# Black PEO Coatings on Titanium and Titanium Alloys Produced at Low Current Densities
## 1 Introduction  
Titanium and its alloys are often employed in many fields, including biomedical, chemical, marine, aeronautics, astronautics, and automotive, due to their low weight, high specific strength, low elastic modulus, biocompatibility, low density, and corrosion resistance [1; 2; 3; 4; 5; 6]. A wide range of titanium-based compounds are also used in decorative coatings. Their primary purpose is to enhance the visual appeal of consumer products [7]. In aeronautics and astronautics, titanium improves equipment stealth and extinction properties [5]. Ceramic coating stands out as a prominent surface coating method, and its importance continues to increase alongside current technological advancements [8]. Ensuring temperature balance in a spacecraft is essential. An effective method to do that is using thermal control coatings (ceramics). These coatings have significant implications for the spacecraft's successful operation in the extreme temperature conditions of space. They are applied to the surfaces of structural materials and are intended to have strong absorption and emission properties, frequently appearing as black coatings [9]. Black coatings are highly desirable in spacecraft internal components, notably electronic housing packages used for thermal regulation [10; 11; 12; 13; 14]. Furthermore, they are widely used in various surface treatments, serving functional and aesthetic purposes. They are commonly applied for decorative purposes on items such as optical devices, heating elements, instrument panels, and sports equipment and are even found in numerous industrial sectors such as 3C (Computers, Communications, and Consumer Electronics) [15] and adiabatic engines [16]. It isworth noting that while we refer to these coatings as \"black\", no material can absorb 100% of light at all angles and wavelengths. Even objects that appear black reflect some light, making ideal black material impossible to obtain [17]. Plasma electrolytic oxidation (PEO), sometimes referred to as micro-arc oxidation [18], has evolved from traditional anodic oxidation methods [19; 20]. It is a surface modification technique that can be applied to various materials, including aluminum, magnesium, titanium, and other valve metal alloys [14; 18; 19; 20; 21]. In contrast to traditional anodizing, PEO offers an environmentally friendly approach [5; 22; 23; 24] that typically employs fewer toxic electrolytes, a method with lower preparation costs, and simplified processing steps [25; 26]. Furthermore, PEO provides distinct advantages, such as the ability to produce thicker coatings while maintaining excellent corrosion resistance, as well as favorable tribological and thermal properties [27; 24; 28]. The increased voltage applied during PEO generates the dielectric breakdown of oxide films, resulting in the formation of plasma discharges on the surface of the treated sample [11]. Coatings are formed due to metal substrate oxidation, electrolysis, electrophoresis processes, and plasma thermochemical reactions. Different compounds, mainly oxides formed by electrolyte components, can be found in the obtained coatings [29]. The microstructure of the PEO coating is usually multi-layer and porous structures [30]. Several parameters can influence the microstructure of PEO coatings, such as the electric regime, the electrolyte composition, the process's length, and the substrate's chemical composition [10; 27; 31]. Among these factors, the electrolyte stands out as the most significant variable influencing coating properties, particularly the colors of the ceramic layers [18; 32]. Furthermore, PEO coatings' multi-layered and porous structure can increase the specific surface area. This improvement helps incorporate desired elements into coatings and develop diverse functional coatings [30; 31; 32; 33].