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Nickel-chromium (Ni–Cr) coatings deposited by magnetron sputtering for accident tolerant nuclear fuel claddings
Surface & Coatings Technology 369 (2019) 69–78
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Nickel-chromium (Ni–Cr) coatings deposited by magnetron sputtering for accident tolerant nuclear fuel claddings
T
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Dmitrii V. Sidelev , Egor B. Kashkarov, Maxim S. Syrtanov, Valery P. Krivobokov Tomsk Polytechnic University, Lenin Avenue 30, Tomsk, 634050, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Nickel-chromium coatings Chromium Nuclear fuel cladding Zirconium alloy High-temperature oxidation Hydrogen uptake
Nickel-chromium coatings were deposited on Zre1Nb alloy using magnetron sputtering systems with «hot» Ni and cooled Cr targets. The effect of coating composition on high-temperature oxidation resistance and hydrogen uptake of Zre1Nb was studied. Hydrogen uptake of the alloy was measured in situ under gas-phase hydrogenation at 633 K. High-temperature oxidation was performed in air atmosphere at 1173–1373 K for 20 min. It was shown that the coating with high Ni content (83 at.%) drastically increases hydrogen uptake of the Zre1Nb alloy and demonstrates low oxidation resistance even at 1173 K. The coatings with Cr content ≥45 at.% have low hydrogen permeability which reduces the rate of hydrogen uptake of the alloy. The oxidation resistance of the NieCr coatings increases with Cr content in the as-deposited coatings. The pure Cr coating exhibits the best oxidation resistance: only 8 μm-thick oxide layer was observed. There is also found the intensive diffusion of nickel into the alloy during high-temperature oxidation of the samples coated by NieCr films with 55 and 17 at. % Ni. The as-deposited NieCr coatings are less brittle than the pure Cr coating, but their mechanical properties degrade stronger than for the Cr coating after the oxidation test.
1. Introduction The Fukushima disaster has strongly effected on nuclear energetics and their progress [1,2]. It pointed to a crucial oxidation of nuclear fuel claddings in light-water reactors in the case of loss of coolant accidents (LOCA). Zirconium alloys used as a cladding material in the pressurized water reactors (PWR) has low thermal neutron cross section and high corrosion and oxidation resistance under normal operation conditions [3]. Nowadays, to improve functional and mechanical properties of the fuel claddings, zirconium alloys with some dopants (Nb, Sn, Fe, and Cr) are developed for the nuclear reactors. However, such materials are intensively oxidized in high-temperature steam during LOCA, when the exothermic reaction can be as follows:
Zr + 2H2 O → ZrO2 + 2H2 + Q.
(1)
The rapid oxidation results in the destruction of fuel claddings that can cause a serious irregularity in operation of the nuclear reactors and make a risk of explosion of hydrogen generated during LOCA. Thus, some strategies of accident tolerant fuel (ATF) materials are elaborately considered to prevent the above-mentioned situation [3–9]. One of the most promising approaches is to deposit an oxidationresistance coating on nuclear fuel claddings [3,6–9]. Up to date, many types of coatings and deposition techniques (chemical or physical vapor
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deposition, spraying, etc.) have been studied. Magnetron sputtering has appropriate process characteristics for deposition corrosion-resistant coatings on fuel claddings. Among them, stability and wide variety of deposition parameters, high adhesion and purity of coatings, no droplet fraction in deposited flow can be highlighted [10]. These features are crucial to develop the coating technology for nuclear fuel claddings. A wide variety of protective coatings have been investigated such as Cr, CrN, TiAlN, AlCrN, Ni, FeCrAl, Ti2AlC, Cr3C2-NiCr, SiC, etc. [3,6–9,11–13]. The analysis of published data and recent reviews shows that the coatings forming chromium oxide demonstrate the highest oxidation resistance in high-temperature steam. Metallic Cr films have high melting temperature (2180 K), excellent corrosion resistance, and thermal expansion coefficient is the same as to the Zr alloy. The deposition of metal films is more ordinary way in comparison with oxide, nitride or carbide coatings. However, pure Cr film is brittle that can result in coating cracking before and during operation of nuclear fuel or in accident conditions [12]. Therefore, various methods to improve mechanical properties of Cr coatings can be considered. The chromium coatings with dopants (Al, Ni, Mo, etc.) or Cr-based metallic compounds are less fragile due to the change of grain size (Hall-Petch effect) and forming intermetallic phases [14,15]. Therefore, these coatings (NieCr, CreAl, etc.) can be more appropriate to improve mechanical properties and corrosion resistance of the Zr alloys in comparison with pure Cr
Corresponding author. E-mail address: [email protected] (D.V. Sidelev).
https://doi.org/10.1016/j.surfcoat.2019.04.057 Received 13 February 2019; Received in revised form 6 April 2019; Accepted 16 April 2019 Available online 17 April 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 369 (2019) 69–78
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Table 1 Deposition parameters of the nickel‑chromium coatings. Sample Ni9Cr NiCr NiCr9 Cr
WNi, kW
WCr, kW
t, min
Ubias, V
jsub, mA/cm2
Ratio of Ni to Cr
1.5 0.8 0.1 –
0.4 2.0 2.9 3.3
111 119 143 145
−50
20 16 21 25
83/17 55/45 17/83 0/100
Note: WNi, WCr – discharge powers for sputtering of “hot” Ni and Cr targets, t – deposition time, Ubias - bias voltage, jsub – ion current density on the substrate.
2.2. Coating characterization The thickness, cross-section and elemental composition of the asdeposited NieCr coatings were measured by scanning electron microscopy (SEM) using Quanta 200 3D with energy dispersive X-ray analysis system (EDS) EDAX ECON IV. The weight gain of the samples after hydrogenation and oxidation tests was measured using analytical balance machine (Sartorius CP124 S) with an accuracy of 10−4 g. Crystalline structure of the samples was studied by X-ray diffraction (XRD) using Shimadzu XRD-7000S in a Bragg-Brentano configuration with CuKα-tube (40 kV, 30 mA). The scanning range was 10–90° with an exposure step of 0.03°. Phase composition was calculated using ICDD 4+ database. After the oxidation test, cross-section and elemental composition of the samples were studied by SEM and EDS. Additionally, a glow discharge optical emission spectroscopy (GDOES, GD-Profiler 2 HORIBA Scientific) was used to evaluate the elemental distribution in the samples before and after high-temperature oxidation. The sputtering depth in these measurements was determined using a three-dimensional optical profilometer Micro Measure 3D Station. The mechanical properties of the deposited coatings were studied by using a scanning nano-hardness tester «Nanoscan-3D». The penetration depth in the measurements did not exceed 10% of the coating thickness.
Fig. 1. The scheme of the ion-plasma installation.
coating. The nickel-chromium coatings are widely used as bond layers of thermal barrier coatings [16,17]. The NieCr coatings have good corrosion resistance and can be used for protection of Zr fuel claddings. However, there is no data about the resistance to corrosion and hydrogen uptake of magnetron deposited NieCr coatings depending on Ni to Cr composition ratio. This research is focused on evaluation of the NieCr coatings as candidate for ATF fuel claddings. The aim of this study is to determine the influence of Ni to Cr ratio on high-temperature oxidation resistance and hydrogen uptake of NiCr-coated Zre1Nb alloy. For it, the investigations of crystal structure and microstructure, weight gain and elemental composition, mechanical properties of the zirconium alloy will be performed.
2. Experimental details
2.3. Calculations of energy flux to the substrate
2.1. Coating deposition
To determine the effect of deposition conditions on coating microstructure and properties the energetic characteristics of deposition are usually calculated [21–25]. The total energy delivered to the growing film ET [23] or the energy per one deposited atom Ea [25] can been used for describing the evolution of microstructure and properties of coatings. The calculations of these parameters are more complicated when using a rotating substrate. In such case, the energy flux delivers to the substrate only when the sample is opposite to the magnetron. For other deposition time, particle flux to the substrate significantly decreases due to “shadowing” effect by other substrates. However, heat loss occurs during all deposition time. Thus, the energy flux delivered to the rotating substrate (F) can be calculated as:
The nickel-chromium coatings (~2 μm thickness) were deposited by magnetron sputtering. The scheme of the ion-plasma installation is shown in Fig. 1. This experimental setup is consisted of vacuum system with a turbomolecular pump, two magnetron sputtering systems with disk (Ø 90 mm) targets, ion source, planetary substrate-holder and substrate biasing system. For chromium deposition, the unbalanced magnetron sputtering system with Cr target (99.95%) was used. The magnetron with “hot” Ni target was used for nickel sputtering due to higher productivity, target utilization and stability of deposition parameters [18,19]. The detailed description of the magnetron system with “hot” Ni target is presented in [18]. The NieCr coatings were deposited on E110 alloy (Zre1Nb, 20 × 20 × 2 mm3) and polished Si (110) substrates for each experimental mode. Such type of the Zr alloy is used as fuel cladding material in Russian PWR reactors [20]. The substrates were prepared by grinding and polishing machine, then rinsed with alcohol and dried by compressed air. Before deposition, the substrates were treated by ion source in At atmosphere to remove surface oxides and contaminations (pressure – 0.15 Pa, acceleration voltage – 2.5 kV, current – 40 mA, time – 20 min). The coatings were deposited at the constant pressure of 0.2 Pa. Other deposition parameters are presented in Table 1. The magnetron and substrate biasing system had direct current (DC) power supplies of APEL-M series (Apelvac, Russia). The base pressure was 2 × 10−3 Pa. The distance from both targets to the substrates was 100 mm. The substrates were planetary rotated during ion treatment and coating deposition.
F = (Fin + Fout ) ∙k − Fout ,
(2)
where Fin – the energy delivered to the unmoved substrate; Fout – heat loss of the substrate; k – the ratio of time that the substrate is opposite to the magnetron to full deposition time (in our case k = 0.13). Knowing the energy fluxes on the unmoved substrate and heat loss, the substrate temperature can be estimated using the equation of energy balance taking in to account the substrate rotation:
Tsub =
((Fin + Fout ) ∙k − Fout ) ∙S∙t − T0, c∙m
(3)
where c – thermal capacitance of Zr (291 J/kg·K); m – mass of the Zr substrate (5.2·10−3 kg); S – area of the substrate surface (4·10−4 m2); t – deposition time, T0 – substrate temperature before deposition (300K). The calculated temperatures of the rotating Zr substrates after deposition are shown in Table 2. To calculate the energy per one deposited atom (Ea) the following 70
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coatings weakly influences on crystal growth of majority component's phase [26,27]. When both Cr and Ni components are high, the crystal growth of each component strongly limits by lower migration of grain boundaries and decreased coalescence. Thus, the dense and uniform microstructure of the NiCr coating is observed (Fig. 2b), while columnar microstructure is formed in other coatings. On the other side, when high Ni-contained coatings were depositing, the discharge power of the magnetron with Ni cathode was enough to operate in «hot target mode» (Ni9Cr, NiCr) [18,28]. In this case, the energy deposited per atom significantly increased from 9.6 to 10.5 to 16.5–27.5 eV (Table 2). The latter causes heating of substrate from 391 to 689 K (Table 2) which enhances the atomic diffusion processes on the surface. Therefore, the adatom mobility increases the crystallite nucleation sites, which leads to the formation of denser microstructure observed for Ni9Cr and NiCr coatings (Fig. 2a and b). The Zre1Nb samples subjected to hydrogenation and oxidation tests is presented in Fig. 3. There is shown that the Ni9Cr coating peeled-off from the Zre1Nb alloy after hydrogenation, which can be associated with hydride precipitation and volume expansion of the alloy compared to the coating. The outer view of other samples did not change after hydrogenation. The uncoated Zre1Nb alloy strongly oxidized even at 1173 K. The white film on its surface indicates zirconium oxidation (ZrOx). The cracks and peelings of ZrOx layers were also observed. At higher temperature (1373 K) the oxidation of the uncoated alloy occurred stronger, and the destruction of this sample was observed. It can be seen from Fig. 3 that color of the coated samples was changed from light gold or silver (as-deposited) to dark green or gray-blue indicating the formation of chromium and nickel oxides on the surface after the oxidation test. The coated samples had unprotected corner part, which intensively oxidized. The Ni9Cr coating partially disintegrated from the Zre1Nb alloy, while other samples were preserved their integrity. The effect of hydrogenation and high-temperature oxidation on the Zre1Nb samples will be described and discussed in detail in the next sections.
Table 2 Energetic characteristics of coating deposition. Sample Ni9Cr NiCr NiCr9 Cr
Fin, W/m2
Fout, W/m2
Tsub, K
Ea, eV/atom
2419.7 1415.4 696.7 731.2
92.5 62.9 53.9 51.4
689 530 391 407
27.5 16.5 9.6 10.5
equation was used [25]:
Ea =
F∙S∙t , Natom
(4)
where F – the total energy flux to the rotating substrate [W/m ]; Natom – the number of the deposited atoms [atom]. For two-component films (e.g., NieCr), the elemental composition should be taken into account: 2
Natom =
mdep , (mNi ∙r + mCr ∙ (1 − r ))
(5)
where mdep – mass of the deposited atoms; mNi – mass of one Ni atom; mCr – mass of one Cr atom; r – the ratio of Ni to Cr in the coating [%]. The results of Ea calculations are shown in Table 2. 2.4. Gas-phase hydrogenation Gas-phase hydrogenation was performed to evaluate hydrogen uptake kinetics of the Zre1Nb alloy. The measurements were carried out using an automated complex Gas Reaction Controller (Advanced Materials Corporation, USA) at the constant temperature of 633 K (normal operation temperature of fuel cladding). Hydrogen (99.9995% purity) was produced by pyrolysis method using a hydrogen generator HyGen 200 (USA). Initially the samples were placed horizontally in the vacuum chamber and heated to 633 K (the heating rate was 6 K/min). Then, the chamber was filled with hydrogen and the samples were maintained during 60 min at the constant pressure of 2 atm. After hydrogenation, the samples were cooled in vacuum (the cooling rate ~2 K/min). Hydrogen concentration was measured in situ during hydrogenation process and additionally measured as weight gain immediately after hydrogenation.
3.2. Resistance to hydrogen uptake Fig. 4a shows the hydrogen sorption kinetics of the Zre1Nb alloy under gas-phase hydrogenation at 633 K. The deposition of the coating with high nickel content (Ni9Cr) leads to increase the rate of hydrogen absorption by the alloy. The absorbed hydrogen concentration (nH) of this sample is higher (~0.3 wt%) than for the samples with the NiCr, NiCr9 and Cr coatings (lower than 0.03 wt%) or the uncoated sample (0.09 wt%). It is known that permeability of hydrogen through pure nickel is high and nickel oxides are easily reduced by hydrogen [29–31]. It is assumed that the chromium content in the Ni9Cr coating is not enough to form a protective chromium oxide layer on the surface; therefore, the coating does not demonstrate the protective properties against hydrogen uptake. The measurements of weight gain after the hydrogenation test are in good correlation with absorption kinetics (Fig. 4b). The maximal weight gain of 2.3 mg/cm2 is observed for the sample with the Ni9Cr coating, while other coated samples have lower 0.1 mg/cm2 (Fig. 4b). Fig. 5 shows the X-ray diffraction patterns of the samples before and after hydrogenation. The reflections of hexagonal closed packed (hcp) α-Zr phase correspond to the Zre1Nb substrate. The as-deposited coatings have face-centered cubic (fcc) γ-Ni and/or body-centered (bcc) α-Cr phases depending on the coating composition. The lattice parameter a = 3.5369 Å of γ-Ni phase is significantly higher than that for pure nickel (3.5240 Å), which indicates the formation of fcc Ni(Cr) solid solution phase in the Ni9Cr coating (Fig. 5a). The phases of γ-Ni and αCr exist in the NiCr coating which causes a strong broadening of the coating reflections (Fig. 5b). According to NieCr phase diagram, the formation of these phases is expected in the NiCr and NiCr9 coatings [32]. However, these phases are difficult to separate due to superposition of its reflections. Therefore, the NiCr9 coating primarily composes of α-Cr phase, and small amount of fcc Ni phase can be
2.5. High-temperature oxidation For high-temperature oxidation two temperatures (1173 and 1373 K) were chosen. A high-temperature atmospheric furnace (ATS 3210, Applied Test Systems Inc.) was used. The samples were fixed in the furnace chamber by tungsten wire and heated from room temperature (the heating rate was ~30 K/min). Then, the samples were exposed during 20 min at 1173 or 1373 K. After this, the chamber was opened, and the samples were naturally cooled to room temperature. 3. Results and discussion 3.1. The as-deposited coatings The elemental composition of the as-deposited NieCr coatings is shown in Table 1. The nickel to chromium ratio (in at.%) in the Ni9Cr, NiCr, NiCr9 and Cr coatings is 83/17, 55/45, 17/83 and 0/100, respectively. The cross-section SEM images of the as-deposited coatings on Si substrates are shown in Fig. 2. The coating thickness is (2.2 ± 0.1) μm. Fig. 2 represents the microstructure of the nickel-chromium coatings of different compositions. In the case of co-deposition, film structure depends on process conditions (energy flux delivered to the substrate) and composition ratio. On the one side, when the NieCr coatings with high content of Cr or Ni are depositing, minority component of the 71
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Fig. 2. The cross-section SEM images of the as-deposited coatings on Si substrate: a – Ni9Cr; b – NiCr; c – NiCr9; d – Cr.
expected. Only α-Cr phase with a = 2.9100 Å exists in the Cr coating (Fig. 5d). After hydrogenation, cubic (δ-ZrH) and tetragonal (γ-ZrH) zirconium hydrides are observed in the sample with the Ni9Cr coating due to high concentration of hydrogen absorbed by this sample (see Fig. 4). On the contrary, the phase composition of the samples coated by the NiCr, NiCr9 and Cr films is unchanged after the same test. This result also indicates better protective properties of the chromium-rich coatings against hydrogen permeation into the zirconium alloy. Due to high hydrogen permeability and corrosion resistance, pure nickel is usually used to improve hydrogen sorption behavior of Zr or Ti alloys and to increase their oxidation resistance [33–35]. Therefore, the NieCr coatings containing pure fcc Ni phase cannot protect the alloy from hydrogen uptake. The result of such exposure is a hydrogen embrittlement of zirconium and its destruction as shown in Fig. 3. For the coatings with lower nickel content (55 at.% and less), where α–Cr phase was formed, the hydrogen absorption by the Zre1Nb alloy at 633 K is negligible. These coatings have low hydrogen permeability and can be used as barrier coatings against hydrogen permeation into the zirconium alloy.
the lowest weight gain compared to other. Fig. 7 presents X-ray diffraction patterns of the samples before and after oxidation at 1173 and 1373 K. The Cr2O3 phase is observed in all samples, while the NiO phase is found only in the sample with the Ni9Cr coating. This is caused by formation of nickel-zirconium (NiZr) phase (Fig. 7b) due to diffusion of nickel into zirconium. Such result is also confirmed by GDOES and EDS measurements (Figs. 8–9). The formation of the intermetallic NiZr phase was observed in the samples with the NiCr and Ni9Cr coatings at 1173 K. At this temperature, the formed Cr2O3 layer on the surface prevented intensive oxidation of the sample. Therefore, nickel migration into the zirconium alloy (see Fig. 8e) with the formation of the NiZr phase was found. At the same time, no intermetallic NiZr compound was observed after oxidation at 1373 K. The relative concentration of nickel at the coating/ alloy interface is significantly lower at higher temperature (Fig. 8e and f), which indicates accelerated nickel diffusion into the zirconium alloy. The analysis of the literature data showed that ultrafast nickel diffusion occurs even in α–Zr phase (diffusion coefficient D = 5 × 10−11 m2/s) and increases with the phase transition αZr → βZr (1136 K) and further temperature rise [36,37]. Thus, the thermal stimulated diffusion of nickel at 1373 K contributes to Ni dissolution in Zr lattice without formation of the intermetallic phase. The formation of NiO occurs only in the coating with low chromium content (Ni9Cr), which is also confirmed by GDOES (Fig. 8b and c). The formation of stable NiO limits the migration of nickel from coating to the substrate, but does not protect the zirconium alloy from high-temperature oxidation. It has been shown that NiO demonstrates weak oxidation resistance at temperatures higher 1073 K [38]. This is the most obvious to the sample with the Ni9Cr coating, where ZrO2 and Zr3O peaks are observed even at 1173 K. In other samples, oxidized hexagonal zirconium (Zr3O) phase is predominantly formed at 1173 K, while tetragonal and monoclinic ZrO2 phases are detected at higher
3.3. Resistance to high-temperature oxidation The samples were oxidized in atmosphere at two temperatures (1173 and 1373 K). The oxidation at 1173 K was studied only by XRD and GDOES. Other measurements (weight gain and SEM) were additionally performed to analyze the samples oxidized at 1373 K. Fig. 6 shows the weight gain (Δm) of the samples after high-temperature oxidation at 1373 K. The uncoated Zre1Nb has the highest value of Δm (46.5 mg/cm2). The weight gain of the coated samples is lower and depends on the coating composition (5.2–35.9 mg/cm2). The best oxidation resistance is observed for the Cr-coated sample having 72
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Zr1–Nb samples with nickel-chromium coatings Ni9Cr NiCr NiCr9 Cr with as-deposited coatings
uncoated Zr–1Nb
after hydrogen test
after high-temperature oxidation at 1173 K
after high-temperature oxidation at 1373 K
Fig. 3. The appearance of the samples after coating deposition and different tests.
completely protect the alloy from high-temperature oxidation since the ZrO2 (˂6 vol%) and Zr3O (11.4 vol%) phases were found. Probably, this is related to the columnar microstructure of the as-deposited Cr coating, which may result oxygen diffusion along the grain boundaries. Thus, the deposition of coatings with dense homogenous microstructure and high Cr content is expected approach to improve oxidation resistance of Zr alloys. Based on XRD and weight gain measurements, the Ni9Cr-coated sample is demonstrated poor oxidation resistance and disintegrated after oxidation test (see Fig. 3). Therefore, cross-section SEM observations and elemental composition measurements were performed only to the oxidized samples with NiCr, NiCr9 and Cr coatings (Fig. 9). From the SEM images, it can be seen that the oxidation of the samples is uniform and the thickness of oxide layer decreases from 86 μm for the
temperature [39]. The quantitative analysis of phase composition (in vol%) of the oxidized samples is shown in Table 3. The volume fraction of the Cr2O3 phase in the oxidized samples is increased with Cr content in the as-deposited NieCr coatings. It is supposed that thicker Cr2O3 layers are formed on the sample surface that results in less oxidation of the zirconium alloy. According to [40], Cr2O3 films can be resistant to oxidation up to 1473 K. Moreover, Ni diffusion from the NieCr coatings to the alloy can adversely affect the corrosion resistance due to formation of structural defects that contribute to fast diffusion of oxygen through the coating. These results are in agreement with the total content of zirconium oxide (∑ZrOx) phases in the samples which is decreased from 54.7 to 17.3 vol%. The pure Cr coating is not fully oxidized since the α–Cr phase (42.8 vol%) exists in the Cr-coated sample after oxidation test. Nevertheless, this coating does not
Fig. 4. The hydrogen sorption kinetics (a) and weight gain (b) at 633 K for the Zre1Nb samples. 73
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Fig. 5. XRD patterns of the Zre1Nb samples with as-deposited coatings (black curves) and after hydrogenation (green curves): a – Ni9Cr; b – NiCr; c – NiCr9; d – Cr. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
plastic deformation resistance (H3/E2) of this coating is lower than for the NieCr coatings. From the data of H/E and H3/E2 ratios, the asdeposited Cr coating is more brittle [41,42]. The addition of nickel to chromium results in both decrease of hardness and elastic modulus. Nevertheless, H/E and H3/E2 ratios for the nickel-chromium coatings are higher that indicates the improvement of cracking resistance of NieCr coatings compared to the pure Cr film. However, addition of Ni to the coatings and higher Ea for the Ni9Cr and NiCr coatings (see Table 2) are not enough to significantly improve cracking resistance of the as-deposited coatings (H/E < 0.1). Thus, it is supposed that the average energy deposited per atom should be higher to increase H/E ratio or deposition of highly elastic over-layer on the brittle Cr coating should be considered [22]. Hydrogenation slightly affects the mechanical properties of all the deposited coatings. However, significant changes in coating characteristics were observed after high-temperature oxidation. All mechanical parameters (H, E, H/E and H3/E2 ratios) were strongly dropped that can be caused by coating oxidation when a porous microstructure is usually formed [43,44]. The degradation of mechanical properties after oxidation is more pronounced for the NieCr coatings compared to the Cr film. The H/E ratio is dropped from 0.058 to 0.031 for the NiCr9 coating, while it is decreased from 0.053 to 0.048 for the Cr coating. Such behavior can indicate denser microstructure of the forming chromia layer and better mechanical properties of the Cr coating after the oxidation test. Thus, together with the above-mentioned results (in Sec. 3.2 and 3.3), the pure Cr coating seems to be better candidate to protect Zr alloys from oxidation at LOCA conditions. However, to obtain highly resistant Cr coating on zirconium alloys, the cracking resistance of the coating should be improved.
Fig. 6. The weight gain of the samples after high-temperature oxidation at 1373 K.
NiCr-coated sample to 8 μm for the Cr-coated sample. 3.4. Mechanical properties A complex manufacturing process of fuel claddings as well as strict requirements for operation and utilization of claddings and their geometry additionally require high cracking resistance and adhesion of the protective coatings. The coatings must be resistant to external mechanical damage, cracking resistant during irradiation growth of zirconium alloy as well as resistant to fretting wear in the coolant flow. Therefore, mechanical properties of the developing coatings are one of the most important parameters determining the possibility of their application as protective coating for fuel claddings. The mechanical properties of the NieCr coatings were measured by nanoindentation (Figs. 10, 11). The as-deposited Cr coating has higher hardness (H) and elastic modulus (E) compared to other coatings. However, the resistance against elastic strain to failure (H/E) and
3.5. Prospects of coating technology for nuclear fuel claddings The high-temperature oxidation resistance of Zr alloys can be 74
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Fig. 7. X-ray diffraction patterns of the coated Zre1Nb before and after high-temperature oxidation: a – Ni9Cr; b – NiCr; c – NiCr9; d – Cr.
on nuclear fuel claddings. On the one hand, fuel claddings have large dimensions (4–6 m in length and low cross-section ˂1 cm2) [47]. On the other hand, light-water reactors are the most widespread reactors, and their quantity increases every year [1]. Therefore, the coating technology should have very high productivity in combination with opportunity to form high cracking and oxidation resistant coatings. This task is complex and can require new approaches to realize. The most obvious solution is deposition of metal or intermetallic compound by using multi-cathode sputtering systems. Such systems are usually consisted of several cathodes from equal or different target material [48,49], where the deposition rate is proportionally increasing with the quantity of cathodes. The choice of metal or intermetallic compound with high oxidation resistance as the coating for ATF is
enhanced by coating deposition. This approach is advantageous due to the possibility of integration of coating technology in the claddings manufacturing process. To develop the coating technology, a large number of deposition techniques are considering [3,6–9,11,45]. Magnetron sputtering is the most perspective due to the following reasons: no droplet fraction in deposition particle flow, minimum of undesirable impurities, high control of process parameters and their stability, high coating uniformity even for large length magnetron (e.g. several meters), great experience in magnetron sputtering techniques to deposit coatings on large-scale substrates, etc. [10]. However, the most sensitive question for industrial coating technology is productivity (coating deposition rate) that is weakness of magnetron sputtering [10,46]. This parameter becomes more significant in the case of coating deposition
Fig. 8. GDOES profiles of elemental distribution in the samples with Ni9Cr and NiCr coatings (a, d – as-deposited, b, e – oxidized at 1173 K; c, f – oxidized at 1373 K). 75
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Fig. 9. Cross-section SEM images and corresponding EDS elemental distribution profiles of the samples oxidized at 1373 K: a – NiCr; b – NiCr9; c – Cr.
horizontal position for both sputtering system and large-scale substrates. This is very complex in view of the fuel cladding geometry. Thus, the sputtering systems with hot target seem to be more appropriate even they have lower productivity relative to the magnetron with molten target. Moreover, the high-powered energy flux to substrate is special feature of these systems. The additional energy flux is formed due to target radiation and evaporated (sublimated) particles reached the substrate [21,28]. It can strongly influence the microstructure and properties of the deposited coatings. Therefore, additional studies of process parameters and coating properties need to perform for magnetrons with hot target. The great interest has the comparison of these characteristics for multi-cathode system and hot target magnetron. There are should be investigated deposition rates, efficiency of deposition process (the cost of coating deposition on a single substrate), coating adhesion and microstructure, resistance to high-temperature oxidation, etc. Such study will be performed by our scientific group in near future. Apart from metal or intermetallic films, ceramic coatings have been strongly considered [3,45,53,54]. At the first sight, these materials have some advantages such as higher oxidation resistance and hardness, lower wear resistance compared to metal coatings. The ceramic coatings can be effective to prevent zirconium oxidation even at lower coating thickness. However, several weaknesses are challenged to their application. Firstly, brittleness of ceramic coatings can result in coating cracking. The second point is lower deposition rate compared to metals and process stability for deposition of stoichiometric ceramic coatings. Lately, some new approaches, based on high power pulsed magnetron sputtering [55,56] can be applied to stabilize reactive process and increase their productivity. However, the additional studies of using this
Table 3 Phase composition (in vol%) of the samples after high-temperature oxidation at 1373 K. Phase
Cr2O3 NiO ZrO2 (m) ZrO2 (t) Ni ZrN CrN Cr2N Cr Zr3O ∑ZrOx
Coatings Ni9Cr
NiCr
NiCr9
17.0 7.5 45.8 8.9 15.4 5.4
42.3 – 46.5 3.2 – 2.8 4.0 1.2 – – 49.7
64.5 – 27.8 1.9 – 2.0 – 3.8 – – 29.7
– – – 54.7
Cr 30.9 – 4.8 1.1 – 9.0 – – 42.8 11.4 17.3
caused by higher deposition rate in comparison with other materials (oxides, nitrides, etc.). It partially hides the low deposition rate of magnetron sputtering. Such approach is confirmed by the recent studies [9,50], including material strategies of industrial nuclear companies and institutions (AREVA NP, Korea Atomic Energy Research Institute, etc.). Other implementation of the coating technology can be based on magnetron sputtering systems with hot or molten targets [46,51,52], where the deposition rate is higher in several times in comparison with the typical sputtering systems with cooled target. In such systems, the target material is evaporated or sublimated additionally to sputtering. The application of the magnetron system with molten target means a 76
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Fig. 10. Hardness (a) and elastic modulus (b) of NieCr coatings deposited on Zre1Nb substrates: (+H) – after hydrogenation; (+T) – after oxidation at 1373 K.
Fig. 11. H/E (a) and H3/E2 (b) ratios of nickel-chromium coatings deposited on Zre1Nb substrates: (+H) – after hydrogenation; (+T) – after oxidation at 1373 K.
as well as accident conditions. However, to develop industrial technology for nuclear fuel cladding tubes, the deposition techniques and type of coating should be attentive considered in view of deposition rate, resistance of the coating to cracking and oxidation. Thus, new approaches for coating deposition can be desired.
technique to the coating deposition for nuclear materials are certainly needed. 4. Conclusions The nickel–chromium coatings with different composition ratio were deposited by magnetron sputtering to increase the resistance of Zre1Nb alloy to high-temperature oxidation and hydrogen uptake. The main conclusions were listed hereafter.
Funding This study was supported by Russian Science Foundation (project №15-19-00026).
1. The NieCr coating with high Ni content (> 83 at.%) causes permeation of hydrogen into zirconium alloy at 633 K and has low high-temperature oxidation resistance even at 1173 K. The increase of Cr content in the as-deposited NieCr coatings improves their protective properties. Higher oxidation resistance is attributed to the formation of Cr2O3 layer which is less permeable to oxygen in comparison with NiO at high temperatures. 2. The diffusion of nickel into Zr alloy occurs during high-temperature oxidation of the nickel-chromium coatings with Ni content of 55 and 17 at.%. This leads to the formation of intermetallic NiZr phase after oxidation at 1173 K. Only solid solution of nickel in zirconium alloy was observed after oxidation at 1373 K due to the fast diffusion of nickel in bcc Zr lattice. 3. The pure Cr film demonstrates the highest oxidation resistance compared to the NieCr coatings. The weight gain of the Cr-coated zirconium alloy after oxidation at 1373 K for 20 min was 5.2 mg/ cm2 that corresponds to 8 μm-thick oxide layer on the sample surface. 4. The as-deposited NieCr coatings exhibit better mechanical properties than the pure Cr coating. The hardness of the NieCr coatings increases from 8.4 to 12.1 GPa with Cr content, but the as-deposited Cr coating is more brittle. Nevertheless, the degradation of mechanical properties is less pronounced for the pure Cr film after high-temperature oxidation. 5. Magnetron sputtering of chromium coatings can be used as the approach to protect zirconium alloys under normal reactor operation
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Nickel-chromium (Ni–Cr) coatings deposited by magnetron sputtering for accident tolerant nuclear fuel claddings
T
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Dmitrii V. Sidelev , Egor B. Kashkarov, Maxim S. Syrtanov, Valery P. Krivobokov Tomsk Polytechnic University, Lenin Avenue 30, Tomsk, 634050, Russia
A R T I C LE I N FO
A B S T R A C T
Keywords: Nickel-chromium coatings Chromium Nuclear fuel cladding Zirconium alloy High-temperature oxidation Hydrogen uptake
Nickel-chromium coatings were deposited on Zre1Nb alloy using magnetron sputtering systems with «hot» Ni and cooled Cr targets. The effect of coating composition on high-temperature oxidation resistance and hydrogen uptake of Zre1Nb was studied. Hydrogen uptake of the alloy was measured in situ under gas-phase hydrogenation at 633 K. High-temperature oxidation was performed in air atmosphere at 1173–1373 K for 20 min. It was shown that the coating with high Ni content (83 at.%) drastically increases hydrogen uptake of the Zre1Nb alloy and demonstrates low oxidation resistance even at 1173 K. The coatings with Cr content ≥45 at.% have low hydrogen permeability which reduces the rate of hydrogen uptake of the alloy. The oxidation resistance of the NieCr coatings increases with Cr content in the as-deposited coatings. The pure Cr coating exhibits the best oxidation resistance: only 8 μm-thick oxide layer was observed. There is also found the intensive diffusion of nickel into the alloy during high-temperature oxidation of the samples coated by NieCr films with 55 and 17 at. % Ni. The as-deposited NieCr coatings are less brittle than the pure Cr coating, but their mechanical properties degrade stronger than for the Cr coating after the oxidation test.
1. Introduction The Fukushima disaster has strongly effected on nuclear energetics and their progress [1,2]. It pointed to a crucial oxidation of nuclear fuel claddings in light-water reactors in the case of loss of coolant accidents (LOCA). Zirconium alloys used as a cladding material in the pressurized water reactors (PWR) has low thermal neutron cross section and high corrosion and oxidation resistance under normal operation conditions [3]. Nowadays, to improve functional and mechanical properties of the fuel claddings, zirconium alloys with some dopants (Nb, Sn, Fe, and Cr) are developed for the nuclear reactors. However, such materials are intensively oxidized in high-temperature steam during LOCA, when the exothermic reaction can be as follows:
Zr + 2H2 O → ZrO2 + 2H2 + Q.
(1)
The rapid oxidation results in the destruction of fuel claddings that can cause a serious irregularity in operation of the nuclear reactors and make a risk of explosion of hydrogen generated during LOCA. Thus, some strategies of accident tolerant fuel (ATF) materials are elaborately considered to prevent the above-mentioned situation [3–9]. One of the most promising approaches is to deposit an oxidationresistance coating on nuclear fuel claddings [3,6–9]. Up to date, many types of coatings and deposition techniques (chemical or physical vapor
⁎
deposition, spraying, etc.) have been studied. Magnetron sputtering has appropriate process characteristics for deposition corrosion-resistant coatings on fuel claddings. Among them, stability and wide variety of deposition parameters, high adhesion and purity of coatings, no droplet fraction in deposited flow can be highlighted [10]. These features are crucial to develop the coating technology for nuclear fuel claddings. A wide variety of protective coatings have been investigated such as Cr, CrN, TiAlN, AlCrN, Ni, FeCrAl, Ti2AlC, Cr3C2-NiCr, SiC, etc. [3,6–9,11–13]. The analysis of published data and recent reviews shows that the coatings forming chromium oxide demonstrate the highest oxidation resistance in high-temperature steam. Metallic Cr films have high melting temperature (2180 K), excellent corrosion resistance, and thermal expansion coefficient is the same as to the Zr alloy. The deposition of metal films is more ordinary way in comparison with oxide, nitride or carbide coatings. However, pure Cr film is brittle that can result in coating cracking before and during operation of nuclear fuel or in accident conditions [12]. Therefore, various methods to improve mechanical properties of Cr coatings can be considered. The chromium coatings with dopants (Al, Ni, Mo, etc.) or Cr-based metallic compounds are less fragile due to the change of grain size (Hall-Petch effect) and forming intermetallic phases [14,15]. Therefore, these coatings (NieCr, CreAl, etc.) can be more appropriate to improve mechanical properties and corrosion resistance of the Zr alloys in comparison with pure Cr
Corresponding author. E-mail address: [email protected] (D.V. Sidelev).
https://doi.org/10.1016/j.surfcoat.2019.04.057 Received 13 February 2019; Received in revised form 6 April 2019; Accepted 16 April 2019 Available online 17 April 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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D.V. Sidelev, et al.
Table 1 Deposition parameters of the nickel‑chromium coatings. Sample Ni9Cr NiCr NiCr9 Cr
WNi, kW
WCr, kW
t, min
Ubias, V
jsub, mA/cm2
Ratio of Ni to Cr
1.5 0.8 0.1 –
0.4 2.0 2.9 3.3
111 119 143 145
−50
20 16 21 25
83/17 55/45 17/83 0/100
Note: WNi, WCr – discharge powers for sputtering of “hot” Ni and Cr targets, t – deposition time, Ubias - bias voltage, jsub – ion current density on the substrate.
2.2. Coating characterization The thickness, cross-section and elemental composition of the asdeposited NieCr coatings were measured by scanning electron microscopy (SEM) using Quanta 200 3D with energy dispersive X-ray analysis system (EDS) EDAX ECON IV. The weight gain of the samples after hydrogenation and oxidation tests was measured using analytical balance machine (Sartorius CP124 S) with an accuracy of 10−4 g. Crystalline structure of the samples was studied by X-ray diffraction (XRD) using Shimadzu XRD-7000S in a Bragg-Brentano configuration with CuKα-tube (40 kV, 30 mA). The scanning range was 10–90° with an exposure step of 0.03°. Phase composition was calculated using ICDD 4+ database. After the oxidation test, cross-section and elemental composition of the samples were studied by SEM and EDS. Additionally, a glow discharge optical emission spectroscopy (GDOES, GD-Profiler 2 HORIBA Scientific) was used to evaluate the elemental distribution in the samples before and after high-temperature oxidation. The sputtering depth in these measurements was determined using a three-dimensional optical profilometer Micro Measure 3D Station. The mechanical properties of the deposited coatings were studied by using a scanning nano-hardness tester «Nanoscan-3D». The penetration depth in the measurements did not exceed 10% of the coating thickness.
Fig. 1. The scheme of the ion-plasma installation.
coating. The nickel-chromium coatings are widely used as bond layers of thermal barrier coatings [16,17]. The NieCr coatings have good corrosion resistance and can be used for protection of Zr fuel claddings. However, there is no data about the resistance to corrosion and hydrogen uptake of magnetron deposited NieCr coatings depending on Ni to Cr composition ratio. This research is focused on evaluation of the NieCr coatings as candidate for ATF fuel claddings. The aim of this study is to determine the influence of Ni to Cr ratio on high-temperature oxidation resistance and hydrogen uptake of NiCr-coated Zre1Nb alloy. For it, the investigations of crystal structure and microstructure, weight gain and elemental composition, mechanical properties of the zirconium alloy will be performed.
2. Experimental details
2.3. Calculations of energy flux to the substrate
2.1. Coating deposition
To determine the effect of deposition conditions on coating microstructure and properties the energetic characteristics of deposition are usually calculated [21–25]. The total energy delivered to the growing film ET [23] or the energy per one deposited atom Ea [25] can been used for describing the evolution of microstructure and properties of coatings. The calculations of these parameters are more complicated when using a rotating substrate. In such case, the energy flux delivers to the substrate only when the sample is opposite to the magnetron. For other deposition time, particle flux to the substrate significantly decreases due to “shadowing” effect by other substrates. However, heat loss occurs during all deposition time. Thus, the energy flux delivered to the rotating substrate (F) can be calculated as:
The nickel-chromium coatings (~2 μm thickness) were deposited by magnetron sputtering. The scheme of the ion-plasma installation is shown in Fig. 1. This experimental setup is consisted of vacuum system with a turbomolecular pump, two magnetron sputtering systems with disk (Ø 90 mm) targets, ion source, planetary substrate-holder and substrate biasing system. For chromium deposition, the unbalanced magnetron sputtering system with Cr target (99.95%) was used. The magnetron with “hot” Ni target was used for nickel sputtering due to higher productivity, target utilization and stability of deposition parameters [18,19]. The detailed description of the magnetron system with “hot” Ni target is presented in [18]. The NieCr coatings were deposited on E110 alloy (Zre1Nb, 20 × 20 × 2 mm3) and polished Si (110) substrates for each experimental mode. Such type of the Zr alloy is used as fuel cladding material in Russian PWR reactors [20]. The substrates were prepared by grinding and polishing machine, then rinsed with alcohol and dried by compressed air. Before deposition, the substrates were treated by ion source in At atmosphere to remove surface oxides and contaminations (pressure – 0.15 Pa, acceleration voltage – 2.5 kV, current – 40 mA, time – 20 min). The coatings were deposited at the constant pressure of 0.2 Pa. Other deposition parameters are presented in Table 1. The magnetron and substrate biasing system had direct current (DC) power supplies of APEL-M series (Apelvac, Russia). The base pressure was 2 × 10−3 Pa. The distance from both targets to the substrates was 100 mm. The substrates were planetary rotated during ion treatment and coating deposition.
F = (Fin + Fout ) ∙k − Fout ,
(2)
where Fin – the energy delivered to the unmoved substrate; Fout – heat loss of the substrate; k – the ratio of time that the substrate is opposite to the magnetron to full deposition time (in our case k = 0.13). Knowing the energy fluxes on the unmoved substrate and heat loss, the substrate temperature can be estimated using the equation of energy balance taking in to account the substrate rotation:
Tsub =
((Fin + Fout ) ∙k − Fout ) ∙S∙t − T0, c∙m
(3)
where c – thermal capacitance of Zr (291 J/kg·K); m – mass of the Zr substrate (5.2·10−3 kg); S – area of the substrate surface (4·10−4 m2); t – deposition time, T0 – substrate temperature before deposition (300K). The calculated temperatures of the rotating Zr substrates after deposition are shown in Table 2. To calculate the energy per one deposited atom (Ea) the following 70
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coatings weakly influences on crystal growth of majority component's phase [26,27]. When both Cr and Ni components are high, the crystal growth of each component strongly limits by lower migration of grain boundaries and decreased coalescence. Thus, the dense and uniform microstructure of the NiCr coating is observed (Fig. 2b), while columnar microstructure is formed in other coatings. On the other side, when high Ni-contained coatings were depositing, the discharge power of the magnetron with Ni cathode was enough to operate in «hot target mode» (Ni9Cr, NiCr) [18,28]. In this case, the energy deposited per atom significantly increased from 9.6 to 10.5 to 16.5–27.5 eV (Table 2). The latter causes heating of substrate from 391 to 689 K (Table 2) which enhances the atomic diffusion processes on the surface. Therefore, the adatom mobility increases the crystallite nucleation sites, which leads to the formation of denser microstructure observed for Ni9Cr and NiCr coatings (Fig. 2a and b). The Zre1Nb samples subjected to hydrogenation and oxidation tests is presented in Fig. 3. There is shown that the Ni9Cr coating peeled-off from the Zre1Nb alloy after hydrogenation, which can be associated with hydride precipitation and volume expansion of the alloy compared to the coating. The outer view of other samples did not change after hydrogenation. The uncoated Zre1Nb alloy strongly oxidized even at 1173 K. The white film on its surface indicates zirconium oxidation (ZrOx). The cracks and peelings of ZrOx layers were also observed. At higher temperature (1373 K) the oxidation of the uncoated alloy occurred stronger, and the destruction of this sample was observed. It can be seen from Fig. 3 that color of the coated samples was changed from light gold or silver (as-deposited) to dark green or gray-blue indicating the formation of chromium and nickel oxides on the surface after the oxidation test. The coated samples had unprotected corner part, which intensively oxidized. The Ni9Cr coating partially disintegrated from the Zre1Nb alloy, while other samples were preserved their integrity. The effect of hydrogenation and high-temperature oxidation on the Zre1Nb samples will be described and discussed in detail in the next sections.
Table 2 Energetic characteristics of coating deposition. Sample Ni9Cr NiCr NiCr9 Cr
Fin, W/m2
Fout, W/m2
Tsub, K
Ea, eV/atom
2419.7 1415.4 696.7 731.2
92.5 62.9 53.9 51.4
689 530 391 407
27.5 16.5 9.6 10.5
equation was used [25]:
Ea =
F∙S∙t , Natom
(4)
where F – the total energy flux to the rotating substrate [W/m ]; Natom – the number of the deposited atoms [atom]. For two-component films (e.g., NieCr), the elemental composition should be taken into account: 2
Natom =
mdep , (mNi ∙r + mCr ∙ (1 − r ))
(5)
where mdep – mass of the deposited atoms; mNi – mass of one Ni atom; mCr – mass of one Cr atom; r – the ratio of Ni to Cr in the coating [%]. The results of Ea calculations are shown in Table 2. 2.4. Gas-phase hydrogenation Gas-phase hydrogenation was performed to evaluate hydrogen uptake kinetics of the Zre1Nb alloy. The measurements were carried out using an automated complex Gas Reaction Controller (Advanced Materials Corporation, USA) at the constant temperature of 633 K (normal operation temperature of fuel cladding). Hydrogen (99.9995% purity) was produced by pyrolysis method using a hydrogen generator HyGen 200 (USA). Initially the samples were placed horizontally in the vacuum chamber and heated to 633 K (the heating rate was 6 K/min). Then, the chamber was filled with hydrogen and the samples were maintained during 60 min at the constant pressure of 2 atm. After hydrogenation, the samples were cooled in vacuum (the cooling rate ~2 K/min). Hydrogen concentration was measured in situ during hydrogenation process and additionally measured as weight gain immediately after hydrogenation.
3.2. Resistance to hydrogen uptake Fig. 4a shows the hydrogen sorption kinetics of the Zre1Nb alloy under gas-phase hydrogenation at 633 K. The deposition of the coating with high nickel content (Ni9Cr) leads to increase the rate of hydrogen absorption by the alloy. The absorbed hydrogen concentration (nH) of this sample is higher (~0.3 wt%) than for the samples with the NiCr, NiCr9 and Cr coatings (lower than 0.03 wt%) or the uncoated sample (0.09 wt%). It is known that permeability of hydrogen through pure nickel is high and nickel oxides are easily reduced by hydrogen [29–31]. It is assumed that the chromium content in the Ni9Cr coating is not enough to form a protective chromium oxide layer on the surface; therefore, the coating does not demonstrate the protective properties against hydrogen uptake. The measurements of weight gain after the hydrogenation test are in good correlation with absorption kinetics (Fig. 4b). The maximal weight gain of 2.3 mg/cm2 is observed for the sample with the Ni9Cr coating, while other coated samples have lower 0.1 mg/cm2 (Fig. 4b). Fig. 5 shows the X-ray diffraction patterns of the samples before and after hydrogenation. The reflections of hexagonal closed packed (hcp) α-Zr phase correspond to the Zre1Nb substrate. The as-deposited coatings have face-centered cubic (fcc) γ-Ni and/or body-centered (bcc) α-Cr phases depending on the coating composition. The lattice parameter a = 3.5369 Å of γ-Ni phase is significantly higher than that for pure nickel (3.5240 Å), which indicates the formation of fcc Ni(Cr) solid solution phase in the Ni9Cr coating (Fig. 5a). The phases of γ-Ni and αCr exist in the NiCr coating which causes a strong broadening of the coating reflections (Fig. 5b). According to NieCr phase diagram, the formation of these phases is expected in the NiCr and NiCr9 coatings [32]. However, these phases are difficult to separate due to superposition of its reflections. Therefore, the NiCr9 coating primarily composes of α-Cr phase, and small amount of fcc Ni phase can be
2.5. High-temperature oxidation For high-temperature oxidation two temperatures (1173 and 1373 K) were chosen. A high-temperature atmospheric furnace (ATS 3210, Applied Test Systems Inc.) was used. The samples were fixed in the furnace chamber by tungsten wire and heated from room temperature (the heating rate was ~30 K/min). Then, the samples were exposed during 20 min at 1173 or 1373 K. After this, the chamber was opened, and the samples were naturally cooled to room temperature. 3. Results and discussion 3.1. The as-deposited coatings The elemental composition of the as-deposited NieCr coatings is shown in Table 1. The nickel to chromium ratio (in at.%) in the Ni9Cr, NiCr, NiCr9 and Cr coatings is 83/17, 55/45, 17/83 and 0/100, respectively. The cross-section SEM images of the as-deposited coatings on Si substrates are shown in Fig. 2. The coating thickness is (2.2 ± 0.1) μm. Fig. 2 represents the microstructure of the nickel-chromium coatings of different compositions. In the case of co-deposition, film structure depends on process conditions (energy flux delivered to the substrate) and composition ratio. On the one side, when the NieCr coatings with high content of Cr or Ni are depositing, minority component of the 71
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Fig. 2. The cross-section SEM images of the as-deposited coatings on Si substrate: a – Ni9Cr; b – NiCr; c – NiCr9; d – Cr.
expected. Only α-Cr phase with a = 2.9100 Å exists in the Cr coating (Fig. 5d). After hydrogenation, cubic (δ-ZrH) and tetragonal (γ-ZrH) zirconium hydrides are observed in the sample with the Ni9Cr coating due to high concentration of hydrogen absorbed by this sample (see Fig. 4). On the contrary, the phase composition of the samples coated by the NiCr, NiCr9 and Cr films is unchanged after the same test. This result also indicates better protective properties of the chromium-rich coatings against hydrogen permeation into the zirconium alloy. Due to high hydrogen permeability and corrosion resistance, pure nickel is usually used to improve hydrogen sorption behavior of Zr or Ti alloys and to increase their oxidation resistance [33–35]. Therefore, the NieCr coatings containing pure fcc Ni phase cannot protect the alloy from hydrogen uptake. The result of such exposure is a hydrogen embrittlement of zirconium and its destruction as shown in Fig. 3. For the coatings with lower nickel content (55 at.% and less), where α–Cr phase was formed, the hydrogen absorption by the Zre1Nb alloy at 633 K is negligible. These coatings have low hydrogen permeability and can be used as barrier coatings against hydrogen permeation into the zirconium alloy.
the lowest weight gain compared to other. Fig. 7 presents X-ray diffraction patterns of the samples before and after oxidation at 1173 and 1373 K. The Cr2O3 phase is observed in all samples, while the NiO phase is found only in the sample with the Ni9Cr coating. This is caused by formation of nickel-zirconium (NiZr) phase (Fig. 7b) due to diffusion of nickel into zirconium. Such result is also confirmed by GDOES and EDS measurements (Figs. 8–9). The formation of the intermetallic NiZr phase was observed in the samples with the NiCr and Ni9Cr coatings at 1173 K. At this temperature, the formed Cr2O3 layer on the surface prevented intensive oxidation of the sample. Therefore, nickel migration into the zirconium alloy (see Fig. 8e) with the formation of the NiZr phase was found. At the same time, no intermetallic NiZr compound was observed after oxidation at 1373 K. The relative concentration of nickel at the coating/ alloy interface is significantly lower at higher temperature (Fig. 8e and f), which indicates accelerated nickel diffusion into the zirconium alloy. The analysis of the literature data showed that ultrafast nickel diffusion occurs even in α–Zr phase (diffusion coefficient D = 5 × 10−11 m2/s) and increases with the phase transition αZr → βZr (1136 K) and further temperature rise [36,37]. Thus, the thermal stimulated diffusion of nickel at 1373 K contributes to Ni dissolution in Zr lattice without formation of the intermetallic phase. The formation of NiO occurs only in the coating with low chromium content (Ni9Cr), which is also confirmed by GDOES (Fig. 8b and c). The formation of stable NiO limits the migration of nickel from coating to the substrate, but does not protect the zirconium alloy from high-temperature oxidation. It has been shown that NiO demonstrates weak oxidation resistance at temperatures higher 1073 K [38]. This is the most obvious to the sample with the Ni9Cr coating, where ZrO2 and Zr3O peaks are observed even at 1173 K. In other samples, oxidized hexagonal zirconium (Zr3O) phase is predominantly formed at 1173 K, while tetragonal and monoclinic ZrO2 phases are detected at higher
3.3. Resistance to high-temperature oxidation The samples were oxidized in atmosphere at two temperatures (1173 and 1373 K). The oxidation at 1173 K was studied only by XRD and GDOES. Other measurements (weight gain and SEM) were additionally performed to analyze the samples oxidized at 1373 K. Fig. 6 shows the weight gain (Δm) of the samples after high-temperature oxidation at 1373 K. The uncoated Zre1Nb has the highest value of Δm (46.5 mg/cm2). The weight gain of the coated samples is lower and depends on the coating composition (5.2–35.9 mg/cm2). The best oxidation resistance is observed for the Cr-coated sample having 72
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Zr1–Nb samples with nickel-chromium coatings Ni9Cr NiCr NiCr9 Cr with as-deposited coatings
uncoated Zr–1Nb
after hydrogen test
after high-temperature oxidation at 1173 K
after high-temperature oxidation at 1373 K
Fig. 3. The appearance of the samples after coating deposition and different tests.
completely protect the alloy from high-temperature oxidation since the ZrO2 (˂6 vol%) and Zr3O (11.4 vol%) phases were found. Probably, this is related to the columnar microstructure of the as-deposited Cr coating, which may result oxygen diffusion along the grain boundaries. Thus, the deposition of coatings with dense homogenous microstructure and high Cr content is expected approach to improve oxidation resistance of Zr alloys. Based on XRD and weight gain measurements, the Ni9Cr-coated sample is demonstrated poor oxidation resistance and disintegrated after oxidation test (see Fig. 3). Therefore, cross-section SEM observations and elemental composition measurements were performed only to the oxidized samples with NiCr, NiCr9 and Cr coatings (Fig. 9). From the SEM images, it can be seen that the oxidation of the samples is uniform and the thickness of oxide layer decreases from 86 μm for the
temperature [39]. The quantitative analysis of phase composition (in vol%) of the oxidized samples is shown in Table 3. The volume fraction of the Cr2O3 phase in the oxidized samples is increased with Cr content in the as-deposited NieCr coatings. It is supposed that thicker Cr2O3 layers are formed on the sample surface that results in less oxidation of the zirconium alloy. According to [40], Cr2O3 films can be resistant to oxidation up to 1473 K. Moreover, Ni diffusion from the NieCr coatings to the alloy can adversely affect the corrosion resistance due to formation of structural defects that contribute to fast diffusion of oxygen through the coating. These results are in agreement with the total content of zirconium oxide (∑ZrOx) phases in the samples which is decreased from 54.7 to 17.3 vol%. The pure Cr coating is not fully oxidized since the α–Cr phase (42.8 vol%) exists in the Cr-coated sample after oxidation test. Nevertheless, this coating does not
Fig. 4. The hydrogen sorption kinetics (a) and weight gain (b) at 633 K for the Zre1Nb samples. 73
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Fig. 5. XRD patterns of the Zre1Nb samples with as-deposited coatings (black curves) and after hydrogenation (green curves): a – Ni9Cr; b – NiCr; c – NiCr9; d – Cr. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
plastic deformation resistance (H3/E2) of this coating is lower than for the NieCr coatings. From the data of H/E and H3/E2 ratios, the asdeposited Cr coating is more brittle [41,42]. The addition of nickel to chromium results in both decrease of hardness and elastic modulus. Nevertheless, H/E and H3/E2 ratios for the nickel-chromium coatings are higher that indicates the improvement of cracking resistance of NieCr coatings compared to the pure Cr film. However, addition of Ni to the coatings and higher Ea for the Ni9Cr and NiCr coatings (see Table 2) are not enough to significantly improve cracking resistance of the as-deposited coatings (H/E < 0.1). Thus, it is supposed that the average energy deposited per atom should be higher to increase H/E ratio or deposition of highly elastic over-layer on the brittle Cr coating should be considered [22]. Hydrogenation slightly affects the mechanical properties of all the deposited coatings. However, significant changes in coating characteristics were observed after high-temperature oxidation. All mechanical parameters (H, E, H/E and H3/E2 ratios) were strongly dropped that can be caused by coating oxidation when a porous microstructure is usually formed [43,44]. The degradation of mechanical properties after oxidation is more pronounced for the NieCr coatings compared to the Cr film. The H/E ratio is dropped from 0.058 to 0.031 for the NiCr9 coating, while it is decreased from 0.053 to 0.048 for the Cr coating. Such behavior can indicate denser microstructure of the forming chromia layer and better mechanical properties of the Cr coating after the oxidation test. Thus, together with the above-mentioned results (in Sec. 3.2 and 3.3), the pure Cr coating seems to be better candidate to protect Zr alloys from oxidation at LOCA conditions. However, to obtain highly resistant Cr coating on zirconium alloys, the cracking resistance of the coating should be improved.
Fig. 6. The weight gain of the samples after high-temperature oxidation at 1373 K.
NiCr-coated sample to 8 μm for the Cr-coated sample. 3.4. Mechanical properties A complex manufacturing process of fuel claddings as well as strict requirements for operation and utilization of claddings and their geometry additionally require high cracking resistance and adhesion of the protective coatings. The coatings must be resistant to external mechanical damage, cracking resistant during irradiation growth of zirconium alloy as well as resistant to fretting wear in the coolant flow. Therefore, mechanical properties of the developing coatings are one of the most important parameters determining the possibility of their application as protective coating for fuel claddings. The mechanical properties of the NieCr coatings were measured by nanoindentation (Figs. 10, 11). The as-deposited Cr coating has higher hardness (H) and elastic modulus (E) compared to other coatings. However, the resistance against elastic strain to failure (H/E) and
3.5. Prospects of coating technology for nuclear fuel claddings The high-temperature oxidation resistance of Zr alloys can be 74
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Fig. 7. X-ray diffraction patterns of the coated Zre1Nb before and after high-temperature oxidation: a – Ni9Cr; b – NiCr; c – NiCr9; d – Cr.
on nuclear fuel claddings. On the one hand, fuel claddings have large dimensions (4–6 m in length and low cross-section ˂1 cm2) [47]. On the other hand, light-water reactors are the most widespread reactors, and their quantity increases every year [1]. Therefore, the coating technology should have very high productivity in combination with opportunity to form high cracking and oxidation resistant coatings. This task is complex and can require new approaches to realize. The most obvious solution is deposition of metal or intermetallic compound by using multi-cathode sputtering systems. Such systems are usually consisted of several cathodes from equal or different target material [48,49], where the deposition rate is proportionally increasing with the quantity of cathodes. The choice of metal or intermetallic compound with high oxidation resistance as the coating for ATF is
enhanced by coating deposition. This approach is advantageous due to the possibility of integration of coating technology in the claddings manufacturing process. To develop the coating technology, a large number of deposition techniques are considering [3,6–9,11,45]. Magnetron sputtering is the most perspective due to the following reasons: no droplet fraction in deposition particle flow, minimum of undesirable impurities, high control of process parameters and their stability, high coating uniformity even for large length magnetron (e.g. several meters), great experience in magnetron sputtering techniques to deposit coatings on large-scale substrates, etc. [10]. However, the most sensitive question for industrial coating technology is productivity (coating deposition rate) that is weakness of magnetron sputtering [10,46]. This parameter becomes more significant in the case of coating deposition
Fig. 8. GDOES profiles of elemental distribution in the samples with Ni9Cr and NiCr coatings (a, d – as-deposited, b, e – oxidized at 1173 K; c, f – oxidized at 1373 K). 75
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Fig. 9. Cross-section SEM images and corresponding EDS elemental distribution profiles of the samples oxidized at 1373 K: a – NiCr; b – NiCr9; c – Cr.
horizontal position for both sputtering system and large-scale substrates. This is very complex in view of the fuel cladding geometry. Thus, the sputtering systems with hot target seem to be more appropriate even they have lower productivity relative to the magnetron with molten target. Moreover, the high-powered energy flux to substrate is special feature of these systems. The additional energy flux is formed due to target radiation and evaporated (sublimated) particles reached the substrate [21,28]. It can strongly influence the microstructure and properties of the deposited coatings. Therefore, additional studies of process parameters and coating properties need to perform for magnetrons with hot target. The great interest has the comparison of these characteristics for multi-cathode system and hot target magnetron. There are should be investigated deposition rates, efficiency of deposition process (the cost of coating deposition on a single substrate), coating adhesion and microstructure, resistance to high-temperature oxidation, etc. Such study will be performed by our scientific group in near future. Apart from metal or intermetallic films, ceramic coatings have been strongly considered [3,45,53,54]. At the first sight, these materials have some advantages such as higher oxidation resistance and hardness, lower wear resistance compared to metal coatings. The ceramic coatings can be effective to prevent zirconium oxidation even at lower coating thickness. However, several weaknesses are challenged to their application. Firstly, brittleness of ceramic coatings can result in coating cracking. The second point is lower deposition rate compared to metals and process stability for deposition of stoichiometric ceramic coatings. Lately, some new approaches, based on high power pulsed magnetron sputtering [55,56] can be applied to stabilize reactive process and increase their productivity. However, the additional studies of using this
Table 3 Phase composition (in vol%) of the samples after high-temperature oxidation at 1373 K. Phase
Cr2O3 NiO ZrO2 (m) ZrO2 (t) Ni ZrN CrN Cr2N Cr Zr3O ∑ZrOx
Coatings Ni9Cr
NiCr
NiCr9
17.0 7.5 45.8 8.9 15.4 5.4
42.3 – 46.5 3.2 – 2.8 4.0 1.2 – – 49.7
64.5 – 27.8 1.9 – 2.0 – 3.8 – – 29.7
– – – 54.7
Cr 30.9 – 4.8 1.1 – 9.0 – – 42.8 11.4 17.3
caused by higher deposition rate in comparison with other materials (oxides, nitrides, etc.). It partially hides the low deposition rate of magnetron sputtering. Such approach is confirmed by the recent studies [9,50], including material strategies of industrial nuclear companies and institutions (AREVA NP, Korea Atomic Energy Research Institute, etc.). Other implementation of the coating technology can be based on magnetron sputtering systems with hot or molten targets [46,51,52], where the deposition rate is higher in several times in comparison with the typical sputtering systems with cooled target. In such systems, the target material is evaporated or sublimated additionally to sputtering. The application of the magnetron system with molten target means a 76
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Fig. 10. Hardness (a) and elastic modulus (b) of NieCr coatings deposited on Zre1Nb substrates: (+H) – after hydrogenation; (+T) – after oxidation at 1373 K.
Fig. 11. H/E (a) and H3/E2 (b) ratios of nickel-chromium coatings deposited on Zre1Nb substrates: (+H) – after hydrogenation; (+T) – after oxidation at 1373 K.
as well as accident conditions. However, to develop industrial technology for nuclear fuel cladding tubes, the deposition techniques and type of coating should be attentive considered in view of deposition rate, resistance of the coating to cracking and oxidation. Thus, new approaches for coating deposition can be desired.
technique to the coating deposition for nuclear materials are certainly needed. 4. Conclusions The nickel–chromium coatings with different composition ratio were deposited by magnetron sputtering to increase the resistance of Zre1Nb alloy to high-temperature oxidation and hydrogen uptake. The main conclusions were listed hereafter.
Funding This study was supported by Russian Science Foundation (project №15-19-00026).
1. The NieCr coating with high Ni content (> 83 at.%) causes permeation of hydrogen into zirconium alloy at 633 K and has low high-temperature oxidation resistance even at 1173 K. The increase of Cr content in the as-deposited NieCr coatings improves their protective properties. Higher oxidation resistance is attributed to the formation of Cr2O3 layer which is less permeable to oxygen in comparison with NiO at high temperatures. 2. The diffusion of nickel into Zr alloy occurs during high-temperature oxidation of the nickel-chromium coatings with Ni content of 55 and 17 at.%. This leads to the formation of intermetallic NiZr phase after oxidation at 1173 K. Only solid solution of nickel in zirconium alloy was observed after oxidation at 1373 K due to the fast diffusion of nickel in bcc Zr lattice. 3. The pure Cr film demonstrates the highest oxidation resistance compared to the NieCr coatings. The weight gain of the Cr-coated zirconium alloy after oxidation at 1373 K for 20 min was 5.2 mg/ cm2 that corresponds to 8 μm-thick oxide layer on the sample surface. 4. The as-deposited NieCr coatings exhibit better mechanical properties than the pure Cr coating. The hardness of the NieCr coatings increases from 8.4 to 12.1 GPa with Cr content, but the as-deposited Cr coating is more brittle. Nevertheless, the degradation of mechanical properties is less pronounced for the pure Cr film after high-temperature oxidation. 5. Magnetron sputtering of chromium coatings can be used as the approach to protect zirconium alloys under normal reactor operation
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