Process of manufacture a nuclear component with metal substrate by DLI-MOCVD and method against oxidation/hydriding of nuclear component

Abstract

Process for manufacturing a nuclear component comprising i) a support containing a substrate based on a metal (1), the substrate (1) being coated or not coated with an interposed layer (3) positioned between the substrate (1) and at least one protective layer (2) and ii) the protective layer (2) composed of a protective material comprising chromium; the process comprising a step a) of vaporizing a mother solution followed by a step b) of depositing the protective layer (2) onto the support via a process of chemical vapor deposition of an organometallic compound by direct liquid injection (DLI-MOCVD).

Claims

1. A process for manufacturing a nuclear component via a method of chemical vapor deposition of an organometallic compound by direct liquid injection (DLI-MOCVD), the nuclear component chosen from a nuclear fuel cladding, a spacer grid, a guide tube, a plate fuel or an absorber rod, comprising: i) a support containing a substrate based on a metal chosen from zirconium, titanium, vanadium, molybdenum or base alloys thereof, the substrate being coated or not with an interposed layer placed between the substrate and at least one protective layer; ii) said at least one protective layer coating said support and composed of a protective material comprising chromium chosen from a carbide of a chromium alloy, a chromium nitride, a chromium carbonitride, a mixed chromium silicon carbide, a mixed chromium silicon nitride, a mixed chromium silicon carbonitride, or mixtures thereof; and the process comprising the following successive steps: a) vaporizing a mother solution containing a hydrocarbon-based solvent free of oxygen atoms, a bis(arene) precursor comprising chromium; and containing, where appropriate, an additional precursor, a carbon incorporation inhibitor or a mixture thereof; the precursors having a decomposition temperature comprised between 300° C. and 600° C.; and b) in a chemical vapor deposition reactor in which is located said support to be covered and the atmosphere of which is at a deposition temperature comprised between 300° C. and 600° C. and at a deposition pressure comprised between 13 Pa and 7000 Pa; introducing the mother solution vaporized in step a), which brings about the deposition of said at least one protective layer on said support.

2. The process for manufacturing a nuclear component according to claim 1, wherein the nuclear component further comprises a liner placed on the inner surface of said support, which is the surface of said support opposite to the medium that is external to the nuclear component; the liner being deposited, at a deposition temperature comprised between 200° C. and 400° C., onto the inner surface of said support by chemical vapor deposition of an organometallic compound (MOCVD) or DLI-MOCVD with, as precursor(s), a titanium amide and further a precursor comprising silicon, a precursor comprising aluminum and/or a liquid additive comprising nitrogen if the material of which the liner is composed comprises, respectively, silicon, aluminum and/or nitrogen.

3. The process for manufacturing a nuclear component according to claim 1, wherein the process further comprises, after step b): c) performing on said at least one protective layer at least one step chosen from a subsequent treatment step of ionic or gaseous nitridation, ionic or gaseous silicidation, ionic or gaseous carbosilicidation, or ionic or gaseous nitridation followed by ionic or gaseous silicidation or carbosilicidation.

4. The process for manufacturing a nuclear component according to claim 1, wherein the mother solution contains the bis(arene) precursor comprising chromium, a precursor comprising silicon as additional precursor; such that, at a deposition temperature comprised between 450° C. and 500° C., the protective material comprising a mixed chromium silicon carbide is obtained.

5. The process for manufacturing a nuclear component according to claim 1, wherein the mother solution contains the bis(arene) precursor comprising chromium, a precursor comprising silicon as additional precursor, a liquid precursor comprising nitrogen as additional precursor being present in the mother solution or a gaseous precursor comprising nitrogen being present in the chemical vapor deposition reactor; such that, at a deposition temperature comprised between 450° C. and 550° C., the protective material comprising a mixed chromium silicon nitride is obtained in the presence of the inhibitor or such that the protective material comprising a mixed chromium silicon carbonitride is obtained in the absence of the inhibitor.

6. The process for manufacturing a nuclear component according to claim 1, wherein the bis(arene) precursor comprising chromium, a bis(arene) precursor comprising vanadium, a bis(arene) precursor comprising niobium or a bis(arene) precursor comprising the addition element comprise an element M, respectively, chosen from chromium, vanadium, niobium or the addition element; the element M being in oxidation state zero (M.sub.0) so as to have a bis(arene) precursor comprising the element M.sub.0.

7. The process for manufacturing a nuclear component according to claim 1, wherein the additional precursor is chosen from a bis(arene) precursor comprising vanadium, a bis(arene) precursor comprising niobium, a precursor comprising aluminum, a mixture of these additional precursors and a precursor comprising silicon and/or nitrogen.

8. In a method for combating oxidation and/or hydriding in a nuclear component in a humid atmosphere comprising water, or for combating hydriding in a nuclear component in an hydrogenated atmosphere comprising hydrogen, the improvement wherein the nuclear component is chosen from a nuclear fuel cladding, a spacer grid, a guide tube, a plate fuel or an absorber rod, and comprises: i) a support containing a substrate based on a metal chosen from zirconium, titanium, vanadium, molybdenum or base alloys thereof, the substrate being coated or not with an interposed layer placed between the substrate and at least one protective layer; ii) said at least one protective layer coating said support and composed of a protective material comprising chromium chosen from a chromium alloy unless the substrate is zirconium-based, a carbide of a chromium alloy, a chromium nitride, a chromium carbonitride, a mixed chromium silicon carbide, a mixed chromium silicon nitride, a mixed chromium silicon carbonitride, or mixtures thereof.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 represents the mass percentage of various chemical elements as a function of the depth obtained by Glow Discharge Spectrometry (GDS) of a Zircaloy-4 wafer covered with a layer of amorphous chromium carbide via the manufacturing process of the invention, after its exposure to ambient air.

(2) FIGS. 2A (overall view) and 2B (magnification of the oxide substrate) are Transmission Electron Microscopy (TEM) images illustrating a cross section of the wafer whose GDS analysis is shown in FIG. 1.

(3) FIGS. 3A and 3B illustrate, respectively, after exposure to dry air and to humid air, the gain in mass as a function of the temperature for Zircaloy-4 wafers covered with various thicknesses of a protective layer of amorphous CrC and for a bare Zircaloy-4 wafer.

(4) FIGS. 3C and 3D illustrate, respectively, after exposure to dry air and to humid air, the gain in mass at 1200° C. as a function of time for Zircaloy-4 wafers covered with various thicknesses of a protective layer of amorphous CrC and for a bare Zircaloy-4 wafer.

(5) FIGS. 4A and 4B illustrate, respectively, after exposure to dry air and to humid air, the gain in mass as a function of the temperature for molybdenum wafers covered with various thicknesses of a protective layer of amorphous CrC and for a bare molybdenum wafer.

(6) FIGS. 5A and 5B illustrate, respectively, after exposure to dry air and to humid air, the gain in mass as a function of the temperature for Zircaloy-4 wafers covered with various thicknesses of a protective layer of partially metastable chromium and for a bare Zircaloy-4 wafer.

(7) FIGS. 5C and 5D illustrate, respectively, after exposure to dry air and to humid air, the gain in mass at 1200° C. as a function of time for Zircaloy-4 wafers covered with various thicknesses of a protective layer of partially metastable chromium and for a bare Zircaloy-4 wafer.

(8) FIGS. 6A and 6B illustrate, respectively, as a function of the temperature and as a function of the time at 1200° C., the gain in mass for a Zircaloy-4 wafer covered with a protective layer of mixed amorphous chromium silicon carbide Cr.sub.xSi.sub.yC.sub.z and for a bare Zircaloy-4 wafer, in dry air and in humid air.

(9) FIGS. 7A (viewed from one side of the wafer) and 7B (particular focus on one corner of the wafer) are Scanning Electron Microscopy (SEM) images illustrating a cross section of a Zircaloy-4 wafer covered with a protective layer of amorphous CrC deposited via the manufacturing process of the invention, subjected to oxidation at 1100° C. followed by quenching in water. FIG. 7C is an SEM image corresponding to FIG. 7A in which wafers each covered with a protective layer of various thicknesses are grouped (left-hand image: thickness of 9 μm, central image: thickness of 5 to 6 μm, right-hand image: thickness of 2 to 3 μm). FIG. 7D illustrates the GDS analysis of the wafer with a thickness of 9 μm.

(10) FIGS. 8A, 8B and 8C represent SEM images illustrating the microstructure of a monolayer coating (8A, 8B) or multilayer coating (8C) of partially metastable chromium deposited at an increasing rate onto a silicon substrate.

(11) FIGS. 9A, 9B, 9C, and 9D represent schematic views in cross section of some of the geometries of a tubular nuclear fuel cladding obtained by the manufacturing process of the invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

(12) The examples of deposition of a coating onto a substrate which follow are performed in a chemical vapor deposition reactor placed in a tube furnace (model sold by the company Carbolite). The reactor is composed of a silica tube: it is connected to the gaseous effluent inlet and evacuation system via leaktight connections and to the mother solution via an injector (Vapbox model sold by the company Kemstream).

(13) In the examples that follow, the substrate is not coated with an interposed layer. However, these examples may be transposed to the case where the substrate is formed from a substrate coated with an interposed layer.

(14) 1. Coating on a Silicon Substrate

(15) As a preliminary experiment, the manufacturing process of the Invention is transposed to a silicon substrate, a square silicon wafer (side length of 1 cm and 300 μm thick) is placed in the reactor so as to be covered with a protective layer of chromium carbide (CrC), chromium (Cr) or mixed chromium silicon carbide (Cr.sub.xSi.sub.yC.sub.z denoted CrSiC) using the following compounds:

(16) chromium precursor: bis(ethylbenzene)chromium, known as BEBC (Cr (C.sub.6H.sub.5Et).sub.2),

(17) silicon precursor: bis(phenyl)silane (H.sub.2Si (C.sub.6H.sub.5).sub.2),

(18) inhibitor: thiophenol (C.sub.6H.sub.5SH),

(19) solvent: toluene,

(20) carrier gas: N.sub.2 at a flow rate of 500 sccm, namely 500 cm.sup.3/min under standard conditions,

(21) evaporator temperature: 200° C.,

(22) deposition time: 20 minutes.

(23) The other conditions for the deposition of the protective coating constituting the outer coating are indicated in Table 1, in particular the injector open time and frequency, and the temperature and pressure in the chemical vapor deposition reactor.

(24) The injection parameters (open time and frequency) act essentially on the deposition rate and thus on the deposit thickness by varying the deposition time. The open frequency is, for example, from 1 Hz to 10 Hz, typically 10 Hz.

(25) The parameters which act on the physicochemical and structural characteristics of the deposit are essentially the deposition temperature (acting in particular on the structure: amorphous or crystalline; dense or porous; equiaxed or columnar) and the composition of the injected solution.

(26) By convention, the silicon substrate covered with coating number N is referred to in Table 1 as sample N: sample 1 thus denotes the silicon substrate covered with coating 1.

(27) TABLE-US-00001 TABLE 1 Total Injection conditions deposition Deposition Coating Frequency Opening time pressure temperature (N) Injected solution (Hz) (ms) (10.sup.−3 Pa) (° C.) CrC (1) BEBC (3.5 × 10.sup.−1 M) + 10 0.5-5 1.3; 6.7 450; 500; toluene (50 ml) 550 Cr (2) BEBC (3.5 × 10.sup.−1 M) + 10 0.5-5 6.7 400; 450 thiophenol (2 mol %) + toluene (50 ml) CrSiC (3) BEBC (3.5 × 10.sup.−1 M) + 4 0.5 6.7 450; 500 bis(phenyl)silane (15 mol %) + toluene (50 ml)

(28) Table 2 collates the result of the structural analyses of these coatings, and also the result of the heat resistance tests detailed in the following examples (thermal stability: temperature at and above which a crystalline phase appears in XRD in situ under argon). The atomic composition expressed as atomic percentage is measured with a Castaing microprobe (known as EPMA, according to the English acronym for “Electron Probe Microanalysis”). Variation of the injection parameters gives several coating thicknesses, which are generally between 0.5 μm and 10 μm: the thicknesses selected for the oxidation tests that follow are indicated in Table 2.

(29) As regards their structure, Table 2 shows that the CrC and CrSiC coatings are amorphous (no crystalline structure), the CrC coating nevertheless being polycrystalline for a deposition temperature of 550° C. As regards the Cr coating, it comprises two crystalline phases: a main phase composed of centered cubic (bcc) chromium crystals in the Im-3m space group (lattice parameter a=2.88 Å) and a phase in very minor proportion composed of chromium crystals of centered cubic crystallographic structure in the Pm-3n space group (lattice parameter a=4.59 Å) when the DLI-MOCVD deposition temperature is less than or equal to 450° C.

(30) TABLE-US-00002 TABLE 2 Deposition Thermal Coating Thickness temperature Atomic stability (N) (μm) (° C.) Structure composition (° C.) CrC (1) 2; 5; 9 400 Amorphous Cr.sub.0.64C.sub.0.33O.sub.0.03 Cr.sub.7C.sub.3 580 500 Cr.sub.3C.sub.2 590 Cr.sub.2O.sub.3 610 550 Polycrystalline Cr.sub.0.61C.sub.0.32O.sub.0.07 Cr (2) 4; 6 400 Polycrystalline, Cr.sub.0.69C.sub.0.12S.sub.0.08O.sub.0.11 Metastable 450 two-phased: Cr phase main phase disappears Cr bcc (Im-3m); at 450° C. metastable phase Cr bcc (Pm-3n). Multilayer structure to increase the density. CrSiC (3) 4 450 Amorphous Cr.sub.0.66Si.sub.0.02C.sub.0.29O.sub.0.03 Cr.sub.7C.sub.3 750 500 CrSi.sub.2 750
2.Evaluation of the Heat Resistance

(31) The coatings must withstand the temperature under an oxidative atmosphere and act as a diffusion barrier.

(32) Their heat resistance under an inert atmosphere is first studied by XRD in situ as a function of the temperature under Ar. The temperature at which a structural change appears (crystallization, phase transformation) is measured. Such structural changes are liable to accelerate the diffusion of oxygen through the coating to the substrate, via the grain joints or microcracks. The coating must therefore be able to maintain its structural integrity at the highest possible temperatures.

(33) The heat resistance results obtained for the wafers coated in the preceding example are collated in Table 2.

(34) 2.1. Heat Resistance of an Amorphous Si/CrC Wafer Under an Inert Atmosphere

(35) A silicon substrate passivated with a layer of a few hundred nanometers composed of amorphous silicon nitride (generally Si.sub.3N.sub.4), which is commercially available, is used to avoid any diffusion with the protective layer deposited on its surface.

(36) Alternatively, a bare silicon substrate may be passivated by thermal CVD with a mixture (SiH.sub.4+NH.sub.3) or by plasma (PECVD) with NH.sub.3.

(37) The passivated silicon substrate is then coated with amorphous chromium carbide according to the conditions of Table 1 (specific conditions: injection at 10 Hz, deposition temperature=500° C., deposition pressure=6700 Pa).

(38) In a chamber flushed with a stream of argon to limit or even prevent any contamination with oxygen, this sample is heated from 30° C. to 800° C. at a temperature increase rate of 5° C./minute while in parallel being analyzed by X-ray diffraction (XRD). After natural cooling to 30° C., a final X-ray spectrum is recorded.

(39) Analysis of the X-ray spectra obtained shows that the sample maintains an amorphous structure up to 570° C. It then crystallizes in the form of Cr.sub.7C.sub.3 at about 580° C., and then Cr.sub.3C.sub.2 at about 590° C., and finally becomes oxidized to Cr.sub.2O.sub.3 at about 610° C. due to traces of oxygen present on account of an airtightness defect of the reactor: these three phases remain up to 800° C. with a proportion of Cr.sub.2O.sub.3 which increases with the temperature.

(40) These results indicate that the amorphous chromium carbide coating preserves its physical integrity up to 800° C. under an inert atmosphere of argon.

(41) 2.2. Heat Resistance of a Partially Metastable Si/Cr Wafer Under an Inert Atmosphere

(42) A silicon substrate passivated as previously is coated with partially metastable chromium according to the conditions of Table 1 (specific conditions: deposition temperature=400° C., deposition pressure=6700 Pa).

(43) It is then placed in the reactor flushed with a stream of argon, in which it is gradually heated from 30° C. to 600° C. at a temperature increase rate of 1° C./second. In parallel, it is analyzed by XRD (acquisitions of 35 minutes each at 30° C. and every 50° C. from 350° C.).

(44) Analysis of these spectra shows that the partially metastable chromium is polycrystalline: it contains a stable phase (Im-3m) and a metastable phase (Pm-3n).

(45) The metastable phase (Pm-3n) disappears after 450° C. in parallel, the stable phase (Im-3m) remains throughout the heat treatment, the corresponding XRD peaks become finer and more intense by virtue of the improvement in the crystallinity.

(46) Above 550° C., chromium oxide Cr.sub.2O.sub.3 appears as a result of the traces of oxygen present in the in situ analysis chamber.

(47) On conclusion of the temperature cycle, the sample is cooled naturally to 30° C.: the metastable phase is no longer present, since it has undergone an irreversible transformation. Only the stable phase remains, along with chromium oxides in small proportion.

(48) These results indicate that the chromium metal coating has a heat resistance under an inert atmosphere in accordance with conventional chromium of centered cubic (bcc) structure which is stable under normal conditions.

(49) Moreover, it is seen that the minor phase of metastable chromium becomes transformed at about 450° C. to form the main stable chromium phase: this is coherent with the literature which states that metastable chromium is transformed into stable chromium after 450° C.

(50) 2.3. Heat Resistance of an Amorphous Si/CrSiC and Steel/CrSiC Wafer Under an Inert Atmosphere

(51) A passivated silicon substrate is coated with an amorphous mixed chromium silicon carbide according to the conditions of Table 1 (specific conditions: mole ratio of diphenylsilane to BEBC=15%, deposition temperature 450° C.), and is then placed in the reactor flushed with a stream of argon. This sample is heated from 30° C. to 70° C. at a temperature increase rate of 1° C./second while in parallel being analyzed by XRD (acquisitions every 50° C. from 400° C.).

(52) In a similar manner to these operating conditions, a 304L stainless-steel substrate coated with an amorphous mixed chromium silicon carbide at a deposition temperature of 500° C. is also analyzed by XRD (acquisitions every 50° C. from 600° C.) during heating from 30° C. to 1100° C.

(53) Comparative analysis of the XRD spectra for these two samples shows similar behavior: the amorphous nature disappears at about 750° C. by crystallization of a chromium carbide Cr.sub.7C.sub.3 and of the compound CrSi.sub.2. Furthermore, it shows that since CrSi.sub.2 appears on the two substrates, this compound is formed with the silicon of the coating and not that of the substrate, the silicon nitride passivation layer applied previously to the surface of the silicon substrate constituting a diffusion barrier.

(54) Above 750° C., other crystalline phases appear: Cr.sub.2O.sub.3 after 850° C. and Cr.sub.3C.sub.2 after 1000° C. It is thus noted that the addition of Si to the chromium carbide coating delays its crystallization toward higher temperatures (750° C. instead of 580° C.).

(55) 3. Evaluation of the Oxidation in Air

(56) 3.1. Resistance to Oxidation in Air at 25° C. of an Amorphous Zr/CrC Wafer

(57) In order to evaluate its resistance to mild oxidation at room temperature, a wafer based on the zirconium alloy Zircaloy-4 (dimensions: 45 mm×14 mm×1 mm) is coated with a single 4 μm protective layer of amorphous chromium carbide in accordance with the deposition conditions indicated in Table 1 for coating 1 (deposition temperature=450° C., deposition pressure=6700 Pa). It resides in air at room temperature for a few days, which results in very mild surface oxidation of the chromium carbide coating due to the passage into the ambient air, as for the majority of metals and alloys.

(58) The elemental composition by mass according to the depth in this sample is then determined for the elements zirconium, chromium, oxygen and carbon by glow discharge spectrometry (GDS). It is shown in FIG. 1 and reveals the presence of a nonzero oxygen content at the interface between the substrate and the coating.

(59) The mean coating composition values found via this technique are in agreement with those measured by microprobe (EPMA, Electron Probe Micro-Analyzer): as weight percentages relative to the total mass of the wafer, 90% of Cr, slightly less than 10% of C and a few % of O, which corresponds in atomic percentages to a composition close to Cr.sub.7C.sub.3.

(60) The Transmission Electron Microscopy (TEM) images shown in FIGS. 2A and 2B are acquired using a microscope (JEM 2100 sold by the company Jeol) equipped with a field emission gun operating at 200 kV: the zones delimited on these images confirm this result of the profile of FIG. 1, among others the presence of a layer of zirconium oxide ZrO.sub.2 (zone b) with a thickness of less than 400 nm, which is at the interface between the non-oxidized substrate (zone a) and the coating (zone c).

(61) These results are confirmed by complementary analyses by Energy Dispersion Spectroscopy (EDS) which indicate a composition, in atomic percentages, of about 40% of O and 60% of Zr on the oxidized interfacial zone as opposed to 5% of O and 95% of Zr on the non-oxidized zone of the substrate.

(62) The formation of a passivation layer at the surface of the Zircaloy-4 is normal in the absence of specific surface stripping. This layer may possibly disappear in the course of the oxidation and temperature increase experiments.

(63) The Zircaloy-4 substrate was already surface-oxidized on receipt. No other source of oxidation could be identified during the phase of deposition of the chromium carbide protective layer. The sample thus coated underwent no additional oxidation on conclusion of storage in air under the ambient temperature conditions.

(64) 3.2. Resistance to Oxidation in Air at 800° C. of an Amorphous Si/CrC Wafer

(65) The silicon wafer covered with amorphous CrC manufactured according to the conditions of Example 2.1 is subjected to aging in air at 800° C. for increasing times of 15, 30, 45, 60, 90, 120 and 180 minutes. For each temperature, the sample is cooled naturally and then analyzed by XRD at room temperature.

(66) Analysis of the XRD spectra shows that, under air at 800° C.:

(67) after 15 minutes of treatment, the same three phases seen previously appear: Cr.sub.7C.sub.3, Cr.sub.3C.sub.2 and Cr.sub.2O.sub.3. The amorphous phase appears to disappear since no broad spread-out lump is visible at about 2 theta=30°;

(68) after 90 minutes at 800° C., there is virtually no more Cr.sub.2O.sub.3, as if the oxygen had consumed all the chromium belonging to the carbides. Residual peaks characteristic of Cr.sub.3C.sub.2 are still present, even after 180 minutes at 800° C.

(69) These results indicate that the amorphous chromium carbide coating withstands oxidation in air at 800° C. for at least 15 minutes. Beyond these conditions, it begins to be oxidized and to crystallize.

(70) 3.3. Resistance to Oxidation as a Function of the Temperature in Dry or Humid Air of Bare or Coated Zr or Mo Wafers

(71) In order to study the resistance to oxidation in dry or humid air as a function of the temperature, thermogravimetric analyses (TGA) are performed on TGA wafers (dimensions: 6 mm×4 mm×1 mm, wafers pierced with a hole 1 mm in diameter and then suspended on the beam of the TGA balance) of Zircaloy-4 or molybdenum coated by the manufacturing process of the invention with protective layers of various compositions (amorphous CrC, partially metastable Cr or amorphous Cr.sub.xSi.sub.yC.sub.z) and various thicknesses (9 μm, between 5 μm and 6 μm, or between 2 μm and 3 μm).

(72) The deposition conditions are those of Table 1 for the corresponding protective layers which cover all the faces of the wafers. These conditions are completed by the following specific parameters:

(73) amorphous CrC: T=450° C. and P=6700 Pa;

(74) partially metastable Cr: T=400° C.;

(75) amorphous CrxSizCy: T=500° C.

(76) These analyses consist in gradually heating each sample from 25° C. to 1200° C. at a temperature increase rate of 40° C./minute, and then in maintaining a temperature of 1200° C.

(77) They are performed with dry or humid air (27.5% relative humidity).

(78) The progress of the oxidation of each sample is evaluated by measuring its gain in mass (due to the formation of oxide).

(79) For comparative purposes, each analysis is repeated on a wafer without a protective layer.

(80) 3.3.1. Results for an Amorphous Zr/CrC Wafer

(81) Zircaloy-4 wafers of various thicknesses (2 μm, 5 μm or 9 μm) each covered with a protective layer of amorphous CrC are subjected to TGA analysis in air between 25° C. and 1200° C. For comparative purposes, an uncoated Zircaloy-4 wafer is subjected to the same analysis.

(82) As illustrated in FIGS. 3A (dry air) and 3B (humid air), during the temperature increase from 25° C. to 1200° C. on a Zircaloy-4 substrate coated with amorphous CrC, the gains in mass change in an equivalent manner in dry air or in humid air, irrespective of the thickness of the protective layer.

(83) On the other hand, in comparison, the extent of the oxidation is much less than that for the bare substrate: the coated substrate gains only about 0.15% in mass as opposed to about 3% mass for the bare substrate, i.e. 20 times more.

(84) During the temperature stage at 1200° C., the three Zircaloy-4 substrates coated with amorphous CrC become gradually oxidized over time. This slowing-down in catastrophic oxidation at and beyond which the substrate disintegrates is smaller for the humid air atmosphere which is the more oxidizing (FIG. 3D) relative to the dry air (FIG. 3C), but advantageously increases with the thickness of the protective layer.

(85) XRD analyses appear to indicate that three crystalline phases coexist in the coating: crystalline Cr.sub.2O.sub.3, amorphous chromium oxides and amorphous chromium carbide CrC.

(86) In comparison, the bare substrate becomes immediately oxidized to ZrO.sub.2.

(87) The oxygen penetration is thus total in the bare substrate, whereas it is blocked or greatly limited up to 900° C. Above this temperature, it becomes partial and gradual in the coated substrates, since the amorphous CrC coating slows down the oxidation kinetics.

(88) The amorphous CrC coating thus indeed protects against and/or delays the oxidation of the Zircaloy-4 substrate during heating from 25° C. to 1200° C., even in humid air. This beneficial effect increases with the thickness of the coating.

(89) 3.3.2. Results for an Amorphous Mo/CrC Wafer

(90) Molybdenum wafers of various thicknesses (2 μm or 5 μm) each covered with a protective layer of amorphous CrC are subjected to TGA analysis in air between 25° C. and 1200° C. For comparative purposes, an uncoated molybdenum wafer is subjected to the same analysis.

(91) As illustrated in FIGS. 4A (dry air) and 4B (humid air), during the temperature increase from 25° C. to 1200° C. on a molybdenum substrate coated with amorphous CrC, the gains in mass change less quickly in dry air than in humid air, which is a more oxidative atmosphere. Moreover, the thicker the amorphous CrC coating, the later the sample becomes oxidized and the more it is capable of withstanding high temperature: up to about 1000° C. for a 2 μm coating, or even more than 1100° C. for a 5 μm coating.

(92) The resistance to oxidation for greater thicknesses of the amorphous CrC coating could not be tested: it should nevertheless change favorably.

(93) In comparison, the bare molybdenum substrate becomes oxidized immediately in dry or humid air: above 600° C. after a gain in mass due to the formation of molybdenum oxide, the bare substrate is then totally destroyed by formation of volatile oxides, which leads to rapid and negative losses of mass.

(94) Since the various samples are rapidly destroyed at a temperature of 1200° C., the corresponding isothermal TGA analysis is not illustrated.

(95) The amorphous CrC coating thus indeed protects against and/or delays the oxidation of the molybdenum substrate during heating from 25° C. to 1100° C., even in humid air. This beneficial effect increases with the thickness of the coating.

(96) 3.3.3. Results for a Partially Metastable Zr/Cr Wafer

(97) Zircaloy-4 wafers are each covered with nine protective layers of partially metastable Cr so as to form multilayer coatings of different thicknesses (4 μm or 6 μm).

(98) The stack of the nine protective layers is made by injecting the mother solution for 15 minutes and then stopping it for 5 minutes: a multilayer coating is thus obtained by repeating this deposition/pause cycle nine times.

(99) The resumption of injection after the 5-minute pause creates an interface which may be visualized by TEM analysis of the multilayer coating.

(100) This avoids the columnar growth and densifies the coatings.

(101) The coated wafers are subjected to TGA analysis in air between 25° C. and 1200° C. For comparative purposes, an uncoated Zircaloy-4 wafer is subjected to the same analysis.

(102) As illustrated in FIGS. 5A (dry air) and 5B (humid air), during the temperature increase from 25° C. to 1200° C. on a Zircaloy-4 substrate coated partially metastable Cr, the gains in mass change in an equivalent manner in dry air or in humid air, irrespective of the thickness of the protective layer.

(103) On the other hand, in comparison, the extent of the oxidation is smaller than that for the bare substrate: the substrate coated with partially metastable Cr gains only about 0.8% in mass as opposed to about 3% mass for the bare substrate (i.e. 3.75 times more), which is nevertheless less protective than the amorphous CrC coating analyzed previously with a gain in mass equal to 0.15%.

(104) During the temperature stage at 1200° C., the two Zircaloy-4 substrates coated with partially metastable Cr become gradually oxidized over time as illustrated by FIGS. 5C (dry air) and 5D (humid air). Compared with what was found with the amorphous Zr/CrC wafer (Example 3.3.1), the multilayer coating of partially metastable Cr shows in the first seconds a delay of oxidation in comparison with the bare substrate, rapidly followed by an acceleration of this oxidation.

(105) The origins of this unfavorable acceleration at 1200° C. relative to the bare substrate have not yet been entirely elucidated. At this stage, various hypotheses are being studied with a view to optimizing the protective effect of the partially metastable Cr coating with respect to oxidation.

(106) However, advantageously, even at 1200° C., the partially metastable Zr/Cr wafer shows less substantial desquamation than the bare Zr wafer, which ensures better mechanical strength.

(107) XRD analyses indicate that, on conclusion of the temperature stage at 1200° C., all of the partially metastable Cr coating is oxidized to Cr.sub.2O.sub.3 and the substrate to ZrO.sub.2.

(108) The partially metastable Cr coating greatly prevents or greatly limits the penetration of oxygen up to about 900° C., above which temperature this penetration becomes partial and gradual.

(109) The partially metastable Cr coating thus indeed protects against and/or delays the oxidation of the Zircaloy-4 substrate.

(110) 3.3.4. Results for a Zr/Cr.sub.xSi.sub.yC.sub.z Wafer

(111) A Zircaloy-4 wafer 4 μm thick covered with a protective layer of amorphous mixed chromium silicon carbide Cr.sub.xSi.sub.yC.sub.z is subjected to TGA analysis in dry or humid air between 25° C. and 1200° C. with a temperature stage at 1200° C. For comparative purposes, an uncoated Zircaloy-4 wafer is subjected to the same analysis, the oxidation in dry or humid air giving, in this case, identical results.

(112) As illustrated in FIG. 6A (25° C. to 1200° C.), the gain in mass changes similarly in dry air or in humid air for the Zircaloy-4 substrate coated with amorphous mixed chromium silicon carbide Cr.sub.xSi.sub.yC.sub.z: the oxidation is delayed up to 700° C. in humid air and 800° C. in dry air. In comparison with the bare substrate, the coated substrate shows better resistance to oxidation.

(113) FIG. 6B (stage at 1200° C.) logically shows better resistance to oxidation in dry air than in humid air of the Zircaloy-4 substrate coated with amorphous mixed chromium silicon carbide Cr.sub.xSi.sub.yC.sub.z, without, however, improving this property relative to the bare substrate.

(114) However, advantageously, even at 1200° C., the Zr/Cr.sub.xSi.sub.yC.sub.z wafer shows less substantial cracking and swelling than the bare Zr wafer, which ensures better mechanical strength.

(115) XRD analyses indicate that, on conclusion of the temperature stage at 1200° C., all of the amorphous mixed chromium silicon carbide Cr.sub.xSi.sub.yC.sub.z coating is oxidized (among others to Cr.sub.2O.sub.3 for chromium) and the substrate to ZrO.sub.2.

(116) The amorphous mixed chromium silicon carbide Cr.sub.xSi.sub.yC.sub.z coating thus indeed protects against and/or delays the oxidation of the Zircaloy-4 substrate. Its resistance to oxidation and its mechanical strength are of intermediate magnitude relative to those of amorphous CrC and of partially metastable Cr.

(117) 3.3.5. Evaluation of the Resistance to Oxidation and to Hydriding After Exposure of a Zr/CrC Wafer to 1100° C. Followed by Quenching in Water

(118) Zircaloy-4 wafers (dimensions: 45 mm×14 mm×1 mm) of three different thicknesses (2 μm to 3 μm, 5 μm to 6 μm, or 9 μm) each covered with a protective layer of amorphous CrC as used in Example 3.3.1 are oxidized in dry air for 14 minutes, and then subjected to quenching in water for 10 seconds.

(119) The Scanning Electron Microscopy (SEM) images of a 9 μm section of the wafer resulting from this treatment are shown in FIGS. 7A and 7B (zoom into a corner of the wafer): although the substrate has a rough finish, i.e. it has a certain surface roughness, the coating is of uniform thickness and it perfectly covers all the unevennesses. The mechanical strength of the wafers is also preserved.

(120) The absence of degradation of the Zircaloy-4 on conclusion of this test at 1100° C. with quenching in water is coherent with the oxidation of Zircaloy-4 which increases as a function of the temperature and is significant only at and above 1200° C. (Example 3.3.1). The lower temperature (1100° C.) and the short time (14 minutes) are such that the oxidation is limited here to the coating (see FIGS. 7A and 7B) which ensures good protection of the Zircaloy-4.

(121) In accordance with GDS profiles not shown herein, three main layers of different chemical composition may be identified on the SEM images of FIGS. 7A and 7B, i.e. going from the external medium to the Zircaloy-4 substrate:

(122) an outer layer of oxidized coating about 4 μm thick comprising a zone A of 2 μm of surface chromium oxides (Cr.sub.2O.sub.3) and a zone B of 2 μm of partially oxidized coating (Cr.sub.3C.sub.2 resulting from the recrystallization of the amorphous CrC during the temperature increase+Cr.sub.2O.sub.3);

(123) an intermediate layer comprising a zone C of non-oxidized coating (amorphous CrC) at the interface with the outer coating and with a zone D formed from chromium and carbon which has diffused into the Zircaloy-4 with which it is at the interface (with possible formation of cubic zirconium carbide ZrC with a lattice parameter of about 4.7 Å, in particular for the thinnest coating of 2 μm to 3 μm);

(124) a layer formed from Zircaloy-4 (zone E).

(125) The amorphous CrC coating thus protects the Zircaloy-4 substrate with respect to oxidation for the three wafers, since no formation of ZrO.sub.2 is detected. Only the coating is partially oxidized to Cr.sub.2O.sub.3.

(126) Table 3 collates the thicknesses evaluated by GDS for zones A, B, C and D for the three Zr/CrC wafers, and also the corresponding overall gains in mass.

(127) TABLE-US-00003 TABLE 3 Zone D: Zone B: Thickness CrCO of Initial Zone A: transition diffusion thickness Thickness zone Zone C: zone in Gain in CrC Cr.sub.2O.sub.3 thickness Thickness substrate mass (μm) (μm) (μm) CrC (μm) (μm) (mg .Math. cm.sup.−2) 9 1.5 2.5 3 15 0.414 5-6 1.5 2.5 2 11 0.543 2-3 1.5 2.5 1 6 0.288

(128) Table 3 is completed by FIG. 7C which groups the SEM images of a cross section for each of the three wafers, given that the total thickness of the oxidized coating is greater than the thickness of the initial protective coating. The SEM image of the wafer 5 μm to 6 μm thick shows a crack resulting from accidental rupture of the initial coating.

(129) Table 3 and FIG. 7C show that the greater the thickness of the initial CrC coating, the thicker the non-oxidized coating (zone C) and the thicker also is the diffusion zone of the non-oxidized coating in the substrate (zone D)._On the other hand, the thickness of the oxidized coating (zone A) does not change: about 2 μm of Cr.sub.2O.sub.3 and 2 μm of partially oxidized transition zone (chromium oxycarbide) (zone B). Since the oxidation phenomenon is superficial, and since the oxidation conditions are the same, it is coherent that the thickness of Cr.sub.2O.sub.3 and of the transition zone should be identical insofar as the CrC coating has not all been consumed. The oxidation is partial depending on the thickness: less than half of the total thickness of the coating is oxidized for the thinnest coating (2 μm to 3 μm) and a quarter for the thickest (9 μm).

(130) The gain in mass is greater for the wafer with a protective layer 9 μm thick, which is also illustrated by FIG. 7C which shows greater progress of the oxidation within the coating. The ratio of the gain in mass between the wafers with a 9 μm coating and a 2-3 μm coating is comparable to the ratio of the oxidized coating thicknesses: 1.44 versus 1.59.

(131) Complementary analyses show that the elemental compositions of the coatings are comparable for each wafer in oxidized and non-oxidized zones, with the curves reaching the same mass percentages and following the same trends. In particular, a diffusion of chromium and of carbon is observed in the Zircaloy-4 substrate, among others in the form of ZrC, which advantageously reinforces the adhesion of the coating to the substrate.

(132) Moreover, the non-oxidized coating zone C (amorphous CrC) maintains a very dense microstructure, formed from small grains of crystalline chromium carbides of submicron size.

(133) As regards the resistance to hydriding, GDS analysis of the 9 μm thick wafer illustrated by FIG. 7D shows a mass percentage of hydrogen of 10 ppm which is not significant in the Zircaloy-4 substrate after oxidation at 1100° C. followed by quenching in water, which confirms the hydrogen-permeation barrier property of the amorphous chromium carbide coating.

(134) Even under oxidative conditions at 1100° C. followed by quenching, a nuclear component obtained via the manufacturing process of the invention can maintain a certain degree of mechanical integrity and have a comfortable residual margin of resistance to oxidation/hydriding.

(135) 3.3.6. Conclusion

(136) Unexpectedly, the inventors have discovered that a nuclear component obtained by the manufacturing process of the invention has improved resistance to oxidation and/or hydriding, in particular at high and very high temperature, among others in the presence of water vapor.

(137) Such properties cannot be anticipated with regard to the chemical and metallurgical specificities of zirconium and of the zirconium alloys used for nuclear applications, among others with regard to their chemical composition, surface state, crystalline texture, final metallurgical state (work-hardened or more or less recrystallized), which are properties that are liable to have an influence on the quality and behavior of the coatings.

(138) In particular, at low temperature, the α phase of a zirconium alloy (denoted “Zr-α”, of compact hexagonal crystalline structure) transforms into the β phase (denoted “Zr-β”, of centered cubic crystallographic structure) in a temperature range typically extending from 700° C. to 1000° C. On passing from the Zr-α structure to the Zr-β cubic structure, the alloy undergoes local dimensional variations. These variations are in principle unfavorable to the mechanical strength of an outer layer which would cover a zirconium-based inner layer, on account among others of the incompatibility of their expansion coefficients. These adhesion difficulties are accentuated by the mechanisms of diffusion of chemical species that are faster in the Zr-β phase than in the Zr-α phase, and which can modify the interface between the substrate and its coating. Now, the various protective layers deposited via the manufacturing process of the invention have shown very good adhesion to a zirconium-based substrate, even under extreme conditions.

(139) 4. Microstructure of a Partially Metastable Si/Cr Monolayer or Multilayer Coating

(140) A passivated silicon substrate is provided with a partially metastable chromium monolayer coating according to the conditions of Example 2.2, or with a multilayer coating of similar thickness and chemical composition while additionally observing a waiting time of 5 minutes between the deposition of each of the nine constituent layers of the multilayer coating.

(141) The microstructure of these coatings is illustrated by the SEM images represented by:

(142) FIG. 8A: monolayer coating obtained with a deposition rate of 5 μm/hour;

(143) FIG. 8B: monolayer coating obtained with a deposition rate of 3 μm/hour;

(144) FIG. 8C: multilayer coating obtained with a deposition rate of 1 μm/hour.

(145) These images show that the monolayer coatings have here a columnar microstructure, whereas the multilayer coating has an equiaxed microstructure. FIG. 8C in particular shows the interface which exists between each of the nine layers which are visualized individually within the multilayer coating.

(146) Moreover, the density of the coating increases gradually while reducing the rate of deposition of the partially metastable chromium by reduction of the injection frequency and time. The density of the multilayer coating is estimated as 7.7±0.6 g.Math.cm.sup.−3 via RBS (“Rutherford Backscattering Spectrometry”) analyses: taking the uncertainties into account, it is thus of the same order of magnitude as the optimum density of 7.2 g.Math.cm.sup.−3 for the stable solid chromium.

(147) The deposition conditions thus have an influence on the microstructure of the coating and on its density. In general, the quality of a coating is inversely proportional to its deposition rate, as shown by the evolution from a porous columnar growth to a dense equiaxed growth obtained by reducing the deposition rate. Reducing the deposition rate may be obtained by reducing the deposition temperature, or by increasing the deposition pressure or the precursor concentration of the mother solution injected into the CVD deposition reactor.

(148) 5. Hardness of the Coatings

(149) In order to measure the hardness of the coatings obtained previously or obtained under similar conditions, nanoindentation experiments are performed. For the amorphous CrC coating, the hardness measurements are performed on a coating obtained from fresh precursor (3.5 μm thick) or from precursor recycled under conditions similar to those presented in Example 6 (1 μm thick).

(150) The nanoindentation machine is equipped with a Berkovich indenter (triangular-based pyramid with an angle of 65.27° between the vertical and the height of one of the faces of the pyramid). The measures are taken in compliance with the rule of the tenth: the indenter drives in by less than one tenth of the thickness of the coating. A measurement cycle takes place in three stages:

(151) increasing load up to the maximum load, in 30 seconds;

(152) maintenance of the maximum load for 30 seconds;

(153) removal of the load for 30 seconds.

(154) The calculations made by the measurement and analysis software take into account a Poisson coefficient of the coating of 0.2.

(155) The results are collated in Table 4.

(156) The parameters “H” and “E” are the hardness and the Young's modulus.

(157) The parameters “H/E” and “H.sup.3/E.sup.2” evaluate the durability of the coating:

(158) H/E: compares the elastic breaking strength or the abrasion resistance between different coatings.

(159) H.sup.3/E.sup.2: characterizes the elastic behavior of a material and is proportional to the resistance to penetration under load and to the plastic deformation.

(160) TABLE-US-00004 TABLE 4 Thickness Coating (μm) Substrate H (GPa) E (GPa) H/E (—) H.sup.3/E.sup.2 (GPa) Cr.sub.xC.sub.y 3.5 Zircaloy-4 22.7 ± 2.3 266.5 ± 25.2 8.5 × 10.sup.−2 1.6 × 10.sup.−1 Cr.sub.xC.sub.y 1.0 Zircaloy-4 28.9 ± 6.4 315.0 ± 41.2 9.2 × 10.sup.−2 3.6 × 10.sup.−1 recycled Cr(S) 3.5 Zircaloy-4  9.7 ± 0.8 344.8 ± 32.0 2.8 × 10.sup.−2 7.7 × 10.sup.−3 columnar Cr(S) 5.5 Zircaloy-4 16.9 ± 0.6 280.1 ± 72.7 6.0 × 10.sup.−2 6.2 × 10.sup.−2 dense Cr.sub.xSi.sub.zC.sub.y 5.5 Zircaloy-4 19.9 ± 2.8 180.8 ± 17.6 1.1 × 10.sup.−1 4.58 × 10.sup.−1  Cr — —  6.1 ± 0.4 224.2 ± 21.4 2.7 × 10.sup.−2 4.5 × 10.sup.−3 solid Zircaloy-4 — —  4.4 ± 0.4 108.8 ± 15.7 4.0 × 10.sup.−2 7.2 × 10.sup.−3 solid

(161) For the various coatings, these results show that:

(162) partially metastable Cr: this coating has a high hardness, of about 17 GPa. This hardness is twice as high as that of the commercially available electrolytic hard chromium. It is in the high hardness range of solid metallic chromium, generally comprised between 6 GPa and 25 GPa depending on the production process.

(163) amorphous chromium carbide CrC: this coating has an appreciably high hardness, comprised between 22 GPa and 29 GPa. The best hardnesses are obtained when the precursors not consumed on conclusion of the manufacturing process of the invention are recycled.

(164) In comparison, the hardness range of a chromium carbide of the prior art generally ranges between 5 GPa and 24 GPa depending on the process used and the deposition conditions.

(165) The high hardness range of the amorphous chromium carbide coating obtained via the manufacturing process of the invention is unexpected: it is generally accepted that a chromium carbide is much less hard in amorphous form than in crystalline form, or even that the presence of carbon would soften a chromium-based coating.

(166) amorphous CrSiC: in comparison with the amorphous CrC coating, this coating has a hardness of about 20 GPa which is smaller, but, advantageously its Young's modulus of 180 GPa is much lower. The amorphous CrSiC coating thus has a very high durability (H/E=11.1×10.sup.−2 and above all H.sup.3/E.sup.2=4.6×10.sup.−1 GPa), which competes with high-durability coatings specially designed for this purpose.

(167) 6. Recycling of the Precursors

(168) The example of a deposition process with recycling which follows is taken from patent application FR 1562862 (reference [11]). It relates to the deposition of an amorphous chromium carbide coating on a silicon substrate and illustrates by analogy the possibility of recycling one or more of the precursors or of derivatives thereof which remain on conclusion of the manufacturing process of the invention. The recycling is performed with a cryogenic trap.

(169) The deposition of an amorphous chromium carbide CrC coating is performed under the following conditions:

(170) Injection conditions: injector open time: 0.5 ms frequency: 10 Hz

(171) Reagent: BEBC (5 g)

(172) Solvent: Toluene (50 mL)

(173) Carrier gas: N.sub.2 (flow rate of 500 sccm, i.e. 500 cm.sup.3/min under standard conditions)

(174) Deposition time: 20 minutes

(175) Deposition temperature: 450° C.; Deposition pressure: 6666 Pa

(176) Evaporation temperature: 200° C.

(177) Temperature of the cryogenic trap: −120° C.

(178) Amount of daughter solution recovered: 30 mL

(179) Two experiments N1 and N2 were performed with a BEBC mother solution. In a third experiment, the two daughter solutions obtained on conclusion of N1 and N2 are recovered to form a recycled mother solution which is used as source of precursor for a third deposition operation N3.

(180) For N1 and N2, the thickness of the coating is typically 5 μm. A coating about 1.5 μm thick is obtained on conclusion of N3. The concentration of BEBC is determined and the yield for N1 and N2 is calculated (see Table 5).

(181) TABLE-US-00005 TABLE 5 [BEBC] in [BEBC] in Yield the Daughter the relative Injected injected solution daughter to the Experiment solution solution recovered solution precursor No. (ml) (g/ml) (ml) (g/ml) (%) N1 55 0.078 30 0.031 60% N2 50 0.083 30 0.034 59% N3 30 + 30 0.040 35 Not N/A measured, since very small

(182) Complementary analyses show that the compositions and amorphous microstructures of the coatings do not depend on the precursor concentration of the injected solution. On the other hand, a daughter solution containing a recycled precursor makes it possible to deposit a coating with a hardness higher than that obtained with the initial precursor of the mother solution.

(183) 7. Geometry of the Nuclear Component According to the Invention

(184) The nuclear component obtained via the manufacturing process of the invention is described on a cross section with reference to FIGS. 9A, 9B, 9C, and 9D in the nonlimiting particular case of a tubular geometry.

(185) When the nuclear component is solid or does not delimit any accessible internal volume, it generally does not comprise any coating deposited on the inner surface of the substrate.

(186) According to a first embodiment of the invention, the cladding illustrated by FIG. 9A comprises a substrate 1 whose inner surface delimits a volume that is capable of receiving the nuclear fuel. The substrate 1 forms a support (not coated with an interposed layer for this embodiment) on which is placed a protective layer 2 composed of a protective material which improves the oxidation resistance of the cladding.

(187) According to a second embodiment illustrated by FIG. 9B, the cladding is further provided with an interposed layer 3 placed between the substrate 1 and the protective layer 2. In this case, the combination of the substrate 1 and of the interposed layer 3 forms the support. The interposed layer 3 is composed of at least one interposed material which is capable of preventing or limiting the diffusion of the protective material of the protective layer 2 toward the substrate 1.

(188) According to a third embodiment illustrated by FIG. 9C, the cladding according to the second embodiment is further provided with an inner protective layer 2B in contact with the internal volume of the cladding and arranged here on the inner face of the substrate 1 (thus not coated with a liner for this embodiment). The inner protective layer 2B completes the protection imparted by the outer protective layer 2A.

(189) According to a fourth embodiment illustrated by FIG. 9D, the cladding according to the third embodiment is further provided with a liner 4 positioned between the inner protective layer 2B and the substrate 1. The liner 4 constitutes a diffusion barrier.

(190) Needless to say, depending on the desired properties, other embodiments are possible as a function of the presence or absence in each embodiment of the interposed layer 3, of the outer protective layer 2A, of the inner protective layer 2B and/or of the liner 4.

(191) The present invention is in no way limited to the embodiments described and represented, and a person skilled in the art will know how to combine them and to make thereof, by means of his general knowledge, numerous variants and modifications.

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