The paper discusses processing and property aspects of oxide films formed on a Ti–6Al–4V alloy by AC plasma electrolytic oxidation (PEO) in aqueous solutions containing aluminate, phosphate, silicate and sulfate anions and some of their combinations. Structure, composition, mechanical tribological and corrosion resistant characteristics of the films formed are studied by SEM, XRD and microhardness analyses, and by scratch, impact, pin-on-disc friction and potentiodynamic corrosion testing. It is found that the films produced from the aluminate–phosphate electrolyte are dense and uniform and are composed mainly of Al 2 TiO 5 and TiO 2 phases of the rutile form. The films possess a beneficial combination of 50–60 μm thickness, 575 kg/mm 2 hardness and high adhesion and provide a low wear rate (3.4×10 −8 mm 3 /Nm) but a relatively high friction coefficient of μ=0.6–0.7 against steel, caused by material transfer from the counterface. A minimum friction coefficient of μ=0.18 is recorded during the testing of softer rutile–anatase films, 7 μm thick, produced from a phosphate electrolyte. Both of these types of film show good corrosion resistance in NaCl and physiological solutions, where the corrosion current is approximately 1.5 orders of magnitude lower than that of the uncoated substrate. SiO 2 /TiO 2 -based films with 70–90 μm thickness and high bulk porosity produced from silicate and silicate–aluminate electrolytes demonstrate better corrosion behaviour in H 2 SO 4 solution, due to the greater chemical stability of the film phase components in this environment.

Characterisation of oxide films produced by plasma electrolytic oxidation of a Ti–6Al–4V alloy

Thick alumina coatings were synthesized on 7075 Al-alloy substrates through the microarc oxidation (MAO) route. Different oxidation times namely 1, 3, 5, 10, 20 and 30 min were employed and the coated samples were subjected to coating thickness and surface roughness measurements. Phase composition of the surface layers of the coatings was evaluated through X-ray diffraction. In addition, the surface features of the coatings were studied using scanning electron microscopy. Influence of coating time on the kinetics of coating formation, surface roughness, microhardness, number and size of the microarc discharge channels was investigated. On the basis of the experimental results, a phenomenological mechanism for the formation of the alumina-based ceramic coatings during the MAO process has been proposed.

Mechanisms underlying the formation of thick alumina coatings through the MAO coating technology

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.

https://iopscience.iop.org/article/10.1088/0022-3727/36/17/314/pdf

Discharge characterization in plasma electrolytic oxidation of aluminium

Titanium alloys have been used with some success in several bioimplant applications. However, they can suffer certain disadvantages, such as poor osteoinductive properties and low corrosive-wear resistance. Attempts to overcome the first of these drawbacks have involved coating the metal with the bioceramic material hydroxyapatite (HA), a primary component of bone and a very good osteoinductor. Since TiO 2 coatings are also known to be effective as chemical barriers against the in-vivo release of metal ions from the implants, a double layer HA–TiO 2 coating on titanium alloys with HA as the top layer and a dense TiO 2 film as the inner layer should possess a very good combination of bioactivity, chemical stability and mechanical integrity. This paper describes efforts to improve implant biocompatibility and durability by applying a hybrid treatment of micro-arc discharge oxidation (MDO) and electrophoretic deposition. The most common structural titanium alloy (Ti-6Al-4V) was used as the substrate material. A phosphate salt solution and an HA powder aqueous suspension were used as the electrolyte for micro-arc oxidation and the solution for HA electrophoretic deposition, respectively. It is shown that a relatively thick and hard TiO 2 coating can be produced by anodic micro-arc oxidation of titanium, and an HA coating incorporated on top of the TiO 2 layer can simultaneously be formed using a combination of plasma electrolysis and electrophoresis, with the suspension held at high values of pH. X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) have been used to investigate the microstructure and morphology of the coatings. The adhesive strength between the coating and substrate has been assessed using scratch adhesion testing. The corrosion resistance of the specimens was examined using potentiodynamic tests in a buffered physiological solution. The results indicate that a hybrid combination of micro-arc oxidation and electrophoretic deposition can provide a phase-pure HA top layer and anticorrosive TiO 2 interlayer, which should show good mechanical and biochemical stability in the corrosive environment of the human body.

Deposition of layered bioceramic hydroxyapatite/TiO2 coatings on titanium alloys using a hybrid technique of micro-arc oxidation and electrophoresis

Porosity in plasma electrolytic oxide coatings

The wear life of components manufactured from Al-based alloys can be drastically increased by the application of ceramic coatings. However, coatings deposited by conventional methods such as vacuum deposition or plasma spray have either insufficient adhesion to Al-based materials or the deposition process causes the component to overheat. A recently developed micro-arc discharge oxidizing (MDO) technique allows for the formation 100–200 μm thick AlSiO coating on the surface of Al alloys. A composite Al2O3SiO2 coating is formed at room temperature as a result of a reactive process between Al in the alloy itself and O and Si supplied by an electrolyte. AlSiO coatings were investigated with XPS, Vickers and nanoindentation hardness tests, ball-on-disk, and block-on-ring friction and wear tests. Coatings were found to consist of at least two phases: a hard Al2O3 phase and a softer aluminasilicate phase. A maximum hardness of 17 GPa was found for coatings with highest content of Al2O3 phase. The tribological properties of AlSiO coatings with different composition are discussed. The lowest friction coefficient was found for the Al0.26Si0.08O0.66 coating and was measured around 0.15–0.25 depending on the test environment. The application of this coating decreased the wear rate of components fabricated from an Al-based alloy by several orders of magnitude and permitted operation of coated friction pairs at 1 GPa contact load.

M.S. Donley

Papers

Characterization of wear protective AlSiO coatings formed on Al-based alloys by micro-arc discharge treatment