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Synthesis of Al2O3-Y2O3 ceramic coatings on FeCr-Al wire via aqueous cathodic plasma electrolysis Yuping Zhang, Chao Chen, Weiwei Chen, Huanwu Cheng, Yingchun Wang, and Lu Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03661 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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Industrial & Engineering Chemistry Research

Synthesis of Al2O3-Y2O3 ceramic coatings on Fe-Cr-Al wire via aqueous cathodic plasma electrolysis Yuping Zhang, Chao Chen, Weiwei Chen*, Huanwu Cheng, Yingchun Wang, Lu Wang

Department of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

Abstract: The Al2O3-Y2O3 coating was successfully prepared by cathodic plasma electrolysis on the FeCrAl wire. Influences of the electrolyte composition and the processing parameters on the microstructure and property of the Al2O3-Y2O3 ceramic coating were systematically investigated. The coating was mainly composed of γ-Al2O3, α-Al2O3 and Y2O3. Experimental results showed that the thickness of the coating was gradually increased with the increase of working voltage, while the bonding became poor. As the increase of the deposition time, the thickness and the electrical resistance of the Al2O3-Y2O3 coating were increased in a certain range. When the Al2O3-Y2O3 coating was deposited at 110 V for 60-90 s, the bonding between the coating and the substrate was superior and the coating microstructure was dense, continuous and uniform with the electrical resistance reaching up to ~40 MΩ. The formation mechanism of the coating was also discussed based on the

*

Corresponding author. Tel: 0086 10 68912709ext109. Email address: [email protected] 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electrochemical mechanism and the plasma discharge. Keywords: Cathodic plasma electrolysis; Al2O3-Y2O3 coating; FeCrAl wire; Electrical insulation; Mechanism.

1 Introduction Fe-Cr-Al alloys have been widely used as the heating resistor materials due to their excellent properties including high resistance and high-temperature oxidation resistance.1,2 The failure of the Fe-Cr-Al resistance-heating wire is mainly caused by high-temperature degradation and the lower strength at high temperatures, leading to a shorter lifetime.3 Nowadays, coating is one of the most promising methods to improve the electrical insulation of Fe-Cr-Al alloys,4-6 extending the lifetime at high temperatures.

The aqueous cathodic plasma electrolysis is a cost-effective and environmental friendly approach for preparing a ceramic coating on the surface of an alloy/metal. The process is a hybrid of conventional electrolysis and plasma process.7-9 In the aqueous cathodic plasma electrolysis process, the work piece as a cathode is usually immersed into electrolyte solution at the ambient temperature. A continuous vapor envelope around the work piece is broken down at a critical voltage, resulting in the generation of plasma discharge in the near-cathode region. Compared with the conventional electrolysis process, the aqueous cathodic plasma electrolysis process can generate new physical-chemical effects and obtain a new coating.10-12

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The Al2O3 coating has superior electrical insulation and shows the optimal protective efficiency for metals. However, the mechanical property of Al2O3 is relatively poor at high temperatures, where Al2O3 is quite sensitive to pores, cracks and other defects.13 It is widely suggested that the Al2O3-Y2O3 coating is one of the most promising coatings to greatly improve the thermodynamic properties of the Al2O3 coating.14-16 Hsu et al. prepared the Al2O3/Y2O3 double-layer films on In617 superalloy, showing the efficient retarding effects on the oxidation (NiO) of In617 superalloy in air at 1000 °C.17 Gao et al. deposited Al2O3 -Y2O3 composite coatings on γ-TiAl based alloy, changing the oxidation behavior of γ-TiAl alloy and improving the thermodynamic properties of the Al2O3 coating.18 The typical methods including thermal spray, sol-gel, electrophoretic and sputtering have been applied to successfully prepare the Al2O3-Y2O3 coating. However, the poor bonding, easy cracking, thickness limitation or high cost of the above methods limit the coating application on the Fe-Cr-Al wire.

In the present work, we applied the cathodic plasma electrolysis for successfully preparing the continuous and uniform Al2O3-Y2O3 ceramic coating on the Fe-Cr-Al wire. Influences of the electrolyte composition and the processing parameters on the microstructure and property of the Al2O3-Y2O3 ceramic coating were systematically investigated. The formation mechanism of the coating was also discussed based on the electrochemical mechanism and the plasma discharge.

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2 Experimental The schematic diagram of the cathodic plasma electrolysis is shown in Figure 1. The substrate was Fe-Cr-Al wire with the diameter of 0.8 mm and the Cr, Al and Fe content of 23%~26%, 4.5%~6.5% and 67.5-72.5 wt.%, respectively. The aqueous electrolyte was a mixture of Al(NO3)3 and Y(NO3)3 with the concentration of 300 g/L and 1.0 g/L, respectively.

As shown in Figure 1, the Fe-Cr-A1 wire was used as the cathode with the graphite electrode as the anode. In order to avoid micro-arc discharge at the solution/air interface, the upper part of the sample was wrapped with polytetrafluorothylene (PTFE) tape. Since the deposition process of the cathodic plasma electrolysis was rapid, a suitable cooling water flow rate was kept. When power supply was switched on, the deposition voltage was quickly adjusted to the required working voltage by a computer-controlled system. The relationship between current (i) and time (t) was recorded during Al2O3 -Y2O3 coating deposition by a computer-controlled software.

The cross-sectional morphologies of the coatings were analyzed using a scanning electron microscopy (SEM). The phase structure of the coating was determined using X-ray diffraction (XRD) with Cu Kα radiation. The diffraction patterns were recorded with the diffraction angle (2θ) ranging from 20o to 80o at a scanning rate of 0.02o s-1. The electrical insulation properties of coatings were detected by VC8900 digital multimeter (DMM).

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3 Results 3.1 Phase structures Figure 2 shows XRD patterns of the Al2O3 and Al2O3-Y2O3 coatings with deposition for 30 s. It was observed that both coatings were mainly composed of γ-Al2O3 and α-Al2O3. Y2O3 was also detected in the Al2O3-Y2O3 coating as shown in Figure 2b. The results show that the deposited coating was evolved from Al2O3 coating to Al2O3-Y2O3 composite coating on FeCrAl substrate by the addition of 1.0 g/L Y(NO3)3.

3.2 Cross-sectional morphologies 3.2.1 Influence of voltages The cross-sectional morphologies of the Al2O3 -Y2O3 and Al2O3 coatings prepared at working voltages of 100 V, 110 V and 120 V were shown in Figures 3 and 4. Given that the deposition was fast, the influence of the deposition voltage was initially investigated based on the short deposition time of 30 s, in order to obtain the optimal deposition voltage. With the increase of the deposition voltage, the thickness of the coating was increased. More pores and cracks were also observed. At the same time, the bonding of the coating seemed poor, and the thickness uniformity of the coating also decreased. Higher arc energy made the deposition more rapid and efficient, so that the thickness of coating gradually increased as working voltage increased from 100 V to 120 V. When the working voltage was excessive, the violently turbulent

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discharge bombardment generated uneven discharge on the coating surface and the deposition rate was inconsistent at different deposition points, resulting in poor bonding and uniformity in thickness. As shown in Figures 3c and 4c, when the working voltage was 120 V, both Al2O3 and Al2O3-Y2O3 coatings were evolved into a severely uneven and porous structure and the bonding of coatings appeared poor. Obviously, the working voltage of 110 V was presently optimal.

It was observed in Figures 3b and 4b that the bonding of the Al2O3-Y2O3 coating seemed better at 110 V. The deposited coating became more dense by adding Y(NO3)3 in the electrolyte. It was suggested that Y2O3 in the coating could effectively decrease the breakdown discharge, so the densification of the coating was improved. The substrate was finally covered by the dense, continuous and uniform Al2O3-Y2O3 coating under the same deposition condition.

3.2.2 Influence of deposition time In order to further study the effect of the deposition time on the growth of Al2O3 -Y2O3 coating, we prepared a series of coatings on the Fe-Cr-A1 wire. Based on the above research, the optimal working voltage was selected as 110 V. Figure 5 shows the cross-sectional morphologies of the coatings prepared at different deposition times. When the deposition time was 2 s, the Al2O3-Y2O3 coating was not continuous and the prototype of the coating initially formed with a high deposition rate, as shown in Figure 5a. With the increase of the deposition time, the thickness of the Al2O3-Y2O3

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coating gradually increased. When the deposition time reached 30 s, the lamellar crystals were observed in the ceramic coating. A relatively dense, continuous and uniform coating was formed on the wire, and the thickness of coating increased as the reaction time increased. After the reaction time reached 60 s, the Al2O3 -Y2O3 coating became more compact, and the bonding between the coating and substrate seemed superior as shown in Figure 5f, possibly due to the ultra-high temperature locally formed during the plasma electrolysis process.7, 19 The quite similar structure was also formed at the deposition time up to 90 s (Figure 5g). In addition, it was studied that once the thickness of the coating increased to a certain value, the electric field strength on the coating also increased,20,21 resulting in breakdown of the discharge. Correspondingly the growth of the coating was hindered as shown in Figure 5h. Therefore, the deposition time was selected to be 60~90 s.

3.3 The electrical insulation properties The mean resistance was measured to investigate the effect of the coatings on the electrical insulation properties of the Fe-Cr-Al wire. During the measurement of the insulation resistance, one testing lead wire of a digital multimeter (DMM) was clamped on the sample site without coating, the other testing lead wire was in contact with the coating. In order to assure the accuracy of the electrical resistance, each specimen was tested for five times, and finally the mean resistance was calculated with an error bar added.

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Figure 6 shows dependence of the electrical resistance on the deposition voltage at the deposition time of 30 s for both Al2O3 and Al2O3-Y2O3 coatings. It was observed that both coatings similarly showed the larger electrical resistance with the increase of the working voltage. Moreover, the resistance of the Al2O3 -Y2O3 coating was lower than that of the Al2O3 coating at 100~120 V, as the conductivity of Y2O3 was better.

Figure 7 shows dependence of the electrical resistance on the deposition time at the deposition voltage of 110 V for the Al2O3-Y2O3 coating. The Fe-Cr-Al wire was a good conductor, so the electrical resistance was much lower than an insulating ceramic. The coating became thicker with the increase of the deposition time (Figure 5). Correspondingly the electrical resistance was increased. The largest resistance was measured to be ~41.90 MΩ at 90 s. When the deposition time reached 120 s, the resistance decreased to 30.17 MΩ due to the decline of the coating thickness. Totally the resistance of the Al2O3-Y2O3 coating was similar in the range of 60~90 s, where the superior deposition could be conducted.

4 Discussion 4.1 i-t curves The i-t characteristics during the cathodic plasma electrolysis are shown in Figure 8. The curve exhibited a change with i initially increased and then declined. It was found in Figures 8 a and b that the plasma current of inflection point rose from 9.4 A to 11.9 A, as the conductivity of the electrolyte was increased due to the addition of Y(NO3)3

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at the same experimental condition. With the increase of the voltage, the current gradually increased and the i-t linear relationship followed the classical Faraday’s law. Meanwhile, the increase of the current caused boiling of the electrolyte and a rapid sparking in scattered gaseous bubbles of the near-electrode region. When the applied voltage exceeded a certain critical value, the current sharply rose. The region deviated the Faraday’s zone with the presence of the unstable gases, accompanied by uniform glow discharge.10 It was suggested from the i-t curve that the decline was observed for i during 5-20 s, possibly attributed to the formation of a continuous gaseous plasma envelope and joule heating at high current densities.21,22 When the time was increased up to 20 s, the current was stabilized at 0.5~0.6 A, and the glow discharge was transformed into intensive arcing where the deposition was begun.

4.2 Plasma electrolysis mechanism The plasma discharge mechanism during the cathode plasma electrolytic deposition is schematically shown in Figure 9. When the working voltage was gradually increased, the hydrogen evolution reaction occurred on the surface of the substrate, which generated a continuous vapor envelope. As the working voltage reached the critical voltage, the breakdown of the vapor envelope resulted in the plasma discharge. In addition, the hydrogen evolution reaction caused an increase in the pH value of the electrolyte, leading to the formation of Al(OH)3 and Y(OH)3.23 The basic reactions steps are suggested as below: i. Hydrogen evolution reaction:

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2H+ + 2e- → H2

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(1)

ii. Formation of hydroxides: Al3+ + 3OH- → Al(OH)3

(2)

Y3+ + 3OH- → Y(OH)3

(3)

iii. Dehydration of the hydroxides: Al(OH)3 → Al2O3 + H2O

(4)

Y(OH)3→Y2O3 + H2O

(5)

There are two kinds of discharge models for the cathode plasma electrolysis: the vapor envelope discharge and ceramic membrane discharge. Once a continuous ceramic membrane was formed on the cathode surface, the dielectric layer was changed from single vapor envelope to double-layer structure including the vapor envelope and the ceramic coating, while the cathode plasma electrolytic deposition was named as gas/solid double-layer discharge.24,25 According to Maxwell-Wagner model, the Egas and the Ecoating of this double-layer structure were not equal to the average E, and the relationship was expressed as follows: Egas = σcoating (dgas + dcoating)E / (σgasdcoating + σcoatingdgas)

(6)

Ecoating = σgas (dgas + dcoating)E / (σgasdcoating + σcoatingdgas)

(7)

where E is electric field strength, σgas is the conductivity of gas layer, σcoating is the conductivity of coating layer, dgas is the thickness of gas layer, and dcoating is the thickness of coating layer.

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As shown in Formulas (6) and (7), it is observed that Egas was larger than Ecoating because the electrical conductivity of the gas layer was lower than that of the coating layer. Meanwhile, the critical breakdown electric field strength (Egas

(breakdown)

≈ 3

MV/m) of the gas layer was much lower than that of the coating layer (Ecoating (breakdown) ≈ 9.9~15.8 MV/m), so the vapor envelope was easy to be punctured by the plasma arc.26 Whether the ceramic coating was punctured and prone to pores or cracks was certainly dependent on the conductivity, as shown in Formulas (6) and (7). Typically, if the conductivity of the ceramic coating is high, like the Y2O3 ionic conductor, the breakdown discharge is difficult, and the coating correspondingly tends to be dense. Otherwise, the coating is prone to breakdown discharge with a porous and cracking structure, such as insulator Al2O3. Meanwhile the breakdown easily damages the coating because of the high arc energy and severe bombardment. The conductivity of the Al2O3-Y2O3 coating was presently improved, leading to lower Ecoating as shown in Formula (7). Finally the densification and uniformity of Al2O3 -Y2O3 coating were greatly improved as shown in Figures 4 and 5.

5 Conclusions The following conclusions could be drawn from the results and analysis in the paper: (1) The working voltage has a significant influence on the morphologies and densification of the Al2O3 -Y2O3 coatings. With the increase of the working voltage, the thickness of coating gradually increased, as well as the pores and cracks. When the working voltage was excessive, the coatings became severely uneven and porous

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and the bonding between coating and substrate was poor. The optimal working voltage was 110V. (2) The addition of Y(NO3)3 had a significant influence on plasma discharge in the deposition process. The dense, continuous and uniform Al2O3 -Y2O3 coating was successfully prepared on the Fe-Cr-Al wire, because the Y2O3 phase in the coating effectively decreases the breakdown discharge by improving the conductivity of coating. (3) With the increase of the deposition time, the coating became thicker, more compact, and the bonding between the coating and substrate seemed superior. The depositing time was suitable in the range of 60~90 s. (4) The electrical resistance was increased with the rise of working voltage, and the resistance of Al2O3 -Y2O3 coating was lower than Al2O3 coating at the same condition. Moreover, with the increase of the deposition time, the resistance of Al2O3-Y2O3 coating initially increased and then declined. The electrical resistance of the coating was similar during 60~90 s, where the superior deposition could be conducted.

Acknowlegements The paper was financially supported by the Basic Research Program of Beijing Institute of Technology (Grant NO: 20150942006). We thank Mr. W.C. Liao for providing the FeCrAl specimens.

References

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Figure Captions: Figure 1 The schematic diagram of the cathodic plasma electrolysis. 1-DC power supply; 2-control system; 3-the cathode (Fe-Cr-A1 wire); 4-the anode (graphite electrode); 5-electrolyte; 6- cooling water; 7-plastic container; 8-PTFE tape Figure 2 XRD patterns of the as-deposited coating on the FeCrAl substrate: (a) Al2O3-Y2O3 coating, and (b) Al2O3 coating. Figure 3 Cross-sectional morphologies of the as-deposited Al2O3 coating prepared in 300 g/L Al(NO3)3 at different working voltages: (a) 100 V, (b) 110 V and (c) 120 V. Figure 4 Cross-sectional morphologies of the as-deposited Al2O3-Y2O3 coating prepared in 300 g/L Al(NO3)3 and 1.0 g/L Y(NO3)3 at different working voltages: (a) 100 V, (b) 110 V and (c) 120 V. Figure 5 Cross-sectional morphologies of the as-deposited Al2O3-Y2O3 coating prepared in 300 g/L Al(NO3)3 and 1.0 g/L Y(NO3)3 at 110 V with different deposition time: (a) 2 s, (b) 5 s, (c) 10 s, (d) 20 s, (e) 30 s, (f) 60 s, (g) 90 s, and (h) 120 s. Figure 6 Dependence of the electrical resistance on the deposition voltage at the deposition time of 30 s for both Al2O3 and Al2O3-Y2O3 coatings. Figure 7 Dependence of the electrical resistance on the deposition time at the deposition voltage of 110 V for the Al2O3-Y2O3 coating. Figure 8 The i-t curves recorded during the cathodic plasma electrolysis at 110 V with deposition time of 30 s: (a) Al2O3 coating, and (b) Al2O3-Y2O3 coating. Figure 9 The schematic diagram of the plasma discharge mechanism in the cathode plasma electrolytic deposition process.

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Figure 1 The schematic diagram of the cathodic plasma electrolysis. 1-DC power supply; 2-control system; 3-the cathode (Fe-Cr-A1 wire); 4-the anode (graphite electrode); 5-electrolyte; 6- cooling water; 7-plastic container; 8-PTFE tape

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300g/L Al(NO3)3+1.0g/LY(NO3)3(a)

☆γ-Al2O3

300g/L Al(NO3)3+ 0 g/L Y(NO3)3(b)

◇α-Al2O3

◇ ♦

♦Y2O3

☆ ♦

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


☆

☆

(a)

◇ ♦

(b)

20

☆

30

◇ ☆

40

◇

50

60

☆

70

80

2θ (degree) Figure 2 XRD patterns of the as-deposited coating on the FeCrAl substrate: (a) Al2O3-Y2O3 coating, and (b) Al2O3 coating.

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Figure 3 Cross-sectional morphologies of the as-deposited Al2O3 coating prepared in 300 g/L Al(NO3)3 at different working voltages: (a) 100 V, (b) 110 V and (c) 120 V.

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Figure 4 Cross-sectional morphologies of the as-deposited Al2O3-Y2O3 coating prepared in 300 g/L Al(NO3)3 and 1.0 g/L Y(NO3)3 at different working voltages: (a) 100 V, (b) 110 V and (c) 120 V.

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Figure 5 Cross-sectional morphologies of the as-deposited Al2O3-Y2O3 coating prepared in 300 g/L Al(NO3)3 and 1.0 g/L Y(NO3)3 at 110 V with different deposition time: (a) 2 s, (b) 5 s, (c) 10 s, (d) 20 s, (e) 30 s, (f) 60 s, (g) 90 s, and (h) 120 s.

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100

300g/L Al(NO3)3+ 0g/L Y(NO3)3

90

300g/L Al(NO3)3+ 1.0g/L Y(NO3)3

80 70

Resistance (MΩ )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


49.57

60 50 40

65.03 30

46.68

57.6 20

49.57

10

38.13 27.28

0 95

100

105

110

115

120

125

Voltage (V)

Figure 6 Dependence of the electrical resistance on the deposition voltage at the deposition time of 30 s for both Al2O3 and Al2O3-Y2O3 coatings.

23



300g/L Al(NO3)3+ 1.0g/L Y(NO3)3

50

40

Resistance(MΩ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

30

39.7

38.13 31.37

20

10

0

41.9 30.17

23.47 26.43 18

0.01

0

20

40

60

80

100

120

Time(s)

Figure 7 Dependence of the electrical resistance on the deposition time at the deposition voltage of 110 V for the Al2O3-Y2O3 coating.

24


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Figure 8 The i-t curves recorded during the cathodic plasma electrolysis at 110 V with deposition time of 30 s: (a) Al2O3 coating, and (b) Al2O3-Y2O3 coating.

25



Figure 9 The schematic diagram of the plasma discharge mechanism in the cathode plasma electrolytic deposition process.

26


Page 26 of 27

Page 27 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35x85mm (300 x 300 DPI)


Synthesis of Al - American Chemical Society

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