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Review

Processes and Properties of Self-Lubricating Coatings Fabricated on Light Alloys by Using Micro-Arc Oxidation: A Review

by
Rui Li
,
Xingyu He
,
Chenyu Li
,
Ruimeng Zhang
,
Fei Meng
,
Hongliang Zhang
*,
Xue Cui
and
Zhisheng Nong
School of Materials Science and Engineering, Shenyang Aerospace University, Shenyang 110136, China
*
Author to whom correspondence should be addressed.
Crystals 2024, 14(10), 845; https://doi.org/10.3390/cryst14100845
Submission received: 6 September 2024 / Revised: 20 September 2024 / Accepted: 24 September 2024 / Published: 27 September 2024
(This article belongs to the Special Issue Microstructure Evolution and Deformation Mechanisms in Lightweight Alloys)
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Versions Notes

Abstract

:
A self-lubricating coating is a kind of coating formed on the surface of the material by various processes that can self-replenish lubricating substances during the friction and wear process. This paper presents a comprehensive review of the processes and properties of self-lubricating ceramic coatings developed through Micro-arc Oxidation (MAO) on light alloys, including aluminum, magnesium, and titanium. Three technical approaches for the preparation of self-lubricating coatings via MAO are recapitulated. The structures and properties of the self-lubricating coatings prepared by each technical route are compared and analyzed, and the future development tendency of this field is also anticipated.

1. Introduction

Aluminum, magnesium, titanium, and their alloys have been extensively utilized owing to their merits of low density, high specific strength and stiffness, and strong fatigue resistance [1,2,3,4,5,6,7,8,9,10,11]. However, their applications have been limited due to the defects of some materials themselves, such as low hardness and poor wear resistance. In addition to adjusting the alloy composition, it has been found that the application of a hard coating can enhance the wear characteristics of the aforementioned materials to a large degree [12,13,14,15,16,17,18,19]. In recent years, many scientists have used different coating techniques to improve the friction and wear properties of light alloys. Chen et al. [20] prepared a ceramic coating on AZ31 magnesium alloy using (Micro-arc Oxidation) MAO in a fluorinated silicate electrolyte supplemented with TiO2 nanoparticles under the bipolar current mode. The test results showed that the surface hardness and wear resistance of the substrate were distinctly enhanced owing to the formation of the ceramic coating with TiO2 nanoparticles dispersed. A Y2O3-modified Ti6Al4V-WC gradient wear-resistant coating was fabricated on the surface of TC4 titanium alloy via laser cladding technology [21]. The average hardness of the coating exhibited a gradient increase. Its wear resistance of the bottom layer and the top layer of the coating were 1.4 times and 3.6 times that of the substrate, respectively. Xiang et al. [22] deposited an AT13-enhanced nickel-based composite self-lubricating coating on the surface of 6061 aluminum alloy using a plasma spraying method and investigated the wear resistance of the composite coating under varying AT13 content and heat treatment conditions. Through friction and wear tests, it was found that the 10%AT13 composite self-lubricating coating exhibited exceptional wear resistance, with a wear rate of approximately 27.2%, which is 99% lower than that of the substrate.
Although these coatings possess higher hardness and excellent wear resistance, they frequently exhibit a higher coefficient of friction (COF), resulting in a significant temperature increase and failure due to excessive wear [23]. It is indispensable to add lubricating oil or grease to mitigate the temperature rise of the equipment to ensure its long-term service. However, the effectiveness of lubricating oil or grease is severely constrained by the working environment, and they still possess deficiencies in load-bearing capacity, volatility, viscostatic temperature performance, and degradation at high temperatures [24]. In the harsh environment characterized by high vacuum and intense radiation in outer space, liquid lubricating oil or grease will gradually volatilize and lose its effectiveness [25]. Thus, it is essential to investigate the preparation and application of self-lubricating coatings on light alloys, which refers to a type of coating that typically offers low friction without influencing the relative hardness. It not only possesses a low COF but also enhances the wear resistance of the materials [26]. The anti-friction mechanism of the self-lubricating coating lies in that when the self-lubricating coating is rubbed against a relatively hard material, the self-lubricating particles transfer, forming a film between the sample and the friction pair, and some self-lubricating particles adhere to the friction pair. Subsequently, the friction will take place between the self-lubricating particles and the film layer, thereby playing a role in anti-friction. For instance, a self-lubricating MoS2 particle possesses a distinctive layered architecture, where weak van der Waals forces bind the layers together. This structure enables MoS2 to slide readily between the layers when subjected to shear forces, thereby obviously lowering the friction coefficient. Simultaneously, the MoS2 particles within the coating can form a lubricating film on the contact surface during the friction process, effectively isolating the direct contact of the friction pair and reducing friction and wear.
At present, a variety of techniques are used to prepare self-lubricating coatings, such as laser cladding (LC) [27,28,29,30,31], Electrodeposition (ED) [32,33,34], Thermal Spraying (THSP) [24,35,36,37], Magnetron Sputtering (MS) [38,39,40], Physical Vapor Deposition (PVD) [41,42], and so on. MAO, also referred to as Plasma Electrolytic Oxidation (PEO) [43,44,45], is an electrochemical surface modification technique, which originated from Anodic Oxidation (AO). During the MAO process, a ceramic-like oxide coating can be fabricated on the surface of certain non-ferrous metals through arc micro-discharge in a specific electrolyte [46]. The oxide coating acquired through this process typically encompasses elements from the substrate and electrolyte solution and possesses fade resistance, high corrosion resistance, and wear resistance [3,47]. The MAO coating is usually composed of an inner dense layer and an outer loose layer. The inner dense layer often plays a role in improving the corrosion resistance and wear resistance of the metal matrix. This process is straightforward, is environmentally friendly, and can be applied to workpieces with complex shapes while fulfilling the requirements of green manufacturing and realizing the vision of both environmental protection and economic benefits [48,49,50,51]. The fabrication of self-lubricating coatings through MAO technology constitutes one of the current research focuses. The frequently utilized self-lubricating materials encompass MoS2 [52,53,54,55,56,57], h-BN [58,59,60], graphite [61,62,63,64], graphene [65,66,67], graphene oxide (GO) [68,69], carbon nanotube (CNT) [70,71], polytetrafluoroethylene (PTFE) [72], and so on. Experimental investigations have demonstrated that the performance of MAO coatings has been notably enhanced subsequent to the introduction of the aforementioned anti-friction materials.
There are three primary approaches for preparing self-lubricating coatings on light alloys based on MAO technology. The first approach involves the in situ synthesis of lubricating phases at an elevated temperature in the electrolyte during the traditional MAO process, thereby forming a composite ceramic coating containing the lubricating phase on the metal surface with the water cooling of the electrolyte. The second method first disperses the self-lubricating particles into the electrolyte by adsorbing a certain amount of anion. The self-lubricating particles are then incorporated to form a composite self-lubricating coating while oxidizing the base metal. The first two methods constitute a one-step approach for achieving the preparation of self-lubricating coatings. The third one is a two-step process. First of all, a porous ceramic coating is fabricated through MAO on the metal surface. Subsequently, lubricating particles are filled into the micropores and micro-cracks of the coating via post-treatment processes. The three preparation process routes mentioned above will be elaborated in detail below in combination with the friction and abrasion properties of the self-lubricating coatings.

2. Three Approaches to Form Self-Lubricating Coatings through MAO

2.1. In Situ Synthesis of Lubricating Phase

In situ synthesis technology refers to the technology that functions in a way that by adjusting the composition of the MAO electrolyte, the specific lubricating particles can be formed in situ during the MAO reaction and dispersed in the coating. In recent years, an increasing number of lubrication phases, such as MoS2 [73,74,75,76], CePO4 [76,77,78], and ZrO2 [78,79], have been synthesized using this technique.
Lubricating MoS2 was in situ synthesized on 6063 Al alloy [73] and AZ31 Mg alloy [75] respectively by one-step MAO process. MoS2 exhibits a layered structure and possesses low shear strength, characteristics that facilitate friction reduction, and the formation of a lubricating protective layer during tribological interactions [80]. The MoS2/Al2O3 coating formed on 6063 Al alloy [73] has a lower COF of around 0.15, which is only 20% of that of the traditional MAO coating without MoS2. Furthermore, the traditional MAO coatings are prone to abrasive wear, leading to a significant delamination of the coating. In contrast, owing to the self-lubricating properties of MoS2, the wear patterns observed in MoS2/Al2O3 coatings exhibit a relatively smooth profile. MoS2 is synthesized mainly from Na2MoO4 and NaS2 dissolved in the electrolyte. Therefore, the ratio of the two salts has become a key factor in the study of the preparation of self-lubricating coatings. When 2.5 g/L Na2MoO4 and 5 g/L Na2S were added, a MoS2/MgO/MgAl2O4 composite self-lubricating coating fabricated on AZ31 Mg alloy exhibited optimal wear resistance, with an average COF of 0.1521 ± 0.007 and a wear volume of 0.0193 ± 0.001 mm3 [75].
Yang et al. [74] performed MAO on TC4 titanium alloy using an electrolyte containing NaS2 and Na2MoO4, resulting in the in situ formation of a ceramic coating containing lubricating MoS2. The influence of the concentration of Na2S on the coating properties was also investigated. The results of the friction and wear test shows that the COF of the MAO coating is 0.2 when the Na2S content is 30 g/L and 40 g/L, which is distinctly lower than that of the MAO coating without lubricating MoS2 (~0.6).
During the fabrication process of the MAO coating on LY12 Al alloy in the electrolyte containing (NaPO₃)6 and Ce(Ac)3, CePO4 is in situ synthesized and distributed uniformly as nano-grains in the coating [77]. Transformation from abrasive wear to adhesive wear is observed with increasing concentrations of CePO4. Owing to the self-lubricating effects rendered by CePO4, the friction coefficient decreases from 0.8–1.0 to 0.1–0.15, and the wear rate is reduced by about 10 times compared to the coating without CePO4.
The MAO coating containing hard phase ZrO2 and lubricating phase CePO4 was in situ fabricated on TC4 alloy by introducing varying concentrations of K2ZrF6 and Ce(Ac)3 into the phosphate electrolyte [78]. The preparation process is depicted in Figure 1. The appropriate quantity of cations in Ce salt and the suitable amount of anions in Zr salt circumvent the competition phenomenon typically observed in traditional polyphase electrolytes, thereby concurrently achieving the combination of a high concentration of the hard phase and the lubricating phase. The coating fabricated in an electrolyte containing 10 g/L K2ZrF6 and 15 g/L Ce(Ac)3 exhibits the best lubrication and wear resistance, with a COF of 0.12 under different loads.
Two self-lubricating phases, MoS2 and CePO4, were simultaneously in situ synthesized in ceramic coating on 6082-T6 alloy using the MAO process [76]. The content of (CH3CO3)3Ce⋅xH2O in the electrolyte has a great influence on the microstructure and properties of the Al2O3/MoS2/CePO4 composite self-lubricating coating. The coating has a maximum thickness of 35.28 μm, a highest average hardness of 961.79 HV1, and a lowest average COF of 0.05 when the addition concentration of (CH3CO3)3Ce⋅xH2O is 7.5 g/L. The COF of the MAO coating without (CH3CO3)3Ce⋅xH2O is about 0.3.

2.2. In Situ Incorporation of Lubricating Phase

In situ incorporation technology involves immersing the metal matrix in an electrolyte suspension containing lubricating particles for MAO. During the oxygen discharge process, these lubricating particles are deposited both within and on the surface of the MAO coating through mechanisms such as electrophoretic adsorption, physical diffusion, and mechanical mosaic [81]. Table 1 lists the details of the in situ incorporated particles and their self-lubricating effect on reducing the COF of the composite self-lubricating coatings.
MoS2 particles were successfully incorporated into theTiO2 coating formed on TC4 Ti alloy during the MAO process [53,54]. As shown in Figure 2, dry sliding tests under a constant normal load of 2N showed that the TiO2/MoS2 composite self-lubricating coating exhibited much lower COF and wear rate than the MAO coating without MoS2 [53]. The enhanced frictional performance can be ascribed to the incorporation of MoS2 particles within the TiO2/MoS2 composite self-lubricating coating, which functions effectively as a lubricant during the sliding test.
A composite self-lubricating coating was fabricated on the surface of AZ91 alloy by dispersing PTFE particles into the phosphate electrolyte [85]. A small amount of surfactant was utilized to enhance the dispersion stability of PTFE particles, and a relatively low energy input was employed to realize in situ incorporation. This can modify the surface polarity of hydrophobic PTFE particles and notably reduce their surface tension, enabling the ultra-low COF of the composite self-lubricating coating. Figure 3 depicts the COF of the coatings subjected to different treatment durations in the PTFE-containing electrolyte. The composite coating treated for 1 min exhibited an increase in the COF to approximately 0.61 during the first 6 m of sliding, followed by a considerable fluctuation within the range of 0.30 to 0.68. The coatings treated for 2 and 3 min exhibited lower and more stable friction coefficients compared with the coating treated for 1 min. The COF of the coating drops distinctly after two minutes of treatment, presumably due to the removal of the surface layer and the release of PTFE particles, which are coated on the metal surface, thereby resulting in a lower COF. The composite self-lubricating coating treated for 5 min exhibits an ultra-low COF of 0.08 during the dry sliding wear test.
Guo et al. [87] successfully fabricated a MAO composite self-lubricating coating on the surface of AZ31 magnesium alloy by incorporating graphite into the alkaline silicate electrolyte. The study reveals that the specimen initially rubs the C-containing ceramic composite self-lubricating coating to form wear marks during sliding, and graphite abrasive particles fill the coating pores to exert a solid lubrication effect. The COF of the coating prepared by adding 25 g/L graphite to the electrolyte is 0.06–0.15, which is lower than that of the MAO coating without graphite (~0.18).
To investigate the tribological characteristics of the three self-lubricating particles, Hu et al. [91] individually incorporated MoS2, WS2, and graphene oxide (GO) into the composite electrolyte of hexametapic acid and fabricated three composite self-lubricating coatings on the surface of 2A12 aluminum alloy through MAO technology. As depicted in Figure 4a, it can be observed from the SEM topography of the wear surface that the wear mechanism of Al/Al2O3 is primarily abrasive wear. However, the incorporation of graphene oxide alters the wear mode between the coating and its counterpart from abrasive wear to adhesive wear. Since graphene oxide exhibits excellent dispersion in water, its negatively charged functional groups can move more readily towards the anode surface under the influence of electric field forces. Consequently, more GO nanoparticles are filled into the pores compared with hydrophobic MoS2 and WS2. It can be observed from the friction and wear curve depicted in Figure 4b that the COF of the Al2O3/GO composite self-lubricating coating is lower and more stable compared with that of Al2O3/MoS2 and Al2O3/WS2, thereby reducing the friction shear stress and enhancing the friction and wear performance of the composite self-lubricating coating.
Gao et al. [59] incorporated h-BN particles into the electrolyte of Na2SiO3 and (NaPO3)6 and successfully fabricated the MAO composite self-lubricating coating on the surface of 2024 aluminum alloy via MAO technology. By observing the trend of the COF under various dry grinding conditions, it was discovered that when the addition of h-BN was 10 g/L, the COF of the composite self-lubricating coating was 0.4, which was 33% lower than that of the coating without h-BN.
Carbon nanotubes (CNTs) and h-BN nanoparticles were simultaneously in situ incorporated into a double-layer coating on TC4 alloy through the MAO technique [98]. Owing to the synergistic lubricating effect of h-BN and CNT, the double-layer coating presents a much lower COF (0.11) under dry grinding conditions, which is substantially lower than that of the traditional MAO coating (0.91). The wear mechanism of the MAO coating, MAO-BN coating, and MAO-BN-CNTs coating are shown in Figure 5. During friction subjected to a 2N load, the COF decreased as h-BN particles and CNTs formed a self-lubricating layer on the surface of the MAO-BN-CNT coating.
Mortezanejad et al. [84] fabricated a MAO nanocomposite coating containing PTFE nanoparticles on the surface of AZ80 magnesium alloy. The test result demonstrated that PTFE nanoparticles effectively sealed the micropores and microcracks of the coating. The MAO composite self-lubricating coating formed in the aluminate electrolyte presented the lowest COF of 0.33 under tribological wear tests on the pin-on-disc with a load of 3N.
Ni et al. [95] incorporated h-BN particles into the MAO coating formed on the surface of TC4 titanium alloy to fabricate the MAO-hBN composite self-lubricating coating. The investigation reveals that the COF of the composite self-lubricating coating reduces with the addition of the h-BN particles in the electrolyte. When the concentration of h-BN particles is 2 g/L, the COF of the MAO-hBN composite self-lubricating coating is 0.4 after the polishing and ball-on-disk test, which is markedly lower than that of the conventional MAO coating (0.7). Meanwhile, the composite self-lubricating coating exhibits outstanding wear properties.

2.3. Two-Step Process: MAO and Post-Treatment

After MAO, the appropriate post-treatment process, such as Impregnation Sintering (IS) [99], Vacuum Impregnation (VI) [100], Hydrothermal Treatment (HT) [101,102], Electrophoretic Deposition (EPD) [103], and Magnetron Sputtering (MS) [104], can effectively fill the micropores on the surface of the MAO coating with lubricating particles and improve the wear resistance of the coating. Currently, the two-step process is a prevalent approach for modifying and optimizing MAO coatings. Table 2 enumerates the details of the post-treatment processes and their effect on reducing the COF of composite self-lubricating coatings.
Ren et al. [99] fabricated a MAO coating on pure titanium. After immersion and sintering, the solid lubricant PTFE was directly deposited on the surface of the MAO coating to prepare the MAO-PTFE composite self-lubricating coating. The study reveals that the COF of the MAO-PTFE composite self-lubricating coating (approximately 0.1) is obviously smaller than that of MAO TiO2 (approximately 0.7), and it remains stable under various loads.
After the treatment of TC6 titanium alloy with MAO, Liu et al. [101] inserted the MAO sample into the hydrothermal solution for hydrothermal heat treatment to obtain the in situ MoSe2 composite self-lubricating coating. The dry friction test demonstrated that the COF and wear rate of the MAO/MoSe2 composite self-lubricating coating decreased by 33.7% and 29%, respectively, compared with the MAO sample, indicating the synergistic effect between the MoSe2 film and the MAO coating. The in situ coating achieved through hydrothermal growth exhibited exceptional density and uniformity. Furthermore, the coating manifested durability within the velocity range of (0.5 cm/s–2 cm/s) and under loads of (1 N, 3 N, 5 N, 7 N) as well as in dry environments and water- and oil-lubricated environments, thereby addressing the limitations associated with MAO coatings.
The MAO coating fabricated on pure titanium was filled with hydrothermally synthesized MoS2 to prepare the MAO/MoS2 composite self-lubricating coating [105]. The tribological properties of the coating were tested and compared with those of the MAO coating. The COF of the composite self-lubricating coating (approximately 0.2) is approximately 66% lower than that of the MAO coating (approximately 0.6). The test shows that the MAO/MoS2 coating possesses a two-layer structure consisting of the upper MoS2 surface layer and the lower MAO-MOS2 interlocking layer. At the onset of the friction test, the GCr15 friction ball initially comes into contact with the upper surface layer. Owing to the abundance of MoS2 synthesized through hydrothermal synthesis, it is facile to form a relatively uniform and continuous self-lubricating MoS2 layer on the wear surface, thereby ensuring the low COF of the composite self-lubricating coating at this stage. Once the upper surface layer is worn out, the GCr15 ball commences to come into contact with the MAO-MoS2 interlock layer. At this juncture, although the self-lubricating MoS2 layer on the worn surface becomes less continuous, it still covers the majority of the worn surface and can be continuously regenerated since the MAO/MoS2 coating possesses a MoS2 reservoir within the MAO hole. Consequently, the MAO/MoS2 coating sustained its self-lubricating attributes throughout the test procedure. Additionally, the worn surface reveals more Al2TiO5, which is beneficial for the wear resistance of the composite self-lubricating coating.
Fu et al. [109] employed MAO technology to fabricate hard ceramic films on the surface of ZL109 aluminum alloy. Subsequently, PAA/WS2 was brushed on the surface of the MAO ceramic film via the spin-coating method to prepare the MAO/PI/WS2 ceramic-based self-lubricating coating. The schematic of the two-step process was illustrated in Figure 6. After testing the tribological properties of the coating under dry sliding conditions, it was found that the COF of the MAO/PI/WS2 coating was the smallest and stable, approximately 0.4, which was approximately 50% lower than that of the MAO coating. Studies have demonstrated that the markedly enhanced tribological properties can be ascribed to the fact that the incorporated WS2 particles possess a layered structure and low shear strength, which glide between the layers under the influence of slight shear forces and subsequently transfer to the surface cracks to form a self-lubricating film and mitigate friction.
A biphase MAO-AND coating was fabricated on TC4 alloy through a two-step method [111]. The MAO coating was achieved on the TC4 substrate initially, and core-shell annealed nanocrystalline diamond (AND) particles were deposited on the MAO coating. The average steady-state COF of the MAO-AND coatings is 0.32, which is 46.67% lower than that of the MAO coating.

3. Summary and Perspectives

Self-lubricating coating is a kind of material with special properties, which is widely used in mechanical, aerospace, and automotive fields to reduce friction and wear and improve the service life of equipment. In this paper, we have reviewed three main technical routes for preparing self-lubricating coatings by MAO technology on light alloys, which mainly centers on adding a certain amount of lubricating materials of specific composition and outstanding performance to prepare self-lubricating coatings. The entry of lubricating particles into the MAO coating can form a lubricating layer during friction, distinctly reducing the COF between contact surfaces and reducing wear loss.
In situ synthesis technology is a one-step method, and some certain lubricant phases (such as MoS2, CePO4) can be prepared by the reaction of ions in the electrolyte and simultaneously enter the composite self-lubricating coating during the MAO process. However, the type of lubricating phase synthesized by this method is relatively limited. More types of lubricant phase particles can be compounded into the MAO coating using the in situ incorporation technology. The key to using this technology is to suspend the lubricating particles in the electrolyte by the adsorption of negative ions. The application of the two-step method makes the structure and types of self-lubricating composite self-lubricating coatings become diversified. The interlocking architecture of the surface layer and the MAO coating enhances the overall adhesion strength of the composite self-lubricating coating, visibly contributing to the improvement of its friction and wear characteristics.
The future research work mainly focuses on several aspects. First, the principle of the chemical synthesis of the lubricating phase in the MAO process has not yet been reported. Further study on the mechanism of chemical formation can provide a theoretical basis for preparing more kinds of lubricating phases. Second, some mechanical components will endure harsh conditions like high temperature and high pressure. Conventional self-lubricating materials cannot stably exist at high temperatures and lose their self-lubricating functionality. Thus, how to prepare self-lubricating composite self-lubricating coatings with high-temperature resistance is an essential research direction for the future. Finally, the application of environmentally friendly MAO electrolytes and subsequent treatment technologies will obviously enhance efforts to mitigate environmental pollution.

Author Contributions

Conceptualization, R.L.; methodology, R.L.; investigation, X.H. and C.L.; data curation, X.H. and R.Z.; formal analysis, R.Z. and F.M.; writing—original draft preparation, R.L.; writing—review and editing, H.Z. and X.C.; supervision, H.Z. and Z.N. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the Fundamental Research Funds for the Universities of Liaoning Province, Basic scientific research project of higher education Department of Liaoning Province (NO. LJKZ0170), Liaoning Provincial Natural Science Foundation of China (No. 2024-BS-152), and College Student Innovation Project of Liaoning Province (No. S202310143030, No. S202410143018).

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram illustrating the preparation of a self-lubricating coating [78].
Figure 1. Schematic diagram illustrating the preparation of a self-lubricating coating [78].
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Figure 2. COF of (a) TiO2 coating and (b) TiO2/MoS2 composite self-lubricating coating [53].
Figure 2. COF of (a) TiO2 coating and (b) TiO2/MoS2 composite self-lubricating coating [53].
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Figure 3. The COF of the coatings during the dry sliding wear test [85].
Figure 3. The COF of the coatings during the dry sliding wear test [85].
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Figure 4. The wear tracks of the MAO coatings: (a) Al/Al2O3, (b) Al2O3/MoS2, (c) Al2O3/WS2, and (d) Al2O3/GO and (e) the COF of MAO coatings with respect to the sliding time [91].
Figure 4. The wear tracks of the MAO coatings: (a) Al/Al2O3, (b) Al2O3/MoS2, (c) Al2O3/WS2, and (d) Al2O3/GO and (e) the COF of MAO coatings with respect to the sliding time [91].
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Figure 5. Wear mechanism for each sample: (a) MAO, (b) MAO-BN, (c) MAO-BN/CNTs [98].
Figure 5. Wear mechanism for each sample: (a) MAO, (b) MAO-BN, (c) MAO-BN/CNTs [98].
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Figure 6. Schematic illustration for the preparation of MAO/PI/WS2 composite self-lubricating coatings [109].
Figure 6. Schematic illustration for the preparation of MAO/PI/WS2 composite self-lubricating coatings [109].
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Table 1. Details of the in situ incorporated particles and their self-lubricating effect.
Table 1. Details of the in situ incorporated particles and their self-lubricating effect.
SubstrateParticleCOFWear RateRef.
TypeSizeConcentrationMatrixMAO CoatingComposite Self-Lubricating CoatingMAO CoatingComposite Self-Lubricating
Coating
TC4MoS20.5–1.0 μm20 g/L-0.80.121.7 × 10−5 mm3/N/m5.5 × 10−6 mm3/N/m[53]
TC4500 nm4 g/L0.580.71–0.740.48–0.546.9–9.3 × 10−4 mm3/N/m0.96 × 10−4 mm3/N/m[54]
6063-6 g/L--0.499–11.46 × 10−7 mm3/N/mm1.09–1.23 × 10−7 mm3/N/mm[82]
TA15PTFE100–200 nm--0.4–0.50.15–0.2--[83]
AZ80200–300 nm10 g/L0.8-0.3--[84]
AZ91180–240 nm20 g/L-0.640.08--[85]
2024 484 nm--0.640.148.21 × 10−7 mm3/N/m7.80 × 10−5 mm3/N/m[86]
AZ31Graphite10–70 μm0–25 g/L0.1–0.20.12–0.180.06–0.32--[87]
Pure-Al0.7 μm10 g/L0.620.650.58--[23]
60610.5–6 μm0.4 g/L-0.12–0.60.06–0.12--[88]
TC4 Graphene10–20 μm0–6 g/L0.710.460.22–0.31--[89]
Mg-Li0.5 μm0.1 g/L0.410.290.11--[90]
2A12GO2 μm2 g/L-0.6–0.650.3–0.34--[91]
Mg-Li-10 mL/L0.520.280.12--[92]
6061CNT-2 g/L-0.680.14--[93]
AZ80-0–4 g/L0.730.450.25–0.42.72 × 103 mm3/N/m1.95 × 103 mm3/N/m[94]
2024 h-BN1 μm0–10 g/L-0.60.4–0.61.29 × 10−3 mm3/N/m4.3 × 10−4 mm3/N/m[59]
TC4 1–2 μm2 g/L, 8 g/L-0.80.4--[95]
2A12 WS22 μm2 g/L--0.4--[91]
TC4 Al2O310 nm0–15 g/L-0.650.2–0.659.37 × 10−4 g/N/m6.19 × 10−4 g/N/m[96]
AZ31BWC80 nm5 g/L--0.14--[97]
TC4 h-BN, CNT-20 g/L, 1 g/L-0.910.11--[98]
Table 2. Details of the post-treatment process and their self-lubricating effect.
Table 2. Details of the post-treatment process and their self-lubricating effect.
Post-TreatmentSubstrateParticleCOFWear RateRef.
TypeSizeConcentrationMAO CoatingComposite Self-Lubricating Coating MAO CoatingComposite Self-Lubricating Coating
ISPure-TiPTFE--0.5–0.650.17.02 × 10−5 mm3/N/m1.34 × 10−5 mm3/N/m[99]
VI2024--0.68–0.80.11--[100]
160–280 nm-0.70.091–0.141.45 × 10−5 mm3/N/m5.1 × 10−6 mm3/N/m[72]
HTTC6MoSe2--0.790.521.3 × 10−3 mm3/N/m1 × 10−3 mm3/N/m[101]
AZ91Graphite5–10 μm5 g/L0.3–0.340.15–0.22--[102]
HT and VIPure-TiMoS2--0.47–0.60.1–0.2--[105]
6063--0.350.2–0.33.7–8.15 × 10−7 mm3/N/mm1.39–2.66 × 10−7 mm3/N/mm[106]
6063---0.22-2.94–3.82 × 10−7 mm3/N/m[107]
EPDAL109MoS240 nm10 g/L-0.45--[103]
Grinding and polishingTC4MoS240 nm-0.80.18--[108]
Spin coatingZL109 WS22 μm-0.80.4--[109]
PECVD2A12 HMDSO--0.3750.253.57 × 10−5 mm3/N/m2.04 × 10−6 mm3/N/m[110]
SinteringTC4 AND--0.16–0.60.3--[111]
MSTA2 DLC---0.2--[104]
Selective sprayingAE44 PTFE120 nm-1.20.2--[112]
Heart treatmentLY12PTFE100–170 nm-0.250.13--[113]
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Li, R.; He, X.; Li, C.; Zhang, R.; Meng, F.; Zhang, H.; Cui, X.; Nong, Z. Processes and Properties of Self-Lubricating Coatings Fabricated on Light Alloys by Using Micro-Arc Oxidation: A Review. Crystals 2024, 14, 845. https://doi.org/10.3390/cryst14100845

AMA Style

Li R, He X, Li C, Zhang R, Meng F, Zhang H, Cui X, Nong Z. Processes and Properties of Self-Lubricating Coatings Fabricated on Light Alloys by Using Micro-Arc Oxidation: A Review. Crystals. 2024; 14(10):845. https://doi.org/10.3390/cryst14100845

Chicago/Turabian Style

Li, Rui, Xingyu He, Chenyu Li, Ruimeng Zhang, Fei Meng, Hongliang Zhang, Xue Cui, and Zhisheng Nong. 2024. "Processes and Properties of Self-Lubricating Coatings Fabricated on Light Alloys by Using Micro-Arc Oxidation: A Review" Crystals 14, no. 10: 845. https://doi.org/10.3390/cryst14100845

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