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Highly Corrosion Resistant and Sandwich-Like Si3N4/Cr-CrNx/ Si3N4 Coatings used for Solar Selective Absorbing Applications Ke Zhang, Miao Du, Lei Hao, Jianping Meng, Jining Wang, Jing Mi, and Xiaopeng Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11607 • Publication Date (Web): 18 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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ACS Applied Materials & Interfaces

Highly Corrosion Resistant and Sandwich-Like Si3N4/Cr-CrNx/Si3N4 Coatings used for Solar Selective Absorbing Applications Ke Zhang, Miao Du, Lei Hao a*, Jianping Meng, Jining Wang, Jing Mi, Xiaopeng Liu Department of Energy Material & Technology, General Research Institute for Nonferrous Metals, No.2 Xinjiekouwai Street, Xicheng District, Beijing 100088, China. ABSTRACT: Highly corrosion-resistant, layer-by-layer nanostructured Si3N4/Cr-CrNx/Si3N4 coatings were deposited on aluminium substrate by DC/RF magnetron sputtering. Corrosion resistance experiments were performed in 0.5, 1.0, 3.0 and 5.0wt% NaCl salt spray at 35ºC for 168h. Properties of the coatings were comprehensively investigated in terms of optical property, surface morphology, microstructure, elemental valence state, element distribution, and potentiodynamic polarisation. UV-Vis-NIR spectrophotometer and FTIR measurements show that the change process in optical properties of Si3N4/Cr-CrNx/Si3N4/Al coatings can be divided into three stages: a rapid active degradation stage, a steady passivation stage, and a transpassivation degradation stage. With the increase in the concentration of NaCl salt spray, solar absorptance and thermal emittance experienced a slight degradation. SEM images reveal that there is an increase in surface defects, such as micro holes and cracks. XRD and TEM measurements indicate that the phase structure changed partially and the content of CrOx and Al2O3 has increased. Auger electron spectroscopy shows that the elements of Cr, N, and O have undergone a minor diffusion. Electrochemical polarisation curves show that the as-deposited Si3N4/Cr-CrNx/Si3N4/Al coatings have an excellent corrosion resistance of 3633.858kΩ while after corroding in 5.0wt% NaCl salt spray for 168h the corrosion resistance dropped to 13.759kΩ. However, these coatings still have an outstanding performance of high solar absorptance of 0.924 and low thermal emittance of 0.090 after corroding in 3.0wt% NaCl salt spray for 120h. Thus, the Si3N4/Cr-CrNx/Si3N4/Al coating is a good choice for solar absorber coatings applied in the high saline environment. Keywords: Si3N4, CrNx, solar absorber coatings, electrochemical corrosion, microstructure

1. INTRODUCTION

1 ACS Paragon Plus Environment


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Solar energy as an inexhaustible energy has been widely exploited 1-2. Solar selective absorbing coatings, as one of the most popular solar thermal transfer materials, have been widely studied and successfully applied in Concentrating Solar Power (CSP) and Domestic Hot Water (DHW) applications 3-5. Many novel coatings are designed to achieve the spectral selectivity, such as semiconductor metal tandem coatings, multilayer interference stack coatings, and cermet composite coatings, etc. Among them, owing to the fact that the multilayer interference stack structure makes full use of optical interference and intrinsic absorption of materials 6, therefore, it possesses a high solar absorptance, such as MgO/Zr/MgO 7

, AlxOy/Pt/AlxOy 8, Al2O3/Mo/Al2O3 9, and W/Ni/Al2O3 10 and so forth. Materials of solar absorber coatings can be metal or dielectric single phase or be metal - dielectric composites. For

instance, silicon nitride (Si3N4) as one of the best candidates for interlayer materials of solar absorber coatings, it has excellent optical property and mechanical strength

11-12

. Firstly, Si3N4 is approximately transparent in the visible range

while opaque in the infrared spectrum. This matches well with the selectivity of the solar absorbing coatings. Secondly, Si3N4 has outstanding oxidation resistance, mechanical strength as well as all-important corrosion resistance 13. Harish C. Barshilia et al. 14 prepared TiAlN/TiAlON/Si3N4 solar absorber coatings, and the coatings were very stable up to 600 °C because of high activation energy and high melting point. In addition, the microstructure has a direct influence on the optical property of Si3N4. It is believed that the fewer defects it has, the better of transmittance it will be. For instance, Sungwook Jung et al.15 adopted the Plasma Enhanced Chemical Vapor Deposition(PECVD) techniques and successfully prepared high pure silicon nitride thin films, and their experimental results confirmed this view. Transition metal chromium has an ideal band gap

16

. After doping of nitrogen or oxygen, the chromium nitride,

chromium oxide, and chromium oxynitride based coatings will become an ideal sublayer of solar selective absorbing coatings. Chenying Yang et al.

17

successfully prepared Cr-containing multilayer coatings, and the coatings showed an

average absorption of ~98% in a broad range of wavelengths from 400 nm to 2000 nm. There are also some other high


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performance and Cr-containing solar selective absorbing coatings, such as Cr2O3/Cr cermet18, chromium oxynitride solar absorber coatings 19, Cr-containing micro and nanostructured surfaces 20, and so forth. Considering the excellent optical property of Si3N4 and high absorptance, low emittance of chromium nitride in a wide range of wavelengths from 300 nm to 3000 nm, we successfully deposited a sandwich-like solar selective absorbing coating –the Si3N4/Cr-CrNx/Si3N4. In addition to highly spectral selectivity, the property of anticorrosion is also very critical for the coatings’ large-scale and long-term application because it directly determines the service life of these coatings. Especially in coastal and saline areas, where the concentration of saline mist in the air is relatively higher than that of some other areas. Much to our regret, there is little literature about the investigation on corrosion of saline mist for solar absorbing coatings. Fortunately, some similar studies have carried out on other coatings. A. Gilewicz et al. 21 studied corrosion resistance of CrN coatings used for biomaterials engineering, and they found that the corrosion resistance of CrN is directly related to the coating structure, especially the micro defects. Xiaoyan Guan et al.

22

investigated Cr/Cr2N nano-multilayer coatings

and pointed out that the principal reason for the reduction in corrosion resistance should due to the porosity and interface effect. Ferdinand Singer et al.

23

polymerised polydopamine layers on Mg surface and found that Mg(OH) 2 was formed

during the corrosion process because electrolyte penetrated the polydopamine layers and resulted in the occurrence of pitting corrosion. With the help of electrochemical impedance spectroscopy, Esam Husain et al.

24

analysed boron nitride

coatings and their results shown that minor crevice corrosion has taken place on the interface of their coatings. In order to develop a kind of solar selective absorbing coatings that can be applied in a complex environment, we have designed a sandwich-like Si3N4/Cr-CrNx/Si3N4/Al structure for high temperature and corrosive ambient applications. This structure exhibits a high solar absorptance of ~0.95 and low thermal emittance of ~0.05 at room temperature. In fact, inevitably, due to the effect of deposition rate, vacuum pressure, bias voltage, and gasses content during the magnetron



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sputtering, many intrinsic defects, such as loose grain boundary and pinholes, will more or less present in the Si 3N4/CrCrNx/Si3N4/Al coatings. 25-27 Therefore, apart from studying its excellent high-temperature thermal stability 28, the corrosion resistance is another essential property for solar absorbing coatings. Thus, in this work, we designed a series of NaCl neutral salt spray tests to figure out the influence of saline mist on optical performance and corrosion mechanisms.

2. EXPERIMENTAL SECTION 2.1. Preparation of Si3N4/Cr-CrNx/Si3N4 Coatings. The sandwich-like Si3N4/Cr-CrNx/Si3N4 coatings were deposited on Al substrate (Reasons for choosing aluminium and its components, please refer to the Supporting Information) using DC/RF magnetron co-sputtering techniques. High purity targets of Cr (99.95%) and Si (99.99%) were used for sputtering deposition. Before deposition, the Al substrates were ultrasonically cleaned for 10 min in acetone; then cleaned by ethanol baths and deionized water. When chamber vacuum pressure was better than 5 × 10–3 Pa and a DC bias of – 600 V was achieved, the Al substrates were anti-splash washed in an argon plasma for 20 min for the sake of removing the oxide layer. Si3N4 (Top) and Si3N4 (Bottom) layers were deposited in argon–nitrogen plasma at a pressure of 0.90 Pa with a negative bias of 100 V, and the flow of argon and nitrogen gases were fixed at 150 sccm and 20 sccm respectively. The deposition time of the Si 3N4 (Top) and Si3N4 (Bottom) layers was 11min and 10min respectively. The Cr-CrNx layer was deposited in argon–nitrogen plasma at a pressure of 0.65 Pa with a negative bias of 80 V while the nitrogen gas declined to 10sccm, and the deposition time of this layer was 6min. Detailed parameters of the preparation of the sandwich-like Si3N4/Cr-CrNx/Si3N4 coatings are listed in Table 1. Corresponding structure sketch is shown in Figure 1. Fabrication Process schedule of the sandwich-like Si3N4/Cr-CrNx/Si3N4 coatings is presented in detail in the Supporting Information. Table 1. Deposition parameters for the Si3N4/Cr-CrNx/Si3N4 solar absorber coatings. Layer

Deposition pressure (Pa)

Substrate bias (V)

N2 flow rate (sccm)

Power (W)

Deposition time (min)

Si3N4(Top )

0.90

-100

20

2000

11


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Cr-CrNx

0.65

–80

10

110

6

Si3N4(Bottom )

0.90

-100

20

2000

10

Figure 1. Structure sketch of sandwich-like Si3N4/Cr-CrNx/Si3N4 solar selective absorbing coatings.

2.2. Salt Spray Corrosion of Si3N4/Cr-CrNx/Si3N4 Coatings. The neutral salt spray (NSS) corrosion test was adopted to simulate the saline mist corrosion of the sandwich-like coatings. This NSS test was performed based on the national standard GB/T 10125-1997 (equivalent to ISO 9227) procedure in a salt spray chamber. The samples were respectively exposed in 0.5, 1.0, 3.0 and 5.0wt% NaCl salt spray ambient with 45° tilted for a series of time of 12, 24, 48, 72, 96, 120, 144 and 168h. After each corrosion experiment, the samples were firstly ultrasonic washed with deionized water for 10min, and then rinsed with deionized water twice to remove surface residues. 2.3. Characterization. The solar absorptance (α) is calculated by the reflectance RVIS(λ) measured by UV/VIS/NIR spectrophotometer (UV3600, SHIMADZU, Japan). Thermal emittance (ε) is calculated based on the infrared reflectance RIR(λ) measured by an FTIR spectrometer (Vertex80，Bruker Optics, Germany). The absorptance and emittance of the solar absorbing coatings are calculated through the following equations 29:

 

3um

0.3um

[1  RVIS ( )]Psun ( )d



3um

0.3um





30um

3um

Psun ( )d

[1  RIR ( )]PB ( )d



30um

3um

PB ( )d

(1)

(2)



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Where Psun(λ) is the normal solar spectral irradiance defined by the ISO standard 9845-1 (1992) on air mass (AM) 1.5, and PB(λ) is the spectral radiance of a black body. The phase structure of the coatings was identified by X-ray diffraction (D/max-B, Rigaku, Japan) using Cu Kα radiation (λ=0.1541 nm), with 2θ angle ranged from 20° - 80° and scanning speed of 3°/min. The surface and cross-sectional morphologies were observed by field emission scanning electron microscope (S4800, Hitachi, Japan). The elemental distribution was analysed by Auger electron spectroscopy (PHI-700, ULVAC-PHI, Japan) with 5KV coaxial Ar+ electron gun, the angle of incidence was 30°, the vacuum pressure was better than 3.9×10 -9 Torr. The analysis of valence state of elements was performed by X-ray photoelectron spectroscopy (Quantera II, ULVAC-PHI, Japan) with monochromatic Al Kα X-ray source (1486.7 eV), operating at 15kV (150 W), and binding energy was calibrated with C1s (284.8eV). 2.4. Electrochemical Measurements. Before electrochemical measurements, the pre-corroded samples were cut into sheets of 1 × 1 cm and the back of aluminium substrate was sealed by paraffin wax. An electrochemical workstation (ZAHNER, Germany) was used to perform the electrochemical experiments in three-electrode configuration, where Pt foil worked as a counter electrode, a calomel electrode as a reference electrode and samples of interests as working electrode. The electrochemical measurements were carried out in 3.0wt% NaCl aqueous solution with a stabilised pH at 6.5. Prior to each measurement, working electrode was soaked in the NaCl aqueous solution for 30min to establish an open circuit potential (OCP). The polarisation curves were obtained by a quasi-static potentiostatic method with a one-way linear sweep at a scanning rate of 1 mV/min from -3V to +1 V (vs. OCP). The voltage step and current range were set at 2 mV and 1µA respectively. Samples and NaCl electrolyte were renewed after each measurement.

3. RESULTS AND DISCUSSION 3.1. Degradation trends of Optical Properties.


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Table 2 shows the optical properties degradation of Si3N4/Cr-CrNx/Si3N4/Al coatings corroded by different concentrations of salt spray (0.5, 1.0, 3.0 and 5.0wt %) for 12, 24, 48, 72, 120 and 168h. Figure 2 exhibits corresponding changes in solar absorptance (α), thermal emittance (ε) as well as the ratio of solar absorptance and thermal emittance. From Table 2, the optical properties do not show a noticeable degradation after corroding in 0.5% and 1.0% NaCl salt spray up to 120h. Even though corroded in 5.0% NaCl salt spray, the Si3N4/Cr-CrNx/Si3N4/Al coatings can also keep a high performance for 72h (α: 0.930, ε: 0.092, Table 2). Generally, if the solar absorptance is greater than 0.90 and the ratio of solar absorptance and thermal emittance is higher than 10.00 after corroding in salt spray, these solar selective absorbing coatings can still be acceptable 30. Thus, in Table 2, these data that above the dash-dot line are still within the acceptable range. In this regard, it clearly indicates that the sandwich-like Si3N4/Cr-CrNx/Si3N4/Al solar absorbing coatings possess an excellent corrosion resistance. The detailed degradation trend of the Si3N4/Cr-CrNx/Si3N4/Al coatings corroded in different concentrations of NaCl salt spray is depicted in figure 2. As shown in Figure 2 (a), with the increase of corrosion time, the solar absorptance decreases faster and faster, and it seems to exhibit an exponential downward trend; meanwhile, the higher of the concentration of NaCl salt spray is, the faster the decline of the solar absorptance will be. The possible reason lies in at the very beginning of the electrochemical corrosion; it takes the time to build the galvanic cells because the electrolyte must first travel through the Si3N4 layer along with the grain boundaries and reach the interface of Cr-CrNx layer or Al substrate. Therefore, there is a gradual decline in solar absorptance at the beginning stage. Then with the growth of corrosion time and electrolyte concentration, the solar absorptance drops faster and faster owing to the increase of micro defects and occurrence of the corrosion products. As for the thermal emittance (Figure 2 (b)), it rose rapidly at the beginning of the salt corrosion (before 24h) due to a sudden corrosion shock. After that, it shows a steady upward trend from 24h to 120h possibly owing to passivation of Cr



metal particles or Al substrate. Finally, from 120h to the ending of corrosion, the thermal emittance increases rapidly again mainly due to the destruction of the passive film. In contrast, the ratio of solar absorptance and thermal emittance (Figure 2 (c)) exhibits a complete contrary change trend. Figure 3 gives the spectra reflectance of the Si3N4/Cr-CrNx/Si3N4/Al coatings after corroding in 5.0wt% NaCl salt spray for 168h. According to Figure 3 as well as equation (1) and (2), it can be clearly seen that the thermal emittance changes more dramatic than solar absorptance, which matches well with the data in Table 2. The possible reason is that the Al substrate is more vulnerable to be corroded than Cr and CrN x 31-32. Thus, according to the degradation rule exhibited in Fig.2, the change process in optical properties of the Si 3N4/CrCrNx/Si3N4/Al solar absorbing coatings can be divided into three stages: a rapid active degradation stage, a steady passivation stage, and a transpassivation degradation stage. During these degradation states, the variation of the optical properties possibly results from the increase of the micro defects such as micro-cracks and holes and the occurrence of the corrosion products such as Al2O3 and CrOx phases, which were mainly induced by the galvanic corrosion. More details on the NaCl salt spray corrosion of the coatings will be systematically investigated in the following discussion. Table 2. Optical properties of Si3N4/Cr-CrNx/Si3N4/Al coatings corroded in different concentrations of NaCl salt spray Time (h)

0.5%

1.0%

3.0%

5.0%

α

ε

α/ε

α

ε

α/ε

α

ε

α/ε

α

ε

α/ε

As-deposited

0.950

0.051

19.000

0.950

0.050

19.000

0.950

0.050

19.000

0.950

0.050

19.000

12

0.949

0.064

14.828

0.948

0.067

14.149

0.948

0.070

13.542

0.947

0.076

12.460

24

0.948

0.068

13.941

0.949

0.071

13.366

0.947

0.075

12.626

0.945

0.081

11.667

72

0.945

0.073

12.945

0.944

0.077

12.259

0.938

0.083

11.301

0.930

0.092

10.108

120

0.935

0.077

12.142

0.933

0.084

11.107

0.924

0.090

10.266

0.908

0.100

9.080

168

0.920

0.086

10.697

0.914

0.095

9.621

0.899

0.106

8.481

0.882

0.121

7.289

(a) 0.13

(b)

20

(c)

0.12

0.5% 1.0% 3.0% 5.0%

0.94 0.11

Thermal Emittance

0.93 0.92 0.91

0.5% 1.0% 3.0% 5.0%

0.90 0.89

Solar Absorptance/Thermal Emittance

0.95

Solar Absorptance

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0.10 0.09 0.08 0.07 0.06

0.88

0.5% 1.0% 3.0% 5.0%

18 16 14 12 10 8

0.05 6

0

24

48

72

96

Corrosion Time (h)

120

144

168

0

24

48

72

96

120

144

168

Corrosion Time (h)

0

24

48

72

96

120

144

168

Corrosion Time (h)


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Figure 2. Optical properties change trend of Si3N4/Cr-CrNx/Si3N4/Al coatings corroded in different concentrations of NaCl salt spray. (a) Change trend of the solar absorptance, (b) Change trend of the thermal emittance, and (c) Change trend of the ratio of solar absorptance and thermal emittance. 1.0

Spectra Reflectance (%)

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0.8

Corroded in 5.0wt% NaCl salt spray for 168h As-deposited

(a)

0.6

0.4

(b)

0.2

0.0 1

10

Wavelength (um)

Figure 3. Spectra reflectance of Si3N4/Cr-CrNx/Si3N4/Al coatings. (a) As-deposited, (b) Corroded in 5.0wt% NaCl salt spray for 168h.

3.2. Changes in Surface Morphology Figure 4 and Figure 5 give the cross-section and surface morphology of Si3N4/Cr-CrNx/Si3N4/Al coatings which were corroded in 5.0wt% NaCl salt spray for12, 24, 72, 120, and 168h. As shown in Figure 4 and Figure 5, before the NaCl salt spray corrosion, the grain size of the coatings was quite uniform, the grains arranged very compact, the surface was quite smooth, and the interface of sublayers was clear and distinguishable (Figure 4 (a), Figure 5 (a)). However, with the increase of corrosion time, the surface of the Si3N4/Cr-CrNx/Si3N4/Al coatings became rougher and rougher, the cross section slightly widened and was hard to distinguish (Figure 4 (c)-(f)), minor grains gradually disappeared and left many holes and micro cracks, such as micro-cracks and holes (Figure 5 (e) and (f)). Additionally, the size of some grains even exceeded 200nm (Figure 5 (c)) that is almost bigger than the thickness of the coatings. This is because the residual energy was left among the grain boundaries after magnetron sputtering, and they would be released during the process of corrosion and some grains priority accessed to this energy and abnormally grew up 33-35. On the other hand, because the surface activation energy of the grains is larger than the volume activation energy, to minimise the total Gibbs free energy, some grains would



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also grow up in varying degrees 33. These abnormal growth grains would conversely generate non-uniform local stressed 36

, and these stresses would further lead to the formation of protuberance on the surface 37 (Figure 5 (d)).

Figure 4. The cross-sectional morphology of Si3N4/Cr-CrNx/Si3N4/Al coatings of as-deposited and the coatings corroded in 5.0wt% NaCl salt spray. (a), As-deposited, (b) Corroded for 12h, (c), Corroded for 24h, (d) Corroded for 72h, (e) Corroded for 120h, and (f) Corroded for 168h.

Figure 5. The surface morphology of Si3N4/Cr-CrNx/Si3N4/Al coatings of as-deposited and the coatings corroded in 5.0% NaCl salt spray. (a), As-deposited, (b) Corroded for 12h, (c), Corroded for 24h, (d) Corroded for 72h, (e) Corroded for 120h, and (f) Corroded for 168h.


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3.3. Influence of NaCl Salt Spray Corrosion on Element Distribution. The watchable changes in morphology indicate that the Si3N4/Cr-CrNx/Si3N4/Al coatings have experienced different extent element diffusion. In order to investigate the distribution of element, an Auger electron spectroscopy was carried out. The element distribution before and after corroding in 5.0wt% NaCl salt spray is given in Figure 6. Contrasting Figure 6 (a) and Figure 6 (b), it can be clearly observed that the boundaries of the sublayers are widened after corroding due to element diffusion. The most noteworthy change is that the oxygen content on the surface and in sublayers is much higher than the one of as-deposited. The oxygen content on the surface has risen to more than 60% from about 10%, especially the oxygen content on the surface of Al substrate is much higher than the one of as-deposited, and meanwhile, the distance that the oxygen diffuses into the Al substrate is much deeper. This change possibly due to the fact that the intercrystalline corrosion has taken place between the interfaces of sublayers 38. This is because during the intercrystalline corrosion, the electrolytic oxygen which diffuses into the Si3N4/Cr-CrNx/Si3N4/Al coatings will also lead to the increase of oxygen content 39

. Besides, from Figure 6 (b), it can also be clearly seen that the nitrogen and chromium obviously diffused toward both

sides because the distribution of nitrogen and chromium is not that steep as the one of as-deposited (Figure 6 (a)). As a result of the element diffusion, the interfaces of sublayers become blurry and illegible (Figure 4(f)). This diffusion of elements indicates that some chemical reactions occurred and these reactions resulted in the formation of new phases which lead to component deviating from optimum design 37.



(a)

100

80

CrNx layer 60

40

Al Cr

N

As-deposited

Si O

20

(b) Si3N4layer CrNx Si3N4layer CrOy (Bottom) (Top) layer

Al Substrate

Si3N4layer (Bottom)

Si3N4layer (Top)

Atomic concentration (%)

100

Atomic concentration (%)

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80

Al Substrate

Al 60

Corroded in 3% NaCl Salt Spray for 168h

N Cr

40

Si

O

20

0

0 0

50

100

150

200

250

300

0

50

100

150

200

250

300

Sputtering Depth (nm)

Sputtering Depth (nm)

Figure 6. Element contrast distribution of Si3N4/Cr-CrNx/Si3N4/Al coatings in 5.0wt% NaCl salt spray for 168h. (a), before

corrosion. (b), after corrosion.

3.4. Changes in the Valence State of Elements. To further figure valence change out during the corrosion process of Si 3N4/Cr-CrNx/Si3N4/Al coatings, X-ray photoelectron spectroscopy (XPS) was conducted. Figure 7 presents the XPS survey spectra of three different depths of Si3N4/Cr-CrNx/Si3N4/Al coating corroded in 5.0wt% NaCl salt spray for 168h. The depth of 40nm corresponds to the top Si3N4 layer, the depth of 80nm corresponds to the Cr-CrNx layer, and the depth of 120nm corresponds to the bottom Si3N4 layer. As shown in Figure 7, at the sputtering depth of 80nm (i.e. the Cr-CrNx layer), comparing with the depth of 40nm and 120nm, there is an O1s peak and an O KLL peak, which means there are some oxides that exist in the Cr-CrNx layer. Also, peaks of Cr are found in the sputtering depth of 40nm and 120nm. So it indicates that the Cr element has diffused into both sides, which is in good agreement with the result of AES analysis. To further investigate the valence state change of elements, Figure 8 exhibits the narrow spectra of four dominated elements from different depths. The peak of carbon located at 284.8eV, which is not shown in this figure, is used as a reference for all binding energies. Table 3 gives the results of the spectra decomposition of the sandwich-like Si3N4/Cr-CrNx/Si3N4/Al coatings corroded in 5.0wt% NaCl salt spray for 168h.


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In Figure 8 (a), O1s core level binding energy can be deconvoluted into three distinct components (530.7 eV, 531.4eV and 535.6 eV) with a full-width half maximum (FWHM) about 2.1 eV. Regarding the spectra from the depth of 40nm (Si3N4 (Top)), the O 1s peak located at a binding energy of 531.4 eV can be assigned to Al-O bond 40 because there is an aggregation of Al in this depth according to AES analysis (Figure 6 (b)). In the depth of 80nm (Cr-CrNx layer), the intensity of Al-O peak (531.4 eV) gets weak while the peak located at 530.7eV becomes the main peak, which can be ascribed to Cr-O bond 41. Nevertheless, in the depth of 120nm (Si3N4 (Bottom)), the Cr-O peak shifts to minor peak and the Al-O peak becomes the main peak because this layer is close to the Al substrate. In each XPS spectra of O1s, there is a peak located at 535.6eV, which can be ascribed to hydroxyl groups (-OH). Because these samples were corroded in NaCl salt spray and dried in air, so it is easy to reserve the residual water or form hydroxide 40. Figure 8 (b) exhibits the binding energy results of N 1s. In the depths of 40nm and 120nm, there is a main peak centred at 397.4 eV, with an FWHM of 1.8 eV, which can be assigned to Si-N bond 42 because of the presence of silicon nitride. In the depth of 80nm, the spectra of N1s can be deconvoluted into two peaks, a small peak located at 396.8eV and a prominent peak located at 397.4 eV (Si-N bond). The small peak located at 396.8eV can be assigned to Cr-N or Al-N bond 43-44

. However, it can also be ascribed to Cr and Al oxynitride, which would correspond to having nitrogen atoms as second-

nearest-neighbor atoms 45. The decrease of intensity of the small peak located at 396.8eV indicates that the Cr-CrNx layer has already reacted with the NaCl electrolyte, this is why it can be observed that many minor grains gradually disappeared in the SEM measurement in Figure 5. As for the XPS spectra of Al 2p (Figure 8 (c)), due to the fact that the Al substrate is easy to be corroded, thus, it enables the formation of Al-O, Al-Si, Al-N, or Al-Al bonds. As shown in Figure 8 (c), the peak centred at 72.8 eV can be ascribed to Al-Si, Al-N or Al-Al bonds, which is in good agreement with other reports obtained for pure aluminium and Al-Si alloys 46-47

. The peak located at the binding energy of 74.5 eV, which is presented in three different depths simultaneously,



corresponds to the formation of Al(OH)3 or Al2O3 compounds 47. However, the peak of 74.5 eV can also be related to OAl-N bond 48, which may occur in depth of 80nm or 120nm for they are close to the surface of Al substrate. Regarding of the Cr 2p spectra shown in Figure 8 (d), the spectra of Cr 2p3/2 and Cr 2p1/2 can be decomposed into two major peaks located at 574.1eV and 583.4eV as well as two minor peaks centred at 576.5eV and 586.3eV. The major peaks can be ascribed to Cr-Cr bond 49, and the minor peaks can be assigned to Cr-N bonds 43 since there are lots of Cr metallic particles and non-stoichiometric CrNx grains in the Cr-CrNx layer. Meanwhile, the minor peaks can also be regarded as CrO bond 41, because the Cr-CrNx is possible to be corroded partially, transformed into hydroxide of chromium, and finally forms the oxides of chromium after drying. So the disappearance of the many minor grains observed by SEM measurement

600

-Si2s -Si2p -Cr3s -Cr3p

-Cr2p1 -Cr2p3

700

-O1s

-Cr2s

-Cr LMM1 -O KLL -Cr LMM2

-Na 1s

Sputtering depth 40nm

-N1s

will make sense.

Intensity (a.u.)

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1200 1100 1000 900

800

500

400

300

200

100

0

Binding Energy (eV)

Figure 7. XPS survey spectra of Si3N4/Cr-CrNx/Si3N4/Al coatings corroded in 5.0wt% NaCl salt spray for 168h.


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Figure 8. XPS spectra in different depths for elements of (a) O1s, (b) N1s, (c) Al 2p and (d) Cr 2p form Si3N4/Cr-CrNx/Si3N4/Al coatings corroded in 5.0wt% NaCl salt spray for 168h. Table 3. Identification of core level binding energies for O 1s, N 1s, Al 2p, and Cr 2p. Depth

40 (nm)

80 (nm)

120 (nm)

Core Level

B.E. (eV)

B.E. (eV)

B.E. (eV)

O 1s

-

530.7

530.7

Cr-O

531.4

531.4

531.4

Al-O

535.6

535.6

535.6

-OH

-

396.8

-

Cr-N; Al-N;

397.4

397.4

397.4

Si-N

-

72.8

72.8

Al-Si; Al-Al; Al-N

74.5

74.5

74.5

Al-O; Al-O-N

-

574.1 (583.4)

-

Cr-Cr

-

576.5 (586.3)

-

Cr-O; Cr-N

Bonding Form

N 1s

Al 2p

Cr 2p3/2 (Cr 2p1/2)

3.5. Analysis of phase structure after NaCl salt spray corrosion. Figure 9 depicts the XRD patterns of Si3N4/Cr-CrNx/Si3N4/Al coatings of as-deposited and these that were corroded in 0.5, 1.0, 3.0 and 5.0wt% NaCl salt spray for 168h. As shown in Figure 9, the coatings of as-deposited exhibits an amorphous structure at room temperature, this is because the deposition temperature is far below the crystallisation temperature of Si3N4 and CrNx. However, with the increase in the concentration of NaCl salt spray, more and more crystallisation phases begin to emerge. These crystallisation phases include the original Si3N4 and Cr-CrNx grains as well as newly generated Al2O3 and CrOx. Due to the fact that it is relative harder to break the lattice of the big crystals than that of the amorphous structures and some minor grains, So, after corroding the original crystallographic Si3N4 and CrCrNx grains begin to emerge their true appearance while the amorphous structures were corroded by the NaCl electrolyte. As for the newly generated Al2O3 and CrOx, it is possible because the Al, Cr, and CrNx react with chloride ions and generate corresponding chlorides, such as AlCl3 and CrCl3 50. These chlorides will hydrolyze to Al(OH)3 and Cr(OH)3 since they are amphoteric compounds. After drying these hydroxides will form dehydrated oxides, such as Al2O3 and CrOx. This result is in good agreement with the XRD patterns because we can find the peaks of (110), (103) and (214) for Al2O3, (111), (200), (220), and (311) for CrO x. This is why we can also find the Al-O and Cr-O bonds even hydroxyl



groups in the XPS spectra. In addition, the intensity of other peaks is very low that means the content of other phases is minuscule, and the possible reason is that during the corrosion process some new generated minor phases transfer to the grain boundaries of other major phases so that they are insufficient to be found.

CrOx(111)

Al(111)

Al2O3(110)

Intensity (arb. units)

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 16 of 27

CrOx(220) Al(220) Al2O3 (214) 64.0

64.5

65.0

65.5

66.0

66.5

CrOx(311)

77.0

77.5

78.0

Al(311)

78.5

79.0

79.5

5.0% 3.0% Al(200) CrOx(200) Al2O3(103)

1.0% 0.5% As-deposited 20

30

40

50

60

70

80

Two theta (degree)

Figure 9. XRD patterns of Si3N4/Cr-CrNx/Si3N4/Al coatings corroded in different concentrations of NaCl salt spray for168h.

To further identify the structure of major phases, a TEM characterization was carried out. The HRTEM micrographs and corresponding selected area electron diffraction patterns obtained from the middle of Cr-CrNx interlayer are presented in Figure 10. The Si3N4/Cr-CrNx/Si3N4/Al coating appears to be amorphous structure before corroding, as shown in Figure 10 (a) and (b). However, after corroding, there are some holes or micro-cracks appear in the top Si3N4 layer. From the result of this figure, more serious is that the corrosive chloride ions even penetrated the Cr-CrNx and bottom Si3N4 layers, reacted with Al substrate and led to corrosion of Al substrate. This means that the Si3N4/Cr-CrNx/Si3N4/Al coating has experienced a pitting corrosion, (more details please refer to the supporting information). Additionally, during the process of the pitting corrosion, the corrosive chloride ions would possibly guide the chromium ions and nitrogen ions diffuse downward and result in the outflow of the Cr-CrNx layer. This is why the Si3N4 bottom layer became wider, and the interface of Cr-CrNx layer became indistinct. Figure 10 (d) gives the detailed phase transformation of Si3N4/CrCrNx/Si3N4/Al coating corroded for 168h in 5.0wt% NaCl salt spray. As shown in the inset of SAED pattern (Figure 10 (d)), there are many crystal phases in the Cr-CrNx layer. Table 4 provides the results of diffraction rings analysis of the


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corresponding phases. In Figure 10 (d), nano-crystalline grains are surrounded by amorphous phases. Precise measurement of the lattice spacing from the SAED patterns shows that the interplanar distances of d1 (4.544Å), d3 (2.273Å), d6 (1.970Å), and d9 (1.391Å) are assigned to the panels of (111), (222), (400) and (440) of γ-Al2O3 51, the interplanar distances of d2 (2.481Å), d4 (2.172Å), and d7 (1.668Å) are assigned to the panels of (110), (113) and (116) of Cr2O3 52, the rest interplanar distances of d5 (2.012Å) and d8 (1.435Å) can be ascribe to the panels of (110) and (200) of metallic chromium 52. The d spacing of experimental deviation from the theoretical values is possibly owing to the residual stress present in the bottom Si3N4 layer. This is because of the volume of Al2O3, Cr2O3 is bigger than that of metallic Al and Cr, and this is why we can observe protuberance on the surface of these corroded coatings.

Figure 10. HRTEM micrographs and corresponding selected area electron diffraction patterns of Si 3N4/Cr-CrNx/Si3N4/Al coatings. (a) Cross-sectional HRTEM image before corroding; (b) HRTEM image of Cr-CrNx layer before corroding; (c) Cross-sectional HRTEM and SAED images after corroding; and (d) Polycrystalline diffraction rings after corroding, (dii) the FFT image of γ-Al2O3, (diii) the FFT image of Cr2O3. Table 4. Theoretical and experimental d-spacing values for the diffraction rings in Figure 10 (d). Ring No.

Phase (hkl)

d theoretical (Å)

d experimental (Å)

1

γ-Al2O3 (111)

4.540

4.544



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

2

Cr2O3 (110)

2.484

2.481

3

γ-Al2O3 (222)

2.280

2.273

4

Cr2O3 (113)

2.175

2.172

5

Cr (110)

2.040

2.012

6

γ-Al2O3 (400)

1.970

1.970

7

Cr2O3 (116)

1.672

1.668

8

Cr (200)

1.440

1.435

9

γ-Al2O3 (440)

1.390

1.391

Page 18 of 27

3.6. Electrochemical Corrosion Reactions mechanism. Since we have identified the structure of the corrosion products, so it is believed that the electrochemical corrosion reactions take place according to the following mechanism: The anodic dissolution of chromium and aluminum: Cr  3e  Cr 3 (3) Al  3e  Al3 (4)

In neutral solution, the cathodic reduction reaction occurs when atmospheric oxygen dissolved in the NaCl solution and generates the corresponding hydroxide radical groups. O2 +2H2 O+4e  4OH  (5)

On the other hand, the penetrative chloride ions turn the insoluble chromium nitride and alumina into soluble chlorides 53

. So Cr3+ and Al3+ ions can react with chloride ions and hydroxide radical and generate corresponding chlorides and

hydroxides. 

Cl CrNx +3xH2O   Cr 3+ +xNH3  +3xOH (6)



Cl Al2O3 +3H2O   2Al3+ +6OH (7)

Cr 3+ +3Cl Ç CrCl3 (8)

Al3  3Cl Ç AlCl3 (9) Cr 3  3OH  Ç Cr( OH)3

(10)

Al3  3OH Ç Al( OH)3

(11)


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When dried in air, these chlorides hydrolyze to hydroxides since they are amphoteric chlorides. 



(12)





(13)

CrCl3 +3H2 O  Cr(OH)3 +3Cl +3H

AlCl3 +3H2 O  Al(OH)3 +3Cl +3H

And these hydroxides finally generate corresponding chromium oxide and aluminium oxide.

2Cr( OH)3  Cr2O3  3H2O

(12)

2Al( OH)3  Al2O3  3H2O

(13)

Overall, the electrochemical corrosion may be explained by introducing pitting corrosion or galvanic corrosion. As illustrated in Figure 11, different ions from NaCl electrolytic solution penetrate into lower sublayers and react with these structures, especially the corrosive OH– and Cl– electrolyte ions react with metallic Cr and Al, and convert them into corresponding chlorides and hydroxides. Additionally, the corrosion reaction occurs mainly at the grain boundaries, in the micro cracks and holes, where porous structures are relatively prone to such degradations.

Figure 11. Schematic illustration of electrochemical corrosion of Si3N4/Cr-CrNx/Si3N4/Al coatings in 5.0wt% NaCl electrolyte.

3.7 Electrochemical Analysis of the Corrosion Process. In order to evaluate the corrosion resistance of the Si3N4/Cr-CrNx/Si3N4/Al coatings and the influence of different concentrations of NaCl salt spray on the corrosion resistance of the coatings, thus, the as-deposited coatings and the coatings that were respectively corroded in 0.5wt%, 1.0wt%, 3.0wt%, and 5.0wt% NaCl salt spray for 168h were used for the potentiodynamic polarization analysis. Figure 12 depicts the potentiodynamic polarisation curves for such corroded



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Page 20 of 27

and as-deposited coatings. The corresponding electrochemical test results are displayed in Table 5. As observed in Figure 12, the corrosion potential (Ecorr) of the Si3N4/Cr-CrNx/Si3N4/Al coatings positively decreases from -1.027 V (Asdeposited) to −1.508 V(5.0wt%) with the increase of the concentration of NaCl salt spray. Meanwhile, the corrosion current density (Icorr) slowly increases from 2.997×10−9 A·cm−2 (As-deposited) to 3.953×10−7 A·cm−2 (5.0 wt%). Accordingly, the corrosion resistance (Rp) has dropped sharply from 3633.858 kΩ cm2 to 13.759 kΩ cm2. This distinctly reveals that the NaCl salt spray will severely impair the corrosion resistance of the Si3N4/Cr-CrNx/Si3N4/Al coatings, and the higher of the concentration of the NaCl salt spray is, the lower of the corrosion resistance will be. In detail, cathodic reactions exhibit almost the same behaviour of the ionisation of the dissolved oxygen (Figure 12). The anodic behaviour is a little complex. It clearly presents that there are some different lengths of passivation regions, especially, the as-deposited coating exhibits the longest passivation region. The occurrence of the passivation mainly because the chromium and aluminium are quite easy to be passivated in neutral solution 54. Regarding to the phenomena of that the length of passivation region gradually reduces over the soar in the concentration of NaCl salt spray (from 0.5wt% to 5.0wt %), mainly due to the increase of corrosive chloride ions, which will obviously block the passivation process of chromium and aluminium 55. During the pre-corrosion process (i.e. the adjusted NSS experiments), with the growth of the concentration of NaCl salt spray, more and more micro defects (such as holes and micro cracks) and corrosion products (such as Cr2O3 and Al2O3) will generate, which will also badly reduce the corrosion resistance of coatings. This is why in the potentiodynamic polarisation curves, the corrosion potential (Ecorr) becomes smaller and smaller and the corrosion current (icorr) gets bigger and bigger. Additionally, from Figure 12, it can be observed that there is a sign of pitting corrosion (Similar to the position of point A). This is possible because there are some micro defects (such as micro cracks, holes, even grain boundaries, etc.) on the surface of Si3N4/Cr-CrNx/Si3N4/Al coatings or the interfaces of the sublayers of the coatings. In the pitting corrosion


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process, the chloride ions selectively absorb on these micro defects, replace the oxygen atoms, combine with cationic and generate soluble chlorides, finally result in the formation of corrosion holes 56. These little holes called the nuclei of pitting corrosion, they can also be understood as the active centres of pitting corrosion, which will further aggravate the intercrystalline corrosion. As for the pre-corroded coatings, due to the fact that they contain more micro defects and corrosion products, therefore, these pre-corroded coatings possibly suffer from the general corrosion, so that their pitting corrosion was covered by the general corrosion 57.

Figure 12. Potentiodynamic polarisation curves for Si3N4/Cr-CrNx/Si3N4/Al coatings of as-deposited and the coatings that pre-corroded in 0.5, 1.0, 3.0 and 5.0wt% NaCl salt spray for 168h. Table 5. The calculated values of Potentiodynamic Polarization Curves for Si3N4/Cr-CrNx/Si3N4/Al coatings of as-deposited and precorroded.

Corrosion

Ecorr (V)

icorr (A/cm2)

ba (V/dec)

bc (V/dec)

Rp (kΩ cm2)

5.0%

-1.508

3.953×10-7

0.029

0.022

13.759

3.0%

-1.425

9.356×10-8

0.035

0.030

75.068

1.0%

-1.275

5.562×10-8

0.048

0.049

189.542

-1.213

2.000×10

-8

0.064

0.066

706.345

2.997×10

-9

0.061

0.043

3633.858

concentration

0.5% As-deposited

-1.027

4. CONCLUSIONS In summary, we have successfully prepared a type of high corrosion resistant sandwich-like Si3N4/Cr-CrNx/Si3N4 solar selective absorbing coatings on aluminium substrate with a high solar absorptance of ~0.95 and low thermal emittance of ~0.05 at room temperature through magnetron sputtering techniques. The influence of different concentrations of NaCl salt



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Page 22 of 27

spray on corrosion properties of the coatings was discussed systematically. The change process in optical properties of Si3N4/Cr-CrNx/Si3N4/Al solar absorbing coatings can be divided into three states: a rapid active degradation stage, a steady passivation stage, and a transpassivation degradation stage. With the increase in the concentration of NaCl salt spray from 0.5 to 5.0wt%, the length of the passivation region becomes shorter and shorter, and finally, the corrosion resistance falls from 3633.858KΩ cm2 to 13.759 KΩ cm2. This indicates that without the protection of passivation film, the corrosion rate will gradually speed up. Regarding surface morphology, after corroding in 5.0wt% NaCl salt spray for 12, 24, 72, 120, and 168h, the surface of the coatings became rougher and rougher, the cross section becomes blur and hard to distinguish, meanwhile minor grains gradually disappeared and left some holes and micro cracks. Regarding phase structure, with the increase in the concentration of NaCl salt spray, more and more crystal phases begin to emerge. This is because the metallic particles of Al and Cr-CrNx react with chloride ions, and it generates the corresponding hydroxide, such as Al(OH)3 and Cr(OH)3. Although the Si3N4/Cr-CrNx/Si3N4/Al coatings have undergone these changes, they still possess a good performance of high solar absorptance of 0.924 and low thermal emittance of 0.090 after corroding in 5.0wt% NaCl salt spray for 120h. In this regard, the Si3N4/Cr-CrNx/Si3N4/Al coating can be recommended as an excellent candidate for solar selective absorber that can be applied in high salinity environments, such as coastal areas and saline areas, and so forth.

ASSOCIATED CONTENT Supporting Information: Reasons for choosing aluminium and its elemental composition; Concrete preparation procedure of Si 3N4/Cr-CrNx/Si3N4 Coatings; Method of Si3N4/Cr-CrNx/Si3N4 TEM samples; Pitting corrosion process on the surface of the Si 3N4/CrCrNx/Si3N4 Coatings. ∗ Corresponding author


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Tel.: +86 010 8224 1241; Fax: +86 010 8224 1294. E-mail: [email protected] (Lei Hao). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge financial support toward this project from the National Key Technology Research and Development Program of the Ministry of Science and Technology of the P.R. China. [Grant number: 2015BAA02B04]. REFERENCES (1) Lewis, N. S. Research Opportunities to Advance Solar Energy Utilization. Science 2016, 351, 6271. (2) Mekhilef, S.; Saidur, R.;Safari, A. A Review on Solar Energy Use in Industries. Renew. Sust. Energ. Rev. 2011, 15,1777-1790. (3) Weinstein, L. A.; Loomis, J.; Bhatia, B.; Bierman, D. M.; Wang, E. N.;Chen, G. Concentrating Solar Power. Chem. Rev. 2015, 115, 12797-12838. (4) Armaroli, N.;Balzani, V. The Future of Energy Supply: Challenges and Opportunities. Angew. Chem. Int. Edit. 2007, 46, 52-66. (5) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.;Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474-6502. (6) Li, P.; Liu, B.; Ni, Y.; Liew, K. K.; Sze, J.; Chen, S.;Shen, S. Large‐Scale Nanophotonic Solar Selective Absorbers for High‐Efficiency Solar Thermal Energy Conversion. Adv. Mater. 2015, 27, 4585-4591. (7) Nuru, Z.; Msimanga, M.; Muller, T.; Arendse, C.; Mtshali, C.;Maaza, M. Microstructural, Optical Properties and Thermal Stability of MgO/Zr/MgO Multilayered Selective Solar Absorber Coatings. Sol. Energy 2015, 111, 357-363. (8) Nuru, Z.; Arendse, C.; Khamlich, S.; Kotsedi, L.;Maaza, M. A Tantalum Diffusion Barrier Layer to Improve the Thermal Stability of Al x O y/Pt/Al x O y Multilayer Solar Absorber. Sol. Energy 2014, 107, 89-96. (9) Thornton, J. A.; Penfold, A. S.;Lamb, J. L. Sputter-eposited Al2O3/Mo/Al2O3 Selective Absorber Coatings. Thin Solid Films 1980, 72, 101-110. (10) Cao, F.; Kraemer, D.; Sun, T.; Lan, Y.; Chen, G.;Ren, Z. Enhanced Thermal Stability of W-Ni-Al2O3 Cermet-Based Spectrally Selective Solar Absorbers with Tungsten Infrared Reflectors. Adv. Energy Mater. 2015, 5, 1-7. (11) Zwickl, B. M.; Shanks, W. E.; Jayich, A. M.; Yang, C.; Bleszynski Jayich, A. C.; Thompson, J. D.;Harris, J. G. E. High Quality Mechanical and Optical Properties of Commercial Silicon Nitride Membranes. Appl. Phys. Lett. 2007, 92, 103125-103125-3. (12) Riley, F. L. Silicon Nitride and Related Materials. J. Am. Ceram. Soc. 2000, 83, 245-265. (13) Zhu, X.;Sakka, Y. Textured Silicon Nitride: Processing and Anisotropic Properties. Sci. Technol. Adv. Mat. 2008, 9, 43-48.



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