**1. Introduction**

Lightweight metals, e.g. aluminum (Al), magnesium (Mg), titanium (Ti) and their alloys are of great importance for applications in various machinery and transportation system, espe‐ cially in aerospace and automobile products due to their high strength-to-weight ratio and superior physical and chemical performances. However, their poor tribological properties, such as low wear resistance, high friction coefficient and difficulty to lubricate, have seriously restricted their extensive applications.

In the past decades, various traditional surface treatments, such as physical vapor deposition [1-2], chemical vapor deposition [3], ion beam assisted deposition [4] and spraying [5], have been applied to metallic substrates to improve their generally poor tribological properties. However, most of the aforementioned methods involve high processing temperature, which may degrade the coatings and/or substrates. Here, a relatively novel technique, plasma electrolytic oxidation (PEO) treatment used to improve the tribological properties of light‐ weight metals was introduced.

#### **1.1. The origin of PEO technique**

Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO) [6], micro-plasma oxidation (MPO) [7], anodic spark deposition (ASD) [8] or micro-arc discharge oxidation (MDO) [9] in modern scientific literatures, is derived from conventional anodizing [10-11]. Anodizing is traditionally carried out using direct current (DC) electrolysis. The workpiece is made anodic in an acid electrolyte (sulfuric acid is most commonly used, but phosphoric, oxalic, chromic and other acids can be used, singly or in combination). Typically, the cell voltage is 20 to 80 V DC and the current density is 1 to10 A dm-2, the process usually being controlled at a constant cell voltage. Plasma electrolytic oxidation (PEO) treatment usually

© 2013 Li et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Li et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

carried out in high voltage condition which is introduced into the high-pressure discharge area from the Faraday region of traditional anodizing. The applied voltage is increased from tens of volts to hundreds of volts, which is the breakthrough of traditional anodizing. The voltage forms developed from DC to continuous pulse, and then to AC, resulting in corona, glow, spark discharge and even micro-arc discharge phenomenon in the surface of the samples [12]. The general comparison between conventional DC anodizing and PEO technique was shown in Table 1.


resulting in the occurrence of micro-arc discharge phenomenon. The process of electrical breakdown of PEO involves many physical (such as crystallizing, melting, phase change at high temperature and electrophoresis, etc.), chemical (such as high-temperature chemistry, plasma chemistry) and electrochemical processes. A variety of models and hypotheses on the electrical breakdown of PEO process were created by many researchers to explain its causes. Therefore, the theory of electric breakdown also experienced different stages of development, such as thermal mechanism, mechanical effects and the mechanism of electron avalanche. The mechanism is so complex that, to date, no theory can give a complete and accurate explain of

1. Insulated enclosure; 2. High voltage AC power supply; 3. Workpiece (light metal substrate); 4. Electrolyte holding tank and counter electrode (stainless steel); 5. Fume extraction vent; 6. Viewing window; 7. Electrolyte mixer; 8. Flow

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Recently, some studies have showed some basic processes of PEO treatment which involves: the formation of space charges in the oxide matrix; gas discharge generated in the pores of oxide; localized melting of the layer material; thermal diffusion; deposition of the colloidal particles; migration of negatively charged colloidal particles into the discharge channels;

The PEO coatings generally consist of a porous top layer, compact intermediate layer and thin inner layer. The intermediate layer and inner layer are dense and adhered well to the substrate [12, 16]. The surface morphologies characterized with many micropores, microcracks and dimples [17]. Previous studies showed that the micropores were formed by molten oxide and gas bubbles thrown out of micro-arc discharge channels and the cracks resulted from the thermal stress due to the rapid solidification of the molten oxide in the relatively cold electrolyte.

The PEO ceramic coatings are composed of not only predominant substrate metal oxides (such as Al2O3, TiO2 and MgO on Al, Ti, Mg and their alloys respectively)[12], but also more complex oxides and compounds which involve the components presented in the electrolytes (such as

the whole PEO process. [14-15].

circulation via chiller and filter

**Figure 1.** Equipment used for PEO treatment [12].

plasma chemical and thermochemical reactions, etc. [14-15].

**1.4. Structures and compositions of PEO coatings**

**Table 1.** General comparison between conventional direct current anodizing and plasma electrolytic oxidation coating technologies [12]

#### **1.2. Process of PEO treatment**

A typical equipment used for PEO treatment is shown in Figure 1. An enclosure (1) is mounted close to a high voltage AC power supply (2). The metal substrate to be PEO coated (3) is im‐ mersed in the electrolyte in a water cooled, insulated electrolyte tank (4) made of stainless steel and also serves as the counter electrode. The tank is insulated from ground and mounted in the safety interlocked enclosure (1), the latter being equipped with fume extraction facilities (5) and a window (6) to allow the PEO process to be viewed. The electrolyte is typically mixed (7) and recycled via a flow circuit containing a heat exchanger/chiller and a 50 to 100 mm filter (8) [12].

Before the PEO treatment, the samples should be ground and polished with abrasive paper, degreased ultrasonically in acetone and cleaned with distilled water. During the treatment, the samples are used as anode plates and immersed in the electrolyte which is cooled by a water cooling system and mechanically stirred by a mixer. After the treatment, the samples should be rinsed with distilled water and air dried [12].

#### **1.3. Mechanism of PEO technique**

When the samples of valve metals [13] or their alloys are placed in the electrolyte, the metal surface immediately generates a layer of insulating oxide film after the energization. The weak parts of the oxide film were broke down after the applied voltage exceeds a critical value,

1. Insulated enclosure; 2. High voltage AC power supply; 3. Workpiece (light metal substrate); 4. Electrolyte holding tank and counter electrode (stainless steel); 5. Fume extraction vent; 6. Viewing window; 7. Electrolyte mixer; 8. Flow circulation via chiller and filter

**Figure 1.** Equipment used for PEO treatment [12].

carried out in high voltage condition which is introduced into the high-pressure discharge area from the Faraday region of traditional anodizing. The applied voltage is increased from tens of volts to hundreds of volts, which is the breakthrough of traditional anodizing. The voltage forms developed from DC to continuous pulse, and then to AC, resulting in corona, glow, spark discharge and even micro-arc discharge phenomenon in the surface of the samples [12]. The general comparison between conventional DC anodizing and PEO technique was shown

**Properties Anodizing PEO technique**

Cell voltage (V) 20-80 120-300 Current density (A/dm2) < 10 < 30

Substate pretreatment Critical Less critical

Coating hardness Moderate Relatively high Adhesion to substrate Moderate Very high

Temperature control critical Not so important

Coating thickness (μm) < 10 < 200

should be rinsed with distilled water and air dried [12].

**1.3. Mechanism of PEO technique**

Common electrolytes Sulfuric, chromic, or phosphoric Neutral/alkaline (pH=7-12)

**Table 1.** General comparison between conventional direct current anodizing and plasma electrolytic oxidation

A typical equipment used for PEO treatment is shown in Figure 1. An enclosure (1) is mounted close to a high voltage AC power supply (2). The metal substrate to be PEO coated (3) is im‐ mersed in the electrolyte in a water cooled, insulated electrolyte tank (4) made of stainless steel and also serves as the counter electrode. The tank is insulated from ground and mounted in the safety interlocked enclosure (1), the latter being equipped with fume extraction facilities (5) and a window (6) to allow the PEO process to be viewed. The electrolyte is typically mixed (7) and recycled via a flow circuit containing a heat exchanger/chiller and a 50 to 100 mm filter (8) [12].

Before the PEO treatment, the samples should be ground and polished with abrasive paper, degreased ultrasonically in acetone and cleaned with distilled water. During the treatment, the samples are used as anode plates and immersed in the electrolyte which is cooled by a water cooling system and mechanically stirred by a mixer. After the treatment, the samples

When the samples of valve metals [13] or their alloys are placed in the electrolyte, the metal surface immediately generates a layer of insulating oxide film after the energization. The weak parts of the oxide film were broke down after the applied voltage exceeds a critical value,

in Table 1.

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coating technologies [12]

**1.2. Process of PEO treatment**

resulting in the occurrence of micro-arc discharge phenomenon. The process of electrical breakdown of PEO involves many physical (such as crystallizing, melting, phase change at high temperature and electrophoresis, etc.), chemical (such as high-temperature chemistry, plasma chemistry) and electrochemical processes. A variety of models and hypotheses on the electrical breakdown of PEO process were created by many researchers to explain its causes. Therefore, the theory of electric breakdown also experienced different stages of development, such as thermal mechanism, mechanical effects and the mechanism of electron avalanche. The mechanism is so complex that, to date, no theory can give a complete and accurate explain of the whole PEO process. [14-15].

Recently, some studies have showed some basic processes of PEO treatment which involves: the formation of space charges in the oxide matrix; gas discharge generated in the pores of oxide; localized melting of the layer material; thermal diffusion; deposition of the colloidal particles; migration of negatively charged colloidal particles into the discharge channels; plasma chemical and thermochemical reactions, etc. [14-15].

#### **1.4. Structures and compositions of PEO coatings**

The PEO coatings generally consist of a porous top layer, compact intermediate layer and thin inner layer. The intermediate layer and inner layer are dense and adhered well to the substrate [12, 16]. The surface morphologies characterized with many micropores, microcracks and dimples [17]. Previous studies showed that the micropores were formed by molten oxide and gas bubbles thrown out of micro-arc discharge channels and the cracks resulted from the thermal stress due to the rapid solidification of the molten oxide in the relatively cold electrolyte.

The PEO ceramic coatings are composed of not only predominant substrate metal oxides (such as Al2O3, TiO2 and MgO on Al, Ti, Mg and their alloys respectively)[12], but also more complex oxides and compounds which involve the components presented in the electrolytes (such as Al2O3 on Ti6Al4V alloy in aluminate solution [18]; mullite on Al alloy in silicate solution [19]; MgF2 on Mg alloy with KF in the electrolyte [20] and TiO2 on Mg alloy in phosphate solution containing titania sol [21] ).

cathode plates located face to face and separated by different distances. Results showed that the anode current was influenced by the distance between the electrodes and had a critical effect on the oxidation efficiency. The anode currents were found to decrease with larger distances. The current flowing through the front surface was higher than that through the back surface. The ball-on-disk tribological tests and corrosion tests revealed that the front surface has better tribological properties and higher corrosion resistance than the back surface.

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R.O. Hussein et al. [27] studied the influence of different current modes on PEO plasma discharge behaviour and alumina coating microstructure. The PEO processes were carried out on pure 1100 aluminum using two different current modes of pulsed unipolar and bipolar in the electrolyte of Na2SiO3 (7 g/L) and KOH (1 g/L). It was found that the plasma temperatures vs. process time were different under different current modes. The plasma temperature spikes were believed to be caused by the strongest plasma discharges initiated at the interface between the oxide coating and substrate. Compared to the unipolar current process, the application of pulsed bipolar current resulted in reducing the high spikes on temperature profiles and the average plasma temperature. The aluminum oxide coating morphology and microstructure were also significantly different under different current modes. The bipolar current mode could improve the coating quality compared with the unipolar current mode, in terms of surface morphology and cross-sectional microstructure. A dense coating morphology could be achieved by adjusting positive to negative current ratio and their timing to eliminate or reduce the strongest plasma discharges and the high temperature spikes, thus resulting in the

Yue Yang et al. [28] investigated the effects of current frequency on microstructure and wear resistance of ceramic coatings embedded with SiC nanoparticles on Mg alloy produced by PEO. The PEO treatments were carried out on AZ91D Mg alloy in the electrolyte containing NaAlO2 (20 g/L), NaOH (3 g/L) and SiC nanoparticles (2 g/L), with the current frequency fixed at 500 Hz, 700 Hz and 900 Hz respectively. Results revealed that with the increasing of current frequency, more SiC nanoparticles randomly dispersed on the surface. The thickness and growth rate of the coatings increased with the increasing of applied current frequency. Furthermore, the ceramic coating embedded with SiC nanoparticles formed at a current frequency of 900 Hz showed the finer microstructure, lower surface roughness and best wear

R.H.U. Khan et al. [29] studied the effects of current density on surface characterization of PEO treated Al alloy. The samples were fabricated by PEO process at different current density of

content of α-Al2O3 tended to increase with increasing current density. Residual stresses in alumina coatings tended to decrease with the increased current density due to increased plasma microdischarge events which promoted stress relaxation through formation of

Ping Huang et al. [30] investigated the effects of different voltages on mechanical properties of titania prepared by PEO. The PEO treatment was carried out on pure titanium, in an aqueous

current density. The largest coating thickness (40 μm) was obtained at 20 A/dm2

density, whereas a thinnest coating (3 μm) was formed at 5 A/dm2

microcrack network and thermal annealing in the coatings.

. It was found that the coating thickness increased with the increased

current

. Furthermore, the relative

improvement of coating qualities.

resistance.

5, 10, 15 and 20 A/dm2

#### **1.5. Influence factors for PEO technique**

It is considered that the PEO treatment is a multifactor-controlled process, which is influenced by many factors, intrinsic or extrinsic. The compositions of substrate materials and electrolyte are considered to be intrinsic factors which play crucial role for the structure and composition of PEO coatings, while the extrinsic factors generally consist of electrical parameters, process‐ ing temperature, oxidation time and additives [14]. Herein, these influence factors for PEO technique will be introduced and discussed briefly.

### *1.5.1. Influence of substrate materials*

The difference of substrate materials plays a crucial role in the components and properties of PEO coatings. The predominant compositions of the PEO coatings depend on the substrate materials, for example, the main content of coatings deposited on Al, Mg, Ti and their alloys are Al2O3, MgO and TiO2 respectively. Therefore, the PEO coatings deposited on different substrate materials generally possess different properties. According to recent studies, the available coating thicknesses are around 300 μm on Al alloy, 150 μm on Mg alloy and 200 μm on Ti alloy respectively. The hardness value ranges for PEO coatings formed on different substrate materials are generally from 300 HV to 2500 HV on Al alloy, from 200 HV to 1000 HV on Mg alloy and from 300 HV to 1100 HV on Ti alloy [14-15].

#### *1.5.2. Influence of electrolytes*

The compositions of electrolyte greatly affect the properties of PEO ceramic coatings. Different electrolytes result in different growth rates, structures, phase compositions and element distribution of the PEO coatings [22-25]. Generally, the electrolytes used for PEO treatment are composed of acidic electrolytes and alkaline electrolytes. The acidic electrolytes including concentrated sulfuric acid, phosphoric acid and other salt solutions etc. are seldom used at present due to their great environmental pollution. While the alkaline electrolytes mainly consist of four systems including sodium hydroxide based electrolytes, silicate based electro‐ lytes, phosphate based electrolytes and aluminate based electrolytes[14].

#### *1.5.3. Influence of electrical parameters*

The whole PEO process and the properties of ceramic coatings are greatly affected by electrical parameters including current modes, current density, current frequency, anodic voltage, cathodic voltage, duty cycle etc. Recently, the effects of electrical parameters on PEO coatings were investigated by many researchers as follows.

The effect of electrode distance on anode current and the influence of anode current on PEO process were investigated by C.B. Wei et al [26]. The PEO processes were carried out on 2024 Al alloy in the electrolyte of sodium silicate with other additives, keeping the anode and cathode plates located face to face and separated by different distances. Results showed that the anode current was influenced by the distance between the electrodes and had a critical effect on the oxidation efficiency. The anode currents were found to decrease with larger distances. The current flowing through the front surface was higher than that through the back surface. The ball-on-disk tribological tests and corrosion tests revealed that the front surface has better tribological properties and higher corrosion resistance than the back surface.

Al2O3 on Ti6Al4V alloy in aluminate solution [18]; mullite on Al alloy in silicate solution [19]; MgF2 on Mg alloy with KF in the electrolyte [20] and TiO2 on Mg alloy in phosphate solution

It is considered that the PEO treatment is a multifactor-controlled process, which is influenced by many factors, intrinsic or extrinsic. The compositions of substrate materials and electrolyte are considered to be intrinsic factors which play crucial role for the structure and composition of PEO coatings, while the extrinsic factors generally consist of electrical parameters, process‐ ing temperature, oxidation time and additives [14]. Herein, these influence factors for PEO

The difference of substrate materials plays a crucial role in the components and properties of PEO coatings. The predominant compositions of the PEO coatings depend on the substrate materials, for example, the main content of coatings deposited on Al, Mg, Ti and their alloys are Al2O3, MgO and TiO2 respectively. Therefore, the PEO coatings deposited on different substrate materials generally possess different properties. According to recent studies, the available coating thicknesses are around 300 μm on Al alloy, 150 μm on Mg alloy and 200 μm on Ti alloy respectively. The hardness value ranges for PEO coatings formed on different substrate materials are generally from 300 HV to 2500 HV on Al alloy, from 200 HV to 1000

The compositions of electrolyte greatly affect the properties of PEO ceramic coatings. Different electrolytes result in different growth rates, structures, phase compositions and element distribution of the PEO coatings [22-25]. Generally, the electrolytes used for PEO treatment are composed of acidic electrolytes and alkaline electrolytes. The acidic electrolytes including concentrated sulfuric acid, phosphoric acid and other salt solutions etc. are seldom used at present due to their great environmental pollution. While the alkaline electrolytes mainly consist of four systems including sodium hydroxide based electrolytes, silicate based electro‐

The whole PEO process and the properties of ceramic coatings are greatly affected by electrical parameters including current modes, current density, current frequency, anodic voltage, cathodic voltage, duty cycle etc. Recently, the effects of electrical parameters on PEO coatings

The effect of electrode distance on anode current and the influence of anode current on PEO process were investigated by C.B. Wei et al [26]. The PEO processes were carried out on 2024 Al alloy in the electrolyte of sodium silicate with other additives, keeping the anode and

containing titania sol [21] ).

78 Modern Surface Engineering Treatments

**1.5. Influence factors for PEO technique**

*1.5.1. Influence of substrate materials*

*1.5.2. Influence of electrolytes*

*1.5.3. Influence of electrical parameters*

were investigated by many researchers as follows.

technique will be introduced and discussed briefly.

HV on Mg alloy and from 300 HV to 1100 HV on Ti alloy [14-15].

lytes, phosphate based electrolytes and aluminate based electrolytes[14].

R.O. Hussein et al. [27] studied the influence of different current modes on PEO plasma discharge behaviour and alumina coating microstructure. The PEO processes were carried out on pure 1100 aluminum using two different current modes of pulsed unipolar and bipolar in the electrolyte of Na2SiO3 (7 g/L) and KOH (1 g/L). It was found that the plasma temperatures vs. process time were different under different current modes. The plasma temperature spikes were believed to be caused by the strongest plasma discharges initiated at the interface between the oxide coating and substrate. Compared to the unipolar current process, the application of pulsed bipolar current resulted in reducing the high spikes on temperature profiles and the average plasma temperature. The aluminum oxide coating morphology and microstructure were also significantly different under different current modes. The bipolar current mode could improve the coating quality compared with the unipolar current mode, in terms of surface morphology and cross-sectional microstructure. A dense coating morphology could be achieved by adjusting positive to negative current ratio and their timing to eliminate or reduce the strongest plasma discharges and the high temperature spikes, thus resulting in the improvement of coating qualities.

Yue Yang et al. [28] investigated the effects of current frequency on microstructure and wear resistance of ceramic coatings embedded with SiC nanoparticles on Mg alloy produced by PEO. The PEO treatments were carried out on AZ91D Mg alloy in the electrolyte containing NaAlO2 (20 g/L), NaOH (3 g/L) and SiC nanoparticles (2 g/L), with the current frequency fixed at 500 Hz, 700 Hz and 900 Hz respectively. Results revealed that with the increasing of current frequency, more SiC nanoparticles randomly dispersed on the surface. The thickness and growth rate of the coatings increased with the increasing of applied current frequency. Furthermore, the ceramic coating embedded with SiC nanoparticles formed at a current frequency of 900 Hz showed the finer microstructure, lower surface roughness and best wear resistance.

R.H.U. Khan et al. [29] studied the effects of current density on surface characterization of PEO treated Al alloy. The samples were fabricated by PEO process at different current density of 5, 10, 15 and 20 A/dm2 . It was found that the coating thickness increased with the increased current density. The largest coating thickness (40 μm) was obtained at 20 A/dm2 current density, whereas a thinnest coating (3 μm) was formed at 5 A/dm2 . Furthermore, the relative content of α-Al2O3 tended to increase with increasing current density. Residual stresses in alumina coatings tended to decrease with the increased current density due to increased plasma microdischarge events which promoted stress relaxation through formation of microcrack network and thermal annealing in the coatings.

Ping Huang et al. [30] investigated the effects of different voltages on mechanical properties of titania prepared by PEO. The PEO treatment was carried out on pure titanium, in an aqueous solution containing calcium salt and phosphate salt, using different voltages from 240 V to 450 V. Results showed that the composition of the PEO coatings was generally anatase and rutile, while at higher voltage of 400-450 V, a new CaTiO3 phase appeared. The pore size of PEO coatings increased with the increase of applied voltage. The samples prepared at 240-350 V had much stronger bonding strength compared to that prepared at higher voltage. The elastic modulus and residual stress both increased with the increasing of applied voltage.

*1.5.5. Influence of oxidation time*

density of 150 mA/cm2

*1.5.6. Influence of additives*

due to the addition of KF.

With the increasing of oxidation time, the coating thickness increases, while the growth rate decreases. Different oxidation time can result in different coating qualities, such as thickness, roughness, adhesion, hardness, wear resistance and corrosion resistance etc.. Therefore, the

Yanhong Gu et al. [34] studied the effect of oxidation time on corrosion behavior of PEO coatings on Mg alloy in simulated body fluid. The samples were fabricated on AZ31 Mg alloy in aqueous solution of sodium phosphate (30 g/L), using applied DC voltage of 325 V, current

oxidation time. Results showed that the coatings mainly consisted of Mg, MgO, MgAl2O4 and Mg3(PO4)2, and the oxidation time had very little influence on the phase compositions. The diameter of the micropores in the PEO coating surface increased with increasing oxidation time. The coating thickness increased with increasing oxidation time until 5 min (20 μm). The sample coated at 5 min showed the thickest layer with a relatively smooth and uniform microstructure with fewer micropores, compared to the other PEO oxidation times. When the oxidation time, however, was increased to 8 min, the coating thickness decreased and the coating surface became rough. The porosity decreased with increasing oxidation time until 5 min (4.40%), and then increased to 6.28% for an oxidation time of 8 min. As a whole, the PEO coating produced at 5 min had the smallest corrosion current density and the largest electro‐ chemical impedance, resulting in the highest corrosion resistance, due to the compact, smooth

Employing different additives in the electrolyte can greatly affect the PEO process, and thus resulting in different properties of the coatings. For example, Jun Liang et al. [20] studied the effect of KF in Na2SiO3-KOH electrolyte on the structure and properties of PEO coatings formed on Mg alloy. It was found that the addition of KF contributed to increase the electrolyte conductivity, decrease the work voltage and final voltage in the PEO process and change the spark discharge characteristics. Furthermore, the addition of KF resulted in a decrease of pore diameter and surface roughness, an increase of the coating compactness and the changes in the phase compositions as well. The hardness and wear-resistance of the coating also enhanced

Employing PEO technique to form ceramic oxide coatings on Ti, Mg, Al and their alloys can significantly enhance the mechanical and tribological properties, such as high hardness, superior wear resistance and good adhesion to the substrate. In recent years, investigations on the phase composition, mechanical and tribological properties of PEO coatings on Ti, Mg, Al and their alloys were done by many researchers. However, the tribological performances of PEO coatings are not only affected by the intrinsic properties of PEO coatings, but also affected

and pulsed frequency of 3000 Hz, with 1, 3, 5 and 8 min different

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oxidation time for PEO treatment should be investigated and optimized.

and uniform morphology of coating surface with lower porosity.

**2. Tribological properties of PEO coatings**

The effects of cathode voltage on structure and properties of PEO ceramic coatings formed on NiTi alloy were investigated by F. Liu et al. [31]. The PEO processes were carried out on nearly equiatomic NiTi alloy in a solution of sodium aluminate and sodium hypophosphite, using a constant voltage mode, with anodic voltage kept constant at 400 V and cathodic voltage controlled at 0, 10, 20, and 30 V respectively. It was found that, the thickness and surface roughness of PEO coatings increased with the increasing of cathodic voltage, the friction coefficient of PEO coatings against GCr15 steel ball also increased, while the bonding strength of the coatings to the substrate and the corrosion resistance of PEO coatings both decreased. The PEO ceramic coatings formed at various cathodic voltages on NiTi alloy were composed of γ-Al2O3 at the only crystalline phase. The crystallinity could be enhanced through increasing the cathodic voltages. As a whole, the cathodic voltage applied for PEO showed a negative correlation with the biocompatibility of the ceramic coatings.

Yuming Tang et al. [32] studied the influences of duty cycle on the bonding strength of Mg alloy by PEO treatment. The duty cycle varied in the range of 10-40% with positive and negative cycle remained equal. It was revealed that the higher duty cycle increased the coating porosity and slightly decreased the thickness of the oxide coating. The PEO coatings mainly consisted of MgO and MgSiO3. And the relative content of MgO in the coatings increased while the content of MgSiO3 slightly decreased with the increase of duty cycle. Furthermore, as the duty cycle increased, the lap-shear strength of the bonding joints increased. The highest lap-shear strength (24.50 MPa) was obtained under the duty cycle of 40%. The reason was attributed to the larger porosity and enhanced mechanical interlocking effect.

#### *1.5.4. Influence of processing temperature*

The electrolyte temperature can greatly affect the PEO process. If the temperature is too low, the oxidation process becomes weak, resulting in less thickness and lower hardness of the PEO coatings. If the temperature is too high, the dissolution of oxide film will be enhanced, and thus cause the coating thickness and hardness to decrease significantly. Therefore, the processing temperature should also be studied and generally controlled in the range of 20-40℃.

H. Habazaki et al. [33] investigated the effects of different electrolyte temperatures on formation and characterization of wear-resistance of PEO coatings on Ti alloy. The PEO processes were carried out on Ti-15-3 alloy in the electrolyte of K2Al2O4 (0.15 mol/L), Na3PO4 (0.02 mol/L) and NaOH (0.015 mol/L), at different electrolyte temperatures between 278 K and 313 K. Results showed that at the lowest temperature of 278 K, the yielded PEO coating contained higher concentration of α-Al2O3 phase in addition to the Al2TiO5 major phase, exhibited lower porosity, uniformity and density, and thus showed more improved wear resistance, compared to that formed at higher temperatures.

#### *1.5.5. Influence of oxidation time*

solution containing calcium salt and phosphate salt, using different voltages from 240 V to 450 V. Results showed that the composition of the PEO coatings was generally anatase and rutile, while at higher voltage of 400-450 V, a new CaTiO3 phase appeared. The pore size of PEO coatings increased with the increase of applied voltage. The samples prepared at 240-350 V had much stronger bonding strength compared to that prepared at higher voltage. The elastic

The effects of cathode voltage on structure and properties of PEO ceramic coatings formed on NiTi alloy were investigated by F. Liu et al. [31]. The PEO processes were carried out on nearly equiatomic NiTi alloy in a solution of sodium aluminate and sodium hypophosphite, using a constant voltage mode, with anodic voltage kept constant at 400 V and cathodic voltage controlled at 0, 10, 20, and 30 V respectively. It was found that, the thickness and surface roughness of PEO coatings increased with the increasing of cathodic voltage, the friction coefficient of PEO coatings against GCr15 steel ball also increased, while the bonding strength of the coatings to the substrate and the corrosion resistance of PEO coatings both decreased. The PEO ceramic coatings formed at various cathodic voltages on NiTi alloy were composed of γ-Al2O3 at the only crystalline phase. The crystallinity could be enhanced through increasing the cathodic voltages. As a whole, the cathodic voltage applied for PEO showed a negative

Yuming Tang et al. [32] studied the influences of duty cycle on the bonding strength of Mg alloy by PEO treatment. The duty cycle varied in the range of 10-40% with positive and negative cycle remained equal. It was revealed that the higher duty cycle increased the coating porosity and slightly decreased the thickness of the oxide coating. The PEO coatings mainly consisted of MgO and MgSiO3. And the relative content of MgO in the coatings increased while the content of MgSiO3 slightly decreased with the increase of duty cycle. Furthermore, as the duty cycle increased, the lap-shear strength of the bonding joints increased. The highest lap-shear strength (24.50 MPa) was obtained under the duty cycle of 40%. The reason was attributed to

The electrolyte temperature can greatly affect the PEO process. If the temperature is too low, the oxidation process becomes weak, resulting in less thickness and lower hardness of the PEO coatings. If the temperature is too high, the dissolution of oxide film will be enhanced, and thus cause the coating thickness and hardness to decrease significantly. Therefore, the processing temperature should also be studied and generally controlled in the range of 20-40℃. H. Habazaki et al. [33] investigated the effects of different electrolyte temperatures on formation and characterization of wear-resistance of PEO coatings on Ti alloy. The PEO processes were carried out on Ti-15-3 alloy in the electrolyte of K2Al2O4 (0.15 mol/L), Na3PO4 (0.02 mol/L) and NaOH (0.015 mol/L), at different electrolyte temperatures between 278 K and 313 K. Results showed that at the lowest temperature of 278 K, the yielded PEO coating contained higher concentration of α-Al2O3 phase in addition to the Al2TiO5 major phase, exhibited lower porosity, uniformity and density, and thus showed more improved wear

modulus and residual stress both increased with the increasing of applied voltage.

correlation with the biocompatibility of the ceramic coatings.

the larger porosity and enhanced mechanical interlocking effect.

resistance, compared to that formed at higher temperatures.

*1.5.4. Influence of processing temperature*

80 Modern Surface Engineering Treatments

With the increasing of oxidation time, the coating thickness increases, while the growth rate decreases. Different oxidation time can result in different coating qualities, such as thickness, roughness, adhesion, hardness, wear resistance and corrosion resistance etc.. Therefore, the oxidation time for PEO treatment should be investigated and optimized.

Yanhong Gu et al. [34] studied the effect of oxidation time on corrosion behavior of PEO coatings on Mg alloy in simulated body fluid. The samples were fabricated on AZ31 Mg alloy in aqueous solution of sodium phosphate (30 g/L), using applied DC voltage of 325 V, current density of 150 mA/cm2 and pulsed frequency of 3000 Hz, with 1, 3, 5 and 8 min different oxidation time. Results showed that the coatings mainly consisted of Mg, MgO, MgAl2O4 and Mg3(PO4)2, and the oxidation time had very little influence on the phase compositions. The diameter of the micropores in the PEO coating surface increased with increasing oxidation time. The coating thickness increased with increasing oxidation time until 5 min (20 μm). The sample coated at 5 min showed the thickest layer with a relatively smooth and uniform microstructure with fewer micropores, compared to the other PEO oxidation times. When the oxidation time, however, was increased to 8 min, the coating thickness decreased and the coating surface became rough. The porosity decreased with increasing oxidation time until 5 min (4.40%), and then increased to 6.28% for an oxidation time of 8 min. As a whole, the PEO coating produced at 5 min had the smallest corrosion current density and the largest electro‐ chemical impedance, resulting in the highest corrosion resistance, due to the compact, smooth and uniform morphology of coating surface with lower porosity.

#### *1.5.6. Influence of additives*

Employing different additives in the electrolyte can greatly affect the PEO process, and thus resulting in different properties of the coatings. For example, Jun Liang et al. [20] studied the effect of KF in Na2SiO3-KOH electrolyte on the structure and properties of PEO coatings formed on Mg alloy. It was found that the addition of KF contributed to increase the electrolyte conductivity, decrease the work voltage and final voltage in the PEO process and change the spark discharge characteristics. Furthermore, the addition of KF resulted in a decrease of pore diameter and surface roughness, an increase of the coating compactness and the changes in the phase compositions as well. The hardness and wear-resistance of the coating also enhanced due to the addition of KF.
