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Characterization and mechanical properties of the duplex coatings produced on steel by electro-spark deposition and micro-arc oxidation
Surface & Coatings Technology 236 (2013) 303–308
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Characterization and mechanical properties of the duplex coatings produced on steel by electro-spark deposition and micro-arc oxidation Salih Durdu ⁎, Salim Levent Aktuğ, Kemal Korkmaz The Department of Materials Science and Engineering, Gebze Institute of Technology, Gebze, Kocaeli, 41400, Turkey
a r t i c l e
i n f o
Article history: Received 13 August 2013 Accepted in revised form 4 October 2013 Available online 16 October 2013 Keywords: Electro-spark deposition (ESD) Micro-arc oxidation (MAO) Duplex coating Mechanical properties
a b s t r a c t In this study, the ESD (electro-spark deposition) and the MAO (micro-arc oxidation) processes were used to improve the mechanical properties and the tribological performance of steel. This study has focused on forming a duplex surface coating on the steel. The ESD method was carried out a titanium alloy (Ti6Al4V) deposit layer on the steel substrates at the first step, and then the MAO process was employed to improve properties of the titanium alloyed layer at the second step. Phase structure, surface morphology, elemental composition, hardness, adhesion strength and tribological property of the ESD and the ESD+MAO coating (the duplex coating) were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDX), micro Vickers tester, micro scratch tester and tribometer, respectively. The XRD results indicated that the duplex coating consists of α-Al2O3 (corundum) and γ-Al2O3 phases while AlFe3, TiN and AlTi3 phases were detected in the ESD coating. The hardness of the duplex coating was significantly improved compared to the uncoated steel and the ESD coating. The adhesion strength of the duplex coating was greater than the ESD coating due to the existence of high hardness and high thickness. In addition, the tribological properties of the ESD and the duplex coatings were significantly improved with compared to the uncoated steel. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys have high strength to weight ratio, great corrosion resistance, excellent mechanical properties and biocompatibility [1–3]. Thus, titanium and its alloys have been used in aerospace, marine, chemical, petrochemical, sports and biomedicine industries because of their outstanding properties [4,5]. However, titanium and its alloys limit to their various applications such as engineering components because they exhibit poor tribological properties such as high and unstable friction coefficients, strong tendency to adhesion, low fretting wear resistance and poor corrosion resistance in some aggressive corrosive environment such as hot chloride solutions [6]. These properties were improved by using some surface engineering methods such as ion implantation [7], thermal oxidation [8], chemical or physical vapor deposition (CVD, PVD) [9,10], magnetron sputtering techniques [11] or the duplex surface treatments [12]. However, these methods are very expensive and difficult to produce complex shaped component. The high cost of titanium and its alloys is another restriction for their use of those applications compared to aluminum and steel alloys which are low cost materials.
The micro-arc oxidation (MAO), also known as plasma electrolytic oxidation (PEO), combines the high voltage spark and electrochemical oxidation. This is a relatively convenient and effective technique to deposit various functional ceramic coatings with porous structures on the surfaces of Ti, Al, Mg and their alloys [13–15]. The MAO coatings on titanium and its alloys provide high hardness, excellent wear resistance and excellent corrosion resistance in terms of high manufacturability and economic efficiency [16–19]. In addition, the MAO forms an oxide film that strongly adheres to a metal substrate with complex geometries [20]. In this study, a duplex coating on St35 steel was produced by using electro-spark deposition (ESD) and micro-arc oxidation (MAO) methods. The ESD method was carried out in order to deposit a titanium alloy (Ti6Al4V) layer on the steel (St35) substrates at the first step. And then, Al2O3 based on a duplex layer was formed to improve mechanical and tribological properties of the titanium alloyed layer on the ESD surface by MAO process at the second step. The mechanical and tribological properties of the ESD and the duplex coatings were investigated in detail. 2. Experimental details
⁎ Corresponding author at: Gebze Institute of Technology, The Department of Materials Science and Engineering, Gebze Institute of Technology, Cayirova Kampus, Gebze, Kocaeli, 41400, Turkey. Tel.: +90 2626052689; fax: +90 2626058490. E-mail addresses: [email protected], [email protected] (S. Durdu), [email protected] (S.L. Aktuğ), [email protected] (K. Korkmaz).
2.1. Materials and methods The Ti6Al4V alloy, which was selected as an electrode, is deposited on a steel (St35) substrate in the ESD process experiments. Tables 1
0257-8972/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.10.004
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S. Durdu et al. / Surface & Coatings Technology 236 (2013) 303–308
Table 1 Chemical composition of St 35 steel substrate (wt. %). C
Si
Mn
Cr
Ni
Mo
Cu
Co
Fe
0.14
0.28
0.81
0.25
0.17
0.02
0.03
0.01
Balance
Table 2 Chemical composition of Ti6Al4V alloy (wt. %). Al
V
Fe
Si
C
N
H
O
Ti
5.5
4.5
0.3
0.15
0.10
0.05
0.015
0.15
Balance
and 2 show the chemical compositions of St35 steel and Ti6Al4V alloys, respectively. The electrode has a cross-section of 3×5mm2 and the steel substrate samples are in the form of rectangular plates with the dimensions of 3 mm × 20 mm × 25 mm. After the ESD process, some of these samples were used for MAO process. In this study, a special electro spark deposition (ESD) machine was operated at the first step. Power consumption was 180 W and the output of stabilized voltage was kept constant at 40 V. The ESD coating system was described in detail in our previous studies [21,22]. The ESD process was performed using a hand-held applicator in unipolar mode under constant temperature as seen in Fig. 1. In the system, the voltage dropped at the inter electrode gap (17 V) and the amount of electricity (3 C) was kept constant. The ESD process was conducted in air with a series of rectangular pulses of duration time of 100 μs, and the amplitudes of current 100 A. The frequencies of pulses were estimated as 100 Hz by considering the requirement of maintaining the constant electricity (3 C) of process. The MAO with the application of bipolar impulses was used for the fabrication of oxide layer on titanium coated steel samples at the second step. Electrolyte solution was prepared by mixing 1.65 g/L Na3PO4, 8 g/L NaAlO2 in distilled water. The MAO process was carried out by transforming metal oxidation on the surfaces of steel samples which were coated Ti6Al4V alloy by using ESD technique. In the experiments of MAO process (Fig. 2), cathodic and anodic voltage were applied as Uc = 160 V and Ua = 550 V, respectively and the electrical energy of flux which was chosen in respect to the area of sample surfaces was 1.8 μF/cm2. The treatment time was 30 min, since the rate of the coating growth was about 0.6–0.7 μm/min.
Fig. 1. Schematic representation of electro-spark deposition (ESD) coating system set up.
Fig. 2. Schematic representation of micro-arc oxidation (MAO) coating system set up.
2.2. Characterization of the coatings The thickness of the ESD and the duplex coatings was measured by using an eddy current method (Fischer Dualscope MP40) at 50 randomly selected locations. The surface roughness measurements were conducted by using a profilometer (SJ-400 Mitutoyo) with a precision of 0.01 μm after the ESD and the MAO processes. The average roughness values (Ra) of five measurements were reported for all surfaces of the samples. Investigation of the surface morphology and its quality (porosity, fracture, etc.) on the ESD and the duplex coatings conducted samples was performed using a scanning electron microscope (SEM, Philips XL 30 SFEG). In addition, the EDX spectrum analyses were performed using the embedded EDX digital controller and software attached to the SEM. The phase composition of steel, Ti6Al4V, the ESD and the ESD+MAO layers were investigated by using XRD (Bruker D8
Fig. 3. The surface morphologies of the coating surfaces: (a) the ESD technique and (b) the MAO process.
S. Durdu et al. / Surface & Coatings Technology 236 (2013) 303–308
305
Fig. 6. The variations of average microhardness of St35 steel substrate, ESD coating and the duplex coating. Fig. 4. The X-ray diffraction patterns of uncoated and the coated surfaces: (a) St35 steel, (b) Ti6Al4V alloy before ESD process, (c) ESD coating and (d) the duplex coating.
Table 3 EDX — elemental composition results of the ESD coating and the duplex coating (at. %). Elements
ESD coating (at. %)
ESD+MAO based duplex coating (at. %)
Fe Ti Al V O Na P
39.06 19.67 2.88 1.48 36.92 – –
0.67 0.31 36.58 – 58.78 0.96 2.69
Advance). The XRD pattern measurements were conducted in the range of 20° to 90°, at scanning speed of 2° min−1. The cross sections of the coatings were polished for hardness (HV) test measurements. These measurements were performed by microhardness tester (Anton Paar MHT-10) at a load of 100 g and a loading time of 5 s. The metallographic analysis was carried out using an optical microscope (Zeiss Axio Imager M1m). The adhesion strengths of the coatings were determined by micro/macro scratch tester (Nanovea Series). During the test, a linearly
increasing load was applied to a static indenter at a loading rate of 60 N/min, while the sample was moved with a table at a speed of 6 mm/min. At the same time, the friction forces were recorded. A Rockwell C diamond indenter with a radius of 200 μm was used at the test, and the scratch load was applied from 0 N and stopped at 60 N during test. A standard ball-on-disk tribometer (CSM Instruments) was employed to investigate the tribological behaviors of steel, Ti6Al4V alloy deposited and micro-arc oxidized samples. Friction coefficient of the coatings was evaluated by the software of the tribometer. The dry sliding wear test experiments were performed under constant pressure at room temperature and at a relative humidity of 50 ± 2%. A WC ball with a diameter of 6 mm was loaded with constant loads of 10 N with the sliding speed of 10 cm/s, the distance of 100 m and amplitude of 17 mm. 3. Results and discussion The surface morphology of a coating processed by the ESD method is shown in Fig. 3a. The irregularity and roughness of the ESD surface prove that the process is successfully done. This is characteristic feature of the ESD process [21]. The surface morphology of a coating processed
Fig. 5. The cross-sectional morphology and line-scan EDX analysis of two layers in the duplex coating produced by ESD and MAO processes: (1) St35 region, (2) ESD coating region, (3) MAO coating region and (4) epofix resin region.
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Fig. 7. The load–distance curves and optical micrograph images for scratch test (a) ESD coating and (b) the duplex coating.
by MAO method is shown in Fig. 3b. There are many pores and crater like structures on surface of the coating produced by MAO. The size of pores increases while the number of pores decreases on surface along the MAO process. Micro discharge channels occur onto the surface of the ESD coating at the MAO. The discharge events appear to be irregular on the surface of the substrate which results in the formation of coating defects like surface roughness, flakes, voids, cavities, and volcano-like features. When a discharge is extinguished, it leaves pores on the coating surface. Thus the MAO coatings usually show a porous surface layer, while the porous surface can be traceable to the high temperature in the discharge channels during the process [23]. As a result, the surface of the micro-arc oxidized coatings is very rough and porous due to the existence of the sparks in micro discharge channels during the MAO process [14] as seen in Fig. 3b. According to the profilometer results, average roughness values (Ra) for the coatings produced by ESD and
ESD+MAO were measured as 4.80 μm and 6.92 μm, respectively. Thus, the duplex coating found to be rougher than the ESD coating. The XRD patterns of the uncoated and coated surfaces are shown in Fig. 4, which reveals the present phases in the coatings. The Ti peaks were detected in the Ti6Al4V alloy. Al and V could not be detected by XRD because they are not a phase structure in Ti6Al4V alloy. According to the XRD pattern of coating obtained by ESD process, the coating layer consists of majorly AlFe3 (aluminum ferrate) and TiN (osbornite) phases, and minorly α-Fe and AlTi3 (aluminum titanate) phases. It is clear that the α-Fe phase belongs to steel substrate and the other phases are accepted as a result of the ESD process. The presence of the AlFe3 phase is related to the alloying of the coating and the steel substrate. It could be thought that the formation of TiN phase is due to the ESD process in open air. It is clear that the ESD+MAO based the duplex coating layer mainly consisting of stable α-Al2O3 (corundum) and
Table 4 Experimental scratch test results obtained at 500 μm radius for the ESD coating and the duplex coating. Sample
The ESD coating The duplex coating
Average thickness (μm)
Average hardness (HV)
Lc1
Lc2
Lc3
Normal load (N)
Frictional force (N)
Distance (mm)
Normal load (N)
Frictional force (N)
Distance (mm)
Normal load (N)
Frictional force (N)
Distance (mm)
17.5 ± 3 32.3 ± 5
510 940
15.199 29.530
3.966 5.493
1.533 2.921
33.196 35.975
8.376 5.945
3.298 3.578
36.123 50.932
7.438 12.849
3.610 5.059
S. Durdu et al. / Surface & Coatings Technology 236 (2013) 303–308
metastable γ-Al2O3 as major phases. And also, the TiN, TiAl2O5 and α-Fe exist as minor phases in the duplex coating structure. Especially, α-Al2O3 contributes to the increase of hardness of the duplex coating as seen in Fig. 5 due to its high hardness [24]. The surface elemental composition results of the ESD coating and the duplex coating are given in Table 3 by EDX analysis. As it is expected, Ti, Al, V and Fe elements present in the ESD alloying layer and also O elements present due to the ESD process. Ti, Fe and O exist as major elements on the surface while the amount of Al and V is very low. Also, Al, O, Ti, Fe, Na and P elements were found in the duplex coating structure. The Al and O as major elements and Na, P and Ti elements were detected in the coating produced in sodium aluminate and sodium phosphate solution by MAO process. The origin of the Na and P could be NaAl2O3 and Na3PO4 based electrolyte during the MAO process. There are two layers (the duplex structure) in the coating produced by ESD and MAO processes as seen in Fig. 5. According to the elemental graph, the amount of Ti and Al decreases from the ESD coating surface to the St35 substrate, while the amount of Fe increases as expected at steel substrate. The main structure is the distribution of Ti, Al and Fe elements. This implies a formation of AlFe3 and AlTi3 based coating in the inner layer. Moreover, the amount of Al and O increases from the ESD coating to the ESD+MAO interface region and it reaches maximum value through the MAO region. The presence of Al and O in the outer layer contributes to the formation of α-Al2O3 and γ-Al2O3 during the MAO process. There are ESD and MAO based two layers in the duplex coating and these layers are nonuniform along the cross section as seen in Fig. 5. The average thickness values of the ESD coating and the duplex coating were evaluated as 17.5 ± 3 μm and 32.3 ± 5 μm, respectively. The cross sectional microhardness results of the St35 steel, the ESD and the duplex coatings is illustrated in Fig. 6. The average values and error variances were calculated based on 10 random measurement points onto cross section of the coatings. The average microhardness values of uncoated St35 steel and coated samples by only ESD process and the ESD + MAO processes were determined as 270 ± 13.5, 510 ± 25.5 and 940 ± 47 HV, respectively. After both coating processes, a significant increase in the hardness is seen as a result of coating processes. The hardness value depends on various parameters such as phase composition and microstructure. The average microhardness values of the ESD and the duplex coatings are greater than uncoated St35 steel, because these coatings mainly consists of phases with high hardness such as AlFe3, AlTi3, TiN, α-Al2O3 and γ-Al2O3. Especially, the average microhardness of the duplex coating was significantly improved to the ESD coating although the coating layer produced by ESD is denser than the one produced by MAO layer. These results are related to the presence of α-Al2O3 (corundum) and γ-Al2O3 phases in the microstructure of the MAO layer. The duplex coating mainly consists of α-Al2O3 phase with high hardness and metastable γ-Al2O3. The corundum phase which has theoretical hardness value with 2500 HV [25] is a stable phase of Al2O3. Therefore, the corundum phase enhances the average hardness of the duplex coating. As a result of this improvement, average hardness of the duplex coating is greater than the one produced by ESD method. Fig. 7a and b illustrate the load–distances curves and optical micrographs at which the failure occur for the ESD and the duplex coatings, respectively. Results of the experimental scratch test are given in Table 4. The initial cracking, extensive cracking and delamination of the coating from the steel surface are represented by Lc1, Lc2 and Lc3, respectively. The failures of the coatings occurred were determined by optical microscopic examination of the scratch track after the test. The critical load values measured by the scratch test are characteristic values for each coating. The critical load values of the duplex coating are greater than the ones of ESD coating because the duplex coating is thicker and harder than the ESD coating. It can be concluded that this critical load values depend on phase structure, thickness and microstructure of the coatings. Therefore, the duplex coating with high hardness and high thickness has high load carrying capacity. As a result, the duplex coating exhibits excellent adhesion strength compared to the ESD coating.
307
Fig. 8. The variations of friction coefficient against to the sliding distance of St35 steel, ESD coating and the duplex coating.
The variations of the friction coefficient with sliding distance for coated and uncoated samples are shown in Fig. 8. At the first stages of wear test, the friction coefficient of uncoated steel quickly increases to create a trace on the surface. After the trace occurs on the surface, it quickly decreases and remains constant with sliding distance during the wear test. Especially, the coated samples exhibit a nonlinear behavior in the friction coefficient during the sliding wear test. This could be connected to the porous and rough structure of the surface. The friction coefficients of coated samples were very high in the initial stage of the test and these values reach a stable condition in final stage. The porous and loose outer layer was removed at initial steps of the wear process. The values of friction coefficient of both the coated samples tend to decrease to a level which the uncoated steel sample is expected because the coated layers are much harder than the steel substrate. 4. Conclusions In this study, the duplex coatings on St35 steel substrate were successfully obtained by using ESD and MAO processes. The following results can be drawn from this research: • After the ESD and the MAO processes, the surface roughness increased compared to uncoated steel substrate. The coating produced by MAO was rougher than the ESD coating due to the existence of micro sparks in the micro discharge channels. • α-Al2O3, γ-Al2O3 and TiN phases were detected in the duplex coating produced by ESD and MAO methods, while TiN, intermetallic-AlFe3 and intermetallic-AlTi3 phases exist in the coating produced by ESD method. • Obtained values from the microhardness tests showed that the average hardness of the ESD layer is 1.8 times higher than St35 steel substrate. However, the average hardness of the duplex coating was 3.5 times higher than St35 steel substrate. • The adhesion strength of the duplex coating was greater than the one of the ESD coating owing to the presence of high coating thickness and the phase with high hardness. • Finally, the duplex coatings with high tribological performance and high wear resistance by using ESD and MAO processes were successfully obtained. References [1] S. Stojadinovic, R. Vasilic, M. Petkovic, B. Kasalica, I. Belca, A. Zekic, L.J. Zekovic, Appl. Surf. Sci. 265 (2013) 226–233. [2] S. Durdu, O.F. Deniz, I. Kutbay, M. Usta, J. Alloys Compd. 551 (2013) 422–429. [3] K.C. Kung, K. Yuan, T.M. Lee, T.S. Lui, J. Alloys Compd. 515 (2012) 68–73.
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[4] X.P. Fan, B. Feng, Y.L. Di, X. Lu, K. Duan, J.X. Wang, J. Weng, Appl. Surf. Sci. 258 (2012) 7584–7588. [5] I. Apachitei, A. Leoni, A.C. Riemslag, L.E. Fratila-Apachitei, J. Duszczyk, Appl. Surf. Sci. 257 (2011) 6941–6944. [6] Y.M. Wang, P.F. Zhang, L.X. Guo, J.H. Ouyang, Y. Zhou, D.C. Jia, Appl. Surf. Sci. 255 (2009) 8616–8623. [7] S. Krischok, C. Blank, M. Engel, R. Gutt, G. Ecke, J. Schawohl, L. Spieß, F. Schrempel, G. Hildebrand, K. Liefeith, Surf. Sci. 601 (2007) 3856–3860. [8] A. Ashrafizadeh, F. Ashrafizadeh, J. Alloys Compd. 480 (2009) 849–852. [9] X. Lin, G. Zhao, L. Wu, G. Duan, G. Han, J. Alloys Compd. 502 (2010) 195–198. [10] P. LeClair, G.P. Berera, J.S. Moodera, Thin Solid Films 376 (2000) 9–15. [11] Y. Xi, H. Fan, W. Liu, J. Alloys Compd. 496 (2010) 695–698. [12] F. Casadei, R. Pileggi, R. Valle, A. Matthews, Surf. Coat. Technol. 201 (2006) 1200–1206. [13] Z.P. Yao, Y.J. Xu, Z.H. Jiang, F.P. Wang, J. Alloys Compd. 488 (2009) 273–278. [14] S. Durdu, M. Usta, Appl. Surf. Sci. 261 (2012) 774–782.
[15] A. Polat, M. Makaraci, M. Usta, J. Alloys Compd. 504 (2010) 519–526. [16] W.B. Xue, C. Wang, R.Y. Chen, Z.W. Deng, Mater. Lett. 52 (2002) 435–441. [17] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Surf. Coat. Technol. 122 (1999) 73–93. [18] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 130 (2000) 195–206. [19] S. Durdu, A. Aytac, M. Usta, J. Alloys Compd. 509 (2011) 8601–8606. [20] S. Durdu, S. Bayramoglu, A. Demirtas, M. Usta, A.H. Ucisik, Vacuum 88 (2013) 130–133. [21] K. Korkmaz, A.V. Ribalko, Kov. Mater. Met. Mater. 49 (2011) 265–270. [22] A.V. Ribalko, K. Korkmaz, O. Sahin, Surf. Coat. Technol. 202 (2008) 3591–3599. [23] R.H.U. Khan, A. Yerokhin, X. Li, H. Dong, A. Matthews, Surf. Coat. Technol. 205 (2010) 1679–1688. [24] G. Sundararajan, L.R. Krishna, Surf. Coat. Technol. 167 (2003) 269–277. [25] M. Gent, M. Menendez, J. Toraño, S. Torno, Powder Technol. 224 (2012) 217–222.
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Characterization and mechanical properties of the duplex coatings produced on steel by electro-spark deposition and micro-arc oxidation Salih Durdu ⁎, Salim Levent Aktuğ, Kemal Korkmaz The Department of Materials Science and Engineering, Gebze Institute of Technology, Gebze, Kocaeli, 41400, Turkey
a r t i c l e
i n f o
Article history: Received 13 August 2013 Accepted in revised form 4 October 2013 Available online 16 October 2013 Keywords: Electro-spark deposition (ESD) Micro-arc oxidation (MAO) Duplex coating Mechanical properties
a b s t r a c t In this study, the ESD (electro-spark deposition) and the MAO (micro-arc oxidation) processes were used to improve the mechanical properties and the tribological performance of steel. This study has focused on forming a duplex surface coating on the steel. The ESD method was carried out a titanium alloy (Ti6Al4V) deposit layer on the steel substrates at the first step, and then the MAO process was employed to improve properties of the titanium alloyed layer at the second step. Phase structure, surface morphology, elemental composition, hardness, adhesion strength and tribological property of the ESD and the ESD+MAO coating (the duplex coating) were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectroscopy (EDX), micro Vickers tester, micro scratch tester and tribometer, respectively. The XRD results indicated that the duplex coating consists of α-Al2O3 (corundum) and γ-Al2O3 phases while AlFe3, TiN and AlTi3 phases were detected in the ESD coating. The hardness of the duplex coating was significantly improved compared to the uncoated steel and the ESD coating. The adhesion strength of the duplex coating was greater than the ESD coating due to the existence of high hardness and high thickness. In addition, the tribological properties of the ESD and the duplex coatings were significantly improved with compared to the uncoated steel. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys have high strength to weight ratio, great corrosion resistance, excellent mechanical properties and biocompatibility [1–3]. Thus, titanium and its alloys have been used in aerospace, marine, chemical, petrochemical, sports and biomedicine industries because of their outstanding properties [4,5]. However, titanium and its alloys limit to their various applications such as engineering components because they exhibit poor tribological properties such as high and unstable friction coefficients, strong tendency to adhesion, low fretting wear resistance and poor corrosion resistance in some aggressive corrosive environment such as hot chloride solutions [6]. These properties were improved by using some surface engineering methods such as ion implantation [7], thermal oxidation [8], chemical or physical vapor deposition (CVD, PVD) [9,10], magnetron sputtering techniques [11] or the duplex surface treatments [12]. However, these methods are very expensive and difficult to produce complex shaped component. The high cost of titanium and its alloys is another restriction for their use of those applications compared to aluminum and steel alloys which are low cost materials.
The micro-arc oxidation (MAO), also known as plasma electrolytic oxidation (PEO), combines the high voltage spark and electrochemical oxidation. This is a relatively convenient and effective technique to deposit various functional ceramic coatings with porous structures on the surfaces of Ti, Al, Mg and their alloys [13–15]. The MAO coatings on titanium and its alloys provide high hardness, excellent wear resistance and excellent corrosion resistance in terms of high manufacturability and economic efficiency [16–19]. In addition, the MAO forms an oxide film that strongly adheres to a metal substrate with complex geometries [20]. In this study, a duplex coating on St35 steel was produced by using electro-spark deposition (ESD) and micro-arc oxidation (MAO) methods. The ESD method was carried out in order to deposit a titanium alloy (Ti6Al4V) layer on the steel (St35) substrates at the first step. And then, Al2O3 based on a duplex layer was formed to improve mechanical and tribological properties of the titanium alloyed layer on the ESD surface by MAO process at the second step. The mechanical and tribological properties of the ESD and the duplex coatings were investigated in detail. 2. Experimental details
⁎ Corresponding author at: Gebze Institute of Technology, The Department of Materials Science and Engineering, Gebze Institute of Technology, Cayirova Kampus, Gebze, Kocaeli, 41400, Turkey. Tel.: +90 2626052689; fax: +90 2626058490. E-mail addresses: [email protected], [email protected] (S. Durdu), [email protected] (S.L. Aktuğ), [email protected] (K. Korkmaz).
2.1. Materials and methods The Ti6Al4V alloy, which was selected as an electrode, is deposited on a steel (St35) substrate in the ESD process experiments. Tables 1
0257-8972/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.10.004
304
S. Durdu et al. / Surface & Coatings Technology 236 (2013) 303–308
Table 1 Chemical composition of St 35 steel substrate (wt. %). C
Si
Mn
Cr
Ni
Mo
Cu
Co
Fe
0.14
0.28
0.81
0.25
0.17
0.02
0.03
0.01
Balance
Table 2 Chemical composition of Ti6Al4V alloy (wt. %). Al
V
Fe
Si
C
N
H
O
Ti
5.5
4.5
0.3
0.15
0.10
0.05
0.015
0.15
Balance
and 2 show the chemical compositions of St35 steel and Ti6Al4V alloys, respectively. The electrode has a cross-section of 3×5mm2 and the steel substrate samples are in the form of rectangular plates with the dimensions of 3 mm × 20 mm × 25 mm. After the ESD process, some of these samples were used for MAO process. In this study, a special electro spark deposition (ESD) machine was operated at the first step. Power consumption was 180 W and the output of stabilized voltage was kept constant at 40 V. The ESD coating system was described in detail in our previous studies [21,22]. The ESD process was performed using a hand-held applicator in unipolar mode under constant temperature as seen in Fig. 1. In the system, the voltage dropped at the inter electrode gap (17 V) and the amount of electricity (3 C) was kept constant. The ESD process was conducted in air with a series of rectangular pulses of duration time of 100 μs, and the amplitudes of current 100 A. The frequencies of pulses were estimated as 100 Hz by considering the requirement of maintaining the constant electricity (3 C) of process. The MAO with the application of bipolar impulses was used for the fabrication of oxide layer on titanium coated steel samples at the second step. Electrolyte solution was prepared by mixing 1.65 g/L Na3PO4, 8 g/L NaAlO2 in distilled water. The MAO process was carried out by transforming metal oxidation on the surfaces of steel samples which were coated Ti6Al4V alloy by using ESD technique. In the experiments of MAO process (Fig. 2), cathodic and anodic voltage were applied as Uc = 160 V and Ua = 550 V, respectively and the electrical energy of flux which was chosen in respect to the area of sample surfaces was 1.8 μF/cm2. The treatment time was 30 min, since the rate of the coating growth was about 0.6–0.7 μm/min.
Fig. 1. Schematic representation of electro-spark deposition (ESD) coating system set up.
Fig. 2. Schematic representation of micro-arc oxidation (MAO) coating system set up.
2.2. Characterization of the coatings The thickness of the ESD and the duplex coatings was measured by using an eddy current method (Fischer Dualscope MP40) at 50 randomly selected locations. The surface roughness measurements were conducted by using a profilometer (SJ-400 Mitutoyo) with a precision of 0.01 μm after the ESD and the MAO processes. The average roughness values (Ra) of five measurements were reported for all surfaces of the samples. Investigation of the surface morphology and its quality (porosity, fracture, etc.) on the ESD and the duplex coatings conducted samples was performed using a scanning electron microscope (SEM, Philips XL 30 SFEG). In addition, the EDX spectrum analyses were performed using the embedded EDX digital controller and software attached to the SEM. The phase composition of steel, Ti6Al4V, the ESD and the ESD+MAO layers were investigated by using XRD (Bruker D8
Fig. 3. The surface morphologies of the coating surfaces: (a) the ESD technique and (b) the MAO process.
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Fig. 6. The variations of average microhardness of St35 steel substrate, ESD coating and the duplex coating. Fig. 4. The X-ray diffraction patterns of uncoated and the coated surfaces: (a) St35 steel, (b) Ti6Al4V alloy before ESD process, (c) ESD coating and (d) the duplex coating.
Table 3 EDX — elemental composition results of the ESD coating and the duplex coating (at. %). Elements
ESD coating (at. %)
ESD+MAO based duplex coating (at. %)
Fe Ti Al V O Na P
39.06 19.67 2.88 1.48 36.92 – –
0.67 0.31 36.58 – 58.78 0.96 2.69
Advance). The XRD pattern measurements were conducted in the range of 20° to 90°, at scanning speed of 2° min−1. The cross sections of the coatings were polished for hardness (HV) test measurements. These measurements were performed by microhardness tester (Anton Paar MHT-10) at a load of 100 g and a loading time of 5 s. The metallographic analysis was carried out using an optical microscope (Zeiss Axio Imager M1m). The adhesion strengths of the coatings were determined by micro/macro scratch tester (Nanovea Series). During the test, a linearly
increasing load was applied to a static indenter at a loading rate of 60 N/min, while the sample was moved with a table at a speed of 6 mm/min. At the same time, the friction forces were recorded. A Rockwell C diamond indenter with a radius of 200 μm was used at the test, and the scratch load was applied from 0 N and stopped at 60 N during test. A standard ball-on-disk tribometer (CSM Instruments) was employed to investigate the tribological behaviors of steel, Ti6Al4V alloy deposited and micro-arc oxidized samples. Friction coefficient of the coatings was evaluated by the software of the tribometer. The dry sliding wear test experiments were performed under constant pressure at room temperature and at a relative humidity of 50 ± 2%. A WC ball with a diameter of 6 mm was loaded with constant loads of 10 N with the sliding speed of 10 cm/s, the distance of 100 m and amplitude of 17 mm. 3. Results and discussion The surface morphology of a coating processed by the ESD method is shown in Fig. 3a. The irregularity and roughness of the ESD surface prove that the process is successfully done. This is characteristic feature of the ESD process [21]. The surface morphology of a coating processed
Fig. 5. The cross-sectional morphology and line-scan EDX analysis of two layers in the duplex coating produced by ESD and MAO processes: (1) St35 region, (2) ESD coating region, (3) MAO coating region and (4) epofix resin region.
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Fig. 7. The load–distance curves and optical micrograph images for scratch test (a) ESD coating and (b) the duplex coating.
by MAO method is shown in Fig. 3b. There are many pores and crater like structures on surface of the coating produced by MAO. The size of pores increases while the number of pores decreases on surface along the MAO process. Micro discharge channels occur onto the surface of the ESD coating at the MAO. The discharge events appear to be irregular on the surface of the substrate which results in the formation of coating defects like surface roughness, flakes, voids, cavities, and volcano-like features. When a discharge is extinguished, it leaves pores on the coating surface. Thus the MAO coatings usually show a porous surface layer, while the porous surface can be traceable to the high temperature in the discharge channels during the process [23]. As a result, the surface of the micro-arc oxidized coatings is very rough and porous due to the existence of the sparks in micro discharge channels during the MAO process [14] as seen in Fig. 3b. According to the profilometer results, average roughness values (Ra) for the coatings produced by ESD and
ESD+MAO were measured as 4.80 μm and 6.92 μm, respectively. Thus, the duplex coating found to be rougher than the ESD coating. The XRD patterns of the uncoated and coated surfaces are shown in Fig. 4, which reveals the present phases in the coatings. The Ti peaks were detected in the Ti6Al4V alloy. Al and V could not be detected by XRD because they are not a phase structure in Ti6Al4V alloy. According to the XRD pattern of coating obtained by ESD process, the coating layer consists of majorly AlFe3 (aluminum ferrate) and TiN (osbornite) phases, and minorly α-Fe and AlTi3 (aluminum titanate) phases. It is clear that the α-Fe phase belongs to steel substrate and the other phases are accepted as a result of the ESD process. The presence of the AlFe3 phase is related to the alloying of the coating and the steel substrate. It could be thought that the formation of TiN phase is due to the ESD process in open air. It is clear that the ESD+MAO based the duplex coating layer mainly consisting of stable α-Al2O3 (corundum) and
Table 4 Experimental scratch test results obtained at 500 μm radius for the ESD coating and the duplex coating. Sample
The ESD coating The duplex coating
Average thickness (μm)
Average hardness (HV)
Lc1
Lc2
Lc3
Normal load (N)
Frictional force (N)
Distance (mm)
Normal load (N)
Frictional force (N)
Distance (mm)
Normal load (N)
Frictional force (N)
Distance (mm)
17.5 ± 3 32.3 ± 5
510 940
15.199 29.530
3.966 5.493
1.533 2.921
33.196 35.975
8.376 5.945
3.298 3.578
36.123 50.932
7.438 12.849
3.610 5.059
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metastable γ-Al2O3 as major phases. And also, the TiN, TiAl2O5 and α-Fe exist as minor phases in the duplex coating structure. Especially, α-Al2O3 contributes to the increase of hardness of the duplex coating as seen in Fig. 5 due to its high hardness [24]. The surface elemental composition results of the ESD coating and the duplex coating are given in Table 3 by EDX analysis. As it is expected, Ti, Al, V and Fe elements present in the ESD alloying layer and also O elements present due to the ESD process. Ti, Fe and O exist as major elements on the surface while the amount of Al and V is very low. Also, Al, O, Ti, Fe, Na and P elements were found in the duplex coating structure. The Al and O as major elements and Na, P and Ti elements were detected in the coating produced in sodium aluminate and sodium phosphate solution by MAO process. The origin of the Na and P could be NaAl2O3 and Na3PO4 based electrolyte during the MAO process. There are two layers (the duplex structure) in the coating produced by ESD and MAO processes as seen in Fig. 5. According to the elemental graph, the amount of Ti and Al decreases from the ESD coating surface to the St35 substrate, while the amount of Fe increases as expected at steel substrate. The main structure is the distribution of Ti, Al and Fe elements. This implies a formation of AlFe3 and AlTi3 based coating in the inner layer. Moreover, the amount of Al and O increases from the ESD coating to the ESD+MAO interface region and it reaches maximum value through the MAO region. The presence of Al and O in the outer layer contributes to the formation of α-Al2O3 and γ-Al2O3 during the MAO process. There are ESD and MAO based two layers in the duplex coating and these layers are nonuniform along the cross section as seen in Fig. 5. The average thickness values of the ESD coating and the duplex coating were evaluated as 17.5 ± 3 μm and 32.3 ± 5 μm, respectively. The cross sectional microhardness results of the St35 steel, the ESD and the duplex coatings is illustrated in Fig. 6. The average values and error variances were calculated based on 10 random measurement points onto cross section of the coatings. The average microhardness values of uncoated St35 steel and coated samples by only ESD process and the ESD + MAO processes were determined as 270 ± 13.5, 510 ± 25.5 and 940 ± 47 HV, respectively. After both coating processes, a significant increase in the hardness is seen as a result of coating processes. The hardness value depends on various parameters such as phase composition and microstructure. The average microhardness values of the ESD and the duplex coatings are greater than uncoated St35 steel, because these coatings mainly consists of phases with high hardness such as AlFe3, AlTi3, TiN, α-Al2O3 and γ-Al2O3. Especially, the average microhardness of the duplex coating was significantly improved to the ESD coating although the coating layer produced by ESD is denser than the one produced by MAO layer. These results are related to the presence of α-Al2O3 (corundum) and γ-Al2O3 phases in the microstructure of the MAO layer. The duplex coating mainly consists of α-Al2O3 phase with high hardness and metastable γ-Al2O3. The corundum phase which has theoretical hardness value with 2500 HV [25] is a stable phase of Al2O3. Therefore, the corundum phase enhances the average hardness of the duplex coating. As a result of this improvement, average hardness of the duplex coating is greater than the one produced by ESD method. Fig. 7a and b illustrate the load–distances curves and optical micrographs at which the failure occur for the ESD and the duplex coatings, respectively. Results of the experimental scratch test are given in Table 4. The initial cracking, extensive cracking and delamination of the coating from the steel surface are represented by Lc1, Lc2 and Lc3, respectively. The failures of the coatings occurred were determined by optical microscopic examination of the scratch track after the test. The critical load values measured by the scratch test are characteristic values for each coating. The critical load values of the duplex coating are greater than the ones of ESD coating because the duplex coating is thicker and harder than the ESD coating. It can be concluded that this critical load values depend on phase structure, thickness and microstructure of the coatings. Therefore, the duplex coating with high hardness and high thickness has high load carrying capacity. As a result, the duplex coating exhibits excellent adhesion strength compared to the ESD coating.
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Fig. 8. The variations of friction coefficient against to the sliding distance of St35 steel, ESD coating and the duplex coating.
The variations of the friction coefficient with sliding distance for coated and uncoated samples are shown in Fig. 8. At the first stages of wear test, the friction coefficient of uncoated steel quickly increases to create a trace on the surface. After the trace occurs on the surface, it quickly decreases and remains constant with sliding distance during the wear test. Especially, the coated samples exhibit a nonlinear behavior in the friction coefficient during the sliding wear test. This could be connected to the porous and rough structure of the surface. The friction coefficients of coated samples were very high in the initial stage of the test and these values reach a stable condition in final stage. The porous and loose outer layer was removed at initial steps of the wear process. The values of friction coefficient of both the coated samples tend to decrease to a level which the uncoated steel sample is expected because the coated layers are much harder than the steel substrate. 4. Conclusions In this study, the duplex coatings on St35 steel substrate were successfully obtained by using ESD and MAO processes. The following results can be drawn from this research: • After the ESD and the MAO processes, the surface roughness increased compared to uncoated steel substrate. The coating produced by MAO was rougher than the ESD coating due to the existence of micro sparks in the micro discharge channels. • α-Al2O3, γ-Al2O3 and TiN phases were detected in the duplex coating produced by ESD and MAO methods, while TiN, intermetallic-AlFe3 and intermetallic-AlTi3 phases exist in the coating produced by ESD method. • Obtained values from the microhardness tests showed that the average hardness of the ESD layer is 1.8 times higher than St35 steel substrate. However, the average hardness of the duplex coating was 3.5 times higher than St35 steel substrate. • The adhesion strength of the duplex coating was greater than the one of the ESD coating owing to the presence of high coating thickness and the phase with high hardness. • Finally, the duplex coatings with high tribological performance and high wear resistance by using ESD and MAO processes were successfully obtained. References [1] S. Stojadinovic, R. Vasilic, M. Petkovic, B. Kasalica, I. Belca, A. Zekic, L.J. Zekovic, Appl. Surf. Sci. 265 (2013) 226–233. [2] S. Durdu, O.F. Deniz, I. Kutbay, M. Usta, J. Alloys Compd. 551 (2013) 422–429. [3] K.C. Kung, K. Yuan, T.M. Lee, T.S. Lui, J. Alloys Compd. 515 (2012) 68–73.
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