<sup2 / > <sub2 / > 2. METHOD FOR FORMING A MONOPHASE OXIDE LAYER (FE, CR)O <ns2:sub> 3 < / ns2:sub> OF RHOMBOHEDRAL STRUCTURE ON A STEEL OR SUPERALLOY SUBSTRATE

Description

TECHNICAL FIELD AND PRIOR TECHNOLOGY

[0001] The present invention relates to a method for forming a single-phase (Fe, Cr) 2 O 3 oxide layer with a rhombohedral structure on a steel or superalloy substrate.

[0002] The invention finds applications in particular in the field of thermal solar absorbers for concentrated solar power plants (CSP for "Concentrating Solar Power Plant").

[0003] A concentrated solar power (CSP) plant is a power plant designed to concentrate the sun's rays using mirrors to heat a heat transfer fluid. This heat transfer fluid then serves as a heat source in a thermodynamic cycle to produce electricity or for direct use. Concentrating solar radiation allows for higher temperatures and significant thermodynamic conversion.

[0004] One of the essential components of a concentrated solar power (CSP) plant is the solar radiation absorber, which absorbs the sun's incoming radiation and converts it into heat. To maximize the absorber's efficiency, it generally incorporates a coating, known as a selective coating or selective treatment. The selective coating allows for maximum absorption of incoming solar energy while minimizing the re-emission of infrared radiation (the blackbody radiation principle). Specifically, such a selective coating is considered ideal if it absorbs all wavelengths below a cutoff wavelength and reflects all wavelengths above that same cutoff wavelength.

[0005] Traditionally, selective coatings are obtained by depositing a metallic layer, which acts as an infrared reflector, followed by one or more ceramic / metal (cermet) layers as absorbers, and finally one or more antireflective layers. However, these layers are generally obtained by vacuum deposition, a relatively expensive technique that increases the manufacturing costs of the solar absorber. Furthermore, these materials do not exhibit good oxidation resistance in air at high temperatures, limiting their use in air to temperatures around 300°C or requiring vacuum deposition. In general, deposition processes produce layers with the desired optical properties but not necessarily with temperature stability or protective properties for the substrate.

[0006] Another solution involves using a chromium-containing steel substrate and oxidizing it to form an intrinsically selective oxide layer. This means the oxide layer is capable of absorbing a maximum of incident solar energy and re-emitting a minimum of infrared radiation. Maximum energy absorption is defined as the surface thin layer absorbing at least 75% of the solar radiation. Minimal infrared re-emission is defined as the surface thin layer's emissivity being less than 25%. Such a layer must be a conformal, single-phase (Fe,Cr)₂O₃ oxide layer with a thickness ≤150 nm. The thicker the oxide layer, the better the absorption in the solar radiation range, but the higher the emissivity of the selective treatment will be in the infrared range. Furthermore, the layer must exhibit good thermal stability.The stability of the oxide layer is also dependent on its thickness; the greater the thickness, the more unfavorable it is.

[0007] In the article by Greef et al. ("The oxidation of industrial FeCrMo steel", Corrosion Science 42 (2000) 1725-1740), the influence of various parameters, particularly heat treatment, on the formation of the oxidation layer is investigated. Oxidation is carried out on an industrial FeCrMo steel at a temperature of 157°C to 758°C, under controlled partial pressure of dioxygen. At temperatures below 400°C, an oxide layer consisting mainly of Fe₂O₃ and a small amount of Cr₂O₃ forms. From 400°C to 600°C, an oxide layer is obtained comprising a mixture of FeO, Fe₂O₃, and Cr₂O₃. At temperatures above 600°C, a Cr₂O₃ layer is formed.

[0008] Various surface preparations of the substrate can be carried out before the oxidation step.

[0009] For example, obtaining a selective optical coating to form a solar absorber by polishing and then thermally oxidizing stainless steel has been studied since the 1980s. The article by VC Sharma et al. ("A comparison of thermal performance of austenitic stainless steel solar absorber plates colored by chemical and thermal oxidation techniques", Energy Vol. 6, pp. 133-138, 1981) describes a chromium-rich steel (18 / 8 Cr / Ni), polished and then subjected to a heat treatment at 770°C for 30 minutes. The best performance obtained was a solar absorbance of 84% and an emittance of 22%.

[0010] B. Karlsson et al. ("Optical constants and spectral selectivity of stainless steel and its oxides", J. Appl. Phys. 53(9), 6340-6346, 1982) studied the optical properties of various stainless steels, previously polished and then subjected to heat treatment. The oxide layer obtained is a mixed oxide layer of Cr2O3 and Fe2O3.

[0011] In document WO-A-2015 / 087021, the solar absorber is obtained from a steel substrate with a chromium content of between 6% and 12.5% ​​by mass. Thermal oxidation, at a temperature ranging from 400°C to 900°C, is carried out on the substrate after it has been previously polished or stretched. The polishing step allows for the formation of substrates with low roughness while limiting the formation of micro-deformations within the substrate. The resulting layer is an iron and chromium oxide, with a thickness between 10 nm and 1000 nm, and preferably between 20 nm and 500 nm.

[0012] In document EP-A-2784172, a selective solar absorber is obtained by oxidation, between 500°C and 1150°C, of ​​a steel with a chromium content between 7.5% and 14% by mass. The substrate can be cold-rolled or cold-drawn before the oxidation step. The oxide formed contains Cr₂O₃ and spinel-type oxides. Lower temperature conditions are also proposed, but it is specified that in this case, the formation of Fe₂O₃ oxide is favored at the expense of Cr₂O₃. However, the oxide obtained is different from the desired one, and therefore the optical and / or thermal stability performance is also different.

[0013] The steel substrate can also be prepared using a Surface Mechanical Attrition Treatment (SMAT) process. This process involves projecting beads with extremely high mechanical energy onto the substrate surface to disrupt its structure to a depth of several hundred microns. During the SMAT process, the substrate temperature remains relatively close to the operating temperature. Heat treatment under air on a substrate prepared in this way leads to the formation of a chromium-rich oxide layer. The presence of numerous defects in the substrate structure promotes chromium migration during the oxidation heat treatment. However, the resulting layer exhibits a nanocrystalline structure due to the high level of mechanical deformation introduced into the substrate's structure, and significant structural variation is observed.The oxide layers formed are very specific to the SMAT process. We generally observe the appearance first of a bilayer (Fe 3 O 4 / FeCr 2 O 4 ) which then transforms during the heat treatment time into a trilayer Fe 3 O 4 / FeCr 2 O 4 / (Fe,Cr) 2 O 3 with a total thickness greater than one micron, as described in the article by Xia et al. ("Improve oxidation resistance at high temperature by nanocrystalline surface layer", Scientific Reports, 5:13027, DOI: 10.1038 / srep13027, 2015).

[0014] The oxidation processes classically implemented in the prior art generally lead to thicker oxide layers (thickness greater than 150nm), in Cr2O3 and / or polyphase.

[0015] However, a layer of Cr₂O₃, a layer of (Fe,Cr)₃O₄, or a polyphase oxide (e.g., two-phase or three-phase) containing a rhombohedral (Fe,Cr)₂O₃ phase does not provide all the desired properties. Only a single-phase layer of (Fe,Cr)₂O₃ can achieve such properties.

[0016] None of these documents describes a process for obtaining a single-phase rhombohedral (Fe, Cr) 2 O 3 oxide layer, with a thickness less than or equal to 150nm, adhering to a steel substrate. DESCRIPTION OF THE INVENTION

[0017] It is, therefore, an object of the present invention to propose a process for forming, on the surface of a steel or superalloy substrate, an oxide layer with both a controlled crystallographic structure (a single-phase oxide of (Fe, Cr) 2 O 3 ) and a controlled thickness (70-150nm).

[0018] This goal is achieved by a process for forming a rhombohedral oxide (Fe, Cr) 2 O 3 layer on a steel or superalloy substrate comprising the following successive steps: a) provision of a steel or superalloy substrate covered by a surface layer, the steel comprising at least 2% by weight of chromium, b) removal of the surface layer until the steel or superalloy substrate is reached, by a subtraction step, advantageously a mechanical subtraction step, under an atmosphere containing at least 0.2 atm of dioxygen, the subtraction step generating a level of micro-deformations in the crystal lattice of the steel or superalloy greater than 1.0 x 10⁻³ and local heating with a kinetic rate greater than 400 °C / s, so as to form a rhombohedral single-phase (Fe, Cr)₂O₃ oxide layer, c) carrying out a heat treatment, under air, at a partial water pressure less than 10,000 ppm, and at a temperature ranging from 400 °C to 1000 °C, so as to grow the single-phase oxide layer rhombohedral formed in step b) up to a thickness ranging from 70nm to 150nm.

[0019] The invention differs fundamentally from the prior art in the specific surface preparation step of the substrate, which leads to the formation of a rhombohedral single-phase oxide layer (Fe, Cr)₂O₃, adhering to the steel or superalloy substrate. Heat treatment allows the oxide layer to grow. The resulting layer is thin (less than or equal to 150 nm). This oxide layer imparts to the substrate optical properties of interest for solar thermal applications, with high solar absorbance (typically greater than 75%) and low emittance (typically less than 20% at 100°C), good resistance to oxidation and corrosion, and good temperature stability up to 600°C in air, for concentrating solar thermal applications, particularly for solar thermal absorber applications.

[0020] The subtraction step is a material removal process that exposes the steel or superalloy by removing the surface layer. This surface layer may contain, for example, contaminants, oxides, and / or carbides. This material removal step simultaneously generates: deformations on the surface of the steel or superalloy substrate and therefore leads to the creation of significant mechanical stresses in the steel or superalloy, by friction, tearing, and / or shearing; a strong temperature gradient of several hundred degrees per second is generated at the material / subtraction means interface.

[0021] This step is carried out under an oxidizing atmosphere (i.e., containing at least 0.2 atm of dioxygen), preferably air, which allows for the instantaneous creation of a thin oxide layer. By atmosphere, we mean a gaseous atmosphere, for example, ambient air, oxygen-enriched air, or air enriched with water vapor. The oxidizing atmosphere contains at least 5% by volume of an oxygen precursor, such as O₂, H₂O, or O₃.

[0022] The combined effect of surface deformation and temperature heating at the surface of the substrate during the removal of the surface layer, under an oxidizing atmosphere, leads to the formation of a single-phase mixed iron and chromium oxide layer (Fe,Cr) 2 O 3 with a rhombohedral structure.

[0023] The oxide is single-phase, meaning it comprises at least 90% by mass of the rhombohedral phase, preferably at least 95%, and even more preferably 100% by mass of the rhombohedral phase. The oxide does not contain other oxide layers, such as the spinel form (Fe,Cr), or Fe-rich forms of the type Fe x O y found in prior art processes such as polishing, SMAT, etc.

[0024] By "it comprises 100% by mass of rhombohedral phase," we mean that the oxide layer contains either only the rhombohedral phase or the rhombohedral phase and a negligible amount of one or more other phases. An amount is said to be negligible if it cannot be detected, for example, if it cannot be detected by X-ray diffraction.

[0025] Optical measurements clearly establish different and systematically more selective properties compared to oxides formed according to the prior art.

[0026] Advantageously, the subtraction step is followed by rapid cooling (typically with a cooling gradient greater than or equal to 100°C / s over the 900°C / 400°C range) to subject the formed oxide to compressive stress. This improves the mechanical strength of the oxide layer and limits the possibility of element migration within the oxide.

[0027] Step c) allows the oxide to grow to a thickness of 70nm to 150nm. The atmosphere used for the heat treatment of growth will be a dry air atmosphere or one containing a partial pressure of H2O between 1 ppm and 10,000 ppm.

[0028] The oxide layer formed is thin (thickness less than or equal to 150nm), dense, continuous, and conforms well. It exhibits good mechanical adhesion to the substrate because the oxide formed is under compression.

[0029] With such a process, the chemical nature and / or mechanical properties of the substrate are little modified.

[0030] Advantageously, the subtraction step of step b) is carried out at a temperature ranging from -10 °C to 400 °C.

[0031] Advantageously, the subtraction step of step b) is carried out at a temperature ranging from -10 °C to 100 °C.

[0032] Advantageously, the water content at step b) is less than 20000 ppm, preferably less than 500 ppm and even more preferably less than 3 ppm.

[0033] Advantageously, the level of micro-deformations generated in the crystal lattice of the steel or superalloy at step b) is greater than 1.5.10 -3< , and preferably greater than 3.10 -3< .

[0034] Advantageously, the local heating kinetics during the subtraction step are between 400°C / s and 900°C / s, and more advantageously between 400°C and 800°C.

[0035] Advantageously, the heat treatment is carried out for a duration ranging from 5 seconds to 2 hours, and preferably from 1 minute to 60 minutes.

[0036] The duration and temperature of the heat treatment will be chosen by a person skilled in the art according to the geometry of the substrate.

[0037] Advantageously, the partial pressure of water, during step c), is less than 600 ppm, and preferably less than 500 ppm.

[0038] Advantageously, the thickness of the rhombohedral oxide layer obtained in step c) ranges from 70 to 150nm.

[0039] Advantageously, the percentage of chromium in the steel ranges from 2 to 25% by weight, and preferably from 5 to 16% by weight.

[0040] The invention also relates to a method for manufacturing a solar absorber comprising the following successive steps: formation of a single-phase (Fe, Cr) 2 O 3 oxide layer with a rhombohedral structure on a steel or superalloy substrate, according to steps a) to c) as defined in the process for forming a single-phase (Fe, Cr) 2 O 3 oxide layer with a rhombohedral structure on a steel or superalloy substrate according to the invention, application of an anti-reflective layer on the oxide layer.

[0041] The invention also relates to a solar absorber obtained by the process described above, comprising a steel or superalloy substrate covered successively by a rhombohedral single-phase (Fe, Cr) 2 O 3 oxide layer, having a thickness ranging from 70nm to 150nm, and by an anti-reflective layer.

[0042] Advantageously, the oxide layer has a thickness ranging from 80nm to 120nm. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The present invention will be better understood on the basis of the following description and the accompanying drawings, in which: THE figures 1A, 1B, 2 and 3 represent different stages of the process of forming a rhombohedral (Fe, Cr) 2 O 3 oxide layer, according to an embodiment of the invention, Figures 4A and 4B are X-ray diffractograms obtained, respectively, on the axis and on the circumference of a steel tube on which a material removal step has been carried out, according to a particular embodiment of the invention, the figure 5 represents, schematically and in three dimensions, a substrate covered by an intrinsically selective oxide layer, obtained by the process of the invention, and by an anti-reflective layer, according to a particular embodiment, the figure 6is a graphical representation of the spectral performance of a selective solar absorber; in particular, curve B is a graphical representation of the absorbance or emittance (left vertical axis, in arbitrary units) and irradiance (right vertical axis, in arbitrary units) as a function of wavelength (horizontal axis in µm) of a selective solar absorber; curve A represents the solar spectrum, and curve C the radiation of a blackbody at 450 °C. figure 7 is a graphical representation of the spectral performance of a polished and oxidized T91-type absorber, according to the process described in document [4] cited at the end of the description. In particular, curve D represents the reflectance of said absorber (vertical axis, in arbitrary units) as a function of wavelength. Curves A and C are the same as those of the figure 6 , there figure 8is a graphical representation of the spectral performance of a T91 type absorber polished and oxidized by the process according to the present invention; in particular, curve E represents the reflectance of said absorber (vertical axis, in arbitrary units) as a function of wavelength; curves A and C are the same as those of the figure 6 , there figure 9 represents diffractograms obtained on a sample prepared according to the process of the present invention, before exposure (diffractogram "R"), after 3000 hours of exposure at 500 °C and in air, respectively, in part I (diffractogram "I"), and in part II (diffractogram "II"), figures 10a and 10b are graphical representations of the evolution, respectively, of solar absorbance and emittance at 450 °C (vertical axis in percentage) as a function of exposure time (horizontal axis in hours) of an oxide substrate according to the process of the present invention.

[0044] The different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible.

[0045] The different possibilities (variants and modes of implementation) should be understood as not being mutually exclusive and being able to be combined with each other. DETAILED DESCRIPTION OF SPECIFIC METHODS OF IMPLEMENTATION

[0046] The process for forming a rhombohedral single-phase (Fe, Cr)2O3 oxide layer on a steel substrate 10 comprises the following successive steps: a) supply of a substrate 10 of steel or superalloy covered by a surface layer 20, the steel comprising at least 2% by weight of chromium ( Figures 1A and 1B), b) removal of the surface layer 20 until the steel or superalloy substrate 10 is reached, by a subtraction step, under an atmosphere containing at least 0.2 atm of dioxygen, the subtraction step generating a level of micro-deformations in the steel crystal lattice greater than 1.0 x 10⁻³ and local heating with kinetics greater than 400°C / s, so as to form a rhombohedral oxide layer 30 (Fe, Cr)₂O₃ ( figures 2 and 3 ), c) carrying out a heat treatment, under air, at a partial water pressure of less than 10,000 ppm, and at a temperature ranging from 400 °C to 1000 °C, so as to grow the rhombohedral oxide layer formed in step b) to a thickness ranging from 70 nm to 150 nm. Substrate 10:

[0047] The substrate 10 provided in step a) is a steel or super alloy substrate.

[0048] The steel substrate 10 comprises at least 2% by weight of chromium.

[0049] A person skilled in the art can, for example, choose a low-alloy, low-alloy, or high-alloy steel. Preferably, the steel or superalloy contains 2% to 25% chromium by weight, and preferably 2% to 20% chromium by weight, and even more preferably 5% to 16% by weight.

[0050] The steels can be steels commonly used in the energy sector, such as steels conforming to European standard EN10216, American standard ASTM A213, ASME construction code or others, containing more than 2% chromium such as 10CrMo9-10 (T22) (1.7380), 11CrMo9-10 (T22) (1.7383), 20CrMoV13-5-5 (1.7779), 7CrWVMoNb9-6 (T23) (1.8201), 7CrMoVTiB10-10 (T24) (1.7378), X11CrMo5 (T5) (1.7362), X11CrMo9-1 (T9) (1.7386), X10CrMoVNb9-1 (T91) (1.4903), X10CrWMoVNb9-2 (T92) (1.4901),

[0051] The steels used can also be Cr-Ni stainless steels chosen from the standards EN10088, AISI, X7Cr13 (1.4003), X10Cr13 (1.4006), X12CrS13 (1.4005), X20Cr13 (1.4021), X30Cr13 (1.4028).

[0052] A nickel-based superalloy and / or a cobalt-based superalloy are advantageous choices. The superalloy could be, for example, an Inconel® type superalloy marketed by Special Metals Corporation or an ASTM-A-494 (625) superalloy.

[0053] The substrate 10 can be of many geometric shapes. For example, it can be flat, concave or convex, tubular, etc. Any shape from which the surface layer 20 can be removed by subtraction of material may be chosen by a person skilled in the art.

[0054] The part may include a through hole or a blind hole, a shoulder, a groove, a channel, or even present several elementary surfaces by the association of a sphere and a cylinder, a plane and / or a cone.

[0055] As depicted on the Figures 1A and 1B The substrate 10 is covered by a surface layer 20. By surface layer 20, we mean that the substrate 10 is covered by a layer that may be partially or totally oxidized and / or carburized / decarburized and / or may contain pollutants. Furthermore, according to the present invention, the surface layer 20 may be of the same chemical nature as the volume of the substrate 10.

[0056] The surface layer 20 covering the substrate 10 provided in step a) does not require a particular surface condition and / or a particular preparation step. A standard clean condition is sufficient; similarly, with regard to oxidation, a standard condition free from pitting is sufficient.

[0057] The surface of the surface layer 20 can be rough or smooth. The substrate 10 can be directly from a casting, hot-rolled or cold-rolled, drawn, hot-forged or cold-forged, or have undergone a prior surface preparation step, for example, chemical or mechanical polishing. These prior shaping or preparation methods are not exhaustive.

[0058] From a general point of view, the surface layer 20 can be in any state since this layer will be removed during step b). Subtraction of the thin surface layer of the substrate 10:

[0059] In step b), the surface layer 20 covering the substrate 10 is removed ( figure 2 ).

[0060] Part of the substrate 10 can also be removed, in addition to the surface layer 20 (for example, by removing the surface layer and the outer part of the substrate, represented by the dotted lines on the figure 1B ).

[0061] The subtraction step allows the removal of pollutants, oxides and / or carbides that may cover the substrate 10.

[0062] The amount of material to be removed to reach the native material of substrate 10 will depend on the surface condition of substrate 10. It will preferably range from 0.05mm to 3mm, and even more preferably from 0.1mm to 1mm.

[0063] The thickness of material removed is advantageously identical or substantially identical at every point of the substrate 10.

[0064] Material removal can be mechanical and achieved through friction, shearing, tearing, etc.

[0065] A person skilled in the art will choose a material removal process that allows both: i. remove the surface layer 20 until the native material of the substrate 10 is reached; ii. achieve a level of microstrain (Lorentz approximation) greater than 1.0 x 10⁻³, preferably greater than 1.5 x 10⁻³, and even more preferably greater than 3.0 x 10⁻³; the level of microstrain can, for example, reach values ​​of 6.6 x 10⁻³ or even higher, depending on the chosen removal process and / or the yield strength of the steel or superalloy. The level of microstrain allows the creation of a dislocation network sufficient for the rapid diffusion of the chromium element towards the surface of the substrate 10; iii.to achieve on the surface of the substrate 10 a minimum temperature, for example, of 400°C and preferably of 500°C to 900°C, the temperature being reached instantaneously or at least almost instantaneously by local heating according to a heating kinetics greater than or equal to 400°C / s, and preferably of 400°C / s to 900°C / s. .

[0066] By "local heating" we mean heating at the very spot of the shrinkage of the surface layer, and almost instantaneously at said shrinkage.

[0067] Conditions i), ii), iii) are cumulative to obtain the desired single-phase oxide layer 30 ( figure 3 ).

[0068] Regarding point ii), depending on the composition of substrate 10, the chromium element will be more or less present in the matrix, and the required dislocation network density will therefore vary. A person skilled in the art will choose the dislocation level based on the mass percentage of chromium in substrate 10.

[0069] The level of micro-deformation can be determined by X-ray diffraction (XRD).

[0070] THE Figures 4A and 4B represent the diffractograms (Co-Kα) of a T91 steel tube (X10CrMoVNb9-1) prepared by a process according to an embodiment of the invention, at the axis of the tube and the circumference of the tube, respectively.

[0071] The substrate 10 obtained after step b) is in biaxial tension. It exhibits an axial stress σ11 = 270 MPa, a circumferential stress σ22 = 320 MPa, and an average microstrain (Lorentz approximation): <ε> = 3.1 × 10⁻³. A broadening of the peak is observed, linked to the formation of microstrains on the surface of substrate 10. The microstrains were measured over depths ranging from 2 to 17 µm, perpendicular to the surface of substrate 10.

[0072] Regarding point iii), the chosen subtraction process, performed in air, delivers a high local deformation force to the surface, which is converted into heat and causes significant temperature variations. These variations are both temporal (very rapid heating under the subtraction tool and very rapid cooling after the tool's passage) and spatial (hot surface on a cold substrate). This rapid transfer of the surface temperature to a high level allows for oxide nucleation. The cooling temperature gradient will be greater than or equal to 100°C / s over the 900°C / 400°C range.

[0073] The process is carried out in a dry atmosphere.

[0074] The temperature at the substrate / subtraction medium interface can be regulated, if necessary, using dry air with a low partial pressure of H₂O below 20,000 ppm, preferably below 500 ppm, and even more preferably below 3 ppm. It has been observed that the use of an aqueous-based coolant will have a negative effect on oxide formation.

[0075] Preferably, the subtraction step is carried out at a temperature below 400°C.

[0076] The material removal step is carried out under an oxidizing atmosphere (in air or any other atmosphere containing at least 0.2 atm of dioxygen) to instantly create on the surface of the substrate 10 a thin, rhombohedral layer of a Cr-rich oxide (Cr x Fe 1-x ) 2 O 3 (with x between 0 and 0.2; for example x = 0.1) of nanometer thickness. This oxide is also denoted (Cr, Fe) 2 O 3 .

[0077] The oxide formed is under compression. Indeed, the oxide has a larger volume than the substrate 10 from which it forms. Its lateral expansion being constrained by the substrate 10, it finds itself under compression. This contributes to the protective nature of the layer by reducing the diffusion coefficients within it.

[0078] The surface of substrate 10, having been subjected to the material removal step meeting criteria i, ii) and iii), under a dry oxidizing atmosphere, is in a state of residual tensile stress. Schematically, this corresponds to the formation of a hot "skin," formed during material removal, contracting upon contact with a substrate 10 that remained cold at its core.

[0079] Conversely, surfaces subjected to a polishing step exhibit residual compressive stresses and do not allow the formation of the desired oxide. Similarly, hot or cold rolling, grinding, electropolishing, or electrical discharge machining (EDM), or even a SMAT process, does not allow for the simultaneous achievement of the desired level of microdeformation and heating. The oxide obtained with such processes will not be a mixed iron-chromium oxide and / or will not be a monophasic oxide and / or will not have the desired final thickness. Therefore, it will not possess the desired optical and / or thermal stability properties.

[0080] The material removal step may be chosen by a person skilled in the art, for example, from grinding, drilling, milling, turning or any other means enabling the simultaneous attainment of the three conditions i, ii), iii).

[0081] For example, depending on the application, a skilled technician might start with a flat X10CrMoVNb9-1 steel substrate in any condition. Using an electric or pneumatic angle grinder, they would select a disc, preferably a continuous disc compatible with the substrate, for example, a corundum or diamond base. The disc's rotation speed, contact pressure, and the applied longitudinal and transverse movements would be chosen to meet criteria i, ii), and iii). The application may be such that continuity is ensured across the substrate surface. Furthermore, to meet the oxidation requirements, the process would be carried out in a controlled atmosphere that meets the previously defined requirements. Applying any tool movement in any atmosphere would not ultimately produce a thin, continuous, and conforming oxide layer.

[0082] By way of non-limiting example, the subtraction step, complying with criteria i), ii) and iii), can be implemented on a steel substrate of the typeX 10 CrMoVNb 9-1, particularly in the form of a flat piece of steel. The initial condition of the steel substrate is of little importance. In other words, the steel substrate can be as-fired, hot-worked, cold-worked, or have undergone at least some preparation. The subtraction step then involves applying contact pressure to the steel substrate using an electric or pneumatic angle grinder (for example, a portable power grinder). The choice of the grinder disc is within the capabilities of a person skilled in the art. The chosen disc is preferably continuous (in other words, without notches) and compatible with the nature of the steel substrate. In particular, the disc can be diamond-based or, preferably, corundum-based.

[0083] The maximum peripheral speed of the disk can be determined based on the diameter, material, and rotational speed of said disk. For example, a 125mm diameter disk of type AS 30 S INOX BF may have a maximum peripheral speed of 80 ms-1.

[0084] During the subtraction stage, the pressure and contact angle of the disc relative to the surface of the steel substrate are adapted so as to meet safety requirements and comply with best practices for the general use of this equipment.

[0085] Finally, to meet the oxidation conditions, the implementation can be carried out under a controlled atmosphere meeting the requirements defined previously (for example, dry air with a low partial pressure of H2O less than 20,000 ppm, preferably less than 500 ppm and even more preferably less than 3 ppm). Growth of the oxide layer 30 (Cr, Fe) 2 O3:

[0086] In step c), a heat treatment is carried out to grow the oxide layer 30 previously formed in step b). The heat treatment temperature ranges from 400°C to 1000°C, and preferably from 400°C to 850°C. The temperature will be chosen by a person skilled in the art based on the composition of the substrate 10. The duration of the heat treatment depends on the geometry of the substrate 10 and the thickness of the oxide layer 30 formed in step b). The duration ranges, for example, from 5 seconds to 2 hours, and preferably from 1 minute to 60 minutes.

[0087] The heat treatment is carried out under an atmosphere of dry air or containing a partial pressure of H2O less than 10,000 ppm, for example between 1 ppm and 10,000 ppm, and preferably less than 600 ppm, for example between 1 ppm and 600 ppm, even more preferably less than 500 ppm, for example between 1 ppm and 500 ppm.

[0088] At the end of step c), the rhombohedral mixed iron and chromium oxide layer 30 of the type (Fe,Cr) 2 O 3 has a thickness ranging from 70nm to 150nm, and preferably from 80nm to 120nm, for example on the order of 100 nm.

[0089] It was observed that the thickness of the single-phase oxide formed at the end of step c) tended towards a maximum of approximately 150 nm, regardless of the duration and temperature of the oxidation treatment. In contrast, the non-single-phase oxide layers could have thicknesses exceeding 400 nm.

[0090] Step c) allows the mechanical stresses of the material to be released.

[0091] The substrate 10 obtained by the previously described process is covered by a stable oxide layer 30. There are no intercalated layers between the steel substrate 10 and the oxide layer 30. The oxide layer 30 exhibits intrinsic optical properties of selectivity, i.e., high solar absorbance and low emittance.

[0092] The substrate 10 obtained by the process described above can, for example, be used to make a solar absorber whose selectivity properties are improved compared to solar absorbers obtained by known prior art techniques.

[0093] In particular and for comparison purposes, the inventors determined the properties of an absorber obtained according to a process known from the prior art and of an absorber obtained by the process according to the present invention.

[0094] In both cases, the aim was to oxidize the surface of a substrate made of T91 type steel (X10CrMoVNb9-1).

[0095] In this regard, the figure 7 reproduces a hemispherical reflectance curve of a T91 (X10CrMoVNb9-1) substrate (called "known sample") polished and oxidized 600°C 1h prepared according to the process described in document [4] cited at the end of the description.

[0096] There figure 8 , for its part, represents the hemispherical reflectance curve of a T91 (X10CrMoVNb9-1) substrate (called "inventive sample") prepared according to the invention and oxidized under the same conditions.

[0097] The reflectance curves for the known sample and the inventive sample allow the calculation of the solar absorbance (α) and emittance (ε) values ​​at 100°C, 300°C, and 450°C for each of these two samples. The method for these calculations is presented in the "Appendix" section of the present invention.

[0098] Thus, Table 1 sets together the absorbance and emittance properties of the known sample, and Table 2 those of the inventive sample. Table 1 α ε (100°C) ε (300°C) ε (450°C) 73,17 6,47 9,50 11,91 Table 2 α ε ( 100°C) ε ( 300°C) ε ( 450°C) 77,70 2,30 3,42 6,89

[0099] Comparing the values ​​in the two tables, we find that for the same type of sample, here T91 (X10CrMoVNb9-1), the process of the invention makes it possible to obtain a tube with a higher level of solar absorbance, 77.70% instead of 73.17%, while having a lower level of emittance, 6.89% at 450°C compared to 11.91%.

[0100] The selectivity of the absorber can be improved by adding one or more anti-reflective layers 40 deposited a posteriori on the oxide layer 30 ( figure 5), or even absorbing layers. The thickness and nature of the layer(s) will depend on the desired antireflective properties and can be easily calculated using an optical simulation tool. For example, the SCOUT optical simulation program (downloadable from the website: www.mtheiss.com) is used by research teams working on the multilayer deposition of selective solar absorbers.

[0101] The solar absorber manufactured according to the process of the invention is rather oriented for medium to high temperatures (concentrating thermodynamic solar).

[0102] The substrate 10 can also be used to make the sensitive part of a flux sensor, or a temperature sensor under irradiation.

[0103] Substrate 10 can also be used for applications in air temperature (below the operating temperature of substrate 10), or for applications requiring enhanced protection against hot oxidation.

[0104] For example, the service temperature of a T91 steel (X10CrMoVNb9-1) is approximately 650°C. Illustrative and non-limiting examples of one embodiment:

[0105] The process was carried out on a steel substrate 10 containing approximately 9.5% Cr (T91 (X10CrMoVNb9-1)). Specifically, the substrate 10 was oxidized at 650°C for one hour under dry air to form the protective layer 30 on its surface. This layer 30 is a 90 nm thick (Fe, Cr)2O3 layer. An antireflective layer of SiO2 was also deposited on a portion of the substrate 10 (referred to as "part I"), leaving another portion of the sample (referred to as "part II") without the antireflective layer.

[0106] The single-phase nature of the oxide layer was verified by X-ray diffraction. The equipment used was a Bruker diffractometer (model: D8 Advance) equipped with a Cu source and a LynxEye linear detector. The X-ray tube's supply voltage and current were 40 kV and 40 mA, respectively. The beam width was 12 mm, and its divergence was 0.45°. Under these conditions, the only phase detected in the oxide was rhombohedral (Fe,Cr)₂O₃, confirming the single-phase nature of the oxide layer.

[0107] A substrate 10 and its protective oxide layer 30 obtained by the process of the invention were tested under operating conditions (500°C in air), and in particular were subjected to 3000 hours of exposure at 500°C in air. In this regard, the figure 9represents the diffractograms obtained on the sample before exposure (diffractogram "R"), after 3000 hours of exposure at 500 °C and in air, respectively, in part I (diffractogram "I"), and in part I (diffractogram "II").

[0108] On each of these three diffractograms, the arrows indicate lines representative of the (Fe, Cr)₂O₃ oxide layer. No significant changes in these lines are detected, thus confirming the excellent stability of the oxide layer. These results therefore confirm that substrate 10 is protected from oxidation.

[0109] The inventors also measured the variation in optical properties, including solar absorbance and emittance at 450 °C, during 3000 hours of aging at 500 °C in air.

[0110] The results obtained are compiled at figures 10a and 10bwhich respectively represent the evolution of solar absorbance and emittance at 450 °C (vertical axis in percent) as a function of exposure time (horizontal axis in hours). These two quantities remain relatively constant during exposure, thus demonstrating excellent stability of the optical properties of the samples tested.

[0111] The stability of the optical properties and of the oxide layer measured by X-ray Diffraction shows the excellent resistance to oxidation in air at 500°C of the samples compared to the samples in documents [5] and [6] cited at the end of the description. APPENDIX

[0112] Absorbance refers to the coefficient by which a body absorbs electromagnetic waves. In this case, solar absorbance corresponds to the absorption coefficient of the solar spectrum by the material, specifically the oxide layer 30. It is calculated from the reflectance spectrum measured by a spectrophotometer and the solar spectrum.

[0113] By "emitterance" we mean the percentage of radiative flux emitted in all directions by a material, in particular the oxide layer 30, at a given temperature T relative to the radiative flux in all directions of the black body at that same temperature (it is understood that a black body has an emittance of 100%).

[0114] Reflectance, or total reflectance, refers to the proportion of light reflected by a surface relative to the incident light flux. Reflectance is considered total when measured in all directions using an integrating sphere. The reflectance spectrum shows reflectance as a function of wavelength.

[0115] Measuring the total reflectance spectrum allows us to calculate the solar absorbance α and the emittance εT at a temperature T. We assume here that Kirchhoff's law is applicable since the material is opaque (those skilled in the art may refer to documents [1] and [2] cited at the end of the description), so that ε ( λ;θ,φ ) = α ( λ;θ , φ ) (1).

[0116] If the total measurement (over the entire space) is for an opaque material without nonlinear effects (which is generally assumed for solar absorbers), the total emittance and total reflectance are related by the following relationship: ε T ( λ ) = 1 - ρ T ( λ ) (2).

[0117] Thus, solar absorptance and emittance at temperature T are defined by the following relationships: α = 1 − ∫ λ = 280 nm 2500 nm ρ T λ × S λ dλ ∫ λ = 280 nm 2500 nm S λ dλ ε T = 1 − ∫ λ = 280 nm 16 000 nm ρ T λ × B T λ dλ ∫ λ = 280 nm 16 000 nm B T λ dλ With : α: total solar absorptance ε(T): emittance at temperature T ρ T (λ): total reflectance at wavelength λ BT (λ): blackbody irradiance at temperature T and wavelength λ S(λ): solar irradiance at wavelength λ λ: wavelength

[0118] The measurement procedure is described in J. Braillon's article et al. [3] cited at the end of the description.

[0119] A solar absorber is said to be selective if it exhibits high solar absorbance and low emittance at its operating temperature. The ideal curve for such a material is given in the article by C. Kennedy [1] cited at the end of the description, and reproduced in the figure 6 . REFERENCES

[0120] [1] C. Kennedy, "Review of mid- to high- temperature solar selective absorber materials," Tech. Rep. NREL / TP-520-31267, NREL, National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393, 2002; [2] J. Palmer, Handbook of Optics, second ed., Part II,, ch. 25, The Measurement of Transmission, Absorption, Emission and Reflection. M. Bass, 1994; [3] J. Braillon et al., Development of optical tool for the characterization of selective solar absorber tubes, AIP Conférence Proceedings 1734, 130004 (2016); [4] FR2976349 ; [5] O. Raccurt et al., "Study of the stability of a selective solar absorber coating under air and high temperature conditions", Energy Procedia, Vol. 69, 2015, p 1551-1557; [6] Raccurt, O. et al., "In air durability study of solar selective coating for parabolic trough technology", (2017), AIP Conférence Proceedings 1850(1), 130010.

Claims

1. Method for forming a layer of single-phase oxide (30) (Fe, Cr)2O3 with a rhombohedral structure on a steel or super alloy substrate (10) comprising the following successive steps: a) supply of a steel or super alloy substrate (10) covered by a surface layer (20), the steel comprising at least 2% by weight of chromium, b) removal of the surface layer (20) to reach the substrate (10), using a subtraction step, advantageously a mechanical subtraction step, under an atmosphere containing at least 0.2 atm of dioxygen, the subtraction step generating a level of microstrains in the steel or super alloy crystalline lattice larger than 1.0 × 10-3 and local heating at a rate of more than 400°C / s, so as to form a single-phase rhombohedral oxide (Fe, Cr)2O3 layer (30), c) performance of a heat treatment under air at a partial water pressure of less than 10,000 ppm, and at a temperature varying from 400°C to 1000°C, so as to make the single-phase rhombohedral oxide layer (30) formed in step b) grow to a thickness from 70nm to 150nm.

2. Method according to claim 1, characterised in that the mechanical subtraction step in step b) is done at a temperature from -10°C to 400°C.

3. Method according to the preceding claim, characterised in that the mechanical subtraction step in step b) is done at a temperature from -10°C to 100°C.

4. Method according to any one of the preceding claims, characterised in that the water content during step b) is less than 20.000ppm, preferably less than 500ppm, and even more preferably less than 3ppm.

5. Method according to any one of the preceding claims, characterised in that the level of microstrains generated in the steel or super alloy crystalline lattice in step b) is more than 1.5 × 10-3, and preferably more than 3 × 10-3.

6. Method according to any one of the preceding claims, characterised in that the heating rate is between 400°C / s and 900°C / s.

7. Method according to any one of the preceding claims, characterised in that the heat treatment is applied for a duration from 5 s to 2 h, and preferably from 1 minute to 60 minutes.

8. Method according to any one of the preceding claims, characterised in that the partial water pressure during step c) is less than 600 ppm, and preferably less than 500 ppm.

9. Method according to any one of the preceding claims, characterised in that the thickness of the rhombohedral oxide layer (30) obtained in step c) varies from 70 to 150nm.

10. Method according to any one of the preceding claims, characterised in that the percentage of chromium in the steel varies from 2 to 25% by weight, and preferably from 5 to 16% by weight.

11. Method for fabrication of a solar absorber comprising the following steps in sequence: - formation of a layer (30) of single-phase oxide (Fe, Cr)2O3 with a rhombohedral structure on a steel or super alloy substrate (10), according to steps a) to c) as defined in claim 1), - application of an anti-reflection layer (40) on the single-phase oxide layer (30).

12. Solar absorber obtained using the method as defined in claim 11, comprising a steel or super alloy substrate (10) covered by a layer (30) of single-phase rhombohedral oxide (Fe, Cr)2O3 with a thickness from 70nm to 150nm, and an anti-reflection layer (40), in sequence.

13. Solar absorber according to the preceding claim, characterised in that the thickness of the oxide layer (30) is from 80nm to 120nm.