A. Yerokhin

Discharge characterization in plasma electrolytic oxidation of aluminium

Digital video imaging of the plasma electrolytic oxidation (PEO) of aluminium has been performed, which allowed evaluation of both dimensional characteristics of individual microdischarges appearing at the oxide–electrolyte interface and their collective behaviour throughout the oxidation process. It has been shown that the microdischarge cross-sectional dimensions vary within the range 0.01–1.35 mm2. In the course of PEO processing, small localized events (<0.03 mm3) always dominate in the microdischarge spatial distribution and the relative proportion of medium-sized to very large microdischarges is gradually redistributed in favour of the latter. Temporal dependences have been found for the fraction of surface area instantaneously experiencing the discharge, as well as for the spatial and current densities of the microdischarge. Discharge mechanisms occurring during PEO are discussed and a model of microdischarge formation is suggested, assuming the possibility of free-electron generation and glow discharge ignition in the gaseous media developed at the oxide–electrolyte interface. First approximation evaluations of thermal processes in the oxide layer under the discharge conditions have been considered. The estimated ranges of the microdischarge current density (50–18 kA m−2) and duration (0.25–3.5 ms) sufficient for initiating phase transitions (e.g. γ–α transformation and melting) in the surface oxide layer are shown to be in good agreement with experimental data.

Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process

In this study, a plasma electrolytic oxidation (PEO) process was used to produce oxide coatings on commercially pure aluminium (1100 alloy) at a pulsed dc power mode. The effects of process parameters (i.e. current density and treatment time) on the plasma discharge behaviour during the PEO treatment were investigated using optical emission spectroscopy (OES) in the visible and near ultraviolet (NUV) band (285–800 nm). The elements present in the plasma were identified. Stark shifts of spectral lines and line intensity ratios were utilized to determine the plasma electron concentrations and temperatures, respectively. The plasma electron temperature profile, coating surface morphology and coating composition were used to interpret the plasma discharging behaviour. The different coating morphologies and compositions at different coating surface regions are explained in terms of three types of discharge, which originate either at the substrate/coating interface, within the upper layer, or at the coating top layer. The high spike peaks on the plasma intensity and temperature profiles corresponded to discharges originated from the substrate/coating interface, while the base line and small fluctuations were due to discharges at the coating/electrolyte interface.

Fatigue properties of Keronite® coatings on a magnesium alloy

In the paper, the feasibility of using the Keronite® plasma electrolytic oxidation process to overcome the problem of fatigue performance reduction caused by anodising treatments in a Mg alloy is studied. Two types of coatings produced using different current regimes, and having two thicknesses of ∼7 and ∼15 μm, were tested using a rotating bending fatigue tester. SEM, XRD and optical microscopy techniques were used to evaluate possible fracture mechanisms involved in the initiation and propagation of the fatigue cracks. The results of the investigation demonstrate that Keronite® coatings may cause no more than a 10% reduction in the endurance limit of the Mg alloy, which is substantially lower than the effect from conventional anodising. A probable cause of that reduction seems to be distortion of the metal subsurface layer rather than structural defects introduced by the oxide film.

Structural characteristics and residual stresses in oxide films produced on Ti by pulsed unipolar plasma electrolytic oxidation

Oxide films, 7–10 µm thick, were produced on commercially pure titanium by plasma electrolytic oxidation in a sodium orthophosphate electrolyte using a pulsed unipolar current with frequency (f) and duty cycle (δ) varying within f = 0.1–10 kHz and δ = 0.8–0.2, respectively. The coatings comprised a mixture of an amorphous phase with nanocrystalline anatase and rutile phases, where the relative rutile content range was 17–25 wt%. Incorporation of phosphorus from the electrolyte into the coating in the form of PO2 –, PO3 2– and PO4 3–, as demonstrated by EDX and FT-IR analyses, contributed to the formation of the amorphous phase. Residual stresses associated with the crystalline coating phase constituents were evaluated using the X-ray diffraction sin2 ψ method. It was found that, depending on the treatment parameters, internal direct and shear stresses in anatase ranged from–205 (±17) to–431 (±27) MPa and from–98 (±6) to–145 (±10) MPa, respectively, whereas the rutile structure is comparatively stress-free.

In vitro biological response of plasma electrolytically oxidized and plasma‐sprayed hydroxyapatite coatings on Ti–6Al–4V alloy

Plasma electrolytic oxidation (PEO) is a relatively new surface modification process that may be used as an alternative to plasma spraying methods to confer bioactivity to Ti alloy implants. The aim of this study was to compare physical, chemical and biological surface characteristics of two coatings applied by PEO processes, containing different calcium phosphate (CaP) and titanium dioxide phases, with a plasma-sprayed hydroxyapatite (HA) coating. Coating characteristics were examined by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, surface profilometry, and wettability tests. The biological properties were determined using the human osteoblastic cell line MG-63 to assess cell viability, calcium and collagen synthesis. The tests showed that PEO coatings are significantly more hydrophilic (6%) and have 78% lower surface roughness (Ra) than the plasma-sprayed coatings. Cell behavior was demonstrated to be strongly dependent on the phase composition and surface distribution of elements in the PEO coating. MG-63 viability for the TiO2 -based PEO coating containing amorphous CaPs was significantly lower than that for the PEO coating containing crystalline HA and the plasma-sprayed coating. However, collagen synthesis on both the CaP and the TiO2 PEO coatings was significantly higher (92% and 71%, respectively) than on the plasma-sprayed coating after 14 days. PEO has been demonstrated to be a promising method for coating of orthopedic implant surfaces.

Influence of current density and electrolyte concentration on DC PEO titania coatings

Abstract Titania films, 5–9 μm in thickness, were produced on commercial purity titanium by a direct current (DC) plasma electrolytic oxidation (PEO) process using 5 to 20 A dm−2 current densities in 5 to 15 g L−1 trisodium orthophosphate electrolytes. Phase analyses (composition and crystallite size) were carried out using X-ray diffraction and TEM techniques. Residual stresses associated with one of the crystalline coating phases (anatase) were evaluated using the X-ray diffraction Sin2ψ method. SEM and TEM techniques were utilised to study the cross-section and surface morphologies and nanofeatures of the PEO titania coatings. Variations in the proportions of anatase and rutile phases, surface morphologies, coating thicknesses and associated internal stresses are correlated with the current density and electrolyte concentration used during the DC PEO process on titanium. Direct and shear stresses in the anatase are found in the range −70 to −331 and −17 to −167 MPa respectively.

Mechanical behaviour of cp-magnesium with duplex hydroxyapatite and PEO coatings.

Hydroxyapatite-magnesia coatings were formed on cp-magnesium by plasma electrolytic oxidation (PEO) followed by cathodic electrodeposition (CED). The static tensile and cyclic fatigue performance of the coated samples were investigated. The cracking behaviour of the coatings during the tensile tests was studied by fracture analysis. The effects of the surface treatment on the fatigue performance of the magnesium substrate were addressed. Tensile strength of cp-Mg was not significantly affected, whereas the fatigue performance was improved by the PEO+CED coatings in the low-cycle region, possibly due to compressive residual stress induced to the metal substrate by the surface treatment. However, reduced fatigue strength was observed in the high-cycle region, which might be attributed to the defects at the coating/substrate interface produced during the surface modification. The in vitro corrosion reduced the fatigue strength in both the low- and high-cycle regions. Finally, the applicability of surface engineered magnesium for biomedical applications was demonstrated from the mechanical standpoint.

Characteristics and in vitro response of thin hydroxyapatite–titania films produced by plasma electrolytic oxidation of Ti alloys in electrolytes with particle additions

Enhanced incorporation of hydroxyapatite nanoparticles in porous titania coating formed by plasma electrolytic oxidation significantly increases surface osteogenic activity.

Evaluation of Residual Stress Development at the Interface of Plasma Electrolytically Oxidized and Cold-Worked Aluminum

Fatigue failure in hard oxide-coated aluminum is usually driven by rapid short crack propagation from the interface through the substrate; mitigation of this is possible by introducing interfacial compressive stresses. Combining cold work with hard oxide coating can improve their performance under conditions of simultaneous wear, corrosion, and fatigue. Three-dimensional strain fields in an aluminum alloy with combined cold work and PEO coating have been measured and mechanisms for stress redistribution presented. These comprise material consumption, expansive growth of oxide layers, and local annealing.

Anodising of light alloys

Abstract: Light metals and alloys can form native oxide films on their surfaces that offer only limited protection to surface degradation. Anodising refers to electrochemical anodic oxide film formation; it is one of the most popular methods to increase wear- and corrosion-resistance of light alloys, especially under light loads. Anodic films are most commonly applied to protect aluminium alloys and are discussed here in detail; magnesium and titanium alloys can also be anodised, which is described at the end of this chapter. Anodised light alloys have been exploited in a wide range of industrial sectors such as automotive, marine and aerospace. More recently, porous anodic alumina films have found their application in nanoscience and nanotechnology.