Coating method

Abstract

The invention relates to a coating method comprising a step of forming a tie coating (200) on a surface of a substrate (100), and a step of forming a barrier coating (300) on the tie coating (200). The tie coating (200) is formed by chemical vapour deposition, with a deposition pressure of between 1 and 5 kPa and a deposition temperature of between 964 and 1135°C, of a precursor comprising silicon, to obtain an average roughness Ra greater than 1 µm.

Description

Description Title of the invention: Coating process Technical Field

[0001] This disclosure relates to the field of coatings and more specifically to coating processes, particularly for protecting ceramic-based substrates from aggressive environments. Previous technique

[0002] Ceramic matrix composites (CMCs), and in particular carbide-based composites (generally referred to as SiC / SiC), have been proposed for numerous applications, notably for the manufacture of gas turbine components such as blades and distributors. Indeed, thanks to their heat resistance properties, these materials make it possible to reduce or even eliminate the cooling typically used in nickel- and / or cobalt-based metallic turbine components, while allowing for higher operating temperatures.

[0003] However, in the corrosive environment of a turbine, SiC / SiC CMCs can undergo oxidation, leading to the formation of silicon oxide, which can then volatilize under the influence of steam. Therefore, for high-temperature applications in oxygen- and steam-rich environments, the application of a protective coating is recommended on ceramic matrix composite parts.

[0004] Due to the particularly aggressive thermomechanical and chemical environment to which the protective coating may be subjected, it should preferably have a coefficient of expansion compatible with that of the substrate, low permeability to corrosive species (which includes both low molecular diffusion directly related to the physical parameter of hermeticity and low ionic diffusion of superoxide and hydroxide ions, intrinsic characteristics of rare-earth silicates), and thermomechanical stability at high temperatures, such as those prevailing in a gas turbine. For this purpose, the protective coating typically comprises a barrier coating or environmental barrier coating (EBC), generally based on rare-earth silicate, and, between the substrate and the barrier coating, a bonding coating, generally silicon-based, to ensure their adhesion. The oxidation of the silicon in the bonding coating can also form an intermediate layer of silica, called thermal growth oxide (TGO), between the bonding coating and the barrier coating.

[0005] Applying protective coatings to substrates with complex shapes, such as turbine blades, can also impose geometric constraints, particularly at thin leading or trailing edges, or cooling ducts or cavities, with very thin coating thicknesses so as not to affect the aerodynamic performance of the blades or their possible cooling.

[0006] Furthermore, although the formation of the intermediate thermal growth oxide layer can help protect silicon carbide-containing substrates from corrosion, it can also have induced thermomechanical effects, such as generating mechanical stresses between the bonding and barrier coatings. This is due to the volumetric increase of silicon as it oxidizes to form silica and the allotropic transformation of silica, which is also associated with a volumetric change. Consequently, beyond a critical thickness of the intermediate thermal growth oxide layer, these mechanical stresses can lead to dome cracking, ultimately resulting in partial or total spalling of the barrier coating and thus the loss of its corrosion protection.

[0007] For these reasons, it is therefore necessary to limit the thickness of the coatings, and in particular the bonding coat. Generally, bonding and barrier coatings are applied by thermal spraying. However, to ensure complete coverage of the substrate, this normally implies thicknesses of at least 75 µm for the bonding coat and at least 100 µm for the barrier. |im for the barrier coating, thicknesses which may be excessive for the reasons mentioned above.

[0008] To provide alternatives, US patent applications 2020 / 0039892 A1 and 2020 / 0039886 A1 proposed coating processes, each comprising a step of forming an adhesion coating on a substrate surface and a step of forming a barrier coating on the adhesion coating. In these processes, the adhesion coating comprises columnar grains of crystalline silicon and is formed by chemical vapor deposition of a silicon-containing precursor. French patent application FR 3 1 14 588 A1 proposed another coating process, also comprising a step of forming an adhesion coating on a substrate surface and a step of forming a barrier coating on the adhesion coating. In this process, the adhesion coating is amorphous and is formed by chemical vapor deposition of a silicon-containing precursor, followed by heat treatment.Although these processes allow for thinner thicknesses than thermal spraying, particularly for the bonding coating, the bonding coating obtained in this way is relatively smooth, which limits the adhesion of the barrier coating. Description of the invention

[0009] This disclosure relates to a coating process comprising a step of forming an adhesion coating on a substrate surface by chemical vapor deposition of a silicon precursor, the adhesion coating comprising columnar grains of crystalline silicon, and a step of forming a barrier coating on the adhesion coating. To obtain a particularly thin adhesion coating with good adhesion to the barrier coating, the adhesion coating formation step can be carried out in a hot-walled reactor with a residence time of the vapor-phase precursor in the reactor between 0.11 and 0.45 seconds, with a deposition pressure between 1 and 5 kPa and a deposition temperature between 964 and 1135 °C, to obtain a The average roughness Ra of the bonding coating is greater than 1 pm. Furthermore, the precursor for the bonding coating formation step may include a low-reactivity precursor, in particular trichlorosilane and / or silicon tetrachloride.

[0010] Thanks to these deposition parameters, it is possible to obtain a specific microstructure of the bonding coating, providing good adhesion of the barrier coating even at very thin thicknesses. Thus, the bonding coating can have a maximum thickness of less than 20 µm, preferably less than 10 µm.

[0011] To protect the substrate from a particularly aggressive environment, the barrier coating may include a rare-earth silicate, especially ytterbium disilicate. To achieve good substrate coverage, even in hard-to-reach cavities, with a thin barrier coating, the coating can be formed by liquid deposition, for example, by dip coating and / or electrophoresis. The barrier coating may have a maximum thickness of less than 40 µm, preferably less than 30 µm.

[0012] The process may include an additional step of forming a protective coating against degradation by calcium-magnesium-aluminosilicates. The substrate may form a turbine component, particularly in a gas turbine engine, and / or comprise a material that is at least partially ceramic, especially a ceramic matrix composite material.

[0013] This disclosure also relates to a coating obtained by the aforementioned process. Brief description of the drawings

[0014] [Fig. 1] Figure 1 is a schematic cross-sectional view of a substrate covered with an adhesion coating and a barrier coating applied according to a coating process according to an embodiment.

[0015] [Fig. 2] Figure 2 schematically represents a gas turbine engine. Description of the implementation methods

[0016] The invention will be well understood and its advantages will become more apparent upon reading the following detailed description of an embodiment represented by way of non-limiting example.

[0017] In a first step of the process according to this embodiment, a surface of a substrate 100 that is at least partially ceramic, which may be, for example, a ceramic matrix composite (CMC), and in particular a SiC / SiC CMC, may first be subjected to chemical vapor deposition to form a bonding coating 200. In this step of forming the bonding coating 200, the precursor used may include, for example, trichlorosilane (HCl3Si) and / or silicon tetrachloride (SiCl4), and the chemical vapor deposition may be carried out at a temperature between 964 and 1135°C and a pressure between 1 and 5 kPa, so as to form a bonding coating 200 with an average thickness t 2Oless than, for example, 20 pm, or even less than 10 pm, but with an average roughness Ra greater than 1 pm. In the context of this disclosure, average roughness Ra is defined as the arithmetic mean of the deviation z of the point thickness of the tack coat 200 from its average thickness t 2O o over the entire surface of the bonding coating 200. At the indicated temperatures and pressures, the deposition rate can be on the order of 1 m / h. The duration of this step can therefore be on the order of 10 hours for the targeted thicknesses. This step can notably be carried out in a hot-walled reactor, to perform the deposition independently of the geometry of the substrate 100. The vapor-phase precursor can circulate in this reactor with a residence time that can be, for example, between 0.11 and 0.45 seconds.

[0018] In a second step of the process according to this embodiment, a barrier coating 300 can be formed on the bonding coating 200 by a known deposition process, such as liquid deposition, for example by dip coating or electrophoresis. This barrier coating 300 may, in particular, comprise a rare-earth silicate, such as, for example, ytterbium disilicate (Yb2Si2O7), as well as other components, such as trivalent iron oxide (Fe2O3), and can have an average thickness t 3O less than 40 pm, or even less than 30 pm. Thus, together, the 200 bonding coating and the 300 barrier coating can have a total thickness of less than 50 pm.

[0019] In a subsequent step, an additional coating (not shown) can be applied to the barrier coating 300 to protect against calcium-magnesium-aluminosilicate (CMAS) degradation. This additional coating may include a rare-earth silicate, and in particular a rare-earth monosilicate such as ytterbium monosilicate (Y₂SiO₅). It can be applied by a known deposition process such as plasma spraying, liquid deposition, or chemical vapor deposition. Its thickness can be, for example, between 5 and 100 µm.

[0020] In a final step, all the superimposed coatings can be subjected to a stabilizing heat treatment.

[0021] The coatings obtained by this process are particularly resistant to aggressive, high-temperature environments and are especially applicable to turbine components exposed to combustion gases, such as the blades and distributors of gas turbine engines. Figure 2 schematically illustrates a gas turbine engine, specifically a turbofan, although the process is also applicable to turbine components of other types of turbomachinery and gas turbine engines, such as turbojets, turboprops, turboshaft engines, or even turbopumps and turbocompressors.In the direction of fluid flow, this gas turbine engine 1 can comprise a blower 2, a low-pressure compressor 3, a high-pressure compressor 4, a combustion chamber 5, a high-pressure turbine 6, a low-pressure turbine 7, and a nozzle 8. The assembly can be surrounded by a nacelle 9. The compressors 3, 4, the combustion chamber 5, and the turbines 6, 7 together form the gas generator 10, which can itself be surrounded by a shroud 11 leading into the nozzle 8. Thus, an air stream 12 from the blower 2 can be defined between the shroud 11 of the gas generator 10 and . an internal wall 13 of the nacelle 9. The high-pressure turbine 6 can be connected to the high-pressure compressor 4 by a first rotating shaft 14 for the drive of the latter, while the low-pressure turbine 7 can be connected to the blower 2 and the low-pressure compressor 3 by a second rotating shaft 15 coaxial to the first rotating shaft 14, in a similar manner.

[0022] In such a gas turbine engine 1, the components of the high-pressure turbine 6 and low-pressure turbine 7, particularly the blades and distributors, are subjected to significant mechanical stresses in the particularly aggressive environment of the combustion gases. They can therefore benefit from the coating process described above.

[0023] Although the present invention has been described with reference to a specific embodiment, it is evident that various modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. Therefore, the description and drawings should be considered illustrative rather than restrictive.

Claims

Demands

1. A coating process comprising the following steps: formation of an adhesion coating (200) comprising columnar grains of crystalline silicon on a surface of a substrate (100), by chemical vapor deposition of a precursor comprising silicon, formation of a barrier coating (300) on the adhesion coating (200), the process being characterized in that the step of forming the adhesion coating (200) is carried out in a hot-walled reactor with a residence time in the reactor of the vapor-phase precursor between 0.11 and 0.45 seconds, a deposition pressure between 1 and 5 kPa and a deposition temperature between 964 and 1135°C, to obtain an average roughness Ra of the adhesion coating (200) greater than 1 pm.

2. A coating method according to claim 1, wherein the precursor for the adhesion coating formation step (200) comprises trichlorosilane and / or silicon tetrachloride.

3. A coating method according to any one of the preceding claims, wherein the barrier coating formation step (300) is carried out by liquid deposition.

4. A coating method according to any one of the preceding claims, wherein the barrier coating (300) comprises a rare earth silicate, in particular ytterbium disilicate.

5. A coating method according to any one of the preceding claims, comprising an additional step of forming a protective coating against degradation by calcium-magnesium-alumino-silicates.

6. A coating method according to any one of the preceding claims, wherein the tack coat (200) has a maximum thickness of less than 20 pm, preferably less than 10 pm.

7. A coating method according to any one of the preceding claims, wherein the barrier coating (300) has a maximum thickness of less than 40 µm, preferably less than 30 µm.

8. A coating method according to any one of the preceding claims, wherein the substrate (100) comprises a material that is at least partially ceramic, in particular a ceramic matrix composite material.

9. A coating method according to any one of the preceding claims, wherein the substrate forms a turbine part (6,7).

10. Coating obtained by the process according to any one of claims 1 to 9.

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