ReviewSurface properties of plasma electrolytic oxidation coating modified by polymeric materials: A review
Introduction
Magnesium is the 8th most abundant element on earth making with extremely light nature as it is 75% lighter than steel, 50 % lighters than titanium, and 33% lighter than aluminum [1]. Light weight metals play a vital role in the reduction of carbon emissions, fuel production and providing green environment. Moreover, this character leads to high strength-to-weight ratios thus making it highly suitable for automotive, aerospace, and electronic fields, leading towards modern technology both industrial and nanoscale level demands, modified versions of devices with reduced weight aspect. This significant mutation in the physical properties of metallic devices is responsible for imparting operational characteristics; by increasing manufacturing efficiency and minimizing power consumption [2]. Less weight/low density leads to fragile nature in general, but Mg is an exceptional metal. The density of Mg (1.7 g cm−3) is comparatively low as compared to various metals aluminum (2.7 g cm−3), titanium (4.6 g cm−3) and steel (7.8 g cm−3) used in aerospace engineering. Moreover, Mg also retains extremely high specific strength and modulus [3]. These unique characters of Mg make it a promising candidate for medical products, especially for cardiovascular and orthopedic applications [4]. Additionally, biomedical applications of Mg are very remarkable because Mg is a natural element of the human body and thus its non-toxicity promotes various biological processes, such as new bone formation both in vitro and in vivo [5].
Nevertheless, Mg has certain disadvantages as it only occurs in a combined form with other materials by having a + 2-oxidation state. The free state of Mg can be produced artificially, but its highly reactive nature results in the formation of a thin oxide layer that partly inhibits reactivity. This reactivity propels Mg and its alloys towards corrosion due to low electrode potentials which create hurdles in their application [6]. Therefore, to endow Mg with corrosion-resistant protective coatings, various surface modification techniques, such as conversion coatings, thermal spraying [7], self-assembled monolayers [8], [9] alkaline heat treatment [10], phosphate treatment [11], organic coating [12], and plasma electrolytic oxidation (PEO) [13] are suggested by several research groups. In aforementioned techniques, PEO holds the advantage of being eco-friendly and the most promising technique for surface engineering [8], [9], [10], [11], [12], [13], [14]. PEO, an electrochemical reaction-based coating, results in the formation of inorganic, hard, consistent, and adhesive protective layer on metal substrates [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. In general, PEO is conducted at potentials above the breakdown voltage, in which the pre-existing passive film is unlikely to withstand the strong electrical field, thus, dielectric breakdown occurred with accompanying the phenomenon of sparking over the substrate [13]. Since a thorough description of the PEO process is outside the purview of this study, further detail should be found elsewhere. Despite useful consequences of plasma discharges during PEO, it also leads to numerous micro defects such as pores, cracks, and discharge channels which allow penetration of corrosive media to the substrate material and as a result cause destruction [13], [23]. To eliminate above all problems, researchers suggest further treatments of PEO coating, such as by filling the open pores with chemically inert substances [24], [25], [26], [27], [28], [29], [30], [31], [32].
Even though the corrosion resistance offered by PEO pores, can be controlled by a variety of substances, the post-treatment done by the incorporation of polymeric substances is considered to be the best one. This can be proved by the fact that the rough coatings of the PEO process with a large surface area demand application of external material with a large molecular structure as to cover the most possible space, which can be most efficiently done by polymers, having clusters of the fullerene molecules [33]. Apart from the tremendous positive features of PEO, the porosity associated with these coatings is a serious problem under discussion. Pores, discharge channels, cracks, and other structural defects not only increase the effective surface area that is exposed to the external environment but also enable the penetration of corrosive medium [34], [35], [36]. Moreover, there is a long range of biodegradable as well as non-biodegradable polymers that are responsible for inculcating unique characteristics in PEO coating. This review, therefore, covers all aspects of externally applied polymers concerning Mg metal as base material treated with PEO coating. In the following detailed explanation, various organic/inorganic polymers are discussed, in both intrinsic and extrinsic manner. By intrinsic it means how the Mg metal encapsulated with polymer molecules, is going to be functionalized after placement in any medical/electrical device, while extrinsic properties deal with operational properties of additional polymeric layer on the substrate metal, in surroundings. Each polymer has its unique structure and thus it induces the corresponding impact, as some lead to tissue regeneration and osteoblast adhesion while other polymer types would enhance the electrochemical properties of implant materials [37], [38], [39], [40].
Section snippets
Formation of polymeric PEO coatings
For the preparation of polymeric PEO coatings, several techniques can be used to apply polymeric material on PEO layers, such as the dipping technique [37], [41], [42], [43] which is further classified as slow or fast dipping depending upon the morphology of applied polymer, spin coating technique [44], spray coating technique [12], a triboelectric method [45], an electrophoretic method [2], and hydrothermal treatment [46].
The full description of these methods is beyond the scope of this
Biodegradable polymeric PEO coating
In the fields of pediatrics, cardiology, and orthopedics for the replacement of damaged areas, artificial metallic implants are used to support tissue growth which is specifically biodegradable as they degrade in the body after their job is done [57], [58], [59]. Moreover, biodegradable implants could interact with the living tissues by producing a biological response that is induced by local tissues, such as local cell proliferation, and systemic cellular migration to a local implantation site
Concluding remarks and future aspects
In this review, we have discussed the fabrication of additional polymeric materials on Mg and Mg alloys coated via PEO. Every polymer has been discussed comprehensively, starting from its initial implementation mechanism on the PEO surface to the final impact on the application site. Focusing on the molecular structure and bonding nature of polymeric components, each polymer has been categorized with its unique application in the human body or industrial media. In the case of biodegradable
Declaration of competing interest
The authors declare that no conflict of interests.
Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) funded by the Korean government (MSIT) (No. 2022R1A2C1006743).
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