X-ray Computed Tomographic Investigation of the Porosity and

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X-ray Computed Tomographic Investigation of the Porosity and Morphology of Plasma Electrolytic Oxidation Coatings Xun Zhang, Sepideh Aliasghari, Aneta N#mcová, Timothy Burnett, Ivo Kub#na, Miroslav Šmíd, George Thompson, Peter Skeldon, and Philip Withers ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00274 • Publication Date (Web): 14 Mar 2016 Downloaded from http://pubs.acs.org on March 19, 2016

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Abstract Graphic

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X-ray Computed Tomographic Investigation of the Porosity and Morphology of Plasma Electrolytic Oxidation Coatings

X. Zhang,a S. Aliasghari,b A. Němcová,b T. L. Burnett,a I. Kuběna,c M. Šmíd,d G. E. Thompson,b P. Skeldon,b* P. J. Withersa,e a

Henry Moseley X-ray Imaging Facility, The University of Manchester, Manchester, M13 9PL, UK Corrosion and Protection Group, School of Materials, The University of Manchester, Manchester M13 9PL. U.K. c Institute of Physics of Materials, AS CR, Žižkova 22, Brno, 616 62, Czech Republic d CEITEC IPM, Žižkova 22, Brno, 616 62, Czech Republic e BP ICAM, The University of Manchester, Manchester, M13 9PL, UK b

*corresponding author e-mail: [email protected]

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ABSTRACT

Plasma electrolytic oxidation (PEO) is of increasing interest for the formation of ceramic coatings on metals for applications that require diverse coating properties, such as wear- and corrosion-resistance, low thermal conductivity and biocompatibility. Porosity in the coatings can have an important impact on the coating performance. However, the quantification of the porosity in coatings can be difficult due to the wide range of pore sizes and the complexity of the coating morphology. In this work, a PEO coating formed on titanium is examined using high resolution X-ray computed tomography (X-ray CT). The observations are validated by comparisons of surface views and cross-sectional views of specific coating features obtained using X-ray CT and scanning electron microscopy. The X-ray CT technique is shown to be capable of resolving pores with volumes of at least 6 µm3. Furthermore, the shapes of large pores are revealed and a correlation is demonstrated between the locations of the pores, nodules on the coating surface and depressions in the titanium substrate. The locations and morphologies of the pores, which constitute 5.7% of the coating volume, indicate that they are generated by release of oxygen gas from the molten coating.

Keywords: titanium, plasma electrolytic oxidation, porosity, X-ray computed tomography, scanning electron microscopy.



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INTRODUCTION

Plasma electrolytic oxidation (PEO) processes are considered to be amongst the most environmental-friendly surface treatments of aluminium, magnesium and titanium alloys, providing good wear, thermal and corrosion resistance.1–4 The coatings have potential for applications in a wide range of industry sectors, including aerospace, automotive, textile processing, electronic components, energy, oil and gas, leisure and sports products.4 The coatings are further being developed for catalysis,5-7 biocompatibility,8,9 drug release systems10, sensors11, decorative finishing12 and as nanostructured and nanocomposite coating materials.13,14 Furthermore, PEO of zirconium and zirconium alloys is of interest for thermal barriers, for improved wear- and corrosion-resistance in nuclear applications and for implants.15-18 Porous ceramic coatings are formed during PEO with the assistance of dielectric breakdown at the sites of short-lived micro-discharges.19,20 The conditions employed for carrying out PEO, such as surface pre-treatment, electrolyte composition, current density, waveform and time of the treatment, have significant effects on the discharge characteristics and, hence, affect the morphology, density and composition of the coating.19,21–24 Typically, the PEO coatings consist of an interfacial barrier-like film, up to 1000 nm thick, an inner denser layer and an outer porous layer containing large cavities.25 PEO processes can form coatings across a wide range of thicknesses up to several hundreds of microns,26–28 commonly with a linear dependence on treatment time.22

Even though PEO coatings provide relatively good corrosion resistance for light alloys, in the long-term the corrosive solution penetrates through coating defects, such as cracks and open pores, and propagation of corrosion may then occur beneath the coating.29,30 Furthermore, PEO treatments appear to affect the mechanical properties of the alloy, due to the influences of the interface roughness and the porous coating structure.31,32 In order to improve the corrosion performance, sealing agents, such as organic coatings or sol-gels, have been used to fill the pores and cracks.33 Porosity also affects the coating hardness,34 the wear resistance35,36 and the thermal conductivity.37,38 It is also an important factor in biocompatible PEO coatings39,40 and in PEO coatings used for joining materials, such as light metals to plastics.41,42 Thus, detailed knowledge


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of the porosity of coatings assists the development of coatings for a wide range of application areas. Curran and Clyne43 evaluated the porosity of a PEO coating formed on AA6082 aluminium alloy by a commercial process, using scanning electron microscopy (SEM), density measurements, isothermal nitrogen adsorption and mercury intrusion porosimetry. From SEM observations, they estimated a porosity level of ~5%. However, at high magnification, a network of sub-micron pores was revealed. Surface-connected porosity of approximately 20% of the coating volume was determined from the other techniques employed, with an average pore size of ~30 nm; most of the pores lay in the size range from 5 nm to 1 µm. However, they had difficulties in assessing the errors of the measurements and also found discrepancies between data obtained from the different techniques.43 Wheeler et al.44 investigated the micromechanical behaviour of coatings on a Ti–6Al–4V alloy prepared by PEO in four different electrolytes; in contrast to Curran and Clyne, they reported that pores ranged from 1 to 10 µm for all four coatings. Moon et al.45 found an epoxy replica technique beneficial for observation of the 3-dimensional network of openpores in a PEO coating on AZ31 magnesium alloy. The replica revealed a network of irregularlyshaped large pores in the middle part of the coating and small pores near the film/alloy interface; in addition, spherical pores were discovered that were connected to the coating surface through channels of a few microns in diameter.

In view of the relatively few detailed reports on the porosity in PEO coatings and the importance of the porosity to the properties of the coatings, the present study examines the potential of X-ray CT for characterizing the coatings. The three-dimensional nature of X-ray CT imaging allows the volumes and shapes of the pores to be quantified46 and their distribution in relation to features at the coating surface and coating thickness variations to be determined. X-ray microtomography has been employed previously to generate 3-dimensional images of the porosity distribution in a PEO coating on aluminium and an aluminium alloy.47 However, the pore sizes and porosity volume fractions were not determined. Neither was the pore imaging validated by an independent imaging method. Furthermore, to the authors’ best knowledge, X-ray microtomography has not been used to study PEO coatings on titanium, which can be more challenging than aluminium as the mismatch in X-ray absorption between the titanium substrate 4 ACS Paragon Plus Environment


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and the PEO coating is much more significant. Unlike the previous X-ray microtomography investigation that employed a synchrotron source,47 the present study employs a laboratory-scale X-ray system. A PEO coating formed on titanium in an aluminate-phosphate electrolyte was chosen for examination. The composition, morphology and structure of the coating have been reported previously;48 furthermore, other work has shown the significant influence of large pores in the inner coating on the failure of the adhesively bonded, PEO-pre-treated substrates.49 The coating exhibits a range of pore size, from sub-micron to several tens of microns, that allow assessment of the resolution of the X-ray CT method. The morphology of selected features within the coating determined by X-ray CT is correlated with observations of the same features by scanning electron microscopy in order to assess the resolution limitations of the technique.

EXPERIMENTAL SECTION

Coating preparation

Commercial purity (99.6%) titanium foil of 125 µm thickness, containing 1500 ppm O, 1500 ppm Fe, 200 ppm C, 120 ppm N, 50 ppm H and Ti balance, was obtained from Goodfellow. Rectangular specimens of dimensions 1 x 5 cm were cut from the sheet, degreased with acetone, rinsed with deionized water and dried in flow of air. A working area ~1.0 cm2 was defined using lacquer (Stopper 45 MacDermid).

An AC PEO treatment was carried out in a volume of 1L of stirred electrolyte contained in a double-walled glass cell, with water cooling to maintain the electrolyte temperature in the range 293 to 303 K. The counter electrode was a sheet of type 304 stainless steel 7.5 x 15 cm in size. The coating was formed for 900 s at a constant RMS current density of 500 mA cm-2, with a square waveform (positive to negative current ratio of -1) and a frequency 50 Hz, using an ACSFB power supply (ET systems electronic GmbH). The electrolyte was prepared by dissolving 10.5 g l-1 NaAlO2 (anhydrous sodium aluminate technical grade, Al2O3: 50-56%, Na2O: 40-45%, Sigma-Aldrich), 4.75 g l-1 Na3PO4.12H2O (sodium phosphate tribasic dodecahydrate, ACS reagent ≥ 98.0%, ≤10 mg kg-1 Cl, Sigma-Aldrich), and 3 g l-1 KOH (Fisher Scientific: analytical 5 ACS Paragon Plus Environment

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grade reagent:

X-ray Computed Tomographic Investigation of the Porosity and

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