Black Electrochemical Coatings for Aerospace and Allied Applications – Part 7 – Black Plasma Electrolytic Oxidation (PEO) Coatings

The PEO process is a rather inexpensive and environmentally friendly technique [19, 20]. The produced coatings possess very good wear and corrosion resistance as well as high thermal stability [21–23]. PEO coatings are employed in various engineering applications where very high corrosion and wear resistance, improved tribological and heat radiation characteristics are required such as pistons, cylinders, and hydraulic gear and variety of automobile and aerospace parts [24–26].

The mechanism of the PEO process can be divided into three stages, all the three stages occur simultaneously. The first stage involves oxide formation at metal-oxide interface. The second stage involves chemical dissolution of the oxide at oxide-electrolyte interface, and the third stage involves dielectric breakdown of the oxide layer at a high voltage. The dielectric breakdown produces millions of short-lived micro-discharges uniformly spread on the surface of the job creating a discharge channel for direct ejection of molten metal which is immediately oxidized, hydrolysed and precipitated on the workpiece [27–30].

At the discharge site chemical, electrochemical, thermo- dynamical, and plasma-chemical reactions occur to modify the structure, composition and morphology of PEO coatings [31]. As a result, mechanical properties such as wear resistance and toughness of the coating are enhanced [32–38]. PEO coating consists of three layers: a porous top layer, a dense intermediate layer with low porosity, and an inner transition layer. The dense inner layer acts as a good barrier layer for corrosion resistance and determines the thermo-mechanical and tribological properties of PEO coatings [5, 11, 29, 39–41]. The coating properties can vary strongly according to the alloy composition and process conditions [41]. The coating growth rate and porosity is dependent on electrolyte components, temperature, and characteristics of applied current.

The coating formation penetrates further into the free metal surface. Subsequent arcs are more likely to occur in other areas, where the oxide film is relatively thin. The discharges therefore have a natural tendency to preferentially thicken the thinnest regions of the film, resulting in fairly uniform film thickness, provided the electric field is fairly uniform. The porosity level in PEO coatings is generally higher. The pores observed are attributed to the entrapment of spherical gas bubbles [42, 43]. The macro- porosity on magnesium-based coatings is of the order of 40 %, however, the bulk of the film, in contrast, is quite dense.

The PEO process is carried out at room temperature in a very dilute and ecologically safe electrolyte. A typical electrolyte includes dilute solution of sodium or potassium hydroxides, silicates, phosphates or aluminates or mixture of these with several additives either to impart specific functional properties in the coating or to improve the coating growth process [44–57]. The PEO coating consists of oxide, silicate, fluoride, aluminate and phosphate, etc., based on the composition of electrolyte. The technological development, microstructural characteristics and applications of PEO process are reviewed elsewhere [3, 20, 24, 39, 41, 58–63].

The surface pre-treatment of PEO is much simpler when compared to anodization. Jobs can be processed for PEO coating just after careful solvent degreasing. The part to be coated is immersed in the electrolyte and is electrically connected in the electrochemical cell, so as to become one of the electrodes. The other counter-electrode is typically made from an inert material such as stainless steel, and often consists of the wall of the bath itself. The process can employ the continuous direct current or pulsed direct current or alternating pulses (pulsed unipolar and pulsed bi- polar). The PEO coating can be grown at the rate of about 1 µm per minute. Over 100 µm coating thickness can be easily grown on most of the substrates. After PEO coating parts are subjected to pores sealing in a demineralized water operating at > 95 °C (boiling) for 20–30 minutes.

Like aluminium anodizing, PEO coating at 15–45 μm thickness can behave as an effective radiator imparting good heat radiation properties. PEO coatings on AA6061 alloy were obtained with different concentrations of sodium silicate using positive uni-polar pulsed DC [64]. The process was optimized to obtain a solar reflector coating for spacecraft thermal control applications [65]. About ~40 μm thick PEO coating was found to provide maximum solar selectivity, where a solar absorptance of 0.37 and a thermal emittance of 0.81 was reported. The coatings were characterized using SEM, EDX, XRD, XPS, and nano profilometry and were subjected to simulated space environment testing.

The solar reflector, low absorbance and high emissivity, PEO coatings on Ti-6Al-4V alloy with silicate electrolytes were prepared by Zhongping Yao [66]. The concentration of silicate and the applied current density influence the thickness and the roughness of the coatings, and consequently the thermal control properties. The optimum results were exhibited with 10 g/L Na₂SiO₃ and 1 g/L NaH₂PO₂·H₂O at a current density of 10 A/dm², frequency: 500 Hz; time: 30 minutes; where a coating thickness of 80–100 µm resulted in a hemispherical emittance of 0.92 and a solar absorptance of 0.39 at 318 K. The coating is mainly composed of O, Si, Ti, P and Na. The PEO coating was porous with some big particles stacking around like large craters. It was not well crystallized and consisted of a large number of amorphous silicates.

The formation of black PEO coating on aluminium alloys using Keronite type process for space applications has been reported by Shrestha et al. [67–69]. A film thickness of 50–70 µm had an average solar absorptance value of 0.82 for AA7075 and 0.89 for AA2219, while the thermal emittance was 0.72. The coatings were subjected to space simulation (UV-exposure, thermal cycling, thermal shock) tests in vacuum and salt spray exposure. In accordance with ASTM D1654-92. The Keronite PEO samples displayed superior protection in salt spray environment (rating 10) than the hard-anodic coatings (rating 9). Wu et al. [70] have produced black PEO thermal control coating in a 0.1 mol/L sodium aluminate electrolyte. Experimental results showed that the coating can reach a solar absorptance value of 0.90 and an infrared emittance value >0.77. The optical properties and phase composition of the coating across the surface to the substrate-coating interface were studied by a spectrophotometer and X-ray diffraction.

The selection of the electrolyte in particular the incorporating of oxides of Fe, Co, Ni, W, Zr, Mn, Cr and V plays an important role in realization of various colours in the PEO coatings [71–84]. The coatings were gray-black and porous when Fe, Co, Ni and W from the electrolyte were incorporated into the coating.

Addition of metavanadate and tungstate in PEO electrolyte was found very encouraging in the formation of black PEO coatings [71–82]. Bayati et al. [72], have reported that the introduction of WO3 compounds into the PEO oxide coating would be effective to achieve the dark black colour WO₃-Al₂O₃ composite films on the aluminium alloys. Surface morphology and topography of the layers were investigated. It was found that the composite layers had a porous structure with a rough surface. The layers consisted of γ-alumina, α-alumina, and tungsten trioxide phases, fractions of which varied with the applied voltage. The band gap of the composite layers was calculated as 3.42 eV using a UV-Vis spectrophotometer. Furthermore, photocatalytic performance of the synthesized composite layers was determined by measuring the decomposition rate of methylene blue solution on the surface of the layers.

A conformal black ceramic layer on aluminium alloy was fabricated by Hwang et al. [73] with an electrolyte containing 0.08 M sodium tungstate, 0.14 M potassium hydroxide and 0.05 M potassium hydrogen phosphate, operating at a current density of 10 A/dm². A PEO coating was synthesized on an aluminium-silicon alloy containing 12 wt% Si with an electrolyte system containing sodium hexametaphosphate, [(NaPO₃)₆] as a conducting salt, and the effect of addition of ammonium metavanadate, [NH₄VO₃] on the surface morphology of PEO coating was investigated [74, 75]. It is difficult to form a uniform passive oxide layer on Al alloys with a high Si content due to the differences in the oxidation behaviour of the silicon-rich phase and the aluminium-rich phase. The addition of ammonium metavanadate to the electrolyte exhibited uniform black colour oxide coating on Si-rich aluminium alloy. The coating was characterized by XPS and confirmed to be a mixture of Al₂O₃, V₂O₃ and V₂O₅.

Li et al. [76] reported that when 6 g/L ammonium metavanadate is added to commonly used phosphate-silicate electrolyte [sodium hexametaphosphate, (NaPO₃)₆, 24g/L and sodium silicate Na₂SiO₃·9H₂O, 15g/L; >40°C; 2 A/dm₂; 15 minutes], the PEO coating changed its white appearance to black. Surface and cross-section morphology of the ceramic layer, in-layer concentration and chemical state of vanadium were investigated. Compared with the inner sublayer, an outer sublayer with a thickness of approximately 4μm formed on the surface which shows higher vanadium concentration. During the PEO process, the formation of metastable oxide phases provides active surfaces for the adsorption of VO₃⁻. The instantaneous high temperature in heat-affected zone around the discharge channel causes the transformation of adsorbed VO₃⁻ to vanadium oxide, such as V₂O₃ and V₂O₅. The formation of V₂O₃ results in the black colour of ceramic layer.

A uniform black PEO film was fabricated on hypoeutectic Al-Si alloy by employing a Na₃PO₄ electrolyte (0.04 mol/L), with different concentration of NH₄VO₃ and Na₂WO₄, and some NaOH (0.06-0.08 mol/L) to control the pH ~10 [77]. A bipolar pulse power supply was used where the positive voltage was increased from 100 to 400 V by nine steps, and the negative voltage was fixed at 30 V. The impulse frequency was 1 kHz, and the duty ratio was 30 %. It was observed that the distribution of the pores left by the discharge channels became more and more uniform with the processing time. The colour of the oxide layer changed from gray to brown gradually, and finally turned to black. The layer looks black due to existence of vanadium oxides and tungsten oxides on its surface. CIELAB colour space mode and colouring analysis demonstrated that the VO₃⁻ plays a more important role than WO₄²⁻ in the colouring of PEO layer. The optimal colorant combination is 0.07 mol/L NH₄VO₃ and 0.06 mol/L Na₂WO₄ and a process time of 14 minutes.

In another method, Ma et al. [78] introduced pre-deposition of Sm_0.5Sr_0.5CoO₃ microparticles to prepare the black PEO coating on AA6061. A layer of microparticles of grain size <10 µm, was first deposited on anodic substrate at 60 °C by gravity and then PEO coating was obtained in the electrolyte consisting of (NaPO₃)₆, 35 g/L and NaOH, 1.5 g/L; pH: 8.5; conductivity: 8.7 mS/cm; temperature: < 50 °C; frequency: 500 Hz; pulse width: 60 µs; current density: 0.8–1.6 A/dm²; time: 5–15 minutes. As the current density or time increased, the α decreased but ε increased. The resultant PEO coatings had a complex multiphase composition consisting of Sm₂O₃, SrAl₄O₇, AlPO₄ and CoAl₂O₄ phases. The film imparted high solar absorptance, ≥ 0.85 and infrared emittance, ≥ 0.90. The surface characteristics of PEO layers on Mg alloy in phosphate electrolytes with and without ammonium metavanadate (NH₄VO₃) were examined by GunKo and co-workers [79]. PEO coatings were formed with KOH: 0.5 mol/L, K₄P₂O₇: 0.15 mol/L with or without NH₄VO₃: 0.08 mol/L; solution conductivity (mS/cm): 61.8 and 55.1 with and without NH₄VO₃; current density: 10 A/dm²; coating time: 200 seconds. The temperature of electrolyte was maintained at 20 °C. With the addition of NH₄VO₃ to the electrolyte, the voltage profile changed significantly with the processing time. After 200 seconds, the size and number of micropores decreased and the colour of the PEO layer altered into black due to the incorporation of vanadium oxides (V₂O₃ and V₂O₅). The potentiodynamic polarization test in the 3.5 wt.% NaCl solution showed that the corrosion resistance of the PEO coatings greatly improved after incorporation of vanadium oxides.

A high absorptance and high emissivity black PEO coatings was prepared on Mg-Li alloy for thermal control application of spacecraft by Li et al. [80]. The PEO coatings appeared black in macroscopic scale, owing to the addition of vanadate in the electrolyte, and the coatings possessed typical porous structures with some protuberances in micron scale. The main element compositions of coatings were Mg, Si, V, O, Na and P, and most of them existed in form of amorphous phase, resulting from the quenching effect during the PEO coating formation process. The oxide of V mainly existed in the form of +3 valence (V₂O₃), which endow coatings with black appearance. With the increase of possessing time and vanadate concentration, the absorptance and emissivity of the coating were improved along with coating thickness. The optimal conditions include, 10 g/L of vanadate content and 10 minutes of process time, where the coatings reached an absorptance and emittance value of 0.964 and 0.951 respectively. The coatings possessed excellent thermal stability and corrosion resistance with promising application in aerospace.

Tu and team [81] have formed black PEO coatings on AZ31 magnesium alloy in an aluminate-tungstate electrolyte; NaAlO₂: 10 g/L + Na₂WO₄·2H₂O: 10 g/L + C₆H₈O₇·H₂O (citric acid): 3 g/L + KOH: 2 g/L; temperature: < 35 °C. Pulsed bipolar and unipolar regimes with a duty cycle of 20 % were employed with average positive and negative current densities being kept at ~22 and ~09 A/dm², respectively. Coatings were fabricated at frequencies of 100, 1000 and 2000 Hz for durations up to 480 seconds. The micro-discharges were investigated by real-time imaging and photomultiplier methods. Coating colour was quantified by the CIELAB method. Blackness of PEO coatings depends on frequency and current regime. Coating‘s blackness was increased with frequency and tungsten incorporation and the blackest coatings obtained under unipolar regime at a frequency of 2000 Hz. Blacker coatings also revealed increased roughness and large internal pores, which were attributed to stronger plasma discharges and greater gas evolution.

A PEO process of flat absorber coating on Ti-6Al-4V alloy in phosphate electrolyte containing sodium tungstate was investigated by Zhongping Yao [82]. The top-notch results were obtained with Na₅P₃O₁₀: 70 g/L, EDTA-2Na: 30 g/L, FeSO₄: 15 g/L, Co(CH₃COO)₂: 15 g/L, Ni(CH₃COO)₂: 15 g/L and Na₂WO₄: 7.5 g/L. A pulsed current density of 1.5 A/cm² was employed in a constant current regime with a working frequency of 1000 Hz for 25 minutes. The reaction temperature was controlled below 30 °C. Under these conditions, a solar absorptance of 0.93 and thermal emittance of 0.88 was achieved with a coating thickness of ~ 40 µm and roughness of ~1.3 µm. The annealing treatment was found to reduce the solar absorbance of the coating but it does not influence the emissivity, which may be associated with the improvement of the crystallization of PEO film.

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