**4. Concluding remarks**

Plasma electrolytic oxidation (PEO) is a relatively novel technique that can be used to form metallurgically bonded ceramic coatings on some valve metals [13], such as Ti, Mg, Al, Nb, Zr and Ta etc. PEO process is now widely used to improve the surface performances of nonferrous metals by virtue of its high effectiveness, more convenience, economic efficiency and envi‐ ronmental performance. Moreover, the PEO ceramic oxide coatings deposited on light metals and their alloys generally exhibit superior wear resistance, large thickness, high microhardness and good adhesion to substrates. Therefore, in many tribological applications, employing PEO treatment to deposit ceramic coatings on surface of light metals can greatly improve the wear resistance, decrease the wear rate, enhance the wear life and reduce the wear damage of workpieces, thus resulting in good economic efficiency. However, it is necessary to realize the following conclusions about the tribological properties of PEO coatings.

Firstly, the thickness of the coatings plays a crucial role to possess a better wear resistance. For at high stress levels, the deformation of the substrate under loading/sliding can cause the cracking and flaking-off of the thin coatings, and thus result in severe failure. So higher thickness of PEO coating can provide a better load bearing capacity and thus possess superior wear resistance and enhanced wear life.

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

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

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

/Nm. It also should be noted that the TiO2/MoS2 composite coating showed better

/Nm to

coefficient from 0.8 to about 0.12 and decreased the wear rate from 1.7×10-5 mm3

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

Plasma electrolytic oxidation (PEO) is a relatively novel technique that can be used to form metallurgically bonded ceramic coatings on some valve metals [13], such as Ti, Mg, Al, Nb, Zr and Ta etc. PEO process is now widely used to improve the surface performances of nonferrous metals by virtue of its high effectiveness, more convenience, economic efficiency and envi‐ ronmental performance. Moreover, the PEO ceramic oxide coatings deposited on light metals and their alloys generally exhibit superior wear resistance, large thickness, high microhardness and good adhesion to substrates. Therefore, in many tribological applications, employing PEO treatment to deposit ceramic coatings on surface of light metals can greatly improve the wear resistance, decrease the wear rate, enhance the wear life and reduce the wear damage of workpieces, thus resulting in good economic efficiency. However, it is necessary

to realize the following conclusions about the tribological properties of PEO coatings.

Firstly, the thickness of the coatings plays a crucial role to possess a better wear resistance. For at high stress levels, the deformation of the substrate under loading/sliding can cause the cracking and flaking-off of the thin coatings, and thus result in severe failure. So higher thickness of PEO coating can provide a better load bearing capacity and thus possess superior

stable friction coefficient of less than 0.2 and low wear rate.

5.5×10-6 mm3

property of solid lubricants.

92 Modern Surface Engineering Treatments

**4. Concluding remarks**

wear resistance and enhanced wear life.

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 than the original PEO coatings and the untreated alloy substrates.

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 which can bring about broad application prospect in tribology.

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

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 radioactive environments, liquid lubricants often do not function.

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 bearing capacity and wear life of the composite coatings will be limited due to the low coating thickness.

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Therefore, a high quality coating is still worth investigating, which has a lower friction coefficient and wear rate, a longer wear life, good mechanical properties and with a simple, cheap, effective fabricating method. More improvements should be done for the PEO techni‐ que which is of great significance in different tribological applications.
