Multifunctional nanoceramic coatings for future generation nuclear systems

Fusion Engineering and Design 146 (2019) 1628–1632

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Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes

Multifunctional nanoceramic coatings for future generation nuclear systems a

D. Iadicicco , M. Vanazzi a b c

a,b

, F. García Ferré

a,1

, B. Paladino

a,b

c

c

, S. Bassini , M. Utili , F. Di Fonzo

a,⁎

T

Center for Nanoscience and Technology @Polimi, Istituto Italiano di Tecnologia, Via Giovanni Pascoli 70/3, 20133, Milano, Italy Dipartimento di Energia, Politecnico di Milano, Via Ponzio 34/3, 20133, Milano, Italy ENEA-FSN-ING Division, C.R. Brasimone, 40032, Camugnano (BO), Italy

A R T I C LE I N FO

A B S T R A C T

Keywords: Alumina Pulsed laser deposition Oxide coatings Tritium permeation barriers Breeding blanket module Nuclear fusion reactor

Several breeding blanket concepts for the DEMO reactor employ the eutectic Pb–16Li as breeder material, namely Helium Cooled Lithium Lead (HCLL), Water Cooled Lithium Lead (WCLL) and Dual Coolant Lithium Lead (DCLL). These three concepts share, with different incidences, three major technological challenges: Tritium containment, steel corrosion and magnetohydrodynamic drag. Here, we describe the ongoing work on multifunctional Al2O3 nanoceramic coatings grown by Pulsed Laser Deposition (PLD) on Reduction-Activation Ferritic-Martensitic (RAFM) steel. In fact, PLD can produce relatively thick (up to tens of μm) high performance coatings with metal-like mechanical properties. The coatings were tested as Tritium permeation barriers with Hydrogen at different temperature (from 350 to 650 °C). Results collected in this way indicate an excellent behavior, with a permeation reduction factor (PRF) close to 105. In addition, it was shown that they are able to maintain similar properties even when Deuterium was employed and also under 2 MeVelectrons irradiation. [1] Moreover, the electrical conductivity of these dielectric coatings was shown to be extremely low even under irradiation. Finally, to evaluate the chemical compatibility of Al2O3 films in liquid eutectic Pb-16Li, PLD deposited samples have been exposed to static corrosion tests up to 8000 h. No corrosive attacks of the steel substrate are detected. To conclude, Alumina coatings deposited by PLD show optimal characteristics in order to tackle the major technological challenges associated to the Breeding Blanket (BB) concepts employing Pb-16Li as breeder materials.

1. Introduction Uncontrolled Tritium permeation as well as the severe corrosive attack due to heavy liquid metal interaction are the main issues related to the development of Breeding Blanket modules (BBs) for future nuclear fusion power plants [1–12]. In particular, the challenge is to reduce radiological hazards as much as possible, while optimizing Tritium balance in the plants. Further issues are related to the eutectic Pb16Li alloy selected as neutron multiplier and Tritium breeder material. In fact, the structural RAFM steel was shown to be heavily corroded when submerged in Pb-16Li, with a corrosion rate close to 400 μm/yr at 550 °C considering a flow rate of 22 cm/s [5–7]. Starting from the 70′, several works have been reporting on these topics, with the aim to select and qualify the most promising materials [13–25]. Only recently, protective coatings were acknowledged to be essential to enhance the performance limits and overcome the technological issues related to BBs, given the difficulty of producing a bulk/structural material having proper characteristics. In particular, thanks to its chemical inertia and

the demonstrated antipermeation properties, the fusion community relies on alumina-based (Al2O3) coatings as the most promising material available. In Table 1 are listed the necessary technological requirements that the candidate materials should satisfy [26,28]. A large number of method are proposed to deposit Al2O3 and/or Al2O3-forming surface alloys layers. The most studied techniques include [26–30]:

• Chemical Vapor Deposition (CVD). • Hot Isostatic Processing (HIP, including Aluminum alloys and FeCrAl alloys). • Hot-Dip Aluminizing (HDA). • Spray techniques (Including vacuum plasma spray, Detonation Jet and VPS). • Sol Gel. • Electrochemical processing. Disadvantages related to these techniques, such as high processing

⁎

Corresponding author. E-mail address: [email protected] (F. Di Fonzo). 1 Current affiliation: ABB Corporate Research, Segelhofstrasse 1 K, 5400 Baden-Deattwil, Switzerland. https://doi.org/10.1016/j.fusengdes.2019.03.004 Received 8 October 2018; Received in revised form 11 February 2019; Accepted 4 March 2019 Available online 14 March 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.

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Eurofer 97 sample (either bare or coated), kept at a given temperature (up to 650 °C) by using PID controlled resistive heaters. During the measurements, the low-pressure section was held at about 10−5 Pa. The high-pressure section was held in the same range of pressure until pure Hydrogen gas (99.9999%) was injected, reaching a pressure of 100 mBar. At this point, Hydrogen permeates through the sample from the high-pressure section to the low-pressure section. Here, a Quadrupole Mass Spectrometer (QMS) registers the corresponding ion current, from which the hydrogen flux is derived.

Table 1 General requirements for BBs coatings. General requirement for BBs High level dose irradiation tolerance Thermal expansion match with the substrate Environmental-friendly and safety-oriented characteristics Materials availability with adequate costs Deposition process compatible with substrate materials (temperature and time) Potential to coat large, complex geometries. Tritium and corrosion properties PRF > 103 (compared to bare Eurofer 97) at the end of life. Compatibility with high temperature Pb-16Li. MHD requirements Insulating properties with an electrical resistivity > 106 Ωcm.

• • • • • • • • •

2.3. Corrosion tests A schematic sketch of the experimental set-up for corrosion tests in stagnant Pb-16Li is reported in reference [1,40]. The tests were carried out inside thermally-insulated stainless-steel capsules (AISI 304, H =480 mm and dint = 134.5 mm) designed for the exposure of samples to stagnant liquid metal alloys. The capsules house alumina crucibles (H =220 mm and dint = 110 mm) acting as liquid Pb-16Li containers to avoid contact between the liquid metal and the steel walls. Heater elements are wrapped around on the outer surface of the capsules to reach the desired Pb-16Li temperature for the test. The flange lid of the capsules is equipped with holes and fittings for the insertion of the components for the test: specimen-holder bars made in W, TC for the monitoring of the Pb-16Li temperature, a connection for the vacuum system (pump) and a connection for the Pb-16Li ingress from the melting tank. For the fittings devoted to TC and specimen-holders bar, ferrules in PTFE were used to provide gas tightness, so that it was possible to remove components before the solidification of the Pb-16Li after the test conserving gas tightness and preventing air contamination in the capsules. The aim of the tests was to provide a simple yet straightforward comparison of the performance of uncoated and coated samples. For this reason, an accurate assessment of the chemical conditions of the liquid melt was not performed systematically. The eutectic Pb-16Li used for the experiment has a melting point of 240 ± 1 °C, determined by DTA (Differential Thermal Analysis). The Pb-16Li (≈ 12 Kg, ≈ 1.3 L) was loaded into the capsules from the molten state using a melting tank operating in slight argon over-pressure (99.9999%, 0.1 ppm mol O2 and 0.5 ppm mol H2O). The outlet section of the melting tank was then connected to the flange lid of the capsules for the transfer. A filter placed in the outlet section of the melting tank allowed to catch oxides or slag contained in the Pb-16Li ingot pieces, preventing their ingress into the capsules. During the melting in the melting tank, the capsules, already containing the specimen in a high position, were heated at first 120 °C and then 180 °C for 2 days and during that atmospheric air was removed by vacuum. The capsules were then heated at higher temperature and then the liquid Pb-16Li inside the melting tank transferred in the capsules under a slight argon overpressure. Finally, coated and bare samples were dipped in the liquid Pb-16Li at about 300 °C and then the Pb-16Li heated up to 550 °C. A plenum of pure Argon at 1.5 105 Pa was used to prevent air ingress during the duration of the tests (2000 h for bare and 8000 h per coated samples) with the above-described procedure, tests might have been carried out, at least in principle, far from oxygen saturation of the melt. In practice, since Li is very prone to oxidation even with very low amount of oxygen contamination, it was likely to occur that oxygen saturation was established during the test. As a consequence, the expected testing conditions likely follow the thermodynamic stability line of formation of Li2O in the Pb-16Li melt, according to the Ellingham’s diagram in Fig. 1 [41]. Here the stability line of Li2O in the Ellingham’s diagram is depicted considering the case of pure Li melt saturated with oxygen (activity of Li and O equal to 1, full line) and the case of Pb-16Li melt saturated with oxygen (aLi < 1 and aO = 1, dashed line). It is also depicted the stability of Al2O3 as pure compound (aAl,O = 1, full line) and the stability of LiAlO2 compound, which is likely to form from the interaction of Al2O3 with Li, in pure Li melt (aLi = 1, full line) and in

temperatures, low adhesive strength, low deposition rates and poor mechanical properties, affected the effectiveness of these materials to achieve the fundamental properties listed in Table 1. In this work, we summarize the unprecedented performance of pulsed lasered deposited nanostructured Al2O3 as protective coatings. The coatings produced by PLD at room temperature show a fully dense and compact morphology [31]. In addition, they possess an unusual combination of antipermeation behavior under electron irradiation [1], metal-like mechanical properties [32], superior radiation tolerance up to 450 dpa [33–35], corrosion resistance in high temperature heavy liquid metal environment [1,34,36], making these coatings particularly interesting for advanced nuclear systems. Thus, the results obtained show that dense and compact nanostructured PLD grown Al2O3 coatings are able to tackle the main issues (from corrosion in heavy liquid metals to very high dose irradiation damage) that will affect the future advanced nuclear power systems. 2. Experimental 2.1. Samples Samples used in this work were Eurofer 97 disks with the diameter of 52 mm (permeation tests) and rectangular plates with the length, the width and the thickness respectively of 40 mm, 8 mm and 3 mm (corrosion tests). The nominal composition of the steel is resumed in Table 2. Prior to the deposition of the Al2O3 protective layer, substrates are grinded with SiC paper (up to P4000) and polished with diamond paste with the size of 1 μm. Finally, samples are rinsed and sonicated with deionized water. The Al2O3 coating was grown in a custom made PLD (manufactured by Kenosistec s.r.l) in a stainless-steel vacuum chamber at low oxygen pressure (0.1 Pa) on just one face of the sample, at room temperature. The energy of the pulse used was fixed at 410 mJ corresponding to a fluence of 3.5 J/cm2 with a repetition rate of 20 Hz. The selected thickness of the coating was 5 μm. The laser used is a Coherent® COMPex 205 F. More details about PLD system are described in the supporting information, Fig. S1, of Passoni et al. [37]. 2.2. Hydrogen permeation test Hydrogen barrier performances were tested using the experimental facility “PERI II” described more in detail elsewhere [38,39]. In particular, the Continuous Flow Method (CFM) was used to evaluate the permeation rate. The facility comprises two different sections (namely low-pressure section and high-pressure sections), separated by an Table 2 Composition of the Eurofer 97 steel substrate in wt%. Cr

W

V

C

Mn

Si

Ni

Fe

8.95

1.08

0.2

0.11

0.048

0.04

0.021

Bal.

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Fig. 3. Spectrum signal obtained during the hydrogen permeation tests (performed at 650 °C) measured by means QMS in the CFM mode. The permeation rate of hydrogen is defined in terms of ion current (A). Fig. 1. Ellingham diagram showing the stability of Li2O, Al2O3 and LiAlO2 compounds in Pb-16Li melt. [41].

Pb-16Li (aLi < 1, dashed line). At the end of the tests, the Pb-16Li in the capsules was cooled down together with the dipped specimen at 1 °C/ min. At about 300 °C, the specimen-holder bars were left up from the melted eutectic to be further cooled down in the in-argon gas plenum. 3. Results and discussion Fig. 2d shows a Scanning Electron Microscopy (SEM) micrograph of the Al2O3 deposited by PLD. The microstructure is fully dense and compact, an essential requirement for high performance protective coatings. The nanostructure of the coatings was investigated by TEM (Fig. 2a–c), and complies a small fraction of crystalline Al2O3 nanodomains randomly dispersed in an amorphous Al2O3 matrix. In addition, room temperature PLD yields Al2O3 layers with metal-like mechanical properties and ceramic hardness (E = 195 ± 9 GPa, ν = 0.29 ± 0.02 and H = 10 ± 1). The combination of these properties results in a high ratio hardness to elastic modulus (H/E≈0.049), that within the shear modulus (G≈75.5 ± 4.3 GPa) are indicators of good wear resistance for a coating in a sliding wear condition [32,36]. The performances as Tritium permeation barriers of the protective PLD-coating were estimated comparing the permeation flux of the bare disk and the coated disk. Hydrogen permeation rates were measured in CFM mode by means of a QMS at different temperature (350 °C, 450 °C, 550 °C and 650 °C). Fig. 3 shows the typical QMS spectrum (650 °C and 100 mBar of H2 partial pressure) in term of ion current (A). Looking more in detail the QMS spectrum, starting at 0 cycles up to the end of the measure, it is clear how the behavior of both bare and coated samples is almost the same. In fact, after the background measurement (at least 500 cycles), pure Hydrogen is injected at 100 mBar, and a new steady state is reached in a

Fig. 4. Comparison of hydrogen flux of bare and coated samples for different temperatures. The curves show clearly the best performance for the coated sample within an effective hydrogen permeation reduction. Table 3 PRF resuming values. 350 °C

450 °C

550 °C

650 °C

554.3 ± 50

3789.6 ± 280.58

12629.2 ± 1035.94

85369.8 ± 2713.55

short time. Overall, the measurements clearly show a lower ion current for the coated samples with respect to the bare ones, indicating a lower Hydrogen permeation, as underlined in Fig. 4. It is worth highlighting that the permeation of increase for increasing temperatures, while it is more restrained for the coated sample. Considering as the principal figure of merit (as engineering performance factor) the Permeation Reduction Factor (PRF) defined by the Eq. (1), we obtain an unprecedented PRF value close to 105 at 650 °C

Fig. 2. A 5 μm thick alumina layer is grown by PLD at room temperature. a–c) Bright-field TEM image revealing that the microstructure of the film is composed by a homogeneous dispersion of Nano-crystalline Al2O3 domains (dark dots) in amorphous Al2O3 matrix (light contrast). d) An SEM cross sectional micrograph underline the full density and compactness of the coating. 1630

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Fig. 5. a,b) Bare sample exposed for 2000 h at 550 °C to corrosion environment. Bare sample suffer dissolution-corrosion due to grain boundary attack. c,d) Coated sample does not suffer of dissolution corrosion even after 8000 h exposition at 550 °C. Fig. 6. a) Cross sectional bare sample exposed for 2000 h hours at 550 °C in Pb-16Li. The steel surface is very rough and clearly suffered of dissolution-corrosion, as typical for Ferriticmartensitic alloy exposed in such environment. b) Instead, coating after 8000 h at 550 °C in Pb16Li exposure is still adherent and compact, protecting the steel below.

PRF =

JBare JCoated

Al2O3 would interact with Li n the Pb-16Li alloy, being Li a strong reducing agent. This could lead the formation of lithium aluminate layer that seems to be stable in liquid Pb-16Li as reported in Ellingham’s diagram in Fig. 1 also acting as protective layer.

(1)

In Table 3 are summarized the PRF values obtained in this test for all temperatures. These results underline the excellent barrier behavior of the PLDgrown Al2O3 coatings in their pristine condition, indeed, this property critically depends on the integrity of the coating under irradiation and its compatibility with the eutectic Pb-16Li. Indeed, we demonstrated [42] that irradiation does not affect the impermeability of the coating to H2. Here we assess its long-term compatibility to static Pb-16Li. Coated and bare Eurofer 97 plates at 550 °C in pure Ar atmosphere for 8000 h and 2000 h respectively. SEM micrographs in Fig. 5a, b and 6 a show the uncoated plate after 2000 h Pb-16Li exposure. This is visibly corroded, with a severe grain boundaries attack, in a typical dissolution-corrosion interaction [43]. On the other hand, coated samples were protected as shown in SEM micrographs in Figs. 5c, d and 6 b. In particular, pristine surface of coated samples is smooth, dense and free of defects. At the same time, after 8000 h Pb-16Li exposure, the coated surface is still smooth, dense and without cracks, delamination phenomena or thickness reduction (Fig. 6b). As reported in previous works [1,32–36], in these experimental conditions, it would be expected that

4. Conclusions Several previous works are carried out demonstrating the protective behavior of alumina-based coating. In particular recent works underline the superior protective properties of PLD-grown alumina in harsh environments such as generation IV and nuclear fusion reactors [1,32–36,40,42,44]. The main features of PLD-grown alumina are highlighted as follow:

• Metal-like mechanical properties [32] • Superior corrosion resistance, even after high dose ion irradiation [34,36] • Extreme dose ions irradiations endurance [33–37] • Electrical insulating properties [43] • Hydrogen permeation behavior [1] 1631

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The combination all of these features with the results obtained in this work (high PRF value and long-term corrosion resistance) and considering that all depositions were done at room temperature, ensures that all points listed in Table 1 are satisfied. This result makes PLD-grown Alumina to all effects a promising solution, not only for the future DEMO fusion reactor but also for all energy technologies (nuclear and not) that will be subjected to severe environments, such as heavy liquid metals and/or facing hydrogen isotypes.

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