On the enhanced antibacterial activity of plasma electrolytic oxidation (PEO) coatings that incorporate particles: A review

Biomaterials are substances of artificial or natural origin that are used to improve, treat, heal or replace the tissue or bone of a human or animal body. Due to the ever-increasing progress and the widespread use of biomaterials for various biomedical purposes, different methods are used to modify their surface characteristics. One of the problems facing biomaterials such as implants, prostheses, and stents is the presence of various bacteria that can cause adverse side effects such as infection, swelling, and tenderness. This raises the issue of their resistance to bacterial infection, a subject that needs to be thoroughly investigated. So far, a variety of methods have been developed to treat or coat biomaterials and make them resistant to bacterial infections. One of the most promising approaches is the plasma electrolytic oxidation (PEO) process. This process is not only successful in the formation of porous, hard, corrosion-resistant, wear-resistant, and biocompatible coatings but also can be easily manipulated to introduce antibacterial agents to the coatings structure. The addition of nano- or micro-sized particles in the electrolytes has been proven to not only modify the composition and structure of the PEO coatings, but also bring about a strong antibacterial activity. In light of recent advances in this field, the following review aims at discussing different aspects of particles addition in PEO electrolytes when the antibacterial activity is the main concern.     

The properties of hydroxyapatite ceramic coatings produced by plasma electrolytic oxidation

Calcium phosphate coatings produced on the surface of Ti6Al4V by plasma electrolytic oxidation (PEO) using different concentrations of hydroxyapatite (HA) in a 0.12 M Na₃PO₄ (NAP) electrolyte solution was investigated. It was found that the amount of calcium phosphate particles infiltrated into the coating layer as well as the thickness and the surface roughness of the coating increased with increasing HA concentration. The porosity of the ceramic coatings indicated an inverse relationship with the concentration of HA particles dispersed in the NAP solution. The result also demonstrates that higher scratch adhesive strength was achieved using 1.5 g/L HA solution, producing a critical load of 2099 mN, while 0 g/L HA only produced a critical load of 1247 mN. The adhesion becomes independent of thickness when the concentration of HA exceeds 1.5 g/L. The failure of the coating was characterized by large periodic hemispherical chipping, while intermittent delamination was noticed with the coating embedded with HA particles. This study demonstrate the viability of using PEO to produce a thin layer of HA ceramic coating on Ti6Al4V suitable for biomedical applications.     

Preparation of a thick sponge-like structured amorphous silica ceramic coating on 6061 aluminum alloy by plasma electrolytic oxidation in TEOS solution

Plasma electrolytic oxidation (PEO) was performed on 6061 aluminum alloy in organosilicon electrolyte using a stepwise constant potential control method for 23 min. The resulting coating was a sponge-like structured amorphous silica ceramic with a thickness of about 130 μm. Its exceptional wear resistance was attributed to the high hardness of the silica ceramic and the low elastic modulus of the sponge-like structure. The corrosion resistance was enhanced by a dense layer of approximately 2 μm between the coating and the substrate. Impressively, the indentation depth of the PEO coating during nano-indentation tests was only 50–60% of that of 6061 aluminium alloy under varying loads, while the recovery depth of the PEO coating after unloading was 2.5–3.1 times greater than that of 6061 aluminium alloy. Due to its special composition and structure, the PEO coating caused serious wear to the high hardness Si₃N₄ friction balls during the friction and wear test. In the electrochemical tests, the coating reduced the corrosion current density from 1.056 × 10⁻⁵A·cm⁻² to 1.239 × 10⁻⁷A·cm⁻², while extending the passivation region from 0.322 V to 1.032 V.     

Investigation on corrosion and wear resistance of MgO-Al2O3 composite coating prepared by plasma electrolytic oxidation

Ceramic coatings were prepared on 6061 Al alloy in a mixed electrolyte with/without MgO powders at different treatment durations. The results of energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) showed that MgO powder was incorporated into the coatings, and Mg species gradually aggregated into coating inside as prolonging the oxidation time. Scanning electron microscopy (SEM) showed that MgO additive had a certain effect on the microstructures and coating thickness. The corrosion behavior tests evaluated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution suggested that at the same treatment time, the addition of MgO powders can improve the corrosion resistance of the coating, and the Mg-rich layer can affect the corrosion resistance of the coating. The tests of mechanical properties showed that the addition of MgO powders improved the stability and hardness of the coating.     

NaF添加剂对铝合金等离子电解氧化陶瓷膜形成的影响

研究了Na2SiO3-KOH电解溶液中添加NaF对铝合金等离子电解氧化陶瓷膜形成的影响。研究表明：电解溶液中加入NaF可加速氧化物陶瓷膜的形成，提高其硬度。NaF的最佳加入量为0．5～0．75g／L。     

Systematic optimization of corrosion,bioactivity, and biocompatibility behaviors of calcium-phosphate plasma electrolytic oxidation (PEO) coatings on titanium substrates

To optimize the corrosion, bioactivity, and biocompatibility behaviors of plasma electrolytic oxidation (PEO) coatings on titanium substrates, the effects of five process variables including frequency, current density, duty cycle, treatment time, and electrolyte Ca/P ratio were evaluated. In our systematic study, a Taguchi design of experimental based on an L16 orthogonal array was used. For this, the coatings characteristics such as the surface roughness, wettability, rutile to anatase and Ca/P ratios, and corrosion polarization resistance were investigated. After determining the optimum process variables for each response, the apatite forming ability in SBF (bioactivity behavior) and MG63 cell attachment and flattening (biocompatibility behavior) for two groups of coatings were examined. The first group was optimized based on the maximum corrosion polarization resistance and the variables were set as the frequency of 2000 Hz, the current density of 5 A/dm², the duty cycle of 30%, the treatment time of 5 min, and the Ca/P ratio of 0.65 at. % in the electrolyte. For the second group, the maximum surface roughness, greatest Ca/P ratio, and highest wettability as well as the minimum rutile to anatase ratio in coatings, could be obtained when the variables were set as the frequency of 10 Hz, the current density of 12.5 A/dm², the duty cycle of 50%, the treatment time of 12.5 min, and the Ca/P ratio of 1.70 at. % in the electrolyte. The results showed that while both groups of coatings indicated a significant apatite forming ability and can serve as bioactive coatings, a proper attachment and flattening of cells and consequently, the favorable biocompatibility properties were seen only in the first group.     

Preparation,scratch adhesion and anti-corrosion performance of TiO2-MgO-BHA coating on Ti6Al4V implant by plasma electrolytic oxidation technique

Bovine hydroxyapatite (BHA) (from cortical bone), was selected as the main electrolyte for plasma electrolytic oxidation (PEO) on Ti6Al4V implant. The prepared PEO coatings were examined by X-ray diffraction, field emission scanning electron microscope and energy-dispersive X-ray spectroscopy. The surface roughness, adhesion strength, wettability, surface energy and corrosion behaviour of the film were also investigated. The results show that the oxide layer (26 μm) formation on the Ti6Al4V was rough and porous. The micro-pores were filled with anatase TiO₂, cubic MgO and hexagonal BHA particles. The porous structures and the compound particles were mainly composed of Mg, O, Ca, P, Ti, Na and Al. Unlike previous coatings produced from calcium and phosphorus inorganic solutions, the coating formation from a newly developed bovine bone-derived HA electrolyte revealed an additional MgO phase in the coating layer. Moreover, higher amount of single phase hexagonal crystalline BHA phase with a Ca/P ratio of 1.1 was achieved with a single PEO process. A film-to-substrate adhesion strength of 1862.24 mN and scratch hardness of about 4.1 GPa was achieved from this method. The TiO₂/MgO/BHA film exhibited better wettability, higher surface energy and superior corrosion resistance compared to the bare Ti6Al4V substrate.     

Black ceramic coatings on Ti alloy with enhanced high absorptivity and high emissivity by plasma electrolytic oxidation

A black ceramic coating with high absorptivity and emissivity was successfully prepared on TA7 (Ti-5Al-2.5Sn) in a hybrid electrolyte solution by plasma electrolytic oxidation for improving the imaging precision of optical system. The influence of electrolyte components and technical parameters on the composition, structure, and optical properties was investigated. The results show the coatings with typically porous structure are mainly composed of O, P, Si, Ti, V, Fe, and Ni. The corresponding amorhous oxide in the outer layer endows the coating with strong absorption in the visible light and infrared areas, and the crystallized TiO₂ indwelling the inner layer contributes to the strong UV absorption property. In addition, the micropores of the coatings have different size ranges corresponding to the wavelengths, facilitating the increase of absorptivity and emissivity in some degree. The absorptivity and emissivity can be adjusted by electrolyte components and technical parameters. The coating presents the best absorptivity of 0.962 and emissivity of 0.950 in the electrolyte solution of 3 g/L NH₄VO₃, 5 g/L FeSO₄, and 5 g/L C₄H₆O₄Ni under 400 V for 10 minutes.     

等离子体电解氧化技术及机理研究进展

等离子体电解氧化技术(PEO)是一种在轻金属表面原位生长陶瓷膜的技术,在材料领域有着广阔的应用前景.本文从PEO技术的作用过程、机理、诊断技术,PEO陶瓷膜性能和影响因素,应用及局限性等方面,概述了PEO技术在国内外的研究进展,并提出了未来的发展方向.     

Incorporation of zirconia nanoparticles into coatings formed on aluminium by AC plasma electrolytic oxidation

Composite ceramic coatings were formed on aluminium by AC plasma electrolytic oxidation (PEO) using Na₆P₆O₁₈ or Na₂SiO₃ · 5H₂O/KOH electrolytes with monoclinic zirconia nanoparticles in suspension. The coatings grown in Na₂SiO₃ · 5H₂O/KOH electrolyte revealed γ-Al₂O₃ and amorphous phase; α-Al₂O₃ and AlPO₄ were additionally produced with the Na₆P₆O₁₈ electrolyte. Higher temperature zirconia phases, possibly tetragonal and orthorhombic, in addition to the monoclinic phase, were indicative of elevated temperatures at sites of microdischarges. Further, local melting resulted in zirconium-rich dendrites in the coating formed in silicate electrolyte. Zirconium was mainly located in the relatively compact, outer layer of the coating, constituting ∼70–90% of the coating thickness. Nanoparticles appeared to be incorporated at the coating surface and following transport to the interface regions between the inner and outer layers along short-circuit paths through the outer coating.     

Investigation of the growth processes of coatings formed by AC plasma electrolytic oxidation of aluminium

The formation of alumina-based coatings on aluminium by AC plasma electrolytic oxidation (PEO) has been investigated using a silicate electrolyte with selective additions of fine zirconia particles. The coatings comprised an amorphous barrier layer, a relatively dense intermediate layer and a more porous outer layer that contained silicon species. Zirconia was incorporated non-uniformly into the outer layer and, to a limited extent, the intermediate layer, as both particles and a component of cellular microstructures. Following treatments firstly in zirconia-containing electrolyte and secondly in zirconia-free electrolyte, the zirconia did not extend beyond about the middle of the intermediate layer, indicating its limited inward mass transfer during microdischarges. The coating efficiency decreased at oxidation times in excess of 40 min due to dissolution of either the substrate or the coating, or physical loss of coating material. The oxidation of aluminium consumed on average ∼29% of the anodic charge; the remainder was used mainly in generation of oxygen gas.     

Strongly enhanced ferroelectric performance in Ca-doped barium titanate coatings produced by plasma electrolytic oxidation

In this study, Ca-doped BaTiO₃ coatings were directly produced on a Ti substrate by a plasma electrolytic oxidation (PEO) process. Aqueous mixtures of 0.2 M Ba(OH)₂ and different amount of CaO were used as electrolytes in the PEO process. The as-synthesized coatings were carefully characterized by different techniques and their ferroelectric performances were investigated. It is found that small amount of Ca doping is beneficial for the improvement of coatings' ferroelectric properties. The ferroelectric performance of the optimized Ca-doping coating is 350 times that of the undoped coating. However, excess Ca doping is harmful for the ferroelectric performances, which is due to the decreased amount of tetragonal phase and the formation of high-Ca compound. This study indicates that the ferroelectric properties of BaTiO₃ coatings can be greatly improved by optimizing the doping amount of Ca. In addition, this study provides new insights of improving ferroelectric and piezoelectric properties of coating materials by chemical doping through the PEO technology.     

电解氧化法制取葡萄糖酸钠

研究了在无隔膜槽中电解氧化葡萄糖制取葡萄糖酸钠的方法，得出了最佳工艺条件，为电解法制取葡萄糖酸钠的工业化提供了依据。     

Potential and morphological transitions during bipolar plasma electrolytic oxidation of tantalum in silicate electrolyte

Surges in the cell potential, due to an increased overpotential for hydrogen evolution, and transitions in ceramic oxide coating morphology during plasma electrolytic oxidation (PEO) of tantalum under a pulsed bipolar current regime at 1000 Hz in a silicate electrolyte are investigated using real-time imaging of gas evolution, analytical scanning electron microscopy, X-ray photoelectron spectroscopy and supplementary potential-controlled electrochemical measurements. The coatings, which contained Ta₂O₅, TaO and incorporated silicon species, revealed a nodular morphology that transformed with treatment time to a “pancake” type and then a “coral reef” type. The first potential surge occurred only in the cathodic potential, coinciding with an increased spark intensity, more vigorous gas evolution, emergence of “pancake” structures and a reduction in the coating porosity. The later increases in both the anodic and cathodic potential, coincided with intensification of the sparking, the establishment of silicon-rich “coral reef” structures, and formation of a comparatively thick coating. The kinetics of coating growth differed significantly between the three morphological stages. Electrochemical measurements showed that anodic discharges increased the overpotential for hydrogen evolution in the subsequent cathodic pulse, which is proposed to be due to gas impeding the coating and at and near the coating surface increasing the resistance to ionic transport.     

Toward rational design of ceramic coatings generated on valve metals by plasma electrolytic oxidation: The role of cathodic polarisation

Plasma electrolytic oxidation (PEO) has attracted significant attention as a promising eco-friendly technology for the deposition of ceramic-like oxide coatings with superior wear- and corrosion resistance as well as biomedical, catalytic, magnetic, dielectric and optical properties. However, the industrial implementation of PEO has been hindered by a poor understanding of the mechanisms underlying plasma-assisted electrochemical coating formation. This study compares the characteristics of PEO processes on different valve metals in a dilute aqueous solution of NaOH and Na₂SiO₃ under pulsed unipolar and bipolar polarisation conditions. The focus is given to the influence of the nature of the valve metal on the transition to the soft sparking regime characterised by reduced energy consumption whilst yielding sintered coating materials enriched with crystalline phases. In-situ dynamic voltammetry analysis was employed to study the effects of the net current ratio and local negative charge injection using diagnostic pulses embedded in the polarisation signal. It has been demonstrated that soft sparking is a characteristic feature of valve metals such as Mg, Al, Zr and Ta that form insulating surface oxides with negligible electronic conductivity, whereas Ti and Nb form semiconducting oxides, which do not exhibit a characteristic voltage drop, although some features of spark softening were detected. This generalisation paves a way for implementation of a model known as ‘digital twin’ to the rational design of the PEO processes for deposition of the functional ceramics.     

Electrochemical corrosion behaviour of plasma electrolytic oxidation coatings on AM50 magnesium alloy formed in silicate and phosphate based electrolytes

PEO coatings were produced on AM50 magnesium alloy by plasma electrolytic oxidation process in silicate and phosphate based electrolytes using a pulsed DC power source. The microstructure and composition of the PEO coatings were analyzed by scanning electron microscopy (SEM) and X-ray Diffraction (XRD). The corrosion resistance of the PEO coatings was evaluated using open circuit potential (OCP) measurements, potentiodynamic polarisation tests and electrochemical impedance spectroscopy (EIS) in 0.1 M NaCl solution. It was found that the electrolyte composition has a significant effect on the coating evolution and on the resulting coating characteristics, such as microstructure, composition, coating thickness, roughness and thus on the corrosion behaviour. The corrosion resistance of the PEO coating formed in silicate electrolyte was found to be superior to that formed in phosphate electrolyte in both the short-term and long-term electrochemical corrosion tests.     

Preparation and characterization of oxide films containing crystalline TiO2 on magnesium alloy by plasma electrolytic oxidation

Oxide films have been produced on AM60B magnesium alloy using plasma electrolytic oxidation process in an alkaline phosphate electrolyte with and without addition of titania sol. The microstructure and composition of the oxide films were analyzed by Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscope (XPS) and X-ray Diffraction (XRD). The corrosion resistances of the oxide films were evaluated using potentiodynamic polarization measurements in 3.5 wt% NaCl solution. It is found that the oxide film containing crystalline rutile and anatase TiO₂ compounds are produced in an alkaline phosphate electrolyte with addition of titania sol. The oxide film formed in electrolyte with addition of titania sol has more uniform morphology with less structural imperfections than that formed in electrolyte without addition of titania sol. The results of potentiodynamic polarization analysis show that the oxide film formed in the present modified electrolyte is successful in providing superior corrosion resistance for magnesium alloy.     

Real-time imaging of coating growth during plasma electrolytic oxidation of titanium

The microdischarge characteristics during dc plasma electrolytic oxidation (PEO) of titanium at 20 mA cm⁻² in orthophosphate electrolyte at 293 K have been investigated by real-time imaging. A relatively constant microdischarge rate of ∼100 cm⁻² s⁻¹ was revealed, with the sizes and lifetimes of microdischarges ranging between ∼70 and 380 μm and ∼35 and 800 ms, respectively. The average lifetime of the microdischarges increased with increased voltage, reaching ∼335 ms at 430 V in the main period of coating growth. The cumulative number of microdischarges during PEO was at least two orders of magnitude lower than the population density of pores on the surface of the final coating. Increased time of oxidation resulted in coarsening of the coating surface, due to the formation of relatively large pores. Microdischarges were observed with a bubble-free surrounding region indicative of generation of shock waves.     

Characteristics of in-situ synthesized Hydroxyapatite on TiO2 ceramic via plasma electrolytic oxidation

The biocompatibility properties of Ti scaffolds can be improved significantly by hydroxyapatite (HA) composite coating. We successfully coated the surface of the Ti substrates by in-situ formation of HA nanocrystals on TiO₂ sublayer under calcium acetate and trisodium phosphate electrolytes through the plasma electrolytic oxidation (PEO) process. The effects of the process parameters and passivation on the characteristics of the coated substrates were studied using X–ray diffraction, Fourier-transform infrared spectroscopy, Field emission scanning electron microscopy, and Energy–dispersive X–ray spectroscopy. The systematically controlled experimental studies indicated that using the higher calcium/phosphorous ratio in the electrolyte enhances the micro arcs power and consequently, thickens the synthesized HA layer. The HA nanocrystals were tailored on the walls and edges of the discharge channels due to the locally concentrated heating zones, which consequently resulted in noticeable amounts of Ca and P dopants in porous TiO₂.     

Growth process of plasma electrolytic oxidation films formed on magnesium alloy AZ91D in silicate solution

In order to get a clear picture for describing the growth process of the oxide film formed on magnesium alloy AZ91D under plasma electrolytic oxidation (PEO) in alkaline silicate solution, the characteristics of PEO films formed at different reaction stages were systemically investigated. The results of morphologies, compositions and electronic properties indicated that the PEO films had a different growth behavior as the PEO treatment proceeding. At the initial stage (before the occurrence of sparking), the growth rate of PEO films was low, the elements (O, Mg, Al and Si) contents were varied obviously and the donor concentration in the film was kept at a high level. After sparking occurred, the PEO films showed a higher growth rate due to the high transfer rate of ions and electrons and the existence of plasma reactions; simultaneously, the films formed on α- and β-phase exhibited different growth rate. With treated time increased, the thickness of PEO films and transfer resistance to ions and electrons were also increased; thereby, the growth rate of the PEO films was decreased gently.