AlCrN coatings at elevated temperatures for low thermal emissivity applications

Applied Surface Science 498 (2019) 143886

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Thermal stability of multi-layered AlCrN/Cr/AlCrN coatings at elevated temperatures for low thermal emissivity applications Q.Y. Lia, S.J. Dongb, Q. Caoc, W.P. Yec, D.Q. Gongd, X.D. Chenga,

T

⁎

a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China c School of Material Science and Engineering, Wuhan University of Technology, Wuhan 430070, China d College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: AlCrN layer Multi-layered coatings Low infrared emissivity Thermal stability Annealing

Multi-layered AlCrN/Cr/AlCrN coatings were designed and deposited on Ni-based substrates by a cathodic arc ion plating system for low thermal emissivity applications. The multi-layered coatings annealed at different temperatures in air and vacuum were systematically investigated by GIXRD, EPMA, XPS, DSC, GDOES, FESEM, HR-TEM and scratch tester as well as the infrared properties evaluation. The as-deposited AlCrN layer was consisted of Cr2N nanocrystallites embedded in AlCrN amorphous matrix, inhibiting element diffusion and presenting higher infrared emissivity. The amorphous matrix began to crystallize as fcc-Cr(Al)N structure at 750 °C. The diffusion of Ni element from the substrate to the middle Cr layer was observed as well as the Cr-rich oxides were substituted by chrome‑aluminum oxides above 750 °C in air. The grain boundaries resulted from the crystallization of amorphous AlCrN layer could promote the penetration of O element from air and the diffusion of Ni element from the substrate. The AlCrN/Cr/AlCrN coatings vacuum-annealed above 750 °C performed lower infrared emissivity than the as-deposited one, while the coatings air-annealed above 750 °C showed higher infrared emissivity than vacuum-annealed ones due to the low extinction coefficient of oxides formed on the coating surface. The multi-layered AlCrN/Cr/AlCrN coating could be proposed for low infrared emissivity applications below 750 °C.

1. Introduction Control of thermal emission from surface has attracted extensive attention due to its architectural and military applications, such as, heat loss control in buildings and protecting vehicles from infrared detection in camouflaging military equipment [1–4]. Infrared radiation from a surface can be spectrally and spatially controlled, and the polarization and coherence of the infrared radiation field can be modified [5]. Low infrared emissivity coatings can modify the thermal emission from surface, which is an attractive way to diminish the radiative heat transfer by either reducing the emission of infrared radiation or reflecting most incident of infrared radiation [6]. In recent years, extensive works on the applications of low infrared emissivity coatings have been reported, such as composited paints [4,7], metallic thin films [1,8,9], and dielectric/metal/dielectric multi-layered coatings [2,3,6,10]. Especially, the multi-layer structured coatings, which have excellent chemical resistance, hardness, wear resistance, corrosion resistance and oxidation resistance, become the hot point of research and

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display well applied prospects [3,10]. The reports about the multi-layer structured coatings mainly focused on the infrared properties at relatively lower temperatures below 600 °C. The corresponding microstructure evolution and the infrared properties of multi-layered coatings with low emissivity above 600 °C have not been reported so far, as well as the degradation mechanisms. Dielectric/metal/dielectric multi-layered coatings with low infrared emissivity are typically prepared by layer stacks, generally comprising one conductive metallic layer embedded between various dielectric layers. This metallic layer is responsible for thermal reflectivity due to its plasmonic resonance frequency character [3]. Normally, copper, silver, platinum and chromium are mostly used as metallic middle layer for reflective function of infrared radiation [10,11]. However, the noble metal Cu, Ag, Au and Pt will suffer from agglomeration after heating, which will degrade the infrared reflection property and increase the infrared emissivity [6,12,13]. Thus, keeping in mind the above constrains and taking in consideration of the cost, Cr can act as a promising candidate for metallic middle layer in the multi-layer structured

Corresponding author. E-mail addresses: [email protected] (Q.Y. Li), [email protected] (Q. Cao), [email protected] (W.P. Ye), [email protected] (X.D. Cheng).

https://doi.org/10.1016/j.apsusc.2019.143886 Received 26 February 2019; Received in revised form 22 August 2019; Accepted 4 September 2019 Available online 05 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

Applied Surface Science 498 (2019) 143886

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coatings for its refractory nature with high melting point (1857 °C) and high reflective property. Generally, the infrared radiation of aircraft mainly comes from engines, plume from exhaust nozzle and ultra-high surface temperature from aerodynamic heating. Considering that individual metallic layer at high temperatures may tend to oxidize, and thus strongly alter the infrared properties, the dielectric layers need to comprise protective functions to prevent the metallic layer from chemical and mechanical damages. Furthermore, these dielectric layers can also act as a barrier to inhibit element diffusion from substrate at high temperature. Such additional layers may comprise non-oxide ceramic materials, such as nitrides of titanium, chromium and aluminum [6,10]. Noteworthy, a ternary nitride, such as, AlCrN, may be helpful to act as a protective layer to withstand at elevated temperatures against the metallic layer oxidation and as a diffusion barrier for corrosion and chemical resistance. In this study, a multi-layered AlCrN/Cr/AlCrN coating with low infrared emissivity was designed and prepared by a cathodic arc ion plating (CAIP) system, which was characterized by high ionization rate, high deposition rate, convenient parameters and high kinetic energy [14]. Moreover, they were heat-treated in air and vacuum conditions, respectively. The effect of heat treatment on the microstructure, composition and infrared properties has been investigated in details. These results would contribute to an understanding of degradation mechanisms of multi-layered AlCrN/Cr/AlCrN coatings with infrared properties during annealing and determine the effective temperature on practice. 2. Experimental 2.1. Coating design Instead of using the noble metals, Cr layer was selected as the infrared reflector to decrease the infrared emissivity. Since the AlCrN layer was demonstrated to exhibit excellent thermal stability as well as high oxidation resistance even at the temperature of 1000 °C [15,16], the AlCrN was selected in this work as the outer-layer and inner-layer attached to the middle Cr layer. Their high oxidation resistant ability and diffusion barrier property could suppress the inter-diffusion from ambient and the diffusion of elements from substrate at high temperature. A schematic of the multi-layered coating consists of an innerlayer (AlCrN), an IR reflective metallic layer (Cr) and an outer-layer (AlCrN) from bottom to top is illustrated in Fig. 1a. As for the thickness of each layer, they were determined as follows. Considering that gradually adding the thickness of Cr layer would lead to a decrease in the infrared emissivity which reached the bulk value as the thickness was about 100 nm [17], Cr layer with a thickness of 200 nm was designed to ensure high infrared reflectivity. The limits of the Cr layer thickness were not always the same for different deposition parameters. The thickness of AlCrN layer was controlled at about 80 nm for multi-layered coatings to maintain the infrared emissivity of about 0.1, because thicker AlCrN layer would increase the emissivity due to that the size effect of dielectric on emissivity was pure volumetric phenomenon [17].

Fig. 1. (a) Schematic illustration and (b) cross-sectional morphology of as-deposited multi-layered AlCrN/Cr/AlCrN coating. Table 1 Detailed deposition parameters of the AlCrN/Cr/AlCrN coatings. Parameters

AlCrN

Cr

Working pressure (Pa) N2 flux (sccm) Ar flux (sccm) Target Target current (A) Substrate voltage (V) Deposition time (min) Duty cycle ratio (%)

0.5 160 0 Al70Cr30 65 −100 2 70

0.5 0 55 Cr 65 −100 30 70

respectively. In addition, single thick AlCrN layers were deposited on JGS1 optical quartz (20 × 20 cm2) with the same parameters (voltage, current, duty cycle ratio, working pressure and gas flow rate), but with an increased deposition time of about 10 min. These samples deposited on quartz were used for the chemical composition analysis. The AlCrN/Cr/AlCrN coatings and single AlCrN layers were annealed respectively using a resistance furnace in air and using a quartz tube in vacuum at 10−5 Pa. Samples were heated-treated at 700–800 °C for 10 h with a heating rate of 5 °C/min and then cooled to room temperature in furnace.

2.2. Coating deposition The multi-layered coatings AlCrN/Cr/AlCrN were respectively deposited on (110) silicon wafers (20 × 20 cm2) and Ni-based superalloy K424 (40 × 30 cm2) substrates using a cathodic arc ion plating (CAIP) system, which was equipped with one Cr (99.99%) target and one Al70Cr30 (99.99%) target on the opposite sides of the chamber. Argon and nitrogen with purity of 99.99% were used for the deposition experiments. Prior to deposition, the chamber was heated up to 200 °C and then pumped down to a base pressure of 5 × 10−3 Pa. The base support was rotated at 5 rpm by using a DC motor. The detailed deposition parameters are listed in Table 1. The thicknesses of Cr and AlCrN layers were controlled to be about 200 nm and 80 nm,

2.3. Coating characterization The phase structure of the coatings was determined by a Grazing Incidence X-ray diffraction (GIXRD, Empyrean) using Cu-Kα radiation (λ = 0.15406 nm) with glazing incidence angles of 0.2° and 1°, by operating at 40 kV and 40 mA. The surface morphology of the coatings was characterized by a Field Emission Scanning Electron Microscope 2

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Fig. 2. Surface morphologies of (a) as-deposited multi-layered AlCrN/Cr/AlCrN coating, and multi-layered coatings annealed at (b) 700 °C, (c) 750 °C and (d) 800 °C.

temperature by the following equation:

(FE-SEM, Zeiss Ultra Plus) system operated at 5 kV. The chemical composition of the coatings was analyzed by an Electron Probe X-ray Microanalyzer (EPMA, JXA-8230) at an acceleration voltage of 20 kV with accuracy lower than 5 at.%. The chemical bonding state was investigated by an X-ray Photoelectron Spectroscopy (XPS, ESCALAB250Xi) with Al-Kα (hν = 1486.6 eV) radiation source at 20 kV voltage and 15 mA current, which was calibrated from the C 1s peak at 284.8 eV. The background was subtracted by the Shirley method and the deconvolution of the spectra was conducted using a non-linear least-square fit with a Lorentzian/Gaussian shape (L/G Mix 30%). The peaks were compared with XPS database compiled by National Institute of Standards and Technology (NIST) X-ray Photoelectron Spectroscopy (XPS) Database Main Search Menu [18]. The microstructure variation of the coatings was analyzed by Raman spectroscopy on the sample surface with a LabRam HR Evolution confocal spectrometer at the range of 100–1100 cm−1, employing a Nd: YAG laser (wavelength 532 nm) for excitation. The Raman signal was detected using a CCD detector attached to a microscope with a hundredfold magnification. The elemental depth profile of the annealed coatings was determined by a Glow Discharge Optical Emission Spectroscopy (GDOES, Spectruma Analytik GmbH-GDA750HP), employing a RF GD Profiler equipped with a 2.5 mm diameter anode. The GDOES tests were carried out at a typical radio frequency discharge pressure of 250 Pa, a voltage of 700 V and a power of 20 W. A Transmission Electron Microscope (TEM, JEOL JEM-2100F) with an operating voltage of 200 kV was used to characterize the microstructure of single AlCrN layer. In this work, the cross-sectional TEM samples were first cut to a size of 1 mm × 1 mm. Then, the pieces were attached and flued with coating face to face by using M-Bond 610 (Micro Measurement, US). The samples were manually grinded to a thickness of 50 μm. At last, the furthering polishing process was carried out on a Precision Ion Polishing System (PIPS, Gatan Inc. 691). A scratch tester (Anton Paar CSM Revetest) was employed to determine the adhesion strength between the substrate and coatings at a load speed of 50 N/min. The spectral reflectance (R(λ, T)) of the coatings was measured by a Bruker Tensor 27 Fourier-Transform Infrared Spectrometer in the wavelength range of 2.5–25 μm. The total normal emissivity was calculated according to the spectral reflectance measured at room

ε=

25μm ∫2.5μm Ib (λ, T)(−R (λ, T ))dλ 25μm ∫2.5μm Ib (λ, T )dλ

(1)

where, R(λ, T) and Ib(λ, T) are the spectral reflectance and black body emissive power at temperature T, respectively. Differential Scanning Calorimetric (DSC, Netzsch STA 449C thermal analyser) measurement was employed to study the thermal behavior. The single AlCrN layer placed in an aluminum oxide crucible was measured and another crucible with substrate was used as blank sample. The samples were heated with a rate of 5 °C/min from room temperature to a maximum temperature of 1000 °C in high purity argon with a constant flowing rate of 20 mL/min. The DSC curve was recorded by subtracting the difference in heat flow between single AlCrN layer and blank sample at the same temperature. 3. Results and discussion 3.1. Thermal stability of multi-layered AlCrN/Cr/AlCrN coatings in vacuum The cathodic arc deposited coating presented a sandwiched or multi-layered structure, the middle Cr layer of which seems to have columnar structure (Fig. 1b). The color difference in contrast between upper and lower AlCrN layers was due to the irradiation of electron beam during the focusing procedure in FESEM measurement. Typical surface morphologies of the vacuum-annealed AlCrN/Cr/AlCrN coatings at different temperatures are given in Fig. 2 and compared with those of the as-deposited one. Both for the as-deposited and vacuumannealed coatings, some spherical particles were dispersedly distributed on the surface, which was typical feature of the cathodic arc deposited coatings [19]. The EPMA compositions of the droplets before and after vacuum-annealing are shown in Table 2, the ratio of Al/Cr suggested that these droplets consisted of Cr2Al alloy. No nitrogen was detected in these droplets after vacuum heating treatment, suggesting that the coatings exhibited good thermal stability. Nevertheless, the asdeposited coating (Fig. 2a) presented a dense surface morphology, 3

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(4)

Cr layer was cubic crystallographic Cr phase. The ICDD cards used in this study were 35-0803 (Cr2N), 11-0065 (CrN) and 25-1495 (AlN), respectively. Besides the Cr phase, some weak peak of chromium nickel (such as Cr7Ni4, Cr7Ni3 and Cr2Ni3) could be observed in the middle Cr layer after vacuum annealing above 750 °C, implying that the Ni atoms from the substrate had already diffused into the middle Cr layer through the inner-AlCrN layer. The grain boundaries in the crystalline AlCrN inner layer were conceivable to serve as quick diffusion channels for the diffusion of Ni from the substrate. As for the vacuum-annealed layers, their characteristic contours had no obvious change as the heat-treatment temperature was below 750 °C (Fig. 3a and b). However, after annealing at 750 °C, a new peak at 2θ = 43.8° located between the standard (200) planes of fcc-CrN and fcc-AlN was detected, suggesting the formation of a solid solution of Cr (Al)N in NaCl type lattice in the outer layer. With further increasing the temperature to 800 °C, the intensity of Cr2N (111) plane decreased and that of Cr(Al)N (200) plane conversely increased. This was related with the transformation of Cr2N into CrN and the crystallization of amorphous Cr(Al)N in the outer layer. Complementary insights into structural changes were obtained by carrying out the relevant XPS core-level spectra of the as-deposited and vacuum-annealed AlCrN/Cr/AlCrN coatings, as shown in Fig. 4. The peak located at about 73.4 eV in the Al 2p spectra (Fig. 4a) could be attributed to AleN bond, indicating that the Al was bonded to N for all the coatings. From the deconvoluted Cr 2p 3/2 spectra in Fig. 4b, OeCr]O, CreN and CreNeCr bonds were observed at 577.3 eV, 575.0 eV and 574.2 eV, respectively. The presence of OeCr]O was probably attributed to the Cr reaction with the residual oxygen during the deposition, heating treatment and even analysis processes, because Cr had a high affinity toward oxygen (ΔHCr2O30 = − 1139.7 kJmol−1). The N 1 s spectra in Fig. 4c could be deconvoluted into N-metals (AleN and CreN) at 396.3 eV and chemisorbed N at 397.3 eV. The bond fraction percentages of Cr element were determined by calculating the area of each peak in Cr 2p 3/2 spectra and listed in Table 4. It was observed that the ratio of CreNeCr/CreN remained at 44/56 up to 700 °C, then gradually decreased to about 34/66 at the annealing temperature of 750 °C and finally sharply dropped to about 29/71 at 800 °C. The evolution of the bond fraction percentage as a function of temperature could confirm the transformation from Cr2N to CrN above 750 °C, formulated as the reverse reaction of Eq. (2). The nitrogen may result from the crystallization of amorphous matrix, since the Cr(Al)N phase was at a sub-stoichiometric concentration. It was recognized that the amorphous would get more energy at higher temperature and the crystalline nuclei would sufficiently grow up, leading to reconfiguration of grains [25]. The reactions occurred at 750–800 °C could be formulated by:

(5)

Cr(Al)N(amorphous) ®Cr(Al)N(crystalline) + N2

Table 2 Chemical composition of the droplets before and after vacuum-annealing as denoted in Fig. 2. Annealing temperature (°C)

Composition (at.%)

As-deposited 700 750 800

Al

Cr

N

22.32 22.36 22.88 22.25

56.82 56.54 55.97 55.82

20.86 21.10 21.15 21.93

which retained till 750 °C (Fig. 2b and c). The dense surface could improve the thermal stability of the coatings by suppressing the outward diffusion of atoms [20]. However, after annealing at 800 °C (Fig. 2d), the coating surface was consisted of packed grains or agglomerated grains, indicating the occurrence of crystallization for the AlCrN outer layer. [1–33]To access the phase composition of each layer in the multilayered AlCrN/Cr/AlCrN coatings and the phase evolution during the heat treatment in vacuum, the GIXRD measurements were carried out with incidence angles of 0.2° and 1°, respectively. It could be seen that two strong peaks related to the (110) and (200) planes of Cr (PDF#851335) and a tiny characteristic peak at 2θ = 42.6° related to Cr2N (111) plane as well as a broad hump corresponding to amorphous phase were presented for the as-deposited coatings both under the two measurement conditions. Since the penetration depths of X-ray varied with different incidence angles [21], it should be distinguished whether the middle Cr layer was detected, i.e., if the Cr phase resulted from the AlCrN layer or the middle Cr layer. The single AlCrN layers were thus vacuum-annealed at 700 °C, 750 °C and 800 °C, respectively, and characterized by EPMA. It could be found that the composition proportions of aluminum, chromium and nitrogen still remained stable at about 37 at.%, 13 at.% and 50 at.%, respectively (Table 3), without visible nitrogen loss after vacuum annealing. The reactions related to nitrogen loss in AlCrN layer can be formulated by:

2CrN = Cr2N + Cr2N = 2Cr +

1 N2 2

(2)

1 N2 2

(3)

The Gibbs free energy changes, ΔG2, and ΔG3, corresponding to Eqs. (2) and (3), could be formulated as follows [22–24]:

ΔG2 = ΔG0 + RT ln(P1/2 N2 ) = 10 − 0.0794T

ΔG3 = ΔG0 + RT

ln(P1/2 N2 )

= 173.1 − 0.0794T

where, R and T are the gas constant and the absolute temperature, respectively. ΔG0 represents the change of the standard Gibbs free energy. It can be predicted from Eqs. (4) and (5) that the CrN and Cr2N in the AlCrN layer were thermodynamically stable in the annealing temperature range of 700–800 °C (ΔG > 0) under vacuum condition. Combined with the GIXRD patterns in Fig. 3a and b, it could be identified that the as-deposited AlCrN outer layer was Cr2N nanocrystallites embedded into AlCrN amorphous matrix, and the as-deposited

Cr2N +

Al Cr N

Annealing temperature (°C) As-deposited

700

750

800

37.33 13.13 49.54

37.61 13.06 49.33

37.12 13.62 49.26

37.58 13.05 49.37

(7)

As evidenced by DSC curve of the single AlCrN layer in Fig. 5, two exothermal peaks centered at 425 °C and 747 °C were observed. According to Lin et al. [26], the exothermal peak at 425 °C could be related to the film recovery and recrystallization processes. In this work, the exothermal peak at 747 °C should be related to the transformation of Cr2N to CrN and the crystallization of amorphous matrix. TEM analysis was also performed for single AlCrN layer samples to further confirm the phase evolution during the vacuum annealing. Fig. 6 illustrates the HR-TEM images, the corresponding Selected-Area Electron Diffraction (SAED) patterns and Fast Fourier Transform (FFT) images of the as-deposited and vacuum-annealed AlCrN layers. As for the as-deposited AlCrN layer, the d values calculated from the SAED patterns (Fig. 6a) were equal to 0.212 nm, 0.149 nm and 0.127 nm, which corresponded to the Cr2N (111), (211) and (113) planes, respectively. Moreover, a broad halo ring was observed, confirming the

Table 3 Chemical composition of AlCrN layers. Composition (at.%)

1 N2 = 2CrN 2

(6)

4

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Fig. 3. GIXRD patterns of vacuum-annealed multi-layered AlCrN/Cr/AlCrN coatings measured with incidence angles of (a) 0.2° and (b) 1°.

amorphous presence and the Cr2N nanocrystallites embedded in amorphous matrix structure of the as-deposited AlCrN layer. Similarly, the SAED pattern in Fig. 6c revealed the presence of diffraction rings related to Cr2N (111), (201) and (113) planes with d values of 0.212 nm, 0.189 nm and 0.127 nm in the AlCrN layer vacuum-annealed at 700 °C. From the FFT transformation image of Cr2N grains in Fig. 6bi and di, the measured d (201) and (101) values of 0.189 nm and 0.305 nm were equal to be 0.189 nm calculated from the corresponding SAED pattern and to be 0.305 nm of the standard lattice distance of Cr2N (101) plane, respectively. Whereas, diffraction reflections of Cr (Al)N (200), (220), (222) and (400) planes could be clearly seen in the SAED patterns for the AlCrN layer annealed at 750 °C (Fig. 6e). The calculated d values were 0.205 nm, 0.146 nm, 0.119 nm and 0.103 nm, between the standard values for the reflections of fcc-AlN and fcc-CrN. As for the AlCrN layer annealed at 800 °C, the SAED patterns also clearly presented a typical polycrystalline structure of Cr(Al)N in the NaCl-structure(Fig. 6g) and the crystalline boundaries were elucidated by yellow solid lines in the HR-TEM image (Fig. 6h). Furthermore, some misfit dislocations and strained areas at the boundaries and within the crystallites were observed. The calculated d values from the FFT transformation image in Fig. 6f and h matched well with the (200), (220), (222) and (400) planes of Cr(Al)N phase. These results agreed

Table 4 Assigned bonds and bond fraction of Cr 2p 3/2. Annealing temperature (°C)

As-deposited 700 750 800

Bond fraction (%) CreNeCr

CreN

OeCr]O

CreNeCr/CreN

36.86 36.93 27.38 21.47

46.50 47.33 52.95 52.70

16.64 15.74 19.67 25.83

44.22/55.78 43.83/56.17 34.08/65.92 28.94/71.06

well with the data shown in XRD patterns and XPS spectra. 3.2. Thermal stability of multi-layered AlCrN/Cr/AlCrN coatings in air With the aim of revealing the chemical composition evolution of the multi-layered AlCrN/Cr/AlCrN coatings annealed in air, XPS measurements were performed. Fig. 7 illustrates the high resolution spectra of Al 2p, Cr 2p and O 1 s of the air-annealed AlCrN/Cr/AlCrN coatings. The deconvoluted Al 2p spectra (Fig. 7a) indicated the presence of AleN and AleO bonds. The peaks in deconvoluted Cr 2p 3/2 spectra (Fig. 7b) could be assigned to CreN and CreO bonds. Furthermore, it was observed that the intensity of metal-O bond increased and that of

Fig. 4. XPS spectra of (a) Al 2p, (b) Cr 2p and (c) N 1 s for deposited and vacuum-annealed multi-layered AlCrN/Cr/AlCrN coatings. 5

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the inner-AlCrN layer was also observed. With further increasing the annealing temperature to 800 °C, as shown in Fig. 9c, the outward diffusion of Ni from the substrate could reach into the middle Cr layer and the inward diffusion of O from air reach to the interface between the AlCrN inner-layer and the middle Cr layer. This was consistent with the GIXRD results in Fig. 3b. The element diffusion between the layers could mainly be achieved along the paths, such as, grain boundaries and dislocations. According to the investigation of Lin et al. [51], the oxidation of CrAlN coating followed a parabolic relation by thermogravimetric. As observed in Fig. 9, the oxidation was controlled by the outer-AlCrN layer and hasn't reached the middle Cr layer, so the parabolic relation could also be applied on the air-annealed AlCrN/Cr/AlCrN coatings. The kinetic constant KT can be calculated from Wagner's parabolic oxidation theory [47]:

Fig. 5. DSC analysis of single thick AlCrN layer.

d = 2 tKT

metal-N bond decreased with the increase in the annealing temperature. The deconvolution of O 1 s spectra of all the air-annealed coatings in Fig. 7c confirmed the presence of AleO and CreO bonds as well as an increase of AleO bond could also be observed with increasing the temperature. To comprehensively detect the microstructure changes of the airannealed AlCrN/Cr/AlCrN coatings, GIXRD and Raman analyses were performed and displayed in Fig. 8. As shown in Fig. 8a, the Raman spectra of all the coatings have two broad bands centered at 260 cm−1 and 750 cm−1 originated from fcc-Cr(Al)N structure [15]. This also indicated the poor crystallinity of AlCrN layer. Up to 750 °C no oxide could be detected in the coatings, while the spectra of the coating annealed at 800 °C suggested the presence of (Al, Cr)2O3 with corundum structure. The GIXRD patterns (Fig. 8b) further confirmed the formation of Cr2O3 phase (PDF#38-1479) for all the air-annealed coatings. However, after annealing at 800 °C, a new phase, Al1.54Cr0.46O3 (PDF#77-2188), was identified, which may arise from the crystallization of Cr2O3. The exact atomic ratio of Al and Cr in (Al, Cr)2O3 was difficult to be identified due to the high solubility of Cr2O3 in alumina [27]. Combined with the results of GIXRD, Raman and XPS analyses, it could be accounted that Cr-rich oxides initially formed on the coating surface and was finally substituted by a chrome‑aluminum oxide layer at 800 °C during air annealing. The possible reactions could be proposed as follows:

2 2 1 Cr2N + O2 = Cr2O3 + N2 3 3 3

(8)

4 2 2 Cr(Al)N + O2 = Cr2O3 (Al2O3) + N2 3 3 3

(9)

where, d is the thickness of the oxide scales from GDOES depth profiles in Fig. 9, and t is the oxidation time. The kinetic constant KT at the temperature T follows the Arrhenius law [28]:

lnKT = lnK 0 −

ΔG8 = ΔG0 + RT

= 0.137T − 670.7

ΔG9 = ΔG0 + RT ln[(P 2/3 N2 )/(PO2)] = 0.089T − 600

Ea RT

(13)

where, K0 is a pre-exponential parameter, R the gas constant and Ea the activation energy of oxidation. Fig. 10 shows the Arrhenius plot lnKT with respect to 1/T for the airannealed AlCrN/Cr/AlCrN coatings, which were compared with the data from previous studies. The activation energy Ea could be calculated from the slope of linear-regression. According to the reported results, the calculated Ea value of 277.69 kJ/mol was acceptable with 150 kJ/ mol for CrN [29], and 375 kJ/mol for Cr0.40Al0.60N [26]. By neglecting the measurement methods and oxidation conditions, the various activation energy values could attributed to the distinctions in morphology, structure and stoichiometry, which strongly depended on the deposition process [24]. Thus, the activation energy of the air-annealed coatings in this study could fall into two oxidation process as 700–750 °C and 750–800 °C according to the structure variations. The activation energy values (Ea) of 383.34 kJ/mol and 185.30 kJ/mol were respectively obtained for the oxidation occurred at 700–750 °C for the amorphous AlCrN layer and 750–800 °C for the crystalline AlCrN layer. It was considered that the higher oxidation activation energy value at 700–750 °C was due to the dense microstructure of amorphous AlCrN layer, which could hinder the element diffusion through grain boundaries. As the crystallization of amorphous AlCrN layer occurred at 750–800 °C, the crystalline boundaries could provide diffusion paths for the inward diffusion of O from air and the outward diffusion of Ni from substrate.

The Gibbs free energy changes, ΔG8 and ΔG9 corresponding to Eqs. (8) and (9), could be expressed by [47]:

ln[(P1/3 N2 )/(PO2)]

(12)

3.3. Adhesion strength of multi-layered AlCrN/Cr/AlCrN coatings annealed in air

(10)

The measurement of adhesion strength for the as-deposited and airannealed AlCrN/Cr/AlCrN coatings were carried out by scratch tester, and the scratch morphologies (each scratch is 3 mm in practical) are shown in Fig. 11. The position of first shedding part was detected and defined as the critical load Lc2 of the coatings. For the as-deposited coating, the first shedding part appeared at 38.2 N. When the annealing temperature increased to 700 °C, the Lc2 of the coatings exhibited an ascending trend, in which it reached its highest value of approximately 43.8 N. The atomic diffusion ability of materials was notably increased at thermal condition, and inevitably resulted in redistribution or annihilation of structural defects incorporated by PVD technology, thus eventually releasing the residual stress and enhancing the adhesion strength [30]. However, after annealing at 750 °C and 800 °C, the Lc2 of the coatings decreased to 34.5 N and 25.1 N, respectively. As mentioned above, the amorphous AlCrN layer began to crystallize at the

(11)

where, R and T are the gas constant and the absolute temperature, respectively. ΔG0 represents the change of the standard Gibbs free energy. The reactions (8) and (9) were thermodynamically favorable as predicted by thermodynamics. It was thus confirmed thermodynamically from Eqs. (10) and (11) that the Cr-rich oxides and chrome‑aluminum oxides could form in the annealing temperature range of 700–800 °C (ΔG < 0) under air condition. GDOES analysis was also employed to perform the elements depth profile analysis of the air-annealed AlCrN/Cr/AlCrN coatings and the results were shown in Fig. 9. Obviously, thin oxide layer formed externally on the outer-AlCrN layer for all the air-annealed coatings at different temperatures. For the coatings air-annealed at 700 °C and 750 °C (Fig. 9a and b), the diffusion of Ni element from the substrate to 6

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Fig. 6. (a) SAED pattern of AlCrN layer, (b) HR-TEM image of AlCrN layer and FFT images of areas i and ii denoted in panel b; (c) SAED pattern of AlCrN layer annealed at 700 °C, (d) HR-TEM image of AlCrN layer annealed at 700 °C and FFT images of areas i and ii denoted in panel d; (e) SAED pattern of AlCrN layer annealed at 750 °C, (f) HR-TEM image of AlCrN layer annealed at 750 °C and FFT images of areas i and ii denoted in panel f; (g) SAED pattern of AlCrN layer annealed at 800 °C, (h) HR-TEM image of AlCrN layer annealed at 800 °C and FFT images of areas i and ii denoted in panel h. 7

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Fig. 7. XPS spectra of (a) Al 2p, (b) Cr 2p and (c) O 1 s for as-deposited and air-annealed multi-layered AlCrN/Cr/AlCrN coatings.

formed localized states, providing the premise for the electron transition and promoting the infrared absorption, thus enhancing the infrared emissivity [33]. Whereas, the air-annealed AlCrN/Cr/AlCrN coatings performed a converse trend of infrared emissivity, compared with the vacuum-annealed samples. As shown in the inset of Fig. 12b, the infrared emissivity of multi-layered AlCrN/Cr/AlCrN coating gradually increased with rising heating temperature. Especially at 750 °C and 800 °C, the air-annealed coatings showed higher infrared emissivity than vacuumannealed coatings. The higher infrared emissivity performance of the air-annealed coatings could be attributed to the low extinction coefficient of oxides formed on the coating surface. The oxidation and diffusion mechanisms of the AlCrN/Cr/AlCrN coatings air-annealed at different temperatures are illustrated in Fig. 13. The crystallization of AlCrN layer at 750 °C and 800 °C resulted in the formation of grain boundaries which could be considered as the penetration of O element and thus corresponded to higher infrared emissivity. Meanwhile, with the crystallization of amorphous AlCrN layer, the Ni element diffusion from substrate into the middle Cr layer at 800 °C would also degrade the reflectivity property of the Cr reflector. Therefore, the multi-layered AlCrN/Cr/AlCrN coating allowed a low infrared emissivity property below 750 °C.

temperature above 750 °C, and the crystalline grains resulted in a loose and porous structure which caused the reduction of adhesion strength. In addition, the penetration of O element through those grain boundaries had a deterioration effect on the adhesion strength of the coatings [31].

3.4. Infrared properties of multi-layered AlCrN/Cr/AlCrN coatings annealed in air or vacuum The reflectance spectra of the as-deposited, vacuum-annealed and air-annealed AlCrN/Cr/AlCrN coatings in the range of 2.5–25 μm are plotted in Fig. 12. It could be observed from Fig. 12a that the reflectance in the 2.5–10 μm range of the crystalline AlCrN layered coatings vacuum-annealed at 750 °C and 800 °C was 20% higher than that of the as-deposited coating with the amorphous AlCrN layer. The coatings vacuum-annealed at 750 °C and 800 °C performed an infrared emissivity of 0.062 as shown in the inset of Fig. 12a, which was much lower than the as-deposited coating and the coating vacuum-annealed at 700 °C. Based on the GIXRD and TEM results of multi-layer AlCrN/ Cr/AlCrN coatings, the AlCrN layer with major amorphous matrix began to crystallize above 750 °C and many grain boundaries could be observed from the HR-TEM images of the annealed AlCrN layer. Namely, the crystallization of the AlCrN layer had a weakened effect on the infrared emissivity. The relative higher infrared emissivity of the amorphous AlCrN layer resulted from the photon emission form the stronger polar vibration due to the higher distortion coefficient [32]. Moreover, the long-range disorder feature in the amorphous phase

4. Conclusions The multi-layered AlCrN/Cr/AlCrN coatings were fabricated on Nibased superalloy K424 substrates by cathodic arc ion plating for low

Fig. 8. (a) Raman spectra and (b) GIXRD patterns (0.2°) of air-annealed multi-layered AlCrN/Cr/AlCrN coatings. 8

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Fig. 9. GDOES depth-profiles of air-annealed multi-layered AlCrN/Cr/AlCrN coatings at (a) 700 °C, (b) 750 °C and (b) 800 °C.

emissivity applications. The multi-layered coatings were annealed at 700 °C, 750 °C and 800 °C in air and vacuum, respectively. The effect of annealing temperature on microstructure and infrared properties had been systematically investigated. The conclusions were summarized as follows: (1) The as-deposited AlCrN layer was consisted of Cr2N nanocrystallites embedded in AlCrN amorphous matrix. The amorphous matrix began to crystallize as fcc-Cr(Al)N structure at 750 °C. Meanwhile, the transformation of Cr2N to CrN also occurred at this temperature. The diffusion of Ni element from the substrate to the middle Cr layer was observed after annealing at 800 °C. (2) The grain boundaries resulted from the crystallization of amorphous AlCrN layer could promote the penetration of O element from air and the diffusion of Ni element from the substrate. The Crrich oxides formed on the coating surface were substituted by chrome‑aluminum oxides above 750 °C in air. (3) The initial increase in adhesion strength of 700 °C air-annealed AlCrN/Cr/AlCrN coating was due to the release of residual stress in the coating. While, the loose and porous crystalline structure of AlCrN layer formed at the temperature above 750 °C resulted in a reduction of adhesion strength. (4) The AlCrN/Cr/AlCrN coatings vacuum-annealed above 750 °C performed lower infrared emissivity than the as-deposited one, while the coatings air-annealed above 750 °C showed higher infrared emissivity than vacuum-annealed ones due to the low extinction coefficient of oxides formed on the coating surface. The multilayered AlCrN/Cr/AlCrN coating could be proposed for low infrared emissivity applications below 750 °C.

Fig. 10. Arrhenius plot lnKT as a function of 1/T in air.

Fig. 11. Scratch morphologies of as-deposited and air-annealed multi-layered AlCrN/Cr/AlCrN coatings.

Acknowledgments This work has been supported by the National Natural Science

Fig. 12. Reflectance spectra of multi-layered AlCrN/Cr/AlCrN coatings annealed in (a) vacuum and (b) air. 9

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Fig. 13. Schematic illustration of multi-layered AlCrN/Cr/AlCrN coating showing the oxidation resistance and diffusion barrier characterization at 700–800 °C.

Foundation of China (Grant Nos. 51875424 and 51402208), the project by State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology) (Grant No. 2016-KF-11) and Shanxi Province Science Foundation for Youths (Grant No. 201801D221126).

org/10.1016/j.tsf.2008.06.050. [17] S. Edalatpour, M. Francoeur, Size effect on the emissivity of thin films, J. Quant. Spectrosc. Radiat. Tansfer 118 (2013) 75–85, https://doi.org/10.1016/j.jqsrt.2012. 12.012. [18] NIST X-ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, Version 4.1. https://srdata.nist.gov/xps/. [19] H.D. Liu, T.R. Fu, M.H. Duan, Q. Wan, C. luo, Y.M. Chen, D.J. Fu, F. Ren, Q.Y. Li, X.D. Cheng, B. Yang, X.J. Hu, Structure and thermal stability of spectrally selective absorber based on AlCrON coating for solar-thermal conversion applications, Sol. Energy Mater. Sol. Cells 157 (2016) 108–116, https://doi.org/10.1016/j.solmat. 2016.05.035. [20] H.D. Liu, Q. Wan, Y.R. Xu, C. Luo, Y.M. Chen, D.J. Ren, G. Luo, X.D. Cheng, X.J. Hu, B. Yang, Long-term thermal stability of CrAlO-based solar selective absorbing coating in elevated temperature air, Sol. Energy Mater. Sol. Cells 134 (2015) 261–267, https://doi.org/10.1016/j.solmat.2014.12.009. [21] X.L. Pang, H.S. Yang, X.L. Liu, K.W. Gao, Y.B. Wang, A.A. Volinsky, A.A. Levin, Annealing effects on microstructure and mechanical properties of sputtered multilayer Cr(1−x)AlxN films, Thin Solid Films 519 (18) (2011) 5831–5837, https://doi. org/10.1016/j.tsf.2011.02.063. [22] F.H. Lu, H.Y. Chen, Phase changes of CrN films annealed at high temperature under controlled atmosphere, Thin Solid Films 398-399 (2001) 368–373, https://doi.org/ 10.1016/S0040-6090(01)01346-3. [23] O.N. Hideki, T. Kenji, U. Tateo, T. Keishi, T. Yuuji, Determination of Standard Gibbs Energies of Formation of Cr2N and CrN, Metall. Mater. Trans. B 32 (6) (2001) 1113–1118, https://doi.org/10.1007/s11663-001-0099-2. [24] Z.B. Qi, B. Liu, Z.T. Wu, F.P. Zhu, Z.C. Wang, C.H. Wu, A comparative study of the oxidation behavior of Cr2N and CrN coatings, Thin Solid Films 554 (2013) 515–520, https://doi.org/10.1016/j.tsf.2013.01.031. [25] K.W. Sun, W.C. Zhou, X.F. Tang, Z.B. Huang, F. Luo, D.M. Zhu, Effects of air annealing on the structure, resistivity, infrared emissivity and transmission of indium tin oxide films, Surf. Coat. Technol. 206 (19-20) (2012) 4095–4098, https://doi. org/10.1016/j.surfcoat.2012.04.001. [26] J. Lin, B. Mishra, J.J. Moore, W.D. Sproul, A study of the oxidation behavior of CrN and CrAlN thin films in air using DSC and TGA analyses, Surf. Coat. Technol. 202 (14) (2008) 3273–3283, https://doi.org/10.1016/j.surfcoat.2007.11.037. [27] G.S. Fox-Rabinovich, G.C. Weatherley, A.I. Dodonov, A.I. Kovalev, L.S. Schuster, S.C. Veldhuis, G.K. Dosbaeva, D.L. Wainstein, M.S. Migranov, Nano-crystalline filtered arc deposited (FAD) TiAlN PVD coatings for high-speed machining applications, Surf. Coat. Technol 177–178 (2004) 800–811, https://doi.org/10.1016/j. surfcoat.2003.05.004. [28] M. Günthner, T. Kraus, A. Dierdorf, D. Decker, W. Krenkel, G. Motz, Advanced coatings on the basis of Si(C)N precursors for protection of steel against oxidation, J. Eur. Ceram. Soc. 29 (10) (2009) 2061-2068. https://doi.org/10.1016/j. jeurceramsoc.2008.11.013. [29] S. Hofmann, H.A. Jehn, Oxidation behavior CrNx and (Cr, Al)Nx hard coatings, Werkst. Korros. 41 (1990) 756–760, https://doi.org/10.1002/maco.19900411222. [30] W. Chen, X. Meng, D. Wu, D. Yao, D. Zhang, The effect of vaccum annealing on microstructure, adhesion strength and electrochemical behaviors of multilayered AlCrTiSiN coatings, Appl. Surf. Sci. 467-468 (2019) 391–401, https://doi.org/10. 1016/j.apsusc.2018.10.171. [31] J. Jeong, C. Lee, Effects of post-deposition annealing on the mechanical and chemical properties of the Si3N4/NbN multilayer coatings, Appl. Surf. Sci. 214 (2003) 11–19, https://doi.org/10.1016/s0169-4332(03)00351-9. [32] M. Al Bosta, K. Ma, H. Chen, The effect of MAO processing time on surface properties and low temperature infrared emissivity of ceramic coating on aluminium 6061 alloy, Infrared Phys. Technol. 60 (2013) 323-334. https://doi.org/10.1016/j. infrared.2013.06.006. [33] Y. Zhang, D. Wen, Infrared emission properties of RE (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) and Mn co-doped Co0.6Zn0.4Fe2O4 ferrites, Mater. Chem. Phys. 131 (2012) 575–580, https://doi.org/10.1016/j.matchemphys.2011.09.003.

References [1] Z. Huang, D. Zhu, F. Lou, W. Zhou, An application of Au thin-film emissivity barrier on Ni alloy, Appl. Surf. Sci. 255 (5) (2008) 2619–2622, https://doi.org/10.1016/j. apsusc.2008.07.185. [2] J. Huang, C. Xiang, S. li, X. Zhao, G. He, Preparation, characterization and performance of Ti1−xAlxN/Ag/Ti1−xAlxN low-emissivity films, Appl. Surf. Sci. 293 (2014) 259–264, https://doi.org/10.1016/j.apsusc.2013.12.146. [3] R. Meszaros, B. Merle, M. Wild, K. Durst, M. Göken, L. Wondraczek, Effect of thermal annealing on the mechanical properties of low-emissivity physical vapor deposited multilayer-coatings for architectural application, Thin Solid Films 520 (24) (2012) 7130–7135, https://doi.org/10.1016/j.tsf.2012.07.086. [4] H.J. Yu, G.Y. Xu, X.M. Shen, X.X. Yan, C.W. Cheng, Low infrared emissivity of polyurethane/Cu composite coatings, Appl. Surf. Sci. 255 (12) (2009) 6077–6081, https://doi.org/10.1016/j.apsusc.2009.01.019. [5] S. Wijewardane, D.Y. Goswami, A review on surface control of thermal radiation by paints and coatings for new energy applications, Renew. Sust. Energ. Rev. 16 (4) (2012) 1863–1873, https://doi.org/10.1016/j.rser.2012.01.046. [6] C. Loka, H.T. Yu, K.S. Lee, The preparation of thermally stable TiNx/Ag(Mo)/TiNx ultrathin films by magnetron sputtering, Thin Solid Films 570 (2014) 178–182, https://doi.org/10.1016/j.tsf.2014.05.029. [7] C. Hu, G. Xu, X. Shen, C. Shao, X. Yan, The epoxy-siloxane/Al composite coatings with low infrared emissivity for high temperature applications, Appl. Surf. Sci. 256 (2010) 3459–3463, https://doi.org/10.1016/j.apsusc.2009.12.053. [8] Z. Huang, W. Zhou, X. Tang, F. Luo, D. Zhu, High-temperature application of the low-emissivity Au/Ni films on alloys, Appl. Surf. Sci. 256 (22) (2010) 6893–6898, https://doi.org/10.1016/j.apsusc.2010.04.107. [9] L. Liu, H.T. Chang, T. Xu, Y.N. Song, C. Zhang, Z.H. Hang, X.H. Hu, Achieving lowemissivity materials with high transmission for broadband radio-frequency signals, Sci. Rep. 4840 (2017). https://doi.org/10.1038/s41598-017-04988-9. [10] C. Loka, K.R. Park, K.S. Lee, Multi-functional TiO2/Si/Ag(Cr)/TiNx coatings for lowemissivity and hydrophilic applications, Appl. Surf. Sci. 363 (2016) 439–444, https://doi.org/10.1016/j.apsusc.2015.12.069. [11] M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Dependence of film composition and thicknesses on optical and electrical properties of ITO–metal–ITO multilayers, Thin Solid Films 326 (1-2) (1998) 67–71, https:// doi.org/10.1016/S0040-6090(98)00520-3. [12] C. Loka, K.R. Park, K.S. Lee, SiO2/TiO2/n-Si/Ag(Cr)/TiO2 thin films with super hydrophilicity and low-emissivity, Jpn. J. Appl. Phys. 55 (2016) 01AA06, , https:// doi.org/10.7567/JJAP.55.01AA06. [13] P. Sangpour, O. Akhavan, A.Z. Moshfegh, M. Roozbehi, Formation of gold nanoparticles in heat-treated reactive co-sputtered Au-SiO2 thin films, Appl. Surf. Sci. 254 (1) (2007) 286–290, https://doi.org/10.1016/j.apsusc.2007.07.095. [14] S. Kappaganthu, Y. Sun, Effect of nitrogen partial pressure and temperature on RF sputtered Fe–N film, Surf. Coat. Technol. 167 (2) (2003) 165–169, https://doi.org/ 10.1016/S0257-8972(02)00910-6. [15] M. Münhlbacher, R. Franz, J. Paulitsch, H. Rudigier, P. Polcik, P.H. Mayrhofer, C. Mitterer, Influence of Fe impurities on structure and properties of arc-evaporated AlCrN coatings, Surf. Coat. Technol. 215 (2013) 96–103, https://doi.org/10.1016/ j.surfcoat.2012.09.063. [16] K. Bobzin, N. Bagcivan, P. Immich, S. Bolz, R. Cremer, T. Leyendecker, Mechanical properties and oxidation behaviour of (Al,Cr)N and (Al,Cr,Si)N coatings for cutting tools deposited by HPPMS, Thin Solid Films 517 (3) (2008) 1251–1256, https://doi.

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