Surface Modification of Titanium – Part 5 – Plasma Electrolytic Oxidation – Mechanism & Microstructure

Plasma electrolytic oxidation imparts improved microhardness, corrosion and wear resistance; tribological and heat radiation characteristics in many aggressive environments, and favourable topography for cellular proliferation [1–8]. The process is carried out in mild aqueous alkaline electrolytes [9–18] and as the coating also grows inwards, an extremely adhesive oxide coating on the substrate is formed. The process provides uniform coverage on complex shapes with well-controlled, predictable growth and superior edge protection. The non-columnar structure of PEO coatings is less susceptible to wear with lower fatigue debit. The process is widely used for growing thick, hard, largely crystalline oxide coating on Al, Mg and Ti alloys [2, 11, 19–27]. The technological development, microstructural characteristics, biocompatibility and applications of PEO for general as well as specific objectives are reviewed elsewhere [1, 5, 12, 19, 28–43]. PEO coatings are used for diverse applications:

• Decorative applications: Depending on substrate composition, electrolyte formulation and processing parameters, the colour of the coatings can be whitish, gray or black. Further by designing the parameters for the porosity of PEO coating, the absorption of colouring agents, dyes, paints, etc. can be enhanced.
• Superior corrosion resistance: PEO coating provides far superior corrosion resistance than the conventional anodizing.
• Superior mechanical properties: Owing to the possession of very high hardness and superior tribological performance, the PEO coatings are used to improve the wear and friction resistance.
• Thermal control applications: As the thermal conductivity of the PEO oxide layer is very low, it can be used as a thermal barrier / thermal shock resistance layer [44, 45]. In addition, the PEO coatings can also be blackened to improve their heat absorptance characteristics for thermal control applications.
• Electrical appliances: As the PEO layer has very high dielectric strength, it is very useful for electrical insulation. The dielectric breakdown occurs at a field of ~10 V per micron. In addition, the PEO coating is very hard, it is suitable for implantation on the inner surfaces of conic, cavital or cylindrical areas.
• Chemical industries: PEO coatings on titanium can be used in chemical industries due to their resistance to acidic and alkaline solutions, as well as for the photo-activity of titania (TiO₂) [46].
• Biomedical applications: PEO coated titanium components are used in biomedical devices, biodegradable materials and dental implants [47–51].
• Super hydrophobic characteristics: The micro- and nanosized TiO₂ particles have been utilized to improve the surface roughness (water contact angle, WCA below 10°) for self-cleaning, antifogging and related applications [52].
• The PEO coatings can be impregnated with polymers such as PTFE to modify their properties for specific applications.

Process Set-Up

An image of a typical plasma electrolytic oxidation plant is shown in Figure 1. A schematic set-up of PEO process is represented in Figure 2. It consists of a stainless-steel tank which contains the electrolyte and a suitable high power supply source. The job is immersed in the aqueous electrolyte and is electrically connected in the electrochemical cell, so as to become an anode. The other counter-electrode (cathode) is typically made from an inert material such as stainless steel, and often consists of the wall of the bath itself. The electrolyte is stirred well either mechanically or by air agitation and cooled through a cold-water circulation unit. The PEO coating is formed by polarizing the titanium part to the dielectric breakdown voltage in a suitable electrolyte. The process is similar to anodizing, but it employs higher potentials, so that discharges occur which leads to partial fusion of an oxide film and the resulting plasma modifies the structure of the layer [44, 45, 54–60]. The mechanism of the PEO process consists of complex physical, chemical, electrochemical and plasma thermo-chemical reactions. Oxide film formation, dissolution of primary film and gas evolution on anode surface constitute the major phenomena.

The process can employ the continuous direct current or pulsed direct current or alternating pulses (pulsed unipolar and pulsed bi-polar). Since the number of electrical parameters that can be regulated in a direct current mode is limited, it is not commonly adopted. Further, the use of AC sources is typically limited by the possible output power (≤ 10 kW) and frequency. The application of pulsed DC current mode has greatly enabled tailoring of PEO coating microstructure and characteristics by efficiently modifying the surface discharge characteristics by simple adjustments of electrical parameters like wave form, pulse width, duty cycle, frequency, etc. This has led to the rapid development of PEO technique.

A unipolar current mode has only the positive component whereas a bipolar current mode consists of both a positive component and a negative component. The use of a hybrid mode of current i.e., a combination of pulsed unipolar and bipolar current modes has also been reported.

Process Mechanism

The PEO coatings on Ti can be obtained with silicate, phosphate, aluminate, silicate-phosphate, phosphate-aluminate based electrolytes. The PEO coating of thicknesses of ~200 μm can be generally achieved on Ti alloy with the corresponding micro hardness values in the range of 300–1100 HV [61]. Table 1 shows details of typical baths:

The chemical reactions for PEO coating of titanium include; anodic dissolution of titanium, formation of titania, and oxygen evolution. The anions (OH^– and AlO₂^–, Al(OH)₄^–, PO₄^3–, SiO₃^2–) depending on the electrolyte composition participate in the reaction at the oxide/electrolyte interface. The chemistry of the whole process can be represented with the following general equations [46].

Metal-oxide interface: anodic process

Ti → Ti⁴⁺ + 4e^–

4OH^– → O_2↑ + 2 H₂O+ 4e^–

Oxide-electrolyte interface

Ti⁴⁺ + 4OH^– → TiO₂ + 2H₂O

3Ti⁴⁺ + 10OH^– → Ti₃O₅ + 5H₂O

Ti⁴⁺ + 4AlO₂^– → TiO₂ + 2Al₂O₃

Ti⁴⁺ + Al(OH)₄^– → TiO₂ + Al₂O₃ + Al(OH)₃ + 5H₂O

TiO₂ + Al₂O₃ → Al₂TiO₅

3Ti⁴⁺ + 4PO₄³⁻→ Ti3(PO₄)₄

Ti⁴⁺ + SiO₃²⁻ + 2OH^– → TiSiO₄ + H₂O

Ti⁴⁺ + 2SiO₃²⁻ → Ti(SiO₃)₂

Tab. 1: Details of typical PEO process on Ti alloys [62]

Aliasghari et al. [63] used the phosphate/silicate electrolyte: sodium silicate (SG,1.5): 10.5 g/L, phosphoric acid: 2 ml/L, KOH: 2.8 g/L; conductivity: 10.2 mS/cm; pH: 12.2; 25 °C, 50 A/dm²; 50 Hz; duty cycle: 50 %; 0 to 310 V for PEO coatings on Ti6Al4V alloy. A coating thickness of 40–50 µm was produced in 60 minutes. Li et al. [64] fabricated 110 µm thick PEO coating on TiAl alloy by the alternating-current in silicate electrolyte. Microhardness of the coating was about three times higher than that of TiAl substrate. The wear rate of coating was only 1/10 of TiAl substrate. Corrosion current density of the coated TiAl alloy was greatly reduced.

Microstructure

The microstructure and phase content of the PEO coatings are related to the process parameters which ultimately influence the wear and corrosion resistance [65, 66]. The PEO coating consists of a porous outer layer with pore diameter ranging from 3 to 8 µm (Fig. 3) [67], dense intermediate layer and a thin inner layer [61]. The surface morphologies are characterized by many micropores, microcracks and dimples [68]. The coatings have non-columnar structure, and are dense near the substrate interface and intermediate layer. The porosity is more near the surface around discharge channels. The micropores are formed by molten oxide and gas bubbles thrown out of micro-arc discharge channels and the cracks are resulted from the thermal stress due to the rapid solidification of the molten oxide in the relatively cold electrolyte. Porous structures in anodic coatings are potentially favourable to some applications where the addition of other material is required to improve the surface properties, for example a solid lubricant material, such as polytetrafluoroethylene (PTFE), or a secondary coating.

Fig. 3: SEM micrographs of PEO coating on Ti (a) overview and (b) detail [67]

PEO technique provides a possibility for the wide variation of composition and structure of the surface oxide film and thus attracts special interest for the corrosion protection and the optimization of friction and wear of titanium alloys. The process also allows the fabrication of relatively rough, micro-porous, thick, titanium oxide coatings containing bioactive compounds (such as hydroxyapatite, calcium phosphate or calcium titanate) to enhance the osseointegration for biomedical applications [69–73].

Solar Selective / High Emittance PEO Coatings

Studies on some solar selective / high emittance PEO coatings on titanium materials for thermal control applications in space environments are reported. Yao et al. [74] fabricated solar reflector, low absorbance and high emissivity, PEO coatings on Ti6Al4V alloy with silicate electrolytes. 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. It was not well crystallized and consisted of a large number of amorphous silicates.

A high-emittance PEO process on Ti6Al4V for spacecraft application was obtained with electrolytes containing sodium silicate and sodium hypophosphate [75]. Average ε_ir and α_s of 5–9 μm thick coating were recorded as 0.85, and 0.76, respectively. The PEO coating was found to be hydrophobic (contact angle ~108°). The characteristic nanohardness (H) and Young’s modulus (E) values were 2.59 GPa and 81.45 GPa, respectively. The corrosion resistance of the PEO coating was evaluated by potentiodynamic polarization and Electrochemical impedance spectroscopy (EIS). The total impedance and corrosion current density (i_corr) were in the order of 1 MΩ and 10-8 µA.cm^–2, respectively. The results indicate superior corrosion resistance of the present PEO coating compared to similar coatings reported earlier in the literature.

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