Chromium coatings deposited by cooled and hot target magnetron sputtering for accident tolerant nuclear fuel claddings

Surface & Coatings Technology 389 (2020) 125618

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Chromium coatings deposited by cooled and hot target magnetron sputtering for accident tolerant nuclear fuel claddings

T

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E.B. Kashkarov , D.V. Sidelev, M. Rombaeva, M.S. Syrtanov, G.A. Bleykher National Research Tomsk Polytechnic University, 634050 Tomsk, Russia

A R T I C LE I N FO

A B S T R A C T

Keywords: Chromium coatings Nuclear fuel claddings Zirconium alloys High-temperature oxidation Hot target Magnetron sputtering

The paper describes the oxidation of E110 alloy with Cr coatings in air atmosphere at 1100 °C for 20 min. The coatings were deposited using magnetron sputtering systems of the classical construction (with cooled targets in a dual configuration) and with a single hot target. The influence of magnetron type on energetic characteristics of the deposition and coating growth was described. The as-deposited and oxidized samples were analyzed by Xray diffraction, scanning electron microscopy and glow discharge optical emission spectroscopy. The oxidation resistance of the Cr-coated alloy is strongly affected by microstructure and thickness of the deposited films. The Cr coatings obtained by hot target sputtering had a columnar microstructure, the mass gain of these samples was decreased from 9.18 to 3.22 mg/cm2 with coating thickness from 1.8 to 4.5 μm. The minimal mass gain (2.86 mg/cm2) and the best protective properties were belong to the Cr coating with a dense microstructure and thickness of 3.1 μm that was deposited by dual magnetron sputtering. The adhesion behavior of Cr-coated Zr alloy strongly depends on oxidation states of Cr coating and Zr substrate as well as interface reactions.

1. Introduction The development of accident tolerant fuel (ATF) for light-water reactors is an actual and emergency task in nuclear science and technologies. The short-term concept of ATF is the deposition of protection coating onto Zr fuel claddings, which should delay or prevent their oxidation and degradation in the case of loss of coolant accident (LOCA) conditions [1–4]. Chromium is considered as a candidate material for this due to its appropriated properties: high melting point, the same thermal expansion coefficient as Zr and high corrosion resistance in atmosphere and water environments [5–8]. From a large number of deposition methods (cold spraying, arc evaporation, etc.), magnetron sputtering is one of the most perspective approach [5,9–12]. However, in spite of a simplicity of the construction, magnetron sputtering systems have a lot of configurations (with disk, planar or cylindrical targets, balanced or unbalanced magnetic fields, single or multi-cathode, direct current (DC) or high power pulsed (HiPIMS) power supplies, etc.) [13–16]. Magnetrons are characterized by wide variety of deposition parameters and the choice of process parameters is a crucial for coating growth and their properties. In the previous research [9] we showed that the Cr coating deposited by a single magnetron had a columnar microstructure, but better protected the E110 (Zr-1%Nb) alloy from air oxidation than the denser Ni-Cr coatings. It has been reported that the

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application of a hybrid (DC + HiPIMS) sputtering system results in the deposition of the Cr coatings with higher oxidation resistance than using DC sputtering [10]. In common way, to deposit the coatings with denser microstructure, magnetrons performing higher energy per one deposited atom can be used [9,17,18]. However, in view of high quantity of functioning nuclear reactors and their needs in ATF claddings, the ways to improve deposition rate of protection coatings can be also considered. The first approach is to increase target power density (up to 30–50 W/cm2) and use of multi-cathode sputtering systems (for example, dual or four magnetron closed-field system) [13,19]. In this case, the deposition rate is proportional to applied power on the sputtered target. Moreover, the magnetrons with closed-field configuration are used for enhancing of ion flux to a substrate, thus the energy per one deposited atom will be higher for such constructions [20,21]. The second way is the use of the magnetron with hot Cr target that has strongly higher deposition rate due to combined sputtering and sublimation mechanisms of Cr target erosion [22,23]. At high target power density, the flux of sublimated particles will be dominant under sputtering species [24], thus the deposited coatings can be porous and have a columnar microstructure. Both deposition strategies can be enabled to the industrial coating technology. Thus, it is important to compare their possibilities to deposit Cr protection coatings onto Zr claddings to increase oxidation resistance at LOCA conditions.

Corresponding author. E-mail address: [email protected] (E.B. Kashkarov).

https://doi.org/10.1016/j.surfcoat.2020.125618 Received 19 November 2019; Received in revised form 21 February 2020; Accepted 10 March 2020 Available online 17 March 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 389 (2020) 125618

E.B. Kashkarov, et al.

next step was the temperature ramp up to 1100 °C with the rate of 30 °C/min. Finally, the samples were isothermally treated for 20 min in air atmosphere. After the test, the chamber was opened and the samples were naturally cooled to a room temperature (~10 °C/min). Then, the samples were visually controlled and photographed. The thickness and microstructure of the as-deposited and oxidized samples were studied by an optical microscopy using AXIOVERT 200MAT (Zeiss, Göttingen, Germany) and scanning electron microscopy (SEM) using Quanta 200 3D (FEI, USA). The crystalline structure of the samples was analyzed by X-ray diffraction (XRD) at the scanning range of 10–90° using XRD-7000S (Shimadzu, Japan). The diffractometer was operated in a Bragg-Brentano configuration with CuKαtube (40 kV, 30 mA). The phase composition was calculated by Rietveld method using ICDD 4+ database. The depth distribution of elements in the oxidized samples was evaluated by a glow discharge optical emission spectroscopy (GDOES) using GD-Profiler 2 (HORIBA Scientific, Japan). The mass gain of the samples was measured using an analytical balance machine (Sartorius CP124 S) with an accuracy of 10−4 g. The adhesion of the deposited coatings after oxidation was analyzed by a scratch method using Micro-Scratch Tester MST-S-AX-0000 (CSEM, Switzerland) with C-029 indenter (Diamond, 100 μm, Rockwell). The maximum load and the loading rate were 30 N and 0.5 N/s, the scratch length was 5 mm.

The aim of this study is to determine the role of deposition approach on the oxidation resistance of Cr-coated Zr-1%Nb alloy. Firstly, the Cr coatings will be deposited by dual magnetron with cooled targets and single magnetron with hot target. Then, the as-deposited coatings will be investigated by analytical methods (X-ray diffraction, scanning electron microscopy, etc.). After it, the Cr-coated Zr samples will be oxidized in air atmosphere at 1100 °C during 20 min, and their properties will be studied. 2. Experimental details 2.1. Coating deposition The Cr coatings were deposited using an ion-plasma installation equipped with magnetron sputtering systems (with Cr disk targets of Ø 90 mm and 99.95% purity), ion source, planetary substrate-holder and substrate biasing system. The detailed description of the installation has been reported in the previous work [9]. The zirconium alloy E110 (Zr–1%Nb, 20 × 20 × 2 mm3) and polished Si (110) were used as substrates. Firstly, the Zr–1%Nb substrates were prepared by grinding and polishing using SiC sandpapers (from P600 to P4000) and then rinsed with alcohol and dried by compressed air for 10 min. The substrates were fixed in the substrate-holder of the installation and evacuated up to 2 × 10−3 Pa. All samples had an uncoated part due to their fixation in the holder (the total area of such part was lower 6 mm2). To decrease the non-uniform coating deposition onto the rectangularshape samples (the edge effect), the edges were beveled. Before deposition, the Zr and Si substrates were etched by ion source with closed electron drift at the following parameters: argon pressure – 0.15 Pa, operation voltage – 2.5 kV, ion current – 40 mA, etching time – 30 min. For coating deposition, a dual magnetron sputtering system (MS) with cooled targets and single magnetron with hot target were used. The dual magnetron sputtering system had a “closed” magnetic field [20] and was equipped with a middle-frequency (MF) power supply (66.6 kHz). For high-rate coating deposition, the hot target magnetron with MF power supply (100 kHz, duty cycle of 0.7) was used. This construction of the magnetron was earlier approved by experiments and calculations [23–25]. The substrates were planetary rotated during etching and deposition processes. The non-uniformity of coating thickness on the rotating substrates was less 7.5%. The target to substrate distance was 10 cm. During the deposition process, the substrates were biased at −50 V (frequency – 100 kHz, pause – 3 μs). The substrate temperature was measured by infrared pyrometer Optris CTlaser 3MH1CF4 (spectral range – 2.3 μm) during coating deposition. The other deposition conditions are shown in Table 1.

2.3. Calculations of energy per one deposited atom The influence of sputtering system (magnetron type) on coating properties can be evaluated by using of energy per one deposited atom (Ea). This is one of the main characteristics of the deposition process and coating growth [17]. For this, the total energy flux (F in W/cm2) should be estimated as overall energy inputs to the given substrate:

P, Pa

Cr-1 Cr-2 Cr-3 Cr-4

Single magnetron with hot target Dual magnetron

29.9 33.0 37.7 31.4

0.2

t, min

25

128

Usub, V

jsub, mA/ cm2

Tsub, °C

h, μm

−50

1.5 2.0 2.3 3.5

240 320 390 280

1.8 3.1 4.5 3.1

(2)

(3)

where Ei – averaged energy of ions delivered to substrate that is equal to ~Ubias∙qe [26], qe – elementary charge. Then, Ea can be calculated for the given coating thickness and substrate area:

Ea =

F∙S∙t , Na

(4)

where S – substrate area (in cm2); Na – number of the deposited atoms (in atom) [9]. The results show that energy per one deposited atom is lower for hot target magnetron sputtering in comparison with the dual MS with cooled targets (see Table 2). Moreover, Ea gradually decreases with target power density for hot target sputtering. This is caused by the significant increase of deposition rate in the case of target sublimation

Table 1 Deposition parameters. Q, W/ cm2

Fdep = Frad + Fcond + Fkin,

Fion = jsub ∙Ei ∙qe ,

The high-temperature (HT) oxidation of the Cr-coated samples was performed using the atmospheric furnace (ATS 3210, Applied Test Systems Inc.). Firstly, the chamber was heated up to 500 °C, then the samples were fixed on the holder using a tungsten wire (Ø 0.5 mm). The

Magnetron type

(1)

where Fdep – the total energy flux to a grounded substrate; Fion – energy flux to substrate due to ion species; Frad, Fcond, Fkin – energy fluxes to substrate due to target radiation, condensation of particles and their kinetic energies, respectively. For this study, the data of Fdep was known from the previous experiments [25], whereas Fion can be determined as the energy of plasma ions delivered to the substrate:

2.2. Oxidation tests and characterization

№

F = Fdep + Fion,

Table 2 Energy per one deposited atom.

Note: Q – target power density, P – operation pressure, t – deposition time, Ubias – bias voltage on the substrate, jsub – ion current density on the substrate, h – coating thickness. 2

Sample

Cr-1

Cr-2

Cr-3

Cr-4

F, W/cm2 Na, 1020 atom Ea, eV/atom

0.71 1.44 8.1

0.92 2.48 6.1

1.07 3.60 4.9

0.93 2.48 31.4

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Fig. 1. SEM images of the as-deposited Cr coatings: Cr-1 (a), Cr-2 (b), Cr-3 (c) and Cr-4 (d).

3.2. Post-test examination results

and lower kinetic energy of sublimated particles (~0.1 eV) versus sputtered species (~10–20 eV) [24,25].

3.2.1. The mass gains For the initial analysis of protective properties, the mass of Crcoated samples was measured before and after the oxidation test. All samples changed the color due oxidation of Zr alloy (to white) or Cr coating (to dark green). The white colored corners of the Cr-coated samples are the uncoated part that can be significantly oxidized during the test. To calculate the mass gain of the Cr-coated sample (WCr), the next expressions were used:

3. Results 3.1. Deposition rate and microstructure of the as-deposited coatings The absolute deposition rate (i.e. for the unmoved substrate) for hot target sputtering increases from 12 to 30 nm/s with the target power density (Q) from 29.9 to 37.7 W/cm2. At the same power density, the deposition rate of the Cr coating for cooled target sputtering using dual MS is > 3 times lower (~4 nm/s). This significant increase of deposition rates was also shown in previous work [23] and the results are well correlated with the numerical calculations [27]. Fig. 1 shows the thickness and microstructure of the as-deposited Cr coatings. The homologous temperature (ratio of Tsub to melting temperature Tm) for hot target sputtering is equal to 0.24–0.31. Thus, the Cr coatings should have a columnar microstructure (zone T) according to the structural zone model (SZM) [28]. It can be seen that the coating microstructure is affected by Q (Fig. 1a–c). The latter is caused by changing of deposition rate, Ea, jsub, Tsub and the ratios between these parameters. At lower Q (29.9 W/cm2), the Cr-1 coating has a dense columnar microstructure. The Cr-2 coating was deposited at higher deposition rate, thus Ea decreased from 8.1 to 6.1 eV/atom. So, the coating microstructure becomes more porous. At Q = 37.7 W/cm2, the Cr-3 coating is less columnar due to higher Tsub and jsub even at the decrease of Ea to 4.9 eV/atom. Moreover, the substrate heats up faster those results in higher adatom mobility and densification of the Cr-3 coating. The homologous temperature for dual magnetron sputtering was 0.26 (zone T). However, this coating has a denser microstructure due to higher Ea of 31.4 eV/atom (Fig. 1d).

WCr =

∆mCr , SCr

(1) (2)

∆mCr = ∆m − Sun ∙WZr ,

where SCr – the area of Cr-coated part of the sample (in cm ); ΔmCr – the mass gain of the Cr-coated part of the sample (in mg); Δm – the mass gain of the sample (in mg); Sun – the area of uncoated part of the sample (in cm2); WZr – the mass gain of uncoated Zr alloy (in mg/cm2). The mass gain diagram shows that the Cr coatings reduce the oxidation of alloy by 5–16 times (Fig. 2). It is clearly demonstrated that the mass gain of the Cr-coated alloy decreases from 9.18 to 3.22 mg/cm2 with coating thickness (1.8 μm → 4.5 μm). The sample with dense microstructure (Cr-4) deposited using dual MS shows the smallest mass gain in comparison with other coatings (2.86 mg/cm2). The comparison of the Cr-2 and Cr-4 series with the same coating thickness shows that denser microstructure of the coating provides better protective properties against air oxidation. 2

3.2.2. Phase composition of the oxidized samples Fig. 3 shows the diffraction patterns of the as-deposited and oxidized Cr-coated samples. The crystalline structure of the as-deposited samples is a body-centered cubic (bcc) α-Cr phase of the deposited coating and closed-packed hexagonal (hcp) α-Zr phase of the substrate. Despite the same thickness of the Cr-2 and Cr-4 coatings, the intensity 3

Surface & Coatings Technology 389 (2020) 125618

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After the oxidation test, the samples have different phase compositions depending on coating series. Oxidation of the Cr-coated Zr alloy occurs with the formation of the rhombohedral chromium oxide (αCr2O3) phase, while some fraction of the residual Cr phase remains after the test (Fig. 3). The monoclinic (m-ZrO2) and tetragonal (t-ZrO2) zirconium oxide phases were only observed in the oxidized Cr-1 sample. Moreover, the zirconium nitride (ZrN) was also found in the Cr-1 sample that is caused by nitrogen diffusion into the Zr substrate. There is also shown the formation of Cr2Zr phase in Cr-2, Cr-3 and Cr-4 samples after HT oxidation. The formation of mixed Cr-Zr region at the interface between coating and substrate has been also observed in the case of high-temperature air oxidation of Cr-Al-C coated Zr702 alloy at 1100 °C [30] as well as steam oxidized at 1200 °C Cr-coated Zr-1%Nb alloy [5]. This phenomenon is associated with the increase of chromium solubility in bcc βZr phase during HT oxidation [10] as well as high diffusion coefficient of Cr in the bcc Zr lattice [31]. Zirconium is not fully oxidized as evidenced by the presence of the oxygen (nitrogen)-stabilized α-Zr(O) and α-Zr(N) (Zr3O phase in XRD patterns). The detected phases in the Cr-2 and Cr-3 samples are similar: Cr2O3, Cr, Cr2Zr and Zr3O (Fig. 3b and c). Moreover, the pure Zr phase was also found after the oxidation of the Cr-4 sample, which has the minimal mass gain (Fig. 3d). For detailed analysis of the phase changes and oxidation degree, the phase composition of the samples was calculated based on the diffraction data (Table 3). The quantitative analysis shows the predominant content of Cr2O3 (~53–81 vol%) and Cr (~12–32 vol%) phases in all oxidized samples. The stoichiometric zirconium oxide only found in the

Fig. 2. The mass gains for the samples after air oxidation tests at 1100 °C for 20 min.

of zirconium peaks is quite different. The deposited coatings have a different density and preferred orientation of crystallites that significantly influence on X-ray penetration depth, especially in the case of Cr-2 and Cr-4 samples. Evaluation of X-ray penetration depending on texture was performed in [29]. The authors showed that texture at large diffraction angles leads to increase in X-ray penetration. Therefore, depth of X-ray penetration in Cr-2 sample was lager in comparison with Cr-4 sample.

Fig. 3. The diffraction patterns of the as-deposited and oxidized Cr-coated zirconium samples (1 – as-deposited; 2 – after oxidation): Cr-1 (a), Cr-2 (b), Cr-3 (c) and Cr4 (d). 4

Surface & Coatings Technology 389 (2020) 125618

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The nitrogen signal is observed up to ~15 μm in the Cr-1 coating (Fig. 4a), the strong maximum – in upper layers of Zr alloy. It shows that nitrogen from air atmosphere diffuses through the coating defects to the coating/metal interface and ZrN phase can be formed [32]. Low nitrogen signals were also detected near the coating/alloy interface in the Cr-2 and Cr-3 samples, while the N signal was minimal in the Cr-4 over the measuring depth. Therefore, it is assumed that nitrogen can diffuse along grain boundaries or structural defects of coatings due to their columnar structure (for Сr-1, Cr-2 and Cr-3 coatings). The chromium signal is detected at depths exceeding the thickness of the deposited Cr. This may indicate the diffusion of Cr into Zr alloy and the formation of an inter-diffusion layer with Cr2Zr phase at the coating/alloy interface. The thickness of such layer is ~2 μm for Cr-2, Cr-3 and Cr-4 samples. For the Cr-1 sample, the strong Cr signal is superimposed with increasing N signal at the depth of 2–3 μm, which may indicate the formation of a Cr2N layer underneath the residual Cr. The formation of Cr2N and Cr2Zr phases are confirmed by XRD.

Table 3 The phase composition of the oxidized samples. Phase content, vol%

Cr-1

Cr-2

Cr-3

Cr-4

Cr2O3 (rhombohedral) Cr (bcc) Cr2N Zr3O (hcp) ZrN (fcc) m-ZrO2 t-ZrO2 Cr2Zr Zr Total content of ZrOx

52.7 12.4 4.2 3.8 15.8 9.9 1.2 – – 14.9

75.6 11.6 – 9.8 – – – 3.0 – 9.8

81.3 13.1 – 2.1 – – – 3.5 – 2.1

62.8 31.9 – 0.4 – – – 4.1 0.8 0.4

Cr-1 sample that is mainly composed of m-ZrO2 and t-ZrO2 phases. The content of Zr3O phase is gradually decreased for the coating series from Cr-2 to Cr-4, whereas the Cr-4 sample with the dense microstructure contains 32 vol% of pure Cr phase, which is approximately 3 times higher than in the sample with columnar microstructure (Cr-2).

3.2.4. Cross-section microstructure of the oxidized samples Fig. 5 shows the cross-section images of the Cr-coated samples after HT oxidation investigated by optical microscopy. For Cr-1 sample, thick (~20 μm) layer of ZrO2 + ZrN and inward penetration (up to 130 μm) of oxygen and nitrogen to Zr alloy forming α-Zr(O) and α-Zr(N) phases are observed (Fig. 5a). The deeper layer of the sample is prior β zirconium phase. The microstructure changes are also detected for Cr-2 and Cr-3 samples. The ZrO2 phases are not observed for these samples, but the α-Zr(O) + α-Zr(N) layer has the thickness of ~80 (Cr-2) and 20 μm (Cr-3). Moreover, the above-mentioned samples show a nonuniform oxidation of the Zr alloy (Fig. 5b and c). The full-scale protection of Zr alloy from HT air oxidation is achieved only by deposition

3.2.3. Distribution of elements by GDOES The non-uniform distribution of the elements through the depth and different oxidation level of the samples is shown in Fig. 4. The high intensity of oxygen signal is observed up to 30 μm in the Cr-1 sample that indicated on oxidation both Cr coating and Zr alloy (Fig. 4a). In other samples, oxygen content is gradually reduced over the Cr coating thickness (up to ~3–4 μm) and only small amount of oxygen is observed in the Zr alloy (Fig. 4b–d). The comparison of oxygen intensities in the alloy region (substrate) indicates the decrease of oxidation from Cr-1 to Cr-4 samples.

Fig. 4. GDOES depth distribution profiles of elements in the oxidized Cr-coated Zr samples: Cr-1 (a), Cr-2 (b), Cr-3 (c) and Cr-4 (d). 5

Surface & Coatings Technology 389 (2020) 125618

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Fig. 5. Optical photographs of the Cr-coated samples after HT oxidation: Cr-1 (a), Cr-2 (b), Cr-3 (c) and Cr-4 (d).

of Cr-4 coating. There are only found the oxidation of outer Cr layer and the prior β‑zirconium phase. Fig. 6 presents the SEM images of the Zr samples after the oxidation test, where the different structures are detected. Four-layer structure is observed for the Cr-1 sample, where the outer (1.8–2.2 μm) Cr2O3 oxide with residual Cr, thin Cr2N, ZrO2 + ZrN (5–20 μm) and α-Zr(O) + α-Zr (N) layers are shown. Other samples have three-layer structure, however, with an inter-diffusion layer instead of ZrO2 + ZrN layer. The outer oxide layer of the Cr-2 sample has 2.3 μm thickness, the second is composed of residual Cr and inter-diffusion Cr-Zr layers, the third layer corresponds to α-Zr(O) + α-Zr(N) phase. The Cr-3 and Cr-4 samples have the similar structure, while the thickness of Cr/Cr-Zr layers is slightly increased.

after high-temperature oxidation. 4. Discussion The hot Cr target sputtering has high deposition rates (up to 30 nm/ s), but the coatings have a columnar microstructure (Fig. 1) due to lower energy per one deposited atom (4.9–8.1 eV/atom). This can be improved by higher power densities, when the substrate temperature increases more rapidly at the equal deposition time. Nevertheless to obtain denser Cr coatings, magnetron sputtering systems with lower deposition rates and higher Ea are still advantageous (as the example – dual MS) that can provide intense surface diffusion of condensed atoms on the substrate during coating growth. The Cr coatings deposited onto the Zr alloy by these types of magnetrons show different behavior during their oxidation in air at 1100 °C for 20 min. Analysis of the obtained results and literature data [32,34–36] indicates the strong effect of nitrogen on oxidation and degradation of Zr alloys at high temperatures (Fig. 8a). For the uncoated zirconium alloy, a non-uniform ZrO2 layer with a thickness of up to 450 μm is observed. Nitrogen can interact with α-Zr(O) alloy that results in the formation of ZrN phase. The latter phase becomes embedded in the oxide matrix during further oxidation stage and then can be re-oxidized by newly available oxygen. The difference in the molar volumes of ZrN (14.8 cm3) and ZrO2 (21.7 cm3) is significant, so the phase transformation of ZrN → ZrO2 occurs with a volume increase of 48% that is the source of high compressive stresses. The stress relaxation can lead to generation of cracks and finally formation of porous oxide layers [35]. Cracking of ZrO2 layer during the re-oxidation stage also results in nonuniformity of the oxide layer due to enhanced oxidation and destruction of zirconium alloy along the formed cracks. The microstructure and thickness of the as-deposited Cr coatings significantly affect the oxidation resistance of Zr alloy that is clearly seen from the weight gain measurements (Fig. 2). Thin and columnar coating (Cr-1) has the worst protection of Zr alloy from high-

3.2.5. Adhesion of the coatings after HT oxidation One of the most important parameters in assessing the protective properties of coatings is the adhesion to the substrate before and after HT oxidation [33], which determines the resistance of the coatings under mechanical damage and thermal oxidation treatment. To evaluate the adhesive properties, three scratch tests for each sample were conducted. According to acoustic emission signals and surveys of the scratch tracks, as-deposited coatings were cracked at the load of 5.2–10.2 N (LC1 - the load of first coating cracking). However, Lc1 significantly decreased after the oxidation test (~0.8–1.5 N, see in Table 4). Fig. 7 shows the elemental maps after the scratch test of the oxidized Cr-1 sample. The analysis of the maps clearly demonstrates a critical load of coating detachment (Lc2). The same procedures for evaluating the adhesive strength were carried out for each sample (Cr-2, Cr-3 and Cr-4). The critical loads of adhesion are presented in Table 4. For the Cr-1 sample, the adhesion strength is sufficiently high (Lc2 > 17 N) relative to other samples. Furthermore, the increase of thickness of the as-deposited Cr coatings leads to significant drop of Lc2 6

Surface & Coatings Technology 389 (2020) 125618

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Fig. 6. The SEM images of the Cr-coated samples after the oxidation test.

the uncoated alloy. Therefore, the non-uniform (5–20 μm) ZrO2 + ZrN layer and thick α-Zr(O) + α-Zr(N) sublayer (up to 130 μm) are found in the Cr-1 sample. Increased Cr coating thickness and denser microstructure result in lower oxygen and nitrogen diffusion to the Zr alloy, preventing the formation of ZrN + ZrO2 layer at the coating/metal interface (Figs. 4 and 5). In this case, diffusion of chromium into the zirconium alloy remains possible (Fig. 6). According to ref. [10, 31], the growth of Cr2Zr phase can occur due to higher solubility and increase of diffusion rate of Cr into β-Zr phase at high temperature. Therefore, the volume fraction of the Cr2Zr phase (from 3.0 to 4.1 vol%) can increase if the oxidation resistance of the Cr coating rises (at less oxidation of Zr). This result is in good agreement with XRD and GDOES measurements. Contrary, the diffusion of Cr to Zr is lower for a thin and columnar Cr coating (Cr-1) because of the rapid formation of oxide/nitride layer at the coating/ metal interface. Under these conditions, the Cr2N layer can be grown by interacting with nitrogen from the re-oxidation reaction. The formation

Table 4 Adhesion of the Cr coatings. Sample

Cr-1

Cr-2

Cr-3

Cr-4

As-deposited Lc1, N

5.2–5.8

7.8–8.4

8.1–10.2

7.5–8.9

After oxidation Lc1, N Lc2, N

0.5–0.9 17.0–30.0

0.7–1.1 8.5–19.4

0.6–1.5 1.9–6.8

0.8–1.2 2.8–3.3

temperature oxidation (Fig. 8b). Nevertheless, the Cr-1 coating provides a reduction in oxidation of the alloy, as evidenced by measurements of the weight gain and thickness of the oxide layer. Due to low solubility of N in chromia [37,38], nitrogen can diffuse through the outer oxide layer (Cr2O3) mainly along the grain boundaries and defects to the metal where it can stabilize the α-Zr phase or react with the α-Zr(O) to form the ZrN phase. Then, the re-oxidation process can occur similar to

Fig. 7. The SEM-image and elemental distribution maps (chromium, oxygen and zirconium) of the surface of the Cr-1 sample after the scratch test. 7

Surface & Coatings Technology 389 (2020) 125618

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Fig. 8. Optical images of the oxidized samples: uncoated (a) and Cr-coated (Cr-1).

required future development for the considered task.

of stoichiometric chromium nitride can occur only at high nitrogen and extremely low oxygen partial pressures [39], so newly available nitrogen can interact with pure Cr at the interface and form a thin Cr2N layer. Above-mentioned results of high-temperature oxidation show the different multilayers structures of the oxidized samples. If the Cr-coatings have dense microstructure and sufficient thickness, the three-layer structure can be formed. Such structure type of the coated Zr alloy after HT oxidation is observed in [10]. In this way, the outer layer is Cr2O3, the next is residual Cr and inter-diffusion Cr-Zr layers, and the third is a Zr-based layer, including α-Zr(O) + α-Zr(N) and prior β-Zr. When the Cr coating has a weak protection, the four-layer structure can be formed: outer Cr2O3 – residual Cr and Cr2N – ZrO2 + ZrN – Zr-alloy (αZr(O) + α-Zr(N) and prior β-Zr). Based on the obtained results, the dense Cr coating with higher thickness seems to be preferable to protect zirconium alloy from oxidation at the given high-temperature conditions. Adhesion of the as-deposited Cr coatings is improved with the thickness (LC1: 5.2 → 10.2 N). This point is very important for coated-Zr alloys which can be used in light-water reactors. Coating cracking is unacceptable in various technological operations. Otherwise Zr alloy will interact with water directly through the cracks. The variety of oxidation behavior of the Cr-coated Zr alloys results in different adhesion properties of the coatings. However, given different post-oxidation states of the samples, a direct comparison between them would be incorrect. Thus, we focused only on the adhesion behavior of the coatedZr alloy in general. Firstly, cracking resistance of coatings is decreased after oxidation due to the formation of hard and brittle Cr2O3 outer layer [40,41] on the sample surface. The second point is a gradual distribution of oxygen and nitrogen through the coating and alloy that can reduce the stresses at the interface. Thirdly, the oxidation and nitriding of Zr increase load bearing capacity of the substrate. Such effect is widely used for surface hardening and to improve coating adhesion, for example duplex technologies [42,43]. The fourth point is the formation of Cr-Zr inter-diffusion layer at the coating/alloy interface with hard Cr2Zr phase [44]. Under these conditions, better adhesion should be expected in the case of substrate hardening that is more pronounced for the Cr-1 sample. Then, the plastic deformation of the substrate is minimal and compressive and buckling spallation ahead of the indenter or elastic recovery-induced spallation behind the indenter are main causes of coating failure [45–47]. The obtained results show that the Cr coatings for protection of Zr alloy should have higher thickness and denser microstructure. For this way, the hot target sputtering can be applied, but it should be improved energetic parameters of coating deposition (for example, increase of Ea by greater ion flux to the substrate). Therefore, this approach is

5. Conclusions The chromium coatings were deposited onto E110 alloy using dual magnetron sputtering system and hot target sputtering. For sputtering of hot Cr target, the increased deposition rate of Cr coatings (12–30 nm/s) by additional target sublimation results in lower energy per one deposited atom (4.9–8.1 eV/atom) and the deposited coatings have a columnar microstructure. The dense Cr coating is obtained using dual magnetron with the deposition rate of 4 nm/s and the energy per one deposited atom of 31.4 eV/atom. The oxidation tests of Cr-coated Zr alloy in air at 1100 °C for 20 min revealed following conclusions. 1. The oxidation resistance of the Cr coatings improves with increasing of their thickness. The mass gain of the samples was reduced from 9.18 to 3.22 mg/cm2 with the change of thickness from 1.8 to 4.5 μm, respectively. 2. The coating with denser microstructure shows better protective properties against air oxidation compared to the coatings with columnar microstructure. No zirconium oxides were found after HT oxidation of the sample coated with dense 3.1 μm Cr using dual magnetron. 3. Thin (1.8 μm) and columnar Cr coatings cannot prevent the diffusion of nitrogen and oxygen to Zr substrate during 20-min oxidation test that causes accelerated degradation of zirconium alloy. 4. The oxidation of Cr coating with higher thickness and denser microstructure is accompanied by a growth of the Cr2O3 outer layer and the formation of an inter-diffusion layer with the ZrCr2 phase. The diffusion of oxygen and nitrogen through the coating results in the formation of oxygen- and nitrogen-stabilized α-Zr(O) + α-Zr(N) phases inside the alloy. 5. The adhesion of the as-deposited Cr coatings increases with their thickness, but the coatings are easily to crack after high-temperature oxidation. Adhesion behavior is strongly influenced by oxidation, nitriding and diffusion processes at the coating/alloy interface. The obtained results show the perspective to enhance the productivity of coating technology based on hot target sputtering, however, the formation of columnar Cr coatings due to low energy per one deposited atom reveals the unsatisfactory protection of Zr alloy and should be necessarily improved in future. Funding The work was supported by the Russian Science Foundation (grant 8

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