Proceedings of the 9th International Symposium on

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modified from the ASTM C633-79 and consisted of bonding a steel dolly to the coated face of the test specimen (for this only one face was coated) and another ...
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SOME PRELIMINARY EVALUATIONS OF BLACK COATING ON ALUMINIUM AA2219 ALLOY PRODUCED BY PLASMA ELECTROLYTIC OXIDATION (PEO) PROCESS FOR SPACE APPLICATIONS S. Shrestha (1), A. Merstallinger (2), D. Sickert (3) and B. D. Dunn (4) (1)

TWI Ltd, Granta Park, Great Abington, Cambridge CB1 6AL, United Kingdom, Email: [email protected] (2) ARC Seibersdorf research GmbH, A-2444 Seibersdorf, Austria, Email: [email protected] (3) Dresden University of Technology, Germany, Email: [email protected] (4) European Space Agency, Keplerlaan 1, NL-2201AZ Noordwijk ZH, The Netherlands, Email: [email protected]

ABSTRACT This paper describes the results of a study of a black coating produced on aluminium AA2219 alloy using a process that involves creation of a hard ceramic oxide layer on the surface of the alloy by plasma electrolytic oxidation (PEO) known as the ‘KERONITE“’ process. Coating microstructure has been examined and the coating characteristics such as porosity, hardness, adhesion and phase composition were measured. The thermo-optical properties such as solar absorptance ‘Ds’ and normal infrared emittance ‘Hn-IR’ of the coating were measured in the ‘as-prepared’ condition and after environmental exposures to humidity, thermal cycling and UV-radiation in vacuum and to thermal shock. Comparison was made with alternative coatings produced using standard black anodising processes. The study also looked at the cold welding and friction behaviours of the coated alloy in vacuum and in an ambient laboratory environment. Standard spacecraft materials tests were conducted on the coated disc against an AISI 52100 steel ball and also against a coated pin using a pin-on-disc apparatus. Parameters such as friction coefficient and wear depth were measured and the cold welding behaviours were investigated. Test results were compared with the data generated for NiCr plated and anodised coatings. Corrosion performance was assessed using a salt spray exposure test and using an accelerated electrochemical test method. In addition, the study looked at the effect of post coating sealing with a sol-gel solution. 1.

INTRODUCTION

Coating application by electrolytic methods such as anodising has been extensively used for improving the wear (abrasion) and corrosion resistance of aluminium alloys. The porous nature of anodised coatings allows production of coloured coatings by deposition of organic dyestuffs or metallic pigments [1]. For space applications, additional coating characteristics such as black colour effect to improve thermo-optical

properties e.g. solar absorptance, infrared emittance and improved surface characteristics to resist against cold welding are considered necessary. A widely used coating technique to date for space applications is black anodising that has been considered to provide aluminium alloys with suitable thermo-optical properties and acceptable resistance to corrosion, wear and cold welding. However, the conventional black anodising process is considered environmentally not safe and the black colour finish for long-term exposures is not stable. Thus, there exists a strong need to identify new coating processes that can meet the requirements of recent environmental legislation and the continual drive for better coating performance. The ‘KERONITE“’ process is one of the relatively new environmentally safe electrolytic coating processes that is applicable to light metals and their alloys in particular Al and Mg. The ‘KERONITE’ process involves the application of a modulated voltage to the component in an electrolytic bath agitated using compressed air. The voltage is sufficiently high to create intense plasma due to microarc generation at the component surface. This results in oxidation of the component surface (plasma electrolytic oxidation) as well as elemental codeposition from the electrolyte solution, which creates a hard ceramic oxide layer on the substrate alloy. A typical Keronite coating consists of a porous outer layer and a low porosity main layer that is the bulk of the coating. The top porous layer is often removed by polishing to expose the main layer. The electrolyte is a low concentration alkaline solution of proprietary composition. Similar coating processes also known as micro arc oxidation (MAO) have been reported elsewhere [2] 2.

EXPERIMENTAL APPROACH

2.1 Materials and coating characteristics An aluminium alloy type AA2219 of composition bal Al, 6.6Cu, 0.3Mn, 0.1Si, 0.01Mg, 0.1Fe, 0.1V and 0.1Zr was selected in this work as a substrate material.

———————————————————————————————————— Proceedings of the 9th International Symposium on Materials in a Space Environment Noordwijk, The Netherlands, 16-20 June 2003 (ESA SP-540, September 2003)

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Coatings to a thickness of 75-80Pm were prepared at Keronite Ltd on disc specimens of various sizes cut from a 6mm rolled AA2219 plate. The disc specimens were ground to 600-grit finish using SiC paper and the edges of the discs were rounded to 2mm radius prior to coating deposition. One face of the coated specimen was polished to a final coating thickness of about 6070Pm after removing a top porous layer. This involved manual polishing of the coating face successively on 360 and 600 grit SiC papers followed by final polishing in 1Pm diamond slurry to Ra ~0.04Pm. The coating after polishing was rinsed in running water and degreased with acetone. These are referred to ‘asprepared’ coatings. In addition, the coatings were subjected to post sealing by dipping in a colourless solgel solution followed by subsequent further polishing to remove the top sol-gel layer. This resulted in the exposure of the Keronite surface while the pores remained impregnated and sealed with sol-gel. The latter coatings are referred to ‘sealed’ coatings. Coating cross sections were prepared, ready for examination using standard metallographic techniques to a 1Pm diamond polishing. A scanning electron microscope (SEM) was used for coating microstructure characterisation. Coating porosity was measured from the polished cross-section using the In2ViewCat-Pro image analyser connected to an optical microscope. Coating hardness was measured from the polished cross section using a DURAMIN Vickers diamond pyramid micro-indenter from Struers Ltd and using a 50g indentation load. Determination of various crystalline phases in the coating was undertaken using the X-ray diffraction (XRD) technique by collecting spectrum from the coating surface using CuKD radiation. Coating adhesion measurements were undertaken following the guidelines as described in the ASTM standard C633-79 (Re-approved 1993) ‘Standard test method for adhesion of cohesive strength of flame sprayed coatings’. The test method was slightly modified from the ASTM C633-79 and consisted of bonding a steel dolly to the coated face of the test specimen (for this only one face was coated) and another identical steel dolly bonded to the rear uncoated face of the test specimen. The test specimen was sandwiched between the two steel dollies using an epoxy film adhesive, type FM73 and heat cured at 120°C. The bonded test specimens were then individually placed in a tensile loading machine (Dartec) with self-aligning devices. Tensile load was increased at 1mm.min-1 and the load at failure recorded. The failure stress was calculated and the nature of failure, whether at the coating/substrate interface, coating cohesion or in the adhesive, was examined. Adhesion measurements were taken also

from the coated specimens that had been subjected to 336 hours of salt spray exposure. The coating peel-off test followed the procedure as described in the European Cooperation for Space Standardization ECSS-Q-70-13A ‘Measurement of the peel and pull-off strength of coatings and finishes using pressure sensitive tapes’ [3]. This test method is based on the controlled peeling of a pressure sensitive tape from the sample surface (‡25mm) using a tensile loading machine. The maximum failure stress was recorded and the sample surface as well as the tape was examined to look at particles detached from the coating surface that may have adhered to the tape. The tape used was double sided 3M-pressure sensitive tape type 600 (670g.cm-1). Details of the test procedure are described in ECSS-Q-70-13A [3]. This method was used to look at dust generation from the as-prepared coating surface and the coating that had been exposed to thermal shock. 2.2 Environmental exposure Humidity During the humidity test coated samples were exposed for one week to 95% relative humidity (RH) at 50°C. The equipment used was a Heraeus Vötsch humidity chamber. Thermal cycling in vacuum The thermal cycling test was performed according to ECSS-Q-70-04A [4]. It comprised 100 cycles of exposure from +100 to –100°C in vacuum. The heating and cooling rate was 10°C·min-1 with the dwell time of 10 minutes at the maximum and minimum temperatures. UV-exposure in vacuum The UV-exposure was carried out in a vacuum chamber at a pressure between 10-6-10-7mbar and at a temperature of 22-25°C. The coating sample was irradiated using a Hamamatsu deuterium lamp having a wavelength spectrum from 115 - 400nm. The distance of UV-source to the coating surface was about 370mm. The exposure period was 167 hours, which is about 6513 equivalent sun hours (ESH). Details of this procedure are described in the literature [5]. Thermal shock Two separate thermal shock exposure tests were undertaken. The first exposure test was conducted at the European Space Agency, which comprised of transferring the coated specimens from a heated oven (+50°C) to a bath of liquid nitrogen (-196°C). This was repeated 10 times with a dwell time of 10 minutes at each temperature. The second exposure test was performed at TWI Ltd. This consisted of alternate

immersion of the coated specimens at two temperature extremes: a boiling de-ionised water bath maintained at +100°C and a liquid nitrogen bath at -196°C. The coating specimens were manually transferred and immersed 50 times in each bath with a dwell time of five minutes in each bath. The maximum time during the transfer between the two baths was 10 seconds. The specimens were degreased with alcohol before and after the thermal shock tests followed by air-drying. Salt spray exposure The corrosion performance of the coated and uncoated test specimens was examined by exposing the specimens to a salt spray environment following the guidelines described in the ASTM standard B117-97 up to 2000 hours of exposure period. The testing comprised of exposing the coating surface up to 2000 hours to a 5wt% NaCl solution (at 35°C) atomised to create a fog within an enclosed chamber. Changes to the coating surface were recorded following periodic observations at 24, 336, 1000 and 2000 hours. The surface quality was given a rating number in accordance with ASTM D1654-92 ‘Evaluation of painted or coated specimens subjected to corrosive environments - procedure B’. Corrosion resistance to a minimum of 336-hour test duration is defined in the Aerospace Materials Specification AMS 2470J (R) ‘Anodic treatment of aluminium alloys - chromic acid process’ as acceptance criteria [6]. Electrochemical corrosion The electrochemical corrosion behaviour was studied using an accelerated potentiodynamic test (anodic polarisation) in a de-aerated 3.5% NaCl solution of pH 8 at 25°C using a standard three-electrode test method in an Avesta cell. The coated surface was exposed to the electrolyte and the rest potential ‘Ecorr’ (also known as the free corrosion potential) was allowed to stabilise for one hour prior to anodic polarisation. The potential value was measured using a reference saturated calomel electrode (SCE) and a plot of the current density as a function to the polarisation potential was recorded using a platinum auxiliary electrode. More detail of the test procedure can be found elsewhere [7]. The collected polarisation plots were used to compare the electrochemical corrosion behaviour of the coatings with uncoated alloys and the integrity of coatings before and after a thermal shock exposure. 2.3 Measurement of thermo-optical properties Solar absorptance ‘DS’ The absolute reflectivity spectrum of the specimen was measured using a Cary-500 UV-VIS-NIR photospectrometer with an integrating sphere accessory and corrected with the help of a reference standard. The solar reflection coefficient was calculated by

59 integrating the measured absolute reflectivity spectra over the solar energy spectrum [8] within the considered wavelength range from 250 - 2500nm. This wavelength band comprises 97% of the energy radiated by the sun. The result of this integration was divided by the total solar energy thus providing the ratio of reflected to incident solar energy (RS). This can be translated into the solar absorptance ‘.S’ by subtracting it from unity for completely opaque samples, i.e. if the transmittance equals zero. More details on this can be obtained in the references [9, 10].

Normal infrared emittance ‘Hn-IR’ The normal infrared reflectivity (Rn-IR) of the coating surface was measured using a Gier-Dunkle DB100 infrared reflectometer. According to Kirchhoff’s radiation law the infrared emittance of a material equals its infrared absorption when maintained at the same temperature. Furthermore the amount of radiation, not reflected by non-transparent materials, equals the absorbed portion of radiation. The measurement device consisted mainly of an internal thermal source that emits infrared radiation towards the coating surface and a detector for the amount of radiation reflected. Applying the above-mentioned relations this value is translated into the normal infrared emittance ‘0n-IR’ by subtracting it from unity. More detailed description of the physical basics can be found in the literatures [9, 10]. 2.4 Cold welding and friction behaviour For the cyclic impact adhesion test, a contact was made between a vertically moving pin. The pin was either AISI 52100 bearing steel of nominal composition bal Fe, 1C, 0.3Si, 0.4Mn, 1.6Cr, 0.3Ni, 0.3Cu or coated aluminium alloy having a radiused contact area against a fixed flat disc (coated or uncoated aluminium alloy) for about 5000 cycles. Each cycle consisted of three steps: i) an impact loading; ii) a static load held for 10 seconds at a vacuum pressure of less than 5.10-8mbar; and iii) unloading with measurement of the separation force, i.e. adhesion force. For the cyclic fretting adhesion test, a contact (without impact) was made followed by a static load, which is held for 10 seconds at a vacuum pressure of less than 5.10-7mbar. During this time fretting is applied: the pin was moved with a sinusoidal frequency at 210Hz and a stroke of 50Pm in each cycle for a total of 5000 cycles. After stopping fretting, the force required to separate the pin from the disc was measured. Tests were conducted on the coated and uncoated test specimens. The uncoated test specimens were fine ground to surface roughness of Ra < 0.03Pm. The coated face was in the polished condition to Ra ~ 0.04Pm.

60 Sliding wear tests were conducted using a vacuum pinon-disc tribometer manufactured by CSEM. The pin (ball) was allowed to slide on the disc surface in unidirectional movement at a sliding speed of 0.1ms-1 and a sliding distance of 1000m. Vacuum pressure was maintained at less than 10-6mbar and the temperature at 25qC. The ball diameter was 7mm and the load was 10N. More details on the testing procedures are given in the literatures [11, 12]. In addition, sliding wear tests were undertaken also in an ambient laboratory environment (22r2qC, relative humidity 35-45%) with a test load of 10N and sliding speed of 0.1ms-1 against a steel ball (AISI 52100) of diameter 10mm for a 1000m sliding distance. This followed the guidelines described in ASTM G99-95a ‘Standard test method for wear testing with a pin-on-disc apparatus’. Friction coefficients and wear track depths where possible were measured.

3.

RESULTS AND DISCUSSIONS

3.1 Coating characterisation prior to exposure A backscattered SEM image of a sealed coating cross section is shown in Fig.1, which shows a relatively dense coating. This image is of the main coating layer after removing the top porous layer (not shown), which also reveals a microstructure comprising of interlocking grains about 20-30Pm in size with occasional very small-sized pores within the grains and a few very fine microcracks. The coating porosity was measured at about