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

the coatings were completely removed under the loads of 30 N and 40 N resulting in contact

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

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

deposited 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

/Nm in reciprocating sliding against ceramic counterpart at a

A/m2

. The as-

Na2SiO3 (30g/L), NaOH (5g/L), with current density controlled to below 103

with the alloy substrate which was probably lubricated by wear debris generated.

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

84 Modern Surface Engineering Treatments

as a potentially low friction and wear reducing material.

wear rate of 3.00-5.00×10-6 mm3

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) composite coatings.

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

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 conditions.

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 within 1.00-8.50×10-9 mm3 /Nm which was reduced to be about 1/1000 compared with that under dry sliding condition registering within 1.00-2.00×10-6 mm3 /Nm. The polished coatings showed excellent wear-resistance in oil-lubricated sliding against steel and Al2O3 ceramic ring and can endure a sliding distance as large as 18.7 km at loads as high as 1400 N.

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 0.11 respectively. The wear mechanism of the PEO coatings changed from abrasive wear in air to mix wear in water, and finally became microploughing wear in oil.

the probability to deposit small sized solid lubricant particles into these micropores to form a binary coating [46]. Herein, a simple and effective method of vacuum impregnation was introduced. In this method, the PEO coating samples are immersed into water-based solid lubricant suspension, and put into a vacuum oven. When deposited for a set time, the samples are heated for a period of time in high temperature for solidification. The solid lubricant particles can impregnate into the micropores of the PEO ceramic coating under the vacuum. With the increasing of deposition time and heat treatment, a compact top film covering the

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Zhijiang Wang et al. [47] investigated the properties of a self-lubricating Al2O3/PTFE duplex coating formed on LY12 Al alloy by PEO treatment combined with vacuum impregnation of PTFE. In their work, the PEO coating samples were immersed into water-based PTFE suspen‐ sion, and putted into a vacuum oven (less than 5×10-3 torr). When deposited for a set time, the samples were heated for 24 h at 200℃. Results showed that the PTFE powder particles with the size in the range of 100-170 nm could deposit into the PEO ceramic coating and covered the rough and porous surface. Tribological tests showed that the friction coefficient and wear mass loss of the Al2O3/PTFE duplex coating decreased sharply. As the cracks and micropores of the PEO coating were filled by solid lubricant, PTFE could form a lubricating film on the frication surface of the steel ball when the steel ball which worked as counter material slided against the coating. With increased sliding distances, the solid lubricant provided continuous supply due to the abundance of PTFE lying in the micropores of the PEO coating. At the same time, the PEO coating could play the role as the wear-resistant substrate to support soft PTFE polymer. As a result, the friction coefficient of the self-lubricating coating could remain at a

The porous feature of the PEO coating opens a good way to introduce solid lubricant into micropores or depositing on the surface of coating. And the presence of pores affords an effective mechanical keying between solid lubricant topcoat and the PEO layer. Spraying is a simple, effective and low cost method as a post treatment to apply solid lubricant on PEO coating to form a self-lubricating duplex coating. The PEO ceramic coating serves as under‐ lying loading layer and solid lubricant top layer plays the roles as friction reducing agent. Y.M. Wang et al. [48-49] have successfully prepared a self-lubricating duplex coating on Ti6Al4V alloy by PEO treatment combined with spraying graphite process. The spraying graphite process forced on the surface of the PEO coating was carried out using a self-made spraying gun with 4 atmosphere pressure, followed by solidification at 180℃ for 15 min. Results showed that the surface of PEO coating was characterized by micropores of different size and shape and covered by graphite lubricant exhibiting a special shape of plate. The duplex coating exhibited good antifriction property, registering friction coefficient of about 0.12,

which is 5 times lower than that of the PEO coating sliding in the similar condition.

C. Martini et al. [50] have successfully fabricated a self-lubricating duplex coating on Ti6Al4V alloy by PEO treatment combined with spraying PTFE process. The PTFE topcoat deposited by spraying a solvent-based aerosol suspension proves to be beneficial in terms of both friction

PEO ceramic coating is formed for anti-friction purpose.

constant with minimal weight loss during the long-term sliding.

*3.2.2. Spraying top coatings*

M.H. Zhu et al. [43] investigated the fretting wear behaviors of PEO coating on LD11 Al alloy sealed by grease. It was found that the friction coefficient of the sealed PEO coating under all test parameters were greatly lower than that of the PEO coating. At the same time, there was a longer stage with low friction coefficient that can be observed in the friction coefficient curves for all test conditions of the sealed PEO coating. It was clear that the sealed PEO coating presented an obvious lubricating action during the fretting wear processes. In partial slip regime, the damage of the two coatings was very slight, and the porous structure was still intact even after 104 cycles. The fretting wear mechanisms of the two coatings in slip regime were main abrasive wear and delamination, but higher proportion of the traces of relative sliding was presented on the scars of the sealed PEO coating. As a conclusion, the sealed PEO coating exhibited a better resistance for alleviating fretting wear and lengthening service life than that of the PEO coating.

The fretting wear behaviour of PEO coatings formed on Ti6Al4V alloy under oil lubricated conditions was studied by Yaming Wang et al. [44]. The fretting wear tests of PEO coatings were conducted on a PLINT fretting fatigue machine under unlubricated and oil lubricated conditions (smear and oil bath lubrication), using 52100 steel ball as vibrated counterpart material and with a small reciprocating amplitude of 60 μm. The results showed that in unlubricated condition, the friction coefficient rapidly increased up to 0.8-0.9 and maintained relatively stable. In smear oil lubricated condition, the friction coefficient showed an obvious higher value within 0.18-0.43 in the range of about 2500-6700 cycles. While in oil bath lubricated condition, the friction coefficient reduced significantly to a low and stable value of 0.15 in the long-term fretting test. This indicated that the coatings with oil lubrication lowered the shear and adhesive stresses between contact surfaces, and consequently alleviated the possibility of initiation and propagation of cracks in the inner layer of the coating or titanium alloy substrate.

#### **3.2. Duplex coatings for improving the tribological behavior of PEO coatings**

Employing liquid lubricants may improve the tribological properties of the PEO coatings. While in rigid and severe working conditions, such as high vacuum, high temperature, chemical and radioactive environments, liquid lubricants often do not function [45]. Further‐ more, liquid lubricants may contaminate the workpieces. Therefore, some multi-step prepa‐ ration methods combined with the PEO process are employed to fabricate PEO-based duplex coatings on the metallic substrates. The duplex coatings are formed by one of the post treatments (mainly including impregnation, spraying and chemical/physical vapour deposi‐ tion) on the yielded PEO ceramic coatings. These duplex coatings can sharply decrease the friction coefficient and improve the wear resistance. Herein, some successful applications for the duplex coatings were introduced briefly.

#### *3.2.1. Impregnation*

Because the PEO ceramic coating is formed on the metal surface via a series of localized electrical discharge events, there are many micropores left in the coating [17]. This provides the probability to deposit small sized solid lubricant particles into these micropores to form a binary coating [46]. Herein, a simple and effective method of vacuum impregnation was introduced. In this method, the PEO coating samples are immersed into water-based solid lubricant suspension, and put into a vacuum oven. When deposited for a set time, the samples are heated for a period of time in high temperature for solidification. The solid lubricant particles can impregnate into the micropores of the PEO ceramic coating under the vacuum. With the increasing of deposition time and heat treatment, a compact top film covering the PEO ceramic coating is formed for anti-friction purpose.

Zhijiang Wang et al. [47] investigated the properties of a self-lubricating Al2O3/PTFE duplex coating formed on LY12 Al alloy by PEO treatment combined with vacuum impregnation of PTFE. In their work, the PEO coating samples were immersed into water-based PTFE suspen‐ sion, and putted into a vacuum oven (less than 5×10-3 torr). When deposited for a set time, the samples were heated for 24 h at 200℃. Results showed that the PTFE powder particles with the size in the range of 100-170 nm could deposit into the PEO ceramic coating and covered the rough and porous surface. Tribological tests showed that the friction coefficient and wear mass loss of the Al2O3/PTFE duplex coating decreased sharply. As the cracks and micropores of the PEO coating were filled by solid lubricant, PTFE could form a lubricating film on the frication surface of the steel ball when the steel ball which worked as counter material slided against the coating. With increased sliding distances, the solid lubricant provided continuous supply due to the abundance of PTFE lying in the micropores of the PEO coating. At the same time, the PEO coating could play the role as the wear-resistant substrate to support soft PTFE polymer. As a result, the friction coefficient of the self-lubricating coating could remain at a constant with minimal weight loss during the long-term sliding.

#### *3.2.2. Spraying top coatings*

0.11 respectively. The wear mechanism of the PEO coatings changed from abrasive wear in air

M.H. Zhu et al. [43] investigated the fretting wear behaviors of PEO coating on LD11 Al alloy sealed by grease. It was found that the friction coefficient of the sealed PEO coating under all test parameters were greatly lower than that of the PEO coating. At the same time, there was a longer stage with low friction coefficient that can be observed in the friction coefficient curves for all test conditions of the sealed PEO coating. It was clear that the sealed PEO coating presented an obvious lubricating action during the fretting wear processes. In partial slip regime, the damage of the two coatings was very slight, and the porous structure was still intact even after 104 cycles. The fretting wear mechanisms of the two coatings in slip regime were main abrasive wear and delamination, but higher proportion of the traces of relative sliding was presented on the scars of the sealed PEO coating. As a conclusion, the sealed PEO coating exhibited a better resistance for alleviating fretting wear and lengthening service life

The fretting wear behaviour of PEO coatings formed on Ti6Al4V alloy under oil lubricated conditions was studied by Yaming Wang et al. [44]. The fretting wear tests of PEO coatings were conducted on a PLINT fretting fatigue machine under unlubricated and oil lubricated conditions (smear and oil bath lubrication), using 52100 steel ball as vibrated counterpart material and with a small reciprocating amplitude of 60 μm. The results showed that in unlubricated condition, the friction coefficient rapidly increased up to 0.8-0.9 and maintained relatively stable. In smear oil lubricated condition, the friction coefficient showed an obvious higher value within 0.18-0.43 in the range of about 2500-6700 cycles. While in oil bath lubricated condition, the friction coefficient reduced significantly to a low and stable value of 0.15 in the long-term fretting test. This indicated that the coatings with oil lubrication lowered the shear and adhesive stresses between contact surfaces, and consequently alleviated the possibility of initiation and propagation of cracks in the inner layer of the coating or titanium alloy substrate.

**3.2. Duplex coatings for improving the tribological behavior of PEO coatings**

Employing liquid lubricants may improve the tribological properties of the PEO coatings. While in rigid and severe working conditions, such as high vacuum, high temperature, chemical and radioactive environments, liquid lubricants often do not function [45]. Further‐ more, liquid lubricants may contaminate the workpieces. Therefore, some multi-step prepa‐ ration methods combined with the PEO process are employed to fabricate PEO-based duplex coatings on the metallic substrates. The duplex coatings are formed by one of the post treatments (mainly including impregnation, spraying and chemical/physical vapour deposi‐ tion) on the yielded PEO ceramic coatings. These duplex coatings can sharply decrease the friction coefficient and improve the wear resistance. Herein, some successful applications for

Because the PEO ceramic coating is formed on the metal surface via a series of localized electrical discharge events, there are many micropores left in the coating [17]. This provides

to mix wear in water, and finally became microploughing wear in oil.

than that of the PEO coating.

86 Modern Surface Engineering Treatments

the duplex coatings were introduced briefly.

*3.2.1. Impregnation*

The porous feature of the PEO coating opens a good way to introduce solid lubricant into micropores or depositing on the surface of coating. And the presence of pores affords an effective mechanical keying between solid lubricant topcoat and the PEO layer. Spraying is a simple, effective and low cost method as a post treatment to apply solid lubricant on PEO coating to form a self-lubricating duplex coating. The PEO ceramic coating serves as under‐ lying loading layer and solid lubricant top layer plays the roles as friction reducing agent.

Y.M. Wang et al. [48-49] have successfully prepared a self-lubricating duplex coating on Ti6Al4V alloy by PEO treatment combined with spraying graphite process. The spraying graphite process forced on the surface of the PEO coating was carried out using a self-made spraying gun with 4 atmosphere pressure, followed by solidification at 180℃ for 15 min. Results showed that the surface of PEO coating was characterized by micropores of different size and shape and covered by graphite lubricant exhibiting a special shape of plate. The duplex coating exhibited good antifriction property, registering friction coefficient of about 0.12, which is 5 times lower than that of the PEO coating sliding in the similar condition.

C. Martini et al. [50] have successfully fabricated a self-lubricating duplex coating on Ti6Al4V alloy by PEO treatment combined with spraying PTFE process. The PTFE topcoat deposited by spraying a solvent-based aerosol suspension proves to be beneficial in terms of both friction and wear resistance, particularly in an intermediate (30-50N) load range. The friction coeffi‐ cient of duplex coating reduced from 0.8-1 to 0.2-0.3, which is attributed to the anti-friction properties of PTFE.

alloy substrate and the PEO alumina coating gave a high friction coefficient of above 0.7 against both counterpart materials. Both the alumina and alumina/DLC duplex coatings exhibited excellent wear resistance, registering the wear rates in a low range of 1.4-1.9×10-6 mm3

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siding against WC-Co. Only the alumina/DLC duplex coatings provided a low and stable friction coefficient in a low range of 0.1-0.22. Whereas the single DLC coating on Al alloy failed

The tribological properties of duplex PEO/DLC coatings on Mg alloy formed by a combined plasma electrolytic oxidation and filtered cathode arc deposition technique were investigated by Jun Liang et al. [55]. The DLC film deposited on PEO coating was not uniform due to surface roughness of interface. The friction and wear properties of the uncoated Mg alloy, the polished PEO coating, single DLC film on Mg alloy and the duplex PEO/DLC coating were evaluated on a reciprocating ball-on-disk UMT-2MT tribometer, under a load of 2 N, with a sliding speed of 0.1 m/s and sliding amplitude of 5 mm, using Si3N4 ball as counterpart material. For the uncoated Mg alloy substrate, the friction coefficient varied in the range of 0.2-0.4 accompanied by severe oscillation. Severe wear and seizing were observed. Even for the DLC deposited Mg alloy substrate, the tribological behavior did not improve significantly. The DLC film failed quickly at around 350 s after starting the sliding test, due to the low load-bearing capacity of soft substrates. The polished PEO coatings showed a very high friction coefficient of around 0.7-0.8. But the deposition of DLC film on PEO coatings improved the dry friction behaviors significantly. The friction coefficient of the duplex PEO/DLC coating remained steady and less

E. Arslan et al. [56] studied the tribological performances of duplex titania/DLC coatings deposited on Ti6Al4V alloy using combined PEO and CFUBMS (closed field unbalanced magnetron sputtering). The thickness of PEO coating and DLC coating were about 10 μm and 6 μm respectively. The tribological properties were evaluated by a pin-on-disk tribometer (Teer-POD2), under a load of 2 N, at a speed of 100 rpm, a relative humidity of 45% and room temperature, using Al2O3 balls as counterpart materials. For the PEO coatings, the friction coefficient was considerably high, approximately above 0.45, and fluctuated, and the wear tracks were quite broad and rough with debris. For the single DLC coatings, the friction coefficient increased abruptly after 420 seconds due to severe failure caused by their low load bearing capacity. While for the duplex PEO/DLC coatings, the friction coefficients were much more stable, and approximately at 0.1. These results indicated that the duplex PEO/DLC coatings exhibited better tribological behaviors than the PEO and DLC coatings deposited on

**3.3. Composite coatings for improving the tribological behavior of PEO coatings**

The duplex coatings fabricated by the aforementioned methods can sharply decrease the friction coefficient and wear rate. However, these coatings are generally comprised of two layers: an inner PEO ceramic layer and an outside solid lubricant layer. When the outside layer is worn through, the tribological properties will be back to its original level. M. Aliofkhazraei et al. have successfully fabricated TiO2/Al2O3 [57] and TiO2/Si3N4 [58-59] nanocomposite coatings by PEO treatment on pure titanium. The PEO processes were carried out in the

quickly only at 25 m sliding distance due to its low load bearing capacity.

than 0.2, independently of the nonuniformity of coating surface.

lightweight metals substrate.

/Nm

89

### *3.2.3. CVD/PVD to form top films*

Chemical vapour deposition (CVD) and physical vapour deposition (PVD) techniques are well known to deposit hard coatings such as TiN, CrN and DLC etc., for providing surfaces with enhanced tribological properties in terms of low friction coefficient and high wear resistance. In recent years, attempts have been made to introduce CVD or PVD coatings on components of machines and engines. However, in practice TiN-, CrN- or DLC-coatings on alloys of light metals formed by various CVD/PVD methods often exhibit limited tribological performance due to the elastic and plastic deformation of the substrates under mechanical loadings, which can result in eventual coating failure, since the coatings are usually too thin to support the heavy loads and protect the substrates in the contact conditions [51]. Deposition of thick (e.g. >10 μm) CVD/PVD coatings usually results in high compressive stresses, and thus low adhesion [52].

Employing PEO technique can deposit thick ceramic coatings which exhibit high hardness, superior wear resistance and excellent load-bearing capacity. However, these PEO coatings generally exhibit high friction coefficients which can limit the wear resistance and cause the wear damage of counterpart materials. Therefore, a multi-step preparation of duplex coatings by PEO and CVD/PVD can integrate the advantages of excellent load-bearing capacity of PEO coatings and low friction coefficient of CVD/PVD coatings. In recent years, successful appli‐ cations of these methods were done by some researchers as follows.

Samir H. Awad et al. [53] studied the tribological properties of duplex Al2O3/TiN coatings on 2A12 Al alloys deposited by a combined plasma electrolytic oxidation (PEO) and arc ion plating (AIP) technique. The thickness of the Al2O3 coatings and TiN coatings were 30-40 μm and 3-5 μm, respectively. The tribological properties were evaluated by ring-on-ring tests at speeds of 0.75 and 1.25 m/s, under loads of 98, 300, 500, and 800 N, using a GCr15 bearing steel ring as rotated counterpart material. Results showed that the duplex coatings possessed very high hardness and wear resistance, and their mechanical and tribological properties were better than those of single TiN coatings, single PEO coatings and the uncoated Al alloy substrate. The Al2O3 intermediate layer played a crucial role in providing the load support essential to withstanding sliding wear at high contact loads.

X. Nie et al. [54] investigated the tribological performances of duplex Al2O3/DLC coatings on Al alloy fabricated by a combined PEO and PI3 (plasma immersion ion implantation) technique. The alumina ceramic coatings with a thickness of 50-60 μm were formed on BS Al-6082 aluminum alloy by PEO treatment and DLC coatings with 2-5 μm thickness were deposited on top of the PEO coatings. All the duplex alumina/DLC coatings exhibited a hardness of over 2000 HK10g. The tribological properties of Al alloy, alumina coating on Al alloy, DLC coating on Al alloy and duplex alumina/DLC coatings on Al alloy were evaluated by pin-on-disc tribological tests, under a 10 N normal load, 0.1 m/s sliding speed and 50% RH, using SAE 52100 bearing steel (BS) or WC-Co (WC) balls as counterpart materials. Both the untreated Al alloy substrate and the PEO alumina coating gave a high friction coefficient of above 0.7 against both counterpart materials. Both the alumina and alumina/DLC duplex coatings exhibited excellent wear resistance, registering the wear rates in a low range of 1.4-1.9×10-6 mm3 /Nm siding against WC-Co. Only the alumina/DLC duplex coatings provided a low and stable friction coefficient in a low range of 0.1-0.22. Whereas the single DLC coating on Al alloy failed quickly only at 25 m sliding distance due to its low load bearing capacity.

and wear resistance, particularly in an intermediate (30-50N) load range. The friction coeffi‐ cient of duplex coating reduced from 0.8-1 to 0.2-0.3, which is attributed to the anti-friction

Chemical vapour deposition (CVD) and physical vapour deposition (PVD) techniques are well known to deposit hard coatings such as TiN, CrN and DLC etc., for providing surfaces with enhanced tribological properties in terms of low friction coefficient and high wear resistance. In recent years, attempts have been made to introduce CVD or PVD coatings on components of machines and engines. However, in practice TiN-, CrN- or DLC-coatings on alloys of light metals formed by various CVD/PVD methods often exhibit limited tribological performance due to the elastic and plastic deformation of the substrates under mechanical loadings, which can result in eventual coating failure, since the coatings are usually too thin to support the heavy loads and protect the substrates in the contact conditions [51]. Deposition of thick (e.g. >10 μm) CVD/PVD coatings usually results in high compressive stresses, and thus low

Employing PEO technique can deposit thick ceramic coatings which exhibit high hardness, superior wear resistance and excellent load-bearing capacity. However, these PEO coatings generally exhibit high friction coefficients which can limit the wear resistance and cause the wear damage of counterpart materials. Therefore, a multi-step preparation of duplex coatings by PEO and CVD/PVD can integrate the advantages of excellent load-bearing capacity of PEO coatings and low friction coefficient of CVD/PVD coatings. In recent years, successful appli‐

Samir H. Awad et al. [53] studied the tribological properties of duplex Al2O3/TiN coatings on 2A12 Al alloys deposited by a combined plasma electrolytic oxidation (PEO) and arc ion plating (AIP) technique. The thickness of the Al2O3 coatings and TiN coatings were 30-40 μm and 3-5 μm, respectively. The tribological properties were evaluated by ring-on-ring tests at speeds of 0.75 and 1.25 m/s, under loads of 98, 300, 500, and 800 N, using a GCr15 bearing steel ring as rotated counterpart material. Results showed that the duplex coatings possessed very high hardness and wear resistance, and their mechanical and tribological properties were better than those of single TiN coatings, single PEO coatings and the uncoated Al alloy substrate. The Al2O3 intermediate layer played a crucial role in providing the load support

X. Nie et al. [54] investigated the tribological performances of duplex Al2O3/DLC coatings on

The alumina ceramic coatings with a thickness of 50-60 μm were formed on BS Al-6082 aluminum alloy by PEO treatment and DLC coatings with 2-5 μm thickness were deposited on top of the PEO coatings. All the duplex alumina/DLC coatings exhibited a hardness of over 2000 HK10g. The tribological properties of Al alloy, alumina coating on Al alloy, DLC coating on Al alloy and duplex alumina/DLC coatings on Al alloy were evaluated by pin-on-disc tribological tests, under a 10 N normal load, 0.1 m/s sliding speed and 50% RH, using SAE 52100 bearing steel (BS) or WC-Co (WC) balls as counterpart materials. Both the untreated Al

(plasma immersion ion implantation) technique.

cations of these methods were done by some researchers as follows.

essential to withstanding sliding wear at high contact loads.

Al alloy fabricated by a combined PEO and PI3

properties of PTFE.

adhesion [52].

*3.2.3. CVD/PVD to form top films*

88 Modern Surface Engineering Treatments

The tribological properties of duplex PEO/DLC coatings on Mg alloy formed by a combined plasma electrolytic oxidation and filtered cathode arc deposition technique were investigated by Jun Liang et al. [55]. The DLC film deposited on PEO coating was not uniform due to surface roughness of interface. The friction and wear properties of the uncoated Mg alloy, the polished PEO coating, single DLC film on Mg alloy and the duplex PEO/DLC coating were evaluated on a reciprocating ball-on-disk UMT-2MT tribometer, under a load of 2 N, with a sliding speed of 0.1 m/s and sliding amplitude of 5 mm, using Si3N4 ball as counterpart material. For the uncoated Mg alloy substrate, the friction coefficient varied in the range of 0.2-0.4 accompanied by severe oscillation. Severe wear and seizing were observed. Even for the DLC deposited Mg alloy substrate, the tribological behavior did not improve significantly. The DLC film failed quickly at around 350 s after starting the sliding test, due to the low load-bearing capacity of soft substrates. The polished PEO coatings showed a very high friction coefficient of around 0.7-0.8. But the deposition of DLC film on PEO coatings improved the dry friction behaviors significantly. The friction coefficient of the duplex PEO/DLC coating remained steady and less than 0.2, independently of the nonuniformity of coating surface.

E. Arslan et al. [56] studied the tribological performances of duplex titania/DLC coatings deposited on Ti6Al4V alloy using combined PEO and CFUBMS (closed field unbalanced magnetron sputtering). The thickness of PEO coating and DLC coating were about 10 μm and 6 μm respectively. The tribological properties were evaluated by a pin-on-disk tribometer (Teer-POD2), under a load of 2 N, at a speed of 100 rpm, a relative humidity of 45% and room temperature, using Al2O3 balls as counterpart materials. For the PEO coatings, the friction coefficient was considerably high, approximately above 0.45, and fluctuated, and the wear tracks were quite broad and rough with debris. For the single DLC coatings, the friction coefficient increased abruptly after 420 seconds due to severe failure caused by their low load bearing capacity. While for the duplex PEO/DLC coatings, the friction coefficients were much more stable, and approximately at 0.1. These results indicated that the duplex PEO/DLC coatings exhibited better tribological behaviors than the PEO and DLC coatings deposited on lightweight metals substrate.

#### **3.3. Composite coatings for improving the tribological behavior of PEO coatings**

The duplex coatings fabricated by the aforementioned methods can sharply decrease the friction coefficient and wear rate. However, these coatings are generally comprised of two layers: an inner PEO ceramic layer and an outside solid lubricant layer. When the outside layer is worn through, the tribological properties will be back to its original level. M. Aliofkhazraei et al. have successfully fabricated TiO2/Al2O3 [57] and TiO2/Si3N4 [58-59] nanocomposite coatings by PEO treatment on pure titanium. The PEO processes were carried out in the electrolyte of sodium-silicate (15 g/L), sodium-phosphate (2 g/L) and potassium hydroxide, adding Al2O3 or Si3N4 fine nanopowder. It was found that due to the incorporation of Al2O3 or Si3N4 nanoparticles in the coatings, the wear resistance and hardness of TiO2/Al2O3 and TiO2/ Si3N4 nanocomposite coatings both increased significantly. However, the friction coefficients were still high or even higher, which could easily cause the wear damage of counterpart materials.

time on the embedding of Si3N4 nanoparticles into TiO2 coatings. Results showed that the wear mass loss rate decreased with the increasing of relative content of Si3N4 in the coatings. And the relative content of Si3N4 in the coatings increased by increasing of concentration, frequency

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Up to now, some researchers have successfully incorporated solid lubricant particles (such as graphite, PTFE, MoS2, etc.) into the PEO ceramic coatings formed on Al, Mg and Ti alloys. In the friction and wear process, the ceramic oxide coating plays the role as wear-resistant substrate while solid lubricant particles act as friction reducing agent during the sliding. Compared with the single PEO coatings, the yielded self-lubricating composite coatings can sharply decrease the friction coefficient and wear loss during the long-term sliding. Further‐ more, the wear damage of counterpart materials can also be reduced greatly due to lower

Xiaohong Wu et al. [60] have successfully incorporated graphite into Al2O3 ceramic coating fabricated on 2024Al alloy by PEO technique in a graphite-dispersed sodium aluminate electrolyte. The thickness of the composite coating produced was in the range 22±1 μm. Ballon-disk tribological tests showed that the self-lubricating composite coating formed in the electrolyte containing 4g/L graphite exhibited a lowest friction coefficient of 0.09, under a normal load of 1 N, with a sliding time of 8 min and linear sliding speed of 0.08m/s, using a

Ming Mu et al. [61] have also successfully incorporated graphite into TiO2 ceramic coating fabricated on Ti6Al4V alloy by PEO technique in a graphite-dispersed phosphate electrolyte. The tribological evaluation was carried out on a ball-on-flat UMT-2MT tribometer, under a constant normal load of 2N, with a frequency of 5 Hz, an oscillating amplitude of 5 mm and a sliding time of 30 min, using AISI52100 steel balls as counterpart materials. And the results of friction and wear tests showed that the friction coefficient of the PEO coating reduced from nearly 0.8 to about 0.15 and the wear resistance improved significantly under dry sliding conditions, due to the presence of the graphite particles in the coating. The specific wear rate of PEO/graphite composite coating decreased significantly, which registered to be around

/Nm and 1.7×10-5 mm3

uncoated alloy showed typical features of abrasive and adhesive wear, which resulted in high wear rate. For the pure PEO coating, the main forms of wear damage were suggested to be abrasive wear and detachment of the TiO2 coating which made its friction coefficient and wear rate high. While for the PEO/graphite composite coating, the worn surface appeared quite smooth and showed no evidence of appreciable detachment of the coating. It was deduced that the graphite particles in the coating could be exposed to the wear track and then smeared on the contact surfaces which acted as solid lubricants in dry sliding wear condition, making the friction coefficient low throughout the sliding test. And consequently, the abrasive wear

Jie Guo et al. [62] tried to introduce PTFE nanoparticles suspension into the electrolyte to fabricated a PTFE-containing multifunctional PEO composite coating on AM60B Mg alloy. The samples were fabricated by PEO treatment in the electrolyte containing Na3PO4 (10.0 g/L),

/Nm, compared with that of the uncoated alloy and pure PEO coating, which

/Nm respectively. The worn surface of

and coating time, while it decreased by increasing of duty cycle and current density.

friction coefficient.

8.6×10-6 mm3

registered to be 5.2×10-5 mm3

and detachment of the coating was effectively reduced.

ball of Si3N4 as counterpart material.

Recently, an alternative approach to obtain PEO coatings with low friction property was to introduce low friction materials into the coating by modifying the electrolytes with the solid lubricants additives. In this approach, solid lubricant particles, such as graphite, PTFE, MoS2, WS2 etc. are added into the electrolyte and dispersed with mechanical stirring to form a suspension. During the PEO process, solid lubricant particles can move from the electrolyte to the surface of the specimen, and be adsorbed on the surface, then be embedded into the ceramic coating.

It is important for this approach that solid lubricant particles should be sufficiently and uniformly dispersed in the electrolyte. So sufficient and constant mechanical stirring is inevitable. What's more, if necessary, a dispersant (such as acetone, ethanol etc.) is used to wet and disperse the solid lubricant particles. A kind of anionic surfactant (e.g. Sodium dodecyl sulfonate, etc.) is also used as additive to help the solid lubricant particles to be negatively charged and be suspended in the electrolyte. But the quantity of added disper‐ sant and surfactant should be controlled and optimized. Too much additive can greatly affect the original properties of electrolyte and the whole coating process, resulting in low qualities of the coatings, such as nonuniformity, high roughness, poor adhesion to the substrate, less thickness, more inclination to breakdown and burn out, etc. However, lower concentration of additive can't wet and disperse the solid lubricant particles in the electrolyte sufficient‐ ly. So the specific and accurate quantity of additives should be decided by different coating processes.

It is generally considered that the embedding of solid lubricant particles into the ceramic coating matrix depends on concentration diffusion and electrophoretic deposition. The embedding of particles may be recognized by the adsorption of particles on the surface of the specimen, so higher concentration can help to enhance the adsorption rate, thus lead to more particles embedded into the ceramic coating. To be negatively charged are beneficial to the electrophoresis of particles in the electrolyte, thus resulting in more particles incorporated into the ceramic coatings. On the other side, the concentration of solid lubricant particles in the electrolyte and the quantity of solid lubricant particles incorporated into the coating should also be controlled and optimized. Too higher concentration of particles in the solution may greatly affect the original properties of electrolyte, causing poor qualities of the coatings. Too much solid lubricant particles incorporated into ceramic coatings may cause the destruction of the original coating structure and less ceramic component which plays the role as wearresistant substrate, resulting in poorer qualities and lower wear resistance of the coatings.

It is also considered that the embedding of nanoparticles into the PEO coatings depends on current density, frequency, duty cycle and coating time. M. Aliofkhazraei et al. [58-59] investigated the effects of concentration, current density, frequency, duty cycle and coating time on the embedding of Si3N4 nanoparticles into TiO2 coatings. Results showed that the wear mass loss rate decreased with the increasing of relative content of Si3N4 in the coatings. And the relative content of Si3N4 in the coatings increased by increasing of concentration, frequency and coating time, while it decreased by increasing of duty cycle and current density.

electrolyte of sodium-silicate (15 g/L), sodium-phosphate (2 g/L) and potassium hydroxide, adding Al2O3 or Si3N4 fine nanopowder. It was found that due to the incorporation of Al2O3 or Si3N4 nanoparticles in the coatings, the wear resistance and hardness of TiO2/Al2O3 and TiO2/ Si3N4 nanocomposite coatings both increased significantly. However, the friction coefficients were still high or even higher, which could easily cause the wear damage of counterpart

Recently, an alternative approach to obtain PEO coatings with low friction property was to introduce low friction materials into the coating by modifying the electrolytes with the solid lubricants additives. In this approach, solid lubricant particles, such as graphite, PTFE, MoS2, WS2 etc. are added into the electrolyte and dispersed with mechanical stirring to form a suspension. During the PEO process, solid lubricant particles can move from the electrolyte to the surface of the specimen, and be adsorbed on the surface, then be embedded into the

It is important for this approach that solid lubricant particles should be sufficiently and uniformly dispersed in the electrolyte. So sufficient and constant mechanical stirring is inevitable. What's more, if necessary, a dispersant (such as acetone, ethanol etc.) is used to wet and disperse the solid lubricant particles. A kind of anionic surfactant (e.g. Sodium dodecyl sulfonate, etc.) is also used as additive to help the solid lubricant particles to be negatively charged and be suspended in the electrolyte. But the quantity of added disper‐ sant and surfactant should be controlled and optimized. Too much additive can greatly affect the original properties of electrolyte and the whole coating process, resulting in low qualities of the coatings, such as nonuniformity, high roughness, poor adhesion to the substrate, less thickness, more inclination to breakdown and burn out, etc. However, lower concentration of additive can't wet and disperse the solid lubricant particles in the electrolyte sufficient‐ ly. So the specific and accurate quantity of additives should be decided by different coating

It is generally considered that the embedding of solid lubricant particles into the ceramic coating matrix depends on concentration diffusion and electrophoretic deposition. The embedding of particles may be recognized by the adsorption of particles on the surface of the specimen, so higher concentration can help to enhance the adsorption rate, thus lead to more particles embedded into the ceramic coating. To be negatively charged are beneficial to the electrophoresis of particles in the electrolyte, thus resulting in more particles incorporated into the ceramic coatings. On the other side, the concentration of solid lubricant particles in the electrolyte and the quantity of solid lubricant particles incorporated into the coating should also be controlled and optimized. Too higher concentration of particles in the solution may greatly affect the original properties of electrolyte, causing poor qualities of the coatings. Too much solid lubricant particles incorporated into ceramic coatings may cause the destruction of the original coating structure and less ceramic component which plays the role as wearresistant substrate, resulting in poorer qualities and lower wear resistance of the coatings. It is also considered that the embedding of nanoparticles into the PEO coatings depends on current density, frequency, duty cycle and coating time. M. Aliofkhazraei et al. [58-59] investigated the effects of concentration, current density, frequency, duty cycle and coating

materials.

90 Modern Surface Engineering Treatments

ceramic coating.

processes.

Up to now, some researchers have successfully incorporated solid lubricant particles (such as graphite, PTFE, MoS2, etc.) into the PEO ceramic coatings formed on Al, Mg and Ti alloys. In the friction and wear process, the ceramic oxide coating plays the role as wear-resistant substrate while solid lubricant particles act as friction reducing agent during the sliding. Compared with the single PEO coatings, the yielded self-lubricating composite coatings can sharply decrease the friction coefficient and wear loss during the long-term sliding. Further‐ more, the wear damage of counterpart materials can also be reduced greatly due to lower friction coefficient.

Xiaohong Wu et al. [60] have successfully incorporated graphite into Al2O3 ceramic coating fabricated on 2024Al alloy by PEO technique in a graphite-dispersed sodium aluminate electrolyte. The thickness of the composite coating produced was in the range 22±1 μm. Ballon-disk tribological tests showed that the self-lubricating composite coating formed in the electrolyte containing 4g/L graphite exhibited a lowest friction coefficient of 0.09, under a normal load of 1 N, with a sliding time of 8 min and linear sliding speed of 0.08m/s, using a ball of Si3N4 as counterpart material.

Ming Mu et al. [61] have also successfully incorporated graphite into TiO2 ceramic coating fabricated on Ti6Al4V alloy by PEO technique in a graphite-dispersed phosphate electrolyte. The tribological evaluation was carried out on a ball-on-flat UMT-2MT tribometer, under a constant normal load of 2N, with a frequency of 5 Hz, an oscillating amplitude of 5 mm and a sliding time of 30 min, using AISI52100 steel balls as counterpart materials. And the results of friction and wear tests showed that the friction coefficient of the PEO coating reduced from nearly 0.8 to about 0.15 and the wear resistance improved significantly under dry sliding conditions, due to the presence of the graphite particles in the coating. The specific wear rate of PEO/graphite composite coating decreased significantly, which registered to be around 8.6×10-6 mm3 /Nm, compared with that of the uncoated alloy and pure PEO coating, which registered to be 5.2×10-5 mm3 /Nm and 1.7×10-5 mm3 /Nm respectively. The worn surface of uncoated alloy showed typical features of abrasive and adhesive wear, which resulted in high wear rate. For the pure PEO coating, the main forms of wear damage were suggested to be abrasive wear and detachment of the TiO2 coating which made its friction coefficient and wear rate high. While for the PEO/graphite composite coating, the worn surface appeared quite smooth and showed no evidence of appreciable detachment of the coating. It was deduced that the graphite particles in the coating could be exposed to the wear track and then smeared on the contact surfaces which acted as solid lubricants in dry sliding wear condition, making the friction coefficient low throughout the sliding test. And consequently, the abrasive wear and detachment of the coating was effectively reduced.

Jie Guo et al. [62] tried to introduce PTFE nanoparticles suspension into the electrolyte to fabricated a PTFE-containing multifunctional PEO composite coating on AM60B Mg alloy. The samples were fabricated by PEO treatment in the electrolyte containing Na3PO4 (10.0 g/L), KOH (1.0 g/L), with the addition of 3 vol.% PTFE nanoparticles suspension (10 wt%). In the PTFE-dispersed suspension, a nonionic surfactant (octylphenol polyoxyethylene ether, with the addition of 1-2 vol.%) and an anionic surfactant (sodium dodecyl sulfonate, with the addition of 2-4 vol.%) were used for PTFE nanoparticles dispersion and surface charge adjustment. Results showed that such PTFE-containing composite coating exhibited superior corrosion resistance, excellent self-lubricating property and better hydrophobic property when compared with pure PEO coatings. The PTFE-containing PEO coating exhibited a low and stable friction coefficient of less than 0.2 and low wear rate.

Secondly, the components of the PEO coating also greatly affect the friction and wear per‐ formances of the coating. It is found that the relative content of α-Al2O3 phase in PEO coatings on Al alloy, Mg2SiO4 phase in PEO coatings on Mg alloy and rutile phase in PEO coatings on

Plasma Electrolytic Oxidation Coatings on Lightweight Metals

http://dx.doi.org/10.5772/55688

93

Thirdly, as the PEO ceramic coatings generally consist of a porous outer layer and a compact inner layer, in many tribological applications, the PEO coatings are polished with abrasive papers to remove the porous outer layer and get a higher hardness and a lower roughness. It is found that the polished PEO coatings generally exhibit improved friction and wear behaviors

And last but not least, although the wear resistance has significantly enhanced, the PEO coatings deposited on the alloy substrates generally exhibit high brittleness and high friction coefficient which have seriously restricted their extensive applications. For the ceramic coatings are hard to bear heavy impingement and mechanical deformation due to their high brittleness. Furthermore, the high friction coefficient of ceramic coatings can easily cause the wear damage of counterpart materials. Therefore, overcoming the challenges of improving the toughness and reducing the friction coefficient of PEO ceramic coatings is of great significance

In recent years, many researchers have done a lot of work to reduce the friction coefficient of PEO ceramic coatings. The successful methods include employing liquid lubricants, introduc‐ ing post treatment such as spraying, vacuum impregnation, PVD and CVD to form a selflubricating duplex coating and one-step preparation of self-lubricating composite coating.

Employing liquid lubricants can decrease the friction coefficient of PEO coatings and conse‐ quently reduce the wear damage of counterpart materials. The wear resistance and wear life of the PEO coatings can also be enhanced in liquid lubricated conditions, ascribed to the antiwear ceramic coatings and friction-reducing liquid lubricants. However, employing liquid lubricants may contaminate the workpieces, especially in precise instruments. Furthermore, in rigid and severe working conditions, such as high vacuum, high temperature, chemical and

The duplex coatings produced by multi-step preparations can not only decrease the friction coefficient and wear rate sharply, but also avoid the shortcomings of liquid lubricants. However, these coatings are generally comprised of two layers: an inner PEO ceramic layer and an outer solid lubricant layer. When the outer layer is worn through, the tribological properties will be back to its original level. Moreover, the coating processes are too complicated and generally employ high temperature which may degrade the coatings and/or substrates.

As for the composite coatings, the solid lubricant micro- and nanoparticles are embedded in the ceramic coatings and play a role as friction-reducing agent during the whole sliding time before complete removal of the coatings. The coating process is simple and convenient, but according to recent studies, the thicknesses of yielded composite coatings are only in the range of 13-23 μm, which are far less than that of the original PEO coatings. As a result, the load

Ti alloy plays a crucial role in presenting higher wear resistance of PEO coatings.

than the original PEO coatings and the untreated alloy substrates.

which can bring about broad application prospect in tribology.

However, none of the methods is so perfect in applications.

radioactive environments, liquid lubricants often do not function.

Recently, Ming Mu et al. [63] have once again successfully incorporated MoS2 into TiO2 ceramic coating fabricated on Ti6Al4V alloy by PEO technique in MoS2-dispersed phosphate electro‐ lyte. The electrolyte was prepared using Na3PO4 (20.0 g/L), KOH (2.0 g/L) in distilled water, with addition of MoS2 particles (20.0 g/L), ethanol (100 ml/L) and an additive (0.5 g/L). Results showed that the TiO2/MoS2 composite coating exhibited improved tribological properties compared with the TiO2 coating under dry sliding condition, which reduced the friction coefficient from 0.8 to about 0.12 and decreased the wear rate from 1.7×10-5 mm3 /Nm to 5.5×10-6 mm3 /Nm. It also should be noted that the TiO2/MoS2 composite coating showed better tribological property than the PEO/graphite composite coating under the same conditions.

From above studies, it is clear that the approach to prepare self-lubricating composite coating was much more effective than the duplex approaches in practice, for the PEO coatings contained low friction materials could be obtained by only one step. Besides, the coatings were expected to integrate the advantages of wear resistance of the PEO coating and low friction property of solid lubricants.
