Plasma Electrolytic Oxidation – Fundamentals, Advances and Applications

This chapter provides a concise historical overview of the main scientific and technological developments that shaped up modern plasma electrolytic oxidation (PEO) technology, which is used to deposit high-performance oxide ceramic coatings on the surfaces of light alloys. It further discusses current trends in the subject area including morphological refinement of PEO coatings on Al through application of pulsed bipolar polarisation waveforms, transition to the soft sparking PEO mode and ongoing work on digitalisation of PEO processes based on application of intelligent polarisation signals incorporating process diagnostics segments. Finally, it discusses recent advances in fundamental understanding of the electrochemical behaviour of PEO coatings under pulsed reversed polarisation conditions.

The plasma electrolytic oxidation (PEO) is an innovative method to endow surface multifunctionalities on Al, Mg, Ti, Zr, Nb, Hf, Ta, and their alloys. While its quality is determined by many factors such as power supply regime, electrical parameters, electrolyte and substrate materials, etc. Herein, from the perspective of PEO process parameters and coating structure optimization, the growth rule and optimization strategies of the PEO coating are revealed by changing the power supply regime, electrical parameters, electrolyte composition and substrate materials. We will first introduce the influence of the power supply output regime on the microstructure of coating, including output mode, electrical parameter and “soft” spark discharge. Then, we highlight the effect of various electrolyte compositions (such as special ions, particles, dopants, etc.) on coating microstructure. Finally, the effect of substrate composition including reaction thermodynamics, alloying element, and metallurgical state on coating microstructure is reviewed, as well as the challenges to improve the coating quality/versatility and future development prospects are summarized.

Sparking phenomena inherent to plasma electrolytic oxidation (PEO) are not fully understood yet and several sparking regimes can occur along a PEO process. In this chapter, sparking discharge regimes and the role of soft sparking are reviewed in the context of PEO. First, the important aspect of the electrolyte–material interaction is discussed based on the behaviour of charges and space charges in semiconductor–electrolyte contact from both electrolyte and semiconductor points of view. The oxide film formation, its stability, and breakdown phenomena are reviewed. The type and mode of electrical discharge used in PEO are introduced by distinguishing DC voltage and current, AC, unipolar-pulsed and bipolar-pulsed regimes. Recent strategies to optimize the quality of the layers and minimize energy consumption are discussed. General features related to the promising soft sparking mode are reviewed, and the influence of some main process parameters on the transition to soft regime is presented. The concept of active zone is briefly exposed to illustrate the mechanisms at the interfaces, which support the soft sparking regime. Finally, new strategies involving the soft sparking regime are discussed. These involve the use of pre-anodized samples or cold-spray treatments prior to PEO, or the incorporation of micro- or nano-particles in the PEO layers.

Through the development of plasma electrolytic oxidation (PEO) technology, many innovative approaches have been developed, which have enriched the PEO processes and have provided new capabilities for the PEO technique. This chapter introduces some selected new approaches, including the two-step/multi-step PEO, laser-assisted PEO, molten-salt PEO, plasma electrolytic fluoridation, ultrasound-assisted PEO, PEO in magnetic fields, cathode PEO, and scanning PEO processes, etc. These innovative PEO techniques are based on the traditional PEO theory but with different modifications of the technical routes. They have shown potential for improving property of the oxide coatings to a certain extent or have widened the application fields of the PEO technique.

This chapter delves into the impact of Plasma Electrolytic Oxidation (PEO) on the mechanical properties of various metallic systems, focusing on aluminum, magnesium, and titanium alloys. PEO is a surface treatment process that forms a hard, ceramic-like oxide layer on metal surfaces, significantly enhancing properties such as hardness, wear resistance, and corrosion resistance. The chapter begins with an introduction to the PEO process, highlighting its versatility and the ability to tailor coatings for specific applications by adjusting process parameters and electrolyte compositions. It then explores the effects of PEO on surface hardness and wear resistance, emphasizing the formation of dense oxide layers and the incorporation of nanoparticles to further enhance these properties. The discussion extends to the challenges posed by microcracks and pores within the oxide layer, which can compromise mechanical performance, and the various posttreatment processes developed to mitigate these issues. Additionally, the chapter addresses the influence of PEO on stress corrosion cracking and corrosion fatigue, underscoring the importance of understanding the interplay between mechanical stress and corrosive environments. By examining the mechanisms and factors that influence PEO coatings, this chapter provides valuable insights for optimizing the process to develop advanced materials with superior mechanical properties for a wide range of industrial applications.

This chapter describes the current understanding of the growth mechanism of plasma electrolytic oxidation (PEO) coatings on valve metals and non-valve metals (carbon steel, brass, and copper). An overview of the historical development of the PEO technique on valve and non-valve metals is first made. The second part discusses the growth mechanism of PEO on valve metals. Various influencing factors and the typical physical—chemical processes and characteristics of PEO coatings on different valve metals are introduced. PEO coatings on valve metals usually grow discretely under repeated discharges. However, soft sparking leads to uniform coating growth on Al. Two growth models are introduced. The third part introduces the PEO of non-valve metals, which is an area still in its infancy. Initially, PEO is thought to be unsuited for non-valve metals. However, recent studies show the successful PEO treatment on some non-valve metals. In these circumstances, stable plasma discharges are proved to be achieved through the deposition of an initial insulating layer of electrolyte species. The substrate participated less in the coating formation. It is still a long way to develop high-quality coatings on non-valve metals.

Plasma electrolytic oxidation (PEO) is an efficient technique for producing a functional ceramic coating on the surface of metal matrix composites (MMCs), which can improve the corrosion resistance, mechanical and thermal protective properties of the material. This chapter focuses on the growth mechanism of PEO on aluminum (Al), magnesium (Mg), and titanium (Ti)-based MMCs under different reinforcement, electrolyte conditions, and power supply modes, respectively. By analyzing the current density–time (j–t) curve and spark discharge characteristics, the correlation between spark discharge intensity and coating growth is revealed. The chapter reviews that the type of enhancers, the composition of electrolytes, and the negative voltage significantly affect the characteristics of spark discharge and the microstructure of the coating. Such as, soft spark discharge under high negative voltage can effectively reduce defects and improve the density of the coating. In addition, the presence of reinforcement plays an important role in the growth kinetics of the coating. Thus, understanding these key factors will provide important references for developing high-performance MMCs and optimizing PEO processes.

Cathodic plasma electrolytic oxidation (CPEO) has been employed to rapidly fabricate oxide coatings on the cathode to enhance their wear and corrosion resistance. Compared with conventional microarc oxidation (MAO) or anodic plasma electrolytic oxidation (PEO), CPEO significantly enhances the growth rate of oxide coating, making it suitable for surface modification on valve and non-valve metals. This chapter provides an overview of the basic principles of the CPEO technique, including the liquid-phase electrical discharge phenomenon, the dependence on voltage and current, the formation and breakdown of vapor-gaseous envelopes (VGE), and the near-surface temperature changes inside the cathode. Optical emission spectroscopic measurements and calculation results for plasma parameters are described, and the CPEO mechanism in VGE plasma discharge around the cathode environment is examined. Subsequently, the morphology, structure, and properties of CPEO coatings on the carbon steel, stainless steel, Mo and TiAl alloys, and the characteristics of the carbon powders suspended in the electrolytic solution are listed in this chapter. Finally, conclusions and an outlook on future development and application of CPEO technology are presented in the final section.

Plasma electrolytic oxidation (PEO) has emerged as a versatile technique for producing ceramic-like coatings on light metals, yet achieving tailored functionalities often requires in situ addition of particles. Functionalising PEO coatings in this manner can provide advanced properties such as enhanced corrosion resistance, improved wear performance, photocatalytic activity, thermal, and biological functionalities, amongst others. To date, a diverse array of particles, including oxides, nitrides, carbides, metallic powders, and more complex two- or three-dimensional nanostructures, have been explored for incorporation into PEO coatings. Approaches to particle incorporation vary widely. Adjusting particle concentration, size, and surface charge can influence their incorporation process. Likewise, controlling the electrical parameters (voltage, current density, frequency, and duty cycle) can promote efficient particle uptake while minimising degradation reactions. Through careful process optimisation, particles may be introduced non-reactively (i.e. inert), partially reactively, or fully reactively, depending on their size, melting point, chemical stability, and structural complexity. Multiple mechanisms have been proposed to explain the incorporation of particles into PEO coatings. Typically, particles migrate towards the coating surface, adsorb onto the growing oxide layers, and become physically entrapped within molten oxide pools. Additional theories point to short-circuit paths and discharge channels as alternate routes for deeper penetration. Overall, these findings pave the way for designing advanced PEO coatings with carefully tuned functionalities and performance characteristics.

The corrosion resistance of plasma electrolytic oxide (PEO) coatings is a function of its thickness, porosity and cracks, type of phases formed and their chemical stability. The presence of pores and cracks is very critical as permeation of the corrosive medium through them decreases the corrosion protective ability of the PEO coatings. Particle incorporation has been explored as one of the strategies to improve the corrosion resistance of PEO coatings. The particles preferentially incorporated at the pores and cracks of the PEO coating serves as a carrier thus preventing the ingress of the corrosive medium. Several other factors such as the type of particles and their concentration, conductivity of the particles, mode of incorporation, change in phase content, densification of the oxide coating following particle incorporation, dependence on the type of anions, process parameters, and surface modification of particles on the level of particle incorporation, could determine the corrosion resistance offered by the PEO composite coatings. This chapter elaborates on how these factors could influence the corrosion resistance of PEO composite coatings.

Plasma electrolytic oxidation (PEO) is a typical surface modification technology that forms ceramic-like films on the surfaces of magnesium, aluminum, and titanium alloys, mainly improving their corrosion resistance and wear resistance. However, the inherent porous structure of the PEO layer is not conducive to long-term corrosion protection. One approach to enhance the performance of the coating is to introduce particles into the electrolyte and achieve particle incorporation during the coating growth process, which can improve the functional properties of the coating. Biomedical implants are one of the application fields of metallic materials. Various biocompatibility problems of bare metals have restricted their further development. This chapter reviewed the research progress of particle-incorporated PEO coatings on metallic materials for biomedical applications. The development prospects of particle-incorporated biomedical-grade PEO composite coatings were prospected.

Plasma electrolytic oxidation (PEO) coatings have emerged as a promising method to enhance the corrosion resistance of light alloys, which are widely employed in the aerospace, automotive, and biomedical industries due to their favorable specific strength and biocompatibility. Recent innovations in PEO coatings emphasize the development of electrical parameters, novel electrolytes, incorporation of nano/micro-sized particles, and application of functional coating systems to improve their anticorrosion properties. This chapter aims to contribute to ongoing research and development efforts focused on optimizing PEO coatings upon Al and Mg alloys by reviewing the state-of-the-art, identifying key challenges, and outlining future directions.

Plasma electrolytic oxidation (PEO) is an advanced electrochemical and plasma-assisted technology employed for the development of multifunctional ceramic coatings on light metals, outstanding aluminum and magnesium alloys. Although these alloys are widely employed in diverse industries, like automotive, aerospace, or biomedical sectors, their low tribological resistance currently limits their use in more demanding applications. In this context, the development of PEO coatings provides an effective solution to these challenges by offering excellent adhesion to the substrate, high hardness, and enhanced wear resistance. Research in recent years has demonstrated the feasibility of PEO technology to improve the tribological properties of Al alloys, with outstanding results even on cast Al-Si alloys, whose composition is challenging. On the other hand, although PEO coating research on Mg alloys has been mainly focused on the improvement of the corrosion resistance, increasing studies are showing the potential of PEO coatings for the tribological improvement of Mg alloys, especially through the incorporation of nanoparticles into the layers. These advances have established PEO technology as a sustainable and versatile surface solution, broadening the application of Al and Mg alloys in high-performance environments.

Plasma electrolytic oxidation (PEO) has emerged as a promising surface modification technique for enhancing the corrosion and wear resistance of titanium and its alloys. Titanium, widely used in aerospace, biomedical, and industrial applications, possesses excellent corrosion resistance but suffers from poor tribological performance in demanding environments. PEO creates a hard, ceramic-like oxide layer with a multilayered microstructure that provides superior protection against mechanical wear and electrochemical degradation. This chapter explores recent advances in anticorrosion and anti-wear PEO coatings for titanium, emphasizing process optimization, electrolyte innovations, and posttreatment strategies. The influence of alloying elements, substrate pretreatment, electrical parameters, and advanced electrolyte formulations on coating microstructure and performance is discussed. Additionally, emerging hybrid and duplex coating approaches and self-lubricating additives are examined for their potential to further enhance durability. The synergy between wear and corrosion mechanisms, tribocorrosion behavior, and long-term performance assessments are analyzed to provide a comprehensive understanding of PEO-coated titanium in real-world applications. Finally, key research gaps and future directions, including AI-driven process optimization, smart coatings, and additive manufacturing integration, are highlighted to guide advancements in this field.

Steel is one of the most substantial alloys used in various industrial applications. This chapter discusses recent advancements in the plasma electrolytic oxidation (PEO) treatment of steel substrates. PEO coating preparation on steel surfaces can be categorized into three methods based on their growth mechanisms: (1) pre-coating the steel with an aluminum layer before PEO, resulting in a coating similar to those formed on aluminum alloys; (2) using specific concentrations of silicate and aluminate electrolytes to form coatings composed mainly of oxide compounds such as SiO₂ or Al₂O₃; and (3) employing cathodic PEO technology to grow protective coatings primarily consisting of compact iron oxides. These methods effectively improve the wear and corrosion resistance of steel by enhancing surface microhardness, coating compactness, and thickness.

Plasma electrolytic oxidation (PEO), also named as micro-arc oxidation (MAO), is popular in enhancing corrosion resistance of valve metals. Unfortunately, the inherent defects ascribed to the fierce reaction during preparation damage the long-term stability of the coating. Recently, researchers have drawn some methods to address this issue by fabricating multilayered coatings and introducing sealing agents based on PEO-coated alloys. However, single passive physical barrier effect seems insufficient to satisfy the prolonged corrosion resistance and stability of the coating systems. Consequently, organic and inorganic corrosion inhibitors are introduced to endow the coatings with an active protective (self-healing) effect. The corrosion inhibitors usually function by forming a precipitation film or chelating on bare substrate. Besides the corrosion inhibitors, layered double hydroxides (LDHs), intrinsic self-healable organic coatings, metal organic frameworks (MOFs), etc., are also applied on the basis of PEO coating to construct multilayer active corrosion protection coatings. This chapter provides an overview of recent research and summarizes characterization methods for active protection, attempting to find new research ideas of active protective coatings on the basis of PEO coatings.

High-temperature thermal protection coatings involve multiple dimensions, including high emissivity radiation heat dissipation protection, low thermal conductivity insulation protection, and multifunctional thermal protection characterized by high-temperature resistance/antioxidant/ablation resistance. Plasma electrolytic oxidation (PEO) coating can be grown in situ on lightweight metal surfaces and is characterized by low thermal conductivity and high binding strength, which is a potential way for preparing high-performance thermal protection coating. Until now, designing and preparing large-area, low-cost PEO coating with high emissivity, low thermal conductivity, and multiple thermal protection characteristics is urgently needed, and it remains a challenge to date. This chapter provides a comprehensive review of the principles of thermal protection and the modulation of infrared emissivity and thermal conductivity in titanium alloys and niobium alloys. The composition, microstructure, and functional properties within PEO coating are designed and modulated for high-performance thermal protection coating by controlling the composition and concentration of electrolytes, electrical parameters, and doping with nanoparticles. Emphasis is placed on the design and preparation of high emissivity PEO coating to break the working temperature limit and ensure the reliability of the hot-end components in service. Furthermore, the future research directions of PEO thermal protection coating while facing higher service temperatures are explored.

Thermal control coatings (TCCs) regulate the temperature of solid surface through spectrally selective absorption and find wide application in the field of aerospace, aviation, and high-power electronics. Plasma electrolytic oxidation (PEO) has emerged as a versatile technique for fabricating these coatings on lightweight metals, enabling precise temperature regulation by tailoring the solar absorption-emission ratio. This chapter provides a comprehensive review of the principles of thermal control and the modulation of solar absorptivity and infrared emissivity in aluminum, magnesium, and titanium alloys. Mechanisms underlying spectrally selective absorption and emission within PEO coatings are discussed, along with strategies for optimizing their composition, microstructure, and functional properties. Emphasis is placed on innovative approaches, including doping with rare-earth elements, multistep processing, and the integration of composite layers, which enhance thermal stability and durability under extreme operational conditions. Furthermore, emerging trends and future research directions are explored, showcasing the potential of PEO coatings to drive the development of next-generation lightweight materials for high-performance thermal control applications.

This chapter concisely explains the catalytic and battery materials applications of plasma electrolytic oxidized (PEO) metals and alloys. The adherent oxide layers developed by PEO on metal supports have attracted significant attention in catalysis research. The catalytic activity of the developed oxide layer can be boosted by various approaches, such as doping and heterojunction formation via in situ integration or post-impregnation of the active components. This chapter discusses PEO-derived electro-, photo-, and photoelectro-catalysts in different applications. PEO-derived anode electrode materials for lithium-ion batteries are also discussed.

This chapter summarizes the results of plasma electrolytic oxidation (PEO) of metals such as Zr, Al, Ti, Zn, Nb, Hf, Ta, Y, and Gd in electrolytes containing Ln₂O₃ (Ln = Eu, Sm, Pr, Tm, Dy, Er, Ho), CeO₂, and Tb₃O₄ particles to form oxide coatings that can be used as photoluminescent (PL) materials. ZrO₂:Eu³⁺, HfO₂:Eu³⁺, Y₂O₃:Eu³⁺, and Gd₂O₃:Eu³⁺ coatings formed by PEO of Zr, Hf, Y, and Gd, respectively, show very high PL intensity at ⁵D₀ → ⁷F₂ transition of Eu³⁺ under UV excitation, which makes these coatings suitable for red solid-state lighting. PEO-based coatings of Eu³⁺-doped ZrO₂, Nb₂O₅, Gd₂O₃, and TiO₂ are acceptable as sensors for luminescence thermometry (noncontact temperature measurement). PEO of Al in an electrolyte containing CeO₂, Eu₂O_3, or Sm₂O₃ is suitable for the formation of Al₂O₃ coatings doped with Ce³⁺, Eu²⁺, or Sm²⁺ ions and exhibiting strong PL emission bands with maxima at about 345 nm, 410 nm, and 688 nm, respectively. PEO coatings (ZrO₂, Ta₂O₅, etc.) doped with Er³⁺/Yb³⁺ and Ho³⁺/Yb³⁺ show up-conversion PL at 980 nm excitation.

Plasma electrolytic oxidation (PEO) with the ability of tunable surface properties (controlled elemental composition, morphology, porosity, and wettability) showed great promise to act as an antibacterial coating. These mechanically robust coatings with antibacterial and corrosion-resistant properties have diverse applications in medical implants, orthopedic instruments, or surgical tools as well as in oil/gas and marine industries. This chapter provides a comprehensive understanding of the influential factors of PEO coating to tune antibacterial properties, which include electrolyte composition, structural properties, and wettability. This chapter further discusses the applications of antibacterial PEO coatings in the field of biomedical implants, and surgical tools as well as in suitable marine structures. Despite the great success of PEO coatings in orthopedic implants, it demands more attention to realize their full potential in controlling marine biofouling.

There are a series of metallic materials such as magnesium, zinc, titanium, and their alloys, considered biomaterials due to their properties, which are very interesting for certain biomedical applications in traumatology, cardiology, dentistry, etc. However, these materials present a series of disadvantages that prevent their more widespread use in biomedicine. Due to the particularities of the coatings generated on metallic substrates through the technique of plasma electrolytic oxidation (PEO), such as the ease of controlling the composition and the addition of particles, their intrinsic porosity, etc., certain aspects can be improved or even new properties can be provided, such as improved biocompatibility, control of the degradation rate of biomaterials when implanted in a biological medium, or the possibility of endowing these materials with bactericidal and antimicrobial properties to reduce the probability of nosocomial infections. This chapter explores all these possibilities, showing the latest advances in the use of PEO coatings for biomedical applications.

Plasma electrolytic oxidation (PEO) coatings are known for their excellent wear and corrosion resistance, but they often develop pores and microcracks during the arc discharge process, which compromise their long-term durability. Therefore, effective sealing treatments are critical to enhancing their performance. This chapter discusses recent advancements in posttreatment sealing strategies for PEO coatings, focusing on three main approaches: physical methods to improve surface properties, chemical sealing techniques that modify the coating’s chemical structure, and hybrid methods that combine both physical and chemical processes. These posttreatment strategies aim to enhance the adhesion, appearance, and overall functionality of PEO coatings. The chapter highlights sealing techniques that address the inherent limitations of PEO coatings, ultimately improving their quality and longevity.