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Applying high voltage cathodic pulse with various pulse durations on aluminium via micro-arc oxidation (MAO)
Accepted Manuscript Applying high voltage cathodic pulse with various pulse durations on aluminium via micro-arc oxidation (MAO)
Mustafa Safa Yilmaz, Orhan Sahin PII: DOI: Reference:
S0257-8972(18)30461-4 doi:10.1016/j.surfcoat.2018.04.085 SCT 23369
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
15 August 2017 25 April 2018 28 April 2018
Please cite this article as: Mustafa Safa Yilmaz, Orhan Sahin , Applying high voltage cathodic pulse with various pulse durations on aluminium via micro-arc oxidation (MAO). The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2018.04.085
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ACCEPTED MANUSCRIPT Applying High Voltage Cathodic Pulse with Various Pulse Durations on Aluminium via Micro-Arc Oxidation (MAO) Mustafa Safa YILMAZ1, * and Orhan SAHIN2 1
Fatih Sultan Mehmet Vakif University, Aluminium Test and Training Centre, Istanbul, Turkey
Gebze Technical University, Department of Materials Science and Engineering, 41400
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1
Kocaeli, Turkey
*Corresponding author: Tel: +90 212 521 81 00 (4336) E-mail: [email protected], [email protected]
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Abstract
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The present study is an investigation of the effect of pulse duration on properties of coatings formed on aluminium alloy (Al) in alkaline solution via using of high voltage cathodic pulses.
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Nine different anodic-cathodic pulse couples with various durations were selected for processing. In order to find out the effect of pulse duration on coating: duty cycle, anodic
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voltage, cathodic voltage, processing time, electrolyte temperature and electrolyte composition were kept constant. Scanning electron microscope (SEM), profilometer, X-ray
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diffractometer and indenter were employed to investigate the microstructure, surface
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roughness, phase distribution and hardness of the coatings. Also, coating thickness measurement was carried out to find out the effect of pulse duration on productivity. Varying of pulse duration caused significant differences among thickness of coating layers. Coating layers with thickness between 45-75 μm and surface roughness between 2.5-5.5 μm were obtained. Coatings consist of two layers, outer porous layer and inner dense layer. The outer porous layer was γ-Al2O3 and the inner dense layer was a mixture of γ-Al2O3 and α-Al2O3. The hardness of inner dense layers was between 1100-1600 Vickers. Keywords: Aluminium, micro-arc oxidation, pulse duration, coating
ACCEPTED MANUSCRIPT 1. Introduction Aluminium and its alloys are important structural materials for automotive, aerospace and transportation industries. The interest in aluminium alloys comes from its high strength to weight ratio, high corrosion resistance and lightweight. Nevertheless, their disadvantages are low wear resistance, low hardness and high friction coefficient, which limit their applications
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[1]. Therefore, to improve their surface properties various surface modifications techniques have been developed. One of these techniques is Micro-Arc Oxidation (MAO), in which a thick and hard alumina layer strongly adhered to aluminium substrate could be fabricated. Mechanisms of oxide layer formation, that is, micro discharge; discharge channel and oxide
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formation have been extensively studied by many researchers [2-6]. MAO processing is a combination of electro-chemical reaction, plasma-chemical reaction and thermal diffusion [2,
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7-9]. The electro-chemical reaction and plasma-chemical reaction take place at metal oxideelectrolyte interface and inside discharge channel respectively.
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The properties of the oxide layer formed as a result of MAO depends on processing
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parameters, such as composition of electrolyte, electrical parameters of pulses, chemical composition of substrate, processing time, etc. [8-15]. The effects of the process parameters,
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especially the electrical parameters of pulses on oxide layer formation are still in the stage of research and development. Electrical parameters include alternative current, direct current and
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various forms of unipolar and bipolar pulses and their frequencies. Currently it had been reported that bipolar current pulse mode forms thicker and more uniform coatings than unipolar current pulse mode [16]. Krishna et al. [17] found that the thickness’ of oxide layer was increased with both increased processing time and pulse current density. Yerokhin et al. [13] found out that frequencies ranging from 1 to 3 kHz significantly increased the growth rate of the oxide layer and reduced the size of the outer porous layer. Dehnavi et al. [14] reported that the coating growth rate increases gradually with decrease in duty cycle at constant frequency.
ACCEPTED MANUSCRIPT Qingbiao et. al. [18] investigated the effect of cathodic voltage on growth rate and wear resistance of coating. During processing anodic voltage was kept constant and it was 520 V. Cathodic voltages was in between 0 V and 190 V. Above a cathodic voltage of 90 V, an oxide layer consisting of two sublayers was obtained. Sublayers were described as compact layer and porous layer. Main component of coatings was γ-Al2O3. Growth rate of coating, thickness
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of compact layer and wear resistance significantly increased with increasing applied cathodic voltage. On the contrary, surface roughness of coating decreased with increasing cathodic voltages.
Hussein et al. [8, 9] studied the effect of unipolar current pulses, bipolar current pulses and
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bipolar current pulse duration on the microstructure of oxide layer. It comes out that the pulse duration of bipolar pulses has a significant improvement on coating quality. The ranges of
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pulse duration used in above study were 400 µs and 400-600 µs for anodic and cathodic pulse respectively. In another study a cathodic voltage of 200V was used, it was showed that the
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processing time and the anodic-cathodic pulse durations have a significant effect on growth rate and properties of oxide layer [19]. The aim of the present study is to extend the
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investigation on the effect of increasing cathodic pulse voltage and its duration on oxide layer
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formation.
ACCEPTED MANUSCRIPT 2. Experimental Procedure 2.1. Instruments Used MAO device used for processing has 2 separate High Frequency Converter (HFC), in which one of them was set to positive and the other one was set to negative polarity. The maximum power of the HFC is 12.5 kW. These HFC’s have the capability to supply voltage up to 800
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V. Device was designed to operate by keeping voltage of anodic and cathodic pulses as constant. Bipolar pulses (anodic-cathodic pulse couples) were used for all experiments. Pulse generator is capable to form bipolar square pulses and adjust parameters of cathodic and anodic parts independently from each other. Pulse parameters, such as pulse amplitude, pulse duration, frequency (that is, number of pulses), pause between cathodic-anodic pulse of a
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pulse couple and pause between anodic-cathodic pulse couples could be set by a computer
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program at the beginning of processing. The pauses between anodic-cathodic pulse couples are identical. Frequency between anodic-cathodic pulse couples was determined according to
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the electricity consumed for the whole process, which is total amount of electricity of anodiccathodic pulse couples. During processing, instantaneous amplitudes of anodic-cathodic
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current pulse couples could be observed on a monitor and these current pulses of processing
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(oscillograms) could be recorded at any time. The schematic diagram of the MAO device was given in Fig. 1. Tektronix-TDS 2024C digital
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storage oscilloscope was used for monitoring the shape of current pulses, pulse amplitude and pulse duration during MAO processing. The cooling unite, which has circulating water in its spiral copper tube, was employed to keep electrolyte temperature as constant. FisherDualscope MP40E-S and Mitutoyo SJ-400 profilometer were used for coating thickness and surface roughness measurements respectively. Brukers D8 (40kW, 40mA) type X-ray diffractometer was used for identification of the phases in oxide layer. X-ray diffraction (XRD) was operated with Cu Kα radiation. Between 20° to 90° were scanned with an increment of 0.02° and account time of 1s. Cross-sectional microstructure of oxide layer were
ACCEPTED MANUSCRIPT analysed by Philips XL30 field emission Scanning Electron Microscopy (SEM). Crosssectional hardness of oxide layer was measured by using Mitutoyo MicroWizhard microhardness tester. 20 g indentation load was preferred for hardness measurement. The elemental analysis of the aluminium substrates was determined with Spectro-Max LMF14. 2.2. Specimens and Electrolyte
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Round bar aluminium alloy were subjected to surface modification. The composition of Al alloy was Al (98.6 %), Si (0.47 %) and Mg (0.46 %). Other elements were quantitatively not significant. The diameter and length of the bars were 1.0 cm and 1.0 cm respectively. The surface area processed was nearly 4 cm2. Before processing, specimens were mechanically
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ground with 200-1200 mesh emery papers and cleaned ultrasonically for 5 minutes in acetone. The electrolyte used for coating was KOH (2 g/l), Na2SiO3.5H2O (9.5 g/l) and its temperature
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was kept at 25°C ±5.
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2.3. Electrical processing parameters
Since the aim of the present study was to determine the effect of cathodic and anodic pulse
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durations on oxide layer formation other electrical parameters, anodic voltage, cathodic
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voltage and duty cycle were all kept constant and they were given in Table I. Furthermore, energy consumed for each experiment was same and it was 6 Coulombs, which includes both
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anodic and cathodic pulses. Equivalence in consumed energy of experiments was obtained by decreasing number of applied anodic-cathodic pulse couples as the duration of pulses increased. That is, frequency of applied pulse couples was highest for the experiment with lowest pulse duration and it was lowest for the experiment with highest pulse duration. This operation is automatically done by instrument. The energies of the pulse couples were given in Table. I. The values of anodic voltage and the duty cycle were decided according to the research data given in [2, 3, 5, 14]. Cathodic voltage of 400 V was intentionally selected in order to
ACCEPTED MANUSCRIPT compare present experimental results with the results of previous studies, in which cathodic voltage was 200V [18, 19]. The main reason of this selection is to understand the influence of increasing cathodic voltage on coating properties. Voltages of anodic and cathodic pulse couples were set to 500 V and 400 V respectively and MAO unit is capable of keeping them as constant during processing. However, anodic and
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cathodic pulse currents and therefore current densities vary depending on the instantaneous resistance of electrochemical process. The pause between anodic and cathodic pulses of the pulse couple was 500 μs and it was same for all experiments. As an example, input of experiments of number 2 and number 5 was given in Fig. 2. Nine different set of pulse
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durations of anodic-cathodic pulse couples were employed. The processing time for all experiments was 20 minutes. For each experiment, oscillograms showing instantaneous
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current of process were recorded after 1, 5, 10, 15 and 20 minutes of processing. That is, 9 experiments, 5 current pulse records for each, it comes up totally 45 records. As an example,
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2 of them, which show the conditions of current pulse after 10 minutes processing, were given in Fig. 3, which belongs to E2 and E3. The durations of anodic-cathodic pulse couples for
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each of nine experiments were given in Table I. For each pulse parameter given in Table I,
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experiment was carried out at least two times.
ACCEPTED MANUSCRIPT 3. Results and Discussions Coating thickness of experiments was measured by the employment of two different methods. Results of both methods of thickness measurement were given in Fig. 4, in which thickness of coating curves were plotted as a function of pulse durations. First method is the direct measurement of thickness by a device that use eddy current (Line-d). The thickness of the
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oxide layer was the average of 20 eddy current measurements from different locations. Second method is a detailed thickness measurement carried out on cross-sectional SEM images of coating layers. Regarding morphological appearance, cross-sectional microstructure was divided to 3 distinct regions. The irregular boundaries between the regions were visually
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marked with great care as in Fig. 5 under high magnification and named as Line-a, Line-b and Line-c. With the help of graph paper mean location of boundaries between regions were
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determined. That is, a straight line, which is parallel to the coating-substrate interface, represents an irregular boundary. The distances between straight lines’ and coating-substrate
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interface were measured to determine thickness of each region. For each experiment, images from 3 different locations were recorded. After marking the boundaries between zones, for
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each image the thickness of the regions were measured and the average thickness of the
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regions were calculated depending on the measurement of 3 different locations of same specimen. As an example, only Fig. 5 was given to show clearly how the boundaries between
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regions were marked. Same measurements were carried out to determine average thickness of regions for all specimens. Regions were named as inner dense layer (below Line-b) and outer porous layer (between Line-b and Line-c). Similarly, depending on morphological appearance, inner dense layer could also be divided two zones. Usually in most of the studies on this kind of coatings, coating layer was named as inner dense and outer porous layer [8, 13, 15, 17]. Therefore, in order to prevent any confusion, the regions of inner dense layer were named as zones. Inner zone next to the substrate has a granular appearance (below Line-a). Other zone between
ACCEPTED MANUSCRIPT outer porous layer and inner zone has a smooth appearance (between Line-a and Line-b). The difference in appearance of zones could be related to the mechanism of molten oxide formation and its solidification. The boundary between two zones and the boundary between inner zone and outer porous layer were marked as in Fig. 5. SiC and α-Al2O3 powders were used for cross-sectional polishing of coatings. Since SiC is
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harder than both α-Al2O3 and γ-Al2O3, SiC polishing produced a smooth cross sectional surface. In case of α-Al2O3 polishing, which is the case of present study, part of the coating adjacent to substrate does not have a smooth surface, which is named as granular zone above, Fig. 5. It is anticipated that granular zone could be consisting of α-Al2O3 and γ-Al2O3. Since,
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hardness of γ-Al2O3 was lower than that of α-Al2O3, γ-Al2O3 could be eroded more than αAl2O3. Therefore, formation of granular appearance was attributed to difference in erosion
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rate of α-Al2O3 and γ-Al2O3. The other zone of inner dense layer probably was mainly consisting of γ-Al2O3 phase. Therefore, erosion was almost uniform. In order to clarify this
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point, XRD analysis was carried out by using a specimen coated under condition of E2. Results of 3 XRD measurements were given in Fig. 6. After the first XRD measurement of as
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coated surface, coating thickness was reduced to 50 μm and later reduced to 28 μm. The
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original as-coated thickness, which was measured by eddy current, was 75 μm. 50 μm and 28 μm thicknesses were preferred for XRD measurement, because they approximately coincide
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with the thicknesses of inner dense layer and inner zone respectively. The α-Al2O3 peaks at 2 positions of 25.7°, 35.2° and 57.6° degrees and similarly γ-Al2O3 peaks at 2 positions of 46.0° degree in Fig. 6, shows that both zones are a mixture of α-Al2O3 and γ-Al2O3 and XRD results clearly shows that volume percent of α-Al2O3 phase is higher in inner zone. This qualitative evaluation based on the difference in height of the α-Al2O3 peaks. At the moment, it has not been investigated yet, either α-Al2O3 is uniformly distributed or across the inner zone, there is a distribution gradient beginning with higher α-Al2O3 concentration at the vicinity of substrate-coating interface.
ACCEPTED MANUSCRIPT As mentioned above the difference in appearance apparently related to the difference between microstructure of zones. The boundary between zones is not straight. In other words, in threedimensional view the interface between the zones is not planar. Because, molten alumina forms intensively at the location of micro channels next to substrate and solid alumina formation, either γ-Al2O3 or α-Al2O3 phase proceeds faster along the micro channels.
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Probably α-Al2O3 intensively forms at the bottom of micro channels. Eventually, a non-planar interface forms between α-Al2O3 rich and γ-Al2O3 rich zones.
Oscillograms (amplitude and duration) of current pulse could be used to judge the developments during processing, which is oxide layer formation. Current densities were
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calculated by dividing experimentally measured current to the processing surface area of specimens, which is 4 cm2. For the experiments E1 to E6, the average value of anodic current
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density was almost stationary during 20 minutes of processing time and it was more or less 8 A/cm2. However, for the experiment E7 to E9 the current densities were also almost
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stationary but 25 % lower than that of previous experiments and it was more or less 5 A/cm2. Similar stationary behaviour was observed for cathodic current density which was more or
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less 16 A/cm2 for the experiments E1 to E6 and it was 10 % lower for the case of E7 to E9
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and it was more or less 14.5 A/cm2. Current densities of all experiments were summarized in Table. I. Shapes of the cathodic current pulses were almost square. However, behaviour of
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anodic pulses was significantly different. At the very beginning of an anodic current pulses, current make a sharp peak and gradually ceased down but never be zero, Fig. 3. This was a general behaviour of anodic current pulses. Probably, during an anodic pulse oxide forms and thickness of oxide layer increases. As thickness of oxide layer gradually grows, the resistance of oxide layer (that is, process) gradually increases. As a consequent of this, anodic pulse current gradually decreases until the end of pulse duration. Same phenomena repeat itself for each of the following anodic-cathodic pulse couples. In case of E7 to E9 significant decrease in both currents could be due to significant increase in resistance of process.
ACCEPTED MANUSCRIPT It could be concluded that anodic-cathodic pulse currents were variable during each process; however variation in current was not significant. Therefore, at the time, no special significance could be attached to this variation in current. As a result, it is obvious that, main variable processing parameter effecting oxide layer formation is anodic and cathodic pulse durations.
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Cross-sectional thickness measurements on images showed that for the short pulse durations, thickness of coatings is slightly above 60 μm (E1and E2). Increasing of pulse duration caused thicker coating layers above 70 μm (E3, E4, E5 and E6) and with further increasing of pulse duration the coating thickness gradually decreased (E7, E8, E9). The total thickness of
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coatings (Line-c), including inner dense layer and outer porous layer, obtained by using SEM images were below the value of thickness obtained by direct measurement of with eddy
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current device (Line-d). The difference could be due to some loss of surface part of porous layer during grinding and polishing of specimens for SEM examination. Nevertheless, the
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tendency of the thickness curves of both methods is same.
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In case of E1 and E2, the thickness of dense layer is slightly above 50 μm and that of porous layer is above 10 μm. In case of E3, E4 and E5, there is a negligible increase in thickness of
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dense layer, if it is compared to that of E1 and E2. However, there is a significant increase in thickness of outer porous layer that is about 10 μm. For the case of E6 the thickness of dense
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layer was slightly lower than 50 μm and thickness of porous layer was slightly higher than that of previous experiments (E3, E4 and E5). The thickness of porous layer is about 27 μm, which is the highest porous layer among the experiments. For the experiments (E7, E8 and E9), the thickness of dense layer gradually decreased down below 30 μm. In case of porous layer, thickness was about 15 μm. Starting with E3, the number and size of porosities gradually increased till E7. In other word, the thickness of porous outer layer starts to increase with E3 and reached its highest value in
ACCEPTED MANUSCRIPT case of E6. Beyond E6, there is no increase in size of the pores however there is a decrease in thickness of both dense and porous layer. The pore formation probably is strongly related to the gas formation during micro charging. The pulse duration in turn pulse energy was lowest for E1 and highest for E9. Increasing in pulse energy probably increases gas formation and as a result of this, for E3 to E6, number and size of pores increases in both outer porous and
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inner dense layers [8, 15]. The amount of gas formation for the E7, E8 and E9 was so intensive, the large pores close to surface were destroyed which cause a decrease in thickness of porous layer.
The increase in coating thickness on E3, E4, E5 and E6 was probably not due to the increase
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in amount of molten oxide formation, but probably due to the increase in number and size of pores. The outer layer of coating is similar to a sponge with larger pores. Whenever the pores
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adjacent to the surface were destroyed, part of porous layer was lost and eventually the thickness of porous layer decreased, as in case of E7, E8 and E9. The weight of the oxide
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layer could had been measured with a precision balance, in order to determine if the increase in thickness as in case of E3, E4, E5 and E6 is due to the oxide formation or due to the
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increasing number and size of pores. Unfortunately this study was not carried out.
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As the pulse duration increased, two simultaneous phenomena took place; larger pores near the surface were destroyed and pore formation gradually extended towards substrate due to
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the increase in the amount of trapped gas. Meantime pore size became larger. Increasing pulse duration caused the thickness of inner dense layer to decrease and pore content of it increase gradually. As seen in Fig. 4, with increasing pulse duration thickness of inner dense layer decreased from 50 μm to below 30 μm (E7, E8 and E9). In a previous study [19], the amplitude of cathodic voltage was 200 V (Half of the present study) and the duration of pulse couples and processing time were same as that of the present study. The thickness of the previous study was approximately 25 μm without regarding pulse
ACCEPTED MANUSCRIPT durations. Comparing previous and present studies with each other shows that three times increase in thickness was obtained by two times increase in cathodic voltage. Surface roughness of coatings was given in Fig. 7. Roughness was obtained by averaging of 12 measurements from different locations. From Fig. 7, it is obvious that the surface roughness and corresponding coating thickness in Fig. 4 have similar behaviour. When
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thickness of outer porous layer increased due to increasing pore size, the surface roughness increased either. As known, the surface roughness usually increases as the coating thickness increases. Similarly when the coating thickness decreases surface roughness decreases either [19]. Same behaviour was observed in the present study.
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Some discharge channels could reach to the substrate through the coating layer as shown in Fig. 8. Molten substrate flows in discharge channels and reacts with the chemicals from
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electrolyte to form molten oxide. Part of molten oxide inside the discharge channel is ejected out through the channel due to high pressure. When it is reached to surface, it is cooled down
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by electrolyte and solidifies on surface in the form a swallow hillock right on top of the
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discharge channel. As a result of solidification of the molten alumina on surface, a porous surface layers with high roughness forms [20].
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The effect of pulse duration on alumina phase formation was investigated by XRD analysis
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and results were given in Fig. 9. Since X-rays were able to penetrate through the oxide layer, diffraction peaks of Al and Mg2Si were obtained from substrate. The intensity of Al and Mg2Si peaks decreased with increasing coating thickness, Fig. 9. The characteristic peaks show that the coatings consist of crystalline γ-Al2O3 and α-Al2O3 phases. The hardness of the oxide layer was the average of thickness through measurements from 3 different locations. For each thickness through measurement, hardness was measured along a line between substrate and the surface of the coating. The distance from the substrate-coating interface was taken as a reference point to calculate average hardness.
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Due to the nature of the process, as mentioned previously the outer layer has a porous structure, which is mostly consisted of γ-Al2O3 phase, while the inner layer is a relatively dense structure and it is a mixture of α-Al2O3 and γ-Al2O3 phases [6]. This could be observed from the hardness versus distance from substrate-coating interface curves, which was given in
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Fig. 10. For specimens E1, E3 and E5 hardness sharply decreases approximately at a distance of 50-55 μm away from substrate-coating interface. Similar behaviour was observed for specimens E7 and E9 at a distance of 33 μm and 24 μm away from substrate-coating interface respectively. According to Fig. 4, thickness of inner dense layer of E1 to E5 was slightly
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above 50 μm and that of E7 and E9 was 35 μm and 25 μm respectively. Comparison of Fig. 10 and Fig. 4 confirms that sharp decrease in hardness correspond to the location of
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boundaries between inner dense layers and outer porous layers. For some experiments, which were not included in present study, coating layers consisting of
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γ-Al2O3 phase without any detectable amount of α-Al2O3 phase were obtained. The hardness
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of these experiments is in between 800 HV and 1200 HV and therefore the average hardness is 1000 HV. On the other hand, it had been reported that under condition of MAO method,
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individual hardness of γ-Al2O3 and α-Al2O3 are 800 HV and 2200 HV respectively [21, 22]. Since hardness of γ-Al2O3 is around 800-1000 HV, part of the hardness curves over 1100 HV
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in Fig. 10 are undeniable evidences of the presence of α-Al2O3 phase in inner dense layer. Since properties of inner dense layer was major objective of the present study, the average hardness of inner dense layers were calculated and given in Fig. 11. Calculations based on data picked up from several points along a line, which is parallel to the substrate-coating interface. The distance between lines and interface was approximately 17±2 µm, 27±2 µm and 37±2 µm. Since, thickness of inner dense layer of E8 and E9 was less than 37 μm the hardness curve of 37±2 µm in Fig. 11 ended with E7. Even though inner dense layer thickness
ACCEPTED MANUSCRIPT of E7 is barely 35 μm its hardness was included in Fig. 11. As seen in Fig. 11, although behaviour of hardness curves is almost same, they do not exactly overlap with each other. Starting with 1300-1500 HV in case of E1 and E2, average hardness in Fig. 11 gradually decreased down slightly below 1100 HV for E4 and then gradually increased up to 1700 HV in case of E7.
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As explained previously inner dense layer is a mixture of α-Al2O3 and γ-Al2O3 phases and the amount of α-Al2O3 could relatively be higher in inner zone next to the substrate-coating interface as observed in Fig. 6. On the other hand, the distribution gradient of α-Al2O3 phase could be evaluated by not only referring Fig. 6 but also Fig. 11. If there has been a
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distribution gradient, hardness of the Line-A in Fig. 11 should be significantly higher than that of the Line-B and Line-C. However, this was not observed. Distance between hardness
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curves is not significant and curves were not clearly separated from each other. Consequently, Fig. 11 shows that the amount of α-Al2O3 content of inner zone next to the interface could
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more or less be higher than the α-Al2O3 content of rest of the coating, but the distribution of α-Al2O3 in inner zone is uniform. In order to clarify this point, an advance study is needed to
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through hardness.
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determine phases in inner dense layer quantitatively and to analyse variation in thickness
The value of hardness depends on both volume percent of phases and porosities [6, 13, 17].
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Although, the present study is not about pores, pores in different number and size were existed in all specimens. Instead of carrying out an advance quantitative porosity analysis including number and size of pores, a simple and quick ImageJ analysis was preferred to determine volume percent of pores in inner dense layers only. The results of the pore measurements were given in Table. 2. Starting with E1 volume percent of pores gradually increases till E5 and beyond E5 it gradually decreases.
ACCEPTED MANUSCRIPT Referring to Fig. 4, Fig. 7 and Table. 2, experiments could be separated as three groups. Group-1 and Group-2 cover experiments E1-E2 and E3 to E6 respectively. Group-3 covers experiments E7 to E9. The distinct property of Group-1 is an inner dense layer with lowest volume percent of porosity. Group-2 has slightly thicker inner dense layer than Group-1. However, comparing with Group-1, Group-2 has significantly higher volume percent of inner
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dense layer porosity. Moreover, Group-2 has significantly thicker outer porous layer than the other groups. When Group-3 is compared with Group-1 and Group-2, the thickness of coatings and inner dense layers are significantly lower than that of other groups. Cross sectional SEM images of all specimens were given in Fig. 12. By a careful visual
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examination of SEM images, one could notice the decrease in coating thickness, decrease in inner dense layer thickness and change in microstructural morphology including pores and
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their distribution. All surfaces have an island-like morphology, which each of islands has a pore inside. Some of these ball shaped islands released gas through an opening on the top or
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sometimes released gas as a result of explosion. Consequently, size and final form of islands
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have a remarkable effect on surface roughness [1]. Pulse durations used in E3, E4, E5 and E6 which are longer than that of E1 and E2, generated
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slightly thicker coating. Further increasing of the anodic-cathodic pulse durations, as in case of E7, E8 and E9, had already caused thinner coating layers. Besides, there is almost no
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granular appearance of inner zone on cross-sectional morphology of E9, Fig. 12. The aim of increasing pulse duration is to increase energy of pulse and therefore to provide more heat for process. It was hoped that increasing amount of heat in turn temperature could promote α-Al2O3 formation. It was reported that α-Al2O3 formation concentrated in inner dense layer of coating next to the substrate, γ-Al2O3 formation distributed all over the coating layer including porous layer [2, 13]. XRD results confirmed that α-Al2O3 and γ-Al2O3 were formed for all experiments. Unfortunately, XRD is not capable of determining the exact
ACCEPTED MANUSCRIPT location of α-Al2O3 grains and exact location of γ-Al2O3 grains. As mentioned earlier in Fig. 6, α-Al2O3 content of inner zone is higher than that of rest of the coating. Another evidence of α-Al2O3 formation is the hardness test results. Average hardness of only dense layers was calculated for each of pulse durations, Fig. 11. As mentioned above, for alumina coating obtained by MAO average hardness of γ-Al2O3 is around 800-1000 HV and that of α-Al2O3 is
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around 2000-2200 HV. In spite of the presence of pores, hardness of the specimens was over 1100 HV. This shows that α-Al2O3 was obtained for the whole range of pulse duration used in the present study. Otherwise, with the presence of porosities, the values of hardness should be below 1000 HV.
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In a previous study 3% porosity in inner layer was reported [13]. In an advanced porosity
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study, it was reported that in a ceramic body hardness decreases 60-70% due to the 20% porosity [23]. If reduction in hardness is directly proportional to the volume percent of porosity, 40% decreases in hardness coincide with 12% porosity as in case of E5.
A
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calculation to find exact value of hardness could not been carried out since volume percent of
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both α-Al2O3 and γ-Al2O3 precisely were not known. Nevertheless, it is reasonable to conclude that significant decrease in hardness of experiment E5 mainly was relatively due to
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the higher volume percent of pores.
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As mentioned above, a gradual increase in volume percent of porosity will cause a gradual decrease in hardness [13]. For short pulse duration (E1 and E2) the average hardness was about 1500 HV. With increasing pulse durations, hardness gradually decreased to 1300 HV in case of E3 and 1120 HV in case of E4, probably not due to decrease in ratio of volume percent α-Al2O3 / volume percent of γ-Al2O3 phases, but due to increase in volume percent of pores. Further increase in pulse duration destroyed larger pores in outer layer of coating. As a result volume percent of the pores per unit volume of coating layer gradually decreased. Therefore, the negative effect of pores on hardness partially eliminated. Moreover, the value
ACCEPTED MANUSCRIPT of average hardness shows that apparently the volume percent of α-Al2O3 gradually and slightly increased. Therefore beyond the specimen of E6, hardness increased up to 1700 HV due to combined effect of increasing volume percent α-Al2O3 and decreasing volume percent
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of pores. 1700 HV is the maximum hardness among the experiments.
ACCEPTED MANUSCRIPT 4. Conclusion The effect of pulse duration on properties of coating was experimentally studied. For the cathodic pulse, range of pulse durations was 200 µs to 2500 µs and in case of anodic pulse it was 300 µs to 3800 µs. It was experimentally proved that short pulse durations are necessary to achieve high quality
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thick inner dense layer. Increasing of pulse durations lead to a significant decrease in thickness of dense layer and a significant increase in pore size.
Hardness of inner dense layer and XRD phase analysis proved that α-Al2O3 was obtained for whole range of pulse durations. The average hardness of inner dense layer was 1500 HV for
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the short pulse durations and it was over 1600 HV for the long pulse durations. Increase in
decrease in volume percent of pore.
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hardness apparently is due to the combine affect of increase in volume percent of α-Al2O3 and
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An inner dense layer of 50 µm was achieved by 20 minutes of processing time that is rate of growth is 2.5 µm/min. Succeeding of this growth rate is an evidence of that the present
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process is energy efficient. Further increase in thickness of inner dense layer could be succeed
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either by increasing voltage of cathodic pulse or by increasing of processing time, but
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absolutely not by increasing of pulse duration. Acknowledgments The authors appreciate Prof. Alexander Ribalco for his contribution in selecting of electrical pulse parameters of the MAO processing, and thanks to technicians Adem Sen and Ahmet Nazim for their kind assistance during XRD and SEM experimental studies. The work was carried out with financial support from the Scientific and Technological Research Council of Turkey (TUBITAK) / Project Name: 2211-C Scholarship of National Priority Subjects Doctoral Thesis Programme.
ACCEPTED MANUSCRIPT References [1] Z.J. Wang, L.N. Wu, Y.L. Qi, W. Cai, Z.H. Jiang, Self-lubricating Al2O3/PTFE composite coating formation on surface of aluminium alloy, Surf Coat Tech, 204 (2010) 3315-3318. [2] R.O. Hussein, X. Nie, D.O. Northwood, An investigation of ceramic coating growth mechanisms in plasma electrolytic oxidation (PEO) processing, Electrochim Acta, 112 (2013) 111-119. [3] L. Yerokhin, L.O. Snizhko, N.L. Gurevina, A. Leyland, A. Pilkington, A. Matthews, Discharge characterization in plasma electrolytic oxidation of aluminium, J Phys D Appl Phys, 36 (2003) 2110-2120.
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[4] A.G. Rakoch, V.V. Khokhlov, V.A. Bautin, N.A. Lebedeva, Y.V. Magurova, I.V. Bardin, Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process, Prot Met+, 42 (2006) 158-169. [5] R.O. Hussein, X. Nie, D.O. Northwood, A. Yerokhin, A. Matthews, Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process, J Phys D Appl Phys, 43 (2010). [6] G. Sundararajan, L.R. Krishna, Mechanisms underlying the formation of thick alumina coatings through the MAO coating technology, Surf Coat Tech, 167 (2003) 269-277.
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[7] B.L. Jiang, Y.M. Wang, Plasma electrolytic oxidation treatment of aluminium and titanium alloys, in: H.Dong (Ed.) Surface Engineering of Ligth Alloys-Aluminium, Magnesium and Titanium Alloys, Woodhead Publishing, Oxford, 2010, pp. 110-146.
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[8] R.O. Hussein, X. Nie, D.O. Northwood, Influence of process parameters on electrolytic plasma discharging behaviour and aluminum oxide coating microstructure, Surf Coat Tech, 205 (2010) 1659-1667.
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[9] R.O. Hussein, D.O. Northwood, X. Nie, The influence of pulse timing and current mode on the microstructure and corrosion behaviour of a plasma electrolytic oxidation (PEO) coated AM60B magnesium alloy, J Alloy Compd, 541 (2012) 41-48.
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[10] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf Coat Tech, 122 (1999) 73-93.
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[11] R.O. Hussein, D.O. Northwood, J.F. Su, X. Nie, A study of the interactive effects of hybrid current modes on the tribological properties of a PEO (plasma electrolytic oxidation) coated AM60B Mg-alloy, Surf Coat Tech, 215 (2013) 421-430.
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[12] M.R. Bayati, A.Z. Moshfegh, F. Golestani-Fard, Effect of electrical parameters on morphology, chemical composition, and photoactivity of the nano-porous titania layers synthesized by pulsemicroarc oxidation, Electrochim Acta, 55 (2010) 2760-2766. [13] A.L. Yerokhin, A. Shatrov, V. Samsonov, P. Shashkov, A. Pilkington, A. Leyland, A. Matthews, Oxide ceramic coatings on aluminium alloys produced by a pulsed bipolar plasma electrolytic oxidation process, Surf Coat Tech, 199 (2005) 150-157. [14] V. Dehnavi, B.L. Luan, D.W. Shoesmith, X.Y. Liu, S. Rohani, Effect of duty cycle and applied current frequency on plasma electrolytic oxidation (PEO) coating growth behavior, Surf Coat Tech, 226 (2013) 100-107. [15] V.S.Egorkin, S.V.Gnedenkov, S.L. Sinebryukhov, I.E. Vyaliy, R.G.C. A.S.Gnedenkov, Increasing thickness and protective properties of PEO-coatings on aluminum alloy, Surface and Coatings Technology, 334 (2018) 29-42. [16] R.O. Hussein, X. Nie, D.O. Northwood, A spectroscopic and microstructural study of oxide coatings produced on a Ti-6Al-4V alloy by plasma electrolytic oxidation, Mater Chem Phys, 134 (2012) 484-492.
ACCEPTED MANUSCRIPT [17] L.R. Krishna, K.R.C. Somaraju, G. Sundararajan, The tribological performance of ultra-hard ceramic composite coatings obtained through microarc oxidation, Surf Coat Tech, 163 (2003) 484-490. [18] Q.B. Li, J. Liang, B.X. Liu, Z.J. Peng, Q. Wang, Effects of cathodic voltages on structure and wear resistance of plasma electrolytic oxidation coatings formed on aluminium alloy, Appl Surf Sci, 297 (2014) 176-181. [19] M.S. Yilmaz, O. Sahin, Effects of Pulse Duration on Structure and Surface Characteristics of Micro-Arc Oxidation Coatings Formed on Aluminum Alloy, Acta Physica Polonica A, 129 (2016) 673-676.
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[20] K. Wang, B.H. Koo, C.G. Lee, Y.J. Kim, S.H. Lee, E. Byon, Effects of electrolytes variation on formation of oxide layers of 6061 Al alloys by plasma electrolytic oxidation, T Nonferr Metal Soc, 19 (2009) 866-870. [21] L.R. Krishna, P.S.V.N.B. Gupta, G. Sundararajan, The influence of phase gradient within the micro arc oxidation (MAO) coatings on mechanical and tribological behaviors, Surf Coat Tech, 269 (2015) 54-63. [22] B.L. Jiang, Wang, Y.M, Surface Engineering of Light Alloys, in: h. Dong (Ed.)2010, pp. 110154.
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[23] J.A. Curran, T.W. Clyne, Porosity in plasma electrolytic oxide coatings, Acta Mater, 54 (2006) 1985-1993.
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Table 1: Electrical processing parameters chosen for depositing MAO coatings at a constant anodic voltage of 500 V, cathodic voltage of 400 V and duty cycle of 8%.
800
600
57
E4
800
800
51.7
E5
1000
800
45
E6
1300
1000
35
E7
1800
1000
28.2
E8
2500
2000
18
E9
3800
2500
12.7
≈8
≈ 16
0.104
≈8
≈ 16
0.116
≈8
≈ 16
0.133
≈8
≈ 16
0.1705
≈5
≈ 14.5
0.213
≈5
≈ 14.5
0.3325
≈5
≈ 14.5
0.473
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Anode Cathode Energy of Current Current Sample pulse pulse Frequency pulse density, Ia density, Ic Code duration duration (Hz) couple (C) 2 2 (A/cm ) (A/cm ) (µs) (µs) 300 200 160 ≈8 ≈ 16 E1 0.0375 400 300 115 ≈8 ≈ 16 E2 0.052
E1 3.8
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Specimen Pore percentage (%)
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Table 2: Volume percent of pore. E2 4.0
E3 6.8
E4 9.5
E5 11.9
E6 9.1
E7 6.2
E8 5.1
E9 3.9
ACCEPTED MANUSCRIPT Figure Captions Fig. 1. The schematic diagram of the MAO device. Fig. 2. The schematic drawing of bipolar pulse couples. Voltage-time pulse input for the experiments E2 and E5. Fig. 3. Current-time pulse output of experiments E2 and E3 after 10 minutes of processing.
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Fig. 4. Effect of pulse durations on the coating thickness. Fig. 5. A cross-sectional SEM image of coating was taken from specimen E1. Fig. 6. XRD results of specimen E2, as coated and after removing part of coating. Fig. 7. Effect of pulse durations on the surface roughness.
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Fig. 8. SEM micrograph of the specimen E6, includes: coating layers and micro discharge channel.
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Fig. 9. XRD results of the MAO coatings prepared at different pulse durations. Fig. 10. Vickers hardness profiles of the specimens E1, E3, E5, E7 and E9.
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Fig. 11. Overall Vickers hardness profiles of the inner dense layers of the all coatings.
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Fig. 12. The cross-sectional SEM images of all coatings.
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Fig. 12.
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Increasing of pulse duration lead to decrease in thickness of dense layer and increase in pore size. The formation α-Al2O3 was depends on pulse duration.
An inner dense layer of 50 µm was achieved by 20 minutes of processing time
α-Al2O3 and -Al2O3 phases existed on all coatings.
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Mustafa Safa Yilmaz, Orhan Sahin PII: DOI: Reference:
S0257-8972(18)30461-4 doi:10.1016/j.surfcoat.2018.04.085 SCT 23369
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
15 August 2017 25 April 2018 28 April 2018
Please cite this article as: Mustafa Safa Yilmaz, Orhan Sahin , Applying high voltage cathodic pulse with various pulse durations on aluminium via micro-arc oxidation (MAO). The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2018.04.085
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ACCEPTED MANUSCRIPT Applying High Voltage Cathodic Pulse with Various Pulse Durations on Aluminium via Micro-Arc Oxidation (MAO) Mustafa Safa YILMAZ1, * and Orhan SAHIN2 1
Fatih Sultan Mehmet Vakif University, Aluminium Test and Training Centre, Istanbul, Turkey
Gebze Technical University, Department of Materials Science and Engineering, 41400
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1
Kocaeli, Turkey
*Corresponding author: Tel: +90 212 521 81 00 (4336) E-mail: [email protected], [email protected]
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Abstract
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The present study is an investigation of the effect of pulse duration on properties of coatings formed on aluminium alloy (Al) in alkaline solution via using of high voltage cathodic pulses.
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Nine different anodic-cathodic pulse couples with various durations were selected for processing. In order to find out the effect of pulse duration on coating: duty cycle, anodic
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voltage, cathodic voltage, processing time, electrolyte temperature and electrolyte composition were kept constant. Scanning electron microscope (SEM), profilometer, X-ray
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diffractometer and indenter were employed to investigate the microstructure, surface
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roughness, phase distribution and hardness of the coatings. Also, coating thickness measurement was carried out to find out the effect of pulse duration on productivity. Varying of pulse duration caused significant differences among thickness of coating layers. Coating layers with thickness between 45-75 μm and surface roughness between 2.5-5.5 μm were obtained. Coatings consist of two layers, outer porous layer and inner dense layer. The outer porous layer was γ-Al2O3 and the inner dense layer was a mixture of γ-Al2O3 and α-Al2O3. The hardness of inner dense layers was between 1100-1600 Vickers. Keywords: Aluminium, micro-arc oxidation, pulse duration, coating
ACCEPTED MANUSCRIPT 1. Introduction Aluminium and its alloys are important structural materials for automotive, aerospace and transportation industries. The interest in aluminium alloys comes from its high strength to weight ratio, high corrosion resistance and lightweight. Nevertheless, their disadvantages are low wear resistance, low hardness and high friction coefficient, which limit their applications
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[1]. Therefore, to improve their surface properties various surface modifications techniques have been developed. One of these techniques is Micro-Arc Oxidation (MAO), in which a thick and hard alumina layer strongly adhered to aluminium substrate could be fabricated. Mechanisms of oxide layer formation, that is, micro discharge; discharge channel and oxide
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formation have been extensively studied by many researchers [2-6]. MAO processing is a combination of electro-chemical reaction, plasma-chemical reaction and thermal diffusion [2,
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7-9]. The electro-chemical reaction and plasma-chemical reaction take place at metal oxideelectrolyte interface and inside discharge channel respectively.
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The properties of the oxide layer formed as a result of MAO depends on processing
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parameters, such as composition of electrolyte, electrical parameters of pulses, chemical composition of substrate, processing time, etc. [8-15]. The effects of the process parameters,
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especially the electrical parameters of pulses on oxide layer formation are still in the stage of research and development. Electrical parameters include alternative current, direct current and
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various forms of unipolar and bipolar pulses and their frequencies. Currently it had been reported that bipolar current pulse mode forms thicker and more uniform coatings than unipolar current pulse mode [16]. Krishna et al. [17] found that the thickness’ of oxide layer was increased with both increased processing time and pulse current density. Yerokhin et al. [13] found out that frequencies ranging from 1 to 3 kHz significantly increased the growth rate of the oxide layer and reduced the size of the outer porous layer. Dehnavi et al. [14] reported that the coating growth rate increases gradually with decrease in duty cycle at constant frequency.
ACCEPTED MANUSCRIPT Qingbiao et. al. [18] investigated the effect of cathodic voltage on growth rate and wear resistance of coating. During processing anodic voltage was kept constant and it was 520 V. Cathodic voltages was in between 0 V and 190 V. Above a cathodic voltage of 90 V, an oxide layer consisting of two sublayers was obtained. Sublayers were described as compact layer and porous layer. Main component of coatings was γ-Al2O3. Growth rate of coating, thickness
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of compact layer and wear resistance significantly increased with increasing applied cathodic voltage. On the contrary, surface roughness of coating decreased with increasing cathodic voltages.
Hussein et al. [8, 9] studied the effect of unipolar current pulses, bipolar current pulses and
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bipolar current pulse duration on the microstructure of oxide layer. It comes out that the pulse duration of bipolar pulses has a significant improvement on coating quality. The ranges of
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pulse duration used in above study were 400 µs and 400-600 µs for anodic and cathodic pulse respectively. In another study a cathodic voltage of 200V was used, it was showed that the
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processing time and the anodic-cathodic pulse durations have a significant effect on growth rate and properties of oxide layer [19]. The aim of the present study is to extend the
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investigation on the effect of increasing cathodic pulse voltage and its duration on oxide layer
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formation.
ACCEPTED MANUSCRIPT 2. Experimental Procedure 2.1. Instruments Used MAO device used for processing has 2 separate High Frequency Converter (HFC), in which one of them was set to positive and the other one was set to negative polarity. The maximum power of the HFC is 12.5 kW. These HFC’s have the capability to supply voltage up to 800
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V. Device was designed to operate by keeping voltage of anodic and cathodic pulses as constant. Bipolar pulses (anodic-cathodic pulse couples) were used for all experiments. Pulse generator is capable to form bipolar square pulses and adjust parameters of cathodic and anodic parts independently from each other. Pulse parameters, such as pulse amplitude, pulse duration, frequency (that is, number of pulses), pause between cathodic-anodic pulse of a
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pulse couple and pause between anodic-cathodic pulse couples could be set by a computer
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program at the beginning of processing. The pauses between anodic-cathodic pulse couples are identical. Frequency between anodic-cathodic pulse couples was determined according to
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the electricity consumed for the whole process, which is total amount of electricity of anodiccathodic pulse couples. During processing, instantaneous amplitudes of anodic-cathodic
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current pulse couples could be observed on a monitor and these current pulses of processing
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(oscillograms) could be recorded at any time. The schematic diagram of the MAO device was given in Fig. 1. Tektronix-TDS 2024C digital
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storage oscilloscope was used for monitoring the shape of current pulses, pulse amplitude and pulse duration during MAO processing. The cooling unite, which has circulating water in its spiral copper tube, was employed to keep electrolyte temperature as constant. FisherDualscope MP40E-S and Mitutoyo SJ-400 profilometer were used for coating thickness and surface roughness measurements respectively. Brukers D8 (40kW, 40mA) type X-ray diffractometer was used for identification of the phases in oxide layer. X-ray diffraction (XRD) was operated with Cu Kα radiation. Between 20° to 90° were scanned with an increment of 0.02° and account time of 1s. Cross-sectional microstructure of oxide layer were
ACCEPTED MANUSCRIPT analysed by Philips XL30 field emission Scanning Electron Microscopy (SEM). Crosssectional hardness of oxide layer was measured by using Mitutoyo MicroWizhard microhardness tester. 20 g indentation load was preferred for hardness measurement. The elemental analysis of the aluminium substrates was determined with Spectro-Max LMF14. 2.2. Specimens and Electrolyte
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Round bar aluminium alloy were subjected to surface modification. The composition of Al alloy was Al (98.6 %), Si (0.47 %) and Mg (0.46 %). Other elements were quantitatively not significant. The diameter and length of the bars were 1.0 cm and 1.0 cm respectively. The surface area processed was nearly 4 cm2. Before processing, specimens were mechanically
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ground with 200-1200 mesh emery papers and cleaned ultrasonically for 5 minutes in acetone. The electrolyte used for coating was KOH (2 g/l), Na2SiO3.5H2O (9.5 g/l) and its temperature
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was kept at 25°C ±5.
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2.3. Electrical processing parameters
Since the aim of the present study was to determine the effect of cathodic and anodic pulse
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durations on oxide layer formation other electrical parameters, anodic voltage, cathodic
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voltage and duty cycle were all kept constant and they were given in Table I. Furthermore, energy consumed for each experiment was same and it was 6 Coulombs, which includes both
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anodic and cathodic pulses. Equivalence in consumed energy of experiments was obtained by decreasing number of applied anodic-cathodic pulse couples as the duration of pulses increased. That is, frequency of applied pulse couples was highest for the experiment with lowest pulse duration and it was lowest for the experiment with highest pulse duration. This operation is automatically done by instrument. The energies of the pulse couples were given in Table. I. The values of anodic voltage and the duty cycle were decided according to the research data given in [2, 3, 5, 14]. Cathodic voltage of 400 V was intentionally selected in order to
ACCEPTED MANUSCRIPT compare present experimental results with the results of previous studies, in which cathodic voltage was 200V [18, 19]. The main reason of this selection is to understand the influence of increasing cathodic voltage on coating properties. Voltages of anodic and cathodic pulse couples were set to 500 V and 400 V respectively and MAO unit is capable of keeping them as constant during processing. However, anodic and
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cathodic pulse currents and therefore current densities vary depending on the instantaneous resistance of electrochemical process. The pause between anodic and cathodic pulses of the pulse couple was 500 μs and it was same for all experiments. As an example, input of experiments of number 2 and number 5 was given in Fig. 2. Nine different set of pulse
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durations of anodic-cathodic pulse couples were employed. The processing time for all experiments was 20 minutes. For each experiment, oscillograms showing instantaneous
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current of process were recorded after 1, 5, 10, 15 and 20 minutes of processing. That is, 9 experiments, 5 current pulse records for each, it comes up totally 45 records. As an example,
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2 of them, which show the conditions of current pulse after 10 minutes processing, were given in Fig. 3, which belongs to E2 and E3. The durations of anodic-cathodic pulse couples for
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each of nine experiments were given in Table I. For each pulse parameter given in Table I,
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experiment was carried out at least two times.
ACCEPTED MANUSCRIPT 3. Results and Discussions Coating thickness of experiments was measured by the employment of two different methods. Results of both methods of thickness measurement were given in Fig. 4, in which thickness of coating curves were plotted as a function of pulse durations. First method is the direct measurement of thickness by a device that use eddy current (Line-d). The thickness of the
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oxide layer was the average of 20 eddy current measurements from different locations. Second method is a detailed thickness measurement carried out on cross-sectional SEM images of coating layers. Regarding morphological appearance, cross-sectional microstructure was divided to 3 distinct regions. The irregular boundaries between the regions were visually
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marked with great care as in Fig. 5 under high magnification and named as Line-a, Line-b and Line-c. With the help of graph paper mean location of boundaries between regions were
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determined. That is, a straight line, which is parallel to the coating-substrate interface, represents an irregular boundary. The distances between straight lines’ and coating-substrate
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interface were measured to determine thickness of each region. For each experiment, images from 3 different locations were recorded. After marking the boundaries between zones, for
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each image the thickness of the regions were measured and the average thickness of the
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regions were calculated depending on the measurement of 3 different locations of same specimen. As an example, only Fig. 5 was given to show clearly how the boundaries between
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regions were marked. Same measurements were carried out to determine average thickness of regions for all specimens. Regions were named as inner dense layer (below Line-b) and outer porous layer (between Line-b and Line-c). Similarly, depending on morphological appearance, inner dense layer could also be divided two zones. Usually in most of the studies on this kind of coatings, coating layer was named as inner dense and outer porous layer [8, 13, 15, 17]. Therefore, in order to prevent any confusion, the regions of inner dense layer were named as zones. Inner zone next to the substrate has a granular appearance (below Line-a). Other zone between
ACCEPTED MANUSCRIPT outer porous layer and inner zone has a smooth appearance (between Line-a and Line-b). The difference in appearance of zones could be related to the mechanism of molten oxide formation and its solidification. The boundary between two zones and the boundary between inner zone and outer porous layer were marked as in Fig. 5. SiC and α-Al2O3 powders were used for cross-sectional polishing of coatings. Since SiC is
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harder than both α-Al2O3 and γ-Al2O3, SiC polishing produced a smooth cross sectional surface. In case of α-Al2O3 polishing, which is the case of present study, part of the coating adjacent to substrate does not have a smooth surface, which is named as granular zone above, Fig. 5. It is anticipated that granular zone could be consisting of α-Al2O3 and γ-Al2O3. Since,
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hardness of γ-Al2O3 was lower than that of α-Al2O3, γ-Al2O3 could be eroded more than αAl2O3. Therefore, formation of granular appearance was attributed to difference in erosion
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rate of α-Al2O3 and γ-Al2O3. The other zone of inner dense layer probably was mainly consisting of γ-Al2O3 phase. Therefore, erosion was almost uniform. In order to clarify this
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point, XRD analysis was carried out by using a specimen coated under condition of E2. Results of 3 XRD measurements were given in Fig. 6. After the first XRD measurement of as
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coated surface, coating thickness was reduced to 50 μm and later reduced to 28 μm. The
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original as-coated thickness, which was measured by eddy current, was 75 μm. 50 μm and 28 μm thicknesses were preferred for XRD measurement, because they approximately coincide
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with the thicknesses of inner dense layer and inner zone respectively. The α-Al2O3 peaks at 2 positions of 25.7°, 35.2° and 57.6° degrees and similarly γ-Al2O3 peaks at 2 positions of 46.0° degree in Fig. 6, shows that both zones are a mixture of α-Al2O3 and γ-Al2O3 and XRD results clearly shows that volume percent of α-Al2O3 phase is higher in inner zone. This qualitative evaluation based on the difference in height of the α-Al2O3 peaks. At the moment, it has not been investigated yet, either α-Al2O3 is uniformly distributed or across the inner zone, there is a distribution gradient beginning with higher α-Al2O3 concentration at the vicinity of substrate-coating interface.
ACCEPTED MANUSCRIPT As mentioned above the difference in appearance apparently related to the difference between microstructure of zones. The boundary between zones is not straight. In other words, in threedimensional view the interface between the zones is not planar. Because, molten alumina forms intensively at the location of micro channels next to substrate and solid alumina formation, either γ-Al2O3 or α-Al2O3 phase proceeds faster along the micro channels.
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Probably α-Al2O3 intensively forms at the bottom of micro channels. Eventually, a non-planar interface forms between α-Al2O3 rich and γ-Al2O3 rich zones.
Oscillograms (amplitude and duration) of current pulse could be used to judge the developments during processing, which is oxide layer formation. Current densities were
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calculated by dividing experimentally measured current to the processing surface area of specimens, which is 4 cm2. For the experiments E1 to E6, the average value of anodic current
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density was almost stationary during 20 minutes of processing time and it was more or less 8 A/cm2. However, for the experiment E7 to E9 the current densities were also almost
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stationary but 25 % lower than that of previous experiments and it was more or less 5 A/cm2. Similar stationary behaviour was observed for cathodic current density which was more or
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less 16 A/cm2 for the experiments E1 to E6 and it was 10 % lower for the case of E7 to E9
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and it was more or less 14.5 A/cm2. Current densities of all experiments were summarized in Table. I. Shapes of the cathodic current pulses were almost square. However, behaviour of
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anodic pulses was significantly different. At the very beginning of an anodic current pulses, current make a sharp peak and gradually ceased down but never be zero, Fig. 3. This was a general behaviour of anodic current pulses. Probably, during an anodic pulse oxide forms and thickness of oxide layer increases. As thickness of oxide layer gradually grows, the resistance of oxide layer (that is, process) gradually increases. As a consequent of this, anodic pulse current gradually decreases until the end of pulse duration. Same phenomena repeat itself for each of the following anodic-cathodic pulse couples. In case of E7 to E9 significant decrease in both currents could be due to significant increase in resistance of process.
ACCEPTED MANUSCRIPT It could be concluded that anodic-cathodic pulse currents were variable during each process; however variation in current was not significant. Therefore, at the time, no special significance could be attached to this variation in current. As a result, it is obvious that, main variable processing parameter effecting oxide layer formation is anodic and cathodic pulse durations.
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Cross-sectional thickness measurements on images showed that for the short pulse durations, thickness of coatings is slightly above 60 μm (E1and E2). Increasing of pulse duration caused thicker coating layers above 70 μm (E3, E4, E5 and E6) and with further increasing of pulse duration the coating thickness gradually decreased (E7, E8, E9). The total thickness of
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coatings (Line-c), including inner dense layer and outer porous layer, obtained by using SEM images were below the value of thickness obtained by direct measurement of with eddy
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current device (Line-d). The difference could be due to some loss of surface part of porous layer during grinding and polishing of specimens for SEM examination. Nevertheless, the
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tendency of the thickness curves of both methods is same.
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In case of E1 and E2, the thickness of dense layer is slightly above 50 μm and that of porous layer is above 10 μm. In case of E3, E4 and E5, there is a negligible increase in thickness of
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dense layer, if it is compared to that of E1 and E2. However, there is a significant increase in thickness of outer porous layer that is about 10 μm. For the case of E6 the thickness of dense
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layer was slightly lower than 50 μm and thickness of porous layer was slightly higher than that of previous experiments (E3, E4 and E5). The thickness of porous layer is about 27 μm, which is the highest porous layer among the experiments. For the experiments (E7, E8 and E9), the thickness of dense layer gradually decreased down below 30 μm. In case of porous layer, thickness was about 15 μm. Starting with E3, the number and size of porosities gradually increased till E7. In other word, the thickness of porous outer layer starts to increase with E3 and reached its highest value in
ACCEPTED MANUSCRIPT case of E6. Beyond E6, there is no increase in size of the pores however there is a decrease in thickness of both dense and porous layer. The pore formation probably is strongly related to the gas formation during micro charging. The pulse duration in turn pulse energy was lowest for E1 and highest for E9. Increasing in pulse energy probably increases gas formation and as a result of this, for E3 to E6, number and size of pores increases in both outer porous and
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inner dense layers [8, 15]. The amount of gas formation for the E7, E8 and E9 was so intensive, the large pores close to surface were destroyed which cause a decrease in thickness of porous layer.
The increase in coating thickness on E3, E4, E5 and E6 was probably not due to the increase
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in amount of molten oxide formation, but probably due to the increase in number and size of pores. The outer layer of coating is similar to a sponge with larger pores. Whenever the pores
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adjacent to the surface were destroyed, part of porous layer was lost and eventually the thickness of porous layer decreased, as in case of E7, E8 and E9. The weight of the oxide
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layer could had been measured with a precision balance, in order to determine if the increase in thickness as in case of E3, E4, E5 and E6 is due to the oxide formation or due to the
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increasing number and size of pores. Unfortunately this study was not carried out.
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As the pulse duration increased, two simultaneous phenomena took place; larger pores near the surface were destroyed and pore formation gradually extended towards substrate due to
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the increase in the amount of trapped gas. Meantime pore size became larger. Increasing pulse duration caused the thickness of inner dense layer to decrease and pore content of it increase gradually. As seen in Fig. 4, with increasing pulse duration thickness of inner dense layer decreased from 50 μm to below 30 μm (E7, E8 and E9). In a previous study [19], the amplitude of cathodic voltage was 200 V (Half of the present study) and the duration of pulse couples and processing time were same as that of the present study. The thickness of the previous study was approximately 25 μm without regarding pulse
ACCEPTED MANUSCRIPT durations. Comparing previous and present studies with each other shows that three times increase in thickness was obtained by two times increase in cathodic voltage. Surface roughness of coatings was given in Fig. 7. Roughness was obtained by averaging of 12 measurements from different locations. From Fig. 7, it is obvious that the surface roughness and corresponding coating thickness in Fig. 4 have similar behaviour. When
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thickness of outer porous layer increased due to increasing pore size, the surface roughness increased either. As known, the surface roughness usually increases as the coating thickness increases. Similarly when the coating thickness decreases surface roughness decreases either [19]. Same behaviour was observed in the present study.
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Some discharge channels could reach to the substrate through the coating layer as shown in Fig. 8. Molten substrate flows in discharge channels and reacts with the chemicals from
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electrolyte to form molten oxide. Part of molten oxide inside the discharge channel is ejected out through the channel due to high pressure. When it is reached to surface, it is cooled down
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by electrolyte and solidifies on surface in the form a swallow hillock right on top of the
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discharge channel. As a result of solidification of the molten alumina on surface, a porous surface layers with high roughness forms [20].
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The effect of pulse duration on alumina phase formation was investigated by XRD analysis
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and results were given in Fig. 9. Since X-rays were able to penetrate through the oxide layer, diffraction peaks of Al and Mg2Si were obtained from substrate. The intensity of Al and Mg2Si peaks decreased with increasing coating thickness, Fig. 9. The characteristic peaks show that the coatings consist of crystalline γ-Al2O3 and α-Al2O3 phases. The hardness of the oxide layer was the average of thickness through measurements from 3 different locations. For each thickness through measurement, hardness was measured along a line between substrate and the surface of the coating. The distance from the substrate-coating interface was taken as a reference point to calculate average hardness.
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Due to the nature of the process, as mentioned previously the outer layer has a porous structure, which is mostly consisted of γ-Al2O3 phase, while the inner layer is a relatively dense structure and it is a mixture of α-Al2O3 and γ-Al2O3 phases [6]. This could be observed from the hardness versus distance from substrate-coating interface curves, which was given in
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Fig. 10. For specimens E1, E3 and E5 hardness sharply decreases approximately at a distance of 50-55 μm away from substrate-coating interface. Similar behaviour was observed for specimens E7 and E9 at a distance of 33 μm and 24 μm away from substrate-coating interface respectively. According to Fig. 4, thickness of inner dense layer of E1 to E5 was slightly
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above 50 μm and that of E7 and E9 was 35 μm and 25 μm respectively. Comparison of Fig. 10 and Fig. 4 confirms that sharp decrease in hardness correspond to the location of
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boundaries between inner dense layers and outer porous layers. For some experiments, which were not included in present study, coating layers consisting of
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γ-Al2O3 phase without any detectable amount of α-Al2O3 phase were obtained. The hardness
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of these experiments is in between 800 HV and 1200 HV and therefore the average hardness is 1000 HV. On the other hand, it had been reported that under condition of MAO method,
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individual hardness of γ-Al2O3 and α-Al2O3 are 800 HV and 2200 HV respectively [21, 22]. Since hardness of γ-Al2O3 is around 800-1000 HV, part of the hardness curves over 1100 HV
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in Fig. 10 are undeniable evidences of the presence of α-Al2O3 phase in inner dense layer. Since properties of inner dense layer was major objective of the present study, the average hardness of inner dense layers were calculated and given in Fig. 11. Calculations based on data picked up from several points along a line, which is parallel to the substrate-coating interface. The distance between lines and interface was approximately 17±2 µm, 27±2 µm and 37±2 µm. Since, thickness of inner dense layer of E8 and E9 was less than 37 μm the hardness curve of 37±2 µm in Fig. 11 ended with E7. Even though inner dense layer thickness
ACCEPTED MANUSCRIPT of E7 is barely 35 μm its hardness was included in Fig. 11. As seen in Fig. 11, although behaviour of hardness curves is almost same, they do not exactly overlap with each other. Starting with 1300-1500 HV in case of E1 and E2, average hardness in Fig. 11 gradually decreased down slightly below 1100 HV for E4 and then gradually increased up to 1700 HV in case of E7.
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As explained previously inner dense layer is a mixture of α-Al2O3 and γ-Al2O3 phases and the amount of α-Al2O3 could relatively be higher in inner zone next to the substrate-coating interface as observed in Fig. 6. On the other hand, the distribution gradient of α-Al2O3 phase could be evaluated by not only referring Fig. 6 but also Fig. 11. If there has been a
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distribution gradient, hardness of the Line-A in Fig. 11 should be significantly higher than that of the Line-B and Line-C. However, this was not observed. Distance between hardness
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curves is not significant and curves were not clearly separated from each other. Consequently, Fig. 11 shows that the amount of α-Al2O3 content of inner zone next to the interface could
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more or less be higher than the α-Al2O3 content of rest of the coating, but the distribution of α-Al2O3 in inner zone is uniform. In order to clarify this point, an advance study is needed to
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through hardness.
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determine phases in inner dense layer quantitatively and to analyse variation in thickness
The value of hardness depends on both volume percent of phases and porosities [6, 13, 17].
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Although, the present study is not about pores, pores in different number and size were existed in all specimens. Instead of carrying out an advance quantitative porosity analysis including number and size of pores, a simple and quick ImageJ analysis was preferred to determine volume percent of pores in inner dense layers only. The results of the pore measurements were given in Table. 2. Starting with E1 volume percent of pores gradually increases till E5 and beyond E5 it gradually decreases.
ACCEPTED MANUSCRIPT Referring to Fig. 4, Fig. 7 and Table. 2, experiments could be separated as three groups. Group-1 and Group-2 cover experiments E1-E2 and E3 to E6 respectively. Group-3 covers experiments E7 to E9. The distinct property of Group-1 is an inner dense layer with lowest volume percent of porosity. Group-2 has slightly thicker inner dense layer than Group-1. However, comparing with Group-1, Group-2 has significantly higher volume percent of inner
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dense layer porosity. Moreover, Group-2 has significantly thicker outer porous layer than the other groups. When Group-3 is compared with Group-1 and Group-2, the thickness of coatings and inner dense layers are significantly lower than that of other groups. Cross sectional SEM images of all specimens were given in Fig. 12. By a careful visual
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examination of SEM images, one could notice the decrease in coating thickness, decrease in inner dense layer thickness and change in microstructural morphology including pores and
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their distribution. All surfaces have an island-like morphology, which each of islands has a pore inside. Some of these ball shaped islands released gas through an opening on the top or
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sometimes released gas as a result of explosion. Consequently, size and final form of islands
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have a remarkable effect on surface roughness [1]. Pulse durations used in E3, E4, E5 and E6 which are longer than that of E1 and E2, generated
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slightly thicker coating. Further increasing of the anodic-cathodic pulse durations, as in case of E7, E8 and E9, had already caused thinner coating layers. Besides, there is almost no
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granular appearance of inner zone on cross-sectional morphology of E9, Fig. 12. The aim of increasing pulse duration is to increase energy of pulse and therefore to provide more heat for process. It was hoped that increasing amount of heat in turn temperature could promote α-Al2O3 formation. It was reported that α-Al2O3 formation concentrated in inner dense layer of coating next to the substrate, γ-Al2O3 formation distributed all over the coating layer including porous layer [2, 13]. XRD results confirmed that α-Al2O3 and γ-Al2O3 were formed for all experiments. Unfortunately, XRD is not capable of determining the exact
ACCEPTED MANUSCRIPT location of α-Al2O3 grains and exact location of γ-Al2O3 grains. As mentioned earlier in Fig. 6, α-Al2O3 content of inner zone is higher than that of rest of the coating. Another evidence of α-Al2O3 formation is the hardness test results. Average hardness of only dense layers was calculated for each of pulse durations, Fig. 11. As mentioned above, for alumina coating obtained by MAO average hardness of γ-Al2O3 is around 800-1000 HV and that of α-Al2O3 is
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around 2000-2200 HV. In spite of the presence of pores, hardness of the specimens was over 1100 HV. This shows that α-Al2O3 was obtained for the whole range of pulse duration used in the present study. Otherwise, with the presence of porosities, the values of hardness should be below 1000 HV.
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In a previous study 3% porosity in inner layer was reported [13]. In an advanced porosity
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study, it was reported that in a ceramic body hardness decreases 60-70% due to the 20% porosity [23]. If reduction in hardness is directly proportional to the volume percent of porosity, 40% decreases in hardness coincide with 12% porosity as in case of E5.
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calculation to find exact value of hardness could not been carried out since volume percent of
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both α-Al2O3 and γ-Al2O3 precisely were not known. Nevertheless, it is reasonable to conclude that significant decrease in hardness of experiment E5 mainly was relatively due to
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the higher volume percent of pores.
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As mentioned above, a gradual increase in volume percent of porosity will cause a gradual decrease in hardness [13]. For short pulse duration (E1 and E2) the average hardness was about 1500 HV. With increasing pulse durations, hardness gradually decreased to 1300 HV in case of E3 and 1120 HV in case of E4, probably not due to decrease in ratio of volume percent α-Al2O3 / volume percent of γ-Al2O3 phases, but due to increase in volume percent of pores. Further increase in pulse duration destroyed larger pores in outer layer of coating. As a result volume percent of the pores per unit volume of coating layer gradually decreased. Therefore, the negative effect of pores on hardness partially eliminated. Moreover, the value
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of pores. 1700 HV is the maximum hardness among the experiments.
ACCEPTED MANUSCRIPT 4. Conclusion The effect of pulse duration on properties of coating was experimentally studied. For the cathodic pulse, range of pulse durations was 200 µs to 2500 µs and in case of anodic pulse it was 300 µs to 3800 µs. It was experimentally proved that short pulse durations are necessary to achieve high quality
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thick inner dense layer. Increasing of pulse durations lead to a significant decrease in thickness of dense layer and a significant increase in pore size.
Hardness of inner dense layer and XRD phase analysis proved that α-Al2O3 was obtained for whole range of pulse durations. The average hardness of inner dense layer was 1500 HV for
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the short pulse durations and it was over 1600 HV for the long pulse durations. Increase in
decrease in volume percent of pore.
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hardness apparently is due to the combine affect of increase in volume percent of α-Al2O3 and
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An inner dense layer of 50 µm was achieved by 20 minutes of processing time that is rate of growth is 2.5 µm/min. Succeeding of this growth rate is an evidence of that the present
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process is energy efficient. Further increase in thickness of inner dense layer could be succeed
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either by increasing voltage of cathodic pulse or by increasing of processing time, but
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absolutely not by increasing of pulse duration. Acknowledgments The authors appreciate Prof. Alexander Ribalco for his contribution in selecting of electrical pulse parameters of the MAO processing, and thanks to technicians Adem Sen and Ahmet Nazim for their kind assistance during XRD and SEM experimental studies. The work was carried out with financial support from the Scientific and Technological Research Council of Turkey (TUBITAK) / Project Name: 2211-C Scholarship of National Priority Subjects Doctoral Thesis Programme.
ACCEPTED MANUSCRIPT References [1] Z.J. Wang, L.N. Wu, Y.L. Qi, W. Cai, Z.H. Jiang, Self-lubricating Al2O3/PTFE composite coating formation on surface of aluminium alloy, Surf Coat Tech, 204 (2010) 3315-3318. [2] R.O. Hussein, X. Nie, D.O. Northwood, An investigation of ceramic coating growth mechanisms in plasma electrolytic oxidation (PEO) processing, Electrochim Acta, 112 (2013) 111-119. [3] L. Yerokhin, L.O. Snizhko, N.L. Gurevina, A. Leyland, A. Pilkington, A. Matthews, Discharge characterization in plasma electrolytic oxidation of aluminium, J Phys D Appl Phys, 36 (2003) 2110-2120.
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[4] A.G. Rakoch, V.V. Khokhlov, V.A. Bautin, N.A. Lebedeva, Y.V. Magurova, I.V. Bardin, Model concepts on the mechanism of microarc oxidation of metal materials and the control over this process, Prot Met+, 42 (2006) 158-169. [5] R.O. Hussein, X. Nie, D.O. Northwood, A. Yerokhin, A. Matthews, Spectroscopic study of electrolytic plasma and discharging behaviour during the plasma electrolytic oxidation (PEO) process, J Phys D Appl Phys, 43 (2010). [6] G. Sundararajan, L.R. Krishna, Mechanisms underlying the formation of thick alumina coatings through the MAO coating technology, Surf Coat Tech, 167 (2003) 269-277.
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[7] B.L. Jiang, Y.M. Wang, Plasma electrolytic oxidation treatment of aluminium and titanium alloys, in: H.Dong (Ed.) Surface Engineering of Ligth Alloys-Aluminium, Magnesium and Titanium Alloys, Woodhead Publishing, Oxford, 2010, pp. 110-146.
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[8] R.O. Hussein, X. Nie, D.O. Northwood, Influence of process parameters on electrolytic plasma discharging behaviour and aluminum oxide coating microstructure, Surf Coat Tech, 205 (2010) 1659-1667.
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[9] R.O. Hussein, D.O. Northwood, X. Nie, The influence of pulse timing and current mode on the microstructure and corrosion behaviour of a plasma electrolytic oxidation (PEO) coated AM60B magnesium alloy, J Alloy Compd, 541 (2012) 41-48.
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[10] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf Coat Tech, 122 (1999) 73-93.
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[11] R.O. Hussein, D.O. Northwood, J.F. Su, X. Nie, A study of the interactive effects of hybrid current modes on the tribological properties of a PEO (plasma electrolytic oxidation) coated AM60B Mg-alloy, Surf Coat Tech, 215 (2013) 421-430.
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[12] M.R. Bayati, A.Z. Moshfegh, F. Golestani-Fard, Effect of electrical parameters on morphology, chemical composition, and photoactivity of the nano-porous titania layers synthesized by pulsemicroarc oxidation, Electrochim Acta, 55 (2010) 2760-2766. [13] A.L. Yerokhin, A. Shatrov, V. Samsonov, P. Shashkov, A. Pilkington, A. Leyland, A. Matthews, Oxide ceramic coatings on aluminium alloys produced by a pulsed bipolar plasma electrolytic oxidation process, Surf Coat Tech, 199 (2005) 150-157. [14] V. Dehnavi, B.L. Luan, D.W. Shoesmith, X.Y. Liu, S. Rohani, Effect of duty cycle and applied current frequency on plasma electrolytic oxidation (PEO) coating growth behavior, Surf Coat Tech, 226 (2013) 100-107. [15] V.S.Egorkin, S.V.Gnedenkov, S.L. Sinebryukhov, I.E. Vyaliy, R.G.C. A.S.Gnedenkov, Increasing thickness and protective properties of PEO-coatings on aluminum alloy, Surface and Coatings Technology, 334 (2018) 29-42. [16] R.O. Hussein, X. Nie, D.O. Northwood, A spectroscopic and microstructural study of oxide coatings produced on a Ti-6Al-4V alloy by plasma electrolytic oxidation, Mater Chem Phys, 134 (2012) 484-492.
ACCEPTED MANUSCRIPT [17] L.R. Krishna, K.R.C. Somaraju, G. Sundararajan, The tribological performance of ultra-hard ceramic composite coatings obtained through microarc oxidation, Surf Coat Tech, 163 (2003) 484-490. [18] Q.B. Li, J. Liang, B.X. Liu, Z.J. Peng, Q. Wang, Effects of cathodic voltages on structure and wear resistance of plasma electrolytic oxidation coatings formed on aluminium alloy, Appl Surf Sci, 297 (2014) 176-181. [19] M.S. Yilmaz, O. Sahin, Effects of Pulse Duration on Structure and Surface Characteristics of Micro-Arc Oxidation Coatings Formed on Aluminum Alloy, Acta Physica Polonica A, 129 (2016) 673-676.
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[20] K. Wang, B.H. Koo, C.G. Lee, Y.J. Kim, S.H. Lee, E. Byon, Effects of electrolytes variation on formation of oxide layers of 6061 Al alloys by plasma electrolytic oxidation, T Nonferr Metal Soc, 19 (2009) 866-870. [21] L.R. Krishna, P.S.V.N.B. Gupta, G. Sundararajan, The influence of phase gradient within the micro arc oxidation (MAO) coatings on mechanical and tribological behaviors, Surf Coat Tech, 269 (2015) 54-63. [22] B.L. Jiang, Wang, Y.M, Surface Engineering of Light Alloys, in: h. Dong (Ed.)2010, pp. 110154.
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[23] J.A. Curran, T.W. Clyne, Porosity in plasma electrolytic oxide coatings, Acta Mater, 54 (2006) 1985-1993.
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Table 1: Electrical processing parameters chosen for depositing MAO coatings at a constant anodic voltage of 500 V, cathodic voltage of 400 V and duty cycle of 8%.
800
600
57
E4
800
800
51.7
E5
1000
800
45
E6
1300
1000
35
E7
1800
1000
28.2
E8
2500
2000
18
E9
3800
2500
12.7
≈8
≈ 16
0.104
≈8
≈ 16
0.116
≈8
≈ 16
0.133
≈8
≈ 16
0.1705
≈5
≈ 14.5
0.213
≈5
≈ 14.5
0.3325
≈5
≈ 14.5
0.473
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E3
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Anode Cathode Energy of Current Current Sample pulse pulse Frequency pulse density, Ia density, Ic Code duration duration (Hz) couple (C) 2 2 (A/cm ) (A/cm ) (µs) (µs) 300 200 160 ≈8 ≈ 16 E1 0.0375 400 300 115 ≈8 ≈ 16 E2 0.052
E1 3.8
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Specimen Pore percentage (%)
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Table 2: Volume percent of pore. E2 4.0
E3 6.8
E4 9.5
E5 11.9
E6 9.1
E7 6.2
E8 5.1
E9 3.9
ACCEPTED MANUSCRIPT Figure Captions Fig. 1. The schematic diagram of the MAO device. Fig. 2. The schematic drawing of bipolar pulse couples. Voltage-time pulse input for the experiments E2 and E5. Fig. 3. Current-time pulse output of experiments E2 and E3 after 10 minutes of processing.
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Fig. 4. Effect of pulse durations on the coating thickness. Fig. 5. A cross-sectional SEM image of coating was taken from specimen E1. Fig. 6. XRD results of specimen E2, as coated and after removing part of coating. Fig. 7. Effect of pulse durations on the surface roughness.
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Fig. 8. SEM micrograph of the specimen E6, includes: coating layers and micro discharge channel.
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Fig. 9. XRD results of the MAO coatings prepared at different pulse durations. Fig. 10. Vickers hardness profiles of the specimens E1, E3, E5, E7 and E9.
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Fig. 11. Overall Vickers hardness profiles of the inner dense layers of the all coatings.
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Fig. 12. The cross-sectional SEM images of all coatings.
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Fig. 12.
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Increasing of pulse duration lead to decrease in thickness of dense layer and increase in pore size. The formation α-Al2O3 was depends on pulse duration.
An inner dense layer of 50 µm was achieved by 20 minutes of processing time
α-Al2O3 and -Al2O3 phases existed on all coatings.
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Figure 1
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