) and P 2p spectrum (d) and XRD spectrum (e) of PEO coatings formed in different electrolytes.
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 7 SEM morphology of aluminum/coating interface of PEO coatings formed in different electrolytes:
Jo ur
Si-coating (a, d), P-coating (b, e) and Si-P-coating (c, f).
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 8 Fracture cross-section morphology and EDS mapping images of PEO coatings formed in different electrolytes: Si-coating (a), P- coating (b) and Si-P- coating (c).
Journal Pre-proof
of
Fig. 9 Fracture cross-section SEM image (a), elemental maps of O, Al, Si and P (b) and mixed elemental
ro
maps of Si and P (c) for PEO coating formed in silicate electrolyte for 45 min and transferred into
Jo ur
na
lP
re
-p
phosphate electrolyte for another 10 min.
Journal Pre-proof
of
Fig. 10 Fracture cross-section SEM image (a), elemental maps of O, Al, Si and P (b) and mixed elemental
ro
maps of Si and P (c) for PEO coating formed in phosphate electrolyte for 45 min and transferred into
Jo ur
na
lP
re
-p
silicate electrolyte for another 10 min.
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Fig. 11 The growth model diagrams for PEO coatings in silicate electrolyte (a), phosphate electrolyte (b) and the mixed electrolyte (c).
Journal Pre-proof
Author contrbutions section
The experimental scheme was framed by S. X. Wang and N. Du. The tests and characterization were carried out by S. X. Wang, X. H. Liu and X. L. Yin. The manuscript was composed by S. X. Wang and X. H. Liu, and revised by S. X.
Jo ur
na
lP
re
-p
ro
of
Wang.
Jo ur
na
lP
re
-p
ro
of
Journal Pre-proof
Graphical abstract
Journal Pre-proof
Research highlights 1. The deposition of silicate dominates the growth of PEO coating in silicate electrolyte.
2. Many loose Si-riching nodules are present on the surface of coating formed in silicate system.
of
3. The growth of PEO coating in phosphate system mainly depends on the oxidation of Al.
ro
4. Obvious “hill”-like protrusions are present at A/C interface of coating formed in phosphate
-p
system.
re
5. The oxidation of matrix accompanying electrolyte deposition forms PEO coating in the
lP
mixed electrolyte.
Jo ur
na
6. The growth model diagrams of PEO coatings in different electrolytes are established.