1.   Introduction

Titanium and its alloys are widely used because of their high specific strength, ductility, corrosion resistance, and bio inertness [1–3]. They are regarded as critical materials particularly in the aerospace industry [4–5]. However, they experience problems such as contact corrosion, cold welding, and surface oxidation in aerospace applications. Microarc oxidation (MAO), which is also known as plasma electrolysis oxidation (PEO) [6–7], is an efficient and advanced surface treatment technology in which nanocrystalline ceramic coatings are grown in situ on threshold metals (such as aluminum, magnesium, and titanium) and their alloys through plasma discharge in an electrolyte at an instantaneous high temperature and high pressure [8–9]. Compared to conventional anodic oxidation processes, MAO uses a specific alternating current (AC) or pulse power supply with a high voltage (~250–750 V), which promotes the generation of microarcs and repeated breakdown of ceramic coatings [10]. The heat generated by microarcs considerably facilitates the crystallization of surrounding oxides, resulting in the in-situ formation of a crystalline ceramic coating. MAO coatings have a large thickness, high hardness, and strong bonding strength. This expands the application range of titanium and its alloys in harsh space environments [6, 11].

Unlike the continuous ion transport mechanism of anodic oxidation, MAO is complex and involves plasma physics, plasma chemistry, electrochemistry, thermochemistry, and acoustics [12]. Therefore, the mechanisms of plasma discharge and MAO coating growth are still unclear, particularly in titanium alloys. In addition to the threshold metal and power supply, the migration of electrolyte ions is important for revealing the growth mechanism of MAO coatings. Extensive research has been performed on silicate-based and phosphate-based electrolytes in this area. Laveissière et al. [13] used electrolytes with increasing complexity, i.e., mono-component, bi-component, and tri-component electrolytes, to study the influence of silicates on MAO coating characteristics. They found that the amorphous phase resulted directly from the presence of silicate in solution, and contained complex Si-based oxides that were difficult to clearly identify. Han et al. [14] controlled the oxidation time in a concentrated silicate electrolyte, and found that the silicate ions decomposed and deposited by plasma discharges contributed significantly to the formation of MAO coatings. Li et al. [15] found that the growth of MAO coatings in silicate electrolytes was dominated by the deposition of silicate oxides and mostly characterized by outward growth. In contrast, the growth of MAO coatings in phosphate electrolytes was dominated by the oxidation of titanium alloy substrates, which led to more inward growth. Aliasghari et al. [16] applied a wide range of duty cycles, current densities, waveforms, and treatment time and found that the silica was present in the veins that penetrate inner titania-rich materials. Cheng et al. [17] studied the kinetics of the formation of a MAO coating on Ti–6Al–4V alloy in a silicate-hexametaphosphate electrolyte and found that the coating growth rate was relatively high in the initial stages of the treatment, and it significantly decreased as the coating thickness increased. Ao et al. [18] used ultrasonic surface rolling to pretreat a titanium alloy and revealed that P from the silicate-phosphate electrolyte showed significant levels of agglomeration. This was correlated with the dissolution of the P-containing oxide, non-uniformity of discharge sparks, and difficult long-distance diffusion of long-chained (NaPO₃)₆. Mortazavi et al. [19] proposed that P-containing ions have low mobility and cannot diffuse through oxides; hence, they remain at the top of MAO coatings. It is currently believed that the use of a mixed silicate and phosphate electrolyte is a feasible optimization strategy to obtain coating with excellent performance, such as high adhesion and improved wear resistance [15]. However, the growth kinetics mechanism of MAO coating on titanium alloy in phosphate/silicate electrolyte is not yet well understood, either as a single electrolyte or a mixed electrolyte with a constant concentration of components.

In this study, binary electrolytes consisting of sodium phosphate and sodium silicate with different amounts of ${\text{PO}}_4^{3 - }$ and ${\text{SiO}}_3^{2 - }$ were designed for the MAO of titanium alloy. The surface morphology, composition, and properties of MAO coatings were investigated to reveal the effect of ${\text{PO}}_4^{3 - }$ and ${\text{SiO}}_3^{2 - }$ on MAO coating growth. The novelty of this method is that the specific effects of different anions on MAO coatings are used to dynamically elucidate the phenomena that are not easily observed in a single electrolyte or a mixed electrolyte with a constant concentration of components.

2.   Experimental

Ti–6Al–4V alloy (Baoji Titanium Industry Co., Ltd., China) samples with a diameter of 10 mm and thickness of 5 mm were used for MAO experiments. The alloy composition is shown in Table 1. MAO was carried out using pulsed electrical power (5005M, ANS Power Supply Co., LTD., Wuxi) at 5 A/dm². The pulse frequency and duty cycle of the power were set to 200 Hz and 10%, respectively. The electrolyte temperature was kept below 30°C by a heat-exchange system during the MAO process. Mixed electrolytes of Na₃PO₄·12H₂O and Na₂SiO₄·9H₂O with different ratios were prepared, and their compositions are shown in Table 2. Five time points (30, 240, 600, 960, and 1800 s) were selected to investigate coating growth based on the voltage–time response of the MAO process.

Table  1.  Chemical composition of Ti–6Al–4V wt%

Ti	Fe	H	O	Al	V
0.1	0.3	0.015	0.2	6.1	4.1

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Table  2.  Molarity of anions in the mixed electrolytes mol·L⁻¹

Group	${\rm PO}_4^{3-}$	${\rm SiO}_3^{2-}$
P10	0.10	0
P7Si3	0.07	0.03
P6Si4	0.06	0.04
P5Si5	0.05	0.05

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The surface and cross-sectional morphology of the MAO coatings were observed by scanning electron microscopy (SEM; Phenom ProX, Funa Scientific Instruments Co. Ltd., Netherlands) and transmission electron microscopy (TEM; JEM-2100F, JEOL Co., Ltd., Japan). The distributions of Ti, O, Al, Si, and P were determined by energy dispersive X-ray spectroscopy (EDS; Phenom ProX, Funa Scientific Instruments Co. Ltd., Netherlands). The thickness of the MAO coating was detected by a coating thickness gauge (DUALSCOPE®MP0, Fischer, Germany), and the coating thickness was measured ten times at different locations and their average value was used as the final measured value of thickness. Cu K_α radiation was used at 40 kV and 30 mA over a 2θ range of 10° to 90° with a scan speed of 0.1°·s⁻¹ to analyze the phase composition of the coating by X-ray diffraction (XRD; AL-2700B, Aolong Ray Instrument Group Co. Ltd., China). The potentiodynamic polarization curves of the samples with MAO treatment were measured in the 3.5wt% NaCl solution using an electrochemical measurement system (CS120, Wuhan Corrtest Instruments Corp., Ltd., China) (current precision 1 nA, voltage precision 500 μm) with a scan rate of 0.1 mV/s, from −0.4 to 0.2 V. Prior to the test, all samples were immersed in the test solution for 4 h to attain a stable open circuit potential (OCP). The OCP measurements were conducted every 5 min during this time.

3.   Results and discussion

3.1   Characteristics of the MAO process

The typical voltage–time response during the MAO of the Ti–6Al–4V alloy with the P/Si electrolytes is shown in Fig. 1. On the basis of the variation in plasma discharge intensity, MAO is divided into three processes, i.e., anodic oxidation, spark discharge, and microarc discharge, which is consistently observed in most studies [15,19–21]. During anodic oxidation, the voltage increased rapidly and linearly in all electrolytes. Bubbles were formed, but no discharge spark was observed. When the processing time exceeded 7 s, a faintly visible and relatively slow crackling discharge of electricity without sparking was observed on the surfaces of the samples. In addition, the voltage growth rate decreased, indicating that the anodized coatings were broken down. The breakdown voltage of the anodized coatings on the titanium alloy was approximately 140 V for all electrolytes. During spark discharge, white sparks were observed on the surfaces of the samples, and voltage increases exponentially. The voltage growth rate increased with increasing ${\text{SiO}}_3^{2 - }$ content. During microarc discharge, the voltage increased linearly, and the discharge sparks were quite violent and bright. The microarc discharge process is divided into stages I, II, and III based on the voltage growth rate. The voltage growth gradually decreased as the stages progress. Additionally, although the voltages at each stage are different in the four electrolytes, the voltage growth rates were similar. The duration of stage I and stage II of microarc discharge decreases as the ${\text{SiO}}_3^{2 - }$ content increases.

Figure  1.  Voltage–time response during the MAO of the Ti–6Al–4V alloy with the P/Si electrolytes and morphologies of the sparks formed in different stages on the sample.

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3.2   Morphology and composition of the MAO coatings

Fig. 2 shows the surface morphology of a coating formed on the Ti–6Al–4V alloy in the P/Si electrolytes. For a treatment time of 30 s, submicron pores and worm-like discharge channels with a width of ~2 μm and a length of ~10 μm were observed on the surface of the P10 sample. This indicates that the first layer of the MAO coating formed during spark discharge was broken down and a second layer was formed gradually during stage I of microarc discharge. As the ${\text{SiO}}_3^{2 - }$ content increased, the worm-like discharge channels changed into circular pores; the size of these pores decreased but their number increased gradually over time. When the treatment time reached 240 s, almost all previous submicron pores on the surface of the P10 sample were replaced by discharge pores with diameters of 2–5 μm. In addition, a part of the molten oxide produced by the secondary discharge was deposited on the surface of the P10 sample. As the ${\text{SiO}}_3^{2 - }$ content increased, the flat topography of the secondary discharge pores changed into a typical crater morphology; the size increased but the number decreased gradually. This is because microarc discharge in phosphate electrolytes is easy and evenly distributed, but that in silicate is difficult and locally concentrated. As the treatment time increased to 1800 s, a few discharge pores with a diameter of 5–10 μm were observed on the surface of the P10 sample. This indicates that the MAO coating was broken down for the third time and a third layer of the MAO coating was gradually formed during stage II. As the ${\text{SiO}}_3^{2 - }$ content increased, the diameter of the discharge pores gradually increased to 10–15 μm and a few microcracks were observed on the MAO coating. Moreover, there was an extremely small number of discharge pores with a diameter of 15–20 μm in the local area of the samples, indicating that the fourth layer of the MAO coating gradually formed during stage III of microarc discharge.

Figure  2.  Surface morphology of the MAO coatings obtained in (a) P10, (b) P7Si3, (c) P6Si4, and (d) P5Si5 electrolytes.

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Fig. 3 shows the surface element distributions of the MAO coatings obtained at 1800 s in the P/Si electrolyte. The elemental compositions of MAO coating were analyzed by EDS. The Si content on the surface of the MAO coating was significantly higher than the P content. This is because amorphous SiO₂ is insoluble in the alkaline electrolyte and easily deposited on the coating surface, whereas P₂O₅ is easily hydrolyzed to ${\text{PO}}_4^{3 - }$ (or ${\text{HPO}}_4^{2 - }$, ${\text{H}}_2 {\rm{PO}}_4^{- }$) [15,22]. These anions participated in the following reactions [14,18,22]:

Figure  3.  EDS map scanning spectra and elemental distribution on the MAO coatings obtained at 1800 s in (a) P10, (b) P7Si3, (c) P6Si4, and (d) P5Si5 electrolytes.

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As the ${\text{SiO}}_3^{2 - }$ content increased, the mass percentage of Si on the coating surface increases linearly and rapidly, whereas the mass percentage of P remained quite low. This indicates that the silicate has a strong effect on the MAO coating morphology, whereas the phosphate has a weak effect.

Fig. 4 presents the cross-sectional morphologies and EDS spectra of the MAO coatings obtained at 1800 s in the P/Si electrolytes. A few supersized pores with a diameter of 10–20 μm were observed in the MAO coatings on the P10, P7Si3, and P6Si4 samples. Previous studies [17–18,22] have proposed that ${\text{PO}}_4^{3 - }$, ${\text{H}}_2 {\rm{PO}}_4^{ - }$, and ${\text{HPO}}_4^{2- }$ in the molten oxide are ionized to P₂O₅ during plasma discharge and then decomposed at high temperature. P enters the molten oxide, which releases oxygen, resulting in the formation of cavities. A dense oxide layer with a thickness of approximately 1 μm was observed at the coating/substrate interface, where there are a few submicron pores (Fig. 4(c)). Herein, the porous layer formed by the external deposition of the molten oxide is referred to as the outer deposition layer, and the dense layer formed by the residual of the molten oxide is referred to as the inner barrier layer [23–26]. As the ${\text{SiO}}_3^{2 - }$ content increased, the number of submicron pores in the inner barrier layer decreased. In addition, the size of cavities in the outer deposition layer first increased and then decreased, and the number of pores decreased gradually. Moreover, plasma discharge in the coating gradually transferred outward. It can be inferred that amorphous SiO₂ and the molten oxide were co-deposited in the outer deposition layer, which blocked parts of the discharge channels. Hence, the second or third discharge transferred to the outer coating. The EDS spectra show that the regions of the coating with more pores contained higher P contents than the dense regions. Moreover, a large amount of Si was detected in the outer deposition layer and a small amount in the inner barrier layer. Generally, ${\text{PO}}_4^{3 - }$ is beneficial for the generation of microarcs and the formation of cavities within the coating, resulting in a thick but porous coating. ${\text{SiO}}_3^{2 - }$ impedes the generation of microarcs, and amorphous SiO₂ fills some of the pores, resulting in a thin dense coating.

Figure  4.  Cross-sectional morphologies and EDS spectra of the MAO coatings obtained at 1800 s in (a) P10, (b) P7Si3, (c) P6Si4, and (d) P5Si5 electrolytes.

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Fig. 5 shows a TEM bright-field image, high-magnification image, selected area diffraction patterns, and EDS spectrum of the MAO coating obtained in the P5Si5 electrolyte. There was an evident layer of fully amorphous TiO₂ with a thickness of approximately 50–100 nm at the coating/substrate interface. After the amorphous layer, there was an intermediate zone consisting of amorphous phase and crystallized anatase and a few rutile TiO₂ grains. Moreover, several large pores surrounded by the amorphous phase with a high P content were observed. This reveals that the transport behavior of ${\text{PO}}_4^{3 - }$ and the formation of pores are related. However, negligible electrolyte elements were detected in the amorphous layer at the coating/substrate interface, indicating that ${\text{PO}}_4^{3 - }$ diffuses into the nanocrystalline region but did not cross the amorphous phase. At the interface between the inner barrier layer and outer deposition layer, a high content of Si was detected, confirming that almost all of the amorphous SiO₂ was co-deposited in the outer deposition layer.

Figure  5.  (a) TEM bright-field image, (b) high magnification image and diffraction patterns of the selected area, and (c) EDS spectrum of the MAO coating obtained in P5Si5 electrolyte.

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The XRD patterns of the coated samples after 1800 s in the P/Si electrolytes are shown in Fig. 6. The broad peak between ~15° and 30° was attributed to the amorphous material. The magnitude of this peak increased with increasing ${\text{SiO}}_3^{2 - }$ content. The TEM and EDS results confirmed that this broad peak was formed by amorphous SiO₂. The phase contents were quantitatively analyzed using the adiabatic method [27] and are shown in Table 3. The content of the rutile phase in the coating was significantly higher than that of the anatase phase, which was due to the transition from the anatase phase to the rutile phase at high temperature (more than 550°C) during plasma discharge. As the ${\text{SiO}}_3^{2 - }$ content increased, the content of the anatase and rutile phases in the coating first increased and then decreased, reaching the maximum for the P7Si3 sample. This is because the growth of MAO coatings is primarily affected by the intensity of microarcs when the ${\text{SiO}}_3^{2 - }$ content is low and by the number of microarcs when the ${\text{SiO}}_3^{2 - }$ content is sufficiently high.

Figure  6.  XRD spectra of the MAO coatings obtained at 1800 s in P/Si electrolytes.

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Table  3.  Mass variation of the phases during the MAO process

Group	Mass variation / %
	Rutile	Anatase	Ti and amorphous
P10	42.57	34.46	22.97
P7Si3	62.88	15.97	21.15
P6Si4	46.71	15.91	37.38
P5Si5	19.70	13.09	67.21

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3.3   Electrochemical behavior of the MAO coatings

The electrochemical responses of the MAO-coated samples in a simulated corrosion process were evaluated by potentiodynamic polarization test in 3.5wt% NaCl solution, which has a similar salinity to that of sea water. Fig. 7 presents the potentiodynamic polarization curves of the MAO-coated samples fabricated in all electrolytes. The anodic/cathodic Tafel slopes (β_a and β_c), corrosion potential (E_corr), and corrosion current density (j_corr) were derived from the data by Tafel extrapolation, as listed in Table 4. The polarization resistance (R_p) was calculated using the Stern-Geary equation [28]. The results of electrochemical tests showed that the polarization resistance of the coating gradually increased with increasing ${\text{SiO}}_3^{2 - }$ content, except for the P7Si3 sample with the thickest coating. This shows that the corrosion resistance is related to the density and thickness of the coatings. The polarization resistance of P7Si3 sample was abnormal, which may be due to the thick MAO coating formed on the sample, which slightly improves the corrosion resistance of the coating (Table 5). Although the coating obtained in P6Si4 electrolyte was denser than the other, it was thicker than that formed in P10 electrolyte, resulting in similar polarization resistances for both samples.

Figure  7.  Potentiodynamic polarization curves of the MAO coatings obtained at 1800 s in P/Si electrolytes.

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Table  4.  Potentiodynamic polarization data of the MAO coatings obtained at 1800 s in P/Si electrolyte

Sample	βa / mV	βc / mV	icorr / (μA·cm−2)	Ecorr / mV	Rp / (kΩ·cm2)
P10	2505.4	3493.6	36.54	–0.09431	17.340
P7Si3	1315.9	2736.8	17.82	–0.12161	21.658
P6Si4	911.2	1407.2	13.65	–0.11736	17.597
P5Si5	1764.6	2276.2	15.15	–0.098378	28.495

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Table  5.  Thickness of the MAO coatings obtained at 1800 s in P/Si electrolytes µm

P10	P7Si3	P6Si4	P5Si5
42.435	47.155	39.873	29.681

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3.4   Growth kinetics and MAO coatings mechanism

As shown in Fig. 8, the thickness (h) of MAO coatings in the P/Si electrolytes showed a piecewise linear increase with the process time (t). The growth of the coatings is described by the following equation:

Figure  8.  Fitted curves of the thickness of MAO coatings obtained in P/Si electrolytes.

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where a is a constant that represents the coating growth rate. The results showed that the value of a in stage I of microarc discharge is approximately the same as that in stage II, but significantly larger than that in stage III. Previous studies [29–30] showed that the incorporation rate of electronic or ionic charges plays a key role in coating growth. The variation in the incorporation rate of electronic charges between stages I and II of microarc discharge is negligible [19], resulting in approximately the same coating growth rate. However, the incorporation rate of electronic charges decreases significantly and then remains stable in stage III of microarc discharge, resulting in a linear increase of the coating at a low rate. Moreover, the duration of stage II of microarc discharge gradually decreases with increasing ${\text{SiO}}_3^{2 - }$ content because a high content of ${\text{SiO}}_3^{2 - }$ can significantly promote the growth of a dense coating. In addition, as the ${\text{SiO}}_3^{2 - }$ content increases, the value of a in stages I and II of microarc discharge first increases and then decreases, whereas it gradually decreases in stage III. This is attributed to the variation in the intensity and number of microarcs, which depend on the ${\text{SiO}}_3^{2 - }$ content, and are illustrated for the different stages of the MAO process in Fig. 1.

Based on an analysis of the process voltage, coating morphology, coating composition, and electrochemical performance, a model of the mechanism of anion migration during the growth of MAO coatings is proposed to verify the test results, as shown in Fig. 9. A large number of micron pores, cracks, and filamentous channels were observed in the MAO coating, which constitute the multichannel network of ion transfer, as shown in Fig. 9(a). ${\text{PO}}_4^{3 - }$ is dehydrated to form P₂O₅, which reacts with H₂O to form ${\text{PO}}_4^{3 - }$, ${\text{H}}_2 {\rm{PO}}_4^{ - }$, and ${\text{HPO}}_4^{2 - }$ [19]. The surface conductivity of the coating increases because these anions are transferred from the outer deposition layer to the inner barrier layer through a multi-channel network in the high intensity electric field, which is beneficial for the generation of microarcs. However, it is difficult for ${\text{SiO}}_3^{2 - }$ to pass through the inner barrier layer containing filamentous channels because SiO₂ formed at a high temperature is insoluble in alkaline electrolytes and typically blocks the networks or deposits on the surface of dense layers. Studies have suggested that the growth of the amorphous TiO₂ layer at the coating/substrate interface is related to the diffusion reaction of O⁻ with Ti⁴⁺ [31–32]. The resistance and voltage in the circuit gradually increases with increasing thickness of the amorphous TiO₂ layer, resulting in filamentous discharge and the breakdown of the inner barrier layer, as shown in Fig. 9(b). The coating and metal melt at the high temperature of the microarcs, which is beneficial for the dehydration reaction of ${\text{SiO}}_3^{2 - }$ and the diffusion reaction of O⁻, ${\text{PO}}_4^{3 - }$, and Ti⁴⁺ into the molten metal. Subsequently, O⁻ reacts with Ti⁴⁺ to form the rutile and anatase phases, and ${\text{PO}}_4^{3 - }$ in the molten metal is decomposed into P and O₂. Here, P contributes to the formation of amorphous TiO₂ and the evolution of O₂ promotes the generation of pores, as shown in Fig. 9(c). In general, P contributes to the formation of pores or discharge channels in the coating, whereas SiO₂ always hinders it.

Figure  9.  Schematic diagram of anion transfers (a) before, (b) during, and (c) after microarcs discharge.

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4.   Conclusions

MAO electrolytes were developed with low ${\text{SiO}}_3^{2 - }$ content and high ${\text{PO}}_4^{3 - }$ content, which revealed the influence of the anions in the electrolyte on the growth of MAO coatings. The major findings are as follows.

(1) ${\text{PO}}_4^{3 - }$ is beneficial for the generation of microarcs and the formation of cavities within the coating, resulting in a thick but porous coating. ${\text{SiO}}_3^{2 - }$ facilitates the block of pores or cavities in the outer deposition layer and impedes the generation of microarcs, resulting in a thin and dense coating.

(2) The thickness, density, phase content, and polarization resistance of the MAO coatings are primarily affected by the intensity of microarcs when the ${\text{SiO}}_3^{2 - }$ content is low, and by the number of microarcs when the ${\text{SiO}}_3^{2 - }$ content is sufficiently high.

(3) The thickness of MAO coatings obtained in the P/Si electrolytes have a piecewise linear increase with increasing process time during the three stages of microarc discharge. ${\text{SiO}}_3^{2 - }$ is beneficial to the density increase of the coating formed in the previous stage of microarcs discharge, but slows down the growth of the coating formed in the next stage.

Acknowledgements

This research is financially supported by China Postdoctoral Science Foundation (No. 2021M700569) and Chongqing Postdoctoral Science Foundation (No. cstc2021jcyj-bsh0133).

Conflict of Interest

The authors declare no conflict of interest.