Plasma electrolytic oxidation

About: Plasma electrolytic oxidation is a research topic. Over the lifetime, 2342 publications have been published within this topic receiving 51355 citations. The topic is also known as: Micro-arc oxidation.

Plasma electrolysis for surface engineering

Abstract: This paper overviews the relatively new surface engineering discipline of plasma electrolysis, the main derivative of this being plasma electrolytic deposition (PED), which includes techniques such as plasma electrolytic oxidation (PEO) and plasma electrolytic saturation (PES) processes such as plasma electrolytic nitriding/carburizing (PEN/PEC). In PED technology, spark or arc plasma micro-discharges in an aqueous solution are utilised to ionise gaseous media from the solution such that complex compounds are synthesised on the metal surface through the plasma chemical interactions. The physical and chemical fundamentals of plasma electrolysis are discussed here. The equipment and deposition procedures for coating production are described, and the effects of electrolyte composition and temperature on ignition voltage, discharge intensity and deposited layer thickness and composition are outlined. AC-pulse PEO treatment of aluminium in a suitable passivating electrolyte allows the formation of relatively thick (up to 500 μm) and hard (up to 23 GPa) surface layers with excellent adhesion to the substrate. A 10–20 μm thick surface compound layer (1200HV) and 200–300 μm inner diffusion layer with very good mechanical and corrosion-resistant properties can also be formed on steel substrates in only 3–5 min by use of the PEN/PEC saturation techniques. Details are given of the basic operational characteristics of the various techniques, and the physical, mechanical and tribological characteristics of coatings produced by plasma electrolytic treatments are presented.

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

Abstract: 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.

Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: Prospects and challenges

Abstract: The development of biodegradable implants is indeed fascinating and among the various types of materials used in this regard, magnesium and its alloys assume significance. However, the rapid corrosion, generation of a large volume of hydrogen gas, accumulation of the hydrogen bubbles in gas pockets adjacent to the implant, increase in local pH of the body fluid, are the major impediments in using them as an implant material. Hence, development of Mg/Mg alloy based degradable implants requires that (i) they should maintain sufficient mechanical strength and integrity until the affected part of body is healed; (ii) they should exhibit good resistance to corrosion in the body fluid during the initial periods of implantation and subsequently corrode in a controlled and uniform fashion; and (iii) the corrosion products should not exceed the acceptable absorption level of the human body. Reducing the rate of corrosion of Mg is the most appropriate strategy and this can be achieved with the use of alloying, surface treatment/coating and mechanical processing. Surface treatment/coating is a viable approach as it not only enables improvement in corrosion resistance but also provides a suitable surface for better bone bonding and cell growth. Among the various surface modification processes, microarc oxidation (MAO) has received considerable attention since the protective oxide layer would delay the rate of corrosion attack during the initial period of implantation and, the decrease in the extent of hydrogen evolution would enhance the primary neo-formation of bone around the implant. The presence of micropores and cracks on the surface of MAO coatings can be considered as an opportunity or a limitation. The presence of a porous outer layer in MAO coatings would significantly improve the mechanical interlocking effect, the bonding area and stress distribution across the adhesive–substrate interface of the joins, resulting in higher bond strength. However, the presence of a higher pore density on the surface of the MAO coatings increases the effective surface area and thus the tendency of the corrosive medium to adsorb and concentrate into these pores. This would facilitate quicker infiltration of the corrosive medium into the inner regions of the coating and subsequently down to the substrate, thus deteriorating the corrosion resistance of the coating by changing its local pH. The pore density, distribution of pores and interconnectivity of the pores with the substrate are the important factors that decide its corrosion protective ability. In spite of the limitation in corrosion rate, MAO coatings exhibit a slow rate of degradation during the first few weeks and an accelerated degradation in later stages of implantation. Nevertheless, the difficulty in achieving a control over the rate of degradation is still a matter of concern in fabricating implant devices with a desired lifetime. Hence, it is not only essential but also mandatory to increase the corrosion resistance of MAO coatings. In this perspective, this review aims to address the various strategies explored to improve the corrosion resistance of MAO coatings on Mg/Mg alloys. This review provides a detailed outline on how the choice of electrolytes, process parameters, pretreatment, additives, incorporation of ceramic particles and, sealing and post-treatment, influence the porosity and corrosion resistance of MAO coatings on Mg/Mg alloys. In addition, the implications of such modifications/choices on the suitability of the resultant coatings for biomedical applications are discussed. The importance of multifunctional approaches in improving the corrosion resistance as well as imparting a controlled drug delivery, better apatite growth, improved bioactivity, etc. is addressed. The important strategies to improve the corrosion resistance and future prospects are summarized.

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

Abstract: 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.

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

Abstract: 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.