Unveiling the Effect of Particle Incorporation in PEO Coatings on the Corrosion and Wear Performance of Magnesium Implants

Abstract

Magnesium has been a focal point of significant exploration in the biomedical engineering domain for many years due to its exceptional attributes, encompassing impressive specific strength, low density, excellent damping abilities, biodegradability, and the sought-after quality of biocompatibility. The primary drawback associated with magnesium-based implants is their susceptibility to corrosion and wear in physiological environments, which represents a significant limitation. Research findings have established that plasma electrolytic oxidation (PEO) induces substantial modifications in the surface characteristics and corrosion behavior of magnesium and its alloy counterparts. By subjecting the surface to high voltages, a porous ceramic coating is formed, resulting in not only altered surface properties and corrosion resistance, but also enhanced wear resistance. However, a drawback of the PEO process is that excessive pore formation and porosity within the shell could potentially undermine the coating’s corrosion and wear resistances. Altering the electrolyte conditions by introducing micro- and nano-particles can serve as a valuable approach to decrease coating porosity and enhance their ultimate characteristics. This paper evaluates the particle adhesion, composition, corrosion, and wear performances of particle-incorporated coatings applied to magnesium alloys through the PEO method.

1. Introduction

In various engineering applications, magnesium and its alloys find extensive use, extending beyond the transportation industry where weight reduction is crucial [1]. They are also utilized in the production of biodegradable orthopedic implants [2]. The distinctive properties of magnesium and its alloys, including a suitable Young’s modulus, high specific strength, biocompatibility, natural degradation, and osteopromotion, position them as revolutionary biometals [3,4]. With a Young’s modulus close to that of bone, around 45 GPa, and a density of approximately 1.74 g/cm³, magnesium and its alloys offer promising applications in bone repair and implant technology [5,6,7].

In contrast, the density of alloys is determined by the elements included, similar to the density of human bones [8]. The good mechanical properties and low density of magnesium alloys make them an excellent choice for biodegradable implants [9]. With degradable implants, patients do not have to undergo a second surgery, which is cost-effective and beneficial for them [10]. The slow load transfer from the implant to the damaged area allows the damaged bone or tissue to heal faster [11]. A biodegradable implant must, however, be controlled so that degradation does not occur faster or slower than the time necessary for healing to ultimately happen [12,13]. As we have already discussed, magnesium-based implants offer all the advantages listed above, making them a suitable material for use as a biodegradable implant [14]. As a result of the efforts of researchers, magnesium alloys have been modified to have improved corrosion and mechanical properties, making them suitable for bone repair [15]. As part of the bone fixation system, there may be several components, including a bone plate, bone pin, bone screw, and so forth, each of which plays a vital role during the healing process [16,17]. Magnesium stents are medical devices made from magnesium alloys and they are used for various vascular applications, particularly in cardiology [18]. These stents are designed to be temporarily implanted in blood vessels to help support and maintain their structural integrity. Over time, magnesium stents gradually degrade and dissolve within the body, eliminating the need for a permanent implant [19]. Figure 1 illustrates a few of the most common magnesium biomedical devices currently used in surgery [20].

Despite magnesium’s excellent biocompatibility, a significant obstacle to its use in biodegradable implants in biomedical applications is its rapid in vivo corrosion [21]. The accumulation of hydrogen gas and the substantial increase in local pH may result in blocking of the bloodstream, necrosis of tissues, or even death due to the release of hydrogen gas [22]. To develop magnesium-based implants, it is crucial to control the rate of corrosion and degradation in the human body [23].

Despite their advantages, the comparatively lower wear resistance of magnesium alloys requires careful scrutiny to ensure the durability and efficacy of implants. Wear resistance is crucial for minimizing complications related to implant wear, such as particle generation and inflammation. This is especially vital in orthopedic applications, particularly in load-bearing joints, where the wear resistance of magnesium implants becomes essential to prevent premature failure and improve patient outcomes [24].

Applying surface coatings and alloying is thought to be very effective at preventing the corrosion and wear rate and for achieving a reasonable service life [25,26]. The low solubility of some alloying elements in the magnesium matrix, although a feasible option to control the degradation rate of magnesium, remains a problem. The surface treatment of magnesium implants has been subjected to various techniques [27,28,29]. Plasma electrolytic oxidation (PEO, also known as micro-arc oxidation (MAO)) coats lightweight metals and alloys with ceramic-like coatings [30,31]. PEO coatings have two layers in their morphology: an amorphous outer film exhibiting pores and coarse morphology and a crystalline inner film showing delicate pores [32,33]. Amorphous elements within the inner film may crystallize due to oxygen production. As a result, it is unlikely that PEO coatings can prevent the formation of high porosity, especially for magnesium alloys, and the behavior of layers is also constrained due to the limited effect of the electrical parameters on the coating composition [34]. Modifying the composition of the electrolyte is another method for optimizing the design and microstructure of coatings to improve their properties [35]. Recently, advances have been made in this field that aim to add particles to the solution, thereby achieving in situ incorporation or sealing porous PEO coatings and providing new features to the coatings [36]. The PEO processes are affected by the addition of particles to the solution. As a result, it may alter the morphology and properties of the layer due to changes in the solution, such as pH, conductivity, and viscosity [37,38]. It is noted as an unmoving incorporation when particles are combined without a new phase being formed or without any reaction taking place. In other words, the particle shapes and sizes have not changed significantly. Another possibility is reactivity or a slight reactivity of the incorporation [39,40]. Because of high-energy discharges in the PEO process and reactions with other matrix and electrolyte elements, this condition may result in the melting of particles [41]. Numerous parameters are involved in this complex process, including substrates, electrolyte compositions, particle concentrations, Zeta potentials, melting points, and energy supply from discharges [42]. This review paper provides an overview and critical analysis of the effects of dissimilar particles on the PEO procedure, composition of coatings, and corrosion and wear performances of magnesium-based implants.

2. Corrosion and Wear Behavior of Magnesium-Based Implants

Magnesium alloys face challenges of low corrosion resistance and rapid degradation, primarily due to physiological solutions containing chloride ions that induce corrosion. Although a magnesium hydroxide film forms as a protective barrier, it is insufficient, leading to substrate exposure and increased corrosion risk, jeopardizing the mechanical integrity of implants and potentially causing premature failure [43,44,45,46]. Rapid degradation can impede correct integration of magnesium implants with bone, as magnesium reacts with water in body fluids, releasing hydrogen gas and hydroxide anions [44,45]. Magnesium ions, released during oxidation, can migrate into cells and tissues, influencing cellular structures and promoting viability, growth, and recovery [46]. The corrosion process, generating hydrogen gas, hydroxide ions, free radicals, and altering pH, can impact immune cells, influencing inflammation and wound healing [47]. Figure 2 illustrates potential reactions on magnesium surfaces [48]. These corrosion-related challenges underscore the need for comprehensive examination to enhance the performance and longevity of magnesium implants in biomedical applications [48].

The wear resistance of magnesium implants directly correlates with their longevity and durability within the human body [49]. Implants subjected to repetitive mechanical stresses, as seen in joints during daily activities, must withstand wear to maintain their structural integrity over an extended period. Enhancing wear resistance ensures a prolonged service life for magnesium implants, reducing the need for early replacements and associated surgical interventions. Their susceptibility to abrasive wear, corrosion-induced wear, and frictional heat generation necessitate innovative solutions [50]. Limited hardness compared with traditional materials contributes to accelerated material loss. Surface deterioration over time and complex loading conditions in the body further accentuate wear concerns. Addressing these weaknesses through advanced materials science, including coatings and surface modifications, is crucial for optimizing the wear resistance of magnesium implants. Overcoming these challenges ensures the longevity, durability, and reliability required for successful orthopedic interventions, making magnesium alloys a viable choice in the dynamic biomechanical environment of the human body [51].

3. PEO Process

As a result of plasma chemical, electrochemical, and thermal chemical reactions, the production mechanism for oxide coatings using PEO is complex [52]. The anodization of a magnesium substrate during the PEO process involves a series of electrochemical reactions. Here are the primary electrochemical reactions that occur during the anodization of magnesium [53]:

At the anode (Oxidation):

Mg → Mg²⁺ + 2e⁻                           Oxidation of magnesium

(1)

This reaction involves the conversion of magnesium on the substrate’s surface into magnesium ions (Mg²⁺) by releasing two electrons. This fundamental reaction leads to the formation of the magnesium oxide (MgO) layer on the surface.

At the cathode (reduction):

O₂ + 4e⁻ + 2H₂O → 4OH⁻                     Reduction of oxygen (O₂)

(2)

Oxygen gas (O₂) can be reduced to form hydroxide ions (OH⁻) by gaining four electrons at the cathode. This reaction typically occurs in the surrounding electrolyte and contributes to the production of hydroxide ions.

2H₂O + 2e⁻ → H₂ + 2OH⁻               Formation of Hydrogen Gas (H₂)

(3)

In addition to the reduction in oxygen, another common cathodic reaction is the formation of hydrogen gas (H₂) through the reduction in water (H₂O) with the gain of two electrons. These reactions collectively drive the anodization process in PEO [54]. At the anode, magnesium undergoes oxidation to create magnesium ions (Mg²⁺), which then react with oxygen ions and hydroxide ions to form the primary component of the coating, magnesium oxide (MgO). At the cathode, the reduction in oxygen and the formation of hydrogen gas help to maintain the charge balance in the system [55,56].

It is important to note that the PEO process may involve more complex reactions and intermediates, such as the production of the oxide coating, the breakdown of the dielectric layer, the dissolution of preexisting layers, and gas anodic evolution [57,58]. It depends on the magnesium alloy composition, the nature of the solution, and the concentrations of the constituents used, along with the current density of the reaction, which determines the dominance of any of these reactions [59]. Concerning the breakdown voltage time, the voltage appears to increase rapidly and linearly. Tiny bubbles of oxygen can be observed on the surface of specimens, and a thin oxide layer indicates the traditional anodizing process [60]. An ignition discharge occurs when the applied voltage exceeds a critical value, which causes the dielectric to break down [61]. During this stage, the current is concentrated within zones of breakdown, resulting in locally thickened oxide coatings. Unlike other areas with less resistance, the new layer is less resistant to current flow, reducing its resistance to leakage [62]. There is an accidental spreading and rapid movement of several tiny white sparks over the entire anode surface in conjunction with a shrill sound [63,64].

Oxide film production is continuous and breaks down, causing fluctuations in the potential. It is possible to produce ceramic oxide coating directly by gasifying the electrolyte and the valve metal [65]. During the growth of the oxide layer, a weak point results in the coating breaking down. The increment of the processing time results in more extensive discharge sparks, changing colors from white to red or orange [66,67]. A micro-arc becomes a strong arc in this zone. When gas is released and intense sparks occur, the film experiences thermal cracking and the formation of large pores. As the voltage declines rapidly, the gas bubbles and sparks disappear, indicating that the PEO process is ending [68].

A two-layer structure has been observed for magnesium, and a thin barrier film characterizes its alloys applied directly to the substrate and a porous outer layer that can be used to apply paints, polymer coatings, or other adhesives to the surface [69]. Despite this, several limitations are associated with the PEO technique, including a limited range of compositions, high porosity, and high energy consumption. In general, the properties of PEO coatings are determined mainly by the design and microstructure, controlled by factors such as the method, solution, and substrate used to create them [70]. Optimizing the electrolyte composition is an effective strategy to enhance the properties of PEO coatings. Changing the design of the solution is an effective method to improve the microstructure and behavior of PEO oxide films. For most research studies, particle addition is just in the form of powders or sols added to solutions [71]. Sols are more malleable and present more substitutions than particles because of their substrate, for example, particle reinforcement within metal matrix composites. Additionally, in situ particle formation may occur during the PEO process if the solubility of a specific compound is exceeded. Particle-bearing solutions are thus considered with these methods. In this case, ensuring that the particles are dispersed evenly in the aqueous solution is challenging [72]. It seems that the predominant mechanism for incorporating fine particles into the coating entails their penetration into surface pores or indentations as a component of the PEO discharge cycle. This cycle occurs after the expansion and collapse of the plasma “bubble”, leading to the replenishment of electrolytes [73]. Figure 3 presents a schematic depiction of a single electrolyte cycle with fine suspended particles in relation to the surface area of the discharge pores [74].

4. The Effect of Incorporated Particles on PEO Process

The incorporation of solid particles into the electrolyte during the PEO process can exert distinct effects on various electrical parameters. These effects are contingent upon factors such as the particles’ type, concentration, and conductivity, as well as overall process conditions [75]. The presence of particles may necessitate voltage and current adjustments to achieve specific coating properties and ensure consistent process outcomes. Moreover, particle concentration can influence the overall electrolyte conductivity, which, in turn, affects the electrical current conduction in the system [76]. The spark discharge characteristics may be altered by particle presence, potentially affecting the energy distribution within the PEO process [77].

Consequently, energy efficiency and consumption may be influenced, necessitating adaptations to maintain optimal efficiency. The system’s electrical impedance and Zeta potential can also be impacted by particle incorporation, with implications for understanding the electrical behavior of the coated surface [78]. Additionally, particle presence can influence sparks’ formation, distribution, and characteristics, leading to variations in the electrical behavior across the coating’s surface. The system’s electrical contact and charge transfer may exhibit differences due to solid particles, affecting resistance and charge transfer processes. These effects highlight the complexity of electrical parameters in PEO and underscore the importance of careful parameter control to attain the desired coating quality and performance [79].

Particles are often added to solutions as powders or sols in many studies. Compared with particles diminishing over the substrate, they have greater flexibility and provides a broader range of options [80]. Additionally, the formation of in situ particles may occur during PEO if the solubility of some compounds exceeds the extent of solubility in the electrolyte. In addition, these methods may also be identified as electrolytes that contain particles. A significant challenge in preparing PEO is achieving uniform particle dispersion in the electrolyte. In solid–liquid interfaces, the zeta potential is an essential factor that explains the charging behavior. The magnitude of the zeta potential measures electrostatic repulsion between adjacent particles in an electrolyte. A particle with a high value of Zeta potential is more stable, which allows it to avoid adhering to other particles and settling in PEO’s electrolyte. Under a specific electrical field, having a high Zeta potential is advantageous as this enhances the movement rate of the particles [81,82].

Adding new ingredients to the electrolyte can affect the PEO process’s electrical response. When particles are added to the PEO treatment bath, it has been demonstrated that the voltage ramp can be slowed down [83]. In the presence of Al₂O₃ particles, researchers observed a delay in the voltage ramp. Others have claimed that PEO processing has a decreased voltage if CeO₂ particles are added to the electrolyte. It should be noted, however, that other researchers have reported conflicting results regarding the voltage response and final voltage. A recent study indicates that PEO coatings can grow more rapidly and reach higher final voltages by adding Al₂O₃ nanoparticles [84].

Moreover, PEO processing is affected by the particle size. A reduction in the size of nanoparticles during PEO treatment can enable a more rapid transition of the voltage/current characteristics than a reduction in the size of the microparticles [85]. According to the study, particle addition has no noticeable impact on the processing of PEO. When ZrO₂ particles were added to the PEO treatment, their effect on the growth rate of the coating and voltage response was minimal. As the pH and conductivity of the electrolyte were not significantly altered after particle addition, this discrepancy was likely caused by the differences in power supplies and electrical parameters utilized during the PEO process [86]. PEO will respond more positively to particles added to the electrolyte in the form of alcosol, as opposed to the addition of particles in the form of sand. As a result, the electrolyte conductivity is decreased due to the primary solvent in alcosol, namely ethanol. Based on the study’s results, it has been found that the breakdown potential, final voltage, and layer growth rate increased enormously with the concentration of sol in the electrolyte [87]. As a side note, it has also been found that such a sol can delay coating growth if added to the electrolyte. When alumina sol was added to the electrolyte, the breakdown and final voltage were significantly reduced [88].

In comparison with the addition of powders to the electrolyte, the effect of particle addition via the sol route appeared to have a more significant influence on the electrical response of the PEO process. PEO’s electrical response is influenced by the electrolyte’s composition, conductivity, and viscosity, which the organic additives affect [89]. The influence of PEO treatment varies greatly depending on the base electrolyte, substrate, particle properties (types and sizes), and power source. Therefore, an accurate picture is not available [90].

5. Incorporation of Particles into the Electrolytes and Mechanisms of Particle Absorption

Numerous recent studies have examined how particles are absorbed and incorporated into PEO coatings. Because the pores on the coating surface are filled with particles following PEO treatment, we considered them pathways for particles to enter the coating [91]. In addition to the concentration of nanoparticles inside the pores, researchers have confirmed that nanoparticles accumulate in the vicinity and inside the pores at a higher rate than in the other zones [92]. A short-circuit path between the outer and inner layers was thought to transfer the particles to the interface between the two layers. Despite the appearance of “soft” sparking, limited inward mass transfer of particles has been observed for three-layered coatings on magnesium, up to 50% of the intermediate layer (Figure 4) [93]. Because of the reduced voltage under the “soft” sparking regime, particles were less likely to be absorbed. There has been a proposal for three significant steps, which led to the incorporation of nanoparticles into coatings: delivering particles to coating growth sites, trapping particles at those sites, and preserving embedded particles during the layering procedure [94]. According to other studies, the incorporation of particles into the coatings on magnesium alloy was primarily caused by electrophoretic deposition and mechanical mixing. PEO coatings on magnesium were also found to incorporate particles through electrophoretic deposition [95]. It has also been suggested that particle transfer from the electrolyte to the layer can be accomplished in two steps, uptake and incorporation. Positive particle uptake before the breakdown potential may be considered a deposition/adsorption process in the presence of anodic dissolution and strong re-deposition of conversion products [96].

It has been observed that particle size plays a significant role in the uptake process. Nanoparticles penetrate deeper into coatings than microparticles, based on the results of other studies [97]. Large particles, on the other hand, cannot penetrate through pores on the surface of the layer. Based on all of the studies related to this subject, it has been determined that particles can be incorporated in the PEO procedure by reactivity, relative reactivity, or infertility based on the electrolyte composition, the employed electrical factors, substrate, melting point, size, and chemical stability of the particle [98]. A mixture that results in no reaction or new phase formation is considered an inert incorporation. Incorporation can also be caused by reactive or relatively reactive responses arising from high-energy discharges that melt particles in the matrix and cause them to react with other components within the electrolyte [99]. A significant role would also seem to be played by the melting point of the particles in the incorporation process. For instance, SiC (2730 °C), CeO₂ (2400 °C), Si₃N₄ (1900 °C), and other high melting point particles were mostly inertly incorporated without considering their sizes. This procedure involved reactively incorporating particles with a kind of lower melting point [100].

Moreover, the particle size influences the mode of particle incorporation, coating properties, and, thus, the PEO process. For instance, ZrO₂ nanoparticles with a size of 150 to 300 nanometers have shown that, despite their high melting points, they react with magnesium to produce Mg₂Zr₅O₁₂. Even though the size of particles has been studied to determine their effects on PEO coatings, there are still many questions about their effectiveness [101]. Specifically, how particles with different melting points and sizes absorb and incorporate remains to be determined. There are also some unknowns and ambiguities regarding the intrinsic mechanism of adding particles to PEO coatings. Some researchers have claimed that the addition of ZrO₂ nanoparticles to the PEO coating causes short-circuiting in the outer film to move the nanoparticles to the interface between the outer and inner films [102]. Alternatively, other researchers found that the electrophoretic mobility of the particles within the magnesium oxide and the mechanical combining of molten magnesium oxides affected the incorporation of particles [103].

Moreover, the applied electrical factors play an essential role in specifying the form of incorporation of particles as the discharge lifetime and intensity directly correlate with the current density and voltage at all stages of the PEO process [104]. The PEO procedure dictates how the particles are absorbed and incorporated into the film based on particle properties (melting point and particle size) and electrical factors applied within the procedure [105]. A recent study revealed that the duties ratio, frequency, current densities, and voltages affect the particles’ absorption within the PEO process. PEO coating can incorporate more particles at lower frequencies and higher duty ratios [106].

In the context of PEO of magnesium, introducing solid particles into the treated surface gives rise to intricate chemical and electrochemical reactions. These reactions occur at multiple interfaces involving the solid particles, the electrolyte, and the magnesium substrate [107]. In the electrolyte, water oxidation yields oxygen gas and hydrogen ions, while water reduction at cathodic sites generates hydroxide ions. When solid particles are immersed in the electrolyte, their dissolution can lead to the release of components into the surrounding solution. Concurrently, chemical reactions occur at the particle–electrolyte interface, forming hydroxides, oxides, or other compounds. The interaction between the solid particles and the magnesium substrate can entail species diffusion, like metal cations, from the substrate to the particle–substrate interface. This can lead to intermetallic compounds’ development or surface chemistry’s transformation [108].

Moreover, electrochemical reactions manifest at the magnesium substrate, with oxidation events leading to the formation of magnesium oxide. Reduction reactions at cathodic sites can involve hydrogen gas generation and oxygen reduction to hydroxide ions [109]. The amalgamation of these reactions contributes to the formation of the PEO coating and influences its chemical composition and microstructure. Furthermore, the responses can significantly impact the coated magnesium surface’s corrosion resistance and other fundamental properties. It is essential to recognize that the precise reactions may vary depending on various factors, such as the type of solid particles employed, the composition of the electrolyte, and the specific process conditions [110].

6. The Effect of Particle Incorporation on the Composition, Microstructure, and Morphology of PEO Coatings

As a result of the particles being introduced into the PEO process, it is clear that the microstructure and morphology of the coated surfaces loaded with particles differ from the particle-free surfaces [85]. The particles are part of the coating formation process. The application of PEO coatings on magnesium and its alloys has been modified using various oxide particles. Small oxide particles with low melting points and small sizes can generally be reactively incorporated much more readily than large oxide particles with high melting points [89]. The amorphous phase is present in the coatings due to the melting of small particles, which causes them to react with other ingredients and become amorphous. As a result, the coating composition obtained from a solution containing micro-sized particles did not differ substantially from that obtained from a solution without micro-sized particles, except for the presence of particles that were inertly incorporated into the composition [102]. PEO coatings are characterized by their microstructure, morphology, and composition, which are determined, in part, by the design of the PEO electrolyte [98].

Plasma electrolytic oxidation (PEO) coatings undergo significant changes in pore characteristics, compactness, and thickness with the introduction of oxide particles, influencing their microstructure and morphology. Incorporating oxide particles into electrolytes reduces the number and size of pores on the coating surface, exemplified by the notably reduced surface porosity observed with the addition of TiO₂ [83]. The efficacy of the sealing process is enhanced when a sol is introduced into the electrolyte, and oxide particles generally do not significantly alter the coating surface or increase porosity [76]. Coatings formed in particle-free electrolytes exhibit a more uniform and compact outer layer, while the impact of particle size on coating thickness lacks a clear trend [80]. Conflicting results exist regarding the effect of oxide particles on coating thickness, with some studies reporting no enhancement or even thinning, while others suggest increased thickness [105]. Changes in voltage/current evolution during PEO processing are associated with variations in the thickness of particle-containing coatings. ZrO₂ nanoparticles, introduced through electrophoretic interaction and mechanical mixing, are found to primarily occupy pores within the magnesium oxide layer, as depicted in Figure 5 [111].

Generally, non-oxide particles, such as metallic, organic, and inorganic particles, are inertly incorporated into PEO layers. There are also some cases in which part-reactive integration can be observed for calcium phosphate- and carbide-based particles when the discharge temperature and lifetime are sufficient for the particles to melt or decompose [112]. Different substrates, base electrolytes, and electrical parameters were used during the PEO process, which can explain the discrepancy. The incorporation of inert particles commonly reduces the porosity of PEO coatings. As a result of the incorporation of PTFE particles into the coating surface, the pores on the coating surface became finer and more homogeneous [113]. It has been reported that particle-containing coatings have a denser cross section as through-going pores and defects are virtually impossible to detect in these coatings. Thicker coatings were formed when SiC nanoparticles containing electrolytes were exposed to constant currents [114]. It depends on the current density applied to the coating to determine how thick it will become. Low current density results in a more significant value when compared with high current density. In contrast, the coatings obtained from particle-free electrolytes under a constant voltage mode were thinner than those produced in PTFE and Si₃N₄-containing electrolytes. The difference in incorporation modes and electrical parameters is likely to be the reason [115].

Regarding cross-sectional morphology, all coatings could be divided into three distinct zones: pore bands, outer films, and inner compact barrier films. PEO layers grow in opposite directions, with growth inwards toward the substrate and outwards toward the interface of the coating and solution, according to the reported growth mechanism. A constant ejection of molten material from the coating surface and its subsequent solidification can be attributed to the inward growth of the coating in thickness [98]. A band of pores was created because the growth dynamics in both directions were rarely identical, resulting in an unfilled zone within the coating. There was a significant difference in thickness between the outer film and the other two parts of the coating, with the outermost film containing more than half of the total coating thickness. Despite this, many observable defects were visible even when magnified relatively lowly [102]. Figure 6 depicts the (backscattered electron images) BSE cross-sections of the CeO₂-incorported coatings on the AM50 magnesium alloy, accompanied by X-ray elemental mapping of Si and Ce [116]. While the oxygen content remained consistent across all coatings, the presence of silicon diminished slightly in the coatings formed using CeO₂-containing electrolytes. As anticipated, cerium was only evident in coatings incorporating CeO₂, and its concentration increased with higher particle concentrations in the electrolyte. EDS point analyses conducted in areas with significant particle agglomeration revealed noteworthy shifts in the content of Ce and Si elements, with the Ce concentration rising to 10 atomic percent and the Si concentration decreasing to 7 atomic percent. The localized variations in coating composition were closely linked to the distinct characteristics of microdischarges and the preferred placement of particles within the coating pores [116].

In the PEO of magnesium, an array of solid particles could be thoughtfully integrated into the electrolyte to yield distinctive enhancements. These particles, harnessed for specific purposes, encompassed diverse functionalities. Ceramic particles, exemplified by hydroxyapatite (HA) or tricalcium phosphate (TCP), are routinely introduced to foster bioactivity and augment osseointegration, emulating the mineral composition of bone tissue and facilitating the integration of bone onto the implant surface [117,118,119,120]. Metallic particles, like Ag or Cu, confer antibacterial properties to the coating. The inclusion of silver particles is renowned for its potent antimicrobial capabilities, effectively safeguarding against bacterial infections, especially in medical implants. Biocompatible nanoparticles, such as TiO₂, find their niche in the realm of PEO for magnesium, elevating the biocompatibility and corrosion resistance of the implant surface [121,122].

Furthermore, alloying elements, such as calcium or strontium, are strategically integrated, bolstering the coating’s mechanical properties and corrosion resistance. Within the purview of PEO, there is also a burgeoning interest in infusing nanoparticles with anti-inflammatory agents or therapeutic drugs, creating coatings that can release these substances to mitigate inflammation and expedite the healing process at the implant site [123,124,125].

Developing hydrophilic PEO coatings on magnesium in biomedical implant applications relies on strategically incorporating specific particles. Examples of particles that enhance the hydrophilicity of PEO coatings include TiO₂ nanoparticles, renowned for their inherent hydrophilic properties, as well as SiO₂ and ZrO₂ nanoparticles, both known for their hydrophilic nature. The introduction of hydrophilic polymers like polyethylene glycol (PEG) can also be employed to increase the coating’s affinity for water. Moreover, calcium phosphate nanoparticles, including HA, play a pivotal role in biomedical coatings, enhancing hydrophilicity and biocompatibility. These hydrophilic particles offer tailored solutions for optimizing the wettability of PEO coatings, ensuring their suitability for biomedical implants where interactions with physiological fluids and tissues necessitate hydrophilic surface properties [126].

Pioneering strides have also been made in tissue regeneration, with nanoparticles bearing growth factors and other bioactive molecules that serve as catalysts for tissue rejuvenation and heighten the implant’s overall performance [127,128,129,130]. The selection of particles is inherently dictated by the PEO process’s unique objectives and the magnesium implant’s intended application. These particles operate in tandem with precise modifications to the coating’s composition, structure, and properties, strategically tailored to fulfill the desired functional requirements. These can encompass vital aspects such as biocompatibility, corrosion resistance, antibacterial efficacy, or bioactivity, all converging to foster enhanced osseointegration and bolster the implant’s clinical utility.

On the other hand, the inner film consists of just a couple of microns in thickness. However, some imperfections may be visible in the film and at the boundaries of the coating and substrate [85]. Hence, it is apparent that the formed coatings adhere firmly to the substrate and are corrosion resistant mainly due to the relatively compact inner layer of protection. In the last few years, it has been hypothesized that adding nanoparticles to the PEO reaction alters the evolution of voltage and current to change the coating thickness [69]. Based on the characteristics of the nanoparticles added, the thickness of the coating can either be enhanced or reduced. Oxide particles can alter or improve a layer’s compactness and thickness. Comparing the coatings formed in particle-free electrolytes with those included in particle-rich electrolytes, the outer layer of the coatings became less compact and uniform. The influence of particles on coating thickness does not appear to be directly correlated [84]. Adding oxide particles to coatings did not enhance their thickness, as coating thickness typically remained the same or decreased. The layers, however, were reported to be a bit thicker when particles were present. During PEO processing, the voltage/current evolution is altered, which changes the thickness of the particle-containing coatings [131].

7. Corrosion Performance of PEO Coatings with Particle Incorporation

The corrosion of magnesium implants is governed by a series of electrochemical reactions at the implant’s surface. At the anode, magnesium undergoes oxidation, releasing magnesium ions (Mg²⁺) and liberating electrons (e⁻), as represented by the following equation [132]:

Mg → Mg²⁺ + 2e⁻

(4)

Simultaneously, at the cathode, where oxygen (O₂) and water (H₂O) are present, reduction reactions occur, leading to the formation of hydroxide ions (OH⁻). This reaction can be expressed as follows:

O₂ + 2H₂O + 4e⁻ → 4OH⁻

(5)

The overall corrosion of magnesium implants can be represented as the combination of these anodic and cathodic reactions, with the formation of magnesium hydroxide (Mg(OH)₂) as a common corrosion product. This corrosion process not only compromises the structural integrity of the implant, but also impacts its biocompatibility. Consequently, researchers are dedicated to developing protective coatings and corrosion-resistant alloys to mitigate the corrosion of magnesium implants and to enhance their long-term performance within the human body, particularly in the context of biodegradable magnesium-based medical implants [132].

As mentioned before, in a ceramic-like PEO structure, micropores and microcracks are inevitable for two reasons [133]. Aside from this, typically, electrical discharges, intense heat, and gas bubbles are formed on the surface of the PEO solution, leading to the formation of many microchannels, some of which do not solidify during the PEO process. It is also important to note that the Pilling–Bedworth (P-B) for magnesium oxide is considerably lower than that for metal, resulting in a loose layer of magnesium oxide forming on the metal’s surface [134]. As a consequence of the quick solidification of molten oxide inside the solution within the cooling process, micro-cracks appear in the structure of oxide coating. Electromagnetic microdischarges produce pores that allow corrosive ions to penetrate the coating, failing the protective layer [135]. The corrosion performance of a PEO coating in biological aggressive media and for long periods depends on several parameters, such as the composition, thickness, porosity, and defects within the coating. When pores and cracks are present in the oxide film, corrosion ions will be able to penetrate in irregular directions, resulting in a decrease in the protective properties of PEO coatings [136,137].

The corrosion behavior of PEO coatings on magnesium and its alloys has been improved using different particles, but controversy has also been raised regarding the results [138]. The increase in corrosion properties can be attributed primarily to the formation of new stable phases (reactive incorporation of particles) and incorporating inert particles into a magnesium oxide coating with a high chemical stability [139]. It has been investigated how the PEO coatings behaved under corrosion and how they performed in terms of corrosion resistance on magnesium and its alloys. Different tests, including open circuit potential (OCP), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) have been conducted. As PDP has been used in most studies, the results of this study will focus on PDP tests. The corrosion potentials (E_corr) and corrosion current densities (i_corr) for PEO-coated samples with and without particles were extracted based on the PDP curves. The extracted data are listed in

Table 1. The comparison of the PDP results of particle-incorporated PEO coatings on magnesium alloys.

Substrate	Incorporated Particle Type	Corrosive Media	Without Particles		With Particles		Ref.
			Ecorr (mV)	icorr (µA.cm−2)	Ecorr (mV)	icorr (µA.cm−2)
Pure Mg	La2O3	SBF	−1770	33.6	−1590	0.68	[92]
AZ91	ZnO	SBF	−1742	6.17	−1386	0.063	[93]
AZ80	CNT	3.5 wt% NaCl	−1380	2.5	−1250	0.56	[103]
AZ91	ZrO2	3.5 wt% NaCl	−1400	0.727	−1300	0.07	[140]
AZ91	CeO2	3.5 wt% NaCl	-	-	−360	0.478	[141]
AZ31	CeO2	3.5 wt% NaCl	−1540	8.6	−1450	0.04	[142]
AZ31	MoS2	3.5 wt% NaCl	−100	2.96	−1300	0.83	[143]
AZ31	SiC	3.5 wt% NaCl	−1485	1.84	−1470	0.13	[144]
AZ31	WC	3.5 wt% NaCl	−1460	31.85	−1451	10.23	[145]
AZ31	GO	3.5 wt% NaCl	−1490	0.124	−1440	0.033	[146]
AZ31	HA	SBF	−1610	4.77	−1540	0.123	[147]
MA8	TiN	3 wt% NaCl	−1370	0.12	−1440	0.14	[148]
AM50	SiO2	0.5 wt% NaCl	−1449	1.2	−1556	0.19	[149]
AM50	Clay	0.5 wt% NaCl	−1477	63	−1542	58	[150]
Mg-Li	Graphene	3.5 wt% NaCl	−1600	1.2	−1512	0.106	[151]
AZ31	HA/GO	SBF	−1598	122.1	−1472	36.43	[152]
ZK60	CeO2/ZrO2-HA	SBF	-	-	−1289	43.84	[153]
AZ31	ZnO	Hank	−1390	0.65	−1461	0.08	[154]
AZ31	Al2O3	3.5 wt% NaCl	−1561	18.89	−1509	0.65	[155]
Mg-Li	TiO2	3.5 wt% NaCl	−1529	4.409	−1.495	1.725	[156]
AZ91	Graphite	0.5 wt% NaCl	−1700	2.0	−1720	0.6	[157]
AZ80	PTFE	0.5 wt% NaCl	−1520	0.878	−1510	0.161	[158]
AZ31	WO3	3.5 wt% NaCl	−1820	0.077	−1670	0.021	[159]

E_corr: corrosion potential; i_corr: corrosion current density; SBF: stimulated body fluid; GO: graphene oxide; HA: hydroxyapatite; CNT: carbon nanotube; PTFE: polytetrafluoroethylene.

.

Corrosion occurrence thermodynamic tendency generally decreases as E_corr increases. i_corr indicates the corrosion rate of samples, and the corrosion resistance is primarily determined by this [138]. As the i_corr increases, corrosion resistance decreases [139]. Compared with uncoated substrates, PEO-coated samples showed a lower i_corr in PDP curves and data in Table 1. Accordingly, the PEO coatings offered higher corrosion resistance than the substrates that were not coated. It can also be seen that corrosion resistance was improved by adding particles to the composition of electrolytes. Upon deposition, the coating inhibited the transfer and penetration of ions from the surface to the substrate by protecting them from electrochemical reactions. A layer of this coating also prevented the substrate from being demolished by electrochemical reactions. Adding particles to the coating enhanced the disorder in the electrochemical reactions by filling the PEO coating defects like porosities/cracks and inhibiting ion transfer from aggressive biological solutions [160].

Ma et al. showed that the addition of TiO₂ nanoparticles (2, 4, and 6 g/L) to PEO coatings on Mg-Li magnesium substrates had a significant effect on their properties [156]. The addition of TiO₂ nanoparticles in the solution at concentrations lower than 4 g/L produced more uniform and thicker coatings with smaller micropores than those that would have been formed in the solution without nanoparticles. It is important to note, however, that TiO₂ nanoparticles can generate thermal solid effects that can decrease the corrosion resistance of coatings as they form cracks, as was highlighted by the addition of 6 g/L of TiO₂ nanoparticles. As a result of Rehman et al.’s investigation, small particles of CeO₂ were observed to affect both the final microstructure of the coating within the solution and the corrosion behavior of the layer [141]. The coatings were formulated with CeO₂ particles that were positioned preferentially in the pores and cracks of the layers. As a result of this process, both the reactive and rising absorbance of CeO₂ were affected, as well as the CeO₂ under local melting, made possible by microdischarges. During the coating process, lower concentrations of CeO₂ lead to an incomplete blocking of pores and cracks. However, an increase in particle concentrations up to 10 g/L resulted in a more complex structure, with more unsealed pores and cracks. This study of the short-term corrosion performance of PEO-coated samples was conducted through an analysis of their microstructures; therefore, PEO-CeO₂-3 g/L displayed the highest overall resistance, and PEO-CeO₂-10 g/L showed the lowest resistance in comparison with PEO coatings without the addition of CeO₂. An experiment conducted by Seyfoori et al. investigated the influence of adding hydroxyapatite (HA) nanoparticles to the AZ31 magnesium alloy (5, 10, and 15 g/L) on the material’s properties [147]. As the uncoated sample was produced while exposed to the atmosphere, a porous oxide film formed on it. According to Nyquist’s diagram, the uncoated sample underwent inductive treatment, as the oxide film of the corrosion-resistant solution traversed the surfaces of the metals and attained the substrate when immersed in the corrosive electrolyte. Because of their low corrosion resistance, this produced an induction behavior. Based on the Nyquist diagram, the coatings formed at various HA concentrations exhibited a similar behavior [147]. The fact that there were two capacitive loops in the coating and an inductive treatment implies that three procedures were present [90]. An inductive procedure revealed the corrosion procedure at low frequencies, with the loop produced at high frequencies connected to the outer porous film. In addition, the loop at high frequencies exhibited the inner dense film at medium frequencies. Inductive analysis revealed that the specimens were subjected to pitting corrosion. It has been demonstrated that an increase in HA NPs from 5 to 15 g/L increased the Nyquist loop diameter, which indicates an increase in the corrosion performance of the coating. The same behavior was reported by Wen et al. with the addition of GO along with HA in the PEO electrolyte. The EIS study results are shown in Figure 7 [152]. The presence of three loops indicated the involvement of three distinct kinetic processes in corrosion. In the case of both the HA/GO coating and the PEO coating, the high-frequency loops (1–10 kHz) were associated with the outer porous region, while the medium-frequency loops (0.1–1 Hz) were attributed to the effects of the inner compact region formed during the PEO process. Within the frequency range of 0.001 to 0.1 Hz, there was a noticeable inductive response indicating the occurrence of pit corrosion in the samples [152].

It was proposed by Lu that particles with different melting points have different effects on the properties of PEO coatings. An alkaline-phosphate-based electrolyte was used to form PEO coatings on AM50, with and without including SiO₂, and clay [149,150]. Several studies have demonstrated that the reactive incorporation of clay particles results in the formation of a dense film, which in turn enhances the corrosion resistance of coatings. To determine the corrosion resistance of PEO coatings over the long term, electrochemical impedance spectroscopy (EIS) measurements were taken as the polarization test only provided limited information. A kinetic analysis of degradation was carried out using EIS to determine how the degradation process unfolded and how corrosion occurred. The Bode plots showed clear indications of a third relaxation procedure at high frequencies when examined in more detail. The response of the outer PEO film at high frequencies can be attributed to this time constant, while the response from the inner barrier film can be attributed to the medium frequency time constant. Furthermore, low-frequency relaxation can be associated with electrochemical activity at the interface between the metal and solution (capacitance of the double layer and resistance to charge transfer). According to Zhao et al., GO nanoparticles added to liquid (0, 1, 2, and 3 g/L, labeled as E0, E1, E2, and E3, respectively) harmed the corrosion properties [146]. The PDP study results are shown in Figure 8a. According to the study findings, the incorporation of GO into PEO coatings led to enhanced corrosion resistance. Coatings produced through the PEO process by adding 2 g/L GO exhibited a noteworthy uniformity, carbon content, and corrosion resistance enhancements. EIS results also revealed that compared with the 0 g/L specimen, the other three samples with GO within their PEO coating showed significantly larger capacitive loops, with the largest circle in the 2 g/L sample. A significant improvement in corrosion resistance was observed upon applying the PEO process, as evidenced by the pronounced increase in impedance. According to electrochemical studies on the coatings, GO incorporation in the coating structures enhanced the corrosion performance of the PEO coatings. The PEO coating generated the highest uniformity, corrosion resistance, and carbon content due to the presence of 2 g/L GO during the PEO procedure. A concentration of GO in the solution resulted in an increased number of micropores on the coating surface and a decrease in the uniformity of the coating structures, thereby decreasing corrosion resistance. However, when the GO concentrations in the electrolyte exceeded 2 g/L, several factors such as an increased electrode surface area, heightened van der Waals forces, and stacking of GO tended to promote aggregation within the electrolyte, leading to coatings that did not exhibit the expected properties. Figure 8b illustrates that the PEO coating on sample E3 exhibited an expanded morphology characterized by a greater amplitude [146].

During all of the electrochemical tests, it was found that the dense inner layers and inner layers of thick coatings offered better corrosion resistance than the porous outer layers. Adsorbent particles on the coating’s surface encapsulated the external porous layers by filling cracks and micropores in the first step [140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160]. During the next step, particles moved to the interface of the internal/superficial layers through some of the pores and discharge channels of the external layer, and filled and sealed the internal layer defects and cracks. In this way, the particles decreased the porosity of the entire PEO coating and raised the resistance of the coating against penetration by corrosive media. Nano-particle-incorporated coatings versus corrosive ions is considered in terms of corrosion protection mechanisms. As several microcracks and micropores are present in coatings without nanoparticles, corrosive ions can penetrate these micropores and cause the substrate metal to rust. The nano-particles, however, have the additional capability of filling microcracks and microporosities in coatings during the PEO procedure, which can diminish the coatings’ poor points concerning corrosion so that the diffusion of corrosive agents to the substrate metal is prevented. PEO coatings on magnesium and its alloys have been modified regarding the corrosion performance using distinct particles, but controversy has resulted from their application. In Figure 9, schematic representations of PEO coatings containing GO illustrate their ability to enhance corrosion resistance by effectively hindering the penetration of corrosive electrolytes into the magnesium surface [146].

Finally, it can be mentioned that incorporating solid particles into PEO coatings on magnesium substantially enhances corrosion protection through diverse mechanisms. These particles instigate a multifaceted transformation in the coating’s properties, shielding the underlying magnesium substrate from corrosive attack. Firstly, solid particles tend to increase coating density by infiltrating micropores and sealing voids, thereby minimizing the pathways for corrosive ions to penetrate the coating [161]. This results in a more uniform, impermeable structure that effectively guards against corrosion. Furthermore, particle incorporation exerts a profound influence on the microstructure of the PEO coating, leading to the refinement of grain boundaries and reduction in the size of defects, including cracks and pores. This finer microstructure not only strengthens the mechanical integrity of the coating, but also impedes the ingress of corrosive species, bolstering the protective qualities of the coating (Figure 10) [93].

Additionally, specific particles, particularly those rich in elements like aluminum or silicon, bring about chemical passivation of the magnesium substrate. This passivation process forms a protective barrier that actively inhibits the corrosion of the underlying magnesium, delivering long-term safeguarding against degradation [162].

Moreover, certain particles can react with the electrolyte and magnesium substrate, forming protective compounds, such as magnesium calcium phosphate. These compounds act as an additional barrier against corrosion, effectively blocking the intrusion of corrosive elements. Solid particles can also generate micro galvanic cells within the coating, leading to localized pH and potential changes, further inhibiting corrosion processes. Moreover, some particles function as pH buffers, stabilizing the local environment and thus offering protection against pH-induced corrosion [123]. The Zeta potential of particles plays a pivotal role in their incorporation into the coating, ensuring their adhesion and overall integrity, thereby minimizing the vulnerability of the layer to corrosion. Increased hardness, another outcome of particle addition, reinforces the PEO coating, making it more resistant to mechanical damage that could otherwise compromise corrosion protection. Finally, particle incorporation can influence the coating’s surface energy and wettability, affecting how liquids interact with the surface. In specific cases, a hydrophobic surface is engineered, limiting the presence of moisture and thereby hindering corrosion processes [163]. It is paramount to recognize that the effectiveness and mechanisms vary depending on the particles’ type, size, concentration, and chemical composition. Therefore, the selection of particles should align with the intended application and environmental conditions to optimize corrosion resistance. Electrochemically, these protective mechanisms can be linked to several reactions [132]. Passivation reactions involving the formation of oxide or compound layers on the magnesium surface play a central role, acting as barriers to corrosion. Redox reactions in the presence of certain particles can lead to the creation of protective compounds or modify the electrochemical behavior of the coating. Oxygen reduction reactions at the coating’s surface may be affected by particle incorporation, influencing the corrosion rate. Solid particles can create micro galvanic cells, altering the local electrochemical behavior and inhibiting corrosion processes. Furthermore, certain particles may affect the local pH of the environment, thereby impacting corrosion rates through electrochemical methods. A comprehensive understanding of these mechanisms and their electrochemical underpinnings is indispensable for optimizing the design of PEO coatings, ensuring superior corrosion protection in diverse applications.

8. Wear Performance of PEO Coatings with Particle Incorporation

While magnesium alloys exhibit notable advantages as potential materials for orthopedic implants, it is essential to acknowledge and address their weaknesses, particularly in terms of wear resistance. Extending on this aspect provides a comprehensive understanding of the challenges associated with magnesium implants. Magnesium alloys are prone to abrasive wear, especially in scenarios involving contact with hard and abrasive surfaces [164]. The inherent softness of magnesium can result in accelerated material loss, leading to concerns about the long-term durability of implants. This susceptibility to abrasive wear emphasizes the need for strategies to mitigate surface damage and enhance wear resistance. Magnesium alloys are susceptible to corrosion in physiological environments. Corrosion products can act as abrasives, contributing to wear and material degradation. The combination of wear and corrosion, known as corrosive wear, poses a substantial challenge for magnesium implants. It necessitates the development of corrosion-resistant coatings or alloy modifications to mitigate the impact of corrosive wear on implant performance [165]. During wear processes, frictional forces generate heat, which can be a concern for magnesium implants. Elevated temperatures may contribute to thermal degradation and compromise the mechanical integrity of the implant. Managing frictional heat is crucial to prevent undesirable consequences such as surface cracking, increased wear rates, and potential inflammatory responses in the surrounding tissues. Magnesium alloys typically have lower hardness compared with traditional implant materials like titanium and stainless steel [166]. This characteristic can lead to higher rates of material removal during wear, particularly in scenarios where implants experience contact with harder surfaces. Improving the hardness of magnesium alloys, either through alloying or surface treatments, is a key consideration for enhancing wear resistance. The surface properties of magnesium implants may deteriorate over time due to wear, potentially impacting their biocompatibility and mechanical stability. Surface modifications, coatings, and treatments must be designed to withstand the dynamic and challenging conditions within the human body, ensuring that the wear resistance of magnesium implants is maintained over the course of their intended service life [167]. Magnesium implants, especially those used in load-bearing joints, are subjected to complex loading conditions in the human body. Variability in loading patterns can contribute to uneven wear distribution, emphasizing the need for comprehensive testing and analysis to understand and address potential weak points in implant design, material selection, and wear-resistant strategies [168].

PEO coatings have gained prominence as a transformative technology for enhancing the wear resistance of magnesium implants. PEO coatings significantly augment the wear resistance of magnesium implants [169]. As mentioned earlier, by subjecting the magnesium surface to controlled plasma discharges, a ceramic oxide layer is formed, imparting superior hardness and wear resistance. Moreover, PEO-coated magnesium implants demonstrate remarkable resilience against abrasive wear [170]. The hardened ceramic layer serves as a robust barrier, minimizing direct contact between the magnesium substrate and abrasive surfaces. This property effectively mitigates material loss and extends the longevity of the implant. Importantly, the microstructure of PEO coatings contributes to effective temperature control during wear processes [171]. This is vital for preventing thermal degradation, maintaining the mechanical integrity of magnesium implants, and ensuring reliable performance under dynamic loading conditions. Furthermore, PEO enables tailoring of the surface characteristics, including the coating thickness, composition, and porosity. This versatility allows for customization based on specific wear requirements, optimizing PEO-coated magnesium implants for diverse orthopedic applications [172].

Incorporating particles into PEO electrolytes leads to composite coatings with enhanced wear resistance for magnesium implants. The introduction of particles, such as ceramics or nanoparticles, fortifies the coating matrix, providing additional hardness and reducing susceptibility to abrasive wear. Furthermore, particle addition allows for the customization of coating composition, enabling the creation of hybrid structures with unique properties. This tailoring process enhances the adaptability of PEO coatings to specific wear requirements, optimizing their performance for diverse orthopedic applications [173]. The incorporation of particles into PEO coatings influences the frictional characteristics during wear. Depending on the nature of the added particles, the coefficient of friction may be modified, contributing to improved tribological properties and better wear performance of the magnesium implants in dynamic conditions. It has also been reported that achieving an optimized dispersion of particles within the PEO coating is crucial for uniform properties across the surface. Proper dispersion ensures that the enhanced wear resistance and other desired characteristics are consistently distributed, promoting the reliability of the composite coating [174]. The choice of particles for addition to PEO electrolytes introduces versatility. Different particle types, sizes, and compositions can be selected to impart specific properties, such as increased hardness, improved lubricity, or tailored biocompatibility, contributing to a multifaceted approach to optimizing wear behavior [175].

Table 2. The data of the wear test results for PEO coatings with and without particles on magnesium alloys.

Substrate	Incorporated Particle Type	Wear Rate of PEO Coatings without Particles	Wear Rate of PEO Coatings with Particles	Reduction of Wear Rate	Ref.
AZ31B	Al2O3	0.45 × 10−6 (g·Nm−1)	0.15 × 10−6 (g·Nm−1)	66.67%	[176]
AZ31		3.09 × 10−4 (mg·Nm−1)	1.55 × 10−4 (mg·Nm−1)	49.84%	[155]
AZ31		4.375 × 10−5 (mm3·Nm−1)	2.5 × 10−5 (mm3·Nm−1)	42.86%	[177]
AZ31B		8.3 × 10−3 (mg·Nm−1)	1.0 × 10−3 (mg·Nm−1)	87.95%	[178]
MA8	ZrO2	4.1 × 10−5 (mm3·Nm−1)	2.9 × 10−5 (mm3·Nm−1)	29.27%	[179]
AZ31		4.1 × 10−5 (mm3·Nm−1)	2.9 × 10−5 (mm3·Nm−1)	29.27%	[180]
MA8	ZrO2/SiO2	4.3 × 10−5 (mm3·Nm−1)	3.2 × 10−5 (mm3·Nm−1)	25.58%	[181]
AM50	CeO2	1.90 × 10−4 (mm3·Nm−1)	6.15 × 10−5 (mm3·Nm−1)	67.63%	[182]
AM50	SiO2	3.7 × 10−3 (mm3·Nm−1)	7.3 × 10−4 (mm3·Nm−1)	80.27%	[149]
MA8		4.1 × 10−5 (mm3·Nm−1)	3.5 × 10−5 (mm3·Nm−1)	14.63%	[179]
AZ31	Ta2O5	1.1 (µm3·Nm−1)	0.1 (µm3·Nm−1)	90.91%	[183]
AZ31	SiC	3.8 × 10−4 (mm3·Nm−1)	3.3 × 10−4 (mm3·Nm−1)	13.16%	[184]
AZ31		14.57 × 10−4 (mg·Nm−1)	8.68 × 10−4 (mg·Nm−1)	40.43%	[144]
AZ31B		24 × 10−4 (mg·mN−1)	13 × 10−4 (mg·mN−1)	45.83%	[185]
AZ31B		5 × 10−3 (mg·m−1)	4 × 10−4 (mg·m−1)	92%	[186]
AZ91D		2.67 × 10−2 (mg·min−1)	1.33 × 10−2 (mg·min−1)	50.19%	[187]
AZ31		22.47 × 10−4 (mg·Nm−1)	9.49 × 10−4 (mg·Nm−1)	57.77%	[188]
AZ31B	WC	28.65 × 10−4 (mg·Nm−1)	6.67 × 10−4 (mg·Nm−1)	76.72%	[189]
AZ31B		15.20 × 103 (mg·m−1)	5 × 103 (mg·m−1)	67.11%	[190]
AZ31B		24.58 × 10−4 (mg·Nm−1)	11.86 × 10−4 (mg·Nm−1)	51.75%	[191]
MA8	TiN	1.1 × 10−5 (mm3·Nm−1)	5.0 × 10−6 (mm3·Nm−1)	54.55%	[192]
MA8		4.3 × 10−5 (mm3·Nm−1)	1.9 × 10−6 (mm3·Nm−1)	95.5%	[193]
MA8		4.3 × 10−5 (mm3·Nm−1)	1.9 × 10−5 (mm3·Nm−1)	55.81%	[194]
Mg–Li	WS2	9.27 × 10−5 (mm3·Nm−1)	4.37 × 10−5 (mm3·Nm−1)	52.86%	[195]
AZ31	MoS2	0.91 × 10−6 (mm3·Nm−1)	4.76 × 10−4 (mm3·Nm−1)	99.8%	[195]
Mg–Li	Graphite	9.27 × 10−5 (mm3·Nm−1)	4.80 × 10−5 (mm3·Nm−1)	48.22%	[194]
AZ31	Graphene	16 × 103 (mg·m−1)	4 × 103 (mg·m−1)	75%	[196]
AZ31	GO	5.62 × 10−4 (mm3·Nm−1)	1.34 × 10−4 (mm3·Nm−1)	76.16%	[197]

summarizes the wear test results data for PEO coatings on magnesium alloys, comparing cases with and without incorporated particles.

Oxide particles contribute significantly to wear resistance. These particles introduce heightened hardness, acting as formidable barriers against abrasive wear. Additionally, their incorporation fosters the development of a dense, stable oxide layer, bolstering the coating’s ability to withstand wear-induced stresses. Furthermore, oxide particles can impart desirable biocompatible characteristics to the coating, aligning with the stringent requirements of orthopedic applications [174]. Al₂O₃, recognized for its exceptional hardness and corrosion resistance, plays a pivotal role in reinforcing PEO coatings for magnesium implants. Moreover, the integration of Al₂O₃ particles significantly boosts the coating’s hardness, providing a robust defense against abrasive wear. This heightened hardness not only minimizes material loss during mechanical stresses, but also contributes to corrosion protection by forming a stable oxide layer [176,177,178]. Similarly, TiO₂, known for its biocompatibility and stabilizing effects, is a key component in tailoring PEO coatings for magnesium implants. When TiO₂ particles are incorporated into the coating, they enhance the biocompatibility of the surface, fostering a harmonious interaction with biological tissues. In addition, TiO₂ contributes to the formation of a stable oxide layer on the magnesium surface, bolstering wear resistance by acting as a protective barrier against abrasive wear. This dual functionality positions TiO₂ as a valuable choice for optimizing the longevity and performance of magnesium implants [198]. Likewise, ZrO₂, characterized by its high hardness and toughness, adds a robust dimension to PEO coatings. The inclusion of ZrO₂ particles enhances the coating’s resistance to abrasive wear, providing a formidable defense against mechanical stresses. ZrO₂-reinforced coatings have shown promise in improving the durability of magnesium implants, especially in load-bearing applications [179,180,181]. Similarly, CeO₂, with its unique combination of high hardness and self-healing properties, contributes to the wear resistance of PEO coatings. The self-healing nature of CeO₂ can mitigate the surface damage caused by abrasive wear, potentially extending the service life of magnesium implants [182,199]. This makes CeO₂ a compelling candidate for enhancing the longevity of orthopedic devices. In investigating the wear mechanism of CeO₂-incorporated PEO coatings on AM50 magnesium alloy substrates, the worn surfaces of steel ball counterparts were examined via SEM, as shown in Figure 11a–f [182]. The analysis revealed flattened surfaces for all balls against PEO coatings, confirming abrasive wear. Deep scoring indicated debris entrapment between the damaged coating and the ball. The steel ball hardness was approximately 900 HV. Testing against PEO-CeO₂ at a 10 N load showed a different wear mechanism compared with PEO alone (Figure 11g). Extensive plowing and debris detachment occurred, with fewer and shallower scores on balls sliding against PEO-CeO₂, suggesting that abrasive wear prevailed over the adhesive mechanism. Coating wear behavior against AISI 52100 steel balls was assessed, and COF versus distance was recorded for 2 N, 5 N, and 10 N loads (Figure 11g,h). Both coatings exhibited similar COF curve shapes at 2 N and 5 N, with COF stabilizing after initial fluctuations. At 10 N, distinct differences emerged. For the CeO₂-free coating, significant COF fluctuations occurred after 20 minutes, followed by a drop to ≈0.35, indicating coating failure. In contrast, PEO-CeO₂ reduced dynamic COF fluctuations, possibly due to debris filling pores, increasing the contact area, and contributing to elevated COF at higher loads. Additionally, YSZ, a stabilized form of ZrO₂ with yttrium oxide as the stabilizing agent, offers exceptional hardness and resistance to thermal and mechanical stresses. Incorporating YSZ particles into PEO coatings imparts improved wear resistance and thermal stability [200]. This is particularly advantageous in applications where implants may experience varying temperatures and mechanical loads. Furthermore, SiO₂ nanoparticles, characterized by their high hardness and biocompatibility, present an intriguing option for PEO coatings on magnesium implants. The addition of SiO₂ contributes to wear resistance while maintaining favorable interactions with biological tissues. This dual functionality positions silica as a versatile candidate for enhancing both the mechanical and biocompatible aspects of magnesium implant coatings [149,179]. Lastly, the incorporation of rare earth oxides, such as La₂O₃, introduces unique properties to PEO coatings. These oxides, known for their high hardness and corrosion resistance, enhance the coating’s ability to withstand wear in aggressive environments. The addition of rare earth oxides presents opportunities for tailoring coatings to specific biomedical applications with stringent wear and corrosion requirements [92].

The inclusion of carbide particles elevates the PEO coating to unprecedented levels of hardness and wear resistance. Furthermore, the inherent toughness of carbide-reinforced coatings provides a robust defense against abrasive wear, making them particularly suited for applications where implants encounter challenging mechanical conditions. Moreover, carbide particles contribute to the formation of a wear-resistant surface, ensuring a sustained performance under cyclic loading and abrasive environments [201]. SiC, renowned for its exceptional hardness and abrasion resistance, is a stalwart choice for reinforcing PEO coatings. In addition, the inclusion of SiC particles significantly elevates the coating’s ability to withstand abrasive wear, providing a formidable defense against mechanical stresses. Additionally, SiC-reinforced coatings have demonstrated promise in improving the longevity and durability of magnesium implants, especially in load-bearing scenarios where wear resistance is paramount [184,185,186,187,188]. WC, celebrated for its hardness and toughness, adds a robust dimension to PEO coatings on magnesium implants. Moreover, the incorporation of WC particles enhances the coating’s resistance to abrasive wear, offering formidable protection against mechanical stresses. Similarly, WC-reinforced coatings exhibit substantial potential in augmenting the wear performance and structural integrity of magnesium implants, particularly in applications where the implants are subjected to dynamic loads and challenging environmental conditions [189,190,191]. TiC, known for its high hardness and wear resistance, is a valuable addition to PEO coatings. Additionally, the incorporation of TiC particles enhances the coating’s ability to withstand abrasive wear, providing a robust defense against mechanical stresses [202]. Furthermore, TiC-reinforced coatings showcase promise in improving the wear resistance and overall performance of magnesium implants, particularly in applications where hardness and durability are critical considerations [203]. Cr₃C₂, recognized for its high hardness and corrosion resistance, contributes significantly to wear resistance in PEO coatings. Similarly, the inclusion of Cr₃C₂ particles enhances the coating’s ability to resist abrasive wear, providing a robust defense against mechanical stresses. In addition, Cr₃C₂-reinforced coatings demonstrate promise in enhancing the longevity and wear performance of magnesium implants, especially in environments where corrosion resistance is as crucial as wear resistance [204].

Incorporating nitrided particles into PEO coatings for magnesium implants offers a unique avenue for enhancing wear resistance and corrosion protection [192]. Nitrided particles, typically compounds containing nitrogen, introduce distinct mechanical and electrochemical properties to the coatings, contributing to their overall performance in challenging biomedical environments [193]. TiN is a notable example of a nitrided particle that has shown promise in reinforcing PEO coatings. Known for its high hardness and excellent wear resistance, TiN particles enhance the coating’s ability to withstand abrasive wear and mechanical stresses. The incorporation of TiN contributes to the overall durability of magnesium implants, particularly in load-bearing applications where wear resistance is crucial [192,193,194]. Si₃N₄ is recognized for its exceptional toughness, high-temperature stability, and corrosion resistance. The incorporation of Si₃N₄ particles into PEO coatings can contribute to enhanced mechanical properties, wear resistance, and corrosion protection. Si₃N₄-reinforced coatings may offer a comprehensive solution to the challenges associated with the dynamic biomechanical environment of magnesium implants [205].

The incorporation of MoS₂ and WS₂ particles into PEO coatings for magnesium implants introduces a fascinating dimension to enhance wear resistance and lubrication properties. These transition metal dichalcogenides exhibit unique structural and tribological characteristics that can significantly impact the performance of PEO-coated magnesium surfaces. MoS₂ is renowned for its lamellar structure and inherent lubricating properties. When integrated into PEO coatings, MoS₂ particles provide a solid lubricating layer, reducing friction between moving parts. This lubricity contributes to improved wear resistance by minimizing material loss during sliding or cyclic motion. MoS₂-reinforced coatings present an intriguing avenue for applications where low friction and enhanced wear resistance are paramount. Similar to MoS₂, WS₂ possesses a layered structure with excellent lubricating characteristics. When incorporated into PEO coatings, WS₂ particles contribute to reduced friction and enhanced wear resistance. The lamellar arrangement of WS₂ particles forms a boundary-lubricating film, offering protection against abrasive wear and reducing the risk of surface damage. WS₂-reinforced coatings hold promise in scenarios where sustained lubrication is critical for the long-term functionality of magnesium implants [195].

Graphite, renowned for its superior lubricating properties, is a pivotal element in magnesium-implant PEO coatings. The introduction of graphite particles revolutionizes these coatings’ tribological dynamics, enhancing wear resistance in orthopedic applications [206]. Graphite’s hexagonal lattice structure enables the formation of a self-renewing lubricating film in PEO coatings, reducing friction and preventing direct metal-to-metal contact. This film acts as a protective shield during mechanical interactions, significantly reducing wear and surface damage. In load-bearing applications, such as joint implants, graphite-infused PEO coatings ensure prolonged durability and functionality [194]. The unique lubrication mechanism of graphite makes it particularly appealing for orthopedic scenarios where minimizing friction is crucial for preventing wear-related issues and complications. The combination of graphite with other reinforcing particles may further enhance the mechanical strength, corrosion resistance, and tailored tribological properties, offering a comprehensive solution for optimizing magnesium-implant performance [194,206]. Graphene particles also play a crucial role in enhancing wear resistance in PEO coatings by showcasing exceptional mechanical strength and unique lubricating properties. Their incorporation forms a reinforcing network, providing an additional defense against wear challenges [196]. Furthermore, the two-dimensional structure of graphene contributes to coating flexibility, minimizing the risk of cracking. This outstanding mechanical strength of graphene fortifies PEO coatings, reducing material loss and improving the durability of magnesium implants, especially in abrasive scenarios [207]. Moreover, graphene not only reduces friction, but also lowers wear rates during motion, benefiting orthopedic implants by preventing accelerated wear and potential tissue complications. The thermal conductivity of graphene efficiently dissipates frictional heat, ensuring implant stability in the dynamic biomechanical environment of the human body [207,208]. Its flexibility allows it to conform to the substrate, thereby reducing the risk of coating cracking or delamination. Additionally, graphene’s biocompatibility supports favorable interactions with biological tissues, which is crucial for the integration and acceptance of magnesium implants within the human body [208]. Moving on to the experimental results, Figure 12a illustrates the friction coefficient evolution during sliding for test specimens [209]. Initially (Stage I), all specimens experienced a rapid increase, followed by stabilization (Stage II). Notably, the Mg-Li alloy showed lower friction due to a smooth surface, but prolonged sliding resulted in higher coefficients (≈0.52). In contrast, coatings with graphene oxide (GO) exhibited lower coefficients (≈0.28, ≈0.12) attributed to fewer micro-pores, higher hardness, compactness, and elemental C presence, enhancing the tribological performance. The relationship between wear track depths/volumes and their 3D topographies was apparent in the as-prepared coatings, showing a significant decrease in both wear width and wear depth compared with the substrate [209]. The GO-containing coating, in particular, demonstrated the most substantial reduction, as illustrated in Figure 12b. Moving on to Figure 12c, which illustrates the abrasion process of the PEO coatings in a schematic manner, it is evident that in the initial stage (Stage I), the dominant model was the “steel-on-protrusions”, emphasizing abrasive wear. During this stage, friction coefficients increased rapidly as the real contact areas expanded, leading to the transfer of wear debris from the stainless-steel ball onto the coating [209]. As sliding progressed (Stage II), asperities wore out, debris entered the micro-pores, and stable friction coefficients were observed. At elevated temperatures, severe adhesive wear became prominent. The performance in terms of friction and wear was influenced by the presence of lubricating materials and hard particles for antifriction, as well as the coating’s hardness, thickness, and roughness for wear resistance. The coating containing GO exhibited lower roughness, higher hardness, and greater compactness, contributing to improved tribological behavior. Despite the presence of elemental carbon on the wear trace, the fine debris in the micro-pores played a role in influencing wear performance [209].

9. Conclusions and Future Directions

As a comprehensive review of the existing literature in this field, the following key conclusions can be drawn:

• Magnesium alloys are highly promising biomaterials in biomedical engineering due to their osteoconductive and antibacterial properties.
• However, their accelerated corrosion in body fluids compromises their mechanical integrity and can impede healing.
• Surface coating and alloying techniques have effectively mitigated magnesium’s corrosion rate, with the plasma electrolytic oxidation (PEO) process proving successful.
• PEO coatings have limitations, including microcracks and pores, that restrict their corrosion resistance.
• Incorporating particles into PEO coatings on magnesium implants effectively enhances the surface morphology, microstructure, and electrochemical properties.
• Modifying electrolyte conditions based on particle presence can reduce porosities and improve coating properties, although achieving uniform particle dispersion can be challenging.
• Zeta potential analysis indicates that most nanoparticles in PEO procedures have a negative charge in common alkaline electrolytes, encouraging their incorporation into the coating.
• Incorporating particles and sealing existing pores modify the coating’s surface structure, resulting in increased corrosion resistance.
• The likelihood of particle incorporation is determined by using high temperature and discharge pressure and by considering particle characteristics.
• PEO coatings on magnesium and its alloys can benefit significantly from adding nano and microparticles to electrolytes, altering the phase composition, microstructure, thickness, and corrosion properties.
• Introducing particles reduces the porosity in PEO coatings, thus improving their microstructure by filling and sealing the micropores.
• Enhanced microstructure prevents destructive ion penetration from the coating into the substrate, contributing to improved corrosion resistance.
• The addition of particles to electrolytes not only influences the thickness and roughness, but also enhances the hydrophobicity of the coating.

There are numerous promising avenues for advancing research and exploration in enhancing magnesium alloy performance through innovative coating techniques. While prior studies have delved into incorporating various particles into PEO coatings on magnesium and its alloys, there has been a noticeable scarcity of research about their corrosion-resistance properties. We anticipate that future investigations will provide heightened attention to this crucial aspect. Further exploration is imperative to gain insight into the diverse effects of different particles on PEO coatings for magnesium and its alloys, considering the impact of particle size and intrinsic properties, particularly concerning corrosion resistance. Additionally, numerous oxide particles, such as strontium oxide, manganese oxide, calcium oxide, potassium oxide, tantalum oxide, and copper oxide, which exhibit a high bioactivity and desired osteoconductivity, have yet to be thoroughly examined within the context of PEO coatings for magnesium implants. Future research in this field should also explore the long-term corrosion behavior of particle-incorporated PEO coatings on magnesium alloys in simulated body fluids, addressing a critical aspect for assessing their suitability for extended implant applications.

In addition to the significant strides that will be gained in understanding the corrosion behavior of PEO coatings on magnesium implants with particle incorporation, it is crucial to address the implications for wear resistance. While much attention has rightfully been devoted to mitigating corrosion challenges, the interplay between particle-enhanced PEO coatings and wear characteristics holds immense importance in the realm of implant performance. Future investigations should delve into the wear behavior of these coatings, exploring how the introduced particles influence friction, material loss, and mechanical stability over extended usage. Moreover, striking a balance between enhanced wear resistance and maintaining other crucial coating properties poses an ongoing challenge. Understanding the synergistic effects of particle addition on both corrosion and wear resistance will be pivotal for advancing the overall durability and functionality of magnesium implants in orthopedic applications.