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Effect of Na2SiO3·5H2O concentration on microstructure and mechanical properties of plasma electrolytic oxide coatings on AZ31 Mg alloy produced by twin roll casting
Author’s Accepted Manuscript Effect of Na2SiO3.5H2O Concentration on Microstructure and Mechanical Properties of Plasma Electrolytic Oxide Coatings on AZ31 Mg Alloy Produced by Twin Roll Casting Salim Levent Aktuğ, Salih Durdu, Işıl Kutbay, Metin Usta www.elsevier.com/locate/ceri
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S0272-8842(15)01764-2 http://dx.doi.org/10.1016/j.ceramint.2015.09.056 CERI11333
To appear in: Ceramics International Received date: 23 July 2015 Revised date: 9 September 2015 Accepted date: 10 September 2015 Cite this article as: Salim Levent Aktuğ, Salih Durdu, Işıl Kutbay and Metin Usta, Effect of Na2SiO3.5H2O Concentration on Microstructure and Mechanical Properties of Plasma Electrolytic Oxide Coatings on AZ31 Mg Alloy Produced by Twin Roll Casting, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.09.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Na2SiO3.5H2O Concentration on Microstructure and Mechanical Properties of Plasma Electrolytic Oxide Coatings on AZ31 Mg Alloy Produced by Twin Roll Casting
Salim Levent Aktuğ1, Salih Durdu1,2, Işıl Kutbay1*, Metin Usta1,3
1
Department of Materials Science and Engineering, Gebze Technical University, Gebze, Kocaeli 41400, Turkey 2
3
Department of Industrial Engineering, Giresun University, Giresun 28200, Turkey
Materials Institute, Marmara Research Center, TUBITAK, Gebze, Kocaeli 41470, Turkey
*
Corresponding Author: Dr. Işıl KUTBAY
Gebze Technical University Department of Materials Science and Engineering Gebze, Kocaeli, 41400, Turkey Tel: +90 262 605 2699 e-mail: [email protected]
Abstract In this study, AZ31 Mg alloys produced by twin roll casting (TRC) were coated by plasma electrolytic oxidation (PEO) in solution of potassium hydroxide (KOH) and different concentration of sodium metasilicate pentahydrate (Na2SiO3.5H2O) electrolytes at 0.140 A/cm2 current density for 60 minutes. The phase structure, morphology, hardness, adhesion strength, surface roughness
1
and wear resistance of the PEO coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), micro hardness tester, micro scratch tester, profilometer and tribometer, respectively. The XRD results indicated that Forsterite (Mg2SiO4) and Periclase (MgO) phases were formed on the surface of the coated magnesium alloys. The surface of the coatings is very porous and rough due to the presence of micro discharges through PEO process. It was seen that the wear rate, hardness and surface roughness of the coatings was increased when the concentration of Na2SiO3.5H2O electrolyte was increased.
Keywords: Plasma electrolytic oxidation (PEO), TRC AZ31 Mg alloy, Mechanical properties, scanning electron microscopy (SEM), X-ray diffraction (XRD).
1. Introduction Magnesium and its alloys have been used in a wide range of applications of aerospace, automotive, aeronautical, transportation, electro-communication, computers and biomedical industries due to their low density, high specific strength, good castability, good weldability, relatively good electrical conductivity, high thermal conductivity, high dimensional
stability and
good
electromagnetic
shielding characteristics
[1-7].
Nevertheless, the magnesium and its alloys have poor corrosion resistance due to the low chemical stability and high negative electric potential [6, 8-10] and low wear resistance [11]. In some researches, the various magnesium alloys have been studied for the practical usage of developing anticorrosive and high wear-resistance strategies in order to improve poor corrosion and wear resistance of its alloys [12, 13]. One of the ways is to form effective ceramic oxide coatings on the surface of magnesium and its alloys by plasma electrolytic oxidation (PEO).
2
Plasma electrolytic oxidation (PEO) known as micro arc oxidation (MAO) developed from the origin of the conventional anodizing process is the production of the hard coatings which form under high voltage with very short-term and complicated reactions around the micro discharge channels on the substrate surface such as aluminum, titanium, zirconium and magnesium and their metallic alloys in the locally elevated temperatures [10, 14-18]. Plasma-chemical, thermodynamical and PEO processes form thick, hard, well-adhered ceramic coatings with high corrosion and wear resistance on the magnesium alloys [6, 10, 19]. The origin of the crystalline and amorphous-based phases formed on the surface of ceramic coatings produced by the PEO process comes from both the substrate and electrolyte solution [19]. Thus, phase compositions and properties of the coatings can be altered by solutions of sodium and potassium hydroxide (NaOH, KOH), sodium silicate (Na2SiO3), sodium phosphate (Na3PO4) and sodium aluminate (Na2Al2O4) electrolytes [19-23]. The composition and concentration of electrolytes play a key role in gaining the required morphological microstructure, mechanical and tribological properties for PEO coatings. In previous work, the effect of treatment time on the PEO coatings on AZ31 Magnesium (Mg) alloy produced by twin roll casting (TRC) was investigated by Durdu et all [24]. In another work, the effect of NaOH, Na2SiO3, sodium fluorosilicate (Na2SiF6) and sodium fluoride (NaF) electrolyte solutions on the mechanical properties for 30 min was investigated by Ahn et all. [25]. Gao and et all. examined the effect of Na2SiO3, KOH, potassium fluoride (KF) and ethylene glycol (CH2OHCH2OH) electrolyte concentrations on the lap-shear strength of adhesive-bonded Mg alloys for 10 min [26]. In this study, unlike the literature, AZ31 Mg alloys produced by twin roll casting (TRC) were coated by PEO in different concentration of sodium metasilicate pentahydrate (Na2SiO3.5H2O) and constant concentration of KOH electrolyte solution for 60 minutes. 3
There is no any study the effect of concentration on mechanical and tribological properties of PEO produced on AZ31 Mg by TRC and it was not investigated in the literature. Therefore, the aim of the study is to investigate the effect of Na2SiO3.5H2O solution concentration on the microstructural, the mechanical and the tribological properties of PEO coatings on TRC AZ31 Mg. The resultant coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), mechanical and tribological measurement, respectively.
2. Experimental Procedure
2.1. Preparation of PEO coatings
AZ31 Mg alloy plates produced by TRC with the size of 50 mm x 25 mm x 5 mm was used as substrate material in this study. The nominal composition of AZ31 Mg alloy is as mass fraction of 2.65% Al, 1.03% Zn, 0.31% Mn, 0.15% Si, 0.003% Fe, 0.002% Sn, 0.0002% Ni and balance Mg. Prior to PEO processes, all samples were polished with abrasive 400, 800, and 1200 SiC papers, washed by distilled water, cleaned by acetone and dried. Chemical reactions between electrodes in an aqueous solution of our study PEO process were illustrated schematically in Fig. 1. PEO system of 100 kW composed of a stainless steel container, cooling and stirring equipment was run with alternative current (AC). The AZ31 Mg sample was used as an anode, while stainless steel container was used as a cathode. Temperature of the electrolyte solution was below 40ºC. The AZ31 Mg alloys were coated by the PEO in the KOH and different concentration of Na2SiO3.5H2O electrolyte solution at 0.140 A/cm2 current density for 60 minutes. The duration time was chosen as 60 minutes which result in denser and thicker coatings. Moreover, the electrolyte 4
solution of KOH with Na2SiO3.5H2O was chosen to obtain oxide ceramics coatings on the AZ31 Mg alloy. Concentration of the electrolyte solutions was given in Table 1.
2.2. Microstructural Characterization of the PEO coatings
Phase structures of all samples were detected by XRD with a Bruker D8 Advance X-Ray Diffraction diffractometer with Cu-Kα radiation at 40 kV/40 mA. Each sample was scanned from 20º to 80º in 2θ with a scanning speed of 1º/min. To determine the phases present, XRD peak positions were compared with JCPDS files.
Surface morphologies of all samples were scanned by using a Philips XL30 SFEG scanning electron microscope (SEM) at 15 kV and under magnifications up to 500X and 2000X.
2.3. Mechanical properties of the PEO coatings
Micro hardness of the substrate and the coatings of AZ31 Mg alloy were measured by using Anton Paar MHT-10 Vickers tester. Adhesion strengths of all samples were measured by using a Nanovea Series scratch tester.
2.4. Tribological measurements of the PEO coatings
Wear rates of all samples and base metal were performed by using a CSM Instrument ball-on-disc mechanism tribometer at room temperature. A bearing ball with the diameter of 6 mm was tightly fixed to the pin as a static friction partner. All samples 5
were reciprocated against Al2O3 ball with the maximum linear speed of 0.3 m/s and amplitude of 17 mm. A normal load of 5 N was applied in all test. Wear tracks generated on the coatings AZ31 Mg alloys and substrate material were measured using a Veeco Dektak 8 profilometer after the wear tests. 3. Results And Discussion
3.1. Phase structure of the PEO coatings
The XRD results showed the phase structures of PEO coatings on TRC AZ31 Mg alloys (Fig. 2). The major phases of Forsterite (Mg2SiO4) and Periclase (MgO) were detected by X-ray diffraction. Similar characteristic peak structure was observed in both solutions. One noticeable difference in the XRD result was the peak intensity of MgO phases, which was greater in concentration 1 than in concentration 2. In Fig. 2(c), the increase of the peak intensity of Mg2SiO4 was observed because phase transformation between SiO2 and MgO into Mg2SiO4 increased with increasing the concentration of Na2SiO3.5H2O electrolyte solution. The results of our XRD study of PEO coatings are in good agreement with the results of other studies [24-29]. The formation mechanism of MgO on Mg alloys by PEO process is similar to the conventional anodization mechanism. Mg2+ ions are immigrated from metal to coatings layer/electrolyte interface and O2- ions are immigrated from electrolytes to metal/coating layer interfaces at the same time under the locally elevated temperature and high voltage. As a result of these reactions, MgO is produced between metal/coating layer and coating layer/electrolyte interfaces [28, 30]. MgO formation mechanism is given below:
6
Mg2+ + O2- → MgO
(1)
In this process, SiO2 was formed because of the hydrolysis of the silicate electrolyte and the following series of reactions. The composition occurred by the dissolution of SiO2 react with the substrate by the aid of the large amount heat resulting from micro discharging [6, 30-32]. SiO32- anions in the silicate electrolyte solution penetrated into the micro discharge channels because of the existence of the electrical field. SiO2 and O2 are produced by losing electrons of SiO32- anions. Both SiO2 and MgO phases melt in PEO process because of the existence of high temperature. Mg2SiO4 phase transformation between SiO2 and MgO occur in the micro discharge channels between substrate and electrolyte solution during PEO reactions at the high temperature [33-35]. As a result of this transformation, the phase formed under the influence of cooling of the electrolyte solution [28, 30, 33, 36] is given below:
SiO32- + 2H+ → SiO2 + H2O
(2)
SiO32- - 2e → SiO2 + ½ O2
(3)
SiO2 + 2 MgO → Mg2SiO4
(4).
3.2. The morphology of the PEO coatings
The surface morphology of the coated AZ31 Mg alloys produced in two different concentration of Na2SiO3.5H2O electrolyte solutions by PEO were shown in Fig. 3. As can be seen, typically, the coatings are very porous and rough surface structure due to the existence of micro sparks through the process. These pores were called as micro discharge channels in PEO process. Pore size and surface morphology depend on various parameters 7
such as the applied voltage, current value, electrolyte solution concentration and duration time. The surface of coatings becomes much porous with decreasing the concentration of Na2SiO3.5H2O electrolyte solution as seen in Fig. 3(a). However, the increasing in the concentration of Na2SiO3.5H2O electrolyte solution changed the pore size significantly in Fig. 3(d) and the number of pores decreased. The results showed that no strong relation between the porosity and the concentration of Na2SiO3 electrolyte solution and some researchers suggested the porosity was not a crucial factor that alters Mg alloy bonding strength [26]. The number of pores on coatings decreased due to growing of micro discharge channels with increasing duration time; however, the size of pores increased. Increasing the diameter of the micro discharge channels was balanced by reducing the pressure in the micro discharge channels through the process [37]. These micro discharge channels were interconnected with pores during PEO process [14]. In Figs. 3(b) and (d), there are some micro cracks seen on the micro discharge channels. These cracks were generated by the PEO process due to thermal stress between the molten oxide and chilly electrolyte during the rapid solidification [38].
3.3. Mechanical properties of the PEO coatings
Average hardness and the critical load (Lc) results measured by micro hardness tester and micro scratch tester for each coating, respectively were given in Table 2. The values of the average micro hardness of the coatings were measured as 734 HV in concentration 1 and 797 HV in concentration 2, while the hardness of TRC AZ31 Mg alloy was 72 HV. The hardness value of the coatings depends on various parameters such as phase structure and microstructure. The micro hardness of the coating increased with increasing Na2SiO3 electrolyte solution because microstructure of the coatings becomes 8
much denser through the process and the amount of Mg2SiO4 phases increased. Mg2SiO4 (forsterite) phase appeared to be greater in the concentration 2 as seen in Fig. 2. Furthermore, the coating layer in concentration 1 has very porous surface, resulting in lower micro hardness values than in concentration 2. Therefore, the coating produced in concentration 2 is harder than the one in concentration 1. Figs 4(a) and 4(b) illustrate the load–distance curves of the PEO coatings produced in concentration 1 and 2, respectively. The critical load, Lc1 that occurs as the first failure corresponds to initial cracking. Lc2 that occurs as the second failure corresponds to extensive cracking. Lc3 that occurs as the final failure corresponds to delamination of the coating from the metal surface. Lc which refers to critical load required to fail the coatings depends on various parameters such as thickness and hardness. The Lc slightly decreased with increasing concentration Na2SiO3 electrolyte solution. Adhesion strength of the coating produced in concentration 1 is greater than the one in concentration 2. It can be concluded that this may be strongly interested thickness of the PEO coating. Increasing in the adhesion strength of PEO coating with increasing thickness may be explained by formation mechanism of PEO coating on AZ31 Mg alloys. A greater volume of metal enters into the oxidation process when the thickness of the coatings increases through PEO process. Moreover, then the adhesion strength of the coating increases more and more [39].
3.4. Tribological properties of the PEO coatings
The wear track results of the coatings and TRC AZ31 Mg alloy substrate material were seen in Table 3. According to the results, wear resistance of the coatings was greater than the substrate material because of the presence of phases with high hard in coatings compared to uncoated AZ31 Mg. Wear rate of the coating produced in concentration 1 was 9
measured as 1.86x10-4 mm3/N.m while it of the coating produced in concentration 2 was measured as 2.89x10-4 mm3/N.m. Wear resistance of the coating produced in concentration 1 is better than the one in concentration 1 whereas the coating produced in concentration 2 is harder and denser than the one in concentration 1. However, the coating produced in concentration 2 is rougher than the one in concentration 2. Wear rate of the coatings depends on various parameters such as surface roughness, thickness, hardness, phase structure and compactness. The wear resistance of the coatings decreases with increasing surface roughness. The contact points in the actual area which will carry the load decrease as the surface roughness increase. Moreover, then wear rate of the coatings increases [40]. Furthermore, the outer layer of the PEO coatings is very loose compared to the inner layer. Therefore, the region of loose layer grows when the surface roughness enhances through PEO process. Cross sectional images of coatings showed that there were three distinct regions produced by PEO process (in Fig. 5). The first region was outer coating layer which had a loose contact area of the coating and these regions were very porous and had micro cracks on the surface. The second coating region was inner coatings layer that had a dense and well-adhered interface between coatings and the substrate metal. The third region was the base metal level called as the substrate. In some research, surface and cross-section morphologies of the oxide coatings formed in different electrolytes were observed by SEM [21, 29, 41]. As the concentration of Na2SiO3 electrolyte decreased in Fig. 5(a), the cross sectional morphology of the coatings become much porous structure and micro cracks. In addition, the coating thickness value is lower than the one shown in Fig. 5(b). There was a greater porosity in the region away from the substrate than in the region close to the substrate as seen in concentration 1 (Fig. 5(a)). It was seen that as the concentration of
10
Na2SiO3.5H2O electrolyte in the coatings was decreased, the coatings wear rate, surface roughness was decreased. Wear tracks of PEO coatings and uncoated AZ31 Mg alloy were given in Fig. 6. Wear track of uncoated AZ31 Mg is wider and deeper than the one of the PEO coatings because AZ31 Mg is with low hardness. There is some plastic deformation on wear tracks of AZ31 Mg. Some cracks were obtained on the surface of PEO coatings after wear test. The size of cracks on PEO coating produced in concentration 1 are greater than the one in concentration 2 because PEO coating produced in concentration 1 has many pores compared to concentration 2.
4. Conclusions
In this study, the PEO coatings on TRC AZ31 Mg alloys were produced in two different concentration of Na2SiO3.5H2O and constant concentration of KOH electrolyte solutions for 60 minutes. The following results were obtained as below: (1) According to XRD analysis results, the coatings produced in two different Na2SiO3 electrolyte solution by PEO consisted of MgO (Periclase) and Mg2SiO4 (Forsterite) phases for 60 min. In XRD result pattern, the intensity of MgO phase is increased while the intensity of Mg2SiO4 decreased with decreasing concentration Na2SiO3 electrolyte solution. (2) According to SEM analysis results, the surface morphology of the coatings was very porous owing to micro sparks during PEO process. The pore size was enhanced with increasing concentration of Na2SiO3 electrolyte solution.
11
(3) The adhesion strength of the coatings showed the similar behaviour to the various concentration of Na2SiO3.5H2O electrolyte solution. There is no perceivable difference between the adhesion strength of the coatings. (4) The micro hardness and the surface roughness of the coatings were increased with increasing concentration of Na2SiO3 electrolyte solution due to the decreased porosity of the surface and the increased in the pore size. (5) The wear rate of AZ31 Mg alloy was higher than that of the coated Mg alloys. Also, wear rates of the coatings increased with increasing concentration Na2SiO3 electrolyte solution due to much more surface roughness.
Acknowledgments
This work was supported by the number of 107M476 TUBITAK-MAG project.
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Fig. 1. Schematic representation of the PEO processes in the electrolyte solutions. Fig. 2. X-ray diffraction patterns of (a) TRC AZ31 Mg alloy, the coated AZ31 Mg alloys in (b) concentration 1 and (c) concentration 2 solutions by PEO. Fig. 3. SEM images of (a) surface morphology, (b) discharge channel morphology in concentration 1 solution and (c) surface morphology, (d) discharge channel morphology in concentration 2 solution of the coated AZ31 Mg alloys by PEO. Fig. 4. Cross section of the coated AZ31 Mg alloy (a) concentration 1 solution and (b) concentration 2 solution by PEO. Fig. 5. Cross section of the coated AZ31 Mg alloy in (a) concentration 1 solution and (b) concentration 2 solution by PEO. Fig. 6. The wear tracks of (a) untreated TRC AZ31 Mg alloy and the coated AZ31 Mg alloys in (b) concentration 1 and (c) concentration 2 solutions by PEO.
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Tables Table 1 Concentration of electrolyte solution Table 2 The values of average thickness, microhardness and microscratch test results of PEO coatings TRC AZ31 Mg alloys Table 3 Wear rates and surface roughness of the coatings and TRC AZ31 Mg alloy substrate material
16
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Table 1
Table 1 Concentration of electrolyte solution. Electrolyte solution
Na2SiO3.5H2O
KOH
Concentration 1 (g/L)
4
1.5
Concentration 2 (g/L)
8
1.5
Table 2
Table 2 The values of average thickness, microhardness and microscratch test results of PEO coatings TRC AZ31 Mg alloys ID Sample
Average
Average
Thickness
Microhardness
(µm)
(Hv)
Lc1 (N)
Normal
Lc2 (N)
Frictional
Lc3 (N)
Normal
Frictional
Normal
Frictional
Load (N) Force (N)
Load (N)
Force (N)
Load (N) Force (N)
Concentration 1 73,3
734
97,726
25,793
128,668
39,484
145,011
46,094
Concentration 2 67,6
797
109,036
23,978
123,252
36,306
143,672
45,792
Table 3
Table 3 Wear rates and surface roughness of the coatings and TRC AZ31 Mg alloy substrate material ID Sample
Surface roughness (µm)
Wear rate x 104 (mm3/N/m)
Untreated AZ31 Mg
0.15
35.0
Concentration 1
5.75
1.86
Concentration 2
6.23
2.89
PII: DOI: Reference:
S0272-8842(15)01764-2 http://dx.doi.org/10.1016/j.ceramint.2015.09.056 CERI11333
To appear in: Ceramics International Received date: 23 July 2015 Revised date: 9 September 2015 Accepted date: 10 September 2015 Cite this article as: Salim Levent Aktuğ, Salih Durdu, Işıl Kutbay and Metin Usta, Effect of Na2SiO3.5H2O Concentration on Microstructure and Mechanical Properties of Plasma Electrolytic Oxide Coatings on AZ31 Mg Alloy Produced by Twin Roll Casting, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.09.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Na2SiO3.5H2O Concentration on Microstructure and Mechanical Properties of Plasma Electrolytic Oxide Coatings on AZ31 Mg Alloy Produced by Twin Roll Casting
Salim Levent Aktuğ1, Salih Durdu1,2, Işıl Kutbay1*, Metin Usta1,3
1
Department of Materials Science and Engineering, Gebze Technical University, Gebze, Kocaeli 41400, Turkey 2
3
Department of Industrial Engineering, Giresun University, Giresun 28200, Turkey
Materials Institute, Marmara Research Center, TUBITAK, Gebze, Kocaeli 41470, Turkey
*
Corresponding Author: Dr. Işıl KUTBAY
Gebze Technical University Department of Materials Science and Engineering Gebze, Kocaeli, 41400, Turkey Tel: +90 262 605 2699 e-mail: [email protected]
Abstract In this study, AZ31 Mg alloys produced by twin roll casting (TRC) were coated by plasma electrolytic oxidation (PEO) in solution of potassium hydroxide (KOH) and different concentration of sodium metasilicate pentahydrate (Na2SiO3.5H2O) electrolytes at 0.140 A/cm2 current density for 60 minutes. The phase structure, morphology, hardness, adhesion strength, surface roughness
1
and wear resistance of the PEO coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), micro hardness tester, micro scratch tester, profilometer and tribometer, respectively. The XRD results indicated that Forsterite (Mg2SiO4) and Periclase (MgO) phases were formed on the surface of the coated magnesium alloys. The surface of the coatings is very porous and rough due to the presence of micro discharges through PEO process. It was seen that the wear rate, hardness and surface roughness of the coatings was increased when the concentration of Na2SiO3.5H2O electrolyte was increased.
Keywords: Plasma electrolytic oxidation (PEO), TRC AZ31 Mg alloy, Mechanical properties, scanning electron microscopy (SEM), X-ray diffraction (XRD).
1. Introduction Magnesium and its alloys have been used in a wide range of applications of aerospace, automotive, aeronautical, transportation, electro-communication, computers and biomedical industries due to their low density, high specific strength, good castability, good weldability, relatively good electrical conductivity, high thermal conductivity, high dimensional
stability and
good
electromagnetic
shielding characteristics
[1-7].
Nevertheless, the magnesium and its alloys have poor corrosion resistance due to the low chemical stability and high negative electric potential [6, 8-10] and low wear resistance [11]. In some researches, the various magnesium alloys have been studied for the practical usage of developing anticorrosive and high wear-resistance strategies in order to improve poor corrosion and wear resistance of its alloys [12, 13]. One of the ways is to form effective ceramic oxide coatings on the surface of magnesium and its alloys by plasma electrolytic oxidation (PEO).
2
Plasma electrolytic oxidation (PEO) known as micro arc oxidation (MAO) developed from the origin of the conventional anodizing process is the production of the hard coatings which form under high voltage with very short-term and complicated reactions around the micro discharge channels on the substrate surface such as aluminum, titanium, zirconium and magnesium and their metallic alloys in the locally elevated temperatures [10, 14-18]. Plasma-chemical, thermodynamical and PEO processes form thick, hard, well-adhered ceramic coatings with high corrosion and wear resistance on the magnesium alloys [6, 10, 19]. The origin of the crystalline and amorphous-based phases formed on the surface of ceramic coatings produced by the PEO process comes from both the substrate and electrolyte solution [19]. Thus, phase compositions and properties of the coatings can be altered by solutions of sodium and potassium hydroxide (NaOH, KOH), sodium silicate (Na2SiO3), sodium phosphate (Na3PO4) and sodium aluminate (Na2Al2O4) electrolytes [19-23]. The composition and concentration of electrolytes play a key role in gaining the required morphological microstructure, mechanical and tribological properties for PEO coatings. In previous work, the effect of treatment time on the PEO coatings on AZ31 Magnesium (Mg) alloy produced by twin roll casting (TRC) was investigated by Durdu et all [24]. In another work, the effect of NaOH, Na2SiO3, sodium fluorosilicate (Na2SiF6) and sodium fluoride (NaF) electrolyte solutions on the mechanical properties for 30 min was investigated by Ahn et all. [25]. Gao and et all. examined the effect of Na2SiO3, KOH, potassium fluoride (KF) and ethylene glycol (CH2OHCH2OH) electrolyte concentrations on the lap-shear strength of adhesive-bonded Mg alloys for 10 min [26]. In this study, unlike the literature, AZ31 Mg alloys produced by twin roll casting (TRC) were coated by PEO in different concentration of sodium metasilicate pentahydrate (Na2SiO3.5H2O) and constant concentration of KOH electrolyte solution for 60 minutes. 3
There is no any study the effect of concentration on mechanical and tribological properties of PEO produced on AZ31 Mg by TRC and it was not investigated in the literature. Therefore, the aim of the study is to investigate the effect of Na2SiO3.5H2O solution concentration on the microstructural, the mechanical and the tribological properties of PEO coatings on TRC AZ31 Mg. The resultant coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), mechanical and tribological measurement, respectively.
2. Experimental Procedure
2.1. Preparation of PEO coatings
AZ31 Mg alloy plates produced by TRC with the size of 50 mm x 25 mm x 5 mm was used as substrate material in this study. The nominal composition of AZ31 Mg alloy is as mass fraction of 2.65% Al, 1.03% Zn, 0.31% Mn, 0.15% Si, 0.003% Fe, 0.002% Sn, 0.0002% Ni and balance Mg. Prior to PEO processes, all samples were polished with abrasive 400, 800, and 1200 SiC papers, washed by distilled water, cleaned by acetone and dried. Chemical reactions between electrodes in an aqueous solution of our study PEO process were illustrated schematically in Fig. 1. PEO system of 100 kW composed of a stainless steel container, cooling and stirring equipment was run with alternative current (AC). The AZ31 Mg sample was used as an anode, while stainless steel container was used as a cathode. Temperature of the electrolyte solution was below 40ºC. The AZ31 Mg alloys were coated by the PEO in the KOH and different concentration of Na2SiO3.5H2O electrolyte solution at 0.140 A/cm2 current density for 60 minutes. The duration time was chosen as 60 minutes which result in denser and thicker coatings. Moreover, the electrolyte 4
solution of KOH with Na2SiO3.5H2O was chosen to obtain oxide ceramics coatings on the AZ31 Mg alloy. Concentration of the electrolyte solutions was given in Table 1.
2.2. Microstructural Characterization of the PEO coatings
Phase structures of all samples were detected by XRD with a Bruker D8 Advance X-Ray Diffraction diffractometer with Cu-Kα radiation at 40 kV/40 mA. Each sample was scanned from 20º to 80º in 2θ with a scanning speed of 1º/min. To determine the phases present, XRD peak positions were compared with JCPDS files.
Surface morphologies of all samples were scanned by using a Philips XL30 SFEG scanning electron microscope (SEM) at 15 kV and under magnifications up to 500X and 2000X.
2.3. Mechanical properties of the PEO coatings
Micro hardness of the substrate and the coatings of AZ31 Mg alloy were measured by using Anton Paar MHT-10 Vickers tester. Adhesion strengths of all samples were measured by using a Nanovea Series scratch tester.
2.4. Tribological measurements of the PEO coatings
Wear rates of all samples and base metal were performed by using a CSM Instrument ball-on-disc mechanism tribometer at room temperature. A bearing ball with the diameter of 6 mm was tightly fixed to the pin as a static friction partner. All samples 5
were reciprocated against Al2O3 ball with the maximum linear speed of 0.3 m/s and amplitude of 17 mm. A normal load of 5 N was applied in all test. Wear tracks generated on the coatings AZ31 Mg alloys and substrate material were measured using a Veeco Dektak 8 profilometer after the wear tests. 3. Results And Discussion
3.1. Phase structure of the PEO coatings
The XRD results showed the phase structures of PEO coatings on TRC AZ31 Mg alloys (Fig. 2). The major phases of Forsterite (Mg2SiO4) and Periclase (MgO) were detected by X-ray diffraction. Similar characteristic peak structure was observed in both solutions. One noticeable difference in the XRD result was the peak intensity of MgO phases, which was greater in concentration 1 than in concentration 2. In Fig. 2(c), the increase of the peak intensity of Mg2SiO4 was observed because phase transformation between SiO2 and MgO into Mg2SiO4 increased with increasing the concentration of Na2SiO3.5H2O electrolyte solution. The results of our XRD study of PEO coatings are in good agreement with the results of other studies [24-29]. The formation mechanism of MgO on Mg alloys by PEO process is similar to the conventional anodization mechanism. Mg2+ ions are immigrated from metal to coatings layer/electrolyte interface and O2- ions are immigrated from electrolytes to metal/coating layer interfaces at the same time under the locally elevated temperature and high voltage. As a result of these reactions, MgO is produced between metal/coating layer and coating layer/electrolyte interfaces [28, 30]. MgO formation mechanism is given below:
6
Mg2+ + O2- → MgO
(1)
In this process, SiO2 was formed because of the hydrolysis of the silicate electrolyte and the following series of reactions. The composition occurred by the dissolution of SiO2 react with the substrate by the aid of the large amount heat resulting from micro discharging [6, 30-32]. SiO32- anions in the silicate electrolyte solution penetrated into the micro discharge channels because of the existence of the electrical field. SiO2 and O2 are produced by losing electrons of SiO32- anions. Both SiO2 and MgO phases melt in PEO process because of the existence of high temperature. Mg2SiO4 phase transformation between SiO2 and MgO occur in the micro discharge channels between substrate and electrolyte solution during PEO reactions at the high temperature [33-35]. As a result of this transformation, the phase formed under the influence of cooling of the electrolyte solution [28, 30, 33, 36] is given below:
SiO32- + 2H+ → SiO2 + H2O
(2)
SiO32- - 2e → SiO2 + ½ O2
(3)
SiO2 + 2 MgO → Mg2SiO4
(4).
3.2. The morphology of the PEO coatings
The surface morphology of the coated AZ31 Mg alloys produced in two different concentration of Na2SiO3.5H2O electrolyte solutions by PEO were shown in Fig. 3. As can be seen, typically, the coatings are very porous and rough surface structure due to the existence of micro sparks through the process. These pores were called as micro discharge channels in PEO process. Pore size and surface morphology depend on various parameters 7
such as the applied voltage, current value, electrolyte solution concentration and duration time. The surface of coatings becomes much porous with decreasing the concentration of Na2SiO3.5H2O electrolyte solution as seen in Fig. 3(a). However, the increasing in the concentration of Na2SiO3.5H2O electrolyte solution changed the pore size significantly in Fig. 3(d) and the number of pores decreased. The results showed that no strong relation between the porosity and the concentration of Na2SiO3 electrolyte solution and some researchers suggested the porosity was not a crucial factor that alters Mg alloy bonding strength [26]. The number of pores on coatings decreased due to growing of micro discharge channels with increasing duration time; however, the size of pores increased. Increasing the diameter of the micro discharge channels was balanced by reducing the pressure in the micro discharge channels through the process [37]. These micro discharge channels were interconnected with pores during PEO process [14]. In Figs. 3(b) and (d), there are some micro cracks seen on the micro discharge channels. These cracks were generated by the PEO process due to thermal stress between the molten oxide and chilly electrolyte during the rapid solidification [38].
3.3. Mechanical properties of the PEO coatings
Average hardness and the critical load (Lc) results measured by micro hardness tester and micro scratch tester for each coating, respectively were given in Table 2. The values of the average micro hardness of the coatings were measured as 734 HV in concentration 1 and 797 HV in concentration 2, while the hardness of TRC AZ31 Mg alloy was 72 HV. The hardness value of the coatings depends on various parameters such as phase structure and microstructure. The micro hardness of the coating increased with increasing Na2SiO3 electrolyte solution because microstructure of the coatings becomes 8
much denser through the process and the amount of Mg2SiO4 phases increased. Mg2SiO4 (forsterite) phase appeared to be greater in the concentration 2 as seen in Fig. 2. Furthermore, the coating layer in concentration 1 has very porous surface, resulting in lower micro hardness values than in concentration 2. Therefore, the coating produced in concentration 2 is harder than the one in concentration 1. Figs 4(a) and 4(b) illustrate the load–distance curves of the PEO coatings produced in concentration 1 and 2, respectively. The critical load, Lc1 that occurs as the first failure corresponds to initial cracking. Lc2 that occurs as the second failure corresponds to extensive cracking. Lc3 that occurs as the final failure corresponds to delamination of the coating from the metal surface. Lc which refers to critical load required to fail the coatings depends on various parameters such as thickness and hardness. The Lc slightly decreased with increasing concentration Na2SiO3 electrolyte solution. Adhesion strength of the coating produced in concentration 1 is greater than the one in concentration 2. It can be concluded that this may be strongly interested thickness of the PEO coating. Increasing in the adhesion strength of PEO coating with increasing thickness may be explained by formation mechanism of PEO coating on AZ31 Mg alloys. A greater volume of metal enters into the oxidation process when the thickness of the coatings increases through PEO process. Moreover, then the adhesion strength of the coating increases more and more [39].
3.4. Tribological properties of the PEO coatings
The wear track results of the coatings and TRC AZ31 Mg alloy substrate material were seen in Table 3. According to the results, wear resistance of the coatings was greater than the substrate material because of the presence of phases with high hard in coatings compared to uncoated AZ31 Mg. Wear rate of the coating produced in concentration 1 was 9
measured as 1.86x10-4 mm3/N.m while it of the coating produced in concentration 2 was measured as 2.89x10-4 mm3/N.m. Wear resistance of the coating produced in concentration 1 is better than the one in concentration 1 whereas the coating produced in concentration 2 is harder and denser than the one in concentration 1. However, the coating produced in concentration 2 is rougher than the one in concentration 2. Wear rate of the coatings depends on various parameters such as surface roughness, thickness, hardness, phase structure and compactness. The wear resistance of the coatings decreases with increasing surface roughness. The contact points in the actual area which will carry the load decrease as the surface roughness increase. Moreover, then wear rate of the coatings increases [40]. Furthermore, the outer layer of the PEO coatings is very loose compared to the inner layer. Therefore, the region of loose layer grows when the surface roughness enhances through PEO process. Cross sectional images of coatings showed that there were three distinct regions produced by PEO process (in Fig. 5). The first region was outer coating layer which had a loose contact area of the coating and these regions were very porous and had micro cracks on the surface. The second coating region was inner coatings layer that had a dense and well-adhered interface between coatings and the substrate metal. The third region was the base metal level called as the substrate. In some research, surface and cross-section morphologies of the oxide coatings formed in different electrolytes were observed by SEM [21, 29, 41]. As the concentration of Na2SiO3 electrolyte decreased in Fig. 5(a), the cross sectional morphology of the coatings become much porous structure and micro cracks. In addition, the coating thickness value is lower than the one shown in Fig. 5(b). There was a greater porosity in the region away from the substrate than in the region close to the substrate as seen in concentration 1 (Fig. 5(a)). It was seen that as the concentration of
10
Na2SiO3.5H2O electrolyte in the coatings was decreased, the coatings wear rate, surface roughness was decreased. Wear tracks of PEO coatings and uncoated AZ31 Mg alloy were given in Fig. 6. Wear track of uncoated AZ31 Mg is wider and deeper than the one of the PEO coatings because AZ31 Mg is with low hardness. There is some plastic deformation on wear tracks of AZ31 Mg. Some cracks were obtained on the surface of PEO coatings after wear test. The size of cracks on PEO coating produced in concentration 1 are greater than the one in concentration 2 because PEO coating produced in concentration 1 has many pores compared to concentration 2.
4. Conclusions
In this study, the PEO coatings on TRC AZ31 Mg alloys were produced in two different concentration of Na2SiO3.5H2O and constant concentration of KOH electrolyte solutions for 60 minutes. The following results were obtained as below: (1) According to XRD analysis results, the coatings produced in two different Na2SiO3 electrolyte solution by PEO consisted of MgO (Periclase) and Mg2SiO4 (Forsterite) phases for 60 min. In XRD result pattern, the intensity of MgO phase is increased while the intensity of Mg2SiO4 decreased with decreasing concentration Na2SiO3 electrolyte solution. (2) According to SEM analysis results, the surface morphology of the coatings was very porous owing to micro sparks during PEO process. The pore size was enhanced with increasing concentration of Na2SiO3 electrolyte solution.
11
(3) The adhesion strength of the coatings showed the similar behaviour to the various concentration of Na2SiO3.5H2O electrolyte solution. There is no perceivable difference between the adhesion strength of the coatings. (4) The micro hardness and the surface roughness of the coatings were increased with increasing concentration of Na2SiO3 electrolyte solution due to the decreased porosity of the surface and the increased in the pore size. (5) The wear rate of AZ31 Mg alloy was higher than that of the coated Mg alloys. Also, wear rates of the coatings increased with increasing concentration Na2SiO3 electrolyte solution due to much more surface roughness.
Acknowledgments
This work was supported by the number of 107M476 TUBITAK-MAG project.
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Fig. 1. Schematic representation of the PEO processes in the electrolyte solutions. Fig. 2. X-ray diffraction patterns of (a) TRC AZ31 Mg alloy, the coated AZ31 Mg alloys in (b) concentration 1 and (c) concentration 2 solutions by PEO. Fig. 3. SEM images of (a) surface morphology, (b) discharge channel morphology in concentration 1 solution and (c) surface morphology, (d) discharge channel morphology in concentration 2 solution of the coated AZ31 Mg alloys by PEO. Fig. 4. Cross section of the coated AZ31 Mg alloy (a) concentration 1 solution and (b) concentration 2 solution by PEO. Fig. 5. Cross section of the coated AZ31 Mg alloy in (a) concentration 1 solution and (b) concentration 2 solution by PEO. Fig. 6. The wear tracks of (a) untreated TRC AZ31 Mg alloy and the coated AZ31 Mg alloys in (b) concentration 1 and (c) concentration 2 solutions by PEO.
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Tables Table 1 Concentration of electrolyte solution Table 2 The values of average thickness, microhardness and microscratch test results of PEO coatings TRC AZ31 Mg alloys Table 3 Wear rates and surface roughness of the coatings and TRC AZ31 Mg alloy substrate material
16
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Table 1
Table 1 Concentration of electrolyte solution. Electrolyte solution
Na2SiO3.5H2O
KOH
Concentration 1 (g/L)
4
1.5
Concentration 2 (g/L)
8
1.5
Table 2
Table 2 The values of average thickness, microhardness and microscratch test results of PEO coatings TRC AZ31 Mg alloys ID Sample
Average
Average
Thickness
Microhardness
(µm)
(Hv)
Lc1 (N)
Normal
Lc2 (N)
Frictional
Lc3 (N)
Normal
Frictional
Normal
Frictional
Load (N) Force (N)
Load (N)
Force (N)
Load (N) Force (N)
Concentration 1 73,3
734
97,726
25,793
128,668
39,484
145,011
46,094
Concentration 2 67,6
797
109,036
23,978
123,252
36,306
143,672
45,792
Table 3
Table 3 Wear rates and surface roughness of the coatings and TRC AZ31 Mg alloy substrate material ID Sample
Surface roughness (µm)
Wear rate x 104 (mm3/N/m)
Untreated AZ31 Mg
0.15
35.0
Concentration 1
5.75
1.86
Concentration 2
6.23
2.89