**2. Tribological properties of PEO coatings**

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 by many extrinsic factors, such as sliding loads, sliding speed, counterpart materials, lubri‐ cated conditions, temperature and humidity etc. Herein, the sliding loads are emphasized and classified into three levels: low loads (0-5 N), medium loads (5-50 N) and heavy loads (above 50 N). And then, the friction and wear behaviors of different PEO coatings in different conditions will be introduced and discussed under different load levels.

**2.2. Friction and wear behavior of PEO coatings under medium loads**

P. Bala Srinivasan et al. [37] studied the dry sliding wear behaviour of PEO coatings with different thickness of 10 μm and 20 μm on cast AZ91 magnesium alloy. The samples were fabricated by PEO treatment in silicate based electrolyte containing Na2SiO3 (10 g/L) and KOH (10 g/L). The dry sliding wear behaviour of the untreated Mg alloy, PEO coated specimen A and B was assessed on a ball-on-disc oscillating tribometer, under three different loads of 2N, 5N and 10N, with an oscillating amplitude of 10 mm and at a sliding velocity of 5 mm/s for a sliding distance of 12 m, using an AISI 52100 steel ball of 6 mm diameter as static friction partner. For the uncoated Mg alloy, the friction coefficients were fluctuating in the range of 0.24-0.40 under different loads. For the 10 μm PEO coating, the friction coefficient reached to a steady value of about 0.78 under 2 N load, dropped to around 0.35 after a sliding distance of about 4 m under 5 N load, while dropped in a very short time under 10 N load. For the 20 μm PEO coating, the friction coefficients did not drop at all loads and remains steady. Moreover, the friction coefficient showed lower value with an increase in load (0.8 at 2 N, 0.68 at 5 N and 0.62 at 10 N). The uncoated Mg alloy under all loads and the 10 μm PEO coating under 5 N and 10 N loads all showed high wear rates. While the 20 μm PEO coating under all loads and the 10 μm PEO coating under 2 N load all showed much lower wear rates. The results indicated that the thickness of coatings played a crucial role in enhancing the wear resistance. At higher initial stress levels, the deformation of the substrate causes the cracking and flaking-off of the coating, especially when it is thin. Under such circumstances the increased thickness of PEO coating provided a better load bearing capacity, thus resulting in a superior wear resistance.

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M. Treviño et al. [38] investigated the wear of coatings on Al 6061 alloy fabricated by PEO treatment in Na2SiO3-KOH electrolyte. The coatings with different thickness of 100, 125 and 150 μm were fabricated and characterized. Composition analysis showed that the coatings consisted of a combination of oxide phases such as mullite, α-Al2O3, γ-Al2O3 and amorphous alumina. It was suggested that the presence of α-Al2O3 phase presented the greatest wear resistance compared with other phases such as mullite, γ-Al2O3 and amorphous alumina which were highly vulnerable for the conditions studied. No difference was detected for the different coatings in hardness values which were of 1556±11 HV50 compared with that of 109±3 HV50 for the substrate. The tribological properties were evaluated by a pin on disc test machine, with a sliding distance of 1 km and a constant linear speed of 13.76 m/min, using 10, 20, 30 and 40 N different normal loads for each coating thickness. Friction and wear tests showed that the friction coefficient changed along the tests, and the weight loss depended on both the thickness of the coatings and the loads applied during the test. The wear mechanisms were suggested to be adhesion and abrasion by hard particles. The thinnest coating of 100 μm exhibited better resistance to wear and showed the friction coefficients which exhibited a continuous increase independently of the applied loads. The friction coefficients for the coating of 125 μm remained constant when loads of 10 and 20 N were used, and reduced their values once a certain distance was achieved when tested with 30 and 40 N. The friction coefficients for the coating of 150 μm were found to increase under a load of 10 N, to remain fairly constant with loads of 20 and 30 N and to reduce their value once a distance of around 500 m was achieved with a load of 40 N. The reduction of friction coefficients for the coatings of 125 μm and 150 μm suggested that

#### **2.1. Friction and wear behavior of PEO coatings under low loads**

The microstructure, mechanical and tribological properties of PEO coatings formed on Ti6Al4V alloy were studied by Y.M. Wang et al [35]. A nanoindentation test showed that the hardness and elastic modulus were about 8.5 GPa and 87.4 GPa for the compact region of the PEO coating, and about 4 GPa and 150 GPa for the Ti6Al4V substrate. The hardness and elastic modulus were mainly constant in the compact region within 33 μm, and decreased remarkably beyond 33 μm to the outer surface. A sheer test showed that the adhesion strength between coating and substrate was about 70 MPa. The tribological behaviors of untreated and PEO coated samples were evaluated by a pin-on-disk tribometer under the normal loads of 0.3, 0.5, and 1 N, with a sliding speed of 0.05 m/s or 0.15 m/s, using SAE52100 steel ball as counterpart material. Results of friction and wear tests showed that the friction coefficient of PEO coating against steel was as low as 0.2-0.3 at loads not more than 1 N and sliding cycles within 2500 times, and gradually increased at the later stage of wear test due to the oxidation and materials transfer wear mechanism.

The investigations of structure, composition, mechanical and tribological properties of PEO coatings formed on AM60B Mg alloy in silicate and phosphate electrolyte have been done by Jun Liang et al. [36] The samples were fabricated in the electrolyte containing Na2SiO3 (10 g/L), KOH (1 g/L) or Na3PO4 (10 g/L), KOH (1 g/L). The coating formed in silicate electrolyte is composed of periclase MgO and forsterite Mg2SiO4 phases while MgO and a little of spinel MgAl2O4 are the main phases of the coating formed in phosphate electrolyte. Generally, the forsterite Mg2SiO4 has a greater hardness than that of the MgO. Therefore, the coating formed in silicate electrolyte exhibits a higher microhardness than that formed in phosphate electro‐ lyte. The friction and wear properties of the PEO coatings were evaluated on a reciprocalsliding UMT-2MT tribometer in dry sliding conditions under a load of 2 N, using Si3N4 ball as counterpart material, with a siding speed of 0.1 m/s and sliding amplitude of 5 mm. The wear life of PEO coatings formed in two different electrolytes was compared with the thin coatings and results showed that the wear life of coating formed in silicate electrolyte is about four times as long as that of coating formed in phosphate electrolyte. The uncoated Mg alloy has a friction coefficient of about 0.3 and exhibits a high wear rate of 3.81×10-4 mm3 /Nm. While for both the oxide coatings, the friction coefficients are in the range of 0.6-0.8 and the wear rates are only in the range of 3.55-8.65×10-5 mm3 /Nm. These evidences demonstrate that the PEO coatings formed on Mg alloy in both electrolytes have greatly enhanced the wear resistance but exhibit higher friction coefficients compared with the uncoated Mg alloy. Furthermore, the oxide coating formed in silicate electrolyte has a higher friction coefficient but exhibit a better wear resistance than that formed in phosphate electrolyte. It also suggests that the structure and phase composition of coatings are indeed the dominant factors which influence the mechanical property and friction and wear behaviors of PEO coatings.

#### **2.2. Friction and wear behavior of PEO coatings under medium loads**

by many extrinsic factors, such as sliding loads, sliding speed, counterpart materials, lubri‐ cated conditions, temperature and humidity etc. Herein, the sliding loads are emphasized and classified into three levels: low loads (0-5 N), medium loads (5-50 N) and heavy loads (above 50 N). And then, the friction and wear behaviors of different PEO coatings in different

The microstructure, mechanical and tribological properties of PEO coatings formed on Ti6Al4V alloy were studied by Y.M. Wang et al [35]. A nanoindentation test showed that the hardness and elastic modulus were about 8.5 GPa and 87.4 GPa for the compact region of the PEO coating, and about 4 GPa and 150 GPa for the Ti6Al4V substrate. The hardness and elastic modulus were mainly constant in the compact region within 33 μm, and decreased remarkably beyond 33 μm to the outer surface. A sheer test showed that the adhesion strength between coating and substrate was about 70 MPa. The tribological behaviors of untreated and PEO coated samples were evaluated by a pin-on-disk tribometer under the normal loads of 0.3, 0.5, and 1 N, with a sliding speed of 0.05 m/s or 0.15 m/s, using SAE52100 steel ball as counterpart material. Results of friction and wear tests showed that the friction coefficient of PEO coating against steel was as low as 0.2-0.3 at loads not more than 1 N and sliding cycles within 2500 times, and gradually increased at the later stage of wear test due to the oxidation and materials

The investigations of structure, composition, mechanical and tribological properties of PEO coatings formed on AM60B Mg alloy in silicate and phosphate electrolyte have been done by Jun Liang et al. [36] The samples were fabricated in the electrolyte containing Na2SiO3 (10 g/L), KOH (1 g/L) or Na3PO4 (10 g/L), KOH (1 g/L). The coating formed in silicate electrolyte is composed of periclase MgO and forsterite Mg2SiO4 phases while MgO and a little of spinel MgAl2O4 are the main phases of the coating formed in phosphate electrolyte. Generally, the forsterite Mg2SiO4 has a greater hardness than that of the MgO. Therefore, the coating formed in silicate electrolyte exhibits a higher microhardness than that formed in phosphate electro‐ lyte. The friction and wear properties of the PEO coatings were evaluated on a reciprocalsliding UMT-2MT tribometer in dry sliding conditions under a load of 2 N, using Si3N4 ball as counterpart material, with a siding speed of 0.1 m/s and sliding amplitude of 5 mm. The wear life of PEO coatings formed in two different electrolytes was compared with the thin coatings and results showed that the wear life of coating formed in silicate electrolyte is about four times as long as that of coating formed in phosphate electrolyte. The uncoated Mg alloy has a friction

oxide coatings, the friction coefficients are in the range of 0.6-0.8 and the wear rates are only

formed on Mg alloy in both electrolytes have greatly enhanced the wear resistance but exhibit higher friction coefficients compared with the uncoated Mg alloy. Furthermore, the oxide coating formed in silicate electrolyte has a higher friction coefficient but exhibit a better wear resistance than that formed in phosphate electrolyte. It also suggests that the structure and phase composition of coatings are indeed the dominant factors which influence the mechanical

/Nm. These evidences demonstrate that the PEO coatings

/Nm. While for both the

conditions will be introduced and discussed under different load levels.

**2.1. Friction and wear behavior of PEO coatings under low loads**

coefficient of about 0.3 and exhibits a high wear rate of 3.81×10-4 mm3

property and friction and wear behaviors of PEO coatings.

transfer wear mechanism.

82 Modern Surface Engineering Treatments

in the range of 3.55-8.65×10-5 mm3

P. Bala Srinivasan et al. [37] studied the dry sliding wear behaviour of PEO coatings with different thickness of 10 μm and 20 μm on cast AZ91 magnesium alloy. The samples were fabricated by PEO treatment in silicate based electrolyte containing Na2SiO3 (10 g/L) and KOH (10 g/L). The dry sliding wear behaviour of the untreated Mg alloy, PEO coated specimen A and B was assessed on a ball-on-disc oscillating tribometer, under three different loads of 2N, 5N and 10N, with an oscillating amplitude of 10 mm and at a sliding velocity of 5 mm/s for a sliding distance of 12 m, using an AISI 52100 steel ball of 6 mm diameter as static friction partner. For the uncoated Mg alloy, the friction coefficients were fluctuating in the range of 0.24-0.40 under different loads. For the 10 μm PEO coating, the friction coefficient reached to a steady value of about 0.78 under 2 N load, dropped to around 0.35 after a sliding distance of about 4 m under 5 N load, while dropped in a very short time under 10 N load. For the 20 μm PEO coating, the friction coefficients did not drop at all loads and remains steady. Moreover, the friction coefficient showed lower value with an increase in load (0.8 at 2 N, 0.68 at 5 N and 0.62 at 10 N). The uncoated Mg alloy under all loads and the 10 μm PEO coating under 5 N and 10 N loads all showed high wear rates. While the 20 μm PEO coating under all loads and the 10 μm PEO coating under 2 N load all showed much lower wear rates. The results indicated that the thickness of coatings played a crucial role in enhancing the wear resistance. At higher initial stress levels, the deformation of the substrate causes the cracking and flaking-off of the coating, especially when it is thin. Under such circumstances the increased thickness of PEO coating provided a better load bearing capacity, thus resulting in a superior wear resistance.

M. Treviño et al. [38] investigated the wear of coatings on Al 6061 alloy fabricated by PEO treatment in Na2SiO3-KOH electrolyte. The coatings with different thickness of 100, 125 and 150 μm were fabricated and characterized. Composition analysis showed that the coatings consisted of a combination of oxide phases such as mullite, α-Al2O3, γ-Al2O3 and amorphous alumina. It was suggested that the presence of α-Al2O3 phase presented the greatest wear resistance compared with other phases such as mullite, γ-Al2O3 and amorphous alumina which were highly vulnerable for the conditions studied. No difference was detected for the different coatings in hardness values which were of 1556±11 HV50 compared with that of 109±3 HV50 for the substrate. The tribological properties were evaluated by a pin on disc test machine, with a sliding distance of 1 km and a constant linear speed of 13.76 m/min, using 10, 20, 30 and 40 N different normal loads for each coating thickness. Friction and wear tests showed that the friction coefficient changed along the tests, and the weight loss depended on both the thickness of the coatings and the loads applied during the test. The wear mechanisms were suggested to be adhesion and abrasion by hard particles. The thinnest coating of 100 μm exhibited better resistance to wear and showed the friction coefficients which exhibited a continuous increase independently of the applied loads. The friction coefficients for the coating of 125 μm remained constant when loads of 10 and 20 N were used, and reduced their values once a certain distance was achieved when tested with 30 and 40 N. The friction coefficients for the coating of 150 μm were found to increase under a load of 10 N, to remain fairly constant with loads of 20 and 30 N and to reduce their value once a distance of around 500 m was achieved with a load of 40 N. The reduction of friction coefficients for the coatings of 125 μm and 150 μm suggested that the coatings were completely removed under the loads of 30 N and 40 N resulting in contact with the alloy substrate which was probably lubricated by wear debris generated.

speed of 0.33 m/s and a contact pressure of 2 MPa. The antiwear life of the polished coating reached 2500 m at a speed of 1.25 m/s and a load of 300 N, and the friction coefficient was more than 0.45 against the steel ring in a Timken tester which was a little lower than that of the out

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The aforementioned studies revealed that the PEO ceramic coatings can sharply increase the wear resistance and decrease the wear rate, compared to the uncoated substrates. However, the PEO coatings normally exhibit higher friction coefficients which can cause not only the wear of sliders, but also the wear damage of counterpart materials in many tribological applications. Thus, it is necessary to fabricate the PEO coatings with both good wear resistance

In order to further improve the tribological properties of the PEO-treated lightweight metals, many attempts to reduce the friction coefficient of the PEO coatings have been made. Herein, three main developments in improvement of tribological properties of PEO coatings are reviewed, which can be categorized as (1) liquid lubrication, (2) duplex coatings and (3)

As there are many micropores, microcracks and dimples on the surface of the PEO coatings [17], these pores, cracks and dimples can act as reservoirs for oil lubricants, which may result in a positive effect to the tribological performance of PEO coatings under boundary-lubricated

Studies on the wear resistance of PEO coatings on 2024 Al alloy under oil-lubricated condition were done by Tongbo Wei et al. [41]. The friction and wear tests were carried on an MRH-3 ring-on-block tester, at a ring linear speed of 2.60 m/s and normal loads from 300 N up to 1400 N, using AISI-C-52100 steel rings and aluminum rings covered with polished PEO coatings as counterpart. Commercial 4838 lubricating oil was used as the lubricating medium. Friction and wear test showed that the friction coefficient of polished coatings was within 0.020-0.060 under oil-lubricated condition which was reduced to about 1/10 compared with that under dry sliding condition registering within 0.20-0.35, and the wear rate of polished coating was

showed excellent wear-resistance in oil-lubricated sliding against steel and Al2O3 ceramic ring

Fei Zhou et al. [42] investigated the friction characteristic of PEO coating on 2024 Al alloy, sliding against Si3N4 balls, in water and oil environments, at different normal loads and sliding speeds. Results showed that, with the increasing of normal load and sliding speed, the friction coefficient of the PEO/Si3N4 tribopair in water and oil decreased from 0.72 to 0.57 and 0.24 to

and can endure a sliding distance as large as 18.7 km at loads as high as 1400 N.

/Nm which was reduced to be about 1/1000 compared with that

/Nm. The polished coatings

**3.1. Liquid lubrication for improving the tribological behavior of PEO coatings**

under dry sliding condition registering within 1.00-2.00×10-6 mm3

**3. Improvements of tribological behavior of PEO coatings**

layer registering more than 0.47.

and low friction coefficient.

composite coatings.

within 1.00-8.50×10-9 mm3

conditions.

#### **2.3. Friction and wear behavior of PEO coatings under heavy loads**

Chen Fei et al. [39] studied the tribological performance of PEO ceramic coatings fabricated on Ti6Al4V alloy in the electrolyte containing Na2SiO3 (10g/L), Na2CO3 (4g/L) and EDTA-2Na (5g/ L). Coatings with a thickness of 10 μm were formed and polished to remove the prominent ceramic particles of the outer surface in order to reduce the effect of roughness on tribological behavior. The tribological behaviors of unpolished coating, polished coating and untreated Ti6Al4V alloy were evaluated on a ball-on-disk tribometer under the dry sliding conditions, using balls of SAE52100 steel as counterpart materials, with normal load of 100 N, rotation speed of 1000 rpm, sliding speed of 0.42 m/s and sliding time of 10 min. For the untreated Ti6Al4V alloy, the long-term friction coefficient is about 0.4, and the worn surface that sliding against steel revealed that the dominant wear mechanism is extensive abrasive and adhesive wear. For the unpolished PEO coating, the friction coefficient exhibited a high value of about 0.5, and the wear track showed severe abrasive wear, also accompanied by severe adhesive wear from the steel counter surface leading to material transfer on the coated surface. The porous surface of the unpolished PEO coatings is very rough due to the scraggy ceramic products. Unlike sliding that usually leads to plastic shearing in materials, the impact caused by the ceramic asperities on the surface results in catastrophic failure, such as cracking and crushing of the contact regions, which leads to faster material removal and the production of the sharp ceramic debris fragments. In contrast, for the polished coating, the friction coefficient exhibited a relatively low and stable value, almost remaining constant at 0.2. As the outer surface was polished to remove the prominent ceramic particles, the initial contact conditions were changed from a rough ceramic/ steel to a smooth ceramic/steel mating surface. Therefore, the cracking and crushing of promi‐ nent ceramic regions due to great vibrations were eliminated. Results showed that the worn surface was relatively smooth, accompanied with fine debris embedded in the edges of contact regions. The good antifriction properties are attributed to the microstructure of the coatings which are mainly composed of rutile and anatase TiO2. TiO2, especially the rutile-type, is known as a potentially low friction and wear reducing material.

Jun Tian et al. [40] investigated the structure and antiwear behavior of PEO coatings on 2A12 Al alloy. The samples were fabricated by PEO treatment in the electrolyte composed of Na2SiO3 (30g/L), NaOH (5g/L), with current density controlled to below 103 A/m2 . The asdeposited coatings were polished with SiC paper to remove 20%, 30%, 40% and 50% of the whole thickness of the coatings as polished coating samples. The results of structural and phase composition analysis showed that the PEO coatings on Al alloys showed two distinct layers, i.e. a porous outer layer consisting predominantly of γ-Al2O3 and a dense inner layer consisting predominantly of α-Al2O3. The inner layer α-Al2O3 has better antiwear ability compared with the outer layer γ-Al2O3. Therefore, with the increasing of the coating thickness, the antiwear life of the outer layer becomes smaller than that of the inner layer. The results of friction and wear tests showed that the polished coating mainly composed of α-Al2O3 registered a lower wear rate of 3.00-5.00×10-6 mm3 /Nm in reciprocating sliding against ceramic counterpart at a speed of 0.33 m/s and a contact pressure of 2 MPa. The antiwear life of the polished coating reached 2500 m at a speed of 1.25 m/s and a load of 300 N, and the friction coefficient was more than 0.45 against the steel ring in a Timken tester which was a little lower than that of the out layer registering more than 0.47.

The aforementioned studies revealed that the PEO ceramic coatings can sharply increase the wear resistance and decrease the wear rate, compared to the uncoated substrates. However, the PEO coatings normally exhibit higher friction coefficients which can cause not only the wear of sliders, but also the wear damage of counterpart materials in many tribological applications. Thus, it is necessary to fabricate the PEO coatings with both good wear resistance and low friction coefficient.
